24/7 Synchronized generators optimized combination 4

[safy2004]

It is required is to install generators to feed loads of 2807 kW + j 1621 kVAR = 3241 kVA on 6.6kV, we propose to use 0.4 kV generators and use step-up transformers to feed the loads with 6.6 kV, my question is what is the best combination of generators for this load, is it better to use:
1- One 4 MVA generator, and one standby 4 MW generator?
OR
2- Two 2 MVA synchronized generators and other two 2 MVA standby generators.
OR
3- Four 1MVA synchronized generators and other Four standby generators.
Or there is another combination of generators to achieve best stability, readability, and efficiency, and at the same time the best fuel consumption.
Taking into account these generators should feed the loads contentiously without any interruption for 24/7 for minimum three continuous years.
Thanks in advanced.

 

[ScottyUK]

Why would you need multiple standby sets?

An N+1 configuration is typical to handle a machine failure, where N is the number of sets required to support the load and one set is either a spinning spare already on the bus at light load, or one stood by ready to start on loss of one of the running sets.

 

[safy2004]

Thanks ScottyUK,
The company’s condition is to have minimum N+2 generators.
But what about the capacity of the generators, what is the best design:
ONE 4 MVA
TWO 2 MVA
FOUR 1 MVA
Or there is another option?

 

[itsmoked]

Six 1 MVA units would provide double backup and one would be shutdown normally. Weekly one is brought down while the current spare is brought up. They all are evenly rotated thru as the weekly stood down spare. This gives a no-rush opportunity to service them cold on their 'week off'. Also importantly it has the staff up to speed on doing set rotations so if one actually goes down there's no panic or mistakes in bringing the spare up and synchronizing it into the system because everyone has done it many times before.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

I'd probably go with your 4MVA + 4MVA arrangement, although that doesn't really meet your N+2 requirement.

At 4MVA you can generate directly at 6.6kV and avoid a load of additional hardware, and you have only two engines to maintain instead of four or eight. The larger set will almost certainly rotate more slowly - probably a six-pole or eight-pole design so quieter, probably more efficient. The larger engines tend to have better instrumentation, more monitoring, better auxiliary components, and so on.

 

[itsmoked]

The company’s condition is to have minimum N+2 generators.

Good point too though Scotty.

I'd bet a six-pack of 1 MVAs will cost 1/4 what two 4 MVAs would because they're more competitive.
MikeL (Catserve) will have a good bead on that I'm betting.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

N+2 standby is a lot of capital tied up in idle metal. Is this a datacentre?

 

[catserveng]

There is a lot of "it depends" in these situations.

The idea of 2 4MW units is pretty simple, but likely won't work well if the "standby" unit has to be spinning reserve, especially with newer high speed engines, as they don't like operating at lower loads. But, as noted above, they could likely operate directly at the voltage you need, which may or may not be a benefit. I like big engines, but they don't always meet the target needs.

It would be helpful if you could provide more information about the type of operation you are running, the expected load profile, and what kinds of loads. Large motor starting loads may require additional capacity online during startup, then dropping some capacity offline when running steady state.

You said for a "minimum of 3 years", what is the maximum projected life of the project? For a three year project your best solution may be to rent multiple 1 or 2MW units at .4kV, but at the end of those three years you may be looking at some pretty significant maintenance costs. What is the expected load profile, you project a maximum but is that really what the site demands 24 hours a day?, if you have a widely varying load profile, again multiple smaller units with a load demand start/stop control system would likely be better, if you a large stable demand than a larger unit could be a better choice.

Your in a load range where you may also want to consider high speed versus medium speed engines if the projected plant life is longer, say 7-10 years. What kind of fuel are you expecting to run? Diesel, MFO, HFO, maybe natural gas? Depending on your load profile and how long you need to run a mixed plant, like a baseload medium speed unit and a few smaller high speed units to manage the load profile, to cover reserve needs and allow for maintenance activities.

If this was a maximum of a three year project running typical "industrial" types loads, like a mine site or even a small island utility, and you wanted to use diesel fuel, then a handful of same size modular units would likely be the best solution, but after about three years that may not meet the longer term needs of the site. But at this point that is a guess. So if you can provide some more details you may get a better answer.

Hope that helps, MikeL.

