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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 22, NO. 1, JANUARY 2007 311

Detrimental Effects of Capacitors in DistributionNetworks in the Presence of Harmonic Pollution

Nicola Locci, Carlo Muscas, Member, IEEE, and Sara Sulis, Student Member, IEEE

AbstractThe main goal of this paper is to analyze the behaviorof an electric power network in the presence of harmonic distor-tion, when capacitors are installed. The work starts from a casestudy, where failure and malfunctioning of an industrial plant aredescribed with the help of experimental measurements. The theo-retical and mathematical details of thephenomena involvedare an-alyzed by suitable computer simulations. The study is performedby considering some important electrical quantities, evaluated ac-cording to the definitions proposed by the standard IEEE 1459.The results put in evidence that using some typical quantities, suchas the power factor, could lead to ambiguous conclusions in evalu-ating the actual quality of the loads, and this can assume specialimportance in virtue of the economic relevance of such quantities.

Index TermsCapacitors, harmonic distortion, power factor,power quality (PQ).

I. INTRODUCTION

CAPACITORS are widely used in distribution power net-

works to obtain reactive compensation of the inductive

loads. The root-mean-square (rms) value of the current in

the power line feeding the customers loads is reduced by

employing capacitors, so that both the generation capability of

the power plant and the losses in the distribution network are

minimized.However, the behavior of such compensation systems is op-

timal only under sinusoidal conditions. These days, the actual

operating conditions of power networks (in particular, distri-

bution networks) may significantly differ from the pure sinu-

soidal steady state, which is the reference condition for which

the plants and the electrical devices are usually designed, real-

ized, and applied.

This paper focuses on some implications of the harmonic dis-

tortion existing in modern distribution networks, when capaci-

tors are installed in some of the nodes, since there is growing ev-

idence that, under nonsinusoidal conditions, the presence of ca-

pacitors is associated with the partial or overall malfunctioning

of the plants (see, for instance, [1] and [2]).Furthermore, let us consider penalties for polluting loads and

power-quality indexes needed to assign responsibility for dis-

turbances in electric power systems. Doubts arise as to how the

role of capacitors should be taken into account when such ac-

knowledged indexes will be used to qualify the polluting be-

havior of loads. In particular, there is a possibility that some in-

dexes may penalize polluting consumers with capacitors in their

Manuscript received April 1, 2005; revised February 13, 2006. Paper no.TPWRD-00185-2005.

The authors are withthe Department of Electricaland ElectronicEngineering,University of Cagliari, Cagliari 9123, Italy (e-mail: sara.sulis@diee.unica.it).

Digital Object Identifier 10.1109/TPWRD.2006.877088

plants [3][5], even if they are used for the reactive compensa-

tion of the fundamental component of the current.

In order to prove the relevance of these topics, we will con-

sider a case study consisting of an industrial plant with signifi-

cant harmonic pollution and the presence of capacitors.

First, the drawbacks that arose during preliminary tests will

be reported. It is shown that the nonsinusoidal conditions of the

network seriously affect the performance of the portion of the

plant where the capacitors are positioned, leading to interven-

tion of the protection relays and, thus, inhibiting the normal

plant operation. Then, the analysis of the network will be car-

ried out by means of proper simulations in order to investigatedifferent possible configurations by evaluating, for each test,

several electrical parameters defined in the IEEE 1459-2000

trial-use standard [6]. In particular, a well known and commonly

used parameter will be considered: the power factor, for which

different definitions are proposed in [6], according to different

measurement purposes and different network conditions. It is

well known that the power factor has important economic im-

plications since its value is related to the penalties applied by

utilities to consumers. The results of this study show that, if the

involved phenomena are not well understood and the proper pa-

rameters are not considered, it is possible to make significant er-

rors in the assignment of the responsibility for low-power factor.

