10.7 MEASURES TO MINIMIZE DAMAGES DUE TO FIRE INCIDENTS OF
POWER TRANSFORMER
Power transformers, being an important and costly equipment of grid system network
must be given priority attention for their safe and reliable operation. During normal
and abnormal service of a power transformer, the chances of its damage or failure
(silent failure or damage with fire out-breakage as it contains bulk quantity of
inflammable oil) cannot be eliminated due to its construction design and operation
characteristics.
As per the local and the worldwide experiences of transformer engineering, the
possible causes of transformer failure are summarized as under for awareness of the
all concerned.
Sr. Causes
of
No. Failure
1
Environment
related
2
System related
3
4
5
6
7
Description & Analysis of Causes
Hurricane, Earthquake, Lightning, Magnetic Storms, Site
conditions, etc.
Over-voltages, Switching operations, Fast front transients,
Ferro resonance, Load rejection, Back-feeding, Shortcircuits, etc.
Operation related Load duty, Overloading, Switching operations, Out-of-phase
synchronization, etc.
Design related
Electrical stresses, Mechanical forces, Transient voltage
distribution, Magnetizing flux, Leakage flux, Circulating
currents, Lead disposition, Cooling arrangements, etc.
Manufacturing
Drying, Processing, Jointing, Connections, Assembly, etc.
related
Materials related Bushings, OLTC, Core steel, Non-magnetic steel,
Conductors, Insulation, Insulating oil, etc
Maintenance
OLTC, Insulating oil, Oil preservation system, Paintwork,
related
Water contamination, Particle contamination, etc.
Note. Transformers handling experience reveals that the most possible location and
component parts of the transformer subjected to failure or damage are magnetic core,
windings, leads, OLTC, bushings and tank.
1
10.8 CRITERIA OF RE-SWITCHING OF THE TRANSFORMERS AFTER
THEIR OUTAGE
During routine functioning of the power system, the transformers are expected to be
disconnected forcedly from the system due to several abnormal conditions within the
transformer or in the connected network. The criteria to re-switch the transformer in
this situation are given below:
- Cases allowing re-switching of the transformer without testing after its outage;
a. Due to operation of over current relays with simultaneous operation of over current
relay of the associated feeders.
b. Due to overloading which resulted from excessive winding temperature (take care
of excessive winding temperature in routine).
c. Due to air accumulated in the Buchholz relay (non-combustible gases)
Note. In any case as above, visual inspection of the transformer and the surrounding
equipment and area to look for any abnormality must be carried out before reswitching. On the operation of any other protection relays, perform the prescribed
tests to look for the cause.
- Cases not allowing in re-switching of the transformer without routine and
detailed diagnostic testing after its outage;
a. Due to operation of differential protection relays (HV, LV, Tertiary windings).
b. Due to operation of Buchholz relays with combustible gases accumulation.
c. Due to operation of pressure relief devices, with or without gushing out of oil from
the transformer.
Note. In any case as above, visual inspection of the transformer and the surrounding
equipment and area to look for any abnormality must be carried out in addition to the
prescribed testing and before re-switching.
Recommended routine and diagnostic tests
- Windings continuity check
- Insulation Resistance Test (Megger Test)
2
-
T.T.R Test
Oil DES Test
Checking of Buchholz relay and OLTC protective relay
Winding Resistance Test
Short Circuit Test
Open Circuit Test
C&DF Test of windings and bushings
DGA (Dissolved Gasses-in-oil Analysis) of oil
OLTC operation
- Fire detection and fire extinguishing arrangements
The following fire detection and fire extinguishing techniques/arrangements are
recommended for an effective fire control for electrical installations:
- Fire detection and water spray system for external fire hazards.
- Nitrogen gas injection explosion and fire prevention system for internal fire hazards
- Measures to be taken to minimize the extent of damages if the fire out-breaks
The following measures are to be taken to minimize the extent of damages to the
transformer itself and the surrounding equipment in case of fire out-breakage in the
transformer:
- De-energize and isolate the affected transformer from the external system (HV & LV
sides).
- De-energize and isolate the surrounding transformers and the other equipment from
the external system.
- Inform fire brigade unit/Rescue service of the local area and start fire fighting with
the local resources and continue efforts till the arrival of fire brigade units.
- Clear the surrounding area of the affected transformer from the fire supporting items
i.e. oil, wood, etc.
Note. Provision of fire protection walls/partitioning walls between the transformers
and oil catchments/sump below the transformers is the effective measures to control
spreading of fire.
3
11. GENERAL APPLICATIONS
11.1 DEVICE FUNCTION NUMBERING
Definition and Function
Device
Device Name Definition and Function
Number
Master Element is the initiating device, such as a control switch,
1
voltage relay, float switch, etc., which serves either directly, or through
such permissive devices as protective and time-delay relays to place an
equipment in or out of operation.
Time-delay Starting, or Closing, Relay is a device, which
2
functions to give a desired amount of time delay before or after any
point or operation in a switching sequence or protective relay system,
except as specifically provided by device functions 62 and 79
described later.
Checking or Interlocking Relay is a device which operates in
3
response to the position of a number of other devices, or to a number of
predetermined conditions in an equipment to allow an operating sequence to proceed, to stop, or to provide a check of the position of
these devices or of these conditions for any purpose.
Master Contactor is a device, generally controlled by device No.1 or
4
equivalent, and the necessary permissive and protective devices, which
serves to make and break the necessary control circuits to place an
equipment into operation under the desired conditions and to take it out of
operation under other or abnormal conditions.
Stopping Device functions to place and hold an equipment out of
5
operation.
Starting Circuit Breaker is a device whose principal function is to
6
connect a machine to its source of starting voltage.
Anode Circuit Breaker is one used in the anode circuits of a power
7
rectifier for the primary purpose of interrupting the rectifier circuit if
an arc back should occur.
Control Power Disconnecting Device is a disconnecting device-such
8
as a knife switch, circuit breaker or pullout fuse block-used for the
purpose of connecting and disconnecting, respectively, the source of
control power to and from the control bus or equipment.
Note: Control power is considered to include auxiliary power
4
9
10
11
12
13
14
15
16
17
18
19
20
21
which supplies such apparatus as small motors and heaters.
Reversing Device is used for the purpose of reversing a machine field
or for performing any other reversing functions.
Unit Sequence Switch is used to change the sequence in which units
may be placed in and out of service in multiple-unit equipments.
Reserved for future application.
Over-Speed Device is usually a direct-connected speed switch which
functions on machine over speed.
Synchronous-Speed Device, such as a centrifugal-speed switch, a slipfrequency relay, a voltage relay, an undercurrent relay or any type of
device, operates at approximately synchronous speed of a machine.
Under-Speed Device functions when the speed of a machine falls
below a predetermined value.
Speed or Frequency, Matching Device functions to match and hold
the speed or the frequency of a machine or of a system equal to, or
approximately equal to, that of another machine, source or system.
Reserved for future application.
Shunting, or Discharge, Switch serves to open or to close a
shunting circuit around any piece of apparatus (except a resistor), such
as a machine field, a machine armature, a capacitor or a reactor.
Note: This excludes devices which perform such shunting operations
as may be necessary in the process of starting a machine by devices
6 or 42, or their equivalent, and also excludes device 73 function
which serves for the switching of resistors.
Accelerating or Decelerating Device is used to close or to cause the
closing of circuits which are used to increase or to decrease the speed
of a machine.
Starting-to-Running Transition Contactor is a device which
operates to initiate or cause the automatic transfer of a machine from the
starting to the running power connection.
Electrically Operated Valve is a solenoid- or motor-operated valve
which is used in a vacuum, air, gas, oil, water, or similar, lines.
Note: The function of the valve may be indicated by the insertion of
descriptive words such as "Brake" or "Pressure Reducing" in the
function name, such as "Electrically Operated Broke Valve".
Distance Relay is a device which functions when the circuit
admittance, impedance or reactance increases or decreases beyond
predetermined limits.
5
22
23
24
25
26
27
28
29
30
31
32
33
Equalizer Circuit Breaker is a breaker which serves to control or
to make and break the equalizer or the current-balancing connections
for a machine field, or for regulating equipment, in a multiple-unit
installation.
Temperature Control Device functions to raise or to lower the
temperature of a machine or other apparatus, or of any medium, when its
temperature falls below, or rises above, a predetermined value.
Note: An example is a thermostat which switches on a space heater
in a switchgear assembly when the temperature falls to a desired
value as distinguished from a device which is used to provide
automatic temperature regulation between close limits and would be
designated as 90T.
Reserved for future application.
Synchronizing, or Synchronism-check, device operates when two
a-c circuits are within the desired limits of frequency, phase angle or
voltage, to permit or to cause the paralleling of these two circuits.
Apparatus Thermal Device functions when the temperature of the
shunt field or
Under-voltage Relay is a device which functions on a given value of
under voltage.
Reserved for future application.
Isolating Contactor is used expressly for disconnecting one circuit
from another for the purposes of emergency operation, maintenance, or
test.
Annunciator Relay is a non automatically reset device which gives a
number of separate visual indications upon the functioning of protective
devices, and which may also be arranged to perform a lockout function.
Separate Excitation Device connects a circuit such as the shunt field
of a synchronous converter to a source of separate excitation during the
Starting sequence; or one which energizes the excitation and ignition
circuits of a power rectifier.
Directional Power Relay is one when function on a desired
value of power flow in a given direction, or upon reverse power
resulting from arc back in the anode or cathode circuits of a power
rectifier.
Position Switch makes or breaks contact when the main device or
piece of apparatus, which has no device function number, reaches a
given position. apparatus, which has no device function number,
6
34
35
36
37
38
39
40
41
42
43
44
45
46
47
reaches a given position.
Motor-Operated Sequence Switch is a multi-contact switch which
fixes the operating sequence of the major devices during starting and
stopping, or during other sequential switching operations.
Brush-Operating, or Slip-Ring Short-Circuiting, Device is used for
raising, lowering, or shifting the brushes of a machine, or for shortcircuiting its slip rings, or for engaging or disengaging the contacts of
a mechanical rectifier.
Polarity device operates or permits the operation of another device on
a predetermined polarity only.
Under-current or Under-power Relay is a device which functions
when the current or power flow decreases below a predetermined
value.
Bearing Protective Device is one which functions on excessive
bearing temperature, or on other abnormal mechanical conditions,
such as undue wear, which may eventually result in excessive bearing
temperature.
Reserved for future application.
Field Relay is a device that functions on a given or abnormally low
value or failure of machine field current, or on an excessive value of
the reactive component of armature current in an AC machine
indicating abnormally low field excitation.
