NEC Delta-Delta Configuration 12.75kV-480V Ungrounded

[MrJLH]

Long story short this facility I'm working has 9 substation with one common utility feeder. 7 of the transformers are 3 x 1-phase in a Delta-Delta configuration, mounted on a concrete pad with no secondary protection. Type THW and RHW feeders run from the transformer secondaries to a "bus gutter" where the feeders are taped to circuit breakers which are then fed to additional switch racks across the facility.

There is no apparent plant ground ring with each substation appearing to have its own isolated ground ring.

The other two transformers are 1 x 3-phase Delta-Delta, and a 1 x 3-phase Delta-Wye transformer solidly grounded.


1. I need help understanding 3 x 1 phase Delta-Delta configuration. How are these transformers typically grounded? The only grounds I can see are the case grounds. Are these grounds tied internally to the transformer wingdings? No ground is ran from the transformer to the switch racks.

2. I have both Delta-Delta and a Delta-Wye (solid grounded), what issues is expected with the facility being configured like this? I'm not exactly sure how the solidly ground transformer is tied.

2. The end goal is to have the facility "unmanned", is there something in the NEC about a facility being Delta-Delta configured? What risks are there to this?

3. Where is the line drawn at ownership of modifying existing electrical equipment that does not meet electrical code? I need to add motors to these existing switchracks, but the racks use a gutter type system where the incoming feeder is tapped to motor starters. I don't want to tie into existing taps as most of the existing installation violates the tap rule.

Thanks All

 

[__dan]

Long story short this facility I'm working has 9 substation with one common utility feeder. 7 of the transformers are 3 x 1-phase in a Delta-Delta configuration, mounted on a concrete pad with no secondary protection. Type THW and RHW feeders run from the transformer secondaries to a "bus gutter" where the feeders are taped to circuit breakers which are then fed to additional switch racks across the facility.

There is no apparent plant ground ring with each substation appearing to have its own isolated ground ring.

The other two transformers are 1 x 3-phase Delta-Delta, and a 1 x 3-phase Delta-Wye transformer solidly grounded.


1. I need help understanding 3 x 1 phase Delta-Delta configuration. How are these transformers typically grounded? The only grounds I can see are the case grounds. Are these grounds tied internally to the transformer wingdings? No ground is ran from the transformer to the switch racks.

2. I have both Delta-Delta and a Delta-Wye (solid grounded), what issues is expected with the facility being configured like this? I'm not exactly sure how the solidly ground transformer is tied.

2. The end goal is to have the facility "unmanned", is there something in the NEC about a facility being Delta-Delta configured? What risks are there to this?

3. Where is the line drawn at ownership of modifying existing electrical equipment that does not meet electrical code? I need to add motors to these existing switchracks, but the racks use a gutter type system where the incoming feeder is tapped to motor starters. I don't want to tie into existing taps as most of the existing installation violates the tap rule.

Thanks All

1. The transformer is not grounded inside the can, most likely. Connecting the winding to ground would be external to the can. This you would check with the manufacturer. If it's not grounded externally, it could have been originally intended to be a floating ungrounded system. This you would check with onsite management and engineering. If no one knows, get someone onsite who does.

2. The issue you can expect is a fatality. It is not correct to say the facility will be unmanned unless you plan to cease all operations and padlock the gate permanently.

What you may have intended to say is there will be no onsite qualified person who knows the layout and dangers of this installation.

Again it is not correct to say this. If you continue operations, your onsite maintenance (mechanics) will end up trying to reset breakers when they trip, wiring things up themselves, and changing fuses repeatedly. Unless you padlock your maintenance staff outside the gate, they will certainly end up touching things that the adequately qualified person would tell you he will not touch (because of the hazards and non compliance).

3. The line is drawn by your many third parties, who all require legal regulatory and code compliance as a condition of doing business with you. The two big ones will be your insurer and your lender. Certainly, your business runs normally by floating notes and borrowing money from the lenders. As soon as you tell your lender or insurer that something does not meet code and you don't want to fix it for any reason, you can expect to find yourself in default and without financing or insurance.

 

[Jraef]

A generation ago, ungrounded delta was actually quite common for large industrials. It was popular with the “Damn the torpedoes, full speed ahead!” crowd who did not want a little nuisance like a ground fault to shut down a process or the entire facility. That’s because in an ungrounded delta system, the FIRST ground fault simply takes you from an ungrounded to a “corner grounded” delta system, but no breakers trip, nothing shuts off. That’s very very useful in a steel mill for example where a ground fault trip on a circuit feeding arc furnaces would shut them down and the ore in the smelter turns into a solid mass.

But starting the the 70s, the NEC began requiring that delta distribution systems have EITHER intentional corner grounding, or a “ground monitoring” system of some sort. Back when I worked at a steel mill in the late 70s, our ground monitoring system was a set of three light bulbs in the Electric Shop connected to the 480V ungrounded delta lines and if one of the bulbs glowed brighter than the other two, THAT phase had a ground fault; somewhere. Many industrial insurance companies will no longer accept that kind of simple system, they will want something more sophisticated and expensive. So to save that cost, many people just elect to do corner grounding if they insist on having a delta system.

The thing is (and I hear the arguments coming), all delta distribution systems (especially over 240V) are essentially deadly to power electronics equipment such as VFDs, servo drivers (robotics), large power supplies, UPS systems, solar inverters, HID lighting, battery chargers etc. But none of these things existed back when delta power systems were popular, yet now they are plentiful. So if your “unmanned” facility has anything inside of it from the modern age, it’s a really bad plan.