07 Transformer

(1)

(2)

Causes of failure:

Environment

System

Mal operation

Wrong design

Manufacture

Material

Maintenance

(3)

Transformer failures classification :

1. Internal failure

Causes: Causes: Causes: Causes:

➢ Winding & terminal faults

➢ Core faults

➢ Onload tap changer faults

➢ Overheating faults

(4)

2. External failure

Causes: Causes: Causes: Causes:

➢ Abnormal operating condition

Abnormal operating condition

➢ sustained or unclear faults

sustained or unclear faults

Transformer failures classification :

(5)

Vector Groups

Phase displacement Phase displacement Phase displacement Phase displacement 0 00 0 Group 1 Group 1 Group 1 Group 1 Phase displacement Phase displacement Phase displacement Phase displacement 180 180 180 180 Group 2 Group 2 Group 2 Group 2

Lag phase displacement Lag phase displacement Lag phase displacement Lag phase displacement 30 30 30 30 Group 3 Group 3 Group 3 Group 3

Lead phase displacement Lead phase displacement Lead phase displacement Lead phase displacement 30 30 30 30 Group 4 Group 4 Group 4 Group 4 Yy0 Yy0Yy0 Yy0 Dd0 Dd0Dd0 Dd0 Zd0 Zd0Zd0 Zd0 Yy6 Yy6Yy6 Yy6 Dd6 Dd6Dd6 Dd6 Dz6 Dz6Dz6 Dz6 Yd1 Yd1Yd1 Yd1 Dy1 Dy1Dy1 Dy1 Yz1 Yz1Yz1 Yz1 Yd11 Yd11Yd11 Yd11 Dy11 Dy11Dy11 Dy11 Yz11 Yz11Yz11 Yz11

(6)

Fault current distribution

Earth fault on Transformer winding

V2 R T2 T1 V1 X Fig.3 If Fig.N

(7)

Fault current distribution

Therefore

Therefore Therefore

Therefore C.T.secondary current C.T.secondary current C.T.secondary current C.T.secondary current ( on primary side of transformer) =, X( on primary side of transformer) =, X( on primary side of transformer) =, X( on primary side of transformer) =, X2222

If differential setting =20% If differential setting =20% If differential setting =20% If differential setting =20% For relay For relay For relay

For relay operation Xoperation Xoperation Xoperation X2222 _20%_20%_20%_20%

Thus X > 59 Thus X > 59 Thus X > 59 Thus X > 59% % % ie% ieieie. 59% of winding is unprotected.. 59% of winding is unprotected.. 59% of winding is unprotected.. 59% of winding is unprotected. Differential relay settingDifferential relay settingDifferential relay settingDifferential relay setting % of winding protected % of winding protected% of winding protected% of winding protected 10 10 10 10% 58%% 58%% 58%% 58% 20 20 20 20% 41%% 41%% 41%% 41% 30 30 30 30% 28%% 28%% 28%% 28% 40 40 40 40% 17%% 17%% 17%% 17% √ √ √ √3333 √ √√ √3333

>

(8)

Differential

Basic Protection

Restricted Earthfault

Overfluxing

(9)

Differential Protection

Where protection co-ordination is difficult /

not possible using time delayed elements

For fast fault clearance

Applied

∗

_{Works on Merz-price current comparison}

principle

∗

_{Relays with bias characteristic should only be}

used

(10)

Differential Protection

Consideration for applying differential

protection

Phase correction

Filtering of zero sequence currents

Ratio correction

Magnetizing inrush during energisation

Overfluxing

(11)

Differential Protection - Principle

R I diff = 0I diff = 0I diff = 0I diff = 0

• Nominal current through the protected equipmentNominal current through the protected equipmentNominal current through the protected equipmentNominal current through the protected equipment I Diff = 0 :

I Diff = 0 : I Diff = 0 :

(12)

Differential Protection

-Principle

• Through fault currentThrough fault currentThrough fault currentThrough fault current

I Diff = 0 : I Diff = 0 :I Diff = 0 :

I Diff = 0 : No trippingNo trippingNo trippingNo tripping

(13)

Differential Protection

-Principle

Tripping Tripping Tripping Tripping

• Internal Fault

Internal Fault

I Diff = 0 : I Diff = 0 : I Diff = 0 : I Diff = 0 :

(14)

Biased differential protection

• Fast operation

• Adjustable characteristic

• High through fault stability

• CT ratio compensation

• Magnetising inrush restraint

(15)

Biased differential protection

1 A 100/50 KV 100 / 1 200 / 1 1 A 0 A LOAD = 200 A

Why bias characteristic ?

