TRANSFORMER PROTECTION

Extent of damage

Fault level

Duration of fault

Winding failures

Voltage regulating load

changers

Transformer faults

Transformer bushing failure

Transformer core problem

Miscellaneous failures

Insulation breakdown

Ageing of insulation

Time

Temperature

Improve cooling system of possible

Condition leading to faults

Over heating due to excitation

Oil contamination%

& oil leakage

Reduced cooling

Reduced load

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

Differential relay

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

132KV/11KV POWER TRANSFORMER

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

D.C trip circuit supervision relay

Trip and lock out relay

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

220/132/11KV AUTO

TRANSFORMER

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

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

500/220KV AUTO

TRANSFORMER

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

Transformer buchhloz relay

Tap changer buchhloz relay

MECHANICAL PROTECTION:

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

Over current Relay Applied to a Transformer

51

HV2

Time

51

HV1

HV1

LV

HV2

51

LV

IF(LV)

1.2IF(LV)

IF(HV)

Current

Use of Instantaneous Over current Protection

Source

50

51

LV

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 magnetizing inrush current.

PROTECTED ZONE

HV

R

LV

·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

Differential Current

I1

BIAS

BIAS I2

I1 - I2

OPERATE

OPERATE

I1 - I2

I1 + I2

2

RESTRAIN

Mean Thro

Current

Bias = Differential (or Spill)Current Mean Through Current

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 Relay

Source

Protected zone REF

Delta winding cannot supply zero sequence current to system

Stability: Consider max LV fault level

Recommended setting: less than 30% minimum

earth fault level

`

Combined Differential and Restricted

Earthfault Protection

A2

A1 a1

a2

P1

P2 S1 S2

P1

P2

S1

S2

REF

P2

P1

S1

S2 To differential relay

Integral Vectorial and Ratio Compensation

Power transformer

Ratio correction

Vectorial correction

Differential

Virtual interposing CT element

Virtual interposing CT

In Zone Earthing Transformer

P1 P2

A1

A2

a1 a2 P2

P1

S2

S1

S2

S1 T2

T1

P1

P2

Three Winding Transformer

63MVA

25MVA

300/5 132KV 11KV

1600/5

50MVA 33KV

1000/5

4.59

2.88

5.51

5

10.33

2.88

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

T1

T2

N A B C

Inter-Turn Fault

E

CT

Shorted turn

Load

Nominal turns ratio Fault turns ratio

Current ratio

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

- 1 / 11,000

Requires Buchholz relay

Buchholz Relay Installation

3 x internal pipe diameter (minimum)

Conservator

5 x internal pipe diameter (minimum)

Oil conservator

3 minimum

Transformer

Buchholz Relay

Petcock

Alarm bucket

Mercury switch

To oil conservator

Trip bucket

Drain plug

Counter balance weight

Oil level

From transformer

Aperture adjuster

Deflector plate

Overfluxing Basic Theory

Causes

V = kf

2m

m

Low frequency

High voltage Ie

Geomagnetic disturbances

Effects

Tripping of differential element (Transient overfluxing)

Damage to transformers (Prolonged overfluxing)

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/Hz Overfluxing Protection

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/H CHARACTERISTIC:

Over-fluxing Relay

Ex

AVR

G

RL

VT

THERMAL OVERLOAD:

· EFFECT OF OVER LOAD ON TRANSFORMER INSULATION LIFE:

Overheating Protection

I load

TD setting

Trip

Alarm

Top oil of power transformer

I load

On

Off

Fan control

On

Pump control

Off

Heater Temp. indication

Thermal

Temperature Local

Remote

replica sensing resistor

Overload Protection

· Overcurrent protection designed for fault condition

· Thermal replica provides better protection for overload

–

Current based

Time

–

– Flexible characteristics Single or dual time constant

–

– Reset facility Non-volatile

Current

Thermal Overload Oil Filled Transformers

Trip time (s)

10000

Single characteristic:

= 120 mins

1000

Dual characteristic

100

Single 10

1

2

3

4

5

6

characteristic:

= 5 mins

Current (multiple of thermal setting) ZA

DIGITAL RELAYS FOR TRANSFORMER THERMAL WINDING PROTECTION