Power Quality Improvement using Asymmetrical Multilevel Inverter for ...

Rev. Téc. Ing. Univ. Zulia. Vol. 38, Nº 3, 47 - 63, 2015

Power quality improvement for grid connected photo voltaic system using asymmetrical multilevel inverter S.Suresh1, S.Kannan2, P.Murugan3 Research Scholar, [email protected], ECE Dept, Kalasalingam University, Krishnankoil, Tamilnadu, India. 2 Professor, [email protected], EEE Dept, Ramco Institute of Technology, Rajapalayam, Tamilnadu, India. 3 Associate Professor, [email protected], ECE Dept, Kalasalingam University, Krishnankoil, Tamilnadu, India. 1

Abstract Power quality (PQ) regulation is a major issue in industrial and domestic power consumption. Left unattended it will directly pollute entire power distribution system through Point of Common Coupling (PCC). As of today, the industry cannot safely run the motive loads without PQ monitoring. For having longer machine life, better machine performance and more profit in transmitting power, the maintenance of PQ at international standard is necessary. This paper presents the implementation of STATCOM built by asymmetric cascaded multilevel inverter (ACMLI) at PCC of power distribution system to improve the PQ. The STATCOM uses regulated DC power, sourced by the Photo Voltaic (PV) system. The performance analysis of a STATCOM using ACMLI is discussed. Synchronous Reference Frame (SRF) method is used. Three-phase asymmetrical cascaded fifty three levels inverter configuration is considered. Fuzzy Logic Control (FLC) is proposed to regulate DC link capacitor voltage of DC converter, which enables the overall reactive power flow control in power distribution system. The simulated results using MATLAB/SIMULINK show that the reactive power demand of the system is compensated by STATCOM with less harmonic distortion. The performance of the proposed FLC has been investigated, through the wide range of operating conditions such as lagging VAr load, leading VAr load, dynamic load and unbalanced load, and with the performance of conventional PI controller. Keywords: Asymmetric Cascaded Multilevel Inverter, Fuzzy Logic Controller, Power Factor, STATCOM, Total Harmonic Distortion, Volt-Ampere reactive.

1. INTRODUCTION Now a days, the power quality (PQ) draws more attention of many researchers on power distribution systems, due to the increased usage of the polluting (non-linear) power electronic equipments in industries and in domestic applications. Due to more number of variable speed drives in industry, the reactive power demand and unbalanced currents result in serious problems in power distribution systems, like increased line losses, decreased power transmission capacity, decreased marginal stability of power system, decreased/ increased system voltage and others (Alper Akdag et al., 2000, Fang Zheng Peng et al., 1997, and Peng.F.Z et al., 1996). Typically, the reactive power is compensated by connecting the fixed capacitors and/or synchronous condensers with the system at the Point of Common Coupling (PCC). The no load/light load condition leads to increase in system voltage and frequency due to the poor voltage regulation of the system and speed of the turbine. The system’s load demands (active power, P and reactive power, Q) increase unnecessarily due to this increase in line voltage. This causes power loss and PQ issues in the line. Similarly, the heavy load/full load condition leads to decrease in system voltage and frequency. This causes line voltage instability and consequent PQ issues like poor performance (saturation of magnetic core) of the motive loads. Hence, the fixed (or stepped) reactive power compensation cannot give a better result under varying load condition. Therefore, the reactive power demand needs to be dynamically met out. The magnitude of unbalance in three phase line currents due to very large number of single phase loads (also with varied power factor) connected in the distribution system, causes the unbalance in the line voltages at tail ends due to the poor line voltage regulation. Again, this unbalance in the line voltages causes poor performance of the three-phase loads and unbalanced real and reactive (P and Q) demand. This unbalanced P and Q demand causes over loading of one line conductor and light loading of other two conductors of the distribution system. This over loading in one particular conductor causes tripping of the total system even though, the average power of three phase connected load is less than the distribution line capacity (Fang Zheng Peng et al., 1997). Hence a dynamic three phase current balancing mechanism is very much essential to maintain balanced system current even for unbalanced (magnitude and phase angle) consumer loads. Today the research on multilevel inverter (MLI) for real power control, reactive power compensation, harmonic current mitigation on power distribution system and electrical drive applications attracts all researchers due to its high voltage applications, simplified decoupled control and less harmonic distorted voltage (Jon Andoni Barren et al., 2008, J.Rodriguez et al., 2002). Hybrid topology based MLI consists of differently rated power semiconductor devices for the construction of individual H-bridge inverter with different capacity and different switching frequency. The number of components used in the topology of Asymmetric Cascaded Multi Level Inverter (ACMLI) has been reduced which simplifies the control system and enables low cost hardware implementation. In this topology, number of levels in output voltage can be increased compared to Symmetrical Cascaded Multi Level Inverter (SCMLI) for same number of H-bridges. This is shown in Table 1 for single phase and three phase system with number of H-bridge inverter units. 47


