07 Transformer

TRANSFORMER PROTECTION

Issue A

Slide 1

Causes of failure:
Environment
System
Mal operation
Wrong design
Manufacture
Material
Maintenance

Issue A

Slide 2

Transformer failures classification :

1. Internal failure Causes:

➢ Winding & terminal faults ➢ Core faults ➢ Onload tap changer faults ➢ Overheating faults

Issue A

Slide 3

Transformer failures classification : 2. External failure Causes:

➢ Abnormal operating condition ➢ sustained or unclear faults

Issue A

Slide 4

Vector Groups Group 1 0 Phase displacement Group 2 180 Phase displacement Group 3 30 Lag phase displacement Group 4 30 Lead phase displacement

Issue A

Yy0 Dd0 Zd0 Yy6 Dd6 Dz6 Yd1 Dy1 Yz1 Yd11 Dy11 Yz11

Slide 5

Fault current distribution

Earth fault on Transformer winding T2

T1

V2

V1

X Fig.N

R Fig.3

Issue A

If

Slide 10

Fault current distribution Therefore C.T.secondary current ( on primary side of transformer) =, X2 √3

If differential setting =20% For relay operation

X2

>

20%

√3 Thus X > 59% 59% ie. ie. 59% of winding is unprotected. Differential relay setting

% of winding protected

10% 10%

58%

20% 20%

41%

30% 30%

28%

40% 40%

17%

50%.

7%

Issue A

Slide 11

Basic Protection
Differential
Restricted Earthfault
Overfluxing
Overcurrent & Earthfault

Issue A

Slide 13

Differential Protection ∗ Works on Merz-price current comparison principle ∗ Relays with bias characteristic should only be used

Applied
Where protection co-ordination is difficult / not possible using time delayed elements
For fast fault clearance
For zone of protection

Issue A

Slide 14

Differential Protection Consideration for applying differential protection
Phase correction
Filtering of zero sequence currents
Ratio correction
Magnetizing inrush during energisation
Overfluxing Issue A

Slide 15

Differential Protection - Principle • Nominal current through the protected equipment I Diff = 0 : No tripping

R I diff = 0

Issue A

Slide 16

Differential Protection Principle • Through fault current

I Diff = 0 : No tripping

R I diff = 0

Issue A

Slide 17

Differential Protection Principle • Internal Fault I Diff = 0 : Tripping

R

Issue A

I diff = 0

Slide 18

Biased differential protection • Fast operation • Adjustable characteristic • High through fault stability • CT ratio compensation • Magnetising inrush restraint • Overfluxing 5th harmonic restraint Issue A

Slide 19

Biased differential protection Why bias characteristic ? 100 / 1

100/50 KV

200 / 1 1A

1A

R

LOAD = 200 A

0A

I1

I2

OLTC Setting is at mid tap Issue A

Slide 20

Biased differential protection 100 / 1

100/50 KV

200 / 1 1A

0.9 A

LOAD = 200 A

R

0.1 A

OLTC SETTING IS AT 10% Differential current = 0.1 A Relay pickup setting = O.2 A, So the Relay restrains Issue A

Slide 21

Biased differential protection 100 / 1

100/50 KV

200 / 1 10 A

9A

2000 A

R

1A

OLTC SETTING IS AT 10% Relay Pickup Setting is O.2 A So the Relay Operates Issue A

Slide 22

Role of Bias 3

2

Operate

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

80

1 Setting range (0.1 - 0.5)

%

pe o Sl

Restrain pe

0

lo 20% S

1

2

4

3

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

Slide 23

High Impedance Principle resistor istor Based on Current operated relay with an external stabilising res • Requires matched current transformers of low reactance design, typically class X or equivalent • Equal CT ratios • NonNon-linear resistor may be required to limit voltage across relay circuit during internal faults • Suitable for zones up to 200 - 300 metres (typically)

Issue A

Slide 24

High Impedance Principle RCT

2RL

M

2RL

A

ZM

RCT

ZM

RCT 2RL M

Issue A

2RL

TC RCTsaturé Slide 25

High Impedance Principle RCT

ZM

2RL

M

A

2RL

RCT

ZM

M

Issue A

Slide 26

High Impedance Principle RCT

ZM

2RL

M

A

2RL

RCT

ZM

M

TC saturé Issue A

Slide 27

High Impedance Principle RCT

ZM

2RL

M

A

2RL

RCT

ZM

M

Issue A

Slide 28

High Impedance Principle RCT

ZM

2RL

M

A

2RL

RCT

ZM

M

TC saturé Issue A

Slide 29

High Impedance Principle RCT

ZM

2RL

M

A

2RL

RCT

ZM

M

Issue A

Slide 30

High Impedance Principle RCT

ZM

2RL

M

A

2RL

RCT

ZM

TC saturé M

Issue A

Slide 31

High Impedance Principle RCT

2RL

M

2RL

A

ZM

RCT

ZM=0

False tripping RCT 2RL M

CT Saturation 2RL

RCT

TC saturé Issue A

Slide 32

High Impedance Principle M RCT

2RL

2RL

RCT

RS A

ZM

ZM=0

RCT 2RL M

2RL

RCT

TC saturé Issue A

Slide 33

High Impedance Principle RCT

2RL

2RL

M

RCT

RS A

ZM

ZM=0

Stabilising resistor

RCT 2RL M

2RL

RCT

TC saturé

Issue A

Slide 34

High Impedance Principle RCT

2RL

2RL

M

RCT

RS A

ZM

ZM

Vset

RCT 2RL M

Issue A

2RL

RCT

Slide 35

High Impedance Principle RCT

2RL

2RL

M

RCT

RS A

ZM

ZM=0

RCT 2RL M

Issue A

ZM = 0

Vset 2RL

RCT

(CT "short circuited" )

