Question

a) In reference to a single phase circuit, briefly explain the response of relay during an in-zone, and through fault conditions.

b) Explain the four factors that reliability of a transformer is dependent upon.

c) List four methods of grounding an Electrical system

d) Briefly, explain with a protection diagram how phase fault and earth fault relay(s)can be used to detect a stator short circuit faults on a polyphase induction motor

Answer 1

a)Response of relay during an in zone and through fault conditions

The differential scheme generates a well defined and closed zone.This zone consists of everything between the two CTs.Thus any fault between the two CTs is termed as internal fault.Faults other than this are "internal faults" or"through faults" Therefore a differential scheme only respond to the internal faults only and restrain from tripping from external faults.Thus an ideal differential scheme has to respond on the smallest internal fault but should restrain from tripping from the largest externl faults.But because of non ideal nature of the various components of the protection system the ideal condition is very difficult to achieve.Two errors affect the functioning of the Cts are ratio error and phase angle error."Through fault "current goes on increasing as various imperfections in the CTs are magnified,because of this spill current current gets increased,That is why as the through current gets increased there comes a stage when the spill current ,due to the difference between the secondary currents of the two CTs exceed the pick up value of over current relay in the spill path.This causes the relay to operate,disconnecting the other equipment from the system.

b)Four factors on which reliability of transformer is dependent upon:

1)development of connection

2)external and internal faults

3)Inrush phenomenon

4)Protection from harmonics

c)

1)Solid neutral earth system

2)Resistance neutral earth

3)resonant earthing system

4)grounding transformer earthing

d)

Protection against faults and ground faults can be provided using fuses or over current relay depending upon voltage rating and size of the induction motor.

8 CT HV Induction motor R. IPS Phase fault PS relays TS TS Trip coil R. Trip battery Rgnd TS Ground fault relay Figure 9.4 Phase fault and ground fault protection of HV induction motor. Induction motor Three units of percentage differential relay

The above fig shows the differential protection of the motor.