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HARMONICS ELIMINATION AND MINIMIZATION TECHNIQUES USED IN

MULTIPULSE CONVERTER FOCUSING ON PEBB BASED VSC FOR FACTS

APPLICATION

S.SHAJIYA BANU P.G. scholar, Dept of EEE, Sri Sai Institute of Technology and Science

Rayachoty,A P D.CHINNA DASTAGIRI

Assistant professor , Dept of EEE, Sri Sai Institute of Technology and Science Rayachoty,A P

V.PRATAPARAO Assistant Professor and HOD of EEE department, Sri Sai Institute of Technology and Science

,Rayachoty,A P

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 elimination modulation 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 output voltage harmonic quality. The harmonic content of the voltage must satisfy the legislation requirements at the point of common coupling (PCC). Multipulse converters are used to improve the output voltage quality without increasing the switching frequency. In high-power applications, full-wave modulation is commonly used, where the switching frequency has the same value of the fundamental frequency of the output voltage. The phase-shifting transformers (PST) are used to change the phase between primary and secondary voltages. A simple PST is the well-known wye-delta transformer. If the primary side is connected in wye and the secondary side is connected in delta, the fundamental component of the voltage in the secondary side is 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.

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Thewye-delta transformers change the phase by 30 or 30 , depending on the connection type. It is possible to obtain different phase-shifting angles by using a zig-zag transformer instead of a wye-delta transformer. In order to cancel out voltage harmonics, a determined phase-shifting angle can be achieved by using zig-zag transformers. Apart from PST, other magnetic devices that are used in order to increase the output voltage quality are the line interface transformer (LIT), interphase transformer (IPT), zero-sequence blocking 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 to reduce 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 output voltage, the dc bus voltage is controlled, exchanging active power with the grid. In [10], a comparison between 24-pulse VSC and quasi 24-pulse VSC is shown. In the first one, 11th, 13th, 35th, and 37th harmonics are cancelled whereas in the second one, they are minimized. These cancellation and minimization techniques are going to be described in Sections III-A and B, respectively. In Section III-C, the ZSBT is explained which is used to cancel out triplen-order harmonics. Harmonic reduction techniques explained before are used to obtain quasi 48 pulses in the output voltage of the studied application that 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-wave modulation. 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 the value of has to be decreased, active power is injected into the grid. The drawback is the relatively slow dynamic response of the output voltage amplitude due to the necessity of exchange active power to control the amplitude of the output voltage [11], [12]. One advantage of this converter configuration is that with full-wave modulation, an output voltage of quasi 48 pulses can be obtained. Another advantage is that the currents of all PEBBs are balanced because they are connected in series at the ac side. This means that magnetic elements, such as IPTs, are not necessary for balancing the power delivered by the PEEBs . III. HARMONIC ELIMINATION IN MULTIPULSE

MODULATION The objective of the harmonic elimination is to cancel or minimize certain undesired harmonics at the output voltage of the converter, using common VSCs connected in multi pulse configuration and working with full-wave modulation. This means that several harmonics could be cancelled out without increasing the switching frequency of the converters

A. Harmonic Cancellation by Using the Phase-

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ShiftingTransformers

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-sequence harmonics. In a three-phase system, the positive-sequence harmonics of lag ; for negative-sequence harmonics, leads ; and for zero-sequence harmonics, the three phases are in phase. Harmonics of order are classified as positivesequence harmonics. Harmonics of order are classified as negative-sequence ones, and harmonics of order are 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 negative sequences can be used to cancel specific harmonics [16]. A wye-delta transformer can be considered as a phase-shifter transformer (PST) because it changes the phase of the harmonics by . However, it is possible to build a PST which shifts the harmonic phase by a desired angle as shown in Fig. 2. In this figure, a zig-zag transformer configuration is depicted where 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 by using the PSTs, in the example shown in Fig. 3, two inverters using full-wave modulation have been connected in series through two PSTs. The output voltage of the first inverter is phase shifted by 15 whereas 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 voltageis 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 PST shifts it by 15 as shown in Fig. 4. The 5th harmonic of the voltages and are in phase opposition and they are cancelled out when they are added. In the same way, the 7th harmonic is phase shifted by 105 and according to (4). The 7th harmonic is a positive sequence and the PST lags it by 15 . The 7th harmonic of the voltages and are in phase opposition and are cancelled out in the sum. Apart from the 5th and 7th harmonics, more harmonics are liminated too. The eliminated harmonics are of order where 1, 3, 5, etc. The 12-pulse converters are based on this harmonic elimination technique.

