# Interview Questions on Power Systems

Q: What is meant by synchronous condenser?

An over excited synchronous motor running on no load is known as synchronous condenser. It is used to improve the power factor of the system.

Q: What is Electric traction?

A:Electric traction means using the electric power for traction system to drive the locomotives.

Q:What is “pu” in electrical engineering?

A:Pu stands for per unit. This is used in single line diagram of power distribution. Power distribution is like a huge electrical circuit with number of components (generators, transformers, loads) with different ratings (in MVA and KV). To bring all the ratings into common platform we use pu concept in which, in general largest MVA and KV ratings of the component is considered as base values.

Q:Describe the operation carried out in Thermal power stations?

A:The water is boiled in the boiler with the help of coal. This generates steam. This steam is allowed to hit the turbine hence the turbine rotates. The turbine is coupled with generator. So the mechanical energy of turbine is converted to electrical energy by Generator.

Q:what is ACSR cable and where we use it?

A: ACSR means Aluminium conductor steel reinforced, this conductor is used in transmission & distribution.

Q:What will happen when power factor is leading in distribution of power?

A:If their is high power factor, i.e if the power factor is close to one:

``````1.losses in form of heat will be reduced,

2.cable becomes less bulky and easy to carry, and very cheap to afford, &amp;

3. it also reduces over heating of transformers.
``````

Q:Why syn. generators are used for the production of electricity?

A:synchronous machines have capability to work on different power factor. Hence syn. generators are used for the production of electricity.

Q:Why we use Vacuum Circuit Breaker in High Transmission System and not Air Circuit Breaker?

A:Vacuum has high arc quenching property compare to air . The dielectric strength of vacuum is equal to 8 times than air . Hence vacuum circuit breaker is used in High voltage line and air circuit breaker is used in low voltage lines.

Q:What is the difference between surge arrestor and lightning arrestor?

A:Lightning Arrestor is installed outside and the effect of lightning is grounded where as surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge.

Q:Explain the working principle of the circuit breaker

A:Circuit Breaker is one which makes or breaks the circuit.It has two contacts namely fixed contact & moving contact. Under normal condition the moving contact comes in contact with fixed contact thereby forming the closed circuit for the flow of current.

During abnormal & faulty conditions(when current exceeds the rated value) relay senses the fault and gives signal to circuit breaker. Hence the moving contact of circuit breaker separates from fixed contact and an arc is produced between them & thereby it forms the open circuit. Arc is extinguished by the Arc Quenching media like air, oil, vacuum, SF6 gas etc.

Q:what is power factor? whether it should be high or low? why?

A:Power factor is the ratio of true power to apparent power. It has to be ideally 1. If it is too low then cable over heating & equipment overloading will occur. if it is greater than 1 then load will act as capacitor and starts feeding the source and will cause tripping.

(if pf is poor ex: 0.17 to meet actual power, load has to draw more current(V constant), result in more losses if pf is good ex: 0.95 to meet actual power, load has to draw less current(V constant),result in less losses).

Q:What is the difference between Isolator and Circuit Breaker?

A: Isolator is a off load device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintenance on downstream circuits. it is manually operated and does not contain any solenoid unlike circuit breaker.

it should not be operated while it is having load. Its specification is given in rated current. But circuit breaker is onload automatic device used for breaking the circuit in case of abnormal conditions like short circuit, overload etc., it is having three specification 1) rated current 2) short circuit breaking capacity and 3) Instantaneous tripping current.

Q:What is buchholz relay and the significance of it in to the transformer?

A:Buchholz relay is a gas based relay which is used for the protection of transformer from its internal faults. whenever any internal fault occurs in a transformer, the buchholz relay at once gives an alarm for some time, if the transformer is isolated from the circuit then it stop its sound itself other wise it trips the circuit by its own tripping mechanism.

Q:What is Ferranti effect?

A: When receiving end voltage is greater than the sending end voltage.

Q: Explain types of Earthing.

A:The two types of earthing are Equipment earthing and system earthing. In Equipment earthing: body ( non conducting part) of the equipment should be earthed to safegaurd the human beings.In System Earthing : neutral of the supply source ( Transformer or Generator) should be grounded.

With this, in case of unbalanced loading neutral will not be shifted so that unbalanced voltages will not arise. We can protect the equipment also. This type of earthing is further classified into directly earthed, Impedance earthing, resistive earthing.

Q:What is the difference between MCB & MCCB, Where it can be used?

