Distorting and Unbalanced Operating Regime A Possible Diagnosis Method?

Transcription

1 Distortig ad Ubalaced Operatig Regime A Possible Diagosis Method? Petre-Maria NICOLAE, Uiversity of Craiova. Faculty of Electrotechics, picolae@elth.ucv.ro, Decebal Blv. 107, Craiova, 00440, ROMANIA Abstract. The paper deals with a sychroous geerator from a eergetic group with distortig ad ubalaced behaviour durig its steady state operatio. Waveforms from mai ad auxiliar geerators ad from the mai geerator excitatio widig were aalyzed. Harmoics determiatios ad powers calculatios were doe. For a low harmoic level i the mai geerator stator, coclusios o the geerator steady state operatio ca be made. Eve harmoics could be caused either by a asychroous thyristors cotrol or by the auxiliar geerator ubalaced operatio. 1. Itroductio The occurace of a distortig regime, either i the mai geerator stator, or i the widig supplied from the auxiliar geerator stator from a eergetic group leads to the apparitio of high harmoic magetic fields. These peetrate the stator ad rotor iro cores, causig a icrease of losses i iro as compared to the sie regime, determied by the fudametal harmoic. Therefore overheatigs of widigs ad iro core, as well as pulsatig couples ad oises ca occur [3]. Moreover, the harmoic currets (maily those from the side toward rotor) ca iduce voltages that ca propagate through the eergetic group metallic parts ad cosequetly they ca have a egative ifluece over the protectio apparatus operatio [4]. The voltages iduced as described before ca accidetaly start the geerator protectios, causig upleasat effects over the etire eergetic system [11].. Waveforms Recordig The recordig of currets ad voltages waveforms was doe usig two data acquisitio systems, with sample frequecy as high as Hz. I this way it was possible to record the first 40 harmoics. The first data acquisitio system was used to record data from the secodary widigs of the curret ad voltage trasformers for the mai geerator (fig.1). The secod data acquisitio system was used to record data correspodig to the time-variatio of the currets from the curret reductors ad the data correspodig to voltages were recorded directly from the auxiliar geerator termials. The secod data acquisitio system was also used to record the waveforms after the rectifier that is full-cotrolled by thyristors. The curret after the rectifier was recorded usig a 60mV shut ad the voltage after rectifier was directly recorded. These quatities waveforms are depicted by fig.. The preseted recorded waveforms correspod to a active power (as idicated by the test apparatus) of 09.6 MW ad respectively to a reactive power of 4.6 MVAr. 3. Experimetal determiatios The proccesig of waveforms was doe i order to determie the eergetic parameters for quality (shape factor, peak factor, distortig coefficiet), the phases active ad reactive powers ad respectively the total three-phase powers. The origial soft relies o the Fourier decompositio usig a Fast Fourier Trasform (FFT) of the experimetal data [6], [7]: 1

2 Fig. 1: The currets ad the voltages at sychroous geerator termials 0 + k si( k t + k ) k = 1 y( t) = ω γ (1) The quality ad eergetic parameters are calculated by meas of the followig equatios [6] - Peak factor max max kv = = () - Shape factor Fig. : The currets ad the voltages at auxiliary sychroous geerator termials ad d.c. voltage ad curret 0 k

3 k = (3) f T T t0 + t0 y( t) dt - Distortig factor k 0 + k d d = = = k 0 THD - Sigle phase fudametal active power P = U I (5) cosϕ1 - Sigle phase active power P = U 0 I U k I k cosϕ k (6) - Sigle phase fudametal reactive power Q = U I (7) siϕ1 - Sigle phase reactive power Q 1 = 1U k I k siϕ (8) k k = - Three-phase active power P = 3P 1 (9) - Three-phase reactive power Q = 3Q 1 (10) 4. Decompositio ad aalysis of voltages ad currets waveforms 4.1. Decompositio ad aalysis of stator waveforms The waveforms correspodig to voltages ad currets from the stator phases were decomposed ito 40 harmoics. Fig. 3 depicts the harmoic cotet of the voltage from phase o. 1 ad Fig. 4 depicts the harmoic cotet of the curret through the same phase. I both cases the iitially decomposed sigal was recostructed usig its harmoics up to the 40-th order. Each time there were revealed small differeces betwee the iitial values ad those obtaied after recostructio. The quality parameters of the phase quatities are as follows: (4) Voltage of phase 1 Xef_u1 =1.415e+004 Voltage of phase Xef_u =1.3571e+004 Voltage of phase 3 Xef_u3 =1.4048e+004 3

