Transmission & Distribution (T&D) Interview Questions

1). What is Skin Effect?

Skin Effect is the phenomenon which states that the tendency of the ac current to be concentrated near the surface of the conductor.

2). Why is Voltage stepped up before Transmission?

Before transmission, voltage is stepped up for several reasons:

• Minimizes relative amount of conductor material
• Increases transmission efficiency
• Decreases percentage line drop

3). What is Corona?

Corona is defined as the violet glow, the hissing noise and the production of the ozone gas which occur in an overhead transmission line.

4). Why is alternating current used for Transmission?

AC is used for transmission because transformers are employed that step it up to high voltage for transmission and step it down to low voltage for distribution and utilization. The power stations output is a rotary turbine which drives electricity generation and by its very design is AC and thus no conversion with power electronics is required.

Moreover, the voltage may be stepped up or down more easily for AC transmission and distribution purposes, the cost of the plant involved in AC transmission (circuit breaker, transformers etc.) is considerably cheaper than the cost of the plant required for DC transmission. 

There are quite a number of technical benefits of using AC transmission. As can be noted, when a fault takes place in the network, huge DC fault current transposes. It becomes ‘fusing’ much easier in an AC system because the sine wave current will naturally tend to become zero at some stage making the current easier to interrupt.

5). What are standard Transmission and Distribution voltages?

• Primary Transmission: Low voltage is 66 kV, Medium voltage 132 KV, High voltage 220 kV and very high voltage 400 KV.
• Secondary Transmission: 33 kV
• Primary Distribution: 25 kV / 11 kV, 3-phase, 3-wire
• Secondary Distribution: 400V between any two phases of a three phase four wire system or 230V between any one phase and the neutral.

6). Explain Transmission and Distribution Systems

Electric-power transmission system is the system by which electrical energy will be transported in bulk from generating power plants to the electrical substations that are situated close to demand centers. 

The different systems of transmission are:

• DC system
• AC system
  • Single phase AC system
  • Two phase AC system
  • Three phase AC system

The part of the power system that supplies electric power for local use is commonly referred to as the Distribution System. The different distribution systems are:

• AC Distribution system
  • Primary distribution system
  • Secondary distribution system
• DC Distribution system
  • 2 wire DC system
  • 3 wire DC system

7). What is the equipment used in sub-station?

The equipments (components) used at the sub-station include:

1. Primary power lines
2. Ground wire
3. Overhead lines
4. Transformer
5. Disconnect switch
6. Circuit breaker
7. Current transformer
8. Lightning arrester
9. Main transformer
10. Control building
11. Security fence
12. Secondary power lines

8). What is the difference between Relay and Fuse?

Relay	Fuse
A relay is a switch; it is not usually intended to open under fault conditions (High current).	A fuse is a protective device designed to die or break in case of an excess current flow in a circuit. When a short circuit occurs, high currents will flow, the fuse link gets soft and hence the circuit gets open.

9). What is PLCC?

Power line communication or power line carrier (PLC), power line digital subscriber line (PDSL), mains communication, power line telecom (PLT), power line networking (PLN), broadband over power lines (BPL) are networking systems used on a conductor used for data communication and also work for electric power transmission. There exists a need for a multiple variety of power line communication technologies for distinct applications such as home and small building automation, Internet access, among others.

10). What are Lightning Arrestors?

Lightning Arrestor is a device that is installed in electrical power systems with the function of protecting insulation in said systems against the calamitous impact of lightning.

11). What is a Bus bar?

A Bus bar is a type of bar that is constantly being charged, creating a circuit carrying an electrical current.

A Bus bar is a solid conductor generally of copper or aluminium through which electricity is taken in a switch board, distribution board, substation or any electrical apparatus. They connect two circuits, an incoming and an outgoing one, or two different circuits. The size of the bus bar defines the maximum current which should be passed through the bus bar as this exceeds the limit the chances of a short circuit increases.