OLTC Setting is at mid tap

R

(16)

Biased differential protection

100/50 KV

100 / 1 _{200 / 1}

0.9 A 1 A

0.1 A

Relay pickup setting = O.2 A, So the Relay restrains

LOAD

= 200 A

OLTC SETTING IS AT 10%

Differential current = 0.1 A

(17)

Biased differential protection

100/50 KV

100 / 1 _{200 / 1}

9 A 10 A

1 A

Relay Pickup Setting is O.2 A

OLTC SETTING IS AT 10%

2000 A

R

Operates

(18)

Role of Bias

Setting range (0.1 - 0.5)

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

Differential current (x In) = I + I + I + I₁ ₂ ₃ ₄ 0 ₁ ₂ ₃ ₄ 1 2 3

Operate

Restrain

80% Slop e 20% Slope

(19)

Based on Current operated relay with an external stabilising res Based on Current operated relay with an external stabilising res Based on Current operated relay with an external stabilising res

Based on Current operated relay with an external stabilising resistoristoristoristor

• Requires matched current transformers of low reactance design,Requires matched current transformers of low reactance design,Requires matched current transformers of low reactance design,Requires matched current transformers of low reactance design, typically class X or equivalent

typically class X or equivalent typically class X or equivalent typically class X or equivalent

• Equal CT ratiosEqual CT ratiosEqual CT ratiosEqual CT ratios

• NonNon----linear resistor may be required to limit voltage across relay NonNon linear resistor may be required to limit voltage across relay linear resistor may be required to limit voltage across relay linear resistor may be required to limit voltage across relay circuit during internal faults

circuit during internal faults circuit during internal faults circuit during internal faults

• Suitable for zones up to 200 Suitable for zones up to 200 ---- 300 metres (typically)Suitable for zones up to 200 Suitable for zones up to 200 300 metres (typically)300 metres (typically)300 metres (typically)

High Impedance Principle

(20)

High Impedance Principle

TC saturé M R_CT Z_M R_CT 2R_L 2R_L A M Z_M R_CT 2R_L 2R_L R_CT

(21)

High Impedance Principle

R_CT Z_M R_CT Z_M 2R_L 2R_L A M M

(22)

High Impedance Principle

R_CT Z_M R_CT Z_M 2R_L 2R_L TC saturé A M M

(23)

High Impedance Principle

(24)

High Impedance Principle

R_CT Z_M R_CT Z_M 2R_L 2R_L TC saturé A M M

(25)

High Impedance Principle

(26)

High Impedance Principle

R_CT Z_M R_CT Z_M 2R_L 2R_L TC sa tu ré A M M

(27)

High Impedance Principle

R_CT Z_M R_CT Z_M=0 2R_L 2R_L R_CT 2R_L 2R_L R_CT A M M

CT Saturation

False tripping

(28)

High Impedance Principle

R_CT Z_M R_CT Z_M=0 2R_L 2R_L TC saturé R_CT 2R_L 2R_L R_CT A R_S M M

(29)

High Impedance Principle

R_CT Z_M R_CT Z_M=0 2R_L 2R_L TC saturé R_CT 2R_L 2R_L R_CT A R_S M M Stabilising resistor Stabilising resistor Stabilising resistor Stabilising resistor

(30)

R_CT Z_M R_CT Z_M 2R_L 2R_L A R_S M M R_CT 2R_L 2R_L R_CT Vset

(31)

