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DESIGN AND IMPLEMENTATION OF SHE METHOD TO ENHANCE HARMONIC

MITIGATION BY USING MULTIPULSE CONVERTER

R.RAVIKIRAN

PG scholar,Vijay Rural Engineering College, Nizamabad, Telanagana, India.

SubashRathod

Associate Professor,Vijay Rural Engineering College, Nizamabad, Telanagana, India

A.SUSHEELA

Associate Professor and HOD, Vijay Rural Engineering College, Nizamabad, Telanagana, India

ABSTRACT-The demand of power-electronic devices is increasing in several applications, and power-electronic building blocks (PEBBs) are a strategic concept to increase the reliability of the power-electronic converters and to minimize their cost. A possible solution to increase the power rate of these converters is the use of series or parallel connected PEBBs in multipulse configurations. Magnetic elements, such as zig-zag transformers, phase-shifted transformers (pst), or zero-sequence blocking transformers(ZSBT), are used to interconnect the PEBBs. In this paper, the operation of multipulse converters will be analyzed, describing the harmonic cancellation and minimization techniques that could be used in these multipulse converters, focusing on the power-electronics flexible ac transmission systems devices installed at the NYPA Marcy substation. In order to improve the dynamic response of this system, the use of selective harmonic eliminationmodulation is proposed and implemented. Index Terms—AC–DC power conversion, power conversion harmonics, power filters, pulsewidth modulation. I.INTRODUCTION IN RECENT years, the development of self-commutated switches and multilevel topologies have allowed increasing the power rate of voltage-source converters (VSCs). Due to the flexibility and controllability of the VSCs, they are used in flexibel ac transmission systems (FACTS) applications, such as STATCOMs or synchronous static series compensators (SSSCs). Some objectives of these kinds of installations are to control the power flow and ensure

voltage stability of the utility grids [1]. Due to the fact that the power rate of the power-electronic devices tends to increase, high-power VSCs are needed. On the one hand, the use of multilevel converters [2]–[4] is a suitable alternative to design high-power electronic converters. On the other hand, the power-electronic building blocks (PEBBs) can be associated, generally by using magnetic elements, in order to increase the power rate of the converters One of the problems in FACTS applications is the outputvoltage harmonic quality. The harmonic content of the voltagemust satisfy the legislation requirements at the point of commoncoupling (PCC). Multipulse converters are used to improvethe output voltage quality without increasing the switchingfrequency. In high-power applications, full-wave modulation iscommonly used, where the switching frequency has the samevalue of the fundamental frequency of the output voltage. The phase-shifting transformers (PST) are used to change thephase between primary and secondary voltages. A simple PSTis the well-known wye-delta transformer. If the primary side isconnected in wye and the secondary side is connected in delta,the fundamental component of the voltage in the secondary sideis going to lead by 30 of the fundamental component of the primary-side voltage. The use of a wye-delta and wye-wye transformers where the wye sides are connected in series is very extended[8] in order to obtain a 12-pulse converter. Thewye-delta transformers change the phase by 30 or 30 ,depending on the connection type. It is possible to obtain differentphase-shifting angles by using a zig-zag transformer

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insteadof a wye-delta transformer. In order to cancel out voltageharmonics, a determined phase-shifting angle can be achievedby using zig-zag transformers. Apart from PST, other magnetic devices that are used inorder to increase the output voltage quality are the line interfacetransformer (LIT), interphase transformer (IPT), zero-sequenceblocking transformers (ZSBTs), and auxiliary magnetic circuits[9]. The FACTS device installed at the New York Power Authority(NYPA) Marcy substation is presented in Section II.Three different techniques are used in this device in order toreduce the output voltage harmonic content: the harmonic cancellation, the harmonic minimization, and the use of ZSBTs.In order to control the fundamental amplitude of the outputvoltage, the dc bus voltage is controlled, exchanging activepower with the grid. In [10], a comparison between 24-pulse VSC and quasi24-pulse VSC is shown. In the first one, 11th, 13th, 35th, and37th harmonics are cancelled whereas in the second one, theyare minimized. These cancellation and minimization techniquesare going to be described in Sections III-A and B, respectively. In Section III-C, the ZSBT is explained which is used to cancelout triplen-order harmonics. Harmonic reduction techniques explained before are used toobtain quasi 48 pulses in the output voltage of the studied applicationthat is described in Section IV.

