Digital Simulation of Multiterminal HVDC Systems for Transient Stability Studies Using a Simplified DC System Representation

As the control mode of operation of the dc system is variable dependent and the operating margin between neighboring modes is narrow, changes in the control mode of operation of the dc system often occur during transient conditions. For transient stability programs that use a static dc network representation, the approach is to cycle through the probable control modes until the computed values of the current operating state are consistent with the assumed mode of operation. With such an approach, the program logic for multiterminal dc system soon becomes complex and time- consuming, because the number of possible combinations of control modes in a multiterminal dc system increases rapidly with the number of converter terminals. A more systematic method that is based on a Linear Programming formulation is presented in this paper. The constraints incorporated in the LP formulation automatically ensure that the solution obtained is feasible. The method is applicable to a general parallel-connected multiterminal dc system and is capable of handling all sorts of converter operating characteristics. Numerical examples illustrating the successful application of the method in handling the operating characteristics of constant current, voltage, power and angle controls, voltage dependent current order limiter, and in simulating the dynamics of power modulation and restart are presented.