A Virtual Energy Analyser For Harmonic Measurements On Discharge Lamps. Charis N. Orfanos, Frangiskos V. Topalis

Transcription

1 A Virtual Eergy Aalyser For Harmoic Measuremets O Discharge Lamps Charis. Orfaos, Fragisos V. opalis atioal echical Uiversity of Athes, School of Electrical & Computer Egieerig 9 Iroo Politechiou Str., Zografou, GR-578, Athes Greece el: , Fax: , topalis@ieee.org, orxa@cetral.tua.gr Abstract- his paper presets a eergy aalyser with the required specificatios for the ivestigatio of the electrical performace of o-liear loads ad especially of discharge lamps. he istrumet is a virtual digital oscilloscope ad a spectrum ad eergy aalyser but the emphasis is give to record the eergy. It has bee developed usig a graphical programmig laguage. All the ecessary measuremets are tae i real time, o lie. A DAQ board through two aalogue iput chaels samples the voltage ad curret waveforms. heir frequecy spectra are determied usig FF ad DF. Moreover, the system performs eergy aalysis. So the total active ad reactive eergy is recorded, as well as the similar eergies of each harmoic compoet. he eergy is recorded i time bads that are idepedet each other so ca be overlapped. Also there is o limit of the umber of bads that ca be used. A time bad ca last from less tha a secod to years. he recordig ca be chose to tae place i every iteratio of the programme, several secods or hours. Withi the bads all the power related parameters are recorded. he maximum ad miimum values of them are also tae. Key words: virtual istrumet, power measuremet, eergy measuremet, power quality, eergy aalyzer I. Itroductio I the last few years, there has bee a icreasig cocer about harmoic distortio ad the effects of harmoics o the power system. oliear loads (such as electroic power supplies, lightig ballasts, battery chargers, motor drives ad UPS systems) are the primary source of harmoic currets. o-liear loads iject harmoic currets ito power systems. Eve if assumed that the source bus cotai oly fudametal frequecy voltage the harmoic currets passig through the impedace of the system cause a voltage drop for each harmoic. his results i voltage harmoics appearig at the load bus. Surveys [] report that the 95% probability level for the 5 th harmoic voltage sustaied a steady icrease from 3% to 5% i.e..67% per years. he discharge lamps (fluorescet, sodium vapour, ad mercury vapour) are such a o-liear loads. hese lamps characterized by their high lumious efficiecy, provide sigificat eergy savig ad last loger tha icadescet lamps. hey operate at low power factor cosumig less active power, providig comparable lumious output to the icadescet lamps []. However, their ballast as well as their arc tube ca be a importat source of higher-order harmoic compoets of curret. hey iduce distorted curret waveform, which ifluece the quality of the supplied power as well as the electrical appliaces [3]. Some istrumets ad o lie moitorig equipmet iclude the capability to sample waveforms ad perform FF calculatios for the record of the harmoic deviatio. he capabilities of these istrumets vary widely ad the user must be careful that the accuracy ad iformatio obtaied is adequate for the ivestigatio [4]. he aim of this project is to develop a adequate experimetal istrumet for recordig the cosumed eergy by the discharge lamps. he cost for the developmet of such a system is very low compared with covetioal oes, which cosist of digital oscilloscopes, spectrum ad eergy aalyzers as well as several measurig istrumets. he cofiguratio of the system is compact ad user friedly. A. Harmoic aalyzer II. Experimetal apparatus he followig are some basic requiremets [5] for harmoic measuremets used to ivestigate a problem: a) Capability to measure both voltage ad curret simultaeously so that harmoic power flow iformatio ca be obtaied. b) Capability to measure both magitude ad phase agle of idividual harmoic compoets (also eeded for power flow ad eergy calculatios). c) Sychroizatio ad a high eough samplig rate for accurate measuremet of harmoic compoets up to at least the 37 th

