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Capacitor Banks and Its Effect on Power System with
High Harmonics Loads
Yatharth Kumar Sharma
Industrial Efficiency Group
The Energy and Resource Institute,
Bangalore, India
y.sharma@teri.res.in
Vijay Mohan R
Industrial Efficiency Group
The Energy and Resource Institute,
Bangalore, India
vijay.mohan@teri.res.in
Abstract -In order to utilize the electrical system effectively,
industries are installing capacitor bank in their power circuit. The
use of power electronic devices has increased in recent years which
resulted in an increase of harmonics in the power system. This has
urged the need to study, understand the behavior of harmonics in
different conditions. This paper assesses the importance of power
factor in electrical system and later explained howharmonics
generated; its effect on the power system and how to reduce
(mitigate) the effect of harmonics. At last effect of capacitor bank
on power system harmonics were explained and concluded the
result with the help of a case study which shows a real-time
example with the help of waveform showing percentage current
and voltage harmonic distortion variation at incomer with respect
to APFC ON/OFF status and harmonic reduction techniques.
Key Worlds: Power Factor, Capacitor Banks, Power System
Harmonics, Harmonic Mitigation Techniques, Series and Parallel
Resonance.
I. INTRODUCTION
Active power (also known as “working power”) is used in all
electrical equipment to perform the task of motion, lighting and
heating etc. It is expressed in kilowatts (kW). Apart from
working power, inductive loads such asa motor, transformer and
chokes also requires reactive power to produce a magnetic field
to operate. This power which doesn‟t perform any work is called
as kilovolt-amperes-reactive (kVAr). Every industry requires
both active and reactive power in order to sustain. These two
powers together results as apparent power, which is expressed in
kilovolt-amperes (kVA).
It is important to calculate the ratio of active power to apparent
power which is known as power factor (PF) in order to know the
health of the electrical power distribution system. A high PF not
only benefits the customer but also benefits the power
distribution companies (DISCOMS). Whereas, low PF indicates
poor utilization of electrical power which causes large current to
flow in power distribution cables in order to deliver required
working power to electrical equipment. Hence it is very
important to maintain the PF near unity.
The most practical and economical method of improving PF is,
to add capacitor bank to the electrical loads in the system which
acts as a reactive current generator that helps to compensate the
reactive power consumed by inductive loads. In most of the
industries, capacitor banks are installed near the PCC to improve
the PF between the industry and electricity grid. However, the
PF measured between the electrical load and the capacitor bank
will remain unchanged. Hence low power factor is observed at
different locations within the industry, which directly increases
the distribution loss and increase kVA demand of the industry.
To reduce the power distribution losses within the industry,
capacitor bank should also be installed at load end (at motor
control centers (MCC) or distribution panel‟s boards).
Generally, Electric power systems are designed to operate at
50/60 Hz frequency. However, some type of loads produce
voltages and currents with frequencies that are integer multiples
of the 50/60 Hz fundamental frequency. This type of higher
frequencies is called ''harmonics''. Harmonics can be produced
within the plant and/or may enter the plant from the grid through
neighboring plants with nonlinear sources.
II. HOW HARMONICS ARE GENERATED
After the 1980s, use of power electronic devices had increased
in industries for various applications like uninterrupted power
supply(UPS), variable frequency drives (VFD) switch-mode
power supply (SMPS), etc. These semiconductor devices distort
the sinusoidal currents which results in harmonic current that
flows through system impedance and creates voltage
harmonics[1]. Figure 1 shows a simple line diagram of electrical
distribution system. VS shows the pure sinusoidal system
voltage, LS represents system impedance and Vpcc shows the
voltage at the PCC.
2018 3rd International Conference for Convergence in Technology (I2CT)
The Gateway Hotel, XION Complex, Wakad Road, Pune, India. Apr 06-08, 2018
978-1-5386-4273-3/18/$31.00 ©2018 IEEE
1
Fig. 1: Single Line Diagram of Electrical Distribution System.
Where, Vpcc can be calculated as shown below:
Vpcc =VSVL= VS LS(diac
dt )
Figure 2 shows the distortion in the voltage waveform at PCC
because of harmonic current.
Fig 2: Voltage waveform at PCC.
Electrical power that is generated from a generating station has a
perfect sinusoidal waveform. But practically at the user end it is
highly impossible to maintain such perfect sinusoidal conditions
at the utility. Components such as transformers, rotating
machinery and other power electronic drives cause some
deviations in the currents (called harmonic current) which may
be different from pure sine wave and thus distorts the supply
voltage to an extent that may be dependent on source impedance
and magnitude of harmonic current. Figure 3 shows the model of
nonlinear loads injecting the harmonic current into the system.
