Ch Bhaskar 1, Ramu Nagapuri 2, Nagaraju Goskula 3 1 Assistant Professor, 2 PG Student, 3 Assistant Professor, KITS(S), Huzurabad.

Transcription

1 Design and ontrol of Filter to Mitigate Zero Sequene Harmonis in Power Distriution Systems h haskar 1, Ramu agapuri 2, agaraju Goskula 3 1 ssistant Professor, 2 PG Student, 3 ssistant Professor, KITS(S), Huzuraad. strat: This projet presents a tehnique to mitigate zerosequene harmonis in power distriution systems. The method is ased on the onept of passive zerosequene harmoni filters. However, its asi onfiguration has een expanded to reate a douletuned filtering feature. This feature makes it possile to trap two harmonis with one filter and is espeially attrative in solving harmoniaused telephone interferene prolems. Furthermore, this projet has shown that ommon utility servie transformers an e used to onstrut the filter. s a result, a pratial and lowost solution to mitigating zerosequene harmonis has een found. method for sizing and loading assessment of filters has also een developed. s an example appliation, the presented filter pakage has een applied to mitigate a telephone interferene prolem. Issues, suh as filter loation, the numer of filters required, and the effetiveness of filtering harmonis produed y distriuted residential loads have een investigated. The results show that the presented filter is a very promising tehnique to redue zerosequene harmonis in primary power distriution systems. Key Words: Power Quality, Harmonis, ZS Filter, VSI I. ITRODUTIO The mass penetration of energyeffiient onsumer eletroni devies has resulted in notieale distortions to the voltage and urrent waveforms in power distriution systems. lthough eah onsumer devie is not a large soure of harmonis, their olletive effet an e signifiant. Home applianes have eome a dominant soure affeting the power quality (PQ) of residential distriution systems. Unlike industrial loads, most of the residential loads are single phase. This fat leads to the reation of zerosequene (ZS) harmoni urrents in threephase distriution systems. It is quite ommon to oserve high levels of ZS harmonis in a distriution system feeding residential loads these days. These harmonis an add up in the neutral ondutor, reating prolems suh as neutral voltage raise. They are also the main ulprit of telephone interferene. There is, therefore, a growing interest y industry to develop solutions to mitigate the ZS harmonis. few passive ZS filters have een suessfully used to mitigate ZS harmonis in ommerial systems. Their variations have also een presented for appliations in primary distriution systems. These filters have various topologies. ll share the same idea reating low ZS impedane to trap the ZS harmonis. The ZS filters an e roadly lassified into two types. The first type has a positive (and negative) sequene impedane so it affets the flow of nonzs harmonis. representative example of these filters is the staronneted apaitors grounded through an indutane. The indutane is tuned to ause a low ZS impedane. The main advantage of this topology is that the apaitors an e tuned to filter positive and negativesequene harmonis as well. The downside is that the filter an affet the 6Hz power flow and an lead to positive or negativesequene resonanes at other frequenies. The seond type of ZS filter is ased on the onept of the grounding transformer. Suh a filter ehaves as an open iruit at positive and negative sequenes so it has no impat on normal power system operation and on nonzerosequene harmonis. One of the examples is the zigzag transformerased filter. drawak of this filter is that it needs a nonstandard transformer, whih an e very expensive for utility ompanies to adopt. nother example onsists of a transformer with a tuned apaitor inserted into the delta winding. However, the invention does not larify whether a speially onstruted transformer is required for the sheme. uilt on the work of, this projet presents a novel douletuned, transformerased ZS filter. Sine the filter an trap two ZS harmonis, it is very attrative to solve prolems that involve two dominant harmonis, suh as the telephone interferene prolem. Telephone interferene is normally aused y the ZS 9th and 15th harmonis. In fat, the presentad filter is oneived originally as a means to solve telephone interferene 424

