04 Transformer

7/29/2019 04 Transformer

1/98

TRANSFORMER PROTECTION

Slide 1Issue A


2/98

Slide 2Issue A

Causes of failure:

Environment

System

Mal operation

Wrong design

Manufacture

Material

Maintenance


3/98

Slide 3Issue A

Transformer failures classification :

1. Internal failureCauses:

Winding & terminal faults

Core faults

Onload tap changer faults

Overheating faults


4/98

2. External failure

Causes:

Issue A Slide 4

Abnormal operating condition

sustained or unclear faults

Transformer failures classification :


5/98

Issue A Slide 5

Vector Groups

Phase displacement0

Group 1

Phase displacement180

Group 2

Lag phase displacement30

Group 3

Lead phase displacement30

Group 4

Yy0Dd0Zd0

Yy6Dd6Dz6

Yd1

Dy1Yz1

Yd11Dy11

Yz11


6/98

Slide 6Issue A

Vector Configurations

12

11

300

1, DRAW PHASE-NEUTRAL VOLTAGE VECTORS

300


7/98

Slide 7Issue A

2. Draw Delta Connection

A

C B

a

c

b



8/98

Issue A Slide 8

3. Draw A Phase Windings

A

C B

a

c

b

a2

a1

A2

A1



9/98

Slide 9Issue A

4. Complete Connections (a)

A

C B

a

c

b

a2

a1

A2

A1

C1

C2

B1

B2

b2b1c1

c2



10/98

Slide 10Issue A

Fault current distribution

Earth fault on Transformer winding

V2

R

T2 T1

V 1

X

Fig.3 If

Fig.N


11/98


12/98

Slide 12Issue A

Fault current distribution

If Transformer star winding is solid earthed,

fault current limited only by the leakage

reactanceof the winding

If asmultiple of

IF.L.

.1 .2 .3 .4 .5 .6 .7 .8 . 9 1.0 x p.u

Delta side

Star side

Fig.Q

10

9

8

7

6

5

4

3

2

1


13/98

Slide 13Issue A

Differential

Basic Protection

Restricted Earthfault

Overfluxing

Overcurrent & Earthfault


14/98

Slide 14Issue A

Differential Protection

Where protection co-ordination is difficult / notpossible using time delayed elements

For fast fault clearance

Applied

Works on Merz-price current comparison principle

Relays with bias characteristic should only be used

For zone of protection


15/98

Slide 15Issue A

Differential Protection

Consideration for applying differential

protection

Phase correction

Filtering of zero sequence currents

Ratio correction

Magnetizing inrush during energisation

Overfluxing


16/98

Slide 16Issue A

Differential Protection - Principle

R I diff = 0

Nominal current through the protected equipment

I Diff = 0 : No tripping


17/98

Slide 17Issue A


Through fault current

I Diff = 0 : No tripping

R I diff = 0


18/98

Slide 18Issue A


Tripping

Internal Fault

I Diff = 0 :

R I diff = 0


19/98

Slide 19Issue A

Biased differential protection

Fast operation

Adjustable characteristic

High through fault stability

CT ratio compensation

Magnetising inrush restraint

Overfluxing 5th harmonic restraint


20/98

Slide 20Issue A


1 A

100/50 KV100 / 1 200 / 1

1 A

0 A

LOAD

= 200 A

Why bias characteristic ?

OLTC Setting is at mid tap

R

I1I2


21/98

Slide 21Issue A

Biased differential protection100/50 KV100 / 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

R


22/98

Slide 22Issue A


100/50 KV100 / 1 200 / 1

9 A10 A

1 A

Relay Pickup Setting is O.2 AOLTC SETTING IS AT 10%

2000 A

R

OperatesSo the Relay


23/98

Slide 23Issue A

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%

Slope

20%Slope


24/98

R RR

Dy1(-30 )

Yd11(+30 )

Interposing CT provides

Vector correction

Ratio correction

Zero sequencecompensation

USE OF ICT


25/98

Dy1(-30 )

RR

R

PROTECTION TRANSFORMATEUR

sur dfaut interne: Protection diffrentielle

Yd11

Vector Group Correction - Static Relays

Vector and Ratio correction by interposing CT

CURRENT DIFFERENTIAL PROTECTION


26/98

RR

R

PROTECTION TRANSFORMATEUR

sur dfaut interne: Protection diffrentielle

Yd11

Vector Group Correction - Static Relays

Vector and Ratio correction by CT Connection

CURRENT DIFFERENTIAL PROTECTION


27/98

87

Yy0 Yd11

0 +30

Yy0, Yd1, Yd5 , Yy6, Yd7, Yd11, Ydy0

0 , -30 , -150 , 180,+150, +30 , 0

Dy1 (-30 )