Fig. 1. Power circuit of the shunt-connected and series-

connected inverters In Section V, the use of the selective harmonic elimination (SHE)modulation is proposed, instead of full-wavemodulation. With SHE modulation, the amplitude of the fundamental component of the output voltage is defined by control, whereas with full-wave modulation, the active power must be exchanged between the VSC and the grid in order to control its amplitude.

II. DESCRIPTION OF A REAL APPLICATION A convertible static compensator implementation at the NYPA Marcy Station is presented in [10]. The nominal power of the converter is 100 MVA and it is connected to a 345-Kv transmission line. Twelve gate turnoff thyristor (GTO) poles are used, and the dc bus voltage is 12 kV. Fig. 1 depicts the VSC configuration used as a FACTS application. It consists of 12 NPC PEBBs connected in parallel in the dc side to a common dc bus. An intermediate zig-zag transformer is used as a PST, eliminating some undesired harmonics. The VSC is connected in shunt or series to the grid depending on the output transformer.When the VSC is connected through the transformer , the FACTS works in shunt connection. Otherwise, if the output transformer is the instead of , it works in a series configuration. The output voltage of the converter has quasi 48 pulses. The amplitude of the output voltage is controlled by changing the voltage amplitude of in the dc side.When has to be increased, active power is consumed from the grid, and if thevalue of has to be decreased, active power is injected intothe grid. The drawback is the relatively slow dynamic response of the output voltage amplitude due to the necessity of exchangeactive power to control the amplitude of the output voltage [11],[12]. One advantage of this converter configuration is that withfull-wave modulation, an output voltage of quasi 48 pulses canbe obtained. Another advantage is that the currents of all PEBBsare balanced because they are connected in series at the ac side.This means that magnetic elements, such as IPTs, are not necessaryfor balancing the power delivered by the PEEBs .

III. HARMONIC ELIMINATION IN MULTIPULSE MODULATION

The objective of the harmonic elimination is to cancel orminimizecertain undesired harmonics at the output voltage of theconverter, using common VSCs connected in multipulse configurationand workingwith full-wavemodulation. Thismeans that several harmonics could be cancelled out without increasing theswitching frequency of the converters

A. Harmonic Cancellation by Using the Phase-ShiftingTransformers

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In a three-phase system, the voltage of each phase can be represented as follows:

푉 = ∑ 푉 푉 = ∑ 푉 ∙ 푒( ∏ . (1) Depending on the harmonic order ( ) of the output voltage,they can be classified as positive-, negative-, or zero-sequenceharmonics. In a three-phase system, the positive-sequence harmonicsof lag ; for negative-sequence harmonics, leads; and for zero-sequence harmonics, the three phases are inphase.Harmonics of order are classified as positivesequenceharmonics. Harmonics of order are classified as negative-sequence ones, and harmonics of orderare classified as zero-sequence harmonics. Note that the phase-shift angle of the phase-to-phase voltage(obtained by subtracting to ), is different for the positive,negative, and zero sequences

(2) For positive-sequence voltages (for example, when ),the phase-to-phase voltage lags by 30 . Nevertheless,for negative-sequence voltages (for example, when 5)leads by 30 . Common-mode voltages are eliminated in(2) because all the triplen harmonics are in phase. This difference in phase shifting between positive and negativesequences can be used to cancel specific harmonics [16]. A wye-delta transformer can be considered as a phase-shiftertransformer (PST) because it changes the phase of the harmonicsby . However, it is possible to build a PST whichshifts the harmonic phase by a desired angle as shown in Fig. 2.In this figure, a zig-zag transformer configuration is depictedwhere the phase-shift angle is defined by the turns ratio N1/N2.

Fig. 2. Zig-zag transformer as a phase-shifted transformer

Fig. 3. Basic configuration of two converters connected in series in the ac side.

