A self coordinating parallel multi-pi control scheme for an HVDC transmission system to accommodate a weak AC system

To transfer bulk electrical power over long distances high voltage direct current (HVDC) transmission is currently being considered more and more because of its superior stability performance compared to AC lines whose stability are limited by their line inductances. There is, therefore, a greater need for thorough studies of dynamic interaction between AC and DC systems for use in system design and performance optimization.; The transients in an HVDC system can temporarily change line currents, voltages, and power substantially from the reference steady-state value, and the behavior of the system depends greatly upon the parameters of the controller. The DC system performance is affected by the AC system characteristics such as the short circuit ratio, and the AC system inertia. A controller which has good performance for a particular operating condition may not have sufficiently good performance under another operating condition or vice-versa. Performance of the controller also depends on the impedance at the rectifier and the inverter bus, which may vary according to the operating conditions of the AC network.; The objective of this research is to develop a control strategy, which would be simple and able to perform better in various operating conditions without the need for any parameter estimation, state measurement, and switching action. This new scheme is called a self coordinating parallel multi-PI controller scheme, where multiple controllers are connected in parallel. A linearized model of the CIGRE HVDC Benchmark system is used to calculate the control parameters.; To evaluate the performance of the proposed controller, a large AC fault in the weak AC system is studied for a two-terminal HVDC system, for a multi-terminal HVDC system, and for a modified WSCC 9-bus system. Results obtained by using the proposed controller and a conventional controller are compared. Fuzzy logic-based self-tuning and gain scheduling controller results are also compared with the proposed controller's simulation results.; Short circuit ratio (SCR) of an AC system plays an important role in the transient stability as well as in the selection of a controller. The proposed controller is tested at a very low SCR---the most critical value for system performance evaluation. The inertia of an AC system also has significant affect on the AC/DC system performance. Comparative evaluation of the proposed controller at both low SCRs and low inertia is presented. All simulations are conducted using the MATLAB/SIMULINK software package.