Unintentional ungrounded 480 delta

[travish]

I am using a 150kva delta/delta 480 to 240 xformer in reverse. so I am feeding 240 to the xformer and getting 480 delta out of the other side. My trouble is the Primary side (which is my secondary side) has no neutral tap. Because it is designed to be the primary. So that leaves me 3 options, Should I ground 1 corner? Buy a ground detector and leave the ground floating? Buy the right xformer? The machine has a 100hp VFD and a 25hp VFD. The machine is only staying at our plant until December we would then ship the machine out to the midwest to one of our manufacturing plants (they have 480 availible). I used the xformer because we had it. We have been runing the machine for about 2 months. Now after thinking about the situation I am scared for safety reasons. If I ground 1 phase and I have a ground fault we get 480 to ground and can the drives deal with the grounded phase? If I don't ground is there any down side (mainly safety)? A new xformer is about $3500

What do you think is the best option

Travis

 

[jim dungar]

Check with your drive vendor, it is possible that they will not like being on an ungrounded system. Heck, they may not even tolerate a corner-ground.

For only two dedicated circuits, If the equipment can handle it I would lean towards ungrounded with ground detectors.

 

[Jraef]

Most drives now will be set up to look for a grounded wye system and the surge protection (MOVs) will be based on that. For some of them, you CANNOT connect a delta power source at all, some will have you completely remove the MOVs because the components are not rated for line voltage and others, like A-B that are designed for the US market, will just have a jumper link on the ground reference that you must remove, because the MOVs ARE designed for line to line voltage even though they were connected in a grounded wye. But again, you cannot universally apply any solution across platforms, you must follow the instructions for YOUR specific drive. If they have none, you must assume they CANNOT be connected to a delta system.

Most of them will not like a corner grounded delta no matter what you do with the MOVs.

 

[jumper]

..... will just have a jumper .

Hey, leave me out of this.

 

[brian john]

Check with your drive vendor, it is possible that they will not like being on an ungrounded system. Heck, they may not even tolerate a corner-ground.

For only two dedicated circuits, If the equipment can handle it I would lean towards ungrounded with ground detectors.

In my expierence you tell them you have an ungrounded or corner grounded and they will tell you a BIG FAT NO, must be a wye.

And I so prefer a grounded wye in this ciscumstance, though the facility does seem to have qualified electricians on the job.


You could always make a ground detection system.

 

[travish]

checked with Danfoss this morning, they said the drive did not care if ground floated or if I corner grounded. from a safety standpoint all of our maint guys understand the floating ground, I don't like it but guess it is OK, I would rather put the ground detection device in and not corner ground. Machine is only here 6 more months, and I will not get caught in this situation again.

Thanks for all your input

Travis

 

[Mike01]

Elevated Voltages

Elevated Voltages

The only item about remaining ungrounded with detection is; What is the procedure when a ground fault occurs? The other phases to ground will be elevated to line to line voltages and no OCPD?s would clear / trip / open the system will continue to operate. Depending on the equipment you are serving are all the components rated for line to line voltage under a fault condition? The insulation and components [MOV?s etc.] my only be rated for line to ground voltages of 277V not 480V. Also depending on how long you plan on allowing the equipment to operate I believe NEMA has guidelines on increased conductor insulation recommendations if the equipment is planning on operating under a ground fault condition for an extended period of time. One solution is to have you ground fault protection also provide the trip of the transformer upstream OCPD. Typically the ground detection lights are connected thru. Three voltage transformers configured in WYE-WYE configuration by providing a WYE-BROKEN DELTA type of configuration you can include a relay that under a ground fault condition could operate the shunt trip of an OCPD. Not sure on this one but would the OCPD serving the transformer be required to be ?fully? rated at 480 and not ?slash rated? 480/277 [if it is indeed a circuit breaker]?

 

[travish]

talked it over with my partner in crime at work and we decided to order a delta/wye 240/480 xformer.

thanks for all the help