In order to show how the harmonics can be eliminated byusing the PSTs, in the example shown in Fig. 3, two inverters using full-wave modulation have been connected in seriesthrough two PSTs.The output voltage of the first inverter is phase shifted by 15whereas the second one is phase shifted by 15 as follows where is the amplitude of the th harmonic of the output voltage of each inverter. Note that the phase shift of the harmonics of order is times the phase shift introduced to the fundamental component by each converter. Each inverter is connected to a PST and the output windings of the PSTs are connected in series. In the first PST, the positive- sequence voltages are phase shifted by and as stated previously, the negative-sequence voltages are phase shifted by 15 . In the second PST, the positive sequence voltages are phase shifted by 15 and the negative-sequence voltages are phase shifted by 15 . Fig. 4 shows the vector diagrams of the 1st, 5th, and 7th harmonics of the voltages. The voltage waveform of the first converter is phase shifted by 15 . The PST lags the fundamental component of this voltage by , so it is positioned at the 0 angle. In the case of the second converter, it is exactly the contrary, so both fundamental components are 0 at the output of the PSTs. The fundamental component of the total output voltage is the sum of these fundamental components, with its amplitude being the double of the fundamental component of the output voltage of each inverter. Analyzing the voltage harmonics, according to (3), the 5th harmonic is phase shifted by 75 at the output of the first inverter, and by at the output of the second inverter. Since

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Fig. 4.Vector diagram of the fundamental and 5th and 7th

harmonics.

this 5th harmonic is a negative-sequence harmonic, the PSTshifts it by 15 as shown in Fig. 4. The 5th harmonic of thevoltages and are in phase opposition and they arecancelled out when they are added.In the same way, the 7th harmonic is phase shifted by 105and according to (4). The 7th harmonic is a positive sequenceand the PST lags it by 15 . The 7th harmonic of thevoltages and are in phase opposition and are cancelledout in the sum.Apart from the 5th and 7th harmonics, more harmonics areliminatedtoo. The eliminated harmonics are of order where 1, 3, 5, etc. The 12-pulse converters are based onthis harmonic elimination technique.

B. Harmonic Minimization by Using the PhaseShifted

Waveforms In the previous paragraph, a method for harmonic optimizationis presented where some specific harmonics are eliminated.There is another method for optimizing the harmonic content ofthe output voltage, where harmonics are minimized instead of eliminated, connecting in series the inverters without using thePSTs. The same modulation is used in both inverters, so the outputwaveform of the inverters is exactly the same . But in thefirst converter the waveform has been shifted by and inthe second converter by (all the angles are expressed in degrees),obtaining the voltage Vx: where the term is called the minimization rate. This minimization rate is the percentage of the th harmonic of the output voltage with respect to the original value of this th harmonic in the waveform. In Table I, the minimization rate for each harmonic, is illustrated for four different values of . The highest minimizations for a given have been highlighted. For example, if two signals are phase shifted by an , respectively, and then they are added, the resulting signal

HARMONICS MINIMIZATION FOR DIFFERENT (IN PER UNIT)

will have minimized harmonics 11th, 13th, 35th and 37th to the13% of the original value.The main drawback of this harmonic minimization methodis that apart from the harmonic components, the fundamentalcomponent is slightly reduced.

C. Blocking of Zero-Sequence Voltage Components by UsingMagnetic Elements

Magnetic elements, such as zero-sequence blocking transformer(ZSBT) or [zero-sequence blocking reactor (ZSBR)],can be used to filter common-mode or zero-sequence harmonics[16]. The impedance that ZSBT imposes to positive or negativesequences is relatively low whereas the impedance for zero sequenceis relatively high In Fig. 5, a commonly used ZSBT with “E”-type magneticcore is shown. The three phases are wired in the central limb.The magnetic flux generated by zero-sequence currents of eachphase flows through the magnetic core. However, magnetic fluxgenerated by differential currents is cancelled out and, therefore,no flux flows through the

IV. QUASI 96-PULSE OUTPUT VOLTAGE The output voltage of the FACTS device shown in Fig. 1 can be a quasi 96-pulse voltage waveform. This voltage waveform is obtained by using previously described harmonic elimination and minimization techniques.

