System Frequency Support Through Multi-Terminal DC (MTDC) Grids
Control of the converter stations in a multi-terminal DC (MTDC) grid to provide frequency support for the surrounding AC systems is the subject matter of this paper. The standard autonomous power sharing control loop for each converter is modified with a frequency droop control loop. The objective is to minimize the deviation from nominal AC system frequency and share the burden of frequency support among the converter stations of the MTDC grid. The effectiveness of the frequency support is demonstrated through nonlinear simulation of a test system consisting of three isolated AC systems interconnected through an MTDC grid with four converter stations. An averaged model of the MTDC grids is developed to carry out modal analysis of combined multi-machine AC-MTDC systems. Modal analysis is used to characterize and substantiate the time domain behavior in presence of frequency droop control. It is established that appropriate droop control loop for the MTDC grid converters could be effective in reducing the deviation from nominal AC system frequency provided the sensitivity of the system eigen-values to changes in control parameters (e.g., droop coefficients) is accounted for a priori through modal analysis.