ieee_harmonicsmitigationinlcifedsmdrives

IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 25, NO. 2, JUNE 2010 369

Harmonics Mitigation in LCI-Fed SynchronousMotor Drives

Bhim Singh, Fellow, IEEE, Sanjeev Singh, Student Member, IEEE, and S. P. Hemanth Chender, Member, IEEE

Abstract—A load-commutated inverter (LCI) fed synchronousmotor (SM) is operated as an adjustable speed drive (ASD) inhigh-power applications. These drives are known as commuta-torless motor (CLM) drives and posses many promising featureslike high efficiency, economic operation, and flexibility of controlin high-power ratings. The CLM drives are used in compressors,blowers, fans, pumps, and mill drives for a range of industries asmining, water treatment plants, chemical, paper, textile, cement,rolling mills, and petrochemical plants. However, the power quality(PQ) problems at ac mains have been the concerns in these drivesas the LCI has front-end thyristor converter injecting harmonicsin the supply. This paper investigates various topologies for themitigation of PQ problems in LCI-fed SM drives using multipulseac–dc converters. A set of hybrid topologies is proposed, whichuse a combination of a passive filter with a multipulse converterto feed CLM. A basis for selection of a suitable ac–dc converter ispresented for PQ improvement at the input mains of the LCI-fedSM drives.

Index Terms—AC–DC power conversion, adjustable speeddrives, load-commutated inverter (LCI), multipulse converters,power quality (PQ) , synchronous motors (SMs).

I. INTRODUCTION

M ANY regulatory standards [1]–[4] regarding the powerquality (PQ) problems have been developed due to in-

creased use of power electronic equipment, and especially acmotor drives in many industrial applications. These drives useconverter–inverter sets consisting of thyristor converters at frontend. These converters are a common source of voltage and/orcurrent harmonics, and create many problems for power utilities[5]–[10] . Therefore, suitable measures are required for mitiga-tion of these harmonics. One of very popular methods is to usepassive or active filters. Some standards [11], [12] have recom-mended the use of filters and transformers.

Synchronous motors (SMs) with speed control are very pop-ular in high-power and variable speed applications as they areeconomic alternative at high-power levels [13]–[15]. A load-commutated inverter (LCI) uses the load voltage with a leadingpower factor (PF) for natural commutation of thyristors [16].

Manuscript received June 02, 2009; revised August 16, 2009 and August 16,2009; accepted November 24, 2009. Date of publication February 08, 2010; dateof current version May 21, 2010. Paper no. TEC-00220-2009.

B. Singh and S. Singh are with the Department of Electrical Engineering,Indian Institute of Technology Delhi, New Delhi 110016, India (e-mail: [email protected]; [email protected]).

S. P. Hemanth Chander is with the Delta Energy Systems (India) Pvt. Ltd.,Gurgaon, Haryana 122001, India (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TEC.2009.2038369

The ideal load for the LCI is a SM operating at a leading PF [17], [18]. Therefore, the load commutation makes the drive systemsimple and reliable. However, the LCI-fed overexcited SM hasproblems at low speeds and at starting due to low back electro-motive force (EMF) across stator terminals. One of the simplestmethods is pulsed starting in which LCI thyristors are commu-tated by interrupting the dc link current [18 ].

The PQ concerns are more prominent in LCI-fed SM drivesbecause of their high-power ratings. The passive wave shapingtechniques are normally used, which are based on magnetics inthree-phase ac–dc converters and one of such systems is knownas multipulse or multiphase converters [19]–[24]. There aremany configurations of multipulse ac–dc converters (MPCs) in12 to large number of pulses [25]–[29].

Recently, many MPCs are reported for PQ improvement. Ituses multiwinding transformers [30]–[34] at the input of the rec-tifier, which results in higher pulses in the dc output, thereby re-duction in the ripple. This ac–dc converter draws a current fromac mains having a number of steps with its waveform close toa sinusoidal. The higher pulses eliminate the need of filteringat the rectifier end, and reduces the problems at the inverterend, namely a high inverter commutation angle and additionalrotor-heating and pulsating electric torque in the motor [30 ],[33].

This paper deals with various solutions for mitigation ofpower quality problems in LCI-fed SM drive and provides abasis for selection of a suitable solution for an application.

II. STATE-OF-THE-ART

The LCI is one of the earliest inverters developed for ad-justable speed drives [35 ]–[38]. It mainly consists of a con-trolled rectifier, which feeds an adjustable dc current , viaa dc inductor to a LCI. Since the thyristor does not haveself-extinguishing capability, it can be commutated by the loadvoltage with a leading PF [36]. Fig. 1(a) shows a basic controlschematic of LCI-fed SM drive operating at a leading PF. Theinverter output current is a quasi-square wave. However, themotor voltage waveform is close to sinusoidal superim-posed with voltage spikes caused by thyristor commutations.Hence, the motor current contains low-order harmonics, suchas the 5th, 7th, 11th, 13th, etc. These harmonic currents causetorque pulsations as well as additional power losses in the motorand associated system [17], [18].

