Voltage Sag And Swell Case Study

found in the literature. These problems have become one of the major concern, especially, with the use of power electronics devices which are sensitive to the variation of power supply. Among these problems Voltage Sag & Swell are identified as major concern to the electrical consumers. Voltage Sag is one of the most occurring power quality problems that cause severe problems and losses to industry. With the power systems restructuring and shifting trend towards distributed and dispersed generation, the concern of power quality has taken newer dimensions. In developing countries like India, where the variation of power frequency and many such other problems related to power quality are themselves a serious question, it is very vital to take
These devices are modeled using PI controller and Discrete PWM pulse generator. The simulations were performed using MATLAB/SIMULINK using SIMPOWER SYSTEM.
Keywords: DSTATCOM (Distribution Static Compensator), Power Quality, DVR (Dynamic Voltage Restorer), Sag, VSC(Voltage Source Converter), PWM
I. INTRODUCTION
Voltage sag, voltage swell and momentary power loss are seen as vital issues from the last decade due to the widespread use of sophisticated power electronic equipment. Due to this the loads are becoming more sensitive and less tolerant to short term voltage disturbances in the form of voltage sag and voltage swell. A great effort has been made to enhance the power quality either
The DVR consists of solid state power electronics switching device either GTO or IGBT, energy storage device and injection transformers. It can maintain load voltage by injecting three phase output voltages whose phase, magnitude and frequency can be controlled [2]. The converter generates the reactive power needed while the taking active power from the energy storage. The energy storage can be different depending on the needs of compensating. The DVR often has limitations on the depth and duration of the voltage sag that it can compensate. The DVR expected time response is about 25millisecond and which is less than some of the traditional methods of voltage correction such as tap-changing transformer. Fig.2. Schematic diagram of DVR

Energy storage device becomes one of the constraint factors in the distribution compensation process, especially for sag and long duration fault due to its capacity. The load impedance Zth depends on the fault level of the load bus. When the system voltage (Vth) drops, the DVR injects a series voltage VDVR through the injection transformer so that the desired load voltage magnitude VL can be maintained. The series injected voltage of the DVR can be written as:

VDVR= VL + Zth IL - Vth (3)
Where, VL: The desired load voltage magnitude; Zth: The load impedance;