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Fig. 5.Zero-sequence blocking transformer with an “E” magnetic core.

Fig. 6.Simplified single-phase circuit.

As stated previously, three techniques are used in order toeliminate harmonics: ZSBT for triplen harmonics elimination,the PST to eliminate harmonics of order n=6k+-1where k=1 3 5 and harmonic minimization for 11th, 13th, 23rd,25th, 35th, and 37th harmonics . In this way, the quasi 48-pulsewaveform voltage is obtained. In order to simplify the analysis of the system, only one phaseis considered. There are four NPC PEBBs for each phase. So,four voltages must be defined which areVa1o ,Va2o ,Vd1o , andVd2o Fig. 6 shows the single-phase diagram of the VSC configurationshown in Fig. 1Vout-a. is the output voltage of the phasea and it is connected in series or in parallel with the grid dependingon the connection of the coupling transformer. The waveform of the voltage at the output of each PEBB isthe sameVx but they are phase shifted by a different angle. On the one hand, the voltageVa1o is obtained by phaseshifting the waveform by and the voltage is obtainedby phase shifting the waveform by as shown inFig. 7. Therefore, (4) can be applied and the resulting voltage,which is , will have minimized the harmonics that areillustrated in Table I. The eliminates common-modeharmonics of and the voltage is obtained, which isin phase with the reference. On the other hand, the voltage is obtained by phase shifting thewaveform by and the voltage is obtained by shifting the waveform by as shown in Fig. 7. Essentially,the idea is the same as in the previous paragraph but allvoltage phases have been shifted by . Hence, the voltagelags the reference by 30 . This voltage is phase shifted by

Fig. 7. Voltages vector diagram. In Fig. 8, the most significant voltage waveforms are displayed.Defining the value of the angle (the angle that correspondsto the period where the voltage is zero) as 7.5 , theharmonics 23rd and 25th are minimized. The angle (thephase-shift angle of the voltage with respect to the reference)is 7.5 and 11th, 13th, 35th, and 37th harmonics are minimizedin and .

Fig. 8.Most significant voltage waveforms. The voltage across the ZSBT is(9)And the voltage is(10)The output voltage is a quasi 48-pulse waveform and it has21 different voltage levels, 10 levels in the positive semi-period,the zero level, and 10 levels in the negative semi-

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period. To summarize: In the voltage , 23rd and 25th harmonicshave been minimized by imposing the angle . Afterthat, the 11th, 13th, 35th, and 37th harmonics are minimizedby setting the angle to 7.5 . Nevertheless, there is anotheroption with which the same output voltage is obtained.The 11th, 13th, 35th, and 37th harmonics of the voltagecan be minimized, giving a value of 7.5 to , asshown in Table I. The 23rd and 25th harmonics can be minimizedby setting the angle to 3.75 .In both cases, the output voltage has the same quasi 96-pulsewaveform.

V. USE OF ADVANCED MODULATION STRATEGIES The convertible static compensator implementation at theNYPA Marcy Station has been described and analyzed in theprevious sections. As has been described, full-wave modulation is used by minimizing the switching power losses of thePEBBs, and harmonic elimination and minimization techniquesare used in order to optimize the harmonic content of the outputvoltage. But the drawback of this modulation strategy is thecontrol of the amplitude of the fundamental output voltage.There are two ways to control the amplitude of the outputvoltage: 1) changing the angle; but the change of this angle meansthat the harmonics are not going to be minimized; 2) changing the dc bus voltage; the dynamic response of theconverter is very slow and the system becomes nonlinearusing this alternative [11], [12]. Therefore, the use of advanced modulation strategies is proposedin this paper in order to maintain the harmonic content ofthe output voltage, controlling the amplitude of the fundamentalcomponent of this voltage. The proposed modulations are basedon the selective harmonic elimination (SHE) methods. In the SHE modulation, the switches of the power convertersare switched several times per period producing notches in theoutput voltage of each PEBB [19]. Controlling the angle atwhich the switches are commutated, the amplitude of several harmonics can be controlled. These degrees of freedom areused to control the amplitude of the fundamental component,and to cancel different harmonics. The following equationdefines the amplitude of the output voltage harmonic for athree-level converter:is the number of notches that has the signal in a quarter ofa period. Each angle gives a degree of freedom with which theamplitude of a harmonic is controlled. The studied application is based on three-level NPC PEBBs.Therefore, SHE modulation is focused on three-level signals.The first quadrant is defined by the three angles whereas thesecond quadrant and the third and fourth quadrants are obtainedby applying quarter-wave and negative half-wave symmetries, respectively. One of the disadvantages of SHE is that nonlinear equationsmust be solved. Moreover, the complexity tends to

