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Failure of Power Factor Improvement Capacitors in Harmonic
Enriched Environment; A Real Case Study Muhammad Umair, Tahir
Mahmood
University of Engineering and Technology
Taxila, Pakistan
Abstract: Harmonic currents are like termite which remains
invisible till the malfunction of expensive
equipment stop different industrial processes. Sometimes, these
harmonic currents affect production
targets as well. In this research paper, industrial environment
of Mustehkam Cement Limited, a cement
manufacturing industry situated near Taxila, Pakistan, have been
considered. To search out the reason of
capacitor failure, many important factors have been
investigated. Equivalent series resistance of failed
and good capacitors is closely observed. Harmonics near big
variable frequency drives is recorded to
analyze its impact on capacitors. A software SOLV is used to
analyze one biggest and most problematic
substation for the impact on its distribution board, substation
transformer and main grid station. Harmonic
resonance which is one of the prominent reasons of premature
capacitor failure is also calculated for the
selected case. On the basis of investigations being observed and
standard practices, different remedial
techniques have been proposed. Of these remedial measures,
merits and demerits are discussed along with
supporting simulations and tests. Besides this, different
critical operating parameters have been searched
out. These parameters are usually ignored while designing and
during operation of an industrial electric
power distribution system (IEPDS). Universal harmonic filter is
recommended as most useful and
economical solution.
Key Words: Harmonics Mitigation, Harmonic Resonance, Capacitor
Protection, Power Factor
Improvement Capacitors, Universal harmonic filter.
Introduction
Mustehkam Cement industry situated near Taxila, Pakistan, has
3000 tons per day production capacity.
This industry has nine (09) small substations with almost 20
megawatt (MW) connected load and 18 MW
sanctioned load from local utility feeder. One 132 kV
sub-transmission line of utility is feeding this
industry grid. The industrys internal grid station has one 26
MVA, 132/6.3kV transformer. The detailed one line diagram including
main substation and nine small substations has been shown in Figure
1. There
are 12 Slip Ring, 6.3 KV motors and one 3.3 KV variable
frequency drive (VFD). There are many
transformers, hundreds of Low Voltage motors, Electrostatic
precipitators with high voltage direct current
(HVDC) system, a number of sensitive instruments and
Programmable Logic Control (PLC) system with
distributed control system (DCS) monitoring and control. In each
substation, there are Low Voltage
Variable Frequency Drives and microprocessor based controllers
for communication and control. A
redundant fiber optic is used as main back bone of communication
network.
For the purpose of power factor improvement in the industry
electrolytic type power capacitors have been
installed on the low voltage bus. But, as stated before,
frequent failure of these capacitors have been
observed. Although the industry constructed brand new
substations with latest electrical equipment
including PLC control system, a number of variable frequency
drives and many squirrel cage and wound
rotor motors. Even then, the failure rate was shockingly high
enough that hundreds of capacitors have
been replaced during last 3 years. It has been assumed that this
failure is due to the harmonic over-
currents/voltages, transients generated during the startup of
Medium Voltage motors or due to the
harmonic resonance between the capacitive and inductive
reactance of the system.
The case was fit for power quality analysis study as the design
specifications were up to the standards and
recommendations of IEC and IEEE were observed. In the subsequent
sections theoretical back ground of
problem in the light of authentic literature, harmonics, and
transient analysis of selected substation,
recording and metering to reach the core of problem is performed
and finally the way out of problem has
been discussed.
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Substation SS 1 SS 2 SS 3 SS 4 SS 5&6 SS 7 SS 8 SS 9
Total
Total Caps.
(25KVAR)
19 12 32 30 22 18 37 16 186
Failure Rate 65% 50% 47% 78% 64% 55% 100% 75% 64%
Burnt Caps. 11 6 14 21 13 10 37 12 119
Table 1: Capacitors replaced in different substations in last 2
years
25 KVAR Capacitors Unit Price Total
Burnt 119 11000/- 13,09,000/-
Power Factor Penalty Imposed during last 2 years 10,50,000/-
Total Loss in PKR: 23,59,000/-
Table 2: Financial loss due to failure of capacitors in 2
years
Substation
No.
1 2 3 4 5 6 7 8 9 10
Name Lime Stone
crusher
Clay
Crusher
Raw
Mill 3
Raw
Mill 4
Kiln Clinker
Cooler
Packing
Plant
Cement
Mill
Coal
Mill
Grid Aux
TF.
