非对称混合多电平逆变器调制策略及功率均衡控制研究
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摘要
随着高压大功率变换领域对逆变器功能和性能的不断需求,多电平逆变器可用耐压较低的功率开关,组合输出谐波含量低、电磁干扰小的多电平高电压,已经成为高压大功率变换技术的研究热点。但目前工程上应用的传统多电平逆变器所需功率器件较多、结构复杂,存在逆变系统体积庞大、价格昂贵、效率较低等敏感问题,严重地限制了多电平逆变器的实际应用范围。而非对称混合多电平逆变器及其控制技术,能以更少的功率器件和直流电源,输出较多的电平数,大大简化多电平拓扑结构、提高功率密度、降低成本,因此成为多电平逆变器的发展趋势。但目前非对称混合多电平逆变器处于初步研究阶段,国内外还没有形成统一结构的拓扑,多是从性价比上对其实用化作出评价。而非对称混合多电平逆变器的运行性能,取决于合适的混合多电平调制策略及相关控制策略,因此本文以非对称混合多电平逆变器为对象,对其调制、控制策略进行研究,具有一定的理论意义和工程实用价值。
本文首先针对非对称混合多电平拓扑中的非对称H桥,深入分析其拓扑结构特点,提出一种对非对称H桥具有通用性的新型载波交错调制策略。对于三种具有较高性价比的五电平非对称H桥,以双向开关非对称H桥为例,深入分析其功率开关在一个基波周期的动作规律,并将这个规律统一成调制方式,确定出正、负交错层叠分布的载波,得到正弦调制波与载波分层、分区脉宽调制的载波交错层叠SPWM调制策略,同样适用于二极管箝位型非对称H桥。最后以FPGA实现三相系统的自然采样法,通过仿真和实验对比分析,载波交错层叠同相位调制性能更佳。
建立电容箝位型非对称H桥拓扑的数学模型,深入分析载波交错层叠同相位调制策略对箝位电容电压的影响,指出箝位电容电压需一个基波周期达到平衡,对电容电压的波动幅度存在不可控的问题。充分利用交错载波的自由度,提出一种改进的控制策略,不仅有效地控制了箝位电容电压波动的幅度,而且均衡了同类功率开关的利用。在此基础上,建立箝位电容容量与负载电流、开关频率和电容电压波动幅度的数学关系,有利于折中确定箝位电容容量与开关频率的选取。通过线电压生成原理分析,针对逆变器输出电压矢量不能满足最近三矢量原则、线电压波形存在电平层交叠的问题,提出一种优化控制方法,进一步提高了线电压谐波性能。
为了扩展输出电压等级,将电容箝位型非对称H桥与传统H桥级联,构成混合H桥级联型多电平逆变器,并建立混合拓扑的开关函数模型。深入分析传统混合调制策略的基本思想,对功率单元进行独立调制,提出载波交错型混合调制策略,提高了混合拓扑输出电压的等效载波频率。在此基础上,扩展了非对称H桥级联的混合调制策略,对于电容箝位型非对称H桥级联多个H桥的混合多电平逆变器具有良好的控制性能。针对混合拓扑中存在的效率问题,提出调制波中注入混合分量的优化控制方法,既提高了直流电压利用率,又降低了高频开关损耗。
针对混合H桥级联型多电平逆变器中的功率单元输出不平衡问题,文中根据瞬时功率推导出功率单元输出有功功率与输出电压基波幅值的关系。深入分析各功率单元输出电压基波幅值与调制度的函数关系,指出低调制度阶段存在功率单元输出分配极端问题,为此提出功率单元输出线性化控制策略,完全解决了输出分配极端问题。为了进一步实现功率单元均衡输出,重构功率均衡曲线,建立功率均衡约束函数模型,进而提出高、低调制度阶段的功率均衡控制策略,实现了全调制范围内功率单元的均衡输出。
With the high voltage and high power fields ceaseless requirements to the functions and performance of the inverter, and the multilevel inverter could output multilevel high voltage, which has lower harmonics and electromagnetic interference with lower voltage power switches, therefore it has been to the research hotspot of high voltage and high power conversion technology. While the present traditional multilevel inverters applied in engineering need more power switches, and their structures are more complicated, so they have the sensitive problems which include that the inversion system has huge volume, more expensive price and lower efficiency. And the actual application range of the multilevel inverter is extremely limited by these problems. But, the asymmetric and hybrid multilevel inverter (AHMI) with its control technology could output more numbers of voltage levels with less power switches and DC supplies. It extremely simplifies the multilevel topology, improves the power density and reduces the cost, thus it will become the developing trend of multilevel inverter. At the present time, the research of AHMI is at the beginning step, there is no unified and recognized topology in nation or abroad yet, thereby, its practicality may only be estimated by cost and performance. While the operating performance of the AHMI, is up to the proper hybrid multilevel modulation strategy and its correlative control strategy. Thereby, the AHMIs are taken as the research objects, then their respective modulation strategies and control strategies are investigated, and it both has the definite theory significance and the engineering practical value.
