reactive power compensation in radial distribution systems using dist flow method project

8/7/2019 REACTIVE POWER COMPENSATION IN RADIAL DISTRIBUTION SYSTEMS USING DIST FLOW METHOD Project

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INTRODUCTION

In India, the distribution systems losses is about 12% of the

generation, which is high when compared to developed countries. In

attempting to reduce distribution system losses, a thorough

knowledge of distribution system losses, a thorough knowledge of

distribution system loss calculation has to be understood.

Presently computers are widely used to solve utility

engineering problems. New programs are being developed for load

flow stability, short circuit, and for control of power systems.

Considering the utility investment in the distribution system, new

tools are essential to save engineering time and to reduce

investment. In solving distribution problems, tedious hand

calculations can be avoided with the help of new algorithms and

preferably with small computing systems.

A power system is an inter connected system composed of

generating stations, which convert fuel energy into electricity.

Substations that distribute electrical power to loads (consumers)

and transmission lines that tie the generating stations and

distribution substations together. According to voltage levels an

electric power system can be viewed as consisting of generating

system, a transmission system and a distribution system.

The distribution system is generally categorized into two

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subdivisions.

1. Primary Distribution

Which carries load at higher than utilization voltages from the

substation (or other source) to the point where the voltage is to be

stepped down to the value at which the energy is utilized by the

consumer.

2. Secondary Distribution

Which includes the part of the system operation at utilization

voltage, upto the meter at the consumer's premises.

Primary distribution system include the following basic types.

i. Radial system and

ii. Loop systems.

Chapter 2 explains in detail about the innovative technique for

load flow calculations reactive power analysis of distribution

networks. A method for reducing a radial network into a single line

equivalent, known as Dist flow method has been developed by

G.B. Jasmon and L.H.C.C. Lee which simplifies lengthy calculations of

an unreduced network. This reduced network also enables the fast

computation of load flow solutions of distribution networks. The

conditions for voltage collapse to occur are easily derived form the

single line equivalent.

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The complete simplification of equations involved are given in

Chapter 2.

Application of this Dist flow method for two test systems is

explained using software programme in Chapter 4 and the results

thus obtained are given in Chapter 5.

1.1 DISTRIBUTION SYSTEMS

1.1.1 Introduction

An electric distribution system, or distribution plant as it

sometimes called, is all of that part of an electric power system

between the bulk power source or sources and the consumer's

service switches. The bulk power sources are located in or near the

load area to be served by the distribution system and may be either

generating stations or power substations supplied over transmission

lines. Distribution system can, in general, be divided into six parts,

namely, subtransmission circuits, distribution substations,

distribution transformers, secondary circuits (or) secondaries, and

consumer's services connections and meters (or) consumer's

services.

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The subtransmission circuits extend from the bulk power

source or sources to the various distribution substations located in

the load area. They may be radial circuits connected to a bulk power

source at only one end or loop and ring circuits connected to one or

more bulk power sources at both ends. The subtransmission circuits

consist of underground cable, aerial cable, or overhead open-wire

conductors carried on poles, or some combination of them. The

subtransmission voltage is usually between 11 and 33 kv, inclusive.

Each distribution substation normally serves its own load area,

which is a subdivision of the area served by the distribution system.

At the distribution system substation the subtransmission voltage is

reduced for general distribution throughout the area. The substation

consists of one or more power-transformer banks together with the

necessary voltage regulating equipment, buses, and switch gear.

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The area served by the distribution substation is also

subdivided and each subdivision is supplied by a distribution or

primary feeder. The 3- primary feeder is usually run out from the

low voltage bus of the substation to its load centre where it

branches into three phase subfeeders and 1- laterals. The primary

feeders and laterals may be either cable (or) open wire circuits.

The distribution plant occupies an important place in any

electric power system. Briefly, its function is to take electric power

from the bulk power source or sources and distribute to deliver it to

the consumers. The effectiveness with which a distribution system

fulfills this function is measured in terms of voltage regulation,

service continuity, flexibility, efficiency and cost.

