ISSN (Print) : 2320 – 3765 ISSN (Online): 2278 – 8875 International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering (An ISO 3297: 2007 Certified Organization) Vol. 3, Issue 9, September 2014 10.15662/ijareeie.2014.0309017 Copyright to IJAREEIE www.ijareeie.com 12001 Cascaded Nine Level H-Bridge Inverter Control of STATCOM for Compensation of Reactive Power and Voltage Stability Enhancement K. Manikanta 1 , Dayakar Kande 2 , S.Sridhar 3 M.Tech (Power Electronics), Dept.of EEE, GIST, Nellore, AP, India 1 Assistant professor, Dept. of EEE, GIST, Nellore, AP, India 2 Associate Professor, Dept. of EEE, GIST, Nellore, AP, India 3 ABSTRACT: This paper deals with the design and implementation of a multi level based STATCOM to maintain voltage stability by compensating the reactive power in the power system. The static synchronous compensator (STATCOM) is the primary shunt connected Flexible AC Transmission System device, in which the power electronic devices are used to control power flow and to improve transient stability on power grids. The control of reactive power is performed by means of a cascaded nine level H-Bridge inverter. This multi level inverter uses forced commutated power electronic devices (GTOs) to synthesize a voltage from a DC voltage source. The control strategy used to control the output of voltage source converter is sinusoidal pulse width modulation technique in which specified harmonic elimination (SHEM) is possible. This paper presents an investigation of transient modulation-index controller and steady-state Modulation-index regulator for good dynamic response in the transient period and to ensure minimal harmonics at steady state. KEYWORDS: Reactive Power Compensation, Voltage Source Converter (VSC), STATCOM, GTO, SHEM, Phase Locked Loop (PLL), Multilevel Inverter, THD, SPWM. I.INTRODUCTION In a long transmission lines due to unbalanced and non linear loads there is a considerable transmission losses. These transmission losses will give rise to power quality issues in electrical power system. Due to sensitive loads present at the consumer side and deregulated market issues, it is necessary to provide quality power to the consumers. In order to meet all power quality problems like voltage sags, voltage swells, we are placing many fact devices like synchronous condenser, static VAR compensator, static synchronous compensator. So the performance of the ac system can improve by all these fact devices. Among all these STATCOM is the one which having better reactive power compensation capability and better control over voltage. In STATCOM there is a GTO or IGBT based 3 phase voltage source inverter, step down transformer and a DC capacitor. FACTS technology is the most advanced and most effective technology in which better voltage control, reactive power flow control, steady state stability, transient stability is possible. It can minimize the operational cost, which leads to reduced per unit cost. II. STATCOM STRUCTURE AND PRINCIPLE OF OPERATION The STATCOM basically consists of a step down transformer with a leakage reactance, a voltage source inverter (VSI), a capacitor in its DC side and a control system. The inverter in conventional STATCOM, switched with a single pulse per period and the transformer is connected in order to provide harmonic minimization and serve as a link between VSI and the system. For purely reactive power flow the three phase Voltages of the STATCOM must be maintained in phase with the system voltages. The variation of reactive power is performed by means of a VSC connected through a coupling transformer.