Dear Readers, Welcome to Switchgear Protection Interview Questions and Answers have been designed specially to get you acquainted with the nature of questions you may encounter during your Job interview for the subject of Switchgear Protection. These Switchgear Protection Questions are very important for campus placement test and job interviews. As per my experience good interviewers hardly plan to ask any particular questions during your Job interview and these model questions are asked in the online technical test and interview of many IT & Non IT Industries.
To detect the fault and initiate the operation of the circuit breaker to isolate the defective element from the rest of the system, thereby protecting the system from damages consequent to the fault.
Whenever a short-circuit occurs, the current flowing through the coil increases to an enormous value. If protective relays are present , a heavy current also flows through the relay coil, causing it to operate by closing its contacts.The trip circuit is then closed, the circuit breaker opens and the fault is isolated from the rest of the system. Also, a low voltage may be created which may damage systems connected to the supply.
Are those which are directly protected by a protective system such as relays, fuses or switchgears.If a fault occurring in a zone can be immediately detected and or isolated by a protection scheme dedicated to that particular zone.
A unit protective system is one in which only faults occurring within its protected zone are isolated.Faults occurring elsewhere in the system have no influence on the operation of a unit system.A non unit system is a protective system which is activated even when the faults are external to its protected zone.
Is the protection in which the fault occurring in a line will be cleared by its own relay and circuit breaker.It serves as the first line of defence.
Is the second line of defence , which operates if the primary protection fails to activate within a definite time delay.
Induction type non-directional over current relay,Induction type directional over current relay & current differential relay.
It refers to the current or voltage which is used to activate the relay into operation.
It is defined as the time period extended from the occurrence of the fault through the relay detecting the fault to the operation of the relay.
It is defined as the time taken by the relay from the instant of isolating the fault to the moment when the fault is removed and the relay can be reset.
Overcurrent relays are those that operate when the current in a line exceeds a predetermined value. (eg: Induction type nondirectional/ directional overcurrent relay, differential overcurrent relay)whereas undercurrent relays are those which operate whenever the current in a circuit/line drops below a predetermined value.(eg: differential over-voltage relay)
Protection of generator & generator transformer unit; protection of large motors and busbars .
The biased beam relay is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected zone. It is essentially an over-current balanced beam relay type with an additional restraining coil. The restraining coil produces a bias force in the opposite direction to the operating force.
The operation of a relay should be fast and selective, ie, it should isolate the fault in the shortest possible time causing minimum disturbance to the system. Also, if a relay fails to operate, there should be suficiently quick backup protection so that the rest of the system is protected. By coordinating relays, faults can always be isolated quickly without serious disturbance to the rest of the system.
In a power transformer, currents in the primary and secondary are to be compared. As these two currents are usually different, the use of identical transformers will give di??erential current, and operate the relay under no-load condition. Also, there is usually a phase difference between the primary and secondary currents of three phase transformers. Even CT’s of proper turn-ratio are used, the differential current may ??ow through the relay under normal condition.
An under frequency relay is one which operates when the frequency of the system (usually an alternator or transformer) falls below a certain value.
Pilot wires refers to the wires that connect the CT’s placed at the ends of a power transmission line as part of its protection scheme. The resistance of the pilot wires is usually less than 500 ohms.
The program time (ie, the time taken by the carrier to reach the other end-upto .1% mile); the response time of band pass filter capacitance phase-shift of the transmission line .
High speed operation; high sensitivity; ability to operate at low voltages; adequate short-time thermal ratio; burden must not be excessive.
Modern alternators are provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the prime mover when a dangerous over-speed occurs.
Fault between phase and ground; fault between phases and inter-turn fault involving turns of the same phase winding.
Phase to phase faults 2, Phase to earth faults 3, Inter turn faults
1, Earth faults
2, Fault between turns
3, Loss of excitation due to fuel failure
1, Over speed
2, Loss of drive
3, Vacuum failure resulting in condenser pressure rise, resulting in shattering of the turbine low pressure casing
1, Fault on lines
2, Fault on busbars
In order to limit the flow of current through neutral and earth a resistor is introduced between them.
External fault or through fault
1, Short circuit in transformer winding and connection 2, Incipient or slow developing faults
Difference in lengths of pilot wires on either sides of the relay. This is overcome by connecting adjustable resistors to pilot wires to get equipotential points on the pilot wires.
Difference in CT ratio error difference at high values of short circuit currents that makes the relay to operate even for external or through faults. This is overcome by introducing bias coil.
Tap changing alters the ratio of voltage and currents between HV and LV sides and the relay will sense this and act. Bias coil will solve this.