Load (KW) 600 360 3950 3412 4300 2762 2290 7000 8110 80
Table 3: Detail of substations and their connected load
The bird eye view of the industry is given in fig. 1 and single
line diagram in fig. 2.
Fig 1: Process picture of considered industry
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Fig 2: Single Line Diagram of industry considered
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Literature Review:
Power factor (PF) capacitor has become a significant ingredient
of any distribution system whether it is
utility or industrial and have completely replaced synchronous
motors for PF correction due to their
simple design and low cost. Their protection have become
essential feature for every distribution facility,
otherwise the loss will be endure in the shape of PF penalties,
less capacity for useful power at the same
KVA, excessive heating causing a permanent stress in
distribution equipment, and premature failure of
equipment. John Houdek et. al. in [1] has discuss possible
failure reason of Metalized polypropylene capacitor, which in its
dry and oil immersed type is typically used in many industries and
working
properly.
Possible Reasons of Capacitor Failure:
According to Houdek Metalized Poly propylene (MPP) capacitors
have self healing phenomenon by
which if some fault occurs it heals up and capacitor continue to
work with relatively lower capacitance
[1]. Due to this phenomenon failure occur gradually and clots of
failed cells are formed increasing the
total equivalent series resistance (ESR). Figure 3 shows the
individual failed element.
Blooming et. al. in [2] has explained another main reason of
capacitor failure that inverter based motors
produce an unpredictable rich ripple current waveform due to
which energy loss is difficult to calculate.
These random currents are attracted by capacitors as they are
susceptible due to frequency matching or
resonance. Another reason explained by Blooming is the heat
transfer attributes of capacitor banks which
depends upon design geometry of capacitor. Those capacitors
which have relatively taller bodies usually
fail more rapidly as their internal I2R losses are relatively
higher. When capacitors absorb higher
frequency (harmonic) currents they quickly become overloaded and
can fail very rapidly. According to
Blooming harmonics are possibly the most detrimental factor in
causing PF capacitor failures [3]. We can
conclude that the capacitor failure occurs when it is compelled
to bear high temperature than its nominal
rating, spikes of currents, over voltages for short and long
durations which first deteriorate its
performance and if it is continuously exposure to these hazards,
it fails eventually.
Fig 3: Self Healing process
Related Standards
Before coming to the case study let us briefly go through the
related standards to compare our results.
IEEE Standard 18-2002 gives continuous overload limits, which
are intended for contingencies and not intended to be used for a
nominal design basis [4]: Maximum voltage allowed is 110% of rated
RMS voltage is allowed as 120% of rated peak voltage. Maximum
current allowed 135% of rated RMS current
(nominal current based on rated KVAR and voltage) and the limit
of KVAR is135% of rated reactive
power.
Second and the most important standard is IEEE Std. 512-1992
which explicate limits of total
harmonic distortion and total demand distortion [5]. The well
known IEEE Standard 519-1992,
Recommended Practices and Requirements for Harmonic Control in
Electrical Power Systems is one of the bases which is considered in
the paper as a test bench and above explained industrial system is
passed
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through the investigation weather it fulfills the limits given
in the table 10.3 of the standard 519-1992. In
the table 2 the limits for current distortion are given for odd
harmonics which have reasonable impact on
system disturbance. In the table ISC is maximum short circuit
current at point of common coupling (PCC)
and IL is maximum demand load current (fundamental frequency
component) at PCC.
Current Distortion Limits for General Distribution Systems (120
V Through 69,000 V)
Maximum Harmonic Current Distortion in Percent of IL (Odd
Harmonics)
ISC/IL
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simplicity one substation which is most affected by the
capacitor failure problem has been analyzed
which comprises five medium voltage motors and 0.8 MW low
voltage load with capacitor banks
37x25KVAR oil filled metalized polypropylene type, all of which
have been replaced within 2 years
period. In this sub-station 5 MV motors (1200KW, 1200KW, 950KW,
950KW and 760KW) are also
installed along the same bus so their starting and stopping can
one of another reason which can be
consider as another cause of capacitor failure hence motor
starting transients are also simulated. A
220KW Variable Frequency Drive for Fan is installed on the low
voltage bus of this substation with no
filter installed at feeder side; therefore its harmonic analysis
is performed.
