Grounding System for 9x 13.8kV generators (30MW total) 1

[mbk2k3]

Hey folks,

I usually deal with genset installations that have between 1 to 4 generators.

In the past, I've used NGRs in each generator's neutral limiting the ground fault current to 5A or 25A (each) perhaps.

With 9 generators, I'm not sure but my mind is telling me that having these many NGRs in parallel might cause some issues. But I can't quite put a finger on it.
Switching the generators in and out of service will cause a big change in ground fault current contributions no?

Should I be looking at a zig-zag transformal/artificial neutral?

Can someone point me to good guides/references/readings on this subject?

 

[xnuke]

C62.92.2-2017 - IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part II--Synchronous Generator Systems

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GroovyGuy]

Howdy mbk,
I have had a similar situation several times over the last few years, albeit on 4160V systems. In all of those cases I have established a 25A (to 50A) 30s NGR connected to the main bus through a zig-zag transformer.
On the last installation I was convinced to add a 2A (continuously rated) NGR to each generator. The idea behind this was to prevent synchronizing a grounded generator to the main bus. It worked fine, but I still wonder if it was worth the additional cost and complexity.
With nine (9) generators I would think thay you should be looking at a NGR via zig-zag connection to the main bus.
BTW, how large are these 9 gensets?
GG


"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[mbk2k3]

@groovyguy,

these are probably going to be 2.5MW each approximately at 13.8kV....

i'm also thinking along the same lines, a zig-zag to be 100% certain, and individual NGRs to limit GF current when the GCBs are open.

 

[GroovyGuy]

Howdy mbk,
Those are relatively small generators (2.5MW each) to be at 13.8kV. I would have thought that 4.16kV would be a more economical choice, but then I don't know of the other considerations.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[mbk2k3]

@groovyguy

existing distribution system is at 13.8kV that we are tying into. a 30-35MVA transformer to step up from 4.16kVA might negate the price of 4.16kV generators maybe? not sure.

 

[waross]

Rather than a zig-zag transformer I would consider three distribution transformers connected in wye-delta.
You won't have to have a transformer specially built and in the event of a failure a replacement should be readily available.
The prinaries must be rated for line to line voltage. The secondary voltage is unimportant.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[xnuke]

You haven't mentioned whether or not all nine generators are directly connected to the same bus, but I think most of us assumed that's what you meant, but advice will be different if each has their own unit transformer, or if three units share a GSU transformer, or another configuration is selected, etc. What configuration are you planning?

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[mbk2k3]

@xnuke, its like your first guess, all 9x 13.8kv generators are connected to the same 13.8kV bus, no individual transformers...

i will have a stepdown transformer for all the engine aux. loads....

 

[waross]

Consider a zig-zag or wye:delta transformers on the bus.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[xnuke]

I agree with waross, put your HRG on the bus. The generators will start up ungrounded, then become high-resistance grounded when they connect to the bus. Just be aware, if a line-to-ground fault happens between the generator and its breaker while the generator is operating at full voltage and ungrounded, bad things are likely to happen - like your generator being damaged due to the high line-to-ground fault current that is likely close to a generator with low zero sequence reactance. Hopefully, you're avoiding the likelihood of that kind of fault by using some type of phase bus conductor system instead of cables, or minimizing the amount of time a generator can run unsynchronized.

Most places I've seen don't typically try to connect that many generators to a common bus. They tend to group them in smaller banks with shared GSU transformers. With that many generators on one bus, the system capacitive charging current could vary widely depending on how many are on the bus. You might want to consider stepped resistances in your HRG system so that the resistance can be adjusted as generators are brought on and off the bus, especially if you don't intend to operate all of them simultaneously all the time. You might want to consider two or even three steps of resistance.




xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[redlinej]

"if a line-to-ground fault happens between the generator and its breaker while the generator is operating at full voltage and ungrounded, bad things are likely to happen - like your generator being damaged due to the high line-to-ground fault current that is likely close to a generator with low zero sequence reactance."-Xnuke I don't understand this statement, can you please explain to me why bad thing will happen.

My thought is, if the line become grounded the generator will continue operator with line being grounded through the frame and since the neutral is not connected off line nothing with happen.Only damage to personnel.

 

[xnuke]

You're right. I wrote something incorrect - I guess I'm too used to people wanting to solidly ground generators and trying to explain to them why they shouldn't unless the generator is built specifically to be able to handle the high ground fault current that can occur. The effects I described occur when solidly grounded generators get a line-to-ground fault, not ungrounded generators. Ungrounded generators with that type of fault see a neutral point shift, so all components connected to them must be rated for line-to-line voltage or risk damage. My mistake; sorry for the confusion.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[RRaghunath]

I have come across 5nos 6MW, 11kV rated machines with a single 100A NGR that can be switched to any machine.
This can be a good option for your case too I think.
100A rating will allow higher capacitance in the 13.8kV system.
If the protection scheme is designed to isolate the faulty outgoing feeder or generator without any delay, 100A fault current magnitude is not expected to cause core damage.

Rompicherla Raghunath

 

[Hoxton]

In the old diesel power stations I grew up with (50's 60's) there old be one neutral resistor and each generator neutral would be connected to a switch. The other ends of the switches would be connected together and then commoned to the resistor. Other end of the resistor connected to station earth.

Operation would be manual with no interlocks. Only one running generator would have its switch closed, others all open. Non operating machines would have their neutral switch closed.

When a stationary set was run up, it would be synchronized, then the neutral switch opened.

When the running set was to be shut down, another set would have its switch closed, just before the running set was stopped.

Later sets had interlocks, auto switches and control sequence panels.

The switches could have technically been isolator so as there should have been no neutral current to switch.