Abstract

An advanced static VAr compensator (ASVC), using a voltage-source-converter (VSC), popularly known as a static synchronous compensator (STATCOM), is generally acknowledged as the next-generation reactive power controller in power systems. Its mode of operation is analogous to that of a rotating synchronous condenser and it provides dynamic voltage compensation, increased transient stability and damping for the power system. Several STATCOMs, based on high power semiconductor switches (e.g. Gate Turn Off thyristors, GTOs or Insulated Gate Bipolar Transistors, IGBTs) and a special zigzag transformer have been put into operation. These STATCOMs have advantages over conventional static VAr compensators (SVCs) of lower harmonic generation, improved performance and a smaller reactor size. However, the zigzag transformers used in these STATCOMs are bulky, expensive and as yet unreliable. An alternative method to eliminate the zigzag transformer and to increase the STATCOM rating with a corresponding reduction in harmonics is to use a multilevel converter, which produces a 'multi-stepped' output voltage waveform in a close approximation to a sine wave. A recent addition to the multilevel converter based STATCOM family has been obtained by connecting a number of VSCs in series on the ac side of the system forming a 'chain' per phase. Each VSC is a 1-phase, full-bridge converter and forms a 'link' of a 'chain'. This STATCOM configuration is referred to as a 'Chain Link STATCOM' (CLS). In this thesis, a GTO is used as a switch for the CLS. However, any other high power semiconductor switch (e.g. IGBT) may also be used instead. Each GTO of the CLS, is switched on/off only once per cycle of the fundamental frequency by employing a sinusoidal pulse width modulation (SPWM) technique. Approximate models of a 3-phase CLS using dq -transformation are derived to design two cascaded controllers for controlling reactive current and ac voltage to stabilize the power system voltage at the point of common coupling (PCC). A novel technique, called the rotated gate signal pattern (RGSP), is used for balancing the voltages of the link do capacitors of the VSC. The performance investigation of the CLS system when used in a radial line transmission system is carried out under steady- and transient-state operating conditions by means of the simulation package EMTP-RV. Index terms. Chain link STATCOM, VSC, SVC, STATCOM, FACTS, EMTP-RV.