Fault Calculation 3-Phase Bolted Fault Current, Single-Line-To-Ground and Line-Line

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todd.toddy@hotmail.com

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Hello everyone,

I have a quick question about fault calculations. Specifically, how I can model an induction motor in a positive sequence reactance circuit, negative sequence reactance circuit and zero sequence reactance circuit in order to perform a 3-phase bolted fault current, single-line-to-ground fault and line-line fault.

The only information that was given to me about the induction motors was its subtransient reactance and its transient reactance.

Can I just assume that the subtransient reactance is a good value to use for its positive sequence reactance? Or can I assume that the transient reactance is a good value to use for its negative sequence reactance? What about the zero sequence reactance?

Thank you.
 
Todd,

1) Yes, you can use the sub-transient reactance for 3-ph and ph-ph faults!

2) The typical induction-motor (i.e., not intentionally grounded) has no zero-sequence impedance!

3) For LV motors the Isc contribution decays in less than 2-cycles, so unless the fault is close-in, contribution is nil!

4) A long, long time ago I developed an equation illustrating how X" decays with time! If it can be helpful, I will try to find it.

Regards, Phil Corso
 
Phil,

Thank you for your help! For the induction motor, if I assume the subtransient reactance to be its positive sequence reactance and the zero sequence reactance to be negligible, what can I assume for its negative sequence reactance?

Again, I really appreciate it.

Todd.
 
Todd,

Negative-Seq is equal to Positive-Seq. Regarding a value for Pos, Neg seq impedances, you can use the starting or locked-rotor impedance.

Except for large motors, the resistance component can be ignored, because locked-rotor PF is low! For example:

Hp LR PF
1-10 0.5
100 0.4
1,000 0.17
10,000 0.1

Regards, Phil
 
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