increasewhen more angles are introduced and when higher order harmonicequations must be solved. Instead of eliminating specific harmonics, they can be reducedby using selective harmonic mitigation (SHM) modulation.This method has the advantage that more than one harmoniccan be reduced for each commutation angle [19], [20].Three possible modulation alternatives are analyzed in the following paragraphs, using selective harmonic elimination or mitigationtechniques with three angles.

A. Elimination of 11th and 13th Harmonics by Applying SHEModulation (SHE I)

SHE modulation with three angles is applied to each PEBBin order to control the fundamental component and to eliminatethe 11th and 13th harmonics. Consequently, the switching frequencyis three times higher compared to full-waveform modulation,but in this case, the fundamental amplitude is controlledby the modulation and not by the level of the dc bus voltage. Another degree of freedom is available that is the angleshown in Fig. 7. Given , the value of 3.75 , 23rd and 25thharmonics is minimized up to 6.5%. As stated previously, all the harmonics of order where , 3, 5, etc are eliminated by the phase shifted transformerT2 of Fig. 1. Therefore, the first significant harmonics ofare harmonics of order 35 and 37.

B. Elimination of 23rd and 25th Harmonics by Applying SHEModulation (SHE II)

As in the previous section, SHE modulation with three anglesis applied to each PEBB output voltage. In this case, the firstangle is used to control the fundamental amplitude and the othertwo angles are used to eliminate harmonics of order 23 and 25. The angle is 7.5 with which harmonics of order 11, 13,35, and 37 are minimized up to 13%. All harmonics of order N=6k+1, where 1, 3, 5, etc. are eliminated by thephase-harmonics shifted transformer T2 of Fig. 1. Thus, the firstrelevant harmonics of output voltage are harmonics oforder 47 and 49.

C. Elimination of 11th and 13th and Minimization of the 23rdand 25th Harmonics by Applying SHE Modulation

(SHE III) In this third alternative, the amplitude of the fundamentalcomponent is not controlled by the SHE modulation angles. Instead,the SHE modulation works with fixed precalculated anglesthat eliminate the 11th and 13th harmonics. Different familiesof angles that eliminate these two harmonics are calculated,and the optimal operation point is selected among all of thesefamilies, choosing the angles that,with a highmodulation index,generate very small amplitude 23rd and 25th harmonics. The“optimum” angles selected in our case generate a fundamentalamplitude of 0.88 p.u., eliminate the 11th and 13th harmonics,and minimize the amplitude of the 23rd and 25th harmonics.Thus, the SHE modulation works at a fixed point. The selected

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three angles of the SHE modulation are shown) 훽1 = 21.08 , 훽2 = 54.08 , 훽3 = 57.08 The amplitude of the fundamental component is controlledby shifting the angle .As stated previously, all harmonics of order where1, 3, 5, etc are eliminated by the phase-shifted transformerT2 of Fig. 1.

D. Comparison of Four Different Modulations Implementedin the Previous Application

Four modulations are compared in the system shown inFig. 1: the original full-waveform modulation changing the dcbus voltage, and the three different options of SHE modulationproposed in this paper (SHE I, SHE II, and SHE III). The lower limit of the amplitude of the fundamental componenthas been set to 0.5 p.u. The 1 p.u of the modulationindex is referred to the amplitude of the fundamental componentthat would be obtained with a square waveform (in the studiedtopology, this would be ). The maximum value

Fig. 9.Va-out voltage THD for different modulations.

