basic transformer protection
TRANSCRIPT
GE Digital EnergyGE Digital EnergyGE Digital EnergyGE Digital Energy
Transformer basics and more…
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Power Transformer as an expensive device
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Power Transformer Name Plate
Generator Step- Up
transformers
Step- Down transmission
substationtransformers
Distribution substation
transformer
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Distribution transforme
r
Location
Transformer types• Generator Step-Up (GSU) transformer
• Transmission step down transformer
• Distribution substation transformer
• Distribution small transformers
• Autotransformers
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• Voltage regulating transformers
• Scott transformers
• Reactors
• Split-Phase autotransformers
GENERATOR STEP- UP (GSU) TRANSFORMERS
52
52
GSU xfmr
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• 1300 MVA
• 343 ±2x1.9% / 25 kV
• 60 Hz
• 3-phase
Aux. xfmr
STEP- DOWN DISTRIBUTION SUBSTATION TRANSFORMER
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• 24 MVA
• 138 / 13.8 kV
• 60 Hz
• 3-phase
• FOA
AUTOTRANSFORMER
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1000 MVA
346 ±17x3.66 kV / 225 kV
60 Hz
3-phase
OA/FA/FA (self-cooled/ forced air cooled)
100kVA, 14.4kV/240V/120V
100kVA, 14.4kV/347VPole-Mount
Single-phase
transformers
500kVA, 14.4kV/600VPlatform-Mount
SMALL DISTRIBUTION TRANSFORMERS
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45kVA, 24.94kV/480V3-phase Pole Mount
14.4kV/240V/120VMini-Pad
Pole-Mount
Three-phase
transformers
Up to 300kVAPrim. voltage - up to 36kV3-phase substation installation
Platform-Mount
2 MVA, 13.8/600kVPad - Mount
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Transformer components – cooling system
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OIL PUMPCOOLING FANS
Transformer components – tap changer
TAP-CHANGER
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BUCHHOLZ RELAY
Transformer components –Buchholtz relayTransformer Basic & More…Transformer Basic & More…Transformer Basic & More…Transformer Basic & More…
SUDDEN PRESSURE
RELAY
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CHANGE PRESSURE RELEIF
DEVICE
Winding connections and phase shiftTransformer Basic & More…Transformer Basic & More…Transformer Basic & More…Transformer Basic & More…
Transformer electric parameters - relations
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SSSPPP PIVIVP === **
S
P
P
S
P
S
I
I
N
N
V
V==
Transformer losses
copper losses -I 2 R
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Stray loss -stray capacitance and leakage inductance lossesApparent loss –loss due to magnetizing current in the primary winding
Iron loss –loss due to varying flux in the core( hysteresis and eddy losses)
- hysteresis loss
Transformer inrush
The steady state flux
lags the voltage by
90° degrees
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The flux builds up from zero, when the voltage is applied at zero crossing, and can reach 2 times the maximum flux. The magnetizing current becomes even higher, if the transformer is energized at zero point of the voltage wave, and there is residual flux
As the flux builds,
the exciting
current grows with
the flux.
Transformer faults and detection
• EXTERNAL FAULTS
– Overloads
– Overvoltage
– Underfrequency
– External system short circuits
• INTERNAL FAULTS
– Incipient faults
• Overheating
• Over-fluxing
• Overpressure
– Active faults
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– Active faults
• Short circuit in wye-connected
windings
• Short circuits in delta windings
• Phase-to-phase faults
• Turn-to-turn faults
• Core faults
• Tank faults
External faults:
OVERLOADSOVERLOADSOVERLOADSOVERLOADSIn most cases, no protection is provided, but an alarm is used to
warn the operating personnel of the conditions. A TOC
protection with definite time delay can be set.
OVERVOLTAGEOVERVOLTAGEOVERVOLTAGEOVERVOLTAGEIt can be either due to short-term transient conditions, or long
term power frequency conditions. Transient over-voltages,
cause end-turn stresses and possible breakdown. The conditions
are detected by Volts/HertzVolts/HertzVolts/HertzVolts/Hertz protection.
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UNDERFREQUENCYUNDERFREQUENCYUNDERFREQUENCYUNDERFREQUENCY
Under-frequency is caused by some system disturbances
resulting in unbalance between generation and load. This low
frequency creates overfluxing in the transformer core, leading to
overheat. Volts/HertzVolts/HertzVolts/HertzVolts/Hertz protection is used with typically 1.1 pu
pickup ratio setting.
EXTERNAL SYSTEM SHORT EXTERNAL SYSTEM SHORT EXTERNAL SYSTEM SHORT EXTERNAL SYSTEM SHORT CIRCUITSCIRCUITSCIRCUITSCIRCUITS
Large external fault currents can cause high mechanical stress
in the transformer windings, with the maximum stress occurring
during the first cycle. The transformers are not protected during
such external conditions. It is a matter of transformer design,
and application, to deal with these conditions.
Incipient transformer internal faults
OVERHEATINGOVERHEATINGOVERHEATINGOVERHEATING
Caused by:
• poor internal connections, in either electric or magnetic circuit
• loss of coolant due to leakage
• blockage of coolant flow
• loss of fans or pumps
Buchholtz relay and thermal elements protections such as Hottest
Spot temperature, Aging Factor and Loss of Life are normally used
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Continuous overfluxing can gradually lead to isolation breakdown. The detection is provided by Volts/Hertz Volts/Hertz Volts/Hertz Volts/Hertz protection
OVERFLUXINGOVERFLUXINGOVERFLUXINGOVERFLUXING
OVERPRESSUREOVERPRESSUREOVERPRESSUREOVERPRESSURE
Overpressure in the transformer tank occurs due to released gases that accompany localized heating. An example is the turn-to-turn fault, that can burn slowly, releasing bubbles of gases, which increase the pressure. Sudden Pressure relay, or Buchholtz relay
Protection elements
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Active internal faults detected by 87T protection
* ** *
D/Y30
WYE connectionWYE connection
I1 prim I2 prim
T60
T60
Phase-to-phase faults
Three-phase faults
Ground faults
Core faults
Tank faults
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i1 sec
i2 sec
T60
DIFFERENTIAL SIGNAL:
I DIFF. = I1COMP + I2COMP
RESTRAINING SIGNAL:
I RESTR. = max ( |I1COMP| , | I2COMP|)
Id, pu
Ir, pu
Min. PKP
B 1 B 2
S 1
S 2
T60 BIASED DIFFERENTIAL(TRIP/NO TRIP DECISION)
Id > PKP
YES
Ir < B1 Id/Ir%>S1YES
NO
NO
NO TRIP
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NO
B1 < Ir < B2 YES
NO
Ir > B2
Id/Ir, % >S2 YES TRIP
Id/Ir,% >S1&S2
YES
NO
NO
YES
Restricted Ground Fault protection
� Fast detection of winding ground faults� Very secure performance on external ground faults� Configurable pickup, slope, and time delay
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Igd, pu
I = max( IR1, IR2, IR0 ), pu
Min. PKP
S lope
gIIIgdρρ
+= 03)2,1,0max( IRIRIRIgr =
Questions
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