1.Electric Power Research Institute, State Grid Shanxi Electric Power Company;2.Xi'3.'4.an Jiaotong University;5.The Hong Kong Polytechnic University
With the increasingly high penetration of renewable energy into the power system, the inertia level of the power system is gradually decreasing and the frequency stability issue has become increasingly prominent. This paper proposes a supplementary control strategy to improve the frequency stability of the renewable energy integrated power system, which estimates the unbalanced power of the system by detecting the rate of change of frequency in the center of system inertia (COI) as well as the equivalent inertia level of the system, and adjusts the referenced power of the integrated converter to provide inertia support for the system. The proposed method for estimating unbalanced power includes two parts: rate of change of COI frequency detection and system equivalent inertia calculation: considering the COI frequency curve passes through the zero points of the second derivative of the local measurement frequency curve, the COI frequency curve can be approximated by the piecewise linear curve connected by the second derivative zero points; the system equivalent inertia considering the effective inertia of the non-synchronized components in the system can be obtained by counting the synchronous inertia of the system, change of active power of generators, and the total unbalanced power. Subsequently, an additional control method that compensates for unbalanced power based on the proportion of new energy installed capacity is proposed to quickly adjust the referenced power value of the renewable energy integrated station when a frequency event occurs to suppress system frequency fluctuations. PSCAD/EMTDC simulation verifies the effectiveness of the proposed additional control strategy.