World Academy of Science, Engineering and Technology 48 2008
Design and Economics of Reactive Power
Control in Distribution Substation
Khin Trar Trar Soe
4. Long and overloaded 11kV and subtransmission lines.
5. Poor voltage regulation on 11 kV and L.T lines, voltage
drops being extended beyond permissible.
6. Under loading of distribution transformers.
7. Absence of shunt compensation in the subtransmission
and distribution system; therefore, necessary to improve the
working of the power distribution systems to reduce the
unfavorable conditions and there by reduce losses, improve
voltage regulation, etc. The system improvement has to be
planned properly with the following objectives in mind.
1. To reduce losses in the distribution and subtransmission
system.
2. To improve the voltage regulation so as to bring it within
the prescribed limit.
3. To improve the power factor in the subtransmission and
distribution system so as to get optimum utilization of
/subtransmission/distribution capacities.
Abstract—An electrical power system consists of three
principle components that are generation station transmission
line and distribution systems. A distribution system connects
all the individual loads in a given to the transmission lines. All
inductive loads require two kinds of power to operate with
active power(kW) and reactive power (kVAR) in design and
operation of alternating current electric power systems. A
significant factor reactive power has been recognized. There is
important interrelation between active and reactive power
transmission. There are not purely sinusoidal wave forms,
especially when it is compensated reactive power. The state
controlled reactive power sources almost always produce
harmonics. In a design of static compensators, harmonics
should be considered individually. For a given distribution of
power, the losses in the system can be reduced by minimizing
the total flow of reactive power stability and voltage control in
reactive control need about the use fixed shunt reactors, shunt
capacitors, series capacitors, synchronous condenses and
modern static compensator needed for reactive power control.
Reactive power compensating mainly transmission system
installed at substation is considered. The location of reactive
power control in distribution substation can be seen that
reactive power control, inrush current, shunt capacitors, series
capacitors, shunt reactors, harmonics effect, ,economical
considerations and selection of using apparatus.
II. SHUNT CAPACITORS
Shunt capacitors can be used on the distribution system to
improve the voltage regulation of the system. The shunt
capacitors, if connected to utilization equipment and switched
on in accordance with the load, reduce the voltage drop in the
distribution system and thus help in obtaining better voltage
regulation. If the utilization equipment draws a current which
is fairly constant, the voltage regulation by the shunt capacitor
is more effective.
Shunt capacitors installed on a distribution system reduce
energy losses in every part of the system between capacitors
and generators. The use of shunt capacitors improve the
voltage regulation of the system,
The size of the shunt capacitor banks varies from individual
units of 5 to 25 kVA connected to the secondary or primary
circuits of a distribution system to a bank of capacitors of
large-size kVA connected to the bus of substation at the
primary voltage side.
Keywords— reactive power control, economical consideration,
inrush current, harmonics effect.
I. INTRODUCTION
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
such as poor voltage regulation, poor power factor, high losses
and poor efficiency, over loading and less reliability for
continuity of supply. The causes for high losses and poor
voltage regulation in the distribution and subtransmission
system are:
1. Low power factor of the consumer installations.
2. Long and over loaded L.T lines.
3. Distribution transformers’ centers located away from the
load centers.
III. SHUNT COMPEMSATION AT THE HT SUBSTATION
The benefits of shunt compensation at the HT substations
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
(iii) release in losses in the subtransmission lines. Benefits
under (iii) can be worked out by considering the improvement
Ms. Khin Trar Trar Soe is student of Mandalay Technological University
(e-mail: khintrartrarsoe@ gmail.com).
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World Academy of Science, Engineering and Technology 48 2008
in power factor and the consequent reduction of the line
current. The revenue due to kWh saved on subtransmission
lines may be calculated at average cost per kWh at 11 kV bus.
The revenue due to release in transformer capacity may be worked
out on the additional kWh.
Imax =
The HT capacitors, 11 to 132 kV may be of the switched
and no switched type, depending on the minimum loading,
maximum voltage conditions of feeders or substations. In case
of no switched capacitors, the switchgear and damping
reactors are not required. It has been found economical to
install fixed capacitors and heavily-loaded 11 kV feeders for
compensation up to 30% of kVAR of average feeder load.