B. Harmonic Minimization by Using the PhaseShifted

Waveforms In the previous paragraph, a method for harmonic optimization is presented where some specific harmonics are eliminated. There is another method for optimizing the harmonic content of the output voltage, where harmonics are minimized instead of eliminated, connecting in series the inverters without using the PSTs. The same modulation is used in both inverters, so the output waveform of the inverters is exactly the same . But in the first converter the waveform has been shifted by and in the 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 the 13% of the original value. The main drawback of this harmonic minimization method is that apart from the harmonic components, the fundamental component is slightly reduced.

C. Blocking of Zero-Sequence Voltage Components by Using Magnetic 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 negative sequences is relatively low whereas the impedance for zero sequence is relatively high In Fig. 5, a commonly used ZSBT with “E”-type magnetic core is shown. The three phases are wired in the central limb. The magnetic flux generated by zero-sequence currents of each phase flows through the magnetic core. However, magnetic flux generated 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 to eliminate harmonics: ZSBT for triplen harmonics elimination, the PST to eliminate harmonics of order n=6k+-1 where k=1 3 5 and harmonic minimization for 11th, 13th, 23rd, 25th, 35th, and 37th harmonics . In this way, the quasi 48-pulse waveform voltage is obtained. In order to simplify the analysis of the system, only one phase is 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 configuration shown in Fig. 1 Vout-a. is the output voltage of the phase a and it is connected in series or in parallel with the grid depending on the connection of the coupling transformer. The waveform of the voltage at the output of each PEBB is the sameVx but they are phase shifted by a different angle. On the one hand, the voltageVa1o is obtained by phase shifting the waveform by and the voltage is obtained by phase shifting the waveform by as shown in Fig. 7. Therefore, (4) can be applied and the resulting voltage, which is , will have minimized the harmonics that are illustrated in Table I. The eliminates common-mode harmonics of and the voltage is obtained, which is in phase with the reference. On the other hand, the voltage is obtained by phase shifting the waveform 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 all voltage phases have been shifted by . Hence, the voltage lags 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 corresponds to the period where the voltage is zero) as 7.5 , the harmonics 23rd and 25th are minimized. The angle (the phase-shift angle of the voltage with respect to the reference) is 7.5 and 11th, 13th, 35th, and 37th harmonics are minimized in and .

Fig. 8. Most significant voltage waveforms.

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The voltage across the ZSBT is (9) And the voltage is (10) The output voltage is a quasi 48-pulse waveform and it has 21 different voltage levels, 10 levels in the positive semi-period, the zero level, and 10 levels in the negative semi-period. To summarize: In the voltage , 23rd and 25th harmonics have been minimized by imposing the angle . After that, the 11th, 13th, 35th, and 37th harmonics are minimized by setting the angle to 7.5 . Nevertheless, there is another option with which the same output voltage is obtained. The 11th, 13th, 35th, and 37th harmonics of the voltage can be minimized, giving a value of 7.5 to , as shown in Table I. The 23rd and 25th harmonics can be minimized by setting the angle to 3.75 . In both cases, the output voltage has the same quasi 96-pulse waveform.

V. USE OF ADVANCED MODULATION STRATEGIES The convertible static compensator implementation at the NYPA 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 the PEBBs, and harmonic elimination and minimization techniques are used in order to optimize the harmonic content of the output voltage. But the drawback of this modulation strategy is the control of the amplitude of the fundamental output voltage. There are two ways to control the amplitude of the output voltage: 1) changing the angle; but the change of this angle means that the harmonics are not going to be minimized; 2) changing the dc bus voltage; the dynamic response of the converter is very slow and the system becomes nonlinear using this alternative [11], [12]. Therefore, the use of advanced modulation strategies is proposed in this paper in order to maintain the harmonic content of the output voltage, controlling the amplitude of the fundamental component of this voltage. The proposed modulations are based on the selective harmonic elimination (SHE) methods. In the SHE modulation, the switches of the power converters are switched several times per period producing notches in the output voltage of each PEBB [19]. Controlling the angle at which the switches are commutated, the amplitude of several harmonics can be controlled. These degrees of freedom are used to control the amplitude of the fundamental component, and to cancel different harmonics. The following equation defines the amplitude of the output voltage harmonic for a three-level converter: is the number of notches that has the signal in a quarter of a period. Each angle gives a degree of freedom with which the amplitude 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 the second quadrant and the third and fourth quadrants are obtained by applying quarter-wave and negative

