World Academy of Science, Engineering and Technology 48 2008

Design and Economics of Reactive Power

Control in Distribution Substation
Khin Trar Trar Soe

Abstract—An electrical power system consists of three 4. Long and overloaded 11kV and subtransmission lines.
principle components that are generation station transmission 5. Poor voltage regulation on 11 kV and L.T lines, voltage
line and distribution systems. A distribution system connects drops being extended beyond permissible.
all the individual loads in a given to the transmission lines. All 6. Under loading of distribution transformers.
inductive loads require two kinds of power to operate with 7. Absence of shunt compensation in the subtransmission
active power(kW) and reactive power (kVAR) in design and and distribution system; therefore, necessary to improve the
operation of alternating current electric power systems. A working of the power distribution systems to reduce the
significant factor reactive power has been recognized. There is unfavorable conditions and there by reduce losses, improve
important interrelation between active and reactive power voltage regulation, etc. The system improvement has to be
transmission. There are not purely sinusoidal wave forms,
planned properly with the following objectives in mind.
especially when it is compensated reactive power. The state
1. To reduce losses in the distribution and subtransmission
controlled reactive power sources almost always produce
system.
harmonics. In a design of static compensators, harmonics
should be considered individually. For a given distribution of 2. To improve the voltage regulation so as to bring it within
power, the losses in the system can be reduced by minimizing the prescribed limit.
the total flow of reactive power stability and voltage control in 3. To improve the power factor in the subtransmission and
reactive control need about the use fixed shunt reactors, shunt distribution system so as to get optimum utilization of
capacitors, series capacitors, synchronous condenses and /subtransmission/distribution capacities.
modern static compensator needed for reactive power control.
Reactive power compensating mainly transmission system II. SHUNT CAPACITORS
installed at substation is considered. The location of reactive Shunt capacitors can be used on the distribution system to
power control in distribution substation can be seen that improve the voltage regulation of the system. The shunt
reactive power control, inrush current, shunt capacitors, series capacitors, if connected to utilization equipment and switched
capacitors, shunt reactors, harmonics effect, ,economical on in accordance with the load, reduce the voltage drop in the
considerations and selection of using apparatus. distribution system and thus help in obtaining better voltage
regulation. If the utilization equipment draws a current which
Keywords— reactive power control, economical consideration,
inrush current, harmonics effect.
is fairly constant, the voltage regulation by the shunt capacitor
is more effective.
I. INTRODUCTION Shunt capacitors installed on a distribution system reduce
energy losses in every part of the system between capacitors
D UE to system expansion without proper and adequate
planning and financial provision for the works in time, a
large number of distribution systems have run into problems
and generators. The use of shunt capacitors improve the
voltage regulation of the system,
The size of the shunt capacitor banks varies from individual
such as poor voltage regulation, poor power factor, high losses
units of 5 to 25 kVA connected to the secondary or primary
and poor efficiency, over loading and less reliability for
circuits of a distribution system to a bank of capacitors of
continuity of supply. The causes for high losses and poor
large-size kVA connected to the bus of substation at the
voltage regulation in the distribution and subtransmission
primary voltage side.
system are:
1. Low power factor of the consumer installations.
III. SHUNT COMPEMSATION AT THE HT SUBSTATION
2. Long and over loaded L.T lines.
3. Distribution transformers’ centers located away from the The benefits of shunt compensation at the HT substations
load centers. are (i)the MVAR loading on the generating stations wherever
it is overloaded is reduced ;(ii) release in transmission system
capacity and reduction in transmission losses is released; and
Ms. Khin Trar Trar Soe is student of Mandalay Technological University (iii) release in losses in the subtransmission lines. Benefits
(e-mail: khintrartrarsoe@ gmail.com).
under (iii) can be worked out by considering the improvement

