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Application of Switched Capacitor banks for Power Factor Improvement and
Harmonics Reduction on the Nigerian Distribution Electric Network
Oodo Ogidi Stephen , Liu Yanli and Sun Hui
Abstracts
The Distribution System in Nigeria is faced with low voltage and high loss, these two
problems of voltage drop and losses in the distribution network varies with the
pattern of loading on the distribution network. Voltage regulation on the
distribution network becomes an important issue due to the presence of many
industrial loads which vary their demand for reactive power. These industrial loads
having lagging power-factor; they absorb reactive power which can deteriorate the
quality of supply on the distribution network. This paper which to examine an
integrated for an optimal cost effective approach to improve the power factor and
harmonic issue by the use of Switched capacitor banks to provide a controllable
and variable amount of reactive power precisely according to the requirement of the
load to such that power factor is improved and harmonics are reduced on the
distribution network , losses are within limits and also satisfy the statutory voltage
limits at customers premises. Capacitor Bank and Reactors devices with mechanical
time control switch can be connected in parallel to the distribution network to
supply the type of reactive power or current needed to counteract the out of phase
component of current required by the inductive load to eliminate or reduce to an
acceptable limit the voltage regulation. These conventional compensators can be
switched in and out of the system by mechanical switches based on the system
loading as it varies throughout the day. The objective shall be achieved by an
evaluation of the operational requirement for power factor requirement and voltage
profile on a typical Nigerian distribution Network
Keywords: Distribution System, Voltage regulation, Power factor, Switched
Capacitor bank, Mechanical Switches.
Introduction.
Voltage Drop in a Distribution System
A basic overview on voltage drop in a distribution system is shown in a one line
diagram in Figure 1. The current I as a function of the load complex apparent power
S = PL jQL and the load voltage U 2 will be [26]
I=
S
P − jQL
= L
U2
U2
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U0
Grid
I
U1
U2
XTX
RLN,XLN
Substation
Feeder
U1
59
PL,QL
Load
jIXLN
U2
I
IRLN
Fig. 1-One line diagram and corresponding phasor diagram for an illustration of the voltage drop in a
distribution system
The voltage drop on the feeder is given by
U1 − U 2 = I ( RLN + jX LN )
=
( RLN PL + X LN QL ) − j ( X LN PL − RLN QL )
U2
(1)
For a small power flow, the voltage angle δ between U 2 and U1 in (3-2) is small, and the
voltage drop ∆U = U1 − U 2 can be approximated by
∆U ≈
RLN PL + X LN QL
U2
(2)
It can be seen from equations (1) and (2) that the load always causes voltage drop. This
voltage is decreasing towards the end of the distribution System.
For a small power flow, the voltage angle δ between U 2 and U1 in (2) is small, and the
voltage drop ∆U = U1 − U 2 can be approximated by
∆U ≈
RLN PL + X LN QL
U2
1.2 Reactive Power Control with Switched Shunt Capacitors
Shunt capacitors inject reactive power to the system according to [26]
Qc = QC ,ratU C2
where
QC is the reactive power injected by the capacitor in Mvar
QC , rat is the Mvar rating of the capacitor
U C is the voltage in pu (relative to the capacitor voltage rating).
The reactive power injected by the capacitor will compensate the reactive power demand
and therby boost the voltage. For example, consider that in Figure 1 a shunt capacitor
injecting reactive power QC is connected to the load bus. The voltage drop on the feeder
can then be approximated as
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RLN PL + X LN (QL − QC )
U2
Which indicates that the capacitor reduces the voltage drop. Further, when the capacitor
properly compensates the reactive power demand, the capacitor will decrease the feeder
current. Theis will in turn decrease the feeder losses PLoss ,
∆U ≈
PL2 + (QL − QC ) 2
I=
U2
PLoss = I RLN
In order to properly compensate the reactive power demand that changes from minimum
to maximum and to be switched off at the load minimum. When the load varies during
the day, the switched capacitors should be properly controlled. Different conventional
controls can be used to control switched capacitors, such as time, voltage and reactive
power. Time controlled capacitors are especially applicable on feeders with typical daily
load profiles in a long term, where the time of the switching-on and off of the shunt
capacitor can be predicted. The main disadvantage of this control is that the control has
no flexibility to respond to load fluctuation caused by weather, holidays, etc. voltage
controlled capacitors are most appropriate when the primary role of the capacitor is for
voltage support and regulation[ 25]. Reactive power controlled capacitors are effective
when the capacitor is intended to minimize the reactive power flow.