1.2 Types of Distribution Systems

1.2.1 The Radial System

The radial type of distribution system, a simple form. It is used

extensively to serve the light and medium density load areas where

the primary and secondary circuits are usually carried over head on

poles. The distribution substation or substations can be supplied

from the bulk power source over radial or loop subtransmission

circuits or over a subtransmission grid or network. The radial system

gets its name from the fact that the primary feeders radiate from

the distribution substations and branch into subfeeders and laterals

which extend into all parts of the area served. The distribution

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transformers are connected to the primary feeders, subfeeders, and

laterals, usually through fused cutouts, and supply the radial

secondary circuits to which the consumer's services are connected.

Fundamentally the advantages of the radial distribution

system are simplicity and low first cost. These result from a straight

forward circuits arrangement where a single (or) radial path is

provided from the distribution substation, and sometimes from the

bulk power source, to the consumer. With such a circuit

arrangement the amount of switching equipment is small and the

protective relaying is simple. Although simplicity and low first cost

account for the wide spread by of the radial system they are not

present in all forms of the system.

The lack of continuity of service is the principal defect of the

radial system of distribution. Attempts to over come this defect

have resulted in many forms and arrangements of the radial

system. Frequently the system is radial only from the distribution

substations to the distribution transformers. Because of the many

system arrangements encountered is some times difficult to

determine in what major type of a system should be classified. To

aid in such classification and to allow more readily the discussion of

radial systems, it should be remembered that a radial system is a

system having a single path over which current may flow for a part

or all of the way from the distribution substation or substations to

the primary of any distribution transformer.

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1.2.2. The Loop System

The loop type of distribution system is used most frequently to

supply bulk loads such as small industrial plants and medium (or)

large commercial buildings, where continuity of service is of

considerable importance. The subtransmission circuits of the loop

system should be parallel (or) loop circuits or a subtransmission

grid. These subtransmission circuits should supply a distribution

substation (or) substations. The reason for this is that as much or

more reliability should be built into the system from the low-voltage

bus of the distribution substation back to the bulk power source (or)

sources as is provided by the loop primary feeders. The use in a

loop system of a radial subtransmission circuit or circuits and a

distribution substation (or) substations, which may not provide good

service continuity, does not give a well coordinated system. This is

because a fault on a subtransmission circuit or in a distribution

substation transformer results in a interruption of service to the

loads supplied over the more reliable loop primary feeders. The

subtransmission circuits and distribution substations are often

common to both radial and loop type distribution systems.

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MODELING OF DISTRIBUTION SYSTEMS

2.1. Load Flow Analysis

Distribution system has not received much attention unlike

load flow analysis of transmission systems. However, some work has

been carried out on load flow analysis of a distribution network but

the choice of a solution method for a practical system is often

difficult. Generally distribution networks are radial and the R/X ratio

is very high. Because of this, distribution networks are ill-

conditioned and conventional Newton Raphson (NR) and fast

decouple load flow (FDLF) method are inefficient at solving such

networks.

Baran and Wu [1], obtained the load flow solution in a

distribution system by the iterative solution of three fundamental

equations representing real power, reactive power and voltage

magnitude. These three equations are very useful, since they deal

to the use in real physical systems than in other traditionally known

forms, in this dissertation work, equations, have been further

developed in which the loss terms in two of the fundamental

equations are grouped and represented in a single line equivalent.

Present work extends the single line equivalent network to be used

for load flow calculations and for deriving the condition for voltage

collapse to occur. Due to simplicity of the single line equivalent

technique, stability analysis based on this equivalence is much

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simplified making it most suitable for use in real time distribution

system monitoring. A further special feature of the method

illustrated in this work is that all voltage terms are eliminated from

the equations for solving the load flows there by simplifying the

equations for iterative solution.

