孤岛型低压微网中改进型无功分配策略

为了解决基于传统下垂控制的孤岛型低压微网无功分配不合理及公共连接点(point of common coupling,PCC)电压降落严重这2个问题,在详细分析传统下垂控制中无功分配机理的基础上,提出一种改进型无功分配策略。通过引入虚拟感抗使得低压微网线路中有功无功解耦以满足所提策略的实现条件,将分布式电源(distributed generation,DG)单元空载电压幅值与PCC电压幅值的差值引入传统下垂控制中以形成闭环控制。对改进型无功分配策略实现过程中用DG单元接入点电压幅值来代替PCC电压幅值所造成的无功分配相对偏差进行了具体的量化分析。仿真结果表明,该改进型无功分配策略既可以实现无功的合理分配,又可以大大降低PCC电压降落。当空载输出电压幅值设定为155.5 V,2个DG单元共同承担负载的条件下,采用传统无功分配策略时PCC电压幅值为141 V,而采用改进型无功分配策略时PCC电压幅值为152 V。该研究可为微网实际运行控制提供参考。

Improved reactive power sharing strategy in islanded low-voltage microgrid

Abstract: Nowadays, more and more DG (distributed generation) and renewable energy sources, e.g. solar, wind, are promoted in vast rural areas. Several DG units can form an islanded microgrid together with storage units and common loads, which can achieve optimal management of electric energy and then better solve the power shortage problem in remote rural areas without electricity. The droop control is always used to realize power sharing in islanded microgrid because of no external communication and high reliability. Though the traditional frequency/voltage droop control technique shares common active loads, its reactive power sharing strategy is plant parameter dependent and does not realize reasonable reactive power sharing. Additionally, PCC (point of common coupling) voltage amplitude drop is serious because of reactive load increasing and inherent characteristics of the traditional droop control. In order to solve the problem of unreasonable reactive power sharing of islanded low-voltage microgrid and serious PCC voltage amplitude drop, a detailed analysis of reactive power sharing mechanism of the traditional droop control is carried out. By the analysis, the inherent limitation of the traditional droop control strategy is revealed and it is proved that the fundamental reason of unreasonable reactive power sharing is a transfer impendence mismatch. Based on the analysis, an improved reactive power sharing strategy is proposed for the microgrid working in islanded mode. Firstly, the proposed reactive power sharing strategy uses a control loop to introduce a virtual inductance at the output end of the DG unit in order to make a transfer impendence of low-voltage microgrid become inductive, which has only the active power and the reactive power decoupled and then meets the condition of realizing the proposed reactive power sharing strategy. Next the difference between the DG unit voltage amplitude at no load and PCC voltage amplitude is fed back to the traditional droop control in a certain way. The proposed reactive power sharing strategy can not only achieve reasonable reactive power sharing under the condition of transfer impendence mismatch, but also greatly improve serious PCC voltage amplitude drop which is caused by increasing reactive loads and the inherent characteristics of the traditional droop control. Through the small signal stability analysis of the proposed strategy, the stability conditions are revealed that are easy to meet in practice. Because the distance from the DG unit to the PCC is far, the acquisition of PCC voltage amplitude need communication, which undermines the advantage of local control of the traditional droop control. In order to keep the advantage of local control, the PCC voltage amplitude is replaced by the access point voltage amplitude of the DG unit, which leads to a reactive power sharing error. Quantitative analysis of the error in reactive power sharing has been carried out thoroughly. By the analysis, it is found that the error is acceptable in the project as long as certain conditions are satisfied. A simulation platform is made up of two parallel connected inverters and a common load. Various simulation results show that the proposed reactive power sharing strategy is very effective and does not interfere with active power sharing.