2 prolems. II. HRMOIS MIGITTIO PPROHES Harmoni distortion in power distriution systems an e suppressed through three asi approahes namely: Passive filter. tive power filter. Hyrid ative power filter.. Hyrid tive Power Filter Previously, majority of the ontrollers developed for PF are ased on analogue iruits. s a result, the PF performane is inherently sujeted to signal drift. Tehnial limitations of onventional PFs an e overome with hyrid PF onfigurations. They are typially the omination of asi PFs and passive filters. Hyrid PFs, inheriting the advantages of oth passive filters and PFs provide improved performane and osteffetive solutions. The idea ehind this sheme is to simultaneously redue the swithing noise and eletromagneti interferene. There are various hyrid PFs reported in literature ut the two most prominent ones are shown in Fig. 2.6.The system onfiguration of the hyrid shunt PF as shown in Fig. 2.6(a). oth the shunt PF and passive filter are onneted in parallel with the nonlinear load. The funtion of the hyrid PF an thus divided into two parts: the loworder harmonis are anelled y the shunt PF, while the higher frequeny harmonis are filtered y the passive HPF. Fig.2.6() shows the system onfiguration of hyrid series PF, in whih the series PF is oupled to the distriution line y an interfaing transformer. The shunt passive filter onsists of one or more singletuned L filter and / or a HPF. The hyrid series PF is ontrolled to at as a harmoni isolator etween the soure and nonlinear load y injetion of a ontrolled harmoni voltage soure. It is ontrolled to offer zero impedane (short iruit) at the fundamental frequeny and high impedane (ideally open iruit) at all undesired harmoni frequenies. This fores all the nonlinear load urrent harmonis to flow into the passive filter, deoupling the soure and nonlinear load at all frequenies, exept at the fundamental. Fig. 2.1 onfiguration of Hyrid PFs (a) omination of shunt PF and shunt passive filter () omination of series PF and shunt passive filter III. PRESETED DOULETUED ZS FILTER It is well known that a transformer an sink ZS urrents. This apaility is the priniple of a grounding transformer. y inserting a apaitor into the delta loop and adjusting the apaitor size, very low ZS impedane an e reated at a desired harmoni frequeny. This pakage is alled a singletuned ZS filter [Fig. 3.1(a)]. Fig. 3.1() depits the presentad douletuned ZS filter, whih an trap two harmonis y using only one transformer. This feature is very attrative for mitigating telephone interferene prolems as they are aused y the 9th and 15th harmonis. The use of only one singletuned ZS filter will have diffiulty in solving these prolems. In Fig. 3.2, a sample ZS impedane frequeny san of the presentad filter that was tuned to the 9 th and 15th harmoni frequenies is shown and ompared with the singletuned version. The theoretial priniples of the presentad filter an e understood y studying this filter s equivalent ZS iruit. Fig. 3.3(a) shows a singletuned ZS filter, illustrating how to derive its equivalent ZS iruit. ZS voltage is applied to the filter. The Kirhhoff s voltage law (KVL) implies that in the seondary side of the transformer 425

3 where is the transformer seondary to primary winding ratio, is the power fundamental frequeny (in radians per seond), is the harmoni order, and are the transformer winding resistane and leakage indutane, transferred to the seondary side, respetively. From (1), the equivalent ZS impedane of the filter at harmoni order an e alulated ext, the equivalent ZS iruit of the filter an e estalished as depited in Fig. 3.3(). Equation (2) shows that the apaitor an e tuned to make the reatane part of the filter impedane zero for a speifi harmoni. In this ase, the filter impedane at the tuned harmoni would e only the transformer winding resistane value transferred to the primary side. For example, to tune the filter to the 9th harmoni (y making the reatane part of (2) zero), the required apaitor size an e alulated as follows: With a similar proedure, the equivalent ZS iruit of the presentad douletuned ZS filter an e derived as shown in Fig. 3.3(). The otained equivalent ZS iruit is very similar to the onventional douletuned filters shown in Fig. 3.4(). Therefore, the proper omponent sizes of the presentad douletuned ZS filter an e also derived y using a similar mathematial approah as follows: Fig ZS filter topologies: (a) single tuned and () doule tuned Fig ZS frequeny san of the designed filters. 426

