MAHALAKSHMI

ENGINEERING COLLEGE

TIRUCHIRAPALLI- 621213.

QUESTION BANK

DEPARTMENT: EEE SEMESTER - V

SUBJECT NAME: Transmission and Distribution

UNIT 5

1. List the various substation equipment?(AUC NOV 2010)

The components of substation equipments are transformers, circuit breakers, isolation

switches, instrument transformer, bus bar, protective relays, lightning arresters and

control room equipments etc.

2. Why are transmission lines 3 phase 3 wire circuits while distribution lines are 3

phase 4 wire circuits? (AUC NOV 2010)

3. What are the advantages of ring main distributor? (AUC NOV 2009)

Due to load variation, the voltage fluctuation is less at the fair end.

Better reliability.

It gives continuity of supply, when fault occurs at any one distributor.

4. What is interconnected system? (AUC NOV 2009)

When the feeder ring is energized by two or more than generating stations or substation

is called as inter- connected system.

5. What are the various methods of earthing in substation? (AUC NOV 2011)

There are several methods of neutral grounding. They are

Isolated neutral

Effectively earthed system.

Resistance earthing.

Resonant earthing.

Grounding transformer.

6. Define the terms feeders and service mains. (AUC NOV 2011)

Feeders are conductors of large current carrying capacity which carries in bulk

to feeding points.

Electrical power service is provided to a consumer from the distribution feeder

through at the service main.

7. Based on what criteria the substation bus scheme are chosen.(AUC MAY 2008)

The choice of bus scheme is necessary for the purpose of safety , reliability, voltage

level, simplicity of relaying, flexibility of operation, least cost, ease of maintains,

available ground area, location of connecting lines, ease of rearrangement and

provision of expansion.

8. State the function of circuit breaker. (AUC MAY 2008)

A circuit breaker can make or break a circuit either manually or automatically under all

condition. Viz., no-load, full load and short circuit conditions.

9. Write down difference between disconnector switch and isolator. (AUC NOV’07)

Whenever maintenance or repair work is to be carried out on an equipment in a

substation, it is disconnected from the supply by the isolator. It is operated under no

load. Isolators are interlocked with circuit breaker and earthing switches. To open

isolator, circuit breakers are to be opened first.

10. Name any one protective device present in substation. (AUC NOV’07)

Circuit breaker.

Lightning arrester.

11. List out the disadvantages of single bus scheme. (AUC MAY’07)

In the event of failure, it can cause serious outages.

During maintenance, discontinuity of supply takes place.

It can be used only where load can be interrupted.

12. What is the role of circuit breaker in power system? (AUC MAY’07)

When fault occurs in the bus bar, the relay schemes sense the fault and give command

signal to the circuit breaker. The circuit breakers disconnected the faulty section,

thereby protecting equipment.

Part B

1. Explain briefly the various types of bus bar arrangement in a substation. (AUC

NOV 2010)

There are many different electrical bus system schemes available but selection of a particular scheme depends upon the system voltage, position of substation in electrical power system, flexibility needed in system and cost to be expensed. The main criteria’s to be considered during selection of one particular Bus – Bar Arrangement Scheme among others

Simplicity of system. Easy maintenance of different equipments. Minimizing the outage during maintenance. Future provision of extension with growth of demand

Optimizing the selection of bus bar arrangement scheme so that it gives maximum return from the system.

Some very commonly used bus bar arrangements are discussed below. Single Bus System

Single Bus System is simplest and cheapest one. In this scheme all the feeders and transformer bay are connected to only one single bus as shown.

Advantages of single bus system

This is very simple in design. Low cost. Very convenient to operate.

Disadvantages of single bus system

One but major difficulty of these type of arrangement is that, maintenance of equipment of any bay cannot be possible without interrupting the feeder or transformer connected to that bay.

The indoor 11KV switchboards have quite often single bus bar arrangement.

Single Bus System with Bus Sectionalizer

Some advantages are realized if a single bus bar is sectionalized with circuit breaker. If there are more than one incoming and the incoming sources and outgoing feeders are evenly distributed on the sections as shown in the figure, interruption of system can be reduced to a good extent.

Advantages of single bus system with bus sectionalizer

If any of the sources is out of system, still all loads can be fed by switching on the sectional circuit breaker or bus coupler breaker. If one section of the bus bar system is

under maintenance, part load of the substation can be fed by energizing the other section of bus bar.

