Comparison of ATP Simulation and Microprocessor

Transcription

1 Elecrical Engineering Research (EER), Volume 3, 15 Comparison of ATP Simulaion and Microprocessor Based Faul ocaion Using DFT H Nouri *1, F Jalili, T Boxshall 3 Power Sysems, Elecronics and Conrol Research aboraory, UWE Brisol, UK 1&3, Sadid Pipe & Equipmen Co, Saveh Road, Tehran, Iran *1 hassan.nouri@uwe.ac.uk Absrac This paper evaluaes he resuls of a microprocessor based faul locaor employed on a laboraory power sysem model wih hose obained wihin simulaions. The Discree Fourier Transform concep is used o exrac he fundamenal frequency componens of he volage and curren waveforms under faul and he reacance based mehod o locae he faul. The resuls sugges a close correlaion beween he experimenal and simulaion sudies for all faul ypes. Keywords Faul ocaion; Discree Fourier Transform; Microprocessor; Inroducion Disribuion line fauls have been a primary concern o neworks. Fauls affec he reliabiliy of he service o he cusomer, he sabiliy of he power sysem and he qualiy of he power delivered. Disribuion line fauls can even cause complee supply ouages. These fauls mus be deeced quickly for fas deacivaion of he fauly secion of line, in order o reroue he power o he cusomer o mee arge resoraion imes. Disribuion line faul locaors can be classified under wo caegories. The firs is he impedance based locaors, which analyse he power frequency (5 Hz or 6 Hz). The second is he ravelling wave based locaors which analyse he ravel ime of high frequency ransiens. Transien based faul locaion mehods require higher sampling raes and more responsive volage or curren ransducers. They also require muliple faul ransien deecors placed hroughou he sysem, which are clocked using GPS echnology and daa sen o a cenral locaion via a communicaion link. The use of GPS can make such a soluion very expensive. Transien based faul locaion echniques for radial, meshed and disribuion lines wih apped load power sysem configuraions are described by a number of researchers (Nouri e al 14, Magnago e al 1998 and Nouri e al 1), respecively. In impedance based mehods, he raio of exraced fundamenal frequency of he volage and curren waveforms yields he impedance seen from he generaor up o he poin of faul. A useful summary of he available impedance calculaions can be found in (Zimmerman 5). The reacance equaion ha will be used o obain he faul locaion (m) in per uni is expressed as: vs IM i s m (1) X 1T where IM defined as soring of he imaginary par of he volage ( v s ) and he curren ( i s ) raio a he sending end of he line, and X1T is he posiive sequence reacance of he disribuion line. Discree Fourier Transform Mahemaical Background In power sysems he volage and curren waveforms are essenially shifed sine or cosine waves of fundamenal frequency. These signals are effecively periodic when he sysem is running under seady sae condiions. Under faul condiions, he fundamenal frequency of he volage and curren ampliude can be exraced from he non 1

2 Elecrical Engineering Research (EER), Volume 3, 15 periodic signals using he Fourier Transform. The basis funcions chosen for he Fourier Transform is he complex j exponenial g () e. The Fourier Transform is defined as follows: j F( ) f( ) e d () On closer examinaion of (), i can be seen ha Fourier Transform inegraion akes place over all of ime, i.e. from o. Therefore, he Fourier Transform should be reserved for periodic signals only. A power sysem conains many devices such as ransformers, loads and swiching circuis. These all creae harmonic disorion under seady sae condiions. The Fourier Transform is useful o analyse he harmonic conen of he signal. However, when he sysem is running under a faul condiion, non periodic signals exis. Consequenly, he Fourier Transform should no be used o analyse faul condiions. An alernaive form of Fourier s analysis echniques should be used, known as he Fourier Series. Unlike he Fourier Transform, he Fourier Series analyses he signal over a finie period of ime. The basic assumpion used in Fourier Series is ha he waveform ha resuls from he faul condiion is assumed o be periodic wihin he inerval from o T, where T is he period of he fundamenal componen. The following equaions are derived from (Wood 1984, PhadKe e al 1998, Johns e al 1995, and Crof e al 1). The volage or curren waveform s() can be represened using he Fourier Series. where he Fourier coefficiens are: And a s() ancosn bnsinn (3) n1 n1 T an s()cos n d (4) T T bn s()sin n d (5) T where n = 1,, 3, for he fundamenal, second, and hird harmonic. I is apparen ha he fundamenal frequency can be seleced by seing n 1. Thus, he Fourier coefficiens for he fundamenal frequency can be deermined: And T a s()cos d (6) 1 T T b s()sin d (7) 1 T In discree analysis he ime of he jh sample is j j and he period of he waveform is T N, where N is he number of samples per cycle of 5Hz. Now (6) and (7) can be deermined in discree ime: N1 N1 a1 s( j)cos j s( j)cos j N j N j N N 1 s( j)cos j (8) j And N1 N1 b1 s( j)sin j s( j)sin j N j N j N N 1 s( j)sin j (9) j The ampliude of he real and imaginary pars of he fundamenal frequency signal are represened by he Fourier coefficiens a 1 and b 1 respecively. I is necessary o deermine he ampliude of he fundamenal frequency in order o apply an impedance equaion o locae he faul,