A: MCB is miniature circuit breaker which is thermal operated for overload condition and magnetically operated for short circuit protection. It is available in small current rating.

MCCB is moulded case circuit breaker which is thermal operated for overload condition and magnetically operated for short circuit protection.Under voltage and under frequency may be inbuilt. Normally it is used where normal current is more than 100 A.

Q:Where should the lighting arrestor be placed in distribution lines?

A:Near distribution transformers and out going feeders of 11kv and incomming feeder of 33kv and near power transformers in sub-stations.

Q:Define IDMT relay?

A:It is an Inverse Definite Minimum Time Relay. IDMT relay. It is inverse in the sense ,the tripping time will decrease as the magnitude of fault current increase.

Q:What is derating?

A:The current currying of cables will change depending upon the site temperature (location of site), type of run (it will run through duct, trench, buried etc.), number of tray, depth of trench, distance between cables.

Considering this condition actual current currying capacity of cable reduce than current currying capacity (which given to cable Catalogue) this is called derating.

Q:What is the difference between over voltage relay and over current relay?

A:The current increases not only for over voltages but also for under voltage(As most of the loads are non-linear in nature). So,the over voltage protection & over current protection are completely different.

Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing. Over current relay meant for sensing any internal short circuit, over load condition ,earth fault thereby reducing the system failure & risk of fire.So, for a better protection of the system.It should have both over voltage & over current relay.

Q:What is reverse power relay?

A:Reverse Power flow relay are used in generating stations’s protection. A generating stations is supposed to fed power to the grid and in case generating units are off,there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.

Q :When we connect the large capacitor bank in series ?

A :we connect large capacitor bank in series to improve the voltage power supply at the load end in balanced transmission line when there is considerable voltage drop along the balanced transmission line due to high impedance of the line.So in order to bring the voltage at the load terminals within its limits (i.e (+ or - %6 )of the rated high terminal voltage )the large capacitor bank is used in series.

Q:Why electricity in India is in the multiples of 11 like 11kv, 22kv, 33kv ?

A:Transformer Induced voltage equation contains 4.44 factor.

E -Induced emf per phase

T -number of turns

f -frequency

Φ -maximum flux per pole

From the equation we see that E is proportional to 4.4 and it is in turn multiple of 11.So always transmission voltage is multiple of 11

Q:Why we use ac system in India why not dc ?

A:Firstly, the output of power stations comes from a rotary turbine, which by it’s nature is AC and therefore requires no power electronics to convert to DC. Secondly it is much easier to change the voltage of AC electricity for transmission and distribution using transformer.

Thirdly the cost of plant associated with AC transmission (circuit breakers, transformers etc) is much lower than the equivalent of DC transmission . When a fault on the network occurs, a large fault current occurs. In an AC system this becomes much easier to interrupt, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.

Q:Which type of motor is used in trains, what is the rating of supply used?

A:Dc series is in the trains to get high starting torque while starting of the trains and operating voltage is 1500v dc.

Q: What is difference between fuse and breaker?

A: (i) Fuse performs both detection and interruption functions. Circuit breaker performs interruption function only. The detection of fault is made by relay system.

(ii) The breaking capacity of fuse is very small compare to that of circuit breaker.

(iii) The operating time of the fuse is smaller than operating time of CB.

(iv) After every operation, replacement is required in the fuse. In case of circuit breaker, replacement is not required.

Q] What is meant by Sag?

The difference in level between points of supports and the lowest point on the conductor is called sag.

[Q] What is corona?

The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is known as corona.

[ Q] How can we reduce the effect of corona?

1. By increasing conductor size:

By increasing conductor size, the voltage at which corona occurs is raised and hence corona effects are considerably reduced.

1. By increasing conductor spacing:

By increasing the spacing between conductors, the voltage at which corona occurs is raised and hence corona effects can be eliminated.

[ Q] What is tariff?

The rate at which electrical energy is supplied to a consumer is known as tariff.

The tariff should include the following items:

(i) Recovery of cost of producing electrical energy at the power station

(ii) Recovery of cost on the capital investment in transmission and distribution systems.

(iii) Recovery of cost of operation and maintenance of supply

(iv) A suitable profit on the capital investment.

[Q] What are the various types of tariff?

1. Simple tariff

2. Flat rate tariff

3. Block rate tariff

4. Two-part tariff

5. Maximum demand tariff

6. Power factor tariff

7. Three-part tariff