Capacitor Type and its Physical Properties:
Metalized polypropylene technology (PPM) has self healing
property. So the area neighboring to the
shorted conductor vaporizes, and removes the shorted circuit.
The capacitor, in this way healed, goes on operating, except with
somewhat lower capacitance [1]. The self healing phenomenon is
apparently
useful, but if the capacitors continue to be operated in
polluted environment, they begin to have many
spots and can lose capacitance more rapidly. This can change the
resonant frequency of the filter network.
Fig 4: Single Line Diagram of considered substation
Equivalent Series Resistance
ESR measurement can lead toward the behavior of failed capacitor
as high ESR is undesirable. If
Capacitor is rapidly healed it starts to accumulate different
slots of bad areas which offer high resistance.
So for testing purpose ESR of 4 good and 4 bad capacitors was
measured which is given in following
table 5.
Calculating Resonance Frequency
The awareness of natural frequency or resonating frequency of
the shunt capacitor banks is important
because if series or parallel resonance occurs at any point it
can lead to serious problems. For example
voltage amplification sets out at drastic level if series
resonance occurs and current multiplication as a
result of parallel resonance can blow the capacitor plates or
the protective devices [8].
H=1/2LC if inductive reactance become 3mH the resonance
frequency will be near 5th harmonic (283 Hz). H=1/2 (Lx131x10-6).
But due to variable system load inductive reactance is changed and
at any certain frequency 925KVAR capacitor can work as tuned filter
to absorb all the current and completely or
partially damage them.
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Fig 5: Measurement of Equivalent Series Resistance of failed
capacitor in lab.
S.No. C (F) KVAR ESR() Comments
1 3x131 25 1.92 High
resistance
indicats
failed
capacitors
2 3x131 25 2.21
3 3x131 25 1.68
4 3x131 25 1.93
5 3x131 25 0.07 Low
Resistance
means
good
Capacitors
6 3x131 25 0.15
7 3x131 25 0.17
8 3x131 25 .03
Table 5: Equivalent Series Resistance Measurements of good and
failed capacitors
Harmonics Analysis of 220KW:
In the consider substation a 220 KW 400V motor is installed,
which is variable speed and an AC variable
frequency drive is controlling its speed. Harmonics of this
motor are recorded using Fluke 41b Harmonic
Analyzer.
Voltage Harmonics measurements
Voltage harmonic contents generated from this 220 KW variable
frequency drive are measured using
Fluke Harmonic analyzer 41B.The results are given in following
table:
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Fig 6: 220KW drive installed in industry
Table 6: Voltage harmonics measurements at incoming bus of 220KW
Classifier Variable Frequency Drive
The meter can measure individual harmonic contents and total
harmonic contents of three phase system
complying both IEC and IEEE standards for THD calculation i-e:
based on fundamental harmonic and
other on Root Mean Square value of all harmonics. The harmonic
contents listed above are well within
the IEEE recommendations about harmonics presence, hence
creating no problem.
Current Harmonics measurements:
Now we will observe the current harmonics weather they have some
impact on the system or not. The
individual and total harmonic currents measured at the incoming
side of 220KW variable frequency drive
are given in following table:
Harm. No. Fund. 2nd 3rd
4th 5
th 6
th 7th 8th 9th 10th 11th 12
th
Magnitude 100 2 3.5 1.5 72.3 1 49.3 0.6 1.5 0.5 13.5 1
Phase 0 5 123 52 22 60 -13 165 170 8 111 -35
Harm. No. 13th 14th 15
th 16
th 17
th 18
th 19th 20th 21st 22nd 23rd 24
th
Magnitude 9.2 0.5 0.5 0 6.5 0.5 3.7 0.1 0.5 0.2 3.2 0.5
Phase -4 58 160 0 60 -91 14 118 158 -22 58 110
Harm. No. 25th 26th 27
th 28
th 29
th 30th 31st
THD
-F%
THD
-R%
VH
(V)
VRM
S (V)
V PK
(V)
2 2.2 12 403 490
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Magnitude 2.6 0 0.5 0 2 0 1.5
Phase -11 0 53 0 45 0 -48
Parameter THD-F% THD-R% IH(A) IRMS(A) IPK(A) KF IHM
Value 86.9 67.9 78 109 211 17 18%
Table 7: Current Harmonics at Classifier Variable Frequency
Drive Incoming
Fig 7 Measuring different parameters of capacitors
The results shows very high harmonics currents present at the
point and as these high frequency currents
flow back to the main distribution board before reaching to
transformer it leave its impact on all the
sensitive devices coming in its path like electronic
controllers, processors and other sensitive devices
which are not electrically isolated from this power bus.
