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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.