Transformer Protection
Transformer Protection Outline Fuses Protection Example Overcurrent Protection Differential Relaying Current Matching Phase Shift Compensation Tap Changing Under Load Magnetizing Inrush Overexcitation Connection Examples Ground Differential Sudden-Pressure Relays (63)
Power Transformer Failure Statistics 1955-2002 (3,112 failures) Winding failures = 31% Tap changer failures = 26% Bushing failures = 12% Cooling equipment failures = 3% Auxiliary equipment failures = 3% Core failures = 1% Leads failures = 1% Other failures = 23% Source: IEEE C37.91-2000
Transformer Fusing Normally used at 10MVA and below Reference transformer damage curves IEEE C37.91 Selected to fit below damage curve Fuses must be coordinated with relays Economical
Fuse Characteristics 1000 100 200 A-AF Maximum Clear OPERATE TIME 10 1 100 A-AF 0.10 0.15 Sec Minimum Melt 0.01 0.5 1 10 100 1K 10K CURRENT
Example: Large Industrial Load 86-2 115kV CS 300/5 49 ALARM 50/ 51 A 51N- 1 63 T1 T2 200/5 30 MVA 87 T XX Primary Protection R G 51N- 2 2000/5 67 86-1 R G 67N POL 67, 67N TRIP DIR 67N OP 87N POL 1/10 ACT 87N OP 51N- 3 51 N.C. 2000/5 13.8 kv 67 POL
Transformer Overcurrent Protection High side overcurrent will not see low side ground faults. CS 50/51 50/51
High Side Overcurrent Coordinate with upstream devices Backup transformer differential / sudden pressure Thermal overload Set above Inrush, 2-8 X Load
Low Side Overcurrent Coordinate with downstream devices (radial) Bus backup scheme Thermal overload
Frequent and Infrequent Operating Limits 10,000 9000 8000 7000 6000 5000 4000 3000 2000 1000 900 800 700 600 500 400 THROUGH-FAULT PROTECTION ON CURVE FOR FAULTS WHICH WILL OCCUR FREQUENTLY (TYPICALLY MORE THAN 5 IN A TRANSFORMER LIFETIME). THROUGH-FAULT PROTECTION ON CURVE FOR FAULTS WHICH WILL OCCUR INFREQUENTLY (TYPICALLY NOT MORE THAN 5 IN A TRANSFORMER LIFETIME). 10,000 9000 8000 7000 6000 5000 4000 3000 2000 1000 900 800 700 600 500 400 300 300 200 200 100 90 80 70 60 50 40 100 90 80 70 60 50 40 T I M E ( S E C O N D S ) 30 20 10 9 8 7 6 5 4 3 30 20 10 9 8 7 6 5 4 3 2 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 12 10 8 7 6 5 4 % OF TRANSFORMER IMPEDANCE FOR FAULT CURRENT FROM 50% TO 100% MAXIMUM POSSIBLE: I 2t =K WHERE I = SYMMETRICAL FAULT CURRENT IN TIMES NORMAL BASE CURRENT (ANSI/IEEE C57.12.00-1980 K = CONSTANT DETERMINED AT MAXIMUM I WITH t = 2 s NOTE: SAMPLE I 2 t = K CURVES HAVE BEEN PLOTTED FOR THIS CURVE MAY ALSO BE USED FOR BACKUP PROTECTION WHERE THE TRANSFORMER IS EXPOSED TO FREQUENT FAULTS NORMALLY CAUSED BY HIGH-SPEED RELAYING 2 3 4 5 6 7 8 910 20 30 40 50 2 3 4 5 6 7 8 910 20 30 40 50 2 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 TIMES NORMAL BASE CURRENT
Transformer Monitor (51TF) Pickup Counter + 1 Alarm - + Start Integration Alarm Counts M M1 + - Get TF(M) 1 from curve dt TF ( M ) 1 + - TF Counter + 1 Reset Integration
Transformer Monitor (51TF)
Transformer Differential Relays Faster More sensitive Eliminates single phasing problem More selective
Percentage Differential PROTECTED ZONE (PHASE A) R1 OP R2 87T
External Fault PROTECTED ZONE (PHASE A) R1 OP R2 87T
Internal Fault PROTECTED ZONE (PHASE A) R1 OP R2 87T
Percentage Differential Characteristic OPERATING CURRENT (IN MULTIPLES OF TAP) 2.33 THRU-CURRENT RESTRAINT SETTING MAXIMUM RESTRAINT CURRENT (IN MULTIPLES OF TAP)
