Cogenerator Grounding

[pgonski]

Here's the setup in question:

Small Cogenerator (300kw) that operates in parallel with the utility source
Generator was designed as a 3 phase 4 wire system
Generator Ties into plant 480V LVSWGR, SWGR is 3 phase 3 wire. There is no "isolation transformer"
Neutral and ground in the plant appear to be bonded just at the utility delta-wye transformer ahead of the LVSWGR service entrance.
There are no 277V loads which is why the neutral isn't used in the plant

The part I am a bit confused on is how the generator neutral should be connected (if at all). Should the frame of the generator be tied back to the LVSWGR ground, or should there be a separate grounding electrode at the generator, and then an EGC tied back to the swgr?

Thoughts?

 

[Smart $]

Here's the setup in question:

Small Cogenerator (300kw) that operates in parallel with the utility source
Generator was designed as a 3 phase 4 wire system
Generator Ties into plant 480V LVSWGR, SWGR is 3 phase 3 wire. There is no "isolation transformer"
Neutral and ground in the plant appear to be bonded just at the utility delta-wye transformer ahead of the LVSWGR service entrance.
There are no 277V loads which is why the neutral isn't used in the plant

The part I am a bit confused on is how the generator neutral should be connected (if at all). Should the frame of the generator be tied back to the LVSWGR ground, or should there be a separate grounding electrode at the generator, and then an EGC tied back to the swgr?

Thoughts?

TTBOMK, the NEC doesn't require the generator neutral to be brought to the LVSWGR.

However, being it is a grounded system, running the neutral will reduce the resistance for ground fault current. Without the neutral, ground fault current has to go through the service neutral, service transformer, back to the LVSWGR on the ungrounded service conductor(s), then through generator's ungrounded conductor(s) in order to trip its OCPD.

As far as grounding goes, under current code there will always be a grounding conductor. What it is called and how it is sized depends on where the [first] disconnecting means is located (genset or SWGR). As a alternate power source (i.e. not a separately derived system), the rules are a bit vague...

 

[__dan]

I'll give an opinion, it's interesting.

If there were a utility system neutral in the SWGR, the generator would be not SDS and the generator neutral would solidly connect to the utility system neutral.

With no utility neutral in the SWGR, I see two choices (three if you consider a HRG).

1. Gen non SDS: You have the choice of carrying the generator neutral as an insulated conductor all the way back to the point where it can connect to the system utility neutral, and depend on the utility / service system bonding jumper for the fault current path.

2. Gen SDS: You can wire the generator as a separately derived system. You would have an EGC back to the SWGR EGC busbar, a GEC back to the facility grounding electrode termination point (I would use an insulated, not in metal raceway, conductor for the GEC), a N to G system bonding jumper at the genset, and an insulated neutral carried only as far as code requires. If you had service supplied neutral connected loads, there would be a fourth pole and switched neutral at the ATS, but with no neutral connected loads and no utility neutral in the SWGR, this may be the cleanest arrangement. Three pole transfer switches and nothing connected to the genset SDS neutral.

If it were me, I would absolutely run it by the factory for their recommendation.

 

[infinity]

Are we talking about a COGEN system where the micro turbine generates and sends power back into the building system? I see nothing about an ATS in this setup so is that an optional installation? Based on the OP I'm not getting why an ATS would be needed.

 

[__dan]

I said ATS as a typical connection to the bus. It is possible the topology is a paralleling breaker that closes to the bus. The method of closing to the bus would not change the choice of SDS or non SDS generator neutral grounding.

It wasn't clear if the facility could operate from generator as an emergency source.

 

[pgonski]

I said ATS as a typical connection to the bus. It is possible the topology is a paralleling breaker that closes to the bus. The method of closing to the bus would not change the choice of SDS or non SDS generator neutral grounding.

It wasn't clear if the facility could operate from generator as an emergency source.

The generator runs off biogas, so it shuts off if the power is off... T

If I were designing this from day 1, I would haveuseda low resistance ground....

I think I should either bond the neutral and the ground at the gen and run an EGC or bond the neural to ground and add a ground rod, plus a egc.

Leaving the neutral ungrounded I would think would make an ungrounded system

 

[__dan]

I'm thinking the electronic controls and sensing for the generator will be three phase, four or five wire, and depend on the neutral sensing, solid neutral to ground reference, for proper operation. If so, referencing of the neutral to ground will be necessary.

If you can be assured through the controls operation that the generator disconnects, opens from the bus, when utility power is lost, then the generator is connecting to a system that is already grounded, at the utility transformer.

The generator manufacturer should be consulted, they must have a scenario for this. If the generator neutral were left unconnected to anything as in a non SDS, I would be concerned about the proper operation of the generator controls because they surely depend on connecting to and sensing of the neutral. I would think the neutral voltage will wobble in the center and smoke the controls if it is not solidly connected to something.

 

[Smart $]

The generator runs off biogas, so it shuts off if the power is off... T

If I were designing this from day 1, I would haveuseda low resistance ground....

I think I should either bond the neutral and the ground at the gen and run an EGC or bond the neural to ground and add a ground rod, plus a egc.

Leaving the neutral ungrounded I would think would make an ungrounded system

As I noted earlier, it is an alternate power source, not an SDS. What that amounts to is you only have one system that is already grounded, i.e. the neutral is [should be] already bonded to earthing electrodes via the main bonding jumper, which only gets done in one location. To bond again at the genset would create a parallel neutral current path.