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【正文】 e source can be neglected ? The subtransient behavior of generators and motors ? This method assumes the following conditions: ? The passive loads and conductor capacitances can be neglected ? The step setting of the transformers do not have to be considered ? The excitation of the generators do not have to be considered ? The time and position dependence of the previous load (loading state) of the work does not have to be considered Fig. : Network circuit with equivalent voltage source a) threephase work, b) equivalent circuit in positive sequence system Superposition method The superposition method is an exact method for the calculation of the short circuit currents. The method consists of three steps. The voltage ratios and the loading condition of the work must be known before the occurrence of the short circuit. In the first step the currents, voltages and the internal voltages for steadystate operation before onset of the short circuit are calculated (Figure ). The calculation considers the impedances, power supply feeders and node loads of the active elements. In the second step the voltage applied to the fault location before the occurrence of the short circuit and the current distribution at the fault location are determined with a negative sign (Figure ). This voltage source is the only voltage source in the work. The internal voltages are shortcircuited. In the third step both conditions are superimposed. We then obtain zero voltage at the fault location. The superposition of the currents also leads to the value zero. The disadvantage of this method is that the steadystate condition must be specified. The data for the work (effective and reactive power, node voltages and the step settings of the transformers) are often difficult to determine. The question also arises, which operating state leads to the greatest short circuit current. Figure illustrates the procedure for the superposition method. Fig. : Principle of the superposition method a) undisturbed operation, b) operating voltage at the fault location, c) superposition of a) and b) Transient calculation With the transient method the individual operating equipment and, as a result, the entire work are represented by a system of differential equations. The calculation is very tedious. The method with the equivalent voltage source is a simplification relative to the other methods. Since 1988, it has been standardized internationally in IEC 60 909. The calculation is independent of a current operational state. In thisbook, we will therefore deal with and discuss the method with the equivalent voltage source. Calculating with reference variables There are several methods for performing short circuit calculations with absolute and reference impedance values. A few are summarized here and examples are calcu
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