Learning symmetrical components can be overwhelming if you consider its mathematical aspect only. It is time to step back and understand this concept from a macro view.
What is symmetrical component?
In simple terms, symmetrical component is a mathematical tool that simplifies the analysis of power system during unbalanced system conditions. It does so by decoupling the one 3-phase system into three 1-phase systems. It is easy and less confusing to calculate currents in a 1-phase network than in a 3-phase network.
Keep in mind that, once decoupled, each 1-phase network has no relationship to phase ‘a’, phase ‘b’ or phase ‘c’ of a 3-phase network. The networks obtained are purely abstract in concept. For example, when phase ‘a’ of the transmission line is de-energized due to single pole circuit breaker operation, the current in that phase drops to zero. However, in the symmetrical world, you will have non-zero current flowing through the open circuit.
How are symmetrical components represented?
The three elements that make up the symmetrical components are
- Positive sequence
- Negative sequence
- Zero sequence
Each one is a vector quantity. What is special about these elements is how they describe an event. For example, see the table below.
|Type of Fault||Positive Sequence||Negative Sequence||Zero Sequence|
|Unbalanced||Line to Ground||X||X||X|
|Line to Line||X||X|
|Double Line to Ground||X||X||X|
|One Open Conductor (due to single pole CB or fuse operation)||X||X||X|
|Two Open Conductor||X||X||X|
The following points can be inferred from the above table.
- Balanced system operation or balanced 3-phase faults have positive sequence elements only.
- Any fault involving ground must have zero sequence elements.
- Negative sequence elements show up in unbalanced systems only.
If you commit these points to your memory then symmetrical components will help make sense of events in a power system. You can analyze data from the power system software or directly from relays and identify if there is a ground fault, phase-to-phase fault, etc. just by checking for symmetrical quantities.
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