The LCI-fed SM drive features low cost and high efficiencydue to the use of low-cost thyristors. The LCI is suitable for largedrives with a power rating in megawatts, where the initial invest-ment and operating efficiency are of great importance. However,the input PF of the drive changes with its operating conditions.

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370 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 25, NO. 2, JUNE 2010

Fig. 1. Isolated converters for LCI-fed SM drive. (a) Six-pulse LCI-fed SMdrive topology with control loop. (b) 12-pulse converter topology with a passivefilter. (c) 18-pulse converter topology. (d) 24-pulse converter topology.

In addition, the rectifier input current is highly distorted, there-fore, a LCI-SM drive should be equipped with harmonic filtersor any other compensation device to reduce line current totalharmonic distortion (THD) [18 ].

The PQ improvement is achieved by using many waveshaping techniques. Usually tuned filters are used in the passivewave-shaping techniques, but these filters cannot be designedfor all the harmonics. Therefore, hybrid wave-shaping methodsand some other passive topologies can be thought of as a fea-sible solution. The passive wave-shaping topologies can furtherbe classified as passive tuned filters and MPCs. Various hybridcombinations of both these topologies can also be used for

PQ improvement. Conventional three-phase SM-fed from twoLCIs using a phase-shifting transformer or six-phase SM-fedfrom two LCIs have also been reported [18] for reduction oftorque ripples and harmonics.

There are several publications on the benefits of LCIs[39]–[46] and MPCs [47]–[53] separately in various ap-plications. However, very few publications [44], [54], [55]are available regarding the PQ improvement using MPCsin LCI-fed SMs. Moreover, there are many applications ofLCI-fed SM drives reported in the literature [56]–[65]. LCIshave also been used to start the SMs driving heavy torque loadso that the ac mains current at the starting could be reduced, andthereafter, the motor runs direct on line [63]. An LCI also findsapplications in ships as a frequency converter, which convertsthe variable generated voltage to a fixed frequency for the ship'smain distributions system [41].

III. MPCS FOR PQ IMPROVEMENT

A MPC consists of magnetics, solid-state devices, and en-ergy storage elements. These MPCs are developed using thyris-tors and magnetics through auto-connected, multiwinding trans-formers, and interphase transformers (IPTs), tapped reactor andadditional thyristors, and capacitors with the concept of pulsemultiplication to get higher pulses starting with 12 pulse to 18,24, 30, 36, 40, etc. [30], [66]–[70].

MPCs use phase shifting through transformers to convert theoriginal three-phase ac supply to multiphase ac supply. This re-sults in higher pulses in dc output, thereby, reduction in rippleand high number of steps in ac mains current close to sinusoidalwith reduced THD. Fig. 1 shows isolated transformer-basedthyristor converter topologies for 6-, 12-, 18-, and 24-pulse con-verters supplying LCI-fed SM drive, and Fig. 2 shows noniso-lated transformer (which are known as autowound transformers)based topologies for 12-, 18-, and 24-pulse thyristor converterssupplying LCI-fed SM drive.

The isolated MPCs mostly use multiwinding transformersat the input of the converter. The ratings of these transformersare equal to load rating; therefore, size and weight are quitehigh. However, by using autotransformers (in nonisolatedconverters), the size, cost, weight, and losses of magneticcomponents can be reduced drastically [30]–[33]. These MPCsuse different winding connections e.g. star, delta, zigzag,polygon, hexagon, T-connection, tapped winding, and plu-rality of winding of transformers so that desired phase shift isachieved to eliminate or reduce harmonics in input ac mainscurrent [30]–[33]. The rating of autotransformers in nonisolatedtopology can be reduced to less than 25% of the load rating [28].Various combinations of star, delta, and zigzag connections areused in the converter topologies shown in Fig. 1, whereas Fig. 2shows various combinations of delta, polygon, and hexagonautotransformer connections.

The techniques of pulse multiplication on dc bus, optimumdc link reactor, and active interphase reactor on dc side, havefurther impact on the rating and size of input transformer. Pulsemultiplication topology [30]–[33], [66]–[70] generates higherpulses in the multiples of 12-pulse, i.e., 24, 36, and 48 pulsesare generated using a reactor and two, three, or four thyristors,

SINGH et al.: HARMONICS MITIGATION IN LCI-FED SYNCHRONOUS MOTOR DRIVES 371

Fig. 2. Nonisolated thyristor converters for LCI-fed SM drive. (a) Six-pulseconverter topology. (b) 12-pulse converter topology with a passive filter. (c)18-pulse converter topology. (d) 24-pulse converter topology. (e) 24-pulse (12� 2) converter topology (pulse multiplication).

respectively [54]. For nonisolated MPCs based on pulse mul-tiplication technique, additional zero-sequence blocking trans-former (ZSBT) and IPT are used [33].