2 harmoic (this requiremet is a combiatio of a high samplig rate ad a samplig iterval based o the 5 Hz fudametal). he harmoic eergy aalyzer cosists of a computer, a data acquisitio multifuctio card PCI- of atioal Istrumets ad the LabView software pacage. he supply voltage ad the curret waveforms are recorded ad trasferred to the computer through aalogue iputs of the card. Both waveforms are aalysed [6] ad their harmoic compoets are computed. he computer cotrols the experimetal procedure usig LabView software pacage. LabView is a graphical programmig laguage that uses icos istead of lies of text to create applicatios. I cotrast to text-based programmig laguages, where istructios determie program executio, LabView uses dataflow programmig, where the flow of data determies executio. I LabView, the user builds the iterface by usig a set of tools ad objects. he user iterface is ow as the frot pael. he the code is added usig graphical represetatios of fuctios to cotrol the frot pael objects. he bloc diagram cotais this code. I some ways, the bloc diagram resembles a flowchart. It has extesive galleries of fuctios ad subrouties for most programmig tass icludig data acquisitio. Virtual Istrumets (VIs) imitate actual istrumets. he developed VI simulates the pael of three physical istrumets: a oscilloscope, a spectrum aalyzer ad a eergy aalyzer. A umber of VIs have especially bee developed for the purposes of the project. B. Data acquisitio card he DAQ board is a PCI- card with maximum samplig rate S/s. he card has output chaels ad 8 differetial (or 6 sigle-eded) iput chaels. he voltage-iput rage is software programmable for -V (uipolar) or ±5 V (bipolar). he card is samplig at 5 Hz o two iput chaels for the recordig of the applied voltage to the lamp ad of the curret of the lamp. his meas that the samplig frequecy is.5 Hz per chael. his frequecy is high eough to prevet aliasig, accordig to yquist theorem, sice the maximum harmoic frequecy of the measured sigals is 95 Hz (39 th harmoic). he asychroous samplig o two iput chaels causes a time delay of /5ms=.4 ms. his is ot a problem because from field measuremets harmoics up to 7 th order have bee detected i the utility voltage. I that case the period of 7 th harmoic is.86 ms so the itroduced by the asychroous samplig error i the phase displacemet betwee the voltage ad curret harmoics of 7 th order is oly 5 o. C. Software. Samplig starts after a aalogue trigger from the software. A specified umber of samples of voltage ad curret waveforms are retrieved through two aalogue iput chaels. A triggered acquisitio ca start multiple time while is avoided the overhead of cofiguratio ad the buffer allocatio each time. his is a timed acquisitio, meaig that a hardware cloc is used to cotrol the acquisitio rate for fast ad accurate timig. It is also a buffered acquisitio because the data are stored i a itermediate memory buffer after they have bee acquired from the DAQ board. Sice a aalogue trigger is used from the software, data are cotiuously acquired while the program is checig whether the itermediate buffer for the aalogue trigger specificatios is beig met. Oce met the program retrieves ad displays data from that buffer after the appropriate amout of data have bee acquired before ad after the trigger. After each iteratio the program waits for the ext trigger, it reads the the same amout of data agai from the same chaels at the same rate. he user ca set up the trigger chael, the polarity of the slope (positive or egative), the hysterisis aroud the trigger level withi which variatios i the aalog sigal level do ot cause triggers. He ca also set the time delay for the trigger ad the umber of scas as well as the umber of pre-trigger scas i.e. the umber of scas before the trigger. he samples of voltage ad curret retur to a x array that is split ito two x arrays, oe for the samples of voltage waveform ad the other for the curret waveform. Each vector of samples is scaled i order to be i accordace with the real values. Whe usig FF or DF the digital sigal is modified to iteger umber of periods i order to avoid leaage of spectral iformatio. he umber of periods is always greater tha three. Additioally, the user may apply smoothig widows such as Haig, Hammig, Blacma, Blacma-Harris, Exact Blacma, Flat op, Geeral Cosie (4 erm B-Harris, 7 erm B-Harris) to improve the spectral characteristics of the sampled sigals [7]. After that, both of the digital sigals are aalysed i the time ad frequecy domai. If the umber of samples is a valid power of, the the subroutie performs FF to reduce the umber of calculatios to 3 log. he maximum legth of FF is the legth of the buffer. he buffer legth is set up to 496 samples. FF is performed by applyig the Split-Radix algorithm, which is similar to the Radix-4 algorithms i additio to the efficiecy of Radix-8 algorithms. he Split-

