3 phase gen set neutral/earth

[Apex13]

Hi guys/gals

I have been asked to install some 230v sockets and lights in an old refrigerated trailer, that would be used as a store on a quarry site.

The trailer has a 3 phase generator built on to the side to originaly work the refrigeration unit, but as everything involved with the refrigeration system is 415v, there is no neutral conductor present but there are earth conductors.
My knowledge of generators is limited at best, but I am wondering if it's simple to alter the supply in a way that there is a neutral so the single phase install can be powered from it?
Cheers

 

[James]

check the wiring of the alternator, it may be possible to get to the neutral terminal if it is a 3 phase 400v star configuration.

if it is 3 phase delta wound, then you will need a transformer with 400v delta windings on the input and 230v star on the output, tie the star point to earth and you have a 3 phase and neutral supply.

 

[David Prosser]

I'll check when I get into work tomorrow but I don't think you are allowed to supply generators with out star winding. As OP has indicated earth connections it would look most likely that a star point is available.

 

[pc1966]

I don't know if you get them for lower powers, but you get "earthing transformers" that simply provide a neutral point from a 3-wire supply and don't have any isolation/voltage transformation action.
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However, I would be a bit surprised if there was not a star point from the alternator anyway.

 

[davesparks]

Of it has earthing then logically there must be a star point in there somewhere.

 

[Apex13]

Thanks for the replies guys, this is the plate from the gen which says s star, (hopefully that's what that means.) Not sure what access I have to the connections though, as it is attached to the underside of the trailer so limiting access quite a bit. I will get a better look in the next day or so when i get back to it though.
If there is access, should it just be a case of running a core from the centre point to the control panel and that's my neutral?

 

[davesparks]

How far have you gone so far with looking for the neutral connection? It may already be there but just not used at the moment because it wasn't needed.

If its not obviously there at all then you've got the manufacturers details and serial number there so give them a call and they should be able to advise.

Also don't forget that three phase generators really don't cope very well with unbalanced loads, you need to get it balanced as far as practicable. With generators I have always worked to a general rule of always having all phases loaded and never being more than 30% out of balance. But I have no technical evidence to support this, it's just something I've picked up somewhere and has always worked for me.
I have seen a few generators where other people have loaded them out of balance and the voltage regulators fail as a result.

 

[Apex13]

Hi Davesparks

So far I have only had a quick look in the control panel and can only see the 3 phase conductors at the main isolator on the panel, with the earth terminal being next to the main switch. I had a quick look and test at a few things and cant find a neutral anywhere in the panel. Everything is either 400v or 12v control circuits from the small panel transformers that are in there as well.

Just above the plate in the photo is a small panel that looks like it would give access to the terminal box but basically straight above what I assume is the terminal box, is the bottom of the trailer so there is no access to the top of the box, just the side so not sure what will be possible from there. I will need to get a better more in depth look though.

As I said in the original post, my knowledge is limited at best, but to my limited knowledge the earth and neutral are one in the generator so wasn't sure if there was any way of checking continuity of the existing earth conductor to the generator if possible and then basically splitting that before the 230v install and making what is essentially a TN-C-S system. I do understand this might be a dumb question but is why I am asking just now before doing anything about it.

Regarding the balancing of the phases, I had considered installing a small 3p n e db to try and balance as much as possible but as it will only be a store, so only has a couple of sockets and a few led lights, I thought It would end up quite hard to balance it to any degree when really the only potential higher load would be on a socket depending on what they plug in. 99% of the time it would just be lights and maybe a cordless drill battery charger that was being used.
Thanks again.

 

[pc1966]

I suspect the degree of imbalance to cause problems is probably 30% of the total capacity, not 30% of any light load. It is rated around 24A per phase so I would aim for less than 8A imbalance.

The other issue is this is a 17kVA generator and you only seem to need a few lights, etc. Not only will it be very inefficient on light load, if you run that way for long periods it will clog up the diesel engine. Search for "wet stacking" for info on that.

Do you need that much power? Could you use 12V/24V DC LED lights and just run the generator for short periods to charge a handful of big batteries (with adequate venting, etc, to make sure no build up of hydrogen) or if running any power tools.

 

[Apex13]

Hi Pc1966,

The imbalance is something that I think I unavoidable to an extent and would only be a temporary thing due to the nature of the install. The lights and possibly a small charger would be the general load over 90% of the time which I cant see any realistic way of balancing this and with the size of the load, I would personally say it's not worth trying to balance anyway.(just my opinion thoigh)
The problem arises when the guys using the store end up using a grinder or drill etc because they have run out of batteries for the cordless tools and need a job done. Even putting the lights on one phase and sockets on another the lights will only be drawing a maximum of about 1.5 Amps and the sockets have a potential to be using up to 3kw say for a short time and then nothing again.

The install really does need to be 230v unfortunately as they do need to use 230v chargers ect and they might have a small compressor just to work an air pump for supplyin oil that will be in the store as well as probably at some point need to use a drill or grinder etc. Anything other than 230v would mean high expense to change their equipment.

I understand the generator will be under used for the majority of the time, but the customer understands this and the guys using the store are mechanics so it's no problem for them to maintain the engine side of it.

 

[pc1966]

If you are only looking at a couple of amps max load I don't think it really matters how it is distributed here!

