ANSI/IEEE Std 492-1974
(Reaffirmed 1981)
IEEE Guide for
Operation and Maintenance of
Hydro-Generators
Published by The Institute of Electrical and Electronics Engineers, Inc 345 East 47th Street, New York, NY 10017, USA
Febtllary .?8?1974
SH049.29
ANSI/IEEE
Std 492-1974
ReaBtirmed 1981
An American National Standard
IEEE Guide for
Operation and Maintenance of
Hydro-Generators
Sponsor
Rotating Machinery Committee
of the
IEEE Power Engineering Society
Reaffirmed September 17, 1981
IEEE Standards Board
Approved August 27,1982
American National Standards Institute
0 Copyright 1974 by
The Institute of Electrical and Electronics Engineers, Inc
345 East 47th Street, New York, NY 10017, USA
No part of this publication m a y be reproduced in a n y form,
in a n electronic retrieval system OT oiYtmwise,
without prior written permissimL of the publisher.
Foreword
(This foreword is not part o f
IEEE Std 4 9 2 - 1 9 7 4 , Guide for Operation and
Maintenance of Hydro-Generators.)
This document was prepared to set forth operating experience and maintenance practices that
have been used successfully over a period of years by several major operators of hydro-electrical
equipment. Accompanying the operating and maintenance background has been the input of manufacturers of large electrical equipment, resulting in a combination of experience factors and design
philosophy that will provide a user with information t o guide\ him in understanding the limits of
his equipment and methods he may employ to reduce down-time to a minimum.
The development of this guide was begun in early 1969, when it was felt that a document similar
to IEEE Std 67-1972, Guide for Operation and Maintenance of Turbine-Generators (ANSI C50.301972), should be available to operators of hydro-electric equipment. Such a document was considered to be of particular value to organizations normally involved only with turbine-generators,
but who had entered the hydroelectric field to meet the increasing energy or peaking needs. To this
end, the guide contains some sections devoted to pumped-storage applications. It is considered desirable that the document be updated and that the pumped-storage sections be expanded as future
needs dictate. Comments are invited on this guide as well as suggestions for additional material that
should be included. These should be addressed to :
Secretary, IEEE Standards Board
The Institute of Electrical and Electronics Engineers, Inc
345 East 47th Street
New York, NY 10017
The guide was prepared by a Working Group of the Synchronous Machinery Subcommittee of the
IEEE Rotating Machinery Committee. The Working Group personnel were:
R. H. Auerbach, Chairman
R. C. Price
A. C. Seidl
E. C. Whitney
Vaine Aare
R. E. Carter
D. R. Green
When this guide was approved on September 6,1973, the IEEE Standards Board had the following membership:
Robert L). Briskman, Chairman
Stephen J . Angello
Saul Aronow
James E. Beehler
Richard Brereton
Warren H . Cook
Louis Costrell
Jay Forster
Joseph L. Koepfinner
William R. Kruesi
Benjamin J . Leon
Donald T . Michael
Voss A. Moore
J . David M. Phelps
Saul W. Rosenthal
Gustave Shapiro
Sava I. Sherr, Secretary
Ralph M. Showers
Robert A. Soderman
Frederick G. Timmel
Leendert van Rooij
Robert V. Wachter
Bruno 0. Weinschel
William T. Wintrinnharr
Contents
SECTION
PAGE
1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.
..
Caution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 . Manufacturer's and User's Responsibility
............................................
5
5
5
4 . Types of Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
5. BasesofRating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1 General Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . ...........................................................
5.2
Temperature Limits
5.3 Mechanical Stress Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4
TemperatureRise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5 Methods of Temperature Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6
Limitations in the Methods of Measuring Temperatures . . . . . . . . . . . . . . . . . . . . .
6
6
6
7
7
8
9
6.
Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1
Relation of Load and Temperature Rise-Stator and Field . . . . . . . . . . . . . . . . . . . . . .
6.2
Loading Within Rating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3
Loading-Power and Reactive Power Relationship . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4
Loading Outside of Nameplate Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.5 General Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
10
10
10
13
15
7.
Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2
Starting and Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3
Braking and Stopping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.4
Load Rejection and Runaway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.5
Automatic Supervision and Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.6
Unusual or Dangerous Operating Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.7
Cooling Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.8 Fire-Protection Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.9
Vibration Detection and Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.10 OperationofGenerator/Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
15
19
21
22
22
22
25
26
26
26
8.
Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.1 General Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2
Frequency of Inspections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3 Inspection and Maintenance Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.4
Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.5 Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.6
Extensive Downtime-Care of Machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.7
Safety Precautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
27
27
28
34
34
34
35
9. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.1 Verification of Protective Relaying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
35
10. Standard References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
. .
11. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
SECTION
PAGE
FIG UR ES
Fig 1
Fig 2
Fig 3
Fig 4
Fig 5
Fig 6
Heat Flow and Typical Temperature Detector Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Typical Temperature Rise versus Stator and Field Current Squared. . . . . . . . . . . . . . . . . .
Typical Hydro-Generator Saturation Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Typical Hydro-Generator Vee Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Typical Hydro-Generator Capability Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Typical Steady-State Power Angle Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
13
14
15
16
19
IEEE Guide for
Operation and Maintenance of
Hydro-Generators
1. Scope
I t is recommended that, if possible, the situation be discussed with the manufacturer of
the machine.
1.1 This guide covers general recommendations for the operation, loading, and maintenance of hydro-generators and generator/
motors (pump/turbines), having salient pole
rotors and conventional cooling systems (as
distinct from conductor-cooling systems). I t
does not apply to generators having cylindrical rotors. In this guide, the term hydro-generator is used to describe a generator driven by
a hydraulic turbine or water wheel.
3. Manufacturer’s and User’s
Responsibility
3.1 Some discussions in the guide involve ope r a t i o n under conditions not covered by
nameplate or specifications conditions. While
the insulation is capable of safe operation up
to the temperature limits set by the standards,
effects of the operating condition on other
parts of the machine must be studied and
evaluated.
1.2 This guide is not intended to supplant
specific or general instructions contained in
the manufacturer’s instruction book or in any
contractual agreement between the manufacturer and the purchaser of a given machine.
3.2 When a generator is operated within its
nameplate rating, the user normally may expect maximum reliability and life. On occasions, consideration must be given to operation beyond the nameplate rating. I n that
case, all the various factors involved must be
carefully considered in determining the user’s
risks.
1.3 Equipment manufacturers have taken
different approaches to the solution of design
problems, and it is not practical to discuss all
the variations of machine design. However,
many of the operational and maintenance
problems are common to all the designs, and
recommendations can be given concerning solutions to these problems. T h e most that the
guide can accomplish is to provide guideposts,
which, on one hand, disclose accepted and
tried values or methods and, on the other
hand, give warnings where hazards might be
encountered. T h e guide is not intended to apply in any way to the prime mover.
3.3 While a nameplate does not signify more
than specified characteristics, it should be recognized that manufacturers must have some
guide in designing a generator which will indicate probable conditions under which the
generator will operate. With this in mind, the
manufacturer’s desire to stay within nameplate rating should be appreciated.
2. Caution
4. Types of Units
2.1 I t must be recognized that loads more severe than those permitted by the nameplate
should not be applied without a thorough
study of the various considerations pertinent
to the specific condition and also the capabilities of the machine and associated equipment.
4.1 This guide applies primarily to vertical
hydro-generators, although horizontal generators are also discussed in less detail. Nearly
all the material concerning hydro-generators
will apply also to generator/motors such as
those used for pumped-storage systems, and
5
IEEE Std
49 2-1974
IEEE GUIDE FOR OPERATION
one section relates various starting methods
used for generatodmotors. The guide applies
to both open and totally enclosed machines.
M a c h i n e s h a v i n g air-to-water h e a t exchangers are described in this guide as waterair-cooled machines.
Vertical hydro-generators are classified by
the position of the thrust bearing with respect
to the rotor, and the following categories are
covered by this guide:
( 1 ) Suspended-thrust bearing located
above the rotor, and machine has two guide
bearings
(2) Umbrella-thrust bearing located below
the rotor, and one guide bearing is located in
close proximity to the thrust bearing
(3) Modified umbrella-thrust bearing below the rotor. Machine has two guide bearings, one above and one below the rotor
mechanical stress, etc, must be considered as
discussed in the following sections.
5.2 Temperature Limits.
5.2.1 T h e stator and rotor windings, stator
core, stator and rotor leads, and generator
bearings are subject to thermal aging or effects
of differential expansion or both. Therefore,
the maximum temperature and temperature
rise limits for most of these parts have been established.
The stator and rotor winding insulation
system classes are defined in ANSI C50.101965, Section 6, and the corresponding temperature rises are listed in ANSI C50.12-1965,
Section 5, and further discussed in Section 5.4
of this guide.
IEEE Std 1-1969, General Principles for
Temperature Limits in the Rating of Electric
Equipment, discusses the general principles
upon which the insulation system temperature limits are based. The test procedures
used to develop the limits are described in
IEEE Std 99-1970, Guide for the Preparation
of Test Procedures for the Thermal Evaluation of Insulation Systems for Electric
Equipment .
5.2.2 For normal life expectancy, generator
loading should be so controlled that the hotspot temperatures of the machine windings do
not exceed the limiting temperatures of the insulation systems.
In most cases the hotspot temperatures cannot be measured directly and therefore realistic additions must be made to the maximum
observable temperatures in order to estimate
the hotspot temperature and maximum allowable generator loading. The generator manufacturer should be consulted to establish relationship between the observable temperature
and the hotspot temperature, particularly if
special loadings, beyond the nameplate rating, are contemplated.
5. Bases of Rating
5.1 General Considerations.
5.1.1 Hydro-generators are generally rated
on a continuous duty basis with the output
rating expressed in kilovolt-amperes available
a t the terminals a t a specified speed, frequency, voltage, and power factor. For rated output, the service conditions specified forthe design of the machine must be observed. One of
the usual service conditions is the requirement that the temperature of the cooling air
will not exceed 4OoC and not be less than
10°C (see ANSI C50.10-1965, General Requirements for Synchronous Machines, Section 3). For general definitions refer to ANSI
C50.10-1965 and for generator definitions to
ANSI C50.12-1965, Requirements for Salient
Pole Synchronous Generators and Condensers.
Special generator capabilities and service
conditions might have been specified a t the
time of manufacture. This information is usually listed in the generator instruction book or
can be obtained from the manufacturer.
5.1.2 It is generally recognized that the actual output capability of a hydraulic turbine
or the load conditions might not match the
nameplate rating of the generator exactly.
When other than rated load and service conditions exist, the limitations of temperature rise,
5.2.3 The thermal aging of insulation systems and generator parts is a function of the
hotspot temperature and the duration of that
temperature. In addition, the changes of load
and temperature and the rate of change make
the windings, bearings, and the surrounding
parts subject to additional differential mechanical stresses. Thus generators with variable load cycles usually show signs of more
rapid aging than the baseload units.
6
IEEE
Std 492-1974
AND MAINTENANCE OF HYDRO-GENERATORS
are usually equipped with a segmental shoetype thrust bearing and with one or two guide
bearings. Horizontal units may be similarly
equipped.
Thrust bearings are designed to carry maximum specified hydraulic thrust and the
weight of rotating parts of the turbine and
generator. They are also capable of operating
a t maximum specified overspeed for certain
specified length of time without excessive
overheating. Usually there is also a requirement that the oil quantities and bearing design must be adequate to operate without
cooling water for a limited time period. T h e
successful operation of bearings is also dependent on supply of clean cooling water in specified quantities and a t temperatures not exceeding the temperature specified for design of
bearings.
Thrust bearing brackets are usually designed for the total steady-state hydraulic
thrust and weight of rotating parts with the
deflection limited to a certain value. Due to
this deflection limitation, stresses in the
bracket are usually low. If, however, high
transient thrust conditions or vibration forces
are present during operation, the bracket and
its component parts should be investigated for
load capability and possible resonance.
5.3.4 Stator Frames. Stator frames and
soleplates are usually designed for maximum
s h o r t - c i r c u i t t o r q u e capability a n d t h e
stresses a t rated load are thus quite low. However, should a three-phase or single-phase
short circuit occur, or if the machine is synchronized more t h a n 30° out-of-phase, the
machine and its foundation should be inspected for possible damage as the stresses on
the stator and its winding become high in
comparison with rated load.
5.3 Mechanical Stress Limits
5.3.1 Coupling, Shaft, Rotor Spider. These
parts are usually designed for maximum specified turbine horsepower and downthrust.
They should also be coordinated in design and
mechanical load carrying capability with the
turbine shaft. (Coupling design reference
ANSI B49.1-1967 (R1971), Integrally Forged
Flange Type Shaft Couplings for Hydroelectric Units.)
T h e design stress limits usually allow for
adequate safety margins to take care of stress
concentrations and dynamic loadings during
short circuits or temporary hydraulic disturbances. However, if loadings above the maximum design power output are possible, the
gate limits on the turbine should be set so that
excessive mechanical overloads are prevented.
If during operation persistent vibrations at
some fixed frequency should occur due to hydraulic forces, the resonant frequencies of rotating parts and associated members (fans,
baffles, brackets) should be investigated because usually no allowance has been made for
this condition in the initial design.
T h e first critical speed of the generator rotating system (the shaft, rotor, exciter, etc) is
usually designed to be above the specified
maximum turbine overspeed and thus outside
any operating speed.