3 Radix algorithm requires the least umber of multiplicatios amog the Radix-, Radix-4 ad Mixed- Radix algorithms [7]. If the umber of samples is ot a valid power of, the the program computes the DF callig a efficiet DF routie. However, DF is slower ad uses more memory tha FF. A. Spectrum aalysis III. Harmoic power ad eergy measuremets he system records the voltage ad curret waveforms as it is show o the frot pael oscilloscope page of the istrumet (Fig.). he it calculates their effective values i the time domai: Y = y ( t) dt () where y(t) represets the voltage or curret sigal v(t), i(t) ad Y their effective (rms) values V ad I respectively. For better accuracy, this procedure is repeated for several periods ad the mea values are determied. Figure. Voltage ad curret waveforms of a 5 W compact fluorescet lamp. he FF (or DF) aalysis of the sigals gives their frequecy spectra i.e. the effective (rms) values of their harmoic compoets Y. herefore, the effective value Y of the sigal is calculated agai with harmoic represetatio: Y = Y is the DC offset, Y the amplitude of the fudametal, Y the amplitude of the harmoic of th order ad the highest harmoic order. he idividual harmoic rate H of each compoet is calculated accordig to the CIGRE defiitio: = Y () Y H = % (3) Y Last but ot least, the total harmoic distortio (HD) of the recorded sigal y(t) is calculated accordig to the IEEE defiitio[8]:

4 Y = HD = % (4) Y he crest factor is also defied as the ratio of the pea value of the waveform to its rms amplitude. he waveform is distorted if this ratio is ot equal to [4]. For a distorted sigal it is importat to determie its harmoic cotet. he waveform should be cleared off the fudametal compoet ad the DC offset. his results to a sigal that is purely the distortio of the waveform. he effective value of the total harmoic cotet Y H is calculated as follows: Y H = Y = (5) B. Power aalysis he active power of the idividual harmoics -icludig the fudametal- is determied from the effective amplitudes V, I of their voltage ad curret: P = V I cosϕ i=,, 3,,. (6) where cosφ is the phase displacemet betwee them. his is the power factor of each idividual harmoic that is also calculated: PF = cosϕ (7) he total active power is the sum of the active powers P of the respective harmoics plus the DC power P : P = = he active power of the recorded sigals is also calculated i the time domai. First, the istataeous power p(t)=v(t) i(t) is calculated ad it is derived the itegral of it. he mea value of the istataeous power is the total active power: P = v( t) i( t) dt (9) he reactive power of the harmoics is determied usig the same as previous method: Q = V I siϕ i=,, 3,,. () As for the total reactive power Q, there is a disagreemet betwee the aalysts o how to defie it i the presece of harmoics. Most of them agree that Q is the sum of reactive powers at each frequecy. I this cocept the total reactive power is be calculated as: Q = = Apparet power is defied as the capacity of the power system required to deliver active power P ad accordig to the IEEE defiitio [8] it is give as the product of the rms values of voltage ad curret: P Q (8) () S = V I () Substitutig the equivalet expressios of V ad I from Eq.() to Eq.() results the followig form for the apparet power: S = = V I + = = Worig similarly with Eq.(6), (8) results the followig form for the active power: V I (3) P = = ( V I cosϕ ) + V I cosϕ V I cosϕ = = + A similar formula is obtaied from Eqs.(), () for the reactive power: Q = = ( V I siϕ ) + V I siϕ V I siϕ = = + (4) (5)