No. of H-bridges 1 2 3

Table 1 Voltage levels and Number of H-bridges Voltage Magnitude Symmetric Cascaded Multilevel Asymmetric Cascaded Multilevel (cumulative) (n) Inverter (SCMLI) Inverter SCMLI ACMLI Single phase Three phase Single phase Three phase (2n+1) (4n+1) (2n+1) (4n+1) (1)V (1)V 3 5 3 5 (1+1)2V (1+3)4V 5 9 9 17 (1+1+1)3V (1+3+9)13V 7 13 27 53

MLI can be directly connected to any existing power distribution system through a coupling inductor without any boosting transformer. It reduces the cost, losses and space requirements. In this paper, among various applications of multilevel inverter, reactive power compensation is discussed with the configuration of ACMLI. Each H-bridge inverter of one phase is connected with different magnitude of DC link voltage, energised by PV system. Fuzzy Logic (FL) is proposed to control the DC voltage of the each DC link capacitor and the overall reactive power. For better control, the DC link voltage should be controlled in an optimistic way to meet out the demanded reactive power of distribution system. The DC link capacitor is charged using the energy obtained from solar PV system through FLC DC converter (Carlo Cecati et al., 2010, E. Latha Mercy et al., 2010, Grain P. Adam et al., 2008, Jayakrishna.G et al., 2010 and S.Saad et al., 2009). The DC converter is controlled by the reference signal obtained from STATCOM control block to maintain DC voltage on the input side of inverter proportional to reactive power demand in distribution system. This paper is organized as follows: Section 2 illustrates with the configuration of Asymmetric Cascaded MLI (ACMLI). Section 3 discusses about the STATCOM connection with power distribution system including its operation and control. Section 4 presents DC link voltage control using FL. Section 5 contains the simulation results and discussions and Section 6 concludes. 2. SYSTEM CONFIGURATION WITH PV ACMLI is selected to implement the STATCOM with separate DC source at different voltage magnitudes for reactive power compensation of the power distribution system. The 3- ( stands for phase) circuit diagram of ACMLI (fifty three levels) is shown in Figure1. In this circuit diagram, three single-phase, twenty seven level inverters are connected in star to implement 3- inverter. In this paper, star configuration has been taken for analysis. In one single phase unit, three H-bridges are connected in series, each having different magnitudes of DC voltage on the input side. The DC link capacitor is frequently charged and maintained at predefined DC voltage by controlled DC converter fed by PV system (Elena Villanueva et al., 2009). This controlled DC voltage is required to compensate the reactive power demand of distribution system. The overall circuit diagram with ACMLI, control block and PV system along with the connection of system is shown in Figure 2. The DC link voltage of SCMLI is equal in all the H-bridges. However, in ACMLI the DC voltage magnitude is in multiples of three i.e., (1, 3, 9). The switching losses are small in this topology due to low switching frequency (Bhim Singh et al., 2007). The combination of switching in various H-bridge inverters of same phase enables the variable switching frequency operation in concerned H-bridge unit shown in figure 4. So more number of voltage levels are possible with less number of H-bridge units.

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A ph

Ca1

Vo1

Ca2

Vo2

Ca3

Vo3

C ph

B ph

Cb1

Cc1

Cb2

Cc2

Cb3

Cc3

n

Figure.1 Circuit diagram of 3- ACMLI (fifty three levels)

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Rs

Ls 3 Reactive Load Solar PV Array

3, 415V, 50Hz Source Vdc

Rc

DC-DC Converter

α1

Lc

Asymmetrical Cascaded Multilevel Inverter

3Vdc

DC-DC Converter

α2

Grid Synchronised Fixed Switching Angle Gate Signal Generation

9Vdc

DC-DC Converter

α3 Figure.2 Grid connected AMLI with PV system

Vdc 0 +1 -1 0 +1 -1 0 +1 -1 0 +1 -1 0 +1 0

Table 2. Output of H-bridge (each) inverter and Output voltage (per phase) level H-bridge inverter having input voltage of Vdc Output voltage (per phase) level 3Vdc 9Vdc Vdc 0 0 +1 +1 +1 -1 -1 -1 0 0 0 +1 +1 +1 0

