Protection of Power Transformer

Mr.Siddharam Kantoli

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.

DIFFERENTIAL PROTECTION SCHEME

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.

Biased Differential Protection Scheme

Construction:

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.

Operation:

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.

Limitations of differential protection of transformer:

1) Due to the magnetization characteristics of the CTs used, the ratio

errors change with respect to the circulating currents.

2) The pilot wires used may vary in length due to which the unbalance in

the secondary circuit parameter (resistance) is created that results in

improper scheme.

3) During heavy short circuit conditions the high currents create

saturation of the flux in core of CTs that lead to abnormal relaying or

unexpected behavior of the relaying circuit.

4) Tap changing may lead to change in settings & improper operation.

5) Inrush of magnetizing current may lead to inadvertent operation &

hence the settings are done for higher values of fault current (higher

imbalance) due to which accuracy of sensing & operation is decreased.

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.

Working Animation

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.

Over current protection for a transformer

Fig. shows two numbers of phase fault over-current relays and one ground-fault over-current relay for providing over-current protection to the star-delta transformer.

The neutral current under load conditions is quite small. The neutral current is essentially because of load unbalance.

EARTH FAULT

The basic working principle of thermal relay is that, when a

bimetallic strip is heated up by a heating coil carrying over

current of the system, it bends and makes normally open

contacts to close and close the Trip Circuit of C.B.

Thank You!!!