Two Honda Inverter generators in Series for 240V

[HomeGen]

Do you think this will work?

I see some chat about using a large transformer (read expensive) rated for generator output to get split phase +/- 120V (for 240V), this might be a less expensive alternative, using a small low cost 1:1 transformer that just provides the 180 degree phase shift required for the second generator to synchronize 180 degrees out of phase with the first one.

https://imgur.com/a/c6I1Wif

 

[steve66]

No.

 

[steve66]

So much is wrong, but one issue (and this would probably apply to any attempt to use 2 generators to create 240 Volts) is that the generators won't be in sync, and the output voltage would fluctuate between 0 volts and 240 volts.

I could list about a dozen other problems with this. I hope you were only asking in a hypothetical sense, and not actually planning on trying something like this.

 

[qcroanoke]

It won't.

 

[synchro]

Many inverter generators can be put in parallel, and the second generator that is started synchronizes with the first. However, the current sharing between them relies on the known output impedance of the particular generator. A small transformer between them would add a substantial impedance, and so I'm not sure how the control loops of the generators would react to that. Also, with the higher impedance in between, inverter generators may not sense they are in parallel, and therefore will not attempt to synchronize. It's possible a small transformer could then be damaged if the overcurrent protection is not adequate for it.

That said, there might be a suitably sized transformer where this could work, but that would be somewhat of a crapshoot.

 

[HomeGen]

Perhaps I should have given more information about these Honda Inverter generators. They are designed to synchronize to each other, they actually have dedicated terminals and a factory umbilical cord specifically for that purpose. So unquestionably they will synchronize to each other. The question is can you trick them to synchronize 180 degrees out of phase instead of at 0 degrees, by using a transformer to provide the required 180 degree phase shift as shown in the original sketch?

I did read "somewhere" can't find the article now, that someone did just that.

I'm not sure how the synchronizing electronics in the inverter work, however it's almost certainly based on sampling of voltage and so if you provide that voltage shifted by 180 degrees then there is a decent chance it will work.

 

[synchro]

I don't think the key issue is that the transformer is wired for the generators to be 180 degrees out of phase with each other. It shouldn't matter whether the transformer is wired for 0 or 180 degree phase shift. It's just that the transformer presents an impedance between the two generators, and so the question is how the control loops of the generators would react to that.

There is a patent from Honda about paralleling generators I saw a few years back. If I remember correctly It showed a current vs. voltage plot with a slope due to an impedance, and how the control loop would use that to appropriately control the two generators. Both voltage and current sensing were utilized.

 

[HomeGen]

I don't think the key issue is that the transformer is wired for the generators to be 180 degrees out of phase with each other. It shouldn't matter whether the transformer is wired for 0 or 180 degree phase shift. It's just that the transformer presents an impedance between the two generators, and so the question is how the control loops of the generators would react to that.

There is a patent from Honda about paralleling generators I saw a few years back. If I remember correctly It showed a current vs. voltage plot with a slope due to an impedance, and how the control loop would use that to appropriately control the two generators. Both voltage and current sensing were utilized.

Well that is a good point, and the reason I started this thread to see if others have either tried or can see an obvious flaw in the concept. I wish I could find the article I read that described it working, IIRC it was written by one of the engineers that was involved with the actual inverter design.

I suppose I can try it and see what happens, while monitoring the 240V output on a scope, unloaded, loaded and with step changes in load. If I try with resistive loads (heaters) and if by chance it does work, then try starting a motor on one leg to see what happens when you add some inductance. I have a couple of HV differential scope probes and several 1500W heaters.

 

[jaggedben]

Many inverter generators can be put in parallel, and the second generator that is started synchronizes with the first. However, the current sharing between them relies on the known output impedance of the particular generator. A small transformer between them would add a substantial impedance, and so I'm not sure how the control loops of the generators would react to that. Also, with the higher impedance in between, inverter generators may not sense they are in parallel, and therefore will not attempt to synchronize. It's possible a small transformer could then be damaged if the overcurrent protection is not adequate for it.

That said, there might be a suitably sized transformer where this could work, but that would be somewhat of a crapshoot.

Can you elaborate on why the bold part would be necessary? Or is it just one way to do it that Honda may have chosen?

Thinking of Enphase IQ8s or Powerwalls and why each one would have to know how many others are in the system. Skeptical that they have to know that. Or maybe I'm misunderstanding something about what you mean by output impedance of the generator.

...

As far as the OP ... I wouldn't be surprised if it works with no load or even a 240V load, but the control loop craps out once you have an unbalanced 120V load.

 

[HomeGen]

...

As far as the OP ... I wouldn't be surprised if it works with no load or even a 240V load, but the control loop craps out once you have an unbalanced 120V load.

Quite possibly, it would be interesting to know how sophisticated the control algorithm is. Thinking about the normal parallel operation in simplest terms if they simply kept the two inverters synchronized then to some extent they would share the load by virtue of the voltage drop from the inverter output stage to the paralleled terminals (as load increases voltage drops and the other inverter picks up a bit more of the load). In actual practice the voltage might be tweaked up as load increases, and that is where the transformer might interact with the control process in unexpected (for the controller) ways.