Table II.

ANGLES OF THREE DIFFERENT SHE MODULATION

of the amplitude of the fundamental component of the outputvoltage that could be achieved with the different modulationstrategies is 0.98 p.u. for the full waveform, 0.97 p.u. for SHE Imodulation, 0.95 p.u. for SHE II, and 0.88 p.u. for SHE III. The proportion between the amplitude of the

fundamentalcomponent and the amplitude of the harmonics is constant alongthe modulation index when full-waveform modulation is used.Thus, the output voltage waveform does not depend on themodulationindex and the THD remains constant. Nevertheless, the proportion between the fundamental amplitudeand the harmonics amplitude depends on the modulationindex when SHE modulations are used. Therefore, the outputvoltage waveform varies along the modulation index, and so does the THD. In Fig. 9, the THD of the voltage by applying the fourmodulations is shown along the modulation index. It can be observed that, in general, the harmonic content isworse with the proposed SHE modulation than using the fullwavemodulation. However, the dynamic response is enhanced,which can be interesting in some applications. In addition, the harmonic content is similar to the high modulation index.

E. Simulation Results With the New Modulation The simulation of the system with the proposed modulationstrategies has been carried out in Matlab Simulink. Theoptimum working point of each modulation, taking into accountthe THD and the modulation index, has been chosenas a working point for the simulation. The angles of the SHEmodulations and the modulation index are illustrated inTable II.

Fig. 10.Va-outvoltage normalized with respect to voltage for different modulations, waveform, SHE I

Fig. 11.Vout -avoltage harmonic content by applying (SHE I)

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Fig. 10 shows the output voltage using the three SHE modulationtechniques. In Fig. 11, the output voltage harmonic content is shown forthese SHE modulations. Note that the harmonic amplitude isrepresented in logarithmic scale.In the chosen working point, the best harmonic content isachieved by using the modulation SHE III. However, this pointis the optimum one for the modulation SHE III, and the harmoniccontent varies considerably with the modulation index as shown in Fig. 9.Fig. 12 shows the voltage harmonic content, which iscomposed by triplen-order harmonics.In the first three sub graphics, the amplitude on the third-orderharmonic is about 4% of the , while in the last subgraphic,the value decreases to 1.7%. Therefore, the magnetic core size ofthe ZSBT can be reduced by using SHE III modulation, becauseless voltage amplitude means less current and, therefore, lessmagnetic flux through the core.

VI. CONCLUSION In this paper, the harmonic cancellation and minimizationin multipulse converters has been analyzed and described, focusingon VSC power-electronics converters using PEBBs forFACTS applications. The convertible static compensator implementationat the NYPA Marcy Station has been described andanalyzed for this purpose. The harmonic elimination and minimizationtechniques used in this multipulse VSC have been explained.In this system, a full-wave modulation strategy is used, and the amplitude of the fundamental component of the outputvoltage is controlled by changing the dc bus voltage. The study can be used as a base to understand the associationof PEBBs by using magnetic elements, and taking advantage ofthis association in order to enhance the harmonic content of theoutput voltage. In addition, three SHE modulation strategies have been proposedand analyzed.With these modulation techniques, the fundamental amplitude of the output voltage is controlled by the modulation technique, improving the dynamic response of thesystem. REFERENCES [1] J. V. Milanovic and Z. Yan, “Modeling of FACTS devices for voltagesag mitigation studies in large power systems,” IEEE Trans. PowerDel., vol. 25, no. 4, pp. 3044–3052, Oct. 2010. [2] J. A. Barrena, L. Marroyo, M. A. R. Vidal, and J. R. T. Apraiz, “Individualvoltage balancing strategy for PWM cascaded H-bridge converter-based STATCOM,” IEEE Trans. Ind. Electron., vol. 55, no. 1,pp. 21–29, Jan. 2008. [3] S. Kouro, M. Malinowski, K. Gopakumar, J. Pou, L. G. Franquelo,B. Wu, J. Rodriguez, M. A. Perez, and J. I. Leon,