Field Circuit Breaker is a device which functions to apply, or to
remove, the field excitation of a machine.
Running Circuit Breaker is a device whose principal function is to
connect a machine to its source of running voltage after having been
brought up to the desired speed on the starting connection.
Manual Transfer or Selector Device transfers the control circuits so
as to modify the plan of operation of the switching equipment or of
some of the devices.
Unit Sequence Starting Relay is a device which functions to start the
next available unit in a multiple-unit equipment on the failure or on
the non-availability of the normally preceding unit.
Reserved for future application.
Reverse-Phase, or Phase-Balance, Current Relay is a device which
functions when the poly-phase currents are of reverse-phase sequence, or
when the poly-phase currents are unbalanced or contain negative phasesequence components above a given amount,
Phase-Sequence Voltage Relay is a device which functions upon a
7
1-9
48
49
50
51
52
53
54
55
56
57
58
59
predetermined value of poly-phase voltage in the desired phase
sequence.
Incomplete Sequence Relay is a device which returns the equipment to
the normal, or off, position and locks it out if the normal starting,
operating or stopping sequence is not properly completed within a
predetermined time.
Machine, or Transformer, Thermal Relay is a device which
functions when the temperature of an a-c machine armature, or of the
armature or other load carrying winding or element of a d-c machine,
or converter or power rectifier or power transformer (including a power
rectifier transformer) exceeds a predetermined value.
Instantaneous Over-current or Rate-of-Rise Relay is a device
which functions instantaneously on an excessive value of current, or on
an excessive rate of current rise, thus indicating a fault in the apparatus or
circuit being protected.
AC Time Over-current Relay is a device with either a definite or
inverse time characteristic which functions when the current in an a-c
circuit exceeds a predetermined value.
AC Circuit Breaker is a device which is used to close and interrupt
an a-c power circuit under normal conditions or to interrupt this circuit
under fault or emergency conditions.
Exciter or D-C Generator Relay is a device which forces the d-c
machine field excitation to build up during starting or which functions
when the machine voltage has built up to a given value.
High-Speed D-C Circuit Breaker is a circuit breaker which starts to
reduce the current in the main circuit in 0.01 second or less, after the
occurrence of the DC over-current or the excessive rate of current rise.
Power Factor Relay is a device which operates when the power
factor in an AC circuit becomes above or below a predetermined
value.
Field Application Relay is a device which automatically
controls the application of the field excitation to an a-c motor at
some predetermined point in the slip cycle.
Short-Circuiting or Grounding Device is a power or stored energy
operated device which functions to short-circuit or to ground a circuit
in response to automatic or manual means.
Power Rectifier Misfire Relay is a device which functions if one or
more of the power rectifier anodes fail to fire.
Overvoltage Relay is a device which functions on a given value of
8
60
61
62
63
64
65
66
67
68
69
overvoltage.
Voltage Balance Relay is a device which operates
on a given difference in voltage between two circuits.
Current Balance Relay is a device which operates on a given
difference in current input or output of two circuits.
Time-Delay Stopping, or Opening, Relay is a time-delay device
which serves in conjunction with the device which initiates the
shutdown, -stopping, or opening operation in an automatic sequence.
Liquid or Gas Pressure, Level, or Flow Relay is a device which
operates on given values of liquid or gas pressure, flow or level, or
on a given rate of change of these values. Buchholz relay, etc.
Ground Protective Relay is a device which functions on failure of
the insulation of a machine, transformer or of other apparatus to ground,
or on flashover of a d-c machine to ground.
Note: This function is assigned only to a relay which detects the
flow of current from the frame of a machine or enclosing case or
structure of a piece of apparatus to ground, or detects a ground on a
normally ungrounded winding or circuit. It is not applied to a
device connected in the secondary circuit or secondary neutral of a
current transformer, or current transformers, connected in the power
circuit of a normally grounded system.
Governor is the equipment which controls the gate or valve opening
of a prime mover.
Notching, or Jogging, Device functions to allow only a specified
number of operations of a given device, or equipment, or a specified
number of successive operations within a given time of each other. It
also functions to energize a circuit periodically,, or which is used to
permit intermittent acceleration or jogging of a machine at low speeds
for mechanical positioning.
AC Directional Over-current Relay is a device which functions on
a desired value of AC over-current flowing in a predetermined
direction.
Blocking Relay is a device which initiates a pilot signal for blocking
of tripping on external faults in a transmission line or in other apparatus
under predetermined conditions, or co-operates with other devices to
block tripping or to block reclosing on an out-of-step condition or on
power swings.
Permissive Control Device is generally a two-position, manually
operated switch which in one position permits the closing of a circuit
9
70
71
72
73
74
75
76
77
78
79
80
81
82
83
breaker, or the placing of an equipment into operation, and in the other
position prevents the circuit breaker or the equipment from being
operated.
Electrically Operated Rheostat is a rheostat which is used to vary the
resistance of a circuit in response to some means of electrical control.
Reserved for future application.
DC Circuit Breaker is used to close and interrupt a d-c power circuit
under normal conditions or to interrupt this circuit under fault or
emergency conditions.
Load-Resistor Contactor is used to shunt or insert a step of load
limiting, shifting, or indicating resistance in a power circuit, or to
switch a space heater in circuit, or to switch a light, or regenerative,
load resistor of a power rectifier or other machine in and out of circuit.
Alarm Relay is a device other than an annunciator, as covered under
device No. 30, which is used to operate, or to operate in connection
with, a visual or audible alarm.
Position Changing Mechanism is the mechanism which is used for
moving a removable circuit breaker unit to and from the connected,
disconnected, and test positions.
DC Over-current Relay is a device which functions when the current
in a d-c circuit exceeds a given value.
Pulse Transmitter is used to generate and transmit pulses over a telemetering or pilot-wire circuit to the remote indicating or receiving
device.
Phase Angle Measuring, or Out-of-Step Protective Relay is a device
which functions at a predetermined phase angle between two voltages or
between two currents or between voltage and current.
AC Reclosing Relay is a device which controls the automatic reclosing
and' locking out of an a-c circuit interrupter.
Reserved for future application.
Frequency Relay is a device which functions on a predetermined
value of frequency-either under or over or on normal system
frequency-or rate of change of frequency.
DC Reclosing Relay is a device which controls the automatic closing
and reclosing of a d-c circuit interrupter, generally in response to load
circuit conditions.
Automatic Selective Control, or Transfer, Relay is a device which
operates to elect automatically between certain sources or conditions in
an equipment or performs a transfer operation automatically.
10
84
85
86
87
88
89
90
91
92
93
94
Operating Mechanism is the complete electrical mechanism or servomechanism, including the operating motor, solenoids, position switches,
etc., for a tap changer, induction regulator or any piece of apparatus
which has no device function number
Carrier, or Pilot-Wire, Receiver Relay is a device which is operated
or restrained by a signal used in connection with carrier-current or d-c
pilot-wire fault directional relaying.
Locking-out Relay is an electrically operated hand or electrically
reset device which functions to shut down and hold an equipment out
of service on the occurrence of abnormal conditions.
Differential Protective Relay is a protective device which functions
on a percentage of-phase angle or other quantitative difference of two
currents or of some other electrical quantities.
Auxiliary Motor, or Motor Generator is one used for operating
auxiliary equipment such as pumps, blowers, exciters, rotating
magnetic amplifiers, etc.
line switch is used as a disconnecting or isolating switch in an a-c or
d-c power circuit, when this device is electrically operated or has
electrical accessories, such as an auxiliary switch, magnetic lock, etc.
Regulating Device functions to regulate a quantity, , or quantities, such
as voltage, current, power, speed, frequency, temperature, and load, at a
certain value or between certain limits for machines, tie lines or other
apparatus.
Voltage Directional Relay is a device which operates when the
voltage across an open circuit breaker or contactor exceeds a given
value in a given direction.
Voltage and Power Directional Relay is a device which permits or
causes the connection of two circuits when the voltage difference
between them exceeds a given value in a predetermined direction
and causes these two circuits to be disconnected from each other
when t h e p o w e r t h e m exceeds a given value in the opposite
direction.
Field Changing Contactor functions to increase or decrease in one step
the value of field excitation on a machine.
Tripping, or Trip-Free, Relay is a device which functions to trip a
circuit breaker, contactor, or equipment, or to permit immediate tripping
by other devices; or to prevent immediate reclosure of a circuit interrupter,
in case it should open automatically even though its closing circuit is
maintained closed.
11
95
96
97
98
99
Note
Used only for specific applications on individual installations where
none of the assigned numbered functions from 1 to 94 is suitable.
Used only for specific applications on individual installations where
none of the assigned numbered functions from 1 to 94 is suitable.
Used only for specific applications on individual installations where
none of the assigned numbered functions from 1 to 94 is suitable.
Used only for specific applications on individual installations where
none of the assigned numbered functions from 1 to 94 is suitable.
Used only for specific applications on individual installations where
none of the assigned numbered functions from 1 to 94 is suitable.
Note: A similar series of numbers, starting with 201 instead of 1,
shall be used for those device functions in a machine, feeder or other
equipment when these are controlled directly from the supervisory
system. Typical examples of such device functions are 201, 205, and
294.
12
11.2 TRANSFORMER’S IMPORTANT
DEFINITIONS
Power transformer
A static piece of apparatus with two or more windings which, by electromagnetic
induction, transforms a system of alternating voltage and current into another system
of voltage and current usually of different values and at the same frequency for the
purpose of transmitting electrical power.
Auto-transformer
A transformer in which at least two windings have a common part.
Note. Where there is a need to express that a transformer is not auto-connected, use is
made of terms such as separate winding transformer, or double-wound transformer.
Booster transformer
A transformer of which one winding is intended to be connected in series with a
circuit in order to alter its voltage and/or shift its phase. The other winding is an
energizing winding.
Oil-immersed type transformer
A transformer of which the magnetic circuit and windings are immersed in oil.
Note. For the purpose of this part any insulating liquid, mineral oil or other product, is
regarded as oil.
Dry-type transformer
A transformer of which the magnetic circuit and windings are not immersed in an
insulating liquid.
Oil preservation system
The system in an oil-immersed transformer by which the thermal expansion of the oil
is accommodated. Contact between the oil and external air may sometimes be
diminished or prevented.
Terminal
A conducting element intended for connecting a winding to external conductors.
Line terminal
A terminal intended for connection to a line conductor of a network.
Neutral terminal
a) For three-phase transformers and three-phase banks of single-phase transformers:
The terminal or terminals connected to the common point (the neutral point) of a starconnected or zigzag connected winding.
b) For single-phase transformers:
The terminal intended for connection to a neutral point of a network.
