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L I G H T I N G D E S I G N& A P P L I C A T I O N

Compact fluorescent lamps (CFLs) provide significant energy saving over incandescent lighting. As a result CFLs are being promoted as part of energy conservation programmes for many electric utilities [3]. Up until now power quality issues associated with CFLs have largely been ignored as the number of these lamps on the system was small and the associated impact difficult to quantify. However, the situation is likely to change in the near future as large numbers of CFLs penetrate the market place and as result harmonic emissions may have to be limited [1], [2].Compact fluorescent lights, like all discharge lights, create harmonics on the supply system because of the control systems limiting the plasma (an electric arc) current, which produces light. These harmonic currents are then injected into the distribution system. Because most electrical systems were designed for linear voltage and current waveforms (i.e. nearly sinusoidal), excessive nonlinear loads can cause serious problems such as overheating conductors, transformer and capacitor failures as well as malfunction of electronic equipment. The magnitudes of harmonics generated by the CFLs vary between manufacturers and between ranges of lamps from the suppliers [1], [3], [4].

Methodology

The research methodology applied was:l Measure the harmonic contribution

for CFL that are currently being used by Eskom as part of the Demand Side Management programme.

l Develop a representation of this type of CFL as a non-linear model for harmonic simulations.

l Investigate the impact of CFLs at the point of common coupling (PCC) with different quantities of CFL loading.

l Results obtained are compared to the IEEE-519 limits which are shown in Table 1.

Theory

Harmonics referred to here are the repetitive electrical disturbances caused by CFLs. Using the Fourier series, a distorted periodic wave shape can be represented by its fundamental and harmonic [4], [5].

It is also common to use a single quantity, the total harmonic distortion (THD) as a measure of the effective value of harmonic distortion [1].A similar equation could be written for a current waveform I(t) and ITHD.

Magnitude of an individual harmonic component as a percentage of the fundamental is also used as a measure of harmonic contribution to the total harmonic distortion. The formula used for the calculation is as follows [1]:

A similar equation could also be written for %IHD.

Experiments

Experiments have been grouped in three models, single, double and triple model (Fig.1). A single model contains 12 CFLs while a double and a triple contains 24 CFLs and 36 CFLs respectively. Measurements were performed to identify the harmonic characteristics for these groups. These characteristics were then used to derive harmonic injection characteristics for households and group of households, including the possible effect of harmonic cancellation.However, the harmonic current generated from CFL groups tends to add up rather than cancel as shown in Fig. 1. This is because only the same type of CFL was used (14 W). The harmonic spectrum representing the 12 CFL group is given in Table 2 .

The main objective of this research is to predict by means of computer modelling and simulations the impact of large number of CFLs on distribution systems.The analysis

uses SuperHarm software to predict the distortion level on a distribution system.

Impact of large numbers of CFLs on distribution systems

by Angula Nashandi and Prof. Gary Atkinson-Hope, Cape Peninsula University of Technology

IEEE 519 limits

PCC voltage Individual VHD%

THD%

Below 69 kV 3,0% 5,0%

Table 1: IEEE standard.

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The double and triple models have similar spectrums. The measurements show that CFLs have large harmonic currents.The harmonic current distortion (%IHD) for the CFL group measured is of the order of 116% as shown by waveform in Fig. 2.Description of the network studiedThe impact of a large number of CFLs on a distribution system was observed by means of computer modeling and simulations of a three-phase, 33 kV network. The network supplies mostly residential customers (BUS4A) and some commercial customers (BUS5A), respectively. The simplified one line diagram of the system studied is provided in Fig. 3.Residential area consisted of 360 homes while the commercial area consisted of 195 factories/offices. Single model was used for residential lighting while triple model was used for commercial lighting. Therefore the total number of lights installed for residential and commercial is 4 320 and 7 020 respectively. The total loading was constrained not to exceed the 5 MVA rating of the transformer (T1). The loads have the following apparent power, S, and power factor, displacement PF, ratings.LOAD1 and LOAD3 in the table below represent when all lights installed are 14 W CFL. The CFLs installed represent 14% of the residential load and 11% of the commercial load. CFLs are modelled to inject harmonic currents (Table 2).

LOAD1 and LOAD2 represent residential loads while LOAD3, LOAD4 and DRIVE1 represent commercial loads. LOAD1 and LOAD3 represent lighting load for residential and commercial respectively. Other loads included computers, TV sets, refrigerators, resistive heating, motors and stoves. For commercial loads it was assumed that there are factories installed with drives that also inject harmonics into the systems. The drives are modelled to inject an ideal harmonic spectrum.The following assumptions were made:l A balanced three phase system was

assumed that means all three phases were equally loaded with single phase loads.

l Simulation was conducted using single phase representation, thus results were obtained from the representation of one phase, assuming the same results for the other two phases.