Disadvantages of single bus system with bus sectionalizer

As in the case of single bus system, maintenance of equipment of any bay cannot be possible without interrupting the feeder or transformer connected to that bay. The use of isolator for bus sectionalizing does not fulfill the purpose. The isolators have to be operated ‘off circuit’ and which is not possible without total interruption of bus – bar. So investment for bus-coupler breaker is required.

Double Bus System

In double bus bar system two identical bus bars are used in such a way that any

outgoing or incoming feeder can be taken from any of the bus.

Actually every feeder is connected to both of the buses in parallel through individual

isolator as shown in the figure.

By closing any of the isolators one can put the feeder to associated bus. Both of the buses are energized and total feeders are divided into two groups, one group is fed from one bus and other from other bus. But any feeder at any time can be transferred from one bus to other. There is one bus coupler breaker which should be kept close during bus transfer operation. For transfer operation, one should first close the bus coupler circuit breaker then close the isolator associated with the bus to where the feeder would be transferred and then open the isolator associated with the bus from where feeder is transferred. Lastly after this transfer operation he or she should open the bus coupler breaker.

Advantages of Double Bus System

Double Bus Bar Arrangement increases the flexibility of system.

Disadvantages of Double Bus System

The arrangement does not permit breaker maintenance with out interruption.

Double Breaker Bus System

In double breaker bus bar system two identical bus bars are used in such a way that any outgoing or incoming feeder can be taken from any of the bus similar to double bus bar system. Only difference is that here every feeder is connected to both of the buses in parallel through individual breaker instead only isolator as shown in the figure. By closing any of the breakers and its associated isolators, one can put the feeder to respective bus. Both of the buses are energized and total feeders are divided into two groups, one group is fed from one bus and other from other bus similar to previous case. But any feeder at any time can be transferred from one bus to other. There is no need of bus coupler as because the operation is done by breakers instead of isolator. For transfer operation, one should first close the isolators and then the breaker associated with the bus to where the feeder would be transferred and then he or she opens the breaker and then isolators associated with the bus from where feeder is transferred.

One and a half Breaker Bus System

This is an improvement on the double breaker scheme to effect saving in the number of circuit breakers. For every two circuits only one spare breaker is provided. The protection is however complicated since it must associate the central breaker with the feeder whose own breaker is taken out for maintenance. For the reasons given under double breaker scheme and because of the prohibitory costs of equipment even this scheme is not much popular. As shown in the figure that it is a simple design, two feeders are fed from two different buses through their associated breakers and these two feeders are coupled by a third breaker which is called tie breaker. Normally all the three breakers are closed and power is fed to both the circuits from two buses which are operated in parallel. The tie breaker acts as coupler for the two feeder circuits.

During failure of any feeder breaker, the power is fed through the breaker of the second feeder and tie breaker, therefore each feeder breaker has to be rated to feed both the feeders, coupled by tie breaker.

Advantages of One and a half Breaker Bus System

During any fault on any one of the buses, that faulty bus will be cleared instantly without interrupting any feeders in the system since all feeders will continue to feed from other healthy bus.

Disadvantages of One and a half Breaker Bus System

This scheme is much expensive due to investment for third breaker.

Main and Transfer Bus System

This is an alternative of double bus system. The main conception of Main and Transfer Bus System is, here every feeder line is directly connected through an isolator to a second bus called transfer bus. The said isolator in between transfer bus and feeder line is generally called bypass isolator. The main bus is as usual connected to each feeder through a bay consists of circuit breaker and associated isolators at both side of the breaker. There is one bus coupler bay which couples transfer bus and main bus through a circuit breaker and associated isolators at both sides of the breaker. If necessary the transfer bus can be energized by main bus power by closing the transfer bus coupler isolators and then breaker. Then the power in transfer bus can directly be fed to the feeder line by closing the bypass isolator. If the main circuit breaker associated with feeder is switched off or isolated from system, the feeder can still be fed in this way by transferring it to transfer bus.

Switching operation for transferring a feeder to transfer bus from main bus without interruption of power

First close the isolators at both side of the bus coupler breaker. Then close the bypass isolator of the feeder which is to be transferred to

transfer bus. Now energized the transfer bus by closing the bus coupler circuit breaker from

remote.

After bus coupler breaker is closed, now the power from main bus flows to the feeder line through its main breaker as well as bus coupler breaker via transfer bus.

Now if main breaker of the feeder is switched off, total power flow will instantaneously shift to the bus coupler breaker and hence this breaker will serve the purpose of protection for the feeder.

