Asymmetric fault

In power engineering, specifically three phase power, an asymmetric or unbalanced fault is a fault which does not affect each of the three phases equally. This is in contrast to a symmetric fault, where each of the phases is affected equally. In practice, most faults in power systems are unbalanced; however, as asymmetric faults are difficult to analyze, analysis of asymmetric faults is built up from a thorough understanding of symmetric faults.

Common types of asymmetric faults, and their causes:

• line-to-line - a short circuit between lines, caused by ionization of air, or when lines come into physical contact, for example due to a broken insulator.
• line-to-ground - a short circuit between one line and ground, very often caused by physical contact, for example due to lightning or other storm damage
• double line-to-ground - two lines come into contact with the ground (and each other), also commonly due to storm damage

Analysis

An asymmetric fault breaks the underlying assumptions used in three phase power, namely that the load is balanced on all three phases. Consequently, it is impossible to directly use tools such as the one-line diagram, where only one phase is considered. However, due to the linearity of power systems, it is usual to consider the resulting voltages and currents as a superposition of symmetrical components, to which three phase analysis can be applied.

The method of symmetric components is perhaps somewhat unintuitive, but can be verified to give correct results. The power system is seen as a superposition of three components:

• a positive-sequence component, in which the phases are in the same order as the original system, i.e., a-b-c
• a negative-sequence component, in which the phases are in the opposite order as the original system, i.e., a-c-b
• a zero-sequence component, which is not truly a three phase system, but instead all three phases are in phase with each other.

To determine the currents resulting from an asymmetrical fault, one must first know the per-unit zero-, positive-, and negative-sequence impedances of the transmission lines, generators, and transformers involved. Three separate circuits are then constructed using these impedances. The individual circuits are then connected together in a particular arrangement that depends upon the type of fault being studied (this can be found in most power systems textbooks). Once the sequence circuits are properly connected, the network can then be analyzed using classical circuit analysis techniques. The solution results in voltages and currents that exist as symmetrical components; these must be transformed back into phase values by using the A matrix.

References

• Glover, J.D., and Sarma, M.S., Power System Analysis and Design, Brooks/Cole, 2002. (ISBN 0-534-95367-0)

• Burton, G.C., power analysis

See also

• Symmetric fault

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