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November 13, 2012

Give the limitations of Merz Price protection.

Figure: Merz-Price protection for star connected alternator.

Since neutral earthing resistances are often used to protect circuit from earth-fault currents, it becomes impossible to protect the whole of a star-connected alternator. If an earth-fault occurs near the neutral point, the voltage may be insufficient to operate the relay. Also it is extremely difficult to find two identical CT’s. In addition to this, there always an inherent phase difference between the primary and the secondary quantities and a possibility of current through the relay even when there is no fault.

November 2, 2012

What are demerits of MOCB (Minimum Oil Circuit Breaker)?

Demerits of MOCB:
  • Short contact life
  • Frequent maintenance 
  • Possibility of explosion
  • Larger arcing time for small currents
  • Prone to restricts

What are the disadvantages of MOCB over a bulk oil circuit breaker?

Disadvantages of MOCB ( Minimum oil circuit breaker) over a bulk oil circuit breaker:
  • The degree of carbonization is increased due to smaller quantity of oil
  • There is difficulty of removing the gases from the contact space in time
  • The dielectric strength of the oil deteriorates rapidly due to high degree of  carbonization.

What are the advantages of MOCB over a bulk oil circuit breaker?

Advantages of MOCB (Minimun Oil circuit Breaker) over a bulk oil circuit breaker:
  • It requires lesser quantity of oil 
  • It requires smaller space
  • There is a reduced risk of fire
  • Maintenance problem are reduced

What are the methods of capacitive switching?

Methods of capacitive switching:
  • Opening of single capacitor bank.
  • Closing of one capacitor bank against another.

What is CPMC in protection system?

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

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.

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.

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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