VECTOR GROUP CORRECTION


28/98

87

Dy11 (+30 )

Yy0

0

Yd1

-30

SELECTION OF SUITABLE

VECTOR CORRECTION FACTOR


29/98

0.875A

87

200/1

1.31 Amps

I = 175A I = 525A 400/1L L

33kV : 11kV10 MVA

1.14 0.76

1A 1A

CT RATIO MISMATCH CORRECTION


30/98

+VE SEQUENCECURRENTSBALANCE

REQUIRE ZEROSEQUENCE

CURRENT

TRAPS FORSTABILITY B CA

ZERO SEQUENCE COMPENSATION


31/98

Slide 24Issue A

Based on Current operated relay with an external stabilisingresistor

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

Equal CT ratios

Non-linear resistor may be required to limit voltage across relaycircuit during internal faults

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

High Impedance Principle


32/98

Slide 25Issue A


TC satur

M

RCT

ZM

RCT2RL 2RL

A

M

ZM

RCT

2RL

2RL

RCT


33/98

Slide 26Issue A


RCT

ZM

RCT

ZM

2RL 2RL

A

M

M


34/98

Slide 27Issue A


RCT

ZM

RCT

ZM

2RL 2RL

TC satur

A

M

M


35/98

Slide 28Issue A


RCT

ZM

RCT

ZM

2RL 2RL

A

M

M


36/98

Slide 29Issue A


RCT

ZM

RCT

ZM

2RL 2RL

TC satur

A

M

M


37/98

Slide 30Issue A


RCT

ZM

RCT

ZM

2RL 2RL

A

M

M


38/98

Slide 31Issue A


RCT

ZM

RCT

ZM

2RL 2RL

TCsatur

A

M

M


39/98

Slide 32Issue A


RCT

ZM

RCT

ZM=0

2RL 2RL

TC satur

RCT

2RL

2RL

RCT

A

M

MCT Saturation

False tripping


40/98

Slide 33Issue A


RCT

ZM

RCT

ZM=0

2RL 2RL

TC satur

RCT

2RL

2RL

RCT

A

RS

M

M


41/98

Slide 34Issue A


RCT

ZM

RCT

ZM=0

2RL 2RL

TC satur

RCT

2RL

2RL

RCT

A

RS

M

M

Stabilising resistor


42/98

Slide 35Issue A

RCT

ZM

RCT

ZM

2RL 2RL

A

RS

M

M

RCT

2RL

2RL

RCT Vset



43/98

Slide 36Issue A

RCT

ZM

RCT

ZM=0

2RL 2RL

RCT

2RL

2RL

RCT

A

RS

M

M

ZM = 0

(CT "shortcircuited" )

Vset



44/98

Slide 37Issue A

A

RCT

ZM

RCT

ZM

2RL 2RL

2RL

RCT

2RL

RCT

RS

M

M

Vset



45/98

Slide 38Issue A


2RL

RCT

2RL

RCT

M

Vset

A

RCT

ZM

RCT

ZM

2RL 2RL

RS

M


46/98

Slide 39Issue A


M

A

RC

T

ZM

RC

T

ZM

2R

L

2R

L

RS

M

Vset

Metrosil may be

required for voltagelimitation

2R

L

RC

T 2R

L

RC

T

M


47/98

Slide 40Issue A

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

6464

64



48/98

Slide 41Issue A

est cted a t au t otect o

P1

S1

P2

S2P1

S1

P2

S2

P1S1

P2S2

P1

P2

S1

S2

Stability level : usually maximum through fault level of transformerREF Case I : Normal Condition

Under normal conditions no current flows thro Relay

So, No Operation



49/98

Slide 42Issue A


REF Case II : External Earth Fault

External earth fault - Current circulates between the phase & neutral CTs;

no current thro the relay

So, No Operation



50/98

Issue ASlide 43

Issue A


REF Case III : Internal Earth Fault

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

So, Relay Operates



51/98

Slide 44Issue A


Restricted Earth Fault ProtectionSetting

1MVA(5%)

11000V 415V

1600/1

RCT = 4.9

80MVA

RS1600/1

RCT = 4.8MCAG14

IS = 0.1 Amp

2 Core 7/0.67mm (7.41/km)100m Long

Setting will requirecalculation of :

1) Setting stabilityvoltage (V

S

)

2) Value of stabilisingresistor required

3) Peak voltage

developed by CTsfor internal fault



52/98

Slide 45Issue A


Example : Earth fault calculation :-

Using 80MVA base

Source impedance = 1 p.u.

