【作者】 魏静；

【导师】 周娟；

【作者基本信息】 中国矿业大学 ， 电气工程（专业学位）， 2020， 硕士

【摘要】 静止同步补偿器(Static Synchronous Compensator,STATCOM)作为当前先进的无功补偿装置,能快速补偿电网中的无功功率,有效降低电能传输损耗,降低电力设备容量,改善电网质量,维持电网电压稳定。级联H桥(Cascaded H-bridge,CHB)STATCOM具有大容量、易扩展、低输出电压谐波等特点,被广泛应用于高压大功率场合。然而,当装置功率较大、要求输出电平数较多时,对子模块数量的需求会很多,这意味着系统中存在大量的直流电容,为支撑直流侧电压缓冲二倍频波动功率,通常选择功率密度高的电解电容。相比较电解电容,薄膜电容寿命长,耐热性好,受环境变化影响不大,为提升级联H桥变换装置可靠性、延长装置寿命以及降低后期电容维护成本,可以考虑采用薄膜电容取代电解电容。但是,薄膜电容功率密度较低,容值相对较小,给级联H桥变换装置的控制带来了较大挑战。因此研究低容值级联H桥变换器直流侧电压控制策略对提高系统可靠性具有重要意义。电容容值降低会导致STATCOM直流侧电压脉动增大,将影响系统在补偿感性无功功率时的补偿容量;导致系统直流侧电压脉动峰值过大,使系统开关器件损耗增加;以及对低容值级联H桥STATCOM的调制策略精确性产生影响。本文针对低容值级联H桥STATCOM存在的问题进行分析并探讨其解决方案。本文分析了级联H桥STATCOM的拓扑结构,建立了级联H桥STATCOM等效模型,详细分析其工作原理;建立级联H桥STATCOM数学模型,并在dq轴对其双环解耦控制策略进行研究,通过仿真实验验证了STATCOM采用前馈解耦控制具有良好的无功补偿性能。分析了影响级联H桥直流侧各模块电压平衡的因素,采用总体、相间、相内三层电压平衡控制策略,控制各H桥子模块直流侧电容电压基波量趋于一致;详细阐述了级联H桥STATCOM装置直流侧电压产生二倍频波动的原因;研究电容取值对级联H桥STATCOM直流侧电压峰值的影响;同时对低容值状况下电容电压大幅波动对载波移相调制策略产生的影响进行分析,并将载波幅值可调PWM技术与单极倍频载波移相调制策略相结合,在满足较高等效开关频率的前提下,使其能够适用于电容电压大幅波动的场合。通过仿真验证了电压平衡控制策略以及单极倍频载波幅值可调PWM技术的有效性。为降低低容值级联H桥STATCOM装置的直流侧电容电压波动,对传统的电压波动抑制策略进行改进,将模型预测控制与注入零序电压抑制电容电压波动的控制方案相结合,提出一种基于模型预测控制的电容电压波动控制策略;控制注入级联H桥的零序电压幅值在剩余调制电压范围之内,避免了系统出现过调制的可能;通过模型预测控制预测注入零序电压后电容电压波动状况,以此来控制级联H桥电容电压达到波动最小以及相间平衡的目标。仿真和实验结果验证了所提出的基于模型预测控制的电压波动控制策略的有效性和可行性。对低容值级联H桥STATCOM直流侧电压波动在不同工况下对补偿容量的影响进行分析,得出系统在容性工况下电容波动电压峰值与级联H桥输出电压波峰对应的关系,以及直流侧电容电压波动对STATCOM补偿容性无功功率的补偿容量并无影响的结论。根据对其电压波动规律的分析,在低容值级联H桥系统补偿容性无功功率时采取抑制电容波动峰值电压的控制策略,通过注入特定三倍频零序电压,控制STATCOM在补偿容性无功功率时电容电压峰值在满足系统调制要求的前提下达到最低。仿真分析验证了LC-CHB STATCOM电容电压波动与系统补偿容量关系的正确性以及容性工况级联H桥直流侧电容电压峰值抑制策略的可行性。结合Matlab/Simulink仿真平台以及搭建的实验平台,对本文提出的电容电压平衡控制,基于模型预测控制的电容电压控制策略,基于三倍频零序电压注入的电容电压峰值控制,单极倍频载波幅值可调PWM技术进行验证,仿真和实验结果表明本文理论推导结果的正确性及所提控制策略的有效性。更多还原

