發(fā)布時(shí)間：2018-08-27 15:22

【摘要】：目前我國(guó)已進(jìn)入全國(guó)大電網(wǎng)互聯(lián)時(shí)代。特高壓直流輸電是實(shí)現(xiàn)我國(guó)“西電東送、南北互濟(jì)、全國(guó)聯(lián)網(wǎng)”戰(zhàn)略中最重要的一環(huán)。在直流輸電系統(tǒng)中,換相失敗故障發(fā)生的幾率很高,它會(huì)帶來(lái)一系列的系統(tǒng)參數(shù)變化,包括電流升高、電壓下降,輸送功率降低等,這將會(huì)使直流系統(tǒng)偏離正常運(yùn)行狀態(tài),損害系統(tǒng)中各元件,如果任由故障發(fā)展,則會(huì)引發(fā)交流系統(tǒng)短路故障,甚至使整個(gè)系統(tǒng)傳輸停止,面臨崩潰。本文基于直流系統(tǒng)的換相過(guò)程,研究了換相失敗發(fā)生原因,得到換相失敗的本質(zhì)是逆變側(cè)熄弧角過(guò)小而不能使晶閘管內(nèi)載流子復(fù)合。隨后分析了系統(tǒng)中熄弧角與各物理量直接的定量關(guān)系,利用此關(guān)系總結(jié)了容易造成換相失敗的幾大原因,并提出了若干防止換相失敗發(fā)生的方法。由于不對(duì)稱故障造成三相線路各物理量變化量不同,故障后換流站中各換相閥發(fā)生換相失敗的幾率也會(huì)不同。本文通過(guò)計(jì)算各種不對(duì)稱故障中系統(tǒng)各物理量變化量的不同來(lái)分析各換流閥換相失敗幾率的差異,并通過(guò)熄弧角γ的變化整合了換相電壓幅值和相位的變化,基于γ的變化分析了過(guò)渡電阻與各閥換相失敗幾率的關(guān)系,并利用PSCAD仿真平臺(tái)驗(yàn)證了結(jié)論的正確性。串聯(lián)電容器可以有效抑制換相失敗的發(fā)生。本文通過(guò)分析換流器串聯(lián)電容前后運(yùn)行狀態(tài)的不同,利用向量圖法分析得出了串聯(lián)電容器抑制換相失敗的幾方面原因,包括:提高換相電壓的幅值,同時(shí)使換相電壓相位滯后;故障后直流電流迅速增大,導(dǎo)致電容器電壓增大,這會(huì)反過(guò)來(lái)抑制直流電流的升高,而且抑制能力與電流的增大程度成正比;串聯(lián)電容器可以抵消一部分換相電抗,使換相時(shí)換相電磁暫態(tài)過(guò)程加快,換相疊弧角μ變小。最后利用PSCAD仿真平臺(tái)分別驗(yàn)證串聯(lián)電容換流器系統(tǒng)可以有效降低各故障的臨界過(guò)渡電阻。為解決換相失敗問(wèn)題,本文提出了一種換相失敗抑制方案,抑制方案由換相失敗的預(yù)測(cè)和串聯(lián)電容器抑制換相失敗兩部分組成。利用最大換相面積與臨界換相面積之差作為衡量是否發(fā)生換相失敗的一個(gè)指標(biāo),若最大換相面積與臨界換相面積之差下降到設(shè)定的安全值以下時(shí)則認(rèn)為將會(huì)發(fā)生換相失敗。此預(yù)測(cè)方法邏輯清晰簡(jiǎn)單,動(dòng)作快速可靠。檢測(cè)頻率密集。經(jīng)仿真實(shí)驗(yàn)驗(yàn)證,此預(yù)測(cè)方法預(yù)測(cè)成功率高。預(yù)測(cè)將會(huì)發(fā)生換相失敗后立即投入串聯(lián)電容器以抑制換相失敗的發(fā)生,經(jīng)仿真實(shí)驗(yàn)驗(yàn)證,串聯(lián)電容器后可以有效避免換相失敗的發(fā)生,并可以有效減小各種故障下臨界過(guò)渡電阻。
[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 緯. Based on the change of 緯, 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.
【學(xué)位授予單位】：昆明理工大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：TM721.1

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本文編號(hào)：2207681