Generator Testing Programs
Industrial applications
Siemens offers a variety of testing
programs for your generators. These
programs are designed to provide vital
information that will help predict the
future performance and reliability of your
equipment. This knowledge allows for
proactive maintenance and outage planning and will help to avoid major shutdowns.
Each of the following tests are directed
toward detecting particular types of issues
or follow long-term trends. Not all tests are
required at every inspection period.
Insulation resistance test
The insulation resistance test measures
the resistance of the electrical insulation
to ground to determine the current condition of the insulation. This test is typically
performed with a MEGGER® or other brand
of megohmmeters. Test voltages are commonly 500, 1,000, 2,500 or 5,000 volts.
The insulation resistance is typically in
the thousand or tens of thousands of
megohms. This test can be used on both
generator rotors and stators, as well as
motor armatures and stators.
Polarization index (DC MEGGER)
Dielectric absorption (polarization index)
tests provide information concerning
the relative condition of the insulation
with respect to moisture and other
contaminants. The polarization index takes
the ratio of the values of an insulation
resistance test at one minute and at 10
minutes.
It provides insight into how the insulation
is absorbing charge or polarizing, and is
used as a rough indication of the condition
of the insulation. As with the insulation
resistance test, this can be used on
generator rotors and stators and on
motor armatures and stators.
Winding resistance
Winding resistance tests on direct
current detect loose connections and
open-circuited windings. The winding
resistance of generator or motor windings,
on both rotors and stators, can be measured with the use of a digital low-resistance ohmmeter (DLRO) or a Kelvin Bridge.
Answers for energy.
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Outages
recommended
Test name
Description
Minor
Major
Insulation resistance test
Insulation quality and performance
Yes
Yes
Destructive Voltage
No
Low
Rotor and stator
Components tested
Polarization index
Insulation quality and performance
Yes
Yes
No
Low
Rotor and stator
Winding resistance
Winding quality and performance
Yes
Yes
No
Low
Rotor and stator
AC impedance
Winding impedance
No
Yes
No
Low
Rotor and stator
Pole balance
Measures shorted turns
Yes
Yes
No
Low
Rotor
Stator
Other tests offered by Siemens
Rarely recommended
Over potential (Hipot) test
Pass/ fail test
Yes
High
Dielectric absorption test
Determines condition of insulation
No
Yes
Yes
High
Rotor and stator
RTD insulation resistance test
Insulation quality and performance
No
Yes
No
Low
Stator and bearings
RTD winding resistance test
Winding quality and performance
No
Yes
No
Low
Stator
ELCID test
Test stator core
Specialized
No
Low
Stator
Loop test
Test stator core
Specialized
Possibly
N/A
Stator
Typically these instruments are intended
to measure very low resistance ranges and
are capable of measuring from 0.000001
Ohms (1 microOhm) to 59.99 Ohms. It is
important to compare the existing resistance to historical measurements taken
during previous outages. If the resistance
has increased compared to previous tests,
it could indicate that a joint or connection
has broken or begun to separate. If the
resistance has decreased, then there could
be a short between layers of the winding.
AC impedance test
The AC impedance test is performed on
generator rotors to determine the winding
impedance at various voltages between
zero and 120 volts AC, or the changes in
winding impedance at a set voltage at
various speeds from zero up to the unit’s
rated speed. It is possible to detect shorted
turns or changes in the number of shorted
turns by tracking changes in impedance
over time. This test is useful as a comparative test where AC impedance data has
been taken and recorded in the past.
Published by and copyright © 2009:
Siemens AG
Energy Sector
Freyeslebenstrasse 1
91058 Erlangen, Germany
Industrial Applications
Wolfgang-Reuter-Platz
47053 Duisburg, Germany
Pole balance
Dielectric absorption test
The pole balance test is used to determine
if there are shorted turns in the generator
rotor winding. A voltage of 120 volts AC
or less is applied to the field winding, and
voltage measurements are taken at the
pole-to-pole connections between poles.
This test relies on the fact that rotor windings need to be symmetrical, both electrically and physically, in order to operate
smoothly. The voltage reading at those
points should be very similar, with no
more than two percent difference. If there
is more than a two percent difference,
then there could be shorted turns in the
generator rotor winding. This is the most
accurate shorted turn test that can be performed on a generator rotor at standstill.
Dielectric over-voltage tests establish that
the winding is capable of withstanding
the applied voltage. Such tests may be
destructive to a greater or lesser extent.
Both AC and DC tests have been widely
used for maintenance over-voltage testing.
For more information, please contact
your local Siemens representative.
Siemens Demag
Delaval Turbomachinery, Inc.
840 Nottingham Way
Hamilton, NJ 08638, USA
Mechanical considerations
The operation and maintenance of
turbine-driven generators require a
thorough knowledge of their construction,
as well as the ability to recognize
the symptoms of impending failure.
Mechanical issues may be detected or
avoided with a comprehensive generator
testing strategy. The electrical tests described in this brochure are typical of those
required during a maintenance outage.
However, Siemens can complete a variety
of additional electrical tests, depending
upon the initial test results or customer
requirements. The above chart briefly
explains further test options for the
various generator components. Most
of these procedures can be performed
either on site or at one of our service
centers across North America.
Oil & Gas and Industrial Applications Service
Order No. E50001-F510-A131-X-4A00
Printed in USA
All rights reserved.
Trademarks mentioned in this document are
the property of Siemens AG, its affiliates, or
their respective owners.
Subject to change without prior notice.
The information in this document contains
general descriptions of the technical options
available, which may not apply in all cases.
The required technical options should therefore
be specified in the contract.
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