0 0 0 0 0 +1 +1 +1 +1 +1 +1 +1 +1 +1 0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 0

50


The switching function and representation of output voltage level in each H- bridge inverter of one phase are given in Table 2. The H-bridge inverters in one phase have input dc voltage of magnitude V dc, 3Vdc and 9Vdc respectively. In this table, +1 represents +Vdc, -1 represents -Vdc and 0 represents 0Vdc of concerned H-bridge inverter. For one quarter, cycle of output per phase the switching function has been given with output voltage level in one phase of inverter. For example to obtain 10V dc magnitude in output voltage per phase, the contribution of switching function in each inverter unit in that phase is +1, 0 and +1 respectively. i.e, Vdc+ 0+9Vdc=10 Vdc. The first inverter unit, which has input dc voltage of Vdc, is switched on, the second inverter unit with input dc voltage of 3Vdc, is switched off and the third inverter with input dc voltage of 9 V dc, is switched on at this switching interval. All H-bridges are connected in series to obtain this 10 Vdc as peak voltage. The STATCOM is connected with power distribution system at PCC through the external inductor for feeding the generated reactive power according to the requirement of demanded reactive power reference obtained from system. The control block separates the reactive power reference signal and calculates the “ac RMS fundamental voltage reference” as control input to STATCOM to generate  VAr at the output side of STATCOM. From this voltage reference, the DC link voltage to be present on the input side of STATCOM is calculated. This dc voltage reference is given to FL controller as DC voltage reference. The FL controller provides the pulse width reference for solar-fed DC converter to regulate and maintain DC link voltage on the input side of the STATCOM. The DC converter receives the variable magnitude dc power from PV system and regulates this voltage into constant DC voltage in magnitude. Here the DC converter is constructed as buck-boost converter. Whenever varying sunlight radiation happens, it functions to generate constant required dc voltage by varying the duty cycle obtained from FL controller. Since the STATCOM needs the exact, calculated DC link voltage, on its input side to produce the required ac RMS fundamental voltage to inject/absorb reactive power into/from the system. Thus, the FL controller plays an important role in this control system. The necessity of maintaining dc voltage across dc link capacitor as same as the DC voltage reference is to supply the required reactive power at PCC. Each H-bridge inverter is producing different level of reactive power as per the output ac fundamental voltage produced by it to contribute to the per phase reactive power requirement. In SCMLI the reactive power supplied by each H-bridge inverter is same due to its input dc voltage is equal in magnitude. However, in ACMLI, the reactive power supplied by each H-bridge inverter depends on the voltage output of concerned H-bridge. In one phase, since all the three H-bridges are connected in series; the common current supplied by them is equal to the phase compensating current. The overall reactive power requirement is satisfied by the summation of all bridge voltages in each phase. Figure 3 shows the phasor diagram representation of STATCOM with system. The phasor representatives of the system without STATCOM and with STATCOM with reactive power injection/absorption are shown. The inverter can produce variable ac voltage for either variable modulation index with constant dc input voltage or variable dc input voltage with fixed modulation index. This paper presents about getting the variable ac voltage for fixed modulation index with variable dc voltage. The modulation index is fixed at an appropriate value at which the system harmonics are minimized. The dc voltage can be regulated to be equal to reference value using FL controller as per the reactive power demand.

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ISTAT Vs θ

Is IL = Is

Vs VSTAT

θ IL

a)

b) c)IL

θ

Is

VSTAT

Vs

ISTAT c) a)

Figure.3 Phasor diagram representing System without STATCOM b) Injection of reactive power into system c) Absorption of reactive power

The magnitude of voltage generated by the inverter should be just greater than the system voltage for injecting the reactive power into the grid. For absorbing reactive power from grid (say, due to increased line capacitance during light load), the inverter voltage should be dynamically adjusted to be just less than the system voltage. The partial output voltage of each H-bridge inverter in one phase and the output voltage of one whole phase are presented in Figure 4.

