DYNAMIC STUDIES OF MULTITERMINAL DC-AC SYSTEMS

In transient stability programs that use a static dc network representation, the procedure to determine the control mode of operation and the solution of the multiterminal dc system is complex and time consuming. A systematic approach that is based on a linear programming formulation is presented in this thesis. The constraints incorporated in the LB formulation automatically ensure that the solution obtained is feasible. It is shown that the method is not only computationally efficient but also versatile in its ability to handle many of the common control characteristics, such as those of the constant angle (extinc­ tion and ignition), constant voltage, constant power and current controls, vol­ tage dependent current order limiter (VDCOL), end-stops, and also simulate the dynamics of power modulation and restart. As some applications require a three-phase detailed representation of the ac/dc system, a technique for detailed simulation of the dc converter and con­ trols is also presented. The developed dynamic simulation program is used to investigate the problem of on-line network flow control using converter controls of a multiter­ minal dc system. In view of fast response of the dc powers to converter con­ trols, a control method is proposed that extends the application of ac network flow control to dynamic situations. Possible applications of the method are to regulate power flows in a select group of ac lines, to smoothly steer the ac/dc system from its present state to some desired state and to enhance the dynamic performance of the ac system by controlling the transient changes in key or ’’backbone” ac lines. CHAPTER