20 years ago I thought I had a good set of "rules of thumb" for this, however with changes in the prime mover and generator iron, increasing complexity of installed power systems, and tighter voltage and frequency requirements for high tech loads sometimes what worked well for a number of years gets challenged a bit these days.
For smaller units with single phase sensing AVR's (like a VR4 or some of the current LS offerings) with SE excitaiton systems, 1 to 1.5 times gen kVA.
For mid to larger size units with three phase sensing SE excitation systems, 1-2 times kVA.
Mid to large size with PM excitation systems, 2-4 times depending on tail end design, some frame sizes did better than others, some PM's were maginally sized and fell off in output, some units were absolute beasts, like KATO and Baldor oil field specific units.
On older CAT systems units with the card type AVR (VR2 as it was called sometimes) seemed to do better, of course they were usually on prime or continuos rated tail ends with lots of iron and copper. The older VR1 (A1 module type AVR) would sometimes have problems with the harmonic content of the inrush and fall off rapidly, found the same issue with VR3's althought not quite as bad.
We did a number of large power module rental projects over the years and quite a few times had power blocks of 5 units with the first unit on line picking up the GSU, then paralleling the rest. Sometimes it worked great, other times we resorted to a dead bus close with the first unit on, or ramping if we had an AVR with the capability.
The CDVR with the excitation disable and soft start features certainly can provide a lot of possibilites, as long as it's properly done and the technicians supporting the project understand what you did.
Here is the key, at least in my opinion, understanding what the real load dynamic is on site. Inrush of a transformer is not a repeatable event, depending on the residual in the core, the installed system, such as cabling, static loads, and the types of loads and disconnect means you have in the distribution all impact how well your generator(s) will pickup the initial inrush and stay on line. Larger systems with MV distribution may also have issues with electrical protections, especially differential protections if not properly restrained and set.
Systems with a GSU at each unit and multiple step down transformers in a system can be especially challenging if a load shed system is not in use.
In general, if you have transformers to pickup in your system, make sure you have a quality three sensing AVR and a PM excitaiton system. My own expereince with AREP systems is mixed, I find they perform better than an SE system but not quite as well as most PM system, however I have recently found PM systems on CAT and other machines I find marginal in providing the needed VA to the AVR to allow a full field forcing condition. An SE machine with series boost is slightly better than SE alone, but at that point you may as well have gone with a PM.
A digital regualtor gives you more flexibility, as I mentioned before, ability to set cut off frequency, volts per hertz slope, soft start and PID tuning can give the ability to make a system work where an analog AVR won't.
Great topic, enjoyed the discussion!
Mike L.
United States / Canada
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