4 Fig (a) ZS filter. () Equivalent ZS iruit of the singletuned ZS filter. () Equivalent ZS iruit of the douletuned ZS filter. Fig (a) Paralleled onventional singletuned filters. () onventional douletuned filter. ording to the equivalent impedanes of two parallel onventional singletuned filters [Fig. 3.4(a)] lose to their tuned harmoni frequenies are almost the same as those of a onventional douletuned onfiguration [shown in Fig. 3.4()], sujet to the following relationships among their omponent parameters: To estalish the mathematial expressions for a douletuned ZS filter tuned to the harmoni orders and, two single tuned ZS filters separately tuned to and an e onsidered with the respetive apaitor sizes of and, as in The two parallel singletuned filters [Fig. 3.4(a)] are supposed to represent the equivalents of the two singletuned ZS filters; therefore, their indutors and apaitors sizes are sujet to the following relationships: The omponent sizes of the douletuned ZS filter an e otained y using the equations set of (4). The only hallenging issue involves, whih is the transformer leakage indutane. y omparing Figs. 3.3() and 3.4(), is oserved to e equal to, on the other hand, aording to (4), has to e To overome this prolem, one an divide the impedane value of every omponent of the douletuned filter y two (i.e., 427

5 divide/multiply the indutane/apaitane values y two) in order to satisfy (7) while keeping the tuned harmoni frequenies of the filter unhanged Finally, omining (4), (6), and (8) will provide the final expressions for deriving the omponent sizes of the presented ZS douletuned filter as follows: where and are the apaitors tuned for the singletuned ZS filters as defined in (5). For the douletuned and singletuned ZS filters, the filter equivalent ZS impedane at the tuned frequenies is equal to the transformer winding resistane transferred to the primary side. For the filter to e effetive, this resistane must e smaller than the system impedane. IV. FILTER OSTRUTIO D DESIG reating a lowresistane transformer does not involve any tehnial diffiulties. However, onstruting a ZS filter y using a speially designed MV transformer is highly uneonomial and must e avoided. distriution utility ompany often has thousands of servie transformers. If suh transformers an e used, the filter ost an e redued signifiantly. Furthermore, the L omponents of the filter an e treated as the loads of the transformer. Installing a filter then eomes the same as adding a new ustomer to the system. Utility ompanies will e muh more reeptive to adopt the tehnology. To ahieve this goal, two prolems must e answered: 1) does the servie transformers have suffiiently small resistane to e used as a ZS filter transformer? and 2) will the transformer e overloaded y the harmonis it traps? To address the first question, the impedane harateristis of ommon servie transformers are investigated. The results show that one threephase or three singlephase servie transformers in the range of total 1 kv to 45 kv are suffiient to reate an effetive ZS filter. The seondary side voltage an e 48 V or 28 V. s a result, the proposed filter eomes a very attrative method for mitigating ZS harmonis. The transformer size an e determined y omparing the system impedane with the transformer resistane (see Setion IV.E). more aurate size determination may e aomplished through Harmoni Load Flow (HLF) studies. One the transformer size is seleted, speifiation of the other filter omponents an e derived y using (3) or (5), (9) (11) for the single tuned or the douletuned versions, respetively. To address the seond question, the transformer loading under harmoni onditions is assessed ased on IEEE Standard n index alled the transformer loading level (TLL) is developed. If the index exeeds 1 p.u. (indiating that the transformer is overloaded), a larger transformer must e utilized. The alulation of TLL eomes a part of the filter design proess. Details are presented in the ppendix. The results have shown that servie transformers have suffiient loading apaility to handle harmonis. dditional issue to address is the loading assessment of the filter apaitors, whih an e done ased on standards. For the apaitors inside a ZS filter, however, their apaitane must not e hanged in ase of overloading (eause the apaitanes are stritly determined y the transformer speifiations as in (9) (1)). Therefore, to alleviate the loading of suh apaitors, one has to inrease their rated voltage and kvar without altering their apaitane. IV. PPLITIO EXMPLE The proposed ZS filter and its design proedure were tested y solving a telephone interferene prolem faed y an atual system. Telephone interferene is proaly the most ommon issue aused y ZS harmonis. 428