The topology characteristics of the asymmetric H bridges which include in AHMIs are analysed deeply, then a novel carriers-staggered modulation strategy is proposed, which is proper to the asymmetric H bridges. According to three kinds of five-level asymmetric H bridges which have higher performance, the bidirection switch asymmetric H bridge is taken as the example. The behavior rules of each power switch during the fundamental period are deeply analysed to form the unified modulation modes, and the carriers which are distributed as the modalities of positive and negative staggered cascades are confirmed, then the carriers- staggered SPWM (CS-SPWM) modulation strategy is obtained with the sinusoidal wave and the carriers which are layered and divisional. This modulation is also proper to the diode-clamped asymmetric H bridge. Finally, the three-phase natural sampling method of the CS-SPWM is implemented by FPGA, and the CS-SPWM with phase distribution has better performance by contrasting analysis of simulation and experimental results.
The mathematics models of the capacitor-clamped asymmetric H bridge are established, and the CS modulation strategy with phase distribution impact on the capacitor voltage is analysed deeply, it is pointed that the clamped capacitor voltage needs a fundamental period to be balanced, and there is a problem that the fluctuating amplitude of the capacitor voltage is uncontrollable. A novel improved control strategy is proposed by means of the staggered carriers’freedoms, and it not only controls the fluctuating amplitude of the capacitor voltage effectively, but also balances the utilization of the homogeneous power switches in the topology. On the base of it, the mathematics relationship of capacitor capacitance and switching frequency versus load current and voltage ripples is established, to ensure the choice between the capacitance and the switching frequency better. By means of the principle of the generated line to line voltage, it is found that the output voltage space vectors couldn’t conform to the principle of the nearest three vectors (NTV), and the line to line voltage exists the problem of overlapped level layers, then the optimization control method is proposed to enhance the performance of the line to line voltage.
In order to increase the level of output voltage, the multilevel inverter with hybrid cascaded H bridges is formed by the capacitor-clamped asymmetric H bridge and traditional H bridges, and its switching function models are established. With the deep analysis of the conventional basic ideas of hybrid modulation strategy, the CS hybrid modulation strategy is proposed with the respective modulation of each power cell, and it increases the equivalent carrier frequency of the output voltage. On the base of it, the hybrid modulation strategy is extended to more asymmetric cascaded H bridges, and it has the better control performance to the inverter with hybrid asymmetric H bridges. In addition, aiming at the efficiency problem of the topology, the optimization control method injecting hybrid components to the modulation waves is proposed, it not only improves the DC voltage utilization, but also reduces the loss of high frequency power switches.
Another problem from the multilevel inverter with hybrid cascaded H bridges is how to balance the output power of each cell. Thus, the function between the active power and the fundamental voltage amplitude is derived from instantaneous power, then the function between the fundamental voltage amplitude and modulation depths is analysed deeply, and it’s found that the output assignment of each power cell has the extremity problem during the lower modulation depths. Therefore, the strategy of linearization control of power cells outputs is proposed to settle the problem completely. In order to make the outputs of the power cells be balanced, the power balance curves are rebuilt, then the restriction functions of power balance is established to propose the power balance control strategies including both higher and lower modulation depths. Finally, the outputs of power cells are balanced during the whole modulation depths.
本文首先针对非对称混合多电平拓扑中的非对称H桥,深入分析其拓扑结构特点,提出一种对非对称H桥具有通用性的新型载波交错调制策略。对于三种具有较高性价比的五电平非对称H桥,以双向开关非对称H桥为例,深入分析其功率开关在一个基波周期的动作规律,并将这个规律统一成调制方式,确定出正、负交错层叠分布的载波,得到正弦调制波与载波分层、分区脉宽调制的载波交错层叠SPWM调制策略,同样适用于二极管箝位型非对称H桥。最后以FPGA实现三相系统的自然采样法,通过仿真和实验对比分析,载波交错层叠同相位调制性能更佳。
建立电容箝位型非对称H桥拓扑的数学模型,深入分析载波交错层叠同相位调制策略对箝位电容电压的影响,指出箝位电容电压需一个基波周期达到平衡,对电容电压的波动幅度存在不可控的问题。充分利用交错载波的自由度,提出一种改进的控制策略,不仅有效地控制了箝位电容电压波动的幅度,而且均衡了同类功率开关的利用。在此基础上,建立箝位电容容量与负载电流、开关频率和电容电压波动幅度的数学关系,有利于折中确定箝位电容容量与开关频率的选取。通过线电压生成原理分析,针对逆变器输出电压矢量不能满足最近三矢量原则、线电压波形存在电平层交叠的问题,提出一种优化控制方法,进一步提高了线电压谐波性能。
为了扩展输出电压等级,将电容箝位型非对称H桥与传统H桥级联,构成混合H桥级联型多电平逆变器,并建立混合拓扑的开关函数模型。深入分析传统混合调制策略的基本思想,对功率单元进行独立调制,提出载波交错型混合调制策略,提高了混合拓扑输出电压的等效载波频率。在此基础上,扩展了非对称H桥级联的混合调制策略,对于电容箝位型非对称H桥级联多个H桥的混合多电平逆变器具有良好的控制性能。针对混合拓扑中存在的效率问题,提出调制波中注入混合分量的优化控制方法,既提高了直流电压利用率,又降低了高频开关损耗。
针对混合H桥级联型多电平逆变器中的功率单元输出不平衡问题,文中根据瞬时功率推导出功率单元输出有功功率与输出电压基波幅值的关系。深入分析各功率单元输出电压基波幅值与调制度的函数关系,指出低调制度阶段存在功率单元输出分配极端问题,为此提出功率单元输出线性化控制策略,完全解决了输出分配极端问题。为了进一步实现功率单元均衡输出,重构功率均衡曲线,建立功率均衡约束函数模型,进而提出高、低调制度阶段的功率均衡控制策略,实现了全调制范围内功率单元的均衡输出。
