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CHAPTER 6

DETUNED CAPACITORS FOR POWER QUALITY

IMPROVEMENT A CASE STUDY

6.1 INTRODUCTION

Reduction of harmonic contents in loads can be done with existing

load equipment. An overexcited transformer can be brought back into

normal operation by lowering the applied voltage to correct range. PWM

drives that charge the dc bus capacitor directly from the line without any

intentional impedance are exception to this problem. Adding a line reactor

or transformer in series will significantly reduce harmonics as well as

provide transient protection benefits. Phase shifting transformer

connections can benefit loads by significantly reducing the fifth and

seventh harmonics. Delta connected transformers can block the flow of

zero sequence harmonics from the line. Automatic power factor correction

system (APFC) is based on fixed and predefined amount of KVAR. However,

the reactive power consumes current capability of the harmonic filter and

hence it is not main priority of APFC.

This chapter presents a practical example of improving the

power factor of the system by an appropriate design of detuned capacitor

filters. The advantage of using detuned capacitors as harmonic filters over

the use of plain capacitors for power factor correction is presented. The

different problems originated by harmonics and how filters prevent them

are reviewed, and a comparison between them is also presented.

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6.2 NEED FOR DETUNING CAPACITORS

Non-linear loads in industry typically contain a high fifth harmonic.

At the fifth harmonic, the tuned filter has a better behavior than the detuned

filter. However, for proper operation, the capacitor bank must be rated to a

higher voltage than the voltage level required for detuned filters. Because

tuned filters absorb more harmonics, they also carry higher harmonic currents

than the detuned filters. These features make tuned filters more expensive as

mentioned by Francisco Ferrandis et al (2003).

Tuned as well as detuned filters either absorb or reject harmonic

distortion and avoid harmonic currents to flow to other equipment or the rest

of the power system. A tuned filter is tuned to a frequency slightly below the

filtered harmonic. On the other hand, a detuned filter is tuned to a frequency

far below the filtered harmonic.

6.3 EFFECTS OF DETUNING CAPACITORS

Bridgeman et al (1998) and Gagaoudakis et al (1998) installed plain

capacitors on a system with a high level of harmonics, it needs to be replaced

with those capacitors to detuned capacitor bank (capacitors and reactors). The

first effect of the reactors is to suppress the risk of resonance with the

harmonics generated by the loads and in turn to protect the capacitors. If

suppressing is not done, the resonance would lead to an over sizing of active

filter connected in the circuit.

Secondly, although the detuned bank is not a passive filter, it will

have some filtering effect and harmonic distortion will be lower than if no

capacitor was installed.

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Calculating the minimum RMS current of the active filter to install

Irms (A) = 0.013(THDiinitial THDitargeted) I1 (6.1)

Based on the appropriate voltage and frequency, active filter with

the RMS current directly superior to the minimum Irms can be used.

Besides the filtering functionality, reactive power compensation is

also possible with the active filter. Compared to traditional capacitor banks,

the reactive compensation of the power quality filter is continuous, fast and

smooth (no transients at switching). The compensation can be either

capacitive or inductive, depending on the load type.

6.4 METHODS OF MODIFYING SYSTEM FREQUENCY

RESPONSE

There are number of methods to modify adverse system

responses to harmonics in Dugan et al (2003).

- Add a shunt filter. This shunt filter eliminates atroublesome harmonic current off the system, but it

completely changes the system response.

- Add a reactor to detune the system. Harmful resonances

generally occur between the system inductance and shunt

power factor correction capacitors. One method is to add a

reactor in series with a capacitor to move the system

resonance without actually tuning the capacitor to create a

filter. Other way is to add reactance in the line.

- Changing the capacitor size is one of least expensive

options for both utilities and industrial consumers.

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- Move a capacitor to a point on the system with different

short circuit impedance or higher losses. New bank causes

telephone interference for utilities, there by moving the

bank to another branch of the feeder may very well resolve

the problem. This is not an option for many of industrial

users because the capacitor cannot be moved far enough to

make a difference.

- Remove the capacitor and simply accept the higher losses,

lower voltage, and power factor penalty. If technically

feasible, this is occasionally the best economic choice.

6.5 DESIGN OF DETUNED CAPACITORS FOR PQ

IMPROVEMENT

The procedure used to convert an existing power factor correction

capacitor into a harmonic filter is shown in Figure 6.1. It is being utilized

for designing suitable detuned capacitor. Power factor correction capacitors

may produce harmonic resonance and magnify utility capacitor switchingtransients. Therefore it is desirable to implement one or more capacitor

banks in a facility as a harmonic filter. System parameters are described as

single tuned notch filter connected to 480 V bus. The load is about

1200 KVA, power factor is 0.75 lagging. Current produced by load is of 30%

harmonics in the fundamental current. Maximum harmonics are 25% of fifth

harmonic current. It is supplied through transformer 1500 KVA with 6%

impedance.

