A Virtual Instrument for the Electric Power Monitoring in the Distributing Network

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1 A itual nstument fo the Electic Powe Monitoing in the Distibuting Netwo Fancesco Adamo, Filippo Attivissimo, Giuseppe Cavone, Anna M.. anzolla aboatoy fo Electic and Electonic Measuements Depatment of Electics and Electonics (DEE) Polytechnic of Bai ia Re David, Bai Abstact - This wo poposes a vitual instument () fo the electic powe quality monitoing aiming to act in eal-time fo detecting, monitoing and ecoding all typical distubances supeimposed on the ideal signal; it is based on both EC standads and [] [] and on the diective fo the Consultation of the talian Authoity fo the Electic Powe and Gas of Apil 6, 005. The latte pesents the peculia featues of an expeimental monitoing system fo the mid voltage electic enegy distibution netwo whose expeimentation should lead the distibuting companies to the espect of the oltage Quality (Q) monitoing obligation. Accoding to the above mentioned egulations, the poposed acquies both line voltage and cuent wavefoms and caies out the innovating powe quality indexes calculations in times compaable with those of othe commecially available instuments; in ode to obtain an accuate estimation of the paametes of inteest a suitable Fast Fouie Tansfom intepolation algoithm is implemented in the.. The laboatoy pototype is based on a low cost data acquisition cad equipped with suitable cuent and voltage sensos; the code is developed using National nstuments abew Pofessional Development System and Mathwos MATAB and achieves vey inteesting pefomances. Keywods: Powe measuement, powe quality, spectal analysis, hamonic analysis. ntoduction n the ecent yeas thee have been emaable changes in the natue of electical loads; electic and electonic appliances equipped with switching powe supply units ae now ubiquitous in domestic, office and industial places; the main consequence of the gowth of this ind of loads is the incease of the hamonic content of the powe netwo and this leads to seious powe quality issues fo public netwos. Recommendations have been issued by the standad oganizations both to limit the injection of hamonics and to define the chaacteistic of voltage supplied by distibuting companies [3]. Consequently, the powe quality monitoing activity has gained a geat impotance in the last few yeas; this objective equies the definition of one o moe powe quality indexes as fai and wide accepted as possible. nfotunately the aleady poposed powe-quality indexes do not ensue the fulfilment of these objectives. n the authos opinion, the use of a vitual instument integating all the data acquisition, pocessing, display and data ecoding tass seems to be the ight oad to an economic, flexible and efficient powe monitoing system. The talian Authoity fo the Electic Powe and Gas (AEEG) with the delibeation no. 4 of Januay 30, 004 (ntegated Text of Povisions of the Authoity fo the Electic Powe and Gas as Regad Electic Powe Distibution Sevice Quality, Measuement and Sale Regulation Peiod ) and following modifications and integations intoduced a new and impotant concept fo final customes connected to the distibution netwo: the Quality Contact. Thans to this diective the distibuting company can daw up a contact with final customes o othe subjects lie electic powe facilities o inteconnected distibuting companies fo minimum quality levels. n taly the deegulation of the electic maet has had a pofound impact on both the electic powe industy and its customes, opening up new pospects and equiing geate attention fo powe quality monitoing; this has been made possible thans to the effots of the AEEG too. n the above-mentioned consultative document of apil 6, 005, the AEEG intoduced the minimum featues of a voltage quality monitoing system on middle voltage netwos; this system has been pototyped and tested duing 005 by CES S.p.a., an italian eseach cente leade in testing and cetification of electomechanical equipment and powe system studies and consultancy.

