聚合恒功率负荷对直流微电网稳定性影响的阻抗法分析

电动汽车等恒功率负荷大量接入直流微电网可能导致系统振荡失稳,对系统的稳定运行提出了挑战。分析了恒功率负荷聚合接入导致系统失稳的机理,并提出了一种基于Nyquist判据的直流微网稳定性估计方法。首先,在负荷动态特性相同的假设下,推导了直流微网等效降阶模型,将含N个恒功率负荷的高阶直流微网等效为含单个负荷接入的微网降阶系统。在降阶系统中,单个恒功率负荷经N倍的线路阻抗和N倍的电源输出阻抗接入恒定电压源,其负荷个数N对稳定性的不利影响称为阻抗倍增效应。然后,为不失一般性,文章进一步讨论了恒功率负荷动态特性不同的情况。在此基础上,文章提出了一种微网稳定性估计方法,可用于衡量恒功率负荷聚合接入带来的失稳风险、并估计系统允许接入的CPLs的最大数量。最后,通过算例分析证明所提方法的有效性。

DC Microgrid StabilityAffected by Aggregated Constant Power Loads Based on Impedance Method

Accession of aggregated CPLs such as electric vehicles may lead to oscillation instability of DC microgrid, which challenges the stable operation of DC system. In this paper, the mechanism of instability risk caused by aggregation of CPLs is revealed, and a stability evaluating method based on Nyquist criteria is proposed. First, based on the assumption that all the dynamic models of CPLs are identical, a reduced-order model of high-order DC microgrid consisting of N CPLs is derived. In the reduced order model, single CPL is connected to a constant DC voltage source via the N times of impedance of tie line and output impedance of source subsystem, while the negative impact of N on stability is defined as Impedance Multiplication Effect. Second, without losing generality, situations of N CPLs with different dynamic models are taken into consideration. Then, based on analysis above, a stability estimation method is proposed, which can quantify the instability risk brought out by aggregation of CPLs and estimate the maximum number of CPLs. Finally, study cases are conducted to demonstrate the correctness of proposed method.