大规模光伏电站经柔性直流并网系统故障穿越策略

Fault Traversal Strategy for Large-scale Photovoltaic Power Station via Flexible DC Grid-connected System

大规模光伏经柔性直流并网系统采用直流架空线路是未来新能源并网的趋势之一。然而,架空线直流短路故障发生概率较大,极易导致光伏电站脱网或换流站电力电子器件损坏。本文首先根据柔性直流输电系统、光伏电站特性,建立了相应的数学模型及仿真系统模型;其次,针对大容量光伏经双极模块化多电平换流器(modular multilevel converter,MMC)的高压直流功率传输系统发生直流架空线路单极短路接地故障工况,分析其故障特性;最后,提出综合考虑直流断路器(direct current circuit breaker,DCCB),换流站控制方式,光伏电站功率出力控制的直流故障穿越的协调控制策略。即故障发生时利用非故障极换流站继续进行功率传输,根据换流站额定功率与光伏电站输出功率计算得到不平衡功率,充分利用光伏阵列自身功率输出特性,优化光伏电站内的直流线路电压,实现控制光伏电站输出功率减载;对直流架空线路在瞬时故障情况下的故障穿越问题,提出了光伏电站减载以及换流站功率前馈增量控制,从而维持系统功率平衡,提高系统的并网稳定性;基于PSCAD/EMTDC的建模仿真,通过故障穿越措施前后的系统参数对比,表明所提方法能够有效地维持光伏电站与柔性直流系统运行特性,平稳实现故障穿越。

Large-scale PV adopts DC overhead lines for flexible DC grid connection is one of the trends of new energy grid-connected in the future. However, the high probability of DC short-circuit fault of overhead line can easily lead to the disconnection of photovoltaic power stations or the damage of power electronic devices in converter stations. To solve this problem, firstly, according to the characteristics of flexible direct current transmission system and photovoltaic power station, the corresponding mathematical and simulation system model were established.Secondly, in view of the single-pole short-circuit grounding fault condition of DC overhead line in the high-voltage DC power transmission system with large-capacity PV through bipolar modular multilevel converter, the fault characteristics were analyzed. Finally, considering the control mode of DC circuit breaker and converter station, as well as the power output of photovoltaic power station comprehensively, the coordinated control strategy of DC fault traversal were proposed. That is, when a fault occurs, the power transmission is continued by using the non-fault pole converter station, and the unbalanced power is calculated according to the rated power of the converter station and the output power of the photovoltaic power station. By making full use of the power output characteristics of the photovoltaic array, the DC line voltage in the photovoltaic power station is optimized, and the output power of the photovoltaic power station is reduced. To solve the problem of DC overhead line fault traversal under instantaneous failure, the load reduction of photovoltaic power station and power feed-forward incremental control of converter station were proposed, so as to maintain the power balance of the system and improve the grid connection stability of the system. Based on the simulations on PSCAD/EMTDC, the comparison of system parameters before and after fault traversal measures shows that the proposed method can effectively maintain the operating characteristics of photovoltaic power station and flexible DC system, and smoothly achieve fault traversal.