Fig 3 Flow of harmonic current in a distribution system.
III. EFFECT OF HARMONICS
Harmonic current during its flow through the utility network
may cause drop in voltage, create stress in the plant distribution
network and also results in malfunctioning of some of the
sensitive electronic equipment. Total voltage distortion is the
summation of individual voltage drops across system impedance
and its magnitude depends on the level of individual harmonic
current. Some of the negative consequences of harmonics on
plant equipment are listed in table below:
TABLE I: Voltage and Current Harmonics and its negative effect on plant
equipment
Equipment
Possible consequences
Effect of Voltage harmonic distortion
Transformers
Premature insulation failure and
increased noise
Electric Motors
Unusual motor overloading
Electronic equipment
Malfunctioning
Effect of Current harmonic distortion
Transformers
Overheating of winding, increase in
copper losses and reduced operating
capacity
Electric motors
reduced operating capacity, operating
efficiency and life of motor
Circuit breakers, fuses
and relays
May get damaged or false operation
Capacitors
Premature damage, resonance effect
IV. INDICATORS OF HARMONIC DISTORTION
Many indicators exist to quantify and asses voltage and current
harmonic distortion in an electrical system such as Power
Factor, Crest Factor, Distortion Power, Frequency Spectrum and
Harmonic Distortion.
Distortion power is the resultant of distorted Voltage and
Current. Total harmonic distortion ofCurrent and Voltage is a
value expressed in percentage gives a clear picture about the
healthiness of plant electrical system. Crest factor is one such
significant indicator to find unwanted peaks in the voltage and
current waveform. For nonlinear load the crest factor is much
greater than 1.5. High crest factor indicate high over current
drawn by the device. Distorter power is the quantity which is the
resultant of distorted voltage and harmonic current.
Also the total harmonic distortion is the primary indicator which
defines degree of the harmonics in the sinusoidal voltage and
current waveforms.
V. HARMONICS ATTENUATION TECHNIQUES
There are many types of solutions that are available to mitigate
or attenuate the effect of harmonics which can be majorly
segregated as „modification of existing electrical system‟ and
2
next one is, use of special devices in the power system (like
chokes, filters and special transformers).
A. Placing the disturbing load upstream in the system.
Harmonic disturbance increases as short-circuit power
decreases, so it is economical and preferable to positioning the
disturbing load as far upstream as possible as shown in figure 4.
Fig. 4 Connect non-linear loads to upstream.
B. Grouping the disturbing loads.
The best practice is to separate the nonlinear loads from other
loads as shown in figure 5. Further attenuation of harmonics can
be done by connecting different type of loads from different bus
bars and if possible to different transformers as shown in figure
6. By doing so sensitive load can be protected from harmonics.
Fig. 5. Grouping of non-linear loads and supply from upstream as far as possible.
Fig. 6. supply of the disturbing loads via a separate transformer.
C. Installing line inductors in series with nonlinear sources.
In electrical system comprising of no linear drives, installation
of line inductors increases the impedance of the supply circuit
such that harmonic current can be suppressed.
D. Harmonic Filters
Even after grouping of non-linear loads or by installing line
inductors, if harmonics exist in the circuit, then filters must be
installed in the electrical system to attenuate the harmonics.
Filters are of three types: Passive Filter, Active Filter and Hybrid
Filter.
Passive filter are also called detuned filters are installed at
stable nonlinear loads that provide both power factor correction
and filter the harmonic current. Whereas active filters are
installed at nonlinear loads (load varying in nature) that provide
attenuation of harmonics over a wide range of frequencies and
also improves power factor. In some special applications,
Hybrid filters are also used which is a combination of active and
passive filter.
VI. EFFECT OF CAPACITOR BANK ON HARMONICS
Nonlinear loads operated in any electrical distribution system
create harmonic currents that flow throughout the system. As the
harmonic order increases the inductive reactance increases
whereas the capacitive reactance decreases. When capacitor
banks are installed in a system, there will be a crossover point
where inductive and capacitive reactance is equal at a given
harmonic frequency. This crossover point is called resonant
point and every system with a capacitor has a parallel resonant
or series resonant point. Harmonic frequencies where parallel
resonance occur, that harmonic current excites into the electric
circuit and this highly amplified current causes severely
distorted voltage. Electrical networks are more sensitive to
harmonic distortion when lightly loaded, so the installation of
fixed capacitors should be carefully reviewed. Figure 7 shows
the simplified parallel resonance circuit diagram.