R_CT Z_M R_CT Z_M=0 2R_L 2R_L R_CT 2R_L 2R_L R_CT A R_S M M

Z

_M

= 0 (CT "short circuited" ) V_set

(32)

A R_CT Z_M R_CT Z_M 2R_L 2R_L 2R_L R_CT 2R_L R_CT R_S M M V_set

(33)

High Impedance Principle

2R_L R_CT 2R_L R_CT M V_set A R_CT Z_M R_CT Z_M 2R_L 2R_L R_S M

(34)

High Impedance Principle

M A R_CT Z_M R_CT Z_M 2R_L 2R_L R_S M V_set Metrosil MetrosilMetrosil

Metrosil may be may be may be may be required for voltage required for voltage required for voltage required for voltage

limitation limitation limitation limitation 2R_L R_CT 2R_L R_CT M

(35)

Restricted Earthfault

Protection

Increased sensitivity for earth faults

REF elements for each transformer winding

CTs may be shared with differential element

Uses high impedance principle

64

64 64

(36)

Restricted Earthfault Protection

P1 P1P1 P1 S1 S1S1 S1 P2 P2P2 P2 S2 S2 S2 S2 P1 P1 P1 P1 S1 S1S1 S1 P2 P2 P2 P2 S2 S2S2 S2 P1 P1 P1 P1 S1 S1S1 S1 P2 P2 P2 P2 S2 S2S2 S2 P1 P1 P1 P1 P2 P2 P2 P2 S1 S1 S1 S1 S2 S2 S2 S2

Stability level : usually maximum through fault level of tran Stability level : usually maximum through fault level of tran Stability level : usually maximum through fault level of tran

Stability level : usually maximum through fault level of transformersformersformersformer

REF Case I : Normal Condition

Under normal conditions no current flows thro’ Relay

So,

No Operation

(37)

Restricted Earthfault Protection

REF Case II : External Earth Fault

External earth fault - Current circulates between the phase & neutral CTs; no current thro’ the relay

(38)

Restricted Earthfault Protection

REF Case III : Internal Earth Fault

For an internal earth fault the unbalanced current flows thro’ the relay

(39)

Restricted Earthfault Protection

Restricted Earth Fault Protection Setting

1MVA (5%) 11000V 415V 1600/1 R_CT = 4.9Ω 80MVA R_S 1600/1 R_CT = 4.8Ω MCAG14 I_S = 0.1 Amp 2 Core 7/0.67mm (7.41Ω/km) 100m Long

Setting will require Setting will require Setting will require Setting will require calculation of : calculation of : calculation of : calculation of : 1) 1) 1)

1) Setting stability Setting stability Setting stability Setting stability voltage (V voltage (V voltage (V voltage (V_S_S_S_S)))) 2) 2) 2)

2) Value of stabilising Value of stabilising Value of stabilising Value of stabilising resistor required resistor required resistor required resistor required 3) 3) 3)

3) Peak voltage Peak voltage Peak voltage Peak voltage developed by developed by developed by

developed by CTCTCTCT’’’’ssss for internal fault for internal fault for internal fault for internal fault

(40)

Restricted Earthfault Protection

Example : Earth fault calculation : Example : Earth fault calculation : Example : Earth fault calculation : Example : Earth fault calculation :----Using 80MVA base

Using 80MVA base Using 80MVA base Using 80MVA base

Source impedance = 1 p.u. Source impedance = 1 p.u. Source impedance = 1 p.u. Source impedance = 1 p.u.

Transformer impedance = 0.05 x 80 = 4 p.u. Transformer impedance = 0.05 x 80 = 4 p.u. Transformer impedance = 0.05 x 80 = 4 p.u. Transformer impedance = 0.05 x 80 = 4 p.u.