3 Elecrical Engineering Research (EER), Volume 3, s a b (1) Also i is required o find he phase angle beween he real and imaginary componens of he fundamenal frequency, 1 b1 s an a1 (11) Pracical Applicaion of DFT A single line diagram of he laboraory power sysem model is shown in Fig. 1, wih a faul locaor sysem conneced o bus A. I can be assumed ha he disribuion line has a lengh of 1 pu. A sysem mus be designed o gaher he phase volages and currens and upload hem o he CPU o analyse he faul condiions. This requires a sensor circui o measure he volages and currens, inerfaces beween he analogue and digial domains, and an exernal microprocessor o conrol he daa gahering process. The chosen exernal microprocessor is he Siemens C164CI on he PHYTEC kicon 164 evaluaion board. A block diagram of he required sysem is shown in Fig.. The ADC and DAC are 1 bi devices. The daa sen beween he C164CI microconroller and he CPU is via an RS 3 bus and he mehod for he C164CI communicaing wih he ADC and DAC is a serial peripheral inerface (SPI) bus. A phoograph of he laboraory power sysem model and microprocessor based faul locaor is shown in Fig. 3. FIG. 1 SINGE INE DIAGRAM OF THE ABORATORY POWER SYSTEM MODE FIG. A BOCK DIAGRAM OF THE DATA ACQUISITION SYSTEM FIG. 3 PHOTOGRAPH OF ABORATORY POWER SYSTEM MODE AND MICROPROCESSOR FAUT OCATOR 3

4 Elecrical Engineering Research (EER), Volume 3, 15 V I V I V Im I m Z T V Im I m Z T FIG. 4 BOCK DIAGRAM OF SOFTWARE FUNCTIONAITY Faul ocaion Sofware The faul locaion sofware is a funcion based program as shown in Fig. 4. There are hree main funcions which deermine he faul locaion: i. The funcion gesamples() uses imer inerrup service subrouine o generae a sampling period of 65μs. A each sampling inerval he funcion collecs he insananeous value of he line volages and currens. I repeas his unil he required number of samples is gahered. ii. This daa is passed on o he funcion DFT(), which exracs he ampliude and phase angle of he fundamenal frequency. iii. The analyse() funcion uses a faul curren hreshold deermined during program iniialisaion o decide which phases are fauly. A variaion of reacance equaion is hen applied o locae he faul depending on he number of fauly phases. The reacance equaion gives a faul locaion in m pu. EMTP ATP Simulaion A line o ground on phase A is simulaed using an simulaion model shown in Fig. 5. The faul occurs in he middle of he disribuion line wih a faul locaion of.5 pu. The model parameers are based on he laboraory power sysem model shown in Fig : 37 Disribuion ine oad FIG. 5 ATP SIMUATION MODE OF A FAUTY 415 V DISTRIBUTION INE Resuls & Discussion EMTP ATP Simulaion A faul is se o occur a =.6 seconds and lass for he remaining duraion of he signal (5 cycles). The profiles of he ATP volage and curren waveforms can be seen for phases A, B and C in Fig. 6 and Fig. 7, respecively. The DFT is used o exrac he ampliude and phase angle of fundamenal frequency line volages and currens from Fig. 6 and Fig. 7. All ransmission line signals are sampled a a rae of 3 samples per 5 Hz cycle, which produces a sampling period of Ts 65s. The DFT fundamenal frequency exracion of phase A for volage and curren can be seen in Fig. 8 and figure Fig. 9, respecively. By examining hese waveforms i can be seen ha here 4

