Merz price protection scheme is used to protect the transformer against earth fault and phase to phase fault. Merz price protection is used in the transformer of rating exceeding above 1 MVA.
Construction :-When this system is employed for the protection of the transformer the following points must be considered :-
1. Since, the primary and secondary winding have different in the magnitude of current depending upon their turn ratio of power transformer. Therefore, CTs of different turn ratio are selected to compensate the above effect.
2. There is usually a phase difference between the primary and secondary current of a 3-phase power transformer depending upon their connections. This effect can be compensated by employing the connections given in the following table of Fig. 1 :-
3. In the case of Tap changing transformer , differential current is flow through the relay evan in normal operating condition ; These effect can be eliminated by adjusting the turn ratio of CTs on the side of the power transformer provides with taps.
4. In normal load condition of transformer , the value magnetising current is very low. But , when the transformer is energised and then it taken out for service , the value of magnetising or in – rush current become very high for short period of time . Then , the relay detect this current as fault current and disconnect the supply.
The above effect can be eliminated by using second harmonic bias features . because , the initial magnetising current contain second harmonic components.
WORKING :-
Merz price protection is utilised to protect the power transformer from any phase to phase or any earth fault . For the given below fig. 2 of the power transformer , which is connected in delta – delta.
To compensate the phase angle of the current , the CTs are connected in star – star , because the winding connection of given power transformer is delta – delta.
In the normal operating condition , the current through the relay coil is zero. Therefore , no operation of the relay coil takes place. But , whenever any fault may be phase to phase fault or earth fault take place in that the corresponding relay coil will operate , which trip the circuit breaker and disconnect the faulty parts.
Advantage :-
- Simple in construction
- Reliable
- Low cost as compared to other schemes
- Accurate and precise
Hustle Engineering (EE) 