Study on Improving Frequency Stability of Long-chain and Weakly Interconnected Power Grids by DC Modulation

Abstract

Tibet Power Grid (TPG) is a typically long-chain and weakly interconnected power grid. It connects with China Northwest Power Grid through single $\pm 400\mathrm{k}\mathrm{V}$ Chaidamu-Lhasa (Chai-Lha) HVDC transmission line, and China Southwest Power Grid through 500 kV AC double circuits. At the same time, TPG is affected by the small installed capacity of conventional power supply and short circuit capacity. So the problems of frequency and voltage stability of TPG are prominent, which seriously limits the AC/DC transmission capacity and renewable energy accommodation. Considering the input of auxiliary frequency control, the frequency fluctuation can be effectively suppressed. In this paper, the auxiliary frequency controller (AFC) of Chai-Lha HVDC system is modeled, and the influence of the control parameters of the DC AFC on the frequency stability when the AC system fails is analyzed, and the simulation study based on the actual power system operation mode is carried out. Through simulation study, suitable parameters of DC AFC for TPG are obtained, so as to improve the power receiving capacity and renewable energy accommodation of the AC interconnected channel of TPG. Finally, the paper checks the influence of DC AFC on DC control itself, three defense lines of power grid, AC transmission capacity and photovoltaic absorption capacity. The research results have guiding value for the formulation of DC modulation strategy in actual power grid operation.