Control system for a PWM-based STATCOM

The always-increasing switching frequency of modern solid-state power switc hes, together with the application of multi-converter topologies, make it p ossible to use Pulse Width Modulation (PWM) in high power applications of S TATCOMs (STATic Synchronous COMpensators), This paper investigates the control system for a PWM-based STATCOM. First o f all. a discrete-time model of the STATCOM is derived to take into account the discrete-time implementation of the controller. Secondly, the control algorithm is detailed. It ensures decoupled control of the real and reactiv e power exchanged between the power converter and the electric-energy syste m, This is necessary to control the d.c. capacitor voltage during transient s of the exchanged reactive power Finally, the control of the capacitor vol tage is explained in detail. The controller is tailored to keep the capacit or voltage almost constant in spite of the fast control of the reactive pow er, This helps to reduce the capacitor size significantly, The main contrib utions will be illustrated using a 15 kVA laboratory prototype.