Aalborg Universitet. Published in: IET Power Electronics. DOI (link to publication from Publisher): /iet-pel Publication date: 2018

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1 Aalborg Universitet Load-Independent Harmonic Mitigation in SCR-Fed Tree-Pase Multiple Adjustable Speed Drive Systems wit Deliberately Dispatced Firing Angles Yang, Yongeng; Davari, Pooya; Blaabjerg, Frede; Zare, Firuz Publised in: IET Power Electronics DOI (link to publication from Publiser): 1.149/iet-pel Publication date: 218 Document Version Accepted autor manuscript, peer reviewed version Link to publication from Aalborg University Citation for publised version (APA): Yang, Y., Davari, P., Blaabjerg, F., & Zare, F. (218). Load-Independent Harmonic Mitigation in SCR-Fed Tree- Pase Multiple Adjustable Speed Drive Systems wit Deliberately Dispatced Firing Angles. IET Power Electronics, 11(4), ttps://doi.org/1.149/iet-pel General rigts Copyrigt and moral rigts for te publications made accessible in te public portal are retained by te autors and/or oter copyrigt owners and it is a condition of accessing publications tat users recognise and abide by te legal requirements associated wit tese rigts.? Users may download and print one copy of any publication from te public portal for te purpose of private study or researc.? You may not furter distribute te material or use it for any profit-making activity or commercial gain? You may freely distribute te URL identifying te publication in te public portal? Take down policy If you believe tat tis document breaces copyrigt please contact us at vbn@aub.aau.dk providing details, and we will remove access to te work immediately and investigate your claim. Downloaded from vbn.aau.dk on: oktober 29, 218

2 IET Power Electronics Special Section: Selected papers from te 8t International Conference on Power Electronics, Macines and Drives (PEMD 216) Load-Independent Harmonic Mitigation in SCR-Fed Tree-Pase Multiple ASD Systems wit Deliberately Dispatced Firing Angles ISSN Received on 6t October 216 Revised on 29t September 217 Accepted on 5t December 217 doi: 1.149/iet.pel.216.xxxx Yongeng Yang *, Pooya Davari *, Frede Blaabjerg *, Firuz Zare * Department of Energy Tecnology, Aalborg University, Aalborg 922, Denmark Power and Energy Group, Te University of Queensland, Brisbane QLD 472, Australia s: yoy@et.aau.dk, pda@et.aau.dk, fbl@et.aau.dk, f.zare@uq.edu.au Abstract: Adjustable Speed Drives (ASDs) are widely used in tree-pase multi-drive applications in industry for energy savings, were low-cost rectifiers (mainly Diode Rectifiers - DRs) are still employed as te front-ends in practice, also for simplicity in control and reliability in operation. However, significant armonic distortions appear at te grid, wic sould be properly tackled according to increasingly stringent standards and/or grid-connection rules. If remain untreated, a poor power quality will cause efficiency drops of te entire system. Currently, communication tecnologies are still not of muc cost-effectiveness, and ence, te use of communication in multiple parallel motordrive systems for armonic control is rarely witnessed in industry. In tis sense, tis paper proposes a armonic mitigation strategy for multiple parallel ASD systems, were anoter type of low-cost rectifiers (i.e., Silicon-Controlled Rectifiers - SCRs) wit boost converters in te dc-link ave adopted to increase te armonic-current controllability. More specific, te SCR firing angles are deliberately dispatced among te SCR drive units, wic results in certain pase sifts of te SCR currents on purpose. In suc a manner, te armonics appearing in te grid current can be mitigated to some extent in multi-drive systems witout communication. Te effectiveness is independent of te power loading of eac drive, wen a number of drives are connected. Simulations are carried out to verify te discussion. Furtermore, to make te most use of te SCR-fed structure for armonic cancellation, a customized modulation sceme as been experimentally demonstrated. Bot simulation and experiment results ave verified te discussion. 