Interview Questions on Transmission & Distribution

Why is alternating current used for transmission?

AC is used for transmission because transformers can step it up to high voltage for long distance, and down to low voltage for local distribution and use. The output of power stations comes from a rotary turbine, which by its nature is AC and therefore requires no power electronics to convert to DC

Also, it is much easier to change the voltage of AC electricity for transmission and distribution and the cost of plant associated with AC transmission (circuit breakers, transformers etc) is much lower than the equivalent of DC transmission. AC transmission provides a number of technical advantages. When a fault in the network occurs, large fault current flows. In an AC system, this becomes much easier to interrupt, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt

What is corona?

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

What is skin effect?

The tendency of alternating current to concentrate near the surface of a conductor is known as Skin Effect

Why is voltage stepped up before transmission?

Ans. Voltage is stepped up before transmission due to the following reasons:

Reduces volume of conductor material
Increases transmission efficiency
Decreases percentage line drop

What are standard Transmission and Distribution voltages?

Primary Transmission: 66 kV, 132 KV, 220 kV and 400 kV

Secondary Transmission: 33 kV

Primary Distribution: 11 kV, 3-phase, 3-wire

Secondary Distribution: 400 V between two phases (3-phase, 4-wire) and 230 V between any one phase and neutral

Explain Transmission and Distribution systems

Electric-power transmission system is the system used for bulk transfer ofelectrical energy, from generating power plants to electrical substations located near demand centers. The different systems of transmission are:

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

The part of power system which distributes electric power for local use is known as 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

What are the equipments used in sub- station?

The elements of a sub- station are

Primary power lines
Ground wire
Overhead lines
Transformer for measurement of electric voltage
Disconnect switch
Circuit breaker
Current transformer
Lightning arrester
Main transformer
Control building
Security fence
Secondary power lines

What is the difference between Relay and Fuse?

A relay is a switch; it is not usually meant to open under fault conditions (high current).

A fuse is a fault protection device. If a short circuit develops, large currents will flow, and the fuse link will melt, causing the circuit to open

What is PLCC?

Power line communication or power line carrier (PLC), also known as power line digital subscriber line (PDSL), mains communication, power line telecom (PLT), power line networking (PLN), or broadband over power lines (BPL) are systems for carrying data on a conductor and are also used for electric power transmission. A wide range of power line communication technologies are needed for different applications, ranging from home automation to Internet access

What are Lightning Arrestors?

A Lightning Arrestor is a device used in electrical power systems to protect the insulation on the system from the damaging effect of lightning

Explain the principle of operation of a Relay and a Circuit breaker

RELAY: An electromagnetic relay works on two principles

Electromagnetic attraction
Electromagnetic induction

The force of attraction produced has two components-one is constant and the other is fluctuating at a frequency, twice the natural frequency. This force in terms of current is represented as-

F= K₁– K₂* I²

K₁=K₂ * I²

Since K₁ and K₂both are constants, so the current I will be constant. This is the condition when the relay is at the verge of operation and F=0.

CIRCUIT BREAKER: Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically-stored energy (using springs or compressed air) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. When a current is interrupted, an arc is generated. This arc must be contained, cooled, and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium in which the arc forms

What is a Bus bar?

A Bus bar is a strip of copper or aluminium that conducts electricity within aswitchboard, distribution board, substationor other electrical apparatus. They form a link between the incoming and outgoing circuits. The size of the bus bar determines the maximum amount of current that can be safely carried

What is Arc formation?

When a short circuit occurs, a heavy current flows through the contacts of the circuit breaker and they are opened by the protective system. At the instant when the contacts begin to separate, the contact area decreases rapidly and large fault current causes increased current density and hence rise in temperature. The heat produced in the medium between the contacts is sufficient to ionise the medium. The ionised air or vapour acts as a conductor and an arc is struck between the contacts.

What is 3-Phase supply?

It is a type of poly phase system and is the most common method used by gridsworldwide to transfer power. In a three-phase system, three circuit conductors carry three alternating currents (of the same frequency) which reach their instantaneous peak values at different times

Why do we require protection against lightning?

The lightning surges may cause serious damage to the expensive equipments in the power system (e.g. generators, transformers etc.) either by direct strokes on the equipment or by the strokes on the transmission lines that reach the equipment as travelling waves

What is a Sub-station?

The assembly of apparatus used to change some characteristics (e.g. voltage, AC to DC, frequency, power factor) of the electric supply is called a sub-station

What is grading of cables?

The process of achieving uniform electrostatic stress in the dielectric of cables is called grading of cables

What is transmission cable made up of?

The principal insulating materials used in cables are rubber, vulcanised India rubber, impregnated paper, varnished cambric and polyvinyl chloride.

Explain the purpose of fuse

When a short circuit or overload occurs, the current through the fuse increases beyond its rated value which raises the temperature and fuse element melts (or blows out), disconnecting the circuit protected by it

What is ACSR cable and where we use it?

ACSR means Aluminium conductor steel reinforced. This conductor is used in transmission & distribution

What will happen when power factor is leading in distribution of power?

If there is high power factor, i.e. if the power factor is close to one

Losses in the form of heat will be reduced,
Cable becomes less bulky and easy to carry, and very cheap to afford, &
It also reduces over heating of transformers.

What are the faults in a three phase line?

The types of faults in a three phase line are

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

What is the approximate power generation capacity in India?

Sector	MW
State Sector	83,563.65
Central Sector	56,572.63
Private Sector	42,553.34
Total	1,82,689.62

Is corona more in AC or DC?

Corona Loss p = .00241[(f+25)/$]*[root of (r/d)]*square of[Vp-Vo]

f= supply frequency

$= air density correction factor

Vp = operating voltage

Vo= critical disprutive voltage

r= radius of conductor

d= distance of separation for fine weather

Now for DC f=0, there is a certain power loss by corona but less than AC corona loss.

What is current chopping?

Current chopping in CB is the incident of arc current interruption before normal current zero is reached. If this current is enough to deliver a voltage which is greater than restriking voltage then the arc will continue again. But gradually the current falls and certainly the voltage also reduces and finally falls down the restriking voltage and ultimately arc is extinguished. It results in voltage oscillation in the line which affects the electrical equipments

Which circuit breaker uses current chopping circuit?

Vacuum circuit breaker uses current chopping circuit

What is SCADA?

SCADA (supervisory control and data acquisition) generally refers to Industrial Control Systems (ICS): computer systems that monitor and control industrial, infrastructure, or facility-based processes

Why do we use a wave trap?

Line trap also is known as Wave trap. It helps in trapping the high frequency communication signals sent on the line from the remote substation and diverting them to the telecom/ tele-protection panel in the substation control room (through coupling capacitor and LMU). The Line trap offers high impedance to the high frequency communication signals thus obstructs the flow of these signals in to the substation Bus bars

Which are preferred – Underground or Overhead cables?

Although many low voltage, local electric distribution lines are buried underground, almost all high-voltage electric transmission lines are tried to be placed as overhead cables due to the cost difference

What is an IDMT relay?

It is an Inverse Definite Minimum Time relay. In IDMT relay, operating is inversely proportional to fault current and also a characteristic of minimum time after which this relay definitely operates

What is a diversity factor?

Diversity Factor = Sum of Individual Maximum Demand/ Maximum Demand on Power Station

Electrical Machine Design

What is the basic structure of a rotating electrical machine?

It mainly consists of the following circuits