POWER TRANSFORMER PROTECTION

By

Bhallamudi SridharDy.Manager, Bangalore SSDate:18-07-2008

Introduction

• The power transformer is the most important links in power system

• A transformer fault will cause a large interruption in power supply and also cause damage to power system stability.

Need of protection

• To limit the damage in case of fault• To limit the damage in case of fault• To isolate the faulty element to protect the

power system.

Types of faults in Transformer

INTERNAL FAULTS

Earth faultsPhase to Phase faults Inter turn faultsCore faults

EXTERNAL FAULTS

Over-load System faults(phase to

phase/ phase to earth) Over voltage Reduced system

frequency.

Classification of transformer protection

• Protection against electrical faults• Protection against abnormal conditions.• Transformer protective devices

Protection against electrical faults

• Unit protection -against internal faults

1.Differential relay

2.Restricted earth fault relay

• Non unit (back up) protection-against external faults

1.Directional over current and earth fault

Differential protection

I1-I2I1 I2

I1 I2

I1 I2

Principle of operation

• During normal condition and external fault condition the protective system is balanced

• This balance is disturbed for internal faults.• The differential current flows through the relay

coil and it operate.

Peculiarity of the Transformer Differential Protection

1. Vector group (e. g. Yd5) 2. Different CT´s, tap changer, magnetising current

vector group adaptation restraint function (stabilising) is necessary

3. Dynamic currents I = f (Irestr.)

Irestr. = |I1| + |I2| inrush current overflux (overexcitation) CT saturation during external faults

blocking via harmonics

saturation detector

trip region

-I2

I1

150°current transformer

tap changer,CT adaptation

(will be eliminated)magnetising current

I

I

ITr, IRestr.

Example of an Inrush CurrentA unit transformer (IN = 396 A) was switched on from the high voltage side

Rush Stabilisation

Recognise inrush condition by evaluating the ratio 2nd harmonic I2HAR to basic wave IDiff.

Time limit for cross-block. Reliable reaction to the inrush condition with cross-block.Trip of a short circuit after the set time delay.

Recognise over-excitation by evaluating the ratio 3rd or 5th harmonic to basic wave

filter window1 cycle L1-block

L2-block

L3-block

Cross-block = No (phase separate blocking)

Cross-block = Yes (blocking of all phases)

Idiff, L2 > trip blocking

O R1

t

t1P 2P 3P

iRUSH = iDiff

15 % setting valueblock

no block

I2HAR

Idiff

Inrush currentin one phase

L1-block

L2-block

L3-block

Idiff, L1 > trip blocking

Idiff, L3 > trip blocking

IDiff > trip blocking for a limited time

&

&

&

Tripping Characteristic

flexible adaptation to various transformers, e.g. with tap changer or different main CTs

high stability against external faults with CT saturation

fast tripping for solid short-circuits within one period

Restricted earth fault protection

• REF is used for protect the earth fault inside the transformer• Protection is provided using bushing core CTs at 400KV,

220KV and neutral.• All the CTs has been connected in parallel• as the net current always zero for normal and through

faults.

• Whenever any earth fault occur in the REF zone of transformer the vector sum of currents come into unbalance and resultant current will flow through REF relay and it will operate

 

 

 

CT ICT

CT  

   

 

 

I1 I2

Directional over current & EF fault

• It is provided as a back up protection to prevent the transformer subjected to prevent high mechanical stresses during through faults.

• Inverse definite minimum time relays with directional elements as well as over current instantaneous elements are used.

• The relay are always made look towards the transformer.

• This protection is to prevent the transformer from high system voltage and low frequency condition.

• The flux density of the transformer is directly proportional to the V/f

• Power transformers are designed to with stand Vn/fn =1.1 where Vn is the normal highest r.m.s and fn is the standard frequency.

Protection against abnormal condition-Over fluxing protection

Over fluxing protection

V/f/Vn/fn 1.1 1.2 1.25 1.3 1.4

Duration of withstand limit(min)

continues

2 1 0.5 0

Transformer protective devices- non electrical

• Besides the electrical relay , a power transformer has protected with following safety and monitoring devices.

1. Buchholz relay2. Pressure relief device3. Winding / oil temperature indicators

Buchholz protection

• This is gas generated relay connected in between the main tank and conservator

• This relay is generally used to detect the following faults:

1. Core faults2. Inter turn faults3. Tank faults4. Hot spot

Pressure relief device protection

• To prevent the pressure rise of inside the transformer above 10 PSI during

1. High over load peaks2. Prolonged overloads3. Arcing faults within oil.• Operation of the PRD will cause either alarm

or instantaneous trip of the transformer.

Winding / oil temperature indicators

• The function of temperature indicator is to indicate and control the temperature of the oil in the transformer.

• Transformer temperature increasing due to over loading or internal faults.

• Typical settings of temperature indicators are as follows.

`

• O T I • 85ºC

• W T I• 95ºC