Pri Sec Distribution System

8/2/2019 Pri Sec Distribution System

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After reading this Module , you should be able to:

Provide classification of distribution systems

Discuss design considerations

Provide an analysis of AC and DC distribution systems


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The part of the electrical-supply system existing between

the distribution substations and the distributiontransformers is called the primary system.

It is made of circuits, known as primary feeders or

primary distribution feeders. The most commonly used

nominal primary voltage is 11 kV.


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The secondary distribution system receives power from the

secondary side of distribution transformers at low voltage

and supplies power to various connected loads via service

lines.The secondary distribution System is the final sub-system of

the power system.

The secondary distribution systems are generally of the

radial type except for some specific service areas such as

hospitals, business centers and military installations, which

require highly reliable service, and therefore, may be of a

grid or mesh type.


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Figure : Radial secondary distribution system configuration


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Good voltage regulation is the most important factor in a

distribution system for delivering good service to the

consumer. For this purpose, careful consideration is

required for the design of feeders and distributor networks.

Feeders are the conductors that connect substations to

consumer ports and have large current carrying capacity.

The current loading of a feeder is uniform along the whole

of its length since no tappings are taken from it. The design

of a feeder is based mainly on the current that is to be

carried.


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Distributors are the conductors, which run along a street

or an area to supply power to consumers.

These can be easily recognized by the number of

tappings, which are taken from them for the supply to

various consumer terminals.

The current loading of a distributor is not uniform and itvaries along the length while its design is largely

influenced by the voltage drop along it.


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Service Main and Sub Main The service mains are the

conductors forming connecting links between distributors

and metering points of the consumer's terminal. Figure

14.1 shows the layout of a distribution system. The term

sub main refers to the several connections given to

consumers from one service main.


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It has been established that 70% of the total losses occur

in the primary and secondary distribution system while

transmission and sub-transmission lines account for only

30% of the total losses.

Distribution losses amount to 15.5% of the generation

capacity and the target level is to bring it down to 7.5%.

Therefore the primary and secondary distribution systems

must be planned with care to ensure losses within

acceptability limits


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Inadequate Size of Conductor :

The rural load is usually scattered and fed byradial feeders.

Feeder Length In practice, 11 kV and 415 V lines in rural

areas are hurriedly extended radially over long distances to

feed loads scattered over large areas. This results in high

line resistance, low voltage, and high current, and

therefore, leads to high I 2R losses in the line.


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Location of Distribution Transformers Often the distribution

transformers are not located centrally in relation to the

intended customer. Consequently, the farthest customers

obtain an extremely low voltage even though a reasonablygood voltage level is maintained at the secondary

transformer.

This again leads to higher line losses.

Therefore, in order to reduce the voltage drop in the line

transmitting power to consumers located farthest from the

line, the DT should be located at the load center to keep the

voltage drop within permissible limits.


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Low Voltage Whenever the voltage applied to an induction

motor deviates from rated voltage, its performance is

adversely affected. A reduced voltage, in case of an induction

motor, results in higher currents drawn for the same output

leading to higher losses. This can be overcome by adjusting

the tap changer at power transformer and at distributiontransformer, if available.


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Use of Over-rated Distribution Transformers

Studies on 11 kV feeders have revealed that often the

rating of distribution transformers (DTs) is much higher

than the maximum kVA demand on the LT feeder. An

overrated transformer produces an unnecessarily high iron

loss.From the above, it is clear that the rating should be

judiciously selected to keep the losses within the permissible

limits.


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Low PowerFactor In most of the LT distribution

systems, it is found that the power factor varies from as

worse as 0.65 to 0.75. A low power factor contributes

towards high distribution losses.

For a given load, if the power factor is low, the current

drawn is high, consequently the losses proportional to a

square of the current, will be more.

Thus, line losses owing to the poor power factor can be

reduced by improving the power factor by using shunt

capacitors for the purpose.


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The following methods are adopted for the reduction of

distribution-system losses:

1. HV distribution system

2. Feeder reconfiguration

3. Grading of conductor

4. Construction of new substation5. . Reactive-power compensation


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1 . HV Distribution System

The low-voltage distribution system contributes a majority of the

total distribution losses because of poor voltage regulation .

The present situation is that LV lines are extended, irrespective of

voltage drops, up to the full capacity of the DT and sometimes

over and above the transformer capacity. Hence, no purpose will

be served by prescribing low kVA-km loading limits for LV lines.

The only practice and feasible solution is to eliminate or

minimize LV lines by switching over to single- phase high voltage

distribution. By adopting HV distribution, the losses in the LV

distribution can be minimized.


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2 . Feeder Reconfiguration

It is defined as the process of altering the topological

structure of distribution feeders by changing the

open/closed status of the sectionalising and tie switches.

Feeder reconfiguration allows the transfer of loads from

heavy loaded feeders to moderately heavy loaded feeders.

Such transfers are effective in improving the voltage

profile along the feeders and effecting reduction in the

overall system power losses.


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3. Grading of Conductor

In normal practice, the conductor used for radial distribution

feeders is of uniform cross-sectional area. However, the load

magnitude at the substation is high and it reduces as we

proceed to the tail-end of the feeder. This indicates that the

use of a higher-sized conductor, which is capable of supplying

load from the source point, is not necessary at the tail-end

point. Similarly, the use of different conductor cross-sectionsfor intermediate sections will lead to a minimum capital

investment cost and line loss.


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4. Construction of New Substation

If a new substation is to be constructed and connected

to an existing network, several possible solutions are to

be studied. These solutions may include various

connection schemes of the substation and several

feasible locations .

The optimum site for a substation is defined as that

location which will result in minimum cost for construction

and minimum losses. Therefore, by constructing a new

substation at load centres, the line osses will be reduced

due to an improvement in the voltage profile and a

reduction in the length of the lines.

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5. Reactive Power Compensation

Voltage-reactive power control function has a pivotal role

to play in the distribution automation.

The problem of reactive power compensation can be

attempted by providing static capacitors.

The method presently used to compensate the reactive

power component is to increase the reactive power by

increasing the terminal voltage of the generator (or), by

increasing the field current of the synchronous machine in

condenser mode at the generating stations. This procedure isnot effective because the power-system losses will be further

increased due to the increase of reactive power in the

transmission system.


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An alternative method for compensating the reactive power isthe use ofcapacitors in distribution systems at customer

points. Shunt capacitors supply the amount of reactive power

to the system at the point where they are connected.

There are two methods of capacitor compensation viz.

1. Series compensation (capacitors are placed in series with line)

2. Shunt compensation (capacitors are placed in parallel with

load)


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The fundamental function of capacitors, whether they are

series or shunt in a power system is to generate reactive

power to improve power factor and voltage, thereby

enhancing the system capacity and reducing losses.

In series capacitors, the reactive power is proportional to

the square of the load current, whereas in shunt capacitors

it is proportional to the square of the voltage.

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Pri Sec Distribution System