Many attempts have been made to reduce size, weight, andcost of magnetics in MPCs. The THD of ac mains current dras-tically reduces if the value of dc link inductor and leakage reac-tance of input transformer is selected optimally [30]. MPC tech-nology is considered superior to pulsewidth modulation (PWM)

converter technology because it not only eliminates specific har-monics but also reduces other harmonics. It also reduces elec-tromagnetic interference (EMI), radio frequency interference(RFI), and switching losses due to low-frequency soft switchingcaused by line commutation, resulting in high efficiency and lownoise level in the converter system [32]. MPCs have been re-ported to reduce THD of ac mains current well below 2% oraround and almost ripple free dc output voltage to feed varietyof loads with various configurations of ac–dc converters [19] .But these MPCs have the drawback of higher dc link voltage ascompared to six-pulse converters, which makes them nonsuit-able for retrofit applications [31].

Passive filters along with controlled rectifiers have been ex-tensively developed [31]–[33], [53], [71] in high-power ratingand already existing installations. The use of tuned passive fil-ters with thyristor MPCs reduces the notches in the input voltagewaveform due to commutation overlap and provides the reactivepower [33], [53]. The reduction in current THD, improved PFat partial loads, and reduced magnetics rating are some advan-tages of passive tuned filters [31], [72].

Various multipulse topologies [30] have been reported forharmonic mitigation and the quantified ac mains current THDvaries from 10.45% to 13.9% for load variation from 100%to 20% in a 12-pulse diode rectifier, whereas, in case of a12-pulse thyristor rectifier the current THD varies from 12.89%to 15.37% for 100% to 20% load variation. Moreover, use ofhigher number of pulses for improvement in current THD andPF is also suggested for variety of applications.

There are many efforts reported [45], [54], [55], [70] for har-monics mitigation using 12-pulse converters, but none of themquantify the THD of ac mains current and PF. However, har-monic spectra of ac mains current have been shown to demon-strate the elimination of 5th and 7th harmonics in case of a12-pulse rectifier as compared to a 6-pulse rectifier. Moreover,improvement in ac mains current THD has been proposed byincreasing the number of pulses from 12-pulse to 36-pulse [54],[55 ]. The harmonic spectrum of ac mains current of a 12-pulserectifier shown in [70] demonstrates around 10% magnitude ofharmonics.

The THD of ac mains current in 12-pulse diode rectifier hasbeen reported [50 ] in the range of 8.2% to 12.15% with PFvariation from 0.975 to 0.976 for load varying from 100% to20%, respectively. To reduce the THD of ac mains current, an18-pulse rectifier is proposed and resultant THD of ac mainscurrent have been presented in the range of 3.8% to 5.5% for100% to 20% load variation, respectively. Moreover, the THDof ac mains current is reported in the range of 8.31% to 18.72%for load varying from 90% to 20% for a 12-pulse converter [69].

For improved THD of ac mains current using 12-pulsethyristor converters, a modified control is used [73], so thatthe synthesized ac mains current is approximately sinusoidal.The resultant current THD is reported less than 1%, but thereis no mention of PF under these conditions. The THD of acmains current have been reported [74] in the range of 10.1%to 17% for load variation of 100% to 20% in a 12-pulse dioderectifier. For improvement in ac mains current THD, use ofa single-phase square wave auxiliary voltage supply in themiddle dc bus has been suggested in the literature [75], [76].


The improvement in the ac mains current THD have beenreported to 4.64% for a 12-pulse thyristor rectifier [75] and4.9% in case of a 12-pulse diode rectifier [76]. In another effortfor reduction of current THD on the ac side of a 12-pulseseries-connected line-commutated ac/dc rectifiers, a controlinvolving switching of two IGBTs for improved shaping of thedc current is presented [77]. Moreover, all these strategies [73],[75]–[77] require additional circuitry/algorithm for the controlof the auxiliary arrangements other than 12-pulse converter.The PF shall still be poor due to reactive power burden at largefiring angles.

Most of the reported paper [30], [44], [54], [55], [69] forPQ improvement of LCI-SM drive use either a 24-pulse con-verter employing pulse multiplication with three-phase SMor a 12-pulse converter with six-phase SM [45]. The MPCswith pulse multiplication have complex control whereas thesix-phase SM is costly and needs special design. This paperuses a combination of the passive filter and a 12-pulse converter[shown in Figs. 1 (b) and 2(b)], which facilitates reduction incurrent THD, improved PF with reactive power compensationthereby reduced voltampere requirement of the converter,control complexity, and the cost of the system. It also has thebenefit of minimum kilovoltampere rating compared to higherpulse converters.