【Abstract】 As the current advanced reactive power compensation device,Static Synchronous Compensator can quickly compensate reactive power in the power grid,effectively reduce power transmission losses,reduce power equipment capacity,improve power grid quality,and maintain grid voltage stability.The cascade H-bridge STATCOM has the characteristics of large capacity,easy expansion,and low output voltage harmonics,and is widely used in high-voltage and high-power applications.However,when the power of the device is large and the number of output levels required is large,the demand for the number of sub-modules will be greater,which means that there are a large number of DC capacitors in the system.In order to support the DC-side voltage buffering double-frequency fluctuation power,electrolytic capacitor with high power density is usually selected.Compared with electrolytic capacitors,film capacitors have a long life,good heat resistance,and are not affected by environmental changes.In order to improve the reliability of the cascaded H-bridge conversion device,extend the life of the device,and reduce the cost of capacitor maintenance in the later period,it can be considered to replace the electrolytic capacitor with film capacitor.However,the thin film capacitor has a low power density and a relatively small capacitance value,which brings great challenges to the control of the cascaded H-bridge converter.Therefore,it is of great significance to study the DC-side voltage control strategy of low-capacitance cascaded H-bridge converters to improve system reliability.Decreasing the capacitance will cause the STATCOM DC-side voltage ripple to increase,which will affect the system’s compensation capacity when compensating inductive reactive power;it will cause the system’s DC-side voltage ripple peak to be too large,which will increase the system switching device loss;and it will affect the accuracy of the low-capacitance cascade H-bridge STATCOM modulation strategy.This thesis analyzes the problems existing in the LC-CHB STATCOM and discusses its solutions.This thesis analyzes the topology of the cascade H-bridge STATCOM,establishes the cascade H-bridge STATCOM equivalent model,and analyzes its working principle in detail;The cascade H-bridge STATCOM mathematical model is established,and its double-loop decoupling control strategy is studied on the DQ axis.Simulation experiments verify that the STATCOM has good reactive power compensation performance using feedforward decoupling control.The factors affecting the voltage balance of the DC-side modules of the cascade H-bridge are analyzed,and the three-layer voltage balance control strategy of overall voltage balancing,clustered voltage balancing,and individual voltages balancing is adopted to control the DC-capacitance fundamental voltage of the H-bridge sub-modules to be consistent;The reasons for the double-frequency fluctuation of the DC side voltage of the cascade H-bridge STATCOM device are explained in detail;the effect of capacitor value on the DC voltage peak of the cascaded H-bridge STATCOM is investigated;At the same time,the impact of large fluctuations in capacitor voltage on carrier phase shift modulation strategy under the condition of low capacitance is analyzed.The combination of adjustable carrier amplitude PWM technology and single-pole frequency doubling carrier phase-shift modulation strategy can make it suitable for occasions where the capacitor voltage fluctuates greatly on the premise of meeting a higher equivalent switching frequency.The effectiveness of the voltage balance control strategy and the unipolar carrier phase shift modulation with adjustable amplitude of carrier is verified by simulation.To reduce the DC-side capacitor voltage fluctuation of low-capacitance cascaded H-bridge STATCOM devices,the model predictive control is combined with the control scheme of suppressing capacitor voltage fluctuations by injecting zero-sequence voltage,and a capacitor voltage fluctuation control strategy based on model predictive control is proposed;Control the amplitude of the zero sequence voltage injected into the cascade H-bridge within the range of the remaining modulation voltage,which avoids the possibility of over-modulation in the system.The model predicts control to predict the fluctuation of the capacitor voltage after the zero-sequence voltage is injected.The cascaded H-bridge capacitor voltage achieves the goals of minimal fluctuations and phase-to-phase voltage balance.Simulation and experimental results verify the effectiveness and feasibility of the proposed voltage fluctuation control strategy based on model predictive control.Analyze the impact of low-capacitance cascaded H-bridge STATCOM DC-side voltage fluctuations on the compensation capacity under different operating conditions,The corresponding relationship between the peak value of the capacitor fluctuation voltage and the peak value of the cascade H-bridge output voltage under the capacitive working condition is obtained.And it is concluded that the DC side capacitor voltage fluctuation has no effect on the compensation capacity of STATCOM to compensate the capacitive reactive power.According to the analysis of its voltage fluctuation rule,when the low-capacitance cascade H-bridge system compensates the capacitive reactive power,a control strategy to suppress the peak voltage of the capacitor fluctuation is used to inject a specific three-frequency zero-sequence voltage to control the STATCOM,make it when compensating capacitive reactive power,the peak value of capacitor voltage reaches the minimum under the premise of meeting the system modulation requirements.Simulation analysis verifies the correctness of the relationshipbetween LC-CHB STATCOM capacitor voltage fluctuation and system compensation capacity,And verifies the feasibility of the peak voltage suppression strategy of the cascaded H-bridge capacitor voltage under capacitive conditions.Combining the Matlab / Simulink simulation platform and the established experimental platform,the capacitor voltage balance control strategy,the capacitor voltage control strategy based on model predictive control,the capacitor voltage peak control based on triple-frequency zero-sequence voltage injection,and the technology of unipolar carrier phase shift modulation with adjustable amplitude of carrier proposed in this paper is used for verification.Simulation and experimental results show the correctness of the theoretical derivation results and the effectiveness of the proposed control strategy.更多还原