Siemens Interview Questions - Power generation and Electrical Engineering

1.What are the general causes of power system faults ?

1.Fault current - Healthy insulation in the equipment subjected to either transient over voltage of small time duration due to switching and lighting strokes,direct or indirect. Failure of insulation may be happened,resulting in very high fault current.This current may be more than 10 times the rated or nominal current of the equipment.

2.Insulation aging Aging of power equipment may cause breakdown of it even at normal power frequency voltage.

3.External Causes- External object such as birds,kite strings, or tree branch are considered as external cause of fault.These object may span one conductor and ground causing single line to ground fault or span two conductors causing phase-phase fault.

2.Fault effects on power system protection?

The faults must be cleared as fast as possible.Many equipment may be destroyed if the fault is not cleared rapidly.The dangers of the faults depend on the type of the fault,as example the three phase short circuit is the most dangerous fault because the short circuit current is maximum.Some of the effect short circuit as listed here

a) Due to overheating and the mechanical force developed by the faults,electrical equipment such as bars,generators,transformers will be generated. b)Negative sequence current arises form unsymmetrical faults will lead to overheating c)Voltage profile may be reduced to unacceptable limits as a result of faults.A frequency drop may lead to instability.

3)What is overcurrent protection?

Overcurrent protection is practical application of magnitude relays since it picks up when the of current exceeds some value. There are four types of over current relays. a)Instantaneous over current relays b)Definite Time over current relay c)Inverse time over current relay d)Direction over current relay

4)What is power swing in power system?

Some of the faults are transient and there is no need to operate the distance relays or to trip circuit breakers.These transients are classified as power swing.Some conditions must be verified to detect power swing which are listed below.

a)No earth fault detection b)No phase fault detection c)Change in impedance with time is three-phase and less than some setting value

5)What are the functions of differential relays?

Different relay is that relay which checks the different between the input and output current for a power system circuit. The difference between the currents may be in magnitude or in phase angle or in both.

For healthy operation,magnitude and angle difference must be zero.If there is a difference and that exceeds some value the relay will operate and associated circuit breaker will trip.

6.What is restricted Earth fault?

Restricted earth fault is that relay which can be defined as half differential relay.

The definition is due to its connection.This is relay compare current flow in a)The neutral path of the current transformer connected on the star side of power transfomer. b)Current flow in the current transformer connected on the star point of the power transformer.

For internal faults (between the two current transformer),there is a differential current which is transformed to volt by a very high shunt resister and this value of volt is sufficient for operation of the relay.

For external faults (outgoing feeder faults) the differential current in this case is zero and the relay will not operate and no tripping for circuit breaker will occur.

7.Definition of breaker failure relays?

Breaker failure relays are called back-up protection devices are applied as back up protection relays for the main relays.

8.What are the main causes of breaker failure ?

In the fault condition of any equipment it is expected that the circuit breaker of this equipment will be tripped by main protection relays of such equipment if there is a failure in tripping circuit of the circuit breaker or in the circuit breaker itself.

In this case circuit breaker will not tripped and so to isolate this fault the bus bar at which the which this equipment is connected must be completely isolated by tripping all the equipment to it by a delayed time.

Then the only connected equipment to this bus bar is the faulted one.In this case the faulted equipment has no current passing through it so,it can be disconnected mainly by isolates.