1 1 1 1

Total impedance = 14 p.u. Total impedance = 14 p.u. Total impedance = 14 p.u. Total impedance = 14 p.u. ∴

∴ ∴

∴ I₁₁₁₁ = 1 = 0.0714 p.u.= 1 = 0.0714 p.u.= 1 = 0.0714 p.u.= 1 = 0.0714 p.u. 14 14 14 14 Base current = 80 x 10 Base current = 80 x 10 Base current = 80 x 10 Base current = 80 x 106666 √ √ √ √3 x 4153 x 4153 x 4153 x 415 = 111296 Amps = 111296 Amps = 111296 Amps = 111296 Amps ∴ ∴ ∴ ∴ I_F_F_F_F = 3 x 0.0714 x 111296= 3 x 0.0714 x 111296= 3 x 0.0714 x 111296= 3 x 0.0714 x 111296 = 23840 Amps (primary) = 23840 Amps (primary) = 23840 Amps (primary) = 23840 Amps (primary) = 14.9 Amps (secondary) = 14.9 Amps (secondary)= 14.9 Amps (secondary) = 14.9 Amps (secondary) 1 P.U. 1 4 I₁ 4 I₂ 4 I₀ 1 1

(41)

Restricted Earthfault Protection

(1) (1) (1)

(1) Setting voltageSetting voltageSetting voltageSetting voltage V V V V_S_S_S_S = = I= = _F_F_F_F (R(R(R(R_CT_CT_CT_CT + 2+ 2+ 2+ 2_RL_RL_RL_RL)))) Assuming Assuming Assuming

Assuming ““““earthearthearthearth”””” CT saturates,CT saturates,CT saturates,CT saturates, R

R R

R_CT_CT_CT_CT = 4.8 ohms= 4.8 ohms= 4.8 ohms= 4.8 ohms 2

2 2

2_RL_RL_RL_RL = 2 x 100 x 7.41 x 10= 2 x 100 x 7.41 x 10= 2 x 100 x 7.41 x 10= 2 x 100 x 7.41 x 10----3333 = 1.482 ohms= 1.482 ohms= 1.482 ohms= 1.482 ohms ∴

∴ ∴

∴ Setting voltage = 14.9 (4.8 + 1.482)Setting voltage = 14.9 (4.8 + 1.482)Setting voltage = 14.9 (4.8 + 1.482)Setting voltage = 14.9 (4.8 + 1.482) = 93.6 Volts = 93.6 Volts = 93.6 Volts = 93.6 Volts (2) (2) (2)

(2) Stabilising Resistor (RStabilising Resistor (RStabilising Resistor (RStabilising Resistor (R_S_S_S_S)))) R R R R_S_S_S_S = = = = {Vs - [VA/(Is^2)]} /Is Where Where Where

Where I_S_S_S_S = relay current setting= relay current setting= relay current setting= relay current setting ∴

∴ ∴

(42)

Restricted Earthfault Protection

3) 3) 3)

3) Peak voltage = 2Peak voltage = 2√Peak voltage = 2Peak voltage = 2√√√2 2 √2 2 √√√VVVV_K_K_K_K (V(V(V(V_F_F_F_F ---- VVVV_K_K_K_K)))) V

VV

V_F_F_F_F = 14.9 x V= 14.9 x V= 14.9 x V= 14.9 x V_S_S_S_S = 14.9 x 936 = 13946 Volts= 14.9 x 936 = 13946 Volts= 14.9 x 936 = 13946 Volts= 14.9 x 936 = 13946 Volts

I_S_S_S_S

For For For

For ‘‘‘‘EarthEarthEarth’’’’ CT, VEarth CT, VCT, VCT, V_K_K_K_K = 1.4 x 236 = 330 Volts (from = 1.4 x 236 = 330 Volts (from = 1.4 x 236 = 330 Volts (from = 1.4 x 236 = 330 Volts (from graph) graph)graph) graph) ∴ ∴ ∴

∴ VVVV_PEAK_PEAK_PEAK_PEAK = 2= 2= 2= 2√√2 √√2 2 √2 √√√330 (13946 330 (13946 330 (13946 330 (13946 ---- 330)330)330)330) = 6kV = 6kV = 6kV = 6kV Thus, Thus, Thus,

(43)

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

(44)

