POWER TRANSFORMERS

PROTECTION

Rating of Some Power transformers used.

1. 220/132 KV 262A,437A 100MVA YNyn0 Auto

2. 132/11.5 KV 350A,2008A 80MVA YNyn0yn0

3. 132/11.5 70A,803A 16MVA YNyn0

The types of faults that the transformers are subjected to

are classified as :-

• Through Faults:- These are due to overload conditions

and external short circuits. Time graded O/C & E/F

relays are employed for external short circuit conditions.

Fuses are provided for Distribution transformers.

• Internal Faults:-

1) Electrical Faults : - Faults which cause immediate

serious damage such as phase to earth or phase to

phase faults, short circuits between turns of HV&LV

windings, etc.

2) Incipient Faults: - Which are initially minor faults,

causing slowly developing damage, such as a poor

electrical connection of conductors or breakdown of

insulation, etc.

The following relays are employed to protect the transformer against internal faults.

i) Differential relays

ii) REF relays

iii) Bucholz Relay

iv) Over fluxing relays

Differential Relays :-

Two basic requirements that the differential relay connections are to be satisfied are :

a) It must not operate for load or external faults.

b) It must operate for internal faults.

Differential Relay Improvement • Differential Relay has improved a lot with

the introduction of numerical relay

• Numerical relays use techniques such as harmonic

blocking, wave shape recognition • In case of electromagnetic and static

relays, in a typical delta-wye power transformer, the current transformer would be connected as wye-delta.

Setting Of 87T

Alarm

Delay Trip Delay

Start Up

10% 10 sec 20% 0 sec

ST 10% 10 sec 20% 0 sec

MT 10% 10 sec 20% 0 sec

Restricted Earth Fault Protection (REF) : -

This relay is operative only for the internal faults of the transformer and thus fast operating time can be achieved.

1. An external fault on the star side will result in current flowing in the line CT of the affected phase and a balancing current in the neutral CT and current in the relay is zero and hence relay is stable. During an internal fault, the line current on the line CT gets reversed and hence relay operates.

2. The arrangement of residually connected CTs on the delta side of a transformer is only sensitive to earth faults on the delta side because zero sequence currents are blocked by the delta winding.

For external faults no current flows through REF unless a CT gets saturated. Hence minimum pickup current setting is adopted (10% or 20% In) on REF relay. Based on the through fault current, the stabilizing resistor is set such that the relay will not operate for external fault when a CT gets saturated. This relay operates only for internal earth faults, instantaneously.

Setting Of 64R

Type Alarm Delay

Trip

Delay

ROA

Start Up HV

Low imp

10%

0 sec

60

ST HV High imp

2 V 5 sec 11V 0 sec

ST LV1&LV2

High imp

10V 5 sec 50V 0sec

MT HV High imp

4V 5 sec 21V 0 sec

MT LV High imp

13V 5 sec 64V 0 sec

Overcurrent Protection:

Step 1 Delay Step 2 Delay

Start up HV

150% 1.4sec 900% 0 sec

Start up LV

150% 1.2 sec 800% 0sec

ST HV 150% 1.4 sec 900% 0sec

MT HV 145% 1 sec 1200% 0sec

Earth fault Protection:

Step 1 Delay

Start up HV

20% 1.4sec

Start up LV

20% 1.2 sec

ST SEF

30A 1 sec

MT SEF LV

20% 1sec

Overfluxing Protection:

1. Overfluxing condition in a transformer can occur during system over voltage and/or under frequency.

Alarm

Delay

Trip Delay

Start up

237.16 KV

5sec 242KV

2000 sec

352KV

.143 sec

Other Protective devices employed

• Pressure Relief Value (PRV) • Winding Temperature• Oil Temperature • OLTC Buchholz

Buchholz Relays : -Whenever a fault in transformer develop slowly,heat is produced locally, which begins todecompose solid or liquid insulated materialsand thus to produce inflammable gas and oilflow. This phenomenon has been used in the gasprotection relay or popularly known as Buchholzrelay. This relay is applicable only to the so-calledconservator type transformer in which the

transformertank is completely filled with oil, and a pipe

connectsthe transformer tank to an auxiliary tank or

“Conservator” which acts as an expansion chamber.

Conservator Alarm

Trip

Transformer Tank

Figure shown as Bucholz relay connected into the pipe leading to the conservator tank and arrange to detect gas produced in the

transformer tank. As the gas accumulates for a minor fault

the oil level falls and, with it a float `F' which operates a

mercury switch sounding an alarm. When a more serious

fault occurs within the transformer during which intense

heating takes place, an intense liberation of gases results.

These gases rush towards the conservator and create a rise in

pressure in the transformer tank due to which the oil is forced

through the connecting pipe to the conservator. The oil flow

develops a force on the lower float shown as "V" in the figure

and overtrips it causing it contacts to complete the trip circuit of

the transformer breaker. Operation of the upper float indicates

an incipient fault and that of the lower float a serious fault.

THANK YOU