Tuesday, 2 July 2019

Interview Questions Related to Faults in Power System

Interview Questions Related to Faults in Power System

Question and Answer

01. What do you mean by fault in a power system ?
Ans. A fault in a line is any failure which interferes with the normal flow of current in the line.

Interview Questions Related to Faults in Power System

02. What are the causes of faults in a power system ?
Ans. A fault may occur on a power system due to a number of reasons such as (a). Lightning (b). High-speed winds (c). Snow (d). Insulation failure (e). Falling of tree (f). Falling of tree on a line (g). Birds shorting the line etc.

03. How does circuit current behave on the occurrence of fault in a power system ?
Ans. The current which flows in different parts of the power system immediately after the occurrence of fault is much greater than that flowing a few cycles later. The exponential decay of fault current from an initial ''transient' value to a final 'steady state' value is due to the chaning impedance offered to the fault by the system.

04. What is the need of reactors in the line of the power system ?
Ans. With the increasing trend towards interconnected system, the fault level (i.e., the power available to flow into a fault) is rising. The circuit breakers which were initially able to handle fault conditions may now be unable to do so. To restore the fault level to a value which the existing circuit breaker can handle, limiting reactors are connected in series with the system at suitable points.

05. What are fault calculation ?
Ans. Fault calculations are made to determine fault currents for various types of faults at various locations in the power system. The data obtained from fault calculations also serves to determine the setting of relays which control the circuit breakers.

06. What is a symmetrical faults ?
Ans. A symmetrical fault occurs when all the three conductors of a three-phase line are brought together into a short-circuit condition. The symmetrical faults rarely occur in the power system. Experience shows that hardly 5% of overhead line faults are symmetrical faults.

07. What is origin of the name symmetrical fault ?
Ans. A symmetrical fault is balanced and symmetrical in the sense that the system remains balanced even after the occurrence of the fault. In other words, when a symmetrical fault occurs, the fault currents in the three lines are equal in magnitude with 120° displacement. For this reason, in symmetrical fault calculations, only one phase is consideres; the conditions in the other two phases being similar.

08. What is an unsymmetrical fault ?
Ans. Unsymmetrical faults involve only one or two phases. When an unsymmetrical fault occurs, the faults currents in the lines become unequal with unequal phase displacement. In other words, unsymmetrical fault causes an unbalance among the phases. For this reason, each phase has to be treated individually for fault calculations.

09. What are the ways in which an unsymmetrical fault may occur ina power system ?
Ans. There are three ways in which unsymmetrical faults may occur in a power system viz
(a). Single line to ground fault.
(b). Line to line fault
(c). Double line to ground fault.

10. Which is the most common fault in a power system ?
Ans. The most common fault in a power system is the three phase short circuit fault (i.e., symmetrical fault). When such a fault occurs, largest fault currents flow in the lines. For this reason, ratings of circuit breakers are circuit breaker are decided on the basis of symmetrical short-circuit calculations.