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comprehensive viva questions and answers for power systems

Electrical Question: What is a System?
Answer: When a number of elements or components are connected in a sequence to perform a specific function, the group of elements that all constitute a System

Electrical Question: What is Control System?
Answer: In a System the output and inputs are interrelated in such a manner that the output quantity or variable is controlled by input quantity, then such a system is called Control System.
The output quantity is called controlled variable or response and the input quantity is called command signal or excitation.

Electrical Question: What are different types of Control Systems?
Answer: Two major types of Control Systems are 1) Open loop Control System 2) Closed Loop Control Systems
Open loop Control Systems:
The Open loop Control System is one in which the Output Quantity has no effect on the Input Quantity. No feedback is present from the output quantity to the input quantity for correction.
Closed Loop Control System:
The Closed loop Control System is one in which the feedback is provided from the Output quantity to the input quantity for the correction so as to maintain the desired output of the system.

Electrical Question: What is a feedback in Control System?

Answer: The Feedback in Control System in one in which the output is sampled and proportional signal is fed back to the input for automatic correction of the error ( any change in desired output) for futher processing to get back the desired output.

Electrical Question: Why Negative Feedback is preferred in the Control System?
Answer: The role of Feedback in control system is to take the sampled output back to the input and compare output signal with input signal for error ( deviation from the desired result).
Negative Feedback results in the better stability of the system and rejects any disturbance signals and is less sensitive to the parameter variations. Hence in control systems negative feedback is considered.

Electrical Question: What is the effect of positive feedback on stability of the system?
Answer: Positive feedback is not used generally in the control system because it increases the error signal and drives the system to instability. But positive feedbacks are used in minor loop control systems to amplify certain internal signals and parameters

Power System
Power System Interview Question and Answers:
[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] Define Power Factor.
The cosine of angle between voltage and the current in an AC circuit is known as power factor.
The value of power factor will always be less than 1.
It is a usual practice to attach a word lagging or leading with the numerical value of power factor to signify whether the current lags or leads the voltage.

[Q] What is the difference between fuse and circuit breaker?
(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.
2. 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

[Q] What are the advantages and disadvantages of nuclear power plant?
Advantages:
(i) The amount of fuel required is quite small.
(ii) It requires less space so it can be located near to the load centre.
(iii) Huge deposits of nuclear fuels available all over the world. So supply of electricity can be ensured.
(iv) It ensures reliability of operation.
Disadvantages:
(i) The fuel used is expensive and is difficult to recover.
(ii) The capital cost on a nuclear plant is very high as compared to other types of plants
(iii) The disposal of the by-products is a big problem. They are radioactive in nature.
(iv) The maintenance charges are high.

Why star delta starter is preferred with induction motor?

Star delta starter is preferred with induction motor due to following reasons:
• Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes less losses.
• Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused.
• In addition, starting torque is increased and it prevents the damage of motor winding.

State the difference between generator and alternator

Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction. Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into dc current for external load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used.

Why AC systems are preferred over DC systems?

Due to following reasons, AC systems are preferred over DC systems:
a. It is easy to maintain and change the voltage of AC electricity for transmission and distribution.
b. Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the equivalent DC transmission
c. From power stations, AC is produced so it is better to use AC then DC instead of converting it.
d. When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.

How can you relate power engineering with electrical engineering?

Power engineering is a sub division of electrical engineering. It deals with generation, transmission and distribution of energy in electrical form. Design of all power equipments also comes under power engineering. Power engineers may work on the design and maintenance of the power grid i.e. called on grid systems and they might work on off grid systems that are not connected to the system.

What are the various kind of cables used for transmission?

Cables, which are used for transmitting power, can be categorized in three forms:
• Low-tension cables, which can transmit voltage upto 1000 volts.
• High-tension cables can transmit voltage upto 23000 volts.
• Super tension cables can transmit voltage 66 kV to 132 kV.

Why back emf used for a dc motor? highlight its significance.