5.3.2 Rotor Rim. Rotor rims are designed
for maximum specified turbine overspeed
which might be as high as twice the rated
speed. Thus, a t the rated load and speed, the
rim stress and expansion are usually quite
moderate. However, as the rim stress and expansion increase with the square of the speed,
even partial overspeeds during load rejections
can cause relatively large values of rim stress
and expansion. If, in addition to overspeed,
additional acceleration forces are present due
to sudden speed increase and possible hydraulic upthrust, some slight movement or
misalignment of the rim stack and the fasteners between the rim and the spider might occur. Usually the design margins are such that
the occasional partial overspeeds are of no
consequence. However, if additional forces are
combined with overspeeds and these occur often, periodic inspections of rim and associated
parts are recommended.
5.3.3 Bearings and Bearing Brackets. T h e
majority of hydro-generators are vertical, and
5.4 Temperature Rise. T h e observable tem-
perature rise of a machine component is defined as the difference between the maximum observable temperature of the component and
the average temperature of the cold coolant
(air). ANSI (30.12-1965, Section 5, specifies
t h e limiting observable temperature rises
which will not be exceeded when the generator
is operating a t nameplate rating. This should
not be construed as specifying allowable operating temperature rises without regard to
rated load. Thus, if the observable temperature rise of a generator is below the standard
7
IEEE SM
492-1974
value at rated load, this does not mean that
the generator load can be increased above the
rated load until the limiting temperature rise
stated in the standard is reached (see Section
6.1.2).
For overloadin.g, refer to Sections 6.4.2 and
7.1.3.
5.5 Methods of Temperature Measurement.
T h e commonly used methods for determining
the temperatures are described below.
5.5.1 Stator Winding. For conventionally
(air) cooled machines, the embedded resistance-type detectors (RTD's) between the upper and lower coil sides (Fig 1) are recommended and most commonly used. The description, size, and location of RTD's is given
in ANSI C50.10-1965, Section 5. While RTD's
are preferred, since they indicate the mean
temperature of the coils over a length of several core packs, the use of thermocouple-type
detectors is also recognized as a n acceptable
practice.
Instrumentation to read and record the detector output is discussed in Section 5.6.1.
5.5.2 Stator Core. For stator cores the thermometer method of measurement is specified
in ANSI C50.12-1965, Section 5, Table 1.This
method is defined in ANSI C50.10-1965, Section 5.1.1 and in IEEE Std 115-1965, Test
Procedure for Synchronous Machines, Section 5.15.05.
Usually the stator core temperatures are
measured during acceptance testing by applying an alcohol thermometer or a thermocouple a t the back of the core. Sometimes a
thermocouple or an R T D has been embedded
permanently in the tooth or yoke portion of
the core by building it into the core during
stacking operations in the factory. In this case
a continuous monitoring of the core temperature is feasible. However, it is generally considered more important to measure and record
the stator coil temperature rather than the
stator core temperature.
5.5.3 Rotor Windings.
5.5.3.1 Field Windings. T h e resistance
method (as defined in ANSI C50.10-1965, Section 5.1.2 and IEEE Std 115-1965, Sections
5.15.15 and 2.10) is used for acceptance testing
and operational monitoring of rotor field
winding temperature. It should be recognized
that this method measures the average tem-
IEEE GUIDE FOR OPERATION
perature of the field winding and not the hotspot temperature.
In order to determine the field temperature
accurately with the resistance method, it is
important to measure the voltage drop across
the field accurately without including the
voltage drop of the brushes supplying the field
current. For acceptance testing, this is usually
accomplished with insulated brushes which
do not carry any field current and do not have
a n established surface film with significant
voltage drop.
For operational monitoring, the insulated
brushes do not always work consistently due
to rapid film buildup. In this case, the other
p r a c t i c a l m e t h o d s of b r u s h d r o p compensation are:
(1) Provide a bias voltage equal to the brush
drop
(2) Calibrate the temperature indicating
device for a value of a resistance equal to the
sum of the expected full-load temperature resistance of the field winding plus the equivalent resistance of the brush voltage drop at
full-load field current
Both of these methods generally have adequate accuracy for day-to-day operation.
5.5.3.2 A m o r t i s s e u r Windings. T h e
amortisseur windings in the pole faces are
generally designed to withstand thermally a
single-phase short circuit with equivalent 1 2 9
equal to or less than 40 (see ANSI C50.121965, Section 6). If temperature measurement
of the amortisseur bars and connection rings
is necessary for some special operating condition(s), this can be adequately accomplished
with special tests by using a series of temperature-sensitive paints painted on the amortisseur. The range and interval of these paints
should be chosen to correspond to the expected temperatures. The maximum design
temperatures for amortisseur windings are established by manufacturer with regard to the
duty cycle and mechanical stresses involved
and could be in the order to 300" C. Thus the
temperature-sensitive paints could be chosen
with approximately 25O C intervals and still
have adequate accuracy for the purpose.
5.5.4 Collector Rings. The thermometer
method after shutdown (see IEEE 115-1965,
Section 5.35) is used to determine the collector
ring temperature. If this is done at various
field current loadings then a curve of collector
IEEE
Std 492-1974
AND MAINTENANCE OF HYDRO-GENERATORS
Most of the common problems and sources
of error are discussed in IEEE Std 119-1973
and in IEEE Std 115-1965, Sections 5.15,5.20,
5.25, and 5.30.
5.6.2 Stator Winding Temperatures by Embedded Detector. T h e detector is separated
from the copper by the insulating wall of the
coil (Fig 1)and therefore will always indicate
somewhat lower temperatures than the hotspot of the copper.
T h e difference between the hotspot temperature and the sensor temperature is a function
of the insulation wall thickness, proportions
of the slot, the amount of heat to be conducted
through the insulation, and several other factors.
These factors have been taken into account
in the design of the machine to assure that the
actual hotspot temperature does not exceed
the limiting temperature of the stator insulation class. If then, as the result, the observable
temperature rise of the detector appears to be
low, with respect to the guaranteed value, it
does not necessarily mean that a margin for
overloading exists.
5.6.3 Stator Core Temperature. T h e limitations in measuring the maximum stator core
ring temperature rise versus field current can
be established as a n operating guide. Infrared
or optical temperature measuring instruments, or temperature-sensitive paints, may
also be useful in determining collector ring
temperatures.
5.5.5 Ambient Temperatures. I n accordance with ANSI (250.12-1965, Section 5.1,
the temperature of the cooling (ambient) air
for open ventilated machines is the temperature of the air entering the ventilating openings of the machine.
For totally enclosed water-air-cooled machines, the temperature of the cooling air is
the temperature of the air leaving the coolers.
Alcohol o r m e r c u r y thermometers or
thermocouples are generally used for acceptance testing. For operational monitoring,
thermocouples, RTD’s, or filled system thermometers are most commonly used.
I n large machines with several ventilating
openings or coolers, the ambient air temperature usually varies with the location. T h e
mean value of the indications is used as the
base for temperature rise calculations and the
indicator reading closest to the mean value
should be used for operational monitoring
(See IEEE Std 115-1965, Section 5.25 and
IEEE S M 119-1974,Recommended Practice
for Temperature Measurement as Applied to
Electrical Apparatus, Section 7).
Fig 1
Heat Flow and Typical
Temperature Detector Location
5.6 Limitations in the Methods of Measuring
Temperatures. I t should be recognized that
there are limitations in the methods of measuring temperatures. These are discussed below.
5.6.1 Instrumentation. For safe operation
of large generators, the reliability and accuracy of temperature indicating and recording devices is of paramount importance.
Whereas the instruments themselves might
be calibrated and accurate, the problems usually arise from the long distance, bad connections, and stray fields between sensing element (thermometer bulb, R T D , or thermocouple) and the indicating or recording panel
instrument.
If acceptance tests are run on a new machine, there is usually a good opportunity to
compare panel instrument indications with
those from the lab instruments used for testing. Any significant differences should be investigated and inaccuracies corrected.
R E S I S TA N C E
TEMPERATURE
DETECTOR
9
IEEE Std
492-1974
IEEE GUIDE FOR OPERATION
temperatures with an embedded detector lie
in the fact that it is very difficult to make provision for mounting of the detector and knowing in advance which part of the core is going
to have the maximum temperature. Also the
stator coil temperature is usually more critical
from the operating point of view than the
core. However, if the unit is operating above
rated voltage or a t low power factor (either
overexcited or underexcited), it can be very
useful to observe the increase in stator core
temperature to avoid excessive increases.
5.6.4 Field Winding Temperature. The field
winding temperature measured by resistance
gives an indication of the average temperature
throughout the winding. Thus, it does not indicate the magnitude of the hotspot temperature with respect to the average temperature.
If overloadings are considered, the hotspot
temperature is the critical one, and proper allowance has to be made for the difference between the average and the estimated hotspot
temperature.
6. Loading
6.1 Relation of Load a n d T e m p e r a t u r e
Rise-Stator and Field. The hydro-generator
nameplate states the output rating and rated
operating conditions for which the generator
has been designed.
6.1.1 When operating within nameplate
condition of electric output and cooling medium, many machines will not reach the guaranteed temperature rises which are stated on
the nameplate. T h e reason for such a situation lies in the fact that the observable temperature rise alone does not constitute the
only basis for the design of a generator. Other
factors which influence design include: hotspot temperature (as discussed in Sections
5.6.2,5.6.3, and 5.6.4). strength of metals and
insulating materials, magnitudes of forces due
to thermal expansion and vibration, margins
for safety, reliability, and design prediction
accuracy. Thus, in a balanced design, the
temperature rise of stator and field winding a t
rated load might be several degrees below the
guaranteed value in order to assure reasonable
life expectancy and trouble-free operation.
6.1.2 If heat runs have been performed on a
generator a t several load points of stator and
field current at rated voltage, then the observed temperature rises could be plotted as a
function of stator and field current squared.
T h e resultant curves are usually close to a
straight line as shown in Fig 2 and provide a
convenient operating guide to estimate the
temperature rises and total temperatures for
various load and coolant conditions.
6.2 Loading Within Rating. Optimum loadings from the standpoint of maximum life expectancy for the generator are those that result in minimum temperature variations a t
the lowest practicable temperature level.
6.2.1 For varying load conditions this is
possible only for totally enclosed water-aircooled machines where flow of cooling water
can be adjusted to regulate the cold air temperature to suit loading conditions and reduce
winding temperature variations. This may
have to be adjusted seasonally, because usually it is not feasible during periods of high cooling-water temperature to increase the cooling
waterflow enough to maintain constant winding temperature a t maximum loads.
6.2.2 Open machines usually do not have
means of controlling the inlet air temperature; however, for machines which have control dampers and means of recirculation of air,
the above discussion applies.
6.2.3 Generator characteristic curves are
useful to determine the relationship between
the field current and the required load setting
and to avoid overloading beyond the rating of
the machine. Typical curves are shown in Figs
3, 4, and 5.
T h e saturation curves (Fig 3) give the relationship of terminal voltage, stator current,
and field current. Load characteristic (Vee)
curves (Fig 4) indicate the apparent power in
kilovolt-amperes and field current relationship for various power factors a t rated voltage.
A capability curve (Fig 5) indicates generator capability in terms of power in kilowatts
and reactive power in kilovars with the indicated limitations for various regions.
6.3 Loading-Power and Reactive Power Relationship. In a system with several generators
in parallel, the principal effect of a change in
excitation on one generator will be to vary the
amount of reactive current supplied by that
machine.
IEEE
Std 492-1974
AND MAINTENANCE OF HYDRO-GENERATORS
80
v
41
az
A T E D STATOR RISE
- -R-------
60
R A T E D F I E L D RISE
W
E
3
Ia
E
40
E
W
c
20
I
0
0
0.25
0.50
NOTE: Values along the abscissa correspond to
(
1.0
0.75
field current
2
or to
(
stator current
rated stator current
Fig 2 Typical Temperature Rise versus Stator and Field Current Squared
Increasing the excitation will tend to make
In the underexcited region the excitation
system might be limiting in terms of its capability of supplying zero or negative excitation.
T h e ultimate stability limit for underexcited
operation can be established by the generator
manufacturer with consideration given to the
excitation system capability. Stability limits
are discussed in Section 7.1.2.2.
6.3.2 T h e relationships between power in
kilowatts and reactive power in kilovars as
discussed above are also apparent from the ca-
a generator supply more overexcited reactive
power and decreasing the excitation will cause
it to supply less reactive power or absorb it.
6.3.1 Typical load characteristic curves related to power factor are shown on Fig 4. As
can be seen from these curves, when a generator is operated a t lower than rated power factor (overexcited), the generator output is limited by field heating and maximum allowable
field current.
ll
IEEE Std
492-1974
IEEE GUIDE FOR OPERATION
1.2
I
R A T E D kVA
A$
1.1
1.0
0.9
-+- 0.8
z
3
U
w 0.7
a
W
c3
1.2
0.E
J
0
>
J 0.:
a
z
-
$
1.0
-f
3
U
w
0.8 5
0.4
W
I-
I-
z
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0.6
E
U
3
0
0.4 w
0.2
2
-
J
0 .I
00
0.2
I
0.5
I .o
I .5
2 .o
2.5
3.0
FIELD CURRENT (PER UNIT)
Fig 3
Typical Hydro-Generator Saturation Curves (0.9 power factor, 1.1 short circuit ratio)
pability curve on Fig 5. The area inside the
curve constitutes the allowable operating region without exceeding the generator rating in
respect to stator or rotor heating.
6.3.3 The characteristic curves and permissible loadings are specific for each generator. It is therefore recommended that the applicable curves for each generator should be
obtained from the instruction book or the
manufacturer.
6.3.4 Special operating conditions may be
encountered when it may not be possible to
stay within the ratings of the machine. In order to minimize the risk of continuing operation through such temporary conditions, it is
essential that the pertinent rating data should
be available to the operator for quick reference.