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Figure 6.1 Design procedure of detuned capacitors for PQ improvement

6.6 SIMULATION RESULTS

This section presents the results of installing detuned filters in an

industrial plant with a significant amount of non-linear loads. For studying the

effect of capacitor on harmonic resonance, various sizes of capacitor with

varied load condition are simulated by MATLAB software.

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The source voltage, current at the PCC and the current at load bus

are analyzed using waveforms. The harmonic distortion in the voltage and

current waveforms are compared for the cases of compensation. From the

figure, the cases to be considered are:

Case 1: With fixed capacitor compensation

The magnitude of harmonic currents in an individual non-linear

load depends greatly on the total effective input reactance, which is comprised

of the source reactance plus added line reactance. In the case of non-linear

load, we can predict the resultant input current harmonic spectrum based on

the input reactance. The value of source reactance and its harmonic content

are inversely proportional. The voltage and current harmonic waveforms with

fixed capacitor compensation are presented.

Figure 6.2 Voltage and current waveforms with fixed capacitor

compensation

Time in seconds

Voltage(V),Current(A)waveforms

of

source,

bus1andbus2

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Figure 6.3 THD of current at PCC (With fixed capacitor compensation)

Case 2: Compensation with detuned capacitor

To avoid these resonances, a reactor is connected in series with the

capacitor, in such a manner that the fundamental reactive power is

compensated but the harmonics are not amplified.

Figure 6.4 Voltage and current waveforms with detuned capacitor

compensation

Time in seconds

Voltage(V),Current(A)waveformsof

source,

bus1andbus2

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The fifth order voltage and current harmonics are 4.1% and 33.3%.

The detuned capacitor bank of 75 KVAR, 525 V with 7% reactor tuned for 5th

order which has given output of 53.76 KVAR @ 415 V.

Figure 6.5 THD of current at PCC (With detuned capacitor compensation)

Table 6.1 Comparison of total harmonic distortions in different

compensation

Total harmonic distortion (THD) in %

Without

capacitor

With fixed

capacitor

Detuned

capacitor

Source voltage 17.05 15.63 1.54

Current at PCC 31.82 12.64 8.91

Current at load bus 33.82 10.15 8.96

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6.7 EXPERIMENTAL RESULTS INDUSTRIAL CASE STUDY

This section presents the results of installing detuned filters in an

industrial plant with a significant amount of non-linear loads. The detuned

capacitor filters installed in Adwaith Textiles Private Limited at Coimbatore,

India. The filters where connected on the point of common coupling (PCC).

The capacity of plant is 1750 KVA / 1100 KW. There are three transformers

with the capacity of 750 KVA, 1000 KVA and 1600 KVA supplying energy

to linear and nonlinear loads.

It is connected with 2 nos. of 25 KVAR fixed capacitors. By replacing the

fixed capacitors by detuned capacitor bank of 75 KVAR, 525 V with 7 %

reactors, the reactors will give an output of 50 KVAR @ 415 V. Hence there

will be a definite reduction in the KW, since the harmonic currents are

reduced and the compensation will be adequate.

Parameters of industrial plant are listed below.

Capacity of the industry : 24,000 spindles.

Transformer capacity : 1) 1000 KVA with impedance : 5.49%

2) 750 KVA with impedance : 4.90%

3) 1600 KVA with impedance : 5.99%

Total sanctioned demand load : 1750 KVA / 1100 KW

Total utilized load 60% @ 40%

power cut : 1064.2 KVA

Average power factor : 0.75

Type of loading : Balanced Load

Total installed load of machines : 3265 KW

Normal loads : 2000 KW

Non linear loads (with drives) : 525 KW

Unutilized loads : 740 KW

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Non linear loads are listed below.

Ring spinning frames : 20 Nos.

Auto-coner frames : 06 Nos.

Carding machines : 23 Nos. (with drives)

Simplex frames : 08 Nos.

Linear loads are listed below.

Draw frames : 04 Nos. (without drives)

Comber frames : 15 Nos.

Compressor : 03 Nos.Blow room machines : 01 No. (without drives)

Humidification plant : 01 No.