With the high voltage and high power fields ceaseless requirements to the functions and performance of the inverter, and the multilevel inverter could output multilevel high voltage, which has lower harmonics and electromagnetic interference with lower voltage power switches, therefore it has been to the research hotspot of high voltage and high power conversion technology. While the present traditional multilevel inverters applied in engineering need more power switches, and their structures are more complicated, so they have the sensitive problems which include that the inversion system has huge volume, more expensive price and lower efficiency. And the actual application range of the multilevel inverter is extremely limited by these problems. But, the asymmetric and hybrid multilevel inverter (AHMI) with its control technology could output more numbers of voltage levels with less power switches and DC supplies. It extremely simplifies the multilevel topology, improves the power density and reduces the cost, thus it will become the developing trend of multilevel inverter. At the present time, the research of AHMI is at the beginning step, there is no unified and recognized topology in nation or abroad yet, thereby, its practicality may only be estimated by cost and performance. While the operating performance of the AHMI, is up to the proper hybrid multilevel modulation strategy and its correlative control strategy. Thereby, the AHMIs are taken as the research objects, then their respective modulation strategies and control strategies are investigated, and it both has the definite theory significance and the engineering practical value.
The topology characteristics of the asymmetric H bridges which include in AHMIs are analysed deeply, then a novel carriers-staggered modulation strategy is proposed, which is proper to the asymmetric H bridges. According to three kinds of five-level asymmetric H bridges which have higher performance, the bidirection switch asymmetric H bridge is taken as the example. The behavior rules of each power switch during the fundamental period are deeply analysed to form the unified modulation modes, and the carriers which are distributed as the modalities of positive and negative staggered cascades are confirmed, then the carriers- staggered SPWM (CS-SPWM) modulation strategy is obtained with the sinusoidal wave and the carriers which are layered and divisional. This modulation is also proper to the diode-clamped asymmetric H bridge. Finally, the three-phase natural sampling method of the CS-SPWM is implemented by FPGA, and the CS-SPWM with phase distribution has better performance by contrasting analysis of simulation and experimental results.
The mathematics models of the capacitor-clamped asymmetric H bridge are established, and the CS modulation strategy with phase distribution impact on the capacitor voltage is analysed deeply, it is pointed that the clamped capacitor voltage needs a fundamental period to be balanced, and there is a problem that the fluctuating amplitude of the capacitor voltage is uncontrollable. A novel improved control strategy is proposed by means of the staggered carriers’freedoms, and it not only controls the fluctuating amplitude of the capacitor voltage effectively, but also balances the utilization of the homogeneous power switches in the topology. On the base of it, the mathematics relationship of capacitor capacitance and switching frequency versus load current and voltage ripples is established, to ensure the choice between the capacitance and the switching frequency better. By means of the principle of the generated line to line voltage, it is found that the output voltage space vectors couldn’t conform to the principle of the nearest three vectors (NTV), and the line to line voltage exists the problem of overlapped level layers, then the optimization control method is proposed to enhance the performance of the line to line voltage.
In order to increase the level of output voltage, the multilevel inverter with hybrid cascaded H bridges is formed by the capacitor-clamped asymmetric H bridge and traditional H bridges, and its switching function models are established. With the deep analysis of the conventional basic ideas of hybrid modulation strategy, the CS hybrid modulation strategy is proposed with the respective modulation of each power cell, and it increases the equivalent carrier frequency of the output voltage. On the base of it, the hybrid modulation strategy is extended to more asymmetric cascaded H bridges, and it has the better control performance to the inverter with hybrid asymmetric H bridges. In addition, aiming at the efficiency problem of the topology, the optimization control method injecting hybrid components to the modulation waves is proposed, it not only improves the DC voltage utilization, but also reduces the loss of high frequency power switches.
Another problem from the multilevel inverter with hybrid cascaded H bridges is how to balance the output power of each cell. Thus, the function between the active power and the fundamental voltage amplitude is derived from instantaneous power, then the function between the fundamental voltage amplitude and modulation depths is analysed deeply, and it’s found that the output assignment of each power cell has the extremity problem during the lower modulation depths. Therefore, the strategy of linearization control of power cells outputs is proposed to settle the problem completely. In order to make the outputs of the power cells be balanced, the power balance curves are rebuilt, then the restriction functions of power balance is established to propose the power balance control strategies including both higher and lower modulation depths. Finally, the outputs of power cells are balanced during the whole modulation depths.
引文
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