7/29/2019 00759846

1/5

Examination of Power Quality in IndustryR. Mienski, R. Pawelek, I. WasiakInstitute of Electrical Power EngineeringTechnical University of LodzLodz. Poland

Abstract-The pap er provid es results of tests of power qualit yperformed for an industrial electrical power network. Severalparameters were identified, like: voltage variation, harmonicsand to tal distortion factor, voltage unbalance. The m easurementaimed at assessment of influence of some loads which areoperated in the plant on power quality and evaluation of qualityof supply from public distribution network. Some results of testsand their analysis are presented in the paper.

This paper presents some selected test results performed i nlow- and medium-voltage distribution networks in A M I C Aco.works in wronki. t aimed at determining whethermalfunctioning of the drivesystem of the press ,nachine iscaused by industrial loads or inadequate quality of supplyfromthe public lectrical power11. TES'I'SPROGRAM

1. INTRODUCTIONI n the condition of increasing awareness of power quality's

role in electrical engineering there is a need to assess thequality of service provided by utilities to their consumers.Many industrial electric consumers now require a higher levelof power quality due to increasing sensitivity of receivers andprocess controls.Quality of supply can be defined[4]as any deviation fromthe characteristic values of the ideal sinusoidal voltagewaveform. Electrical power quality is then determned by thefollowing supply voltage parameters which are measured atthe connecting point of the common coupling with publicnetwork (PCC):- slow voltage variation,- voltage fluctuation- flicker severity,- voltage harmonics (values of higher harmonics and THDfactor),- voltage unbalancein three-phase systems.The above mentioned parameters for low- and medium-voltage distribution networks are defined and standardised inEN 50160 European Standard [ l ] which is being put inpractice in Poland at the moment.In recent years negative phenomena resulting from electricalpower quality deregulation have been observed morefrequently. The electrical equipment characterised by a non-linear voltage/cui-rent e.g. power electronics), widely appliedin industry. causes harmonic disturbances. Some industrialintermttent loads. like arc furnaces are the sources of voltagefluctuations resulting in annoying voltage flicker. Thephenomena may be strain-effective for electrical power usersand may defective operation of regulating devices, control andsignalling systems and protective relays. Distortions comngfrom power electronic drive systems sometimes prevent theirown control systems from proper functioning. Suchphenomena have started examination of power quality inindustrial environment.

A. Description ojlhe Examined NetworkFig. 1 shows the schemeof power supply to AMlCA Co.

works in Wronki from the public electrical power networkand presents a part of the low- and medium-voltage industrialdistribution networks which has been examined.

Q ;,..:,,

Fi g. Schemc afpawcr supply io AMIC'A C ' oSupply voltage parameters have been measured on the busof 15 kV switching station and on the bus of section I of

0.4kV switching station in SO-3 station. In low-voltagcswitching station scheme the receivers which influencedelectrical power quality have been distinguished. They areasfollows: the drive system of the press forming machine, therectifier station for charging battery-operated truck and the0-7803- 5105- 3/98/ $10.00 1998IEEE

94

7/29/2019 00759846

2/5

capacitor bank for reactive power compensation. The bank issupplied with automatic power control system in the functionof power factor. What has not been examned was theinfluenceof the receivers installed in the boiler house andothers shownas ,,other receivers" group on the scheme.

Filter characteristic:Filter frcqucncy:Band width:Averaging timc of voltagc calculationresul ts:Voltage measurement accuracy:

B. Measurement System

rectanglei,S0Hz

(0,02+0.022) f,from 2 SCC. to I 5 min.

consistent with EN 61000 [3]

The measurements have been carried out by means ofcomputer - operated measurement system. which has beendesigned and constructed in the Instituteof Electrical PowerEngineering of Technical University of Lodz, basically toevaluate supply voltage parameters according to EN 50160European Standard demand. Table I illustrates basicinformation which refers to measurement system data. Whiletaking measurementsin 0.4 kV switching station, the systeminputs were connected directly to three phase voltages,however, in case of 15 kV switching station, they wereconnected to three delta voltages by means of voltagetransformers. Test results were registered on hard discof thecomputer at frequency given in the Table11.