4 Kv_u1 =1.3809e+000 Kf_u1 =1.0873e+000 UTHD1: Kd_u1 =4.5943e+000 Kv_u =1.3907e+000 Kf_u =1.0888e+000 UTHD: Kd_u = e+000 Kv_u3 =1.3964e+000 Kf_u3 =1.0890e+000 UTHD3: Kd_u3 =5.345e+000 Curret of phase 1 Xef_i1 = e+003 Kv_i1 =1.4170e+000 Kf_i1 =1.0951e+000 ITHD1: Kd_i1 =.6564e+000 Curret of phase Xef_i = 4.888e+003 Kv_i =1.459e+000 Kf_i =1.0971e+000 ITHD: Kd_i =.5506e+000 Curret of phase 3 Xef_i3 =5.1868e+003 Kv_i3 =1.44e+000 Kf_i3 =1.0937e+000 ITHD3: Kd_i3 =.637e+000 The computed total active ad reactive powers are: P =09.5 [MW]; Q = 4.1 [MVAr]. Fig. 3: Harmoic cotet of voltage u1 Fig.4: Harmoic cotet of curret i1 4.. Decompositio ad aalysis of waveforms for rotor Decompositio ad aalysis of waveforms at the auxiliary geerator The waveforms correspodig to voltages ad currets from the stator phases at the auxiliary geerator were decomposed ito 40 harmoics. Fig. 5 depicts the harmoic cotet of the voltage from phase o. 1 ad Fig. 6 depicts the harmoic cotet of the curret through the same phase. I both cases the iitially decomposed sigal was recostructed usig its harmoics up to the 40-th order. Each time there were revealed small differeces betwee the iitial values ad those obtaied after recostructio. The quality parameters of the phase quatities are as follows: Voltage of phase 1 Xef_u1 =4.31e+00 Kv_u1 =1.5084e+000 Kf_u1 =1.117e+000 UTHD1 Voltage of phase Xef_u =4.1511e+00 Kv_u =1.587e+000 Kf_u =1.145e+000 UTHD Voltage of phase 3 Xef_u3 = 4.160e+00 Kv_u3 =1.680e+000 Kf_u3 =1.1408e+000 UTHD3 4

5 Kd_u1 =1.8565e+001 Kd_u =.177e+001 Kd_u3 =.5835e+001 Curret of phase 1 Xef_i1 =1.3995e+003 Kv_i1 =1.3637e+000 Kf_i1 =1.1957e+000 ITHD1 Kd_i1 =.8137e+001 Curret of phase Xef_i =1.3894e+003 Kv_i =1.797e+000 Kf_i =1.089e+000 ITHD Kd_i =.800e+001 Curret of phase 3 Xef_i3 =1.3734e+003 Kv_i3 =1.833e+000 Kf_i3 =1.1973e+000 ITHD3 Kd_i3 =.710e+001 Fig. 5 : Harmoic cotet of voltage u1 for auxilliary geerator Fig. 6 : Harmoic cotet of curret i1 for auxilliary geerator 4... Decompositio ad aalysis of waveforms after cotrol rectifier with thyristors The harmoic decompositios of the excitatio voltage ad curret (o the d.c. side) are depicted by figures 7 ad 8. Fig. 7 : Harmoic cotet of excitatio voltage Fig. 8 : Harmoic cotet of excitatio curret The d.c. values of voltage ad curret after rectifier are: Uex0= V; Iex0= A After rectifier, the active power of the d.c. compoets is P0 = 345kW ad the total active power is Pexcitatio 343kW. 5

