Study on frequency oscillation controlling strategies for islanded operation mode of HVDC transmission systems

Islanded operation mode of HVDC sending terminal is the condition that generators are only connected to DC system, disconnected with main AC networks. When disturbed, an islanded system is more prone to be frequency instable than a connected system. In this paper, a type of frequency controller based on phase compensation principle is proposed. By changing the closed-loop dominated poles to expected values, the controller can make the system frequency return back to normal in a few seconds. The characteristics of islanded systems are introduced. A practical UHVDC project is introduced, based on which, a detailed module is set up in PSCAD/EMTDC. The proposed frequency controller is added to the module, and four typical faults are conducted to verify the effectiveness of the controller. A proportion-integral (PI) controller of optimal parameters is designed to make a comparison with the proposed controller. The reported results demonstrate that the proposed frequency controller is a better choice than PI controller in eliminating frequency oscillation, and it can improve the steady state stability of islanded systems.