The distribution STATCOM for reducing the effect of voltage sag and swell

The concepts of flexible alternating current transmission systems (FACTs) and custom power have been widely studied. FACTs use power electronic devices and methods to control the high-voltage side of the network for improving power flow. Custom power is for low-voltage distribution, and improving poor power quality and reliability of supplies affecting factories, offices and homes. Power quality and reliability are becoming important issues for critical and sensitive loads after the introduction of the term of custom power by Hingorani in the early 1980s. Custom power devices is classified into three categories by their structures: dynamic voltage restorers (DVRs), distribution STATCOMs (DSTATCOMs) and unified power quality compensators (UPQCs). Among these devices, the main purpose of a DVR that injects voltage in series with a distribution feeder is reducing the effect of short-term voltage sags, dips, swells and momentary interruptions. The DVR is capable of generating and absorbing voltage independently controllable real and reactive power. It consists of a three-phase voltage source inverter, injection transformer, DC link and rectifier for charging the DC link or battery. As is known, rectifiers generate harmonic problems in distribution lines. Rectifiers or devices for charging the DC link are useless in this proposed system due to their structure. The proposed system has the function of generating and absorbing voltage using a self-charging control technique. This system has three states: (1) normal operation; (2) charging operation; and (3) recharging operation. The paper discusses control issues and the proposed control algorithm. The proposed control technique is applied to a DSTATCOM for protecting voltage sags, swells and momentary interruptions. The dynamic performance is analysed and verified through simulation