摘要
近年来相继发生的多起大面积停电事故,充分暴露了传统大规模电网的弊端。在能源危机和环境污染日益加剧的严峻形势下,世界各国开始关注基于可再生清洁能源的分布式发电技术。分布式发电具有许多优点,但大量分布式电源接入对大电网产生的不利影响制约了其发展,微电网的出现很好地解决了这一问题。微电网将分布式电源DG、负荷、储能单元及控制装置组合成一个单一可控的系统,既可与大电网并联运行,也可在大电网故障时孤岛运行。良好的运行控制是微电网实现诸多优势的前提,微电网的惯性小,容易受到扰动而产生振荡,所以对其稳定性的研究必不可少。
基于下垂控制的微电网可以实现孤岛运行时电压和频率的自动调节,无需借助于通信,因而可以提高系统的可靠性,易于实现微电源和负荷的即插即用,而且运行方式变化时微电源不用改变控制方法,可以实现微电网运行方式的无缝切换。
本文首先在分析微电源的输出功率传输特性和下垂控制原理的基础上,对下垂控制器进行了设计,针对低压微电网中线路阻抗呈阻性的特点,利用频域法对电压电流双环控制环节的传递函数进行分析,通过PI控制参数的设计使逆变电源等效输出阻抗呈感性,从而满足P-f、Q-U下垂特性。
其次,利用Matlab/Simulink软件搭建了下垂控制器的仿真模型并设计仿真算例进行了验证,在此基础上建立了采用对等控制策略的微电网仿真模型,对其孤岛、并网及运行方式切换时的运行特性进行了仿真研究,验证了下垂控制方法的有效性和对等控制策略的控制效果。
最后,建立了微电网孤岛运行时较完整的小信号状态空间模型,利用特征值法对孤岛微电网小信号模型的状态矩阵进行分析,得出不同参数变化时对微电网稳定性的影响：下垂特性系数增大会降低微电网系统的稳定性,线路阻抗参数较小时微电网系统容易失去稳定,并通过仿真验证了微电网小信号稳定性分析结论的正确性。
In recent years, large area power failures have occurred so frequently that fully exposed the malpractice of traditional grid. In the grim situation of increasing energy crisis and environmental pollution, people start to pay attention to distributed generation technologies based on renewable clean energy. Distributed generation has many advantages, but the adverse effects that the connection of a large number of distributed sources brings to the grid restrict its development, micro-grid is a good solution to this problem. Micro-grid combines distributed sources, loads, energy storage units and control devices into a single manageable system, which can both operate in grid-connected mode and islanding mode. Good operational control is the premise of micro-grid to achieve its advantages. The small inertia of micro-grid makes it vulnerable to disturbance and easily to oscillate, so it's essential to study on its stability.
Micro-grid based on droop control can achieve automatic adjustment of voltage and frequency without resorting to communication when operating in islanding mode, so it can improve reliability of the micro-grid system, and can easily implement the plug-and-play of micro-sources and loads, moreover, micro-grid can change its operation mode without changing the control method of micro-sources, so that it can achieve seamless switching of operation mode.
Firstly, based on the analysis of output power transmission characteristics of micro-sources and droop control theory, droop controller is designed. For the line impedance is resistive in the low voltage micro-grid, the transfer function of the voltage and current double loop control is analyzed using frequency domain method, and the equivalent output impedance of inverter sources is made inductive though design of PI control parameters, thus it can meet P-f, Q-U droop characteristics.
Secondly, simulation model of the droop controller is built in Matlab/Simulink software and verified by a designed simulation example, based on this, a micro-grid simulation model using peer-to-peer control strategy is established, then simulation research on operating characteristics of islanding mode, grid-connected mode and operation mode switching is conducted to verify the effectiveness of droop control method and the control effect of peer-to-peer control strategy.
Finally, complete small-signal state-space model of micro-grid in islanding operation mode is established, then the influence of different parameters'changing on micro-grid stability is concluded by analyzing the state matrix of micro-grid small-signal model though eigenvalue method:the increase of droop characteristic coefficient will reduce the stability of micro-grid system, and the micro-grid system will easily lose stability when the line impedance parameters become small. The conclusions of micro-grid small-signal stability analysis are demonstrated to be correct by simulation result.
引文
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