M easurement range:Calculation method:Averaging time of calculation results:Measurement accuracy offlicker severi ty:

TABLE IBASIC CHARACTERISTICSOF MEASUREMENT SYSTEM

Operating modeThe system operates in real-time. without intervals between data windows.measuring, analysing and recording measurement results on the discautomatically

(0,OOlS +0.2)U"consistent with EN 60868 121

(2+ I S) min.consistent with EN 60868 121

Input

M easurement system charartcr isticsData window. 1 rcctaneular IWindow span: 2sec.Sampling frequency: 4095 Hz IConsecutive data windows: I are in contactVoltnge curve analysisCalculatins method: I FFT

C. Tests RangeThe performed tests aimed mainly at assessment 01 'influence of some loads, which are operated in AMICA Co.Wronki. Special attention was paid to the drive systemof the

pressing machine and the mutual interaction of rectifiers andcapacitor bank. Another purpose was estimation quality ofsupply from public distribution network. So the examnationscope referred to measurement of selected voltage parametersin SO-3 15/0.4 kV station (which supplied energy to the pressmachine) cairied out on both the busesof 15kV and 0.4kVand the analysisof results obtained.A number of measurement cycles have been performed.andthree of them have been chosen to analyse. Table I 1 illustratestheir characteristics. In each cycle the following suppl)voltage parameters, defined according to [11, have been tcstcdat a given measurement point:0 voltage changes in relation to its declared value for eachphase,

where: U,,- data window rm valueof supply voltage (phaseor line-to-line), U,. -declared voltage at the connecting pointof common supply,0 asymmetry factor of three-phase system,

ASYM, =5 00 %U1where: U,-- data window rms value of negative sequencccomponent of supply voltage basic harmonic, U, - datawindowrms value of positive sequence component of supplyvoltage basic harmonic.

relative valuesof voltage higher harmonics for each phase.

where: Ulf data window rms value of h - voltage haiinonic(h= 1 , ...., 0). b0 THD factor for each phase voltage.7

The following values have been accepted as declaredvoltage:- for0.4 kV - U, =220V (phase voltage of the network).- for I 5 kV - U,=100V (secondary voltage of voltagetran sforiners)

95

7/29/2019 00759846

3/5

Measurementplace Load characteristicsSO-3: 0.4 kV During measurement cyclc. aftcr all the receivers had bcen putotT. the press

machine was switched on and the starting and braking of thc drive systein 01'this press were tested several times.During the cycle thcrc was a simultaneous pcrtormanceol' the hoilcr slalioiireceivcrs. rectifier station and capacitor hank In autoniatcd control niodc.Thc switching on and off of subsequent steps of thc bank was l iwxd.During the cycl e the plant operated under normal night-characteristic load.

SO-3; 0,4 kV

SO-3; IS kV

Date,Cycle No. time of measurement

26. O . 199717:59 I O sec.26f27.10.199722:05 2 min.

Number ofsamplcs. averagingI

111. MEA S~JREM ENTES~J L IS

~start-up time26. O 997 I O015:59 10 sec.

A. General Remarks

No.

As the time of canying out measurements was limted, theaveraging time of measurement results was forced to beshortened (in reference to EN 50160 Standard [I]) from I Omn. to 10 sec. (at night time to 2 mn.). It enabled to examnethe influence of dynamic states of the press machineperformance (starting and braking) on the power quality and itgave the adequate number of measurement samples insubsequent measurement cycles as well. For these samples,statistical characteristic values were determned, basically themaximum, mnimum and mean value and the so-called95%level, i.e. the value, which is not acceded by 95%measurements during a given cycle.

Total harmonic Asym-Index distortion factor of the Supply voltage level metry

B. Influence of the Press Machine Drive System on PowerQuality

I .

Table111 presents the basic results of measurement of powerquality parameters obtained in measurement cycle I. For thesame cycle changes in time and systematic graph of T H Dfactor of phase voltages have been illustrated in the Fig. 2.And Fig. 3shows the spectrum of harmonics distorting phasevoltage curves (for L I phase).

.. .voltage factorValue THDLI THDLI THDL~ ULI U L ~ UL; ASYM% % % Yo % % Yomaximum 10,17 10,98 11,12 109.73 109.55 109.75 1.54

2.3.4.

minimum 1.58 1.42 1.68 107,40 107.34 107.44 1.44mean 2.74 2.74 2.86 109.16 108.99 109.20 1.48

95%level 8.79 9.71 9.82 109.63 109.43 109.66 1.51Measurements were carried out under the conditions ofhigher supply voltage (TI transformer tap switch in - 5%position) and of the lack of other loads switched on theexamned section. When analysing the test results, we can

conclude that the drive system of the press machine.especially during transient processes which take place instarting and braking, has a significant influence on supplyvoltage distortion. The evidence for this is in high maximumvalues and the values of95% level of THD factor, comngupto 11% and9% respectively. Such an apparent supply voltagcdistortion is seen in the wide range of higher harmonics(Fig.3) which arise during examned dynamic states, basicall)so-called harmonics characteristicof 6-pulse -?-phase rectifierbridge.12w, I

b)I1.M

10w8.W-s0 bW

cI 2.Mn io LO w U) .w 611 711 xo vm I I I I

timc 1%1Fig. 2. THD factor for three phascs.

a) changes intimc graph. b) systematic graph

96

7/29/2019 00759846

4/5

4.00

2.w

0.00

No.

