High U.P.S. Ground Currents

[ghostbuster]

My customer has a 3 phase U.P.S. system.On the 480 volt delta input side,the phase currents are approx. 40 amps balanced on all 3 phases.The ground current on the 480 volt input side is approx. 5 amps.The U.P.S. supplier has performed a field investigation and claims this 5 amps of ground current is normal.They claim this current is from their filtering capacitors connected phase to ground on the 480 volt delta input side.This 5 amps of ground current appears to be extremely high.I am also concerned it may cause other problems in the overall electrical system at this site.

 

[rdelauter]

Could you provide us with more info about the UPS system. How large is it, what % of load is on the system now? I don't know of any system that have thier input filters connected Y, most I have seen are Delta, but I have not seen them all. Is there a G.E.C. pulled to the UPS and is this were the 5 amps is or is it on the earth ground?

 

[ghostbuster]

Could you provide us with more info about the UPS system. How large is it, what % of load is on the system now? I don't know of any system that have thier input filters connected Y, most I have seen are Delta, but I have not seen them all. Is there a G.E.C. pulled to the UPS and is this were the 5 amps is or is it on the earth ground?

The U.P.S. is approx. 60% loaded.The ground current was measured on the incoming green insulated wire.This current was then compared to the filter capacitor ground currents for correlation purposes by the U.P.S. supplier.I agree,I would have expected these filter capacitors to be connected phase to phase.The U.P.S. supplier(one of the big three) refuses to change this filter connection arrangement.

 

[dereckbc]

They claim this current is from their filtering capacitors connected phase to ground on the 480 volt delta input side.

Pardon me for Saying, DUH!
There will be current with this configuration. Only way to change it is lower the value of the capictors, remove them, or install them P-P-P.

What the manufacture has vitually done by adding capacitors from P-G on the input has turned it into a wye input transformer. Other than FCC requirement for noise reduction I cannot think of any other reason they would do that. It is a poor design.

 

[ghostbuster]

"" It is a poor design.""

To add to these comments:

I should also mention during situations where the U.P.S. is powered down for P.M.:
When the U.P.S. is powered up again(re-energize the 480 volt input feed) the upstream fuses blow each time (due to high capacitor filter inrush current to ground).The only way to get this U.P.S. powered up, is to first disconnect this capacitor filter,then energize the 480 volt circuit to the U.P.S.,,after everything has been stabilized then reconnect this filter.Yes,I agree this is an extremely poor design,but the U.P.S. technical department refuses to change this design for my customer(they have signed a service contract).

 

[dbuckley]

When the U.P.S. is powered up again(re-energize the 480 volt input feed) the upstream fuses blow each time (due to high capacitor filter inrush current to ground).The only way to get this U.P.S. powered up, is to first disconnect this capacitor filter,then energize the 480 volt circuit to the U.P.S.,,after everything has been stabilized then reconnect this filter.

(they have signed a service contract).

In twenty plus years in IT with UPS sizes from toys to megawatts I've never known a UPS you can't switch on.

You're the wrong person to fight the battle - convince your customer who has the signed contract that they've got an unacceptable UPS (i.e. they've been had), and they can then harass the UPS supplier. I certainly wouldn't accept it.

How big is this UPS?

 

[rdelauter]

"" It is a poor design.""

To add to these comments:

I should also mention during situations where the U.P.S. is powered down for P.M.:
When the U.P.S. is powered up again(re-energize the 480 volt input feed) the upstream fuses blow each time (due to high capacitor filter inrush current to ground).The only way to get this U.P.S. powered up, is to first disconnect this capacitor filter,then energize the 480 volt circuit to the U.P.S.,,after everything has been stabilized then reconnect this filter.Yes,I agree this is an extremely poor design,but the U.P.S. technical department refuses to change this design for my customer(they have signed a service contract).

I think you may have some problems here, and not just with the UPS design. If applying power after a PM caused the input fuses to open, why don't they open after return of power after an outage? What is different?

Is there a circuit breaker that allows you to disconnect the input filter from the input of the UPS? On most of the new designs I have seen there is not an option to disconnect the input filter manually. One system I know off will do it automatically if on gen. or the load is below a certian %.

Also, appling AC voltage to the input filter of a UPS system does not normally cause a large input current rush, this is normally caused by an input transformer. You can review onlines of most UPS system and notice that there is nothing to limit the input current rush. Lots of times there are chokes in series and caps in parallel with rectifer.

You may want to be sure you input fuses are the correct type and size.

 

[ghostbuster]

I
You may want to be sure you input fuses are the correct type and size.

Thanks for the heads up.The U.P.S. guys have been supplying all the fuses and turning on/off the cap filter each time.(on double and triple time hourly rates).The bottom line--this U.P.S.will have to be replaced or re-designed.

 

[steve66]

It can't be redesigned or modified without violating the UL listing. It is UL listed right? Are you sure the filter caps go to the ground? That's just not right.

Does the feed to the UPS have a neutral? What kind of feeder do you have to the UPS? Is it from a Delta, or a Wye, and whats the voltage? Finally, are you sure it is installed correct? (The UPS guys should have noticed by now if it is, but they are apparently worthless.)

Have you tried powering it on with the load removed from the output? If the fuses hold then, maybe there is a problem with the load.

Steve

 

[steve66]

One other thought, maybe the fuses are too small. Some people try to size the fuses from the KVA rating of the UPS. That's a mistake since you also have to count the UPS charging current and other things. The manufacturer should have a table of breaker and fuse and cable sizes for the UPS.

And why fuses? Why not circuit breakers? Maybe you need fuses with a longer time delay.

Steve

 

[ghostbuster]

It can't be redesigned or modified without violating the UL listing. It is UL listed right? Are you sure the filter caps go to the ground? That's just not right.

Does the feed to the UPS have a neutral? What kind of feeder do you have to the UPS? Is it from a Delta, or a Wye, and whats the voltage? Finally, are you sure it is installed correct? (The UPS guys should have noticed by now if it is, but they are apparently worthless.)

Have you tried powering it on with the load removed from the output? If the fuses hold then, maybe there is a problem with the load.

Steve

Steve
The unit is UL listed and the caps have been visually inspected (they are connected to ground , this has also been verified by the U.P.S. mfr.(one of the big 3)).As mentioned in earlier posts ,this is fed 480 volts .delta.With no U.P.S. loads,the fuses still blow only with the cap filter connected.Hope this helps.