Slide 36

High Impedance Principle RCT

2RL

2RL

M

RCT

RS A

ZM

ZM

RCT

RCT 2RL

2RL M Vset

Issue A

Slide 37

High Impedance Principle RCT

2RL

2RL

M

RCT

RS A

ZM

ZM

RCT

RCT 2RL

2RL M

Vset

Issue A

Slide 38

High Impedance Principle RCT

2RL

2RL

M

RCT

RS A

ZM

Metrosil may be required for voltage limitation

RCT 2RL

M

ZM

RCT 2RL

M

Vset

Issue A

Slide 39

Restricted Earthfault Protection
Uses high impedance principle


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

64

64

Issue A

64 Slide 40

Restricted Earthfault Protection REF Case I : Normal Condition transformer sformer Stability level : usually maximum through fault level of tran P1

P2

S1

S2 P1 S1

P1

S1

P2

S2

P2 S2 P1

P2

S1

S2

Under normal conditions no current flows thro’ Relay So, No Operation Issue A

Slide 41

Restricted Earthfault Protection REF Case II : External Earth Fault

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

So, No Operation Issue A

Slide 42

Restricted Earthfault Protection REF Case III : Internal Earth Fault

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

So, Relay Operates Issue A

Slide 43

Restricted Earthfault Protection Restricted Earth Fault Protection Setting 1MVA (5%) 11000V 415V

1600/1 RCT = 4.9Ω

Setting will require calculation of : 1) Setting stability voltage (VS)

80MVA

2) Value of stabilising resistor required 1600/1 RCT = 4.8Ω

RS

2 Core 7/0.67mm (7.41Ω/km) 100m Long

Issue A

MCAG14 IS = 0.1 Amp

3) Peak voltage developed by CT’ CT’s for internal fault

Slide 44

Restricted Earthfault Protection Example : Earth fault calculation ::Using 80MVA base Source impedance = 1 p.u. 1 P.U.

Transformer impedance = 0.05 x 80 = 4 p.u. 1 1

1

4 I1

1

4 I2

∴ I1 = 1 = 0.0714 p.u. 14 Base current = 80 x 106 √3 x 415 = 111296 Amps

4 I0

Issue A

Total impedance = 14 p.u.

∴ IF = 3 x 0.0714 x 111296 = 23840 Amps (primary) = 14.9 Amps (secondary) Slide 45

Restricted Earthfault Protection (1) Setting voltage VS = IF (RCT + 2RL) Assuming “earth” earth” CT saturates, RCT = 4.8 ohms 2RL = 2 x 100 x 7.41 x 10-3 = 1.482 ohms ∴ Setting voltage = 14.9 (4.8 + 1.482) = 93.6 Volts (2) Stabilising Resistor (RS) RS = {Vs - [VA/(Is^2)]} /Is Where IS = relay current setting ∴ RS = {93.6 - [ 1/0.1^2]}/0.1

Issue A

= 836 ohms

Slide 46

Restricted Earthfault Protection 3) Peak voltage = 2√ 2√2 √VK (VF - VK) VF = 14.9 x VS = 14.9 x 936 = 13946 Volts IS For ‘Earth’ Earth’ CT, VK = 1.4 x 236 = 330 Volts (from graph) ∴ VPEAK = 2√ 2√2 √330 (13946 - 330) = 6kV Thus, metrosil voltage limiter will be required.

Issue A

Slide 47

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

Issue A

Slide 48

Magnetising Inrush Transformer Magnetising Characteristic Twice Normal Flux

Normal Flux

Normal No Load Current No Load Current at Twice Normal Flux Issue A

Slide 49

Magnetising Inrush Inrush Current + Φm

V

Φ Im

STEADY STATE - Φm Im

2 Φm

Φ V

Issue A

SWITCH ON AT VOLTAGE ZERO - NO RESIDUAL FLUX

Slide 50

Magnetising Inrush

Issue A

Slide 51

Magnetising Inrush Effect of magnetising current

• 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

Issue A

Slide 52

Magnetising Inrush

IR IS

P1

P2

S1

S2 P1

IT

S1

P2 S2 P1

P2

S1

S2

IR + IS + IT = 3Io = 0 Issue A

Slide 53

Magnetising Inrush Effect of magnetising current

Example of disurbance records with detail

Issue A

Slide 54

Magnetising Inrush Restrain 2nd (and 5th) harmonic restraint • Makes relay immune to magnetising inrush • Slow operation may result for genuine transformer faults if CT saturation occurs

