【作者】 龚宇雷；

【导师】 李庆民；

【作者基本信息】 山东大学 ， 电力电子与电力传动， 2011， 博士

【摘要】 作为可再生能源与分布式发电的有效组织方式,微电网既可并入大电网运行,也可自成体系而独立运行。独立运行的微电网必须满足功率平衡以支撑其电压与频率的稳定性,这就对并入微电网的各种微电源以及储能装置提出了较高要求.与传统的大型发电机不同,微电源的输出特性各异,且具有典型的间歇性和随机性特征,多数基于电力电子逆变拓扑并入微电网。因逆变拓扑控制与微电网参数的交互耦合影响,将给微电源并网带来一些亟待解决的新问题,包括并网逆变器的同步相位检测方法、三相逆变桥路的暂态保护技术等,这些问题的有效解决可提高微电源并网运行的可靠性。同时,为实现微电网内的功率平衡以及电压与频率稳定,要求各个微电源及储能装置必须按照调度分配的指定功率进行发电,这迫切需要针对各种微电源和储能装置的输出特性,研究相应的定功率输出控制策略。探索这些关键技术问题的解决方案,即构成了本文研究的主要内容。多数微电源采用电流控制型电压源逆变器并入微电网。因线路电感与功率器件开关过程的交互作用,将在并网接入点电压上叠加一定幅值的开关纹波电压,导致实际过零点时刻的多过零点现象,造成较大的电压相位跟踪误差。基于动态特性分析和实验,本文系统研究了线路电感对接入点电压波形的影响机制,证明采样电压的多过零点区域关于实际微电网电压的过零点具有严格的左右对称特征,并藉此提出一种改进的电压过零点相位检测算法,适用于单开关周期内的多个过零点情形。针对开关电压纹波较大的工况,将锁相环技术和傅立叶级数计算相结合,提出一种准确跟踪微电源并网接入点电压相位的新方法,并针对滞环电流控制方法,论证了基于傅立叶级数法计算电压幅值和相位的准确度,并不受逆变器直流侧电压、线路电感及开关电压的影响。为进一步提高相位跟踪的实时性,并有效抑制外来干扰以及开关电压纹波的影响,研究提出了基于“过采样”技术的改进型三相软件锁相环算法,利用工频周期的8k分解和加权均值处理,可有效抑制不利因素的影响,特别适用于三相三线制系统。仿真分析和物理实验都表明上述相位检测与跟踪方法的有效性。针对三相逆变桥路的暂态保护问题,通过建立基于器件杂散参数的等效电路模型,对微电源并网逆变桥的桥臂直通现象进行了深入分析,揭示了瞬间冲击尖峰电流的产生机理,推导出尖峰电流幅值与瞬时变化率的计算公式。在理论分析和实验研究的基础上,基于LCRD组合网络,提出一种微电源并网逆变桥的瞬态冲击保护新拓扑,详细分析了其工作机理,并藉此给出了拓扑参数的有效选择准则。实验研究表明,提出的冲击保护拓扑可抑制功率器件开关瞬态过中的冲击电流幅值以及瞬时电流与瞬时电压变化率,有效防止开关器件出现误触发和损坏,实现三相逆变器的正常工作和微电源的可靠并网。微电网正常运行的前提条件是保证发电和用电设备之间的有功与无功功率平衡,以实现微电网内的电压与频率稳定,这要求对微电源及储能实施定功率控制。本文基于理论分析和实验,以太阳能光伏发电阵列、蓄电池储能装置和风力发电装置为主要受控对象,系统研究了微电源及储能装置的定功率输出控制问题。基于光伏电池单元的基本参数关系,导出了光伏电池阵列输出特性的单调函数关系,这为研究相应的定功率输出快速跟踪控制方法奠定了理论基础。提出了基于高阶构造函数的光伏电池阵列定功率输出控制方法,所构造的函数在光伏阵列给定输出功率点的导数为零,且二阶导数小于零,使得光伏电池阵列由当前输出功率向给定功率自适应逼近,可大幅度提高功率跟踪的响应速度。基于参考短路电流法和开路电压法,推导出高阶构造函数中各参数的计算公式。进一步建立了基于上述方法的非线性控制系统模型,基于小电压扰动和线性化方法分析了影响系统稳定性的关键因素,并给出控制参数的取值准则与范围。针对所提出的定功率输出跟踪控制方法,开展了仿真和实验研究,验证了该方法的有效性和快速性。针对微电网独立运行时储能蓄电池的定功率充放电拓扑结构,基于坐标变换推导出三相交流侧指令电流的计算方法,并将滑模变结构控制理论引入到输出电流的跟踪控制中,基于离散趋近率函数参数的自适应变化,提出一种自适应的离散滑模变结构电流控制方法,可有效实现蓄电池充放电电流的实时跟踪与柔性控制。通过对控制系统的深入分析,获得了控制参数应满足的临界条件与定量关系式,可用于控制参数的优化设计。构建了储能蓄电池的定功率充放电系统仿真模型与实验样机,仿真与实验结果都表明该控制方案可较好地实现蓄电池的定功率充放电功能,且系统对外界干扰和参数摄动具有较强的鲁棒性。针对微电网内的定桨距双馈风力发电单元,通过改变风轮机叶尖速比,建立电磁输出功率的有效控制方法。进一步研究了双馈风力发电机在风速突变时输出功率的波动现象,提出一种基于超级电容器缓冲的实时功率补偿方法,可将功率波动控制在较小范围内。通过仿真分析佐证了补偿方法的有效性。基于光伏发电单元、风电模拟单元、蓄电池储能单元以及控制单元,本文研制了微电网的实验平台系统,并开展了物理模拟实验研究,验证了前述理论分析结果与控制方法的有效性。本文工作结果进一步丰富了微电源并网与定功率输出控制方面的理论基础和分析方法,对发展微电网技术具有参考价值。更多还原

【Abstract】 As an effective way to organize renewable sources and distributed generation, microgrid can be connected to large power system or run as an independent power system. While operating independently, microgrid should keep power balance so as to ensure voltage and frequency stability, resulting in high technical requirements for micro sources and energy storage devices connected to the microgrid.Unlike traditional large generators, micro sources have various output characteristics and present typical intermittence and randomness. The majority of micro sources are connected to microgrid through power electronic inverters. Due to the coupling effects between the inverter topology control and the parameters of microgrid, new issues are raised with grid-connection of the micro sources, such as phase detection method of the grid voltage as the CCVSI (Current Controlled Voltage Source Inverter) is connected to microgrid, as well as transient protection technology for three-phase inverter bridge. Preferable solutions to these problems can improve the reliability of grid-connection of the micro sources. Meanwhile, in order to realize power balance as well as voltage and frequency stability, each micro source and energy storage device should generate electricity according to designated quota by the power dispatcher. Therefore, corresponding designated power output control strategy is indispensable and should be analyzed based on the output characteristics of the micro