Distribution Systems Questions and Answers

Answer: A

The output of the transformer used for primary distribution is 11 kV. Electricity is generated at about 11 kV, stepped up to 132 kV or more for transmission, and then stepped down at the receiving station to 33 kV for sub-transmission and to 11 kV for primary distribution, which feeds the distribution transformers that supply 400 V (or 230 V) to consumers. The values 33 kV, 66 kV and 132 kV belong to sub-transmission and transmission levels, not to primary distribution.

Answer: A

A step-up transformer is not a part of the distribution system. Step-up transformers are used at the generating station to raise the voltage for transmission, while the distribution system lowers the voltage again with step-down transformers.

Feeders (which carry power from the substation to the distribution area), distributors (from which the consumers are served) and service mains (the connection to the consumer's premises) are the parts of distribution. So the first option is the answer.

Answer: C

Boosters are basically Transformers.

A booster in an AC system is a booster transformer: its secondary is connected in series with the line, so it adds (or subtracts) a controllable voltage to correct the voltage drop along a feeder.

Inductors and capacitors are used for reactive compensation, and the synchronous motor run on no load as a synchronous condenser is also a reactive power device. They influence voltage indirectly through reactive power, but none of them is a booster.

Answer: C

The preferred location is At the intermediate point.

A booster transformer corrects the voltage drop along a line. Placed at an intermediate point it lifts the voltage where the drop has become significant, so consumers on the far part of the line receive close to normal voltage while the near part is not over-boosted.

At the sending end the boost would be applied where the voltage is already highest. At the receiving end the consumers along the line would see no benefit. The location is chosen from the voltage profile, so 'Anywhere in the line' is wrong.

Answer: A

Earth electrode resistance is resistance of an earth electrode to earth.

It is the resistance between the electrode and the general mass of earth, made up mainly of the resistance of the soil immediately around the electrode. It determines how much current can flow to earth for a given voltage, and so how well the earthing protects people and equipment.

The resistance to a bus bar, a conductor or the neutral is the resistance of the earthing conductors and connections, not of the electrode to earth.

Answer: B

The potential of earth is conventionally taken as 0.

The earth is a very large conductor whose potential changes negligibly when charge flows into or out of it, so it is used as the reference level for electrical potential and assigned the value zero. All other potentials are measured relative to it.

1, 0.1 and infinity are not used as the earth's potential. In electrostatics the zero of potential is sometimes placed at an infinite distance, but that is a choice of where zero lies, not a value of infinity; the earth itself is taken at 0.

Answer: A

Grounding is the Connection of current carrying parts to ground.

In the usage this question follows, grounding means connecting a current-carrying part of the system, typically the neutral of a generator or transformer, to earth. It stabilises the system voltage with respect to earth and lets earth fault protection operate.

Connecting non-current-carrying metal parts such as equipment frames and enclosures to earth is called earthing, which protects people from shock. Interconnecting earth pits is part of building an earthing system, not the definition of grounding.

Answer: D

The one that is not a type of earthing is Balance earthing.

Earthing is classified by purpose. System earthing connects the neutral of the supply system to earth. Equipment earthing connects the exposed metal parts of apparatus to earth. Static and lightning protection earthing provides a path to discharge static charge and lightning current safely.

'Balance earthing' is not one of the recognised categories, so it is the odd one out.

Answer: D

The types of electrode are All of the above.

Common earth electrodes include plate electrodes (a copper or galvanised iron plate buried vertically), pipe electrodes (a galvanised iron pipe driven into the ground, the most common in India) and rod electrodes (copper or steel rods driven deep). Strip or wire electrodes laid in trenches are also used.

Since plate, pipe and rod electrodes are all standard types, no single one is the full answer.

Answer: A

A ring main is preferred because the voltage drop in the feeder is less.

In a ring main each distributor is fed from both ends, so the load current divides between two paths and every consumer is closer, electrically, to a supply point. The voltage drop is therefore smaller and the voltage at the far consumers is better. A fault on one section can also be isolated while the rest of the ring stays supplied, so continuity is higher than in a radial system.

A ring main costs more, not less, because it needs more conductor and switchgear. It does not change the load power factor, and a very high voltage drop is the drawback of the radial system, not the advantage of the ring.