Magnetizing inrush current appears wherever a transformer is energized on its primary side producing harmonics. No current will be seen by the secondary. CT’s as there is no load in the circuit. This difference in current will actuate the differential relay. A harmonic restraining unit is added to the relay which will block it when the transformer is energized.
It is restricted earth fault relay. When the fault occurs very near to the neutral point of the transformer, the voltage available to drive the earth circuit is very small, which may not be suffcient to activate the relay, unless the relay is set for a very low current.
Hence the zone of protection in the winding of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay.
If the turns ratio of the transformer is more than 1:1, there will be higher core loss and the capability of the transformer to withstand this is limited to a few minutes only. This phenomenon is called over fluxing.
Fault level at busbar is high
The stability of the system is affected by the faults in the bus zone.
A fault in the bus bar causes interruption of supply to a large portion of the system network.
When a fault occurs in an alternator winding even though the generator circuit breaker is tripped, the fault continues to fed because EMF is induced in the generator itself. Hence the field circuit breaker is opened and stored energy in the field winding is discharged through another resistor. This method is known as field suppression.
Failure of support insulator resulting in earth fault
Flashover across support insulator during over voltage Heavily polluted insulator causing flashover Earthquake, mechanical damage etc.
Large number of circuits, different current levels for different circuits for external faults.
Saturation of CT cores due to dc component and ac component in short circuit currents. The saturation introduces ratio error.
Sectionalizing of the bus makes circuit complicated.
Setting of relays need a change with large load changes.
It is a relay in which measurement or comparison of electrical quantities is made in a static network which is designed to give an output signal when a threshold condition is passed which operates a tripping device.
During switching of lines or wrong synchronization surges of real and reactive power flowing in transmission line causes severe oscillations in the voltage and current vectors. It is represented by curves originating in load regions and traveling towards relay characteristics.
A static relay may have one or more programmable units such as microprocessors or microcomputers in its circuit.
It is the method of connecting a resistance in parallel with the contact space(arc). The resistance reduces the restriking voltage frequency and it diverts part of the arc current. It assists the circuit breaker in interrupting the magnetizing current and capacity current.
When interrupting low inductive currents such as magnetizing currents of the transformer, shunt reactor, the rapid deionization of the contact space and blast effect may cause the current to be interrupted before the natural current zero. This phenomenon of interruption of the current before its natural zero is called current chopping.
High resistance interruption:-the arc resistance is increased by elongating, and splitting the arc so that the arc is fully extinguished Current zero method:-The arc is interrupted at current zero position that occurs100 times a second in case of 50Hz power system frequency in ac.
It is the transient voltage appearing across the breaker contacts at the instant of arc being extinguished.
The power frequency rms voltage appearing across the breaker contacts after the arc is extinguished and transient oscillations die out is called recovery voltage.
Air break circuit breaker Oil circuit breaker
Minimum oil circuit breaker Air blast circuit breaker
SF6 circuit breaker
Vacuum circuit breaker
When the contacts of the breaker are separated, an arc is struck between them. This arc delays the current interruption process and also generates enormous heat which may cause damage to the system or to the breaker itself. This is the main problem.
Short contact life
Frequent maintenance Possibility of explosion
Larger arcing time for small currents Prone to restricts
It absorbs the arc energy to decompose the oil into gases, which have excellent cooling properties
It acts as an insulator and permits smaller clearance between line conductors and earthed components
It requires lesser quantity of oil
It requires smaller space
There is a reduced risk of fire
Maintenance problem are reduced
The breaker can be tested for desired transient recovery voltage and RRRV.
Both test current and test voltage can be independently varied. This gives flexibility to the test
The method is simple
With this method a breaker capacity (MVA) of five time of that of the capacity of the test plant can be tested.
The over voltage of the power system leads to insulation breakdown of the equipments. It causes the line insulation to flash over and may also damage the nearby transformer, generators and the other equipment connected to the line.
In this method the breaker is first tested for its rated breaking capacity at a reduced voltage and afterwards for rated voltage at a low current.This method does not give a proper estimate of the breaker performance.
Using a short circuit generator as the source.
Using the power utility system or network as the source.
It consists of overvoltage withstand test of power frequency lightning and impulse voltages.Testa are done for both internal and external insulation with switch in both open and closed conditions.
It has 3 vectors equal in magnitude and displaced from each other by an angle 120 degrees and has the phase sequence in opposite to its original phasors.
Fuse is a low current interrupting device. It is a copper or an aluminium wire.Circuit breaker is a high current interrupting device and it act as a switch under normal operating conditions.