Case studiesFour case studies were conducted with different CFL group loadings, to analyse their impact on a distribution system.Case study 1 (CS1)This is the worst-case scenario in terms of lighting and other load demands. The total load is assumed to be at full load. Lighting loads installed is assumed to be 60 W incandescent bulbs for both residential and commercial buildings. This can be during high lighting demand in the night when all lights are ON.Case study 2 (CS2)Case study 1 is repeated, this time all the incandescent bulbs are replaced with 14 W CFLs and all are ON.Case study 3 (CS3)A diversity factor of 0,6 is widely used [1]

therefore, for this case study all lighting and other loads for both residential and commercial are reduced to 60% of full load. This case was selected to represent realistic conditions. A reactor is added in series with the capacitor as a harmonic filter to reduce the %VTHD which is 14,0% to 5,74517%.Case study 4 (CS4)For this case study it is assumed that only 60% residential CFLs and 10% of commercial CFLs are ON. Other loads are assumed to be at 10% of full load. This case study represents nighttime around 20h00.ResultsTo comply with the IEEE 519, harmonic penetration results were obtained at the point of common coupling (BUS3).Harmonic impedance scan was conducted in CS3 at Bus PCC as shown in Fig. 4.From the scan results, resonance occurs close to the 5th harmonic order.AnalysisThe total harmonic distortion results are displayed in Fig. 5.To evaluate the impact of CFLs penetration on a distribution system, CS1 and CS2 are compared. CS1 is a base case condition representing distortion levels due to existing load characteristics. It is found that promotions of CFLs

Harmonic order

Magnitude (%)

Angle (degrees)

3rd 77,2 173

5th 50,1 3

7th 39,7 -164

9th 32,7 27

11th 19,4 -157

13th 8,2 24

Table 2: Measured harmonic spectrum for 12 CFL group.

Fig. 1: Harmonic levels for the three models.

Fig. 2: Measured CFL group current waveform.

Fig. 3: Single line diagram for the network studied.

Name Apparent power (kVA)

Power factor

LOAD1 21,804 0,9

LOAD2 132 0,85

LOAD3 35,432 0,9

LOAD4 187,2 0,85

DRIVE1 +CAP 108 0,97

Table 3: Load data.

Harmonic order

Magnitude (%)

Angle (deg)

3rd 33,3 0

5th 20 0

7th 14,28 0

9th 11,11 0

11th 9,09 0

13th 7,69 0

Table 4: Drive 1 - Harmonic current spectrum.

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installation in CS2 result in an increase of 1,38% voltage distortion, which is a significant change. The results obtained exceed the specified IEEE-519 limits in CS3. The VTHD obtained in this case was 5,74519%. Although the capacitor improves the power factor, it causes harmonic resonance. Therefore the mitigation solution is included in CS3 by tuning a passive filter to 5,7th harmonic. They are used for shunting harmonic currents in a power system.ConclusionsFrom the results obtained it can be concluded that the distortion level that can be expected depends on the type of CFL used and the distribution parameter. The currents distortion for the CFL used is very high, that even when CFL is 10% of the total load, can result in an unacceptable voltage distortion at the point of common coupling.The use of CFL with highly distorted current should be discouraged and those with low distorted current should be encouraged.

Index CS1 CS2 CS3 CS4

V1 6167,17 6175,42 6246,39 6319,7

%V3 0,99255 0,83815 2,96818 0,26067

%V5 0,97265 1,69527 4,14631 0,49566

%V7 0,95647 2,22744 0,9488 0,56234

%V9 0,93897 0,52756 1,89477 0,33273

%V11 0,91936 2,0427 1,06091 0,49496

%V13 0,89785 0,85566 1,17857 0,08985

%VTHD 2,31928 3,70419 5,74517 0,99681

VRMS 6168,83 6179,65 6275,29 6081,7

Table 5: Harmonic penetration results at PCC.

Fig. 4: Impedance scan at PCC for case study 3.

Fig. 5: VTHD comparison at PCC.

Future research should be done with an industrial distribution network.AcknowledgementThis paper was presented at the recent 2007 ICUE Conference in Cape Town, and is republished with permission.References[1] Theodore, W.: “Electrical Machines,

Drives and Power Systems,” 4th Edition, Prentice-Hall, New Jersey, 2000, pp. 809-821.

[2] Emmanuel E, Gentile T J, Pileggi D J, Gulachenski E M, Root C E: “The Effect of Modern Compact Fluorescent Lights on Voltage Distortion”, Presented at the IEEE/PES 1992 Summer Power Meeting, Seattle, WA, July, 1992.

[3] Dwyer, R, Khan, A K, McGranaghan, M, Tang L, McCluskey, R K, Sung, R, Houy, T: “Evaluation of Harmonic Impact from Compact Fluorescent Lights on Distribution Systems”, IEEE Transactions on Power Delivery, Vol. 10, No. 4, November 1995.

[4] Henderson, R: “Harmonics of Compact Fluorescent Lamps in the Home”, Domestic Use of Electrical Energy Conference, 1999

[5] IEEE Std. 519-1992, “IEEE recommended practices and requirements for harmonic control in electrical power systems”, IEEE std.519-1992, IEEE, Institute of Electrical and Electronic Engineers, USA, April 12, 1993.

[6] Sawiki, J and Galewsi, M, “Economical definition of distortion power”, Proceeding of the spring seminar on Nonsinusoidal Systems, Zielona Gora University, Poland, 1999, pp. 31.

[7] Porges, F, “The Design of Electrical Services for Buildings”, 3rd Edition, Chapman and Hall Ltd, New York, 1989, pp. 84-86.

[8] SuperHarm Electrotek Concepts, User’s Guide, Version 4.3.0, USA, October, 2004, pp. 1.0-4.35.

C o n t a c t A n g u l a N a s h a n d i , CPUT, gnashandi@webmail.co.za; Gary Atkinson-Hope, atkinsonhopeg@cput.ac.za D