What is CPMC in protection system?

It is combined protection, monitoring and control system incorporated in the static system.

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What is over fluxing protection in transformer?

If the turns ratio of the transformer is more than 1:1, there will be higher core loss and the capability of the transformer to withstand this is limited to a few minutes only. This phenomenon is called over fluxing.

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What are the problems arising in differential protection in power transformer and how are they overcome?

1. Difference in lengths of pilot wires on either sides of the relay. This is overcome by connecting adjustable resistors to pilot wires to get equipotential points on the pilot wires.

2. Difference in CT ratio error difference at high values of short circuit currents that makes the relay to operate even for external or through faults. This is overcome by introducing bias coil.

3. Tap changing alters the ratio of voltage and currents between HV and LV sides and the relay will sense this and act. Bias coil will solve this.

4. Magnetizing inrush current appears wherever a transformer is energized on its primary side producing harmonics. No current will be seen by the secondary. CT’s as there is no load in the circuit. This difference in current will actuate the differential relay. A harmonic restraining unit is added to the relay which will block it when the transformer is energized.

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What are the causes of over speed and how alternators are protected from it?

Sudden loss of all or major part of the load causes over-speeding in alternators. Modern alternators are provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the prime mover when a dangerous over-speed occurs.

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What is Electrical Fault? Write down the causes & classification of fault.

Fault:

A fault in an electrical equipment is defined as a defect in its electrical circuit due to which the current is diverted to the un-intended path.

Causes:

Generally caused by breaking of conductors or failure of insulation

Other causes:

• Mechanical failure

• Accidents

• Excessive stresses

Fault Classification:

• Open circuit - most common fault
• Single line to ground fault - most common fault
• Line to line fault
• Double line to ground fault
• Three phase fault – Dead short circuit
• Three phase to ground fault – Rare fault

* Faults cannot be eliminated but can be minimized

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