Symmetrical Fault

Short-circuit conditions are the result of the majority of symmetrical fault on the three-phase power system. A strong current (known as short circuit current) flows through the apparatus under such circumstances, seriously damaging it and interrupting service to the users. The issue of determining short circuit currents under fault conditions is probably the one that electrical engineers care about the most. Practically every piece of equipment in the power system, from the apparatus chosen to the design and arrangement, is influenced by short-circuit current considerations.

A symmetrical fault is one that results in symmetrical fault currents, or equal fault currents in the lines with a 120° displacement, on the power system.

When all three conductors of a three-phase line are simultaneously brought together into a short-circuit condition, as depicted in Fig. 17.1, the symmetrical faults occurs. Equivalent fault currents with a 120° displacement are produced by this type of fault and are symmetrical currents, L e. Thus, with reference to Figure 17.1,. The fault currents IR, ly, and IB will have a 120° separation between them and be of equal magnitude. Due to the fault’s balanced nature, only one phase needs to be taken into account in calculations. Because the conditions in the other two phases will be similar.

The following points may be particularly noted :

1. As the majority of faults are unsymmetrical in nature, symmetrical faults are uncommon in practice. To help the reader understand the issues short circuit conditions pose to the power system, symmetrical fault calculations are discussed in this chapter.
2. The most serious fault is symmetrical, which places a heavier burden on the circuit breaker.

Limitation of Fault Current

The impedance of the system up to the point of fault limits the short-circuit current when it happens at any point in a system. Referring to Fig. 17.2, if a symmetrical fault on a three-phase system occurs on the feeder at point F,. The short circuit current from the generating station will have a value. That is constrained by the impedance of the generator and transformer as well as the line’s impedance between the generator and the point of the symmetrical fault. This demonstrates how crucial it is to understand the impedance of the various devices. And circuits in the system’s line in order to accurately determine short-circuit currents.

Transformers, reactors, and generators are a few examples of the impedances. That frequently play a large reactive role in limiting the current caused by symmetrical faults on three-phase systems. Although cables and lines are typically resistive, the latter is typically ignored in calculations. Where the total reactance is greater than three times the resistance. This assumption will result in an error that is no greater than 5%.