Transformer Protections
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TRANSFORMER
PROTECTIONS
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The Types of faults that the transformers are
subjected to are classified as:
INTERNAL FAULTS
EXTERNAL FAULTS
( THROUGH FAULTS)
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INTERNAL FAULTS
FAULTS ARISING IN THE TRANSFORMER ITSELF
ELECTRICAL FAULTS: Faults which cause
immediate serious damage such as
1. Phase to earth fault
2. Phase to phase faults on HV or LV windings
3. Short circuit between turns on HV and LV
winding (inter turn faults)
4. Problem in tap changer gear
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INCIPIENT FAULTS: Which are initially minor
faults, causing slowly developing damage,
such as1. Poor electrical connection of conductors
2. Breakdown of insulation of laminations, core
bolt faults, clampings.
3. Coolant failure
4. Continuous uneven load sharing betweentransformers in parallel causing overheating
due to circulating current.
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EXTERNAL FAULTS (through faults)
The faults in the system to which the
transformer is connected Due to external faults (like phase-phase, phase
to earth) the effect is excessive over currentand electromagnetic stresses proportional tosquare of short circuit current.
1. Short circuits: Phase to phase, phase to earth
2. High Voltage disturbance3. High Frequency disturbance
4. Earth faults
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PROTECTION PRINCIPLE
1. Circuit breaker
2. Relay
3. Trip coil of C.B
5.Battery
6.Relay contacts
7.Bus bars
8.Current T/F9.Pilot-wires
a.Aux.switch of C.B
x.Protected Power T/F
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THE FOLLOWING RELAYS ARE EMPLOYED TO PROTECTTHE TRANSFORMER AGAINST DIFFERENT FAULTS
1. Over load
2. Over current3. Over voltage
4. Restricted Earth Fault
5. Differential protection
6. Buchholz relay
7. Pressure relief value (PRV)
8. Winding temperature
9. Oil temperature10. OLTC buchholz (oil surge relay)
11. Over fluxing
12. Over/Under frequency
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OVER LOAD
This protection is required for continuous over load ofthe transformer
Protection is based on measurement of windingtemperature which is measured by thermal imagetechnique.
Thermal sensing element is placed in small pocketlocated near the top transformer tank in the hot oil. Asmall terminal of one phase is also located in thispocket and produced a local temp, rise, similar to thatof main winding and proportional to copper losses,
above general temp of oil. Winging temperature high alarm/trip is provided
through mercury switches in the winding temp,indicators.
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OVER LAOD PROTECTION The permissible overload and their duration
depends upon the type of cooling andinsulation class of transformer. Higheroverloads are permissible for a shorterduration.
Hence for most of the transformers, overloadprotection is generally arranged to initiatealarm.
Overload % 125 150 175 200 300
Duration
(minutes)
125 45 15 10 1
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THERMAL OVER HEATING PROTECTION
Thermo couples or RTD are kept near each
winding to detect the temperature. When thetransformer temperature increases cooling
system comes in to picture.
Fans get ON : 60 0C OFF : 50 0C
Alarm : 90 0C
Trip : 104 0C
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OVER CURRENT
This protection is required for short circuit
faults and earth faults IDMT relays should be chosen
Instantaneous trip facility for high speedclearance of terminals short circuit should beprovided
Setting depends on transformer reactance or
percentage impedance, faults MVA. Setting of over current relays can be slightly
higher than rated full load current (say 120percent of FL) with proper discrimination.
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EARTH FAULT PROTECTIONThis protection is required for earth faults
Combined over current and earth fault protection
(un-restricted earth fault)
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The increase in current of phases causes correspondingincrease in respective secondary currents. The secondarycurrent flows through respective relay units. Very oftenonly two phase relays are provided instead of three,
because in case of phase faults current in at lease any twophases must increase. Hence two rely units are enough.The earth fault relay is residually connected
Over current and Earth fault protections are used as a mainprotection for medium transformers where differentialprotection is not provided. Differential protection isgenerally uneconomical for power transformers below 1MVA. In such cases, over current protection is employed asmain protection against phase faults. For transformersabove 1 MVA, if differential protection is used as a mainprotection, over current protection is used in addition, as abackup for sustained through faults. Earth fault protectionis provided in addition to phase fault protection.
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RESTRICTED EARTH FAULT PROTECTION
Transformer earth fault protection should notoperate for faults beyond the transformerprotection zone. This is ensured by the restrictedearth fault protection. This protection operateson the principle of Kirchoofs law and requiresCTs of identical ratio and ratings as the phasesand neutral earth connection. For external earthfault, the associated CTs have dissimilar polaritiesforming a series connection. Thus, the resulting
current through the relay is negligible. Forinternal fault, however, the CTs have similarpolarities, forming a parallels connection, thisadding up the current in the relay branch. Thisensures positive operation of the relay.
