Why high voltage is preferred for power transmission?
As voltage increases, current flow through the line decreases and I2R loss reduces. So transmission efficiency increases. Therefore high voltage is preferred for power transmission.
State the disadvantages of HVDC transmission.
i) High cost of terminal equipment
ii) Converters require considerable reactive power.
iii) Harmonics are generated which requires filters.
iv) Converters do not have overload capacity.
Why the transmission lines 3 phase 3 wire circuits while distribution lines are 3ϕ, 4 wire circuits?
A Balanced 3 phase circuit does not require the neutral conductor, as the instantaneous sum of the 3 line currents are zero. Therefore the transmission lines and feeders are 3 phase 3 wire circuits. The distributors are 3 phase 4 wire circuits because a neutral wire is necessary to supply the 1 phase loads of domestic and commercial consumers.
Define the terms feeders and service mains.
Feeders:
It is a conductor which connects the substation to the area where power is to be distributed. Generally, no tappings are taken from the feeder.
Service mains:
A service main is generally a small cable which connects the distributor to the consumer’s terminal.
List out the advantages of high voltage A.C transmission.
i) The power can be generated at high voltages.
ii) The maintenance of ac substation is easy and cheaper.
What is ring main distributor?
In ring main distributor, the distributor is in the form of a closed ring. The distributor ring may be fed from one or more than one point.
List the types of HVDC links.
i) Monopolar HVDC link
ii) Bipolar HVDC link
iii) HVDC back to back tie link
iv) Homopolar HVDC link
v) Multi terminal HVDC link
What is a distributor?
A distributor is a conductor from which tappings are taken for supply to the consumers.
List out the objectives of FACTS.
i) To increase the power transfer capability of transmission systems.
ii) To keep power flow over designed routes.
What are the components of a power system?
i) Generating station
ii) Transmission system
iii) Distribution system
State Kelvin’s law for size of transmission conductor.
The annual expenditure on the variable part of the transmission system should be equal to the annual cost of energy wasted in the conductor used in that system.
Write the primary distribution voltages in India.
The primary distribution voltages in India are 33KV and 11KV.
State the limitation of high transmission voltage.
i) The increased cost of insulating the conductor.
ii) The increased cost of transformers ,switch gears and other terminal apparatus.
Distinguish between a feeder and a distributor.
Feeders:
Feeders, radiating from the distribution substation supply power to the distributors.
No consumer is directly connected to the feeder.
Distributor:
From the distributors, the tappings are taken for the supply to the consumers.
Hence they are used to transfer power from distribution centre to the consumers.
Define feeder and distributor.
Feeders:
It is a conductor which connects the substation to the area where power is to be distributed. Generally, no tappings are taken from the feeder.
Distributor:
A distributor is a conductor from which tappings are taken for supply to the consumers.
State the applications of HVDC transmission.
Long distance bulk power transmission
ii) Underground or sub marine cables.
iii) Asynchronous connection of AC system with different frequencies.
iv) Control and stabilize the power systems with power flow control.
What do you understand by distribution system?
The distribution of electric power from load centres to the consumer premises as per the requirement.
Mention the transmission voltages that are followed in India.
AC: 765KV, 400KV, 230KV, 220KV, 132KV, 110KV and 66KV
DC: 500KV and 400KV
Define skin effect.
The steady current when flowing through the conductor, does not distribute uniformly, rather it has the tendency to concentrate near the surface of the conductor. This phenomenon is called skin effect.
On what factors does the skin effect depend?
i) Nature of the material
ii) Diameter of the wire
iii) Frequency and shape of the wire.
What are the advantages of using bundled conductors?
i) Increase in capacitance
ii) Increase in power capability of the line
iii) Reduce the voltage surface gradient
iv) Reduce corona loss
v) Reduce surge impedance
vi) Increases surge impedance loading.
What is the need of transposition?
The effect of transposition is that each conductor has the same average inductance. After transposition, the voltage drops are equal in all conductors.