5 Elecrical Engineering Research (EER), Volume 3, 15 are 64 pre faul samples ( 64 o ) and 16 samples afer he ime of faul occurrence. During he pre faul samples, he fundamenal frequency volage and curren ampliude is 1 pu. The applicaion of a faul on he line causes he curren a he sending end o increase o approximaely 11 pu. Also, here is a decaying exponenial DC offse of shor duraion. I should be noed ha he DFT ignores his DC offse and jus racks he ampliude of he fundamenal frequency, as required. The fundamenal frequency volage ampliude does no change by a significan amoun. Va() Vb() Vc() Ia() Ib() Ic() Volage(pu) Curren(pu) Time Time FIG. 6 ATP PHASE VOTAGES WAVEFORMS Time FIG. 7 ATP PHASE CURRENTS WAVEFORMS FIG. 8 EMTP ATP FUNDAMENTA FREQUENCY VOTAGE EXTRACTION USING DFT FIG. 9 EMTP ATP FUNDAMENTA FREQUENCY CURRENT EXTRACTION USING DFT These resuls show ha he DFT can exrac he fundamenal frequency in approximaely 3 samples, i.e. a complee cycle of 5 Hz. The resulan faul locaion in pu can be found by using he reacance equaion (discussed earlier) as shown in Fig. 1. This waveform correcly shows ha he faul locaion converges o.5 pu afer he applicaion of he faul. The resulan faul locaion in per uni can be found using he reacance equaion as show in Fig Va() VA() Experimenal.5 Volage(pu) Pre-faul Faul Pre-faul Faul Samples FIG. 1 EMTP ATP FAUT OCATION WAVEFORM FIG. 11 MICROPROCESSOR BASED FUNDAMENTA FREQUENCY VOTAGE EXTRACTION USING DFT 5

6 Elecrical Engineering Research (EER), Volume 3, Ia() IA() Experimenal Experimenal Curren(pu) Pre-faul Faul Faul ocaion(pu) Pre-faul Faul m() Samples FIG. 1 MICROPROCESSOR BASED FUNDAMENTA Samples FIG. 13 MICROPROCESSOR FAUT OCATION WAVEFORM FREQUENCY CURRENT EXTRACTION USING DFT Verificaion of Resuls Using he Microprocessor Based Faul ocaor Resuls produced by simulaion are verified using he microprocessor based faul locaor on he laboraory power sysem model. The DFT is used o exrac he fundamenal frequency, ampliude and phase angle for all hree line volages and currens. Similarly o he simulaion a sampling condiion of 3 samples per 5 Hz cycle is used. The DFT fundamenal frequency exracion of he volage and curren waveforms can be seen in Fig. 11 and Fig. 1, respecively. Conclusions The resuls produced in he environmen are compared wih experimenal resuls produced by microprocessor based faul locaor on a laboraory power sysem model. The DFT exracs he fundamenal frequency in less han a complee cycle of 5Hz and when combined wih he reacance equaion produces very accurae faul locaion. The DFT effecively ignores DC offse especially when presen in heavily inducive lines. A collapse in line volage or an increase in line curren is a useful indicaor of a fauly phase. The faul locaor allows quick deecion for fas deacivaion of he fauly line. This allows he power o be reroued o he cusomer o mee arge resoraion imes. Typically, disribuion line fauls have a faul impedance of unknown quaniy, causing error in faul locaion. The reacance mehod significanly reduces his error o a olerable amoun. Generally, faul impedance is resisive in naure. The reacance equaion ignores all resisances in he sysem and locaes he faul purely based on reacive impedance. The DFT requires a large amoun of processing power due o he need for i o analyse a complee 5 Hz cycle a every sample period. I has he major advanage over oher mehods of exracing he fundamenal frequency ha no prior disribuion line informaion is required apar from he reacive par of he line impedance. REFERENCES [1]. Crof, R. Davison, and M. Hargreaves, Engineering Mahemaics: A Foundaion for Elecronic, Elecrical, Communicaions, and Sysems Engineers: Prenice Hall, 1. []. G. Phadke and J. S. Thorp, Compuer relaying for power sysems: John Wiley & Sons, Inc. New York, NY, USA, [3]. J. Wood and B. F. Wollenberg, Power generaion, operaion, and conrol: Wiley New York, [4]. T. Johns and S. K. Salman, Digial Proecion for Power Sysems: Peer Peregrinus d, [5]. Wang, ʺMehodologies & Algorihms for Faul ocaors in Modern Power Sysems,ʺ PhD Thesis, UWE Brisol,. [6]. F. H. Magnago and A. Abur, ʺFaul locaion using waveles,ʺ Power Delivery, IEEE ransacions on, vol. 13, pp ,

7 Elecrical Engineering Research (EER), Volume 3, 15 [7]. H. Nouri and C. Wang, A Fas Faul Classificaion Technique for Power Sysems, Inernaional Journal of Advanced Research in Elecrical, Elecronics and Insrumenaion Engineering, Vol. 3, Issue 11, November 14. [8]. H. Nouri, C. Wang, and T. Davies, ʺAn Accurae Faul ocaion Technique for Disribuion ines Wih Tapped oads Using Wavele Transform,ʺ Proc. 1 IEEE Power Tech, Poro, vol. 3, pp. 1 13, 1. [9]. K. Zimmerman and D. Cosello, ʺImpedance based faul locaion experience,ʺ Proecive relay Engineers, 5 58h Annual Conference for, pp. 11 6, 5. 7