II. CASE STUDY

A. Plant Layout

The experimental investigation was performed on the

network supplying the forge unit of a metallurgic plant for man-

ufacturing steel pieces. Fig. 1 shows the general scheme of the

plant. The industrial plant is supplied by two identical 1-MVA

medium-voltage (MV)/low-voltage (LV) transformers (referred

to as Tr1 and Tr2 in the following), having a common primary

MV (15 kV) bus. In the studied situation, the low-voltage

outputs of the transformers are independent of each other: the

feeder leaving from Tr1 is dedicated to the forge division (bymeans of its distribution center, named DC1 in Fig. 1), while

Tr2 supplies the other loads of the plant (distribution centers

DC2 and DC3).

The supply line from Tr1 to the distribution center of the forge

unit is 140 m long and is made by an EPR cable ( mm

for each phase plus the neutral conductor).

The power supply to the forge is provided by a three-phase

rectifier bridge, realized by thyristors, followed by a dc/ac high-

frequencyconverter (10 kHz) realized by means of a high-power

insulated-gate bipolar transistor (IGBT) inverter.

The nominal current of the device is 870 A, with a rated

voltage of 380 V. This nonlinear load is named Nlin in Fig. 1.

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Fig. 1. Plant layout.

From the same distribution center, a different feeder supplies

a linear load (Lin) representing the service circuits of the unit.

These circuits mainly include lighting circuits, consisting of

groups of 400-W metal halide lamps, equipped with capacitors

for reactive power compensation. The protection for the lighting

circuits is guaranteed by automatic circuit breakers (CBs).

During the preliminary tests, performed before starting the

normal operation of the industrial process, overload conditions

always occurred in the lighting circuit, thus causing the auto-

matic CBs to open in less than a minute after the forge was

triggered, even if CBs were designed and installed according to

standard requirements. Such problems have prevented the plant

from becoming operational.

B. Voltage and Current Measurement

To examine the nature and origin of the reported malfunc-

tioning, both the voltages and the currents in the crucial points

of the plant have been measured. A digital phosphor oscillo-scope Tektronix 3014 (sample rate 1.25 GSamples/s on each

channel, 9 b for the vertical resolution) was used to acquire the

waveforms. Voltage transduction is guaranteed by an active dif-

ferential probe Tektronix P5200 with a bandwidth ( 3 dB) up

to 25 MHz and accuracy 5%. Current transduction for currents

up to 100 A is ensured by a Hall effect clamp-on probe Tek-

tronix A622 with a frequency range up to 100 kHz and accu-

racy . The current feeding the distribution center

DC1 (whose rated value is 870 A) was directly measured on the

output of transformer Tr1, acquiring the voltage at the output ter-

minals of the current transformer installed in the power center.

Theaccuracy of themeasurement systemis quite low, owingtothe uncertainty introduced by both the transducers and the ver-

tical channels of the oscilloscope. According to Standard IEC

61000-4-30 [6], the instrument specifications could be marked

as class B performance.However, this accuracy class canbe con-

sidered sufficient for the troubleshooting purpose of these mea-

surementsand for the technical analysis reported in Section II-C.

As for the evaluation of the power quantities described in [7] and

discussed in Section III, more accurate data are necessary. To

calculate such quantities, the experimental tests have been sub-

stituted with suitable computer simulations that reproduce with

good approximation theactual situation. In this way, thedata pro-

cessed are not corrupted by any measurement uncertainty.

Fig. 2 shows the acquired waveform of the current supplyingthe forge distribution center and its spectral content.

Fig. 2. Current absorbed by the forge: (a) waveform and (b) frequency spec-trum.

Fig. 3. Line-to-neutral voltage in the division distribution center.

The significant distortion of the signal can be clearly ob-

served. The relevant total harmonic distortion (THD) is about

23%.

Fig. 3 shows the line-to-neutral voltage waveform measured

on the terminals of the feeder supplying the forge in the distri-

bution center. We can observe the significant distortion arising

fromthe highly distorted current absorbed by the nonlinear load.

