98/00377 Design of reactive current and voltage controller of static condenser

The STATic CONdenser (STATCON) is a shunt connected voltage source converter using self-commutating devices like GTOs which can be used for reactive power control. Their principle of operation is similar to that of a synchronous condenser. A typical application of a STATCON is for voltage regulation at the midpoint of a long transmission line. In this paper, a two stage structure for the control of voltage is investigated. This uses a closed loop reactive current controller whose reference is set by an outer voltage controller. For reactive current current control, the use of a PI controller and a fuzzy logic PI controller is investigated. It is found that the fuzzy logic controller overcomes the problem of oscillatory instability encountered when a conventional PI controller is used. An important observation is that the plant transfer function for voltage control (by reactive current injection) is generally of the nonminimum phase type. This precludes the use of large gains in a PI controller as there is a risk of instability of an oscillatory mode. Eigenvalue analysis using a linearized model is carried out to design a compensator in cascade with an integral controller to overcome this problem.