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1 Research Paper A NOVEL CONTROL SCHEME FOR THE MANAGEMENT OF OUTPUT VOLTAGE REGULATION WITH IMPROVED POWER QUALITY OF A MODULAR MULTI LEVEL INVERTER WITH FUZZY LOGIC CONTROL SYSTEM 1 A. V. Antony Albert, 2 Dr. V. Rajasekaran Address for Correspondence 1 Associate Professor, Department of ECE, SBM College of Engineering and Technology, Dindigul Professor and Head, Department of EEE, PSNA College of Engineering and Technology, Dindigul ABSTRACT Modular Multi Level Converters (MMC) are used for high voltage high power DC to AC conversion. The MMCs with increased number of levels offer close to sine wave operation with reduced THD on the AC side. In this paper a novel voltage control scheme for the regulation of the output voltage of a nine level MMC using Fuzzy Logic Control is discussed. The performance of the FLC in the face of load side and source side disturbances are studied and compared against the recorded performance of a PI controller in a similar situation. The real power pumping into the grid is forced to be at unity power factor with increased power quality ensured by the FLC managed average current mode control scheme The FLC based voltage regulation with unity power factor on the source side with sinusoidal source current as applied to a Nine Level MMC was simulated using MATLAB / SIMULINK. Key Words: Modular Multi level Converter ( MMC), Fuzzy Logic Controller (FLC), Average Current mode control. INTRODUCTION Modular Multi Level Converters (MMC) are suitable for high voltage and high power applications especially useful in the large power transaction systems like HVDC. Modular Multi Level Converters are also useful in static power filters and compensator in power systems [1,2,3,4,10,11]. With single DC source if a multi level inverter is opted for then the use of the cascaded H bridge inverter scheme will be ruled out as it need isolated DC sources [7]. The other options for constructing multi level inverters with single DC source will be the diode clamped or the flying capacitor type of multi level inverters. The uneven or unequal distribution of energy sourcing by each level of the converter leads to the uneven charge discharge characteristics and this leads to power quality problems [6,9]. With the modular multi level inverter the problem of capacitor voltage balancing as that found in the diode clamped inverter is eliminated. Another added feature with the MMC is that any number of MMC units or levels can be easily augmented [8,12,16]. In pumping large power into the three phase AC grid, from a huge DC source like the DC bus of the HVDC system or from that of a large DC power pool derived from large Photo Voltaic farm, the three important factors to be considered are that the terminal voltage is maintained in accordance with the grid, the incoming source should be in compliance with the power quality requirements as applicable to the grid and finally for the maximum utility of the MMC the power fed into the grid from the MMC should happen with unity power factor. As and when the command for real power demand to be met by the MMC is received the controller should govern the MMC in association with the existing PWM techniques like typical Multi Carrier Sinusoidal PWM or the Space Vector PWM such that the aforesaid power transaction requirements are met with. Since the inception of the MMC in [5, 13,14, 15] there has been a number of developments happening around the MMC. One of the two important directions of research, in the pursuit of ideal power support with the MMC, lead towards improved methods of PWM techniques like the SPWM with many styles of multi carrier methods and the SVPWM. The other direction of research points towards the novel control techniques exploiting the advantages of the modern digital systems like Digital Signal processor, Digital Signal Controller the VLSI technologies. The availability of high band width digital control schemes lead to the design of modern control systems like average current mode control system Fuzzy Logic Control system etc. A new modular voltage source inverter topology has been proposed in [14]. In [15], the authors proposed a new single-phase ac/ac multilevel converter for traction vehicles operating on ac line voltage In [16], the authors have discussed a prototype of multiphase modular-multilevel-converter with 2 MW power rating and 17-level-output-voltage. A new ac/ac multilevel converter family has been presented in [17]. The trends and scenario of multilevel VSC technologies for power transmission has been explained in [18]. New transformer less, scalable modular multilevel converters for HVDCtransmission is proposed in [19]. In [20] the authors analyze the control and performance of a transformer less cascade PWM STATCOM with star configuration. A transformer less energy storage system based on a cascade multilevel PWM converter with star configuration has been proposed in [21, 22]. The Novelty of this circuit is that it combines the features of the average current mode control and the Fuzzy Logic controller. Use of Fuzzy Logic Control system simplifies the design procedure while the average current mode controller leads to more closer a sinusoidal current through the load. The paper is arranged as follows. Next to this introduction under CHAPTER I, the Structure of the MMC and its modeling are given in CHAPTER II. The average current mode control scheme and the fuzzy logic control scheme are discussed in CHAPTER III. The sub systems of the MATLAB / SIMULINK based simulation is discussed in CHAPTER IV. The results and the related discussions are presented in CHAPTER V followed by the conclusion. CHAPTER II Structure of the MMC Figure 1 shows the structure of a modular multi level converter for three phase applications. The Nine Level MMC is fed from a single DC source Vdc. There are three legs with three nodes from where the out to the AC load is drawn.

