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由柔性电流输电系统图得到的等效电路如图3所示。
图 3 柔性直流输电系统等效电路图
Figure 3. Equivalent circuit diagram of flexible direct current transmission system
由图3所示,PCC为交流系统发电设备的简称,Req、Leq为发电设备到整流设备之间的等效阻抗,VSC的交流母线电压的基频分量Us和交流输出电压的基频分量Vc将会作用于变压器和电抗器两端,设变压器和电抗器整体的等效电抗为Xc,VSC和交流输电系统交换的有功功率和无功功率可以按照以下公式求导,其中Xeq为系统的等效阻抗。
$$ {P}=\dfrac{{U}_{{\rm{S}}}{V}_{{\rm{C}}}}{{X}_{{\rm{eq}}}}\mathrm{sin}\delta $$ (1) $$ {Q}=\dfrac{{U}_{{\rm{S}}}{({U}_{{\rm{S}}}-V}_{{\rm{C}}}\mathrm{cos}\delta )}{{X}_{{\rm{eq}}}} $$ (2) 由计算公式可知,有功功率P和无功功率Q都和VSC的交流母线电压的基频分量Us成正相关,且和Us、Uc和之间的夹角δ、系统的等效阻抗成负相关,且由公式可知,P和Q可以实现四象限运动。P和Q的运行图如图4所示。
图 4 VSC和交流输电系统交换的有功功率和无功功率关系图
Figure 4. The relation diagram of active power and reactive power exchanged by VSC and AC transmission system
根据Q正负的不同,可以分为不同的无功功率形式,当Q<0时,为容性无功,当Q>0时,为感性无功;根据P正负的不同,可以区分IGBT元件不同的工作模式,当P<0时,为逆变模式,即直流向交流的转变,当P>0时,为整流模式,即交流电转变为直流电的模式。由此可见,柔性直流系统可以在四象限运行,运行灵活性较高。
将N个VSC模块串联组合,就组成了模块化多电平(Modular Multi-Level Converter,MMC)的拓扑方式。
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由图3可知,单个的VSC元件由上下2个IGBT元件组成,我们将上方的IGBT元件命名为D1,下方的为D2,通过控制D1、D2的导通或者关断,即可控制VSC模块的投入或退出。
如图5所示,当D1开通,D2关断,则电容器C0投入,子模块输出高电平。
而当D1关断、D2导通时,电容器C0被旁路掉,子模块则输出低电平。
由图5、图6所示,单个VSC元件有运行、退出2种不同的运行模式,而柔性直流输电将N个VSC模块级联,可以通过控制桥臂上每个VSC模块的退出和投入,实现交直流电源的柔性互换。
Research on Flexible HVDC Transmission Technology and Strategies for Improving the Operational Stability of IGBT Components
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摘要:
目的 随着清洁能源的战略地位不断提高,越来越多的新能源发电设备接入电网,但其电能质量不稳定,对交流输电网产生了较大的冲击。柔性直流输电网络可以有效隔离交流网络和直流网络,具有较好的发展前景。柔性直流输电的关键部件是由全控型IGBT(Insulate-Gate Bipolar Transistor,绝缘栅双极晶体管)元件为核心的换流阀,因此IGBT元件的安全稳定运行对整个柔性直流系统的稳定起到重要作用。为了提高IGBT元件的安全性和运行稳定性,必须控制其运行电流和电压,避免承受过流、过压损害,且需要控制环境的温度、湿度以及设备防腐性,改善运行条件。 方法 采用内外两方面措施:内部设置启动电阻、电抗器、避雷器等,进而控制回路的电流和电压;外部设置空调,提高房间严密度,设备防腐处理等保证工作环境的良好。 结果 内部回路配置启动电阻、电抗器、避雷器等设施的措施,限制回路电流、电压,进而控制极端情况下作用于IGBT元件上的过电流倍数、过电压倍数;外部通过控制IGBT所在房间的温度、湿度及其设备防腐性,为IGBT元件的运行提供一个适宜的环境,对提高IGBT元件的安全性及运行可靠性有所帮助。 结论 柔性直流输电可以很好地解决电能质量较差的新能源发电设施的电能传输问题。通过优化柔性直流输电系统回路的内部措施及提高IGBT元件运行环境适宜度的外部措施,提高IGBT元件乃至整个柔性直流系统的安全性和运行可靠性。 Abstract:Introduction With the continuous improvement of the strategic position of clean energy, more and more new energy power generation equipment is connected to the power grid, but its power quality is unstable, which has a great impact on the AC (Alternating-Current) transmission grid. Flexible DC (Direct-Current) transmission networks can effectively isolate AC and the DC networks, and have good development prospects. The key component of flexible DC power transmission is the converter valve. The key component of flexible DC power transmission is the converter valve with fully controlled IGBT components as the core. Therefore, the safe and stable operation of IGBT (Insulate-Gate Bipolar Transistor) components plays an important role in the stability of the entire flexible DC system. In order to improve the safety and operational stability of IGBT components, it is necessary to control their operating current and voltage to avoid overcurrent and overvoltage damage. At the same time, it is necessary to control the temperature and humidity of the environment, equipment corrosion resistance to improve operating conditions. Method Therefore, both internal and external strategies were adopted. Starting resistors, reactors, lightning arresters, etc. were installed internally to control the current and voltage of the circuit. Externally, air conditioners were set, the room tightness was improved, and equipment anti-corrosion treatment was equipped to ensure a good working environment. Result By configuring starting resistors, reactors, lightning arresters and other facilities in the internal circuit, the circuit current and voltage are limited, and then the overcurrent multiple and overvoltage multiple acting on the IGBT components are controlled under extreme conditions. By controlling the temperature and humidity of the room where the IGBT is located and the corrosion resistance of the equipment, a suitable environment is provided for the operation of the IGBT components, which is helpful to improve the safety and operational reliability of the IGBT components. Conclusion Flexible DC transmission can well solve the power transmission problem of new energy power generation facilities with poor power quality. And by optimizing the internal strategy of the flexible DC transmission system circuit and the external strategy to improve the suitability of the operating environment of the IGBT components, the safety and operational reliability of the IGBT components and even the entire flexible direct current system are improved. -
Key words:
- Flexible DC /
- IGBT Components /
- operating stability /
- loop current /
- loop voltage /
- operating environment
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