【作者】 张震；

【导师】 马瑞卿；

【作者基本信息】 西北工业大学 ， 电气工程， 2019， 博士

【摘要】 光伏并网逆变器作为分布式光伏发电系统与电网之间的“桥梁”,需要可靠的控制策略方可保证逆变器输出的电能质量以及设备运行的稳定性。为避免电网发生故障时,大量电力电子变换器型发电设备脱网而导致负载功率缺额,电网波动加剧,故障规模扩大,多国都在新的电网标准中要求并网功率变换器具备低电压穿越(LVRT)的能力,甚至在短暂电网电压跌落至零时,还需要具备零电压穿越(ZVRT)的能力。目前关于LVRT和ZVRT的研究多是基于三相发电系统,如何在单相低压分布式电网中利用有限的电网信息,实现高效率、高可靠性的光伏并网发电故障穿越,已成为当前亟待解决的技术问题。本文以非隔离型单相光伏并网逆变器为研究对象,旨在设计、评估、验证可靠的控制策略,以实现对电网故障的准确监控和并网逆变器故障穿越的平稳控制。在单相并网逆变器中,电网同步单元对电网电压故障的快速、精准监测是保证平稳故障穿越控制的先决条件。论文对单相并网逆变器中的三种典型电网同步锁相环(T/4周期延迟锁相环T/4 delay PLL、增强型锁相环EPLL、二阶广义积分锁相环SOGI-PLL)进行了详细的结构分析,并给出了影响动态性能的参数设计方法。重点对EPLL和SOGIPLL在电网故障过程中的电网状态检测性能和精度进行了仿真分析,并通过实验结果进行了对比和评估。针对其中零电压穿越过程频率估计稳态误差不为零的问题,提出并设计了一种单相电网故障穿越的改进型锁相环结构,提高了零电压穿越过程中对电网电压频率的估计精度,保证了故障穿越的稳定性。针对结构简单、易于实现的T/4 delay PLL在应用中不具备频率偏移适应性以及没有谐波抑制能力的问题,提出了一种单相并网逆变器定长时间延时的自适应锁频环(TDAFLL),实现了对电网频率的自适应检测,给出了稳定性数学证明,仿真与实验结果表明,所设计的TD-AFLL可以有效地避免频率和相位偏移误差以及二倍基波频率扰动,具有动态响应快和零稳态误差的明显优势。针对TD-AFLL谐波抑制能力不突出的问题,设计了一种基于高阶自适应观测器的前置滤波器,提高了其在分布式电网中使用的适用性。另外,并网逆变器电流控制的快速性和稳定性也是保证可靠故障穿越的关键。论文将传统有限状态集模型预测控制(FCS-MPC)引入单相并网逆变器中,并针对其在电流控制中因开关状态匮乏、开关频率不固定,导致输出并网电流波动误差大、谐波频谱分散的问题,提出了一种单相并网逆变器定频模型预测电流控制算法(CSF-FCS-MPC),在单个采样控制周期内,构造了组合开关状态集,并给出了详细的组合开关状态时间分配计算方法。仿真和实验结果表明,所提出的CSF-FCS-MPC具有良好的动态性能,有效地减小了输出并网电流的波动误差,实现了逆变器开关状态的定频控制,使得并网电流谐波频谱分散的问题得到了显著地改善。在并网逆变器中,功率控制环的控制效果直接决定着其故障穿越性能,实现了对输出有功功率和无功功率的实时调节。为保证故障穿越控制的可靠性,论文以单相电网瞬时功率理论为基础,针对并网逆变器在电网电压跌落时,需及时提供无功电流支撑电网恢复的技术需求,对基于αβ-静止参考坐标系和dq-旋转参考坐标系的功率控制策略进行了结构分析,提出了基于功率因数角相移调节的功率控制策略。通过仿真分析和实验验证,分别对三种功率控制策略在LVRT和ZVRT两种电网电压故障穿越过程的性能进行了对比分析和性能评估。结果表明,三种控制策略均可以实现低电压穿越,但基于αβ-静止参考坐标系的功率控制不具备零电压穿越的能力,基于功率因数角移相调节的功率控制具有更好的动态性能。更多还原

