Transformer Protection

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TRANSFORMER PROTECTION CAUSES OF FAILURE ENVIRONMENTAL SYSTEM

OPERATION DESIGN MANUFACTURE MATERIALS MAINTENANCE

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TRANSFORMER PROTECTION

SYSTEM CAUSES OF FAILURE OVERVOLTAGES FAST TRANSIENTS LOAD REJECTION BACKFEEDING SHORTCIRCUITS

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TRANSFORMER PROTECTION

OPERATIONAL CAUSES OF FAILURE LOAD DUTY OVERLOADING SWITCHING OPERATIONS OUT-OF-PHASE SYNCHRONISATION

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TRANSFORMER PROTECTION

BASIC PROTECTION DIFFERENTIAL RESTRICTED EARTHFAULT

OVERFLUXING OVERCURRENT & EARTHFAULT

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TRANSFORMER PROTECTION

DIFFERENTIAL PROTECTION WORKS ON MERZ-PRICE CURRENT COMPARISON PRINCIPLE RELAYS WITH BIAS CHARACTERISTIC SHOULD ONLY BE USED RELAY SHOULD BE STABLE DURING MAG-INRUSH & OVERFLUXING CONDITIONS

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TRANSFORMER PROTECTION

WHY BIAS CHARACTERISTIC ? 100 / 1

100/50 KV

200 / 1 1A

1A

O

0A

OLTC SETTING IS AT MIDTAP

LOAD = 200 A

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TRANSFORMER PROTECTION 100 / 1

100/50 KV

200 / 1 1A

0.9 A

O

0.1 A

OLTC SETTING IS AT 10% RELAY PICKUP SETTING IS O.2 A SO THE RELAY RESTRAINS

LOAD = 200 A

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TRANSFORMER PROTECTION 100 / 1

100/50 KV

200 / 1 10 A

9A

O

1A

OLTC SETTING IS AT 10% RELAY PICKUP SETTING IS O.2 A SO THE RELAY OPERATES

2000 A

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TRANSFORMER PROTECTION 100 / 1

100/50 KV

200 / 1 10 A

9A B

2000 A

B O

1A

MODIFIED PICKUP DUE TO A BIAS SETTING OF 30% IS 30% OF ((10+9)/2), THAT IS, 2.85 A BUT DIFF. CURRENT IS 1 A, SO THE RELAY RESTRAINS

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TRANSFORMER PROTECTION 100 / 1

100/50 KV

200 / 1 0A

9A

2000 A B

B O

9A

MODIFIED PICKUP DUE TO A BIAS SETTING OF 30% IS 30% OF ((0+9)/2), THAT IS, 1.35 A BUT DIFF. CURRENT IS 9 A, SO THE RELAY OPERATES

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TRANSFORMER PROTECTION

MAGNETISING INRUSH TRANSIENT CONDITION - OCCURS WHEN A TRANSFORMER IS ENERGISED

NORMAL OPERATING FLUX OF A TRANSFORMER IS CLOSE TO SATURATION LEVEL RESIDUAL FLUX CAN INCREASE THE MAG-CURRENT IN THE CASE OF THREE PHASE TRANSFORMER, THE POINT-ON-WAVE AT SWITCH-ON DIFFERS FOR EACH PHASE AND HENCE, ALSO THE INRUSH CURRENTS

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TRANSFORMER PROTECTION INRUSH CURRENT

+ m

V



Im

STEADY STATE - m

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TRANSFORMER PROTECTION INRUSH CURRENT

+ m

V



Im

STEADY STATE - m Im

2 m

 V

SWITCH ON AT VOLTAGE ZERO - NO RESIDUAL FLUX

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TRANSFORMER PROTECTION

MAGNETISING INRUSH IF SYMMETRICAL WAVE -- ONLY ODD HARMONICS

- 3RD AT LOW SATURATION LEVELS - HIGHER HARMONICS AT HEAVY SATURATION IF OFFSET WAVE -- MAX. INRUSH; 2ND & 3RD HARMONICS INITIAL INRUSH LIMITED BY AIR CORE INDUCTANCE INITIAL DECAY RATE HIGH -- REDUCES AS IRON COMES OUT OF SATURATION

