Test: Power Systems- 4 - Electrical Engineering (EE) MCQ

The advantages of DC transmission system over AC transmission system are:

1) DC system is economical for long line

2) DC needs only two wires of transmission, while a 3 phase AC may need up to 4 wires

3) The corona loss is absent in DC, while for AC it increases with its frequency

4) The skin effect is also observed in AC, leading to problems in transmission conductor designs

5) No inductive and capacitive losses

6) No proximity effect

Whenever an inductive load is connected to the transmission line, power factor lags because of lagging load current. To compensate this, a shunt capacitor is connected which draws current leading the source voltage. The power factor can be improved.

Series capacitors are used to compensate the inductance of transmission line. They will increase the transmission capacity and the stability of the line. These are also used to share the load between parallel lines.

Series reactors are used as current limiting reactors to increase the impedance of a system. They are also used to limit the starting currents of synchronous electric motors and to compensate reactive power in order to improve the transmission capacity of power lines.

A shunt reactor is an absorber of reactive power, thus increasing the energy efficiency of the system.

The string efficiency is defined as the ratio of voltage across the string to the product of the number of strings and the voltage across the unit adjacent string.

For the adequate performance of transmission line, it is essential that the voltage distribution across the line should be uniform. This can be achieved by the following methods.

1) Use of longer cross arm

2) Capacitive grading

3) By using grading rings or static shielding

During no load condition, the current flowing is only charging current due to line capacitance. It increases the capacitive var in the system.

Since the line is under no load the line inductance will be less. Therefore, the capacitive var becomes greater than inductive var during no load or light load condition.

Due to this phenomenon the receiving end voltage becomes greater than sending end voltage. This effect is also called Ferranti effect.

Series capacitors are used to compensate the inductance of transmission line. They will increase the transmission capacity and the stability of the line. These are also used to share the load between parallel lines.

A shunt reactor is an absorber of reactive power, thus increasing the energy efficiency of the system.

The effect of wind pressure is more predominant on supporting tower than transmission lines.

The towers do have a reasonable surface area that would receive the force generated by any constant wind. It would also have the forces at the suspension point generated by the wind on the transmission lines. The transmission towers have flat surfaces whereas the transmission lines are circular. Then there is the possibility of the wind being just at the right speed to cause the towers to oscillate.

Because of the following benefits we use cross linked polyethylene as insulation for cables.

1) Works with a variety of voltage ranges 600 V to 35 kV

2) Provides mechanical protection

3) Can withstand extreme pressure

4) Resists underground damage

5) Weather resistant

6) Thermal resistant

7) Permits high conductor operating temperatures

8) Reduces short circuit and overload levels

8) Moisture resistant

Insulation resistance is temperature sensitive. When temperature increases, insulation resistance decreases and vice versa. Insulation resistance changes by a factor of two for each 10°C change.

Insulation resistance is inversely proportional to the length. Hence it decreases with the increase in length of the cable.

In radial power distribution system, different feeders radially came out from the substation and connected to the primary of distribution transformer. It is represented by only open paths.

But radial electrical power distribution system has one major drawback that in case of any feeder failure, the associated consumers would not get any power as there is no alternative path to feed the transformer. In case of transformer failure also, the power supply is interrupted.

In an AC transmission line, difference in phase of voltage at two ends of the line is due to the inductive and capacitive reactance of the transmission lines.

A synchronous motor can be made to take either a lagging or leading current from the line by alternating its excitation. Idle running synchronous motors are sued in connection with power plants to correct for low power factor of the load and thus reduced the current and power losses in the feeders and generators.

Synchronous phase modifier differ from the ordinary synchronous motor is as much as they are built for the highest economical speeds and provided with smaller shafts and bearings.

The shunt fault involves short circuit between conductor and ground or short circuit between two or more conductors.

The shunt faults are characterized by increase in current and fall in voltage and frequency.

Shunt faults are classified as follows:

1) Single line to Ground fault (LG fault)

2) Line to Line fault (LL fault)

3) Double line to Ground fault (LLG fault)

4) Three phase fault

In a line to ground fault,

Fault current, 

An isolator is a manually operated mechanical switch which separates a part of the electrical power. These separates a part of the system from rest for safe maintenance works.

Isolators are used to open a circuit under no load. Its main purpose is to isolate one portion of the circuit from the other and is not intended to be opened while current is flowing in the line. Isolators are generally used on both ends of the breaker so that repair or replacement of circuit breaker can be done without any danger.

Hence current rating is not necessary in the case of isolators

A fuse is an electrical safety device that operates to provide overcurrent protection of an electrical circuit. Its essential component is a metal wire or strip that melts when too much current flows through it, thereby interrupting the current. Hence it acts as safeguard to the circuit against heavy currents.

Arc is a sparking current in the circuit breaker when the two contacts of it are open. There are two methods of arc extinction in circuit breaker as follows:

1) High resistance method: In high resistance method, the arc resistance is made to increase gradually with the time so that the current force in between the two contacts is insufficient to maintain the arc. Hence the current is interrupted by with high resistance the arc is extinguished without creating any damage to the circuit breaker compartment. The different methods to increase the resistance are:

a) Increase the length of the arc

b) Reducing the cross section area of arc

c) Arc cooling method

d) Arc splitting method

2) Low resistance method: In this method the arc resistance is kept low until the current magnitude becomes zero. The arc extinguish naturally and is prevented by restricting by making the medium dielectric strength high compared to voltage between contacts. The different methods to achieve are

a) Increase the length of the contact gap

b) Cooling the ac medium

c) High pressure in arc chamber

d) Blast effect of ions

A fixed connection of resistance in parallel with the contact space or arc is called the resistance switching. Resistance switching is employed in circuit breakers having a high post zero resistance of contact space. The resistance switching is mainly used for reducing the re-striking voltage and the transient voltage surge.

SF₆ circuit breaker is most suitable for interrupting short line fault without resistance switching.

The induction disc type relay is based on the same principle as that of an ammeter or a volt meter, or a wattmeter or an energy mater. In induction relay the deflecting torque is produced by the eddy currents in an aluminium or copper disc by the flux of an AC electromagnet.

It works only on application AC voltages.

A Mho relay measures a component of admittance |Y|∠θ. But its characteristic when plotted on the impedance diagram is a circle passing through the origin.

The relay is called Mho relay because its characteristic is a straight line, when plotted on an admittance diagram.

It is inherently a directional relay as it detects the fault only in the forward direction.

A surge protector is an electrical device that is used to protect equipment against power surges and voltage spikes while blocking voltage over a safe threshold (approximately 120 V). When a threshold is over 120 V, a surge protector shorts to ground voltage or blocks the voltage.

Hence we can say that, surge protector provides high impedance to normal voltage and low impedance to surge.