In India, all the 11 KV rural distribution feeders are radial and

too long. The voltages at the far end of many such feeders are very

low with very high voltage regulation more preferable. Another

advantage of the distflow method is that it requires less computer

memory. Convergence is always guaranteed for any type of

practical radial distribution network with a realistic R/X ratio while

using the distflow method. Loads in the present formulation have

been represented as constant power. However, the dist flow method

can easily include composite load modelling. Several practical rural

radial distribution feeders in India have been successfully solved

using the dist flow method. The data of various radial systems can

be obtained using the On line production of load data in substations

[10] system described by Schrock and K.C.Kwong[10].

2.2. On Line Production of Load Data in Substations

The acquisition of statistical data on a power system is a

necessary part in its operation and planning. The essential

information, such as average daily load curve and maximum

demand is often derived by a combination of pulse summation and

computer analysis. A microprocessor based system has been

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developed which captures and analyses voltages and currents on

feeders and produces a set of reduced data which fully describes

the recent load pattern locally in the substation. Such a system is

described by F.S.Schroder and K.C.Kwong.

The load on a power system is characterised by the fact that it

is determined by the consumer and not the electricity authority.

Consequently, it must be metered firstly so that it can be controlled

and secondly so that a record is kept for later analysis. A

microprocessor systems is chosen so that the load data could be

fully analysed locally in the substation and only the minimum

amount of data consistent with the provision of all the necessary

information to describe what loads have occured is outputed. A

number of such statistical metering schemes are available

commercially. In the system developed by the author[10] the

statistical data is on-line and could be displayed on the screen of a

Video Display Unit or on a printer at any time. This data is also in a

much condensed format and is immediately usable for planning and

system operation purposes from the hard-copy printer output.

Alternatively the data could be transferred onto a cassette tape for

archive and

for further analysis.

The system as developed is aimed for the acquisition of

statistical data in a medium size substation. It caters for the

measurement of both balanced and unbalanced feeder loads using

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a flexible combination of input channels.

The data returned by the data acquisition system can be

accessed for functions such as

detection of power failures (when the voltage drops to below

20% of the rated value).

measurement of time when the voltage value falls outside the

+6% limits.

detection of maximum demands and their times of

occurrence. to predict the voltage collapse (point of

occurrence).

This system is a joint effort between the South East

Queensland Electricity Board & The Capricornia Institute of

Advanced Education.

2.3 Methodology

Distflow Method

2.3.1. Mathematical formulation of Techniques

Governing equations of a single-line system.

Before proceeding to the actual system we first derive the

equations that characterize the behaviour of a single-line system.

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Consider the single line in fig. 1 which has the following

parameters.

Where

P : Injection of real power

Q : Injection of reactive power

r : Resistance of the line

x : Reactance of the line

PL : Real load

QL : Reactive load

V : Voltage magnitude

From fig. (1) the real and reactive power equations have been

derived as

P = resistive loss in the line + real load

i.e. P = I2r+PL

the current through the line, I in terms of P and Q is

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I2 =2

22

V

QP +

Therefore the equation becomes

P = L2

22

PV

QPr +

+ (1)

Similarly the reactive power

Q = L2

22

QV

QPx +

+ (2)

From equations (1 and 2), we can eliminate the

+2

22

V

QPterms.

From equation (1)

P = L2

22

PV

QPr +

+

r

PP

V

QP L2

22 =

+ (3)

and from the equation (2)

Q = L2

22

QV

QPx +

+

X

QQ

V

QP L2

22 =

+ (4)

From equations (3) and (4) the resultant equation can be written as

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2

22LL

V

QP

x

QQ

r

PP +=

=

x

QQ

r

PP LL =

)PP(x L = r(Q-QL)

By rearranging5

)PP(x L = r(Q-QL)