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TRANSFORMER PROTECTION

TYPICAL DECAY TIMES FOR MAG INRUSH MVA RATING

NO. OF CYCLES TO REACH 50% OF FIRST PEAK

0.5 -- 1.0

8 TO 10

1.0 -- 10

10 TO 60

> 10

60 TO 600

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TRANSFORMER PROTECTION OVERFLUXING CONDITION

DIFFERENTIAL ELEMENT SHOULD BE BLOCKED FOR TRANSIENT OVERFLUXING-+ 25% OVERVOLTAGE CONDITION

OVERFLUXING WAVEFORM CONTAINS VERY HIGH 5TH HARMONIC CONTENT

43% 5TH HARMONIC CONTENT

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TRANSFORMER PROTECTION

BIASED DIFF. PROTECTION FAST OPERATION ADJUSTABLE CHARACTERISTIC HIGH THROUGH FAULT STABILITY CT RATIO COMPENSATION MAGNETISING INRUSH RESTRAINT OVERFLUXING 5TH HARMONIC RESTRAINT

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TRANSFORMER PROTECTION

DDT 12/32 FOR TRANSFORMERS UPTO 5 MVA INDUCTION DISC TYPE BIAS 20,30,40% PICKUP CURRENT 40-100% TIME DELAYED

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TRANSFORMER PROTECTION DTH 31/32

FOR TRANSFORMERS ABOVE 5 MVA STATIC HIGH SPEED WITH 2ND & 5TH HARMONIC RESTRAINT

BIAS 15%, 30%, 45% PICKUP CURRENT 15%

UNBIASED HIGHSET

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TRANSFORMER PROTECTION

OPERATING TIME (CYCLES)

NEED FOR HIGHSET UNIT 6 LOWSET WITH HAR. RESTRAINT

4

2

0 10 20 30 40 FAULT CURRENT (XIn)

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TRANSFORMER PROTECTION

OPERATING TIME (CYCLES)

NEED FOR HIGHSET UNIT 6 LOWSET WITH HAR. RESTRAINT

4

2

INST. HIGHSET ELEMENT 0 10 20 30 40 FAULT CURRENT (XIn)

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TRANSFORMER PROTECTION

MBCH BIASED DIFF. RELAY MANY NOVEL FEATURES MBCH 12 FOR 2 WNDG TFRS MBCH 13 FOR 3 WNDG TFRS MBCH 16 FOR OTHER APPLICATIONS

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TRANSFORMER PROTECTION

TYPICAL MAG INRUSH WAVEFORMS

A B C

1/3 Cycle minimum

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TRANSFORMER PROTECTION

COMPARITIVE CHARACTERISTICS (WITHOUT HIGHSET) OPERATING TIME (CYCLES)

6

RELAY WITH HAR. RESTRAINT 4

2 MBCH 0

10

20

30

FAULT CURRENT (XIn)

40

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TRANSFORMER PROTECTION

ROLE OF BIAS

3

2

Operate

Differential current (x In) = I1+ I2 + I3 + I 4 1

Restrain

Setting range (0.1 - 0.5) 0

1

2

3

4

Effective bias (x In) = I1 + I 2 + I 3 + I 4 2

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TRANSFORMER PROTECTION SATURATION DETECTOR NO SATURATION

I1 I2 I 1 + I2 I1 - I2 I1

I2

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TRANSFORMER PROTECTION SATURATION DETECTOR NO SATURATION

SATURATION DUE TO THRO’ FAULT

I1 I2 I 1 + I2 I1 - I2 I1

I2

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TRANSFORMER PROTECTION SATURATION DETECTOR NO SATURATION

SATURATION DUE TO THRO’ FAULT

I1 I2 I 1 + I2 I1 - I2 I1

I2

INTERNAL FAULT

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TRANSFORMER PROTECTION USE OF ICT Dy1(-30 )

Interposing CT provides  Vector correction Yd11(+30 )

 Ratio correction  Zero sequence compensation

R

R

R

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TRANSFORMER PROTECTION

VECTOR GROUP CORRECTION Dy1 (-30 )

Yy0 0

87

Yd11 +30

Yy0, Yd1, Yd5 , Yy6, Yd7, Yd11, Ydy0 0 , -30 , -150 , 180,+150, +30 , 0

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TRANSFORMER PROTECTION

SELECTION OF SUITABLE VECTOR CORRECTION FACTOR

Dy11 (+30 )