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

Total impedance = 14 p.u.

I1 = 1 = 0.0714 p.u.14

Base current = 80 x 106

3 x 415

= 111296 Amps IF = 3 x 0.0714 x 111296

= 23840 Amps (primary)= 14.9 Amps (secondary)

1 P.U.1 4

I1

4

I2

4

I0

1

1



53/98

Slide 46Issue A


(1) Setting voltage

VS = IF (RCT + 2RL)

Assuming earth CT saturates,

RCT = 4.8 ohms2RL = 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 - 1IS IS

2 Where IS = relay current setting

RS = 93.6 - 1 = 836 ohms

0.1 0.22



54/98

Slide 47Issue A


3) Peak voltage = 22 VK (VF - VK)

VF = 14.9 x VS = 14.9 x 936 = 13946 VoltsIS

For Earth CT, VK = 1.4 x 236 = 330 Volts (from graph) VPEAK = 22 330 (13946 - 330)

= 6kV

Thus, metrosil voltage limiter will be required.

Magnetising Inrush


55/98

Slide 48Issue A

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

Magnetising Inrush


56/98

Slide 49Issue A

Transformer Magnetising Characteristic

TwiceNormal

Flux

Normal

Flux

NormalNo Load Current

No Load Currentat Twice NormalFlux

Magnetising Inrush

Magnetising Inrush


57/98

Slide 50Issue A

Magnetising Inrush

m+

SWITCH ON AT VOLTAGE

ZERO - NO RESIDUAL FLUX

m-

m2

STEADY STATE

V

mI

mI

V

Inrush Current

Magnetising Inrush


58/98

Slide 51Issue A

Magnetising Inrush

Magnetising Inrush


59/98

Slide 52Issue A

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

Magnetising Inrush


60/98

Slide 53Issue A

P1

S1

P2

S2P1

S1

P2

S2P1

S1

P2

S2

IR

IS

IT

IR + IS + IT = 3Io = 0

Magnetising Inrush

Magnetising Inrush


61/98

Slide 54Issue A

Effect of magnetising current

Example of disurbance recordswith detail

Magnetising Inrush

Magnetising Inrush Restrain


62/98

Slide 55Issue A

2nd (and 5th) harmonic restraint

Makes relay immune to magnetisinginrush

Slow operation may result for genuine

transformer faults if CT saturationoccurs




63/98

Slide 56Issue A

Biasdifferential

threshold

TripDifferential

comparator

T1 = 5ms T2 = 22ms

Differential input

Comparator output

Trip

T2 Reset

T1


Overfluxing - Basic Theory


64/98

Slide 57Issue A

Overfluxing Basic Theory

Low frequency

High voltage

Geomagnetic disturbances

Causes

Overfluxing = V/F

Overfluxing - Basic Theory


65/98

Slide 58Issue A

Overfluxing Basic Theory

Transient Overfluxing - Tripping of differential

element Prolonged Overfluxing - Damage to transformers

Effects

m2

Ie

m

V = kf

Overfluxing - Condition


66/98

Slide 59Issue A

Ove u g Co d t o

Differential element should be blockedfor transient overfluxing-+

25% OVERVOLTAGE CONDITION

43% 5TH HARMONIC CONTENT

Overfluxing waveform

contains very high 5th

Harmonic content

Overfluxing - Protection


67/98

Slide 60Issue A

V Kf

Trip and alarm outputs for clearing prolonged overfluxing

Alarm : Definite time characteristic to initiate correctiveaction

Trip : IT or DT characteristic to clear overfluxing condition

g

BUCCHOLZ PROTECTION


68/98

Slide 60Issue A

Oil conservator

Bucholz Relay

BUCCHOLZ PROTECTION


69/98

Slide 60Issue A

Buchholz Relay Installation

5 x internal pipediameter (minimum)

3 x internal pipediameter (minimum)

Transformer

76 mm typical

To oil conservator

BUCCHOLZ PROTECTION


70/98

Slide 60Issue A

Buchholz Relay

Petcock

From

transformer

Deflector plate

Trip bucket

To oilconservator

Mercury switch

Alarm bucket

BUCCHOLZ PROTECTION


71/98

Slide 60Issue A

Accumulation of gazOil LeakageSevere winding faults

Buccholz Protection Application

BUCCHOLZ PROTECTION


72/98

Slide 60Issue A

Interturn faults

Winding faults to earth with low

power (fault close to neutral forexample)

