【摘要】：針對低壓微電網(wǎng)特性及非線性因素、三相負載不平衡日益凸顯的問題,對其控制方式和逆變系統(tǒng)的研究意義重大。本文對兩級式拓撲結(jié)構(gòu)的逆變系統(tǒng)進行了分析,前級通過Buck-Boost電路進行直流電壓變換,跟隨分布式電源電壓波動影響來完成對電壓的調(diào)整,以滿足后級逆變器工作需要,后級并網(wǎng)逆變器采用LCC(電感-電容-電容,inductor-capacitor-capacitor)逆變電路將前級的輸出電壓轉(zhuǎn)換成滿足并網(wǎng)要求的交流電。文中首先利用狀態(tài)空間平均法針對Buck-Boost電路進行分析建模,得到交流小信號的線性傳遞函數(shù),針對系統(tǒng)的不穩(wěn)定問題,設(shè)計了比例-積分補償器,使控制系統(tǒng)在輸入電壓以及負載出現(xiàn)階躍擾動情況下,能夠有效抑制低頻擾動并迅速跟隨給定電壓。然后分析建立了輸出電壓基波僅由輸入的基波電壓決定的電壓型LCC逆變系統(tǒng),在非線性負載和負載不平衡下均具有良好的輸出電壓質(zhì)量。文章在LCC逆變系統(tǒng)優(yōu)良特性的基礎(chǔ)上,重點研究了LCC逆變系統(tǒng)的電壓控制策略。針對LCC逆變器的開環(huán)控制策略存在啟動超調(diào)大和無法抑制輸入電壓擾動,采用一種輸出電壓的實時滯環(huán)控制策略,有效減小了啟動超調(diào),同時實現(xiàn)了對電壓擾動的快速調(diào)節(jié)。通過對LCC逆變器接入三相不平衡負載和非線性負載情況的分析,根據(jù)LCC逆變器系統(tǒng)的模型,設(shè)計了擾動分量的補償器,引入一個補償鏈抵消非線性負載的影響。針對調(diào)制過程中死區(qū)效應(yīng)引起的輸出電壓幅值下降問題,通過對死區(qū)時刻單相橋臂的工作模式分析,設(shè)計了死區(qū)效應(yīng)的消除抑制策略。同時,在闡述傳統(tǒng)Pf-QV下垂控制原理的基礎(chǔ)上,針對低壓微電網(wǎng)設(shè)計了引入阻性虛擬阻抗的頻率電壓協(xié)調(diào)控制的改進的PV-Qf下垂控制策略。當逆變器向電網(wǎng)饋能時,設(shè)計了一種并網(wǎng)開環(huán)控制策略,仿真結(jié)果說明開環(huán)控制在理想情況下是可行的,而對于存在擾動的情況,采用比例-諧振控制策略將LCC逆變系統(tǒng)同步并入電網(wǎng),控制系統(tǒng)能夠無相位偏差的跟蹤50Hz的正弦信號,同時對其他諧波具有衰減效果。最后,在MATLAB/Simulink中搭建了含兩個LCC逆變系統(tǒng)并聯(lián)的微網(wǎng)仿真模型并進行孤島和并網(wǎng)模式下的仿真分析。在孤島模式下,驗證了改進的針對低壓微電網(wǎng)的下垂控制策略能實現(xiàn)兩臺逆變器的功率分配和平滑投切。在并網(wǎng)模式下,對設(shè)計的比例諧振控制器進行仿真,驗證了并網(wǎng)電流能實現(xiàn)對給定電流的跟隨和單位功率因數(shù)運行。此外,對LCC逆變器與LCL逆變器并網(wǎng)電流的波形質(zhì)量進行了比較,LCC逆變器并網(wǎng)電流的穩(wěn)態(tài)誤差與諧波畸變率均小于LCL逆變器,進一步說明了LCC逆變器的優(yōu)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.
【學(xué)位授予單位】：西華大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：TM464

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