Mains Disconnected and parallel connected generation switchgear

[simma]

Hi, I am new so I need to get some posts going. I am not a registered spark I am more of a technical contracts engineer. I am looking at if it possible to have a generator which backs up the mains supply (auto change over) but also is connected in parallel so can export to the grid. I am not sure if this is even legal in the UK. But I have found that Eaton / CAT make a change over relay which does just this.

The XLM type. This does the exact function described above. One of the main things about this is that you must be covered under G59, but you are also operating a switched alternative. How is this system earthed?

Any one have any thoughts on this?

I cant seem to post a link but the switchgear is on the CAT website.

 

[Engineer54]

It can be done, it will involve the inclusion of quite a bit of extra protection monitoring etc. The thing is though, it will not be a cost effective method of supplying power to the grid, ...it'll probably be costing you money in the medium to long term, if not also in the short term!!! lol!!

Normally only the larger industrial factories with their own small power stations will find this feasible/workable where they divert power to the grid during out of hours working. These prime power generator (or other, like steam turbines) power stations are/would be running 24 hours a day, not on a stand-by basis as in your case. Another thing is, that stand-by and prime power generators have very different duty characteristics , and can rarely be interchanged with each other.

 

[Knobhead]

As to earthing, look at this. You can not use the DNO earth for you generator.

 

[Engineer54]

We use the general earthing system of the main installation (via a cable link to the local earth field to the generator) to reference the neutral to generators we have on our projects. It shouldn't matter a jot with 4 pole change-overs and Breakers (MCCB/ACB etc).

If you think about it, it would be almost impossible to isolate separated earthing systems, as when in Generator mode virtually everything the generator will be supplying past the change-over switch, will be solidly linked to the main earthing system of the installation.

EDIT....

Thinking about it further, you could have separate earthing to the generator, and leave it behind . (ie.... used for N-E and frame connection only) The generators supply will then be using the installations earthing system. Any SWA cables used from gen-set via MCCB/ACB and control panel to change over device can be isolated at the Gen-Set side. Not sure i'd go to all the trouble though!!

 

[Knobhead]

I'm just thinking about the poor schmuck down a hole cutting armouring not knowing some fool was using it as an earth for their generator.
In you're situation E54 it's "your" earth system and the dangers will be known.

 

[johnboy6083]

We run a seperate single core cable from the MET of the installation to the Genny. The Genny has its own rods anyway, and all metalwork that is accessible from the Genny is also bonded to it.
we use standard disconnect links on the earth bar. To be honest in practice, the DNO earth is never disconnected.

 

[Knobhead]

I'm not saying disconect the DNO earth John, it should not be used as the sole means of earthing for the generator.

 

[Engineer54]

Think you misread what i was saying. All generator positions should be supplied with an earth mat or rods that in turn should be connected to the N-E point of the generators alternator and frame. Whether you link the earth mat to the installations MET/EMT of the installation or not, shouldn't really matter, we do as a matter of course. I certainly wouldn't rely on the Armouring of a SWA cable as the sole source of earthing-out a generators installation.

 

[Edd]

Nothing to add, intresting thread thx

 

[simma]

I don't disagree that it would not be cost affective. But If you had a large CHP which you wanted to run after mains supply failure, I am thinking down these lines.

 

[simma]

Thanks everyone for the responses

 

[Engineer54]

I don't disagree that it would not be cost affective. But If you had a large CHP which you wanted to run after mains supply failure, I am thinking down these lines.

Better if you leave the Generator purely as the stand-by option, as runing a stand-by rated generator, in what would be a prime power mode will also not be a good idea as far as longativity of the Gen-Set goes. As i stated earlier, it's generally only industrial sites that have they're own small generating power stations, running 24/7 where it makes any sense to back feed onto the grid during factory/site down times. The other thing is, these type of installations would also be feeding the grid at MV (11/33KV) rather than at LV. You say a ''large CHP'' so depending on your generators KVA output, it may well be too large to feed into the local DNO LV network....