half-wave symmetries, respectively. One of the disadvantages of SHE is that nonlinear equations must be solved. Moreover, the complexity tends to increase when more angles are introduced and when higher order harmonic equations must be solved. Instead of eliminating specific harmonics, they can be reduced by using selective harmonic mitigation (SHM) modulation. This method has the advantage that more than one harmonic can be reduced for each commutation angle [19], [20]. Three possible modulation alternatives are analyzed in the following paragraphs, using selective harmonic elimination ormitigation techniques with three angles.

A. Elimination of 11th and 13th Harmonics by Applying

SHE Modulation (SHE I) SHE modulation with three angles is applied to each PEBB in order to control the fundamental component and to eliminate the 11th and 13th harmonics. Consequently, the switching frequency is three times higher compared to full-waveform modulation, but in this case, the fundamental amplitude is controlled by the modulation and not by the level of the dc bus voltage. Another degree of freedom is available that is the angle shown in Fig. 7. Given , the value of 3.75 , 23rd and 25th harmonics is minimized up to 6.5%. As stated previously, all the harmonics of order where , 3, 5, etc are eliminated by the phase shifted transformer T2 of Fig. 1. Therefore, the first significant harmonics of are harmonics of order 35 and 37. B. Elimination of 23rd and 25th Harmonics by Applying

SHE Modulation (SHE II) As in the previous section, SHE modulation with three angles is applied to each PEBB output voltage. In this case, the first angle is used to control the fundamental amplitude and the other two 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 the phase-harmonics shifted transformer T2 of Fig. 1. Thus, the first relevant harmonics of output voltage are harmonics of order 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 fundamental component is not controlled by the SHE modulation angles. Instead, the SHE modulation works with fixed precalculated angles that eliminate the 11th and 13th harmonics. Different families of angles that eliminate these two harmonics are calculated, and the optimal operation point is selected among all of these families, choosing the angles that,with a highmodulation index, generate very

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small amplitude 23rd and 25th harmonics. The “optimum” angles selected in our case generate a fundamental amplitude 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 three angles of the SHE modulation are shown) 훽1 = 21.08 , 훽2 = 54.08 , 훽3 = 57.08 The amplitude of the fundamental component is controlled by shifting the angle . As stated previously, all harmonics of order where 1, 3, 5, etc are eliminated by the phase-shifted transformer T2 of Fig. 1.

D. Comparison of Four Different Modulations Implementedin the Previous Application

Four modulations are compared in the system shown in Fig. 1: the original full-waveform modulation changing the dc bus voltage, and the three different options of SHE modulation proposed in this paper (SHE I, SHE II, and SHE III). The lower limit of the amplitude of the fundamental component has been set to 0.5 p.u. The 1 p.u of the modulation index is referred to the amplitude of the fundamental component that would be obtained with a square waveform (in the studied topology, 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 output voltage that could be achieved with the different modulation strategies is 0.98 p.u. for the full waveform, 0.97 p.u. for SHE I modulation, 0.95 p.u. for SHE II, and 0.88 p.u. for SHE III.

The proportion between the amplitude of the fundamental component and the amplitude of the harmonics is constant along the modulation index when full-waveform modulation is used. Thus, the output voltage waveform does not depend on themodulation index and the THD remains constant. Nevertheless, the proportion between the fundamental amplitude and the harmonics amplitude depends on the modulation index when SHE modulations are used. Therefore, the output voltage waveform varies along the modulation index, and so does the THD. In Fig. 9, the THD of the voltage by applying the four modulations is shown along the modulation index. It can be observed that, in general, the harmonic content is worse with the proposed SHE modulation than using the fullwave modulation. 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 modulation strategies has been carried out in Matlab Simulink. The optimum working point of each modulation, taking into account the THD and the modulation index, has been chosen as a working point for the simulation. The angles of the SHE modulations and the modulation index are illustrated in Table II.

