Transformer protection.ppt

TRANSFORMER PROTECTION

Target Audiance

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TRANSFORMER PROTECTION

Introduction

Transformers are a critical and expensive component of the

power system. Due to long lead time for

repair and replacement of transformers, a

major goal of

transformer protection is limiting the damage to a faulted transformer. The comprehensive transformer protection provided by multiple function protective

relays is appropriate for critical transformers of all applications.

TRANSFORMER PROTECTION

The type of protection for the transformers varies depending on the application and importance of the transformer.

Transformers are protected primarily against faults and

overloads. The type of protection used should minimize the time of disconnection for faults within the transformer and to reduce the risk of catastrophic failure to simplify eventual repair.

TRANSFORMER PROTECTION Any extended operation of the transformer under abnormal condition such as faults or overloads

compromises the life of the transformer, which means

adequate protection should be provided for quicker isolation of the transformer under such condition.

The various possible transformer faults are, _{1. Overheating} _{2. Winding faults} _{3. Open circuits} _{4. External faults} _{5. Over fluxing}

TYPES OF FAULTS

They are due to

 Sustained overloads and short circuits

 _{Failure of cooling system}

Protection

 _{The thermal overload relays and temperature relays, sounding}

the alarm are used

 _{The thermocouples or resistance temperature indicators are}

also provided near the winding.

 _{These are connected in a bridge circuits.}

 When the bridge balance gets disturbed for more than the

permissible duration circuit breaker trips.

The winding faults are called internal faults. The overheating or mechanical shocks deteriorates the winding insulation. If the winding insulation-is weak, there is a possibility of

 _{Phase to phase faults}

 Earth faults

 _{Inter turn faults}

Persistent fault would have a possibility of oil-fire. PROTECTION

 _{Differential protection}

 Over current protection is also used as a backup protection

 _{For earth fault protection, the restricted earth fault protection}

system, neutral current relays or leakage to frame protection system is used.

_{The open circuit in one of the three phases causes the}

undesirable heating of the transformer.

_{Open circuits are much harmless compared to other faults.}

_{Transformers can be manually disconnected from the system}

during such faults.

 _{Through faults are the external faults which occur outside the}

protected zone.

 _{These are not detected by the differential protection.}

 _{If faults persists the transformer gets subjected to thermal and}

mechanical stresses which can damage the transformer. PROTECTION

 _{The overcurrent relays with undervoltage blocking, zero}

sequence protection and negative sequence protection are used to give protection

 _{The setting of the overcurrent protection not only protects}

transformer but also covers the station busbar and portion of a transmission line.

 _{The flux density in the transformer core is proportional to the}

ratio of the voltage to frequency i.e. V/f

 _{Transformers are designed to work with certain value of flux}

density in the core. In the generator transformer unit, if full

excitation is applied before generator reaches its synchronous speed then due to high V/f the overfluxing of core may occur. PROTECTION

 _{V/f relay called volts/hertz relay is provided to give the}

protection against overfluxing operation.

Other faults

_{Tap changer faults}

_{High voltage surges due to lightning}

_{Switching surges}

Importance of Protection

_{Detect and isolate fault}

Maintain system stability

_{To limit the damage in adjacent equipment}

Minimize fire risk

A B C _c b a a2 a1 b1 b2 B1 B2 A2 A1 C2 C1 c1 c2 A B C C2 C1 A2 A1 B2 B1 a b c c2 c1 a2 a1 b1 _b2

“Clock face” numbers refer to position of low voltage phase - neutral vector with respect to high voltage phase - neutral vector.

Line connections made to highest numbered winding terminal available. Line phase designation is same as winding.

Phase displacement 0 Group 1 Phase displacement 180 Group 2

Lag phase displacement 30

Group 3

Lead phase displacement 30 Group 4 Yy0 Dd0 Zd0 Yy6 Dd6 Dz6 Yd1 Dy1 Yz1 Yd11 Dy11 Yz11

 _{“Clock Face” numbers refer to position of low voltage}

phase-neutral vector with respect to high voltage phase neutral vector

 _{Line connections made to highest numbered winding}

terminal available

 _{Line phase designation is same as winding}

Example 1 : Dy 11 Transformer B1 B2 b1 b2 a2 a1 A1 A2 C1 C2 c1 c2 High Voltage Windings Low Voltage Windings A Phase Winding s B Phase Winding s C Phase

Transformer Connections

Requirements

 _{Fast operation for primary short circuits}

 _{Discrimination with downstream protections}

 _{Operation within transformer withstand}

 _{Non-operation for short or long term overloads}

 _{Non-operation for magnetising inrush}

→

50 51

50 set to 1.2 - 1.3 x through fault level

Concerns relay response to offset waveforms (DC transient) Definition

I

- I

I

x 100

I

I

D.

