Dear Readers, Welcome to Transmission and Distribution Interview Questions and Answers have been designed specially to get you acquainted with the nature of questions you may encounter during your Job interview for the subject of Transmission and Distribution. These Transmission and Distribution Questions are very important for campus placement test and job interviews. As per my experience good interviewers hardly plan to ask any particular questions during your Job interview and these model questions are asked in the online technical test and interview of many IT & Non IT Industries.
Answer: Series capacitance is provided in EHV lines to artificially reduce the series reactance of the line so as to improve stability, voltage regulation and transmission efficiency
Shunt compensation is provided to artificially reduce the line susceptance so as to improve the voltage regulation under light load condition
Answer: ACSR conductors are employed.
Answer: The problems associated with EHV transmission are corona loss and radio interference, requirement of heavy supporting structures and insulation requirments
Answer: SIL varies as the square of the operating voltage, so SIL inceases with increase in voltage level
Answer: Some of the factors which limits the maximum power transfer are:
Electrical phase shift
Thermal effects in the line
Introducing or adding of new transmission lines in to the system to avoid the overloading of the existing lines
Application of devices such as series capacitors in the right location of power system which helps in increase in the power transfer capability
Up gradation of the existing transmission system
Adoption of multi-voltage levels an multi-circuits in the existing ac transmission system
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.
The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is known as Corona.
The tendency of alternating current to concentrate near the surface of a conductor is known as Skin Effect.
Ans. Voltage is stepped up before transmission due to the following reasons:
i. Reduces volume of conductor material
ii. Increases transmission efficiency
iii. Decreases percentage line drop
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
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:
i. DC system
ii. Single phase AC system
iii. Two phase AC system
iv. 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
The elements of a sub- station are:
Primary power lines
Transformer for measurement of electric voltage
Secondary power lines
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.
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.
A Lightning Arrestor is a device used in electrical power systems to protect the insulation on the system from the damaging effect of lightning.
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
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.
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.
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.
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.
The process of achieving uniform electrostatic stress in the dielectric of cables is called grading of cables
The principal insulating materials used in cables are rubber, vulcanised India rubber, impregnated paper, varnished cambric and polyvinyl chloride.
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.
ACSR means Aluminium conductor steel reinforced. This conductor is used in transmission & distribution
The heat can be dissipated by
It is the ratio of the average ??ux density over the pole pitch to the maximum ??ux density. It is also known as flux distribution factor
The different types of ratings are:
Short time duty
Intermittent periodic duty
Intermittent periodic duty with starting
Intermittent periodic duty with starting and braking
Continuous duty with intermittent periodic duty
Continuous duty with starting and braking
Continuous duty with periodic speed changes
It mainly consists of the following circuits
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 ??ow of these signals in to the substation Bus bars.