At last the operating personnel open the isolators at both sides of the main circuit breaker to make it isolated from rest of the live system.

So it can be concluded that in Main & Transfer Bus System the maintenance of circuit breaker is possible without any interruption of power. Because of this advantage the scheme is very popular for 33KV and 13KV system.

Double Bus System with Bypass Isolators

This is combination of the double bus system and main and transfer bus system. In Double Bus System with Bypass Isolators either bus can act as main bus and second bus as transfer bus. It permits breaker maintenance without interruption of power which is not possible in double bus system but it provides all the advantages of double bus system. It however requires one additional isolator (bypass isolator) for each feeder circuit and introduces slight complication in system layout. Still this scheme is best for optimum economy of system and it is best optimum choice for 220KV system.

Ring Bus System

The schematic diagram of the system is given in the figure. It provides a double feed to each feeder circuit, opening one breaker under maintenance or otherwise does not affect supply to any feeder. But this system has two major disadvantages. One as it is closed circuit system it is next to impossible to extend in future and hence it is unsuitable for developing system. Secondly, during maintenance or any other reason if any one of the circuit breaker in ring loop is switch of reliability of system becomes very poor as because closed loop becomes opened. Since, at that moment for any tripping of any breaker in the open loop causes interruption in all the feeders between tripped breaker and open end of the loop.

Advantages:

Each branch requires only one breaker.

Initial cost is low.

Switching operation is done by breaker.

It does not use main bus.

Disadvantages

Automatic reclosing and protective relaying circuit is complex.

It requires potential devices in all the circuit for synchronizing or voltage

indication

2. Discuss briefly each of the following.(AUC NOV 2010)

i. Feeders.(4)

ii. Radial distribution.(6)

iii. Ring main distribution(6)

i. Feeders.

A feeder is a circuit carrying power from a main substation to a secondary

subsatation.

Since the area of cross section of the feeder are designed based on current

capacity it is assumed to be same for normal voltage and with increased

voltage.

Feeder are classified into three types

a. Parallel feeder

b. Ring feeder

c. Loop feeder.

ii. Radial Distribution

If a distributor is connected to the supply system from one end only, it is

called as radial distributor.

Disadvantages

Distributors nearest to substation are heavily loaded.

Due to load variation, voltage fluctuation is more at far ends.

If any faults occur, there is no continuity of power supply.

iii. Ring main distributor:

A ring main distributor is arranged to form a closed loop. It may have one or

more feeding points. It employs a distributor which covers the whole area of

supply finally returning to the substation.

Advantages:

Due to load variation, the voltage fluctuation is less at the far end.

Better reliability.

It gives the continuity of supply, when fault occur at any one

distributor.

3. Explain the different types of grounding system. (AUC NOV 2010)

Substation Grounding/ Earthing

The sole purpose of substation grounding/earthing is to protect the equipment

from surges and lightning strikes and to protect the operating persons in the substation.

The substation earthing system is necessary for connecting neutral points of

transformers and generators to ground and also for connecting the non current carrying

metal parts such as structures, overhead shielding wires, tanks, frames, etc to earth.

Earthing of surge arresters is through the earthing system. The function of substation

earthing system is to provide a grounding mat below the earth surface in and around

the substation which will have uniformly zero potential with respect to ground and lower

earth resistance to ensure that

To provide discharge path for lightning over voltages coming via rod-gaps, surge

arresters, and shielding wires etc. .

To ensure safety of the operating staff by limiting voltage gradient at ground level in the

substation

To provide low resistance path to the earthing switch earthed terminals, so as to

discharge the trapped charge (Due to charging currents even the line is dead still

charge remains which causes dangerous shocks) to earth prior to maintenance and

repairs.

Earth Resistance

Earth Resistance is the resistance offered by the earth electrode to the flow of current in to the

ground. To provide a sufficiently low resistance path to the earth to minimize the rise in earth

potential with respect to a remote earth fault. Persons touching any of the non current carrying

grounded parts shall not receive a dangerous shock during an earth fault. Each structure,

transformer tank, body of equipment, etc, should be connected to earthing mat by their own

earth connection.

Generally lower earth resistance is preferable but for certain applications following earth

resistance are satisfactory

Large Power Station s– 0.5 Ohm

Major Power Stations - 1.0 Ohm

Small Substation – 2.0 Ohm

In all Other Cases – 8.0 Ohm

Step Potential and Touch Potential

Grounding system in a electrical system is designed to achieve low earth resistance and also

to achieve safe ‘Step Potential ‘and ‘Touch Potential’.