2
2. Effect of Voltage Variation on Nigerian Distribution System
The Voltage profile, system loading, and reactive power control presented in this paper is
tested on an 11 kv 7.5Mvar distribution System fed from a 33kv 30Mva injection
Substation Shown in Fig 2, With its detail specification presented in Table 1, the daily
load profile is adapted from a graph of measurement at the distribution grid of the
network in Nigeria [25]. This feeder experiences peak loading period which affects the
voltage stability on the system [22].
Fig. 2-.A Typical Nigeria 33kv /30MVa Distribution Network(33kv line)
3
Load Assessment of a The 11 kv Feeder
An assessment of the load flow, quality, reliability, and voltage profile of 33/11 KV
Injection Substation in the network was performed and the system load flow was
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observed for the system lagging reactive loads and the determined values where plotted
on a curve as indicated in Fig.3 below. The Curve indicated that the minimum reactive
power demand of 600 Kvar on the system occurs at light load period around 3am when
the system load is 3333kw, then there is a gradually raise in the demand for the reactive
power up to around 7pm in the night and is maximum of 2980 kvar at peak load of 5667
kW.
Fig. 3- Daily load profile of System under study
3.1 Load Flow Studies
The load flow studies stimulated 2 varied conditions:
1— Lightly loaded Network
2— Peak Loaded Network.
The load flow indicated the voltage level on the feeder for the network at light and
peak load period which occurs during the early hours of the morning around 4am and at
night around 7-8 pm respectively. The load flow for the 2 period is as indicated in table 1
below:
Voltage (kV)
Loading (kW)
Light Load
32.8
PF
3333
0.987
5667
0.89
Peak load
29.6
Table 1 Load flow of a 33 kv network
The Q-T curve in Fig 3 and the Load flow analysis in Table 1 indicate the Voltage and
Voltage Change quantities that determine the optimum control set point which can be
explain as follows. The reactive power demand progressively increase due to the system
loading, the system voltage and power factor begins to drop and the voltage under peak
load was noticed to be about 29.6 kV, this was regarded as voltage instability. The system
voltage and power factor is best at the lightly load period around 5am. Where the power
factor is given by [25]:
Active power
(3)
PF =
Apparent power
For lightly loaded, the PF calculated is as:
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PF = 3333 / 5667 2 + 600 2 = 0.987 ;
This also corresponds to a voltage level of 32.89 kV at the feeder, and corresponds to a
drop of about 1.5% drop in the feeder voltage. he PF of the peak load period is given by:
PF = 5667 / 5667 2 + 28002 = 0.89 .
And this corresponds to a voltage level of 29.6 kV which represents about a 12%
drop in voltage from the nominal level and this occurs around 7 to 7pm in the night.
4
Distribution Network Performance Analysis
Power factor Analysis and sizing of the capacitor are performed using the Power point
and Matlab Software to determine the daily fixed and varying reactive power
requirement. It was observed that the size required was 600kvar for the fixed .The 2380
kvar which is varying with time remaining will be met by switched capacitors. Capacitors
were selected to match the remaining load characteristics from hour to hour. Matalab
software was used to determine the Size of the switched capacitors
Time-Hours
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
1
2
Q-kvar
600
600
700
800
900
1000
1100
1200
1100
1000
1100
1200
1300
1400
2500
2700
2900
2900
2500
1900
1400
1300
900
700
Load-KW
3333.009
3333.009
3434.478
3535.956
3637.434
3738.912
3840.390
3941.868
3840.390
3738.912
3840.390
3941.868
4043.346
4144.824
5261.082
5464.038
5666.991
5666.991
5464.038
4652.274
4144.824
4043.346
3637.434
3434.478
Pf
0.984
0.984
0.979
0.975
0.970
0.966
0.961
0.956
0.961
0.966
0.961
0.956
0.952
0.947
0.902
0.896
0.889
0.889
0.896
0.925
0.947
0.952
0.970
0.979
Table 2- Hourly Load flow and Reactive Power Demand
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5. Capacitor bank selection
Sizing of the capacitor banks using MATLAB: at the peak load, the PF raised from
0.889 to 0.95 with total 1200 kvar reactive power switched on by the capacitor banks.
