13.8kV Generator/System Grounding

[Alfminator]

Being a European engineer I am not familiar with American practices, but the following has really surprised me:
A client's 13.8kV system is being designed with three generators in parallel supplying a board with some outgoing feeder. The 13.8 kV generators are high resistance grounded limiting earth fault currents to 5A. And the 13.8kV board is medium resistance grounded through a zigzag xformer limiting earth faults to 400A.
I can't se how this system would work since I now have 2 paths to earth and I would have thought that any earth fault will go through the low impedance route?

Any help would be much appreciated

 

[Aquarius]

The zig-zag is for grounding purposes and to allow earth fault detection.
The generators are high resistance grounded to limit third harmonic currents which are same as zero sequence current.
Even if the generators are identical there is possibility of 3rd harmonic circulating current being present due to unequal loading.

The system will work, the e/fault current will in reality flow only via zig-zag since the other path is of a very high resitance. If you draw a zero sequenc network it is a parallel, two leg system and the e/fault current will be distributed as a ratio of grounding impedances.

 

[Kiribanda]

Hi Alfminator,

While fully agreeing with you, it is surprising me too as there are two paths for the zero sequence current to flow during an earth fault. I believe that it is nothing to do with US practice but merely a wrong application. As a result it seems to me that the main intention of having a high resistance grounding for the 13.8 kV generator(s) is totally lost. Without any doubt the system will work. But the objectives!

If you really dig into the problem more and able to investigate the history of the installation, you MAY BE able to reveal that on a latter stage somebody must have recommended to connect a zig-zag and a NGR with a let-through current of 400 A, due to the reason of detecting & clearing difficulties of high resistance ground faults by the maintenance crew of the installation on the 13.8 kV level. Although the HR resistance grounding is a good method on the paper, from my experience (and from other’s too) it leads to transient voltage problems above 5 kV systems which sometimes difficult to troubleshoot.

So if you get any clue later, the forum really appreciate to know them.

Regard!

Kiribanda

 

[dpc]

I have run into this kind of hodge-podge before. It typically occurs when a generator is added to a system without consideration of the existing system grounding.

The existing low-resistance grounding will defeat the benefits of high-resistance grounding of the generators, since a ground fault in the stator will now see 400A instead of the expected 5 amps.

If the 13.8 kV system is always grounded via the grounding transformer, the generators do not really need to be separately grounded, in my opinion. The high-res ground on each generator will provide a ground reference for the brief period of time while they are being synchronized, but once tied to the bus, the generator grounding systems are largely irrelevant.

As others have said, the system as installed will function and will be safe, but will not accomplish what was most likely intended.

 

[Modula2]

So, for a ground fault, what currents will flow through each part of the system? The generators will have ground fault current related to 5A, which is good for them and the system will have ground fault current related to 400 A. Why is that bad or good on this 13.8 KV system?

 

[busbar]

An applicable reference for ANSI practices may be IEEE C62.92.2-1989 …Neutral Grounding in Electrical Utility Systems Part II—Grounding of Synchronous Generators

 

[gordonl]

Modula2,

I don't think there is an issue of damage on the 13.8kV system, the problem, or perceived problem, is that because the system is low resistance grounded the damage to the machines will be increased due to the 400A ground fault source should there be a stator fault. If the 13.8kV system is larger than just the two machines I'don't think there is much choice.

I have a similar system in service, and would appreciate some thought or suggestions. Two generators limited to 1000A of ground fault current, and Y point of a transformer also limited to 1000A, for a total of 3000A of ground fault current. The system could be running with only one machine and no utility connection (no wye point grounding), but typically with all three. The 11kV system consists of three busses and about 15 feeders which use co-ordinated ground protection, geographically covering about 2-3 km's of industrial load. I've been asked to begin looking at rewinding the machines (uprate), replace the switchgear, and new excitation, so there will be oppurtunity to change the system if there is signifigant justification.

Thank You,
Gord

 

[alehman]

The zig-zag transformer eliminates the problem of having a common neutral, which during a fault would elevate the potential of the windings of any off-line generators - possibly hazardous to maintenance personnel. The high-resistance grounds at the generators may help to reduce transient votlages in their windings when running off-line for maintenance, etc.

 

[dpc]

Generator stator damage from 400A will be much greater than 5A. This is why the insurance industry is asking IEEE to re-examine grounding standards for generators in industrial facilities. They are tired of paying for costly restacking of stator cores for ground faults.

IEEE is working on an update of recommended grounding practices for industrial generator grounding that will address the problems associated with low-resistance grounding.

 

[Aquarius]

The system is correctly grounded. Just draw a zero sequence diagram and you will see the flow of earth fault = 3xzero sequence current in the system.
There are of course other schemes but they would be more expensive and complicated in operation.

Solid grounding is used for low and transmission high voltage transmission systems. Medium (2.4 to 15 kV) systems seldom uses a solid grounding because the possible high earth fault currents would damage the rotating machinery operating at these voltages.
A 2.4 to 15 kV ungrounded system would be a subject of a severe overvoltages. There are however some disadventages i.e. if ground fault is not removed immediately second ground fault causes disconnection of more production equipment.

dpc - stator ground faults may be due to a partial discharge (deterioration of insulation) and if this is a case a simple on line PD couplers would detect problem by trending the PD activities.

 

[Modula2]

Alfminator, what is the size of the generators?