Step Potential:

Step potential is the potential difference between the feet of a person standing on the floor of

the substation, with 0.5 m spacing between the feet (one step), through the flow of earth fault

current through the grounding system.

Touch Potential:

Touch potential is a potential difference between the fingers of raised hand touching the faulted

structure and the feet of the person standing on the substation floor. The person should not get

a shock even if the grounded structure is carrying fault current, i.e, The Touch Potential should

be very small.

Step Potential and Touch Potential

Types of Grounding:

Un earthed Systems:

It is used no more. The neutral is not connected to the earth, also called as insulated neutral

system.

Solid grounding or effective grounding:

The neutral is directly connected to the earth without any impedance between neutral and

ground.

Resistance grounding:

Resistance is connected between the neutral and the ground.

Reactance grounding:

Reactance is connected between the neutral and ground.

Resonant Grounding:

An adjustable reactor of correctly selected value to compensate the capacitive earth current is

connected between the neutral and the earth. The coil is called Arc Suppression Coil or Earth

Fault Neutralizer.

It is necessary to earth a power system at a suitable point by a suitable method as it offers

many advantages as

1. It provide Safety to the electrical equipments against over-current

2. It provides better safety

3. It reduces the maintenance expenditure

4. It improves the service reliabilty

5. It provides improved lightning protection

Solid Earthing

When the neutral of the power transformer and generator is directly connected to the earth,

then the system is said to be solidly earthed. The solidly earthing does not make a zero

impedance circuit as generator or transformer would have its own reactance in series with the

neutral circuit. The direct earthing of a generator without external impedance causes earth fault

current from the generator to exceed the maximum 3-phase fault current if the impedance of

the generator is too low. This results in Stator winding damage as the short circuit current

during fault will exceed the short circuit rating of the winding for which it was designed. For this

system of earthing, it is necessary that the earth fault current shall be in the range of 25% to

100% of the 3-phase fault current to prevent the development of high transit over voltages.

Resistance Earthing

In resistance earthing the neutral of the generator or transformer is connected to the earth

trough a resistance in series.

Advantage Of Resistance Earthing are:

1. It reduces the line voltage drop caused when earth fault occurs

2. It reduces electric shock hazards to the persons, caused by stray earth fault currents in

the return path

3. It reduces the mechanical stresses in the circuit carrying fault current

4. It reduces the effect of burning of faulted electrical equipment

The magnitude of the resistance to be used should be such that it should limit the earth fault

current to a value which will reduce minimum damage at the point of the fault.

Type of Earth Resistance Methods

Reactance Earthing:

In reactance earthing a reactor is connected in between the neutral of the machine and earth.

A low reactance is connected in series with the neutral of the machine to limit the earth fault

current through the generator. This current should not be greater than the 3-phase fault current

of the generator. The earth fault current of the earthed system should not be less than 25% of

the 3-phase fault current in order to minimize the transient voltages.

5. Explain the following substation bus scheme.

i. Double bus with double breaker.

ii. Main and transfer bus.

Double Bus System

In double bus bar system two identical bus bars are used in such a way that any outgoing

or incoming feeder can be taken from any of the bus.

Actually every feeder is connected to both of the buses in parallel through individual

isolator as shown in the figure.

By closing any of the isolators one can put the feeder to associated bus. Both of the buses are energized and total feeders are divided into two groups, one group is fed from one bus and other from other bus. But any feeder at any time can be transferred from one bus to other. There is one bus coupler breaker which should be kept close during bus transfer operation. For transfer operation, one should first close the bus coupler circuit breaker then close the isolator associated with the bus to where the feeder would be transferred and then open the isolator associated with the bus from where feeder is transferred. Lastly after this transfer operation he or she should open the bus coupler breaker.

Advantages of Double Bus System

Double Bus Bar Arrangement increases the flexibility of system.

Disadvantages of Double Bus System

The arrangement does not permit breaker maintenance with out interruption.

Double Breaker Bus System

In double breaker bus bar system two identical bus bars are used in such a way that any outgoing or incoming feeder can be taken from any of the bus similar to double bus bar system. Only difference is that here every feeder is connected to both of the buses in parallel through individual breaker instead only isolator as shown in the figure. By closing any of the breakers and its associated isolators, one can put the feeder to respective bus. Both of the buses are energized and total feeders are divided into two groups, one group is fed from one bus and other from other bus similar to previous case. But any feeder at any time can be transferred from one bus to other. There is no need of bus coupler as because the operation is done by breakers instead of isolator. For transfer operation, one should first close the isolators and then the breaker associated with the bus to where the feeder would be transferred and then he or she opens the breaker and then isolators associated with the bus from where feeder is transferred.