Neutral point
The point of a symmetrical system of voltages which is normally at zero potential.
Corresponding terminals
13
Terminals of different windings of a transformer, marked with the same letter or
corresponding symbol.
Winding
The assembly of turns forming an electrical circuit associated with one of the voltages
assigned to the transformer.
Note. For a three-phase transformer, the 'winding' is the combination of the phase
windings (see phase winding).
Tapped winding
A winding in which the effective number of turns can be changed in steps.
Phase winding
The assembly of turns forming one phase of a three-phase winding.
Note. The term 'phase winding' should not be used for identifying the assembly of all
coils on a specific leg.
High-voltage winding
The winding having the highest rated voltage.
Low-voltage winding
The winding having the lowest rated voltage.
Note. For a booster transformer, the winding having the lower rated voltage may be
that having the higher insulation level.
Intermediate-voltage winding
A winding of a multi-winding transformer having a rated voltage intermediate
between the highest and lowest winding rated voltages.
Auxiliary winding
A winding intended only for a small load compared with the rated power of the
transformer.
Stabilizing winding
A supplementary delta-connected winding provided in a star-star-connected or starzigzag-connected transformer to decrease its zero-sequence impedance (see also
tertiary winding).
Note. A winding is referred to as a stabilizing winding only if it is not intended for
three-phase connection to an external circuit.
Common winding
The common part of the windings of an auto-transformer.
Primary winding
The winding which receives active power from the supply source in service is referred
to as a primary winding.
Secondary winding
The winding which delivers active power to a load is referred to as a secondary
winding.
14
Note. These terms have no significance as to which of the windings has the higher
rated voltage and should not be used except in the context of direction of active power
flow.
Tertiary winding
A further winding in the transformer, usually with lower value of rated power than the
secondary winding, is then often referred to as tertiary winding (see also stabilizing
winding).
Series winding
The part of the winding of an auto-transformer or the winding of a booster
transformer which is intended to be connected in series with a circuit.
Energizing winding The winding of a booster transformer which is intended to
supply power to the series winding.
Rating
Those numerical values assigned to the quantities which define the operation of the
transformer in the conditions specified and on which the manufacturer's guarantees
and the tests are based.
Rated quantities
Quantities (voltage, current, etc.), the numerical values of which define the rating.
Note 1. For transformers having tappings, rated quantities are related to the principal
tapping, unless otherwise specified. Corresponding quantities with analogous
meaning, related to other specific tappings, are
called tapping quantities.
Note 2. Voltages and currents are always expressed by their r.m.s. values, unless
otherwise specified.
Rated voltage of a winding (Ur)
The voltage assigned to be applied, or developed at no-load, between the terminals of
an untapped winding, or of a tapped winding connected on the principal tapping. For a
three-phase winding it is the voltage between line terminals.
Note.1. The rated voltages of all windings appear simultaneously at no-load when the
voltage applied to one of them has its rated value.
Note.2. For single-phase transformers intended to be connected in star to form a threephase bank, the rated voltage is indicated as phase-to-phase voltage, divided by √3 for
example Ur = 500/√3 kV.
Note.3. For the series winding of a three-phase booster transformer which is designed
as an open winding the rated voltage is indicated as if the winding were connected in
star, for example Ur =23/√3 kV.
Rated voltage ratio
The ratio of the rated voltage of a winding to the rated voltage of another winding
associated with a lower or equal rated voltage.
Rated frequency (fr)
15
The frequency at which the transformer is designed to operate.
Rated power (Sr)
A conventional value of apparent power assigned to a winding which, together with
the rated voltage of the winding, determines its rated current.
Note.1. Both windings of a two-winding transformer have the same rated power which
by definition is the rated power of the whole transformer.
Note.2. For a multi-winding transformer, half the arithmetic sum of the rated power
values of all windings (separate windings, not auto-connected) gives a rough estimate
of its physical size as compared with a two-winding transformer.
Rated current (Ir)
The current flowing through a line terminal of a winding which is derived from rated
power Sr and rated voltage Ur for the winding.
Tapping
In a transformer having a tapped winding, a specific connection of that winding,
representing a definite effective number of turns in the tapped winding and,
consequently, a definite turns ratio between this winding and any other winding with
fixed number of turns.
Note. One of the tapping is the principal tapping, and other tapping are described in
relation to the principal tapping by their respective tapping factors.
Tapping factor (corresponding to a given tapping)
The ratio:
Ud
Ud
(tapping factor) or
x100 (% tapping factor)
Ur
Ur
where
Ur is the rated voltage of the winding ;
Ud is the voltage which would be developed at no-load at the terminals of the
winding, at the tapping concerned, by applying rated voltage to an untapped winding.
Note. This definition is not appropriate in relation to a series winding of a booster
transformer, and in that case the percentage notation would be referred to the voltage
of the energizing winding or of the winding of an
associated system transformer.
Principal tapping
The tapping to which the rated quantities are related.
Plus tapping
A tapping whose tapping factor is higher than 1.
Minus tapping
A tapping whose tapping factor is lower than 1.
16
Tapping step
The difference between the tapping factors, expressed as a percentage, of two adjacent
tapping.
Tapping duty
The numerical values assigned to the quantities, analogous to rated quantities, which
refer to tapping other than the principal tapping.
On load tap changer (OLTC)
A device for changing the tapping connections of a winding suitable for operation
whilst the transformer is energized or on load. Generally it consists of a diverter
switch with a transition impedance and a tap selector which can be with or without a
change-over selector, the whole being operated by the driving mechanism. In some
form of on load tap changers, the functions of diverter switch and the tap selector are
combined on a selector switch.
Tap selector
A device designed to carry, but not to make or break, current, used in conjunction with
a diverter switch, to select tapping connections.
Diverter switch
A switching device used in conjunction with a tap selector to carry, make and break
currents in circuits which have already been selected.
Note. Diverter switches of spring-operated type include an independent means of
storing energy for their operation.
Selector switch
A switching device capable of making, carrying and breaking currents, combining the
duties of a tap selector and a diverter switch.
Change-over selector
A device designed to carry, but not to make or break, current, used in conjunction with
a tap selector or selector switch to enable its contacts, and the connected tappings, to
be used more than once when moving from one extreme position to another.
Coarse change-over selector
A change-over selector connecting the tapped winding to either the coarse winding or
the main winding.
Reversing change-over selector
A change-over selector connecting one or other end of the tapped winding to the main
winding.
Transition impedance
A resister or reactor consisting of one or more units, bridging the tappings in use and
tapping next to be used, for the purpose of transferring load from one tapping to the
other without interruption or appreciable change in the load current, at the same time
limiting the circulating current for the period that both tappings are used.
Driving mechanism
17
The means by which the drive of the on load tap changer is actuated.
NOTE. The mechanism may include an independent means of storing energy to
control the operation.
Set of contacts
A pair of individual fixed and moving contacts or a combination of such pairs
operating substantially simultaneously.
Main contacts (diverter switch and selector switch)
A set of through-current carrying contacts which has no transition impedance between
the transformer winding and the contacts and does not switch any current.
Main switching contacts (diverter switch and selector switch)
A set of contacts which has no transition impedance between the transformer winding
and the contacts and makes and breaks current.
Transition contacts
A set of contacts which is connected in series with a transition impedance and makes
and breaks current.
NOTE. In the case of reactor transition on load tap changers, this set of contacts is
used, in many instances, to carry the through-current in the full tap position.
Circulating current
The part of the current which flows through the transition impedance at the time when
two tapping are bridged during a tap change operation and which due to the voltage
difference between the tapping.
Switched current
The prospective current to be broken during switching operation by each set of main
switching or transition contacts incorporated in the diverter switch or selector switch.
Recovery voltage
The power-frequency voltage which appears across each set of main switching pr
transition contacts of the diverter switch or selector switch after these contacts have
broken the switched current.
Tap-change operation
A complete sequence of events from the initiation to the completion of the transition
of the through-current from one tap of the winding to an adjacent one.
Cycle of operation
The movement of tap-changer from one end of its range to the other and the return to
its original position.
Insulation level
The withstand values of the impulse and the power-frequency test voltages to earth
and where appropriate between the phases, and between those parts where insulation
is required.
Rated through-current (IU)
18
The current flowing through the tap-changer towards the external circuit, which the
apparatus is capable of transferring from one tapping to the other at the relevant rated
step voltage and which can be carried continuously while meeting the requirements of
this standard.
Note. See relationship between a rated through-current and the step voltage.
Maximum rated through-current (Ium)
The rated through-current for which both the temperature rise of the contacts and the
service test apply.
Rated step voltage (Ui)
For each value of rated through-current, the highest permissible voltage between
terminals which are intended to be connected to successive tappings of a transformer.
Note. If a rated step voltage is given in connection with a rated through-current, it is
called "relevant rated step voltage".
Maximum rated step voltage (Uim)
The highest value of rated step voltage for which the tap-changer is designed.
Rated frequency
The frequency of the alternating current for which the tap changer is designed.
Number of inherent tapping positions
The highest number of tapping positions for half a cycle of operation for which a tapchanger can be used according to its design.
Number of service tap-positions
The number of tapping positions for half a cycle of operation for which a tap-changer
is used in a transformer.
NOTE. 1 These terms are generally given as the ± values of the relevant numbers, e.g.
±11 positions; they are in principle valid also for the motor-drive mechanism. When
using the term "number of tapping positions" in connection with a transformer, this
always refers to the number of service tapping positions of the tap-changer.
Type test
A test made on a tap-changer or the components of a tap-changer, or a range of tapchangers or components all based on the same design, to prove compliance with the
standard.
NOTE. A range of tap-changers is a number of tap-changers based on the same design
and having the same characteristics, with the exception of the insulation levels to earth
and possibly between phases, the number of steps and the value of the transition
impedances.
Routine tests
A test made on each completed tap-changer, the design of which has been verified by
type test, to establish that the tap-changer is without manufacturing defects.
Motor-drive mechanism
A driving mechanism which incorporates an electric motor and control circuit.
19
Step-by-step control
Electrical and mechanical devices stopping the motor-drive mechanism after
completion of a tap-change, independently of the operating sequence of the control
switch.
Tap position indicator
An electrical and/or mechanical device for indicating the tap position of the tapchanger.
Tap-change in progress indication
A device indicating that the motor-drive mechanism is running.
Limit switches
Electro-mechanical devices preventing the operation of the tap-changer beyond either
end position, but allowing operation in the opposite direction.
Mechanical end stop
A device which physically prevents operation of the tap-changer beyond either end
position, but allows operation in the opposite direction.