2 Figure 1: Structure of three phase modular multi level converter Figure 2: Chopper cell unit In both the upper and lower arms of each leg of the MMC we have four numbers of chopper cells. The chopper cell units are as shown in figure 2. In each chopper module or cell we have two IGBTs and these two IGBTs are operated in a complementary fashion with the help of a NOT gate. Within each module is a capacitor connected across the two series connected IGBTs. The number of voltage level decides the sub modules in MMC. Equation 1 shows the relation between sub-modules voltage level: N n 2 1 V (1) where, Nv is the number of voltage levels, n is thetotal number of sub-module in the system. For phase R, V rn is the output voltage of phase-r with respect to the neutral point V r Vr0 Vdc Vru Vrl Vdc (2) 2 2 Vru Vrl Vdc (3) where, V ru and V rl, equivalent voltage sources of upper arm and lower arm. V dc is the DC voltage. The three phases are in symmetry. Hence, the current relationship is shown in Equation 4& iru idc ir (4) irl idc ir (5) 3 2 where i ru is the current of upper arm and i rl is the lower, i dc is the input current at the dc-side, i r is the output current at the ac-side. Figure 1 shows the structure of the system under consideration where the source is DC and it is converted into AC through the MMC and feeds an RL load. Voltage and Current measurements are carried out on both source and load sides. In between the source and the load a passive filter unit. CHAPTER III Average Current Mode Control The average current mode control is a technique used to ensure that the source current is sinusoidal and that the power factor on the source side is unity. In the case of a DC to AC power conversion system typically of the voltage source inverter it is essential that the modulation process leads to a sinusoidal output voltage. With suitable modulation procedures like the Sinusoidal PWM or the Space Vector PWM operated at fairly high frequency switching and with the aid of passive filters the output voltage waveform of the VSIs can be made sinusoidal. The basic objective of producing a sinusoidal voltage output is actually to drive the fundamental sinusoidal current alone through the loads demanding sinusoidal currents. In addition to using PWM techniques such as the SPWM or the SVPWM the average current mode control is a direct control technique that ensures that the current sourced by the inverter is as close to a sine wave as possible. A double stage control scheme is actually implemented in which one controller checks the error between the magnitude of the actual terminal voltage and the reference voltage. In addition to this a current controller is also implemented based on the fact that if a preset power is to be delivered to the load at the prescribed voltage then the current should be of the sinusoidal form in phase with the source voltage with appropriate amplitude to ensure the required power delivery to the load. Going by an example, in a star connected system of load, if 1 MW real power P is to be supplied to the load at a terminal phase voltage of V rms, at unity power factor, then the current to be supplied to the load will be given by the basic power formula. P = 1.732*VL*IL*Cos ф (6) Where VL and IL are RMS values. IL = P/ (1.732*VL*Cos ф) = P/ (3*VPh*Cos ф) with Cos ф = 1 (7) IL = = P/ (3*VPh) (8) If P = 1MWand Vph = 480v then IL = (1e^6)/480 amps. If the phase and frequency of the sinusoidal voltage source ф = 0 and f= 50 Hz then the instantaneous current to be drawn by the load can be given as (1e^6)/480 Sin((2*pi*50*t) + 0). The objective of the controller is to compare in real time, the instantaneous actual current with the instantaneous values of the set current. A PI controller with sufficiently large band width can be used here to track the actual load current and force it to be as close as the expected sinusoidal current. It is possible that the source current with the average current mode current with voltage regulation is of