Table 8: Current Harmonic Distortion at 220KW VFD towards the
motor
In the said variable frequency drive a built in harmonic filter
is present at the motor side of drive which
protects motor from harmonic currents exposure to ensure it
smooth operation. But no protection is
present to prevent the back flow of this big amount of harmonic
current to the system. Table 6 shows
outgoing feeder of this drive which is going towards the motor
side. It is very clear from the table that the
impact of these current become almost zero. K-factor given in
the table 5 and 6 give the expected ratio of
harmonic current and the value is important while designing the
transformer.
ITHD-F% ITHD-R% IH(A) IRMS(A) IPK(A)
9.2 6.7 8.8 78 114
Table 9: Current Harmonic Distortion at auxiliary
transformer
The incandescent lighting, air conditioning and other irregular
load had generated 11 Amp harmonic
current which feeds to the system causing unnecessary heating
and other bad effects. If we closely
observe this data we can reach to the point that capacitor banks
are prey of harmonic distortion generated
ITHD-F% ITHD-R% IH(A) IRMS(A) IPK(A) KF VTHD-F% VTHD-R%
2.2 2.3 3 155 210 12 0.5 1.41
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by different frequency currents. The results dont comply IEEE
standard 519-1992. Hence a detail analysis of this drive is
performed using Miruss SOLV software.
Simulation in SOLV Software:
Ideal parameters of this drive are simulated using simulating
software SOLV.
Fig 8: One Line Diagram in Simulator SOLV
Graph 1: Simulated Current Harmonics at PCC-1 Graph 2 Distorted
Current Waveform due to harmonics at PCC-1
Fig 9: Results at 132KV Transformer (PCC-2), 6.3KV Transformer
(PCC-1) and Distribution Board
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Graph 3: Simulated Current Harmonics at LT Capacitor Bank Graph
4 Distorted Current Waveform due to harmonics at LT Capacitor
Bank
Determination:
After all these tests and close observations we have reached to
the outcome as under
1. Geometrical and physical aspects have ignorable impact, as
the length of capacitor is moderate and internal I
2R losses are negligible.
2. ESR of bad capacitors depicts that they have undergone
stresses (high temperature, heavy currents, voltage or current
surges etc). Different capacitors showed the failure mode different
from each other.
3. Resonance study has explained that if the inductive reactance
of the considered substation becomes equal to 0.05 uH resonance
occurs at 5
th harmonic, which was measured higher in the system, which
is very dangerous for the system.
4. The THD-I (total harmonic distortion in current waveform) is
greater than the allowable range of IEEE which is reflected in both
simulations. Also in the results recorded in the field. Hence due
to
steady flow of high frequency current capacitors undergo
premature failure. If Inductive reactance at
any frequency becomes equal capacitor banks natural frequency
major break down occurs.
5. Medium voltage motors have no major impact as the banks are
thoroughly monitored during start and stop of these motors and the
current rise is not evident and the spikes flows backward towards
main
grid.
Solutions suggested
The market is thoroughly searched and it is observed that many
solutions are [9], [10] available. A few of
them are AC line reactor, DC link Choke, Both AC line reactor
and DC link choke, Isolation
transformers, K-Factor transformers, Tuned harmonic filters
(fixed capacity or automatic switched
multiple banks), IGBT based fast switched harmonic filters, Low
pass harmonic filters, 12 & 18 pulse
rectifiers, Phase shifting transformers, Active harmonic
filters, and Universal harmonic filter (by MIRUS
Int.)
Pros and Cons of these techniques: Generally the placement of
these treatments will be near VFD, this way we can save not only
capacitors
but sensitive equipment like electronic controllers,
computerized machines, PLCs etc.
