Asymmetrical voltage swells during recovery of a short-circuit fault lead to fluctuations in the dc-link voltage of a renewable energy conversion system(RECS),and may induce reversed power flow and even trip the RECS. This paper studies characteristics of both typical causes resulting in the practical asymmetrical voltage swell and the voltage at the point of common coupling(PCC)during the fault recovery. As analyzed, the fault recovery process can be divided into two continuous periods in which different control strategies have to be applied. Also protective measures are necessary in the transient period of the process. Additionally, the asymmetrical high-voltage ride-through capability and the controllability criteria of the RECS are analyzed based on eliminating the fluctuations. Furthermore, an asymmetrical control scheme is proposed to maintain the controllability of the RECS and ride through the entire recovery process. As verified by the simulation, the scheme can promise the RECS to deal with the practical fault recovery period and mitigate the dc-link voltage fluctuations, which improves the reliability of the RECS and the power system.
Along with the increasing penetration of distributed generation with voltage-source converters(VSCs),there are extensive concerns over the potential virtual rotor angle stability, which is characterized by oscillations of power and frequency during the dynamic process of synchronization in the grid. Several control strategies have been developed for VSCs to emulate rotating inertia as well as damping of oscillations. This paper classifies these strategies and provides a small-signal modeling framework including all kinds of VSCs in different applications for virtual rotor angle stability. A unified perspective based on the famous Phillips–Heffron model is established for various VSCs. Thus, the concepts of equivalent inertia and the synchronizing and damping coefficients in different VSCs are highlighted, based on the similarities with the synchronous generator(SG) system in both physical mechanisms and mathematical models. It revealed the potentiality of various VSCs to achieve equivalence with the SG. This study helps promote the unity of VSCs and traditional SGs in both theories and methods for analyzing the dynamic behavior and enhancing the stability. Finally,future research needs and new perspectives are addressed.
基于简化Bernardi模型的锂离子电池热模型包含内阻R、开路电压Uocv、热容mCH及换热系数k等参数。其中,R与Uocv是电池荷电状态(state of charge,SOC)、电流I和电池温度Tbat的非线性函数,mCH与k是常数。由于上述参数所表征电池特性不同,迄今的估计方法往往需要进行多组不同实验以分别激励对应特性,耗时耗力。该文的热模型参数估计方法仅由一组恒电流实验和相应数据处理流程组成。一组多环境温度下的恒电流实验用于在短实验时间内得到足够数据;基于电池机理的实验数据处理流程则解决了参数的非线性特性以及受到的SOC与Tbat耦合变化的影响。与现有方法相比,该方法具有短时间内同时且准确估计出R(SOC,I,Tbat)、Uocv(SOC,Tbat)和常数mCH与k等优点,易于工程实现。实验验证了方法的上述特点。
