Stability problems if three-phase systems with bidirectional energy flow are compensated using the FBD-method

A variety of power compensator circuits and even more methods to control them have already been published. However, most of these publications do not treat the critical case of loads changing abruptly from generating to consuming and vice versa without prior notice. Control algorithms well suited for handling passive loads may lead to instabilities in case of generating loads. The paper discusses this issue on the basis of a three-phase system containing a parallel compensator (shunt controlled filter). The control of the filter is based on the calculation of an equivalent conductance according to the so-called FBD-method. In the basic version the control scheme may lead to instability in case of generating loads. Simple countermeasures preventing instability are analysed theoretically and verified using simulations. Additionally a modified algorithm for the calculation of the equivalent conductance is described, which considerably reduces the required energy storage capability of the compensator in the worst case of stepwise changes of the load power