The light-load aspect is a concern, but if it is only needed for a short period that is tolerable. You can get load banks to run at high power for short periods to try and clear the engine, though a 15kW 3-phase heater might be cheaper and easier to get (just point it outside on a dry day). MachineMart sell them.

 

[Andy-1960]

If the generator is ok to run, why don’t you run it and measure the voltage between a phase and the earth connection to see if it is a combined neutral earth? If you don’t get a voltage reading you will know that the star point of the winding is floating.

 

[pc1966]

It might be better just to continuity check it when not running. I expect you will always find some voltage to earth even if star floating!

Just to add to my earlier comment, slapping a full 15kW load on suddenly might just stall it, so you might want to get a smaller one (9kW or similar) and/or something that allows a couple of different power settings so you can step it up over several seconds as the engine governor adjusts fuel flow (danger with that is it is a choice of 1, 2, or 3 of the phases in use for cheapness of switching).

 

[Apex13]

If the generator is ok to run, why don’t you run it and measure the voltage between a phase and the earth connection to see if it is a combined neutral earth? If you don’t get a voltage reading you will know that the star point of the winding is floating.

Hi Andy,
Sorry if I should have mentioned that before but I have measured this and I am getting 230v between the earth and phase conductors. Does this mean that the earth conductor can be used safely to do what I spoke about above and split the supply before the 230v install?
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It might be better just to continuity check it when not running. I expect you will always find some voltage to earth even if star floating!

Just to add to my earlier comment, slapping a full 15kW load on suddenly might just stall it, so you might want to get a smaller one (9kW or similar) and/or something that allows a couple of different power settings so you can step it up over several seconds as the engine governor adjusts fuel flow (danger with that is it is a choice of 1, 2, or 3 of the phases in use for cheapness of switching).

That makes sense, I will definitely suggest this to them. Thanks.

I will check for continuity between the earth coming in to the panel and the generator tomorrow, I am just keeping my fingers crossed I can get access to the terminals.

 

[pc1966]

They may not be bothered about engine life if it is a short-term solution using old stuff, but at least if they do end up using it further (as many "temporary" systems end up doing) they should have ideas on how to get the best out of it and minimise service/repair costs.

 

[davesparks]

I suspect the degree of imbalance to cause problems is probably 30% of the total capacity, not 30% of any light load. It is rated around 24A per phase so I would aim for less than 8A imbalance.

No, 30% of the actual loading is what I have always worked to.
The voltage regulators can get very upset if a light load is running imbalanced in my experience.
They seem to be more tolerant of imbalance once they've got a bit of load on them.

I once arrived on an event site to start setting up and found one of the pair of 200KVA generators on and hunting. The site guys had started it up themselves and plugged in a few single phase leads to run their tools, kettle etc, mostly on the same phase.
The cause of the hunting was the AVR having gone kaput likely caused by the unbalanced light loading.

As I said, this is just personal experience so may not be technically correct.

 

[pc1966]

That is interesting, so not really an issue for the alternator but something odd with the AVR? Will bare that in mind!

 

[davesparks]

Sorry if I should have mentioned that before but I have measured this and I am getting 230v between the earth and phase conductors. Does this mean that the earth conductor can be used safely to do what I spoke about above and split the supply before the 230v install?.

No, just no, don't do it!

The earth is unlikely to be the correct size. Also it may not go directly to the star point, it probably has half a dozen connections along the way to various bits of metal which may or may not have slightly high resistances, you could end up with current flowing through metalwork that was never intended to have current flowing through it, or random voltages on exposed metalwork.
Not to mention the disaster that will result if that connection to the star point were to fail.
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That is interesting, so not really an issue for the alternator but something odd with the AVR? Will bare that in mind!

Out of balance can cause plenty of issues for the alternator and the engine too, especially on big generators with big out of balance conditions.
But older AVRs (I don't think recent ones suffer so badly) seem to be somewhat over-dramatic.

 

[Apex13]

No, just no, don't do it!

The earth is unlikely to be the correct size. Also it may not go directly to the star point, it probably has half a dozen connections along the way to various bits of metal which may or may not have slightly high resistances, you could end up with current flowing through metalwork that was never intended to have current flowing through it, or random voltages on exposed metalwork.
Not to mention the disaster that will result if that connection to the star point were to fail.
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Out of balance can cause plenty of issues for the alternator and the engine too, especially on big generators with big out of balance conditions.
But older AVRs (I don't think recent ones suffer so badly) seem to be somewhat over-dramatic.

Ok, thanks, I understand that the earth conductor may not be going straight to the alternator as it probably does tag the trailer chassis/frame etc on the way and therefore isn't able to be used as the neutral but being that there is 230v present between the phase and earth conductors am I right in assuming that the earth does in some way return to the alternator and therefor should be possible for me to run a new conductor or better, a full new supply from there to the panel, giving me my neutral?
If this is possible, does this mean the existing earth core would be better to be disconnected as we are not sure where it runs to, or is it then creating potential problems without re-earthing everything that it may be tagging along the way again? I would be expecting to bond the frame of the trailer etc as part of the new install anyway but not knowing what else the original earth is tagging along the way is it better left intact if it goes back to the alternator eventually anyway?
Thanks