One and a half Breaker Bus System

This is an improvement on the double breaker scheme to effect saving in the number of circuit breakers. For every two circuits only one spare breaker is provided. The protection is however complicated since it must associate the central breaker with the feeder whose own breaker is taken out for maintenance. For the reasons given under double breaker scheme and because of the prohibitory costs of equipment even this scheme is not much popular. As shown in the figure that it is a simple design, two feeders are fed from two different buses through their associated breakers and these two feeders are coupled by a third breaker which is called tie breaker. Normally all the three breakers are closed and power is fed to both the circuits from two buses which are operated in parallel. The tie breaker acts as coupler for the two feeder circuits.

During failure of any feeder breaker, the power is fed through the breaker of the second feeder and tie breaker, therefore each feeder breaker has to be rated to feed both the feeders, coupled by tie breaker.

Advantages of One and a half Breaker Bus System

During any fault on any one of the buses, that faulty bus will be cleared instantly without interrupting any feeders in the system since all feeders will continue to feed from other healthy bus.

Disadvantages of One and a half Breaker Bus System

This scheme is much expensive due to investment for third breaker.

Main and Transfer Bus System

This is an alternative of double bus system. The main conception of Main and Transfer Bus System is, here every feeder line is directly connected through an isolator to a second bus called transfer bus. The said isolator in between transfer bus and feeder line is generally called bypass isolator. The main bus is as usual connected to each feeder through a bay consists of circuit breaker and associated isolators at both side of the breaker. There is one bus coupler bay which couples transfer bus and main bus through a circuit breaker and associated isolators at both sides of the breaker. If necessary the transfer bus can be energized by main bus power by closing the transfer bus coupler isolators and then breaker. Then the power in transfer bus can directly be fed to the feeder line by closing the bypass isolator. If the main circuit breaker associated with feeder is switched off or isolated from system, the feeder can still be fed in this way by transferring it to transfer bus.

6. Discuss and compare radial and ring main distribution system. What the role is of interconnected in distribution system.

Another system of distribution which eliminates the disadvantages of the radial system

is used in practice called ring main distribution system.

In such system, the feeders covers the whole area of supply in the ring fashion and

finally terminates at the substation from where it is started. The feeder is in closed loop from

and looks like a ring hence the name given to the system as ring main system. This is shown in

the Fig. 1.

The feeder in the ring fashion is divided into number of sections as AB, BC, CD, DE and

EA. The various distributors are connected at A, B, C, D and E. Each distributor is supplied by

the two feeders and hence the design is similar to the two feeders in parallel on different paths.

Hence if there is any fault on any part of the feeder, still the consumers will keep on getting the

continuous supply. For example, if the fault occurs at point P in the section AB of the feeder

can be isolated and repaired. The feeder can be fed at one or more feeding points. Thus the

disadvantages of radial system are eliminated in this system. The great saving in copper is

another major advantage of the ring main system.

It has been mentioned that in ring main system, the cable is arranged in the loop or ring

fashion. In most simple case, the ring distributor is fed at only one point.

But sometimes the ring main system is used to supply a large area and hence voltage drop

across the various sections may become large in such case. Hence to compensate for such

excessive voltage drops, the distant points of ring distributor are joined together by a

conductor. This is called an interconnector. The Fig. 1 shows a ring main system with an

interconnector.

Fig. 1

The points D and G are joined by an interconnector.

Such a case is generally analysed using Thevenin's theorem.

Let us briefly revise the steps to use the Thevenin's theorem.

The steps to use Thevenin's theorem :

1. Remove an interconnector DG.

2. Find the voltage VDG without an interconnector, which is

Thevenin's voltage denoted as Eo.

3. Determine the equivalent resistance as viewed through the

terminals D and G, i.e. where an interconnector is to be

connected. This is Thevenin's equivalent resistance denoted as

RTH.

4. Knowing the resistance of an interconnector DG, the Thevenin;s

equivalent can be drawn as shown in the fed Fig. 2.

b

1. The current I through an interconnector then can be obtained as,

c

d. Once this current is known, current in all the sections and the voltages at

load points can be determined