Parallel control device
Electrical control devices to move, in the case of parallel operation of several
transformers with tappings, all tap-changers to the required position and to avoid
divergence of the respective motor-drive mechanisms.
Note. Such devices would be necessary also in the case single-phase transformers
forming a three-phase bank when each single-phase tap-changer is fitted with its own
motor-drive mechanism.
Emergency tripping device
An electrical and / or mechanical device for stopping the motor-drive mechanism at
any time in such a way that a special action must be performed before the next tap
changer operation can be started.
Over current blocking device
An electrical device preventing or interrupting operation of the motor-drive
mechanism for the period in which an over current exceeding a preset value is flowing
in the transformer winding.
Note. Where diverter switches are actuated by spring energy system, interruption of
the operation of the motor-drive mechanism will not prevent operation of the diverter
switch if the spring release has been actuated.
Restarting device
A mechanical and/or electrical device restarting the motor-drive mechanism after an
interruption of the supply voltage and thus completing a tap-change operation already
initiated.
Operation counter
A device indicating the number of tap-changing accomplished.
20
Manual operation of motor-drive mechanism: Operation of the tap-changer manually
by a mechanical device, blocking at the same time operation by the electric motor.
Motor-drive cubical
A cubical housing the motor-drive mechanism.
No-load loss
The active power absorbed when rated voltage (tapping voltage) at rated frequency is
applied to the terminals of one of the windings, the other winding or windings being
open-circuited.
No-load current
The r.m.s. value of the current flowing through a line terminal of a winding when
rated voltage (tapping voltage) is applied at rated frequency, the other winding or
windings being open-circuited.
Note.1. For a three-phase transformer, the value is the arithmetic mean of the values
of current in the three phases.
Note.2. The no-load current of a winding is often expressed as a percentage of the
rated current of that winding.
For a multi-winding transformer this percentage is referred to the winding with the
highest rated power.
Load loss
The absorbed active power at rated frequency and reference temperature, associated
with a pair of windings when rated current (tapping current) is flowing through the
line terminals of one of the windings, and the terminals of the other winding are shortcircuited. Further windings, if existing, are open-circuited.
Note.1. For a two-winding transformer there is only one winding combination and one
value of load loss. For a multi-winding transformer there are several values of load
loss corresponding to the different two-winding combinations (see clause 6 of IEC
60606). A combined load loss figure for the complete transformer is referred to a
specified winding load combination. In general, it is usually not accessible for direct
measurement in testing.
Note.2. When the windings of the pair have different rated power values the load loss
is referred to rated current in the winding with the lower rated power and the reference
power should be mentioned.
Note.3. See general requirements of routine, type and special tests—any ambient
temperature between 10 ºC and 40 ºC and with cooling water (if required) at any
temperature not exceeding 25 ºC---reference correction temperature is 75 ºC for oilimmersed transformers.
Total losses
The sum of the no-load loss and the load loss.
Note. The power consumption of the auxiliary plant is not included in the total losses
and is stated separately.
21
Short-circuit impedance of a pair of windings
The equivalent series impedance Z = R + jX, in ohms, at rated frequency and
reference temperature, across the terminals of one winding of a pair, when the
terminals of the other winding are short-circuited and further windings, if existing, are
open-circuited. For a three-phase transformer the impedance is expressed as phase
impedance (equivalent star connection). In a transformer having a tapped winding, the
short-circuit impedance is referred to a particular tapping. Unless otherwise specified
the principal tapping applies.
The relative value is also equal to the ratio between the applied voltage during a shortcircuit measurement which causes the relevant rated current (or tapping current) to
flow, and rated voltage (or tapping voltage). This applied voltage is referred to as the
short-circuit voltage of the pair of windings. It is normally expressed as a percentage.
Voltage drop or rise for a specified load condition
The arithmetic difference between the no-load voltage of a winding and the voltage
developed at the terminals of the same winding at a specified load and power factor,
the voltage supplied to (one of) the other winding(s) being equal to:
– its rated value if the transformer is connected on the principal tapping (the no-load
voltage of the former winding is then equal to its rated value);
– the tapping voltage if the transformer is connected on another tapping.
This difference is generally expressed as a percentage of the no-load voltage of the
former winding.
Note. For multi-winding transformers, the voltage drop or rise depends not only on the
load and power factor of the winding itself, but also on the load and power factor of
the other windings (see IEC 60606).
Zero-sequence impedance (of a three-phase winding)
The impedance, expressed in ohms per phase at rated frequency, between the line
terminals of a three-phase star-connected or zigzag-connected winding, connected
together, and its neutral terminal.
Note.1. The zero-sequence impedance may have several values because it depends on
how the terminals of the other winding or windings are connected and loaded.
Note.2. The zero-sequence impedance may be dependent on the value of the current
and the temperature, particularly in transformers without any delta-connected
winding.
Note.3. The zero-sequence impedance may also be expressed as a relative value in the
same way as the (positive sequence) short-circuit impedance.
Temperature rise
The difference between the temperature of the part under consideration and the
temperature of the external cooling medium.
22
Highest voltage for equipment Um applicable to a transformer winding The
highest r.m.s. phase-to-phase voltage in a three-phase system for which a transformer
winding is designed in respect of its insulation.
Rated insulation level
A set of standard withstand voltages which characterize the dielectric strength of the
insulation.
Standard insulation level
A rated insulation level, the standard withstand voltages of which are associated to Um
( tables 2 and 3 of IEC 60071-1).
Uniform insulation of a transformer winding
The insulation of a transformer winding when all its ends connected to terminals have
the same rated insulation level.
Non-uniform insulation of a transformer winding
The insulation of a transformer winding when it has a neutral terminal end for direct
or indirect connection to earth, and is designed with a lower insulation level than
assigned for the line terminal.
Star connection (Y-connection)
The winding connection so arranged that each of the phase windings of a three-phase
transformer, or of each of the windings for the same rated voltage of single-phase
transformers associated in a three-phase bank, is connected to a common point (the
neutral point) and the other end to its appropriate line terminal.
Delta connection (D-connection)
The winding connection so arranged that the phase windings of a three-phase
transformer, or the windings for the same rated voltage of single-phase transformers
associated in a three-phase bank, are connected in series to form a closed circuit.
Open-delta connection
The winding connection in which the phase windings of a three-phase transformer, or
the windings for the same rated voltage of single-phase transformers associated in a
three-phase bank, are connected in series without closing one corner of the delta.
Zigzag connection (Z-connection)
The winding connection in which one end of each phase winding of a three-phase
transformer is connected to a common point (neutral point), and each phase winding
consists of two parts in which phase-displaced voltages are induced.
Note. These two parts normally have the same number of turns.
Open windings
Phase windings of a three-phase transformer which are not interconnected within the
transformer.
Phase displacement of a three-phase winding
23
The angular difference between the phasors representing the voltages between the
neutral point (real or imaginary) and the corresponding terminals of two windings, a
positive-sequence voltage system being applied to the high-voltage terminals,
following each other in alphabetical sequence if they are lettered, or in numerical
sequence if they are numbered. The phasors are assumed to rotate in a counterclockwise sense.
Note. The high-voltage winding phasor is taken as reference, and the displacement for
any other winding is conventionally expressed by the 'clock notation', that is, the hour
indicated by the winding phasor when the H.V winding phasor is at 12 o'clock (rising
numbers indicate increasing phase lag).
Connection symbol
A conventional notation indicating the connections of the high-voltage, intermediatevoltage (if any), and low-voltage windings and their relative phase displacement(s)
expressed as a combination of letters and clock-hour figure(s).
Routine test
A test to which each individual transformer is subjected.
Type test
A test made on a transformer which is representative of other transformers, to
demonstrate that these transformers comply with specified requirements not covered
by routine tests.
Note. A transformer is considered to be representative of others if it is fully identical
in rating and construction, but the type test may also be considered valid if it is made
on a transformer which has minor deviations of rating or other characteristics. These
deviations should be subject to agreement between the manufacturer and the
purchaser.
Special test
A test other than a type test or a routine test, agreed by the manufacturer and the
purchaser.
Monthly average temperature
Half the sum of the average of the daily maxima and the average of the daily minima
during a particular month – over many years.
Yearly average temperature
One-twelfth of the sum of the monthly average temperatures.
Bushing
Bushing is a device that enables one or several conductors to pass through a partition
such as a wall or a tank, and insulates the conductors from it. The means of attachment
(flange or fixing device) to the partition forms part of the bushing.
Note. 1 The conductor may form an integral part of the bushing or be drawn into the
central tube of the bushing.
Note. 2 The bushing may be of the types as mentioned below.
24
Liquid-filled bushing
Bushing in which the space between the inside surface of the insulating envelop and
the solid major insulation is filled with oil.
Compound-filled bushing
Bushing in which the space between the inside surface of the insulating envelop and
the solid major insulation is filled with an insulating compound.
Liquid-insulated bushing
Bushing in which the major insulation consists of oil or another insulating liquid.
Gas-filled bushing
Bushing in which the space between the inside surface of the insulating envelope and
the solid major insulation is filled with gas (other than ambient air) at atmospheric
pressure or higher.
Note. This definition includes bushings which are intended to form an integral part of
gas-insulated equipment, the gas of the equipment being in communication with that
of the bushing.
Gas-insulated bushing
Bushing in which the major insulation consists of gas (other than ambient air) at
atmospheric pressure or higher.
Note. 1 This definition includes bushings which are intended to form an integral part
of gas-insulated equipment, the gas of the equipment being in communication with
that of the bushing.
Note. 2 A bushing which contains solid insulating materials other than the envelope
containing the gas (e.g. support for conducting layers or insulating cylinder), is a
combined insulation bushing.
Gas-impregnated bushing
Bushing in which the major insulation of a core wound from paper or plastic film
(GIF) and subsequently treated and impregnated with gas (other than ambient air) at
atmospheric pressure or higher, the space the core and the insulating envelope being
filled with the same gas.
Oil-impregnated paper bushing (OIP)
Bushing in which the major insulation of a core wound from paper and subsequently
treated and impregnated with an insulating liquid, generally transformer oil.
Note. The core is contained in an insulating envelope, the space between the core and
insulating envelope being filled with the same insulating liquid as that used for
impregnation.
Resin-bonded paper bushing (RBP)
Bushing in which the major insulation of a core wound from resin-coated paper.
Note. 1 During the winding process, each paper layer is bonded to the previous layer
by its resin coating and the bonding achieved by curing the resin.
25
Note. 2 A resin-bonded paper bushing can be provided with an insulating envelope, in
which case the intervening space can be filled with an insulating liquid or another
insulating medium.