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

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Fig. 11. Vout -avoltage harmonic content by applying (SHE I) Fig. 10 shows the output voltage using the three SHE modulation techniques. In Fig. 11, the output voltage harmonic content is shown for these SHE modulations. Note that the harmonic amplitude is represented in logarithmic scale. In the chosen working point, the best harmonic content is achieved by using the modulation SHE III. However, this point is the optimum one for the modulation SHE III, and the harmonic content varies considerably with the modulation index as shown in Fig. 9. Fig. 12 shows the voltage harmonic content, which is composed by triplen-order harmonics. In the first three subgraphics, the amplitude on the third-order harmonic is about 4% of the , while in the last subgraphic, the value decreases to 1.7%. Therefore, themagnetic core size of the ZSBT can be reduced by using SHE III modulation, because less voltage amplitude means less current and, therefore, less magnetic flux through the core.

VI. CONCLUSION In this paper, the harmonic cancellation and minimization in multipulse converters has been analyzed and described, focusing on VSC power-electronics converters using PEBBs for FACTS applications. The convertible static compensator implementation at the NYPA Marcy Station has been described and analyzed for this purpose. The harmonic elimination and minimization techniques 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 output voltage is controlled by changing the dc bus voltage. The study can be used as a base to understand the association of PEBBs by using magnetic elements, and taking advantage of this association in order to enhance the harmonic content of the output voltage. In addition, three SHE modulation strategies have been proposed and analyzed.With these modulation techniques, the fundamental amplitude of the output voltage is controlled by the modulation technique, improving the dynamic response of the system.

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[3] S. Kouro, M. Malinowski, K. Gopakumar, J. Pou, L. G. Franquelo,B. Wu, J. Rodriguez, M. A. Perez, and J. I. Leon, “Recent advances and 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-bridge STATCOM 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 UPQC for 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 source converter for FACTS applications with low effects on the transmission line,” IEEE Trans. Power Electron., vol. 27, no. 12, pp. 4868–4879, Dec. 2012. [8] X. Zhengping and S. Bhattacharya, “STATCOM control and operation with 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, passive 24-pulse AC-DC converter with inherent load balancing,” IEEE Trans. Power Electron., vol. 21, no. 2, pp. 430–439, Mar. 2006. [10] Convertible Static Compensator (CSC) for New York Power Authority EPRI, Palo Alto, CA, and New York Power Authority, White Plains, NY: 2001. 1001970. [11] P. S. Sensarma, K. R. Padiyar, and V. Ramanarayanan, “Analysis and performance evaluation of a distribution STATCOM for compensating voltage fluctuations,” IEEE Trans. Power Del., vol. 16, no. 2, pp. 259–264, Apr. 2001. [12] C. Schauder, “Vector analysis and control of advanced static VAr compensators,” 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 a STATCOM,” IEEE Trans. Ind. Electron., vol. 55, no. 2, pp. 655–664, Feb. 2008. [14] C. Ben-Sheng and H. Yuan-Yih, “An analytical

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approach to harmonic analysis and controller design of a STATCOM,” IEEE Trans. Power Del., vol. 22, no. 1, pp. 423–432, Jan. 2007. [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.

S.SHAJIYABANUMailid:shajiyabanu.275@gmail.com currently pursuing her M.Tech in Power ELECTRONICS From Sri Sai Institute of Technology and Science affiliated to JNTUAananthapur. she received her B.Tech degree from Sri sai institute of technology and science , Affiliated to JNTU ananthapur , in 2011,and her field of interest includes Power electronics..

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D. CHINNA DASTAGIRI received the B.Tech degree in Electrical & Electronics Engineering from Sri Sai Institute of Technology and Science affiliated to JNTU ananthapur in 2006, the M.TECH degree in Power systems from NIE MYSORE, in 2009 ,he was currently working as Assistant Professor at Sri Sai institute of technology and science, Rayachoty, Andhra Pradesh, India. his area of interest include power system.

V.PRATAPARAO received the B.Tech degree in Electrical & Electronics Engineering from R.G.M college of engineering and technology ,affiliated to JNTUH University, in 2003, the M.TECH degree in Power Systems from AITS, Rajampet, affiliated to JNTUA University, and presently he is interested to reach topics includes power system especially in ELECTRICAL DISTRIBUTION SYSTEM, he was currently working as Assistant Professor and HOD of EEE department at Sri Sai institute of technology and science, Affiliated to JNTUA university, Rayachoty, Kadapa (dst), Andhra Pradesh, India.