Lecture
The general expressions for short-circuit current obtained in Section 2 make it possible to determine its value with high accuracy at any instant of the transient process in a circuit fed by a single generator. The structure of these expressions shows that, even under such simple conditions, their application requires a large amount of computational work.
When moving to circuits with several generators, the task of accurately calculating the transient process becomes sharply more complicated. Even disregarding the issues of generator swinging and the behavior of connected loads, it is enough to recall that the changes in the free currents of each generator are mutually interrelated. With automatic excitation control, a similar relationship also exists in the increments of the forced currents.
Using operational-calculus techniques to compute transient processes in complex circuits involves overcoming very cumbersome and laborious calculations. The order of the characteristic equation increases rapidly as the number of machines in the circuit under consideration grows. Therefore, even despite the extensive capabilities of modern computers, this method, as well as numerical integration methods, can be regarded as a benchmark for evaluating other approximate calculation methods.
The main requirement that a practical method must satisfy is simplicity of execution, which above all prevents the possibility of errors. However, the simpler the method, the more assumptions it is based on, and consequently the lower its accuracy.
In addition to the assumptions mentioned earlier, practical short-circuit calculations further assume that:
the law of variation of the periodic component of the short-circuit current, established for a circuit with a single generator, can be used for an approximate evaluation of this component in a circuit with an arbitrary number of generators;
the aperiodic component of the short-circuit current can, in all cases, be accounted for approximately;
the rotor of each machine is symmetrical, i.e., the machine parameters are the same for any rotor position.
This last assumption makes it possible to operate with EMFs, voltages, and currents without resolving them into direct- and quadrature-axis components. At the same time, it excludes consideration of the second-harmonic current arising from the aperiodic component of the short-circuit current with an asymmetrical rotor.
The difference between practical methods for calculating the short-circuit transient process consists mainly in the different approaches to computing the periodic component of the short-circuit current. This approach is set by the requirements and purpose of the particular calculation. The premises and assumptions that can be used in a calculation whose task is limited to determining the current at the fault location, and especially at a greater electrical distance from it, turn out to be unsuitable if it is necessary to find the current distribution among the individual branches of the circuit, as is usually required when solving relay-protection problems.
No less stringent requirements apply to calculations performed in fault analysis
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