HT Panel meter readings

Voltage harmonics THD %

R Phase = 6 % , Y Phase = 6 % , B Phase = 5 %

Current harmonics THD %

R Phase = 14 %, Y Phase = 14 % , B Phase = 13 %

At transformer no : 1

1000 kVA at secondary side of the meter at the main PCC Panel.

Current harmonics :

PHASE 3rd

[Peak] 5th

[Peak] 7th

[Peak]

R 18.8 18.8 18.7

Y 17.6 17.3 17.4

B 21.3 21.4 17.2

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Voltage harmonics :

PHASE 3rd

[Peak] 5th

[Peak] 7th

[Peak]

RY 3.1 @ 6.2 V 3.2 @ 12 V 3.2 @ 4.6 V

YB 3.1 @ 1.1 V 3.0 @ 11.4 V 3.1 @ 5.6 V

BR 3.2 @ 0.8 V 3.3 @ 12.5 V 3.3 @ 5.5 V

Comments : Heavy duty capacitors would be fine. 525 V to be suitably

derated.

Fixed Capacitor Bank + 7 % Reactor connected in series can be erected at the

power house itself.


750 kVA reading could not be taken because of No Load.


1600 kVA at secondary side of the meter at the main PCC Panel.

Current harmonics :

PHASE 3rd

[Peak] 5th [ Peak ] 7 th [ Peak ]

R 9.8 10.0 10.4

Y 9.7 10.4 10.9

B 10.3 10.4 10.4

Voltage harmonics :

PHASE 3rd

[Peak] 5th[Peak] 7

th[Peak]

RY 2.4 @ 0.6 V 2.3 @ 8.9 V 2.5 @ 4.0 V

YB 2.4 @ 0.8 V 2.4 @ 9.4 V 2.3 @ 3.6 V

BR 2.4 @ 0.7 V 2.5 @ 9.4 V 2.4 @ 3.4 V

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It is connected with 2 nos. of 25 KVAR fixed capacitors. By

replacing the fixed capacitors by detuned capacitor bank of 75 KVAR, 525 V

with 7 % reactors, the reactors will give an output of 50 KVAR @ 415 V.

Hence there will be a definite reduction in the KW, since the harmonic

currents are reduced and the compensation will be adequate.

Based on the industrial load, the measurements in installing the

filters before and after compensation are presented together with waveforms.

The comparison is made in Table 6.2.

Figure 6.6 THD of three phase voltage waveform

Figure 6.7 Spectrum of 5th

order voltage harmonics values

Equivalent line voltage in volts

Harmonic order

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Figure 6.8 Spectrum of 5th

order current harmonics values

Figure 6.9 voltage and current waveforms after detuned compensation

Figure 6.10 Harmonic spectrum after detuned compensation

Time in seconds

Harmonic order

Harmonic order

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Power factor is calculated based on the capacitor rating with respect

to reactive power multiplier factors. The power factor before and after

compensation was 0.75 and 0.99. The voltage THD satisfies IEEE limit and

the appropriate reduction in current THD from 57.5 to 48.8. With effect of

detuning capacitors, there is a significant reduction in power consumed about

20.16%.

6.7.1 Comparison of Results

Table 6.2 Industrial system parameters of fixed and detuned capacitors

System parametersWith detuned

capacitor bank [off]

With detuned

capacitor bank [on]

Total RMS current demand 177 Amps 142 Amps

Average kW required 94.5 kW 94.5 kW

Average kVA required 126 kVA 101 kVA

Average power factor 0.75 0.93

5th

order voltage harmonics 10.5 4.27

5th

order current harmonics 53.2 40.1

Voltage THD 18.2 4.7Current THD 57.5 48.8

Table 6.3 Reduction in industrial system parameters

Reduction in system parameters

35 amps 19.77%

25 kVA 19.84%

By implementing the detuned capacitor banks in the above system,

there is a reduction in average current consumption from 172 A to 142 A.

Similarly the average kVA required is reduced from 126 kVA to 101 kVA.

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The reduction in system parameters shows potential improvement in terms of

efficiency as well as economical savings.

6.8 CONCLUSION

This chapter presented an industrial case study. The plant is having

a significant number of non linear loads. The installation of detuned filters

under harmonic conditions shows improvement in power factor. It establishes

a practical and economical way to recover p.f. It is found that the voltage

THD satisfies IEEE limit but the current THD reduces from 57.5 to 48.8.

With effect of detuning capacitors, there is a significant reduction in kVA

required about 19.84%. The power factor was increased from 75% to 99%

after installing detuned filters. The individual THD increases as TDD

decreases. The overall conclusion is that detuned filter ensures less power

requirement, but the effect in harmonic reduction is not so significant.

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11 Chapter 6