At present, short-circuit fault currents in electric power systems are calculated in accordance with national standards, which use a number of assumptions. One of these assumptions is neglecting possible out-of-phase operation of synchronous generators in the mode preceding the short-circuit fault. At present, the constantly growing level of short-circuit currents is a factor aggravating the problem of ensuring consistency between the equipment installed in the electric power systems and these currents. Accordingly, more stringent requirements are posed to the accuracy of short-circuit fault analysis methods. The aim of this work is to estimate the effect the out-of-phase operating conditions of synchronous generators have on the short-circuit current and to decide whether it is advisable to introduce changes in the short-circuit current calculation procedure stipulated by the relevant standards. To this end, the effect the out-of-phase operating conditions of synchronous generators have on the current at the initial moment of a short-circuit fault is studied taking into account the parameters of the mode preceding the short-circuit fault. To analyze the influence of out-of-phase operating conditions of the generators on the short-circuit fault currents, a mathematical model was developed, and case calculations were carried out using the Mathcad software package. The study was carried out for a synchronous generator operating in parallel with the electric power system; factors reflecting the design features of the synchronous generators, their power in the mode preceding the short-circuit fault, and the electrical remoteness of the synchronous generator from the grid were varied. The parameters of the electrical equipment installed in the electric power system of the Republic of Tajikistan were used as initial data. The effect the static out-of-phase operating conditions of synchronous generators have on the short-circuit current periodic component value has been evaluated. It is shown that calculations of short-circuit currents carried out according to the methods stipulated by the standards that are currently in force, in which out-of-phase operation of synchronous generators is not taken into account, yield overestimated short-circuit current values, and the calculation error can be as much as 14%. It has been recommended to take into account the static out-of-phase operating conditions of synchronous generators in solving matters concerned with coordination of short-circuit current levels, and to improve the methods for calculating short-circuit currents in power systems. The out-of-phase operation of synchronous generators should be taken into account in substantiating the need of replacing the electrical equipment of power plants and substations in view of growing levels of short-circuit currents.
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Oct 19, 2021
X Zhang + 2
Open Access