(Q-QL) = )PP(r

xL

Q = LL QPPr

x+ )(

Squaring on both sides,

)(2)( 22

222

LLLL PPQr

xPP

r

xQQ ++=

( )r

)PP(xQ2PP2PP

r

xQQ LLL

2L

2

2

22L

2 +++=

Now this Q2 substitute in equation (1)

( ) LLLL2L222

2L

2

2P

r

)PP(xQ2PP2PP

r

xQP

V

rP +

++

+=

+

+= L2

22L2

22

2

22L

22

PP2r

xP

r

xP

r

xPP

V

r

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LLLL P

r

PxQ2

r

PxQ2+

The voltage at the sending end is the reference voltage and

its magnitude is kept constant, and in this case V2=1.

Therefore the equation becomes

0PPr

Qrx2Px

r

Qrx2xP2P

r

xrPQPr L

L2L

2L

2L

2222

L =+

+

++

From this equation a quadratic equation in terms of P is

obtained as follows:

0rPP)PQrx2Px()Qrx2xP2(P)xr(PrQ LLL2

L2

L2

L2222

L =++++

0rPPrQPrxQ2Px)rxQ2xP2(P)xr(p L2LLL

2L

2L

2L

222 =++++

Finally we get,

0rPPrxQ2rQPx)rrxQ2xP2(P)xr(PLLL

2

L

2

L

2

L

2

L

222 =+++++

From the above equation, the expression for P can be obtained as

( ) ( ) ( )

( )

+

++

=22

222

LL2

LL2

xr2

xr4rrxQ2Px2rrxQ2Px2

P

( )}

( )

+

++22

2/1

LLL2L

22L

2

xr2

rPQrxP2PxQr (6)

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Similarly for the reactive power Q,

Rearranging the equation (5) and eliminating P in equation (2)

( ) ( )LL QQrPPx =

( ) ( )L1 QQx

rPP =

P = ( ) LL PQQx

r+

Squaring on both sides,

( ) ( )LL2

L2

22L

2 QQx

rP2QQ

x

rPP

+

+=

( ) ( )LLL2L222

2L

2 QQr

xQ2QQ2QQ

x

rPP

++

+

Substitute this value of P in equation (2)

( ) ( ) L2LLL2L222

2L2

QQQQr

xP2QQ2QQ

x

rP

V

xQ +

+

++

+=

=

++

2L

2L

L2

2L2

L2 x

Qr

x

rP2Q

x

Qr

x

rP2QP

V

x

L22L

2

QQ

x

QQr2+

+

+++=

2

2L

2LL

2

22L2

L2 x

Qr

x

rPQ2

x

Qr

x

rQP2P

V

x

L2

2L

2

QQx

QQr2+

+

Q =

+

++ 2 L

2

L2

22

22L2 x

rxP2rQ2Q

x

xrQP

V

x

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L2

LL2L

2

Qx

PrxQ2Qr+

The above equation can be written as

+

++

2L

2L

2

2222

L2 x

rxP2rQ2Q

x

xrQP

V

x

0QQx

PrxQ2QxL2

LL2L

2

=+

+

+

+ xrxP2xQ2

V

Q

x

xr

V

Q

PV

x L2

L

2

22

2

22

L2

0QQx

PQrx2Qx

V

1L

LL2L

2

2=+

(Since V2=1).

Therefore the equation becomes,

++

x

rxP2rQ2Q

x

xrQPx L

2L

2222

L

+ 0QQx

PQrx2QrL

LL2L

2

=

0

x

QxxQPrxQ2QrrxP2rQ2QxrQxP LLL2L

2L

2L

2222L =

++++

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( ( ( 0xQxQPQrx2QrrxP2rQ2QxrQxP LLL2L2L2L2222L =++++

( ) ( ) ( ) 0xQrQPrxQ2PxxrxP2rQQxrQ L2LLL2L2L22L222 =++++++

This is the quadratic equation interms of Q.

