Question

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三相短路电流计算是电力系统规划、设计、运行中必须进行的计算分析工作。目前，三相短 路电流超标问题已成为困扰国内许多电网运行的关键问题。然而，在进行三相短路电流计算 时，各设计、运行和研究部门采用的计算方法各不相同，这就有可能造成短路电流计算结论 的差异和短路电流超标判断的差异，以及短路电流限制措施的不同。
如果短路电流计算结果偏于保守，有可能造成不必要的投资浪费；若偏于乐观，则将给系统 的安全稳定运行埋下灾难性的隐患。因而，在深入研究短路电流计算标准的基础上，比较了不同短路电流计算条件对短路电流计算结论的影响，以及能为电网短路电流的计算和限制提供更切合实际的方法和思路。
1短路电流计算方法
经典的短路电流计算方法为：取变比为1.0，不考虑线路充电电容和并联补偿，不考虑负荷 电流和负荷的影响，节点电压取1.0，发电机空载。短路电流计算的标准主要有IEC标准和ANSI标准，中国采用的是IEC标准。国标规定了短路电流的计算方法、计算条件。国标推荐的三相短路电流计算方法是等值电压 源法，其计算条件为：①不考虑非旋转负载的运行数据和发电机励磁方式；②忽略线路电 容和非旋转负载的并联导纳；③具有分接开关的变压器，其开关位置均视为在主分接位置 ；④ 不计弧电阻；⑤ 35 kV及以上系统的最大短路电流计算时，等值电压源取标称电压的1 .1（计算中额定电压的1.05 pu），但不超过设备的最高运行电压。
对于电网规划、运行部门，三相最大短路电流计算是主要的计算内容。计算中，各电网、电 网内的不同部门可能采用不同的计算条件。差别主要集中在变压器变比、节点电压的选取上 。变压器变比有取1.0，有取实际运行变比的；节点电压可能取1.0，也可能取1.05。这两者 的不同组合均有所采用，显然，这将影响短路电流的计算结果。问题的根源在于计算人员往 往根据计算程序的固有设置来计算，而计算程序又缺乏足够的灵活性所致。其实，若了解短 路电流计算的要求，计算人员是可以对所得的计算结果进行适当的加工的。有的商业软件也 提供了灵活的短路电流计算条件设置功能。�
2影响短路电流计算结果的因素
美国PTI公司的PSS/E程序提供了基于潮流的短路电流计算和采用经典方法、ANSI标准或IEC 标准进行短路电流计算的选择。采用IEC标准时，允许用户任意设定短路电流计算的初值条 件。可设定的选项包括：①变比选择：1.0或正常变比；②考虑充电电容与否；③计及并联 补偿与否；④节点电压值；⑤发电机功率因素。为此，利用PSS/E程序对其自带的算例进行了不同初值设置条件对短路电流计算结果的影响分析。

Answer 1

Three-phase short-circuit current calculation of power system planning, design, operation must be carried out in the calculation of analytical work. At present, the three-phase short-circuit current has become troubled over many of the key issues of power system operation. However, during the three-phase short-circuit current calculation, the design, operation and research departments of the different calculation methods, which may cause short-circuit current calculation of the conclusions of the differences and short-circuit current to determine the difference between standard and short-circuit current restrictions different.

If short-circuit current calculations are somewhat conservative, it may result in unnecessary waste of the investment; if biased optimism, the system will be safe and stable operation of the catastrophic dangers buried. Thus, in-depth study of the standard short-circuit current calculation based on comparing the different conditions of short-circuit current calculation of short-circuit current calculation of the impact of the conclusions, as well as for grid computing and short-circuit current limit to provide a more realistic approach and thinking.

1 short-circuit current calculation method

Classic method of calculating short-circuit current: The variable ratio of 1.0, does not consider the line charging capacitance and parallel compensation, regardless of load current and the effects of load, the node voltage from 1.0, no-load generator. Short-circuit current calculation of the standard IEC main standards and ANSI standards, China is the IEC standards used. GB provides short-circuit current calculation method, calculation conditions. GB recommended three-phase short-circuit current calculation method is the equivalent voltage source method, the calculation conditions are as follows: ① not consider non-rotating load data and run the way of excitation; ② neglect and non-revolving line load capacitance of the parallel admittance; ③ with the transformer tap, the switch position will be deemed to be the main tap location; ④ excluding arc resistance; ⑤ 35 kV and above the maximum system short-circuit current calculation, the equivalent voltage source taking a nominal voltage of 1.1 ( Calculation of rated voltage 1.05 pu), but not exceeding the maximum operating voltage equipment.

For network planning, operation department, the largest three-phase short-circuit current calculation is the calculation of the main content. Calculation, the power grid, power grids of the different departments may use different calculation conditions. Differences mainly concentrated in the transformer ratio, the selection of the node voltage. Variable transformer check than 1.0, the change from the ratio of the actual operation; node voltage may take 1.0, 1.05 may also be taken. Both are used in different combinations, it is clear that this will affect the outcome of short-circuit current calculation. Root of the problem is often calculated based on the inherent computational procedures set up to calculate, and calculation procedures due to lack of sufficient flexibility. In fact, if I understand the requirements of short-circuit current calculation, calculation of staff is calculated on the results obtained from the appropriate processing. Some commercial software also provides a flexible conditions for short-circuit current calculation capabilities. �

2 the impact of short-circuit current calculation factors

United States PTI's PSS / E to provide a trend based on the short-circuit current calculation and the use of classical methods, ANSI standards or IEC standards, the choice of short-circuit current calculation. The use of IEC standards, allows the user to arbitrarily set the initial value calculation of short-circuit current conditions. The options can be set include: ① variable than the choice: 1.0 or normal ratio; ② consider whether or not charging capacitor; ③ or, taking into account the parallel compensation; ④ voltage node; ⑤ generator power factor. To this end, the use of PSS / E program of its own examples of the different initial conditions on the results of short-circuit current calculation of the impact analysis.