Paralleled 120 Vac/ 60 Hz phase angle 1

[aegcmac]

I'm in unfamiliar territory on this particular matter. I have a case involving a privately owned, off grid power system and we want to parallel two 120 Vac sources; the AC main (sine wave) and a small sine wave inverter. We only need 100 to 200 watts from this 120 Vac inverter. I've got control of the DC input to limit the output of the inverter, and I've got control of feeding or not feeding the main if the main goes down. I've got the two voltages within 4 to 5% and frequency within 1 - 2%, but I'm not sure the easiest way to auto-eliminate the phase angle between these two AC supplies.

As I understand it, if I can get the phase angle to 5% or less, then it will close a sync check relay and lock them in and they'll stay locked. The issue is getting them to as close to 0 phase angle as possible since they're both on their own clock and not ticking at the same time - so to speak. You guys understand this better than I do. I understand they used to use light bulbs to get two AC alternators in sync years ago.

The time to do this is not at issue even if it took several minutes, etc, but it has to happen without manual adjustments. I'm trying to do it without a series of more complex components (more complex to me). One of my thoughts was a time/relay/switch inserted on the inverter's PCB that activated the inverter's AC output, where it would be On for a half second and if the synch check relay doesn't close then it turns Off a half second and turns On again, etc. thinking that at some point it would turn ON and I'd have less than 5% phase angle. When the sync check relay eventually closes this event would be able to also stop the timer/switch from turning off the inverter. Of course, I have no idea how long it would take to trip the sync relay, since it would be like a dice roll or random event. So I don't think this would work very well.

I was also wondering if at some point the phase angle would smooth on it's own and eventually trip the (500ms) sync relay or if there's a severe 50 degree angle would it always be at more/less 50 degrees if both sources remain active?

Any thoughts, references or known circuits of a relatively simple way to get rid of most or all of the phase angle would be appreciated. I know of an auto-synchronizers for like $1000, but I was hoping to do this myself and learn something from it.

 

[OperaHouse]

Not sure I understand the aplication. Do you only need to supply 200 watts? I know one manufacturer that runs the inverter all the time so it doesn't have to sync and just powers the inverter from the line.

 

[walker1]

I might also go for the UPS-like version OperaHouse mentions.

Getting one of the cheaper off-the-shelf inverters to sync to the mains grid is usually not an option.

Also, they usually create a 'semi-sine wave' output.
Something like an '0 - positive DC - 0 - negative DC' sequence, I suspect.

What kind of load would you be connecting to the output ?
Light bulb, computer, stereo, power tool or .. ?

If the load can accept DC (lighting (not neon) or off-line switch-mode supply) there may be other options available.

 

[aegcmac]

Thanks for the input. This is a unique application that requires I bring the small sine wave inverter on line periodically vs. just solely powering dedicated load(s)with the inverter or having the inverter always on line, etc. It falls between the cracks of UPS systems, stand alone battery input based inverters and those inverters that are solely designed to sync and back feed into the grid, but cannot operate a load directly since their input is variable and can drop off significantly at any given time.

Yes, I'm trying to parallel connect 100 to 200 watts to the Main and this small inverter does produce true sine wave because of the devices it has to operate whether paralleled on Main line or used remotely as a independent power supply. I can't state the exact devices it operates due to very serious non-disclosure issues, but the small inverter does have to output into a 120 Vac environment vs. seeking a point where DC could be used.

I'm still wondering if they'll come into sufficient phase sync on their own for a brief .25 to .50 second, but I don't yet understand whether the phase angle "moves" and gets worse only to eventually come around to a point of sync before continuing to move on and get worse, etc or whether it's a fixed relationship that can only be changed with intervention.

 

[Chuckles159]

Unless the frequency is absolutely identical, there will be phase drift over time which will cause all sorts of grief. I miche consider using a PLL syncing off the AC line to contreol the inverter. Then, when the AC line goes away for a short time, the frequency of the inverter should only drift a bit. The catch is that when the AC line came back on, there may be a sync problem. If the AC line went off, a relay should drop out. When the AC line comes back on, the relay won't connect it until the PLL has rematched the phase of the inverter with the AC line. Once the phase is established correctly then the relay can connect the AC mains to the load again.

You will need to modify the inverter to run off a PLL and you will have to monitor the LOCK line of the PLL, if it has one, to allow the mains to only connect to the inverter when the AC line and inverter are in sync. This is not a trivial job to accomplish and if it does not work right, there will be smoke and fireworks.

 

[aegcmac]

Thanks, Chuckles159.

I just saw this phase drift from two non-identical frequencies you mentioned while dual-tracing two AC sources on an O'scope. At first I thought they would lock phase and freqeuency once the sync check breaker tripped if the phase and frequency were about 2% of each other, but now I see they likely won't lock and stay locked without additional control like the PPL. It'll just drift off and open the sync check relay. I'll probably smoke a small sine wave inverter just to be 110% sure I have to go with a PLL. It'll be worth it to know 110%. The PLL is a lot more complicated for me to configure, and probably beyond my expertise...

 

[Rodmcm]

Talk to Vectec or ASEA in Calafornia. Both are manufacturers of inverter systems that synch to runninmg supplies for the marine industry *shore power converters. The synching is not the problem, the problem is that the inverter has to have a droop characteristic if it is to remained locked to the grid. The problem is that as the grid surges there is an attempt to backfeed into the inverter. This results in a high DC voltage and a loss of the inverter