Grounding an ungrounded delta
Grounding an ungrounded delta
(OP)
We recently had a costly incident in our mill that resulted in the destruction of 6 VFDs. These drives are in a section of the mill that is supplied by a transformer with a ungrounded 480 V delta secondary. It appears that the damage resulted from over voltages created by an intermittent arcing fault in an underground feeder.
In order to prevent future damage from similar incidents, I believe that we need to get the system grounded. My first inclination is to simply corner ground the transformer and monitor for ground current so that we know if the B phase gets grounded elsewhere. However, I am also considering using a Zig-zag transformer to create a neutral point.
Outside of the issue of resistance grounding, are there any advantages of the Zig-zag transformer over a corner grounded system? Are there any unforeseen problems that could result from corner grounding?
In order to prevent future damage from similar incidents, I believe that we need to get the system grounded. My first inclination is to simply corner ground the transformer and monitor for ground current so that we know if the B phase gets grounded elsewhere. However, I am also considering using a Zig-zag transformer to create a neutral point.
Outside of the issue of resistance grounding, are there any advantages of the Zig-zag transformer over a corner grounded system? Are there any unforeseen problems that could result from corner grounding?






RE: Grounding an ungrounded delta
As a minimum, you will want to check with the VFD manufacturers, since I believe most VFD front-ends expect a grounded neutral supply.
I don't have a lot of experience with corner-grounded delta systems - and neither will anyone else at the plant, I suspect. It will introduce a lot a new ways to make mistakes and bad assumptions by anyone not familiar with your system.
A grounded neutral system will be familiar to a journeyman electrician and will be much easier to maintain and troubleshoot.
As for grounding transformer, a standard delta-wye transformer can also be used as grounding transformer - a zig-zag unit is not required. If you go with neutral grounding, I'd recommend high-resistance grounding. But again, verify with VFD supplier that this will not be an issue with the drive.
David Castor
www.cvoes.com
RE: Grounding an ungrounded delta
Beside that there is no real advantage that I know of. But a balanced system with a neutral would be more familiar to operators and electrician, who are used to a grounded wye systems, from maintenance and testing points of view.
Corner grounding is perhaps cost effective but requires a little more care in maintenance and trouble shooting. It is also becoming very uncommon.
Rafiq Bulsara
http://www.srengineersct.com
RE: Grounding an ungrounded delta
RE: Grounding an ungrounded delta
I would prefer to go with a grounded neutral setup but I thought that corner grounding would be a quick easy and inexpensive way to solve the problem. Also, none of the electrical suppliers that we use had ever heard of a zig-zag transformer, so I was not sure where to get one.
Dpc, thanks for pointing out that a standard delta-wye transformer would work in place of a zig-zag transformer. I assume that with high resistance grounding that you would limit the ground fault current to a few amps and therefore you would not need a very large transformer. Is this correct?
RE: Grounding an ungrounded delta
David Castor
www.cvoes.com
RE: Grounding an ungrounded delta
But seriously, you'll have nightmares with corner grounded delta, it causes wierd ground currents to flow in unexpected ways. If we renovate anything with corner grounded delta, the power company will not connect to it again until we fix it. sometimes we've had to put in huge 480Y:480Delta transformers before the power company would even touch us.
Establish a neutral that's your best bet.
SparksRfun
"The truth will set you free, but first it will make you madder than a wet bobcat"
RE: Grounding an ungrounded delta
RE: Grounding an ungrounded delta
With neutral grounding(grounding transformer or Y_D transformer) you can use symmetrical components for calculation
and you will have problem with corner grounding.
RE: Grounding an ungrounded delta
Poor man's solution.
I prefer davidbeach's suggestion. Wye delta transformer.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Grounding an ungrounded delta
Understand that using a Y-Δ transformer, if the wyepoint is grounded through a resistor, and the Δ left floating, then you have a high-resistance-grounded system. A good starting point for offsetting charging current is to size the resistor at 1 amp/MVA of transformer capacity.
RE: Grounding an ungrounded delta
While I do like the idea of the wye-delta transformer suggested by davidbeach and others, I have also looked into the grounding transformer and resistor package units from several manufacturers. They all are pushing their own version of pulser units. These units are supposed to aid in locating ground faults by creating a higher current pulse that can be seen on an amp meter. The basic idea behind them seems simple and I have no doubt that they work as advertised, however, I am not sure that the procedure used to locate the fault it practical.
Have any of you had any experience with these systems? Are they worth the added expense?
RE: Grounding an ungrounded delta
But I wouldn't consider a high-resistance grounded 480 V system without some type of fault location device.
David Castor
www.cvoes.com
RE: Grounding an ungrounded delta
A good IEEE paper on the practical aspects of high-resistance grounding is http://neiengineering.com/pdfs/paper1JN.pdf