6 Fig. 4. Shemati view of the studied distriution feeder.. System Desription Fig. 4 shows the shemati of the atual distriution feeder involved in the telephone interferene prolem. The feeder is loated in Edmonton, anada and is supplying two large residential neighorhoods (eah neighorhood is enompassed y a irle in the figure laeled as the upstream and downstream harmoni loads). telephone line is experiening severe disturing noises indued from the feeder s adjaent overhead setions. The setions in parallel with the overhead power lines are shown y the dashed lines in Fig. 4 (The effet of the underground setions in noise indution is negligile sine the neutral ondutor is very lose to the phase ondutors in an underground ale and also arries the whole returning urrent, aneling out the overall generated eletromagneti field). The main harateristis of the feeder as provided y the utility ompany are listed in Tale I. TLE I SYSTEM PRMETERS. System Model and Simulation Method omprehensive simulation platform was estalished to perform harmoni load flows on the studied system. The ottom up proailisti modeling approah introdued was employed to develop aggregated harmoni models for representing the distorting residential loads. The modeling tehnique replaes eah group of a servie transformer and its supplied residential ustomers y equivalent harmoni soures and impedanes. Multiphase PI models were used for all the overhead lines and underground ales (inluding the single and doulephase setions) inside the feeder. neutral wire was also inluded y employing multiphase models. In order to evaluate the filters effet of on the noises indued on the telephone lines, two sample parallel ondutors were also inluded in the model to represent the involved telephone line setions (the dashed lines in Fig. 4). Finally, the developed models were employed in a multiphase HLF program to perform the simulation studies. Sine the models are timevarying during a day, the otained results are for different times of the day.. Mitigation Ojetive The ZS omponent of the harmoni urrents in an overhead powerline is the dominant soure of the noise indution on the adjaent 429

7 telephone lines [4]. Most of the ZS omponent of a harmoni urrent is dominated y the triplen harmoni orders suh as 3rd, 9th, 15th. s well, for the telephone interferene, the message fators determine the ontriution of eah harmoni frequeny to the disturing noise [4]. The fator is very small for the 3rd harmoni ompared to that for the higherorder ones. Moreover, sine the magnitude of the harmoni orders higher than 15th in residential distriution systems is negligile, the ZS 9th and 15th harmoni orders an e identified as the main ontriutors to a telephone interferene prolem. Therefore, the proposed douletuned ZS filter will e tuned to the 9th and 15th harmoni frequenies. s reommended y, the atual interferene level an e quantified y multiplying the indued voltage of eah harmoni frequeny on the ommuniation line y its orresponding message fator. Therefore, an index alled message weighted voltage (MWV) an e defined as follows: where and are the indued voltage of the nth harmoni frequeny on the telephone line and its orresponding message weighting fator, respetively. ext, the total value of the MWV as defined in (13) an exhiit the total interferene level: D. Filter Loation In order to effetively mitigate a telephone interferene prolem, installation of harmoni filters must eliminate the troulesome ZS harmoni urrents in the powerline setions ausing the disturane. This setion investigates the required numer of filters and their appropriate plaement in the studied feeder (shown in Fig. 6). During the normal system ondition, most loadgenerated harmoni urrents flow to the sustation due to the low supply system impedane. The ideal loation for installing the filter to mitigate the telephone interferene prolem is at the downstream side of the telephone line suh as loation F1 shown in Fig. 4. The filter is expeted to sink the harmonis produed y the downstream loads. Thus, these harmonis will not travel upstream and interfere with the telephone line upstream. However, an important issue must e addressed here: Will the filter also attrat the harmonis from the upstream loads? If the filter does so, it may not e ale to redue the telephone interferene level. HLF simulation results an determine whether this issue will e a prolem or not. If a prolem is aused in the mitigation, then two filters will e required, and the seond one will need to e installed at the loation etween the telephone line and the upstream loads (laelled y in Fig. 4). E. Filter Design Results For seleting the transformer size, some of the typial sizes of the threephase transformers used y the utility ompany in their 25 kv voltage level distriution systems are listed in Tale III. The ZS impedane of the filter at the tuned harmonis (whih is, in fat, the transformer resistane) otained y eah of the transformer sizes is also provided in the last olumn. On the other hand, the system model was also used to estimate the system equivalent ZS impedane at the 9th and 15th harmoni frequenies at the proposed filter loation (in Fig. 4). The otained minimum values during the day are and 41.6 for the 9th and 15th harmoni orders, respetively. eause of these values, the transformer size of 225 kv is adopted as the minimum transformer size to e used in the ZS filter design proedure. In order to estalish a sound understanding of the disussed ZS filters performane, the HLF simulation results for the following ases are also presented and disussed in this setion. ase 1) normal system ondition efore installing any filter; ase 2) one singletuned version of the designed ZS filter (with the 45 kv transformer in Tale IV) is installed in loation F1; ase 3) two singletuned versions of the designed ZS filter (with the 45 kv transformer in Tale IV) are installed in loations F1 and F2; ase 4) one douletuned version of the designed ZS filter (with the 45 kv transformer in Tale IV) is installed in loation F1; ase 5) one douletuned version of the designed ZS filter ut with a larger transformer size of 1 kv is installed in loation F1; ase 6) two douletuned versions of the designed ZS filter (with the 45 kv transformer in Tale IV) are installed in the loations F1 and F2 (This ase presents the reommended filter set to ompletely fulfill the mitigation ojetive). Individual demand distortion (IDD) of the dominant sequene harmoni urrents alulated at the Monitoring Loation shown in Fig. 4; The frequeny spetrum of the total indued voltage on the parallel ondutor representing telephone line; The message weighted values of the indued noise voltages ( and, whih were used for defining the mitigation ojetive); Powerline IT at the Monitoring Loation shown in Fig. 4. Instead of presenting and analyzing the whole 24h profiles of the simulation results, the daily 95% perentile value is adopted. For eah index, the daily 95% perentile value means that 95% of 43