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in power factor and the consequent reduction of the line C1

current. The revenue due to kWh saved on subtransmission Imax = 2 × EN × (4)
L1
lines may be calculated at average cost per kWh at 11 kV bus.
The revenue due to release in transformer capacity may be worked Where, C1 = equivalent capacitance of the circuit in µF
out on the additional kWh. L1 = equivalent inductance between the energized
banks and bank to be energized in µF
IV. HT SHUNT CAPACITORS’ INSTALLATION REQUIREMENT EN = line to neutral voltage
The HT capacitors, 11 to 132 kV may be of the switched Thus it may be desirable to install parallel capacitor bank
and no switched type, depending on the minimum loading, with series reactors. The most important point to check is that
maximum voltage conditions of feeders or substations. In case such capacitors must have matched voltage rating with respect
of no switched capacitors, the switchgear and damping to reactors. Series reactors are normally installed to limit
reactors are not required. It has been found economical to inrush currents and to prevent excessive harmonic voltages.
install fixed capacitors and heavily-loaded 11 kV feeders for Series reactors chosen with respect to harmonics are large
compensation up to 30% of kVAR of average feeder load. enough that inrush currents cause no problems for capacitors
For switched capacitor banks, the switching and damping of and circuit breakers.
inrush currents and the suppression of harmonics need special
consideration. In the case of single capacitor banks, the VI. REACTORS
damping reaction is not normally required from the Reactors, like capacitors, are basic to and an integral part of
consideration of inrush currents at the time of switching. The both distribution and transmission power systems. Depending
system reactance including that of the transformer at which the on their function, reactors are connected in shunt or in series
capacitor bank is installed is adequate enough to bring down with the network; singularly (current limiting reactors, shunt
the value of inrush currents within safe limits of the capacitor reactors) or in conjunction with other basic components such
on switchgear. When a number of capacitor banks are used in as power capacitors (shunt capacitor switching reactors,
parallel, it may become necessary to use series reactors for capacitor discharge reactors filter reactors). Reactors are
limiting the inrush currents. utilized to provide inductive reactance in power circuits for a
wide variety of purposes. These include fault current limiting,
V. INRUSH CURRENT inrush current limiting for capacitors and motors, harmonic
The maximum peak inrush current can be approximately filtering, VAR compensation, reduction of ripple currents.
given by the formula: Reactors may be installed at any industrial, distribution, or
Imax = IC1 [ 1+XC1/XL1 ] (1) transmission voltage level. Shunt reactor compensation is
Where, IC1 = Capacitor’s rated current (fundamental wave) typically required under conditions that are the opposite of this
rms requiring shunt capacitor compensation.
XC1 = Capacitor reactance (fundamental wave)
XL1 = Total inductive reactance of the system including VII. LT CAPACITORS’ INSTALLATION
capacitor bank LT capacitors are installed on the distribution system on
(fundamental wave) individual lines or consumers motors to reduce system losses
The inrush current comprises a steady component of system losses and improve the system voltage and capacity. In
forced oscillation at supply frequency and a free oscillation of addition, they provide other advantages for the consumer, such
frequency. as reduction in kVA demand, losses and stable voltage . The
Inrush current frequency optimum benefit desired from the capacitors largely depends
1 1 R2 on the correct positioning of the capacitor in the system.
f0 = × (2)
2π L 1C 1 4 L21 VIII. SERIES AND SHUNT CAPACITORS
2
Neglecting terms, R because R is very small as
4 L 21 Capacitors aid in minimizing operating expenses and
compared to L1 allow the utilities to serve new loads and consumers with a
1 1 minimum system investment. Series and shunt capacitors in a
f0 = × (3) power system generate reactive power to improve power factor
2π L 1C 1
and voltage, thereby enhancing the system capacity and
In use of parallel banks, which already energized, the reducing the losses. In series capacitors the reactive power is
inrush current is mainly governed by the momentary discharge proportional to the square of the load current, whereas in shunt
energized capacitor bank and since the impedance between the capacitors it is proportional to the square of the voltage. There
energized capacitor bank and the capacitor bank to be are certain unfavorable aspects of series capacitors.
energized may be small, it may result in high peak inrush Generally the cost of installing series capacitors is higher
current. The maximum peak current is given by the expression: than that of a corresponding installation of a shunt capacitor.