For switched capacitor banks, the switching and damping of
inrush currents and the suppression of harmonics need special
consideration. In the case of single capacitor banks, the
damping reaction is not normally required from the
consideration of inrush currents at the time of switching. The
system reactance including that of the transformer at which the
capacitor bank is installed is adequate enough to bring down
the value of inrush currents within safe limits of the capacitor
on switchgear. When a number of capacitor banks are used in
parallel, it may become necessary to use series reactors for
limiting the inrush currents.
Reactors, like capacitors, are basic to and an integral part of
both distribution and transmission power systems. Depending
on their function, reactors are connected in shunt or in series
with the network; singularly (current limiting reactors, shunt
reactors) or in conjunction with other basic components such
as power capacitors (shunt capacitor switching reactors,
capacitor discharge reactors filter reactors). Reactors are
utilized to provide inductive reactance in power circuits for a
wide variety of purposes. These include fault current limiting,
inrush current limiting for capacitors and motors, harmonic
filtering, VAR compensation, reduction of ripple currents.
Reactors may be installed at any industrial, distribution, or
transmission voltage level. Shunt reactor compensation is
typically required under conditions that are the opposite of this
requiring shunt capacitor compensation.
The maximum peak inrush current can be approximately
given by the formula:
(1)
Imax = IC1 [ 1+XC1/XL1 ]
Where, IC1 = Capacitor’s rated current (fundamental wave)
rms
XC1 = Capacitor reactance (fundamental wave)
XL1 = Total inductive reactance of the system including
capacitor bank
(fundamental wave)
The inrush current comprises a steady component of
forced oscillation at supply frequency and a free oscillation of
frequency.
Inrush current frequency
1
L 1C 1
R2
4 L21
2
Neglecting terms, R
4 L 21
VII. LT CAPACITORS’ INSTALLATION
LT capacitors are installed on the distribution system on
individual lines or consumers motors to reduce system losses
system losses and improve the system voltage and capacity. In
addition, they provide other advantages for the consumer, such
as reduction in kVA demand, losses and stable voltage . The
optimum benefit desired from the capacitors largely depends
on the correct positioning of the capacitor in the system.
(2)
VIII. SERIES AND SHUNT CAPACITORS
because R is very small as
Capacitors aid in minimizing operating expenses and
allow the utilities to serve new loads and consumers with a
minimum system investment. Series and shunt capacitors in a
power system generate reactive power to improve power factor
and voltage, thereby enhancing the system capacity and
reducing the 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. There
are certain unfavorable aspects of series capacitors.
Generally the cost of installing series capacitors is higher
than that of a corresponding installation of a shunt capacitor.
compared to L1
f0 =
1
×
2π
(4)
VI. REACTORS
V. INRUSH CURRENT
1
×
2π
C1
L1
Where, C1 = equivalent capacitance of the circuit in µF
L1 = equivalent inductance between the energized
banks and bank to be energized in µF
EN = line to neutral voltage
Thus it may be desirable to install parallel capacitor bank
with series reactors. The most important point to check is that
such capacitors must have matched voltage rating with respect
to reactors. Series reactors are normally installed to limit
inrush currents and to prevent excessive harmonic voltages.
Series reactors chosen with respect to harmonics are large
enough that inrush currents cause no problems for capacitors
and circuit breakers.
IV. HT SHUNT CAPACITORS’ INSTALLATION REQUIREMENT
f0 =
2 × EN ×
1
L 1C 1
(3)
In use of parallel banks, which already energized, the
inrush current is mainly governed by the momentary discharge
energized capacitor bank and since the impedance between the
energized capacitor bank and the capacitor bank to be
energized may be small, it may result in high peak inrush
current. The maximum peak current is given by the expression:
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World Academy of Science, Engineering and Technology 48 2008
3. Harmonic studies to determine the series and parallel
resonance points in the system with connection of filter banks.
Resonance occurs whenever an electrical circuit’s inductive
and capacitive reactance connected either in parallel or series
are equal at some frequency. The frequency at which a circuit
is in resonance is called the natural frequency of the circuit. A
shunt capacitor bank forms a resonant circuit with system
inductive elements. This resonance condition can be excited by
remote system disturbances such as remote bank switching or
sources of harmonics current. Resonance can cause excessive
over-voltages and currents possibly resulting in failure of
equipment such as capacitors, surge arresters, instrument
transformers, and fuses.