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RELAY DOES NOT OPERATE FOR EXTERNAL EARTH FAULT
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RELAY OPERATES FOR INTERNAL FAULT
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EARTH FAUALT PROTECTION
of transformers feeding through cables by zero
sequence relay (core balance currenttransformer)
In absence of an earth fault in the transformeror the supply cable, the current in thesecondary of zero sequence CT is negligible.Under balanced load and without earth fault
Ir = (Ia + Ib + Ic ) = 0 During tan earth fault (Ia + Ib + Ic ) 0 flux
in the core 0
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CORE BALANCE CT
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DIFFERENTIAL PROTECTION
This protection is also for internal faults of the transformerand responds to the vector difference between two
similar quantities. Two basic requirements that the differential relay
connections are to be satisfied are:
It must not operate for external faults It must operate for internal faults
As on load tap change facilities are invariably provided inthe grid transformer may departure from the nominal tapposition will result in spill currents in the relay circuits.
Further the CTs are often of different types and havedissimilar magnetization characteristics, again resulting inspill current during heavy through fault conditions.
To avoid unwanted relays operation under the above twoconditions a percentage bias differential relays is used.
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DIFFERENTIAL PROTECTION
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DIFFERENTIAL PROTECTIONSTAR/STAR TRANSFORMER
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DIFFERENTIAL PROTECTIONDELTA/STAR TRANSFORMER
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The current flowing through the operating coil of the relayshould be nearly zero during normal operating conditionsand when external short circuit occurs.
While setting the differential relay on a transformer, the( mismatch) current through differential element at normaltap and positive and negative extreme taps are to becomputed . Differential element pickup setting and/ orbias setting is adopted based on maximum percentagemismatch adding some safety margin.
In this connection the CT secondarys are connected instar or delta and pilot wires are connected between the CTsof each end. The CT connections and CT ratios are such
that currents fed into the pilot wires from both the endsare equal during normal conditions and for through faults.During the internal faults such as phase to phase or phaseto ground, the balance is disturbed
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The out of balance current (I1- I2) flows throughthe relay operating coils (O.C). To avoidunwanted operation on through faults,
restraining bias coils are provided in series withpilot wires. The ampere-turns provided bias coilor restraining coil or proportional to (I1- I2)/2, I1and I2 are vectors. Three sets of CTs primaries areconnected on primary side and 3 sets onsecondary side. CT secondarys on STAR side ofmain transformer are connected in DELTA , CTsecondarys on DELTA side of main transformerare connected in STAR. This ensures correct flowof circulating currents in pilot wires.
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COMBINED DIFFERENTIAL PROTECTION
for Generator Main Transformer
(Overall differential)The zone of combined differential
protection may include generator statorwinding, main step up transformer and Unitauxiliary transformer. In addition to normaldifferential protection of generator, overall
biased differential protection relay isconnected to protect the unit. This is asupplementary protection for individualdifferential protection of the generator.
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OVER VOLTAGE PROTECTION
Two stage protection is provided. Thedelayed trip is set at 110 percent of the rated
voltage with two second time delay and
Instantaneous setting is kept at 115-120
percent of the rated voltage. During voltage
fluctuations the AVR (Automatic Voltage
Regulator ) will care to avoid over voltagecondition if fluctuations are within its
operating limits (for generator transformers).
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SURGE ARRESTER PROTECTIONSurge arresters (lighting arresters) are connected
between phase and earth near the terminal of each
phase of the transformer. The surge arrester providesprotection against Lightning surges and switchingsurges. Lightning surges occur due to lightningstrokes on transmission line conductor. The surgetravels up to transformer and can damage thetransformer winding insulation. The surge arresterintercepts the surge and diverts it to the earth and
protects the transformer. Switching surges originatein the system while opening of inductive currentsand closing of transmission lines. The surge arresterintercepts the switching surge and diverts it to the
earth and protects the transformer.
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POWER FREQUENCY TEMPORARY OVERVOLTAGEPROTECTION
When load or line is thrown off (switched off )the power frequency (50 Hz) overvoltage occursat the bus bars of receiving substation. The onload tap changers act and bring down the voltageto permissible value. If overvoltage does notdrop in short time the transformers may fail. Thep.f voltage above 1.1 per unit is harmful totransformer. Inverse Definite Minimum Time
over voltage relay is connected to secondary ofprotective bus PT. This relay is arranged to tripthe circuit breakers feeding the transformersinstantaneously for overvoltage of 1.3 p.u. Forover voltages of 1.2 p.u time delay provided.
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OVERFLUXING PROTECTIONThe magnetic flux in the transformer core is
proportional to V/f, where V is the voltage and f is the
frequency. With rated voltage = Vr and ratedfrequency fr the V/f ratio is Vr / fr = 1 p.u . Transformercore can tolerate the V/f ratio of 1.1 p.u for longduration without harm. If V/f ratio is higher than 1.1the condition of over fluxing arises. The core gets hotand core insulation is damaged. The core loss4esincreases. The transformer becomes hot. The overfluxing condition in generator transformers and unitauxiliary transformers if the generator excitation is
increased before reaching synchronous speed. Overfluxing relay is connected in generator excitationsystem. This relay does not allow the excitationcurrent of generator till synchronous speedcorresponding to the rated frequency is reached.
