直流输电线路换相失败机理和抑制措施分析

发布时间：2018-08-27 15:22

【摘要】：目前我国已进入全国大电网互联时代。特高压直流输电是实现我国“西电东送、南北互济、全国联网”战略中最重要的一环。在直流输电系统中,换相失败故障发生的几率很高,它会带来一系列的系统参数变化,包括电流升高、电压下降,输送功率降低等,这将会使直流系统偏离正常运行状态,损害系统中各元件,如果任由故障发展,则会引发交流系统短路故障,甚至使整个系统传输停止,面临崩溃。本文基于直流系统的换相过程,研究了换相失败发生原因,得到换相失败的本质是逆变侧熄弧角过小而不能使晶闸管内载流子复合。随后分析了系统中熄弧角与各物理量直接的定量关系,利用此关系总结了容易造成换相失败的几大原因,并提出了若干防止换相失败发生的方法。由于不对称故障造成三相线路各物理量变化量不同,故障后换流站中各换相阀发生换相失败的几率也会不同。本文通过计算各种不对称故障中系统各物理量变化量的不同来分析各换流阀换相失败几率的差异,并通过熄弧角γ的变化整合了换相电压幅值和相位的变化,基于γ的变化分析了过渡电阻与各阀换相失败几率的关系,并利用PSCAD仿真平台验证了结论的正确性。串联电容器可以有效抑制换相失败的发生。本文通过分析换流器串联电容前后运行状态的不同,利用向量图法分析得出了串联电容器抑制换相失败的几方面原因,包括:提高换相电压的幅值,同时使换相电压相位滞后;故障后直流电流迅速增大,导致电容器电压增大,这会反过来抑制直流电流的升高,而且抑制能力与电流的增大程度成正比;串联电容器可以抵消一部分换相电抗,使换相时换相电磁暂态过程加快,换相叠弧角μ变小。最后利用PSCAD仿真平台分别验证串联电容换流器系统可以有效降低各故障的临界过渡电阻。为解决换相失败问题,本文提出了一种换相失败抑制方案,抑制方案由换相失败的预测和串联电容器抑制换相失败两部分组成。利用最大换相面积与临界换相面积之差作为衡量是否发生换相失败的一个指标,若最大换相面积与临界换相面积之差下降到设定的安全值以下时则认为将会发生换相失败。此预测方法逻辑清晰简单,动作快速可靠。检测频率密集。经仿真实验验证,此预测方法预测成功率高。预测将会发生换相失败后立即投入串联电容器以抑制换相失败的发生,经仿真实验验证,串联电容器后可以有效避免换相失败的发生,并可以有效减小各种故障下临界过渡电阻。
[Abstract]:At present, our country has entered the era of national power grid interconnection. UHVDC transmission is the most important part of the strategy of "power transmission from west to east, from north to south, and from the whole country". In HVDC transmission system, the probability of commutation failure is very high, which will bring about a series of system parameters change, including current rising, voltage decreasing, transmission power decreasing and so on, which will make DC system deviate from normal operation state. If the components in the system are damaged, if the fault is allowed to develop, it will lead to the short circuit fault of the AC system, and even make the whole system transmission stop and face collapse. Based on the commutation process of DC system, the cause of commutation failure is studied in this paper. The essence of commutation failure is that the inverter side extinguishing arc angle is too small to recombine the carrier in thyristor. Then the direct quantitative relationship between the extinguishing angle and the physical quantities in the system is analyzed. By using this relationship several main reasons for the commutation failure are summarized and some methods to prevent the commutation failure are put forward. Because of the different physical quantities of the three-phase line caused by asymmetric fault, the probability of commutation failure in the converter station is also different. In this paper, the difference of commutation failure probability of each commutation valve is analyzed by calculating the variation of system physical quantities in various asymmetric faults, and the amplitude and phase of commutation voltage are integrated by the change of extinction angle 纬. Based on the change of 纬, the relationship between the transition resistance and the probability of commutation failure of each valve is analyzed, and the correctness of the conclusion is verified by using PSCAD simulation platform. Series capacitors can effectively restrain the occurrence of commutation failure. In this paper, by analyzing the different operation states of series capacitor of converter before and after, by using vector graph method, several reasons for suppression of commutation failure of series capacitor are obtained, including: increasing the amplitude of commutation voltage, at the same time making the phase of commutation voltage lag; The DC current increases rapidly after the fault, which increases the voltage of the capacitor, which in turn suppresses the increase of the DC current, and the suppression ability is proportional to the increase of the current. The series capacitor can counteract part of the commutation reactance. The commutation electromagnetic transient process is accelerated and the commutative arc angle 渭 is reduced. Finally, the PSCAD simulation platform is used to verify that the series capacitor converter system can effectively reduce the critical transition resistance of each fault. In order to solve the problem of commutation failure, a commutation failure suppression scheme is proposed in this paper, which consists of two parts: the prediction of commutation failure and the suppression of commutation failure by series capacitor. The difference between the maximum commutative area and the critical commutative area is used as an index to judge whether the commutative failure occurs. If the difference between the maximum commutative area and the critical commutative area falls below the set safe value, the commutation failure will occur. This prediction method is clear and simple in logic and fast and reliable in action. The detection frequency is dense. The simulation results show that the prediction method has a high success rate. It is predicted that the series capacitor will be put into series immediately after the commutation failure occurs to restrain the commutative failure. The simulation results show that the series capacitor can effectively avoid the commutative failure. And it can effectively reduce the critical transition resistance under various faults.
【学位授予单位】：昆明理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TM721.1

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