TRANSFORMER PROTECTION

Extent of damage

Fault level

Duration of fault

Transformer faults

Winding failures

Voltage regulating load

changers

Transformer bushing failure

Transformer core problem

Miscellaneous failures

Condition leading to faults

Insulation breakdown

Ageing of insulation

Time

Temperature

Improve cooling system of possible

Reduced load Over heating

due to excitation

Oil contamination%

& oil leakage

Reduced cooling

FAULT OF TRANSFORMER

Earth fault on H.V external connection Phases to phase fault on H.V external connection Internal earth fault on H.V windings Internal phase to phase fault on H.V windings. Short circuit between turns L.V windings. Earth fault on L.V external winding Phase to phase fault on L.V external connection. Internal earth fault on L.V windings Internal phase to phase fault on L.V winding Short circuit b/w turn L.V windings Earth fault on tertiary windings Short circuit b/W turns tertiary windings Sustained system phase to phase fault Sustained system earth fault

132KV/11KV POWER TRANSFORMER

Differential relay

L.V side three phase over current and earth fault relay

H.V side three phases over current and earth fault relay

D.C trip circuit supervision relay

Trip and lock out relay

220/132/11KV AUTO TRANSFORMER

Percentage biased Transformer

differential relay

High REF impedance Circulating current

differential protection

HV side over current and earth fault relay

LV side over current and earth fault relay

Over excitation relay

Thermal over load relay

Overall percentage biased differential

relay

L.H&H.V Connection circulating protection

Tertiary over current protection

D.C trip circuit supervision relay

500/220KV AUTO TRANSFORMER

Percentage biased Transformer

differential relay

High REF impedance Circulating current

differential protection

HV side over current and earth fault

relay

LV side over current and earth fault

relay

Over excitation relay

Thermal over load relay

Overall percentage biased differential

relay

L.H&H.V Connection circulating protection

Tertiary over current protection

D.C trip circuit supervision relay

Over voltage relay

MECHANICAL PROTECTION:

Transformer buchhloz relay

Tap changer buchhloz relay

Winding Temperature

0IL temperature

Pressure relieve valve

• Factors:

• The shape, magnitude and duration of the inrush current depend on the factors:

• Size of power transformer

• Source Impendence

• The magnetic properties of the core i.e. saturation density

• The remanence of core

• Resistance in power system from source to transformer.

• The moment when transformer is switch on.

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 operate

• The vector group shows the connection of windings of transformer and numerical index (hour numbers) for displacement of vector of two star voltages.

• Capital Letter DY11 Small letter ( clock dial reference)

• The first capital letter donates the connection of high voltage winding of transformer

• The small letter represent the connection of low voltage secondary winding of transformer

• Yy0d5

•

• The first capital letter Y is referred to H.V or primary winding, the second letter y is referred as secondary winding and third letter is referred as tertiary winding.

• Primary winding is taken as phase referred ‘O’ means that phase angle b/W H.V and M.V winding is zero. Whereas ‘5’ denotes that phase angle b/W H.V and tertiary winding is 150 (5x30)

OVER CURRENT PROTECTION

As it names implies, relay will pick up when it exceeds its present value

TYPES:

The types of over current relay are based on the relay characteristics over can be classified into three groups.

• Definite current or instantaneous

• Definite time

• Inverse time

Vécurent Relay Applied to a Transformer

HV2

51 51 51

LV HV1

Time LV

HV1 HV2

Current IF(LV)

1.2IF(LV) IF(HV)

Use of Instantaneous Overcurrent Protection

Source LV

50

51

Differential Protection

• 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 thro’ 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 inrush current.

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

• (3) Zero sequence currents

Biased Differential Scheme

Bias = Differential (or Spill)

Current Mean Through Current

BIAS BIAS I2 I1

OPERATE I1 - I2

Differential

Current

OPERATE

RESTRAIN

Mean Thro Current

I1 + I2

2

I1 - I2

Restricted E/F Protection Low Voltage Windings (1)

A B C N

LV restricted E/F

protection trips both HV and LV breaker

Recommended setting : 10% rated

Restricted E/F Protection Low Voltage Windings (2)

A B C N

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

Delta Winding Restricted Earth Fault

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 level

Protection of Auto-Transformer

by High Impedance Differential

Relays (2)

(b) Phase and Earth Fault Scheme

A

B

C

a b c

87 87 87

n

`

Combined Differential and Restricted Earthfault Protection

To differential relay S2

S1

P1

P2 REF

S2

S1 P1

P2

S2 S1 P1 P2 a2 a1 A1 A2

Integral Vectorial and Ratio Compensation

Power transformer

Differential element Virtual interposing CT

Vectorial correction

Ratio correction

Virtual interposing CT

In Zone Earthing Transformer P2 P1

a2 a1

A2 A1 P1 P2

S1 S2

P2 P1 S1 S2 T1 T2

Three Winding Transformer 25MVA 11KV

63MVA 132KV

1600/5 300/5

50MVA 33KV

1000/5

4.59

2.88

10.33

2.88

5.51

5

5

All interposing C.T. ratio’s refer to common MVA base (63MVA

Transformer Magnetising Characteristic

Twice

Normal

Flux

Normal

Flux

Normal No

Load Current

No Load Current at

Twice Normal Flux

Parallel Transformers A N T1

T2

C B

Inter-Turn Fault

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

Buchholz Relay Installation

5 x internal pipe

diameter (minimum)

3 x internal pipe

diameter (minimum)

Transformer

3 minimum

Oil conservator

Conservator

Buchholz Relay

Petcock

Counter balance weight

Oil level

From transformer

Aperture adjuster

Deflector plate Drain plug

Trip bucket

To oil conservator

Mercury switch

Alarm bucket

Low frequency

High voltage

Geomagnetic disturbances

m

Tripping of differential element (Transient overfluxing)

Damage to transformers (Prolonged overfluxing)

2

I e

m Causes

Effects

V = kf

Overfluxing Basic Theory

EFFECTS OF OVER FLUXING:

• Increase in magnetizing current

• Increase in winding temperature

• Increase in noise and vibration

• Overheating of laminations and metal parts (cause by stray flux)

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

V/Hz Overfluxing Protection

V/H CHARACTERISTIC:

RL AVR

Ex

VT

G

Over-fluxing Relay

THERMAL OVERLOAD:

• EFFECT OF OVER LOAD ON TRANSFORMER INSULATION LIFE:

I load

TD setting

I load Top oil of power transformer

Heater

Thermal replica

Temperature sensing resistor

Remote

Temp. indication Local

Off

On

Off

On

Alarm

Trip

Pump control

Fan control

Overheating Protection

• Overcurrent protection designed for fault condition

• Thermal replica provides better protection for overload

– Current based – Flexible characteristics – Single or dual time constant – Reset facility – Non-volatile

Current

Time

Overload Protection

ZA

10000

1000

100

10

1 2 3 4 5 6

Trip time (s)

Current (multiple of thermal setting)

Single characteristic:

= 120 mins

Dual characteristic

Single characteristic:

= 5 mins

Thermal Overload Oil Filled Transformers

DIGITAL RELAYS FOR TRANSFORMER THERMAL WINDING PROTECTION