Wye-Wye

[LarryFine]

The first ones I saw were three sets of two 220 volt lamps connected in series and then each set connected in a wye with the center grounded.

 

[jim dungar]

Sure it's easy, but perhaps not the best solution in some cases.

Why not?

 

[LarryFine]

Why not?

Yeah, wye not?

 

[electrofelon]

Why ask wye? Drink bud dry.

 

[electrofelon]

Why not?

The general consensus on this forum, and from the industry in general is always to avoid ungrounded systems if possible. I know you know what the issues are, but to recap for others reading: the vast majority of people are just not familiar with these systems, will not know what the fault indicators are for or what they mean, will probably not recognize or rectify a fault. For a facility with dedicated qualified personnel I don't see an issue, but that is often not the case. I would not install an ungrounded system in 99% of the places I work in for that reason.

 

[winnie]

The inverters in question can run on an ungrounded delta but not on a corner grounded service.

OK, but we are telling you that NEC 250.21(B) will require ground detectors for your ungrounded 3-wire 480V delta system, so you'll need to figure that out.

Cheers, Wayne

Sure it's easy, but perhaps not the best solution in some cases.

If the inverters can run on an ungrounded system but not on a corner grounded system, then the first question to ask is 'what happens if a fault occurs and the system becomes inadvertently corner grounded?'

If the answer is 'the inverters get confused by the L-G voltages and shut down', then a simple set of ground fault detection lights is IMHO sufficient. A fault occurs, the inverters shut down, and the lights tell the story to the tech that goes out to repair things.

If the answer is 'the magic smoke starts pouring out of the inverters', then something that detects and shuts down the system in the event of a ground fault is more in order.

Another approach is to add a grounding transformer to derive a neutral and create a proper 480V neutral grounded system. This grounding transformer can be much smaller than the delta supplying the service, because it only has to carry small 'charging' current and transient fault currents.

-Jonathan

 

[electrofelon]

If the inverters can run on an ungrounded system but not on a corner grounded system, then the first question to ask is 'what happens if a fault occurs and the system becomes inadvertently corner grounded?'

If the answer is 'the inverters get confused by the L-G voltages and shut down', then a simple set of ground fault detection lights is IMHO sufficient. A fault occurs, the inverters shut down, and the lights tell the story to the tech that goes out to repair things.

If the answer is 'the magic smoke starts pouring out of the inverters', then something that detects and shuts down the system in the event of a ground fault is more in order.

Another approach is to add a grounding transformer to derive a neutral and create a proper 480V neutral grounded system. This grounding transformer can be much smaller than the delta supplying the service, because it only has to carry small 'charging' current and transient fault currents.

-Jonathan

My thinking is for an ungrounded system, the inverter can't really make any decisions based on the phase to ground voltage. It could certainly have that capability programmed in, where it could test the stability of the phase to ground voltage using some low impedance measurements from different phases, but seems to me that is not an inverter's job. Is my reasoning sound?

 

[winnie]

My thinking is for an ungrounded system, the inverter can't really make any decisions based on the phase to ground voltage. It could certainly have that capability programmed in, where it could test the stability of the phase to ground voltage using some low impedance measurements from different phases, but seems to me that is not an inverter's job. Is my reasoning sound?

I would think that if the inverter can run on a truly ungrounded system it should be ignoring L-G voltage (within limits). But perhaps the capability of using an ungrounded secondary actually depends on some internal ground referencing, and the assumption of roughly similar L-G voltages. It is plausible to me that a solidly corner grounded system would be interpreted as a fault but a floating system would function.

The 'ground detector lamps' actually create a high impedance wye ground, which may be sufficient for the inverter to be happy.

 

[ggunn]

Might be better to just corner ground it? If an inverter can run on an ungrounded system, does that mean it can run on a grounded system? My first thought is yes but I would confirm.

Corner grounded delta systems are not supported by the inverter company.

 

[electrofelon]

I would think that if the inverter can run on a truly ungrounded system it should be ignoring L-G voltage (within limits). But perhaps the capability of using an ungrounded secondary actually depends on some internal ground referencing, and the assumption of roughly similar L-G voltages. It is plausible to me that a solidly corner grounded system would be interpreted as a fault but a floating system would function.