Accumulation of Gaz

BUCCHOLZ PROTECTION


73/98

Slide 60Issue A

Inter-Turn Fault

Nominal turns ratio

Fault turns ratio

Current ratio

: 11,000 / 240

: 11,000 / 1

: 1 / 11,000

Shortedturn

Load

Primary Secondary

CTE

BUCCHOLZ PROTECTION


74/98

Slide 60Issue A

Nominal turns ratio

Fault turns ratioCurrent ratio

: 11,000 / 240

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

CTE

Shortedturn

Primary Secondary

Inter-Turn Fault

BUCCHOLZ PROTECTION


75/98

Slide 60Issue A

Interturn Fault Current / Numberof Turns Short Circuited

5 10 15 20 25

Turn short-circuited(percentage ofwinding)

Primary current(multiples ofrated current)

Fault current(multiples ofrated current)

100

80

60

40

20

BUCCHOLZ PROTECTION


76/98

Slide 60Issue A

Interturn Fault Current / Numberof 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

operated relays

BUCCHOLZ PROTECTION


77/98

Slide 60Issue A

Interturn faults

Winding faults to earth with low

power (fault close to neutral forexample)

Accumulation of Gaz

BUCCHOLZ PROTECTION


78/98

Slide 60Issue A

Earth Fault Current / Number ofTurns Short Circuited

5 10 15 20 25

Turn short-circuited(percentage of

winding)

Primary current

Fault current

100

80

60

40

20

multiples ofmax fault current

BUCCHOLZ PROTECTION


79/98

Slide 60Issue A

Operating principle

Accumulation of Gaz

BUCCHOLZ PROTECTION


80/98

Slide 60Issue A

Buchholz Relay

Accumulationof gaz

BUCCHOLZ PROTECTION


81/98

Slide 60Issue A

Accumulationof gaz

Buchholz Relay

BUCCHOLZ PROTECTION


82/98

Slide 60Issue A

Accumulationof gaz

Buchholz Relay

BUCCHOLZ PROTECTION


83/98

Slide 60Issue A

Color of gaz indicatesthe type of fault

White or Yellow :Insulation burnt

Grey :

Dissociated oil

Accumulationof gaz

BUCCHOLZ PROTECTION


84/98

Slide 60Issue A

Accumulation

of gaz

Gaz can be extractedfor detailled analysis

Buchholz Relay

BUCCHOLZ PROTECTION


85/98

Slide 60Issue A

After oil maintenance, false

tripping may occur because Oil

aeration

Effects of Oil Maintenance

Bucholz relay tripping inhibited duringsuitable period

Need of electrical protection

BUCCHOLZ PROTECTION


86/98

Slide 60Issue A


Bucholtz Protection Application

BUCCHOLZ PROTECTION


87/98

Slide 60Issue A

Oil LeakageBuchholz Relay

BUCCHOLZ PROTECTION


88/98

Slide 60Issue A


BUCCHOLZ PROTECTION


89/98

Slide 60Issue A


BUCCHOLZ PROTECTION


90/98

Slide 60Issue A


BUCCHOLZ PROTECTION


91/98

Slide 60Issue A


Buccholz Protection Application

B hh l R l

BUCCHOLZ PROTECTION


92/98

Slide 60Issue A

Severe winding faultBuchholz Relay

B hh l R l

BUCCHOLZ PROTECTION


93/98

Slide 60Issue A


B hh l R l

BUCCHOLZ PROTECTION


94/98

Slide 60Issue A


3 3kVScheme exemple Up to 1MVA

CONCLUSION


95/98

51

3.3kV

200/5 P1215051

1MVA3.3/0.44kV1500/5

1500/564

P120

MCAG14

N50N

51N

Scheme exemple Up to 1MVA

11kVScheme exemple 1 - 5MVA

CONCLUSION


96/98

11kV

P1215051

5MVA11/3.3kV1000/5

1000/564

P120

MCAG14

51N

MCAG1464

3.3kV

33KV

Scheme exemple Above 5MVACONCLUSION


97/98

33KV

P1415051

200/5

P120

10MVA33/11KV 51

N 87600/5P631

MCAG1464

600/5

5/5A

Three Winding Transformer25MVA63MVA

CONCLUSION


98/98

25MVA11KV

63MVA132KV

1600/5300/5

50MVA

33KV

1000/5

4.59

2.88

10.33

2.88

5.51

5

5

All interposing C.T. ratios referto common MVA base (63MVA)

04 Transformer

Documents

Transcript of 04 Transformer