Types of Transformer Protection

Before going into the actual topic that is types of transformer protection, let us see why it is an important topic to discuss.

Why Transformer protection is an important aspect?

The main objective of transformer protection is to detect abnormal conditions and protect the transformer from internal faults and external faults nothing but through faults.

Transformer Protection Types:

Transformer protection basically divided into two types. One is Electrical Protection and it is designed based on Electrical parameters like Current, Voltage, Frequency, and Impedance. The second type of protection is Mechanical Protection and it is designed based on Mechanical parameters like Temperature, Pressure, Density, etc.

1. Transformer-Electrical Protection Types:

• Over Current/Earth Fault-50/51Protection

• Under Impedance/Distance  relay Protection-21

• Differential Current Protection-87

• Restricted Earth fault Protection-64H

• Three Phase Overload Protection-49

• Over Fluxing Protection-24

• Over Voltage-59/Under Voltage-27 Protection

2. Transformer-Mechanical Protection Types:

• Oil Temperature Indicator-26

• Winding Temperature Indicator-49

• Oil Pressure Relief-PRV-63PRD

• Gas Accumulation using Gas Accumulation Relay (Buchholz Relay)-63

Transformer Electrical Protection:

1. Over Current/Earth Fault Protection:

 HRC fuses are used for small transformers rating below 10 MVA. Above that overcurrent relays are used as primary protection if differential protection is not used. If differential protection is used Overcurrent protection is used as backup protection.

Overcurrent Earth fault protection operates when the current in any one of the phases is exceed the maximum value of the setting.

Types of OC protection:

50 – Instantaneous Overcurrent

51 – Time Overcurrent Phase

50G – Ground Fault Instantaneous

51G – Time Ground Fault/ Standby Earth Fault

Numerical Overcurrent (50)/Earth fault (51) protection relay settings

Overcurrent relays should not trip in the following conditions.

1. During the period of Magnetising current inrush– During switching of load, the load take sudden surge of magnetising inrush current.
2. Short time load- Cold load pickup-To avoid tripping during switching on Circuit Breaker.
3. Through Faults-For load side faults usually 3-phase short circuit OC relay should not pick up before operating any other protection relays.
4. Three phase over load– For permitted values of three phase overloads the Over current rely should not pickup.

Various Overcurrent/Earth fault connection schemes

2. Under Impedance/Distance  Relay Protection:

OC protection is not suitable for providing backup protection for transformers connected to networks. In this case Under Impedance or Distance relay protection is required when there is a large difference between the maximum and minimum short circuit fault MVA.

3. Differential Current Protection:

Differential protection relay compares the phase currents on both sides of the transformer to be protected. If the differential current of the phase currents in one of the phases exceeds the setting of the stabilized operation characteristic or instantaneous protection stage of the function, the relay provides an operating signal.

Over-current protection is used as a backup for differential protection.

Differential protection protects the transformer from

1. Winding short circuit faults.

2. Inter-turn faults.

The application of Differential current protection for Generator Transformer (GT) and Unit transformers (UT) is shown in below fig.

A detailed explanation of Differential protection using a relay is explained here.

Differential protection Relay

4. Restricted Earth Fault Protection:

Differential protection provides Protection for only 80 percent of the winding and the rest of the protection will be provided by REF. It protects the transformer winding from earth fault.

REF provides protection in the zone between Transformer Star side winding and its Neutral Terminal which is earthed. It senses the fault current only in this particular zone so it is called Restricted protection.

• The earth fault current is sensed from the bushing CTs of the phases and neutral side CT which may be either bushing CT or by the external CT in case of resistive earthing.

• REF protection is implemented using a set of Phase Current Transformers and Neutral Current transformer along with Metrosil and Stabilising Resistors explained in detail here.

High Impedance Restricted Earth Fault Protection [64H]

  5. Three Phase Overload Protection :

Provides protection against possible damage during overload conditions.

Overload protection is provided by a Thermal overcurrent relay (49). This relay uses the Thermal time constant of the transformer and maximum allowable continuous overload current (I_MAX) in the inverse time characteristic.

6. Over Fluxing Protection:

Over Fluxing or Over Excitation leads to transformers overheating and getting damaged. A V/Hz Over excitation relay is used for transformers that are likely to operate at a too high voltage or at a too low frequency. Generator transformers can be overexcited during acceleration or deceleration of the turbine. The maximum allowable V/F ratio is 1.1 times the rated V/F ratio.

7. Over Voltage Protection:

Over and Under voltage protection relay offers protection against abnormal phase voltage conditions.

The complete protection details using Over(59)/Under(27) voltage relay

Transformer Mechanical Protection:

1. Oil Temperature Trip Relay:

This relay senses oil temperature when an internal fault like insulation failure in one phase of winding and shorted or a local hotspot is generated, so the produced temperature is transferred to oil, and oil gets heated. When the oil temperature increases and exceeds a set value, this oil temperature relay operates.

2. Winding Temperature Trip Relay:

Winding temperature indicator indicates temperature, which is proportional to load current plus top oil temperature of the transformer. The indicator is provided with two/four mercury switches. One is used for alarm, second is used for the trip, third for fan control, and fourth for Pump. The temperature settings of the switches are different from each other for discrimination.

When the temp exceeds the max. Set temp the trip contact actuates and de-energizes the transformer.

3. Pressure Relief Valve (PRV):

In case of a severe fault in the transformer, the internal pressure may build up to a level, which may result in an explosion of the tank. To avoid such a contingency a Pressure relief valve is fitted on the transformer.  Normally, the pressure relief device valve PRD will be mounted on top of the transformer. 

If pressure arises inside a transformer and exceeds a pre-set pressure limit, the pressure safety valve PRD opens its valve clap, which is held by a spring and releases the internal pressure until it declines. After a decrease in the pressure, the pressure valve clap moves back to its original position and closes completely.

4. Buchholz Relay:

It is basically a Gas-actuated relay. During the normal operation of the transformer, the Buchholz relay is completely filled with oil. In the event of any internal fault, gas bubbles are produced in the transformer tank and gas is accumulated in the relay chamber on the way to the conservator.

This is a double element relay, which detects minor or major faults in a transformer. The first (alarm) element will operate after a specified volume of gas has accumulated. The second (trip) element will operate if a rapid loss of oil occurs in the event of serious faults inside a transformer.