Note. 3 A rubber compound covering may be applied directly on to the bushing major
insulation.
Resin-impregnated paper bushing (RIP)
Bushing in which the major insulation of a core wound from untreated paper and
subsequently treated and impregnated with a curable resin.
Note. 1 A resin-impregnated paper bushing can be provided with an insulating
envelope, in which case the intervening space can be filled with an insulating liquid or
another insulating medium.
Note. 2 A rubber compound covering may be applied directly on to the bushing major
insulation.
Ceramic, glass or analogous inorganic material bushing:
Bushing in which the major insulation consists of a ceramic, glass or analogous
inorganic material.
Note. A rubber compound covering may be applied directly on to the bushing major
insulation.
Cast or moulded resin-insulated bushing
Bushing in which the major insulation consists of a cast or moulded organic material
with or without an inorganic filter.
Note. A rubber compound covering may be applied directly on to the bushing major
insulation.
Combined insulation bushing
Bushing in which the major insulation consists of a combination of at least two
different insulating materials.
Capacitance graded bushing
Bushing, in which a desired voltage grading is obtained by an arrangement of
conducting or semi-conducting layers incorporated into the insulating material.
Indoor bushing
Bushing, both ends of which are intended to be in ambient air at atmospheric pressure,
but not exposed to outdoor atmospheric conditions.
Outdoor bushing
Bushing, both ends of which are intended to be in ambient air at atmospheric pressure
and exposed to outdoor atmospheric conditions.
Outdoor-indoor bushing
Bushing, both ends of which are intended to be in ambient air at atmospheric pressure.
Note. One end is intended to be exposed to outdoor atmospheric conditions, and the
other end not to be exposed to outdoor atmospheric conditions.
Indoor-immersed bushing
26
Bushing, one end of which is intended to be in ambient air but not exposed to outdoor
atmospheric conditions and the other end to be immersed in an insulating medium
other than ambient air (e.g. oil or gas).
Note. This definition includes bushings operating in air at temperatures above
ambient, such as occur with air-insulated ducting.
Outdoor-immersed bushing
Bushing, one end of which is intended to be in ambient air and exposed to outdoor
atmospheric conditions and the other end to be immersed in an insulating medium
other than ambient air (e.g. oil or gas).
Completely immersed bushing
Bushing, both ends of which are intended to be immersed in an insulating medium
other than ambient air (e.g. oil or gas).
Bushing for separable connector (plug-in-type bushing)
Bushing, one end of which is immersed in an insulating medium and the other end
designed to receive a separable insulated cable connector, without which the bushing
cannot function.
Highest voltage for equipment (Um)
Highest r.m.s. value of phase-to-phase voltage for which the equipment is designed in
respect of its insulation as well as other characteristics which relates to this voltage in
the relevant equipment standard.
Rated phase-to-earth voltage
Maximum r.m.s. value of the voltage which the bushing withstands continuously
between the conductor and the earthed flange or other fixing device, under the
specified operating conditions.
Rated current (I r)
Maximum r.m.s. value of current which the bushing can carry continuously under the
specified operating conditions, with exceeding the specified temperature rise limits.
Rated thermal short-time current (I th)
R.M.S. value of a symmetrical current which the bushing withstands thermally for the
rated duration (t th) immediately following continuous operation at rated current with
the maximum temperatures of ambient air and immersion media.
Rated dynamic current (I d)
Peak value of a current which the bushing withstands mechanically.
Temperature rise
Difference between the measured temperature of the hottest spot of the metal parts of
the bushing which are in contact with insulating material and the ambient air
temperature.
Rated frequency (Fr)
Frequency at which the bushing is designed to operate.
27
Minimum operating pressure of gas for insulation
Minimum pressure, referenced to 20oC, assigned by the bushing supplier, at which
the rated insulation level applies.
Maximum internal operating gas pressure
Pressure, when the bushing is in operation, carrying rated current at the specified
highest temperatures.
Maximum external operating gas pressure
Maximum pressure of the gaseous insulating medium in which the bushing is
partially or completely immersed when in operation.
Design pressure (of the enclosure)
Pressure used to determine the thickness of the enclosure.
Leakage rate of gas-filled, gas-insulated, gas-impregnated and gas-immersed
bushings
Quantity of dry gas at a given temperature that flows through a leak per unit of time
and for a known difference of pressure across the leak.
Note. The basic SI unit for leak rate is "Pascal cubic meter per second (Pa x m3/s)".
The derived units "Pa x cm3/s and bar x cm3"are used commonly, as they better
conform with the orders of magnitude used in common industrial practice. It should be
remembered that; 1Pa x m3/s = 106 Pa x cm3/s = 10 bar x cm3/s.
Insulating envelope
Hollow insulator which is open from end to end, with or without sheds.
Note. An insulating envelope may consist of one insulator unit or two or more
permanently assembled insulator units.
Creepage distance
Shortest distance along the surface of an insulator between two conductive parts.
Note. 1 The surface of cement or of any other non-insulating jointing material is not
considered as forming part of the creepage distance.
Note. 2 If high-resistance coating is applied to parts of the insulating part of an
insulator, such parts are considered to be effective insulating surfaces and the distance
over them is included in the creepage distance.
Arcing distance
Shortest distance in air external to the insulator between metallic parts which
normally have the operating voltage between them.
Note. The term "dry arcing distance" or "taut string distance" are also used.
Test tap / measuring tap / tan δ tap
Connection, accessible from outside the bushing, insulated the flange or other fixing
device, made to one of the outer conducting layers of a capacitance graded bushing in
order to allow measurement of dissipation factor, capacitance and partial discharge
whilst the flange of the bushing is earthed.
Note. 1 This connection should be earthed directly when it is not used.
28
Note. 2 When the test tap is used for condition monitoring, in service, care should be
taken to avoid an open circuit.
Voltage tap / potential tap / capacitance tap
Connection, accessible from outside the bushing, insulated the flange or other fixing
device, made to one of the outer conducting layers of a capacitance graded bushing in
order to provide a voltage source whilst the bushing is in operation.
Note. 1 This connection should be earthed directly when it is not used.
Note. 2 This tap can also be used for the measurement of dissipation factor,
capacitance and partial discharge.
Rated voltage of the voltage tap
Maximum voltage at which the tap is designed to supply the associated equipment
with the rated load connected thereto, when the rated phase-to-earth voltage is applied
to the bushing at the rated frequency.
Composite bushing
Bushing with an insulating envelope consisting of a resin impregnated fibre tube with
or without a rubber compound covering.
Main capacitance (C1) of bushing
Capacitance between the high-voltage conductor and the test tap or the voltage of a
capacitance-graded bushing.
Tap capacitance (C2) of bushing
Capacitance between the test tap or the voltage tap and the mounting flange of a
capacitance-graded bushing.
Capacitance (C) of bushing
Capacitance between the high-voltage conductor and the mounting flange of a bushing
without a voltage tap or test tap.
29
11.3 CIRCUIT BREAKER’ IMPORTANT DEFINITIONS, OPERATING
TIME AND CHARACTERISTIC QUANTITIES
Circuit breaker class E1
The circuit breaker with basic electrical endurance not falling into the category of
class E2 is referred as class E1.
Circuit breaker class E2
The circuit breaker designed so as not to require maintenance of the interrupting parts
of the main circuit during its expected operating life, and only minimum maintenance
of its other parts (circuit breaker with extended electrical endurance)
Note. 1 The minimum maintenance may include lubrication, replenishment of gas and
cleaning of external surfaces, where applicable.
Note. 2 This definition is restricted to distribution circuit-breakers having a rated
voltage above 1 kV, and up to and including 52 kV. See annex G for rationale behind
introduction of class E2.
Circuit breaker class C1
The circuit breaker with low probability of re-strike during capacitive current breaking
as demonstrated by specific type tests
Circuit breaker class C2
The circuit breaker with very low probability of re-strike during capacitive current
breaking as demonstrated by specific type tests
Circuit breaker class M1
The circuit breaker with normal mechanical endurance (mechanically type tested for
2000 operations) not falling into the category of class M2.
Circuit breaker class M2
The frequently operated circuit-breaker for special service requirements and designed
so as to require only limited maintenance as demonstrated by specific type tests
(circuit-breaker with extended mechanical endurance, mechanically type tested for
10000 operations).
Note. Combination of the different classes of circuit-breakers with regard to electrical
endurance, mechanical endurance and the re-strike probability during capacitive
current breaking is possible. For the designation of these circuit-breakers the notation
of the different classes are combined following an alphabetical order, for example C1M2.
Circuit breaker class S1
The circuit breaker intended to be used in a cable system.
Circuit breaker class S2
The circuit breaker intended to be used in a line-system or in a cable system with
direct connection (without cable) to overhead lines.
30
Opening time
Opening time of a circuit-breaker defined according to the tripping method as stated
below and with any time delay device forming an integral part of the circuit-breaker
adjusted to its minimum setting:
a) For a circuit-breaker tripped by any form of auxiliary power (DC trip), the opening
time is the interval of time between the instant of energizing the opening release, the
circuit-breaker being in the closed position, and the instant when the arcing contacts
have separated in all poles;
b) For a self-tripping circuit-breaker (AC trip), the opening time is the interval of time
between the instant at which, the circuit-breaker being in the closed position, the
current in the main circuit reaches the operating value of the over-current release and
the instant when the arcing contacts have separated in all poles.
Note. 1 The opening time may vary with the breaking current.
Note. 2 For circuit-breakers with more than one interrupting unit per pole, the instant
when the arcing contacts have separated in all poles is determined as the instant of
contact separation in the first unit of the last pole.
Note. 3 The opening time includes the operating time of any auxiliary equipment
necessary to open the circuit breaker and forming an integral part of the circuitbreaker.
Arcing time (of a multi-pole switching device)
The interval of time between the instant of the first initiation of an arc and the instant
of final arc extinction in all poles
Break time
The interval of time between the beginning of the opening time of a mechanical
switching device and the end of the arcing time
Closing time
The interval of time between energizing the closing circuit, the circuit-breaker being
in the open position, and the instant when the contacts touch in all poles
Note. The closing time includes the operating time of any auxiliary equipment
necessary to close the circuit breaker and forming an integral part of the circuitbreaker.
Make time
The interval of time between energizing the closing circuit, the circuit-breaker being
in the open position, and the instant when the current begins to flow in the first pole
Note. 1 The make time includes the operating time of any auxiliary equipment
necessary to close the circuit breaker and forming an integral part of the circuitbreaker.
Note. 2 The make time may vary, e.g. due to the variation of the pre-arcing time.
