4.3 交直流混合配电网的电压控制研究
相比于低压交流配电网,低压直流配电网具有线路损耗小、节约成本等特点,且户用光伏及储能也更易于接入,同时具有更好的供电可靠性和电能质量[68],一些文献对于直流配电网的拓扑结构,规划以及控制问题进行了相关的讨论[69-71]。但是,将当前的交流网络直接转换成直流网络尚存在巨大的政策、资金以及技术挑战,因此,低压交流配网向低压交直流混合配网过渡成为当前网络发展的趋势之一,并且有可能在未来长期保持下去。文献[72]提出一种基于交直流互联的低压配网转供模型,如图8所示,利用AC/DC换流器将不同负荷特性的馈线末端连接起来,并且通过变下垂系数策略调节换流器流过功率的大小和方向,使得负荷特性不同的馈线功率均衡,可以有效化解电压风险。
文献[73]则提出了交直流线路并行架设的网络结构,如图9所示,交直流接口同时入户,传统交流网络的并网负荷、控制设备和控制方式不变,户用光伏以及电动汽车则通过直流馈线并网;直流馈线通过首端的储能控制,还可以稳定首端交流母线的电压,极大的降低了网络运行风险和设备投资。
图8 基于交直流混合低压配电网转供模型
图9 交直流线路并行架设结构
4.4 多目标多场景的电压控制研究
当前多数控制方法的控制目标和场景比较单一。一方面,这些文献重点解决光伏并网后所引起的电压越上限问题,对于传统的夜间重负荷所引起的欠电压问题以及光伏功率波动所引起的网络电压波动问题考虑还有所欠缺,即控制并没有充分考虑光伏的运行场景。另一方面,也鲜有文献考虑控制策略对于网络损耗和网络功率因数的影响,即控制并没有充分考虑网络运行指标。因此,有必要在含有高比例户用光伏发电的低压配电网中建立多场景、多目标的控制模型,兼顾网络风险的抑制及网络运行指标的优化。
4.5 考虑经济性因素的电压控制研究
当前,政策性因素是推进户用光伏并网的主要动力,但最终将会被市场和价格因素所取代。在此背景下户用光伏消纳问题的利益主体将变得多元化,基于一定的价格要素考虑光伏的并网以及网络电压控制等问题会更加合理,如可以建立不同利益主体间光伏并网和辅助服务的博弈及合作模型。在此背景下低压配网的电压控制将被赋予经济和价格属性,相关的研究将会对光伏的并网和设备的高效利用起到促进作用,也会对并网政策的完善提供参考和依据。
4.6 光储虚拟同步机建模及电压控制研究
与经过逆变器并网的电源不同,同步发电机的并网功率具有很大的惯性,避免了并网功率的快速波动。通过对光储逆变器的参数设置以及阻抗匹配,可以使分布式光储系统具有与同步发电机类似的并网特性[74-76]。当前文献对光储虚拟同步机建模的研究还有所不足,多采用简化的网络模型并且没有充分考虑低压网络的参数特点,光储虚拟同步机对网络电压的影响及调节作用的研究也较少。因此,相关的研究还有待补充和完善。
4.7 新型电力电子设备的研制
一些新型电力电子设备在低压配电网中的应用正在受到关注。固态分接头变压器的分接头变化不再需要进行机械性的调整,图10是固态分接头的结构示意图,通过对晶闸管的开合操作即可调整变压器的变比,避免了分接头的磨损,使得变压器的变比可以频繁调节[51],这将极大增强网络的电压调节能力。类似设备在低压配网中的应用还有待补充和完善。
图10 含固态分接头变压器的控制原理图
5 结语
本文围绕含高比例户用光伏低压配电网电压控制问题,总结了户用光伏在低压配电网的并网特征及其引起的电压问题,基于电压灵敏度理论介绍了当前低压配电网的电压控制原理。并以此为基础,结合低压配电网的通信条件,综述了基于有功、无功以及分接头的电压控制方法,同时对多类设备的协调控制方法也进行了介绍,总结了不同控制方法的使用场景、优势与不足之处。最后,对解决低压配电网电压问题的新技术进行了展望,以期能为该领域将来的研究提供一些参考。
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