For the same reason the limiting values for
power, reactive power, voltage, and power fac-
12
IEEE
Std 492-1974
AND MAINTENANCE OF HYDRO-GENERATORS
I. 10
I .o
0.90
c
I-
0.80
z
3
a
0.70
v
I-
5ix
0.60
E
3
W
0.50
z-1
o 0.40
a
0.30
0.2c
0.IC
0
0.5
I .o
1.5
2 .o
2.5
FIELD CURRENT (PER UNIT)
Fig 4
Typical Hydro-Generator Vee Curves (0.9 power factor, 1.1 short circuit ratio)
quency. Refer to Section 6.4.1. ANSI C50.121965 does not provide for this condition
(3) Above rated kVA but normal voltage
and frequency. Refer to Section 6.4.2 and
ANSI C50.12-1965, Table 1
(4) Combination of (1) and (3).Refer to Section 6 - 4 2
(5) Short-time abnormal conditions. Refer
to Section 6.4.3 and ANSI C50.12-1965
tors should be established in order to avoid
overheating and instability.
6.4 Loading Outside of Nameplate Rating. It
may be necessary to operate a generator under
the following conditions:
(1) At rated kVA but other than rated voltage. Refer to Section 6.4.1 and ANSI C50.121965
(2) At rated kVA but other than rated fre-
13
IEEE Std
492-1974
IEEE GUIDE FOR OPERATION
0.8
0
0.6
W
k
0
X
W
U
W
>
o 0.4
0.2
c
k
z
3
U
W
a
Y
U
a
3
0.2
E
0.4
k
0
X
W
U
W
c3
z
3
0.6
0.8
/
Fig 5
Typical Hydro-Generator Capability Curves (0.9 power factor, rated voltage)
14
IEEE
Std 492-1974
AND MAINTENANCE OF HYDRO-GENERATORS
6.4.1 All generators built in accordance
with ANSI C50.12-1965 are capable of operating a t rated kVA with a voltage variation of
5 percent above or below normal. If operations
beyond these limits are necessary, the manufacturer should be consulted.
Operation a t other than rated frequency is
quite unusual and any large deviation (more
than 2 percent of rated frequency) should be
discussed with the manufacturer.
6.4.2 Large and high-voltage hydro-generators built in accordance with ANSI C50.121965 for Class B and 60° C rise may be operated up to 115 percent load a t rated power factor, frequency, and voltage with the stator
and rotor temperature rise in excess of normal
for these machines. For this operation it is recommended that the ambient be kept as low as
possible to assure maximum insulation life,
but above the loo C minimum. Also see Section 7.1.3.1. Sometimes other t h a n t h e
C50.12-1965 standard temperature rises and
loading conditions have been specified and
the 115 percent loading provision might not
apply.
As a general rule, and since all hydro-generators have somewhat differing characteristics
and actual temperature and stress margins, it
is advisable to discuss any planned operation
beyond the rating of the generator with the
manufacturer to establish safe limits. Additional tests might have to be conducted to substantiate the possibilities for higher than
rated loadings.
6.4.3 Short-circuit capabilities are specified in ANSI C50.12-1965, Section 6.
For other unusual conditions such as shorttime overloadings, unbalanced load operations, harmonic current loadings, etc, the Industry Standards do not make any specific
provisions. Each such case should again be
discussed with the manufacturer to establish
safe limits with regard to expected life of the
machine.
dampers are provided, to avoid extremely low
temperatures and condensation.
In case of extremely low cooling water temperature for totally enclosed water-cooled machines it would be necessary to throttle the
waterflow to avoid excessive condensation a t
the coolers.
7. Operation
7.1 General. This portion of the guide describes a practical method of operating hydrogenerators.
7.1.1 Requirements for Operation. Operation of a hydro-generator requires consideration of several different factors, some of
which are as follows:
7.1.1.1 S t a tor Wind in g Tempera tu re.
Stator winding temperature indications are
useful in providing a continuous record of the
temperature history of the unit. Any trend
away from past temperature performance is
an indication of a change in machine condition and should be investigated.
(1) Instruments used a t the switchboard for
measuring the temperature of the stator winding by resistance detector operate on the basis
of change in resistance of the detector element
and may be made to record the temperatures
of a large number of detectors on a continuous
recording meter or indicate the temperatures
on a common meter with a selector switch to
connect the individual RTD’s. It is common
practice to provide multipoint recorders with
adjustable alarm contacts.
7.1.1.2 Field Winding Temperature. Operation a t power factors less than the rated
value in the overexcited region is limited by
rotor winding temperature. In this region the
full rated kVA of the generator cannot be realized due to this limitation. Any operating
method must recognize this limitation.
7.1.1.3 Stator Core Temperature. Operation in the underexcited region is sometimes
limited by overheating in the extreme ends of
the stator core where temperatures are not detected by usual stator winding temperature
detectors. Excessive heating of the core ends is
more of a problem in machines which have
magnetic clamping fingers, but even in some
machines with nonmagnetic fingers, flux concentrations around the stator end turns may
6.5 General Considerations.
6.5.1 Low Ambient Air and Cooling-Water
Temperatures. As discussed in Sections 5.2.2,
5.4, and 5.6.2, allowance must be made for increased difference between copper and detector temperatures when exceeding nameplate current a t low ambient temperatures.
For open ventilated generators it may be
necessary to throttle the cooling airflow, if
15
IEEE Std
492-1974
cause undesirable core heating. If a machine
is to be operated underexcited for long periods
of time, a heat run should be made and core
end temperatures checked.
7.1.1.4 Collector Ring Temperature.
Brush and collector ring performance are dependent on collector ring temperatures. Periodic checks for abnormal temperatures are
therefore recommended.
A record of field temperature can also be
useful in detecting collector ring brush problems. Indications of erratic changes of the
temperature can be caused by brush arcing
due to uneven or low brush pressure. Incipient
collector ring flashover may be indicated. See
Sections 8.3.4 and 8.3.5 concerning brush
maintenance.
7.1.1.5 Stator Differential Expansion.
T h e capability of the stator winding is limited
not only by total temperature and winding vibration, but also by the effects of differential
expansion between the stator coils and the
stator core. Differential expansion is a function of the total temperatures of the winding
and the core. This factor is more critical in
machines with long cores (over 70 in). During
load changes the copper temperature changes
more rapidly than the core temperature, thus
accentuating this differential expansion problem. It is desirable that the maximum load on
the machine be limited so as to reduce differential expansion and that changes in load be
made slowly so as to minimize differential expansion.
F r e q u e n t variations in differential expansion may result in damage to the stator
coil insulation and occasionally cause stator
core looseness or movements and increased
maintenance.
7.1.2 Use of Capability Curves. Typical
generator operating curves are shown in Fig 5.
Similar curves applicable to the particular
generator should be used to guide its operation.
T h e operation of the generator according to
the capability curves may be accomplished by
the use of instruments to measure power in
watts, reactive power in vars, terminal voltage
in volts, line current in amperes, and when
available, field current in amperes. Generators are usually operated between rated power factor overexcited and unity power factor.
In this range the generator can be controlled
IEEE GUIDE FOR OPERATION
by the terminal voltage and line current since
the stator winding heating limits the load.
Standard alternating-current generators will
operate successfully a t rated kVA load within
plus or minus 5 percent of rated terminal voltage a t safe temperatures.
7.1.2.1 Method o f Determining Overexcited and Underexcited Limits. T h e overexcited portion of the capability curve in the
region between the unity power factor and
rated power factor lines is determined by the
stator winding temperature rise. T h e overexcited portion between the rated power factor line and zero power factor is determined by
the field winding temperature rise.
T h e underexcited portion of the curve is
limited by four factors: stator winding temperature rise, stator core end heating as discussed
in Section 7.1.1.3, minimum excitation conditions, and the system stability limit discussed
in Section 7.1.2.2.
7.1.2.2 Methods of Computing System
Stability Limit. T h e system stability limit discussed in Section 7.1.2.1 and the system stability limit curve shown on Fig 5 can be computed in the following manner.
T h e steady-state power angle curve (Fig 6)
may be calculated by means of Eq 1 below for
a machine connected to an infinite system
through a series reactance such as a transformer and a small line reactance. This is the
most common terminal condition. Steadystate stability for other terminal conditions
such as a large induction motor load or transient stability is beyond the scope of this
guide, and for these situations a text on the
subject should be consulted.
where
P = per unit power (kilowatts), where 1.0 =
rated kVA
Ed = per unit excitation
- (actual excitation)
excitation at rate
voItage on the
air gap line
E = per unit system voltage
Xd = per unit direct-axis synchronous reactance
X, = per unit quadrature-axis synchronous
reactance
?
IEEE
Std 492-1974
AND MAINTENANCE OF HYDRO-GENERATORS
2 .o
I .6
-!=
z
3
1.2
W
a
v
5
G
E
z
0.8
a
0.4
0
30
90
60
120
150
180
ANGLES I N DEGREES -6
Fig 6
Typical Steady-State Power Angle Curve
citation, some load may be carried due to saliency (second term of the equation). A small
negative excitation can be used when this capability is available in the excitation system,
but when the first term is equal and opposite
to the second term the machine will slip a pole
with no power input and the excitation controls are likely to force the excitation to a
negative ceiling with no corrective action
available except tripping the unit and restarting.
Unfortunately the turbine output is often
nonuniform enough a t very small outputs so
that negative excitation does not often provide satisfactory operation. Minimum excitation devices should be set in accordance
with the manufacturer’s recommendations
X, = per unit reactance between machine
and infinite system
6 = angle between machine rotor and infinite system
Steady-state underexcited capability follows Eq 1 with the excitation Ed reduced to a
fraction of the no-load value. As the peak of
the power angle curve approaches the output
of the turbine, the margin to ride through
small system disturbances decreases to zero.
Transient capabilities may allow momentary
performance beyond this point, but will not
permit continuous performance without shifting excitation and or terminal voltage to a
stable steady-state condition. At zero ex17
IEEE Std
492-1974
IEEE GUIDE FOR OPERATION
c h a n i c a l design of t h e hydro-generator
coupled with the particular type supplied by
the manufacturer makes it impractical to attempt to cover all details of thrust bearing operation and maintenance. It is important to
carefully follow the instructions and recommendations of the manufacturer. The thrust
bearing is regarded as of prime importance in
the mechanical considerations of the vertical
hydro-generator, and much attention has
been given to its design and manufacture to
insure efficient and reliable service.
7.1.4.2 Principal types of thrust bearings
used for vertical hydro-generators are the adjustable pivoted shoe, self-equalizing pivoted
shoe, spherical, and spring supported. Thrust
bearings support the weight of the rotating
parts of the generator and the turbine plus the
load imposed by the water passing through
the hydraulic turbine. T h e principal elements
of the thrust bearing are the rotating runner
and the stationary shoes. T h e runner receives
the load through a massive hub called the
thrust block. Depending on the type of hydrogenerator the thrust block may be integral
with the shaft or is attached to the shaft and
suitable for removal when disassembling the
machine.
7.1.4.3 Considerations which apply to
operation and maintenance of all types of
thrust bearings are:
(1) Use of the specified lubricating oil
(2) Maintaining clean uncontaminated oil
(3) Maintaining the proper oil level
(4) Periodic checks of bearing insulation
(5) Maintaining cooling medium flow
(6) Temperature of bearings, see Section
7.1.4.8
7.1.4.4 T h e care required to maintain
clean oil is of special importance for bearings
equipped for high-pressure lubrication during
the starting period since the oil is fed directly
to the center of the bearing shoe and small
particles of foreign material could be forced
into the oil film.
7.1.4.5 Most vertical generators are
equipped with high and low oil level indicators which function to give alarm or shutdown of the unit if the proper oil level is not
maintained. High oil levels are usually caused
by water leaks in the oil cooler mounted in the
oil reservoir. A leak in the oil cooler can also
cause a low oil level when the discharge water
which will provide some steady-state margin
for normal situations.
A second method of computing the stability
limit is given in the reference listed in Section
11.5.
7.1.3 Operation at 115 Percent Rating.
Large and high-voltage hydro-generators and
reversible generator-motor units built in accordance with ANSI C50.12-1965 may be operated a t 115 percent load a t rated power factor, frequency, and voltage with temperature
rises in excess of normal standards for these
machines. I n some cases values for stator and
field winding temperature rises may be agreed
upon in the contractual stage.
When operated a t loads above rated conditions it is recommended that the ambient temperature be kept as low as possible by use of
maximum cooling water to assure maximum
useable life.
7.1.3.1 Correlation o f Overloading with
Reduction of Insulation Life. Operation a t
overload conditions with consequent higher
temperatures than those specified in the appropriate standards can cause a n appreciable
reduction in service life.
Temperature is not the only criterion since
the life of the insulation system is also influenced by other factors such as endurance under electric and mechanical stress, vibration,
exposure to moisture, chemicals, etc.
Other factors being equal, however, thermal
degradation is accelerated as the temperature
is increased. For most of the insulating materials currently in use in hydro-generators the
life is a n exponential function of the reciprocal of the absolute operating temperature over
a limited range of temperatures. Evaluation of
thermal aging by a system of temperature indices is advocated in IEEE Std 1-1969, General Principles for Temperature Limits in the
Rating of Electric Equipment.
Prior to that standard a rough rule in assessing the effects of operation a t overload
conditions was t h a t the insulation life would
be decreased by one-half for each loo C rise in
temperature above the standard value for the
class of apparatus involved.
7.1.4 Bearings. Another major factor to be
considered in operation of a hydro-generator
is the performance of the bearings.
7.1.4.1 T h e close coordination of the
thrust bearing with other elements in the me18
AND MAINTENANCE OF HYDRO-GENERATORS
piping is arranged so that there is a suction
head a t the cooler.
7.1.4.6 Thrust bearing insulation fitted
in the bearing base is provided to prevent the
circulation of possibly damaging current
through the bearings. A principal source of
such currents is induced shaft voltage. Shaft
voltages are most often a result of some assymmetry in the magnetic circuit of the machine which causes some net flux linkage with
the rotor shaft that induces a voltage in it.
