4160V Solid grounded system

[HamidEle]

I went through the code requirements in terms of the grounding requirements and haven't seen any limitation on Solid grounded system at any voltage levels.
We have a temeperary application with no load testing. There is not much point to ask for a high resistance grounding system for that. The problem is, we would see a 2kA short circuit at 4160V level which will require a huge size of Neutral conductor. Anybody can shed some light on me?
Thanks in advance.

 

[dpc]

There are a lot of solidly-grounded 4160 V systems in continuous duty. They are all bad ideas, but they exist nevertheless. 2000 A sounds very low for a solidly-grounded 4160 V system. Did you mean 20 kA?

The ground wire size required depends on how long you plan to let the fault last. 480 V systems have very high fault currents when solidly-grounded and the ground wires survive, although much smaller than the phase conductors.

 

[dpc]

Just to clarify - I was referring to industrial and commercial systems.

For utility distribution feeders, solid grounding may make sense, and there are still some 4160 V overhead systems around. It is not a good idea for industrial systems.

 

[rbulsara]

Hamid:

Think a little further..if the neutral size depends solely on the scc current, what would be the size of the phase conductors? They would also see the same current during a fault!



Rafiq Bulsara

http://www.srengineersct.com

 

[HamidEle]

It is fed from 2MW genset. 480V/4160V Step-up transformer. That's why the S-C is ~2KA. The problem is, we only prepared 3 conductor cables and we would have to use multiple cables to provide the additional Neutral if they are available.

 

[jghrist]

If you don't have any Ø-grd load, then you don't need a neutral. You need a ground conductor for fault currents, but it could be separate from the cable.

 

[rbulsara]

You seriously need to involve a senior engineer in whatever you are doing.

Rafiq Bulsara

http://www.srengineersct.com

 

[pashwuee]

If you want to solidly ground a step up transformer of 480/4160 V and fault level is 2 KA then 25X3 GI strip is sufficient for the neutral.

 

[Pikoplatt]

If your 4160V Transformer winding has its star point solidly grounded, then using a CT on the star-point connection to ground in conjunction with a 50G protection relay, you can trip the generator breaker on the 480V side of the transformer within 50 msec. This would allow you to select your star-point-to-ground cable size such that its I^2 X t value for short circuit damage is greater than the I^2 X t of the fault current flow = (2000A)^2 X .050 sec. You will find the I^2 X t damage curve for different size cables in the IEEE Red Book. If you need to coordinate the Generator breaker with 4160V feeder breaker ground fault relays on the transformer MV side, then you can select a 51G Relay instead of a 50G relay for the transformer star-point. You can set the relay pickups(50G or 51G) at 200A.

The neutral cable should be sized equal in size to the phase cables if you have a 3-phase, 4-wire system with single-phase 2400V loads. The neutral conductor will be protected from Line-to-neutral fault damage by a 51 Relay on each of the phases of the transformer 4160V winding. Again ensure that I^2t damage limits of the neutral and phase cables are greater than the I^2t of the fault current(2000A) and the relay trip time(t). The 51 phase relay pick-up can be set at 1.25 X Transformer secondary full load amps and wired to trip either Generator main breaker or Transformer secondary main breaker.

Since relay protection is a safety system, then I suggest you get a licensed P.E to look at your application and make the necessary recommendations, with the appropriate documentation, as Rafiq alluded to in an earlier post.

 

[pkelly54]

I agree with DPC a solidly grounded medium voltage system is a bad idea. According to the NEC, the ground wire needs to be sized so it is protected by the circuit protection available. If you have a three wire load and ground fault protection you can reduce the size of the ground wire if the cable protection is maintained. The I^2t of the all the cables must not be exceeded.

Note that a 12 ohm neutral resistor, on the neutral of the step-up transformer, will reduce the ground fault current to 200 amps or less. This assumes the 4160 V supply is a "Wye" configuration with a grounded neutral.

480V systems are solidly grounded because a slight resistance in the fault can reduce the ground fault current to values less than the feeder protection so the fault may not be cleared. Solidly grounded medium voltage system can give you a severe arc flash hazard. I recommned some resistance grounding.

 

[murray112]

Connection to star point by earth conductor must be greater or equal to phase conductor.
Resistive (impedance) earthing will limit fault current, BIG BUT,This will allow dangerous voltages to appear exactly where you do not want them during a fault.
You are in fact dicing with death.Balance the equation;
Cost of neutral conductor Versus Death damage and injury.
A no brainer I hope.
Keep away from resistance earthing and let the protection do its job as quickly as possibe.

 

[dpc]

murray112,

I'm afraid I have to disagree with virtually everything in your post.

Neutral to earth conductor does not have to be equal in size to phase conductor. It must be sized appropriately for the expected current it will see and the duration of the current.

Despite your dramatic phrases, resistance grounding is accepted and recommended practice for medium-voltage industrial systems.

 

[davidbeach]

What dpc said. murray112 obviously doesn't know what he is talking about.

 

[ppedUK]

murray112

Resistance earthing in an MV system provides the exact opposite of what you have said in your post. The idea is to (a) limit earth fault current to minimise equipment damage under fault and (b) limit the rise of earth potential at the point of the earth fault by dropping the majority of the phase-earth volts across the NER.

 

[murray112]

Ok, let me in order, reply,
Mr DPC, For a single phase to earth fault, the return must be equally capable of conducting as the phase conductor to maximise scc and clear the fault asap.
As faults are unpredictable it makes sense to prepare for worst scenario.
Mr david beach and mr DPC, Utility companies have decided, in certain circumstances, where the provision of a return path is economically unviable, to reduce the fault current by introducing an impedance. This is justifiable where security of supply takes more precedence over spurious short circuits due to contact with vegetation, etc.
I can only assume from the quoted voltage level that this is not distribution per se but an industrial location.
However,at a distribution level, I know that in one case, a lady who went to see why one of her cattle was not moving, also died.
The point being that limiting current by impedance causes voltage rises where they are most damaging.
Mr David Beach; I have worked in the electrical industry for a speck off thirty years. I have commissioned plant at 10, 15, 20,110,220 and 400kV.I have also in my time, been a protection engineer.
This was my first post, and my last.