31
Pre-arcing time
The interval of time between the initiation of current flow in the first pole during a
closing operation and the instant when the contacts touch in all poles for three-phase
conditions and the instant when the contacts touch in the arcing pole for single-phase
conditions
Note. 1 The pre-arcing time depends on the instantaneous value of the applied voltage
during a specific closing operation and therefore may vary considerably.
Note. 2 This definition for pre-arcing time for a circuit-breaker should not be confused
with the definition for pre-arcing time for a fuse.
Open-close time (during auto-reclosing)
The interval of time between the instant when the arcing contacts have separated in all
poles and the instant when the contacts touch in the first pole during a reclosing cycle
Dead time (during auto-reclosing)
The interval of time between final arc extinction in all poles in the opening operation
and the first re-establishment of current in any pole in the subsequent closing
operation
Note. The dead time may vary, e.g. due to the variation of the pre-arcing time.
Reclosing time
The interval of time between the beginning of the opening time and the instant when
the contacts touch in all poles during a reclosing cycle
Re-make time (during reclosing)
The interval of time between the beginning of the opening time and the first reestablishment of current in any pole in the subsequent closing operation
Note. The re-make time may vary, e.g. due to the variation of the pre-arcing time.
Close-open time
The interval of time between the instant when the contacts touch in the first pole
during a closing operation and the instant when the arcing contacts have separated in
all poles during the subsequent opening operation
Note. Unless otherwise stated, it is assumed that the opening release incorporated in
the circuit-breaker is energized at the instant when the contacts touch in the first pole
during closing. This represents the minimum close-open time.
Make-break time
The interval of time between the initiation of current flow in the first pole during a
closing operation and the end of the arcing time during the subsequent opening
operation
Note. 1. Unless otherwise stated, it is assumed that the opening release of the circuitbreaker is energized one half-cycle after current begins to flow in the main circuit
during making. It should be noted that the use of relays with shorter operating time
may subject the circuit-breaker to asymmetrical currents that are in excess of the
specified one.
32
Note. 2 The make-break time may vary due to the variation of the pre-arcing time.
Pre-insertion time
The interval of time during a closing operation in any one pole between the instant of
contact touch in the closing resistor element and the instant of contact touch in the
main breaking unit of that pole
Note. For circuit-breakers having series connected breaking units, the pre-insertion
time is defined as the interval of time between the instant of the last contact touch in
any closing resistor element and the instant of the last contact touch in any main
breaking unit.
Minimum trip duration
The minimum time the auxiliary power is applied to the opening release to ensure
complete opening of the circuit-breaker
Minimum close duration
The minimum time the auxiliary power is applied to the closing device to ensure
complete closing of the circuit breaker
Normal current
The current which the main circuit of a circuit-breaker is capable of carrying
continuously under specified conditions of use and behaviour
Peak factor (of the line transient voltage)
The ratio between the maximum excursion and the initial value of the line transient
voltage to earth of a phase of an overhead line after the breaking of a short-line fault
current
Note. The initial value of the transient voltage corresponds to the instant of arc
extinction in the pole considered.
First-pole-to-clear factor (in a three-phase system)
When interrupting any symmetrical three-phase current the first-pole-to-clear factor is
the ratio of the power frequency voltage across the interrupting pole before current
interruption in the other poles, to the power frequency voltage occurring across the
pole or the poles after interruption in all three poles
Amplitude factor
The ratio between the maximum excursion of the transient recovery voltage to the
crest value of the power frequency recovery voltage
Main circuit
All conductive parts of the switchgear constituting wires to be opened and closed.
Control circuit
A circuit that controls energy for operation of the switchgear in response to signals
from outside the GCB.
Operating circuit
A circuit used to operate the switchgear.
33
Auxiliary circuit
Generic terms for all circuits except for the main circuit, operation circuit, and control
circuit of the switchgear.
Ambient air temperature
The temperature of the air surrounding the switchgear as measured under certain
conditions.
Switch
Mechanical switch capable of opening and closing wires under normal conditions
and under specified overload running conditions, of flowing currents, or of flowing
currents in a short-circuit or in a specified abnormal circuit state for a specified period.
Terminal
A conductive part designed to connect the GCB electrically to external circuits.
Earth Terminal
A terminal designed to ground part of the switchgear when connected to the ground in
a specified manner.
Contact
The conductive portion of the switchgear designed to constitute a conductive part by
getting into contact when closed, and move relatively to open and close the circuit
involved.
Fixed contact
A contact mounted in the fixed region of a conductive part constituting a circuit.
Moving contact
A contact mounted in the movable region of a conductive part constituting a circuit.
Main contact
Contact provided in the main conductive part to supply current to the main circuit
when the circuit is closed
Arcing contact
Contact provided to prevent arcing to the main contact by inducing an arc by opening
or closing
a-contact
A contact in an auxiliary switch designed to close when the main contact of the
switchgear closes and opens when the main contact opens.
b-contact
A contact in an auxiliary switch designed to close when the main contact of the
switchgear opens and open when the main contact closes.
Conductive part
The current-conductive portion of the GCB.
Main conductive part
The conductive portion where the main circuit current flows.
34
Arc-control device
Device that contains the arc contacts, and restrains the arc in the container to promote
arc extinguishing.
Control device
Equipment that receives instructions or signals from outside the switchgear, selects
from among them, and controls the energy level for running the switchgear.
Control switch
A mechanical switch that controls the operation of the switchgear.
Pilot switch
A control switch that gets activated automatically when a specified operating
condition is reached.
Pressure switch
A detection switch that gets activated when the gas or liquid pressure reaches a
specified level.
Push-button switch
A switch that interlocks with the operation of the switchgear and produces displays.
Auxiliary switch
Switch used for interlock indication by linking with the switching equipment.
Change-over switch
A control switch that switches between two or more circuits.
Contactor
A mechanical switch that always returns to a state before operation when a feed of
operation energy is turned off and which is operated by means other than manual and
frequently opens and closes wires in normal state including overloads.
Control
The action of energizing, de-energizing, or locking a solenoid valve, coil, or other
equipment designed to activate the switchgear operator according to the purpose of
operation.
Control voltage
Voltage applied to a terminal of the controller.
Operation
The action of moving a movable contact of the switchgear from one position to
the one next to it.
Operating voltage
Voltage added to a terminal of the solenoid valve, motor, or other equipment on
the operator.
Closing operation
The action of closing an open contact.
Opening operation
The action of opening a closed contact.
35
Manual operation
An operation conducted by manual means alone without using any other source of
motive power.
Seal-in feature
It is desirable to have the control circuit ensure that the circuit breaker will fully close
each time that closing operation is initiated. If the circuit breaker is closed by a simple
switch, the operator could be open the switch to cut off the closing power before the
circuit breaker was fully closed and latched. In this event, the circuit breaker will fall
open. In most circuit breakers a relay with one set of contacts used to seal-in itself is
used to ensure a complete closing stroke once initiated.
Trip free
When contact of a contact or an arc between contacts has caused the main circuit to be
active, the tripper can operate to trip the circuit breaker even if the closing command
is being effective. Alternatively, even if a command for the closing device to conduct
closing is still given after completion of tripping, the closing operation is conducted
when the current closing command has been cancelled and another closing command
has been given.
When closing a circuit breaker, the closing coil (for example, the solenoid in solenoidoperated circuit breaker) is energized and the plunger operates through the linkage to
close the circuit breaker contacts. At the end of the closing stroke, appreciable time is
required to de-energize the solenoid coil. In the event that the circuit breaker has been
closed on a faulted circuit, it must be reopened as quickly as possible. If a circuit
breaker can trip automatically upon receiving a trip signal before closing operation is
complete, it is said to be “trip free”.
Various arrangements are provided to obtain trip-free action. Solenoid-operated circuit
breakers are sometimes provided with collapsible linkage. Pneumatically-operated
circuit breaker may be equipped two latches, one of which is unlatched during a
normal trip operation and the other is only unlatched for trip signal while the circuit
breaker is closing. Other circuit breakers use a large dump valve to quickly exhaust
the air under the closing piston. Many motor-operated circuit breakers obtain a fast
trip-free action by use of a relay energized from the trip circuit to open the closing
circuit. These methods would be known as mechanically trip free, pneumatically trip
free and electrically trip free respectively.
Anti-pumping
Preventing re-closing as long as the closing command is continuously effective upon
completion of tripping.
When a circuit breaker is closed and a trip-free operation results, the close and trip
stroke will be completed in a very short time. For a modern pneumatically –operated
high voltage circuit breaker, the complete operation will take less than one-half of one
second, thus, it is quite likely that the operator will still have the control switch in the
36
closed position. Means must therefore be provided to prevent the circuit breaker from
closing a second time, even though the operator is still holding the control switch in
the closed position. This is usually accomplished by the use of a sealed-in relay (antipumping relay) which can only be released by opening the closing control switch.
When this feature is incorporated in the control circuit, the circuit breaker is said to be
“pump free” or “anti-pumping”. Following a trip-free operation of the circuit breaker,
the operator must open the control switch before a second attempt to close the circuit
breaker can be made.
Anti-slam
When a circuit breaker is in the closed position, energizing the closing coil could
cause the solenoid plunger to move. Since the circuit breaker is in closed position,
there is a very little to retard this movement and the plunger will “slam” at the end of
its stroke. The control circuit should prevent slamming of the mechanism when the
circuit breaker is in the closed position.
Trip/release
Control operation for opening the circuit of the switching equipment upon receiving
the command from an external device.
Operating current
Minimum current for operating the tripper.
Interlock
Regulating the operation of the switching equipment by relating to the location
or operation of other device.
Breaking current
Current which flows through each pole during opening operation of the switching
equipment and is indicated by a value at a moment of arcing.
Standard operating sequence
Operating sequence used as a standard for defining the closing performance and
opening performance of the circuit breaker and switch.
O - 0.3s - CO - 3min - CO / CO - 15s - CO
37
11.4 PROPERTIES OF OIL, THEIR SIGNIFICANCE AND TEST METHODS (
Mineral insulating oils
These oils are obtained by refining the naturally available crude petroleum. They are
mixtures of many compounds known as hydrocarbons in different proportions which
depend on the source of crude petroleum.
Hydrocarbons can be divided into three classes, naphthenes, paraffins and aromatics.
Naphthenes and paraffins are saturated hydrocarbons and are chemically stable. They
differ from one another by their molecular structure and their physical and chemical
properties. Aromatics are not saturated and as a result they are less stable and more
chemically active. The generally used classifications of naphthenic and paraffinic do
not imply that these oils contain naphthenes or paraffins exclusively but refer to the
dominant characteristic of one of these classes in a mixture of naphthenic, paraffinic
and aromatic hydrocarbons. The proportions vary according to the source of the crude
petroleum and the refining process.