Fig. 7: Parallel Resonance.
Series resonance is the result of fixed capacitors at load centers
or with capacitors that are switched with motors [3] and occur
because of combination of inductive and capacitive reactance
that provides low impedance path to harmonic current at natural
frequency of the power system. The effect of a series resonance
can be a high voltage distortion level between the inductance
and capacitance [4]. Figure 8 shows simplified circuit of series
resonance.
Linear loads
Non linear loads-1
Z1
Z2Non linear loads-2
3
Fig. 8: Series Resonance.
VII. CASE STUDY: - EFFECT OF CAPACITOR BANK ON
HARMONICS
This case study involves an automobile industry in India which
runs continuously day and night. The plant receives electricity
from grid at 33kV voltage level. It consists of loads such as
variable speed drives, punching machines, shot blasting, chain
conveyors operated with VFD, plasma cutting and other robotic
equipment. All the loads are connected to 415V bus of single
main PCC. The plant is surrounded by many other plants with
arc furnaces and induction furnaces etc., due to which supply
voltage entering the plant consists of harmonics. Plant has
installed 2x250 kVAr of Automatic Power Factor Controller
(APFC) at the 415V bus of main PCC. The single line diagram
of the plant is presented below.
Fig. 9: Single line diagram of the plant
During the study period, the peak load recorded was 1066 kW.
The average load was 804 kW and minimum load recorded was
532 kW. The average monthly PF is 0.87 (Lag). Figure
10shows the load variation and power factor trend at the
33kVincomer.
Fig. 10: Power factor trend at group incomer (24 hours logging).
Individual voltage and current harmonics studied at the main
incomer when APFC was ONLINE. The details are given in
table 2.
TABLE II: Individual Voltage & current harmonics measured at 33kV group
incomer when APFC is Online
Current Harmonic (%)
Voltage Harmonic (%)
R
Y
B
R
Y
B
Ah1
100
100
100
Vh1
100
100
100
Ah3
0.1
1.8
1.8
Vh3
0.0
0.0
0.0
Ah5
1.9
2.1
1.6
Vh5
1.9
2.1
2.2
Ah7
2.8
3.5
2.7
Vh7
1.1
1.0
0.8
Ah9
1.6
0.8
0.7
Vh9
0.0
0.0
0.0
Ah11
17
17
13.6
Vh11
4.3
3.3
3.5
Ah13
2.4
2.0
2.5
Vh13
0.8
0.8
1.0
Ah15
0.1
0.1
0.2
Vh15
0.0
0.0
0.0
Note: The values in the table are shown for the peak load of the
plant.
It can be observed from the above table that 11th harmonic of
both current and Voltage is significant in the plant electrical
system. The total fundamental current value is 1750 Amps of
which 11th harmonic current is around 280 Amp. The variation
of total harmonic distortion of Voltage (%Vthd) and Current
(%Ithd) during the study period when APFC is in ON condition
is shown in figure 11.
Linear and
non linear
plant loads
250 kVAr
APFC-1 Linear and
non linear
plant loads
250 kVAr
APFC-2
Bus coupler
Transformer-1
33/0.415 kV Transformer-2
33/0.415 kV
33kV Supply
from grid
415V bus
0.83
0.85
0.87
0.89
0.91
0.93
0.95
0.97
0
200
400
600
800
1000
1200
Power Factor
Power (kW)
Time
Load and Power factor variation at main plant
incomer
kW
PF
4
Fig. 11: Variation of %Vthd & %Ithd with respect to plant load when APFC is
ONLINE.
It can be seen from the above graph that voltage and current
harmonic distortion percentage was on higher side. Hence, to
understand the effect of harmonic level variation due to APFC
operation, a trail was taken by switching off the APFC banks. It
was found from the study that, immediately current harmonic
level has come down to 4.5% as shown in figure12.
Fig. 12: Variation of %Vthd & %Ithd when APFC is OFFLINE.
Individual voltage and current harmonics studied at the main
incomer when APFC was OFFLINE. The details are given in
table 3.