This is because the protective equipment for a series capacitor
is often more complicated. The factors which influence the
choice between the shunt and series capacitors are summarized
in Table 1.
TABLE1. SERIES AND SHUNT CAPACITORS
Preference
Sr.No.
Objective
1
2
Series
capacitor
Shunt
capacitor
Improve power factor
Second
First
Improve voltage level in an
overhead line system with a
normal and low power factor
First
Second
3
Improve voltage level in an
overhead line system with a
high power factor
Not used
First
4
Improve voltage in an
underground line system with a
normal and low power factor
First
Not used
5
Improve voltage in an
underground line system with a
high power factor
Not used
Not used
6
Reduce line losses
Second
First
7
Reduce voltage fluctuations
First
Not used
A. Harmonic Resonance
Capacitor banks may resonate with harmonic currents
produced else where on the system. Harmonic current flow
into the capacitor bank may excite parallel resonance between
the system inductance and bank capacitance. Parallel
resonance causes high oscillating current between inductive
and capacitive energy-storage elements. High oscillating
currents cause excessive voltage distortion.
Installing current-limiting reactors in series with the shunt
capacitor bank can tune the bank to the offending harmonic’s
frequency and eliminate parallel resonance. Parallel resonance
is avoided since harmonic current cannot flow between the
system inductance and the bank’s capacitance.
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 same voltage improvement, the rating of a shunt capacitor
will be higher than that of a series capacitor. Thus a series
capacitor stiffens the system, which is especially beneficial for
starting large motors from an otherwise weak power system,
for reducing light flicker caused by large fluctuating load, etc.
The degree of compensation being decided by an economic
point of view between the capitalized cost of compensator and
the capitalized cost of reactive power from supply system over
a period of time. In practice a compensator such as a bank of
capacitors (or inductors) can be divided into parallel sections,
each Switched separately, so that discrete changes in the
compensating reactive power may be made, according to the
requirements of the load.
Reasons for the application of shunt capacitor units are
because of
1. Increase voltage level at the load
2. Improves voltage regulation if the capacitor units ar
properly switched.
3. Reduces I2R power loss in the system because of
reduction in current.
4. Reduces I2X kVAR loss in the system because of
reduction in current.
5. Increases power factor of the source generator.
6. Decrease kVA loading on the source generators and
circuits to relieve an overloaded condition or release capacity
for additional load growth.
7.By reducing kVA loading on the source generators
additional kilowatt loading may be placed on the
generation if turbine capacity is available.
8. To reduce demand power is purchased. Correction to
100 percent power factor may be economical in some cases.
IX. RESONANCE AND HARMONIC
For capacitor banks connected to high-voltage system series
reactor must be used (a) for limiting the inrush current on
energisation of bank and (b) to suppress the harmonics in order
to prevent harmonic overloading of the bank as well as to
avoid undesirable parallel resonance with the system
reactance. It is therefore advisable for economic reason, to
combine the power factor correction and harmonic filtering in
the same bank. However depending on the most prominent
harmonics in a particular installation, a number of banks may
be necessary and needs to be determined by following system
studies.
1. Short circuit study to evaluate the range of various of
system impedance at the point of connection of compensation
equipment.
2. Load flow study to evaluate the range of vibration of
system voltage.
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World Academy of Science, Engineering and Technology 48 2008
as small as possible to achieve maximum economy for a given
amount of energy supplied. Assuming a fixed maximum power
requirement, this can only achieved by power factor
correction.
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
factor correction equipment this is never economically
justified. Hence the load is partially compensated ( i.e. | Qγ| <
|QL| ) the degree of compensation being decided by an
economic between the capital cost of the compensator ( which
depends on it’s rating) and the capitalized cost of obtaining the
reactive power from the supply system over a period of time.
9. Reduces investment in system facilities per kilowatt of
load supplied.
kV A R
Q2
P
φ1
kW
φ2
S2
Q2
Q1
S1
XII. ESTIMATION OF MOST ECONOMICAL POWER FACTOR
Fig 1. Phasor Diagram of Improving Power Factor
p.f1=φ1
p.f2 = cos φ2
-1
φ1 = cos (p.f1)
Q1 = P tan φ1
φ2 = cos-1(p.f2)
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
working temperature.