Yy0 0

87

Yd1 -30

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TRANSFORMER PROTECTION

CT RATIO MISMATCH CORRECTION

200/1

33kV : 11kV 10 MVA IL = 175A

I L = 525A

0.875A

400/1

1.31 Amps 1A

1A

1.14

0.76

87

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TRANSFORMER PROTECTION

ZERO SEQUENCE COMPENSATION +VE SEQUENCE CURRENTS BALANCE REQUIRE ZERO SEQUENCE CURRENT TRAPS FOR STABILITY

A

B

C

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TRANSFORMER PROTECTION HIGH IMPEDANCE REF

 Increased sensitivity for earth faults  REF elements for each transformer winding  CTs may be shared with differential element

64

64

64

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TRANSFORMER PROTECTION

LIMITATION OF E/F CURRENT 1.0 Current (x full load)

0.2

IF

Source

IF

IS

IS

R

1.0 0.2 Fault position from neutral

R

Differential relay setting

% of Star winding protected

10% 20% 30% 40% 50%

58% 41% 28% 17% 7%

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TRANSFORMER PROTECTION REF CONNECTIONS

3 Phase 3 wire

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TRANSFORMER PROTECTION REF CONNECTIONS

3 Phase 3 wire

R CAG14

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TRANSFORMER PROTECTION REF CONNECTIONS

3 Phase 3 wire

Delta winding

R CAG14

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TRANSFORMER PROTECTION REF CONNECTIONS

3 Phase 3 wire

Delta winding

R CAG14

3 Phase 4 wire

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TRANSFORMER PROTECTION REF CONNECTIONS

3 Phase 3 wire

Delta winding

R CAG14

3 Phase 4 wire

Alt’ 3 phase 4 wire V

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TRANSFORMER PROTECTION

REF CASE I : NORMAL CONDITION P1

P2

P2

64

P1

UNDER NORMAL CONDITIONS NO CURRENT FLOWS THRO’ RELAY

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TRANSFORMER PROTECTION

REF CASE II : EXTERNAL EARTH FAULT P1

P2

P2

64

P1

FOR AN EXTERNAL EARTH FAULT, THE CURRENT CIRCULATES BETWEEN THE PHASE & NEUTRAL CTs; NO CURRENT THRO’ THE RELAY

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TRANSFORMER PROTECTION

REF CASE III : INTERNAL EARTH FAULT P1

P2

P2

64

P1

FOR AN INTERNAL EARTH FAULT THE UNBALANCED CURRENT FLOWS THRO’ THE RELAY

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TRANSFORMER PROTECTION

HIGH IMPEDANCE DIFF. PROT. FOR AUTO TRANSFORMER

R

R

R

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TRANSFORMER PROTECTION

OVERFLUXING -- BASIC THEORY V = kf Causes  Low frequency  High voltage  Geomagnetic disturbances

2m

m Ie

Effects  Tripping of differential element (Transient overfluxing)  Damage to transformers (Prolonged overfluxing)

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TRANSFORMER PROTECTION

OVERFLUXING PROTECTION V

K  f

Trip and alarm outputs for clearing prolonged overfluxing Alarm : Definite time characteristic to initiate corrective action

Trip : IDMT or DT characteristic to clear overfluxing condition

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TRANSFORMER PROTECTION

BACKUP PROTECTION FOR TRANSFORMERS INFLUENCING FACTORS : VECTOR CONFIGURATION TYPE OF EARTHING INFEED CONDITIONS

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TRANSFORMER PROTECTION

STAR -- DELTA UNGROUNDED

~

LOAD

CAG 37 50 CDG 31

51 CDG 31

51 64 67 N CDD 21

VDG 14

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TRANSFORMER PROTECTION WHERE DO WE NEED 3 O/C ?

2:1:1 DISTRIBUTION FOR PH--PH FAULT IF MIN. F.C > 4 X FULL LOAD, THEN 2 O/C CAN BE USED

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TRANSFORMER PROTECTION

UNRESTRICTED E/F PROTECTION 3 x 51 51N

PROVIDES BACKUP PROTECTION FOR SYSTEM TIME DELAY REQUIRED FOR COORDINATION

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TRANSFORMER PROTECTION

STAR -- STAR INTERCONNECTED

CAG 17 50

50 CAG 17

CDD 21

67

67 CDD 21

67 N

67 N

CDD 21

CDD 21

H.S CAN BE APPLIED ON BOTH SIDES DIREC. O/C & E/F RELAYS ARE PREFERRED

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TRANSFORMER PROTECTION

OVERLOAD PROTECTION RATING OF A TFR IS BASED ON THE ALLOWABLE TEMPERATURE RISE ABOVE AN ASSUMED MAX. AMBIENT TEMPERATURE SHORT PERIODS OF OVERLOAD ARE PERMISSIBLE TO AN EXTENT DEPENDING ON PREVIOUS LOADING

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TRANSFORMER PROTECTION

OVERLOAD PROTECTION

(CONTD..)

