摘要
为了减少电力系统正常运行时电流和电压的不对称性,原则上高压架空线路应采用换位措施。但设计和运行经验证明,高压线路完全换位会减弱线路的机械强度,并增加其建造和运行的费用。对于同杆双回架设的高压线路,由于导线与导线之间存在着电磁和静电的耦合关系,更加难以实现完全换位。
随着我国用电负荷的强劲增长及城市化进程的不断加快,地区电网中出现大量的短甚至超短的220kV同杆多回输电线路,且普遍采用不换位架设。近年来因不换位同杆双回线路引起系统三相电流不平衡问题日益突出,不平衡电流除了增大电网输电元件的损耗之外,过大的不平衡电流可能导致发电机负序保护或者线路零序保护误动,从而造成大面积停电。因此,研究高压不换位线路参数不对称所引起的三相电流不平衡问题具有重要意义。
对于三相参数不对称线路,三相参数变换到序坐标下仍不能解耦,因此在研究不换位高压线路的不对称问题时一般在相坐标下对线路三相参数进行建模。本文首先从卡森模型出发,对不换位架空线路进行分相参数建模,详细介绍了线路各相电感和电容的计算方法以及架空地线和分裂导线的处理方法,并针对220kV线路提出了研究不平衡电流的化简模型。在此基础上分析线路塔型、导线参数、相序排列等因素对不平衡电流的影响。
对于同杆并架双回线路,本文从互感矩阵的不对称性的角度来研究线路的参数不对称问题,阐述了利用调整相序手段降低线路电流不平衡度的数学原理,并从36种相序排列方式中归结出6种影响电流不平衡度的相序排列方式。研究在线路长度、负荷等变化的情况下,该6种相序排列方式对各种电流不平衡度的影响,从中寻找具有普遍适用性相序排列方式以降低线路不平衡电流。
结合实际电网的环网条件,研究电源相角和线路潮流等因素对不平衡电流的影响。最后以EMTDC仿真为手段对深圳电网典型线路的不平衡电流进行再现,并根据前文的研究结论提出相应的解决方法并仿真验证。
In order to decrease the asymmetry of current and voltage of the power system, the high voltage transmission lines should be transposed in principle. Design and operation experience has shown that, the transposition of high voltage transmission lines would weaken the mechanical strength and increase the cost of its construction and operation. For the double circuit transmission line, it is more difficult to be transposed completely because of the coupling of electromagnetic and electrostatic almong the lines.
With the strong growth of electricity load and accelerating of the urbanization process, lots of short or ultrashort 220kV double-circuit trasmission lines in regional power grid emerge, which are without transposition generally. In recent years, the imbalance of three-phase current caused by double-circuit trasmission lines without transposition is increasingly prominent.The imbanlance of three-phase current increases the loss of network components or even may lead to maloperation of generator negative sequence protection or zero-sequence protection,and then brings about large area lackouts. Therefore, the research of the imbanlance of three-phase caused by the asymmetry of transmission line parameters is important.
For the asymmetric three-phase parameters transmission lines, under the sequence domain the parameters are still coupling. So the modeling of three-phase parameters must be done under the phase domain.This paper research individual phase modeling of un-transposed high voltage transmission lines consulted Carson model. It introduces the caculationg method of inductance and capacitance of each phase and the processing method of overhead ground wire and bundle conductor, and a simplified model against 220kV transmission lines is proposed. On this basis, the influences to unbalance current of line tower type, wire parameters and phase sequence arrange are analyzed.
For the double-circuit trasmission lines, this paper studies the asymmetric parameters by studying the asymmetry of mutual inductance matrix and elaborates the principia mathematica of decreasing current unbalanced degree by phase sequence arrange.From 36 kinds of phase sequence arranges,6 kinds of phase sequence arranges are attributed. When the length of lines, loads etc are changing, the impacts on the current unbalanced degree of these 6 kinds of phase sequence arranges are compared, and the kind of it which is general and applicable to decrease the unbanlance current is found.
On the background of ring network in the power system, the impacts on unbalance current of power phase and powerflow of lines are researched. Finally, the unbalanced current in Shenzhen power grid is reproduced by simulation with EMTDC. Based on conclusions of earlier studies, a solution is proposed and it is proved by simulation.
引文
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