TABLE III: Individual Voltage & current harmonics measured at 33kV group
incomer when APFC is OFFLINE
Current Harmonic (%)
Voltage Harmonic (%)
R
Y
B
R
Y
B
Ah1
100
100
100
Vh1
100
100
100
Ah3
1.2
1.8
1.0
Vh3
0.0
0.2
0.0
Ah5
2.8
3.1
2.6
Vh5
1.4
1.3
1.8
Ah7
2.0
1.8
1.8
Vh7
0.5
0.6
0.1
Ah9
0.1
0.0
0.2
Vh9
0.0
0.0
0.0
Ah11
3.7
3.4
3.1
Vh11
5.0
4.4
4.9
Ah13
1.2
1.1
1.2
Vh13
0.7
0.7
0.8
Ah15
0.0
0.0
0.0
Vh15
0.0
0.0
0.0
From the above table it can be seen that when APFC is
OFFLINE, the total harmonic current distortion value has
significantly came down. It clearly shows that parallel resonance
effect has taken place at the plant incomer where capacitors
were coupled to the 415V bus. Also it can be concluded that
APFC installed at the incomer is not able to improve the power
factor above the set value (i.e; 0.95 lag) which is mainly because
of this parallel resonant effect.
Based on the nature of installed equipment in the plant, proper
measures and expert suggestions need to be taken before
installation of power factor correction equipment in the plant.
IEEE STD 1531 2003, is an IEEE guide that defines the
application and specification criteria to be considered in
designing, controlling and protection of harmonic filters. The
details of Voltage and current harmonic level at different
sections of the plant are given in table 4.
TABLE IV:Voltage and current harmonic level at different sections of the plant.
Description
I
% Vthd
% Athd
PF
Incomer
1750
4.5
17.1
0.87
DG panel
196
3.9
19.6
0.81
Power
distribution
board
199
4.1
5.8
0.70
Shot blast
254
3.8
6.2
0.5
Air compressor
303
8.1
9.6
0.78
Punching
Machine
86.4
6.8
15.9
0.86
Roll Former
102
8.4
62
0.75
From the above table, it can be seen that power factor is low at
most of the feeder end level and also %Vthd & %Athd are
higher in few areas. As per the IEEE STD 519 -1992, harmonic
voltage distortion (%Vthd) on power systems 69kV and below is
limited to 5.0% total harmonic distortion (THD) with each
individual harmonics limited to 3%. The current harmonic limits
vary based on the short circuit strength (Isc/IL) of the system
they are injected into at the point of common coupling (PCC).
VII. CONCLUSION
The use of nonlinear loads is continuously increasing in a power
system network which has drawn the attention to understand the
nature of harmonics and their mitigation techniques. Capacitors
installed in an industry helps to maintain the PF close to unity
0
5
10
15
20
25
30
0
5
10
15
20
25
30
35
40
%Vthd
%Ithd
Variation of Voltage current total harmonic
distortion at plant incomer when APFC was in ON
condition
%Ithd
%Vthd
5
but when installed in a harmonic rich environment, they create a
low impedance path and magnify the magnitude of current and
voltage harmonics in a system resulting in parallel resonant
effect. Hence, in order to reduce the adverse effect of harmonics
and to ensure proper operation and maintenance of plant
machinery, it is required to do a power quality study and install
power factor improvement or harmonic mitigating filters based
on expert suggestion. To ensure system compatibility with
international standards as IEEE STD 519-1992 [5] harmonic
study is required. According to IEEE STD 399 [6], when the
harmonic loads are about 30 % or more with respect to total
plant loads, the harmonic impacts needs to be examined.
A case study was presented which shows percentage voltage and
current harmonic distortion level at the incomer with respect to
APFC ON/OFF status. It clearly shows that parallel resonance
effect has taken place at the point of capacitor coupling and it
can be concluded because of resonant effect APFC is not able to
improve the power factor above the set value (i.e; 0.95 lag).
REFERENCE
[1] Harmonics in power system-cause, effect & control, Siemens, 2013.
[2] Eng. Osamah Saleh, “Harmonics Effects in Power System”, Int. Journal of
Engineering Research and Applications www.ijera.com ISSN: 2248-9622,
Vol. 5, Issue 2, (Part -5) February 2015, pp.01-19.
[3] The Origin, Effect, and Suppression of Harmonics in Industrial Electrical
Networks, Square D product data bulletin, Bulletin no.- 0140PD9502
March, 1997.
[4] Power System Harmonics – “A Reference Guide to Causes, Effects and
Corrective Measures,” an Allen-Bradley Series of Issues and Answers,
2001.
[5] Recommended Practice for Industrial and Commercial Power Systems,
ANSI/ IEEE standard 399-1997, Chapter 10, PP. 265-312.
[6] Recommended Practice and Requirements for Harmonics Control in
Electrical power Systems, ANSI/ IEEE 519-1992.
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