In particular, the sudden commutations occurring in the cur-

rent cause voltage drops in the inductive impedance of the sup-

plying network, thus leading to the noticeable voltage spikes in

the waveform of Fig. 3. The THD of this waveform is 14%. Inorder to get a more exhaustive view over the voltage profile in

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Fig. 4. Line-to-neutral voltage on the low-voltage terminals of the transformerTr1.

Fig. 5. Line-to-neutral voltage on the low-voltage terminals of the transformerTr2.

the overall plant, Fig. 4 shows the voltage waveform acquired

directly on the low-voltage side of Tr1.

The voltage distortion, caused by the harmonic current

flowing into the equivalent series impedance of both trans-

former Tr1 and supply network, is reduced with respect to the

voltage at distribution center DC1, which was also affected by

the voltage drop on the cable. This is confirmed by the fact that

the THD of this waveform is 11.6%.

Finally, Fig. 5 shows the voltage on low-voltage (LV) termi-

nals of transformer Tr2. This waveform is even less disturbed

than the one shown in Fig. 4, since it is not influenced by thedistorted voltage drop caused by the harmonic currents in Tr1.

As a consequence, the voltage on the LV bars of Tr2 has a THD

equal to 3%.

C. Failure Report and Analysis

In the distribution center DC1, as well as in the overall plant,

the protection for the lighting circuits was designed following

the usual rule of thumb. Malfunctioning and inefficiency of the

protection plant occurred during the tests as untimely operation.

The experimental acquisitions, along with the following consid-

erations, allowed us to explain the situation. Each 400-W metal

halide lamp of the lighting circuits is compensated with a 40- Fcapacitor and the combination absorbs 1.7-A rms line current

Fig. 6. Current absorbed by the three lamps lighting circuit.

with rated voltage 220 V. Each feeder supplies a group com-

posed of six lamps directly from the division distribution center.

The applied voltage is the one represented in Fig. 3.

In order to perform the experimental survey without causingthe untimely intervention of the protection, three of the six

lamps supplied by a single feeder where purposely bypassed.

Fig. 6 shows the current absorbed by this reduced load. The

rms current is 15 A, while the fundamental harmonic has an

amplitude of 5 A. There are current peaks up to 50 A, with

THD .

Therefore, the actual current in the feeder with the original

load (six lamps) is about 30 A rms, with peaks up to 100 A.

These values should be compared to the rms current that would

be absorbed by the same load under sinusoidal conditions,

which is .

One can notice that the ratio between the rms value of the dis-torted current and the nominal one

is around three.

Let us consider a thermal-magnetic CB, following the nor-

malized curve C (according to [9]) for the modular CBs, with

rated current . The same breaker in a polluted system

works with a ratio and, therefore, in this condition,

trips in 1 min. These considerations correspond perfectly with

that which was experimentally observed.

III. EVALUATION OF IEEE 1459 QUANTITIES

A. Simulations

The aim of this section is to closely examine the situation il-

lustrated in Section II by evaluating a few significant parameters

defined in the IEEE standard [7] in different realistic operating

conditions. Therefore, different network configurations should

be tested to compare the results. These tests were performed by

simulating the distribution plant under examination by means of

the PSCAD/EMTDC program, produced by Manitoba HVDC.

As stated before, the use of simulations also allows possible

problems arising from inadequate measurement accuracy to be

avoided in the evaluation of the meaningful parameters.

Nominal data from manufacturers, as well as experimental

results, were used to obtain a realistic model of the actual

system, so that a very good agreement exists between the ac-quired voltage and current waveforms and the simulated ones.

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In particular, each lamp was modeled with a

model (where , ,

and F). The nonlinear load was modeled by means

of an equivalent circuit consisting of a linear load shunted with

harmonic current generators, whose values were extracted from

the experimental waveforms.

The IEEE Trial Use Standard 1459 provides definitions forthe measurement of electric power quantities under sinusoidal,

nonsinusoidal, balanced, or unbalanced conditions. Besides

the mathematical expressions that were used in the past, this

standard defines new expressions aimed at accounting for the

important changes that have occurred in the electric distribution

network during the last decades. In the introductive note of

the document, there is the claim that the new definitions

were developed to give guidance with respect to the quantities

that should be measured or monitored for revenue purposes,

engineering economic decisions, and determination of major

harmonic polluters.

