1MW at 208V is nearly 3000A. Why would you want more current when a higher voltage, lower current installation will almost certainly be less expensive?
Are you using the wild leg of a 120/240 Volt four wire delta generator"
That's the only place that I can think of where you will find both 208 and 240 on a generator.
The sets that I have seen, (a lot of them)
have a an allowable V/Hz of over 2.3 V/Hz.
That allows the individual nominal 120 Volt windings to safely produce over 138.5 Volts.
138.5 Volts x 2 = 277 Volts.
That allows two nominal 120 Volt windings to be connected in series for 277 Volts when 277/480 Volts is needed.
Bill
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"Why not the best?"
Jimmy Carter
Looking at a Stamford model S6L1D-H4 Wdg.312,
KVA 1489 Volts 416
KVA 1561 Volts 440
KVA 1638 Volts 460
KVA 1694 Volts 480
This is a wye connected set.
Reconnected in delta the corresponding voltages will be 240 V, 254 V, 266 V and 277 V.
You could dial this down to 208 Volts with a corresponding loss in KVA and kW.
For use at 120/208 Volts you would need an artificial neutral.
Yes, you could, but for the reasons given by DavidBeach you probably would not.
Do you have an application?
Bill
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"Why not the best?"
Jimmy Carter
What voltages do you need?
If you can find a 10 lead or 12 lead generator the chances are good that it may be reconnected for the lower voltage even if the manufacturer doesn't list the connection.
Expanding on David's suggestion, use an autotransformer.
Bill
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"Why not the best?"
Jimmy Carter
A few companies made them, mostly to support the rental market.
CAT had them for several years as an SER (Special Engineering Request), we had two 1500kVA 120/208 rental setts mostly used for entertainment rentals at a dealership I worked for, but utilization dropped off and they got sold.
KATO made them up to 1000kW as I remember, but after CAT bought them I think the larger 120/208 options were dropped.
Aggreko had a larger 120/208 package for awhile, Cummins engine with a Stamford tail end, again a rental market application.
Baylor made a special 1200kVA 120/208 tail end for offshore applications, usually coupled to a CAT prime mover for certain oil and gas, mainly rig service housekeeping operations.
When we ran across needs for a larger 120/208 system it pretty much always ended up as paralleled smaller units.
Talk to some suppliers for a special order.
That will be a lot less cost than a transformer.
A 480 Volt machine will be a six lead machine.
A conversion to 120/208 Volts requires each of the three windings to be split and six more leads brought out.
The capacity will be reduced in the ratio of 208/240 or 87% of the 480 V capacity.
A smaller prime mover is allowed.
BUT
Will you be able to source a transfer switch to handle < 1000 KVA?
If an ATS is available, then check the possibility of a special order.
If you go this route first check with generator end manufacturers such as Leroy Somer as to the cost.
If an ATS will be difficult to procure, then consider:
Split the load; Each part of the load on its own gen-set and ATS.
Consider a higher incoming service voltage and use the transformer for both normal and standby service.
I suggest that it may be better to avoid the extra complexity of synchronizing multiple standby sets.
Bill
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"Why not the best?"
Jimmy Carter
Alright- I can tell folks here have never seen a need for 1000kw of backup power at 120/208 so I will include an example of where a 600kw and 400kw unit was used to meet the need:
Most tax funded building and some hospitals will either have a very large 120/208Y service or have many 12kv-120/208Y substation scattered throughout the building or campus.
Have you actually spoken to reps of the genset manufacturers to see if they will build it the way you want it perhaps? No need to limit yourself to stock models, which are probably built to order anyway, for a set of this magnitude.
Well, maybe, "Alright- I can tell folks here have never seen a need for 1000kw of backup power at 120/208 so I will include an example of where a 600kw and 400kw unit was used to meet the need:"
Frankly your design isn't all that unique, worked in lots of facilities that had multiple generators supplying groups of ATS switches and other loads. If the loads aren't critical then that system may be ok, but you have no redundancy. I recently worked on a system that started out like what you have drawn, and then was modified to add additional ATS switches to try and cross connect certain circuits, got to be a bit of a nightmare. Because of some seismic issues the whole facility is going to get a major rework with a completely new power system that will be based on an N+2 "Power Block" generator system with a mixed bag of diesel and natural gas units.
Not sure how old you are or where in the world you work, but this was a fairly common issue for the generator suppliers in the late 80's up until around Y2K. Older healthcare facilities trying to grow without making massive changes to their infrastructure, telecom facilities shifting from analog phone lines to digital with increasing power needs, and growth of older data centers that started with 120/208 systems and kept trying to grow them without huge changes.
You may also live in an area where things like runtime limits and strict air quality regulations don't have an impact on your design criteria, but they may.
One of the primary reasons companies like CAT and other generator set suppliers don't make bigger 120/208 sets is that with better control and protection systems now available a lot of emergency systems are going to smaller multiple generators for various reasons.
Take a typical healthcare facility in my state, following OSHPD guidelines for sizing you end up with a generator that is usually way oversized from actual connected loads, but you may need that capacity for energizing transformers, starting motors and recharging UPS systems. Then after a short time your big generator is running at loads well below its rating. With older design engines it wasn't really much of a problem, but newer high output lower emissions engines with lots of turbocharging don't do too well running at lower loads, if you have to deal with exhaust after-treatment systems it really gets difficult.
A large number of the systems I work on these days use multiple units, when an outage occurs, they all start and come online, power system gets restored, then as everything stabilizes and loads level out, the control system using a load demand strategy takes excess capacity off line.
You may find this an interesting read, not the only company that does it, but one of the better technical descriptions I have found,
But have been seeing a fair number of systems by them landing in our area in the last couple years.
To answer another of your questions, yes, most manufacturers will provide a non-standard tail end, but the one off structural and torsional engineering will likely make the cost way higher than the standard packages. There may also be some certifications involved depending on where this will land, so those costs will factor in as well. Some genset dealers excel in providing custom engineered packages, but again cost will be higher that a standard model, and hard to get someone to pay the extra money unless you're dealing with an industry that is used to paying more for exactly what they want. In my experience standby system for government and healthcare are almost always driven to the lowest bidder.
I've noticed that too- smaller parellel sets are becoming the norm. I'd also note that around 750kw you get the most kw per dollar- so in theory many smaller sets are more economical than one or two giant sets.
Regarding parelleling I like the concept however I'd argue it isn't always worth the money. Paralleling creates single mode and common mode failure points. Personally I like to divide non critical loads across multiple gensets (when demand/reliability calls for it) while having critical loads on ATSs in series- ie one utility feed with two gens- or 3 sources. IMO much greater flexibility and reliability. I also like to disperse gensets around a facility rather than having them in one location.
Down side is as you say larger gens (in some cases, in general) for inrush- but IMO its worth the advantages.