IV. LCI-FED SYNCHRONOUS MOTOR DRIVE

The conventional LCI-fed SM drive has a six-pulse converterfeeding to a three-phase SM [78]–[80]. However, the multi-pulse concept of current source converters can easily be used fora LCI-fed SM drive, where the ac source can be replaced bythree-phase SM and the natural commutation is substituted byload commutation [18]. However, the operation is limited to a12-pulse converter configuration as the triggering control cir-cuit becomes complicated for higher number of pulses. Therecan be two topologies for 12-pulse LCI operation. One topology[shown in Fig. 3(a)] uses phase-shifting transformers, whichcombines the 2 six-pulse LCI outputs to get a three-phase supply(12-pulse converter) for a conventional three-phase SM. Thesephase shift transformers are very expensive but has an advan-tage of being used with a conventional three-phase SM. Othertopology [shown in Fig. 3(b)] uses an asymmetric six-phase SMin which two sets of three-phase windings displaced at an angleof 30 are employed [18], [45]. These topologies of SM can befed from any rectifier set discussed earlier to meet the PQ stan-dards, provided the cost, control complexity, and efficiency arewithin acceptable limits.

V. DESIGN OF MULTIPULSE AC–DC CONVERTERS FOR

LCI-SM DRIVE

An n-pulse ac–dc controlled converter operates on the prin-ciple of harmonic elimination by allowing the flow of harmoniccurrents through transformers (isolated or nonisolated) requiredby one bridge to be supplied by another, however, the individualharmonic current of each bridge converter remains the same[31]–[33 ]. The minimum order of harmonics in an n-pulse con-verter is nK 1, where K is a positive integer and n is the numberof rectification pulses per cycle of the fundamental voltage. Thephase shift required for a converter having x number of six-pulse

Fig. 3. Six-pulse converter-fed 12-pulse LCI-SM drives. (a) Three-phase con-verter topology. (b) Six-phase converter topology.

converters is 60 /x [19]. The fact that the negative-sequencevoltages and currents are shifted in the opposite sense to posi-tive-sequence values also provides a mechanism to cancel har-monics in pairs.

A. 12-Pulse AC–DC Converters

A 12-pulse ac–dc converter requires phase shift of 30 ,which can be achieved by two configurations, namely isolatedstar–delta transformer having 0 and 30 phase shift or isolatedstar–zigzag transformer combinations with phase shift of –15

and 15 w.r.t supply voltage. The isolated star–zigzagtransformer combination is balanced compared to the star–deltatransformer combination [32]. The turns ratios for Y/Z – 1and Y/Z – 2 transformers to provide 15 and –15 phaseshift, respectively, are given by and

, where is number of turns perphase in star winding, and N , N are number of turns of thezigzag winding. The detailed design of these transformers hasbeen given in Appendix B [see Figs. 1(b) and 2(b)].

For nonisolated topologies, the autotransformer may havevarious connections for 15 phase shift e.g., polygon,delta-polygon, zigzag, and T-connected transformers [31]–[33].The delta-polygon connected autotransformer (detailed designhas been given in Appendix C) is used in this investigation.

B. 18-Pulse AC–DC Converters

The transformer design for an isolated 18-pulse ac–dc con-verter configuration requires 20 , 0 , and –20 phase shift,which is achieved by the use of two zigzag and a star windingin the secondary side of the transformer. The turns ratio of pri-mary star winding to secondary star winding is 3:1, whereas, theturns ratio for Y/Z – 1 and Y/Z – 2 transformers to provide 20and –20 phase shift, respectively, is given by

and , where is number of turnsper phase in star winding, and N , N are number of turns of the


zigzag winding. The detailed design is given in Appendix B [seeFigs. 1 (c) and 2(c)].

To achieve 20 phase shift required for an 18-pulse con-verter with a nonisolated topology, the autotransformer mayhave various connections for e.g. polygon, delta-polygon,zigzag, and T-connected transformers [31]–[33]. Thedelta-polygon connected autotransformer (detailed designhas been given in Appendix D) is used in this investigation.

C. 24-Pulse AC–DC Converters

A 24-pulse ac–dc converter requires –15 , 0 , 15 , and30 phase shift among four sets of voltages. For isolated trans-

former design, it is achieved by the use of two zigzag, one star,and one delta configurations in the secondary windings. Theturns ratio for Y/Z – 1 and Y/Z – 2 transformers to provide 15and –15 phase shift, respectively, is similar to a 12-pulse con-verter phase-shift transformer, for which the detailed design isgiven in Appendix B [see Figs. 1(d) and 2(d)].