sources and energy storage devices. Exploration of possible solutions to these crucial technical problems forms the basis of this thesis.Most micro sources are connected to microgrid through current controlled type voltage source inverters (CCVSI). Due to the interaction of the line inductances and the power device switching process, switched voltage ripples will superimpose onto the voltage waveform at the grid-connection point, leading to the appearance of multiple zero-crossing phenomenon at the actual zero-crossing moment, resulting in large deviation in phase detection of the grid voltage. Based on theoretical analysis and experiment study of the dynamic behaviors, the impact of line inductances on the voltage waveform at the grid-connection point is analyzed, which proves the multiple zero-crossings area of the sampled voltage presents strict symmetrical feature regarding to the actual zero-crossing of the microgrid voltage waveform. Based on the above analysis, an improved zero-crossing based phase detection method is proposed, which can be applied to the situation with multiple zero crossing points in a single switching cycle. To deal with large switching voltage ripples, a novel method that combines PLL technology and Fourier series analysis is further developed, so as to accurately track the phase of the grid voltage at the connection site. Based on the hysteresis current control, the calculation accuracy of the voltage amplitude and phase by Fourier series is validated, which is not influenced by the voltage on DC side of the inverter, the grid inductance and the switching voltage. In order to improve the real-time performance of phase tracking and restrain the extraneous interference as well as the influence of switching ripple voltage, a method based on improved three-phase software PLL algorithm is brought forward, which adopts 8k decomposition and weighted mean technique in a primitive period, and thereby can restrain negative effects while especially applicable for three-phase three-wire system. Both simulation analysis and experimental results have verified the method.To solve the transient protection problem of typical CCVSI bridge, an equivalent circuit model is established based on stray parameters of the devices. Short through phenomenon of CCVSI bridge arm is thoroughly studied and the mechanism for surge current generation is elucidated, in addition, quantitative formula to determine the current amplitude and rate of change is also deduced. Through theoretical analysis and experimental research, a novel transient protection topology for CCVSI bridge arm is presented based on a LCRD combinational network. The operating principle of the protection topology is analyzed in details and the effective criterion to choose the typology parameters is also given. Experiments indicate that the new protection typology can effectively limit amplitude of the surge current and rate of change of the instantaneous current and voltage during power device switching process, hence, false trigging and damage of power devices can be avoided effectively, and normal operation of the three-phase inverter and reliable connection of micro sources to microgrid can be achieved.The balance of both active and reactive power with a microgrid among the generating devices and the utilization equipment is the precondition for reliable operation of the microgrid. In order to maintain voltage and frequency stability of the microgrid, designated power control over micro sources and energy storage devices is indispensable. Based on theoretical analysis and experimental study, designated power output control strategies