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Figure.4 Output voltage of each H-bridge inverter unit and Output voltage of one whole phase The mode of control is named as capacitive or inductive. The equivalent circuit diagram (per phase) is shown in Figure 5. It shows the connection of STATCOM to the system through an inductor.

Lc

Rc

Vs

Ic Vc

System/ Source

Inverter/ STATCOM

Figure.5 Per phase equivalent circuit diagram Several methods are discussed about getting reference reactive power component from system voltage and current for compensation (Alper Akdag et al., 2000, Fang Zheng Peng et al., 1997, and Peng.F.Z et al., 1996). Synchronous Reference Frame (SRF) transformation is used to separate reactive current component. The dq values of system current give the details about real and reactive power components.

V  2 1 V    3 0  

1

1

2 3

2

2



Va      V 3   b 2 Vc 

(1)

The grid voltage is assumed to be Va=Vm coswt, Vb=Vm cos(wt-120˚), Vc=Vm cos(wt+120˚) 53


After calculation of αβ plane parameters, the dq values are calculated using equation (2) as

Vd   cos  V     q   sin 

sin   V    cos   Vβ 

(2)

The inverter is synchronized to act as STATCOM for supplying or consuming reactive power only by fixing the active reference current as Icd* = 0 and the reactive reference current Icq* is calculated using equation (3),

QL  3VsphI sph sin   Vsd I sq

(3)

I 0

(4)

* cd

and

I   I sq * cq

Where Vsph, Isph are system voltage (per phase) and current (per phase) respectively and θ is the phase angle between the voltage and the current. I*cd, I*cq are dq compensating current components. Vsd, Isq are system dq voltage and current components. Thus, the reactive power component is calculated and given to control block as one of the control parameters. A simple three phase system has been taken for study. The system parameters (per phase) are given in Table 3. The inverter is designed with 1000VAr capacity. The proposed CI acts as STATCOM in an appropriate way.

Table 3. System parameters for case study System parameter Value of the parameter System phase voltage, Vsph 230V RMS System frequency, f 50 Hz VAr rating, QSTAT ±1 kVAr Current rating, ISTATCOM 2.5 A [Leading/Lagging] Coupling inductance, Lc 5 mH (5% drop) Capacitors Ci1,Ci2,Ci3;i = phase a, b and c 4700, 3300, 2200 μF

3. FUZZY LOGIC CONTROLLER FOR DC LINK VOLTAGE REGULATION FLC is used in this system to regulate the DC link voltage as per the reference command received from the control block. Purpose of this STATCOM is to generate leading/lagging VAr for the required value to meet the VAr demanded by the load. This is possible by controlling the magnitude of inverter output voltage to be just greater/ lesser than the magnitude of system voltage. For this, the DC voltage on the supply side of the inverter should be monitored and controlled to generate the required AC voltage (V c) on the output of inverter. So, the DC voltage is to be controlled and maintained at constant level. Formerly PI controller was used to control the STATCOM operation in dynamic mode, which needed complex control algorithm and mathematical representation. Here, for simplicity, the FLC is implemented in the control algorithm using MATLAB/SIMULINK tool in the STATCOM in a closed loop manner. FL helps to represent uncertain and imprecise knowledge of the system. Fuzzy control enables taking a decision even if inputs/outputs are found to be having uncertain parameters. FLC does not need any mathematical representation and is simple to implement. The capacitor voltage V dc (feedback) is sensed and compared with the desired reference value Vdc*. The error (E) signal (error = Vdc*-Vdc), the differentiation of error signal which is termed as the change in error (CE) are used as input for fuzzy processing. The output of FLC modulates the pulse width of gating signals of DC converter to regulate DC voltage of capacitor. The real power, from photo voltaic system, charges the DC link capacitor to maintain DC voltage at constant level at predetermined value, because the stored energy in the capacitor is frequently discharged to meet the switching/conduction (heat) losses occurring in the switches of H bridges. FL Control has two inputs and one output. The inputs are represented as Gaussian membership function with five sets (NB negative big, NS negative small, Z zero, PS positive small, PB positive big) in each and the output is represented as triangular membership function with five sets. The minimum operator is implemented to convert the crisp value into linguistic variable. For defuzzification the centroid method is used. The various operators are given below.

