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 Post subject: utility fault current
PostPosted: Mon Mar 29, 2010 9:23 am 
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This may be a stupid question, but can the utility companies ever reduce the amount of fault current they supply? I know their transformer sizes are fixed but can't they do something from the substation? And if so, will they?
(just curious)


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PostPosted: Mon Mar 29, 2010 9:30 am 
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uriah1 wrote:
This may be a stupid question, but can the utility companies ever reduce the amount of fault current they supply? I know their transformer sizes are fixed but can't they do something from the substation? And if so, will they?
(just curious)


Please be advised that lower Fault current may not result in lower incident energy. It depends on clearing time as well.


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PostPosted: Mon Mar 29, 2010 12:18 pm 
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The utility could install reactors at the substation to reduce the fault current. I seriously doubt that they would, not only because of the cost, but because it would make voltage regulation worse for their other customers.

The utility might be able to serve you from a different circuit with a longer distance from the substation. This could also be done during line outages, so the lower fault level might need to be considered in arc flash calculations.


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PostPosted: Mon Mar 29, 2010 7:16 pm 
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Utilities distribute power with the thought of conservation of cash. There are technical steps the utility could do to reduce fault current, like reactors, or even better higher impedance transformers at the users, or secondary protection in the transformer, but they won't. It may be better that they don't, as all utility costs are passed to the consumers with a 15% markup.


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PostPosted: Tue Mar 30, 2010 1:00 pm 
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haze10 wrote:
Utilities distribute power with the thought of conservation of cash. There are technical steps the utility could do to reduce fault current, like reactors, or even better higher impedance transformers at the users, or secondary protection in the transformer, but they won't. It may be better that they don't, as all utility costs are passed to the consumers with a 15% markup.


There is not just cost implications regarding why high fault levels are preferred among utilities. Lower fault levels will affect the systems ability to regulate the voltage within required limits. This is especially true when there are relatively large industrial loads in the area with poor power factor.


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PostPosted: Tue Mar 30, 2010 6:28 pm 
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From my experience with our local utility, high fault current is furnished reflexively. In my opinion, this is for a couple of reasons:

Convenience. An account representative can pull fault current values from a chart, based on default impedance values and infinite primary. Engineering is a separate department and isn't as accessible. Sometimes it takes some effort just to get a response from customer service.

Liability. A high fault current is conservative with respect to specifying withstand and interrupting ratings for the customer's equipment. (Avid forum readers know that lower utility fault currents are more limiting with respect to arc flash in many cases. )


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PostPosted: Tue Mar 30, 2010 9:15 pm 
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A utility could have a high fault current delivery system on the primary, but if they employed a high impedance transformer then only the secondary side (the user) would have a voltage issue. Typically, this might not be a big issue for large users as they have a 50% load factor or better, so its just a matter of setting the taps properly. The pole transformers usually are in the 2 to 4% Imp and the padmounts are 5%. They could be wound at 8%. Something like this would not eliminate arcs or injury but have an effect in the positive direction. Downside would be those customers with large AC motors that start across the line. But one motor would have to represent 30% of the transformer value.


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PostPosted: Wed Mar 31, 2010 3:00 pm 
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haze10 wrote:
A utility could have a high fault current delivery system on the primary, but if they employed a high impedance transformer then only the secondary side (the user) would have a voltage issue. Typically, this might not be a big issue for large users as they have a 50% load factor or better, so its just a matter of setting the taps properly. The pole transformers usually are in the 2 to 4% Imp and the padmounts are 5%. They could be wound at 8%. Something like this would not eliminate arcs or injury but have an effect in the positive direction.


The lesser the fault current, the more difficult it is to coordinate OCPDs, and the longer (typically) it takes to open them.

I'm not sure lowering the fault current on the secondary of the service transformer is a good rule to follow for everyone.


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PostPosted: Wed Mar 31, 2010 7:12 pm 
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Unless you have a specific need, like starting large motors across the line, or a process that loads your transformer 10% to 90%, there is little benefits to high fault current. The fault current will still be high enough to get breakers to operate in their inst zone as long as they are size appropriately. The concept of having 'high fault current' in order to have the breakers operate faster is a 'two edged sword' as IE values will also be higher.


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