The sources of naphthenic crude are becoming rare therefore the tendency to
manufacture insulating oils from paraffinic crude petroleum is increasing. This does
not cause any particular problem except from the point of view of the freezing point,
and it may become necessary to use special additives. Mineral insulating oils are used
universally but with necessary fire protection arrangements depending upon the
installation conditions of the electrical equipment.
Mineral insulating oils of different manufacturers are mutually mixable/ miscibility in
any proportion, but it is not possible to restore used oil to a renewed condition by
mixing. The use of oxidation inhibitors to retard ageing is recommended in
exceptional cases such as in very large transformers subjected to severe operating
conditions.
Synthetic insulating oils
Insulating oils other than the mineral insulating oils are known as synthetic insulating
oils. The most common synthetic insulating oils derived from silicones are used only
as special applications such as in power capacitors, etc.
DEFINITIONS
Transformer oil
The mineral insulating oil for transformers and similar electrical equipment where
normal oxidation resistance is required is termed as transformer oil.
Low temperature switchgear oil
The mineral insulating oil for oil-filled switchgear for outdoor application in very cold
climatic conditions is termed as low temperature switchgear oil.
38
Additive
A suitable chemical substance which is deliberately added to mineral insulating oil in
order to improve certain characteristics is termed as an additive.
Note. Examples include antioxidants, pour point depressants, electrostatic charging
tendency depressants such as benzotriazole (BTA), anti-foam agents, refining process
improvers, etc.
Antioxidant additive
An additive incorporated in an insulating oil to improve oxidation stability is termed
as an antioxidant additive.
Note. A large number of antioxidant additives are available. For this standard, these
are limited to those identified in IEC 60666.
Uninhibited oil
The mineral insulating oil, containing no antioxidant additives, but which may contain
other additives is termed as uninhabited oil.
Trace inhibited oil
The mineral insulating oil containing up to 0.08 % antioxidant additive together with
other additives as included in the category of antioxidant additives.
Inhibited oil
The mineral insulating oil containing a minimum of 0.08 % and a maximum of 0.40 %
antioxidant additive together with other additives as included in the category of
antioxidant additives.
Note. In certain countries the inhibited mineral insulating oil is defined as the mineral
insulating oil containing at least 0.15% by mass, but not more than 0.40% by mass of
2.6-di-tert-butyle-paracresole (DBPC) or 2.6-di-turt-butyle-phenol (DBP).
Unused mineral insulating oil
The mineral insulating oil as delivered by the supplier is termed as unused mineral
insulating oil.
Note. Such oil has not been used in, nor been in contact with electrical equipment or
other equipment not required for manufacture, storage or transport. The manufacturer
and supplier of unused oil will have taken all reasonable precautions to ensure that
there is no contamination with polychlorinated biphenyls or terphenyls (PCB, PCT),
used, reclaimed or dechlorinated oil or other contaminants.
Reclaimed oil
The mineral insulating oil used in electrical equipment which has been subjected to
chemical and/or physical processing to eliminate soluble and insoluble contaminants
Note. A blend of unused and reclaimed oil in any proportion is regarded as being
reclaimed.
Viscosity
Viscosity influences heat transfer and therefore the temperature rise of the equipment.
The lower the viscosity, the easier the oil circulates leading to improved heat transfer.
39
At low temperatures the resulting higher viscosity of oil is a critical factor for the cold
start of transformers with ON cooling (no circulation and therefore possible
overheating at the hot spots) and negatively influences the speed of moving parts, such
as in power circuit breakers, switchgear, on-load tap changer mechanisms, pumps and
regulators. The viscosity at the lowest cold start energizing temperature (LCSET) shall
not exceed 1800 mm2/s (respectively 2500 mm2/s at -40°C). This lowest cold start
energizing temperature (LCSET) for transformer oils is defined in this standard as
being –30 °C. Other LCSET (see Table 1) can be agreed between supplier and
purchaser.
Low temperature switchgear oil should have a lower viscosity at LCSET: max. 400
mm2/s. The standard LCSET of low temperature switchgear oil is defined with -40 °C
but other LCSET may be agreed between supplier and purchaser.
Table 1- Maximum viscosity and pour point of transformer oil at lowest cold start
energizing temperature (LCEST)
LCEST °C
Maximum Viscosity mm2/s
Maximum Pour Point °C
0
1800
-10
-20
1800
-30
-30
1800
-40
-40
2500
-50
Note.1 For more details concerning ON cooling (natural oil circulation without
pump).
Note.2 There is no lower viscosity limit set in this IEC 60296, but under certain
conditions oil with a viscosity less than 7 mm2/s /40°C can be considered to be a
potential hazard.
Viscosity shall be measured according to ISO 3104, viscosity at very low temperature
according to IEC 61868
Pour point
Pour point of mineral insulating oil is the lowest temperature at which the oil will just
flow. It is recommended that the pour point should be minimum 10 K below the
lowest cold start energizing temperature (LCSET). If a pour point depressant additive
is used, this should be mentioned by the supplier to the user. Pour point shall be
measured in accordance with ISO 3016.
Water content
A low water content of mineral insulating oil is necessary to achieve adequate
electrical strength and low dissipation losses. To avoid separation of free water,
unused insulating oil should have a limited water content. Before filling the electrical
equipment, the oil should be treated to meet the requirements of IEC 60422. Where
requested by the purchaser, the supplier of oil shall demonstrate that after treatment to
40
remove solid particles, humidity and dissolved air by a vacuum procedure (see note),
the oil shall have a high dielectric strength of minimum 70 kV break-down Voltage.
Water content shall be measured in accordance with IEC 60814.
Note. This laboratory treatment referred to consists of filtration of the oil at 60 °C by
vacuum (pressure below 2.5 kPa) through a sintered glass filter (porosity 4).
Breakdown voltage
Breakdown voltage of transformer oil indicates its ability to resist electrical stress in
electrical equipment. Breakdown voltage shall be measured in accordance with IEC
60156.
Dielectric dissipation factor (DDF)
DDF is a measure for dielectric losses caused by the oil. Increased DDF can indicate
contamination of the oil by moisture, particles or soluble polar contaminants or poor
refining quality. DDF shall be measured in accordance with IEC 60247 or IEC 61620
at 90 °C. In case of dispute, IEC 60247 at 90 °C should be used.
Note. By agreement between parties, DDF may be measured at temperatures other
than 90 °C. In such cases the temperature of measurement should be stated in the
report.
Appearance
A visual inspection of insulating oil (oil sample in transmitted light under a thickness
of approximately 10 cm and at ambient temperature) indicates the presence of visible
contaminants, free water or suspended matter.
Acidity
Unused mineral insulating oil should be neutral and free from any acidic compound.
Acidity should be measured following IEC 62021-1.
Interfacial tension (IFT)
Low IFT sometimes indicates the presence of undesirable contaminants. IFT shall be
measured in accordance with ISO 6295.
Sulfur content
Different organo-sulfur compounds are present in transformer oils, dependent on the
crude oil origin and the degree and type of refining. Refining treats sulfur and
aromatic hydrocarbons. As some sulfur compounds have an affinity to metals, they
may act as copper passivators or they may promote corrosion.
Sulfur content should be measured following BS 2000 Part 373 or ISO 14596.
Corrosive sulfur
Some sulfur compounds, e.g. mercaptans, are very corrosive to metal surfaces, i.e.
steel, copper and silver (switchgear contacts) and shall not be present in new oil.
Corrosive sulfur should be measured following DIN 51353.
Antioxidant additive content
Antioxidant additive (inhibitor) slows down the oxidation of oil and therefore the
formation of oil sludge and acidity. It is important to know whether and in what
41
proportion antioxidant additive has been added in order to monitor additive depletion
during service. 2.6-di-tert-butylp-cresol (DBPC) is the most commonly used
antioxidant, but others are also used. Detection and measurement of defined
antioxidant additives shall be determined in accordance with IEC 60666. The type and
quantity of each antioxidant additive present in the oil shall be stated in the quality
certificate. If co-stabilizers are used during the refining process, their presence shall be
agreed between the supplier and the purchaser.
Oxidation stability
Oxidation of oil gives rise to acidity and sludge formation and can be minimized as a
result of high oxidation stability leading to longer service life time by minimizing
sludge deposition, electrical losses, metal corrosion, electrical faults and maximizing
insulation life. Oxidation stability is measured in accordance with method C of IEC
61125. There is an option for stricter limits for special applications. In some countries
more stringent limits and/or additional requirements and tests may be requested.
Gassing
Gassing tendency of mineral insulating oil, i.e. the gas absorbing property of an oil
under electrical stress, is only necessary and important for special transformers like
HV (high voltage) transformers and is a measure of the rate of absorption or evolution
of hydrogen into oil under prescribed laboratory conditions. Gas absorption properties
are related to oil aromaticity which is subject to indirect control by the oil's oxidation
requirements. Gassing tendency is measured using method A of IEC 60628. Gassing
tendency is a specific requirement.
Electrostatic charging tendency (ECT)
ECT of oil is an important property for certain designs of HV and EHV transformers
which have oil pumping rates that can give rise to the build-up of electrostatic charge.
This charge can result in energy discharge causing transformer failure. A method to
measure ECT is proposed by CIGRE SC12. ECT is a specific requirement.
Flash point
The safe operation of electrical equipment requires an adequately high flash point that
is measured in accordance with ISO 2719 (Pensky-Martens closed cup procedure).
Density
Density of oil shall be low enough to avoid, in cold climates, that ice resulting from
the freezing of free water is floating on the oil surface and possibly leading to fault
conditions developing in flashover of conductors. Density shall be measured in
accordance with ISO 3675 (reference method) but ISO 12185 as well is accepted.
Polycyclic aromatics (PCA)
Some PCAs are classified to be carcinogens and therefore need to be controlled to an
acceptable level in mineral insulating oil. PCAs are defined so as to be detectable by
extraction with DMSO (Dimethylsulfoxide) under the conditions of BS 2000 Part 346.
42
Polychlorinated biphenyls (PCB)
Unused mineral insulating oil shall be free from PCB. The reference method is IEC
61619. The detection limit for a single peak is 0,1 mg/kg.
Note. The total limits are given by national regulations.
Furfural and related compounds (2-FAL)
2-FAL and related compounds in unused mineral insulating oils can result either from
improper re-distillation after solvent extraction during refining or from contamination
with used oil.
Unused insulating oils should have a low level of 2-FAL and related compounds and
measurement should be done according to IEC 61198.