“Recent advancesand industrial applications of multilevel converters,” IEEE Trans. Ind.Electron., vol. 57, no. 8, pp. 2553–2580, Aug. 2010. [4] N. Hatano and T. Ise, “Control scheme of cascaded H-bridgeSTATCOM using zero-sequence voltage and negative-sequence current,”IEEE Trans, Power Del., vol. 25, no. 2, pp. 543–550, Apr. 2010. [5] T. Ericsen, “The second electronic revolution (it’s all about control),”IEEE Trans. Ind. Appl., vol. 46, no. 5, pp. 1778–1786, Sep./Oct. 2010. [6] B. Han, B. Bae, S. Baek, and G. Jang, “New configuration of UPQCfor medium-voltage application,” IEEE Trans. Power Del., vol. 21, no.3, pp. 1438–1444, Jul. 2006. [7] J. Chivite-Zabalza, M. A. Rodriguez, P. Izurza, G. Calvo, and D.Madariaga, “A large power, low-switching frequency voltage sourceconverter for FACTS applications with low effects on the transmissionline,” IEEE Trans. Power Electron., vol. 27, no. 12, pp. 4868–4879,Dec. 2012. [8] X. Zhengping and S. Bhattacharya, “STATCOM control and operationwith series connected transformer based 48-pulse VSC,” in Proc. 33rdAnnu. IEEE Ind. Electron. Soc. Conf., 2007, pp. 1714–1719. [9] F. J. Chivite-Zabalza, A. J. Forsyth, and D. R. Trainer, “A simple, passive24-pulse AC-DC converter with inherent load balancing,” IEEETrans. Power Electron., vol. 21, no. 2, pp. 430–439, Mar. 2006. [10] Convertible Static Compensator (CSC) for New York Power AuthorityEPRI, Palo Alto, CA, and New York Power Authority, White Plains,NY: 2001. 1001970. [11] P. S. Sensarma, K. R. Padiyar, and V. Ramanarayanan, “Analysis andperformance evaluation of a distribution STATCOM for compensatingvoltage fluctuations,” IEEE Trans. Power Del., vol. 16, no. 2, pp.259–264, Apr. 2001. [12] C. Schauder, “Vector analysis and control of advanced static VArcompensators,” in Proc. Int. Conf. AC DC Power Transm. , 1991, pp.266–272. [13] C. Ben-Sheng and H. Yuan-Yih, “A minimal harmonic controller for aSTATCOM,” IEEE Trans. Ind. Electron., vol. 55, no. 2, pp. 655–664,Feb. 2008. [14] C. Ben-Sheng and H. Yuan-Yih, “An analytical approach to harmonic analysis and controller design of a STATCOM,” IEEE Trans. PowerDel., vol. 22, no. 1, pp. 423–432, Jan. 2007.

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[15] F. Z. Peng, Q. Wei, and C. Dong, “Recent advances in multilevel converter/inverter topologies and applications,” in Proc. Power Electron.Int. Conf,, 2010, pp. 492–501.

. R.RAVIKIRAN currently pursuing his M.Tech in Power Eelectronics from Vijay Rural Engineering College, Rochis Valley, Manikbhandar, Nizamabad, JNT University, Hyderabad B.E degree from Vasavi College of Engineering affiliated

to Osmania University, Hyderabad in 2012.

SUBASHRATHOD has completed his B.Tech Electrical & Electronics Engineering in 2004 from Kamala institute of technology and sciences, JNTUH University, Hyderabad and M.TECH in PowerElectronics and Drives in 2008 from NIT Warangle, working

as Associate Professor at VIJAY RURAL ENGINEERING COLLEGE, Nizamabad, Telanagana, India. His area of interest includes power system, power electronics and drives also Facts.

A.SUSHEELA has completed her B.Tech Electrical & Electronics Engineering in 2005 from ARKAY College of Engineering JNTUH , M.TECH in Electrical Power Systems in 2008 from JBIT and presently , working as Associate Professor and Head of EEE department at Vijay Rural Engineering

College, Nizamabad, Telanagana, India. Her area of interest include power system, power electronics and high voltage engineering.