One of the topics we covered in an introduction to power systems class was using zero/positive/negative sequence networks to analyze faults in networks. For many problems, this requires setting up positive and negative sequence networks and their equivalent impedance.

My understanding is that balanced/static (?) components have equal positive/negative sequence impedance, while this can be different for generators (I think we're limiting ourselves to synchronous generators).

On to the real question: in one of the examples we are working out the fault current of a SLG fault, and a table is given with different negative/positive sequence impedance for the generator (as I would expect) but according to the solutions, the equivalent negative sequence impedance is the same as the equivalent positive sequence impedance (even though the generator impedance is used to calculate it).

So, what gives? Is this a special case for SLG faults, an approximation, or perhaps just a mistake?

(pretty much the entire faculty seems to be on vacation, and as a Physics major I don't know that many people that know this stuff... thanks in advance!)

  • \$\begingroup\$ the web search engines never go on holidays ... learn how to learn more efficiently. this is the goal of uni education with theory to practice your learning skills, cdn.selinc.com/assets/Literature/Publications/White%20Papers/… try p10 \$\endgroup\$ Aug 12 '17 at 19:39
  • \$\begingroup\$ Thank you for that document, which I have already found on my own searches... After all, that is why I have come to StackExchange. Specifically, the example in this document does exactly what I suspect should happen: different equivalent impedances because the generator has different impedance for neg. and pos. sequence. That still does not answer my question whether there is something special about this - is it an acceptable approximation to ignore the difference for SLG faults, or otherwise specific to this type of fault? \$\endgroup\$ Aug 12 '17 at 19:52
  • \$\begingroup\$ I believe it depends proximity of network earth ground to ground fault impedance. If the problem is defined a solid ground or near trans. then ... yes \$\endgroup\$ Aug 12 '17 at 20:41

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