考虑控制系统的LCC-HVDC直流电流暂态特性分析与交流短路电流近似解析方法

Analysis of DC Current Transient Characteristics of LCC-HVDC Considering Control System and Approximate Analytical Method of AC Short-circuit Current

短路电流是电网规划的关键依据之一,特别是在交直混联系统中。当混合输电系统的交流侧发生故障时,直流电流的变化对交流短路电流有着直接影响,此外由于LCC-HVDC的直流控制系统对故障后直流电流暂态过程有着重要影响,因此该文针对LCC-HVDC受端交流系统故障,提出计及控制系统的直流电流暂态全过程的时域分段近似解析计算方法。首先分析受端交流母线发生三相短路故障后逆变侧换流器的换相失败暂态过程,并阐述故障后直流系统向逆变侧交流系统贡献直流电流的原理。其次,根据直流控制系统的结构和暂态响应特性,分段建立其传递函数模型。最后,将受控量的频域表达式经过拉式反变换,得到时域下的直流电流分段解析式,并进一步得到考虑直流电流贡献的逆变侧交流短路电流的近似解析结果,通过与实际仿真进行对比,验证提出的直流电流计算方法的正确性。该近似解析结果能够较为准确描述故障后逆变侧交流电流的暂态全过程,为线路保护定值和断路器相关参数的选取和整定提供一定的依据。

Short circuit current is one of the key basis of power grid planning,especially in AC-DC hybrid system.When the AC side of the hybrid transmission system fails,the change of DC current has a direct impact on AC short-circuit current.In addition,the DC control system of LCC-HVDC has an important impact on the DC current transient process after the fault.Therefore,a time domain piecewise approximate analytical calculation method considering the whole DC current transient process of the control system was proposed when the AC system fails at the receiving end.Firstly,the transient process of commutation failure on the inverter side after three-phase short-circuit fault of receiving AC bus was analyzed,and the principle that the DC system contributes DC current to the AC system on the inverter side after fault was described.Secondly,according to the structure and transient response characteristics of DC control system,the transfer function model was built section by section.Finally,the frequency domain expression of the controlled quantity was transformed by inverse Laplace transform to obtain piecewise analytical expression of DC current in time domain,therefore the approximate analytical results of the AC short-circuit current on the inverter side considering the contribution of DC current was obtained.The correctness of the DC current calculation method given in this paper was verified by comparing with the simulation.The approximate analytical results can accurately describe the whole transient process of AC current on the inverter side after fault,and provide a certain basis for the selection and setting of line protection setting value and circuit breaker related parameters.