Quote:
Originally Posted by flattracker
I have not seen any post on this site about it, but you can in fact connect ordinary generators in parallel. The must meet requirements are that both are running very close to the same speed (and same frequency as a result) The second generator needs to have a slightly higher voltage.
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This is technically completely correct but I'm concerned it might be a little misleading to someone unfamiliar. At work paralleling generators is a routine task for me, although I generally am working with generators a little bigger than our buses use - typically anywhere from 25kW to 12.5MW, for electrical plants up to around 50MW which is the biggest I've dealt with so far - and that was 6 generators, at least 4 of which had to be running in parallel to make full rated power. The principles are all the same but the mistakes get more spectacular with the bigger ones. We often parallel multiple generators, and occasionally generators of different sizes can run in parallel too (with the appropriate setup). The way it works is that there is generally a bus connected to at least one online generator. The bus is supplying power to whatever electrical load needs to be powered, so all the electrical "combining" happens there. To parallel a second generator, once it is stable (running, up to temp, etc.), the voltages are adjusted so that the unloaded (offline) generator is a few volts higher, and the speed (measured in Hz/frequency of the AC wave output) is a few RPMs higher. It varies but to give you an idea, something like 1-2% difference in speed and voltage. Then the unloaded generator is connected precisely as the power output is in phase. This is critical because if they are out of phase you get an instantaneous torque loading which tries to instantaneously align the two machines which may be up to 180 degrees apart in their rotations. If you are lucky it just knocks the oncoming generator back off the bus. If you are less lucky it knocks the online generator off too, or even worse breaks something important, like a crankshaft or some piston pins. Anyway, assuming that doesn't happen, you have now got your generator paralleled to the bus. If they are running in parallel, then by definition the voltages and frequencies are identical, but your work is not finished. Despite voltages and frequencies being in parallel, the loading is likely unequal. To visualize this, picture two people pushing a stalled car. If the car is rolling and both people have hands on the bumper, they are "in parallel". But you want both people contributing equal effort, not one doing all the work (risking an overlaod) and one walking effortlessly alongside. So because different generators and different governors have different droop characteristics, your oncoming generator might be taking too much or too little of the load. This is adjusted by trimming speed and frequency up or down. Since all generators on the same bus are now locked together in phase, by increasing speed setting you are mostly just increasing the load assumed by that particular generator. Once you get that sorted out, you might have your generator ouputs equal, but voltage and frequency might be higher or lower than what is nominal. So adjustments are then made to correct that.
In modern versions of the systems I work with this is all handled automatically with the entire sequence initiated by one or two button pushes. Honda and Generac and the other little inverter generators that offer parallel capability can avoid a lot of that through their DC-AC conversion, because there is no phase compatibility to worry about with DC power, but the AC-DC-AC conversion also introduces a lot of inherent inefficiency. That's the tradeoff for simplicity I suppose.
I have never tried to parallel a generator that was not meant to be paralleled so I cannot speak from direct experience. With that caveat... If you have small enough, similar enough generators I think you MIGHT be able to just wire them together with no precautions and it MIGHT work out just because the components and therefore inertial forces are small enough that they MIGHT handle that instantaneous torque and resultant acceleration without ripping apart or overloading and shutting down, but I think that would be leaving a lot to chance and the risks range from destroying your equipment to everything that comes with really sketchy electrical wiring practices. Perhaps someone has actually tried it and had success but I'd still be pretty hesitant to attempt with my own gear, and if I did try it I would definitely be wearing safety glasses. Of course it is possible to engineer proper controls just like big generators have, but by that point you might be economically much better off just buying the proper generator.
Anyway, that's only my $0.02. I'm not an expert on generators or electrical installations by any means, but I have manually paralleled many different generators a few hundred times. I hope I have provided food for thought if anyone is considering creative ways to parallel their generators. FWIW I'm probably eventually going to replace my 10kW Onan with a Honda EU2000 series, or perhaps a similar model from one of the cheaper brands. And supplement with solar.
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