How does an inverter synchronise to a generator?

[Browse]

So the setup would change to this:


The other option would be to use the output of the Honda to power an AC to DC converter/charger, connected directly to the battery bus bars, but this would be less efficient and would require additional purchases of the charger(s).

 

[timselectric]

Your AIO can't charge from AC, unless the loads are powered from AC passed through.

 

[Browse]

Your AIO can't charge from AC, unless the loads are powered from AC passed through.

So, it's either inverting AC to DC for charging, or it's inverting DC to AC for powering loads, but it can't do both at the same time...

 

[RCinFLA]

How does this affect the loads attached to the inverter, are there any loads that will not like the slow phase change (eg electronics / motors)

That is one reason the phase tracking is slow, so it does not disturb the AC motors too much. If phasing is changed too quickly the AC motor will create a surge current, either push or pull, depending on phase/freq shift direction.

For AC coupling of a PV GT inverter to output of hybrid inverter, this phasing slew rate limit also limits the frequency shifting power control on PV GT inverter. This creates a potential problem in quickly responding to a sudden PV GT overproduction backfeed, like might happen when a large house AC load is shut off. If grid is not present to absorb excess PV GT inverter's power, the hybrid inverter must suck it up and push it to battery.

 

[RCinFLA]

What if the inverter is set to SBU in setting 1? Will it convert the incoming AC to DC like it normally does in charging mode?

Solar before Utility is a software managed feature.

On HF AIO's, the PV power is delivered to inverter's HV DC internal bus. The inverter's sinewave PWM H-bridge AC converter can pull from this HV DC bus and the battery to HV DC converter can pull from HV DC bus to charge battery.

There is some design variations between different HF AIO inverters on how AC input is rectified and supplied to HV DC bus. This may allow AC charging and PV charging supplementing at the same time or not. It may also allow PV battery charging and AC output supplementing or not.
Cheaper units often only allow one or the other. It can also affect the AC input charging power factor.

Most HF AIO have a battery to HV DC converter mode switching to change from sourcing HV DC from battery or pulling from HV DC bus to charge battery. This takes a little time to switch modes so if it is charging battery, it cannot instantly switch to supplementing AC output for a feature like load shaving. SolArk and Deye have massive HV DC capacitor bank to supply power during the battery to HV DC converter mode change.

 

[Zwy]

So, it's either inverting AC to DC for charging, or it's inverting DC to AC for powering loads, but it can't do both at the same time...

There is a thread where SUB was used on a EG4 it did show charging occurring for the batteries, PV was producing and AC was inputted. https://diysolarforum.com/threads/i...500-and-other-eg4-inverters.53312/post-680073

 

[timselectric]

There is a thread where SUB was used on a EG4 it did show charging occurring for the batteries, PV was producing and AC was inputted. https://diysolarforum.com/threads/i...500-and-other-eg4-inverters.53312/post-680073

His unit doesn't have SUB mode.

 

[timselectric]

So, it's either inverting AC to DC for charging, or it's inverting DC to AC for powering loads, but it can't do both at the same time...

Correct

 

[timselectric]

Generator powering a separate charger is your best option. Sized for your Generators best efficiency.

 

[Zwy]

His unit doesn't have SUB mode.

You are correct, didn't catch that it was a GW 12K LF inverter. Manual shows SOL, Uti, or SBU.

 

[RCinFLA]

I've always assumed it's not as difficult to convert generator (variable) AC -> DC Charging since there's no sine-wave to match and I image that variances in voltage are less critical.

Is this true? or is it in a similar relm of sophistication as discussed above in the hybrid case?

That assumes the AC to DC conversion is done by simple rectifiers. Most hybrid inverters do synchronous rectification of AC input. This gives charging from AC input a good power factor. It requires the inverter to sync to AC input to synchronize the synchronous rectifiers.

Some very cheap HF inverters use IGBT body diodes for simple rectifiers during charging. They have poor AC sourced charging power factor.

