480 3 wire delta

[gregoryfast]

I work for a small utility company. We have changed managers. The old manager would not allow ungrounded 3 wire Delta. So, "B" phase was always tied to ground. There were a few installations where we had ungrounded 3 wire Delta. He made us ground them. I understood it to be that there was some way that line voltage could end up on one phase. I had an article that explained it, but I can't find it. He is thinking that he might have us disconnect all of the grounded 3 wire Delta.

Would someone please help me to understand this subject?

I have always appreciated the collective wisdom of this board.
Thank you,
Gregory

 

[dereckbc]

Re: 480 3 wire delta

So what is it you want to know?

They are mostly used in industrial process like blow film, refinery, electric furnaces, etc where a power outage would cause extensive downtime and tool damage.

A delta system is not referenced to ground, if one phase is grounded nothing happens. If a second phase is grounded the relays trip the power.

There is more to it, but that is the short version.

 

[gregoryfast]

Re: 480 3 wire delta

Is there a safety difference between 480 ungrounded and grounded?

The system was installed for power quality. The customer receives 480 3 wire grounded delta. There is a 480 to 208Y/120 transformer to isolate the building from the utility. Now the customer wants to use the 480 as a distribution voltage in their building. This building is on stell pilings over the water. All of the pilings had to be replaced due to electrolysis. At that point, the customer installed an isolation power system. Now that they want to use the 480, it has been suggested that if we remove the ground from the grounded phase, they will still be isolated from our system.

Thanks,
Gregory

 

[bennie]

Re: 480 3 wire delta

Gregory: This is an interesting system. I have seen it before when corrosion was a big problem.

By supplying an ungrounded secondary to the premises, the multi-neutral ground is eliminated. This make the facility a system without external connections, except through magnetic coupling. This facility is a true separately derived system. The earth grounding, of the system, is to be according to 250.30.

The only negative side effect of an ungrounded 480 volt secondary to the primary of the customer transformer is the absence of through fault protection.

Ground lights are for notification of the failure of one level of possible failure in a 2 level system.

Ungrounded 480 volts will read 277 volts to ground through capacitance coupling. The open circuit voltage is from a high impedance source and will be extremely low on a closed circuit.

 

[madmike]

Re: 480 3 wire delta

Gregory,

I think your old boss was afraid of ground faults which can drive the nominal 277v phase-to-ground voltages to a full 480 or higher on the ungrounded phases. The transients thus generated can momentarily reach 6 or 7 times the line-to-line voltage. Look at some of the technical papers on the Post Glover web site. There is a couple by David Shipp that explain the behavior very well. Something for you to explore would be a High-Resistance Grounding system. This would give better safety than a straight delta system, and it also gives better voltage stability. By not having a grounded leg, or neutral, you will also eliminate the corrosion problems.

 

[gregoryfast]

Re: 480 3 wire delta

Mad Mike,
Would you please post the web site for the Post Glover web site.

I need to be able to explain the transient voltages reaching 6 or 7 times the line to line voltage.

Thank you to everyone,
Gregory

 

[hurk27]

Re: 480 3 wire delta

Wouldn't it be better if they installed a 480 delta to 480 delta and then just bond the "B" phase on the secondary to the building grounding and the "B" phase on the primary to the shore grounding? This would keep the isolation but would it even be necessary, as the main service transformer could be set up to keep the isolation.

 

[madmike]

Re: 480 3 wire delta

gregoryfast,

The site is http://www.postglover.com/representative_leads/scalifornia_form.html

Unfortunately, you now have to fill out their form before you can access the technical papers. Once you get there though, any of the David Shipp papers, or the Meister paper would be good ones to read. If you have any trouble, I have them in PDF, send me an e-mail, mcclellandm@sbcglobal.net and I will send some to you.

I suspect if you ground any phase on the secondary, it will make quite a difference in the available fault current as well as current leakage to ground. This might require a re-evaluation of your breaker coordnation.

 

[mclain]

Re: 480 3 wire delta

We run a 600 volt and 480 volt ungrounded systems. I have worked with these type systems for 20+ years and have seen nothing unsafe about them. As long as you have the proper ground detection system and trained personnel. I have read about the high resistive grounding systems at the post glover and downloaded most of there information. I can?t see where the advantages of a high resistive grounding system warrant the additional cost. I think you would still have trouble with electrolysis since the high resistive grounded system still has a grounded neutral. It just passes through a resistor to limit the current when a fault occurs. The transit voltage that reaches 6 to 7 times the line to line voltage is from what is described as a rotating fault or repetitive fault, such as a motor winding making contact with a ground point as it turns.