The power factor definitions used here are as follows, with

the variables as defined in the Appendix:1) fundamental (50/60 Hz) power factor ;

2) total power factor .

To achieve the desired quantities, suitable signal processing has

been performed in the LabVIEW software package. One period

of the steady-state waveforms has been extracted from the simu-

lations to avoid spectral leakage problems in the frequency anal-

ysis performed by means of a discrete Fourier transform (DFT)

[9].

B. Results and Discussion

As shown in Fig. 1, the network has a linear load (Lin),namely the lighting circuits, and a strongly nonlinear load

(NLin), the forge, connected to the bus. In this specific situ-

ation, both loads are managed by the same customer, but, for

the purposes of this work, it is interesting to consider them as

if they would be representative of different users supplied by

the same bars, which could play the role of a point of common

coupling (PCC).

The measurement of both applied voltages and absorbed cur-

rents has been achieved at the departure of the relevant feeders

for each load.

The first tests were conducted by considering only one load

connected at a time.As for the linear lighting circuits, the two power factor defi-

nitions lead, as was expected, to the same numerical result.

In particular, when capacitors are not present, the inductive

nature of the load is clearly evidenced by a low power factor

, while the reactive compensation

achieved with the capacitors increases this value up to 0.954

for both indexes.

As for the nonlinear load, it is characterized by the following

values: and .

These results should be compared to the ones obtained when

both loads are supplied at the same time, which is the normal

operating condition of the plant.

When the forge is on, the noncompensated lighting circuitsfeature and . The similar values of

the two quantities can be explained by the fact that the induc-

tive lamps smooth the harmonic currents and, therefore, the har-

monic powers have little effect on the total power quantities.

In any case, the value is so low that the use of capacitors for

power factor correction seems to be mandatory. However, this

correction is valid for quantities at the fundamental frequency,

whereas it can introduce dramatic collateral effects, dependingon the strong increase of the harmonic distortion on the network,

as clearly shown in Section II-B. This is confirmed by the

results: the fundamental power factor is increased again

up to 0.954, whereas the total power factor falls down

to 0.180.

To complete the survey, it should be considered that on the

forge side the effects of the reactive compensation on the lamps

are significantly less noticeable: when the lighting circuit is not

compensated, the forge features and ,

while the introduction of the capacitors leads to

and .

The results achieved on this simple network emphasize the

need for carefully stating the most suitable definitions to be im-plemented in measurement instruments designed to analyze the

behavior of electric systems under nonsinusoidal conditions, es-

pecially when such results are used to establish penalties for

loads that contribute to the power-quality (PQ) degradation.

IV. CONCLUSION

This work focuses on an engineering problem related to the

effects of the capacitors in electric power networks in the pres-

ence of harmonic pollution. Starting from a real case in an ac-

tual low-voltage industrial plant, the study remarks that, owing

to the pervasiveness in the use of capacitors in such systems,

strong nonlinear loads could have serious implications on theoperation of neighboring plants.

In addition, by evaluating quantities purposely defined to be

measured for either revenue purposes or determination of major

harmonic polluters, it has been shown that customers with linear

loads, besides suffering from malfunctioning of their plants,

could also be penalized for the low-power factor, depending on

the definition implemented in the utility measurement station.

These considerations could be more complex when both linear

and nonlinear consumers take responsibility for harmonic pollu-

tion. In the assumption of sinusoidal conditions, the authorities

enforce penalties to avoid the low-power factor in the network.

Under nonsinusoidal conditions, the same penalties should be

enforced according to parameters solidly acknowledged, whose

recognized reliability would make the possible economical ef-

fects acceptable from the customers point of view.

