Corner Grounded Delta Madness

[mark32]

Recently I've read a number of posts here talking about corner grounded delta sytems, which intrigues me for I have never seen such a configuration before. My thoughts on this arrangement, at first, were that one phase would be grounded and the other two ungrounded conductors would be used to power single phase machinery/equipment/etc. However, after viewing a .gif online (I tried uploading it but an error message said it wasn't the correct dimensions and it looked horrible once I brought it into said specs. So I am forced to describe the picture) I see that I am wrong. In this pic there is a transformer showing two secondary ungrounded phases going into a disconnect with the third phase going to ground as well as to the disconnect. Inside the disconnect the grounded phase goes to a ground bar and then from the bar another conductor runs along to a motor which is then bonded to the motor frame, which I assumed it would. However, from the ground bar another conductor is run to the third terminal on the disconnect itself, which then goes on to feed the motor!! I can see how many would freak when faced with such a sight out in the field, I know I would. In this diagram the ungrounded phases have fuses on them (In the disconnect) but no fuse on the "Grounded" conductor? (Looks like a piece of wire connects the line and load terminals which is fashioned in such a way that all conductors will make and break simultaneously) It's pure madness!

 

[bphgravity]

It is a little wacky, but it works, and it is safe, if you are properly trained.

 

[rattus]

Is this any different in principle from any other grounded conductor? I have seen old houses with a fuse in the grounded conductor.

There is no need for a fuse in the grounded conductor. In fact it is undesirable.

 

[ptonsparky]

We see them often enough in this area. Usually a 240V grounded delta but on rare occasion 480v. Unfortunately they are usually rural and feeding equipment, wells, that have seen little maintenance in 30 years. Can get kind of wary when you see the broom stick handle next to the pump panel to start things up with.

 

[hardworkingstiff]

So the voltage readings from ungrouded phase to ungrounded phase would be the same as the voltage reading from either ungrounded phase to ground (aka, grounded phase).

Wouldn't that just make you think if you were expecting a single phase center tap voltage?

 

[mark32]

Thanks for the replies. Rattus, if we go back to that discussion about running a 208/230 compressor on a high leg of 208v to ground most said it was a bad idea to use the high leg for a couple of reasons, but aside from that the motor would still operate on one hot and one grounded conductor. However, LarryFine wrote "This motor is rated to be run from line-to-line voltage" I agreed with this thinking. The same logic is what was throwing me off in the case of this three phase motor running on two ungrounded conductors and one grounded, figuring all three lines needed to be ungrounded, but I guess my logic was flawed.

 

[rattus]

So the voltage readings from ungrouded phase to ungrounded phase would be the same as the voltage reading from either ungrounded phase to ground (aka, grounded phase).

Wouldn't that just make you think if you were expecting a single phase center tap voltage?

Stiff, not so. With a CT, the voltages would be 240V L1-L2, 120V L1-G, and 120V L2-G.

Consider an ungrounded delta, the line to ground voltages will float. Connect a Lo-Z meter between any line and ground and that floating voltage will be pulled to ground.

Grounding one corner has no effect on the L-L voltages. You could do the same thing with an ungrounded wye, but I see no reason for it.

 

[infinity]

The same logic is what was throwing me off in the case of this three phase motor running on two ungrounded conductors and one grounded, figuring all three lines needed to be ungrounded, but I guess my logic was flawed.

What difference would there be if the system were 3 phase ungrounded or 3 phase with one phase grounded? Either way the supply to the 3 phase equipment would still be three phase.

 

[e57]

What difference would there be if the system were 3 phase ungrounded or 3 phase with one phase grounded? Either way the supply to the 3 phase equipment would still be three phase.

OCP - on an ungrounded system if one phase gets accidently grounded it becomes a fire and shock hazard, unless there is really good ground fault protection, and that did not really exist way back. If you had two different systems in a building it could be much worse. In a grounded system you have something to short to, and will be able to trip a thermal magnetic breaker. Some of the older guys will tell you about fault lights that will light if there was a short... If you weren't there to see the lights, it would run just fine, but will have current from the short back to the transformer on the ground. If you had two different systems in a building it could be much worse.

Also, I think... Had a problem once, and cant remember for the life of me now - some motors wont run on a corner grounded delta. If the motor is wye connected internaly I thinK it doesnt work?

 

[rattus]

Also, I think... Had a problem once, and cant remember for the life of me now - some motors wont run on a corner grounded delta. If the motor is wye connected internaly I thinK it doesnt work?

True, if the wye neutral is grounded also. Don't do it!

 

[iwire]

OCP - on an ungrounded system if one phase gets accidently grounded it becomes a fire and shock hazard,

How so?

It becomes a corner grounded delta and will be perfectly fine until a second fault happens.

 

[infinity]

How so?

It becomes a corner grounded delta and will be perfectly fine until a second fault happens.

As Bob mentioned this is the reason you would use a ungrounded system in the first place. An accidental ground on one phase would allow the system to operate normally as a corner grounded system. This may keep a facility up and running until the problem can be fixed.

 

[hardworkingstiff]

Consider an ungrounded delta, the line to ground voltages will float. Connect a Lo-Z meter between any line and ground and that floating voltage will be pulled to ground.

Hi Rattus,

Your comment is confusing, I thought we were talking about a corner grounded delta, in which the line to ground voltage should be the same as the line to line (and stable), no?

 

[winnie]

The same logic is what was throwing me off in the case of this three phase motor running on two ungrounded conductors and one grounded, figuring all three lines needed to be ungrounded, but I guess my logic was flawed.

