transformer protection

(1)

TRANSFORMER

PROTECTION

(2)

TRANSFORMER FAULTS

 EARTH FAULTS  PHASE FAULTS

 INTER TURN FAULTS  CORE FAULTS

 TANK FAULTS AND COOLING FAILURE  TAP CHANGER FAULTS

(3)

TRANSFORMER ABNORMAL

 OVERLOAD

Increase copper loss & Temperature rise

 SYSTEM FAULTS  OVERVOLTAGE  OVERFLUXING

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EARTH FAULT CURRENT DEPENDS ON

 Method of Earthing

 Impedance of the winding

(5)

TYPES OF EARTHING

EFFECTIVE EARTHING

 Solid and low impedance earthed systems  During Earth fault healthy phase voltage is

below 80% of nominal

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TYPES OF EARTHING

NON - EFFECTIVE EARTHING

 High resistance and Reactance earthed systems

 Systems are subjected to high over voltage

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Inter-turn Faults

 Degradation of winding insulation due to lightning or switching  70 to 80% of the Transformer failures

(10)

Core Faults

 Breakdown in the insulation b/w silicon steel

core lamination

 Breakdown in the core bolt insulation

 Causes excessive eddy currents to flow and

(11)

Tap Changer Faults

Due to

 Flashovers

 Selector or Divertor failure  Contact failure

(12)

System Short Circuit

Transformer Fault Permitted

Reactance Current Duration (Sec)

4 25 2

5 20 3

6 16.6 4

7 14.2 5

 Limited by self reactance  Severe mechanical stress

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Over Voltages

Transient Surge Voltage

Power frequency Voltage

 Arise from switching & Lightning disturbances  Cause interturn faults

 Load throw off condition

 Increase stress on the insulation  Damage the core bolt and coil

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OVERALL TRANSFORMER

PROTECTION REQUIREMENT

 SENSITIVITY FOR INTERNAL FAULT

 FASTER OPERATION FOR INTERNAL

FAULT

 THROUGH FAULT STABILITY  MAGNETIC INRUSH STABILITY  OVER FLUXING STABILITY

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 OVER CURRENT AND EARTH FAULT  HIGH IMPEDANCE DIFFERENTIAL  BIASED DIFFERENTIAL

 RESTRICTED EARTH FAULT

 WINDING / OIL TEMPERATURE  BUCHOLZ GAS

(16)

Biased Differential

 CT error

 Tap Changer variation  Magnetizing Inrush

 Through fault stability

 Through fault stability - CT saturated condition  Overfluxing Stability

(17)

OPERATE RESTRAIN | I1 - I2 | (A) | I1 + I2 | / 2 (A) 0.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 4.5 3.0 2.5 2.0 1.5 1.0 0.5 3.5 4.0

Measured Point under Maximum Load Highset Differential at 12x

(12A)

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Supply

Figure 8. Restricted Earth Fault

R Y B SETTING RESISTOR SETTING RESISTOR METROSIL REF RELAY METROSIL REF RELAY R Y B K EARTHING RESISTOR I_F I_P

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15 20 3 6 9 12 100 80 60 40

Earth Fault Current - I_F

K - PERCENTAGE OF WINDING FROM NEUTRAL AT WHICH EARTH FAULT OCCURS

Primary Current - I_P

CURRENT AS A MULTIPLE OF FULL LOAD

100 5 20 40 60 80 30 25 20 15 10

RESTRICTED EARTH FAULT RELAY

DIFFERENTIAL RELAY

FAULT SETTING - PERCENTAGE OF FULL LOAD

PERCENTAGE OF WINDING PROTECTED

Protection against Earth Faults Variation of Earth Fault Current with position (solidly earthed)

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Protection Overview

 Current amplitude correction (0.25 - 3.0)

 Vector group compensation

 Proven biased differential protection

 Low or high impedance REF/SEF

 Backup IDMTL and DTL overcurrent and earth fault

 Overfluxing protection

(23)

 4-stage Undervoltage / under-frequency load shedding

 Neutral voltage displacement

 Trip circuit supervision

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Biased Differential Protection Settings

 Differential, Initial setting (10 - 200% xIn)

 Differential, Bias Slope (0 - 70%)

Setting > maximum percentage unbalance (tap mismatch, CT mismatch)

 Differential, Bias Slope Limit (1-20 xIn)

Aids stability for heavy external faults accompanied by CT saturation

Setting > transformer maximum 3phase through fault current/2

(25)

Biased Differential Protection Settings

 Differential, Highset (1-30 xIn)

Setting > max through fault, and Setting > max magnetizing inrush

 Mag. Inrush Restraint Level (10-50% x Id)

Determines level of even harmonic that will cause relay to be inhibited

(26)

Single phase line diagram

Mag inrush detector RMS converter 5% of In Initial setting Bias slope Bias limit RMS converter RMS converter Inrush level 0.5 + -+ + -I1 I2 I1-12 I1+12 I1 + 12 2 Trip from other phases Other phase inhibits Inhibit enable Phase inhibit Trip -+ Restraint Operate

(27)

Biased Differential Characteristic

Highset Differential Set to x7

O p er ate C u rr e nt (I 1 - I 2 ) Restraint Current ( |I₁| + |I₂| ) / 2 Initial Setting Bias Slope

Setting Bias Slope Limit Set at x4

RESTRAIN OPERATE 0 2 4 6 8 10 2 4 6 8

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7.5 MVA; 66 / 11 KV; Dyn11; HVCT ratio = 100/1; LVCT ratio = 450/1;

HV full load current = 7.5MVA / (1.732 x 66KV)

= 65.61A

HV CT secondary current = (65.61 x 1) / 100

= 0.656A

LV full load current = 7.5MVA / (1.732 x 11KV)

= 393.65A

LV CT secondary current = (393.65 x 1) / 450

= 0.875A

Tap changer range = -10% to +10%

(32)

HV ICT multiplier = 1 / 0.656 = 1.52

LV ICT multiplier = 1 / 0.875 = 1.14

(33)

DUOBIAS - M Relay SETTINGS:

Initial setting = 20%

Bias setting = 20%

Bias slope = 4 x In

High set over current = 8 x In

HV ICT vector connection = Ydy0, 0

HV ICT multiplier = 1.52

LV ICT vector connection = Yd1, 30