 

[synchro]

Thinking of Enphase IQ8s or Powerwalls and why each one would have to know how many others are in the system. Skeptical that they have to know that. Or maybe I'm misunderstanding something about what you mean by output impedance of the generator.

I believe both of these use data communication between them to enable paralleling of their low impedance outputs while also achieving good current sharing between them.

As I'm remembering what the Honda patent described, no data communication between inverters generators was necessary. There was a sequence needed so that one had to be started first to act as the master. Then when the other one was started, it determined that because voltage was already present at its output, parallel operation would be needed. Then it would do some sensing of current vs. voltage so that it could adapt to effectively share the load. Synchronization was a separate function.

With the smarts of the Enphase IQ8s and Powerwalls no such sequencing is necessary. Certainly inverter generators could be made this way, but Honda has had generators that can be paralleled since 1996 and so they had to use simpler technology. Perhaps there are some inverter generators that communicate data between themselves and use that for real-time control, but there may not be a business case for having that capability.

 

[jaggedben]

I believe both of these use data communication between them to enable paralleling of their low impedance outputs while also achieving good current sharing between them. ...

Hmm. I doubt it. Enphase PLC sure seems to be way too slow to respond to changes in load that way, for example.

I surely just don't understand the issues mathematically, but I would have thought that as long as all inverters work to maintain the correct voltage then current sharing would happen, er, naturally.

 

[synchro]

Hmm. I doubt it. Enphase PLC sure seems to be way too slow to respond to changes in load that way, for example.

I surely just don't understand the issues mathematically, but I would have thought that as long as all inverters work to maintain the correct voltage then current sharing would happen, er, naturally.

We'll all I know is what Enphase has to say about it:
"Our newest IQ8 Microinverters are the industry's first grid-forming microinverters with split-phase power conversion capability to convert DC power to AC power efficiently.

The brain of the semiconductor-based microinverter is our proprietary application-specific integrated circuit (ASIC) which enables the microinverter to operate in grid-tied or off-grid modes. This chip is built in advanced 55nm technology with high-speed digital logic and has super-fast response times to changing loads and grid events, alleviating constraints on battery sizing for home energy systems."

Without any technical details it's hard to say what that means. I suspect that is a mixed-mode (analog and digital) ASIC, perhaps with A/D and D/A converters on it. It's kind of hokey because the reason for using a 55nm device gate length process wouldn't be for speed but for a smaller chip area, and therefore a lower cost.

 

[wwhitney]

I surely just don't understand the issues mathematically, but I would have thought that as long as all inverters work to maintain the correct voltage then current sharing would happen, er, naturally.

Working this out as I go, but:

Say your voltage source is modeled as an idealized source of voltage V and a series resistance R. Say you have two sources 1 and 2, with the same voltage (and in sync), but different series resistance R1 and R2. And say you parallel the sources, which means paralleling the two V-R series circuits and then connecting that to a load.

Then in order for there to be a single defined voltage across the (aggregate) load, the voltage drops across R1 and R2 must be equal, since the idealized sources have equal voltage. That means that the current through the load divides between the two sources in inverse proportion to the resistances. I.e. if R2 is twice R1, then voltage source 2 sees half the current that voltage source 1 sees.

Cheers, Wayne

 

[OldBroadcastTech]

Do you think this will work?

I see some chat about using a large transformer (read expensive) rated for generator output to get split phase +/- 120V (for 240V), this might be a less expensive alternative, using a small low cost 1:1 transformer that just provides the 180 degree phase shift required for the second generator to synchronize 180 degrees out of phase with the first one.

https://imgur.com/a/c6I1Wif

1) I know NOTHING about the generators in question
2) or the method of synchronizing them

But it would seem to me that they would be sync'd by feeding a low-voltage sample of the output of Unit 1 to the inverter controls in Unit # 2. to 'lock' them in sync.
If so, it should be possible to insert a small 1:1 transformer in the SYNCHRONIZING jumper, inverting the sync signal, and thereby 'inverting' the output of unit # 2.

Just a SWAG, and worth exactly what you paid for it, $ 0.00.

 

[jaggedben]

Working this out as I go, but:

Say your voltage source is modeled as an idealized source of voltage V and a series resistance R. Say you have two sources 1 and 2, with the same voltage (and in sync), but different series resistance R1 and R2. And say you parallel the sources, which means paralleling the two V-R series circuits and then connecting that to a load.

Then in order for there to be a single defined voltage across the (aggregate) load, the voltage drops across R1 and R2 must be equal, since the idealized sources have equal voltage. That means that the current through the load divides between the two sources in inverse proportion to the resistances. I.e. if R2 is twice R1, then voltage source 2 sees half the current that voltage source 1 sees.

Cheers, Wayne

So if the only difference between R1 and R2 is a few feet of wire, the current should be pretty equally shared. Pretty much what I thought. I suppose if the generators had really different internal characteristics then this could be different but if they are all the same it's not really an issue?