1 Introduction As an almost standardized solution to motor drive applications, low-cost Diode Rectifiers (DRs) are still popular as te front-end apparatus of tree-pase Adjustable Speed Drive (ASD) systems [1]-[3], as it is exemplified in Fig. 1(a). Altoug DRs ave merits including simple structure and control, small volume, low cost, and ig reliability during operation, tey will introduce large armonics to te grid [2], [4]-[6]. Te power quality issue sould be addressed properly according to demands and standards [7]. Oterwise, te overall system efficiency will be degraded, and oter equipment connected to te Point of Common Coupling (PCC) may malfunction. Te approaces to alleviate te armonic distortions can be categorized as a) adding passive filters (e.g., dc or ac cokes) [3], b) employing multi-pulse transformer-based front-ends [8], [9], c) using active power filtering tecniques [1], [11], and d) ybrid scemes [11]. In respect to te cost-effectiveness and complexity, te use of active Power Factor Correction (PFC) circuits [2] in te dc-link as sown in Fig. 1(a) as enabled muc flexibility in te control of te grid current quality. In te case of single-pase drive systems, te employed PFC systems ensure tat te grid current drawn by te rectifiers can be saped as muc close to purely sinusoidal as possible [13]-[]. Tis contributes to a significant power quality improvement in single-pase motor drive applications, and it is almost becoming a standardized solution. Similarly, in tree-pase ASD systems, te power quality can also be enanced by incorporating PFC circuits (e.g., boost dc-dc converters) [2], [16]-[18], as it is demonstrated in Fig. 1(a). In tat case, te rectified currents (e.g., i rd in Fig. 1(a)) can be controlled [19]. Tat is to say, by properly regulating te rectified currents i rd troug te boost dc-link, te grid currents i sabc can be saped, and tus te armonic currents appearing in te grid can be controlled. Notably, te dc-link is to emulate an infinite inductor. It sould be pointed out tat, altoug te PFCs in ASD increase te controllability of te power quality to some extent, te performance cannot compete wit tat in singlepase applications. Tis is mainly owing to te inerent beaviour of rectifiers in te case of a Continuous Conduction Mode (CCM) [18]. Tat is, te pase current conducts wit a period of 12. Tus, it is not possible to acieve a purely sinusoidal current wit te topology sown in Fig. 1(c) [2]. One possibility is to use interleaved boost converters as te PFC stage [2]. Neverteless, te configuration in Fig. 1(a) enables te possibility to sape te currents drawn from te grid wit a focus on reducing te distortion level, as it as been presented in [21], [22]. 1

3 On te oter and, in te case of a multi-drive system (consisting of DR units), as demonstrated in Fig. 1(b) tat is more commonly seen in practice, te current quality will be almost constant (current THD is around 31%) wen boost converters are adopted in te dc-link. However, according to te superposition principle, if te currents drawn by certain ASDs can be pase-sifted, te quality of te vector summation of all currents at te PCC will be improved. To enable tis pase control freedom, te DRs can be replaced by anoter type of rectifiers te Silicon-Controlled Rectifiers (SCRs). In tat case, te power quality will be enanced by means of a pase-sifted current control [21], [23], [24] and te boost dc-link control/modulation. Wen certain pase-sifts are introduced to parallel SCR-drive systems, te total currents drawn from te grid will become multi-level, and certain armonics of interest (e.g., te 5torder armonic) will be eliminated teoretically. It tus contributes to an improved quality of te grid current, and also possibilities to optimize te firing angles among te drives. However, te implementation of te optimal firing angles relies on costly communication, wic is not desired in practice. In ligt of te above issues, tis paper proposes a armonic mitigation strategy for SCR-fed tree-pase parallel ASD systems in 2 witout communication, were te basic control structure of suc drives is also presented. Te proposed armonic mitigation strategy takes te strengt of te pase-sifted current control [21], [23], and ten te firing angles are deliberately dispatced (e.g., linear increase) in suc a way tat te grid current becomes multi-level wit no communication, leading to an enanced power quality. Te armonic mitigation effectiveness is also independent of te loading of eac drive wen a number of drives are connected. Simulations on multi-drive systems ave been performed in order to verify te effectiveness of te proposed solution, and te results are presented in 3. Furtermore, to fully exploit te controllability of armonics enabled by te boost-based dc-link, a customized selectively armonic cancellation modulation sceme is applied to a drive system in 4, and te performance is also verified experimentally. Finally, concluding remarks are given in 5. 