Q3. A star-connected, 3-phase, 10 MVA, 6.6 kV alternator isprotected by a differential current protection scheme using1000/5 A current transformer. The star point of the alternator isearthed through a resistance of 7.5 Q. If the minimum operatingcurrent for the relay is 0.5 A, Calculate the % of each phase ofthe stator winding which is unprotected against earth-fault whenthemachineisoperatingatnormalvoltage.

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Answered: current for the relay is 0.5 A,…

current for the relay is 0.5 A, Calculate the % of each phase of the stator winding which is unprotected against earth-fault when the machine is operating at normal voltage.

Power System Analysis and Design (MindTap Course List)

6th Edition

ISBN:9781305632134

Author:J. Duncan Glover, Thomas Overbye, Mulukutla S. Sarma

Publisher:J. Duncan Glover, Thomas Overbye, Mulukutla S. Sarma

Chapter14: Power Distribution

Section: Chapter Questions

Problem 14.8P

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Concept explainers

Fault Analysis

In an electrical engineering system, a fault is an abnormal electric current flowing through the system, and fault analysis is the study of these faults. For instance, a short-circuit is a fault where the live wire touches the ground or neutral wire. If a circuit is interrupted by a failure of a current-carrying wire then an open circuit fault occurs. The open-circuit faults are also called series faults. One or more phases or ground may be involved in a three-phase fault or may occur only between two phases. Current flows into the earth in case of a ground fault or earth fault. For most situations, the prospective short circuit current of a fault that is predictable can be calculated. To limit the loss of service because of a failure, the protective devices can detect fault conditions and operate circuit breakers and also other devices. All phases equally can be affected in a poly-phase system which is known as symmetric electrical fault. The asymmetric fault becomes more complicated to analyze if only some phases are affected. By using methods such as symmetrical components, the power system analysis of these types of faults is often simplified. The main objective of power system protection is to design, detect, and interrupt power system faults.

Question

Q3. A star-connected, 3-phase, 10 MVA, 6.6 kV alternator is
protected by a differential current protection scheme using
1000/5 A current transformer. The star point of the alternator is
earthed through a resistance of 7.5 Q. If the minimum operating
current for the relay is 0.5 A, Calculate the % of each phase of
the stator winding which is unprotected against earth-fault when
the
machine
is
operating
at
normal
voltage.

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