From the above equation, the expression fro reactive power Q can

be obtained as

( ) ( )

( ) ( )( )22

LLL2L22

L222

2

LL2

LL2

xr2

x QQr x P2QrPxxr4

xr x P2Qr2xrx P2Qr2

Q+

+++

++

= (7)

2.3.2. Power Flow Equations:

Consider the radial network

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Fig. 4: Online Diagram of a radial Network

We represent the line with impedance iii jxrZ += and loads as

constant power sinks LLL jQPS += power flow in a radial distribution

network can be described by a set of recursive equations that are

structurally rich and conductive for computationally efficient

solutions. Those power flow equations are called DISTFLOW Branch

equations, that use the real power, reactive power and voltage

magnitude at the sending end of a branch i.e., P i, Q i, Vi respectively

to express the same quantities at the receiving end of the branch as

follows:

1iP + =

+

2

2

i2

iii

V

QPrP - ILiP + (8)

1iQ + =

+

2

2

i2

iii

V

QPxQ - ILiP + (9)

2IiV

+ = ( ) ( )

++++

2

i

2

i2

i2i

2

iiiii2

iV

QPxrQxPr2V (10)

Hence if P1, Q1, V1 at the first node of the network is known or

estimated, then the same quantities at the other nodes can be

calculated by applying the above branch equations successively. We

shall refer to this procedure as a FORWARD UP DATE.

Dist flow branch equations can be written backward too, i.e.,

by using the real power, reactive power and the voltage magnitude

at the receiving end of a branch Pi, Qi, Vi to express the same

quantities at the sending end of the branch. The result is the

following recursive equations, called the BACKWARD branch

equations.

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1iP =( ) ( )

Li2

i

21

i

21

iii P

V

QPrP +

++ (11)

1iQ + = ( ) ( ) Li2i

21

i1

iii Q

V

QPxQ +

++ (12)

1iQ = ( ) ( )( ) ( )

+++++

2

i

21

i

21

i2i

2

i1

ii1

ii2

iV

QPxrQxPr2V (13)

Where 1iP = Lii PP +

1

iQ = Lii QQ +

The procedure is referred as BACKWARD UPDATE. Similar to

forward update, a Back ward update can be defined.

Start updating from the last node of the network assuming the

variables nnn V,Q,P at that point are given and proceed backward

calculating the same quantities at the other nodes by applying (11),

(12) and (13) successfully. Updating process ends at the first node

(i.e., at node 1) and will provide the new estimate of the power

injections into the network P1 and Q1.

Note that by applying backward and forward update schemes

successively one can get a power flow solution.

2.3.3. Reduction of Real Network to a Single Line

Equivalent

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In this section we will show how a given power distribution

network can be reduced to a single line equivalent.

The real and reactive power flows in any line are given by

1iP + =

+

2

i

2

i2

iii

V

QPrP - 1LiP +

1iQ + =

+

2

i

2

i2

iii

V

QPXQ - 1LiQ +

The real and reactive loss terms in the above equations are

iLP =

+2

i

2

i2

ii

V

QPr (14)

iLQ =

+2

i

2

i2

i

i V

QP

x (15)

Using equation (2.14) the ratio of real losses (LPi) between

branch i and proceeding branch i+1 can be computed as,

i1i

LPLP + =

+

+

+

+++

2

i

2

i2

i1

2

1i

2

1i2

1i1i

V

QPr

V

QPr

= ( )2i

2

ii

2

1i2

1i1i

QPr

QPr

+

+ +++

+2

1i

2

i

V

V (16)

By considering the current flow in the branch i,

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+2

i

2

i2

i

V

QP=

( ) ( )

+++

+

++++2

1i

2

1Li1i2

1Li1i

V

QQPP

From the above equation the voltage ratio between branches i and

i+1 is

+2

1i

2

i

V

V=

( ) ( )

+++

+

++++2

1Li1i2

1Li1i

2

i2

i

QQPP

QP (17)

equation (2.17) can be submitted in equation (2.16) to get the ratio

of real losses

i

1i

LP

LP +=

( ) ( )