2 Harmonic Issues and Mitigation 2.1 Current Control and Harmonic Caracteristics As aforementioned, te use of PFC circuits enables a flexible control (modulation) of te dc-link current (i.e., te rectified current i rd in Fig. 1(a)). Figs. 1(c) and 1(d) present te detailed ardware scematic and te control structure of te dc-link current for an SCR-drive system, respectively. Assuming tat te rectified current i rs is controlled as a purely dc current (denoted as I rs) troug te PFC according to Fig. 1(d), te currents drawn from te grid will tus be a rectangular wave wit a conduction angle of 12. Tis means tat te incorporated dc-dc converter is expected to emulate an ideal inductor at te dc-link. It also indicates tat it is not possible to acieve a sinusoidal current wit one boost converter in te dc-link. Taking te pase-a grid current as an example, it can be expressed in a compact format as [25] i t i t I sin t ga sa rs f n (1) N Grid Z g i sabc v rd i rd DR L Point of Common Coupling i rd dc-link Inverter were = 6n±1 wit > being te armonic order, n =, f is te firing angle of te SCR unit. For te DR system, te armonics appearing in te grid current can be obtained by setting f = in (1). According to (1), te amplitude of te -t armonic component appearing in te grid current (e.g., i sa) can be given as 2 3 Isa Irs (2) wit being te -t armonic current amplitude. Equation (2) indicates tat te armonic component amplitude will be inversely proportional to te armonic order once te dclink rectified current is controlled as a purely dc current. Furtermore, te amplitude of te resultant armonic components in (2) is independent of te firing angle f. Accordingly, te Total Harmonic Distortion (THD) level of te grid current can be obtained as well as te armonic distribution, wic is sown in Fig. 2. It can be observed in Fig. 2 tat te rectangular current wit a conduction period of 12 by controlling te rectified current to be dc brings ig armonics (e.g., te 5t armonic is 2% of te fundamental) to te grid, leading to a poor THD of approx. 31%. Tis current THD is irrelevant to te firing angle f in C dc Drives (ac-dc-ac) a Grid Motors M M M... M Z g i gabc v od M Motor i sabc ac i rd,1 ac i rd,2 ac i rd,3 ac i Drives dc dc dc... rd,n dc ac ac ac ac Grid v dc - i ga * + Voltage v dc a #1 #2 #3 #n b c i rs controller v rs L S * i rs + b c i rs - d C dc + v dc Current controller Motor Gate signal S Fig. 1 Examples of te DR-fed drive systems: (a) single-drive unit wit a boost dc-dc converter in te dc-link, (b) multi-drive systems (n drive units), (c) ardware scematic of an SCR-fed tree-pase drive system wit a boost converter in te dc-link, and (d) control structure of te boost dc-link stage. 2

4 Fig. 2 Harmonic caracteristics of te pase-a current drawn by a SCR drive system: (a) f f and (b) armonic distribution. a single-scr-fed drive system. Connecting suc apparatus to te grid comes wit a penalty on te owners, since te grid operator as to pay for te power quality compensation. Furtermore, it is implied in Equation (1) tat te firing angle f can control te average rectified output voltage v rs (and tus te dc-link voltage v dc troug te control of te PFC), as well as te pase of te grid current i ga (i.e., i sa). Tis furter explains tat te grid currents can be controlled by adjusting te firing angle in terms of sapes and pases. Notably, it is not possible to acieve a sinusoidal sape only by controlling te single boost converter due to te commutation caracteristics in tree-pase rectifiers (i.e., te conduction periods of 12 ). 