SUSTAINED EXCESSIVE OVERLOAD LEADS TO DETERIORATION OF INSULATION TEMP. OF 98*C GIVES NORMAL LIFE SPAN OF 20-30 YRS

LIFE SPAN IS HALVED FOR EVERY 6*C TEMP. INCREASE PROTECTION -- DEFINITE TIME DELAYED O/C

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TRANSFORMER PROTECTION TYPICAL PROTECTIONS FOR TRANSFORMER

51

51

64 51N

50N

ICT

50 87

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TRANSFORMER PROTECTION

CT REQUIREMENTS RELAY

VK >

IMAG <

DDT

2 IF ( RCT+2RL+RB )

@ VK/4, 3%In

DTH

40 In ( RCT+2RL )

@ VK/4, 3%In

MBCH

24 In ( RCT+2RL )

@ VK/2, 10%In

CAG 14

2 IF ( RCT+2RL )

@ VK/2, 3%In

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TRANSFORMER PROTECTION

CT REQUIREMENTS RELAY

VK >

IMAG <

DDT

2 IF ( RCT+2RL+RB )

@ VK/4, 3%In

DTH

40 In ( RCT+2RL )

@ VK/4, 3%In

MBCH

24 In ( RCT+2RL )

@ VK/2, 10%In

CAG 14

2 IF ( RCT+2RL )

@ VK/2, 3%In

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TRANSFORMER PROTECTION

CT REQUIREMENTS RELAY

VK >

IMAG <

DDT

2 IF ( RCT+2RL+RB )

@ VK/4, 3%In

DTH

40 In ( RCT+2RL )

@ VK/4, 3%In

MBCH

24 In ( RCT+2RL )

@ VK/2, 10%In

CAG 14

2 IF ( RCT+2RL )

@ VK/2, 3%In

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TRANSFORMER PROTECTION

CT REQUIREMENTS RELAY

VK >

IMAG <

DDT

2 IF ( RCT+2RL+RB )

@ VK/4, 3%In

DTH

40 In ( RCT+2RL )

@ VK/4, 3%In

MBCH

24 In ( RCT+2RL )

@ VK/2, 10%In

CAG 14

2 IF ( RCT+2RL )

@ VK/2, 3%In

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TRANSFORMER PROTECTION

CT REQUIREMENTS FOR O/C RELAYS ELEMENT

CLASS

OUTPUT VOLTAGE

INST.

5 P10

TO DRIVE 2 X ISET

IDMT

5 P 10

TO DRIVE 10 X ISET

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TRANSFORMER PROTECTION

CT REQUIREMENTS FOR O/C RELAYS ELEMENT

CLASS

OUTPUT VOLTAGE

INST.

5 P10

TO DRIVE 2 X ISET

IDMT

5 P 10

TO DRIVE 10 X ISET

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TRANSFORMER PROTECTION

SETTING CRITERIA BIASED DIFF. CURRENT PICKUP IS SET JUST ABOVE THE STANDING UNBALANCE CURRENT DUE TO OLTC, CT MISMATCH AND ERRORS BIAS SLOPE SHOULD TAKE CARE OF SATURATION DUE TO HEAVY THRO’ FAULT

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TRANSFORMER PROTECTION

SETTING CRITERIA (CONTD...) HIGH IMPEDANCE DIFF. & REF LOWEST POSSIBLE SETTING STAB. RESISTOR = IF (RCT + 2RL)/Is - (VA BURDEN)/ Is2 WHERE IF = RCT = RL = Is =

MAX. FAULT CURRENT CT SECONDARY RESISTANCE ONE-WAY LEAD RESISTANCE SETTING CURRENT

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TRANSFORMER PROTECTION

SETTING CRITERIA (CONTD...) OVERFLUXING RELAY CHARACTERISTIC SHOULD MATCH THE TFR WITHSTAND

O/C & E/F TO BE CO-ORDINATED WITH DOWNSTREAM RELAYS

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TRANSFORMER PROTECTION

INSTANTANEOUS ELEMENT SETTING

R

IF

RELAY SETTING > IF AND MAG INRUSH PICKUP = 1.3 X MVA / ( 1.732 X KV X pu IMP.)

THANK YOU

50

51

WORLD LEADER IN PROTECTION & CONTROL

abcd

24

87 64

67 67N

99