2 Following the same appoach, the AEEG also activated two othe initiatives: a) a sample monitoing campaign fo the talian Electic Powe National Tansmission Netwo (in collaboation with the talian Tansmission System Opeato, delibeation n. 50/04); b) the standadization of the voltage quality egistation obligations between distibution and tansmission; in the nea futue, this will lead to the extension of voltage quality (Q) paametes egistation obligations also to high voltage electic powe distibuting companies. This appoach paves the way to new and moe complex instuments fo powe measuement and monitoing which will have to povide flexibility, expandability and high-speed pefomances.. oltage quality and powe quality indexes Fo powe quality assessment, the AEEG poposed the obsevation of the following paametes []: i) souce voltage amplitude fluctuations; ii) souce voltage inteuptions; iii) voltage holes; iv) cuent and voltage hamonics; v) shot tem (P st ) and long tem (P lt ) flice intensity; vi) voltage lac of balance. Howeve, the pevious paametes do not always ensue a full account of the powe quality; in fact, following the esults of independently made measuement expeiences, seveal othe National Enegy Regulatos have adopted definitions, limitations, and indicatos that disagee with those given [] and []; besides, these diffeent appoaches, combined with diffeent netwo chaacteistics, maes both the analysis and the hamonization vey had. Really, in the last few yeas, a lot of additional powe quality indexes have been poposed by many authos; they, when compaed with the limits of the pevious ones, mae it possible a moe in-depth and caeful analysis of the powe quality state. This is the case, as an example, of the distotion index (DN) that substitutes the total hamonic distotion (THD) [4]; this new paamete pemits to ovecome the poblem of measuements on fequency modulated signals that do not have a fundamental component. The instument pesented in this wo measues and/o computes, beyond those indicated by the AEEG, an expanded set of powe quality indexes that, in the authos opinion, ae impotant fo a ight evaluation of the electic powe quality. The following indexes ae computed fom acquied data:. the voltage/cuent distotion index (DN and DN espectively) defined as [5]: DN v v, DN i i being v ( i ) the RMS values of the th hamonics of voltage/cuent espectively.. the genealized voltage/cuent distotion factos and given by whee: - and ae the voltage and cuent collective RMS values espectively, defined as: being: n and n

3 - and cuent espectively; (j,,..., n) the ms values of the zeo-sum line voltages and the line - and the collective voltage/cuent RMS values of the fundamental components only of voltage and cuent espectively; - n the numbe of phases of the powe system. 3. the genealized voltage/cuent distotion facto being and the collective positive sequence voltage/cuent RMS values of the fundamental components only of voltage and cuent espectively. ndexes and 3 ae computed to obtain a quantification of both distotion and unbalance; a compaison between the values of and (o between and ) gives indication on what is the main cause of powe quality (distotion o unbalance). 4. Coefficients η and η : η η which povide infomation about the location of the souce poducing distotion. 5. The genealized active (P ) and appaent powe (S ) given by: T n (T is the peiod of the voltage and cuent wavefoms); 0 - () () P u t i t dt - S ; P 6. The genealized powe facto λ ; S P 7. The supply and loading quality index ξslq P which gives infomation about the majo souce of distotion/unbalance: - ξ slq > when the distotion and/o unbalancing effects of the supply pevail ove the load distoting and unbalancing effects: - ξ slq < when the load distoting and/o unbalancing effects pevail ove the supply voltage distotion and/o unbalance; 8. the hamonic global index whee ξ, HG S