However, for nonisolated converter to generate 15 and30 phase shift, a hexagon-connected autotransformer (detaileddesign has been given in Appendix E) is selected among variousother connections, e.g., star, delta, hexagon, and T-connectedconfiguration.

D. 24-Pulse (12 2) AC–DC Converter

A 24-pulse ac–dc converter shown in Fig. 2(e) using pulsemultiplication requires a 12-pulse ac–dc converter (as discussedearlier) followed by a pulse multiplication or ripple reinjectioncircuit. The pulse multiplication circuit mainly consists of aZSBT and an IPT. The ZSBT is smaller in size, volume, andweight as it contains only triple frequency components. It offersvery high impedance to the zero-sequence current and helps inindependent operation of two rectifier bridges. An IPT is themain component for pulse doubling, which uses two thyristorsonly. The detailed design of ripple injection circuit is given inAppendix F [see Fig. 2(e)].

E. 12-Pulse AC–DC Converter With Tuned High-Pass Filter

A combination of a second-order damped passive filter tunedto 11th order harmonics and a high-pass shunt passive filteris used at the input of a 12-pulse ac–dc converter to feed theLCI-SM drive. The passive filters have been designed for 11thorder harmonic and higher order harmonics separately andconnected in parallel in case of 12-pulse converter. For higherpulse converters, passive filter tuned to other frequencies shallbe used, e.g., 17th and high-pass filter for an 18-pulse converterand 23rd and high-pass filter in case of 24-pulse converter.The design of a 12-pulse ac–dc converter is same as discussedearlier, whereas, the high-pass filter elements are designedusing equations given in Appendix G.

VI. MODELING OF LCI-FED SM DRIVE

The adjustable speed LCI-fed SM drive system includes acurrent-control loop inside a speed-control loop, where the dccurrent is controlled by the current-control loop to follow a cur-rent reference given by the speed-control loop. Therefore, themodeling of the speed controller and a current controller forms

a vital part of the drive modeling, and thereby, the response ofthe drive system.

The speed controller is a proportional and integral (PI) con-troller. Thus, the speed error – is transformed intocurrent reference through transfer function

(1)

where and are the speed con-troller gain and T is the speed controller integral time constant.

The current controller is also a typical PI controller, throughwhich the current error is transmitted to the rectifier throughtransfer function

(2)

where , and are the current con-troller gain and T is the current controller integral time con-stant.

The integral time constant of the current controllershould be considerably smaller than that of the speed controller

, approximately).The output of the current controller ( ) is used to control the

firing angle of the controlled rectifier. The operation of LCI-SMdrive can be controlled using constant commutation lead angleand constant margin angle strategies [17] , [18]. However, theoperation of the LCI-SM drive at the minimum margin angleneeded for safe commutation results in highest PF at the motorterminals and the best utilization of its windings [32]. There-fore, constant margin angle control of LCI-SM is used in thisstudy for performance evaluation of various PQ improvementtopologies. Moreover, for starting of these drives, a pulsed con-trol scheme is used, which is switched on to LCI mode whenthe motor attains sufficient speed such that the back EMF of themotor reaches suitable value capable of load commutation [14].

VII. PERFORMANCE OF VARIOUS CONVERTER TOPOLOGIES

The proposed converter topologies are designed and mod-eled for an 85 kW synchronous motor drive (data is given inAppendix A) in the MATLAB/Simulink environment. Fig. 4(a)shows the supply current waveform and harmonic spectrum ofthe six-pulse isolated thyristor converter-fed LCI-SM drive. Itshows 28.82% THD of ac mains current at rated load with 1.37crest factor (CF). Their current THD reduces sharply with in-crease in converter pulses and reaches 1.22% at rated load with1.41 CF for an isolated 24-pulse converter. Fig. 4 shows thesupply currents and their harmonic spectra of the LCI-SM drivefor 6, 12, 18, 24-pulse controlled converters at rated load. Fig. 5shows the supply currents and their harmonic spectra of theLCI-SM drive for nonisolated MPCs with rated load. The THDof supply current reduces to 5.09% for a 12-pulse converter caseat rated load and reaches 3.98% in case of a nonisolated 24-pulseconverter.

The performance of various topologies shows that thesix-pulse converter (isolated and nonisolated) based LCI-SMdrive is having a poor power quality, both in terms of currentharmonic distortion and PF during the total operating range.