5 Observig Eq.(3)-(5) becomes obvious that the sum P +Q is ot equal to S for distorted sigals v(t) ad i(t). his would be true oly i case of pure siusoidal voltage ad curret waveforms i.e. without harmoics. For distorted sigals the apparet power cosists ot oly of active ad reactive power but also of a ew quatity that shall be added i order to fulfil the equatio: S = P + Q + D (6) his quatity is equal to the sum of the cross products of the harmoics of differet order V I ( ). It is called distortio power D sice these harmoics do ot produce wor ad yield o average power. D is calculated from the followig formula that is obtaied from Eq.(3)-(6): D = = = + ( V I cos V I cosϕ ) + ( V I si ϕ V I si ϕ ) ϕ (7) = = + Fially, the power factor is determied usig the well-ow expressio: P PF = (8) S he variatio of the istataeous ad average value of each power-related parameter i a time bad is also displayed as well as the miimum ad maximum values. he pael with the measuremets of the power-related parameters is show i Fig.. C. Eergy aalysis. Figure. Istataeous powers of a 5 W compact fluorescet lamp he istrumet ca measure the active ad reactive eergy that cosumed due to the voltage ad curet waveforms as well as their commo harmoic compoets. For the calculatios is used the geeral formulae: Eergy = Σ(Power X time) he time iterval of a program executio cycle is always cosidered as time. he eergies as well as the powers are recorded i time bads. he user ca select so may time bads he wats for his experimet without limit. Also there is ot limit about the duratio of a time bad that ca be less tha a secod up to years. he time bads ca be overlapped (Figure 3). hus the user ca tae iformatio for the variatio of the variables i a time bad that is icluded i other or there is commo time with aother. here is Boolea idicatio for the status of the time bads (recordig or ot)

6 he user has also the possibility to choose the frequecy of recordig. It ca be every,, 3, 4, 5,, 3 secods,, 5, 3 miutes,, 6,, 4 hours ad so o. hus is give the possibility to be recorded simultaeously hudred of variables. For example: Ivestigatig the behavior of a discharge lamp feed by a slightly distorted voltage there are about 4 plots that ca draw. 7 plots display the variatio of the total active power ad the active power of the harmoics while the other 7 display the total reactive power ad the reactive power of the harmoics. 8 eergy parameters are plotted per time bad. Fially plots are draw for recordig them i 4 time bads. I geeral there is o limit from the program but oly from the capability of the computer. Reducig the recordig frequecy dimiishes the ecessary computer capability. All the above metioed plots ca be i oe graph. he user has the possibility to choose those that he wats to examie. Figure 3 Eergies cosumed by a 5W compact fluorescet lamp recorded i 4 overlapped time bads. IV. Coclusios he developed virtual istrumet (hardware ad software tools) facilitates experimets o eergy harmoic aalysis. he mai feature of this istrumet is the give possibility to the user to choose the time bads without restrictios of the umber of them, the duratio ad permittig the overlappig. he advatage of this istrumet as a virtual oe is its expadability. It is easy to modify the software if it is required to measure a ew quatity durig the ivestigatio of the behaviour of a o liear load. he cost of the system is very low compared with a covetioal oe cosistig of a umber of meters as oscilloscope power ad eergy aalyser etc. he system facilitates the performace evaluatio of various appliaces for distorted supply voltages. Refereces [] H.J. Koster, F. Weiel, Voltage ad curret harmoics i low ad medium voltage etwors, Eletrizitatwirschaft, Vol. 88, pp , 989. [] F.V. opalis, Efficiecy of eergy savig lamps ad harmoic distortio i distributio systems, IEEE ras. o Power Delivery, Vol. 8, r. 4, pp. 38-4, 993. [3] D.J. Pileggi, E.M. Gulachesi, C.E. Root,.J. Getile, A.E. Emauel, he effect of moder compact fluorescet lights o voltage distortio, IEEE ras. o Power Delivery, Vol. 8, r. 3, pp , 993. [4] IEC Stadard 6-4-7, Electromagetic compatibility (EMC) - Part 4-7: estig ad measuremet techiques - Geeral guide o harmoics ad iterharmoics measuremets ad istrumetatio for power supply systems ad equipmet coected thereto, d ed., August

7 [5] C. Melhor, M.F. McGraagha, Iterpretatio ad aalysis of power quality measuremets, Electrote Cocepts, Ic. Koxville, eessee. [6] IEEE Stadard 59, IEEE Recommeded Practice for Moitorig Electric Power Quality, ovember 995. [7] atioal Istrumets Corporatio, LabView Aalysis VI Referece Maual, Austi, 996. [8] IEEE Stadard 35, IEEE Recommeded Practice: est procedure for utility-itercoected static power coverters, 989.