C

.

Transient Overreach

HV 2 5 1 5 1 5 1 _{HV LV} 1 Tim e LV HV1HV2 Curren t I I

Instantaneous High Set Overcurrent

Relay Applied to a Transformer

I3Ø

I3Ø

I3Ø

I3Ø

0.866

I3Ø

0.866

_I3Ø

0.4 sec

LV relay

Grid supply Feeders 51 51 67 67 51 51

Parallel Transformers

Directional Relays (1)

51 51 51 51 Bus Section

Parallel Transformers

Directional Relays (2)

1 p.u. turns 3 p.u. turns x I_P PR R IF Protective Relay : 3 ratio turns Effective . 3 . current Fault : at fault a For . x 3 current, primary Thus, values load full to current E/F limits Resistor F.L. 2 F.L. F.L. P               secondary C.T. then r, transforme of current load full on based is side) primary (on ratio C.T. If 2

 IF 51 Overcurrent Relay I_f as multiple of I_F.L. 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Star Side Delta Side 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 p.u..

Overcurrent Relay Sensitivity

to Earth Faults (1)

10 9 8 7 6 5 4 3 2 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 p.u.. I_f as multiple of I_F.L. Star Side Delta Side

Overcurrent Relay Sensitivity

to Earth Faults (2)

 IF

51 Overcurrent

 I_F I_P I_N 10 9 8 7 6 5 4 3 2 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0  p.u.. I_N I_P I_F

Overcurrent Relay Sensitivity

to Earth Faults (3)

I_f as multiple of I_F.L.

Differential Relay Setting % of Star Winding Protected

10% 58%

20% 41%

30% 28%

40% 17%

Earth Fault on Transformer Winding

I

Differential Relay

Setting = I_S



For relay operation, I > I_S

e.g. If I_S = 20%, then _{> 20% for}

operation

i.e.  > 59%

Thus 59% of winding is not protected

2

 3

I =

2

 Provides back-up protection for system

 _{Time delay required for co-ordination}

Unrestricted Earthfault Protection (1)

5 1 5 1 5 1 51 N

 _{Can provide better sensitivity}

(C.T. ratio not related to full load current) (Improved “effective” setting)

 _{Provides back up protection for transformer and system}

5 1 5 1 5 1 51 N 51 N

Protected Zone

RE F

 _{Relay only operates for earth faults within protected zone.}  _{Uses high impedance principle.}

 Stability level : usually maximum through fault level of

LV restricted E/F protection trips

both HV and LV breaker Recommended setting : 10% rated

Restricted E/F Protection

A B C N

LV restricted E/F protection trips both HV and LV breaker Recommended setting : 10% rated

Protected zone REF

Source

 _{Delta winding cannot supply zero sequence current to}

system

 _{Stability : Consider max LV fault level}

 _{Recommended setting : less than 30% minimum earth fault}

Protected Circuit R Z M R_CT R_L Z M R_CT R_L I_F I_S R_S T V_S R_L R_L

High Impedance Principle

Voltage Across Relay Circuit V_S = I_F (R_CT + 2R_L)

Stabilising resistor R_ST limits spill current to I_S (relay setting)  R_ST = - R_R where R_R = relay burden

CT knee point V_S

 _{During internal faults the high impedance relay circuit}

constitutes an excessive burden to the CT’s.

 _{A very high voltage develops across the relay circuit and}

the CT’s.

 _{Causes damage to insulation of CT, secondary}

winding and relay.

 Magnitude of peak voltage V_P is given by an approximate formula (based on experimental results)

V_P = 2  2V_K (V_F - V_K)

Where V_F = Swgr. Fault Rating in amps x Z of relay

circuit CT ratio

@ setting

Metrosil required if V > 3kV

I_OP R_ST V_S V_M R_R Metrosil Characteristic V = C I 

 Suitable values of C &  chosen based on :

 _{Max secondary current under fault conditions}

1MVA (5%) 11000V 415V 1600/1 R_CT = 4.9 80MV A R_S 1600/1 R_CT = 4.8 MCAG14 I_S = 0.1 Amp 2 Core 7/0.67mm (7.41/km) 100m Long Calculate : 1) Setting voltage (V_S) 2) Value of stabilising resistor required 3) Effective setting 4) Peak voltage developed by CT’s for internal fault

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  I_F = 3 x 0.0714 x 111296 = 23840 Amps (primary) = 14.9 Amps (secondary) 1 P.U. 1 4 I₁ 4 I₂ 4 I₀

Solution (1)

(1) Setting voltage

V_S = I_F (R_CT + 2_RL)

Assuming “earth” CT saturates, R_CT = 4.8 ohms 2_RL = 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 (R_S) R_S = V_S - 1 I_S I_S2 _{Where I}

S = relay current setting

 R_S = 93.6 - 1 = 836 ohms 0.1 0.12

(4) Peak voltage = 22V_K (V_F - V_K)

V_F = 14.9 x V_S = 14.9 x 936 = 13946 Volts I_S

For ‘Earth’ CT, V_K = 1.4 x 236 = 330 Volts (from graph)

 V_PEAK = 22 x 330 x (13946 - 330) = 6kV

Thus, Metrosil voltage limiter will be required.