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AND – Intersection OR –Union NOT –Complement

:μA∩B= min[μA(x),μB(X)] :μAUB= max [μA(x), μB(X)] : μA= 1- μA(X)

The rules used in the proposed controller are shown in Table 4. Fuzzy logic is used to take decision about the pulse width modulation in DC-DC converter, to maintain the voltage across the DC link capacitor as constant and same as the reference value. The FLC provides the necessary pulse width modulation reference (α). This is shown in Figure.6. Table 4. Rule Base for Fuzzy Logic Control Error Change in error NBCE NSCE ZCE PSCE PBCE

NBE

NSE

ZE

PSE

PBE

NB NB NS NS Z

NB NS NS Z PS

NS NS Z PS PS

NS Z PS PS PB

Z PS PS PB PB

Separate FLCs are used to control and regulate the DC voltage across each DC link capacitor connected with DC-DC converter. i.e. Vdc, 3Vdc and 9 Vdc are to be maintained in one phase. Due to switching losses in inverters, the DC link capacitor voltage reduces intermittently from the predefined level. If it deviates from its expected value, then the STATCOM cannot be able to compensate for the reactive power. Therefore, it is very essential to regulate the DC voltage as per reference value. Whenever there is a drop in the magnitude of DC voltage across the DC link capacitor, power is drawn through DC converter from PV system to regulate and restore the DC voltage level for the normal operation of STATCOM.

Figure.6 FL controlled DC link voltage control blocks

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4. SIMULATION RESULTS AND DISCUSSION The 3- ACMLI (fifty three levels) along with the control block is modelled and simulated using MATLAB/SIMULINK. The source parameters (per phase) are given in Table 3. The simulation is carried out for various cases. Results are explained in detail. In this section, 4 types of load have been taken for comparing the performance of STATCOM with PI controller and FL controller through lagging load, leading load, and unbalanced load. 4.1 Case i) Lagging reactive load The RL load is taken for case study, with Rph= 92Ω and Lph = 0.2455H, which consumes the reactive power of approximately 855 VAr from the system. The source voltage, source current, load current and inverter current with a scaling factor of 1:35, are shown in Figure 7 (for all phases). The STATCOM is not connected with the system at PCC up to t = 0.05 sec, load current lags system voltage at an angle θ. So no compensation is made by STATCOM and current drawn/delivered by STATCOM is zero. At time t = 0.05 sec, the STATCOM is connected with the system. After getting connected STATCOM draws leading current from system i.e., it starts to act as capacitor and injects reactive power into system through PCC. The source current is made in phase with the source voltage due to appropriate level of reactive power injection from STATCOM. The required reactive current for load is supplied from STATCOM. So, the combination of the load and STATCOM is seen by the source as a resistive load. The current distortion is very less at PCC. During the reactive power injection, the RMS (Root Mean Square) magnitude of voltage of STATCOM is ensured to be greater than the system voltage as shown in Figure 8. This enables the STATCOM to inject the reactive power as demanded. The required real power has been taken from the PV system to make the inverter voltage dominate the system voltage.

Figure.7 Waveform of phase parameters a) System voltage (scale used is 1:35) and inverter current b) System voltage, load current and inverter current c) System voltage and system current 56


Figure.8 Waveform of phase parameters a) System phase voltage, STATCOM phase voltage (with slightly higher amplitude) and (the leading) inverter current b) RMS value of Voltage (phase) of the system as well as STATCOM voltage (slightly higher level) 4.2 Case ii) Dynamic lagging load During dynamic loads, the inverter should supply the required reactive power to the load/system to meet the transient requirements. It is noticed from Figure 9 that the load change takes place at time t = 0.3 sec, the inverter works in an expected way and responds very quickly in supplying reactive power because at time t = 0.3 sec, the inverter voltage level is promptly made somewhat greater than its own previous voltage level to supply additional reactive power. Figure 9 shows three phase voltages and currents. In all phases, the current is in phase with source voltage. Figure 10 shows the DC link voltage across each H-bridge inverter unit. It is essentially equal to 30V, 90V and 270V respectively. The three differently rated (in terms power) DC converter in one phase receive power from three different solar arrays which have different voltage and current ratings to suit the input specifications of the DC converters for their optimum performances. The input DC voltage rating of each DC converter (Buck-Boost) must be approximately equal to the output DC voltage rating of respective DC converters for maximum efficiency of each DC converter and to optimize the cost of solar arrays.