6 5. Coclusios The presece of some harmoics of secod ad third order from the mai geerator stator voltages ad currets waveforms, eve though their values are small, demostrate that the mai geerator operates i a ubalaced maer. Most probably the ubalace is caused by iteral causes (a possible fault betwee some loops from the excitatio widig) [8],[9]. The presece of some pulses i the voltages ad currets waveforms from the auxiliar geerator phases owig to the thyristors cotrol leads to the apparitio of some eve harmoics, multiple of 6 i the rectified voltage waveform. Sigificat amouts of harmoics with the orders 6, 1 ad 18 ca be oticed. This leads to a value of 08.4V for the d.c. compoet after the rectifier, ad the excitatio voltage RMS value is as high as 317V. The presece of these pulses results ito a overlappig of some harmoic compoets multiple of 6 i the rectified curret waveform. Moreover, oe ca otice a sigificat value for the fudametal harmoic of 50Hz i the rectified curret waveform. The d.c. active power for the mai geerator excitatio supply is approximately 345 kw, whilst the total active power after the rectifier is approximately 343 kw. Actually this idicates a reversed power circulatio acrross the harmoics from the mai geerator excitatio toward the fully cotrolled rectifier. The cause could be a possible ubalaced operatio of the mai geerator excitatio. The peaks with differet magitudes of the curret pulses correspodig to harmoics multiple of 6 proove a ubalaced operatio of the fully cotrolled rectifier. The eve harmoics presece i the voltage waveforms from certai phases prooves a ubalaced cotrol of the thyristors from the fully cotrolled rectifier supplied from the auxiliar geerator termials [10]. This could be caused by a asychroous cotrol of the thyristors placed o the same phase i derivatio from the auxilliary geerator, but oe ca also cosider aother cause, amely the ubalaced operatio of the auxiliar geerator that could be caused by some widids costructive o-symmetrical features. The auxiliary geerator aalysis prooved sigificatly differet distortig factors for phases voltages. They could also ifluece a erroous cotrol of thyristors. Refereces [1] Albert, K. The History of Power Factor, Proceedigs of 4 th It. Cof. O Harmoics i Power Systems, Budapesta 90, p [] Budeau, C., Rolul fizic al marimilor istataee i feomeele de coservare, Eergetica, 86, p [3] Depebrock, M., The FDB-Method a Geerally Applicable Tool for Aalyzig Power Relatios, IEEE Tr. o Power Systems, vol.8, o./93, p [4] Emauel, A.E., a.o., New Cocepts of Istataeous Active ad Reactive Powers i Electrical Systems with Geeric Loads, IEEE Tr. o Power Del., o.3/93, p [5] Nicolae, P.M., Istataeous Real ad Imagiary Powers at Three-Phase Networks with Balaced Loads that Fuctio uder Distortig Regime,, RRST Serie Electr. et Eerg., o. 3/ 95, p [6] Nicolae, P.M., Calitatea eergiei electrice i sisteme electroeergetice de putere limitata, Ed. Tehicá, Bucuresti, 1998 [7] Puscasu, S., Marcovici, J., Márimi si regimuri electrice esiusoidale, Ed. Scrisul Romaesc, Craiova, 74 [8] Tugulea, A., Cosideratii privid efectele eergetice i regimuri deformate, Eergetica, o. 1/ 86, p.7-31 [9] Tugulea, A., Factorul de putere i regim deformat si esimetric, Eergetica, o. 9/ 86, p [10] Tugulea, A., Criteria for the Defiitios of the Electric Power Quality ad its Measuremet Systems, ETEP, vol.6, o.5/ 96,, p [11] IEEE Workig Group o Nesiusoidal Situatios Practical Defiitios for Powers i Systems with Nosiusoidal Waveforms ad Ubalaced Loads, IEEE Tr. o Power Del., o.1/96, p