2.2 Proposed Harmonic Mitigation Strategy In order to avoid te penalty, te current quality sould be improved, wen suc armonic sources (i.e., te SCR- or DR-fed drives) are connected to te grid. Mixing non-linear loads [24] offers muc convenience to reduce te armonics wile being size- and cost-effective in contrast to prior-art solutions (e.g., 12-pulse and 18-pulse transformer rectifiers). Fig. 1(b) gives a layout of multi-drive systems. In tat case, te THD level of te total grid currents can be lowered and te targeted armonics (e.g., te 5t armonic) can be minimized, as long as te firing angle of eac SCR-fed drive system is properly designed [21], [23], being te pasesifted current control. Hereafter, a two-scr-drive system (#1 and #2 in Fig. 1(b)) is considered to illustrate te grid current quality improvement (i.e., lowering THD level) by te pase-sifted current control. First, te -t order armonic current pasors of te drive units (#1 and #2) can be given as I I sa,1 sa,2 I j sa,1 sa,1e j sa,2 sa,2e in wic, are te magnitudes, and, are te pases of te -t armonic component for te corresponding SCR-fed drive unit. Tey can be calculated according to te Fourier analysis [25]. Alternatively, te magnitudes and pases can be obtained troug Equations (1) and (2). Neverteless, based on te superposition principle, te pasor of te -t order armonic component of te total grid current (pase-a) can be given as I sa,1 j ga sa,1 sa,2 sa,1 sa,2 j sa,2 (3) I I I I e I e (4) Subsequently, te amplitude of te -t order armonic of te grid current is obtained as 2 2 ga sa,1 sa,2 sa,1 sa,2 I I I 2I I cos (5) 1 2 wit being te amplitude of te -t order armonic of te grid current and = - -. It is tus indicated in Equations (4) and (5) tat, if te -t armonic component of te grid current sould be cancelled (i.e., = ), te two drives ave to: 1) Draw te same level of te armonic currents from te grid (i.e., = ) and 2) Have a pase difference of 18 o between te two armonic currents (i.e., - = ). In practice, it is not easy to always ensure te currents drawn by te two drive units are at te same level, being te main drawback of te pase-sifted current control [21]. An advanced communication tecnology can be adopted to enance te control performance, since it provides te loading information [26]. However, te communication can be more complicated in multiple ASD systems (more tan two drives), were it may become costly (te total expenses may exceed te initial outlay). Accordingly, a cost-effective solution for armonic mitigation in multiple ASD systems is proposed in te following, were its implementation is independent of te loading condition. Specially, in te proposed armonic mitigation strategy, te firing angles of te SCR-fed drive units are linearly designed witin a specified range ( f, to f, max) in te consideration of te resultant power factor. Tat is to say, te firing angle for eac SCR-fed drive is deliberately dispatced as k f f, k 1 f, max f, (6) N 1 in wic represents te firing angle of te SCR-fed drive unit #k, k is te drive number, and N is te total number of drives forming te multi-drive system. It is implied in Equation (6) tat te armonic mitigation strategy does not require any communication among te SCR-fed drives, but it can acieve a satisfactory performance in terms of a low THD of te grid current and a relatively ig power factor. Altoug te implementation of te proposed sceme does not rely on expensive communication, te number of te drives N tat ave been connected to te PCC sould be available according to Equation (6). Moreover, a central control unit tat governs all te parallel drive systems is also essential to implement te proposed armonic mitigation strategy. Tose make te proposed sceme especially suitable for te applications in industrial production lines. In addition, te more te SCR drives are connected to te PCC, te better te performance will be. In tat case, te grid current will ave more levels, and its sape will be close to a sinusoidal waveform, leading to a better quality (a lower THD). It is noted tat te firing angle range (i.e., f, to f, max) sould not be very wide to maintain a satisfactory power factor (e.g., iger tan.95); oterwise power factor compensation equipment (e.g., power factor correction capacitor banks) as to be installed. 3 Verification of te Proposed Approac In order to evaluate te proposed armonic mitigation strategy for multi-drive systems, numerical simulations in MATLAB ave been performed firstly, were te currents drawn by te SCR-fed multiple drive systems ave been programmed according to Equation (1) so tat te armonic caracteristics can be studied. Fig. 3(a) sows te numerical 3

