AdvancedTopic 73 of 106Electrical Engineering
Symmetrical components
Decompose unbalanced phasors into positive, negative and zero sequences.
In depth
Fortescue's method splits unbalanced three-phase phasors into positive, negative and zero sequence sets, each analysed with its own network. Fault types connect sequence networks differently. Negative sequence indicates unbalance; zero sequence indicates earth faults.
Key points
- a=1∠120° operator
- Transformers affect zero sequence
- Basis of protection relays
ExampleA single line-to-ground fault connects the three sequence networks in series.
What is Symmetrical components?
Decompose unbalanced phasors into positive, negative and zero sequences.
Why is it important to understand symmetrical components?
Symmetrical components convert unbalanced three-phase quantities into simpler balanced parts, so unbalanced faults can be analysed with standard methods.
What are the three sequences?
Positive, negative and zero sequence components, which together represent any set of unbalanced three-phase phasors.
Why is zero sequence important?
Zero sequence current flows only through earth or neutral paths, so it is needed for ground fault studies and grounding design.
How is Symmetrical components used in practice?
Typical applications include calculating the fault current for a single line to ground fault, analysing the unbalance caused by a broken conductor and estimating the negative sequence heating in a generator under unbalanced load.
What should I learn before Symmetrical components?
Study "Fault analysis" first (topic 72 of 106 in Electrical Engineering). It covers the groundwork this topic builds on.
What are the key points of Symmetrical components?
a=1∠120° operator. Transformers affect zero sequence. Basis of protection relays.
Can you give an example of Symmetrical components?
A single line-to-ground fault connects the three sequence networks in series.
What comes after Symmetrical components?
The next topic is "Protective relays", which builds directly on this one.
How difficult is Symmetrical components?
It is classed as Advanced in Electrical Engineering (topic 73 of 106). It is an advanced topic. It assumes you are comfortable with the intermediate topics in Electrical Engineering.