If the capacitor banks are designed in two sets and switched on separately, there are
two banks with two alternatives as indicated in table 3 below
Fig. 3-The Biu Shani Distribution network Single line Diagram with Capacitor Banks
time
Q
Alternative one
(bank 1 fixed, bank 2 switched)
No
600
1200
prefered
CB
kvar
kvar
PF
24
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
900
780
750
600
630
760
820
900
1050
1180
1100
900
1100
1480
1700
1850
0.971
0.975
0.977
0.984
0.983
0.977
0.975
0.971
0.963
0.958
0.961
0.970
0.961
0.942
0.934
0.926
0.997
0.999
0.999
1.000
1.000
0.999
0.998
0.997
0.993
0.989
0.992
0.997
0.992
0.978
0.971
0.964
0.994
0.990
0.997
0.999
0.999
1.000
1.000
0.999
0.998
0.997
0.993
0.989
0.992
0.997
0.992
0.978
0.994
0.990
16
17
18
19
20
21
22
23
2250
2620
2900
2980
2980
2880
2620
2300
0.911
0.903
0.890
0.885
0.885
0.891
0.902
0.907
0.949
0.939
0.927
0.922
0.922
0.928
0.938
0.946
0.978
0.968
0.958
0.954
0.954
0.959
0.968
0.976
0.978
0.968
0.958
0.954
0.954
0.959
0.968
0.976
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Alternative two
(both banks switched)
600
1200
Prefered
kvar
kvar
PF
0.971
0.975
0.977
0.984
0.983
0.977
0.975
0.971
0.963
0.958
0.961
0.970
0.992
0.978
0.971
0.964
0.992
0.978
0.971
0.964
0.978
0.968
0.958
0.954
0.954
0.959
0.968
0.976
0.978
0.968
0.958
0.954
0.954
0.959
0.968
0.976
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p f im p ro ve m e n t
2980
2620
2250
Ql
1850
1700
1480
1200
1100
900
760
600
a lt e r n a t i ve 2
a l t e r n a t ive 1
0
1
5
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
t im e
Fig. 4- Q-T Curve Showing the Fixed and Switched capacitors network in Nigeria- Biu
Shani
6. Calculation of Modeled components in PSCAD
The first approach taken towards simulation is the calculation of the parameters of
the components of the Model. Care must be taken in calculating the parameters in order
to achieve accuracy.
6.1 Calculation of the size capacitor banks.
The source is Y connected and the line Voltage is 11kv,
11kV
Phase Voltage, Vph =
= 6.35 V
3
Capacitor bank rating Qc = 600 kvar.
600 kvar
Capacitor bank rating per phase Q cph =
= 200kvar .
3
Impedance of capacitance per Phase,
V 2 ph (6.35kv) 2
X Cph =
=
= 201.61 Ω
Q cph
200kvar
1
=> C = 15.78 µF ,
2π × f × C
Where f= 50 Hz, is the power system frequency.
Assuming severest conditions of switching the value of L in Henry is given by
1
L=
= 0.64 H .
(2π × f) 2 × C
Xcph =
15.78 × 10 −6
= 44.59 amps .
0.64
Line XL: Aerial line with Conductor of size 100mm2
Length of line = 5m.
Line XL = 0.5 Ω / km.
Total Line XL is 0.005 × 0.5 = 0.0025 Ω .
Inrush Current = 2 × 6.35 × 10 3 ×
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6.2 Series reactor Rating and Selection
In order to avoid resonance at all odd harmonics the minimum value of series reactor
reactance (XL) should to be from (1-12)% of the capacitive reactance (Xc). Moreover, to
keep the effective capacitive reactance same as ohms the 201.61 ohms capacitive
reactance of capacitor bank should be increased by the same amount of the series rector
reactance. The value of series reactor reactance is, assuming XL= 0.01 × 0.64=0.0064H,
the simulation rector values of 0.0064H, 0.0194H, 0.0384H, 0.0576, 0.0768H
corresponding to 1, 3, 6, 9 and 12 percents respectively and where used with all these
values, the next approach is to construct the model In PSCAD.