5 Total number of drives (N) a Fig. 3 Simulation results of a multi-drive system consisting of 2 SCR-fed drive units (N = 2) under a random loading condition wit linearly f (a) typical waveforms of te pasea voltage v an and currents i ga, (b) armonic distribution of te grid current, (c) loading of te multi-drive system (rectified power -, rectified current - ), and (d) dispatced firing angles Total number of drives (N) b Fig. 2 Performance of a multi-drive system considering various number of drives under different loading conditions wit linearly dispatced firing angles (Top: THD level of te total current at PCC, i.e., te grid current; Bottom: resultant power factor): (a) f (b) f tere is no pase-sifting (all DR-fed drives), a THD of 31% and a power factor of.96 will be obtained. simulation results of a multi-drive system under a random loading condition wit te proposed metod, were te firing angles are deliberately dispatced ( f, = and f, max = 3 ). Notably, for Test- (T), te currents drawn by te SCR-fed drive units are at te same level for comparison, wile for te rest tests (T1 to T6), te loading is random. More specifically, in te simulations, te loading of eac drive is generated randomly level of te rectified current of eac drive can be calculated accordingly. Finally, te currents drawn by eac drive are summed up, becoming te grid current of multi-level. It is clear tat in some cases te loading of certain drives can be far below te rated. It can be observed in Fig. 3(a) tat te resultant THD level of te total current (i.e., te grid current) is around 14-18% and a power factor of around as been acieved. Tis indicates tat te current quality as well as te power factor is independent of te loading conditions Fig. 4 Simulation results of a multi-drive system consisting of 2 SCR-fed drive units (N = 2) under a random loading condition wit linearly f (a) typical waveforms of te pasea voltage v an and currents i ga, (b) armonic distribution of te grid current, (c) loading of te multi-drive system (rectified power -, rectified current - ), and (d) dispatced firing angles. as discussed in 2, wen te proposed metod is adopted. It is furter implied in Fig. 3(a) tat wit more parallel drive units, te THD level of te grid current and te power factor tend to be bounded (i.e., becoming more independent of te loading condition). In te worst cases, all drives are operating at te rated power (or only one drive is operating), te THD level of te total grid current will become 31%. Following, te firing angle range as been extended to 6 (i.e., f, = and f, max = 3 ), were for Test T, te currents drawn by te SCR-drives are at te same level. Te results are presented in Fig. 3(b). It can be observed in Fig. 3(b) tat te THD level of te grid can be brougt to around 8% wen te firing angle range for dispatcing is extended. Similar trends can also be seen from Fig. 3(b) te THD level and te power factor variations are bounded, wen te number of drives increases. However, as it as been mentioned, te THD improvement is acieved at te cost of power factor (see Fig. 3(b)). Hence, a trade-off between te power quality and te power factor as to be made wen te 4

6 Table 1 Parameters of a multi-drive system (referring to Fig. 1(b)) consisting of four SCR-fed units Parameter Boost converter inductor dc-link capacitor Grid frequency Grid pase voltage (rms) Grid impedance Proportional gain of GPI(s) Integral gain of GPI(s) Hysteresis band Symbol L Cdc fg vabcn Zg (Lg, Rg) kp ki - van vbn Value 2 mh 47 F 5 Hz 22 V vcn a iga A igb igc Table 2 Optimized firing angles for te four-drive system (P = [.5, 6, 4, 2] kw). Optimization range (º) [ ] 8 [ 3] f [ 45] f, f,2 Optimal firing angles f,n (º) f,3 2.1 f,4 Grid current THD (%) Power factor Notes: Subscripts (i.e., 1, 2, 3, and 4) represent te drive number. f b 1 f Fundamental THDig = 18.6% 6 4 5t arm.: 13.6% 7t arm.: 9.9% Harmonic order c Fig. 5 Simulation results of te multiple parallel ASD system consisting of four SCRf (a) grid pase voltages vabc,n [4 V/div], (b) total grid currents [4 A/div], time [1 ms/div], and (c) te Fast Fourier Transform (FFT) of te pase-a grid current iga. proposed strategy is employed in multi-drive systems. Oterwise, it may incur more investments into compensating te resultant power