【Abstract】 As the "bridge" between the photovoltaic(PV)distributed power generation systems(DPGs)and the power grid,the power quality and the stability of the grid connected PV inverters should be guaranteed by using the reliable control strategies.If a mountain of power electronics converters are off-grid when a grid fault occurs,the shortage of the power supply for the load demand would exacerbate the grid voltage fluctuations,and the grid fault scales would be expanded.Many countries require grid-connected power inverters with the low-voltage ridethrough(LVRT)capability in their updated grid code.Moreover,in extreme cases,i.e.,the grid voltage dips to zero,the zero-voltage ride-through(ZVRT)capability is also required.At present,many attempts have been made in the literature for the three-phase power system.And by using few grid information to realize single-phase grid-connected PV generation fault ride-through(FRT)operations with high-efficiency and high-reliability in the single-phase lowvoltage DPGs,that has become an urgent problem to be solved.In this thesis,taking singlephase grid-connected transformerless PV inverters as the research object,aim at designing,evaluating and verifying reliable control strategies,to achieve high precision grid fault monitoring and steady FRT operation.In the single-phase grid-connected inverters,the accuracy and dynamics of the grid synchronization unit have a direct impact on the performance of FRT operations in terms of stability.Thus,the most commonly-used and recently-developed PLL synchronization methods(T/4 delay PLL,EPLL,SOGI-PLL)for single-phase grid-connected inverters have been explored,and the comprehensive design guidelines to fine-tune their dynamic performance parameters are given.The simulation and experimental results are presented to assess the performances of the prior-art PLL methods in response to grid faults in terms of detection precision and dynamic response.However,the estimated frequency outputs have steady-state errors for ZVRT operation.In this case,a control strategy by modifying the single-phase grid system PLL scheme is then introduced,to improve the estimation accuracy of grid voltage frequency for the stability of the FRT operation.The transfer delay-based PLL(TD-PLL),i.e.the T/4 delay PLL,is still the favorites due to the simplicity,and easy implementation.However,it suffers from phase-offset errors and doublefrequency oscillatory errors,when the grid frequency drifts away from its nominal,or under distorted conditions.To tackle these issues,a transfer delay-based adaptive frequency locked loop(TD-AFLL)is proposed,A mathematic proof indicates that the proposed TD-AFLL can reject both phase offset errors and double-frequency oscillatory errors.Moreover,fast dynamics and relatively high accuracy of the TD-AFLL are achieved due to the adaptive frequency estimation based on the transfer delay structure.Notably,the harmonic immunity of the TDAFLL is poor,and it can be enhanced by adding a high-order adaptive observer-based filter,thus consolidating the applicability of the TD-AFLL in the DPGs.Furthermore,it becomes essential to combine a fast and stable current control mechanism with a dedicated control strategy for the FRT operations.In the conventional FCS-MPC for singlephase grid-connected inverters,with limited switching states and variable switching frequency,while these lead to current ripples,tracking errors,and scattered harmonic spectrums.Thus,a constant switching frequency FCS-MPC(CSF-FCS-MPC)method specifically suitable for single-phase grid-connected inverters is proposed.The CSF-FCS-MPC synthesizes the switching states in one sampling period.In addition,and the time allocation method is presented in detail.The performance of the proposed CSF-FCS-MPC is experimentally compared with the conventional FCS-MPC.The benchmark verifies the effectiveness of the proposal in terms of constant switching frequency MPC,and thus resulting in an almost ripple-free output current with high quality(i.e.,low harmonics).The power control loop is accurately and rapidly to regulate the output active and reactive powers of the grid-connected inverters in case of the FRT operations.According to the current active grid requirements,the inverters should provide reactive current injection to support the grid recovery.In this thesis,based on the single-phase instantaneous power theory,two flexible power control methods(i.e.,the αβ-stationary reference frame power control and dq-rotating reference frame power control)are explored,and a power phase-angle shifting power control method is proposed.All the three power control methods are evaluated for grid-connected single-phase PV systems in the case of LVRT and ZVRT operations.Simulation and experimental results are presented,which verifying that three methods can help the PV systems to temporarily ride-through the grid low-voltage faults.However,the αβ-stationary reference frame power control method is not suitable for use in ZVRT operations.The power phase-angle shifting control method has a better dynamic response.更多还原