AC LINE REACTOR:
Being relatively low cost they are usually preferred. It
comprises of an AC reactor installed in series with
each phase of VFD at feeder side and has high impedance due to
which it absorbs switching and/or fast
changing loads transient over-voltages but often can introduce
troublesome voltage drops if a separate
suitable capacitor is not in series with this reactor. So it is
less reliable but is in use frequently due to
being cheap.
DC LINK CHOKE
DC choke is installed inside the VFD at DC link and it is
relatively more effective in reducing harmonic
currents than the ac reactor and does not cause an AC voltage
drop. But it has less immunity to absorb
over-voltage spikes.
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BOTH AC LINE REACTOR AND DC LINK CHOKE:
If we connect in series an AC reactor at the rectifier input
(AC) and a DC choke at output (DC) [11] it is
another well known solution. It is more effective than using a
single device as it reduces ITHD in half
(30% to 40% approx.). It takes the advantages of both AC reactor
and DC choke hence better from both.
It has relatively low cost then the modern techniques and is
considered best among conventional
techniques. It is available in the products of good VFD vendors
built in inside the package.
ISOLATION TRANSFORMERS
The isolation transformer attenuates the harmonic frequencies in
delta configuration by offering leakage
inductance of appropriate values of circuit impedance. The
inductive reactance is low at the fundamental
frequency to pass fundamental current easily, but with the
increases in frequency it increases
proportionally.
TUNED HARMONIC FILTERS
As the name indicates they require tuning to a specific harmonic
frequency and have fixed capacity or
automatic switched multiple banks. The tuning means that they
offer very low impedance for the tuned
harmonic. For many frequencies multiple tuned filters required
which makes the circuit more and more
complex. Different types are fixed, automatic and hybrid. They
are only useful at fixed magnitude
harmonics [12]. Their main drawback is that they can produce
resonance.
K-FACTOR TRANSFORMER
The K-factor transformer is one of the best techniques to handle
harmonic current but its cost is very high
which is almost four times higher than that of ac line reactor.
K-factor is a constant that indicates the
ability of the transformer to handle harmonic currents and heat
generated due to them. K-factor
transformer is specially designed to balance the temperature
rise caused by current harmonics in the
transformer windings. It is performed by doubling of neutral
conductor at secondary side. It filters out
triplen harmonics.
IGBT BASED FAST SWITCHED HARMONIC FILTERS:
It can automatically switch the capacitor bank using soft
switching phenomenon without generating
voltage spikes, so it is very useful when the reactive power
changes irregularly over time. Its capability to switch without
transients and to respond in real time, to dynamically changing
load conditions are main
advantages. It is very high cost and requires rapid
replacement.
LOW PASS HARMONIC FILTERS
It consists of an L-C circuit with a big series inductor. It can
reduces total current harmonic distortion up
to 12% .But it requires large capacitor bank. Under light loads
leading power factor is created which is
undesirable and cause generator compatibility problems. Due to
big inductor and capacitor its cost is high
[13].
PHASE SHIFTING TRANSFORMERS:
Different Phase shifting transformers are available in market.
For example 12-pulse PST uses 300 phase
shift to cancel 5th, 7
th, 17
th and 19
th. Similarly18-pulse PST uses 20
0 phase shift to cancel 5
th, 7
th, 11
th and
13th
and 24-pulse PST uses 100 and 30
0 phase shift to cancel 5
th, 7
th, 11
th, 13
th, 17
th and 19
th. But it is not so
much cost effective.
12-18 PULSE RECTIFIERS
Twelve and eighteen pulse rectifiers can filter 11, 13, 23, 25
harmonics and 17, 19, 35, 37 Harmonics
respectively. It will work if the currents drawn by each
rectifier bridge would be balanced and source
voltages for all phases are same, if these conditions dont
fulfill miss-operation can occur. Total harmonic current distortion
reduction depends upon which pulse system is operated. Mostly <
12% for 12 pulse and
< 8% for 18 pulse. The costs increases by increasing power
rating of motor.
ACTIVE FILTERS
Active filters are parallel connected before drive rectifier. It
measures real time value of harmonic
currents in the system and inducts currents of opposite polarity
which cancels the generated harmonics.
This reduction depends upon size of filter and harmonic current
present up to < 5%. It uses intelligent
processor to detect the harmonics quick enough for effective
treatment. It is very costly and complex.