Fig. 4. Supply current waveforms and harmonic spectra of isolated converter-based LCI-fed SM running at rated speed and rated torque. (a) Six-pulse con-verter [see Fig. 1(a)]. (b) 12-pulse converter [see Fig. 1(b)]. (c) 18-pulse con-verter [see Fig. 1 (c)]. (d) 24-pulse converter [see Fig. 1(d)].

Fig. 6 shows current waveforms at ac mains and its harmonicspectra for 12-pulse converters (isolated and nonisolatedtopologies) with shunt passive filter feeding LCI-SM driveoperating at full load and half load. Amongst, different isolatedand nonisolated MPCs, the 12-pulse converter is able to achievethe power quality within limits of international standards[1]–[4] with a passive filter only. However, the 18 and 24-pulseconverters have resulted in lower current harmonic distortion atac mains. But the PF of ac mains during fractional speeds stillremains poor. The performance of these converter topologiesis summarized in Tables I and II in terms of PQ indices forrated and half the rated load. The performance evaluation ofthe proposed converter topologies show consistent results inthe wide range of speed and meet the desired PQ specifications.However, the results have been recorded at rated and halfthe rated speeds with rated torque just for the performancecomparison of various converter topologies at some commonreference.

To achieve improved power quality in LCI-SM drive, the12-pulse converter topology has been simulated at rated loadwith passive tuned filters and it is observed that this topologyshows consistent improved power quality in wide range of op-eration. The design of the shunt passive filter has been aimed tosupplement the reactive power requirement of the drive duringwide speed range. The variation of reactive power from lightload to full load is reasonably large and a 12-pulse converterwith a passive filter shows consistently improved PQ in thetotal load range.

Fig. 5. Supply current waveform and harmonic spectra of nonisolated con-verter-based LCI-fed SM running at rated speed and rated torque. (a) Six-pulseconverter [see Fig. 2(a)]. (b) 12-pulse converter [see Fig. 2(b)]. (c) 18-pulse con-verter [see Fig. 2 (c)]. (d) 24-pulse converter [see Fig. 2(d)].

Fig. 7 shows the variation of the current THD and PF withload at ac mains for isolated 12-pulse converter topology withand without shunt passive filters. The detailed power quality pa-rameters with load variation (rated torque and variable speed)are also given in Tables III and IV for a 12-pulse converter withand without passive filters, respectively. It is observed that thecurrent THD at ac mains of a 12-pulse converter with shuntpassive filter remains below 8% in the 20%–100% load range.The PF variation of the 12-pulse converter with shunt passivefilter shows consistent improvement as compared to results ofthe same topology without filter and it remains in the range of0.85–0.94 from light-load to full-load condition. The voltageTHD is also reduced to less than 1% with the 12-pulse convertertopology using passive filters. The rms current at ac mains is alsoreduced drastically at light load, however, the reduction is ob-served in the wide load range, i.e., 36.5–136.5 A from light-loadto full-load condition against very high values (122–143 A fromlight load to full load) for the same topology without filter asshown in Tables III and IV . Therefore, a 12-pulse convertertopology with passive shunt filter is considered a good optionfor constant torque loads operating in variable speed range.

VIII. APPLICATION POTENTIAL

The multipulse ac–dc converter-fed LCI-SM drive has avast application potential in various large rating adjustablespeed drives. The major applications include roller mills, largecompressors, crushers, conveyors, industrial fans and pumpsin cement, steel, pulp and paper, petrochemicals, mining,


Fig. 6. Supply current waveform and harmonic spectra of 12-pulse converter-based LCI-fed SM drive with shunt passive filter. (a) Isolated topology at ratedload [see Fig. 1(b)]. (b) Isolated topology at half of the rated load [see Fig. 1(b)].(c) Nonisolated topology at rated load [see Fig. 2(b)]. (d) Nonisolated topologyat half the rated load [see Fig. 2(b)].

TABLE IPOWER QUALITY INDEXES OF VARIOUS MPC-BASED LCI-SM DRIVE AT

RATED SPEED (1500 ��) AND RATED TORQUE

TABLE IIPOWER QUALITY INDEXES OF VARIOUS MPC-BASED LCI-SM DRIVE AT HALF

THE RATED SPEED (750 ��) AND RATED TORQUE

Fig. 7. Variation of PF and current THD with load for isolated 12-pulse con-verter-fed LCI-SM drive at rated torque and 1000 r/min speed.