4 - - is a vecto whose components ae the collective RMS values of the hamonic and sequence components associated with active powes eflected bacwad fom the load to the souce; is a vecto whose components ae the collective RMS values of the hamonic and S sequence components associated with active powes flowing fom the souce towads the load. 9. the -facto of evey phase [6] which is a weighting of the hamonic load cuents accoding to thei effects on tansfome heating. Howeve, fo the most pat of these new indexes, thee ae not available pecise ules of use; this is mainly due to the novelty of them and, consequently, to the absence of pe-established limits. Finally, it is woth undeling that the citical pat of question is the implementation of a both on-line had eal time and accuate estimation algoithm.. Measuing system The design of the measuement system has been made in many steps; fist of all a tial vesion of the was developed to veify both the involved models and algoithms using numeically synthesized coupted signals. This simulation has been developed with the aim to pecisely simulate a theephase system behavio. The final objective of this phase was to see if the developed algoithm was able to discove all the powe souce impefections that, thans to the geat cae of the simulation, can be finely contolled by the opeato. n the simulation thee cuent and voltage wavefoms wee numeically geneated supeimposing to the theoetical ones all the distubances defined in the CE EN 5060 Standad [3]: inteupts, voltage holes, ovevoltages, flice, hamonics up to 40 th, inte-hamonics and noise. Then, afte esults veification, the techniques elevant to both acquisition and optimization of signals have been developed to obtain eal-time measuements. Encouaged by the simulation esults, a pototype system useful to both monito and analyze the powe quality in thee-phase systems has been implemented on a Pentium pesonal compute with.4 GHz cloc, GBytes of RAM equipped with an eight channel simultaneous sampling analog-to-digital convesion boad with 4 bit esolution at a maximum sampling ate of.5 MSample/s (National nstument PC-633 DAQ [7]). Real cuent and voltage wavefoms of the monitoed thee-phase system have been acquied using Hall effect-based tansduces chosen with bandwidth equal to 00 Hz, lineaity of 0.% and having a delay time less than μs..a The instument pefomances Fo a quantitative evaluation of the measuement uncetainties the AEEG poposes to use TeamWae s Wally-RT Electic powe quality station [8]; it is a thee-phase powe quality analyze which accomplishes voltage, cuent, fequency, hamonic distotion and electical enegy consumption measuements; it has a sampling fequency of 6.4 Sample/s and uses a phase loced synchonous ealtime sampling technique, is declaed class A instument whit egad to methodology and class B instument with egad to uncetainty esults. Really, it gives an uncetainty less than 0.5 % fo input values 0% FS (full scale input), and less than % when the input signal vaies in the ange -0 % of FS. t is impotant to note that the measuement bandwidth of this instument does not comply with the EC minimum bandwidth equiements, which is at least 9 Hz. The maximum sampling ate of the DAQ cad used in this poject is geatly ove the equiements fo the paticula application; the actual sampling ate used fo the pesented was Sample/s and this ensues a total uncetainty less than 0.5% fo input values anging -0 % FS and below 0.% fo signals of 0% FS and ove. The ealized powe quality monitoing device pototype measues and shows besides the measuements equied by the AEEG all the indexes defined in section. All the measuements made by the in a 0 minutes wide aggegation window (as established in [3]) ae ecoded on files; even the distubances (inteupts, voltage holes and ove voltages) ae ecoded on suitable files in a eal-time basis. Moeove, the pesence of the ecoding files pemits to ceate an achive containing all the infomation with egad to the electic powe quality evolution. Even if it is not specifically equested by the AEEG, the instument monitos the ove voltages which ae significant fo voltage quality. To obtain the eal-time elaboation of acquied data, the aggegation window is subdivided in many subintevals on which the double buffeing technique is used to pipeline the elaboation of samples acquied in the pevious subinteval and the acquisition of new data in the cuent one; this avoid to lose peiods of input signals in which significant distubances may happen. Also this pemits to avoid the use of an expensive synchonizing system between the input signals and

5 the digitizing hadwae. The ealized device, being based mainly on high level softwae, is a low-cost, pogammable, extendable, use-fiendly instument which does not equie futhe softwae setup fo measuement data teatment as the most pat of othe devices commecially available. n fact, as all the vitual instuments, its behavio can be changed accoding to the futue developments of the powe quality theoy and the pobable consequent new AEEG s diectives. Finally an in-depth analysis of the uncetainties poduced by the instument, showed that the expeimental uncetainties ae lowe than the bounds indicated in the same standad fo the Class A instuments; this means that the pesented instument could be placed in this class without poblems and encouages to continue the expeiments and to add impovements; so in the authos opinion, the poposed would be integated with a GSMbased data tansmitte fo the measuement data dispatch to some centalized data acquisition oganism (lie the CES) with the aim of identification of the coelated data..b The use inteface The image in Fig. shows one of the many sections of the Font Panel of the developed ; to simplify the use inteface and to mae it moe usable, the Font Panel has been oganized as a collection of pages gouping vaious subsections of coelated contols and indicatos. The fist section (Fig. ) ( the setup section) is devoted to the basic equiements of unning the poposed instument (setting of DAQ gain, choice of the esolution, selection of FFT analysis paametes and so on) and to the statement of both path and file name output. The second section is the page of esults whee the poposed quality indices ae displayed. The thid section is devoted to distubances analysis and chaacteization (flice, distotion, etc). Fo sae of bevity, the desciption of the othe mino sections, useful to a complete chaacteization of powe systems ae hee omitted. Fig. Font Panel of the developed : Fist page of the instument.c The spectal analysis algoithm The caies out a vey accuate spectal analysis with bette amplitude, fequency and phases estimates in compaison with the commecial analyze which ensues accuate measuements only fo coheent sampling. Paticulaly, the pototype developed in this wo intepolates between the DFT (Discete Fouie Tansfom) spectal ows by evaluating the spectal shift factional bin δ as: w( ) { w( ) w( ) } { w( ) w( )} { w( ) w( )} δ P and obtaining the scalloping loss S( δ ): S ( δ ) ( πδ ) δ sin πa l ( ) () l l 0 a δ l ()