 

[TheCats]

I also used to think that the parallel kits were involved in sophisticated communication.

The paralleling kits for portable inverter generators are nothing more than shielded banana plugs and wires. Some kits have the wires joining at a higher amperage receptacle, while others are just a cable connecting the two generators, with one of the generators providing a higher amperage connection (the Honda "companion model") .

Internally the parallel outputs are powered by the same wires leading to the receptacles, with various circuit breaker schemes in use. The inverters limit their own output power, so the circuit breakers are either redundant or simply protecting the wiring/outlets.

All 'communication' is done by phase/frequency, similar to wide area grids. An unloaded inverter generator free-runs slightly faster than 60Hz. As it reaches its maximum output, it drops the output frequency to run slightly below. The clever bit is that the 'slave' inverter is connected before its prime mover (engine) is started. It recognizes that there is already a frequency reference and matches the phase as it starts up. (Non-inverter generators require physical matching frequency and exact phase, else Boom.) The inverter potentially modifies its behavior, either frequency or response, to avoid shuffling the load cyclically between generators.

 

[steve66]

I have seen an electrician test an ATS that was hooked up incorrectly.

When he pressed the "transfer to generator", the load got disconnected, while the generator was connected in parallel with the utility source.

I fully expected that to blow a fuse on the main service disconnect, but the generator synched with the utility all on its own and ran perfectly happy until the EC realized what was going on.

But in that case the utility supply was much larger and stiffer than the generator. So that might make a difference.

Looking back at our diagram with the 2 generators and the transformer, the transformer has a primary and a secondary. When one gen on the primary side is started, the transformer secondary side becomes a voltage source. That voltage source is in parallel with the other generator.

In general, its bad to parallel two voltage sources - their voltages can't be exactly equal, and the difference in voltage causes a current to flow that is typically only limited by the resistance (or impedance) of the source, which is typically low. (And don't forget - we can't start both generators at exactly the same time, so for a while, the current is only limited by the impedance of the generator windings).

So generators in parallel not only have to sync, and have similar impedances, but their output voltage must match very closely.

But we can imagine that as one generator puts out more current, there will be more torque on the shaft. That will in turn slow it down slightly, or maybe just cause its phase angle to lag the other genrator by a little bit. That reduces the current from that generator, and increases the current from the other generator. So we can see that their voltage and timing will tend to synch together, even if they don't specifically have a circuit made to do this.

But I wouldn't try this at home. Even if it would work in theory, I still think there are other practical issues (aka - Safety Issues) with this setup. So please don't try this at home.

 

[jaggedben]

I have seen an electrician test an ATS that was hooked up incorrectly.

When he pressed the "transfer to generator", the load got disconnected, while the generator was connected in parallel with the utility source.

I fully expected that to blow a fuse on the main service disconnect, but the generator synched with the utility all on its own and ran perfectly happy until the EC realized what was going on.

Is that something that happens naturally? How did you know it was synced? Did you scope it?

...

But we can imagine that as one generator puts out more current, there will be more torque on the shaft. That will in turn slow it down slightly, or maybe just cause its phase angle to lag the other genrator by a little bit. That reduces the current from that generator, and increases the current from the other generator. So we can see that their voltage and timing will tend to synch together, even if they don't specifically have a circuit made to do this.

But these are inverter generators so shaft torque has got to be rather irrelevant.

But I wouldn't try this at home. Even if it would work in theory, I still think there are other practical issues (aka - Safety Issues) with this setup. So please don't try this at home.

I wouldn't try it at home, but I don't think I would be too afraid of trying it in a removed outdoor location with proper preparations. (That is, to be clear, if I was hellbent on satisfying curiosity and didn't care about the generator warranties or cost of transformer.)

Worst that could happen I think is that the transformer catches fire. I think the problem is calculating how big a transformer you actually need for that not to happen. Which probably isn't possible to calculate correctly without proprietary information, unless you just go big (full VA of generators) which is what he wanted to avoid. Assuming the inverters can be tricked at all.

 

[wwhitney]

Looking back at our diagram with the 2 generators and the transformer, the transformer has a primary and a secondary. When one gen on the primary side is started, the transformer secondary side becomes a voltage source. That voltage source is in parallel with the other generator.

Right, but we know these inverter generators are designed to be paralleled, and on startup they check for the presence of another generator to sync to. So the question from the OP is how that syncing works, and whether the inverted voltage waveform provided by the transformer is something that the second generator can sync to.

As for current sharing, that's only an issue for 240V loads on this 120/240V system that is trying to be created, but at first glance I haven't wrapped my head around how that is going to work out. Any 120V load will be on one generator or the other.

This has been an interesting question, as prior to this thread I just assumed that parallelable 120V inverter generators would support this inverse syncing configuration to create 120/240V split phase right out of the box. Just because it seems like an obvious application and I assume the electronics to do it would be simple.

Cheers, Wayne