These voltages are alternating a t a multiple of
system frequency and are low, usually less
t h a n 5 V. If there is a continuous low-impedance path from the shaft ends through the
bearings and base the current produced by the
shaft voltage may be many amperes and
enough to damage the bearing. Terminals for
checking the installed bearing insulation resistance are usually provided on the bearing
bridge. Generators with no bearings above the
rotor do not require bearing insulation. However, any fittings or apparatus a t the top of
the machine which contact the shaft must be
insulated.
7.1.4.7 Should the occasion arise where
disassembly of the generator is required to the
extent of dismantling the thrust bearing it is
recommended the work be done only by qualified and experienced erectors in accordance
with the manufacturer’s recommendations.
Loads on the segmental shoes must be equalized on all but the self-equalizing and springsupported type.
7.1.4.8 After startup and initial running,
the bearings will usually operate a t a constant
temperature which will vary slightly with
variation in the cooling water temperature.
Any increase in bearing operating temperatures could be a sign of metallic contact
which may precede bearing failure, change
taking place in bearing oil, cooling system, or
hydraulic thrust and should be investigated as
soon as possible.
7.2 Starting and Loading. This section does
not provide complete instructions on starting
hydro-generators but details some important
aspects of this phase of operation.
7.2.1 Normal Startup. I n starting, sufficient breakaway torque (turbine gate opening) should be applied to start the machine
rapidly to establish a n oil film. Thereafter,
IEEE
Std 492-1974
the rate of speed increase to full speed is a
matter of concern only in respect to the hydraulic turbine and is immaterial in respect to
the generator. Machines having thrust bearings equipped with a high-pressure lubrication system are usually capable of a slow
start, such as a synchronous start of a generator/motor (pumped-storage) unit.
7.2.1.1 Generators which differ as to
method of cooling and as to excitation require
proper establishment and sequence of operation of the associated auxiliary equipment in
accordance with the manufacturer’s recommended procedures. Sequences such as release
of brakes, establishment of the high-pressure
oil film to the thrust bearings when so equipped, and in some instances concern for the
start of waterflow downstream are other important considerations.
7.2.1.2 For the first start of a new machine or if any work has been done on any
connections which involve synchronizing, initial establishment of correct phase rotation is
required.
7.2.1.3 Accuracy in the act of synchronizing. T h e standards do not require generators to be designed to withstand currents and
mechanical forces due to incorrect phasing or
incorrect synchronizing. I n general the synchronizing accuracy should be such t h a t the
phase angle between the generator and bus
voltage be not greater than 15O a t the moment
of closing of the synchronizing breaker. In
judging the synchronizing point the breaker
closing time must be considered as well as the
difference in frequency between the generator
and the bus. I t is preferable t h a t the generator
be ahead of the bus at the moment of synchronizing.
7.2.2 Emergency Starting. Under ordinary
circumstances a generator is shut down after
the load has been reduced to zero and the generator is disconnected from the system. An
emergency start would be considered a start
immediately after a normal shutdown or after
a shutdown and lockout due to a protective relay operation.
7.2.2.1 T h e principal concern of a n
emergency start of a hydro-generator is the
operation of the thrust bearing. When the
thrust bearing is not equipped with a highpressure oil lubrication system, the conservative practice is to avoid a start immedi-
IEEE Std
492-1974
IEEE GUIDE FOR OPERATION
tion resistance tests are in IEEE Std 43-1961,
Guide for Testing Insulation Resistance of
Rotating Machines
(2) When dryout of the windings is indicated, the procedure detailed in NEMA MG
5.2-1972, Installation of Vertical Hydraulic-Turbine-Driven Generators and Reversible Generatorhlotors for Pumped-Storage Installations, should be followed
If the suitability of the winding is in question, a dielectric test may be performed. T h e
winding may be tested using either ac or dc
high potential equipment. See Section 8.5.
T h e dielectric test voltage applied should be in
accordance with IEEE Std 56-1958, Guide for
Insulation Maintenance for Large AC Rotating Machinery (ANSI C50.25-1972) and the
method employed in making the test should
be in accordance with IEEE Std 115-1965.
For rotating exciters the dielectric test voltage applied should be in accordance with the
ANSI C50.5-1955, Rotating Exciters for Synchronous Machines, and. the method employed in making the dielectric test should be
in accordance with the IEEE Std 113-1973,
Test Code for Direct-Current Machines.
7.2.4 Recommended Rates of Loading.
Hydro-generators are capable of rapid loading
rates and advantageous flexibility in load adjustment. When needed to meet system demands, the generator can be loaded a t very
rapid rates limited only by the action of the
governor, the hydraulic turbine, and hydraulic conditions.
7.2.4.1 The primary consideration in the
loading rate of the hydro-generator is the rate
of change of temperature of the stator and
field coils and Its effect on insulation.
7.2.4.2 A conservative method of loading
the generator is to apply the load in increments so as to attain full rated load in 45 to
60 min after the initial load application. This
allows a reasonably steady and distributed increase of heating of the generator parts and
tends to lessen thermal shock to individual
parts. While it cannot be calculated with absolute accuracy, the conservative method of
loading of a generator will increase the operating life of the windings of the generator and
is a recommended procedure for normal operation. The ideal method of operation of a generator is to maintain a steady load and relatively constant temperature of the windings.
ately after shutdown from prolonged operation with the bearing hot, since hot oil or distorted shoe surfaces may interfere with establishment of an oil film. Some types of thrust
bearings should be allowed to cool for an hour
or two before restarting the hydro-generator.
The manufacturer should be consulted for
more specific recommendations.
7.2.2.2 The precautions regarding starting a unit while the thrust bearing is hot do
not apply to a unit equipped with a high-pressure lubrication system since an oil film is established with the application of the highpressure oil between the bearing shoe surface
and the bearing runner.
7.2.3 Starting After Extensive Maintenance
or Downtime. After an extensive shutdown period it is desirable to make a thorough inspection of the machine and associated equipment before starting. Preventative maintenance checks on operation of auxiliary
equipment such as breakers, protective relays,
water control valves, brakes, and similar devices will be helpful in returning the generator
to reliable service promptly. A test of the generator windings should be made to be certain
the insulation resistance is satisfactory or if a
dryout of the windings is required before energizing the machine.
7.2.3.1 If the unit is not equipped with a
high-pressure lubrication system, the unit
should be lifted momentarily off the thrust
bearing with the jacks to reestablish an oil
film on the bearing shoes and the runner prior
to starting.
7.2.3.2 Insulation resistance tests of the
windings may serve as a n indication of the
condition of the generator insulation and its
suitability for dielectric testing or normal operation. Insulation resistance test results can
be used to determine if dryout of the windings
is necessary before proceeding with testing or
operation.
Tests are most useful in the establishment
of a historical record of the insulation resistance of the windings. Periodic tests made a t a
fixed winding temperature are helpful in indicating progressive or unusual deterioration
of the generator insulation. They also serve as
a guide for the desired insulation resistance
level for a machine which may have had an
extended downtime period.
(1) Procedures for performance of insula-
20
AND MAINTENANCE OF HYDRO-GENERATORS
7.3 Braking and Stopping. Similar to the considerations for starting, the stopping or shutdown procedures for hydro-generators are influenced by requirements of the hydraulic turbine.
7.3.1 Normal Stopping. Normal stopping
sequences proceed after the load has been removed from the generator and the generator
has been disconnected from the system. The
hydraulic-turbine gates are closed and the
generator allowed to decelerate to 20 to 50 percent speed. The brakes are then automatically
applied either continuously, or in on-and-off
cycles of about 10 seconds duration.
7.3.1.1 When the unit has been braked to
approximately 10 percent speed, the brakes
are applied continuously until the unit comes
to rest. Generators equipped with direct-connected exciters usually decelerate with field
excitation applied since the main exciter field
breaker remains closed through the complete
stopping period. I n some instances water leakage through the gates is sufficient to cause the
generator to creep or revolve very slowly after
it has been brought to rest and the brakes
have been released. T o prevent creeping the
brakes would have to be kept energized during
t h e shutdown period. Generators can be
equipped with creep detectors designed to give
a n indication when this condition exists.
7.3.1.2 Generators equipped with a highpressure oil lubrication system on the thrust
bearing usually have that system restarted
when the generator speed drops to 25 to 30
percent speed and kept on until the generator
comes to rest. One important consideration of
the braking operation is the periodic rnaintenance required to prevent a n accumulation
of residue from the brake shoe surface wear on
the interior parts of the generator. Since the
brake ring and brake shoe mechanisms are
usually'located in the interior of the generator, some of the brake residue caused by the
braking operation is picked up by the rotor
fans and carried to windings of the machines.
If allowed to accumulate over a prolonged period, it may affect the cooling of the generator
windings.
7.3.2 Emergency Stopping. Emergency
stops which usually are initiated through operation of protective relays are combined with
prompt rejection of load and rapid closing of
the turbine gates. T h e gate closure time is a
IEEE
Std 492-1974
function of the governor setting, the permissible penstock pressure rise, and other hydraulic considerations. Application of the
brakes occur as in the normal stopping sequence. When the generator is being stopped
with the gates closed and with the turbine immersed in water, as with some high-head
Francis or Kaplan turbines, the water assists
in stopping the generator. A Pelton turbine
might be equipped with a braking jet to assist
in stopping; however, stopping time of the
unit is usually longer than with other types of
hydraulic turbines.
7.3.3 Operation of Brakes and Jacks. On
vertical hydro-generators t h e function of
braking and jacking often is served with one
assembly. T h e braking operation uses compressed air in the range of 100 psi fed to the
brakes. For normal operation the braking of
the generator is a n automatic operation initiated by speed switches on the generator.
7.3.3.1 T h e jacking function uses oil supplied from either a manually operated pump
or a motorized pump. Release of the jacks permits the oil to flow back to the oil sump. T h e
manufacturer's instructions detail the permissible maximum jacking lift of the rotor.
This value is relatively small, in the order of
0.375 to 0.625 in, and therefore requires careful attention when jacking the rotor to prevent damage to the machine.
7.3.4 Dynamic Braking. Dynamic braking
can be used when very rapid stopping of a generator is a usual requirement. Dynamic braking consists of loading the generator with a resistance load after the unit is separated from
the system and the shutdown sequence of the
generator is initiated. T h e rotational energy
of the generator is dissipated in the resistance
load connected to the generator terminals.
The rate of deceleration is a function of the
value of the resistance load and pertinent generator characteristics. Most probable use of
dynamic braking for a hydro-generator unit is
to make a rapid change of generation to
pumping mode in a pumped-storage installation. The electrical forces incident to dynamic
braking are less than those associated with
short circuits and are well within the capability of the generator. Dynamic braking can also
be used with hydro-generators for improvement of the stability by controlling the rate of
divergence during the transient power system
IEEE Std
492-1974
IEEE GUIDE FOR OPERATION
(2) Balanced and unbalanced faults up to
and including the generator breaker
(3) Insulation failure of the winding
7.5.3 Protective devices for the field winding which may be used to initiate the remova!
of the generator from the system or to give a n
alarm only are usually provided for the following conditions:
(1) Open circuits in the field winding (loss
of field relay)
(2) Short circuits in the field winding (excessive vibration relay)
(3) Ground in the field circuit
7.5.4 Other conditions for which protective
devices may be provided to remove the generator from the system or to give a n alarm are as
follows:
(1) Overcurrent in stator or field
(2) Overtemperature in stator or field
(3) Overvoltage in generator stator
(4) Overvoltage in field
(5) Excessive unbalanced stator current
(negative sequence)
(6) Excessive unbalance of current in stator
parallel circuits
(7) Fire
(8) Loss of cooling water for air or oil coolers
or both
(9) Overtemperature of bearings
(10) Overtemperature of cooling air
(11) Low or high oil level for bearings
(12) Machine vibration
swings. T h e generator remains connected to
the system and the additional resistance load
is added a t the proper time during system disturbances.
7.4 Load Rejection and Runaway. T h e characteristics of hydro-generator operation under
conditions of load rejection or runaway are
determined by the hydraulic turbine characteristics, the governor settings, and the combined WR' of the installation. While the
hydro-generator is designed to withstand the
runaway speed of the turbine, it is recognized
t h a t prolonged operation at this speed is undesirable and should be avoided. Under ordinary
conditions of load rejection by generators, the
governor acts to close the turbine gates rapidly
enough to hold the speed of the generator to
well below full runaway speed. I n the event of
governor or wicket gate failure, the penstock
(head) gate must be closed to prevent sustained runaway.
7.5 Automatic Supervision and Protection.
Hydro-generators are well suited to completely automatic supervision of operation
and protection. Although the range of hydrogenerator ratings in terms of apparent
power in kilovolt-amperes, voltage in volts,
revolution rate in revolutions per minute,
power factor, and other characteristics is very
large, the inherent reliability of its construction and operation makes the automatic and
remote control of the machine practical.
7.5.1 Suitable protective devices are available to prevent or minimize damage to the
generator under all operating conditions.
These devices are arranged to give a n alarm or
to initiate a n emergency shutdown of the generator when specific limits of operating criteria are reached. T h e extent of the protection
desired for a particular generator depends
upon such factors as importance of the machine, age, type, and rating. Usually a suitable
compromise can be made.
7.5.2 Protective devices for the stator winding which will initiate removal of the generator from the system, deenergize the excitation, and in some cases begin the rapid
shutdown sequence of the hydraulic turbine
are usually provided for these conditions:
(1) Balanced and unbalanced faults beyond
the generator breaker (backup relay operation)
7.6 Unusual or Dangerous Operation Conditions. Operation under the following abnormal conditions requires careful observation of
performance of the machine so that it can be
shut down immediately, if necessary, to prevent damage.