UNIVERSAL HARMONIC FILTER (Recommended)
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It is a purely passive series connected device. It consists of
3-phase reactor design consisting of multiple
windings on a common magnetic core which needs much smaller
capacitor bank typically 3-4 times lower
than that of conventional filters. Cost and space requirements
reduced along with getting rid from leading
power factor. It also eliminates switching of capacitor. It has
simple and small design. Tuned filters
require a detuning reactor in series with the supply feeder,
else they will be overloaded by attracting
harmonics from upstream sources which introduce a voltage drop
at the dc bus. Issue is resolved using
multiple winding in UHF. A comparison of different Solution
described above is given [13].
Simulation of the 220KW frequency drive analyzed above using
Results after adding Universal Harmonic
Filter.
Fig 10: Results using universal harmonics filter
A comparison of these techniques is given in [14] which is a
very provides clear advantage of universal
harmonic filters over other techniques both in the durability
and cost saving, given in following table:
Table10: Comparison of different techniques about harmonics
issues
Type of
Correction
Reactor Tuned
Filter
Low pass
Filter
Multi
Phased
Phase
Shifting
Active
Filter
UHF
Current
deformation
< 35% < 15% < 12%
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The UHF ensures a fail proof system which can reduce day bay day
cost of capacitors and electronic
equipments prone to high frequency noise hence recommended.
FURTHER SUGGESTIONS:
ESR of all the capacitors should be checked on monthly basis to
check their performance and take necessary steps of preventive
maintenance.
To verify manufacturing fault tests and standards should be
thoroughly investigated.
Instead of simple switching zero crossing switches for the
capacitor banks can reduce their self transients, especially for
those capacitors which are frequently switched.
Conclusion:
The case study of power quality impacts on LT capacitor bank in
the presence variable frequency drives
and MV motors gives a standard analyzing procedure. Equivalent
series resistance is a basic parameter to
know the current situation of every individual capacitor. To
avoid series or parallel resonance upon
capacitors without meeting the under-voltage issue on gate and
for getting a stable power factor beside the
others universal harmonic filter is a best solution exits in
market which is economical also.
References
[1] John Houdek, P. a. C. C. Extending the Life of Power Factor
Capacitors. (2009)
[2] Thomas M. Blooming, Capacitor Failure Analysis Oct 2006
[3] Tony Hoevenaars, P. E. W. E. Corp. Power Factor Correction
Capacitors. (2012)
[4] IEEE Standard-18, for Shunt Power Capacitors, 2002.
[5]IEEE Standard-519. Recommended Practices and Requirements for
Harmonic Control in Electrical
Power Systems. (1992)
[6] Thomas M. Blooming, and Daniel J. Carnovale Capacitor
Application Issues (Aug 2008)
[7]Q & ESR Explained A Johansson Technology Primer.
California (2004)
[8] C. SANKARAN, Power Quality, Published by CRC Press LLC
(2001)
[9] Tony Hoevenaars, P. E. A New Solution for Harmonics
Generated by Variable Speed Drives. Power
Quality Assurance (1999)
[10] B. Prokuda, Applying low voltage harmonic filters,
revisited, in Power Syst. World, Power Quality
Conf., Chicago, IL, (Nov. 1999)
[11] Mirus; A Revolutionary New Universal Harmonic Filter for
Variable Speed Drives, MIRUS
International Inc. (2002)
[12] Daniel J. Carnovale, P.E. Eaton, Cutler-Hammer Moon, Price
and Performance Considerations for
Harmonic Solutions (2005)
[13] D. J. Carnovale, Power factor correction and harmonic
resonance: A volatile mix. EC&M Magazine,
pp. 1619, (Jun. 2003)
[14] MIRUS International Inc. LINEATOR Advanced Universal
Harmonic Filter for VFDs (2010)
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UTILIZATION OF RESEARCH RESULTS
Case study, results and suggestions given in the thesis can
become a useful example in Power Factor
Improvement Capacitor designing / selection in huge Industries
where POWER QUALITY issues are
severe due to presence of POWER ELECTRONICS equipment, Medium
Voltage Motors and Sensitive
Electronics Equipment installed in a close proximity and are in
operation simultaneously. E.g.: Big
process Industries likes Cement, Fertilizer, sugar, Textile, Oil
and Gas sector, and big utilities etc. can use
the paper as a reference while proposing new capacitor
banks.