Traditional Large Transformer

Protection Package

 _{Overall differential protection may be justified for}

larger transformers (generally > 5MVA).

 Provides fast operation on any winding

 _{Measuring principle :}

 _{Based on the same circulating current principle as the}

restricted earth fault protection

 _{However, it employs the biasing technique, to maintain}

stability for heavy through fault current

 _{Biasing allows mismatch between CT outputs.}

 _{It is essential for transformers with tap changing}

facility.

 _{Another important requirement of transformer}

differential protection is immunity to magnetising in rush current.

BIAS BIAS _I 2 I₁ OPERATE I₁ - I₂ Differential Current OPERATE RESTRAIN Mean Through Current I₁ + I₂ 2 I₁ - I₂

LV

R HV

PROTECTED ZONE

Correct application of differential protection requires CT ratio and winding connections to match those of transformer.

CT secondary circuit should be a “replica” of primary system. Consider :

(1) Difference in current magnitude

(2) Phase shift

P1 P2 A2 A1 a1 a2 P2 P1

A _{B C}

R R R P1 P2 A2 A1 a1 a2 P2 P1 S2 S1 P1 P2 Interposing CT provides :  _{Vector correction}  Ratio correction S1 S2 S2 S1

Use of Interposing CT

S2 S1 P1 P2 A1 A2 a1 a2 P2 15MVA 66kV / 11kV 150/5 800/5 P1 Dy1 S1 S2

Differential Protection

Given above: Need to consider

-(1) Winding full load current (2) Effect of tap changer (if any) (3) C.T. polarities

Assuming no tap changer

Full load currents:- 66kV: 131 Amp = 4.37 Amps secondary 11kV: 787 Amp = 4.92 Amps secondary However, require 11kV C.T.’s to be connected in 

Differential Protection

It is usual to connect 11kV C.T.’s in and utilise a / interposing C.T. (this method reduces lead VA burden on the line C.T.’s)

Current from 66kV side = 4.37 Amp

Thus, current required from winding of int. C.T. = 4.37 Amp Current input to winding of int. C.T. = 4.92 Amp

Required int C.T. ratio = 4.92 / 4.37 = 4.92 / 2.52 3

Effect of Tap Changer

e.g. Assume 66kV +5%, -15%

Interposing C.T. ratio should be based on mid tap position

Mid Tap (-5%) = 62.7 kV

Primary current (15 MVA) = 138 Amp

Secondary current = 4.6 Amp

 Interposing C.T. ratio required = 4.92 / 4.6 3

( / ) = 4.92 / 2.66 May also be expressed as : 5 / 2.7

To differential relay S2 S1 P1 P2 REF S2 S1 P1 P2 S2 S1 P1 P2 a2 a1 A1 A2

Combined Differential and

Restricted Earth Fault Protection

25MV A 11KV 63MV A 132KV 1600/5 300/5 50MV A 33KV 1000/5 4.59 2.88 10.33 2.88 5.51 5 5

All interposing C.T. ratio’s refer

Dy1(-30°) R R R Yd11(+30°) Interposing CT provides : _{Vector correction}  _{Ratio correction}

Differential element Numeric Relay Virtual interposing CT Vectorial correction Ratio correction Virtual interposing CT

Integral Vectorial and Ratio

Compensation

Power transformer

Twice Normal Flux Normal Flux Normal No Load Current No Load Current

Transformer Magnetising

Characteristic

m



m



+

-

V

m

I

Magnetising Inrush Current

Steady State

m





_m

2

V

I

Magnetising Inrush Current

Switch on at Voltage Zero - No residual flux

Transformer Differential Protection

Effect of Magnetising Current

 Appears on one side of transformer only

 _{Seen as fault by differential relay}

 _{Normal steady state magnetising current is less than}

relay setting

 _{Transient magnetising inrush could cause relay to}

Transformer Differential Protection

Effect of Magnetising Inrush

 SOLUTION 1 : TIME DELAY



Transformer Differential Protection

Effect of Magnetising Inrush

 SOLUTION 2 : 2ND (and 5TH) HARMONIC RESTRAINT





A

C

B

Typical Magnetising Inrush

Waveforms

Nominal turns ratio Fault turns ratio Current ratio

- 11,000 / 240 - 11,000 / 1 - 1 / 11,000

Requires Buchholz relay

CT E Shorted turn Load

Inter-Turn Fault

100 80 60 40 20 0 5 10 15 20 25 10 8 6 4 2 Fault current in short

circuited turns

Primary input current Fault current (multiples of rated current) Primary current (multiples of rated current)