7.6.1 Unbalanced Loading. Unbalanced
stator currents may be the result of a n unbalanced load, of unequal induced voltages in
the stator, or of a long unbalanced transmission line between the machine and its
load. A proposed standard currently being
considered for publication would require that
a generator be capable of withstanding without injury the effects of continuous current
unbalance corresponding to a negative sequence current not exceeding 10 percent for
salient pole generators with connected amortisseur windings and not exceeding 5 percent
for salient pole generators with nonconnected
amortisseur windings. T h e percentage values
22
IEEE
AND MAINTENANCE OF HYDRO-GENERATORS
are in percent of stator current a t rated kVA.
Additional provisions are that rated kVA
should not be exceeded and that the maximum current does not exceed 105 percent of
rated in any phase.
While this standard does not apply to generators currently in operation, it does give some
guide to the magnitude of unbalance which
could be tolerated. Specific cases should be referred to the manufacturer for his recornmen2
dations.
If there have been unbalanced currents in
the stator for a long period of time, or a n unbalanced fault exceeding I& of 40, then inspection of amortisseur connections and
joints a t the pole and of the ends of the dovetail should be made to determine if damage
has occurred. This inspection is particularly
important for machines having unconnected
damper (amortisseur) windings.
7.6.2 Operation with Stator Coils Cut Out.
Some generators may be operated successfully
with one or more stator coils cut out and bypassed. Slower speed generators having several hundred coils with a large number of coils
per parallel circuit may operate satisfactorily
with as many as 8 or 10 coils cut out, though
this is usually a temporary condition and
should be closely monitored. Stator and rotor
vibration may increase and should be checked
carefully. T h e unbalanced magnetic pull may
cause a vertical machine rotor to pull or skate
more to one side than is normal with consequent possible effects on guide bearings. I n a
multipath generator this is more likely to occur if the individual paths are concentrated in
circumferential sectors instead of being interleaved or distributed around the bore. Differe n t i a l relays may require moderate desensitizing in order to allow increased path
and circulating currents to flow; however,
such relays will continue to provide good protection. T h e embedded RTD's may not respond to the temperatures associated with the
increased currents if they are not located advantageously. The conditions which brought
about cutting out coils may justify limiting
the machine load to less than rated kVA or restricting the load cycling service or both.
7.6.3 Operation with Winding Paths Cut
Out. A few generators may be operated successfully with a winding path cut out. Naturally, only generators having several paths per
Std 492-1974
phase lend themselves to this. Such operation
should only be attempted when compelling
maintenance problems prevent immediate full
repairs. The manufacturer's advice should be
obtained concerning whether derating will be
greater with a path cut out or with a path in
service lacking a coil or two. (Circulating currents affect the latter.) Current transformers
associated with some split winding differential relay schemes may require auxiliary
current transformers to achieve acceptable
balanced conditions. Comments in Section
7.6.2 may apply in some cases.
7.6.4 Asynchronous Operation (Field Maintained). Operation of a generator out-of-synchronism with part or full-field excitation
maintained, places the most severe type of
duty on the unit. Such operation produces
heavy surge currents in the stator windings
whose magnitude may exceed those associated
with the machine short-circuit requirements
of ANSI C50.12-1965 and cause serious damage to the winding. Such operation also produces torque reversals which create in many
parts of the unit high mechanical stresses of
magnitudes that may be several times those
produced by rated torque. High induced voltages and currents in the field may cause flashover of the collector rings and of the commutator of an associated exciter.
For these reasons, although it may be difficult to detect the out-of-synchronism machine, it must be identified promptly and the
condition remedied. Possible corrective action
includes removal of the unit from the system.
7.6.5 Loss of Field Excitation. Complete
loss of excitation on a n operating generator results in dangerous overheating of its rotor
within a very short time unless the machine is
disconnected from the system. The degree to
which this heating will occur depends on the
initial load on the generator and the manner
in which the generator is connected to the system. When excitation is lost, the generator
tends to overspeed and operates as a n induction generator. This overspeed normally
results in a reduction in load due to the characteristics of the turbine governor, an increase
in stator current associated with low voltage
a t the generator terminals, and high rotor currents. These rotor currents will flow through
the field winding (provided the field circuit is
not open), and also through the amortisseur
IEEE Std
492-1974
winding and rotor pole faces. T h e amortisseur
winding and rotor pole face currents will
cause high and possibly dangerous temperatures in a very short time.
Some users provide a loss-of-field relay to
trip the generator breaker removing the unit
from the system and in some cases also to
close the turbine water inlet valves or gates,
while others provide alarm indication only.
Time can often be saved by running back the
turbine inlet water valves or gates to a no-load
speed position and following precautionary
checks t o restore excitation t o a n d resynchronize the system.
Where neither loss-of-field tripping nor
alarm indication is provided, the operator
must recognize the condition and manually
perform the functions described for the relay
above. If the loss-of-field condition has persisted for some considerable or unknown
length of time, the rotor should be inspected
a t the earliest opportunity.
7 . 6 . 6 O p e r a t i o n w i t h Field Circuit
Grounded. Usually the field winding and all of
its excitation supply circuit is operated as a
completely ungrounded system. On such a n
ungrounded system the existence of a single
ground a t any point in the system will not interfere with the normal operation of the generator. Its presence can be detected and indicated by suitable ground relay and alarm.
Upon indication of this initial ground, it is advisable to shut down the machine and correct
the trouble promptly.
If the initial ground should occur a t some
point in the generator field winding, a second
ground in the excitation circuit may prove
serious. When a double ground exists, part of
the field winding will be shorted out through
the shaft forging and pole pieces. This condition will cause a magnetic or thermal unbalance or both whose severity may result in
serious vibration of amplitude sufficient to
wreck the machine if allowed to exist too long.
The use of vibration detection and protection
equipment would assure instant knowledge of
such a condition and take the machine off the
line more quickly t h a n a n alert operator could
be informed and take corrective action. Lossof-field relay protection cannot be depended
upon to trip the generator in case part of the
field winding is shorted by a double ground because
IEEE GUIDE FOR OPERATION
(1) its operation is based upon the principle
of reactive power
(2) it is not sensitive to either magnetic or
thermal unbalance in the rotor or the resultant vibration
(3) the change in reactive power caused by
the partially shorted field may not be sufficient to actuate a loss-of-field relay.
7.6.7 Field Forcing by Voltage Regulator.
Continuous-acting fast-response voltage regulators used with exciters having high ceiling
voltages are employed by many users to improve power system stability. At times of prolonged system low voltage this automatic
equipment can impose a severe overload upon
both the field and stator windings. For any
particular case, the manufacturer should be
consulted to determine the maximum time the
overload condition can be permitted. Automatic means should be provided to relieve the
overload a t the end of this time and return the
machine to its maximum permissible continuous load.
7.6.8 Operation In Cold Ambient Temperatures. Allowance must be made for increased
difference between hotspot and detector temperatures when exceeding nameplate current.
Maintaining a low ambient temperature results in lower total temperature of the windings at all loads, less distortion of the metal
parts, and less drying out of the gaskets. In
the case of generators cooled with air supplied
from out-of-doors, it sometimes is not readily
practicable to control the temperature of the
cold coolant, which means that at low ambient temperature, it may be necessary to
throttle the cooling airflow to avoid condensation on the generator surfaces exposed
to warm room air.
I n the case of totally enclosed generators, it
may be found advantageous to regulate the
raw waterflow to the heat exchanger in such a
manner as to result in cold coolant temperatures lower than the normal value. (In this
guide, the term raw water will be used to designate the water used to take heat away from
heat exchangers. )
With either manual or automatic regulation, the control points should be chosen in
accordance with the manufacturer's recommendations. It should be borne in mind that a
cold coolant temperature which is too low
may be detrimental to some insulation sys-
AND MAINTENANCE O F HYDRO-GENERATORS
tems and may produce undesirable effects on
the mechanical balance of the unit.
In view of the lower losses a t low generator
loads, the generator hotspot temperature variation will be reduced by varying the ambient
temperature inversely with the load. This
method of operation would effect some saving
in cooling waterflow a t low loads. This is not
to be interpreted as a recommendation to operate the generator a t constant hotspot temperature. See Section 6.1.1.
I n order to maintain all parts of open-ventilated air-cooled generators at a safe temperature, it may be found advantageous to furnish heaters in the generator during periods of
shutdown. As the generator is loaded and
heated by its losses, the heaters should be removed from service.
Where the raw water supply temperature is
below the rated raw water temperature of the
coolers, care must be taken to prevent formation of excessive condensation which could
occur if the dewpoint of the cooling air is higher than the temperature of the coolant. T o
avoid condensation, a recirculation system to
raise the temperature of the raw water may be
provided. Dehumidifying-type dryers are
available which can be connected into the
generator cooling circuit and are suitable for
drying air under any pressure a t which the
cooling system operates. In these driers a desiccant absorbs the moisture which then is
driven off outside the cooling system. Some
operators are providing for continuous supervision of dewpoint through the installation of
dewpoint recorders provided with maximum
alarm contacts. Care should be exercised
when using very low temperature coolant that
a sudden drop in load will not cause the cooling air to reach dewpoint temperature which
will result in condensed moisture being deposited.
7.6.9 Hazards of Operation During Initial
Installation. A number of conditions should
be carefully observed during the initial installation period including the following:
7.6.9.1 Insulation dielectric strength
should be checked, particularly for moisture,
by means of insulation resistance checks and
dielectric absorption tests. High potential
proof or acceptance tests are usually performed before the machine is placed in operation.
If necessary, the machine should be dried out
IEEE
Std 492-1974
by means of space heaters or controlled lowvoltage current. Means should be provided for
expelling the moisture-laden air from the machine housing.
7.6.9.2 Rotor unbalance should be measured and corrected, if necessary, by the addition of appropriate balancing weights.
7.6.9.3 Machine vibration should be observed to be sure it is within normal limits.
7.6.9.4 Machine temperatures including
stator winding, rotor winding, and bearing
lube oil should be observed. If any are out of
normal limits, such things as cooler operation,
blocked ventilation passages, and lube-oil system operation should be checked.
7.6.9.5 Unusual noises should be investigated for possible mechanical defects or looseness and corrected immediately to prevent
more serious damage.
7.7 Cooling Systems. Generators having cooling systems should have the following taken
in to considera tion:
7.7.1 Initial Filling of Coolers. When initially filling the coolers, the flow of raw water
should be controlled so that the entire cooler
will be vented and filled with water at a rate
such that the design pressure will not be exceeded due to water hammer.
7.7.2 Normal Cooler Operation. The flow of
water to coolers consisting of several sections
should be controlled so that the entire cooler
will be filled with water by continuous venting
and so t h a t design pressure will not be exceeded. With multisection coolers which are
in parallel for waterflow, which is the usual
arrangement, care must be exercised to equalize the waterflow through the sections. This
should be done by the use of flowmeters or
pressure gages on the waterflow to the sections.
7.7.3 Automatic Cooling Water Systems.
There is no inherent lower limit in the power
loading of a generator itself, although the
prime mover and associated equipment may
impose such a limit. When operating a t loads
varying from a low value up to rated full load,
the desired degree of cooling system control
depends on the time duration and extent of
the load swings. Where load swings are more
or less unpredictable and irregular, some form
of automatic regulation of the raw water is advantageous.
IEEE Std
492-1974
T h e object of such regulation is to reduce
temperature variations and maintain a more
uniform total temperature, particularly in the
stator winding, and thus prolong the life of
various machine parts. Automatic systems
have been used in which the raw water supply
header is equipped with a motor-operated
valve that opens and closes in response to temperature detector variations. A disadvantage
of this system is that the cooler has normal
water volume and flow only when the valve is
fully open and therefore is subject to scaling
and deposits in the tubes as well as unpredictable cooling characteristics. Other systems have been used in which waterflow
through the cooler is kept constant, and temperature is changed by mixing cooler discharge water with varying amounts of raw water through a two-way valve. T h e temperature
detector which operates the automatic controls for these systems may be a stator winding R T D or other temperature-sensing element.
7.8 Fire-Protection Systems. Fire-protection
systems are usually provided for generators to
keep the damage caused by a n internal fire to
a minimum.
7.8.1 Carbon Dioxide System. Carbon dioxide is generally used as the fire quenching medium. It is admitted to the generator housing
through piping and nozzles to fill the housing.
T h e carbon dioxide may be admitted either by
manually actuated valves or by automatically
actuated valves. The automatically actuated
valves may be operated by a signal from temperature-sensitive thermostats or by relay action such as the generator differential relays.
7.8.2 Other Systems. Water sprinkler systems can be installed on both open and totally
enclosed air-cooled generators. As with carbon dioxide systems, they can be manually actuated or operate automatically in response to
fire detecting devices and differential relays.
Their operation, whether inadvertent or legitimate, is likely to result in a generator
dryout being necessary. Inadvertent operation can cause unnecessary electrical failures
of the generators.
7.9 Vibration Detection and Correction.
7.9.1 Detection Devices. Some generators
might be equipped with a vibration detection
device (usually seismic weight principle)
IEEE GUIDE FOR OPERATION
mounted on a vibration sensitive stationary
part such as bearing bracket. This device can
be set a t a vibration g-level slightly higher
than the normal operating vibration.
If any unusual mechanical or magnetic
forces cause higher than normal vibration, the
device will close a contact giving alarm or
shutting the unit down, depending on the design of control circuits.
7.9.2 Causes of Vibration. Vibrations attributable to the generator can be caused by
abnormal mechanical or electrical conditions
either in stator, rotor, or the combination of
the two (for example, air gap variation).