APPENDIX

Let us indicate the rms value of the line to neutral voltage

as , the rms value of the line to line voltage as , the

rms value of the line current as , and the rms value of the

neutral current as , the fundamental positive-sequence voltage

component as , the fundamental positive-sequence current

component as , and the relevant phase angle as . On these

base meanings, the standard IEEE 1459 provides the following

definitions for power factors in three-phase nonsinusoidal andunbalanced four-wire systems.

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1) Fundamental positive-sequence power factor

where

is the fundamental positive-sequence apparent power

is the fundamental positive-sequence active power

is the fundamental positive-sequence reactive power.

2) Total power factor

where is the total active power and

is the effective apparent power.

The rms effective current and voltage are defined as

REFERENCES

[1] M. Mamdouh Abdel Aziz, E. El-Din Abou El-Zahab, A. M. Ibrahim,and A. F. Zobaa, Effect of connecting shunt capacitor on nonlinearload terminals,IEEE Trans. Power Del., vol.18,no.4, pp. 14501454,Oct. 2003.

[2] R. H. Simpson, Misapplication of powercapacitorsin distribution sys-tems with nonlinear loads-three case histories,IEEE Trans. Ind. Appl.,vol. 41, no. 1, pp. 134143, Jan./Feb. 2005.

[3] D. Castaldo, A. Ferrero, S. Salicone, and A. Testa, An index for as-sessing the responsibility for injecting periodic disturbances, in Proc.6th Int. Workshop Power Definitions and Measurement Under Non-Si-nusoidal Conditions, Milano, Italy, Oct. 2003.

[4] E. J. Davis, A. E. Emanuel, and D. J. Pileggi, Evaluation of single-point measurements method for harmonic pollution cost allocation,

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quality indexes for harmonic distortion in the presence of capacitors,in Proc. IEEE IMTC, Ottawa, ON, Canada, May 1719, 2005, pp.16001605.

[6] Electromagnetic Compatibility ( EMC)Part 4: Testing and Measure-ment TechniquesSection 30: Power Quality Measurement Methods ,IEC Std. 61000-4-30, 2003.

[7] Trial-Use Standard: Definitions for the Measurement of Electric PowerQuantities Under Sinusoidal, Nonsinusoidal, Balanced or UnbalancedConditions, IEEE Std. 1459-2000, Jan. 2000.

[8] Electrical AccessoriesCircuit-Breakers for Overcurrent Protectionfor Household and Similar InstallationsPart 1: Circuit-Breakers fora.c. Operation, IEC 60898-1, 2003.

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Nicola Locci received the Laurea degree in mechanical engineering from theUniversity of Cagliari, Cagliari, Italy, in 1974.

Currently, he is Associate Professor of electrical measurements with the De-partment of Electrical and Electronic Engineering, University of Cagliari. Hewas Professor of communication systems and his research topics are coding,photovoltaic systems, losses measurement in power electronics, and high-fre-quency (HF) transformers, variable reluctance motor parameters measurement,

and nonactive energy compensation. His research interests include measurementon power systems with distorted waveforms, transducers performance improve-ment, and accuracy evaluation in signal processing of data-acquisition systems.

Carlo Muscas (M98) wasborn in Cagliari, Italy, in 1969. He received the M.S.degree in electrical engineering from the University of Cagliari, Cagliari, Italy,in 1994.

He was Assistant Professor in the Electrical and Electronic MeasurementsGroup with the University of Cagliari from 1996 to 2001. Currently, he is Asso-

ciate Professor of electrical and electronic measurement. His research activitymainly focuses on the study of power-quality phenomena, including the de fini-tion of electrical quantities used to characterize the behavior of power systemsunder nonsinusoidal conditions, along with the metrological qualification of therelevant measurement processes. He is author or co-author of many scientificpapers.

Sara Sulis (S04) received the degree in electrical engineering from the Uni-versity of Cagliari, Cagliari, Italy, in 2002 and is currently pursuing the Ph.D.

degree in industrial engineering at the University of Cagliari.Her main research activity is in the field of power-quality measurements, with

particular attention to the definition and measurement of electrical quantities inpower systems under nonsinusoidal conditions and to the metrological qualifi-cation of the measurement processes involved.