Direct Ground & Xfrm Xo bonding

[haze10]

Need some help with Siemens RLE Low voltage switchgear trip programmer.

Feed to the gear is via a 12.4KV - 480/277 2000KVA Xfrm. Three phase conductors and 50% neutral are carried to the switchgear. The switchgear has a neutral bus to land the neutral conductors. This bus also has a 2000:5 CT around it with leads to the MCB trip unit. There are NO neutral feeder breakers, nor is the neutral bus exposed in the branch feeder cabinets. So, zero neutral current under normal operation.

The neutral bus is connected via a grounding jumper to the ground bus in the switchgear gear. This is what I believe is called: Direct Ground Fault Protections.

Direct Ground – In this scheme, the phase currents are not used in detecting and processing ground faults. The trip unit executes the ground fault protection function based on data from a ground current sensor. This sensor is located on the neutral connection to ground at the service entrance and is connect to the appropriate input terminals on the trip unit.

So this arrangement would only be looking for a ground fault (phase to ground) inside the switchgear. The branch feeders have only the three phase CT's and uses an unbalanced % current setting for ground fault. Any internal phase to ground would cause current from the phase to want to flow back to the xfrm via the neutral, and thus energize the neutral CT and trip the MCB open.

But I have a couple of questions maybe you can help with.

1) The switchgear has bare ground wires from the cement slap and building steel attached to it. The xfrm which is 200ft away, as the factory Xo bonding strap bonded to the transformer tank. There are no physical ground wires from the transformer to the switchgear. So under a switchgear ground fault condition, while the neutral conductors become the low resistance path back to the xfrm, the earth becomes the high resistance path as well. I would think removing the bonding strap from the Xo bushing at the xfrm would eliminate this. But not sure if I should do it. What do you think?

2) Under normal operations there should be zero current on the neutral CT as absolutely nothing is connected to the neutral for loads. The MCB has a long time setting of 3000 amps. So one might think 20 to 30% of this value would be acceptable if it was a conventional three phase CT setup. But in this case any amount of current on the neutral indicates leakage current to ground that should not be happening. So I'm tempted to set it really low, like 100 or 200 amps just to eliminate any noise on the CT from causing a false trip. Where would be a typical setting for this type of ground fault on a 3000A main.

any and all advise appreciated.

 

[dpc]

Location? If in the US, and if the transformer belongs to the utility, the "50% neutral" is probably a combined neutral/ground - this is normal in a service entrance. If this is your transformer, you probably need to add a ground wire.

The jumper from the neutral bus to the ground bus is the main bonding jumper and must be there regardless of the ground fault protection scheme (again, in the USA). If you don't care about coordination with downstream breakers, you can set the ground fault pickup on the main breaker to be quite sensitive. But normally, you will want to set it fairly high (and slow) to improve the odds of coordination with downstream breakers. Ideally, each of the feeder breakers will also have ground fault protection - these can be set to be faster than the main breaker ground fault.

 

[haze10]

Yes, this is in USA. The transformer belongs to us, not the utility. We take power off the pole as a primary metered customer to 15KV switchgear and then distribute to our building transformers that are 12.4KV-480/277. This xfrm is 2000KVA.
This setup is new to me. I'm thinking there would need to be some coordination with branch breakers. All the branch breakers have ground fault, which is a 3CT differential type, typically set at 40% of Long Time. So the branch breakers are only measuring phase current. Only the MCB has the neutral CT. I imagine that if a branch breaker feeder has a ground fault, it would flow through earth ground, and find the low resistant path back through the neutral CT.
But this would be difficult to ascertain how much of the current would flow through the neutral CT. A branch feeder to ground would have to take two paths back to the transformer. One would be through the building steel, and earth, back to the Xo bonding jumper. The other would be through building steel and building ground wires, back to the switchgear neutral bus, then the low resistance path back through the neutral conductors of the xfrm. That could be quite difficult to predict magnitude. If say the branch breakers have 800A Long Time, and GF is 40% or 320 amps, would the MCB GF just be set above this point? It seems to two different types of GF, the branch breakers 3CT differential, and the MCB Direct Neutral CT would be difficult to precisely coordinate.
Thoughts?

 

[dpc]

I do agree that you have a problem. The transformer secondary is a separately derived system in NEC language. The X0 to Ground connection at the transformer is the bonding jumper from neutral to ground for the system. You can't have another one at the panel.

 

[unclebob]

It is OK if your bonding jumper at the XO and the neutral-ground link in the switchgear are upstream of the neutral CT. The ground fault have may have multiple paths in the ground but have only one return path in the neutral where the CT is.

 

[dpc]

The bonding jumper between neutral and ground can be at the transformer, OR it can be in the switchgear. But only one is allowed.

 

[wroggent]

2017 NEC 250.30(C) says that if the source is located outside of the building or structure supplied, a grounding electrode connection must be made at the source location.

250.30(A)(1) says that if a source is located outside the building or structure supplied, a system bonding jumper shall be installed at the grounding electrode connection.

2350.30(A)(1) Exception No.2 says if a building or structure is supplied by a feeder from an outdoor separately derived system, a system bonding jumper at both source and first disconnecting means is allowed if doing so does not establish a parallel path for the grounded conductor. A path through earth shall not be considered a parallel path.

My interpretation of that is if your source is outdoors you can have two system bonding jumpers, but one must be at the transformer.

If you'll never have any load connected to neutral perhaps consider a high resistance ground?