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Figure.9 System parameters: System voltage (phase) (scale used is 1:35) and current (phase) for a) Phase A b) Phase B c) Phase C d) System parameter: Current (Three phase system)

Figure.10 FL controlled DC link capacitor voltage across each H-bridge inverter of one phase

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4.3 Case iii) Lagging load to leading load Figure 11 shows the inverter operation during change over of the reactive load from inductive to capacitive. The inverter responds abruptly to change the polarityof the VAr output on getting the refernce from control block. If the load is RL (reactive power consuming load) then the inverter has to supply the required reactive power (+QSTAT) to the system. Similarly if the load is RC (reactive power injecting load) then the inverter has to absorb the reactive power (-QSTAT) from the system. Depending upon the load condition, the references have been accordingly changed in STATCOM control block to produce the required output voltage of the inverter to respond instantaneously to supply reactive power to the load or consume reactive power from the system.

Figure.11 a) System voltage (scale used is 1:35) and inverter current b) System voltage, load current and inverter current c) System voltage and System current during change of load from lagging power factor to leading power factor 4.4 Unbalanced load The STATCOM performance has been investigated for an unbalanced (VAr load) condition. STATCOM supplies required quantity of reactive current to individual phase of the load and maintains the current (hence VA) and phase angle balance in the system. Figure 12 shows the system voltage and current in all phases.

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Figure.12 System voltage (scale used is 1:35) and system current for a) Phase A b) Phase B c) Phase C d) Three phase system current for an unbalanced VAr load. In Table 5, Response of the inverter is analyzed and compared with conventional PI controller. Table 5. Comparison of Response of FLC with conventional PI controller Type of controller Case study Dynamic load change condition ( from +0.78 kVAr lagging to +0.94 kVAr lagging (difference of 0.16 kVAr) Lagging VAr to Leading VAr (from +0.78 kVAr lagging to -0.78 kVAr leading (difference of 1.56 kVAr)

Settling time for current to reach steady state (sec) PI controller

Fuzzy Logic Controller

0.08

0.05

0.05

0.02

The response of inverter [while supplying (as well as consuming) reactive power] is better with FLC than with conventional PI controller as shown in Figure 13 and Figure 14. The tracking of reference-reactive current of inverter while using PI is shown in Figure 15a. In addition, the tracking of reference-reactive current of inverter while using FLC is shown in Figure 15b. Better performance of FLC is due to regulation of capacitor voltage.

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Figure.13 Source currents with conventional PI controller

Figure.14 Source currents with proposed fuzzy controller

a)

b) Figure.15 Inverter current (q-axis) following the reference current a) For PI controller b) For FLC