2 5 8 I I I 4 17 10 U 1 29 31 J 5 38Harmonicsnumher

Total harmonic :\syn1-Index distortion lsclor of thc Supply voltage level inctryvoltaec litclor

Fig. 3. Spectrumof harmonics in L I phase voltage,0 maximum value. - 95% level

1.

C. Interactionof Rectifiers and Capacitor Bank.L Value THDLI THDL? TH11i.1 U. , I UL: UI ASYM

% % % % I 940 %maximum 3.24 3.52 3.5s 100.85 I 99.82 100.03 1.26To illustrate the phenomena which take place during

reactive power compensation in the network with non-linearreceivers. the results of cycle I I measurement were chosen.The non-linear receiver is represented by the group of three-phase rectifiers, designed for loading battety-operated trucks,which performs practically under constant load. The switch-on of subsequent steps of capacitor bank in automatic controloperating mode was forced (by adequate positioning of coscpregulator). The bank had 23 steps of adjustment, each of 20kVAr. In the process of switchingon the capacitor bank,appearing of resonance phenomena was observed successivelyfor 13, 1 I , 7and 5 harmonics, i.e. for harmonics characteristicfor 6-pulse 3-phase rectifier bridge. The analysis was carriedout for such a capacitors power, when individual harmonics(from 5, 7, I 1 and 13 sets) reached the maximum values.Fig.4 illustrates the above mentioned phenomenon presentingvalues of harmonics in dependenceon capacitor bank power(Q

7/29/2019 00759846

5/5

EN 50160 standard [l], but in order to confirm thisconclusion, additional one-week examnation with theaveraging time of measurement results consistent with thestandard requirements should also be carried out.

k0.0

Fig. 6. Spectrum of harmonics in the supply voltage (SO-3. I S kV)U -maximum value, -95% level

IV . CONCLUSIONS1 . Testsof power quality carried out in AMlCA Wronki Co.works enabled to :

- evaluate the influence of non-linear receivers withfrequent variable load on the voltage parameters inindustrial network,- examne resonance phenomena which take place inreactive power compensation when rectifier typereceivers are present,- estimate generally the quality of electrical powersupply by public distribution system.2. Transient processes taking place during starting andbraking the drive system of the press machine,significantly increase the voltage distortion in examnedlow-voltage industrial network. This distortion comprisesa wide rangeof higher harmonics, and at the same time thecrucial significance is put to so-called harmonicscharacteristic for 6-pulse 3-phase rectifier bridge.

3. In networks with non-linear receivers, reactive powercompensation using a capacitor bank with automaticcontrol of the power, may lead to resonance for somehigher harmonics. The higher accuracy of regulation is.the higher probability of this phenomenon occurringexists.

4. Computer based measurement system, which has beendesigned and constructed at the Institute of ElectricalPower Engineering of Technical University of Lodz.makes it possible to evaluate three-phase supply voltagei naccordance with EN 50160 European Standard [ I ] .Moreover, regulated time of averaging the results in therange from 2 sec. to I S mn. enables to shorten theexamnation time, what allows to observe the phenomenawhich take place in dynamc states of devices niorcprecisely.V. REFERENCES

[11 Europcan Standard EN SO 160: I .~'ohgechuruc/eris/ics yf e/ec/rrcrJl,121 European Standard EN 60868: /-'/ic&enrre/er - /'irncfiorrd ( i d W , V J[3] European Standard EN 61000: f?/ecfromugne/ic coriipohdifr /E\ / ( ' / .[4] Guide to Quality of Electrical Supply for Industrial Installations. I J l l i .

sirppliedhypuhlic clistrihir/rong W w r s . 1994.specficcitrons. 1993.I 993I 994

Dr Rozmystww Miedskireceived M.Sc. and 1'h.D. degrees lion1 'I'cchnical Untvcrsity 01' I-otlz. A1present hc is a scnior lecturer at thc Institutc of Electrical Power I.:npinccringof Technical University of Lodz. His area of interest is powcr quality idA C N power network simulator.Dr Rysmrd Pawelekwas born in 1952 in Chocz. Poland. IIc received M.Sc. and I'h I ) dcyw.;from Technical llniversity of Lodz. At prcscnt he is a senior lecturer at thclnstitutc of Elcctrical Power Engineeringof Technical llnivcrsity of I . i d 7His field of interest I Spowcr quality.Dr l rcna Wasiakgraduatcd from the Tcchnical University of Lodz (Poland) i n 1976. Thcrcshe received the Ph. D. egrcc in clcctrical power engineering. Prcscntl> slicis a senior lecturer at the Institute of Electrical Power En_ri iiccring. Tcc1iritc;ilUniversity of I.odz. I Icrarcaof intcrcst includes modelling and stmulntlon 0 1'transienis 111power systems. and powcr supply quality

98