 

[edison123]

Another question. Why would you buy 0.4 KV generators and then step up to 6.6 KV when you can go with 6.6 KV generators? At 1 to 4 MVA, the current handling becomes an issue (busbars/multiple cables, breakers). If your load is mainly 6.6 KV with minor 0.4 KV loads, then a 6.6 KV/0.4 KV step-down transformer to suit the lower voltage makes more sense.

Muthu

http://www.edison.co.in

 

[FreddyNurk]

If you've already got personnel capable of carrying out switching operations, then one of the reasons for going 415V is negated.

Lead times for medium speed versus smaller higher speed units may also be a factor in which way you go. Consider as well, that if you're running your sets baseloaded, the acceptable load factor may require larger sets than what you'd expect. The amount of spinning reserve you carry for a varying load is also a big factor in oversizing the sets in order to be able to support sudden load increases without dropping the generation plant.

ScottyUK is right though, you don't need 'standby' sets, rather the ability to account for sets being unavailable due to failure or maintenance.

1MVA is probably a bit small but likely available for a 6.6kV alternator, 1.5MVA is definitely available in 6.6kV, and from what I can remember, tends to tie in with the upper size of most high speed offerings.

EDMS Australia

 

[safy2004]

Thanks catserving, please see attached the total load and the continuous load details, the fuel is diesel.

 

[davidbeach]

Was once somewhat involved with a major bank data center/call center that, if I remember somewhat correctly, had four 2MW units (12.47kV) and a normal load a bit over 3 MW. These were nominally standby units that could parallel with the utility.

If the facility lost utility power all four would start and parallel. If all four started and synched one would be dropped after a few minutes.

At that point there’d be three units carrying a load that two could could carry. As long as those three ran all was well, if one shut down the idle unit started and synched with the others.

If the load became low enough that one unit could carry it the running unit count would reduce to 2.

 

[catserveng]

David,

We called those systems "Load Demand Start/Stop", Woodward and others have that logic already built into some of the new controllers like the EasyGen. Have also done systems with LDSS that if a large load is being called, like at a large pump station, when the pump start is requested it will check to see if sufficient capacity is on the bus before actually starting the pump, and if needing bring on additional unit(s), after pump is online and the system stable, the load demand becomes active again and will adjust number of units online to meet new needs.

I'm at a site today with 5 2MW standby units at 12kV, site load varies between 3-5MW. Units can be paralleled to grid for testing (as long as no power is exported) and the system can also do a closed transition transfer from utility to the generators for demand response. On a loss of utility all five units 'scatter start" come on line and take on system loads, after all feeder breakers reclose and a settable time delay has passed for system stabilization, the load demand scheme will look at site load and units on line and start dropping units until appropriate number of units are online to meet load and reserve requirements. If an online unit goes into alarm, the system will replace that unit with an available unit, put the unit in alarm on standby at the lowest priority and alert the operators. If the load increases rapidly, or a unit drops off unexpectedly the system also has a bus 27/81U device that will call all available units back online to steady things up, then go back into load demand after system stabilizes.

When utility returns this system will parallel to grid and drop the generators to prevent export. We have some sites that the utility allows for a ramped return to grid power depending on the local system.

There are a number of these systems in place, mostly hospitals, data centers and sites like this, large biotech facilities. Also have some larger sewage and water treatment plants with similar systems.

About 5 years ago we did a similar type project for the construction of a water storage dam project in SoCal, driving force in that was due to emissions, easier to manage a power plant than a bunch of smaller units scattered over the site, had 6 modular 2MW units, a switchgear module, a capacitor bank and a 600 kW "weekender" unit that only came on when site loads were real low and no major construction activity was going on. After the project, 4 of the units and the switchgear ended up being the standby system for the water treatment plant built for the project, and was later modified to grid parallel for a utility demand response program. The load demand control on that system was easily modified from managing an island mode power plant to a grid connectable standby system when the site requirement changed.

MikeL.

 

[catserveng]

To the OP, still not quite enough info to provide better suggestions, at least from me. You have what looks like a main-tie main bus and 4 large motor loads of the same size, so I'd guess a pump stations maybe? Do you intend on supply one side of the bus from your temp system and having a utility feed come in on the other side later in the project? Will your engine driven power system go away and be replaced by another utility feed or stay on site as a standby system?

MikeL