Most commonly, the vibrations originate in
the rotor due to mechanical unbalance (which
causes shaft runout) or magnetic unbalance
caused by out-of-roundness of the rotor or stator, or shorted turn(s) of a field pole. In addit i o n , abnormal hydraulic conditions can
cause vibrations to be transmitted from the
turbine through the shaft or foundation to the
generator. For additional information, see
Sections 8.3.2, 8.3.3, and 8.3.7.
7.9.3 Correction of Vibration. Any permanent increase of vibration indicates change
in the generator-turbine unit, and the cause of
it should be immediately investigated. Depending on the severity of the vibration increase and the cause of it, decision should be
made whether or not the generator should be
taken out of service for immediate corrective
action.
For procedures for alignment check and rebalancing, see Section 8.3.7. For unusual operating conditions, see Section 7.6.
7.10 Operation of Generator/Motors. Hydrogenerators may be reversible in nature and
used as motors in pumped-storage applications. The turbine becomes a pump, and the
generator becomes a motor to replace the water taken from the upper reservoir during the
generating mode.
7.10.1 Starting Methods. One of the problems unique to generator/motors is starting as
a motor in preparation for the pumping mode
of operation. Among the means of starting the
motor are the following:
7.10.1.1 Wound-Rotor Induction Starting Motor. A wound-rotor motor, large
enough to overcome the losses of the main
unit and accelerate it to synchronous speed, is
mounted on the shaft of the main unit.
IEEE
Std 492-1974
AND MAINTENANCE OF HYDRO-GENERATORS
7.10.1.2 Synchronous Start. There are
several variations of this method, all utilizing
one machine operating as a generator in parallel with a second machine operating as a
motor. Among the variations are:
(1) Both machines are paralleled a t standstill, separated from the power system, and
have their field windings energized. T h e pair
are brought up to speed, one acting as a generator and the other as a motor. They are synchronized to the system, and the generating
machine is then shut down
(2) The same as (1)except the motor unit
does not have its field energized and operates
as a n induction motor during starting. T h e
field is applied after the motor has been
brought near synchronous speed
(3) Similar to (1) or (2) except that the generator is a smaller nonreversible accessorytype turbine generator
(4) Similar to (1)or (2) except the units are
not paralleled until after the generator unit
has been brought to some intermediate speed
7.10.1.3 Full-Voltage Induction Start.
T h e main unit has a heavy amortisseur winding which is used to start the main unit as a n
induction motor across the line.
7.10.1.4 Reduced - Vol tage Induct ion
Start. The unit is started as an induction motor on reduced line voltage using either taps
on the main transformer, a n autotransformer,
reactors in the leads, or delta-wye shift of the
main transformer connections to provide the
reduced voltage.
7.10.1.5 Reduced - Voltage Reduced - Frequency Start. T h e generatodmotor to be
started from rest is electrically connected to a
decelerating generator of comparable size.
T h e energy of the rotating deceleration generator is electrically transferred to the generator/motor with reduced voltage and varying
frequency. T h e units are synchronized a t a
subsynchronous speed and then accelerated to
rated speed with the generator turbine. Synchronization to the system is accomplished in
the normal manner.
7.10.1.6 Variable-Frequency ReducedVoltage Start. T h e generatodmotor is started
from a static variable frequency source capable of supplying starting power from zero
frequency to greater than 60 Hz and a t a value
of reduced voltage suitable to the application.
7.10.1.7 Accessory Turbine. A small tur-
bine is mounted on the shaft of the main unit
with the opposite rotation to the main turbine
and is used to accelerate the motor pump to
synchronous speed.
T h e water in the pump cavity is generally
depressed by the use of compressed air during
the starting period for any of these starting
met hods.
7.10.2 Synchronization. When operating a
unit in the pumping mode, the motor must be
synchronized to the system after reaching
synchronous speed for any of the starting
methods with the exception of the full voltage
induction and reduced voltage methods. T h e
synchronizing may be accomplished either
manually or through the use of automatic
synchronizing equipment.
8. Maintenance
8.1 General Considerations. Parts of a hydrogenerator such as collector ring brushes are
subject to wear and require periodic adjustment and replacement. Other parts such as
coolers and the ventilating system may accumulate dirt or foreign material and require occasional cleaning. Winding insulation deteriorates with age and requires careful inspection
and maintenance to obtain reliable service.
All parts are exposed to continual vibration,
stress, and temperature changes and may in
time become loose or develop fractures. As a
consequence, it is desirable to observe the operation of the unit frequently and to investigate any unusual changes in the performance,
sound, temperature, vibration, or appearance.
I n addition, a regular schedule of inspection,
testing, and maintenance should be adopted
so t h a t minor troubles may be eliminated before they develop into major ones.
8.1.1 T h e expense of inspecting and maintaining large or important machines in good
condition, rather than operating the machine
continuously until failure occurs in service,
can be justified on the basis of increased reliability and lower over-all cost. Furthermore,
there is a distinct advantage in maintaining
machines a t periods when outages can be
planned so as not to interfere with peakloads
on the system.
8.2 Frequency of Inspections. Visual in-
spections vary widely in how extensive they
27
IEEE Std
492-1974
IEEE GUIDE FOR OPERATION
Efforts should be made to periodically examine all of the machine parts mentioned
above. T h e sliprings, commutators, and
brushes should be inspected frequently as is
discussed later. For the other parts of the generator, yearly partial inspections are recom mended. Later, depending on the troubles
found, partial inspections may be prudently
scheduled every two years or more and may be
limited in scope.
8.2.2 Complete Inspection. A complete inspection can best be accomplished with removal of the rotor. Great care should be exercised in removing the rotor to avoid damage
to the armature punchings, stator winding,
bearings, and other parts. All parts removed
should be carefully identified so as to assure
their reinstallation in precisely the same
places. Reassembly should include scrupulous
attention to cleanliness, removal of all foreign
objects such as tools and rags, and positive securing or locking of all assemblies (fans, baffles, and plates).
8.3 Inspection and Maintenance Procedures.
Care should be exercised by all workmen and
inspectors to assure that no damage ensues
from the activities of disassembly, inspection,
and maintenance. All tools and inspection
aids such as mirrors and lights should be secured unless retrieval is certain. Complete descriptions of findings (good and bad) and repairs should be made a t the time or promptly
thereafter.
8.3.1 Stator.
8.3.1.1 Inspection of Winding, Bracing,
Wedging, and Ties. T h e stator winding should
be examined closely for signs of electrical and
thermal degradation and deterioration from
mechanical forces. Corona attack on coil end
turns will be evidenced by paint discoloration
or paint erosion, often in spots and small
areas. In severe cases, outer protective tapes
or even portions of the ground wall may be
eroded. Such deterioration is likely to commence in areas where adjacent end turns are
a t markedly different potentials in service.
Semiconducting surface paints and tapes
which have special resistivities to limit harmful corona are often applied to the end turns.
Conducting varnishes and tapes are also used
in ties and bracings for the same purpose. Patterns of corona may indicate breaks in these
special surface treatments or reveal areas
are and, consequently, in how much work and
time they involve. Many manufacturers and
users believe that major rotating electrical
machinery should be completely dismantled
and thoroughly inspected near the end of the
first year of service. Thereafter, complete disassembly and visual inspection may be warranted only a t long intervals. In such instances, periodic partial inspections should be
considered because incipient disabilities often
can be discovered and corrected during short
partial inspections. Listed below are some of
the factors which may have bearing in determining the time between visual inspections
and the degree of disassembly and inspection:
( 1) Findings a t previous visual inspections
(2) Results of previous electrical tests
(3) Results of electrical tests made during
current outage (some owners decide which
electrical tests to apply after making a visual
inspection)
(4) History of duplicate and similar machines
(5) Special recommendations from manufacturers
( 6 ) Concurrence of outages on other associated equipment such as turbines and transmission lines
(7) Upcoming service circumstances such as
frequency of starts, load cycling, and overloading
(8) Anticipated remaining service life of
whole unit
(9) Availability of manpower
(10)Weather-if outdoor exposure is involved
(11) Severe or unusual operating conditions
8.2.1 Partial Inspection. Simply by shutting the machine down, it is possible to inspect the sliprings, commutators, and brushes. On some machines, removal of manholes
or handholes may provide limited but useful
views of stator winding end turns, fans, and
airgaps. More extensive inspections can be
made by removing housing endbells, end covers, or plates thus affording access to stator
end turns, parallel rings and leads, airgaps,
field connections and leads, fans, baffles, and
internal bushings. Other housing covers may
have to be removed to provide access to terminals, bushings, current transformers, back of
stator core iron, ventilation passages, and
soleplates.
28
IEEE
AND MAINTENANCE OF HYDRO-GENERATORS
where deterioration has occurred or the original design is inadequate.
End turns should be examined for cracks in
the outer tapes and should be felt by hand to
judge the condition of the ground wall. Characteristics such as dryness, firmness, and
sponginess should be looked for. Long-bore asphalt compounded coils may develop tape separation or girth cracks a t or near the iron if
they are tight in the slots and are subjected to
frequent severe cycling. Such a girth crack
may not be evident visually but may be felt as
a shallow circumferential groove beneath
smooth appearing outer or armor tapes. End
wedges, even very tight ones, can be removed
to afford much greater inspection access to the
top coil where it leaves the iron.
Swelling of coils and migration of compound should be noted. General cleanliness or
oiliness should be observed.
Chafing of coils against each other and
against blocks, braces, and ties should be looked for. Packing or bedding behind and between end turns should be inspected.
E n d t u r n distortion from short circuit
stresses should be noted.
All leads, jumpers, and parallel rings should
be inspected for signs of corona and looseness.
T h e wedges in the slots should be checked
for tightness in all three directions. Wedges
which sound hollow when tapped with a n object such as a pocket knife should be noted.
Slots, core ventilation passages, and end turns
should be examined for dust as a n indication
of looseness of coils, side fillers, wedges, or
core. Red dust a t or near wedge grooves may
be iron oxide due to loose wedges or a loose stator core.
Small mirrors and borescopes aid in making
inspections of inaccessible areas.
8.3.1.2 Maintenance and Testing of
Various Insulation Systems. T h e principal
maintenance aim should be to keep the windings and leads mechanically tight, for often a
winding succumbs to troubles arising from
looseness and vibration rather than from thermal or electrical degradation. Therefore, all
loose ties, blocks, wedges, supports, etc, should
be secured. Repair materials can include varnish, glass cord, fabric reinforced phenolics'
and epoxy. The materials should be com' S u c h a s Micarta.
Std 492-1974
patible with the original materials in chemical composition, temperature class, and abrasion characteristics. Slot, rewedgmg with additional filler should be done where necessary so t h a t all coils are tight radially. This is
essential to preclude vibration and pounding
in t h e slots. I n vertical units, downward
movement of wedges and filler should be prevented by suitable stops. Care must be exercised to avoid damaging the stator coils
while removing and reinstalling slot wedges
and fillers.
Corona attack in the end turns can best be
eliminated by judicious application of semiconducting finishes and tapes to the coil surfaces and the blocks between. Unless the user
has experience in this field, the manufacturer
should be consulted, because ill-designed
changes in surface coverings may harmfully
move corona activity to other sites in the end
turns. Temporary protection of the basic
ground wall insulation can be supplied by applying insulating varnishes as wearing surfaces, but the owner should realize that these
will be eroded in time if the corona damage activity pattern is not changed. Periodic revarnishing may suffice in places where corona
damage is very slow.
Wedges which are merely loose radially can
sometimes be adequately secured by an application of thick varnish or a n epoxy cement.
Dust may be blown out with compressed air.
Care must be exercised in the applied pressure
so as not to damage the insulation, and the
usual safety precautions for handling compressed air should be observed. Since condensation often occurs in air lines, it should be
ascertained that the air is free from moisture
before being directed into the generator. A
moisture separator or filter may be necessary.
All exposed surfaces may be wiped with clean
cloths to remove any remaining oil or dirt.
Crushed corncobs propelled by compressed air
provide a scrubbing medium with which excessively oily and caked surfaces can be cleaned. I t may be necessary to use a cleaning solution recommended by the manufacturer in order to effectively remove the dirt. Stoddards
solvent and 1,1,1 trichloroethane are effective
as solvents and may be used by wiping the
winding with solvent-moistened rags. Care
should be exercised in using cleaning fluids because of their toxic effect and possible ex-
IEEE Std
492-1974
plosion hazard. Gasoline, carbon tetrachloride, and similar solvents should not be used.
Special attention should be given to ventilation for protection of personnel. Any cleaning fluid is more or less a solvent for insulating
compounds, hence, the application of these
fluids in large quantities should be avoided.
Also, they should not be allowed to stand in
contact with the winding any longer t h a n necessary to remove the oil and dirt.
After the windings and core have been
cleaned, they should be inspected carefully for
any signs of deterioration. If required, one or
two very thin coats of insulating varnish recommended by the manufacturer may be applied. Compatibility between new and existing
varnishes should be checked. T h e unnecessary and frequent application of coats of varnish often results in more harm than benefit.
For proper maintenance of the machine, it
is necessary to combine tests of proven significance with visual inspection. T h e use of either
visual inspection or any presently known test
procedures alone is not sufficient for a proper
understanding of the condition of the machine. Both visual inspection and proper test
procedures must be utilized and coordinated
in order to reach a sound conclusion. Electrical tests which may prove helpful are discussed in Section 8.5. Additional information
concerning insulation maintenance may be
found in IEEE Std 56-1958 (ANSI C50.251972).
