transformer protection

TRANSFORMER PROTECTION

TRANSFORMER FAULTS  EARTH FAULTS  PHASE FAULTS  INTER TURN FAULTS  CORE FAULTS  TANK FAULTS AND COOLING FAILURE  TAP CHANGER FAULTS

TRANSFORMER ABNORMAL  OVERLOAD

Increase copper loss & Temperature rise

 SYSTEM FAULTS  OVERVOLTAGE  OVERFLUXING

EARTH FAULT CURRENT DEPENDS ON  Method of Earthing  Impedance of the winding  Position of the fault on the winding

TYPES OF EARTHING EFFECTIVE EARTHING  Solid and low impedance earthed systems  During Earth fault healthy phase voltage is below 80% of nominal  Used in higher voltage level

Solidly Earthed Systems

TYPES OF EARTHING NON - EFFECTIVE EARTHING

 High resistance and Reactance earthed systems  Systems are subjected to high over voltage  Suitable for Lower voltage distribution networks

Impedance Earthed Systems

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

Inter-turn Faults

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 hot spots formed in Transformer core

Due to

Tap Changer Faults

 Flashovers  Selector or Divertor failure  Contact failure  Burn out of divertor resistors

Transformer Reactance

Fault Current

Permitted Duration (Sec)

4

25

2

5

20

3

6

16.6

4

7

14.2

5

System Short Circuit  Limited by self reactance  Severe mechanical stress

Over Voltages Transient Surge Voltage  Arise from switching & Lightning disturbances  Cause interturn faults Power frequency Voltage  Load throw off condition  Increase stress on the insulation  Damage the core bolt and coil insulation

OVERALL TRANSFORMER PROTECTION REQUIREMENT  SENSITIVITY FOR INTERNAL FAULT  FASTER OPERATION FOR INTERNAL FAULT  THROUGH FAULT STABILITY  MAGNETIC INRUSH STABILITY  OVER FLUXING STABILITY

TYPES OF PROTECTION  OVER CURRENT AND EARTH FAULT  HIGH IMPEDANCE DIFFERENTIAL  BIASED DIFFERENTIAL  RESTRICTED EARTH FAULT  WINDING / OIL TEMPERATURE  BUCHOLZ GAS

Biased Differential  CT error  Tap Changer variation  Magnetizing Inrush  Through fault stability  Through fault stability - CT saturated condition  Overfluxing Stability

4.5

Highset Differential at 12x (12A)

4.0 3.5

| I1 - I2 | 3.0 2.5 (A)

OPERATE

2.0 1.5

RESTRAIN

1.0

Measured Point under Maximum Load

0.5 0.5

1.0

1.5

2.0

2.5

| I1 + I2 | / 2

3.0

3.5

(A)

Biased Differential Protection

4.0

R

R

Y

Y Supply

B

B

SETTING RESISTOR

REF RELAY

METROSIL

IP

K

IF SETTING RESISTOR

EARTHING RESISTOR

Figure 8.

Restricted Earth Fault

REF RELAY

METROSIL

Restricted Earth Fault

100

15

RESTRICTED EARTH FAULT RELAY

PERCENTAGE OF WINDING PROTECTED

60 DIFFERENTIAL RELAY 40

20

5

10

15

20

25

FAULT SETTING - PERCENTAGE OF FULL LOAD

30

CURRENT AS A MULTIPLE OF FULL LOAD

12

80

9

6

3

Earth Fault Current - I F

Primary Current - I P

20 40 60 80 100 K - PERCENTAGE OF WINDING FROM NEUTRAL AT WHICH EARTH FAULT OCCURS

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

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



DTL Overvoltage

Protection Overview  4-stage Undervoltage / under-frequency load shedding  Neutral voltage displacement  Trip circuit supervision

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

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

Single phase line diagram Mag inrush detector

RMS converter

+ -

Inrush level + -

5% of In

Phase inhibit

Inhibit enable

Other phase inhibits

Initial setting Bias limit

I1

+

I1-12 -

Operate

Bias slope RMS converter

Trip Restraint

I2

+-

I1+12

0.5

I1 + 12 2

RMS converter

Trip from other phases

Biased Differential Characteristic 8

Operate Current (I1 - I2)

Highset Differential Set to x7

6 OPERATE 4

2

RESTRAIN

0

2 Initial Setting

Bias Slope Setting

4

6

8

Bias Slope Limit Set at x4

Restraint Current ( |I1| + |I2| ) / 2

10

A.C. line diagram

Duobias - Line currents

Duobias - Output relays

Transformer Details: 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%

HV ICT multiplier

= 1 / 0.656 = 1.52

LV ICT multiplier

= 1 / 0.875 = 1.14

Transformer Impedance

= 12.5%

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

LV ICT multiplier

= 1.14.