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Does reactive power of a machine impact on fault levels?

asked 2012-01-11 04:35:47 -0500

chiangm gravatar image

updated 2012-01-12 05:30:51 -0500

JervisW gravatar image

Hi,

My understanding is fault levels are calculated based on the pre-fault voltage & impedances of machine,transformers,lines & shunts. But my question is why when I try to increase the MVAR output of a machine, the fault level changes?

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Very good question, my guess is there are some second order affects in play here.

jtrain gravatar imagejtrain ( 2012-01-11 05:48:47 -0500 )edit

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answered 2012-01-11 06:03:07 -0500

jtrain gravatar image

I know that fault levels are based on pre-fault voltage and that this relationship is proportional.

I = V / X

where X is the system impedance as seen at the fault.

Reactive power too is strongly related to voltage, such that increasing reactive power tends to increase the local voltage in an area.

Combining those two effects, increasing the reactive power in some instances will increase your pre-fault voltage. And if you are running PSS/E ASCC analysis by default this higher prefault voltage will give a linearly higher fault level.

That is, increasing your pre-fault voltage by 5% can increase your fault level by 5%

To avoid the reliance on pre-fault voltage The Australian Standard for fault calculations recommends that all simulations be based on a nominal 1 p.u pre-fault voltage (no variance) and a fudge factor of 1.1 be applied to the final result to account for the fact that power lines are often run at higher than nominal 1 p.u voltage.

psspy.ascc_2(sid=1,
             status1=1,      # line to ground faults.
             status4=1,      # impose nominal 1.0 p.u prefault voltage.
             # ... rest of arguments.
)

or using the pssarrays module

short_circuits = pssarrays.ascc_currents(
             sid=1,
             fltlg=1,        # line to ground faults.
             flat=1)         # impose nominal 1.0 p.u prefault voltage.
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This is a very interesting topic. I hope to work on this in the near future.

amaity gravatar imageamaity ( 2012-02-24 10:42:02 -0500 )edit

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Asked: 2012-01-11 04:35:47 -0500

Seen: 767 times

Last updated: Jan 11 '12