Transient stability and optimal control of parallel A.C.-D.C. power systems

For a parallel ac-dc power system, the effect of a controlled dc link on system stability and damping depends on the system operating conditions.Thus a controller which improves stability at one operating condition may have a detrimental effect at other system operating conditions. Further, when the machine power angle to the ac line angle is high, improvement in stability is difficult to obtain solely by the control of a d.c. link. The variation with operating conditions of the influence of a dc link control on system performance indicates that a predictive or adaptive control mode would be needed to improve system stability over a wide range of system performance. The paper, therefore, examines both the conventionaI and optimal control of the link for large disturbance performance of a parallel ac-dc power system and digital simulation results show interesting features of optimal control. From a practical implementation point of view, full state optimal feedback law is suitably modified to be a linear combination of reduced state variables by an eigenvalue grouping technique and the performance of the actual nonlinear system with the suboptimal controller is quite encouraging.