Protection of Power Transformer

Prepared by Sumeet Ratnawat NCA Indore

DIFFERENTIAL PROTECTION SCHEME

This scheme is employed for the protection of transformers against internal short circuits. It provides the best overall protection for internal faults.It compares the current entering the transformer with the current leaving the element.

If they are equal there is no fault inside the zone of protection.

If they are not equal it means that a fault occurs between the two ends.

Ip - primary rated current of transformer Is - secondary current of transformer CT of ratio Ip/1A at primary side and CT of ratio Is/1A at secondary side of the transformer.

3- Phase Transformer Differential Relay Connections

The secondaries of these both CTs are connected together in such a manner that secondary currents of both CTs will oppose each other. In other words, the secondaries of both CTs should be connected to same current coil of differential relay in such a opposite manner that there will be no resultant current in that coil in normal working condition of the transformer.

But if any major fault occurs inside the transformer due to which the normal ratio of the transformer disturbed then the secondary current of both transformer will not remain the same and one resultant current will flow through the current coil of the differential relay, which will actuate the relay and trip both the primary and secondary circuit breakers.

To correct phase shift of current because of star – delta connection of transformer winding in case of three phase transformer, the current transformer secondaries should be connected in delta and star.

PROTECTION AGAINST INCIPIENT FAULTS

INCIPIENT FAULTS: Faults which are not serious at the beginning but which slowly develops into serious faults are known as incipient faults.

BUCHHOLZ RELAY

It is a gas actuated relay. When a fault develops slowly,it produces heat, thereby decomposing solid or liquid insulating material in the transformer. The decomposition of the insulating material produces inflammable gases. The Buchholz relay gives an alarm when a specified amount of gas is formed. The analysis of the gas collected in the relay chamber indicates the type of the incipient fault.

There is a chamber to accommodate Buchholz relay,in between the transformer tank and the conservator.The Buchholz relay is a slow acting device,the minimum operating time is 0.1 s and the average time is 0.2 s.

The Buchholz relay also has another set of contacts operated by a float. These contacts stay open when the transformer tank is filled with oil. However, in case of leakage of oil or decomposition of oil, the float sinks causing the contacts to close.

Loss of oil will no doubt cause the transformer temperature to rise but does not warrant immediate tripping. Hence, normally these contacts are wired to an alarm which alerts the operator.

RESTRICTED EARTH FAULT PROTECTION

A percentage differential relay has a certain minimum value of pick up for internal faults. Faults with current below this value are not detected by the relay.Winding-to-core faults, which are single phase to ground type, involving high resistance,fall in this category.Therefore for such type of faults RESTRICTED EARTH FAULT PROTECTION is used.

An external fault in the star side will result in current flowing in the line CT of the affected phase and at the same time a balancing current flows in the NCT, hence the resultant current in the relay is therefore zero. So this REF relay will not be actuated for external earth fault.

But during internal fault the NCT only carries the unbalance fault current and operation of Restricted Earth Fault Relay takes place. This scheme of REF protection is very sensitive for internal earth fault of electrical power transformer. 

The protection scheme is comparatively cheaper than differential protection scheme.

Restricted earth fault protection is provided in  power transformer for sensing internal earth fault of the transformer. In this scheme the CT secondary of each phase of power transformer are connected together as shown.

Then common terminals are connected to the secondary of a Neutral Current Transformer or NCT. The CT connected to the neutral of transformer  is called Neutral Current Transformer .

OVER CURRENT PROTECTION

In an over current relay, there would be essentially a  current  coil. When normal  current flows through this coil, the magnetic effect generated by the coil is not sufficient to move the moving element of the relay, as in this condition the restraining force is greater than deflecting force.

But when the  current  through the coil increased, the magnetic effect increases, and after certain level of current, the deflecting force generated by the magnetic effect of the coil, crosses the restraining force, as a result, the moving element starts moving to change the contact position in the relay.

PROTECTION AGAINST OVERFLUXING

The magnetic flux increases when voltage increases. This results in increased iron loss and magnetizing current. The core and core bolts gets heated and the lamination insulation is affected.

Protection against overfluxing is required where overfluxing due to sustained overvoltage can occur.The reduction in frequency also increases the flux density and thus has the same effect of overfluxing.

The expression for flux in a transformer is given by Φ = K E/fWhere Φ = flux E = applied voltagef = frequency K is a constant.

To control flux, the ratio E/ f is controlled. When the ratio exceeds a threshold value, it has to be detected.

Electronic circuits with suitable relays are available to measure this ratio. Overfluxing does not require high speed tripping and hence instantaneous operation is undesirable when momentary disturbances occur. But the transformer should be isolated in one or two minutes at the most if overfluxing persists.

Thank you