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5. CONCLUSION A 3 asymmetric cascaded fifty three levels inverter proposed in this paper regulates the transport of reactive power by supplying/consuming the VAr at PCC according to the reference value obtained from the system. The inverter performance has been analyzed in a way for supplying/consuming reactive power in a distribution power system during dynamic load changes. This inverter becomes suitable for high voltage (HV) grid direct connection as it provides less THD in output voltage (due to fifty three levels). FLC enables faster and reliable DC link capacitor voltage control than PI control. FLC also smoothen the inverter current flow. If the number of levels is increased beyond this then i) the output voltage waveform produced by the inverter will be close to a sinusoidal waveform and ii) free from lower order harmonics. Thus the FLC improves the performance of ACMLI based STATCOM of a power distribution system by controlling the DC link capacitor voltage provided by PV system. References Alper Akdag, Susumu Tadakuma, Hideaki Minakata,“Load Balancing Control by Symmetrical Coordinates Frame for PWM Inverter Based Reactive Power Compensator”, Tran. IEE, Japan, 2000, vol. 121-D, no.1, pp. 43-51. Bhim Singh, R. Saha, “Modeling of 18-Pulse STATCOM for Power System Applications”, Journal on Power Electronics, vol. 7, no. 2, April 2007, pp. 146-158. Carlo Cecati, Fabrizio Ciancetta, Pierluigi Siano, “A Multilevel Inverter for Photovoltaic Systems with Fuzzy Logic Control”, IEEE Transactions on Industrial Electronics, vol. 57, no. 12, Dec 2010, pp. 4115-4124. E. Latha Mercy, R.Karthick, and S.Arumugam., “Fuzzy Controlled Shunt Active Power Filter for Power Quality Improvement”, International Journal of Soft Computing,(1816-0953), 2010, vol.5, no.2, pp.35-41. Elena Villanueva, Pablo Correa, José Rodríguez, and Mario Pacas, “Control of a Single-Phase Cascaded H-Bridge Multilevel Inverter for Grid-Connected Photovoltaic Systems”, IEEE Transactions on Industrial Electronics, vol. 56, no. 11, November 2009, pp. 4399-4406. Fang Zheng Peng, and Jih-Sheng Lai., “Dynamic Performance and Control of a Static Var Generator Using Cascade Multilevel Inverters”, IEEE Tran. on Industry Applications, May/June 1997, vol. 33, no. 3, pp. 748-755. Grain P. Adam, Stephen J.Finney, Ahmed M.Massoud, Barry W.Williams, “Capacitor Balancing issues of the Diode Clamped Multilevel Inverter operated in a Two-State mode”, IEEE Tran. on Industrial Electronics, August 2008, vol.55, no. 8, pp. 3088-3099. Jayakrishna.G, Anjaneyulu. K.S.R., “Fuzzy Logic Control based Three Phase Shunt Active Filter for Voltage Regulation and Harmonic Reduction”, International Journal of Computer Applications (0975 – 8887), November 2010, vol. 10, no.5, pp. 13-19. Jon Andoni Barren, Luis Marroyo, Miguel angel Rodriguez and Joseramon Torrealdaya Praiz, “Individual Voltage Balancing Strategy for pwm Cascaded H-bridge Converter based STATCOM”, IEEE Tran. on Industrial Electronics, Jan 2008, vol.55, no.1, pp. 21-29. J.Rodriguez, J-S. Lai, F. Z. Peng, “Multilevel Inverters: A survey of Topologies, Controls, and Applications”, IEEE Tran. on Industrial Electronics, Aug. 2002, vol.49, no.4, pp. 724-738. Peng.F.Z, Lai.J.S, McKeever. J. W. and Van Coevering.J., “A Multilevel Voltage-Source Inverter with Separate DC Sources for Static Var Generation”, IEEE Tran. on Industry Applications, Sep./Oct. 1996,vol. 32, pp. 11301138. S.Saad, L.Zellouma, “Fuzzy Logic Controller for Three-level Shunt Active Filter Compensating Harmonics and Reactive Power”, International Journal on Electric Power Systems Research, vol. 79, no.10,2009, pp. 13371341. S.Suresh received B.E degree in Electrical and Electronics Engineering in 2001 from Manonmanium Sundaranar University and M.E. degree in Power Electronics and Drives in 2007 from Anna University, India. He is currently the research scholar in the Department of ECE, Kalasalingam University, Krishnankovil, Virudhunagar, Tamilnadu. He is awarded as Young Scientist Fellowship Award (YSFA) for the year 2009, awarded by Tamilnadu State Council for Science and Technology Govt. of Tamilnadu. His research interests include Power Electronics, FACTS Devices and Power Quality. He is a life member of ISTE.

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[1]. S. Kannan received his B.E., M.E., and Ph.D Degrees from Madurai Kamaraj University, India in 1991, 1998 and 2005 respectively. His research interests include Power System Deregulation and Evolutionary Computation. He was a visiting scholar in Iowa State University, USA (October 2006-September 2007) supported by the Department of Science and Technology, Government of India with BOYSCAST Fellowship. He is working as Professor and Head of Electrical and Electronics Engineering Department, Ramco Institute of Technology, Rajapalayam, Tamilnadu, India. He is a Sr. Member of IEEE, Member in IET, Fellow of IE (I), Sr. Member in CSI, Fellow in IETE, Life member SSI, and Life member of ISTE.

P. Murugan received his B.E., (Electrical and Electronics Engineering) degree from Thiagarajar College of Engineering, Madurai- 625 015 affiliated to Madurai Kamaraj University, Madurai- 625 021, Tamilanadu, India in 1990 and M.E.,(Power Systems Engineering) degree from Thiagarajar College of Engineering (Autonomous), Madurai625 015, Tamilnadu, India in 1992. He is working as Associate Professor in ECE Department of Kalasalingam University (Formerly known as Arulmigu Kalasalingam College of Engineering (1984-2007), affiliated to Anna University) Krishnankoil, Srivilliputhur, Tamilnadu, India from 2004 onwards. He is working towards his Ph.D. His area of interest includes Power System Planning, Optimization and Evolutionary Computation. He is a life member of ISTE (LM14477).

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