The 'ground detector lamps' actually create a high impedance wye ground, which may be sufficient for the inverter to be happy.

I guess the within limits part is what I am not sure of. Although the only un- grounded system I have worked on did indeed float right about at "center", I don't think that can be relied on, so where do you draw the line?

 

[jim dungar]

The general consensus on this forum, and from the industry in general is always to avoid ungrounded systems if possible. I know you know what the issues are, but to recap for others reading: the vast majority of people are just not familiar with these systems, will not know what the fault indicators are for or what they mean, will probably not recognize or rectify a fault. For a facility with dedicated qualified personnel I don't see an issue, but that is often not the case. I would not install an ungrounded system in 99% of the places I work in for that reason.

So you are really expressing disagreement on the entire ungrounded system, rather than just the GF section scheme I mentioned.

 

[wwhitney]

If the inverters can run on an ungrounded system but not on a corner grounded system, then the first question to ask is 'what happens if a fault occurs and the system becomes inadvertently corner grounded?'

My guess is the inverter sees the fault and shuts down. I.e. it is going to be monitoring the voltage L-G, and if it sees 0V it considers that a fault.

In other words, corner grounded systems are not supported because you can't program it to accept 0V L-G on a particular designated line conductor.

Just a guess.

Cheers, Wayne

 

[electrofelon]

So you are really expressing disagreement on the entire ungrounded system, rather than just the GF section scheme I mentioned.

Probably. I would try to avoid installing an ungrounded system in places where unqualified people will be. I may make an exception depending on what type of location it is and what the increased cost and hassle of going grounded was. I am guessing I would be in the majority y here with that opinion.

 

[jim dungar]

Probably. I would try to avoid installing an ungrounded system in places where unqualified people will be. I may make an exception depending on what type of location it is and what the increased cost and hassle of going grounded was. I am guessing I would be in the majority y here with that opinion.

I had no problem with ungrounded installations which fed a single piece of equipment/machine. I would not design ungrounded systems feeding multiple items except under the NEC definition of 'supervised industrial' installations.

 

[electrofelon]

I had no problem with ungrounded installations which fed a single piece of equipment/machine. I would not design ungrounded systems feeding multiple items except under the NEC definition of 'supervised industrial' installations.

And that is certainly a factor. We were talking PV systems here, so it kind of depends on how many inverters, what sort of combining equipment is needed, how far the system needs to run.

 

[electrofelon]

First, you are not a utility system do not try to act like one.

My parents always told me a could be anything I wanted to be, you are shattering my dreams.

 

[ggunn]

Probably. I would try to avoid installing an ungrounded system in places where unqualified people will be. I may make an exception depending on what type of location it is and what the increased cost and hassle of going grounded was. I am guessing I would be in the majority y here with that opinion.

Believe me, I would never have designed the system this way. My client had never before connected 480V inverters to 208V services, so they asked me for a recommendation for this system. I specified a xfmr with sufficient kVA, a 208V delta primary, and a 480V/277V wye secondary, which I have used many times and which is shown as such on the electrical drawings. What they bought and installed is big enough, but it has a 208/120V wye primary and a 480V delta secondary.

 

[electrofelon]

Believe me, I would never have designed the system this way. My client had never before connected 480V inverters to 208V services, so they asked me for a recommendation for this system. I specified a xfmr with sufficient kVA, a 208V delta primary, and a 480V/277V wye secondary, which I have used many times and which is shown as such on the electrical drawings. What they bought and installed is big enough, but it has a 208/120V wye primary and a 480V delta secondary.

Its crazy how often this happens with people getting the wrong transformer. I guess in this case I wouldn't have an issue with the ungrounded, just put it on them that because they got the wrong transformer they need ground detectors and will have to be responsible for monitoring them and and understanding what the lights mean.

 

[ggunn]

So is the consensus that if I float the neutral on the 208/120V primary and install ground detectors on the 480V delta secondary it will be safe and NEC compliant?

 

[electrofelon]

So is the consensus that if I float the neutral on the 208/120V primary and install ground detectors on the 480V delta secondary it will be safe and NEC compliant?

Seems ok to me. I believe you still need a grounding electrode system for the SDS, there will just not be a SBJ.