Diesel generators, paralleling and load sharing

[sberbece]

Hello!

I have to design an island power plant, maximum 45 MW, with a relatively high fluctuating load. Say continuously maximum 30 MW. The only fuel available is diesel.
I am proposing a plant configuration based on 6 x 7.4 MW (N+1) = 37 MW medium speed diesel generators and 6 x 1.6 MW (N+1) = 8 MW high speed engines, all machines are working in parallel on the same 11 kV bus.

This configuration, I believe, will offer the advantage of base loading the medium speed engines (7.4 MW) and use the small engines (1.6 MW), which can be fast brought online and synchronized, to manage the fluctuating loads.

Does anyone see a problem operating 7.4 MW machine in parallel with 1.6 MW machines, from load shedding point of view? I believe that latest technology governors shall overcome any motoring issue.
Are there any operation / maintenance problems that could appear, operating 1.6 MW and 7.4 MW diesel engines in parallel?

Thank you in advance for your support!

Regards,
Stefan

 

[rbulsara]

With proper design and controls there should not be any issues. Was not an issue even with older 'technologies' to parallel units of different sizes.

As for maintenance and all, if depends on lot of local factors.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

I consult for a very small plant that used to use a mix of 350 KW and 600 KW generators. The small machines were most used when the load was low. We would run through the night with one 350 KW machine. As the morning load grew we would switch to a 600 KW set. Then add a 350 KW, then go to two 600's.
I would suggest N+2. And eight large sets.
Simplicity
Cost
Rebuilds.
After about two years your sets will be in need of overhauls. You will almost always have a set down for rebuild.
If your running philosophy is N+1, you will have lots of time to bring a large set online as the load grows.
Running different size sets in parallel is doable, but with an almost 5:1 size ratio it may be more trouble than it is worth.
Are there any gas turbines on the market which will accept your available grade of diesel?
For a plant that size, combined cycle may be an option.
Re:fuel costs. The 350's and 600's are gone replaced with a pair of 1200 KW machines. Only one is run at a time.
The saving in fuel costs between new generation diesel engines and the old sets is enough to make the payments on the new sets.
I have a gut feeling that when rebuild costs are compared, in 5 or 10 years, you may be replacing small sets with larger ones.
You should be able to find a load control panel that will run the machines in parallel.
But with a 4.6:1 size ratio, it will work but why??

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Hoxton]

Do not forget the downtime due to maintenance / overhauls, someone once told me that for a true "island", you should divide the load by 5 to get the genset rating and install 7 sets. 5 or fewer on load; one on standby, one for maintenance.

The rest depends on load profile against time and future growth projections.

My gut feeling is that after a few years the smaller sets will revert to emergency use, due to the relatively high cost of operation against the larger slower speed sets.

 

[ruggedscot]

This sort of set up opens up a few things that you need to take into consideration.

Maximum load step ! Working out in bosnia we had a kitchen feed breaker trip on the run up to feeding time. kitchen worker saw the switch panel open and saw the tripped breaker and put it back on. But by this time the generators had backed off and shut down leaving the base load being supplied by two sets. Of course switching the breaker back on increased the load demand and the generators could not deal with this step. Two sets were bouncing about and the other sets were sitting unable to syncronise to the bus. Id be very very wary of the set up and have a think about how the load demand on the sets are controlled. Ensure that you can never have a load application automatically to the bus that would overload the availible capacity. Ensure that you have proceedures on how to recover an unstable system. Ensure you have proceedures in place on how to re-set any distribution trips without causing instability !