TABLE IIIPOWER QUALITY INDEXES WITH LOAD VARIATION (VARIABLE SPEED AND

RATED TORQUE) FOR A 12-PULSE CONVERTER-FED LCI-SM DRIVE WITH

PASSIVE FILTER

TABLE IVPOWER QUALITY INDEXES WITH LOAD VARIATION (VARIABLE SPEED AND

RATED TORQUE) FOR A 12-PULSE CONVERTER-FED LCI-SM DRIVE WITHOUT

PASSIVE FILTER

chemical and process industries [56]–[65]. The combination ofthe multipulse converter system with passive filters providesimproved PQ at ac mains while fulfilling the load requirement.The 12-pulse converter with a passive filter has magnetics(i.e., voltampere rating) comparable with the 18- and 24-pulseconverters, whereas the control complexity and the cost of theconverter are reduced considerably. It is used in a wide range ofapplications including large industrial loads, e.g., heating, ven-tilating and air conditioning (HVAC)/air compressors, crushers,grinders, rollers, dryers, conveyors, water/fluid/effluent pumps,and induced draft (ID)/forced draft (FD) fans.


IX. CONCLUSION

The detailed design of converter topologies for power qualityimprovement in LCI-SM drives has been presented to providea clear perspective on various aspects of these drives to the re-searchers and engineers working in this field. Multipulse con-trolled converters with isolated and nonisolated topologies havebeen found capable of providing the desired power quality forlarge rating industrial loads. A combination of passive filters and12-pulse converter has been proposed for the LCI-SM drive,which shows improved performance with reduced THD andmagnitude of ac mains current. The 12-pulse converter withshunt passive filter has added advantages of simple control andconsistently improved PF in the wide operating speed range ofthe drive. These converter topologies may be a good candidatefor many applications in near future with cost-effective solution.It is hoped that this investigation on various PQ improvementtopologies for LCI-SM drives is a useful reference to the usersand manufacturers.

APPENDIX IAPPENDIX

A. Synchronous Machine Parameters Used for Simulation

Nominal power: 85 kVA, nominal voltage: 400 V, nominalfrequency: 50 Hz, no-load field current: 10 A, stator armatureresistance: 0.055 ohm, stator leakage inductance: 0.3595 mH,d-axis mutual inductance: 12.82 mH, q-axis mutual induc-tance: 5.692 mH, field resistance referred to stator: 0.03634ohm, field leakage inductance: 1.302 mH, damper-windingparameters: the d-axis resistance ohm,leakage inductance mH, the q-axis resistance

ohm, leakage inductance mH,inertia , friction factor 0.07 Nm·s, pole pairs

2, rotor type: salient pole, wound field. Source impedance:0.03 pu, transformer leakage impedance: 0.03 pu, dc linkinductor: 15 mH. Gains of PI controllers: ,

s, , s.

B. Design of Phase-Shifting Transformers

Y/Z – 1 transformer: From phasor diagram shown in Fig. 8(a)

for (3)

and

(4)

In a balanced system , so the relation becomes

(5)

Similarly, the following relation can be derived usingFig. 8. Schematic and phasor diagrams of various components of MPCs. (a)Y/Z – 1 topology. (b) Y/Z – 2 topology. (c) Delta-polygon ( �15 ) auto-transformer topology. (d) Delta-polygon (�20 ) autotransformer topology. (e)Hexagon autotransformer topology. (f) IPT for pulse doubling. (g) Passive fil-ters.


TABLE VTURN RATIOS OF STAR/ZIGZAG TRANSFORMERS FOR VARIOUS PHASE-SHIFT

ANGLES

(6)

These ratios are summarized in Table V for a given value of.

Y/Z – 2 transformer: Following the similar procedure as inprevious case, we can get from the phasor diagram of Fig. 8(b)

for (7)

and

(8)

In a balanced system , so the relation becomes

(9)

Similarly, the second relation can be derived as follows:

(10)

Table V summarizes the turn ratios for a given value of . Theratio ( ) has been taken as two, three, and four for 12-,18-, and 24-pulse transformers, respectively, in this paper.

C. Design of Delta-Polygon Connected Autotransformer for12-pulse Converter Operation

Fig. 8(c) shows connection and phasor diagrams of a delta-polygon connected autotransformer for producing desired phaseshifts. The number of turns (shown in Fig. 8(c) as , and

) required for achieving these phase shifts among differentphases can be calculated by considering the voltages of phase“a ” given by two sets of equations as follows:

(11)

(12)

where , ,– , , , V is rms value of

phase voltage and 2K K K 1. These equations resultin , , and .

D. Design of Delta-Polygon Connected Autotransformer for18-pulse Converter Operation

Fig. 8(d) shows connection and phasor diagrams of a delta-polygon connected autotransformer for producing desired phaseshifts. The number of turns required for achieving these phaseshifts among different phases as shown in Fig. 8(d) are given by

, , and . The input phase, and are connected to the output directly as one set

of voltages. The remaining two sets of voltages for phase “a”are given by (11) and (12), where ,

, – , , ,V is rms voltage/phase.