+++

+

+

+

++++

+++2

1Li1i2

1Li1i

2

i2

i2

i2

i

2

1i2

1i

i

1i

QQPP

QP

QP

QP

r

r

i

1i

LP

LP +=

( ) ( )

+++

+

++++

+++

21Li1i21Li1i

2

1i2

1i

i

1i

QQPP

QP

r

r

Similarly for the ratio of reactive losses

i

1i

LQ

LQ +=

+

+

+

+++

2

i

2

i2

ii

2

1i

2

1i2

1i1i

V

QPx

V

QPx

.

=(( )2

i2

ii

2

1i2

1i1i

QPx

QPx

+

+ +++

+2

1i

2

i

V

V

i

1i

LQ

LQ +=

( ) ( )

+++

+

++++

+++2

1Li1i2

1Li1i

2

1i2

1i

i

1i

PPPP

QP

X

X (19)

For a given distribution network the total injected real and reactive

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powers are:

P = Lii PLP + (20)

Q = Lii QLQ +

(21)

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From the equation (18) and (19) it can be seen that the losses

in the distribution network are ratios of the losses in the preceding

branch of the network.

Hence

P = ( ) Li22eq PQPr ++ (22)

Q = ( ) Li22eq QQPx ++ (23)

Since (V2 = 1)

Where

req is the equivalent resistance of the single line

and xeq is the equivalent reactance of the single line

Hence we have now reduced the real distribution network consisting

of many branches into a system with only one line.

The values of req and xeq can be obtained by

eqr = ( )2i2

iQP

TLP

+ (24)

xeq = ( )2i

2

iQP

TLQ

+ (25)

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Where

TLP = LPi is the total real power losses in the system with a

power injection of ii jQP + .

TLQ = LQi is the total reactive power losses in the system with a

power injection of ii jQP + .

Power Factor: Power factor is defined as Ratio of active power (in

KW) to the apparent power (in KVA).

22 QP

P.f.p

+

= (26)

Where p is active power.

q is reactive power

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REACTIVE POWER COMPENSATION

INTRODUCTION

Shunt and series reactive compensation using capacitors has

been 3 widely recognized and powerful method to combat the

problems of voltage drops, power losses, and voltage flicker in

power distribution networks. The importance of compensation

schemes has gone up in recent years due to the increased

awareness on energy conservation and quality of supply on the part

of the Power Utility as well as power consumers. This article (in two

parts) amplifies on the advantages that accrue from using shunt and

series capacitor compensation. It also tries to answer the twin

questions of how much to compensate and where to locate the

compensation capacitors.

3.1 SHUNT CAPACITOR COMPENSATION IN

DISTRIBUTION SYSTEMS:

Fig. 1 represents an a.c. generator supplying a load through a

line of series impedance (R+jX) ohms, fig. 2(a) shows the phasor

diagram when the line is delivering a complex power of (P+jQ) VA

and Fig. 2 -(b) shows the phasor diagram when the line is delivering

a complex power of (P+jO) VA i.e. with the load fully compensated.

A thorough examination of these phasor diagrams will reveal the

following facts. which is higher by a factor of

2

Cos

1

compared to

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the minimum power loss attainable in the system.

2. The loading on generator, transformers, line etc is decided by

the current flow. The higher current flow in the case of

uncompensated load necessitated by the reactive demand

results in a tie up of capacity in this equipment by a factor of

Cos

1i.e. compensating the load to UPF will release a capacity

of (load VA rating X Cos ) in all these equipment.

3. The sending-end voltage to be maintained for a specified

receiving-end voltage is higher in the case of uncompensated

load. The line has bad regulation with uncompensated load.

4. The sending-end power factor is less in the case of an

uncompensated one. This due to the higher reactive

absorption taking place in the line reactance.