Issue A

Slide 55

Overfluxing - Basic Theory Overfluxing = V/F

Causes
Low frequency
High voltage
Geomagnetic disturbances Issue A

Slide 57

Overfluxing - Basic Theory V = kfΦ

2Φm

Φm Ie Effects
Transient Overfluxing - Tripping of differential element
Prolonged Overfluxing - Damage to transformers Issue A

Slide 58

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 Issue A

Slide 59

Overfluxing - Protection V

KΦ α 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

Issue A

Slide 60

BUCCHOLZ PROTECTION Oil conservator

Bucholz Relay

Issue A

Slide 60

BUCCHOLZ PROTECTION Buchholz Relay Installation To oil conservator 3 x internal pipe diameter (minimum) 5 x internal pipe diameter (minimum)

76 mm typical Transformer

Issue A

Slide 60

BUCCHOLZ PROTECTION Buchholz Relay Petcock Alarm bucket

Mercury switch To oil conservator From transformer

Trip bucket

Deflector plate Issue A

Slide 60

BUCCHOLZ PROTECTION Buccholz Protection Application Accumulation of gaz Oil Leakage Severe winding faults

Issue A

Slide 60

BUCCHOLZ PROTECTION Accumulation of Gaz Interturn faults Winding faults to earth with low power (fault (fault close to neutral for example) example)

Issue A

Slide 60

BUCCHOLZ PROTECTION Inter-Turn Fault

E

CT Load

Shorted turn

Nominal turns ratio : 11,000 / 240 Fault turns ratio Current ratio

: 11,000 / 1 :1 / 11,000 Primary

Issue A

Secondary Slide 60

BUCCHOLZ PROTECTION Inter-Turn Fault

E

CT Shorted turn

Nominal turns ratio : 11,000 / 240 Fault turns ratio Current ratio

: 11,000 / 1 :1 / 11,000 Primary

Issue A

Secondary Slide 60

BUCCHOLZ PROTECTION Interturn Fault Current / Number of Turns Short Circuited Primary current (multiples of rated current) 100 Fault current (multiples of rated current)

80

60

40

20

5

Issue A

10

15

20

25

Turn shortshort-circuited (percentage of winding) Slide 60

BUCCHOLZ PROTECTION Interturn Fault Current / Number of Turns Short Circuited Primary current (multiples of rated current) 100 Fault current (multiples of rated current)

80

60

Fault current very high

40

Detected by Bucholz relay

20

Primary phase current very low

5

Issue A

10

15

20

25

Not detected by current operated relays Slide 60

BUCCHOLZ PROTECTION Accumulation of Gaz Interturn faults Winding faults to earth with low power (fault (fault close to neutral for example) example)

Issue A

Slide 60

BUCCHOLZ PROTECTION Earth Fault Current / Number of Turns Short Circuited multiples of max fault current Primary current 100

80 Fault current 60

40

20

5 Issue A

10

15

20

25

Turn shortshort-circuited (percentage of winding) Slide 60

BUCCHOLZ PROTECTION Accumulation of Gaz Operating principle

Issue A

Slide 60

BUCCHOLZ PROTECTION

Buchholz Relay Accumulation of gaz

Issue A

Slide 60

BUCCHOLZ PROTECTION

Buchholz Relay Accumulation of gaz

Issue A

Slide 60

BUCCHOLZ PROTECTION

Buchholz Relay Accumulation of gaz

Issue A

Slide 60

BUCCHOLZ PROTECTION

Accumulation of gaz

Color of gaz indicates the type of fault White or Yellow : Insulation burnt Grey : Dissociated oil

Issue A

Slide 60

BUCCHOLZ PROTECTION Buchholz Relay Accumulation of gaz

Issue A

Gaz can be extracted for detailled analysis

Slide 60

BUCCHOLZ PROTECTION Effects of Oil Maintenance

• After oil maintenance, false tripping may occur because Oil aeration Bucholz relay tripping inhibited during suitable period

Need of electrical protection

Issue A

Slide 60

BUCCHOLZ PROTECTION Bucholtz Protection Application Accumulation of gaz Oil Leakage Severe winding faults

Issue A

Slide 60

BUCCHOLZ PROTECTION Buchholz Relay Oil Leakage

Issue A

Slide 60

BUCCHOLZ PROTECTION Buchholz Relay Oil Leakage

Issue A

Slide 60

BUCCHOLZ PROTECTION Buchholz Relay Oil Leakage

Issue A

Slide 60

BUCCHOLZ PROTECTION Buchholz Relay Oil Leakage

Issue A

Slide 60

BUCCHOLZ PROTECTION Buccholz Protection Application Accumulation of gaz Oil Leakage Severe winding faults

Issue A

Slide 60

BUCCHOLZ PROTECTION Buchholz Relay Severe winding fault

Issue A

Slide 60

BUCCHOLZ PROTECTION Buchholz Relay Severe winding fault

Issue A

Slide 60

BUCCHOLZ PROTECTION Buchholz Relay Severe winding fault

Issue A

Slide 60