Define the term critical disruptive voltage.
The potential difference between conductors, at which electric field intensity at the surface of the conductor exceeds the critical value and generates corona is known as critical disruptive voltage. It is minimum phase to neutral voltage at which corona occurs.
What is proximity effect?
The alternating magnetic flux in a conductor caused by the current flowing in a neighbouring conductor gives rise to a circulating current which cause an apparent increase in the resistance of the conductor .This phenomenon is called as proximity effect.
Define visual critical voltage.
It is the minimum phase to neutral voltage at which corona glow appears all along the line conductors. In case of parallel line conductors corona glow does not begin at critical disruptive voltage, Vc but at a higher voltage, Vv which is called as visual critical voltage. This is because the dielectric breakdown of air requires a finite volume of over stressed air.
What is meant by transposition on overhead line?
Transposition means changing the positions of the three phases on the line supports twice over the total length of the line .the line conductors in practice ,are so transposed that each of the three possible arrangements of conductors exit for one-third of the total length of the line.
Distinguish between self and mutual GMD.
Self GMD:
It is the geometrical mean of the distance between the conductors belonging to same phase. Hence it is otherwise called as geometrical mean radius. It depends only upon the size & shape of the conductor and is independent of spacing between conductors.
Mutual GMD:
It is the geometrical mean of the distance between the phases. It depends only upon the spacing and is independent of the exact size, shape and orientation of the conductor.
What is corona?
The phenomenon of violet glow, hissing noise and production o ozone gas in an overhead line is called corona.
How will you reduce corona loss?
By increasing the conductor size and conductor spacing.
State the different types of overhead line conductors.
i) Stranded conductors
ii) ACSR conductors
iii) Bundled conductors
iv) Composite conductors.
What is Ferranti effect?
When a transmission line is lightly loaded or open circuited, the voltage at the receiving end becomes greater than the sending end voltage, due to substantial quantity of charging current. This is called Ferranti effect.
What is shunt compensation?
Shunt compensation is the use of shunt capacitors and shunt reactors is the line to avoid voltage instability.
Define voltage regulation in connection with transmission line.
Voltage regulation is defined as the change in voltage at the receiving (or load) end when the full-load is thrown off, the sending-end (or supply) voltage and supply frequency remaining unchanged.
What is the range of surge impedance in case of underground cables?
Surge impedance loading of underground cables is 40 Ω
What is the range of surge impedance in case of overhead transmission lines?
Surge impedance loading of overhead transmission lines is 400Ω
What is meant by natural loading of transmission line?
It is the power transmitted when a lossless line is terminated with a resistance equal to surge impedance of the line. This indicates the maximum power that can be delivered and it is useful in transmission line design.
Define transmission efficiency.
It is defined as the ratio of receiving end power to the sending end power of the transmission line.
Why the control of reactive power is essential for maintaining a desired voltage
profile? (ND14)
The reactive power transferred over a transmission line is proportional to the line voltage drop and is independent of power angle. Transfer of reactive power over the line is the main cause for voltage drop across the line. Thus it is needed to control the reactive power for maintaining the desired voltage profile.
What is meant by surge impedance and surge impedance loading of transmission line?
Surge impedance:
The characteristic impedance of a lossless line is known as surge impedance.
Surge impedance loading:
It is the power transmitted when a lossless line is terminated with a resistance equal to surge impedance of the line. This indicates the maximum power that can be delivered and it is useful in transmission line design.
What is the importance of voltage control?
i) Lightning loads are very sensitive to change in voltage
ii) Voltage variations will cause erratic operation in inductive motors.
iii) Large variations of voltage cause excessive heating of distribution transformers.
Mention the various methods of voltage control in transmission line.
i) By excitation control
ii) By using tap changing transformers
iii) Auto transformer tap changing
iv) Booster transformer
v) Induction regulators
vi) By synchronous condenser
States the condition for maximum power delivered and draw the power angle diagram.
