Sizing of Neutral Grounding Resistors

[ksps100]

Dear ALL,
We have installed Neutral Grounding Resistors on the Neutral of a 34.5kV/480V Delta / Star Transformers.
Its rated as 400A,10Secs.
Kindly advise what is the sizing criteria for NGR's.
What are the applicable Standards and codes for sizing NGR's.
Can I get some reference materials for the above.
Thanx

 

[DickDV]

There are others that are doubtless more knowledgeable about the code requirements regarding this but, briefly, there are two approaches to neutral grounding. The first is low resistance grounding and the intent is to simply limit ground fault current to a level that will release the current limiting devices while controlling the damaging effects of the fault to acceptable levels. This type of neutral grounding is desireable and poses no safety or reliability issues.

The second type is high resistance grounding and involves adding so much resistance in the ground connection that the network can continue to operate even with one phase tied tight to ground (sometimes called corner grounding). This essentially converts a wye source network into a floating network similar to a delta source as far as grounding protection and detection is concerned. This grounding system is being promoted as a way of continuing operations with one ground fault in the network until a convenient time when the fault can be cleared. There are a number of grievous problems with this arrangement, some of which involve hazards that, in my view, are not being honestly addressed. First, corner grounding of a 460V network forces two phases to operate 460V above ground resulting in 460V stress of equipment grounding insulation rather than the normal 277V stress. Equipment in perfect condition can tolerate this but anything that is worn, moist, or marginal in other ways will either leak current to ground or flash over with damaging effects. Second, equipment such as DC drives and AC drives require a ground reference to properly operate their ground-fault detection features. When powered by an unbalanced-to-ground network or a network with large ammounts of ground leakage, these drives will either not work properly or be damaged by the leakage currents. Third, whereever solid state components are involved, normal isolation techniques do not guarantee safety since these devices leak in to open condition. This causes serious risks particularly for maintenance people. For example, some years ago, I was inspecting an old analog DC drive and, with all disconnect switches open and all meters reading zero, I climbed up on the machine with my left hand and reached into a box for some drawings with my right hand and got 700VDC arm-to-arm. I am lucky to be here to talk about it. (The delta network was operating corner grounded with one phase in a pool of water, it turns out)

Bottom line, low resistance grounding is a good idea. High resistance grounding should be outlawed. When asked to commission drives on floating networks or high resistance grounded networks, I require a drive isolation transformer with a grounded wye secondary or they can find someone else to start it up. I won't be party to these levels of hazard.

 

[davidbeach]

Sorry to disagree, DickDV, but a properly installed and maintained high resistance grounded system can be quite safe. A high resistance grounded wye is very much different from a corner grounded delta. I will agree that corner grounded delta systems can be bad news.

A high resistance grounded wye is the system that will allow critical process loads to continue to function while a single ground fault is found and taken care of. Allowing a ground fault to persist is sheer stupidity as a second ground fault will be a line-to-line fault and has the potential for severe damage.

Low resistance grounding certainly limits the current in a ground fault, but possibly making it more difficult to clear the fault in a timely manner, but not limiting the fault current to a value low enough to not need tripping to avoid damage.

To me, a (new) system should either be solidly grounded or high resistance grounded. Ungrounded is truly bad, and low resistance grounded combines the disadvantages of both solidly grounded and high resistance grounded without gaining the full benefits of either. Extensions of existing systems should follow the existing precedent and not change it unless the entire system, and all of the protection, can be changed at the same time.

 

[dpc]

I generally agree with David Beach, but would add that a low-resistance grounded system does allow for selective coordination and clearing of ground faults. A high-resistance grounded system generally does not. The latest IEEE recommendations for generator grounding at industrial facilities recommends a combination of the two, I believe.

With modern relays, the low-resistance grounding system can be limited to 50A, perhaps even less. You still have to trip, but equipment damage is much less.

For a large, extended medium-voltage system, high-resistance grounding would not be my first recommendation. But for 480V systems, I do like the high-res grounding. It also greatly lowers the arc-flash hazard (although you don't get any credit for this when it comes to the PPE required).

 

[DickDV]

to david beach and dpc, I raised several specific concerns in my post above with regard to high resistance grounded and floating delta networks. While you claim that high-resistance grounding is desireable, you offer no explanation as to how it is safer or different than a floating delta system.

I would like it and probably learn something too if you would explain specifically how the hazards are reduced with regard to those I raised in my post.

Additionally, the ability to generate an alarm when a phase ground occurs is essentially the same in both systems. That's nice when you have a responsible owner but in the world I live in, that "more convenient" time to go find and remove the phase ground never comes. The result is a system running corner grounded for months with all of the hazards and operational problems listed above.

That is not the case when you have a low-resistance grounded system since the clearing of the current-limiting device presents enough nuisance value to get the ground fixed right away.

 

[dpc]

Dick,

The big improvement of high resistance grounding over ungrounded delta is the elimination of transient overvoltages during arcing faults. In an ungrounded (really capacitance-grounded) system, repetitive restrikes of an arcing fault can have a voltage multiplying effect due to the charge stored in the system capacitance, resulting in voltages that are 4 to 6 times nominal, or even higher. With a resistance grounded system, this can't happen because resistance will prevent this charge buildup.

High-resistance grounding is much safer than ungrounded systems and has a really good track record.

What it does share in common with the ungrounded system is the need to agressively seek out and repair ground faults on the system to avoid really nasty line-line faults.

For a commercial facility without an electrician, I would always go with solid grounding.

 

[davidbeach]

dpc, well stated.

 

[DickDV]

I see your point, dpc, and thanks. I had never considered stored charge and its effect on transient voltages.

But your point about getting after phase grounds is central to the safety of network and needs to be enforced.

I think we are seeing this the same way.