新能源发电设备对电网的支撑能力需得到提升,因此构网型控制受到广泛关注,其中虚拟同步发电机(VSG)已成为研究前沿,并在工程领域中得到示范应用.以VSG作为同步环节的电压源型变换器(VSC)根据控制结构有无电流环分为电压电流双环控制和直接电压控制,控制结构的差异对暂态特性具有较大影响.为研究两种构网型VSC暂态特性的差异,基于“功率激励-内电势响应”关系模型分别建立其暂态模型,对比分析内电势形成机制和暂态特征.VSG模拟同步机运行特性,故在机电尺度下解析得到VSC的等效惯量和等效阻尼,并对比分析其暂态特性.结果表明:直接电压控制VSC的等效惯量和阻尼为常数,电压电流双环控制VSC的等效惯量和阻尼具有时变特征,且数值上小于直接电压控制.最后,通过电磁暂态仿真验证了理论分析的正确性.

As the support capacity of renewable energy generation equipment for the power grid needs enhancement, grid-forming control has attracted extensive attention, among which the virtual synchronous generator (VSG) has emerged as a key research frontier and is already being applied in engineering demonstration. Voltage source converter (VSC) with VSG as the synchronization link can be classified into voltage and current dual loop control and direct voltage control according to whether there is a current control loop in the structure. The difference in the two control structures has a significant impact on the transient characteristics of VSC. To study the difference between transient characteristics of two kinds of VSCs, the transient models are developed based on the “power excitation-internal voltage response” model, and the formation mechanism of internal voltage and transient characteristics are comparatively analyzed. Since the VSG simulates the operation characteristics of the synchronous machine, the equivalent inertia and equivalent damping of the VSC are analytically obtained at the electromechanical scale, and their transient behaviors are compared. It is found that the equivalent inertia and damping of a VSC with direct voltage control remain constant, while those of a VSC with voltage and current dual loop control exhibit time-varying characteristics and are numerically smaller than of the direct voltage control system. Finally, the validity of the theoretical analysis is confirmed by electromagnetic transient simulation.