8.3.1.3 Replacing Individual Coils or
Half-Coils. Replacing individual coils is feasible on many machines. Success depends
upon the coils not being too tightly fitted in
the slots circumferentially, the insulation not
being excessively dried out, and provision of
sufficient space around the bore of the machine to enable loosening or fanning out of
many coils. Access for fanning out coils sometimes can be provided by removing several
field poles, but in other instances the entire
generator rotor must be removed. I t is important to have proper access to the face of the
bore in order to avoid damaging additional
coils. In order to replace a coil, it is necessary
to gently move forward all the coils lying over
it. This inevitably involves also gently moving
forward a number of adjacent coils. Distortion
of each coil should be kept to an absolute minimum, for there can occur both ground-wall
damage and turn-to-turn damage if the coils
IEEE GUIDE FOR OPERATION
are twisted or bent. Thus, there is a risk t h a t
the very act of replacing a coil may create other defective coils. Once a coil is replaced, the
ones moved out must be put back just as carefully as they were moved forward. Consequently, many connections must be taken
apart and remade. T h e work is tedious but
when successful, will restore a winding to its
full rating. Windings which are relatively new
and consequently have resilient ground wall
insulation and windings which have been
compounded with mica-asphalt combinations
lend themselves more readily to coil movement without damage. Heat aids in making
asphalt coils more limber and also may assist
some of the thermosetting insulation systems,
but care should be taken to not overheat the
coils. Some owners test coils individually for
turn-to-turn shorts during repairs of this sort.
Highpotential tests are advisable after the
coils are set down and wedged but before the
connections are made.
Half-coils or bars are much more readily replaced, either front ones or back ones, for distortion of whole coils can be avoided completely. Half-coils equipped with connections
a t both ends lend themselves to removal and
replacement easily. Single-turn coils also can
be replaced, since connecting them into the
winding is a simple matter. One-half of multiturn coils can be replaced but with considerably more work at the knuckles. Care must
be exercised to restore the turn and strand insulation and the correct wire-to-wire connections a t both knuckles. The finished size
and appearance of the knuckles a t the ends, in
such a case, may be markedly different, but
this is of no concern if the knuckles are properly insulated, braced, and ventilated.
8.3.1.4 Bypassing Stator Coils. A coil to
be bypassed should be cut a t the connection
(lead) end, physically separated from the rest
of the winding, and suitably insulated. At the
opposite end, the coil should also be cut and
insulated so as to eliminate circulating currents should a short develop later between
turns in the idle defective coil. Extreme care
should be exercised in correctly identifying
the other end of the defective coil. Jumpers to
complete the winding connections, whether
series or group connections, can be made up of
regular wire stocks by bundling enough
strands to provide the cross section needed to
match the rest of the winding. Where the
IEEE
Std 492-1974
AND MAINTENANCE OF HYDRO-GENERATORS
and maintenance, they should be protected
from dirt and moisture and efforts made to
keep their temperature a few degrees above
that of the surrounding air, particularly if
compounded with asphalt, as such windings
are more susceptible to moisture pickup in the
ground-wall insulation. If the machine cannot be kept dry, then the normal procedure
recommended by the manufacturer should be
followed to determine whether dryout is needed before the machine is returned to service.
Advice concerning dryout procedures will be
found in instruction books and in IEEE Std
43-1961 (Reaff 1971) (ANSI C50.22-1972).
8.3.2 Stator Core and Frame. T h e airgap
side of the stator core iron should be examined
for shorts between laminations, evidenced by
overheating or by appearance as if peened.
T h e core fingerplates and structural parts
should be inspected for looseness and hotspots. A suitable epoxy glue may be used to secure loose laminations and restore interlaminar insulation. Air baffles and the structures holding the parallel rings or connections
should be inspected and tightened where necessary. T h e back of the core should be examined similarly including the foundation bolts
and such major frame support pieces as can be
seen. Occasional very careful inspections of
the frame are in order so as to assure that very
slow long-time changes are not affecting the
main structural strength and roundness of the
stator core. Broken welds should be repaired,
and, if they persist in recurring, the manufacturer should be consulted. Stator throughbolts should be inspected and retightened, if
n e c e s s a r y , t o m a n u f a c t u r e r ’ s suggested
torque.
8.3.3 Rotor. Connections and coils should
be inspected for looseness and signs of overheating or shorts. On salient pole machines,
poles having shorted turns usually can be repaired without difficulty if the poles are removed from the rotor frame. Ground insulation washers should be examined and renewed
if necessary. General cleanliness should be
noted and suitable cleaning and repainting
carried out if required. On some vertical generators the rotor poles are keyed to a laminated rim which is in turn supported by small
ledges at the bottom of the rotor spider. Such
ledges are occasionally found to have failed
due to fatigue and ought to be inspected a t intervals because breakage of one or more of
winding design requires preservation of strand
insulation through the connections, the jumpers should do this also. Connections that are
brazed will have longer life. Mica-mat insulation and glass tape coverings, together with
p a i n t , will provide long-lasting jumpers.
Lashing and ties can be added where required.
Experienced winders or engineers can manage
this repair without reference to the manufacturer’s winding diagrams.
On rare occasions it may be desirable to arrange for a sound coil to serve in a winding
path different from that wherein it is installed, as a consequence of several coils having been cut out of a particular path. I n this
event, it is important that a winding diagram
be available; for otherwise, it is possible to
make serious mistakes in rearranging the
group or phase path connections. Jumpers
properly insulated and braced can be used as
described above once the proper connections
have been decided upon.
T h e front half of a bypassed coil can be removed from the machine readily if dissection
and inspection of it a t close hand is desirable.
This usually can be done merely by pulling
one of the rotor poles to provide working
space. T h e missing half-coil, in such a n instance, can be replaced with a wood beam or
block, wedging it in place to provide support
for the half-coil in the bottom of the slot. End
turn blocks and lashings must be adapted to
bridge across the vacant space.
8.3.1.5 Interchanging Main and Neutral
Connections. Many hydro-generators are constructed so t h a t the stator winding main and
neutral lead connections to the circuit rings
may be interchanged. T h e lowest dielectric
gradient across the stator-coil ground wall insulation occurs a t the neutral end of the winding, and the highest gradient (line-to-ground
voltage) is at the main lead end. When the insulation begins to deteriorate with service as
evidenced by tests or visual observation, main
lead end coils are often in poorer condition
t h a n neutral end coils because of the greater
dielectric stress to which they have been subjected. Therefore, it may be possible to extend
life of t h e winding appreciably by interchanging the connections so that the weaker
coils are near the neutral end and under less
dielectric stress.
8.3.1.6 Moisture Protection and Dryout.
While the windings are exposed for inspection
31
IEEE Std
492-1974
IEEE GUIDE FOR OPERATION
these ledges might allow the rim to slump and
the poles to lean outward toward the stator.
T h e r e should be close attention paid to
whether the individual poles are solidly bearing against the rim of the rotor. Feeler gauges
may be used to check for a possible gap between the inner surface of the pole body and
the rim. Such parts of the rotor as are involved
in the fan structures should be examined
closely for breakage and the security of all
bolts. Cracked blades should be looked for as
well as overheating due to circulating currents
a t fan blade tips or a t baffle mating surfaces.
Amortisseur bars and the iron along the bars
should be examined closely. Evidence of iron
heating may indicate a break in a bar. The
pole-to-pole connections, deterioration of
which may not show up in pole drop tests,
warrant inspections and in some designs
should be replaced with ones of more flexible
types.
8.3.3.1 Airgap Measurements and Variations. I t is important to measure the air gap
in detail on each machine early in its life. This
should include measurements a t top and bottom of every pole, if possible. T h e position of
the rotor relative to the stator should be
noted. It is well to also determine the precise
roundness of the rotor. In later years the airgap should be measured occasionally a t every
900.
Many manufacturers recommend that rotor
airgaps vary no more t h a n plus or minus 5
percent from the average. Users find in many
cases that plus or minus 10 percent is about as
good as can be reasonably achieved. Variations over 10 percent should be investigated.
Unequal airgaps may have serious effects on
the structural strength of the rotor as well as
the stator.
8.3.4 Rotating Exciter. Moisture and dirt
are the causes of most breakdowns. The exciter should, therefore, be kept as free as possible from metal dust, dirt of any description,
oil, or water. Wiping with rags, a vacuum
cleaner, or low-pressure dry compressed air
may be used to remove dirt. T h e air must be
free of water. T h e insulation resistance of the
exciter windings should be checked annually
and not allowed to fall below the manufacturer’s recommendation. If oil should become mixed with the dust and dirt, it will be
necessary to clean with a solvent designed for
the purpose. T h e solvent should be used only
in a n area that is free from open flames and is
well ventilated. T h e solvent used should have
a high flashpoint, low toxic characteristics,
and should have proper solvent action on
grease and oil but a minimum effect on insulating varnishes. Care should be taken to prevent the solvent from contacting the commutator surface since the surface films are
sensitive to solvents.
Inspect the commutator frequently to be
sure that a polished surface is maintained. Ordinarily, the commutator will require only a n
occasional wiping with a piece of dry canvas
or other nonlinting material, but if blackening
or sparking appears and grows worse, the
cause must be determined and corrected. T h e
brushes must move freely in the holders and,
a t the same time, make firm, even contact
with the commutator. Check the pressure and
keep it to the proper value as the brushes
wear. Refer to manufacturer’s instructions for
details concerning proper brush pressure, distance between brush holder and commutator,
and adjusting methods. Replace worn brushes
with brushes of the same grade; do not mix different grades of brushes.
8.3.5 Collector Rings, Brushes, and Rigging. Inspect the collector ring frequently
since trouble from defective brushes can develop suddenly. The brushes should move freely
in the holders, and the brush faces should
have a polished surface. As noted above for
the commutator, replacement brushes should
be of the same grade. When installing new
brushes, they should be fitted to the curvature
of the collector ring. This can be done by putting a piece of sandpaper under the brush and
moving it in the direction of rotation while
pressing down on the brush. Check that the
recommended brush pressure is held as the
brushes wear, and the proper distance between brush holder and collector ring is maintained. Keep the collector ring, brushes, and
rigging clean and dry. Dry dirt or dust can be
cleaned with a vacuum. If oil is mixed with
the dirt or dust, it will be necessary to clean
with a solvent designed for the purpose. (Note
the precautions for using a solvent given under Section 8.3.4 above.)
8.3.6 Bearings.
8.3.6.1 Guide Bearing Clearance Checks.
Guide bearing clearances are normally only
32
IEEE
AND MAINTENANCE OF HYDRO-GENERATORS
checked during a n overhaul. One means used
for checking the bearing clearance is to jack
the shaft to one side and set a dial indicator.
Then jack the shaft in the opposite direction
and read the travel. This travel is the total
bearing clearance. The same measuremeqts
are also taken 90° around the shaft. An alternative method is to check and record the guide
bearing clearance at each bearing shoe. While
investigating bearing clearances, it is advisable to check clearance of guide bearing seals
which, if excessive, could result in oil or vapor
leaks or both.
8.3.6.2 Bearing Insulation Checks. T h e
bearings above the rotor of vertical generators
are usually insulated from the frame to prevent circulating current through the bearing
surfaces, causing damage to the surfaces. T h e
bearing insulation should be checked annually by means of a n ohmmeter. If the insulation resistance is less t h a n the manufacturer’s recommendation, the cause of the
trouble should be investigated. I t is good practice to keep a record of these readings for comparison so that a problem can be detected and
corrected before it develops into a failure.
8.3.6.3 H i g h-Pressure Lube S y s tern
Checks. A high-pressure lubrication system
requires very little maintenance since it is
only used during the starting and stopping period, and operates for a short time. The most
common problem is failure of a high-pressure
line. Most systems are equipped with a pressure switch which gives a n alarm or prevents a
start if the proper oil pressure is not obtained.
8.3.6.4 Lubrication Testing. A lubricant
testing program is suggested to detect changes
in the lubricant which should be investigated.
Laboratory tests for viscosity and total acidity, which is a n indication of oil oxidation,
are normally used. T h e atomic absorption
spectrometer test may be used to determine
the concentration of metals in the lubricant.
Annual or semiannual sampling and testing
of the oil in a lubricating system is normally
adequate. Water in the oil may indicate a leak
in the oil cooler. One of the important items in
a testing program is to have a good recordkeeping system so the test results can be compared and changes detected.
8.3.7 Alignment and Balancing. Initial
alignment of the shaft or rotating parts is of-
Std 492-1974
ten done in the factory before shipment. Additional adjustments are made by the manufacturer near the completion of field installation so that the machine is satisfactorily
aligned and balanced when it is turned over to
the user and placed in service.
After a major inspection or repair involving
removal of the rotor, realignment is necessary.
Alignment of a vertical hydro-generator is a n
involved process requiring precise measurements of shaft plumbness and runout. A logical pattern of recording the data must be used
so that a correct interpretation of the readings
may be made. Detailed explanations of alignment and balancing techniques are available
from sources listed under Section 11.
8.3.7.1 Acceptable Vibration and Shaft
Runout. Acceptable runout for rotational
checks is defined in NEMA MG 5.2 by the formula 0.002 x L / D , where L is the distance in
inches from the thrust surface to the point of
runout measurement and D is the thrust bearing outside diameter in inches. No standards
for acceptable maximum vibration have been
developed.
I t has been found that rebalancing may be
used to correct difficult situations of misalignment. Hydraulic and electrical forces on
a unit under load may produce excessive runouts which can be countered by balancing
more easily t h a n by adjustments to the bearings or other parts.
8.3.8 Cooling Systems. T h e effectiveness of
coolers will be reduced due to accumulation of
sludge or other matter in the cooler tubes. If
water conditions are such as to cause the accumulation of foreign material in the cooler,
periodic cleaning should be carried out. Strainers in the cooling water supply, if used,
should be cleaned regularly. Dirt in the cooler
or deterioration of the cooler performance is
indicated by a n increase in temperature difference between the water and air temperature for the same load condition.