Transformer Magnetising Characteristic

Twice Twice Twice Twice Normal Flux Normal Flux Normal Flux Normal Flux Normal NormalNormal Normal Flux FluxFlux Flux Normal Normal Normal Normal No Load Current No Load Current No Load Current No Load Current No Load Current at No Load Current at No Load Current at No Load Current at Twice Normal Flux Twice Normal FluxTwice Normal Flux Twice Normal Flux

(45)

Magnetising Inrush

m

Φ

+

SWITCH ON AT VOLTAGE

ZERO - NO RESIDUAL FLUX

m

Φ

-m

Φ

2 STEADY STATE

V

Φ

m I m I V

_Φ

Inrush Current

(46)

(47)

Magnetising Inrush

• Appears on one side of transformer only

- Seen as fault by differential relay

- Transient magnetising inrush could cause

relay to operate

• Makes CT transient saturation

- Can make mal-operation of Zero sequence

relay at primary

Effect of magnetising current Effect of magnetising currentEffect of magnetising current Effect of magnetising current

(48)

P1 P1 P1 P1 S1 S1 S1 S1 P2 P2 P2 P2 S2 S2 S2 S2 P1 P1P1 P1 S1 S1S1 S1 P2 P2P2 P2 S2 S2 S2 S2 P1 P1P1 P1 S1 S1S1 S1 P2 P2P2 P2 S2 S2 S2 S2 IR IR IR IR IS IS IS IS IT ITIT IT

IR + IS + IT = 3Io = 0

Magnetising Inrush

(49)

Effect of magnetising current Effect of magnetising currentEffect of magnetising current Effect of magnetising current

Example of Example of Example of

Example of disurbancedisurbancedisurbancedisurbance records records records records with detail

with detail with detail with detail

(50)

2nd (and 5th) harmonic restraint

• Makes relay immune to magnetising

inrush

• Slow operation may result for genuine

transformer faults if CT saturation

occurs

(51)

Overfluxing - Basic Theory

Low frequency

High voltage

Geomagnetic disturbances

Causes

Overfluxing = V/F

(52)

Overfluxing - Basic Theory

Transient Overfluxing - Tripping of differential

element

Prolonged Overfluxing - Damage to transformers

Effects

m

Φ

2 Ie

m

Φ

V = kfΦ

Φ

(53)

Overfluxing - Condition

Differential element should be

blocked for transient overfluxing-+

25% OVERVOLTAGE CONDITION

Overfluxing waveform

contains very high 5th

Harmonic content

(54)

Φ

V

α

K

f

• Trip and alarm outputs for clearing prolonged overfluxing

• Alarm : Definite time characteristic to initiate corrective

action

• Trip : IT or DT characteristic to clear overfluxing condition

(55)

Oil conservator Oil conservator Oil conservator Oil conservator Bucholz BucholzBucholz

Bucholz RelayRelayRelayRelay

(56)

Buchholz Relay Installation

5 x internal pipe 5 x internal pipe 5 x internal pipe 5 x internal pipe diameter (minimum) diameter (minimum) diameter (minimum) diameter (minimum) 3 x internal pipe 3 x internal pipe 3 x internal pipe 3 x internal pipe diameter (minimum) diameter (minimum) diameter (minimum) diameter (minimum) Transformer TransformerTransformer Transformer 76 mm typical 76 mm typical 76 mm typical 76 mm typical To oil conservator To oil conservator To oil conservator To oil conservator

BUCCHOLZ PROTECTION

(57)

Buchholz Relay

Petcock Petcock Petcock Petcock From From From From transformer transformer transformer

transformer Trip bucketTrip bucketTrip bucketTrip bucket

To oil To oilTo oil To oil conservator conservatorconservator conservator Mercury switch Mercury switch Mercury switch Mercury switch Alarm bucket

Alarm bucketAlarm bucket

Alarm bucket

(58)

Accumulation of gaz

Oil

Oil Leakage

Leakage

Severe

Severe winding

winding

winding faults

winding

faults

Buccholz

Buccholz Protection Application

Protection Application

(59)

