Bus Bar Protection Interview Questions and Answers:

1. Why use of single bus-bar arrangement is restricted to small and medium sized substations and small power stations ?

Ans. The glaring drawback of this system is that in case of fault on the bus-bars, whole of the supply is affected and all the healthy feeders are disconnected. Moreover, when maintenance is to be carried out on any of the feeder sections or on a part of bus-bar the whole supply is to be disrupted. Thus such an arrangement provides least flexibility and immunity from total shutdown. That is why single bus-bar arrangement is restricted to small and medium sized substations and small power stations.

2. Why is circuit breaker preferred over an air-break switch or an isolator for sectionalizing the bus-bars ?

Ans. At times air-break isolators were used in place of circuit breakers as bus-sectionalizer due to economy, but it must be remembered that any isolation affected by them must be affected under off-load conditions otherwise it may cause spark. It will be preferable to provide circuit breaker as a sectionalizing switch so that uncoupling of bus-bar may be carried out safely during load transfer. A double isolation is however necessary when the circuit breaker is employed as sectionalizing switch so that the maintenance work can be carried out on circuit breaker while the bus-bars are alive.

3. For what a bus coupler is used in main and transfer bus arrangement ?

Ans. In main and transfer bus arrangement, a bus coupler is usually used so that the change-over from one bus-bar to the other can be carried out under load condition.

4. Why double bus double breaker arrangement is rarely used at substations ?

Ans. Because of its very heavy initial as well as maintenance costs, double bus double breaker arrangement is rarely used.

5. Name the most commonly used schemes for bus-bar protection.

Ans. The most commonly used schemes for bus-bar protection are : 1. Backup protection, 2. Frame leakage protection and 3. Differential overcurrent protection.

6. Why is it necessary to isolate the switchgear framework from lead cable sheaths, cable boxes and conduit fittings in frame leakage protection of bus-bars ?

Ans. It is necessary to isolate the switchgear framework from lead cable sheaths, cable boxes and conduit fittings so that when a leakage to the framework occurs, the only path for leakage current is through the connection from the framework to the earth.

7. What is the drawback of differential overcurrent protection for bus-bars and how is it overcome ?

Ans. The main drawback of differential overcurrent protection is the difference in the magnetic conditions of the iron-cored CTs which may cause false operation of the relay at the time of an external fault. Even with identical CTs having large iron cores to avoid the saturation with maximum fault currents the dc transient component creates problem due to its slow decay. Biasing of differential relays improves the stability considerably but does not solve the problem completely.

Better discrimination between internal and external faults can be had if high impedance bus differential relay is used in place of usual low impedance relay. High impedance relay is an overcurrent relay with a series resistance. Such a relay remains stable against spill currents due to external faults or CTs inaccuracies.