I don't understand why. I have built several corner grounded and center grounded deltas for CP&L, now Progress energy.
Corner Grounded Delta
- Thread starter Misha80
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I don't understand why. I have built several corner grounded and center grounded deltas for CP&L, now Progress energy.
hardworkingstiff
Senior Member
- Location
- Wilmington, NC
- Location
- Massachusetts
Never seen one in my area either.
kwired
Electron manager
- Location
- NE Nebraska
If you have a single load that does not require a neutral it makes a lot of sense, or did at some time. You are not running a fourth conductor that serves no purpose except for equipment grounding.
POCO's in this area have done away with all their corner grounded systems AFAIK. They were primarily irrigation wells and farmers and wiring don't go together well, the last thing needed is to increase the voltage to ground (to 480 on those systems). I believe voltage to ground and number of non qualified people that have worked on them was their biggest motivator to switch them all to 3 phase 4 wire systems.
That is about the point we are at now. 480 center grounded banks are what we build now, as it cuts down on confusion. I know a place that has their GFCI disabled because it shows ground fault. All three phases come through the GFCI, and it is a corner grounded bank. I'm not sure, and may be straightened out in a few moments, but I think if the bank was changed to a center ground, the GFCI would work properly.
Are all your delta loads fed from a wye? When we feed a load requiring 480 with a PMT, the delta loads have to be fed from a wye XF, we don't buy delta PMT's.
I guess I've lived a sheltered life.
This was back about 17-22 years ago, mostly for commercial loads and mills.
- Occupation
- Licensed Electrician
Well now that all depends on how you are judging. I have seen some farmer wiring that most certainly made me say, "Wow"!
kwired
Electron manager
- Location
- NE Nebraska
The grounded conductor is probably grounded on load side of GF device whether intentional or not.
GF protection is only required on solidly grounded wye systems with a voltage of 150 or more to ground BTW See 230.95
kwired
Electron manager
- Location
- NE Nebraska
Around here there are farmers that are legal electricians and do electrical work when they are not farming. I was not counting them.
A lot of work from the others does make me say "wow" but not necessarily because I am impressed with how good the work is.
- Occupation
- Licensed Electrician
That is exactly what I meant. Resourceful might be another good description.
Even if it is a star (wye) connected transformer and the load required only three phase, you still wouldn't need to run a fourth conductor.
That's all I've come across - someone forgot to float the Neutral. 150-600V and 1,00A and our local codes require a qualified testing firm, manufacturers' instructions, and a report for the inspector. If there's a GF turned off, a testing firm's report should tell them what's heading to ground.
For the OP; you own the land directly above your building. Did you grant an easement for their equipment? Aside from SEC's feeding you, there shouldn't be a standing eminent domain/right of way. Sounds like an option only if reason fails. City Hall must be pretty lax in your area. Is this in Mexico???
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Actually you do, kind of.
The NEC requires us to run the X0 conductor from the utility to the service equipment. After the service entrance the '4wire' does not need to be run to any location that does not need it.
Thanks Jim.
Being a Brit, I am largely ignorant of the NEC code. But I'm learning. Your X0 would be the star point?
I was thinking in pragmatic terms. If you don't need the fourth wire, why bother? If you need a ground reference then doing that at the transformer would seem to be a simple solution and reduce the cost of conductors.
Obviously, if rules preclude it, rules preclude it.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Yes.
While other parts of the NEC allow the ground->neutral connection to be made in a few different places, it must occur at the 'utility service entrance' equipment. The This 'quirk' has been known to cause some confusion in regards to the requirements for Ground Fault on services.
However, if the X0 (star point) is allowed to float, and never ever be referenced to ground, then it does not need to be taken to the service equipment. This type of utility provided ungrounded service is extremely rare, maybe to the point of being non-existent.
- Location
- Illinois
- Occupation
- retired electrician
Yes, XO is the star point. If the utility grounds any conductor at their transformer, that conductor is required to be run to the service disconnect. It will be connected to the grounding electrode conductor(s) and to the equipment grounding conductors to provide a low impedance path for fault current.
kwired
Electron manager
- Location
- NE Nebraska
Yes. But we are not talking wye systems here.
With corner grounded delta you have X, X2, X3. One of them happens to be grounded. Run all three to service disconnect, install main bonding jumper on the grounded one and separate grounded and grounding from that point on just like any other grounded system.
Here, for LV residential services (400/230V) it is usual for the earth (grounding) to be done at the local distribution transformer. The is usually an 11kV delta to 400/230V star unit feeding a number of residences.
A slight aside....
The ROC made a visit our local transformer. I happened to he home at the time so I asked them what they were doing. They were replacing the neutral to ground connection with aluminium.
Thieves (pykies) had stolen the copper. A dreadful but not unique occurrence. Another similar crime is the theft of drain covers and manhole covers. They are cast iron and they are being stolen for their scrap metal value. Perhaps a sign of the economic times. Still doesn't excuse it in my book.
Floating is not good.
As you probably know, we do power electronics.
For AC to DC conversion on some larger systems we sometimes use 24-pulse rectification.
Two supply transformers, each with two secondaries. One star, the other delta. That gives a 30deg shift between the secondaries. The 15 deg shift between the two transformers is done on the primary. The primaries are delta with a bit sticking out from each corner of the delta. The bit sticking out is in different directions for the two transformers thus giving all the secondaries required for the 24-p rectification.
As I noted, floating is not good. But how to ground a system with multiple secondaries, all interconnected, isn't quite straightforward.
Our approach is to put a balanced voltage divider across the DC and tie the centre point to earth (ground) and then sense the voltage balance across each half.
We sell a version of this to some companies making traction rectifiers.
Yes, I know that. I addressing this point you made:
In practical terms, that load could be also be supplied with just three conductors from a star connected transformer with the neutral connected to ground at the transformer.
I've seen it here in UK on industrial applications.
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