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This is because the protective equipment for a series capacitor 3. Harmonic studies to determine the series and parallel
is often more complicated. The factors which influence the resonance points in the system with connection of filter banks.
choice between the shunt and series capacitors are summarized Resonance occurs whenever an electrical circuit’s inductive
in Table 1. and capacitive reactance connected either in parallel or series
are equal at some frequency. The frequency at which a circuit
TABLE1. SERIES AND SHUNT CAPACITORS is in resonance is called the natural frequency of the circuit. A
shunt capacitor bank forms a resonant circuit with system
Preference inductive elements. This resonance condition can be excited by
Sr.No. Objective
Series Shunt remote system disturbances such as remote bank switching or
capacitor capacitor sources of harmonics current. Resonance can cause excessive
1 Improve power factor Second First over-voltages and currents possibly resulting in failure of
equipment such as capacitors, surge arresters, instrument
Improve voltage level in an transformers, and fuses.
2 overhead line system with a First Second
normal and low power factor A. Harmonic Resonance
Improve voltage level in an Capacitor banks may resonate with harmonic currents
3 overhead line system with a Not used First produced else where on the system. Harmonic current flow
high power factor into the capacitor bank may excite parallel resonance between
the system inductance and bank capacitance. Parallel
Improve voltage in an
4 underground line system with a First Not used resonance causes high oscillating current between inductive
normal and low power factor and capacitive energy-storage elements. High oscillating
currents cause excessive voltage distortion.
Improve voltage in an Installing current-limiting reactors in series with the shunt
5 underground line system with a Not used Not used
high power factor
capacitor bank can tune the bank to the offending harmonic’s
frequency and eliminate parallel resonance. Parallel resonance
6 Reduce line losses Second First is avoided since harmonic current cannot flow between the
system inductance and the bank’s capacitance.
7 Reduce voltage fluctuations First Not used
X. THE DEGREE OF COMPENSATION
Due to various limitations in the use of series capacitors,
shunt capacitors are widely used in distribution systems. For The degree of compensation being decided by an economic
the same voltage improvement, the rating of a shunt capacitor point of view between the capitalized cost of compensator and
will be higher than that of a series capacitor. Thus a series the capitalized cost of reactive power from supply system over
capacitor stiffens the system, which is especially beneficial for a period of time. In practice a compensator such as a bank of
starting large motors from an otherwise weak power system, capacitors (or inductors) can be divided into parallel sections,
for reducing light flicker caused by large fluctuating load, etc. each Switched separately, so that discrete changes in the
compensating reactive power may be made, according to the
IX. RESONANCE AND HARMONIC requirements of the load.
Reasons for the application of shunt capacitor units are
For capacitor banks connected to high-voltage system series because of
reactor must be used (a) for limiting the inrush current on 1. Increase voltage level at the load
energisation of bank and (b) to suppress the harmonics in order 2. Improves voltage regulation if the capacitor units ar
to prevent harmonic overloading of the bank as well as to properly switched.
avoid undesirable parallel resonance with the system 3. Reduces I2R power loss in the system because of
reactance. It is therefore advisable for economic reason, to reduction in current.
combine the power factor correction and harmonic filtering in 4. Reduces I2X kVAR loss in the system because of
the same bank. However depending on the most prominent reduction in current.
harmonics in a particular installation, a number of banks may 5. Increases power factor of the source generator.
be necessary and needs to be determined by following system 6. Decrease kVA loading on the source generators and
studies. circuits to relieve an overloaded condition or release capacity
1. Short circuit study to evaluate the range of various of for additional load growth.
system impedance at the point of connection of compensation 7.By reducing kVA loading on the source generators
equipment. additional kilowatt loading may be placed on the
2. Load flow study to evaluate the range of vibration of generation if turbine capacity is available.
system voltage. 8. To reduce demand power is purchased. Correction to
100 percent power factor may be economical in some cases.

