【作者】 陈燕东；

【导师】 罗安；

【作者基本信息】 湖南大学 ， 电气工程， 2014， 博士

【摘要】 微电网是在新能源分布式发电基础上新兴的前沿技术,与大电网互为支撑,是提高分布式发电供能效益的有效方式。微电网具有并网和离网(孤岛)两种运行模式。在并网工作模式下,微电网与中低压配电网并网运行,互为支撑,实现能量的双向交换。在外部电网故障或计划孤岛情况下,微电网可转为离网运行模式为微电网内部重要负荷供电。由于光伏、风力、燃料电池等分布式微源和储能装置大多通过逆变器在公共连接点并联接入微电网,因此,微电网多逆变器并联普遍存在。但由于逆变器的输出阻抗差异、连接阻抗不同、负载的非线性、控制方式不同以及微电网内部结构的多元化,在微电网逆变器并网控制、多逆变器并联控制和微电网电能质量主动控制等方面仍存在诸多技术难题,亟待突破。本文在国家自然科学基金重点项目“微电网多逆变器并联及电能质量控制方法研究”、国家重点基础研究发展计划项目(973计划)“分布式发电供能系统相关基础研究”等资助下,在并网运行模式下微电网逆变器并网控制技术、孤岛模式下微电网多逆变器并联环流分析与均流控制技术、微电网逆变器的电能质量主动控制技术方面开展研究,解决了微电网中逆变器并网与多逆变器并联运行关键技术难题,为微电网示范与推广提供理论依据与实践指导。主要技术创新点如下：1、在光伏并网控制中,延时和滤波电感量变化会影响系统响应速度、稳定性及并网电流畸变率。对此,本文提出一种功率前馈的鲁棒预测无差拍并网控制方法。通过引入功率前馈控制加快系统的响应速度。鲁棒预测无差拍控制方法被提出用于并网电流控制,以增强系统的鲁棒性,降低因控制延时和电感量偏差对并网电流造成的畸变。分析了带延迟环节的z域无差拍控制模型,并讨论了延时和电感量取值对系统稳定性的影响。给出了鲁棒预测无差拍控制的设计方法,并根据z域内的传递函数对其进行了稳定性分析,确定了控制参数的选取范围。仿真与实验结果证明了所提控制方法的有效性。2、通过改进逆变器并网控制策略,使微电网和分布式发电DG具备一定的无功补偿和电压频率调整功能是一种解决微电网电能质量问题的有效途径,可进一步降低分布式发电并网门槛。对此,本文针对分布于电网末梢的单相光伏并网发电系统,提出了兼具无功补偿功能的并网功率控制方法,使系统在向电网和本地负载快速提供有功电能的同时,也能提供负载所需的无功电能。在分析系统结构基础上,通过构建两相正交电流和利用瞬时无功电流ip-iq算法,检测出单相负载的无功电流的直流分量,通过直流侧电压外环PI控制输出得到有功电流的直流分量,从而获取并网指令电流。结合鲁棒预测无差拍控制,该系统无需增加额外的功率器件,可并接于电网末梢,在实现并网发电的同时,实现了对本地负载的动态无功补偿,提高了供电质量。为了解决构造单相无功电流带来的延时问题,进一步提高系统响应速度和电流控制精度,提出了一种无延时的兼具无功补偿功能的并网功率控制方法,该方法由无功电流检测、电压电流双环控制、功率前馈及电网电压前馈构成。提出了对负载电流求导来构建两相正交电流的方法,实现了无延时单相无功电流检测,与传统延时方法相比,解决了传统单相无功电流检测因采样数据不具备同时性以及负载电流跟踪存在滞后造成的较大延时问题,提高了系统稳定性。电流内环采用准谐振PR控制实现并网电流的零稳态误差控制,并降低电网频率偏移对电流的影响；引入电网电压前馈降低电压畸变或扰动造成的电流畸变。给出了并网功率控制系统的设计,分析了准谐振PR控制中不同控制参数对系统性能的影响,并选取了合适的设计参数。仿真与实验结果验证了所提方法的有效性。3、微电网中并联逆变器的等效输出阻抗和线路阻抗差异对功率分配和环流抑制存在较大影响。本文从逆变器并联的功率传输特性出发,分析了现有下垂控制对阻性逆变器并联功率分配的影响,并在分析阻性逆变器环流特性的基础上,提出了一种鲁棒下垂多环控制方法,包括功率外环和电压电流内环构成。提出功率外环鲁棒下垂控制来减少阻抗差异对功率精确分配的影响；通过引入含阻性分量和感性分量的虚拟复阻抗,将逆变器的等效输出阻抗设计呈阻性；采用准谐振PR控制实现在较宽频带内逆变器输出电压的零稳态误差控制,进而减少逆变器间输出电压偏差并抑制环流。采用输出电压前馈和电容电流比例控制提高系统暂态响应和电流抗扰动能力。深入对比分析了控制方式和控制参数对等效输出阻抗的影响,并择优选取了控制参数。仿真与实验结果验证了所提方法的可行性与有效性。4、在低压微电网多逆变器并联系统中,负荷突变会导致微电网电压波动,逆变器具备快速无功支撑能力是维持电压稳定的必要途径。本文通过将阻容性虚拟复阻抗引入到逆变器输出电流反馈中,提出了一种快速无功支撑的阻容性逆变器(RC型逆变器)及其并联功率分配方法。该逆变器将其等效输出阻抗设计成阻容性,可实现微电网在公共连接点处的无功功率快速支撑,从而保持系统电压稳定,并可抑制逆变器输出阻抗和电网阻抗间的谐振,进一步降低电压畸变。在对阻容性逆变器进行等效建模基础上,通过设计阻容性虚拟复阻抗,给出了该类逆变器并联的多环功率精确分配方法,包括功率下垂控制外环,虚拟阻抗中间环及输出电压控制内环。分析了虚拟复阻抗参数对并联环流的影响,并选取了合适的控制参数。仿真和实验验证了控制方法的有效性。更多还原

【Abstract】 Microgrid is an emerging frontier technology based on distributed generation of new energy, and is also an effective way to improve energy supply efficiency of distributed power generation for supporting each other with the grid. Microgrid has two operation modes:grid-connected mode and off-grid (island) mode. In grid-connected mode, microgrid and the low and medium voltage distribution network operate grid-connected and support each other to realize bidirectional exchange of energy. In the situation of external power fault or planned island, microgrid can be converted to off-grid operation mode and supply energy for the critical loads in microgrid. Distributed generation microsources such as photovoltaic, wind, and fuel cells and energy storage devices mostly connect through the inverters to the microgrid at the public point of coupling, so the multi-inverters are universal in microgrid. Due to the difference among the output impedance and the coupling impedance of the inverters, non-linearity of loads, difference among the control methods, and diversification of the internal structure of microgid, there are still many technical problems in the aspects of grid-connected control of microgird inverters, operation control of parallel multi-inverters and power quality active control in microgrid, which are urgent to breakthrough.This dissertation is funded by the National Natural Science Foundation of China "research of multi-inverter parallel operation