VOLTAGE SAG MITIGATION USING DYNAMIC VOLTAGE RESTORER WITH MULTI-FEEDBACK CONTROL by G. Weerasekera This thesis is submitted to the department of Electrical Engineering in partial fulfillment of the requirement for the Degree of Master of Engineering Supervised by: Prof. H. Y. R. Perera & Dr. H. M. Wijekoon Department of Electrical Engineering University .of Moratuwa Sri Lanka 2005 82707 Abstract Voltage sag is one of the most serious power problems that the industrial customers are facing nowadays. Voltage sag is a momentary reduction of rms voltage. These momentary reductions are, sometimes, sufficient to cause tripping of sensitive equipment of an industrial installation. One such tripping can cause production loss worth of several hundred thousands to few millions of rupees depending on the nature of the industry (e.g. Glass industry). Voltage sags can be remedied at 'system-level' as well as 'device-level'. The system level solutions are costly because controlling the voltage sag events of a power system involves large amount of money and effort. On the other-hand, every customer. fed by a power system is not affected by voltage sags. Only few customers have sensitive equipment such as process controllers- which need protection from sags. Therefore a device-level solution provided at the customers' doorstep is more attractive in economic terms. Dynamic Voltage Restorer (DVR) is one of such 'device-level' mitigating devices that could be used to protect a customer from voltage sags. The basic theory behind the DVR is the series voltage compensation. In case of a sag of the incoming supply voltage to a customer installation, the DVR injects the balance of voltage in series with the incoming voltage so that the load does not see any abnormal condition. Since a sag continues within a few fractions of a second, the DVR has to respond fast enough to compensate the sag. Therefore, the control method adopted in the DVR has a vital role in its satisfactory performance. In this research project, three different control options, namely open-loop control system, close-loop control system and multi-feed back control system were designed, analysed and digitally simulated. This thesis contains the details of those design, analysis and simulation. Out of the three control options, the multi-feed back controlled DVR has shown superior performance compared with the other two options and therefore, this option is recommended for practical use in protecting an important load. From the analysis and also the digital simulations, it has been proved that the multi-feed back controlled DVR is capable of protecting any load up to 5 MVA (power factor from 0.6 to unity) against system voltage sags. Declaration I certify that this thesis has not been previously prepared in whole or part to any Un1vers1ty or Institution for a higher degree ··~···· G. Weerasekera Jan 2005 Supervisor .. ,,: " ~ CONT Contents Acknowledgements r\bbr~viations Abstract C hapte r I INTRODUCTION 1.0 Pm' er Qual it) 1.1 i'vlotiYation for Re<;earch 4 1.2 Obj~cti\CS or Research 5 ... Chapter 2 VOL TAG!:: SA(j~ \t'\1) ~Ahill Assoeiated \\ ith \ullage ~ags 14 Ill . .: "' NTS Page \I \'11 VIII 01 07 --- 2.6 Sag Compensation tvkthod!-. 17 2.6.! Reduct ion of Number of Faults 17 2.6.2 Structure Changes to the Power System 18 2.6.3 Reduction of Fault Clearing lime IS 2.6.4 Installation ofCompensJtion DC\'H:es at the Interfacing Point 20 2.!>.5 lmpto\ement of t~quipm..:mlmmunity to \'olta~c ~ags 21 Chaptet· 3 VOLTAGE SAG C0).1PENS!\ fiON USING DYNAMIC \{)] I'V.1L RL">I II , 1ll\ RJ ~ 0 Introduction to D\'R 2~ ~ I ~rructure of OVR 23 3 1.1 Lnergy Storage Device 23 I ~ 1.~ Volt Rl :--.1 I I~ .; (I ln trodUL't iO!l )j :\latlab Sunulink ~lode: 51 ... I I :-..-tude ling or Supply Vnlt.tgL' \\'a"~finm '' ith a '>ac 52 :- ! 2 111\crtel ,:ild l'ubc Cir~t:1; ~2 1\ .,., JO --- 51 5.1.3 DC' Link Voltage 52 5.2 Digital Simulation of Open-Loop Controlled DVR 61 5.3 Digital Simulation of Close-Loop Controlled DVR 61 5.4 Digital Simulation of DVR with Multi-feedback Control System 57 5.5 Simulation of Sag Compensation Methods of D\'R 61 5.51 Pre-Sag Compensauon 62 5.5.2 In-phase Compcnsatll)ll 62 Chapt«c>r CO~CLL.'SIO\: :\~0 Rl!C0~1~1El\DATIO~S Appendix A Computer models of 0Jftl:!rcnt 0\ R Configuration~ Appendix B Examples for \ oltagc Sags in Sri Lankan Power Syste;~~ Reference ... \ ' -- 6-t 67 71 78 Ackno1Niedgements I The guidance given by Prof. H. Y. R. Perera in doing this research project amidst his busy schedules of work as the Head of the Department of Electrical Engineering as well as a senior lecturer was an enormous support to me. My heartiest thank goes to him for his invaluable service. Secondly, my sincere thank goes to Or. H. M. Wijekoon for his valuable advices and guidance given at all times and at every step of this research study. It with his great support that I could expose myself to thfs newly developing field of study. Finally, I thank the three Deputy General Managers of Ceylon Electricity Board, Mr. H. A. G. Pushpakumara, Mr. Nalin Samaranayake and Mr. U. K. W. De Silva for giving me their assistance officially as well as personally during the period of doing this research project. G. Weerasekera ..... vi ABBREVIATIONS AC Alternating Current l DC Direct Current DVR Dynamic Voltage Restorer Fig. Figure IEC International Electromechanical Commission IGBT Insulated Gate Bipolar-junction Transistor kV Kilo-Volt mH milli-Henry ms milli-second MV Medium Voltage MVA Mega Volt Ampere .. PCC Point of Common Coupling p.f. Power Factor pu per unit PWM Pulse Width Modulation rms Root Mean Square sec. second THO Total Harmonic Distortion UPS Uninterrupted Power Supply v Volt -VA Volt-Ampere VSI Voltage Source Inverter 0 Ohm. VII