Maximum power can be delivered when
i) δ or power angle is 90·
ii) By increasing sending end or and receiving end voltage magnitude
iii) By reducing the reactance X of the line
Define voltage stability.
The ability of the system to maintain the voltage level within its acceptable limits is called as voltage stability.
Mention the significance of surge impedance loading.
The voltage and current are equal and are in phase at all points along the line.
No reactive power is generated or absorbed at the line ends.
Name any four insulating materials used for cable.
i) Rubber (or) Vulcanized Indian Rubber
ii) Impregnated paper
iii) Varnished cambric
iv) polyvinyl Chloride
v) Cross linked poly Ethylene
Name the methods for improving string efficiency.
i) By using longer cross arms
ii) By grading the insulator units
iii) By using a guard ring
Define string efficiency.
The ratio of voltage across the whole string to the product of number of discs and the voltage across the disc nearest to the conductor is known as string efficiency.
What is the necessity of grading of an underground cable?
Distribution of dielectric material is such that the difference between Emax and Emin is reduced is known as grading of cables. Thereby a cable same size can be operated for high voltage or for the same operation voltage, the size of the cable can be reduced.
Enumerate the methods of grading of cables.
i) Capacitance grading
ii) Intersheath grading
Define grading of cable.
The process of achieving uniform electrostatic stress in the dielectric of cables is known as grading of cables.
What is meant by birdage?
Short – circuiting of the line conductor to earth through the large birds or similar objects is known as birdage.
How does grading ring improve string efficiency?
Guard ring introduces capacitance between metal fittings and line conductors. These capacitances are greater for lower units and thus the voltage across that bottom unit is reduced. Hence the string efficiency is improved.
What are the advantages of string insulators?
i) For higher voltages these are cheaper than pin insulators.
ii) This type of insulators provide greater flexibility to the line
iii) The string is suspended and is free to swing in any direction. So it takes the position so that mechanical stresses on the line are minimum.
What is shackle insulator?
It is used in low voltage distribution line (11KV). This can be used either in a vertical position or horizontal position. In this type of construction, the conductor is placed between the clamp and the insulator and is fixed along the groove of the insulator using soft binding wires of same material as the conductor.
What is meant by dielectric stress in a cable?
Dielectric stress occurs, when the insulation of a cable is subjected to electrostatic forces under normal operating conditions. Dielectric stress at any point in a cable is the electric field intensity at that point.
Why are insulators used with overhead lines?
i) To provide necessary insulation between line conductors and supports
ii) To prevent any leakage current from conductors to earth
iii) To provide necessary mechanical support for the conductor
What are the factors to be considered while selecting a cable for a particular service?
i) Operating voltage
ii) Current carrying capacity
iii) Voltage drop
iv) Site requirement
v) Economic evauation
Define safety factor of insulator. Why it is desired to have this value be high?
It is defined as the ratio of puncture strength to the flashover voltage. It is desirable to have high safety factor so that flashover takes place before the insulator gets punctured. For pin type insulators, the value of safety factor is above 10.
State the limitation of solid type cables. How are these overcome in pressure cables?
For voltages beyond 66KV, solid type cables are unreliable, because there is no danger of breakdown of insulation due to the presence of voids. But in pressure cables, voids are eliminated by increasing the pressure of the compound by using oil, gas etc.
What is the main purpose of armouring?
The purpose of armour is to protect the cable from mechanical injury while laying it and during the course of handling. It consists of one or more layers of galvanized steel wire or steel tape.
Specify the different types of insulators.
i) Pin type insulators
ii) Suspension type insulators
iii) Strain insulators
iv) Shackle insulators
v) Stay insulators
What are the tests performed on the insulators?
i) Mechanical test
ii) Electrical insulation test
iii) Environmental test
iv) Temporary cycle test
v) Corona and radio interference test
Classify the cables used for three phase service.
i) Belted cables (up to 11KV)
ii) Screened cables (up to 33KV)
iii) Pressure cables (beyond 66KV)