Proc. of Int. Conf. on Control, Communication and Power Engineering 2010 281 © 2009 ACEEE Direct and Indirect Control Strategies of Dynamic Voltage Restorer K.Sandhya 1 , Dr.A.Jaya Laxmi 2 , Dr. M.P.Soni 3 1&3 Department of Electrical and Electronics Engineering, VNRVJIET, Bachupally, Hyderabad, INDIA Email: sandhyakp_msr @yahoo.co.in 2 Department of Electrical and Electronics Engineering, College of Engineering, JNTU Hyderabad, Hyderabad, INDIA Abstract— This paper presents direct and indirect control strategies of Dynamic Voltage Restorer (DVR). With the development of information and automation techniques, dynamic voltage problems are once again in spot light. As an important custom power device, DVR is powerful in solving these power quality problems. The program investigated was the MATLAB/SIMULINK. Simulation results were presented to illustrate and understand the performances of DVR with direct control and with indirect control strategy. The reliability and robustness of these control schemes in the system response to the voltage disturbances due to system faults or load variations are proved in the simulation results. Index Terms—DVR, Custom Power Devices, Dynamic Power Quality, Control Strategy. I. INTRODUCTION Quality power supply is essential for proper operation of industrial processes which contain critical and sensitive loads. For Power Quality improvement, the developments of power electronics devices such as FACTS and Custom Power Devices have introduced an emerging branch of technology providing the power system with versatile new control capabilities. Voltage sags and swells in the medium and low voltage grid are considered to be the most frequent type of Power Quality problems. Their impact on sensitive loads is severe. Different solutions have been developed to protect sensitive loads against such disturbances. Among these DVR is most effective device. A DVR injects a voltage in series with the system voltage to correct the voltage fluctuations. II. CUSTOM POWER TECHNOLOGY The concept of custom power was introduces in 1995 by N.G. Hingorani. Like Flexible AC Transmission Systems (FACTS) for transmission systems, the new technology known as Custom Power pertains to the use of power electronics controllers in a distribution systems [1], [2]. Just as FACTS improves the power transfer capability and stability margins, custom power makes sure consumers get pre-specified quality and reliability of supply. Some of these Custom Power Devices are: Series- connected compensator like DVR (Dynamic Voltage Restorer), Shunt-connected compensator like DSTATCOM (Distribution Static Compensator), Series and shunt compensator like UPQC (Unified Power Quality Conditioner) and SSTS (Solid State Transfer Switch). Among these, the DVR is an effective custom power solution which is based on the VSC principle [9] can deal with voltage sags and swells. III. DYNAMIC VOLTAGE RESTORER(DVR) A DVR is a device that injects a dynamically controlled voltage V inj (t) in series to the bus voltage by means of a booster transformer. The Dynamic Voltage Restorer employs series boost technology using solid state switches to correct the load voltage amplitude as needed [7],[8]. A DVR consists of a voltage source converter and is shown in fig.1. There are three single phase transformers connected to a three phase converter with energy storage system and control circuit [11]. The objective is to avoid voltage sags/swells in the network. Moreover, it is consider that the DVR acts only during fault period. On the contrary, it is considered by-passed. Figure1. Structure of DVR An equivalent circuit diagram of the DVR and the principle of series injection for sag/swell compensation is depicted in fig.2. The load voltage is given by V L = V S + V inj (1) Where V S is the supply voltage and V inj is the voltage injected by the mitigation device. Under nominal voltage conditions, the load power on each phase is given by S L =V L I L * = P L -jQ L (2) Where I L is the load current, P L and Q L are the real and reactive power taken by the load respectively during sag/swell.