For a particular power system, voltage is constant and
current is limited by the losses. Therefore, the volt-ampere (or
kVA) has a maximum value and from P =VI cos φ the greater
the value of cos φ
 the greater the power transmitted. It is thus
an economical to work with low power factor since the power
transmitted by the apparatus is reduced. It is also advantages,
when a given amount of energy is to be transmitted, that this is
done at lower power level over a long period of time, i.e. with
a high load factor. Thus kVA loading is reduced by having
both the high load factor and a high power factor.
In order to induce consumers to work with minimum kVA
and also to make those pay most who make the most demand
on the power system, a two tariff may be used. A consumer’s
annual cost is there of the form(AM + keU) kyats
Where A = kyats per annum per kVA maximum demand
M= maximum demand
Ke = a charge ( kyats) per kWhr for each energy
consumed
U = energy consumed in a years ( average load)
From the above expression, the consumer should make ‘M’
Fig
2. Power diagram for Estimation of Power Factor
P = VI cos φ1 = VIa
(5)
Consider a load of power (kW) per phase at a lagging
power factor of cos φ. It is required to correct the power factor
at the consumer’s terminal by connecting power factor
correction capacitance ‘C’ to give the most economical power
factor.
Since the load power is constant, only the reactance
component of I2 (the current taken from the supply after power
factor correction) is variable.
Before power factor correction,
Annual cost (1) = AM1 + keU
(6)
After power factor correction,
Annual cost (2) = AM2 + B (kVAR1 –kVAR2) + keU (7)
where B = the annual charge per kVAR of the power factor
correction equipment
kVAR1 = reactance power from the supply before power
factor correction
kVAR2 = reactance power from the supply after power
factor correction
The power triangle before and after power factor correction
may be drawn as show below.
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World Academy of Science, Engineering and Technology 48 2008
A. Configuration
Fig 3.Triangle Diagram
From the above two equations,
Annual saving = cost (1) – cost (2)
= A [M1-M2] – B [kVAR1- kVAR2]
(8)
= A [P sec φ1 - P sec φ2] – B [P tan φ1 - P tan φ2]
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
sin φ2 =
B
A
XIV. COMPENSATED CONDITION TO INCREASE THE VOLTAGE
OF SITTAUNG 33 KV TO 32.488 KV
(9)
is the condition for minimum cost. Hence φ2 will be the
most economical power factor.
Rs =8.15993Ω
Xs = 19.14005Ω
P =5 MW
QL = 3.096 MVAR
Reactive power supply from system is
Qs = 1.49572MVAR
Q γ = 1 MVAR
XIII. REACTIVE POWER CONTROL IN DISTRIBUTION
SUBSTATION FROM BAYARGYI TO SITTAUNG
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
The compensator current, I γ =j 0.0307kA
The current flow into the load = I L=0.199 ∠ -39.488 kA
Power factor =cos16.6678 = 0.9579 (lag:)
Voltage regulation = 1.575%
Fig 4. One line diagram from Bayargyi to Sittaung
TABLE 2.CALCULATION RESULT FOR REACTIVE POWER CONTROL(POWER
FACTOR CORRECTION) AND VOLTAGE REGULATION WITH CAPACITOR BANKS
From
To
kV
Length
(km)
Conductor
size
(mcm)
code
Rdc(
per mile)
Rac
XL
Z (Ω)
Bayargyi
Sittaung
33
9.75
28.19
12.73
ACSR
397.5
ACSR
397.5
ACSR
397.5
Ibis
0.2306Ω
Ibis
0.23033Ω
Ibis
.023.32Ω
1.57163 Ω
3.6024Ω
1.571+j3.60
4.5387Ω
10.8342Ω
4.538+j10.83
2.0496Ω
4.7.345Ω
2.049+j4.703
XV. CONCLUSION
One of the reasons for improving the power factor is to
decrease the reactive power. Another reason for improving for
the power factor is to avoide poor voltage regulation. Power
factor improvement may be achieved the use of synchronous
motor. But this paper use the capacitor banks because it has no
moving parts , initial cost is low, reaction in failures. This
paper will help and give the knowledge of the power factor
correction for distribution substation and calculation of size of
capacitor banks to improve p.f and voltage regulation. We can
calculate the economics about power factor correction
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World Academy of Science, Engineering and Technology 48 2008
(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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