In power engineering, specifically three phase power, a symmetric or balanced fault is a fault which affects each of the three phases equally. This is in contrast to an asymmetric fault, where each of the phases is not affected equally. In practice, most faults in power systems are unbalanced. With this in mind, symmetric faults can be viewed as somewhat of an abstraction; however, as asymmetric faults are difficult to analyze, analysis of asymmetric faults is built up from a thorough understanding of symmetric faults.

Analysis

Symmetric faults can be analyzed via the same methods as any other phenomena in power systems, and in fact many software tools exist to accomplish this type of analysis automatically (see power flow study). However, there is another method which is as accurate and is usually more instructive.

First, some simplifying assumptions are made. It is assumed that all generators in the system are in phase, and operating at the nominal voltage of the system. Motors can also be considered to be generators, because when a fault occurs, they usually supply rather than draw power. The voltages and currents are then calculated for this base case.

Next, the location of the fault is considered to be supplied with a negative voltage source, equal to the voltage at that location in the base case, while all other sources are set to zero. This method makes use of the principle of superposition.

To obtain a more accurate result, these calculations should be performed separately for three separate time ranges:

• subtransient is first, and is associated with the largest currents
• transient comes between subtransient and steady-state
• steady-state occurs after all the transients have had time to settle