transformer protection

TRANSFORMER PROTECTION

By Y. k. Pandharipande.

CLASSIFICATION

1. Protection against Electrical FaultsUnit Protection (against Internal Faults)Differential Protection.REF Protection.

Non-Unit Protection (against uncleared external faults)IDMT O/C, E/F Relays.Definite Time O/C, E/F Relays.

2.Protection against Abnormal Conditions Over Load protectionOver Fluxing Protection

3. Protective Devices (Non-electrical)Buchholz (Main Tank / OLTC)Oil / Winding Temperature Indicators / RelaysPressure Release ValveOil Level Gauges / Indicators



DIFFERENTIAL RELAY – OPERATING PRINCIPLECurrent Distribution (External Fault)

Battery Analogy (External Fault)


DIFFERENTIAL RELAY – OPERATING PRINCIPLE.

Current Distribution ( Internal Fault)

Battery Analogy (Internal Fault)


DIFFERENTIAL RELAY – NEED FOR BIAS

Transformer ratio = 100/50KV F.L. Current = 100A (on 100 KV side) = 200A (on 50 KV side) Tap Range = (+/-) 10% % Impedance (Zt) = 10%


DIFFERENTIAL RELAY – NEED FOR BIAS.

Current Distribution on Load on Normal Tap


DIFFERENTIAL PROTECTION – NEED FOR BIAS

CURRENT DISTRIBUTION ON LOAD ON + 10% TAP


DIFFERENTIAL PROTECTION – NEED FOR BIAS.

CURRENT DISTRIBUTION ON THROUGH FAULT ON + 10%TAP.


DIFFERENTIAL RELAY – NEED FOR BIAS.

Diff. Pick-up = 15% (0.15A), Bias = 30% (Assumed) Modified Diff. Pick-up due to Restraining Effect of Bias Id mod = 0.30 x (10+9)/2 + 0.15 = 2.85 + 0.15 = 3.0A. Actual Diff. Current = 1.0A


DIFFERENTIAL PROTECTION – BIAS CHARACTERISTIC.


DIFFERENTIAL PROTECTION – MAGNETISING INRUSH.

STEADY STATE At zero volts, flux is “(-) ɸm” since flux in quadrature with voltage. Total flux change in half the voltage cycle is “2ɸm” (-ɸm to +ɸm).SWITCHING AT VOLTAGE ZERO. Initial flux zero as against (-) ɸm corresponding to steady state. Total flux changes from zero to 2ɸm. Flux doubling causes extreme saturation of core.


DIFFERENTIAL PROTECTION- MAGNETISING INRUSH.

Magnetising Impedance drastically reduces due to core saturation causing heavy inrush current.

Inrush appears on excited winding (primary) without any balancing current on secondary and unstabilises differential protection.


DIFFERENTIAL PROTECTION – INRUSH STABILITY

Inrush current has predominant 2 nd harmonic content (typical value, up to 60% max. of Fundamental depending upon instant of switching).

Dominant 2 nd harmonic component is used to restrain the protection on inrush condition.


BIASED DIFFERENTIAL PROTECTION- INTERNAL CIRCUIT.


DIFFERENTIAL PROTECTION- CT CONNECTIONS.STAR / STAR TRANSFORMER


DIFFERENTIAL PROTECTION – CT CONNECTIONS.DELTA / STAR TRANSFORMER (Dy11)


DIFFERENTIAL PROTECTION – STAR / STAR TRANSFORMER

Instability With STAR CONNECTED CTs On Up-stream Earth Fault.


DIFFERENTIAL PROTECTION – STAR / STAR TRANSFORMER.

Delta / Delta CTs filter Zero Seq. Current & ensure Stability.


REF PROTECTION

Current Distribution on External Earth Fault.


REF PROTECTION

Current Distribution on Single Phase load.


REF PROTECTION

Current distribution on Internal Fault.


STABILISING RESISTOR SETTING CRIETERIA

If B-ɸ CT fully active while NCT fully saturates, maximum Voltage across Relay Branch (Vs) = If/n (Rct + 2Rl). If Relay current setting is “Is” and Ohmic burden is “Zr”, Stab. Resistor Setting (Rs) = Vs/Is – Zr Ohms.


BACKUP PROTECTION

INFLUENCING FACTORS

Vector Group

Infeed conditions (Single or both end infeeds)

Type of Neutral Earthing


BACKUP PROTECTION

50 – High Set Inst. O/C Relay ( Delta side S.C. protection) 64 – REF Protection (Delta side REF protection) 51 – IDMT O/C Relay (time graded backup for down-stream S.C.) 51N – IDMT E/F Relay (time graded backup for down-stream E/F)


BACKUP PROTECTIONE/F PROTECTION ON DELTA SIDE.

Delta side E/F relay inherently immune to Star side earth faults but can respond to Delta winding earth faults. Hence sensitive, high speed E/F relay (64) in High Impedance mode can be used on Delta side.


BACKUP PROTECTION

50 – High Set Inst. O/C Relay (Primary Star side S.C.) 51- IDMT O/C Relay (time graded backup for down-stream S.C.) 67N – Directional IDMT E/F Relay (backup to TFR/down-stream E/F.) 51N–IDMT E/F relay (time graded backup against down-stream E/F)


BACKUP E/F PROTECTION- STAR / STAR TRANSFORMER

Directional IDMT E/F relay (67N) looking towards transformer do not respond to up-stream E/F due to current out-flow from TFR terminal into Grid, thus requiring co-ordination with down-stream E/F protections only.


OVER LOAD PROTECTION1. Definite Time Over Load Relay

Definite Time O/C relay used for Alarm.

Set above permissible over load with small time delay(say 5 seconds)

Relay should have adjustable current setting, time delay (built-in or external) and high reset ratio.

Some times, extended tripping is provided with additional time delay.


OVER LOAD PROTECTION2. Thermal Over Load Relay Set to match over load withstand of transformer. Pre-trip Alarm with lower pick-up threshold. Operating time for trip (t) t = T. ln {(I/K.In)² – (I pre/K.In)²} / (I/K.In)²- 1} Where I = Over Load current. K.In = Max. Permissible Load Current. I pre = Load current prior to Over Load. T = Thermal Time Constant (minutes)


OVER FLUXING PROTECTIONINDEX OF OVER FLUXINGV α Φ.f, Hence Φα V/f Tandem variations in voltage & frequency most

likely to occur in case of Generator Transformer during startup or coasting down (due to AVR malfunction or manual control) requiring Over Fluxing Protection.

It is necessary for Grid TFRs as well, under certain conditions (e.g. TFR fed by long transmission line subjected to high voltage due to Ferranti Effect when off-loaded).


OVER FLUXING PROTECTION1. Definite Time Over fluxing relay. Adjustable V/f setting (typically 1 – 1.5 of 110V / 50Hz). Adjustable trip time delay (typically 10 – 120S) Pre-trip Alarm (fixed or variable delay)2. IDMT Over Fluxing Relay Adjustable V/f setting. Inverse time characteristic to match with TFR with-stand

characteristic. Pre-trip Alarm and accelerated trip for severe over

fluxing condition.


BACKUP EARTH FAULT PROTECTION

NCT operated Standby E/F Relay responds to internal earth faults as well, besides providing backup against external faults.

Stand by E/F relay offers better sensitivity since NCT ratio not tied up with load and lower ratio, based on E/F infeeds, is possible.

transformer protection

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