3 better quality as compared to the case with simple voltage regulation. Fuzzy Logic control Fuzzy Logic control scheme is a technique to deal with approximate data to arrive at agreeable results. If the system model cannot be derived precisely, as required for the design of a PI controller design then the FLC can be used with competitively good results. The design of an FLC demands the experience of the user or the operator. The experience gained by the operator will be used as a knowledge base actually used in the FLC as a set of rules called the rule base. Importantly the meant here is not necessarily the same of the operator with the system under consideration. The experience of the user, designer or operator with other similar systems also counts in this situation. A person capable of driving one car, may in case of necessity drive another car with a little consideration and gets adopted for the new car. The main advantage of the FLC is that it can be used even under situations where the parameters of a system under control are not precisely known but where the experience of the user put in the mathematical form can make use of the experience in adapting to the task at hand. FLC is an intelligent control scheme as it is designed based on the human experience. While the FLC is fairly robust and is easier to design because of less mathematical overheads, it is not guaranteed that it should give better results as compared to the classical control systems like the PI variants. FLC can give practically acceptable results under linear and non linear control requirements and its major advantage is that it does not require a precise mathematical model of the system to be controlled. Fuzzy logic is a technique that deals with data presented in the form of linguistic variables rather than arithmetic variables. The entire universe of discourse or range of a variable is first normalized in a scale typically from -100 to 100 percentages and then this scale is divided into a number of segments. Each segment is assigned a name typically like Negative Big, Negative Small, Zero, Positive Small and Positive Big. The value of a variable after normalization will be accommodated in any of the segments with a certain degree of membership in that segment. With the fuzzy logic control the decision making process for the calculation of the control data is carried out in two phases. In the first phase a rule matrix is used to relate the segments of error and error rate and the segment of the output is found out. In the second phase considering the degree of membership of error and error rate in their respective segments the actual value of the output in its segment is found. In this application the universe of discourse of the error, the error rate and the output have been segmentised into five segments. There are 25 rules related in a slide rule matrix as shown in the figure. 3 and 4. The MATLAB SIMULINK based screen shots related to the Fuzzy Logic Controller are shown in figures Figure 5 The error and error rate input and the output functions as in Fuzzy Tool box in MATLAB SIMULINK. Figure 6 Membership function for Error. Figure 3. Typical universe discourse segmentised into number of segments Error NB NS ZE PS PB Error rate NB NB NB NB NS ZE NS NB NB NS ZE PS ZE NB NS ZE PS PB PS NS ZE PS PB PB PB ZE PS PB PB PB Figure 4. The rule matrix as used in the proposed application Figure 7 The Rule Base. Figure 8 The error erate and output surface.

4 Figures 13 and 14 respectively. The fuzzy logic unit is also shown in the figure 15. Figure 9 The Rule Base editor. CHAPTER IV MATLAB SIMULINK Simulation A three phase MMC with nine levels was simulated in the MATLAB / SIMULINK environment. The main system and the sub systems are as shown in figures Figure 13: Triangular carrier based PWM Figure 14: Sinusoidal carrier based PWM Figure 10: Main MATLAB simulink system of MMC Figure 11: A single cell structure Figure 15: FLC controller sub system Experimental Verification In order to validate the proposed system hardware based experimental verification unit was also fabricated and tested. A three phase MMC was designed to deliver 120 watts in each phase to a three phase star connected RL load. The chopper cells of the MMC were constructed using MOSFETS IRF 840. The control system was implemented in PIC 16 F 877A micro controller. The four sinusoidal wave carrier signals were developed using analog circuits. The reference signal was a sine wave of 50 Hz and for each phase the reference sine wave was generated from a three phase sinusoidal wave generator with facility to amplitude frequency and phase controls. Operational amplifier comparators were used for the staged comparison of the level shifted four carriers. Transistorized inverters were used to invert the gating pulses for the second MOSFET in each modular unit. The PIC 16F877A micro controller was mainly used for generating a control data based on the error that is based on the actual output voltage and current with respect the reference voltages and currents. The photograph is shown in Figure 16. Figure 12: Three phase load system Sinusoidal PWM with multiple sinusoidal carrier as well as triangular carriers were used. It was found that, under open loop operation with various modulation indices, the output of the sinusoidal carrier based PWM technique outperformed the triangular carrier based PWM. Therefore with the closed loop controller the sinusoidal carrier based PWM was considered. The subsystems of the triangular carrier based PWM and sinusoidal based PWM units are shown in Figure 16: Experimental setup for proposed system CHAPTER V

5 Results and Discussions The results of MATLAB / SIMULINK simulation are analyzed from different angles. Firstly the performance in open loop with triangular carrier based PWM has been compared against the sinusoidal carrier based PWM. The second comparison is that the performance of a simple voltage control scheme has been compared with the performance of the case with additional average current mode control scheme. The various waveforms associated with the proposed system and the existing systems have all been compared in figures 17 and 18 and table1. Triangular waveform (a) Line voltage (b) Neutral voltages (a) Line voltage (c) phase current (b) Neutral voltage (d) Sinusoidal PWM carrier (c) Phase current (d)triangular PWM carrier (e) FFT Figure 17: MMC results based on Triangular carrier PWM (e) FFT Figure 18: MMC results based on Siusoidal carrier PWM Conclusion The design and performance study of the Modular Multi Level inverter with a fuzzy logic controller and an average current mode controller has been presented in this paper. It has been observed by both simulation and experimental verification that the use of sinusoidal multi carrier PWM technique with fuzzy logic control scheme incorporated with average current mode control scheme is promising in terms of reduction in terminal voltage harmonics and source current harmonics. References [1] S. Mori, K. Matsuno, T. Hasegawa, S. Ohnishi, M. Takeda, M. Seto, S. Murakami, and F. Ishiguro, Development of a large static var generator using selfcommutated inverters for improving power system

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