8 the time, the index is elow this value. The following are the key oservations from the simulation results: y lowering the total MWV from 56.5 to 12 V, the goal of a 75% derease in the interferene level was ahieved (Fig. 5). Powerline IT is also dereased from around 1 to 2 in this ase, demonstrating the effetiveness of the filter in the redution of telephone interferene. The douletuned ZS filters were found to e effetive in mitigating oth the 9th and 15th harmonis. lthough the singletuned versions ould suppress the 9th harmoni, they were unale to mitigate the 15th harmoni. The 15th harmoni omponent was oserved to inrease in the singletuned filter ases (II & III) resulting in mitigation failure (See Figs. 5 and 6). This phenomenon is likely due to a parallel resonane etween the filter and the system at the 15th harmoni frequeny. t this harmoni frequeny, the filter is indutive and the system is apaitive eause of the shunt apaitane of the underground ales. Suh oservations one again onfirm the need to use the proposed douletuned ZS filter to mitigate a telephone interferene prolem. V. SIMULTIO RESULTS a Disrete, Ts = 5e 5 s. a a a Fig.5.1 Simulation model ase

9 Fig.5.2 Simulation results ase 1 a Dis rete, Ts = 5e 5 s. a a a Fig.5.3 Simulation model ase

10 Fig.5.4 Simulation results ase 2 a Disrete, Ts = 5e5 s. a a a Fig.5.5 Simulation model ase

11 Fig.5.6 Simulation results ase 3 a Disrete, Ts = 5e 5 s. a a a Fig.5.7 Simulation model ase Fig.5.8 Simulation results ase 4 434

12 a Disre te, Ts = 5e5 s. a a a Fig.5.9 Simulation model ase Fig.5.1 Simulation results ase 5 435

13 a Disrete, Ts = 5e5 s. a a a Fig.5.11 Simulation model ase Fig.5.11 Simulation results ase 6 VI. OLUSIO This projet has presented a novel and effetive filter to mitigate ZS harmonis in the power systems. The design and onstrution 436