12). What is Arc Formation?

In the case of a short circuit, high current starts to circulate through the contact of the circuit breaker and it is operated by its protection scheme. The contact area decreases rapidly when the contacts start to part and huge fault current increases the current density and hence increases in temperature. 

This heat generated at the contact spaces ionizes the medium between the contacts sufficiently enough. The ionized air or vapor functions both as a conductor and when the contacts are brought close to each other an arc is produced.

13). What is 3-Phase Supply?

It is a type of poly phase system and is one of the most common practices adopted by most grids across the world in transmitting power. In a three-phase system, there are three circuit conductors through which three supply currents of the same frequency flow but at different instants of time attain the optimum steady-state values.

14). Why do we require Protection against Lightning?

The lightning surges may result in severe damages to the costly equipment in the power system for instance generators, transformers etc. either through the direct effects of the bolt on the equipment or through the effects of the bolt on the transmission line in the form of traveling waves.

15). What is a Substation?

This is a collection of equipment to alter some properties of the electric power supply such as 

• Voltage, 
• Current type, 
• Frequency , 
• Power factor 

of electricity supply, is called a substation.

16). What is Grading of Cables?

Grading of cables means the cabling system has to be checked periodically and examined if it is still up to the required industry standard, capable of carrying the required load, or whether it is gradually deteriorating.

The technique used to maintain a uniform electrostatic stress across the dielectric of cables is known as cable grading.

17). What is a Transmission Cable made up of?

The major types of insulating materials employed in cables are 

• Rubber, 
• Vulcanized Rubber, 
• Insulated papers, 
• Varnished cambric, and 
• Polyvinyl chloride.

18). Explain the purpose of Fuse

If there is a short circuit or excessive loading the current through the fuse rises beyond its capacity which makes the temperature and hence the fuse element to melt or blow out thus disconnecting the circuit served by it.

19). What is an ACSR cable and where do we use it?

ACSR stands for Aluminium Conductor Steel Reinforced. It is mainly applied in transmission and distribution.

20). What will happen if the power factor leads the way in power distribution?

If the power factor is high, in other words, when the power factor is close to 1.

• Heat loss trading will be minimized,
• Cable becomes less bulky and easy to handle, they become cheaper to obtain and be affordable.
• It also minimizes over heating of transformers and thus comes up with a good saving on power.

21). What are the faults in a three phase line?

In a three phase line faults are classified as

• Phase-To-Earth Fault
• Phase-To-Phase Fault
• Phase-To-Phase-To-Earth Fault
• Three-Phase Fault
• Three-Phase-To-Earth Fault
• Phase-To-Pilot Fault
• Pilot-To-Earth Fault

22). What is Current Chopping?

Current chopping in CB is the process of interrupting the arc current before normal current zero takes place. If this current is sufficient to maintain and give a voltage which is more than restriking voltage then the arc will continue once more. But as time passes the current and of course the voltage also decreases and finally the restriking voltage comes down, thus the arc is broken. It leads to voltage fluctuation in the line that has an impacts on the electric appliances

23). What is SCADA?

SCADA (Supervisory Control And Data Acquisition) generally refers to Industrial Control Systems (ICS): industrial, infrastructure or facility networked controls, that is, program based systems used to manage industrial or facility based processes.

24). Why do we use a Wave Trap?

Line trap also is known as Wave trap. It assists in receiving the high frequency communication signals transmitted on the line from the remote substation to the telecom/tele-protection panel in the substation control room (through coupling capacitor/LMU). 

The Line trap impedes the flow of the high frequency communication signals by presenting high impedance to these signals and in so doing it hampers the flow of these signals into the substation Bus bars.

25). Which are preferred: Underground or Overhead cables?

Because installing cables underground is more expensive, most high-voltage electric transmission lines are tried to be established as overhead electric cables even though many low voltage local electric supply cables are installed underground.