Turn short-circuited (percentage of winding)

Interturn Fault Current / Number

of Turns Short Circuited

Non-electrical Protection

Buchholz Protection

Pressure Relief Protection

Sudden Pressure Protection

_{Winding Temperature Protection}

Oil Temperature Protection

Buchholz Protection

_{The function of the relay is to detect an abnormal condition}

Buchholz operate when:

_{Gas produced from the transformer due to fault}

_{An oil surge from the tank to the conservator}

Pressure Relief Device

Sudden Pressure Protection

_{Used to detect sudden increase in gas pressure caused by}

internal arcing

Oil Temperature Protection

_{Typically sealed spiral-bourdon-tube dial indicators with}

5 x internal pipe diameter (minimum) 3 x internal pipe diameter (minimum) Transformer 3 minimum Oil conservator Conservator

Petcock Counter balance weight Oil level From transformer Aperture adjuster Deflector plate Drain plug Trip bucket To oil conservator Mercury switch Alarm bucket

Buchholz Relay

Transformer protection



_{Over fluxing protection}

Over fluxing arises principally from the

following system conditions.



_{High system voltage}



_{Low system frequency}



_{Geomagnetic disturbances}

The latter result in low frequency earth

currents circulating through a transmission

system.

Overfluxing

 _{Generator transformers}  _{Grid transformers}

Usually only a problem during run-up or shut down, but can be caused by loss of load / load shedding etc.

Flux



 _{Effects of overfluxing :}

 Increase in magnetising current

 Increase in winding temperature

 Increase in noise and vibration

 Overheating of laminations and metal parts (caused by stray flux)

 _{Protective relay responds to V/f ratio}

 Stage 1 - lower A.V.R.

m



2

Ie

m



V = kf



Overfluxing Basic Theory

 _CAUSES







 _EFFECTS





V f



K

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

Settings

Pick-up 1.5 to 3.0 i.e. 110V x 1.05 = 2.31 50Hz

DT setting range 0.1 to 60 seconds

1000 100 10 1 1 1.1 1.2 1.3 1.4 1.5 1.6 M = Operating time (s) V Hz K = 63 K = 40 K = 20 K = 5 K = 1

t = 0.8 + 0.18 x K

(M - 1)2

V/Hz Characteristics

Over temperature

 _{Generally regarded as overload protection also deals}

with failure of or interference with pumps and fans or shutting of valves to pumps

 _{Winding hot spot temperature is the main issue, but both}

oil and winding temperature are usually measured and used to:

 _{… initiate an alarm}

 _{… trip circuit breakers}

Over temperature

 _{Two temperatures must be monitored:}

 _{Winding temperature (‘WTI’) -(short}

 thermal τ) this can rise rapidly, without

 _{much of an increase in oil temperature}

 …Oil temperature (‘OTI’) -(long thermal τ)

 this can rise slowly to a critical point

 without an unacceptable winding

Typical alarm and trip levels



_{winding alarm - 90ºC to}

110ºC



_{„ winding trip - 110ºC to}

135ºC



_{„ oil alarm - 80ºC to 95ºC}



_{„ oil trip - 95ºC to 115ºC}

I load TD setting I load Top oil of power transformer Heater Thermal

replica Temperaturesensing resistor

Remote Temp. indication Local Off On Off On Alarm Trip Pump control Fan control

Overheating Protection

Typical Transformer Protection to

Feeders

Protection of Parallel

Transformer Feeders

2 1 _{2 stage}SBEF _{2 stage}SBEF 1 2

FEEDE R PILOT S PW P W

Transformer Feeders

 _{For use where no breaker separates the transformer from the}

feeder.

 _{Transformer inrush current must be considered.}

 Inrush is a transient condition which may occur at the instant of transformer energisation.

 _{Mag. Inrush current is not a fault condition}

 Protection must remain stable.

 _{MCTH provides a blocking signal in the presence of inrush}

Dy11 P2 P1 II III I S1 S2 MVTW 02 MFAC PILOTS _MBCI MBCI MCTH MCTH 23 24 C B A ii iii i P2 P1 S2 S1 C B A 25 27 26 28 23 24 25 27 26 28 27 18 19 + 1 17 17 17 19 18 19 17 19 23 24 25 27 26 28 23 24 25 27 26 28 18 17 S1

Protection

of Transformer Feeders

Power transformer

P540

Scheme