factor. In addition, Figs. 4 and 5 sow te results of one case of Fig. 3 in details respectively in order to demonstrate te effectiveness of te proposed strategy. It can be seen in Figs. 4(b) and 5(b) tat te THD of te grid current iga is significantly reduced in contrast to tat of a single-drive system sown in Fig. 2. Furtermore, if te firing angles are simulation results. It demonstrates tat te multiple ASD systems employing te proposed armonic mitigation strategy can operate smootly during load transients. At te same time, te THD and te power factor variations are witin certain bands (i.e., THD: 18.6% 16.8%, power factor:.94.92), wic is in agreement wit te previous discussions (see Fig. 3). However, large oversoots in te dclink voltages and slow dynamics ave been observed, wic requires furter parameter-tuning. Neverteless, te above ave validated te effectiveness of te proposal in terms of armonic mitigation in multi-drive systems, were communication is not mandatory. However, it sould be noted tat, once te communication is available, te grid current quality could be significantly enanced. In tat case, te firing angles can be optimally dispatced according to loading conditions [27]. Table 2 gives an example of te optimized firing angles for te four-drive system wit known power levels, were te particle swarm optimization algoritm is adopted. Clearly, compared to te case of linearly dispatced firing angles (see Fig. 6), te THD level can be furter reduced once te optimization firing angle range is above [ 3º], as sown in Table 2. However, te power factor will be poor if te optimization range is too wide. In addition, tere is a possibility to improve te grid current quality by operating certain drives in partial-loading conditions wit optimal firing angles and optimal power levels. Tis is out of te cope of tis paper, and implementing of te optimized results is anoter callenging issue, wic also relies on communication. Moreover, in order to validate te performance of multiple drive systems wit deliberately assigned firing angles, experimental tests ave been performed. Notably, te experimental tests were carried out under low voltage conditions (i.e., input pase-a voltage: 85 V in root-meansquare; output voltage: 25 V) and te rectified current levels are assumed at te same level. Due to te line impedance and te small number of drives (i.e., N = 3 and 4), te firing angles are not exactly linearly assigned in te experiments. Control systems ave been implemented according to Fig. 1(d). Te f THD level becomes even smaller, were te sape of te grid current is very close to sinusoidal, as it is sown in Fig. 5(a). However, it also introduces a large power factor angle tat sould be compensated in practice. In a word, te simulations ave verified te effectiveness of te proposed strategy for multi-drive systems. Apart from te above numerical studies, more simulations ave been carried out in MATLAB/Simulink referring to Fig. 1, were four SCR-fed drive systems (P = [.5, 6, 4, 2] kw) are considered. A Proportional Integral (PI) controller GPI(s) as been adopted to control te dc-link voltage (i.e., vdc), were te reference dc-link voltage as been set as = 65 V. A ysteresis controller as been employed as te current controller to regulate te rectified dclink current (i.e., irs). Te system and controller parameters are listed in Table 1. Simulation results are sown in Fig. 6. Fig. 6 demonstrates te simulation results of te multidrive system, were te firing angles are linearly dispatced witin a small range (i.e., f = [, 1, 2, 3 ]) in consideration of te resultant power factor. It can be seen tat te grid currents igabc are not purely rectangular wen comparing to tose sown in Fig. 2(a) but more close to a sinusoidal sape (more levels due to te pase-sifts to te SCR drives). Tus, te current quality is improved in terms of a lower THD, as it is given in te armonic distribution plot of Fig. 6(c). Furtermore, te power factor in tis case as been calculated as.91, were compensating devices can be adopted. Additionally, te dynamic performance of te multidrive system is studied, were te system power levels ave experienced step canges from P = [.5, 6, 4, 2] kw to P = [1.5, 2, 4, 6] kw; wile, te firing angles remained te same as te designed (i.e., f = [, 1, 2, 3 ]). Fig. 7 sows te 5