for micro sources and energy storage devices are investigated in the thesis, specifically with photovoltaic array, battery energy storage device and wind power generation device as the controllable objects.Based on the fundamental relationship between parameters of a photovoltaic battery cell, the monotony characteristics of theⅠ-Ⅴcurve and dP/dV curve of the photovoltaic arrays are deduced, which lays theoretical foundation for further development of tracking control method at designated power output. A new method based a high order constructed function to implement designated power tracking control is presented, where the first derivative of the operation voltage-based function at the designated reference point is zero while the second derivative is less than zero. This method guarantees that the photovoltaic battery array can self-adaptively approach the designated power point from the present status, and the response speed of the designated power tracking is significantly raised as well. Based on short-circuit current method and open-circuit voltage method, calculation formulas for key parameters in the constructed function are deduced. A nonlinear control system model based on the above proposed method is established and key factors influencing the system stability are analyzed based on small voltage perturbation and linear smoothing, then selection criterion and reliable range of the control parameters are also given. Simulation and experimental results validate effectiveness and speediness of the control method.For designated power control strategy of battery storage devices within a independent microgrid, the calculation method for instruction current on the three-phase AC side is deduced based on coordinates transformation, while slide-mode variable structure control theory is used in the output current tracking and control. Based on adaptive changes of the parameters in the discrete reaching law function, an adaptive discrete slide-mode variable structure current control method is brought forward, which can effectively achieve real-time tracking and flexible control of both charging and discharging currents of the batteries. Critical condition and quantitative formula are deduced, which can be used in the optimization design of the control parameters. Simulation model and experimental prototype of the battery storage device for designated power charging/discharging are established. Simulation and experimental results demonstrates effectiveness of the proposed method, with preferable robustness to exterior interference and parameter perturbation.With regard to the double-feed wind power generation unit in the microgrid, an effective method to control the output power by changing the tip speed ratio of the wind turbine is proposed. Further studies are carried out specifically on power fluctuation phenomenon while a sudden step change of the wind speed happens, and a real-time compensation method based on controllable supercapacitor buffer is presented. Simulations are also given to verify effectiveness of the proposed method.Based on photovoltaic arrays unit, wind power generation unit, battery storage unit and central control unit, an experimental platform for microgrid study is constructed, and multipurpose experiments for physical simulation are implemented to verify the theoretical analysis and the proposed control methodologies.This proposed research enriches the theoretical foundation and analytical methods for systematic study on grid-connection and designated power output control of micro sources, which provides referential basis for further development of the microgrid technologies.更多还原