E. Design of Hexagon-Connected Autotransformer for24-pulse Converter Operation

Fig. 8(e) shows connection and phasor diagrams of ahexagon-connected autotransformer for producing desiredphase shifts. The number of turns (shown in Fig. 8 (e) as

, and ) required for achieving these phase shiftsamong different phases can be calculated by considering thevoltages of phase “a ” given by four sets of equations as follows:

(13)

(14)

(15)

(16)

where , , ,, , ,

and V is rms value of phase voltage. These equations result in, , , and

.

F. Design of Pulse-Multiplication Circuit

Fig. 8(f) shows the schematic diagrams of pulse-doublingcircuit for thyristor converters using an IPT connected to twothyristors [33].

The voltage appearing across the reactor winding is anac voltage ripple of six times the source frequency, resultingin smaller size weight and volume of the IPT [30], [31], [33].When , the thyristor is forward biased and can befired. In this mode, the current and MMF relationship are givenby following equations:

(17)

(18)

(19)


where is the total number of turns in the IPT, whereas, and are the number of turns as shown in

Fig. 8(f). From the aforementioned relations, the output cur-rents of the two bridges can be given as follows:

(20)

(21)

where ; k 0 signifies the center-tapped trans-former and 12-pulse converter operation. When , thethyristor is forward biased and can be fired. The output cur-rents of the two bridges are given as follows:

(22)

(23)

The thyristors and are fired in a specific sequence[38] in coordination with the firing of converters. The thyristors

and constitute three legsof first and second converter, respectively. The ratio k is kept at0.246 to eliminate the harmonics up to 23rd order [33].

G. Design of High-Pass Filter

Fig. 8(g) shows the schematic diagrams of second-orderdamped high-pass filter [31].

The impedance of the filter at any harmonic h is given by

(24)

(25)

where Q is the quality factor of the passive filter. The value ofQ has been taken as 1 in this paper, however, for high-pass filterQ varies between 0.5 to 5.

The capacitance is decided by the reactive power requirementin the circuit, where as the inductance is decided by the fre-quency at which the passive filter is tuned. The resistance forhigh-pass filter can be calculated by

(26)

In this paper, passive filters are designed for 11th harmonictuned shunt filter ( H, F) and high-passfilter (L 0.1 mH, C 600 F, and R 0.4083 ) for a12-pulse converter.

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Bhim Singh (SM’99–F’10) was born in Rahamapur,India, in 1956. He received the B.E. degree in elec-trical from the University of Roorkee, Roorkee, India,in 1977, and the M.Tech. and Ph.D. degrees from In-dian Institute of Technology (IIT) Delhi, New Delhi,India, in 1979 and 1983, respectively.

In 1983, he joined the Department of ElectricalEngineering, University of Roorkee, as a Lecturer,and in 1988 became a Reader. In December 1990, hejoined the Department of Electrical Engineering, IITDelhi, as an Assistant Professor, where he became

an Associate Professor, in 1994 and a Professor, in 1997. His current researchinterests include power electronics, electrical machines and drives, activefilters, flexible AC transmission system (FACTS), high voltage DC (HVDC),and power quality.


Dr. Singh is a Fellow of the Indian National Academy of Engineering, theNational Academy of Science, India, the Institution of Engineers, India, and theInstitution of Electronics and Telecommunication Engineers, a Life Member ofthe Indian Society for Technical Education, the System Society of India, and theNational Institution of Quality and Reliability.

Sanjeev Singh (S’09) was born in Deoria, India, in1972. He received the B.E. degree in electrical fromAwadhesh Pratap Singh University, Rewa, India, in1993 and the M.Tech. degree from Devi Ahilya Vish-wavidyalaya, Indore, India, in 1997.

In 1997, he joined as a Project Officer with theNorth India Technical Consultancy Organisation,Chandigarh, India, in 1997. In 2000, he joined as aLecturer with the Department of Electrical and In-strumentation Engineering, Sant Longowal Instituteof Engineering and Technology, Sangrur, Punjab,

India. His current research interests include power electronics, electricalmachines and drives, energy efficiency, and power quality.

Mr. Singh is a Life Member of the Indian Society for Technical Educationand the System Society of India.

S. P. Hemanth Chender (M’10) was born in My-laram, Warangal, Andhra Pradesh, India, in 1985.He received the B.Tech. degree in electrical andelectronics from Jawaharlal Nehru TechnologicalUniversity, Hyderabad, India, in 2006 and theM.Tech. degree from Indian Institute of Technology(IIT) Delhi, New Delhi, India, in 2008.

In 2008, he joined as an R&D Engineer, DeltaEnergy Systems (India) Pvt. Ltd., Gurgaon, India.His current research interests include power elec-tronics, electrical machines and drives, as well as

switch-mode power supply design for custom design, telecom, network, serverand storage power supplies.

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