7 Load canged 5 van vbn -5 5 vcn igc iga igb -5 7 vdc,2 vdc,3 65 vdc,1 vdc,4 6 2 irs,4 irs,2 irs,3 1.5 irs, Time (s) Fig. 7 Transient performance of te multi-drive system consisting of four SCRf = [, 1, 2, 3 ], were te subscripts (i.e., 1, 2, 3, and 4) represent te drive number. proportional and integral gains of te PI controller for te dclink voltage are.1 and.1, respectively; te ysteresis band for te ysteresis current controller is 2 A. Te control and modulation algoritms are implemented in digital signal processors. Experimental results are sown in Fig. 6. As it can be seen in Fig. 6, wen more drives are connected and te firing angles are almost linearly dispatced, te THD level of te total grid current can be significantly reduced. In tat case, te grid current will become multilevel. Compared to te single-drive unit, te THD level as been reduced to 1.9% and 8.3% for te tree-drive and four-drive system, respectively. It sould be pointed out tat, in practice, te rectified current levels are not te same (i.e., te loading is not te same among te parallel drives). Tis will inevitably affect te armonic mitigation performance of te system. However, once te number of drives (e.g., N = 1 or 2) is large, te THD of te total current will become independent of te loading wen te firing angles are linearly assigned. In general, te experimental tests demonstrate te effectiveness of te armonic mitigation sceme for multiple drive systems by deliberately dispatcing te firing angles. Fig. 6 Performance (experimental results) of a multi-drive ASD system wit te pase-sifted current control (grid current i ga [5 A/div]; Bottom: Fast Fourier Transform FFT analysis of te grid current i ga [% of fund., 2%/div])): (a) f1 = º -f2 f3 = 42.7º, power factor:.91) and (b) f1 = º -f2 f3 = 33º, f4 = 5º, power factor:.92). armonics at te PCC. Te performance is also compared wit te conventional solution (i.e., only wit te pasesifted current control and te rectified current is controlled as a flat current). Fig. 9(a) sows te sape of te demonstrative modulation signal and its armonic distribution, were it is observed tat te modulation signal selectively mitigates te 7t and te 13t order armonics, wen applied to a single motor drive wit a boost converter in te dc-link (see Fig. 1(a)). However, te total armonic distortion is iger compared to tat of te grid current witout modulation (see Fig. 2(b)). It is also seen in Fig. 9(a) tat te 5t order armonic and te armonics of fivefold te fundamental frequency are te major armonic distortions. Hence, a pase-sift of 36º is applied to te SCR unit, wic sould mitigate te 5t order armonics and all te armonics of fivefold te fundament frequency in te grid currents. Te resultant current sape is sown in Fig. 9(b), were it can be seen tat all te armonics of fivefold te fundament frequency ave been mitigated, and tus a THD of 12% is acieved. It means tat te SCR wit a boost converter in te dc-link can improve te power quality in multiple motor drive systems. In order to furter verify te effectiveness of te armonic mitigation by te SCR-fed drive system wit te customized dc-link modulation sceme, experimental tests ave been 4 DC-link Current Modulation for Selective Harmonic Mitigation Altoug te use of SCRs and boost converters in te dc-link as improved te power quality (e.g., from 31% to around % if f te f potential of te boost dc-link to furter lower te power quality is not exploited. Hence, in tis section, a customized dc-link current modulation is demonstrated on a two-drive system (a DR- and a SCR-fed drive) to furter mitigate te 6

8 Fig. 8 Performance of a two-drive ASD system wit boost converters in te dc-link: (a) armonic caracteristics of te customized dc-link modulation sceme for selective armonic cancellation (i.e., te 7 t and te 13 t armonics) and (b) armonic caracteristics of ideal current sapes of te two-drive system (i.e., a DR-fed and a SCR-fed drive) wen te dc-link modulation sceme (see Fig. 9(a)) is applied to te two drives (a pase-sift of 36º as been applied to te SCR-fed drive). carried out on a two-drive system (a DR- and a SCR-fed drive). Te system parameters are sown in Table 1. Controller parameters are te same as tose for te tests in Fig. 8. Te experimental results are sown in Fig. 1, were one motor is connected to te