Furfural Concentration in Transformer Oil as an Indicator of Paper Aging
The concentration of the chemical furfural in transformer oil is well known to be an
indicator of the extent to which the paper insulation of the winding has deteriorated.
The two major insulating materials used in transformers are mineral insulating oil and
paper. Transformer insulation is subjected to electrical and thermal stresses during
service. Life of the transformer is mainly decided by the condition of the insulation
because the operation, loading and reliability of the transformer is dependent on the
state of the insulation.
The windings of oil-filled transformers use paper insulation and if this becomes weak
the consequence of a fault current may be severe, the inevitably high forces on the
windings may result in mechanical failure of the insulation and consequent inter-turn
short circuits. The reduction in strength of paper as it deteriorates is thus of
considerable importance. An important by-product of the degradation of paper is 2furfuraldehyde of ‘furfural’ whose presence can as an indicator of the progress of
deterioration.
Insulating paper degrades with time at rates which depends on factors like
temperature, amount of air and moisture present. If the cellulose paper is heated above
100 °C, it begins to degrade more rapidly and the useful service life of the transformer
is reduced as a result. Laboratory experiments and studies show a direct relationship
between the degree of polymerization (DP) value and the amount of furanic
concentration in the oil. The studies reveal the lower DP values indicate increased
furfural content when paper samples were subjected to heating over a long period.
Some reports suggest the end of life of the cellulose insulation would be reached when
total furan content reaches 10 PPM for a corresponding DP value of 15-200.
It has been observed that the products of degradation of cellulose material in
transformer appear in the oil in the form of furanic compounds commonly referred to
as forans; 2-Furfural, 2-Fururalalcohal, 5-Hydroxymethyl-2-furfural, 2-Acetylfuran,
Methyl-2-furfural.
43
11.5 TREATMENT OF OIL (DEHYDRATION/RECONDITIONING,
REGENERATION/RECLAIMING, RE-FINING)
Treatment of transformer oil on site by continuous circulation through the oil filtration
and dehydration plant without completely emptying the transformer tank is in general,
satisfactory for routine maintenance. However where the presence of sludge is known,
it may be necessary to remove the oil completely for filtration, so that the transformer
itself may be inspected and cleaned. As sludge tends to harden when exposed to
atmosphere, it is recommended that the cleaning be carried as soon as possible after
the removal of oil and of the core and windings from the tank.
Filtration of the oil for removal of sludge must be carried out in the cold, since,
although sludge tends to dissolve in hot oil, it comes out of the solution when the oil is
cooled. Water, too, is much more soluble in hot oil than in cold, but this contaminant
can be rapidly and virtually completely removed by vacuum treatment of the hot oil,
and the most sophisticated modern oil treatment plants employ high-vacuum
equipment in combination with fine filtration. These plants are capable of removing
almost completely, the dissolved air and water from transformer oil, particles of free
solids such as fiber, as well as any free water. During a typical servicing period these
types of oil treatment plants will reduce in a single pass, the dissolved air content of
oil from 11% to less than 1% by volume and dissolved moisture from 30ppm to less
than 5ppm. Simultaneously, the DES can be increased from under 30kV to over 70kV.
The most prominent factors causing oil deterioration are the presence of air and the
effect of temperature.
The complete exclusion of air may be achieved by the use of sealed transformer tanks
having suitable oil expansion arrangements, or by providing nitrogen or other inert gas
cushions in the tank. However, a major exclusion of air is obtained by the use of oil
conservators, particularly in the case of large transformers.
Reconditioning
This is a process which eliminates, by physical means only, solid particles from the oil
and decreases water content to an acceptable level.
The physical means that are used for removing water and solids from oil include
several types of filtration, centrifuging and vacuum dehydration techniques.
If vacuum -treatment is not employed it is advisable to limit the temperature to 60 °C.
If vacuum is employed, a higher temperature may be advantageous. However, at the
vacuum used, the initial boiling point of that oil should not be exceeded, to avoid
undue loss of lighter fractions. If this information is not available, it is recommended
44
that the oil should not be vacuum treated at temperatures over 70 °C. If it is desired to
reduce sludge or free water, cold treatment may be appropriate.
Filters deal efficiently with solid impurities, but are generally capable of removing
only small quantities of water such as may be found in oil from equipment housed in
buildings. Where relatively large quantities of water are present, most of it can, and
shall, be drained away before filtration of the oil.
Centrifugal separators are, in general satisfactory for removing free water from oil and
can in any case deal also with any finely divided solid impurities.
If oil is purified hot its viscosity is reduced and the throughput with certain types of
purifier is greater. On the other hand, sludge and free water are more soluble in hot oil
than in cold; sludge and free water are, therefore, more effectively removed by cold
treatment. Dissolved and suspended water is effectively removed by hot vacuum
treatment.
If the oil contains solid matter, it is advisable to pass it through some kind of filter
before processing it under vacuum.
Note.
- Processing inhibited mineral oil under vacuum and at elevated temperatures may
cause partial loss of oxidation inhibitors; the common inhibitors, 2.6-di-tert-butylparacresol and 2.6-ditert - butyl - phenol are more volatile than mineral insulating oil.
The selectivity for removal of water and air in preference to loss of inhibitor and oil is
improved by use of a low processing temperature.
Conditions that have been found satisfactory for most inhibited mineral oil processing
are:
Temperature °C
40
50
60
70
80
Pressure pa
5
10
20
40
100
Filters in Reconditioning equipment
These are generally based on the principle of forcing oil under pressure through
absorbing material such as paper or other filter media. Filters of this type are
preferentially used in removing contaminants in suspension. (The filter medium
should be capable of removing particles larger than a nominal 10 rim.) These devices
do not de-gas the oil.
The water-removing ability of a filter is dependent upon the dryness and quantity of
the filter medium. When filtering oil containing water, the water content of the filter
medium rapidly comes into equilibrium with the water content of the oil. A
45
continuous indication of the water content of the outgoing oil is useful to monitor the
efficiency of the process.
Care should be taken that paper filters are of the correct grade to ensure that no fibers
are shed by them.
Centrifuges
In general, a centrifuge can handle a much greater concentration of contaminants than
can a conventional filter but cannot remove some of the solid contaminants as
completely as a filter.
Consequently, the centrifuge is generally found in use for rough bulk cleaning where
large amounts of contaminated oil are to be handled.
Frequently the output of the centrifuge is put through a filter for the final clean-up.
Vacuum dehydrators
The vacuum dehydrator is an efficient means of reducing the gas and water content of
a mineral insulating oil to very low values. There are two types of vacuum dehydrator;
both function at elevated temperature. In one method the treatment is accomplished by
spraying the oil into a vacuum chamber; in the other, the oil flows in thin layers over a
series of baffles inside a vacuum chamber. In both types the objective is to expose
maximum surface and minimum thickness of oil to the vacuum.
In addition to removing water, vacuum dehydration will degas the oil and remove the
more volatile acids.
Application to electrical equipment
Direct purification
The oil is passed through a purifier and then stored in suitable clean containers. When
the electrical equipment is to be refilled the oil is passed through the purifier again and
then directly into the equipment. This method should be used for switchgear. It is
suitable, too, for the smaller transformer, but care is needed to ensure that the core, the
windings, the interior of the tank and other oil-containing compartments are
thoroughly cleaned. The oil-containing compartments of all equipment should also be
well cleaned, by means of oil from the purifier.
Purification by circulation
The oil is circulated through the purifier, being taken from the bottom of the tank of
the electrical equipment and re-delivered to the top. The return delivery should be
made smoothly and horizontally at or near the top oil level to avoid, as far as possible,
mixing cleaned oil with oil which has not yet passed through the purifier. The
circulation method is particularly useful for removing suspended contaminants, but all
adhering contaminants will not necessarily be removed.
46
Experience has shown that it is generally necessary to pass the total volume of oil
through the purifier not less than three times, and equipment of appropriate capacity
should be chosen with this in mind.
The final number of cycles will depend on the degree of contamination, and it is
essential that the process be continued until a sample taken from the bottom of the
electrical equipment, after the oil has been allowed to settle for a few hours, passes the
breakdown voltage test. The circulation should be performed with the electrical
equipment disconnected from the power source, and this is essential when using a
purifier which aerates the oil. In all cases, and especially when aeration has occurred,
the oil should be allowed to stand for some time in accordance with the manufacturer's
instructions before the equipment is re-energized.
Another technique is sometimes used for transformers, in which oil is continuously
circulated during normal service through an adsorbent, such as molecular sieve, thus
keeping both oil and windings dry and removing many oil oxidation products; this is a
specialized method not further considered in this guide.
Reclaiming
This is a process which eliminates soluble and insoluble contaminants from the oil by
chemical and adsorption means in addition to mechanical means, in order to restore
properties as close as possible to the original values.
Reclaiming is a process often performed by an oil refiner but, since in some countries
reclaiming is performed by the user on site.
This is a process which eliminates, by chemical and adsorbent means, the acidic and
colloidal contaminants and products of oil deterioration from the oil, to obtain oil with
many characteristics similar to those of a new product.
Safety; Hygiene and environmental precautions
The oils with which this code of practice is concerned are mineral hydrocarbon
(petroleum) oils. Although no special risks are involved in the handling and use of
mineral insulating oils, attention is drawn to the need for personal hygiene (washing of
skin and clothing which has come into contact with oil) by those working with these
products.
When mineral oil has to be disposed of, certain precautions are necessary to avoid risk
of environmental pollution, and legal requirements may apply. Normally, if the
precautions and regulations applicable to the handling and disposal of industrial and
other lubricants (e. g. automobile crank-case draining) are applied to mineral
insulating oils, no problems should arise.
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12. Revision Table for SOP (M)
Revised Number
SOP(M)-TSG001/R0
SOP(M)-TSG001/R1
Contents of Reason for the Date
revision
revision
revision
New issue
Aug. 2008
Revision
Up-gradation
May. 2013
of Person in
in-charge
Mirza
Muhammad
Akram
Mirza
Muhammad
Akram
48
13. REFERENCES
The following books/literatures have been consulted for preparation of this booklet of
SOPs:
1. ONTARIO HYDRO/CIDA CANADA Training manuals
2. WAPDA Specifications
3. ABB Switchgear manuals
5. IEC Standards/Specifications
6. JICA/AKC TOT Training manuals
7. Instruction Manuals of grid system equipment in NTDC/DISCOs/WAPDA
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Mailing Address of TSG
Chief Engineer (TSG) NTDC
TSG Training Center,
220kV Grid Station, NKLP, Feroze Pur Road Lahore.
Telephone: 042 35821418
Fax: 042 35821498
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