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Over fluxing can occur in transformers of generator unit. Thiscondition arises during abnormal operating conditions i.e heavy
voltage fluctuations at lower frequency conditions. This condition is
experienced by the transformer during heavy power swings,cascade tripping of the generator sets and HT line in the grid,
interstate system separation conditions and due to AVR
malfunctioning during start up or shutting down in case of
generator transformers. The power frequency over voltage cause
both stress on insulation and proportionate increase in the
magnetizing flux inside the transformer due to which the iron losses
are increased and the core bolts get maximum component of flux,
thereby rapidly heating and damaging its own insulation and coil
insulation. Reduction in frequency during high voltage fluctuation
has the same effect.
V/f over fluxing factor 1.1 1.2 1.25 1.3 1.4
Duration of withstand
limit (minutes)
Contin
uous
2 1 0.5 0
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BUCHHOLZ RELAYThe incipient faults in transformer tank below oil
level actuate Buchholz relay so as to give an alarm. The
arc due to fault causes decomposition of transformeroil. The product of decomposition contain more than70% of hydrogen gas. Which being light, rises upwardsand tries to go into the conservator, The Bnuchholzrelay is fitted in the pipe leading to the conservator.The gas gets collected in the upper portion of theBuchholz relay, thereby the oil level in the Buchholzrelay drops down. The float , floating in the oil in thebuchholz relay tilts down with the lowering oil level.
While doing so the mercury switch attached to thefloat is closed and the mercury switch closes the alarmcircuit. Buchholz relay gives an alarm so that thetransformer can be disconnected before the incipientfault grows in to a serious one.
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BUCHHOLZ RELAY
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ARRANGEMENT OF BUCHHOLZ RELAY
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PRESSURE RELIEF VALUE (PRV)This is different from rate of rise pressure relay.
Pressure relief valve is mounted on transformer tank.It releases gas pressure to the atmosphere during
1. High overload peaks
2. Prolonged overloads
3. Arcing faults within oil
The PRV is spring loaded and has a seal seat. Whenpressure inside the tank increases above a certain
value, the force on movable sub assembly exceeds thespring force and valve operates. The contacts areclosed. After release of PRV the valve may be resetmanually.
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PRESSURE RELIEF VALUE (PRV)
DURING NORMAL POSITION
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PRESSURE RELIEF VALUE (PRV)
DURING INTERNAL PRESSURE
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OIL TEMPERATURE
An oil thermometer, which is common with all oilfilled transformers, can be considered as a partiallyeffective protective device when equipped with alarmcontacts connected to give remote warning ofabnormally high oil temperature. Its location is suchthat it naturally monitors the hottest fluid that exists inthe transformer. The same thermo meter is often usedto start fan motors ON and OFF.
WINDING TEMPERATURE
The thermometer bulb is located in a pocket near
the winding. The bulb is surrounded by hot circulatingoil. The bulb is also heated by a small heater connectedacross CT secondary . Thereby the heat given to thebulb is a function of load current as well as thetemperature of oil near winding.
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PROTECTION CHART OF POWER TRANSFORMER
Sl.
No.
ABNORMAL CONDITION PROTECTION REMARKS
1. Incipient fault below oil level Buchholz relay (Gasactuated relay
Buchholz relay fortransformers of rating 500
kVA and above
2. Large internal faults phase to phase, phase to
ground
1.Buchholz relay trips the
circuit breaker
2. Percentage differential
Protection
3.High speed high set over
current relay
Buchholz relay slow and
less sensitive
Percentage differential
protection for
transformers above 5
MVA
3. Saturation of Magnetic circuit 1.Overfluxing protection
with
voltage regulator
2. Over voltage protection
of
Bus bars
For generator transformer
and unit auxiliary
transformer
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Sl.
No.
ABNORMAL CONDITION PROTECTION REMARKS
4. Earth faults 1. Differential Protection
and
2. Earth fault relay in
residually connected
circuit
For transformers of and
above 5 MVA
(a) Instantaneous
restricted E/F
Relay
(b) Time lag E/F relay
5. Through faults (External faults feeding through
the transformer)
1. Graded time lag over
current relay
2. HRC fuses
Protection of transformers
Small distribution
transformers up to 500
kVA
6. Over loads 1. Thermal over load
relays
2. Temperature relays
3. Over current relayswith
inverse characteristic
Fans started at certain
temperature. Alarm and
tripping actions later
7. High voltage surges due to lightning switching 1. Horn gaps
2.Surge arresters
3.R.C surge suppressors
Not favored for important
transformers
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