DR unit according to Fig. 1 and a resistive load is connected to te SCR unit. It can be seen in Fig. 1(a) tat wit te SCR-fed drive included in te multi-drive system, te armonics of te total grid currents at te PCC can be mitigated to some extent by pase-sifting te current drawn by te SCR unit. Here, in te experiments, a pase-sift of 36º as been applied to te SCR unit, wic sould in teory mitigate te 5t order and all te oter armonics of fivefold te fundamental frequency. However, due to te line impedance and quantization errors, te 5t order armonic is not completely cancelled out but lowered significantly by te pase-sift control of te SCR-fed unit, as sown in Fig. 1(a). As a consequence, te total armonic distortion level as been significantly reduced to 16% in comparison to te DR-fed drives (eiter a single-drive or a multi-dr drive system, wose armonic distribution is sown in Fig. 2(b)). Additionally, in order to improve te current quality, a customized modulation sceme tat aims to mitigate te 7t and 11t order armonics as been applied to te two drive units, and te experimental results are sown in Fig. 1(b). Notably, considering te line impedance, te firing angle is sligtly increased to 38 to cancel out te 5t order armonic and all armonics of fivefold te fundamental frequency. Observations from Fig. 1(b) indicate tat te SCR-fed unit togeter wit a customized modulation sceme for te boost dc-link can improve te power quality, like te case sown in Fig. 9(b). Te effectiveness lies in two points: Te SCR-fed system enables te pase-sift control tat can cancel out certain armonics in te total grid currents (e.g., te 5t order and all armonics of fivefold te fundamental frequency), and 7 Fig. 9 Performance (experimental results) of te ASD system wit boost converters in te dc-link (Top: grid voltage v an [2 V/div], grid current i ga [1 A/div], DR input current [1 A/div], SCR input current [1 A/div]; Bottom: Fast Fourier Transform FFT analysis of te grid current i ga [% of fund., 2%/div]): (a) only wit te pase- f = 36º, te total power is 6.63 kw and te PF is.93) and (b) wit te pase-sifted f = 38º) and te customized modulation sceme (designed to mitigate te 7 t and 11 t armonics), were te total power is 6.86 kw and te PF is.94. Te boost dc-link results in te possibility to modulate te rectified currents (and tus to sape te grid currents) in order to mitigate specific armonics (e.g., te 7t and 11t order armonics). As a consequence, a THD of 1% as been acieved, as sown in Fig. 1(b). In bot cases, a power factor above.9 (i.e.,.93 and.94, respectively) as been obtained, and as aforementioned, power factor correction devices can be installed per demands. In addition, it sould be mentioned tat, if more drives wit boost converters in te dc-link are considered and customized modulation scemes are employed, te THD level of te grid currents can furter be lowered. If properly designed, te resultant power quality can be competitive to tat acieved by multi-pulse transformer based rectifiers. 5 Conclusion In tis paper, te armonic issues in multiple parallel motor drive systems ave been briefly investigated, and accordingly, a cost-effective armonic mitigation strategy as been proposed for te SCR-fed ASD systems, were te firing angles for te SCR units are designed witin a certain

9 range and ten deliberately dispatced among te parallel drives. Wit te proposed armonic mitigation metod, te THD level of te total grid current as been reduced, wile te power factor can be maintained at a satisfactory level. Simulations and experimental tests ave validated te effectiveness of te proposal, being of load-independent, wic can practically be implemented on a production line witout communication. More important, since boost converters ave been employed in te dc-link in order to increase te controllability of te grid current armonics, customized dc-link current modulation scemes can ten be used to selectively cancel out te armonics. In tat case, te current quality can be improved. Te effectiveness of tis armonic mitigation sceme as also been demonstrated by simulations and experiments. If te customized modulation scemes are applied to multiple drive systems, a low THD level of te grid current can be acieved. 6 References [1] P. K. 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