【作者】 尚荣艳；

【导师】 罗隆福；

【作者基本信息】 湖南大学 ， 电气工程， 2008， 博士

【摘要】 我国地域幅员辽阔,能源分布和负荷需求发展很不平衡,远距离大容量输电以及区域电网互联势在必行。在此方面,直流输电具有技术上和经济上的独特优势,必将对我国电力工业的可持续发展起到十分重要的作用。基于新型换流变压器及其滤波系统的直流输电系统(简称新型直流输电系统)采用了一种全新的滤波方案,称为感应滤波技术。它既有别于传统的无源滤波方式,也有别于现有的有源滤波方式。这项技术在新型换流变压器阀侧绕组的中间抽头处接入特征谐波滤波器,使得大部分谐波电流通过滤波支路短路,而在网侧绕组中流过的部分很小,可以忽略不计。本文的理论分析与现场测试均表明,新型直流输电系统的滤波效果明显优于传统直流输电系统;通过换流变压器网侧绕组的谐波很小,考虑谐波的影响而计入额定容量的设计裕度减小,有利于降低换流变压器的制造成本和运行损耗;此外,新型换流变压器的振动速度下降到传统换流变压器的1/3左右,在减振降噪方面有一定的优势。进一步考虑,新型直流输电系统采用感应滤波技术后,对换相会产生什么样的影响?和传统直流输电系统相比,前者在换相方面有什么潜在的优势?针对这些问题,本文主要完成以下几方面的基础研究工作:(1)计算直流输电稳态运行参数时,传统的方法要么非常复杂,一旦考虑相关滤波装置,在计算与分析方法上都容易遇到困难;要么过于简化,导致计算误差很大。针对此问题,本文将换流变压器阀侧相电流进行了完全傅里叶分解,并建立了计算直流输电系统运行参数的详细数学模型。模型有利于计入滤波器时的分析与计算,能够十分精确地反映出各种运行参数在稳态运行时的变化规律,而且计算过程特别简单。(2)通过深入研究发现,交流系统耦合电抗及换流变压器漏抗的存在是导致直流输电逆变器换相失败的重要因素之一;通过滤波,新型直流输电系统与传统直流输电系统均能消除耦合电抗对逆变器换相的不良影响,但前者能够大大削弱变压器漏抗的消极作用,改善了换相特性,后者却无法避免变压器漏抗对逆变器换相的消极作用。在此基础上,本文进一步分析得出新型直流输电系统能够改善换相的内在原因,即相比于传统直流输电系统,换相电抗大幅减小。(3)通过系统研究表明,未接入滤波器时,新型换流变压器换相电抗的计算方法与传统计算方法完全一致,但是接入滤波器后,传统的求解方法不再适用于新型换流变压器换相电抗的计算。因此,本文提出全新的计算换流变压器换相电抗的方法,认为换相电抗的大小与换相电流的基波及谐波分量流通回路的等值电感以及各分量的占有率密切相关。(4)传统换流变压器的换相电抗与短路电抗基本相同,主要由换流变压器漏抗决定。因此,换流变压器漏抗必须足够大,以限制短路电流,但又不宜过大,否则无功损耗增加,且造成换相失败。本文研究发现,新型换流变压器及其滤波系统在换相电抗减小的同时,短路电抗并无明显变化,两者并不矛盾。(5)可行性研究表明,传统直流输电系统难以应用于弱交流系统,而新型直流输电系统接入滤波器时所需换相压降大幅减小,因而对交流系统的强度要求大大降低,具有应用于弱交流系统的前景。本文通过系统而深入的研究,建立了新型直流输电系统在换相特性方面的理论研究体系。新型直流输电系统改善了换相特性,还可能应用到弱交流系统中。它在诸多方面体现出明显的优越性,具有良好的应用前景。更多还原

【Abstract】 Although our country is vast in territory, the energy distribution and the load demand development is not very balanced. So the remote and high-capacity power transmission systems as well as the interconnected local power networks are imperative. The DC transmission system will certainly play a very vital role on the sustainable development of our country’s power industry for its technical and economical superiority.DC transmission system based on the novel converter transformer and its filter system, for short, novel DC transmission system, has adopted a kind of brand new filtering plan, which is named inductive filtering technology. It is not only different from the traditional passive filter way but also different from the existing active filter way. It adds the characteristic harmonic filter into the middle tap place of valve-side winding in the novel converter transformer. So the majority of harmonic current is short-circuited through the filter leg while few part of it can pass through the net-side winding, which can be ignored. Both the theoretical analysis and the field tests in this article indicate that the filtering effect in the novel DC transmission system is much superior to that in the traditional system. The design allowance to its rated capacity can be reduced with considering the influence of harmonics, which is in favor of reducing the manufacturing cost of the converter transformer and the operation loss. What’s more, the vibration velocity of novel converter transformer drops to about one third of traditional converter transformer and the novel DC transmission system has certain superiority in lowering the vibration velocity and noise in the converter transformer.Furthermore, how does the novel DC transmission system influence commutation? What potential advantages does the novel system have by contrast with the traditional one? The contents of the paper are as follows:(1) Traditional computation method of stable state parameter in DC transmission system is either complex, especially when the related filter considered, or too simple to avoid large calculation error. In view of this question, we carry on Fourier decomposition to phase current of valve-side of converter transformer completely, and then establish a detailed mathematical model to calculate the operation parameters of the DC transmission system. This model will be help of analysis and calculation in the case of the DC system with filter. And it is able to reflect precisely the changing law of various operation parameters in the stable state. The computation process is very simple too.(2) Through the thorough research, the coupling reactance of AC system and leakage reactance in the converter transformer can lead to commutation failure. Although both the novel DC transmission system and the traditional system can eliminate the bad effects of coupling reactance to inverter commutation, the former can greatly weaken the negative effects of transformer leakage reactance and improve the commutation characteristic while the later cannot avoid these negative effects. Furthermore, this paper finds out the intrinsic reasons, namely the commutation reactance has greatly reduces in the novel DC transmission system.(3) Through systematic study, when filter is not added to the novel DC transmission system, the computation method of commutation reactance is the same as the traditional method. Contrarily, the traditional method is no longer suitable.Therefore, we propose a new computation method and prove that the commutation reactance is related to the loop equivalent inductance of the fundamental and harmonic wave components of commutation current and these components’occupancy.(4) The commutation reactance of traditional converter transformer is basically identical with the short circuit reactance and mainly determined by the leakage reactance of the converter transformer. Thus, the leakage reactance should be large enough to limit the short-circuit current, but it should not too oversized. Otherwise, the reactive loss increases and commutation fails. Our research shows that the short-circuit reactance of the novel DC transmission system doesn’t change obviously while the commutation reactance reduces.(5) The feasibility studies prove that it is difficult for the traditional DC transmission system to be applied to the weak AC system, but the novel system has a certain application prospect. The commutation voltage required in the novel system has been greatly reduced and the intensity of the AC system reduces as well.This paper analyzes the fundamental theory about the commutation characteristic in the novel DC transmission system. Results show that the novel system can better the commutation and probably be used to the weak AC system. It has great advantage and wide application prospect.更多还原