【作者】 潘武略；

【导师】 徐政；

【作者基本信息】 浙江大学 ， 电力系统及其自动化， 2008， 博士

【摘要】 新型直流输电的高损耗是其广泛应用的主要障碍,所以开展新型直流输电降损措施的研究具有明显的工程应用价值和现实意义。本文采用理论分析和仿真验证相结合的研究手段,重点研究了直流输电的损耗特性和降损措施,研究的主要内容如下:(1)研究了直流输电损耗的评估方法,提出了一种基于曲线拟合理论的电压源换流器功率损耗计算方法。通过分析换流器IGBT器件的开关特性,并考虑反向并联二极管的影响,推导出IGBT损耗与电压、电流、结温、死区时间等参数的关系式。关系式中的各种参数,可从厂商提供的参数和特性曲线中获得。该方法能够有效利用厂商提供的器件特性参数,可操作性强,与物理模型和功能模型的建模方法相比,更适合于实际工程应用。基于本文方法开发的VSC-HVDC换流器损耗分析程序,可计算各种工况下的换流器功率损耗,为电压源换流器的降损设计提供了有力工具。(2)研究了常见多电平结构的换流器特性,提出了一种基于直流电压注入的低损耗的新型电压源换流器结构。此换流器由一个12脉动换流器和一个附加电路组成。附加电路通过变压器向12脉动换流器中点注入直流电压,将12脉动换流器变为60脉动换流器。换流器输出电压、电流的谐波水平很低,因此不需要滤波器就可以满足系统的谐波要求。换流器主电路采用基频触发,降低了开关频率;直流电压的注入,显著降低了开关电压,所以该换流器损耗低、开关电压应力低,更适合于高电压、大功率的应用场合。分析了该换流器的拓扑结构、工作原理和控制策略,并进行了仿真验证,为将来的实机制造提供了参考。(3)研究了多种调制方式的损耗和暂态特性,提出了一种降低新型直流输电系统损耗的混合调制方式,可满足输电系统暂态响应和稳态损耗特性的需求。混合调制方式配置SPWM和最小开关损耗两种调制方式,当系统受到扰动或处于暂态时,采用响应速度快的SPWM调制,提高系统的暂态控制能力,改善暂态响应特性;系统处于稳态运行时,则采用最小开关损耗控制,提高系统运行的稳态特性和经济性。使用状态监测器,监视系统状态,据此动态地选择适当的调制方式。描述了混合调制方式的基本原理,并对该调制方式的控制效果进行了仿真验证。仿真表明,所提出的混合调制方式具有良好的控制性能和稳态损耗特性。(4)研究了利用混合直流输电降损的可行性。理论分析了基于LCC和VSC型混合直流输电的局限性,并提出一种新型电流源型混合直流输电拓扑。该新型混合直流输电的逆变侧采用基于可关断器件的CSC换流器,整流侧采用传统LCC换流器。本文建立了该型混合直流输电的数学模型、触发方法、控制策略,并仿真了其在各种常见故障下的响应特性。仿真结果表明:当直流线路发生接地故障时,直流系统能够通过换流器控制来清除故障,并可以快速重启动;在整流侧或逆变侧交流系统发生严重交流短路故障而导致交流电压跌至30%时,该混合直流输电系统均可持续运行,并保持一定的直流功率,有利于逆变侧交流系统的稳定;当故障被清除后,该型直流系统能够快速恢复至故障前的稳定状态。所以,该混合直流输电系统具有工程实用性,是新型直流输电在远距离、大功率应用领域的一种可行的改进方案。更多还原

【Abstract】 The relative higher converter loss is one of the main barriers for the application of VSC-HVDC in bulk power transmission systems. Thus to research the loss evaluation and reduction approaches is of evident application value and practical significance. In this dissertation, the loss evaluation and reduction approaches of VSC-HVDC are investigated by means of theorical analysis and the simulation validatins. The main contents and related conclusions of this dissertation are as follows:(1) The loss evaluation of VSC-HVDC is investigated and an innovative generalized loss calculation method based on the curve fitting theory is proposed. By analyzing the switching characteristics of IGBT and the anti-parallel diode, the functions of IGBT losses with voltage, current, junction temperature and dead time are derived. The parameters in the functions can be obtained from the datasheet supplied by the manufacturers. Compared with the physical model and functional model, this proposed method is more suitable for practical engineering. A universal VSC-HVDC loss calculation module is established in PSCAD/EMTDC and it provides a powerful tool for VSC-HVDC loss reduction.(2) The characteristic of the multi-level converter is investigated and an innovative multi-pulse voltage source converter with low loss ratio is proposed. The proposed converter is composed of a 12-puls converter and an auxiliary circuit. The auxiliary circuit injects DC voltage via the mid point of the 12-puls converter. Proper injection ratio and frequency parameters are selected to convert the standard 12-pulse into 60-pulse configuration without using PWM or increasing the number of bridges, thus the voltage and current can fulfill the THD limit without conventional filter. As the main bridges operate under at fundamental frequency, the switching lose is relatively low. This converter may be widely used in the HVDC and FACTS system. The principle and control strategy of the proposed converter are described. Its operational feasibility is also verified by digital simulation using the PSCAD/EMTDC package.(3) The loss characteristics and control performance of the common modulation methods is analyzed and an innovative hybrid modulation method, which can reduce the VSC-HVDC loss and meet its static and dynamic performance requirement, is proposed. The new hybrid modulation method is equipped with two different PWM methods, i.e, SPWM and Minimum Switching Losses PWM. When the VSC-HVDC transmission system is under disturbance or transient states, the SPWM method, which has faster response, will be used; otherwise, in static state, the Minimum Switching Losses PWM method will be used. The system states can be obtained by the "Disturbance Detector", then the system states is used to select the proper modulation method dynamically. The operational principle of the hybrid modulation method is demonstrated in this paper and its operational feasibility is also verified by digital simulation using the PSCAD/EMTDC package. The results show that this hybrid modulation method can fulfill the static and dynamic performance and can be used in the control of VSC-HVDC.(4) The feasibility of loss reduction using Hybrid HVDC is investigated. The limitations of the Hybrid HVDC based on LCC and VSC is analyzed and an innovative Hybrid HVDC topology is proposed. In this new Hybrid HVDC, the traditional LCC is used at rectifier side and the CSC is used at inverter side. The mathematical model, firing scheme and control strategy of this Hybrid HVDC is set up and carefully designed. The system responses following the common faults are analyzed by simulation. The simulation results show that: (1) This Hybrid HVDC can clear DC line faults its converter controllers and can restart quickly after fault clearness; (2) When the AC voltage drops to 0.3 pu because of severe faults both at rectifier and inverter side ac systems, the hybrid HVDC can still transmit certain amount power to inverter side AC system and is helpful for the stability of the inverter side system. When the AC fault has been cleared, the Hybrid HVDC can recover quickly to its steady states before fault. Therefore, this Hybrid HVDC system has a promising potential for the bulk power transmission.更多还原