Control Strategies of STATCOM during System Faults

The possibility of generating or absorbing controllable reactive power with various power electronic switching converters has long been recognized. Alternating Current Transmission Systems (FACTS) controller is the key element for flexible, controllable, and secure transforming transmission and distribution (T&D) system. Advances in power electronics technologies, such as the gate-turn-off thyristors (GTOs), modular voltage source converter (VSC) technology and digital control technology, promote the implementation of these new electronic concepts in transmission systems.;This dissertation is dedicated to a comprehensive study of multilevel voltage source converter based STATCOM and its application, especially its operation during system fault. To solve STATCOM operation problem during system faults, "Emergency PWM" is proposed. When a system fault is detected, "Emergency PWM" (EPWM) is implemented with angle control until the fault is removed. By this way, the switches are working in the 60Hz during normal time; there is no extra system loss. EPWM can prevent over-current and trips in the VSC during and after system fault, and ensures that the STATCOM supplies required reactive power.;Based on normal three-phase PLL, "Instantaneous PLL" (IPLL) is proposed. By using the voltage vector angle as the output of PLL, IPLL considers not only positive sequence, but also negative sequence, which is generated by system faults. System with IPLL has the same performance as system with normal PLL, and the system performance is improved by IPLL during system faults. It means a system can implement normal PLL for normal operation, and switch to IPLL at the fault.;Ice accumulated on power transmission lines in winter can cause severe damage to power system. VSC based STATCOM provides good asset utilization during the majority of the time when ice-melting is not required. By changing Iq reference under Iq regulation, fiving fixed angle for angle control to keep charging DC capacitors and changing DC voltage according the demanded DC current through transmission conductors, the STATCOM can perform the ice melting function when it is needed. The incremental cost for ice-melting capability is relatively small. The changeover procedure is simple and can be accomplished by remote control.;The integration and control of energy storage systems (ESSs), such as Supercapacitor (Ultracapacitor - UCAP) into a D-STATCOM (Distribution system STATCOM) is developed to enhance power quality and improve distribution system reliability. This dissertation develops the control concepts to charge/discharge the UCAP by the DSTATCOM, and validate the performance of an integrated D-STATCOM/UCAP system for improving distribution system performance under all types of system related disturbances.