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Forum Index : Electronics : Inverter synchronising
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RFburns Regular Member  Joined: 21/07/2017 Location: AustraliaPosts: 43 |
Well I don't often post here; being "Strong like horse and smart like tractor  " but inverter sync is nothing new .Maybe this will help- volt sync or this sync 2 Oh forgot to add for those that prefer a hardware solution there plenty of off the shelf zero crossing detectors Strong like horse smart like tractor! |
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| Solar Mike Guru  Joined: 08/02/2015 Location: New ZealandPosts: 1123 |
Back when I used to work at the BCNZ radio transmitter site, the 500KVA 3Ph generator was tested on full site load every month. We had to start it (compressed air start) and manually sync it with the incoming mains feed, then gradually take the full load and finally trip out the incoming breaker to be mains free. The controls were two buttons for generator speed (frequency), Up,Down; two buttons for alternator voltage Up,Down; a rotating 360 degree synchroscope meter and the incoming load and generator load meters. Procedure was always after start, adjust frequency via sync-scope so was slightly faster than mains, set voltages same. At zero crossing of 'scope close genset contactor then adjust speed (freq) so back in sync, then with short stabs of the Up voltage button allow the genset to start taking load off the mains, observing the two power meters and scope; all going well over a minute or so mains load would drop to virtually nothing and genset would increase; then the mains contactor would be opened. After a couple of hours testing the reverse would be done to re-sync with the mains and smoothly switch back with hardly a blip in the lights. So if two inverters were to share the load for any period, only one would require active adjustment, both would have to remain in phase lock and a control system would have to monitor the current load on each and continually adjust the voltage on one so that they both shared the load. Cheers Mike |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
We did pretty much the same thing down in the Antarctic, where all of our power came from diesel alternators. Running two machines in parallel to share the load is no big deal, as was starting up a second diesel, running both together in parallel, then shutting down the first one for regular routine servicing without any interruption of power. The whole grid works that way too, with power supplied from different kinds of generation sources in very different physical locations. During periods of peak load, extra machinery can be brought on line, for example from hydro power, or in Melbourne gas turbines. In other places it might be wind generators. How all this works is not usually understood by people that have never done it, or been shown how it all works. But a synchronous alternator has a dc field winding and it can be run either as an alternator or equally well as a three phase motor. Its completely bi directional in power flow. So when you close that main circuit breaker and connect two machines in parallel its EXACTLY like linking the two machines together with a mechanical clutch. So what you must do, is the exact procedure Mike speaks of above ^^^. Get both machines running very close to the same speed, adjust the voltages to be the same. And at the exact instant when both are exactly in phase close the circuit breaker. The two machines are now solidly linked together electromechanically. The first machine is still supplying all the load. All of the torque, and all of the amps. And the second no load at all. You could open the circuit breaker again if you wanted to, and sever the coupling. Now the trick is to transfer some load onto the second machine. Either half of it, or perhaps all of it. The way that is done is to transfer the alternator drive TORQUE across, and the amps will follow. Very gradually open the throttle on the second machine and drive torque will transfer smoothly across. The frequency may rise very slightly. When all of the load is off the first machine, you can open the circuit breaker and shut it down. Many people are surprised to learn that its not the voltage adjustment that is used to transfer the load across. With a dc circuit that would be the only way to do it, but with ac synchronous machines, its the throttle and the drive torque that must be adjusted. Cheers, Tony. |
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Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
@RFburns, That is very helpful thank you. The article on syncing 2 or more inverters is designed for syncing inverters where the frequency differences or very slight. So just syncing the zero crossing is adequate, with a generator frequency can and will change a lot more than the very small changes between 2 electronic inverters of the same type. I think Poidas frequency syncing is on the right track. @Solar Mike, That is a good description of the process, in that situation you have the advantage that the generator will fall into line with the mains AC if it is close enough. Once they are connected together the generator can act as a motor or generator to a point to keep in sync. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
If you are going to try to sync an inverter to a small portable generator, it absolutely must be one of the newer inverter generator types that has a constant stable output frequency. The older type of dc excited alternators will have an output frequency that changes directly with rpm. And rpm goes up and down with load changes, no matter how good the mechanical speed governor. Even the slightest phase difference will cause a massive load shift, even if the voltages are well controlled. It will be just about impossible to do any load sharing with sudden load changes, it may end up becoming highly unstable and start surging until either a fuse blows or breaker trips. Another thing that does not work, and can never work is using SCR or Triac phase control on the output of a small generator, unless its the newer inverter generator type. The SCR gate trigger is usually some fixed delay after the previous zero crossing. As the frequency falls triggering becomes earlier, and the load increases. As the frequency rises triggering becomes later, and the load decreases. A mechanical speed governor just cannot cope with that situation, and it will surge horribly no matter what you try to do. I have seen engineers that should have known better spend days with boxes of governor springs and weights, and resistors and capacitors trying to make this work. And it never does. It would be nice to be able to synchronise your inverter to a put-put, and it may even be possible if the whole thing is thought right through. But there are evil combinations that can never work, and you will be wasting your time trying to fix it with clever software. Cheers, Tony. |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
Trace had no problem syncing to a "put-put". Another is the Invetek sold by Jaycar. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| BenandAmber Guru Joined: 16/02/2019 Location: United StatesPosts: 961 |
I'm new to all this and this is probably just showing my ignorance but can that generator not be rectified into DC and then used I am either going to build a gasoline powered battery bank charger or if converting my old Onan into the DC will work I don't have to build anything be warned i am good parrot but Dumber than a box of rocks |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
Yes, you can rectify the AC output and then feed the DC into a grid-tie inverter, but that's the easy way. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Even a sophisticated grid tie inverter may have an attack of the horrors if it tries to synchronise with a crude wobbly old put put. Anyhow, the way I see it, there are two fundamental reasons to have a standby put put with an off grid system. 1/ Independently power something really nasty such as a stick welder that may give the main home inverter a heart attack. 2/ Recharge the main battery fairly quickly when the sky goes awful. And as Ben says, a put put and a BIG dc battery charger with real balls, can restore the main battery to almost fully charged condition fairly quickly without having to synchronise with anything. That is how I intend to do it with a 5Kva Wisconsin with electric start powered from natural gas. It will have two outputs, 230v single phase and a nominal 100 volts dc. Cheers, Tony. |
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