14 issues of the filters are fully investigated. The main findings and ontriutions of this work an e summarized as follows.. The presented filter an trap two harmonis simultaneously. It eomes a very attrative method to mitigate telephone interferene prolems.. The filter an e onstruted using regular servie transformers and low voltage L omponents. s a result, it is a very osteffetive filter. Installing a filter is as simple as adding a ustomer to the distriution system.. pratial design proedure has een developed for the filter. This projet also presented a method for the loading assessment of the filter transformer. D. The presented filter pakage and its design method have een tested through omputer simulation studies. The results have shown that the filter is quite effetive to trap ZS harmonis and is an attrative solution to the telephone interferene prolem. Field tests on the presented filter are presently eing pursued. The results will e presented in the future. nother appliation of the filter inludes mitigating the harmoniaused neutral voltage/urrent rise. REFEREES [1] P. agheri, Methods to mitigate harmonis in residential power distriution systems, M.S. dissertation, Dept. Elet. omput. Eng., Univ. lerta, Edmonton,, anada, 213. [2] T. Q. Tran, L. E. onrad, and. K. Stallman, Eletri shok and elevated EMF levels due to triplen harmonis, IEEE Trans. Power Del., vol. 11, no. 2, pp , pr [3] T. M. Gruzs, survey of neutral urrents in threephase omputer power systems, in Pro. Ind. ommerial Power Syst. Teh. onf., May 7 11, 1989, pp [4] IV.. Power line harmoni effets on ommuniation line interferene IEEE working group on power system harmonis, IEEE Power Eng. Rev., vol. PER5, no. 9, p. 56, Sep [5] W. Jewell, W. Miller, and T. asey, udile telephone noise from small harmoni soures on singlephase distriution, in Pro. 8th Int. onf. Harmonis Qual. Power, Ot , 1998, vol. 2, pp [6] W. Xu and V. Sharma, Evaluation of the use of zero sequene harmoni traps in ommerial uildings, E, Projet rep. 311D937, [7] P. Rodriguez, I. andela, S. ogarra, R. Teodoresu, and F. laajerg, anellation of neutral urrent harmonis y using a fourranh star hyrid filter, in Pro. IEEE 8th Power Eletron. Speialists onf., Jun , 28, pp [8] Passive power filter tunale at two simultaneous frequenies, one for zerosequene and another on for positivesequene, (in Spanish) Spanish patent ES225395, Jan. 27. [9] W. Jewell, W. L. Miller, and T. asey, Filtering dispersed harmoni soures on distriution, IEEE Trans. Power Del., vol. 15, no. 3, pp , Jul. 2. [1]. Wu, J. hiang, S. Yen,. Liao, J. Yang, and T. Guo, Investigation and mitigation of harmoni amplifiation prolems aused y singletuned filters, IEEE Trans. Power Del., vol. 13, no. 3, pp. 8 86, Jul [11] H. Jou, J. Wu, K. Wu, W. hiang, and Y. hen, nalysis of zigzag transformer applying in the threephase fourwire distriution power system, IEEE Trans. Power Del., vol. 2, no. 2, pt. 1, pp , pr. 25. [12] E. Larsen, Filter for removing harmoni urrent from a neutral ondutor, U.S. Patent , Jun. 22, [13] J. rrillaga and R. W. eville, Power System Harmonis, 2nd ed. Hooken, J, US: Wiley, 23. [14] IEEE Standard for Shunt Power apaitors, IEEE Standard 1822 (Rev. of IEEE Standard ). [15] D. Salles,. Jiang, W. Xu, W. Freitas, and H. E. Mazin, ssessing the olletive harmoni impat of modern residential loads Part I: Methodology, IEEE Trans. Power Del., vol. 27, no. 4, pp , Ot [16]. Jiang, D. Salles, W. Xu, and W. Freitas, ssessing the olletive harmoni impat of modern residential loads Part II: ppliations, IEEE Trans. Power Del., vol. 27, no. 4, pp , Ot [17] W. Xu, J. R. Marti, and H. W. Dommel, multiphase harmoni load flow solution tehnique, IEEE Trans. Power Syst., vol. 6, no. 1, pp , Fe [18] IEEE Reommended Pratie for Indutive oordination of Eletri Supply and ommuniation Lines, IEEE Standard , Fe. 8, 212. [19] IEEE Reommended Praties and Requirements for Harmoni ontrol in Eletrial Power Systems, IEEE Standard , [2] J. M. Frank, Development and design of fator transformers, in Pro. IEEE Ind. ppl. So. nnu. Meeting onf. Re., Denver, O, US, 1994, pp [21] R. D. Henderson and P. J. Rose, Harmonis: The effets on power quality and transformers, IEEE Trans. Ind. ppl., vol. 3, no. 3, pp , May/Jun [22] IEEE Reommended Pratie for Estalishing LiquidFilled and Dry Type Power and Distriution Transformer apaility When Supplying onsinusoidal Load urrents, EEE Standard (Revision of IEEE Standard ), ug. 15, 28. [23] D. E. Rie, djustale speed drive and power retifier harmonistheir effets on power systems omponents, IEEE Trans. Ind. ppl., vol. I22, no. 1, pp , Jan./Fe