5. The excitation requirements on the generator is severe in the

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case of uncompensated load. Under this condition, the

generator is required to maintain a higher terminal voltage

with a greater current flowing in the armature at a lower

lagging power factor compared to the situation with the same

load fully compensated. It is entirely possible that the required

excitation is much beyond the maximum excitation current

capacity of the machine and in that case further voltage drop

at receiving-end will take place due to the inability of the

generator to maintain the required sending-end voltage. It is

also clear that the increased excitation requirement results in

considerable increase in losses in the excitation system.

It is abundantly cleat from the above that compensating a

lagging load by using shunt capacitors will result in

i. Lesser power loss everywhere upto the location of capacitor

and hence a more efficient system

ii. Releasing of tied-up capacity in all the system equipments

thereby enabling a postponement of the capital intensive

capacity enhancement programmes to a later date.

iii. Increased life of eqipments due to optimum loading on them

iv. Lesser voltage drops in the system and better regulation

v. Less strain on the excitation system of generators and lesser

excitation losses.

vi. Increase in the ability of the generators to meet the system

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peak demand thanks to the released capacity and lesser

power losses.

Shunt capacitive compensation delivers maximum benefit

when employed right across the load. And employing compensation

in HT & LT distribution network is the closest one can get to the

load in a power network. However, various considerations like ease

of operation end control, economy achievable by lumping shunt

compensation at EHV stations etc will tend to shift a portion of shunt

compensation to EHV & HV substations. Power utilities in most

countries employ about 60% capacitors on feeders, 30% capacitors

on the substation buses and the remaining 10% on the transmission

system. Application of capacitors on the LT side is not usually

resorted to by the utilities.

Just as a lagging system power factor is detrimental to the

system on various counts, a leading system pf is also undesirable. It

tends to result in over-voltages, higher losses, lesser capacity

utilisation, and reduced stability margin in the generators. The

reduced stability margin makes a leading power factor operation of

the system much more undesirable than the lagging p.f operation.

This fact has to be given due to consideration in designing shunt

compensation in view of changing reactive load levels in a power

network.

Shunt compensation is successful in reducing voltage drop

and power loss problems in the network under steady load

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conditions. But the voltage dips produced by DOL starting of large

motors, motors driving sharply fluctuating or periodically varying

loads, arc furnaces, welding units etc can not be improved by shunt

capacitors since it would require a rapidly varying compensation

level. The voltage dips, especially in the case of a low short circuit

capacity system can result in annoying lamp-flicker, dropping out of

motor contactors due to U/V pick up, stalling of loaded motors etc

and fixed or switched shunt capacitors are powerless against these

voltage dips. But Thyristor controlled Static Var compensators with a

fast response will be able to alleviate the voltage dip problem

effectively.

3.2. SERIES CAPACITOR COMPENSATION IN

DISTRIBUTION SYSTEMS:

Shunt compensation essentially reduces the current flow

everywhere upto the point where capacitors are located and all

other advantages follow from this fact. But series compensation

acts directly on the series reactance of the line. It reduces the

transfer reactance between supply point and the load and thereby

reduces the voltage drop. Series capacitor can be thought of as a

voltage regulator, which adds a voltage proportional to the load

current and there by improves the load voltage.

Series compensation is employed in EHV lines to 1) improve

the power transfer capability 2) improve voltage regulation 3)

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Series capacitors, with their inherent ability to add a voltage

proportional to load current, will be the ideal solution for handling

the voltage dip problem brought about by motor starting, arc

furnaces, welders etc. And, usually the application of series

compensation in distribution system is limited to this due to the

complex protection required for the capacitors and the consequent

high cost. Also, some problems like self-excitation of motors during

starting, ferroresonance, steady hunting of synchronous motors etc

discourages wide spread use of series compensation in distribution

systems.

