低压微电网中两级LCC逆变系统及其并网控制策略研究

发布时间：2019-06-25 12:02

【摘要】：针对低压微电网特性及非线性因素、三相负载不平衡日益凸显的问题,对其控制方式和逆变系统的研究意义重大。本文对两级式拓扑结构的逆变系统进行了分析,前级通过Buck-Boost电路进行直流电压变换,跟随分布式电源电压波动影响来完成对电压的调整,以满足后级逆变器工作需要,后级并网逆变器采用LCC(电感-电容-电容,inductor-capacitor-capacitor)逆变电路将前级的输出电压转换成满足并网要求的交流电。文中首先利用状态空间平均法针对Buck-Boost电路进行分析建模,得到交流小信号的线性传递函数,针对系统的不稳定问题,设计了比例-积分补偿器,使控制系统在输入电压以及负载出现阶跃扰动情况下,能够有效抑制低频扰动并迅速跟随给定电压。然后分析建立了输出电压基波仅由输入的基波电压决定的电压型LCC逆变系统,在非线性负载和负载不平衡下均具有良好的输出电压质量。文章在LCC逆变系统优良特性的基础上,重点研究了LCC逆变系统的电压控制策略。针对LCC逆变器的开环控制策略存在启动超调大和无法抑制输入电压扰动,采用一种输出电压的实时滞环控制策略,有效减小了启动超调,同时实现了对电压扰动的快速调节。通过对LCC逆变器接入三相不平衡负载和非线性负载情况的分析,根据LCC逆变器系统的模型,设计了扰动分量的补偿器,引入一个补偿链抵消非线性负载的影响。针对调制过程中死区效应引起的输出电压幅值下降问题,通过对死区时刻单相桥臂的工作模式分析,设计了死区效应的消除抑制策略。同时,在阐述传统Pf-QV下垂控制原理的基础上,针对低压微电网设计了引入阻性虚拟阻抗的频率电压协调控制的改进的PV-Qf下垂控制策略。当逆变器向电网馈能时,设计了一种并网开环控制策略,仿真结果说明开环控制在理想情况下是可行的,而对于存在扰动的情况,采用比例-谐振控制策略将LCC逆变系统同步并入电网,控制系统能够无相位偏差的跟踪50Hz的正弦信号,同时对其他谐波具有衰减效果。最后,在MATLAB/Simulink中搭建了含两个LCC逆变系统并联的微网仿真模型并进行孤岛和并网模式下的仿真分析。在孤岛模式下,验证了改进的针对低压微电网的下垂控制策略能实现两台逆变器的功率分配和平滑投切。在并网模式下,对设计的比例谐振控制器进行仿真,验证了并网电流能实现对给定电流的跟随和单位功率因数运行。此外,对LCC逆变器与LCL逆变器并网电流的波形质量进行了比较,LCC逆变器并网电流的稳态误差与谐波畸变率均小于LCL逆变器,进一步说明了LCC逆变器的优良特性。
[Abstract]:In view of the characteristics and nonlinear factors of low voltage microgrid and the increasingly prominent problem of three-phase load imbalance, the research on its control mode and inverter system is of great significance. In this paper, the inverter system with two-stage topology is analyzed. The front stage converts the DC voltage through Buck-Boost circuit, and follows the influence of distributed power supply voltage fluctuation to complete the voltage adjustment in order to meet the working needs of the later stage inverter. LCC (inductor-Capacitor, inductor-capacitor-capacitor) inverter circuit is used to convert the output voltage of the front stage into AC current which meets the requirements of grid-connected. In this paper, the state space average method is used to analyze and model the Buck-Boost circuit, and the linear transfer function of AC small signal is obtained. Aiming at the instability of the system, a proportional integral compensator is designed so that the control system can effectively suppress the low frequency disturbance and follow the given voltage quickly when the input voltage and load appear step disturbance. Then the voltage source LCC inverter system, which is only determined by the input fundamental voltage, is analyzed and established, which has good output voltage quality under both nonlinear load and load imbalance. Based on the excellent characteristics of LCC inverter system, the voltage control strategy of LCC inverter system is studied in this paper. Aiming at the open-loop control strategy of LCC inverter, which has large start-up overshoot and can not restrain input voltage disturbance, a real-time hysteretic control strategy with output voltage is adopted, which effectively reduces the start-up overshoot and realizes the fast regulation of voltage disturbance. Based on the analysis of three-phase unbalanced load and nonlinear load of LCC inverter, according to the model of LCC inverter system, the compensator of disturbance component is designed, and a compensation chain is introduced to counteract the influence of nonlinear load. In order to solve the problem of output voltage amplitude decline caused by dead-time effect in modulation process, the elimination and suppression strategy of dead-time effect is designed by analyzing the working mode of single-phase bridge arm at dead time. At the same time, on the basis of expounding the principle of traditional Pf-QV droop control, an improved PV-Qf drooping control strategy with frequency and voltage coordination control with resistive virtual impedance is designed for low voltage microgrid. When the inverter fed energy to the power grid, a grid-connected open-loop control strategy was designed. The simulation results show that the open-loop control is feasible under ideal conditions. For the case of disturbance, the proportional resonance control strategy is used to synchronize the LCC inverter system into the power grid, and the control system can track the sinusoidal signal of 50Hz without phase deviation, and at the same time has attenuation effect on other harmonics. Finally, a microgrid simulation model with two LCC inverter systems in parallel is built in MATLAB/Simulink, and the simulation analysis in isolated island and grid-connected mode is carried out. In isolated island mode, it is verified that the improved drooping control strategy for low voltage microgrid can realize the power distribution and smooth switching of the two inverters. In the grid-connected mode, the designed proportional resonance controller is simulated, and it is verified that the grid-connected current can follow the given current and run per unit power factor. In addition, the waveform quality of grid-connected current of LCC inverter and LCL inverter is compared. The steady-state error and harmonic distortion rate of grid-connected current of LCC inverter are lower than those of LCL inverter, which further explains the excellent characteristics of LCC inverter.
【学位授予单位】：西华大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TM464

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