T h e cooler tubes should be examined for
traces of moisture which may indicate leaks.
Accumulation of sludge, or other matter, in
the cooler tubes will increase corrosion. New
tubes should be installed if necessary.
Coolers operated a t reduced waterflow may
accumulate sludge and should be periodically
flushed with full waterflow. This will reduce
IEEE Std
492-1974
the frequency a t which they will have to be removed for cleaning.
8.3.9 Brakes and Jacks. Brakes and jacks
normally require very little maintenance. T h e
brakeshoes should be inspected occasionally
for wear. Air-operated systems should also be
inspected for leaks and lubrication of the cylinder packing.
8.4 Cleaning. T h e machine should be cleaned
as often as operating conditions make it necessary. A vacuum cleaner is recommended for
cleaning. Low-pressure compressed air can be
used, but it must be dry. Cleaning fluids may
be needed to clean oil, but the user should be
certain they do not attack the electrical insulation or varnish. Consult the manufacturer
for suitable cleaning fluids. Caution is required when using any cleaning fluid; the area
must be well ventilated, and smoking should
be prohibited (See Section 8.3.1.2).
8.5 Testing. A great deal can be learned about
the condition of a generator by a complete and
thorough visual inspection. Mechanical wear
or degradation in particular are often manifested by external signs. However, external
signs are not reliable in judging condition of
some machine components, such as insulation, and specialized tests are necessary to
make a realistic evaluation of the condition of
these components.
8.5.1 I n the maintenance of generators, a
user is interested in tests which will accomplish the following:
(1) Provide a basis for determining the serviceability of a generator, or corrective action
necessary, after a suspected or actual failure,
or severe duty
(2) Indicate long-term trends in behavior
(3) Anticipate possible service failures
Tests which have proved to be useful in accomplishing these objectives are listed below.
8.5.2 T h e method of making most of these
tests and the interpretation of the test results
are discussed in greater detail in IEEE Std 431961 (Reaff 1971) (ANSI C50.22-1972).
8.5.2.1 Insulation Resistance Tests at
Low- Voltage Direct Current. Primarily to detect grounds and wet or dirty insulation. Includes tests a t 500 to 5000 V.
8.5.2.2 Insulation Resistance Tests at
High-Voltage Direct Current. T o reveal char-
IEEE GUIDE FOR OPERATION
acteristics which may not be disclosed by the
low-voltage test.
8.5.2.3 Dielectric Absorption Tests. T o
furnish information concerning the relative
condition of the insulation.
8.5.2.4 Dielectric Overvoltage Tests. T o
establish at the time of test t h a t the winding is
capable of withstanding the applied voltage.
Both alternating- and direct-current tests
have been used for maintenance overvoltage
testing. T h e dc high-potential test method has
the following advantages over the ac method:
(1) Direct potential testing causes less deterioration of the insulation up to within a few
percent of the breakdown point
(2) T h e small leakage current drawn can be
readily measured and tendencies to increase
or decrease noted
(3) Much less damage is done if a breakdown occurs since the capacity of the equipment is small
(4) T h e test equipment is relatively small
and light since it needs to be only large enough
to supply the insulation leakage current
A special precaution to be observed with the
dc method is the long time required to drain
off the charge after a test is made.
8.5.2.5 Insulation Power Factor Tip-Up
Tests. Sometimes used to detect moisture and
voids in the insulation and indicate amount of
ionization.
8.5.2.6 Slot Discharge Test. T o detect
surface discharge which may be injurious.
8.5.2.7 Surge Comparison Tests. T o test
turn-to-turn insulation.
8.5.2.8 Corona Probe Test. T o indicate
and locate unusual ionization about the insulation structure.
8.5.2.9 W i n d in g R e s i s t a n c e T e s t s
-Direct Current. T o detect loose connections
and open-circuited windings.
8.5.2.10 Field- Winding Impedance and
Pole Drop Tests. T o detect and assist in locating turn-to-turn faults.
8.5.2.11 Ducter (Low-Range Voltmeter).
T o check interpolar connections of the field
and amortisseur windings.
8.5.2.12 In t erla m i n ar Insul a t ion Tes ts.
T o detect and locate damaged areas in the stator core.
8.6 Extensive Downtime-Care of Machine.
8.6.1 Temperature and Moisture Consid-
IEEE
Std 492-1974
AND MAINTENANCE OF HYDRO-GENERATORS
tection, and grounds of the electrical components.
Verification of the reliability of these devices would include manual operation of the
actuating mechanism to be certain it operates
freely and an inspection and cleaning of the
device with special attention to the electrical
contacts to make certain they are in satisfactory condition.
Most of these devices are rarely operated in
the normal service life of a hydro-generator.
They are subject to a n accumulation of dust
or dirt and the dryout or hardening of lubrication. Instances of failure to perform their
function have been traced to accumulation of
rust, oxidation, and foreign material. Detailed
maintenance instructions for these protective
devices are usually covered in the instruction
books supplied by the manufacturer of the
hydro-generator.
eratwns. Due to their large size and weight,
hydro-generators are very vulnerable to moisture condensation from temperature variations. This may cause rusting and affect
winding insulation. Most generators are provided with heaters to keep the generator temperature abova the surrounding ambient to
avoid condensation. Particular attention has
to be paid to this feature when extensive shutdown is planned. The heater should be suitably controlled to avoid overheating.
8.6.2 Cooling Water Supplies. All cooling
water should be shut off completely and lines
preferably drained to avoid condensation,
particularly in the bearing oil reservoirs.
8.6.3 Restart of the Machine. Depending on
the extent of downtime, the components of the
machine have to be checked out, particularly
the bearings and windings as explained in
Sections 7.2.3, 7.6.9, and 8.3.
8.7 Safety Precautions. Due to the large physical size of hydro-generators including the associated gear, such as exciters, field breakers,
etc, it is usually the responsibility of the operating personnel to establish detailed safety
procedures regarding access to rotating and
high voltage parts, door interlocks, operating
procedures, etc.
The generator instruction book may contain some of the special warning notes and
safety precautions which are intimately associated with the machine.
10. Standard References
The following American National, NEMA,
and IEEE Standards were used as references in
preparing this guide and apply to the machines
described in it:
A N S I B49.1-1967 (R1971), Integrally
Forged Flange Type Shaft Couplings for Hydroelectric Units
ANSI (30.10-1965, General Requirements
for Synchronous Machines
ANSI (30.11-1965, Requirements for Synchronous Motors
A N S I C50.12-1965, Requirements for
Salient Pole Synchronous Generators and
Condensers
ANSI C50.5-1955, Rotating Exciters for
Synchronous Machines
NEMA MG 5.1-1969, Large Hydraulic-Turbine-Driven Synchronous Generators
NEMA MG 5.2-1972, Installation of Vertical
Hydraulic-Turbine-Driven Generators and Reversible Generatorhlotors for Pumped Storage Installations
IEEE Std 1-1969, General Principles for
Temperature Limits in the Rating of Electric
Equipment
I E E E S t d 43-1961, (Reaff 1971) (ANSI
9. Miscellaneous
9.1 Verification of Protective Relaying. It is
desirable to include a check of the operation of
the protective relays and devices during the
scheduled preventative maintenance inspections of a hydro-generator. Most relays
which operate in the metering electrical circuits of the hydro-generator are routinely
checked by the relay specialists.
Protective relays are devices which are easily overlooked since they are not metered and
are not intended to function unless a n abnormal condition exists include switches for overspeed of the generator, bearing oil level, rotor
lift limit, excessive vibration, creep detection,
bearing temperatures, cooling water, fire pro-
35
LOOPER, F. C. Performance of Generator
Thrust Bearings a t Hoover Power Plant.
AIEE Transactions, vol 77, part 111, pp
1216-1223.
OLSEN, 0. A. Shaft Currents in A-C Machinery. Power Generation, Nov 1949, p
102.
ROTH, H. H. Hydro-Generator Thrust Bearings. Allis-Chalmers Engineering Review,
vol33, no 1, 1968.
C50.22-1972), Guide for Testing Insulation
Resistance of Rotating Machinery
IEEE S t d 56-1958 (ANSI C50.25-1972),
Guide for Insulation Maintenance for Large
AC Rotating Machinery
IEEE Std 95-1962, Guide for Insulation
Testing of Large AC Rotating Machinery
with High Direct Voltage
IEEE Std 99-1970, Guide for the Preparation of Test Procedures for the Thermal Evaluation of Insulation Systems for Electric
Equipment. Published for Trial Use, June
1957
IEEE S M 100-1972 (ANSI C42.100-1972),
Dictionary of Electrical and Electronics Terms
IEEE Std 113-1973, Test Code for DirectCurrent Machines
IEEE Std 115-1965, Test Procedures for
Synchronous Machines
IEEE Std 119-1974, Recommended Practice for Temperature Measurement as Applied
to Electric Apparatus
IEEE Std 286-1968, Recommended Practice for Measurement of Power-Factor Tip-Up
of Rotating Machinery Stator Coil Insulation
IEEE Std 421-1972, Standard Criteria and
Definitions for Excitation Systems for Synchronous Machines
11.2 Brakes.
ROTH, H. H. Braking Vertical Generators.
Allis-Chalmers Engineering Review, vol 32,
no 3, 1967.
11.3 Protective Relaying.
AIEE Committee on Generator Protection of
AIEE Relay Committee. Relay Protection
of AC Generators AIEE Transactions, vol
70, p 275.
11.4 Alignment and Balancing.
Alignment of Vertical Shaft Hydro Units.
Power O&M Bulletin no 2.
Field Balancing of Large Rotating Machinery. Power O&M Bulletin no 13A.
Both references above are available from:
United States Department of the Interior
Bureau of Reclamation
Building 67, Denver Federal Center
Denver, Colorado 80225
11. Bibliography
11.5 Stability.
P H I L L I P S and RUBENSTEIN Operation of
Large Synchronous Generators in the Dynamic Stability Region with a Modern Amplidyne Voltage Regulator. AIEE Transactions, vol75, part 111,p 762.
11.1 Bearings.
ALGER, P. L., and SAMSON, H. W. Shaft
Currents in Electrical Machines. AIEE
Transactions, 1924, p 1325.
BAUDRY, R. A. Evolution of Large Thrust
Bearing Design. A IEE Transactions, vol77,
part 111, pp 502-507.
BAUDRY, R. A., KUHN, E. C., and COOPER, G. D. Performance of Large Waterwheel Generator Pivoted-Pad Thrust Bearing Determined by Tests Under Normal Operating Conditions. AIEE Transactions, vol
78, part IIIB, pp 1300-1309.
JOHNSON, R. C. Loading Vertical Thrust
Bearings. Allis- Chalmers Engineering Review, vol33, no 4, 1968.
11.6 General.
CONCORDIA, C. Synchronous Machines.
New York: Wiley, 1951.
BROWN and OTTE. Electrical Design Considerations i n P u m p e d Storage Hydro
Plants. IEEE Transactions, vol82, p 625.
CONCORDIA, BROWN, MILLER, and
WUOSMAA. Synchronous Starting of
Motor From Generator of Small Capacity.
IEEE Transactions, vol86, no 10, p 1215.
36
IEEE Standards on
Rotating Machinery
IEEE
Std
43-1961
Title
(Reaff 1971) Testing Insulation Resistance of Rotating Machinery
(ANSI (250.22-1972)
56-1958
(Reaff 1971) Insulation Maintenance for Large AC Rotating Machinery (ANSI C50.25-1972)
543-1958
Induction Motor Letter Symbols
66-1969
Short Circuit Characteristics of DC Machinery
67-1972
Guide for Operation and Maintainance of Turbine Generators (ANSI
C50.30-1972)
85-1973
Airborne Sound Measurements on Rotating Electrical Machinery
86-1961
Definitions of Basic Per Unit Quantities for AC Rotating Machines
95-1962
Insulation Testing of Large AC Rotating Machinery with High
Direct Voltage
112A-1964 Test Procedure for Polyphase Induction Motors and Generators
113-1973 Test Code for Direct-Current Machines
114-1969 Single Phase Induction Motor Tests (ANSI C50.21-1972)
115-1965 Test Procedure for Synchronous Machines
116-1958 Test Code for Carbon Brushes
117-1974
Standard Test Procedure for Evaluation of Systems of Insulating Materials
121-1959 Guide for Measurement of Rotary Speed
251-1963 (Reaff 1971) Test Procedures for DC Tachometer Generators
252-1963 Test Procedures for Polyphase Induction Motors with Liquid in the
Magnetic Gap
275-1966
(Reaff 1972) Test Procedure for Evaluation of Systems of Insulating
Materials for AC Electric Machinery Employing Form-Wound Preinsulated Stator Coils
286-1968 Recommended Practice for Measurement of Power-Factor Tip-Up of
Rotating Machinery Stator Coil Insulation
288-1969 Guide for Induction Motor Protection (ANSI C37.92-1972)
290-1969
Electric Couplings
303-1969
Auxiliary Devices for Motors in Class 1 - Groups A, B, C. and D,
Division 2 Locations
304-1969 Test Procedure for Evaluation and Classification of Insulation Sys-
tems for DC Machines
329-1971 Synchronous Motor Protection Guide (ANSI C37.94-1972)
421-1972 Criteria and Definitions for Excitation Control Systems for Synchro-
nous Machines
429-1972 Evaluation of Sealed Insulation Systems for AC Electric Machinery
Employing Form-Wound Stator Coils (ANSI C50.26-1972)
434-1973 Guide for Functional Evaluation of Insulation Systems for Large
High-Voltage Machines
492-1974 Guide for Operation and Maintenance of HydroGenerators
Binder:
A special binder is available for IEEE Standards on Rotating
Machinery