6 whee P is the Rife-incent window ode, ( ) is the spectum pea value and ( ) ( ) ae the spectum samples immediately adjacent ( ) w e w w [9]. n the paticula case of a coheent sampling δ 0 and S( δ ) so that the intepolation is not necessay. Once nown the δ and S( δ ) values it is simple to estimate the amplitudes of evey single hamonic of inteest as: X (0) A0 e A CG N S( δ ) X( F) CG N S( δ ) being A 0 and A the DC and the -th hamonic component, espectively; in the same way, both the -th fequency and phase ae estimated espectively as: X im ( F) f F ( δ ) and X ( F) ag X ( F) actg (4) X F e ( ) Even if the actual standadization doesn t equie the voltage fluctuations amplitude measuements but only the flice evaluation with statistic indexes lie P st and P lt, the poposed ealizes the automatic envelope of the voltage fluctuations amplitude; moeove, it offes the possibility to monito the extent of the phenomena useful fo a moe in-depth analysis of this poblem.. Conclusions The potential economic consequence of an uneliable distibution netwo foces powe distibuting companies to an accuate and continuous monitoing of meaningful powe quality paametes; so powe monitoing system will evolve to povide still moe featues fo distibuto and use. Fom this point of view, the authos pesent a completely digital method fo the fast and accuate monitoing of the electical powe quality useful to poduce eal time quality/distubance epots. n the pape, the mathematical basis of the poposed estimation algoithm is discussed in tems of eliability and uncetainty. Finally, the vitual instument ealized implementing this technique detects, assess and stoes coectly tansient distubances occuing on the netwo. The fist simulation shown that the esults obtained using the poposed pototype do agee with the theoetical one and encouage to futhe develop the poject in depth.. Refeences [] EC, Standad EC , Testing and measuement techniques Geneal guide on hamonics and intehamonics measuements and instumentation, fo powe supply systems and equipment connected theeto, 00. [] EC, Standad EC , Testing and measuement techniques Powe quality measuement methods, Fist edition, [3] CENEEC, EN 5060, oltage chaacteistics of supplied by public distibution systems, 999 [4] G. Heydt: Electic Powe Quality, Stas in a Cicle Publications, Second Edition, Scottsdale AZ, 995. [5] Y. Shin,M. Gady, A. Aapostathis, Powe Quality ndices fo Tansient Distubances, EEE Tansactions on Powe Delivey, ol., No., Januay 006, pp [6] S.C. Wang, M. Devaney: A Time-Domain Appoach to the Measuement fo -facto, Applied Powe Electonics Confeence and Exposition, Confeence Poceedings of APEC '94, 994, ol., pp [7] National nstuments Measuement and Automation Catalog 007, pp. 50-5, National nstuments Cop., 500 N Mopac Expwy, Austin TX , P/N N-0. [8] TW_Teamwae S..l. web site, [9] G. Andia, M. Savino, A. Totta: Windows and intepolation algoithms to impove electical measuement accuacy, EEE Tans. On nstum. Meas., vol. 49, pp , 000. w (3)

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