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9. Reduces investment in system facilities per kilowatt of as small as possible to achieve maximum economy for a given
load supplied. amount of energy supplied. Assuming a fixed maximum power
kV A R requirement, this can only achieved by power factor
correction.
Q2 It is possible to correct low power factor up to unity power
factor, thus making the power factor in kW and kVA loading
on a power system equal. But owing to the cost of power
P kW factor correction equipment this is never economically
φ2 justified. Hence the load is partially compensated ( i.e. | Qγ| <
φ1
|QL| ) the degree of compensation being decided by an
S2 economic between the capital cost of the compensator ( which
Q2
depends on it’s rating) and the capitalized cost of obtaining the
reactive power from the supply system over a period of time.

Q1 S1
Fig 1. Phasor Diagram of Improving Power Factor XII. ESTIMATION OF MOST ECONOMICAL POWER FACTOR

p.f1=φ1 p.f2 = cos φ2

-1
φ1 = cos (p.f1) φ2 = cos-1(p.f2)
Q1 = P tan φ1 Q2 = P tan φ2

Size of capacitor to improve power factor from p.f1 to p.f2

Qc = Q1- Q2
= P tan φ1 - P tan φ2
= P(tan φ1− tan φ2)

XI. ECONOMIC OF REACTIVE POWER CONTROL

The electrical loading on electrical apparatus in power
systems is a kVA loading. Such apparatus is designed to work
at a definite voltage and not to exceed a definite maximum
current. Both the operating voltage and the current, core losses
and there together must not exceed the power which the
apparatus can dissipate without exceeding its maximum Fig 2. Power diagram for Estimation of Power Factor
working temperature.
P = VI cos φ1 = VIa (5)
For a particular power system, voltage is constant and
Consider a load of power (kW) per phase at a lagging
current is limited by the losses. Therefore, the volt-ampere (or
power factor of cos φ. It is required to correct the power factor
kVA) has a maximum value and from P =VI cos φ the greater
at the consumer’s terminal by connecting power factor
the value of cos φ the greater the power transmitted. It is thus
correction capacitance ‘C’ to give the most economical power
an economical to work with low power factor since the power
factor.
transmitted by the apparatus is reduced. It is also advantages,
Since the load power is constant, only the reactance
when a given amount of energy is to be transmitted, that this is
component of I2 (the current taken from the supply after power
done at lower power level over a long period of time, i.e. with
factor correction) is variable.
a high load factor. Thus kVA loading is reduced by having
Before power factor correction,
both the high load factor and a high power factor.
Annual cost (1) = AM1 + keU (6)
In order to induce consumers to work with minimum kVA
After power factor correction,
and also to make those pay most who make the most demand
Annual cost (2) = AM2 + B (kVAR1 –kVAR2) + keU (7)
on the power system, a two tariff may be used. A consumer’s
where B = the annual charge per kVAR of the power factor
annual cost is there of the form-
correction equipment
(AM + keU) kyats
kVAR1 = reactance power from the supply before power
Where A = kyats per annum per kVA maximum demand
factor correction
M= maximum demand
kVAR2 = reactance power from the supply after power
Ke = a charge ( kyats) per kWhr for each energy
factor correction
consumed
The power triangle before and after power factor correction
U = energy consumed in a years ( average load)
may be drawn as show below.
From the above expression, the consumer should make ‘M’

