Interview Question On Power System Part 4

Q. What is a load curve?

A. The curve showing the variation of load on the power station with respect to (w.r.t) time is known as a load curve.

Q. What is the difference between base load and peak load?

A. Base load:  The unvarying load which occurs almost the whole day on the station is known as base load.
Peak load:  The various peak demands of load over and above the base load of the station is known as peak load.

Q. What is power factor?

A. The cosine of angle between voltage and current in an a.c. circuit is known as power factor.

Q. What are the disadvantages of low power factor and how to improve it?

A. Disadvantages: Large KVA rating of equipment, Greater conductor size, High copper losses, Poor voltage regulation
The low power factor is mainly due to the fact that most of the power loads are inductive and, therefore, take lagging currents.  In order to improve the power factor, some device taking leading power should be connected in parallel with the load.  One of such devices can be a capacitor. The capacitor draws a leading current and partly or completely neutralizes the lagging reactive component of load current. This raises the power factor of the load.

Q. Why is a.c transmission preferred over d.c transmission?

A.     1. The power can be generated at high voltages.
2. The maintenance of a.c. sub-stations is easy and cheaper.
3.  The a.c. voltage can be stepped up or stepped down by transformers with ease and efficiency.Although a.c transmission is preferred due to the above reasons d.c transmission is superior (2 conductors instead of 3, no inductance, capacitance, no corona, no skin effect etc).

Q. Why is electric power transmitted at higher voltages?

A.     Higher the transmission voltage lesser will be the volume of conductor required.
Efficiency will be high at higher voltages.
Percentage line drop will be less.

Q. What are the different types of insulators in Overhead lines?

A. Pin type insulators, Suspension type insulators, Strain insulators, Shackle insulators.

Q. What is string efficiency?

A. The ratio of voltage across the whole string to the product of number of discs and the voltage across the disc nearest to the conductor is known as string efficiency.

Q. What are the different methods used to improve string efficiency?

A. String efficiency can be improved by using longer cross arms, by grading the insulators and by using guard rings.

Q. What is corona?

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

Q. What are the advantages and disadvantages of corona?

A. Advantages:
1. Due to corona formation, the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased.  The increased diameter reduces the electrostatic stresses between the conductors.
2. Corona reduces the effects of transients produced by surges.Disadvantages:
1. Corona is accompanied by a loss of energy.  This affects the transmission efficiency of the line.
2. Ozone is produced by corona and may cause corrosion of the conductor due to chemical action.
3. The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line.  This may cause inductive interference with neighboring communication lines.

Q. How to reduce the corona effect?

A. Corona effect can be reduced by
1. Increasing the conductor size
2. Increasing the conductor spacing

Q. What is sag? Explain its importance?

A. The difference in level between points of supports and the lowest point on the conductor is called sag.
If the conductors are too much stretched between supports in a bid to save conductor material, the stress in the conductor may reach unsafe value and in certain cases the conductor may break due to excessive tension.  In order to permit safe tension in the conductors, they are not fully stretched but are allowed to have a dip or sag.

Q. What is skin effect?

A. The tendency of alternating current to concentrate near the surface of a conductor is known as skin effect.

Q. Why is the need for transposition of conductors?

A. When the 3-phase line conductors are unsymmetrically spaced, the flux linkages and inductance of each phase are not the same.  A different inductance in each phase results in unequal voltage drops in the three phases even if the currents in the conductors are balanced.  Therefore, the voltage at the receiving end will not be the same for all phases.  In order that voltage drops are equal in all conductors, we generally interchange the positions of the conductors at regular intervals along the line so that each conductor occupies the original position of every other conductor over an equal distance.  This is called transposition.