Tuesday, July 19, 2011

Off Grid Power Synchronization

I saw a question on an off-grid generator forum of which I am a member. The question went something like this: "I've seen a lot of information about solar (PV) systems, wind generators, and other offgrid power sources, but I haven't found much information about store-bought or home-built synchronizers for these power sources, beyond a few expensive inverters that have built-in synchronization. Where can I find more information on this subject?" I contributed my 0.02 there, then decided the subject was worth touching upon here as well. So, how do I synchronize my offgrid power sources?

That's what the inverter does. Solar, wind and micro-hydro energy is stored as DC in a battery bank, then drawn out as needed by the inverter(s), which inverts it to regulated AC. There are only two scenarios I am aware of that require synchronization: combining two or more generators when the output of one alone is insufficient for a particular load, or combining a pair of inverters for 120/240 volt single phase operation. As noted, inverters which can do this are expensive, so most North American off-gridders just run the generator when they need 240 volts.

Synchronizing a pair of generators can be a good idea, though. Let's say for example that you have a typical off-grid home that only needs a 5KW generator to power all its normal loads, whether it be the primary power source or backup to wind/solar/whatever. But let's say you also have a shop with a welder, air compressor, etc. You may need 8KW or more when you use the shop, even if you don't try to weld and run the compressor at the same time, and even if you totally disconnect the house loads from the generator while you are welding. Of course you could buy a generator big enough to power the shop, and you should if you plan to use the shop very much. Even better, you could have both a big genny for powering the shop, and a smaller one for efficiently powering the house. These could be on completely separate systems. That is a great way to do it for a couple of reasons. First, generators work better and use less fuel if they are operated at 75-80% of capacity. A generator uses a certain amount of fuel any time it is running. The greater the load on the generator, the smaller the percentage of total fuel usage is expended just keeping the generator online. It's kind of like a car: if it is not moving, it is getting zero mpg. Up to a certain point, the more load you put on a generator, the more efficiently it operates. A 5KW generator powers a 4KW total load quite efficiently, but a 10KW generator powering a 4KW load is wasting fuel. Second, even a very good quality 5KW generator is less expensive to buy, maintain and service than a 10KW generator. So why not put most of the wear on the smaller genny, and conserve the big one for the occasion that its capacity is really needed?
But the point here is generator synchronization, and if you have a generator that is big enough to power your heaviest load, you don't need to synchronize them.
Consider this scenario: instead of buying a big generator, you buy two smaller generators, each sized to power your normal household load. They can act as backups for each other, and you can periodically change which generator is active and which is backup, to equalize the wear on both generators. If one generator develops a problem that requires major service or even replacement, the homestead can continue to function normally on the second generator while the problem generator is in the shop. And here is where synchronization comes in: with a pair of 5KW generators, when you need to run the welder or other heavy load, you can run both generators to almost double the output of one generator alone. I say "almost," because nothing is 100% efficient, and you will incur some losses when you connect the two generators together.

So, how do you synchronize a pair of generators? The easiest way to do it is to buy a pair of generators that are designed to synchronize together, and use the factory synchronization cable. Some Honda generators are designed this way, and I'm sure there are others, as well. But it is possible to synchronize a pair, or even three or more, generators that lack the factory synchronization hardware.
One would assume that generator controllers would be available to the off-gridder who wishes to parallel two or more generators. Unfortunately, that is not the case, and for a very good reason: an automatic generator paralleling controller is a very complicated and expensive piece of equipment, and a manual controller requires a level of understanding of how generators work that the average generator owner simply does not possess. With that in mind, here is a description of how it can be done. Do not use this as a set of instructions. This is not enough information to constitute instructions. You should only attempt this if you are confident in your knowledge of generators and electricity and understand exactly what you are doing. If you do this and something goes wrong, I am not responsible.
With the forgoing in mind, here is how it works. Do not think of this as paralleling two identical generators; think of it as adding generators to a live bus. The live bus is one or more generators that are already running. To begin with, it will be one generator, and it should be the largest generator if they are not the same size. You will have to match the voltage, frequency, and most importantly, the phase of the generators before you connect them. To do this you will need three AC voltmeters. One goes on each generator, and one goes from one leg of the fist generator to one leg of the second generator. You will also need a frequency readout on each generator. You can get by with a multimeter that has a frequency readout, and use it to measure the output of each generator individually. But for the phasing meter (the one that connects between the two generators) you really need an analog meter that is capable of displaying double the voltage of the generators. An analog meter allows you to track the voltage changes; a digital meter doesn't. Some people have successfully used a pair of light bulbs connected in series across the two generators, but this is a pretty imprecise method.
A better way to monitor the generators, instead of a multimeter, is to use a dual-trace oscilloscope. If you do this, you should still use an analog voltmeter across the generators, just to make sure you are getting it right.
Another thing you will need is a voltage (field) control circuit on at least one generator, and governor speed control of the generator you will be adding to the live bus.
Once you have the two generators connected to the load, you will need a means of measuring the current each generator is supplying.

Start both generators and allow them to warm up. Connect the most powerful generator to the bus, and adjust its voltage and frequency to the correct levels. With the second generator running but disconnected, adjust its speed so that its frequency exactly matches that of the live bus. Then adjust its voltage to also be the same as the live bus.
Now manipulate the governor control of the unconnected generator so that its frequency and voltage lags that of the bus. What will be happening at this point is best described in racetrack terms: the bus will be "lapping" the unconnected generator.
Watch the analog voltmeter that is connected across the two generators. The voltage will be rising to the combined voltages of the two generators (double the voltage of one leg) when the generators are 180 degrees out of phase, and falling to zero when the generators are in phase, then rising again. Gradually increase the speed of the unconnected generator so that this rise and fall is as slow as possible. Then when the voltage is at zero, connect the second generator to the bus. At this point, you can begin adding loads to the bus.
Monitor the current drawn from each generator, and try to maintain equal load percentages on each generator. For example, if the first generator can handle 7,500 watts, the second generator can handle 5,000 watts, and the load is using 3,600 watts at 240 volts (15 amps), the governor of the second generator should be adjusted so that it supplies approximately 6 amperes, with the remaining 9 amps being supplied by the larger generator. Theoretically, that should allow the two generators to respond similarly when you apply a larger load. It will not be precisely what the calculations say it will be, but so long as you monitor it to prevent either generator from overloading, it will be fine.
If necessary, you can add a third and even fourth generator to the bus in the same manner. Just keep in mind that losses increase and efficiency goes down with each generator you add.

There is one other way to increase capacity of your setup while actually increasing the efficiency in the low-power mode. I will address that in a later article.

No comments: