直流电网阻抗建模与振荡机理及稳定控制方法

维持直流母线电压稳定是直流电网高效、安全、稳定运行的前提。多个电压源变换器(Voltage Source Converter,VSC)经下垂控制接入直流电网,下垂控制等效于在VSC直流端口增加虚拟电阻,实现多VSC之间的功率分配的同时增加了系统阻尼。但研究发现,采用直流电压下垂控制的VSC,其输出阻抗会在电压控制带宽外呈现负阻性,该负阻抗会与线路阻抗、负载输入电容及恒功率负载(Constant Power Load, CPL)相互作用,引起直流系统振荡。针对此问题,建立了典型的单母线直流电网系统小信号模型,分析了直流系统的稳定性。提出一种虚拟阻容性阻抗的稳定控制方法,使得VSC输出阻抗在电压控制带宽外保持较大的正阻性,抑制直流系统的振荡。同时所提方法能够增大VSC直流端口的容性阻抗,增强直流系统的惯性,提升母线电压抗负载波动的能力。最后,仿真验证了所提控制方法的正确性和有效性。

Impedance-based modeling, stability analysis and virtual damping approach in DC grid

Maintaining DC bus voltage stability is a prerequisite for efficient,safe and stable operation of DC power grid.Multiple Voltage Source Converters(VSCs)integrates into DC grid with voltage droop approach.Voltage droop approach is equivalent to install a virtual resistor in series to VSC’s DC port.In general,droop approach realizes the power sharing between VSCs and improves the damping of DC system.However,research finds that when VSC is running with DC voltage droop approach,there is a negative resistance component in the output impedance of the VSC.The DC-side voltage is easy to produce high-frequency oscillations,because of the negative resistance,the line parameters and the complex impedance interactions by Constant Power Load(CPL).For this issue,small-signal impedance-based model of the DC power system is modelled and the stability of the DC system is analyzed in this paper.Next,a virtual capacitive resistance impedance control approach is proposed to reshape the output impedance of the VSC,the proposed approach keeps the positive impedance outside the bandwidth of voltage loop and it can suppress the oscillations of the DC grid.Meanwhile,the proposed approach improves the capacitive component of the output impedance of VSC;hence,the dynamic response of the DC bus is smoothed and the inertia of the system can be improved.Finally,simulation results validate the correctness and efficiency of this proposed control approach.