3.3. SHUNT CAPACITOR INSTALLATION TYPES:

The capacitor installation types and types of control for

switched capacitor are best understood by considering a long feeder

supplying a concentrated load at feeder end. This is usually a valid

approximation for some of the city feeders, which emanate from

substations, located 4 to 8 Kms away from the heart of the city. Ref

Figs 3 & 4.

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Absolute minimum power loss in this case will result when the

concentrated load is compensated to up by locating capacitors

across the load or nearby on the feeder. But the optimum value of

compensation can be arrived at only by considering a cost benefit

analysis.

The reactive demand of the load varies over a day and a

typical reactive demand curve for a day is given in fig. 5.

It is evident from fig.5 that it will require a continuously

variable capacitor to keep the compensation at economically

optimum level throughout the day. However, this can only be

approximated by switched capacitor banks. Usually one fixed

capacitor and two or three switched units will be employed to match

the compensation to the reactive demand of the load over a day.

The value of fixed capacitor is decided by minimum reactive

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demand as shown in Fig 5.

Automatic control of switching is required for capacitors

located at the load end or on the feeder. Automatic switching is

done usually by a time switch or voltage controlled switch as shown

in Fig 5. The time switch is used to switch on the capacitor bank

required to meet the day time reactive load and another capacitor

bank switched on by a low voltage signal during evening peak along

with the other two banks will maintain the required compensation

during night peak hours.

3.4 ECONOMIC JUSTIFICATION FOR USE OF

CAPACITORS:

The increase in benefits for 1 kVAR of additional compensation

decrease rapidly as the system power factor reaches close to unity.

This fact prompts an economic analysis to arrive at the optimum

compensation level. Different economic criteria can be used for this

purpose. The annual financial benefit obtained by using capacitors

can be compared against the annual equivalent of the total cost

involved in the capacitor installation. The decision also can be based

on the number of years it will take to recover the cost involved in

the Capacitor installation. A more sophisticated method would be

able to calculate the present value of future benefits and compare it

against the present cost of capacitor installation.

When reactive power is provided only by generators, each

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system component (generators, transformers, transmission and

distribution lines, switch gear and protective equipment etc) has to

be increased in size accordingly. Capacitors reduce losses and

loading in all these equipments, thereby effecting savings through

powerless reduction and increase in generator, line and substation

capacity for additional load. Depending on the initial power factor,

capacitor installations can release at least 30% additional capacity

in generators, lines and transformers. Also they can increase the

distribution feeder load capability by about 30% in the case of

feeders which were limited by voltage drop considerations earlier.

Improvement in system voltage profile will usually result in

increased power consumption thereby enhancing the revenue from

energy sales.

Thus, the following benefits are to be considered in an

economic analysis of compensation requirements.

i. Benefits due to released generation capacity.

ii. Benefits due to released transmission capacity.

iii. Benefits due to released distribution substation capacity.

iv: Benefits due to reduced energy loss.

v. Benefits due to reduced voltage drop.

vi. Benefits due to released feeder capacity.

vii. Financial Benefits due to voltage improvement.

Which are the benefits to be considered in capacitor

application in distribution system? Capacitors in distribution system

will indeed release generation and transmission capacities. But

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when an individual distribution feeder compensation is in question,

the value of released capacities in generation and transmission

system are likely to be too small to warrant inclusion in economic

analysis. Moreover, due to the tighty inter-connected nature of the

system, the exact benefit due to capacity release in these areas is

quite difficult to compute. Capacity release in generation and

transmission system is probably more relevant in compensation

studies at transmission and subtransmission levels and hence are

left out from the economic analysis of capacitor application in

distribution systems.

3.4.1. Benefits due to released distribution substation

capacity:

The released distribution substation capacity due to

installation of capacitors which deliver Qc MVARs of compensation

at peak load conditions may be shown to be equal to

c2

c

c

2/1

2

c

22

cc S1

S

SinQ

S

CosQ1S

+

=

In general and SinQS cc when1 0

SQ CC

reactive power compensation in radial distribution systems using dist flow method project

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