Fig. 5- Single line diagram
of reactive power compensation using capacitor bank -The
Biu-Shani network
7 Economic benefits
In Nigeria like all over the world, Electrification is carried out by the national or
regional government, with the ongoing privatization of the sector voltage stability is
being addressed by the utility companies.
Some the benefits from the installation of capacitor will result in.
7.1 Benefit due to release capacity
Assume load installation to of-----------------------5667 kW at peak period.
Power factor @ Peak period-------------------------0.89
Required power factor with CB installation-------0.98
Maximum load at @ 0 89 PF------------------------6367.41 kVA
Maximum load @ 0.98 PF---------------------------5553.68 kVA
Reduction in Maximum load-------------------------813 kVA.
Which will corresponds to an improvement in revenue for the utility company; also
it is the typical daily kVA requirement for a rural community in Nigeria is about 300kVA
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7.2 Benefits due to Voltage profile Improvement
The improvement on the voltage profile will reduce to a minimum the use of
independent small private electric generators used by several industries. This will reduce
noise and tones of CO2 emission from the environment, the estimated reduction of CO2
emissions will be about 246,506 tones.
Secondly the monthly running cost of these diesel Generators at a minimum of 12
hours daily is as indicated in table 4 below:
Component Unit cost Quantity Yearly Present Value
(USD)
Cost
Of Life-Cycle
(USD) (USD)
50kVA Engine $110,000 2 (rotation
$220000
each
of 12
hours/day)
Maintenance and 10% per
spare
year of
$264000
parts(including initial
$22000
engine oil and
capital
filters)
cost.
Market Fuel
$1.19/liter, 14600
Price
February liters
$17,374
$193000
2011
year**
Total per kVA
$13540 per annum
Table 4-Diesel Generator life cycle Cost Projection System Lifetime 12 years
*Addition of off-grid electricity supply generators, running on diesels or gasoline located at
the premises of industries or households. Generator power varied per KW as required by
House-hold or Industries and Commercial, but equals or exceed the KW estimated Load.
8. Future work and Conclusion.
Shunt capacitors and reactors may be connected to prevent low voltage during peak load
conditions, during light conditions to reduce or remove the capacitive reactive power of
the line to prevent high voltages. It has continuously adjustable characteristics and has
sufficiently rapid response that can effect changes on power systems to improve the
voltage stability of the system they are designed to vary the average value of the voltage
up and down rapidly and correct the momentary over voltages or voltage dips. The
capacitor Banks are usually switched during peak power flow periods and reduces the
steady power transfer capacity to the line during this period, High speed mechanical
switches capable of connecting the capacitor banks to the network are available for
operator directed steady state voltage control. These conventional Switches gears and
control can not reconnect fast enough to prevent or suppress under voltage in sudden Line
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Loading, or voltage dip during first swing period, repeated operation of the switchgears
and control may result in prohibitive wear and tear of the switching mechanism and
contactors. For repeatedly switched on and off at precise times. The switching can be
more reliably achieved with Thyristors switches than with conventional mechanical
switchgears[25].
The Filters absorb harmonic generated by the Capacitor bank structure and large
industrial loads. Equipments such as electric motors overheats when working under low
voltage, several and had to be replaced, also incandescent lamps have to be replaced
frequently this is also a direct increase in the running cost of such companies, an improve
voltage profile will remove. The reduction in max load during Peak loading period
implies capacity release on the network, this means additional revenue for the utility
company and more homes will be provided with electricity. Secondly the removal of the
running cost of these generators from the production line of the industries will reduce the
production cost of the goods produced; the resultant effect will be a reduction in the
production cost of such goods, and the multiplying effect on the reductions of price of
goods and services provided by such industries. The used engine oil runs into 100 of
tones of liters every year improper disposal of these used oil have led to several cases of
water, crops and land pollution in these rural areas, vast piece of land have been known to
be destroyed by waste oil from these generators.
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