International Journal of Computer Applications (0975 – 8887) Volume 111 – No 7, February 2015 23 Dynamic Voltage Restorer with New Hybrid Cascade Multilevel Inverter using Fuzzy Controller Varsha Singh N.I.T Raipur Electrical Engineering Department Raipur, India S.Gupta N.I.T Raipur Electrical Engineering Department Raipur, India S.Pattnaik V.N.I.T Nagpur Electrical Engineering Department Nagpur, India ABSTRACT Inverters are used in many applications now-a-days. One among the major applications is Dynamic Voltage Restorer (DVR), it is a custom power device to guard sensitive loads against voltage disturbances such as voltage sag and voltage swell, both considered to be important parameters of power quality. This paper attempts to study and analyze the performance of a DVR based on a new 11-level symmetrical multilevel inverter, using minimum required numbers of switches with fuzzy logic controller to compensate voltage sag and swell to regulate the load voltage to the best possible extent. The proposed multilevel inverter topology also reduces the cost and complexity in controlling of inverters. Keywords Multilevel Inverter, DVR, Voltage sag, Voltage Swell. 1. INTRODUCTION In modern Industrial setups, several electronic devices are used to enhance production. These electronic devices are susceptible to failure due to sudden change in quality of power supply. Power quality is the set of limits of electrical properties that allows electrical/electronic system to function in their intended manner without significant loss of performance or life. Voltage sags & voltage swell, poor voltage and frequency regulation, harmonics and switching transients are frequently encountered power quality issues. Voltage sag is a sudden reduction of utility supply voltage from 90% to 10% of its nominal value, whereas, voltage swell is a sudden rise of supply voltage from 110% to 180% of its nominal value. The main requirement of any system is to maintain the load side voltage at constant level. Dynamic Voltage Restorer (DVR) provides effective solution to the power quality problems such as voltage sag and voltage swell.[1] DVR is mostly used for Low Voltage (LV) and Medium Voltage (MV) applications. DVR protects a sensitive load from distortions in the supply side during faults or overload in power system by inserting a voltage of required magnitude and frequency; the series compensator can restore the load side voltage to the desired amplitude and waveform even when the source voltage is unbalanced or distorted In LV application generally two level VSI are used and if two level VSI are used in MV application, the switches of VSI must block high voltages. A closed loop control method is used to maintain a constant load voltage in the event of disturbances in supply [2]. In MV applications it is more appropriate to implement DVR with Multilevel VSI to reduces the stress on switches and lower the losses at switches[3][4]. In DVR, Voltage Source Inverter (VSI) is used to generate required amount of voltage to maintain the load voltage at constant level. Recently several multilevel inverter are used in DVR for compensating the voltage sag and swell [5][6]. The main disadvantage of multilevel inverter is that, it requires more numbers of power switches which increases the losses and control complexity of inverter [7][8][9]. Sometimes the output voltage is not limited to DC voltage source similar to traditional cascade multilevel inverter and can be increased with Z network shoot-through state control [10]. Fuzzy logic controller is implemented in DVR to respond quickly on the request from abruptly changing reference signals [11]. As proposed by authors, (Ramasamy.M et.al.) a fuzzy logic based PV-DVR can be deployed to diminish the power quality problems during peak load and off load hours at distribution system thereby also doing away with the shortcomings of conventional DVR and UPS[12]. The addition of fuzzy logic control to conventional PI control gives added advantage of faster response as compared to the conventional one .The STATCOM, which is a vital device in field of power quality, has several applications such as power factor corrector, harmonic compensator and voltage mitigation [13]. To improve the performance of DVR the fundamental frequency control method is used instead of pulse width modulation- based methods, these results in lowering the switching losses. [14]. By using a ac/ac converter, and eliminating the dc-link, the topology for DVR is further simplified, this topology can be extended to n-phase systems also without compromising the basic principles, to restore voltage sags and swells of both balanced and unbalanced conditions [15]. In this proposed work a fuzzy logic controller to control symmetrical Hybrid Cascade Multilevel inverter with less numbers of power switches is used for DVR to compensate Voltage sag/ swell. Out of the different power quality problem i.e. flicker, transient, harmonic distortion voltage sag/swell the important power quality problems voltage sag/swell are discussed. Voltage Sag: Definition: According to the IEEE defined standard, IEEE Std. 1159, 1995 Voltage Sag is defined as a decrease of RMS voltage from 10% and 90% of nominal, for duration of 0.5 cycles to 1 minute. Decrease in the RMS value of the voltage ranging from a half cycle to a few seconds and voltage sags often caused by starting of large induction motors, energizing a large capacitor bank and faults such as single line to ground fault, three phase to ground fault, double line to ground fault on the power distribution system. Voltage Swell: Definition: As per IEEE standard, is defined as the increase in the RMS voltage level to 110% - 180% of nominal, for durations of 0.5 cycles to 1 minute. Increase in the RMS value of the voltage ranging from a half cycle to a few seconds causes Voltage swells that are usually associated with system faults condition just like voltage sags but are much less common. This is mainly true for ungrounded or floating delta systems, where the sudden change in ground reference results in a voltage rise on the ungrounded phases. In the case of a voltage swell due to a single line to ground (SLG) fault on the system, the result is a temporary voltage rise on the un-faulted phases, which last for the duration of the fault.