Interturn

Interturn faults

faults

Winding

Winding faults

faults

faults to

to

to earth

earth

earth with

with

with low

with

low

power (

power (fault

fault

fault close to

fault

close to

close to neutral

close to

neutral

neutral for

for

example

example))))

Accumulation of Gaz

(60)

Inter-Turn Fault

Nominal turns ratio

Fault turns ratio Current ratio : 11,000 / 240 : 11,000 / 1 : 1 / 11,000 Shorted turn Load Primary Secondary CT CTCT CT E E E E

BUCCHOLZ PROTECTION

(61)

Nominal turns ratio

Fault turns ratio Current ratio : 11,000 / 240 : 11,000 / 1 : 1 / 11,000 CT CT CT CT E E E E Shorted turn Primary Secondary

Inter-Turn Fault

BUCCHOLZ PROTECTION

(62)

Interturn Fault Current / Number

of Turns Short Circuited

5 5 5

5 10101010 15151515 20202020 25252525

Turn short Turn shortTurn short

Turn short----circuited circuited circuited circuited (percentage of (percentage of (percentage of (percentage of winding) winding)winding) winding) Primary current Primary current Primary current Primary current (multiples of (multiples of (multiples of (multiples of rated current) rated current) rated current) rated current) Fault current Fault current Fault current Fault current (multiples of (multiples of (multiples of (multiples of rated current) rated current) rated current) rated current) 100 100 100 100 80 80 80 80 60 60 60 60 40 40 40 40 20 20 20 20

BUCCHOLZ PROTECTION

(63)

Interturn Fault Current / Number

of Turns Short Circuited

5 10 15 20 25 Primary current (multiples of rated current) Fault current (multiples of rated current) 100 80 60 40 20

Fault current very high

Primary phase current very low

Detected by Bucholz relay

Not detected by current

(64)

Interturn

Interturn faults

faults

Winding

Winding faults

faults

faults to

to

to earth

earth

earth with

with

with low

with

low

power (

power (fault

fault

fault close to

fault

close to

close to neutral

close to

neutral

neutral for

for

example

example))))

Accumulation of Gaz

(65)

Earth Fault Current / Number of

Turns Short Circuited

Turn short Turn short Turn short

Turn short----circuited circuited circuited circuited (percentage of

(percentage of (percentage of (percentage of

Primary current Primary currentPrimary current Primary current

Fault current Fault currentFault current Fault current 100 100100 100 80 80 80 80 60 60 60 60 40 40 40 40 20 20 20 20 multiples of multiples of multiples of multiples of max fault current max fault current max fault current max fault current

(66)

Operating

Operating principle

principle

Accumulation of Gaz

(67)

Buchholz Relay

Accumulation

of

of gaz

gaz

(68)

Accumulation

of

of gaz

gaz

Buchholz Relay

(69)

Accumulation

of

of gaz

gaz

Buchholz Relay

(70)

Color ColorColor

Color of of of of gazgazgazgaz indicates indicates indicates indicates the type of fault

the type of faultthe type of fault the type of fault

White or Yellow : White or Yellow : White or Yellow : White or Yellow : Insulation burnt Insulation burnt Insulation burnt Insulation burnt Grey : Grey : Grey : Grey : Dissociated oil Dissociated oil Dissociated oil Dissociated oil

Accumulation

of

of gaz

gaz

(71)

Accumulation

of

of gaz

gaz

Gaz GazGaz

Gaz can be extracted for can be extracted for can be extracted for can be extracted for detailled

detailleddetailled

detailled analysisanalysisanalysisanalysis

Buchholz Relay

(72)

• After oil maintenance, false

tripping may occur because Oil

aeration

Effects of Oil Maintenance Effects of Oil Maintenance Effects of Oil Maintenance Effects of Oil Maintenance

Bucholz BucholzBucholz

Bucholz relay tripping inhibited during relay tripping inhibited during relay tripping inhibited during relay tripping inhibited during suitable period

suitable period suitable period suitable period

Need of electrical protection Need of electrical protectionNeed of electrical protection Need of electrical protection