Exact analysis of a multipulse shunt converter compensator or Statcon .1. Performance

As the converter we take a voltage sourced inverter bridge with perfect switches. There is a single capacitor on the d.c. side and the output is connected to an a.c. system through an inductive link. By changing the phase, theta(o), of the operation of the inverter switches relative to phase of the a.c. system, the voltage across the capacitor can be controlled thus controlling the magnitude of the fundamental of the inverter a.c. output voltage. The difference between the inverter output voltage and the a.c. system voltage determines the flow of reactive power through the linking inductor to or from the system. Analysis shows that the reactive power is approximately given by the simple relationship Q approximate to -V-2/X[q(1) +q(2) theta(ob)] where X is the reactance of the inductive link, R is the combined resistance of the link and bridge and theta(ob) = 2X theta(o)/R is a control parameter. The coefficients q(1) and q(2) are functions mainly of root(XY) where Y is the susceptance of the capacitor at supply frequency and of the number of pulses in the inverter; q(1) is small and q(2) similar or equal to 0.5 for a well-designed equipment. There is a size of the passive elements below which the performance of the compensator is poor and above which it is not greatly enhanced. The required passive components are smaller than are needed for a compensator formed from switched capacitors and thyristor controlled inductors. Increasing the number pulses in the inverter by using phase shifting transformers, reduces the size of the passive components that are required as well as reducing the magnitude of the harmonic currents in the output.