and power quality control methods in microgrid", and the National Basic Research Program of China (973program)"basic research related to energy supply system of distributed generation". It carries out these researches on the inverter grid-connected control technology in grid-connected mode of microgrid, circulating current analysis and its sharing control technoque of parallel inverters in island mode of microgrid, and power quality active control technoque of microgrid inverters. It has solved the key problems of grid-connected inverter control and multi-inverter parallel operation in microgrid, and provided theoretical basis and practical guidance for demonstration and promotion of microgrid. The main technical innovations are as follows:1. In photovoltaic grid-connected control system, the delay and the filtering inductance variation affect the system response speed, stability, and the current distortion. In this paper, a robust predictive deadbeat grid-connected control method based on power feed-forward is proposed. Power feed-forward control is introduced to speed up the system response. For the grid-connected current controller, a robust predictive deadbeat control method is proposed to enhance system robustness and to reduce the current distortion due to the control delay and inductance deviation. A delayed z-domain deadbeat control model was analyzed, and the delay and inductance effects on system stability were discussed. The design of the proposed robust predictive deadbeat control was given. In z-domain, the system transfer function was derived, and the system stability was analyzed, and the control parameters were selected. Simulation and experimental results verified the validity of the proposed control method.2. By improving the grid-connected control strategy of the inverter, microgrid and distributed generation (DG) have the ability of reactive power compensation and voltage and frequency regulation, which is an effective way to solve the power quality problems of microgrid and can further reduce the threshold of distributed generation. For the single-phase photovoltaic grid-connected generation system installed at the end of the grid, a grid-connected power control method with reactive power compensation was proposed in this paper. By this way, the system can quickly provide not only active power for the grid and the local loads but also the required reactive power for the loads. Based on the analysis of the system structure, the dc component of single-phase load reactive power can be detected by constructing two-phase orthogonal current and using the ip-iq algorithm based on the instantaneous reactive current, and the dc component of active current can be obtained by the PI outer voltage loop, thus the grid-connected reference current is derived. Combined with robust predictive deadbeat control, the system need not the additional power devices and can be installed at the end of the grid, which can realize dynamic reactive power compensation of the local loads and gird-connected power generation. As a result, the power quality is improved. To solve the delay of single-phase reactive current detection, and to improve further system response and current control accuracy, a grid-connected power control method with the function of reactive power compensation without delay is proposed. The method consists of reactive current detection, the voltage and current dual control loop, power feedforward, and grid voltage feedforword. The method of constructing two-phase quadrature current by doing the derivation over the load current is proposed to realize the detection of single-phase reactive current without delay. Compared