I have just had a look at the list of PLC/design jobs that I have done over the last 10 years. There are no less than 132 generator jobs done in that time. They are all PLC controlled. There have been with Cummins, CAT, Perkins, MTU, MAN, Ruston, Detroit, English Electric, Allen, Dorman, Paxman sets and a few others.
Have never had a problem with a proper design. Most systems have been multiple engine with synchronising and load sharing (use specialised modules for this and then a PLC bypass system can be designed in), SCTT (soft transfer), Co-generation, base load power stations, 415V generation, 415V with step up to 11kV, 11kV generation, laod control, load shedding, capacity control, auto start up after a shut down (base load power stations), building load control (up to 11 networked PLC's) - you name it.
In all this time the only real PLC problems I have had is an occassional faulty power supply.
SCADA systems are nearly always used over the top of complex systems. I always put an "automatic return" button on the control cubicle in case Mr Gates operating system does what it does best. That way they can put the building, or whatever, back to normal without the computer.
Have used remote I/O etc (usually Device Net). The largest distributed I/O system I did for one of these jobs was 92 Device Net I/O blocks into 1 PLC plus a total of 23 Allen Bradley Powermonitor 3000 devices, also on Device Net into the same PLC. There were 9 Device Net scanners on the backplane. Works like a charm. The AB PM3000 units are addressed by explicit messaging. Data is taken into the PLC and then extracted by Citect SCADA on an Omron network. Updates to the SCADA from each of these devices is better than twice a second. Awesome.
Have used AB, Mitsubishi, Hitachi, Siemens, IDEC (Izumi), Schneider, GE 90-30, and a couple of others over the years.
The systems are very reliable and run on battery (for obvious reasons). Batteries are not a problem at all provided proper maintenance checks are carried out. We recommend that every 3 months a discharge test is carried out on all battery systems. The data is then entered into an Excel spreadsheet for each battery bank. If there is a noticeable drop in performance from a battery set this can be shown in Excel as a chart and the client can then decide whether to replace the batteries or not. I have not experienced a catostrophic battery failure at ANY site where this has been the practice. Have seen plenty of catostrophic battery failures where the client has decided to "save a buck"!!!!
During these years I have also formed an opinion about the most reliable PLC DC power supplies, due to monitoring any failures. I will not go into that here but suffice it to say I now have a favourite brand for lots of reasons, not the least of which is reliability.
I cannot remember the last time I did a generator job without a PLC, including several fairly large base load power stations. Nearly all output cards are transistor with diode protected relays to drive load. A lot easier to replace an external relay than have to pull a relay out of a PCB in the middle of the night. Yes, Murphy was an optimist - it always happens in the middle of the night.