1.Department of Electrical Engineering, Tsinghua University, Beijing 100084, China;2.State Key Laboratory of Power System Operation and Control (Tsinghua University), Beijing 100084, China;3.Zhejiang Provincial Energy Group Co., Ltd., Hangzhou 310007, China;4.Zhejiang Zheneng Technology Research Institute Co., Ltd., Hangzhou 310000, China;5.Zhejiang Zheneng Jiaxing Offshore Wind Power Co., Ltd., Zhejiang 314211, China

In the grid-connected photovoltaic system, the maximum power point tracking (MPPT) control will introduce massive interharmonics to DC-side voltages, which may cause forced oscillations. Traditional research focuses on the relationship between the frequency of disturbance sources and the frequency of the inherent weak damping mode of the system, with less attention paid to the effect of converter control characteristics on exacerbating the forced oscillation. This paper takes the virtual synchronous generator (VSG) control as an example to construct a power input-output frequency-domain closed-loop transfer function model of the single-stage grid-connected photovoltaic system. It is pointed out that the interharmonics introduced by the MPPT control will cause forced low-frequency oscillation in the single-stage grid-connected photovoltaic system based on the VSG control. Then, the proposed model is compared with the the forced oscillation analysis method based on the traditional second-order time-domain differential equation. The equivalence between the proposed closed-loop transfer function and the traditional second-order differential equation is verified. Thus, the influence of disturbance sources and converter control characteristics on the forced oscillation can be analyzed from the perspective of the frequency-domain input-output relationship. Finally, the influence mechanisms of the VSG overshoot effect and dynamic coupling effect of active and reactive power on the forced oscillation are analyzed. The characteristics of the influence of the VSG control overshoot and power coupling on the exacerbation of forced oscillation are verified by simulation cases.

This work is supported by National Natural Science Foundation of China (No. U22B20100) and State-funded Postdoctoral Researcher Program (No. GZC20231212).