Abstract
Virtual inertia and damping control (VIDC) improve the stability of DC-MG. However, the potential positive feedback aggravates low-frequency oscillation induced by the interaction insides control loops. A multi-timescale impedance modelling framework is proposed to clarify stability mechanism of VIDC and the low-frequency oscillation of VIDC controlled DC-MG. In the proposed modelling tool, control loops of different timescales are visualized as multi-independent loop virtual impedance elements. An impedance circuit considering the constant power load (CPL) is formed, rather than an all-in-one impedance as the external dynamic representation of power converters. Concrete impedance analysis is performed on loops virtual impedances (LVIs) to explain the impedance-shaping effect of control loops intuitively, and the physical impedance nature of control parameters. The interaction between subsystems, which illustrates the stability mechanism of VIDC, is also analyzed. The low-frequency oscillation (LC impedance interaction) in voltage- and inertia-loop is elaborated by RLC circuits of LVIs. Besides, the potential instability factors, resulting in poor damping against voltage oscillation, are also revealed. Based on this observation, a dynamic stability enhancement method is further proposed to compensate for the negative damping caused by positive feedbacks of VIDC and CPL. A super-capacitor is added to alleviate rapid voltage changes. Accordingly, the passivity property of system impedance is strengthened, and the stability can be evaluated by Nyquist plot. The proposed impedance model framework and stability enhancement method are validated by the switching model simulation and hardware in loop experiment.
Published Version
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