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A. Configuration

Fig 3.Triangle Diagram

From the above two equations,

Annual saving = cost (1) – cost (2)
= A [M1-M2] – B [kVAR1- kVAR2]
= A [P sec φ1 - P sec φ2] – B [P tan φ1 - P tan φ2] (8)
Differentiating the saving with respect to the variable φ2 and
equating to zero for maximum saving,
A P [sec φ1 tan φ2] + P B [-sec2 φ2] = 0
A tan φ2 = B sec φ2
B XIV. COMPENSATED CONDITION TO INCREASE THE VOLTAGE
sin φ2 = (9)
A OF SITTAUNG 33 KV TO 32.488 KV
is the condition for minimum cost. Hence φ2 will be the Rs =8.15993Ω
most economical power factor. Xs = 19.14005Ω
P =5 MW
XIII. REACTIVE POWER CONTROL IN DISTRIBUTION QL = 3.096 MVAR
SUBSTATION FROM BAYARGYI TO SITTAUNG Reactive power supply from system is
Qs = 1.49572MVAR
Q γ = 1 MVAR
One parallel capcitor banks 1 MVAR is applied in this
distribution substation.
After compensation
Supply voltage= E = V + ∆V
= 33.7526+j 2.57
= 33.85 ∠ 4.3542 kV
The total current in the supply line,
Is = 0.1606 ∠ -16.6698 kA
Fig 4. One line diagram from Bayargyi to Sittaung The compensator current, I γ =j 0.0307kA
TABLE 2.CALCULATION RESULT FOR REACTIVE POWER CONTROL(POWER
FACTOR CORRECTION) AND VOLTAGE REGULATION WITH CAPACITOR BANKS The current flow into the load = I L=0.199 ∠ -39.488 kA
Power factor =cos16.6678 = 0.9579 (lag:)
From Bayargyi Voltage regulation = 1.575%
To Sittaung
kV 33
Length 9.75 28.19 12.73 XV. CONCLUSION
(km) One of the reasons for improving the power factor is to
Conductor ACSR ACSR ACSR decrease the reactive power. Another reason for improving for
size 397.5 397.5 397.5 the power factor is to avoide poor voltage regulation. Power
(mcm) factor improvement may be achieved the use of synchronous
code Ibis Ibis Ibis motor. But this paper use the capacitor banks because it has no
Rdc( 0.2306Ω 0.23033Ω .023.32Ω moving parts , initial cost is low, reaction in failures. This
per mile)
paper will help and give the knowledge of the power factor
Rac 1.57163 Ω 4.5387Ω 2.0496Ω
correction for distribution substation and calculation of size of
XL 3.6024Ω 10.8342Ω 4.7.345Ω capacitor banks to improve p.f and voltage regulation. We can
Z (Ω) 1.571+j3.60 4.538+j10.83 2.049+j4.703 calculate the economics about power factor correction

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(reactive power control) .If we know about the consumer kVA,

those pay demand on the power system, a two tariff ,we can
calculate the maximum economy for amount of energy
supplied. This paper can see and calculate for minimum cost
and the most economical power will be φ2

ACKNOWLEDGMENT
Firstly, the author would like to express her indebtedness
and gratitude to her beloved parents, for their kindness,
support, understanding during the whole course of this work
and encouragement to attain ambition without any trouble. The
author is indebted to all her teachers who give her knowledge
from M.T.U and Y.T.U in Myanmar.

REFERENCES
[1] T.J.E.MILLER, 1982. “Reactive Power Control in Electric Systems”
1982 by Jihn Wiley & Sons Inc.
[2] R.K.Mukhopashyay and T.Choudhury, S.P. Choudhury,
Samiran Choudhuri, F.I.E, Power System for the year 2000 and
beyond.
“Reactive Power Compensation in Industrial Power Distribution System”
[3] A.S pabla, “Electric Power Distribution”
(Fourth edition) Tata McGraw-Hill Pubkishing Company Limited.
[4] Williiam D. Stevenson, Jr, “Elements of Power System Analysis”
(Third ediion) 1955,1962,1975 by Mc Graw-Hill, Inc.
[5] ]Bernhardt G.A.SKROTZKI,. “Electric Transmission & Distribution.”
1954 Jersey Central Power and Light Cmpany.
[6] Glen Ballou, "Electrical Engineering HandBook”. 1999.
[7] Ed LL.Grisby Boca Ratton,.” Electrical Power Engineering.” 2001
[8] .R.S.ARORA. “Handbook of Electrical Engineering.” 2004. (Fourth
edition), New Dehli.
[9] A.Johnson, “Electrical Transmission and Distribution Reference
Book”. Oxford & IBH publishing Company.

Ms. Khin Trar Trar Soe received her M.E degree in Electrical Power
Engineering from Yangon Technological University, and then following three
months training in industry; joined the Department of Electrical Power
Engineering at Technological University (Loikaw, Myanmar) where she
taught courses in Transmission and Distribution for five months. Her interests
include Transmission and Distribution in Station and substation..She is a
student of Mandalay Technological University.

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