Test: Power Systems- 1 - Electrical Engineering (EE) MCQ

Unit type schemes protect a specific area of the system i.e. a transformer, transmission line, generator or bus bar. The unit protection schemes are based on Kirchhoff’s Current Law (the sum of the currents entering an area of the system must be zero). Any deviation from this must indicate an abnormal current path. In these schemes, the effects of any disturbance or operating condition outside the area of interest are totally ignored and the protection must be designed to be stable above the maximum possible fault current that could flow through the protected area.

Positive sequence component: It represents three equal phasors, phase displaced by 120° and has phase sequence same as the original phasors.

Negative sequence component: It represents three equal phasors, phase displaced by 120° with each other and has phase sequence opposite to that of the original phasors.

Zero sequence component: It represents three equal and parallel phasors with zero degree phase displacement.

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 in order that repair or replacement of circuit breaker can be done without and danger.

A fuse is a short length of wire designed to melt and separate in the event of excessive current. Fuses are always connected in series with the components to be protected from over current. When the fuse blows, it will open the entire circuit and stop current through the components.

A fuse is a short length of wire designed to melt and separate in the event of excessive current. It must have low resistance, high conductivity, low ohmic losses and low melting point.

A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in vacuum. This is suitable for medium voltage application. Resisting capacity of this type of circuit breaker is very high.

Air blast circuit breakers are used in arc furnace duty, traction systems, and used for the railway electrification. Because they are suitable for repeated duty.

When low inductive currents such as magnetizing current of transformer, shunt reactors are interrupted by breaker, there is rapid deionization of contact space and may cause the current to be interrupted before its neutral zero. This phenomena is called current chopping. For average values of load current, it occurs frequently in Air blast circuit breakers.

Earth fault relays are typically **non-directional relays**. These relays are designed to detect faults to ground in electrical systems regardless of the direction of current flow. Therefore, the correct answer is:

1. non-directional relays

According to the principle of operation and construction, the relay may be classified as follows

1) Electromagnetic Relays

2) Electromagnetic Induction Relays

3) Electrodynamic Type Relays

4) Moving Coil Type Relays

5) Electro-thermal Relays

6) Static Relays

Impedance relay is used for the phase faults in medium transmission line

Reactance relay is used for the phase faults in short transmission line

Mho relay is used for the phase faults in long transmission line

Correct Answer :-D

Explanation : The CT ratio is the ratio of primary current input to secondary current output at full load.

A thermal switch is a device which normally opens at a high temperature and re-closes when the temperature drops. The thermal switch may be a bimetallic strip, often encased in a tubular glass bulb to protect it from dust or short circuit.

A thermal switch is usually reusable, and is therefore suited to protecting against temporary situations which are common and user-correctable. Thermal switches are used in power supplies in case of overload, and also as thermostats in some heating and cooling systems.

We know that,

Here t = 24 hours. 

Peak load or maximum demand, 

Electrical diversity factor is the ratio of the sum of the individual maximum demands of the various electrical components of a grid to the maximum demand of the grid. The value of the diversity factor of an electrical grid usually exceeds one.

Under no load condition or light load condition, medium and long transmission lines may operate at leading power factor due to capacitance effect. This leading power factor can be changed to a lagging power factor by using a shunt reactor. By using a shunt reactor, it will compensate the effect of capacitance and changes the power factor.

The spacing between the conductors in an overhead line is directly proportional to inductance and inversely proportional to capacitance. Hence as spacing of overhead line is increase, the line capacitance decreases but line inductance increases.

A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. These devices achieve this because their neutron economy is high enough to breed more fissile fuel than they use from the fertile material, such as uranium-238 or thorium-232. Its conversion ratio is greater than one.

The main objective of the governor system in power systems for all types of turbines is to control the frequency. A governor protects the prime mover from over speed and keeps the prime mover speed at or near the desired revolutions per minute. When a prime mover drives an alternator supplying electrical power at a given frequency, a governor must be used to hold the prime mover at a speed that will yield this frequency.