Transformer Differential Limitations Unequal secondary currents, because of the different turns ratios of the power transformer windings and the CTs Phase shift of wye-delta banks Tap changing under load Magnetizing inrush
Current Matching 5 RESTRAINT COILS OPERATING COIL R 1 5 R 2 10 5
Current Matching MATCHING TAPS RELAY INPUT CTS ELECTRONICS OPERATE RESTRAINT COMPARATOR
Phase Shift Compensation 87T
Phase Shift Compensation / Zero Sequence Trap 1 3 3 1 C 2 A B 3 A 2 3 3 3 0 0 B C 3 0 3 3 0 0 3 3 C B A
Two Kinds of Delta Connections A B C A B C A B C B I A I A I B I C I C I B I A I C I A I B I C I C I A I A I B I B I C I
Percentage Differential Characteristic OPERATING CURRENT (IN MULTIPLES OF TAP) 2.33 60% THRU-CURRENT RESTRAINT SETTING MAXIMUM RESTRAINT CURRENT (IN MULTIPLES OF TAP)
Tap Changing Under Load I O P - ( M U L T. O F T A P ) MARGIN 60% MARGIN 15% XMFR EXCITING CURRENT MAXIMUM I - (MULT. OF TAP) R
Magnetizing Inrush Ø i e Ø R Ø R Time Transformer Deenergized at This Point Transformer Reenergized at This Point
Magnetizing Inrush i s 1 Max i e R + Max R Time Transformer Deenergized at This Point Max Transformer Reenergized at This Point
DEAD SPOT Inrush Waveform
Unbalanced Inrush
Differential Setting Review If the transformer connection is a delta-wye the angles of the two currents will not be 180 degrees apart, and must be compensated by 30 degrees To compensate with the CT connection, reverse the connection; for delta-wye transformers connect the CT s wye-delta Digital relays can be connected wye-wye and set the compensation inside the relay
Differential Setting Review Ratio of taps=ratio of currents
Differential Setting Review Set slope low for low mismatch and high quality CTs Increase slope setting for tap changer transformers, poor quality CTs or poorly matched CTs Use of transient monitor to detect the effect of CT saturation during through-fault => enhance security
Differential Setting Review Use of 2 nd harmonic sharing for 2 nd harmonic inhibit => Superior method to enhance security
Overexcitation OPEN A) G S B) S LONG LINE OPEN S - POWER SYSTEM
Relay restrains over the voltage range of 104-138% of rated excitation % 100 M a g n e t i z i n g c u r r e n t I m a n d i t s h a r m o n i c c o m p o n e n t s 80 60 40 20 I (% of I ) 1 m I (% of I ) 3 1 I M (% of I n ) I (% of I ) 5 1 I (% of I ) 7 1 100 110 120 130 140 150 160 Voltage in percent of nominal voltage
Bus/Transformer Application CS 0P R 1 R 2 R 3 R 4 R 5
Bus/Transformer Application 1 2 3 4 30A 20A 50A 20A 20A R1 20A 0P 0 R2
CT Error Produces Incorrect Operation 20A O P E R A T I N G OPERATING ZONE OPERATING POINT 20A MAXIMUM RESTRAINT
Ground Differential 87N
Neutral Overcurrent 51N
Sudden Pressure Protection Operates on rate of change in gas or oil pressure Detects incipient low magnitude faults Protects for faults differential may not see Backs up differential for high magnitude internal faults Users are split between using sudden pressure for trip or alarm
Sudden Pressure Relay Mounted in Gas Space 2 1 SUDDEN PRESSURE RELAY 1 2 TRANSFORMER TANK 7 5 4 8 3 INSULATING OIL LEVEL 4 MAIN PORT 5 BELLOWS 6 6 GAS CUSHION 7 SNAP SWITCH 3 8 EQUALIZER PORT
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