Generator for open delta system

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blainebenson

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Is it possible to use a back-up generator on a 120/240V 3 phase open delta system? Is it possible to obtain a generator (special order?) one that will work for a open delta system?

Or do I have to use 2 generators - one for the 3 phase delta connected loads and 1 for the single phase loads?

The service I have is one 75 kVA transformer and one 37.5 transformer banked together. Overhead service is #4/0. According to the utility the peak demand in the last two years was 52 kW.

All of my experience has been with wye connected generators - open delta service is a new one for me. Thanks for any help.
 
wirenut1980 said:
A regular closed delta generator will work just fine.
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This is correct. In a manner of speaking, it's the open Delta that is the weird thing here, not the load or the generator.
 
I'd like to add that I have had problems in the past because some generators (Kohler to be sure) only monitor 2 phases for voltage control/stabilisation. If the Hi-leg is not one of the monitored phases all kinds of inbalance and voltage fluctuations can occur.
I recently had 1 generator producing 267V on the high leg which made a 3 phase motor VERY noisy. because the customers engineers refused to accept that this was the problem they ended up paying me $27,000.00 to install a delta/wye transformer to cure the problem.
 
Limey Pete said:
(Kohler to be sure) only monitor 2 phases for voltage control/stabilisation.
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I haven't had any real problems but have always wondered about this engineering. Are you just supposed to stand there and switch the monitoring between L1-L2, L2-L3 and L1-L3 until your hand gets tired?:smile:
 
LarryFine said:
Pete, are you talking about monitoring the utility voltages or the generator output?
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Larry.
I meant the monitored generator , in the installation I was referring to the utility voltage was fine. Kohler informed me that the generator could be modified to monitor and control all 3 phases but The Fortune 500 Company engineers told me this was not the problem and that I should basically install a 240/240 volt deta/wye transformer after the generator output. It did help the situation but was a very expensive solution.
 
Okay, now I have it.

First, it looks like the genny meters are merely for visual monitoring, and have no effect on voltage stability.

Second, with a Delta genny, there's no way to derive the neutral tap for 120v loads, unless the genny has it.

Third, there.s no way to avoid imbalanced loading of the genny, since open Deltas lightly-load the high leg.
 
LarryFine said:
Okay, now I have it.

First, it looks like the genny meters are merely for visual monitoring, and have no effect on voltage stability.

Second, with a Delta genny, there's no way to derive the neutral tap for 120v loads, unless the genny has it.

Third, there.s no way to avoid imbalanced loading of the genny, since open Deltas lightly-load the high leg.
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This generator did have a neutral tap but the big problem was as you say: not being able to balance the High-leg as this was always lightly loaded. Worse than normal even, because the 120V loads were high (multiple highload machines) and the only 3 phase was one 2hp pump motor. All six A/C units were single phase 240Volt and Engineers did not want to use the high-leg as part of this supply as they thought the A/C control voltage was 120V and they would fry. Using a Delta/Wye transformer with some jiggling of the taps, and re-balancing the 120V loads across all 3 phases cured the problem to within about 3 volts difference. I suspect that adding a 3rd coil for voltage stabilization to the generator would have been much cheaper though, but not my decision to make.
 
If I have a 120/208 generator is it possible to engineer the system to run off of it? Could I use the three phase of the generator to connect to the transfer switch in delta then feed a buck boost transformer to go from 208 3 phase to 240 3 phase then use a dry type transformer with a 240 primary to derive the 120 V loads? (I would feed my 3 phase loads from the 240 delta boost tranformer).

I know this sound like a Rube Goldberg approach. There is a long convulted story on why my company wants to use an existing 60 kW 120/208 portable generator to back up a 240 open delta service.

Can it technically be done? If so, is there a better way to do it?

Thanks for the help. I feel like I have painted myself into a corner trying to come up with a solution before I tell upper management it can't be done with the equipment we have.
 
blainebenson said:
If I have a 120/208 generator is it possible to engineer the system to run off of it? Could I use the three phase of the generator to connect to the transfer switch in delta then feed a buck boost transformer to go from 208 3 phase to 240 3 phase then use a dry type transformer with a 240 primary to derive the 120 V loads? (I would feed my 3 phase loads from the 240 delta boost tranformer).

I know this sound like a Rube Goldberg approach. There is a long convulted story on why my company wants to use an existing 60 kW 120/208 portable generator to back up a 240 open delta service.

Can it technically be done? If so, is there a better way to do it?

Thanks for the help. I feel like I have painted myself into a corner trying to come up with a solution before I tell upper management it can't be done with the equipment we have.
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I got a litle confused as to what you want to change, if I understood correctly then;

If your generator is 12 lead reconfigurable you need nothing, if not you just need one 208/240 volt transformer, 3 pase primary and 3 phase and neutral secondary

Seems too simple I guess I misunderstood.
 
I am not sure what you mean about a 12 lead generator - wouldn't it be 4 leads (3 phases and a neutral?)

Limey Pete - so I would connect the 3 phases of the generator to the delta of a transformer and the secondary would be 240 wye?

How common are 208 delta to 240 wye generators? Are these special order items? I didn't see one last night while I was looking on Square D's site.

Thanks.
 
I just had a Kohler Genset converted from open delta service to single phase by eliminating all of the 3-phase loads. I also had the utility changed. The generator service compnay was able to easily reconfigure the alternator. All of the leads, I think there were 12 just needed to be reconfigured. It also derated the genset ~ 30%. From 30Kw to 20 KW.

Check with your local Generator service contractors to see if the installed alternators can be reconfigured as these were.

Good luck!
 
blainebenson said:
I am not sure what you mean about a 12 lead generator - wouldn't it be 4 leads (3 phases and a neutral?)

Limey Pete - so I would connect the 3 phases of the generator to the delta of a transformer and the secondary would be 240 wye?

How common are 208 delta to 240 wye generators? Are these special order items? I didn't see one last night while I was looking on Square D's site.

Thanks.
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Count the number of leads exiting the generator housing, (not the number entering the disconnect) if it is twelve you can reconfigure it, let me know and I'll send you a Kohler diagram. I'm not sure how common such a transformer is, I had to wait 2 months for a rare beast: 240/240V wye/delta. I should think 208/240 would be easier.
 
On my 12 wire Katolight generator, you can wire it from 240 volt single phase to 440 three phase,

At one time I had it wired for 240 single phase service, and now have it wired for 240 three phase delta, (the POCO service I have is open delta 240 three phase),

yes the generator loads will be "unbalanced" as normally most of the load is on the single phase side of the windings, In a open delta situation, and thus it may be wise to treat the generator down rated as if it was on single phase, (note the post above) 2/3 derating, actually it may be wise to consider it 1/2 as most of the load will be on set of the windings, not spread out over the three sets of windings, except for the three phase loads.
 
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