with the traditional delay method, the big delay caused by asynchronism of sampled data in traditional single-phase reactive current detection and the lag in load current tracking is solved, which has improved the system stability. The quasi-PR control is adopted in the inner current loop to realize the zero steady error control of grid-connect current and reduce the influence on the current due to the grid frequency offset. Grid voltage feed-forward was introduced to reduce the current distortion due to the distortion and disturbance of the grid voltage. The design of the grid-connected power control system is given. The effects on the system performance were analyzed under the different control parameters of the quasi-PR controller, and the proper parameters were selected. Simulation and experimental results verified the validity of the proposed control methods.3. For parallel multi-inverters in island microgrid, the difference of equivalent output impedance and line impedance affects greatly on power sharing and circulating current restraining. From power transmission characteristics of parallel inverters, the influence of power sharing among resistive inverters was analyzed in this paper, where the conventional droop control was applied. Based on the analysis of circulating current characteristics of resistive inverters, a robust droop multiple loop control method was proposed, which included the outer power loop and the inner voltage and current loop. In the outer power loop, a robust droop controller is adopted to reduce the effects on accurate power sharing due to the impedance difference. Introducing virtual complex impedance including resistive component and inductive component, the equivalent output impedance of inverters is redesigned as pure resistance. Quasi proportional-resonant (QPR) control is applied to realize zero steady-state errors control of the output voltage with wide bandwidth for parallel inverters, which will reduce further the deviation of output voltage and restrain circulating current. The output voltage feedforward control and the proportional control of the capacitor current are adopted to improve the transient response and current disturbance. The effects on the equivalent output impedance in the different control mode and parameters were analyzed comparatively, and the proper parameters were selected. Simulation and experimental results show the correctness and validity of the proposed control method.4. For parallel multi-inverters in low voltage microgrid, load mutation will lead to voltage fluctuation, and providing rapid reactive power for inverters is a necessary way to maintain the system voltage stability. Thus, an inverter using resistive-capacitive output impedance (RC-inverter) was proposed in this paper. The equivalent output impedances of RC-inverter were designed as resistive-capacitance by introducing resistive-capacitive virtual complex impedance into the feedback of the output current. RC-inverters cannot only provide rapid reactive power for low-voltage microgrid to maintain the system voltage stability, but also restrain high frequency resonance between output impedance of inverters and the grid impedance. Based on the equivalent modeling of RC-inverter, a multi-loop power sharing control method for parallel inverters was presented, which mainly includes the outer power droop control, the virtual impedance, and the output voltage control. The effects of the virtual complex impedance on parallel circulating current were analyzed, and the appropriate control parameters were selected. Both the simulation and experimental results verify the effect of the control method.更多还原