【作者】 李国栋；

【导师】 毛承雄；

【作者基本信息】 华中科技大学 ， 电力系统及其自动化， 2007， 博士

【摘要】 近年来,节能成为人们日益关注的焦点,我国提出了“能源开发与节约并举,把节约放在首位”的方针。在我国电力系统中有大约21%的装机容量用于驱动水泵、风机及压缩机等需要节能调速的负载,能源浪费严重。本文所研究的高压变频器通过改变高压电动机的电源输入频率实现电动机的无级调速,是目前最有效的节能方式之一。本文介绍了高压变频器的发展背景及大功率电力电子器件的发展,分析、比较了目前常用的两电平型、三电平型及串联多重化型高压大功率变频器的拓扑结构,并着重研究了基于IGCT串联的二极管箝位型三电平高压大功率变频器。基于IGCT及二极管的di/dt特性,研究了变频器直流环节(包括限流电感、钳位电容及限流电阻)对变频器安全运行的影响。通过分析三电平结构的开关暂态过程,建立了变频器暂态开关数学模型,并设计了直流环节部分的限流电抗。在综合考虑三电平的开关过程、直流限流电感的大小及IGCT的过压能力基础上,研究了直流钳位电容对变频器逆变器持续性直流电压的影响,并推导了直流钳位电容的设计要求。根据目前的IGCT耐压水平,为了得到我国数量众多的6kV高压电动机所需的输入电压需要两只IGCT及相应的阻容吸收回路的串联使用。本文根据IGCT的实测波形,提出了IGCT关断过程中等效为可变电流源的设想,并基于该假设研究IGCT串联动态均压问题。斩波电路的实测结果及数值仿真表明了该思路的有效性。通过对二极管中心点箝位型三电平变频器的研究,讨论了三电平结构中一相桥臂暂态开关状态及相应的流过三电平二极管(包括箝位二极管,IGCT反并联二极管及直流环节限流二极管)的反向恢复di/dt。进一步,分析了三相桥臂输出电压和电流方向、开关时间及开关操作对相应的二极管暂态高恢复di/dt影响。基于控制多电平调制策略及鼠笼式异步电动机的恒U/f控制,分析了载波比及变频器工作频率对di/dt的影响,并推导了6k次载波比条件对抑制箝位及IGCT反并联二极管关断时的高di/dt以防止器件的损坏的作用。进一步讨论了死区补偿问题,旨在提高变频器的电压利用率。本文分析了多脉波整流器对电网的谐波污染,研究了多脉波整流技术与直流母线电压谐波之间的关系,分析了多脉波整流变压器短路阻抗对变频器轻载时直流母线电压的影响,给出了整流变压器的容量计算方式。进一步,本文给出了三电平逆变器的共模模型,并提出了一种限制电动机输入共模电压的方法。基于本文所提出的研究成果,设计了6kV/2.5MW及6kV/1.0MW的高压变频器,其中6kV/2.5MW高压变频器于2006年7月在武汉高压研究院通过了6kV重载试验。基于IGCT应用于6kV高压变频器,本文介绍了双三电平型高压大功率变频器拓扑结构。在不增加开关器件数量的前提下,该结构应用于6kV场合时无需IGCT的串联使用,理论上能够有效地提高变频器效率,改善变频器输出波形,同时使得变频器结构更为紧凑。进一步,根据三电平开关换流分析,本文提出了改进型的三电平直流环节以有效地保护开关器件。更多还原

【Abstract】 Recently, energy saving have been the focus of attention, and developing new energy and energy saving are both proposed as the policy of our government, while the energy saving is treated as the primacy. However, there exists huge waste in those areas, including fans, pumps and compressors, which cost about 21% of the total installed capacity of our power grid and need speed adjust for energy saving. The medium voltage drives (MVDs) discussed in this paper can adjust the input frequency of the high voltage motor to achieve the variable speeds, which is one of the most effective energy saving modes.Firstly, the background of the MVDs and the development of the high power high voltage power devices are introduced in this paper. Then, the general topologies of the MVDs, including two-level, three-level and cascaded multi-level inverters, are analyzed, and the diode clamped three-level inverter based on the IGCTs’series connection is emphasized in this paper.Based on the IGCT used in neutral point clamped (NPC) three-level MVDs and the characteristics of the fast recover diode (FRD), the contribution of the DC-Link (including DC-Link choke, DC-Link resistor and clamped capacitor) to the MVDs safe operation is studied in this paper. Through the analysis on the turn-on and turn-off transience of NPC three-level MVD, the corresponding mathematical models are proposed, based on which the DC-Link choke is designed. With the DC-Link choke, switching transient state and device’s over-voltage capability taken into accounts, the relationship between the clamped capacitor and inverter’s DC over-voltage is studied.Due to the rating voltage of the IGCT, series connections of two IGCTs and its corresponding dynamic and static snubbers are necessary to satisfy the 6kV motor, which is very popular in our country. According to the test waveforms of the IGCTs, it is treated as one variable current source during its turn-off transients, based on which the transient voltage sharing is analyzed. The test results of the chopper circuit and its mathematical simulation results show its efficiency.Through the researches on the diodes neutral point clamped (NPC) three-level inverter, the transient switching states of one phase leg and its corresponding reverse recovery current decay rates through the fast recovery diodes are presented. Consequently, according to the directions of the output voltage and current, switching time and switching operation in three phase legs, the turn-off transient high di/dt are discussed. With the multi-level modulation strategy used and constant U/f control applied to the squirrel asynchronous motor, the contributions of carrier ratio and the working frequency of the MVDs to the turn-off di/dt are deduced. Accordingly, compared to the modulation modes, the multi-level modulation strategy with carrier ratio equal to 6n is recommended aiming at suppressing the high turn-off di/dt. Furthermore, the dead time compensation is proposed for improving the DC voltage utilization ratio.For suppressing the harmonic pollution to the power system, the multi-pulse rectifier is used, and the harmonic contents of the DC voltage are analyzed. Accordingly, the influence of the short-circuit impedance of the multi-pulse rectifier transformer to the DC voltage is described under different operating mode, based on which its corresponding capability of the rectifier transformer is deduced. Then, a high frequency model of the common-mode model of three-level inverter is established, and the method for suppressing the common-mode voltage of the motor is presented.With the research results in this paper, 6kV, 2.5MW and 6kV, 1.0MW MVDs are designed, and 6kV, 2.5MW MVDs has passed the 6kV, large load experiments.Based on the IGCTs used in the 6kV MVDs, one novel topology based on the double three-level inverter proposed in this paper, in which the 6kV outputs can be achieved without the series connection of IGCTs. Without increasing the power devices, the MVDs will be more compact. Furthermore, higher efficiency and better output waveform can be obtained. According to the commutations among the double three-level inverters phase legs, an improved DC-Link of double three-level inverters is proposed to protect the switching devices effectively.更多还原