4 算例分析

采用PSCAD仿真软件对某一简单并网海上风电系统进行仿真分析。风电场采用等值模型表示，电场额定容量300MW，电能通过35KV电缆传送至陆地，再经35/110KV升压变连接至等值电网，故障点设置在电能接入点PCC处，如图6所示。

图7所示为故障发生时双馈电机的响应特性。故障类型为三相接地短路，持续时间0.15s。发生短路后，PCC处电压降落接近0，定子三相电压降低，定子三相电流迅速升高，直流电容电压升高，保护电路作用，转子电流降低。故障结束后，变频器开始工作，PCC处电压逐步恢复，30ms后达到1.0p.u，定子三相电压、三线电流恢复至稳定状态，转子三相电路恢复正常。此期间，电磁转矩降低，DFIG输出有功功率降低。有图b可知，故障后DFIG发出无功功率最高约80Mvar用于支持接入点电压。有图7可以看到故障期间，PCC处电压、定子电压电流、转子电流以及电机无功输出变化均符合风电机组并网要求，DFIG电机表现出良好的低电压穿越特性。

随着风力发电及风电并网技术的快速发展，风电在电网的比重会越来越大，电网规程对风电并网运行的要求将越来越高。电网故障期间，风力发电场将不能仅仅为了自身安全而切断与电网的联系，而应当承担起维护电网稳定运行的任务。基于DFIG的风电场网在电网故障期间在适当的控制策略和装置作用下，能够发出无功给电网提供电压支持，实现一定的低电压穿越功能。

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