Your logic is not flawed; and in fact there _is_ a difference between a 'center grounded' and a 'corner grounded' system that _may_ cause problems.

While it is quite true that as far as the _load_ is concerned, there is no difference between grounding at any point on the transformer, or even having a totally ungrounded electrical system, there is more than the load to consider. Most importantly, the voltage to ground affects the insulation system. There may also be differences seen in capacitive coupling to ground, or with any protective devices connected between line and ground (eg. breakdown devices used for surge suppression).

A corner grounded delta system will put more stress on the insulation between windings and ground. In addition, if you have any sort of rectification stage, for example in a VSD, then the DC rail voltages relative to ground will be very different. While the rail-rail DC component of the rectifier output is the same in both wye and corner-grounded delta systems (that is the load), the rail-ground voltage shows an AC component. In a wye connected system, this rail-ground AC component is at 360 Hz and is a fraction of the rail-rail voltage; in a corner grounded system the rail-ground voltage goes as low as 0V or as high as the full DC rail-rail voltage. Again, this could show up as insulation system stress or as increased capacitive current between the DC bus and ground.

But for common low voltage (<600V) three phase motors connected directly to the line, there is probably no significant decrease in reliability with a corner grounded delta system. The insulation in these motors needs to tolerate the full line-line voltage anyway, and the slot lining insulation relative to ground is probably perfectly happy with the increased stress in a corner grounded system.

I don't know if such motors are regularly tested or _listed_ for operation on corner grounded supplies, however. They are simply commonly used in such a fashion.

-Jon

 

[rattus]

Hi Rattus,

Your comment is confusing, I thought we were talking about a corner grounded delta, in which the line to ground voltage should be the same as the line to line (and stable), no?

True Lou, I am saying that the line to ground voltages in an ungrounded delta are iffy even if measured with a Hi-Z meter. Use a Lo-Z meter and you have in effect a corner grounded delta.

The phase voltages are the same in either case. When you ground a corner you are simply establishing a ground reference for two of the phases.

Right?

 

[hardworkingstiff]

Right?

Right!

 

[e57]

How so?

It becomes a corner grounded delta and will be perfectly fine until a second fault happens.

What I meant by that was... If it were unintentional it might not exactly end up being grounded were you want it to be grounded. (Just like any other unintentional neutral ground connection.) If you have another grounded system in the building, and you just might if there is lighting or other types of 120 loads in the building, you're gonna have some other interesting effects if say the grounds of both systems connect elsewhere. - like on a trapeeze or metal studs in a wall..

 

[don_resqcapt19]

e57,

What I meant by that was... If it were unintentional it might not exactly end up being grounded were you want it to be grounded. (Just like any other unintentional neutral ground connection.) If you have another grounded system in the building, and you just might if there is lighting or other types of 120 loads in the building, you're gonna have some other interesting effects if say the grounds of both systems connect elsewhere. - like on a trapeeze or metal studs in a wall..

It doesn't matter where the connection to ground is made...it only matters that there is a single connection to ground for that system. It also doesn't matter if there are other grounded systems in the building. There is no current path between the secondarys of different transformers.
Don

 

[brian john]

Something some electricians do not understand is EARTH/GROUND, does not know, does not care that somewhere someone is bonding anything to building steel, a water pipe, driven electrodes, a Ufer.....on and on.

Take a simple control transformer 480/120, HI-H2 are the primary at 480 VAC,
major dilemma, on the secondary do I ground X1 or X2. It does not matter as far as our selected ground cares. Ground X1 and voltage from X2 to ground is 120 VAC. Ground X2 and voltage from X1 to ground is 120 VAC. No problem.
Do not ground either X1 and X2 and voltage at no load is typically low "0" VAC, with a load and some wiring voltage may be 60 VAC from X1 and X2 to ground.

Take it a step further a typical transformer, 480 Delta to 208/120 Wye make sure XO is not grounded and ground any phase X1, X2 or X3, now read voltages. with X1 grounded from X2, X3 you have 208VAC to ground, from X0 you would have 120 VAC to ground. Not code compliant and not are a recommended practice, but it will work.

AND GROUND/EARTH or some body serving in place of said Earth does not care.

 

[mark32]

Thanks again for all the insight, especially winnie, rattus and brian. Please allow me to summarize my thoughts on this corner grounded topic. First, if one phase is intentionally grounded the load will still see this as a three phase supply. Secondly, if one were to touch an enclosure that was bonded with said grounded conductor, the chance of getting shocked is slim because the equipment's potential is very low because it is bonded to the frame of the transformer, ges, and connected to one of the transformer's windings and will open the ocpd when one of the ungrounded conductors shorts to ground. Thirdly, in an ungrounded system, one should not get shocked by touching properly grounded equipment that contains a single fault because it's potential is also kept low due to being bonded, via the equipment ground, to the frame of the transformer and grounding electrode system. However, because there is no connection to the source the ocpd will only open when a second fault occurs. Is what I have stated above correct?

I enjoy asking questions but in this case I feel rather dumb for I think I should know all of this stuff by now. I asked one of the more experienced electricians at work last night about corner grounded delta, and I described what I had seen in the diagram I mentioned in my first post. He had never seen one but voiced concerns about shock hazards from bonding the grounded phase conductor to the equipment. This guy has around 25 years in the trade, a number of those years as a foreman so I no longer feel too bad about asking this question. I've been in the trade around eight years now and haven't come across these particular systems yet either and because I was somewhat green on the commercial jobs I've worked they just let me run pipe and never touched transformers. Now I just want to familiarize myself with them, I love theory and read a lot of books but sometimes they just don't answer all my questions.