The induced emf developed when the rotating conductors of the armature between the poles of magnet, in a DC motor, cut the magnetic flux, opposes the current flowing through the conductor, when the armature rotates, is called back emf. Its value depends upon the speed of rotation of the armature conductors. In starting, the value of back emf is zero.

What is slip in an induction motor?

Slip can be defined as the difference between the flux speed (Ns) and the rotor speed (N). Speed of the rotor of an induction motor is always less than its synchronous speed. It is usually expressed as a percentage of synchronous speed (Ns) and represented by the symbol ‘S’.

Explain the application of storage batteries.

Storage batteries are used for various purposes, some of the applications are mentioned below:

• For the operation of protective devices and for emergency lighting at generating stations and substations.
• For starting, ignition and lighting of automobiles, aircrafts etc.
• For lighting on steam and diesel railways trains.
• As a supply power source in telephone exchange, laboratories and broad casting stations.
• For emergency lighting at hospitals, banks, rural areas where electricity supplies are not possible.

Q1. What is the Power Station ?

A. Power Station is a place where Power is Generated and Transmitted , eg. Tharmal power station, Hydel power station, Nuclear power station etc.

Q2. What is Slack Bus ?

A. Slack Bus is the responsible for the delivering losses in the power system. in this bus the magnitude of voltage and phase angle are specified. This bus is first to respond to a changing load condition.

Q3. what is critical disruptive voltage.

A. The minimum voltage at which the breakdown of the insulating properties of air occurs and corona start is called D C V .

Q4. How many types of fault occurs in the 3 P power system ?

A. Faults in the 3 P system can be classified under the following :-

a. single line to ground (LG) fault
b.line to line (LL) fault
c. Double line to ground (LLG) fault
d. Thee Phase short circuit (LLL) fault
e. three phase to ground (LLLG) fault

Q5. How many methods uses for the load flow solution ?
A.
Gauss-seidel method using Ybus
Newton- Raphson method (NR)
Power flow through line and slack bus power
Decouple load flow method
Fast Decouple load flow method.
Q6. What is skin effect ?

A. When the AC passed through the conductor the density of the current is not uniform over the cross-section of the conductor, the density of the current is very high near the surface of the conductor and very low at middle this effect is called skin effect. this effect is only in the ac.

Q7. What is the Bundle conductor and why is is used ?

A. A bundle conductor consists of two more parallel subconductors at a spacing of several diameters.this is use to reduce the corona and skin effect.


Electrical Circuit Theorems:

To solve problems in electrical networks, there are number circuit theorems have been developed like
(1) Kirchhoff's laws
(2) Superposition theorem
(3) Thevenin theorem
(4) Norton Theorem
(5) The maximum power transfer theorem

In this post we have listed all these theorems. We believe that this post will serve as a ready reference for you.

Kirchhoff's laws:
(a) Current Law:
At any junction in an electric circuit the total current flowing towards that junction is equal to the total current flowing away from the junction, i.e. I =0

(b) Voltage Law:
In any closed loop in a network, the algebraic sum of the voltage drops (i.e.products of current and resistance) taken around the loop is equal to the resultant e.m.f. acting in that loop.

Superposition Theorem:
In any network made up of linear resistances and containing more than one source of EMF, the resultant current flowing in any branch is the algebraic sum of the currents that would flow in that branch if each source was considered separately, all other sources being replaced at that time by their respective internal resistances.

Thevenin Theorem:
The current in any branch of a network is that which would result if an EMF. equal to the potential differential across a break made in the branch, were introduced into the branch, all other EMFs being removed and represented by the internal resistances of the sources.

Norton Theorem:
The current that flows in any branch of a network is the same as that which would flow in the branch if it were connected across a source of electrical energy, the short-circuit current of which is equal to the current that would flow in a short-circuit across the branch, and the internal resistance of which is equal to the resistance which appears across the open-circuited branch terminals.

The Maximum Power Transfer Theorem:
The power transferred from a supply source to a load is at its maximum when the resistance of the load is equal to the internal resistance of the source