 

[Browse]

Strange, in the manual this SbU mode indicates that it will use solar first, then battery, then utility only if battery falls below a specified level:



One would think that if a generator was connected to the inverter input, that the inverter would use battery power for the loads and the generator for charging, but I guess that's not the case if it can't invert AC to DC and DC to AC at the same time.

 

[timselectric]

It would be a nice option. But, I guess that the price would be higher.

 

[RCinFLA]

What would happen if you ran a 240VAC inverter generator into a 240VAC battery inverter while the battery inverter was providing power to a load center? Would the battery inverter pass through the generator power or could you force it to use the input power for battery charging only? I have a Growatt 12k LF inverter and a Honda EU7000is inverter generator and was wondering about how best to recharge the batteries while the inverter is powering the house. The way I have it set up now, it's either the inverter powering the house or it's the Honda powering the house, with the inverter taking some of that power to recharge while it's output is disconnected. This severely limits how much load I can have on when using only the generator. It would be cool to use the Honda as auxiliary only to reduce the draw on the batteries while the Growatt is powering the loads.

View attachment 130608

This setup is a bit dangerous.

If you have inverter AC input plugged in, synchronized to AC input with pass-through relay closed, and pull AC input plug, it can take some time before inverter's pass-through relay releases. This means the inverter's AC output will be connected to AC input through pass-through relay and AC voltage will be present on the male plug for the time until inverter recognizes AC input has been removed.

Zero AC input current is a legitimate condition and does not cause inverter to release pass-through relay. Open AC input is sometimes hard for inverter to detect.

If you have AC load due to charging battery it will release pass-through relay fairly quickly.

If you have near zero AC input current (like battery fully charged), the inverter pass through relay will stay engaged and inverter will gradually wander off in AC frequency until it hits out of frequency range limit and disconnects pass-through relay. This can take some time (possibly tens of seconds). Meanwhile, the exposed AC input plug male prongs have 240vac on them.

 

[Mattb4]

Strange, in the manual this SbU mode indicates that it will use solar first, then battery, then utility only if battery falls below a specified level:

View attachment 130633

One would think that if a generator was connected to the inverter input, that the inverter would use battery power for the loads and the generator for charging, but I guess that's not the case if it can't invert AC to DC and DC to AC at the same time.

Can be confusing but if you read it enough times you will find that no where does it say it will charge from utility (AC in) when operating inverter from PV or battery. It states, "Utility provides power to loads (this includes any battery charging) only when battery voltage drops to either low level warning voltage or setting point in program 12 is reached." (sic) I don't have this particular manual in front of me but I am guessing program 12 is the battery to utility transfer setting.

 

[JohnWhitcraftPE]

You are referring to a battery equipped hybrid inverter that accepts AC input from grid or generator.

Hybrid inverter has inverter permanently connected to AC output. There is a connect relay between AC input and inverter. Relay is normally open. When an AC input voltage is detected and within acceptable voltage/frequency range the inverter starts a slow phase tracking adjusment to match the AC input phase. The inverter also is adjusted to match AC input voltage which is the easy part. Once AC phase and voltage of inverter matches AC input the input relay closes putting the inverter in parallel with AC input (grid or generator).

PWM inverter can modify its AC voltage slightly when running in parallel wth grid. If inverter puts out a little more voltage then AC input voltage the inverter pushes out power. If inverter is adjusted to out a little less voltage it sucks power in from AC input for charging batteries.

There is a three way connection node. AC input, AC output, and InverterAC. With current sensing the inverter can manage the AC current flow between the three branch node. This means inverter can totally power loads by itself, supplement incoming AC input power for output AC loads (load shaving), or even supply loads and push power out backwards into grid (export to grid). It can also suck power from grid or generator to charge battery. AC coupling allows a PV grid tied inverter connected in parallel with hybrid inverter output to push power into AC out to either push power through to grid or through inverter to charge battery. For AC coupling the hybrid inverter acts as as surrogate grid for PV grid tied inverters when grid goes down.

Generator AC must be stable. A common problem with 3600 rpm engine generators is their freq stabilty wobbles because of unstable governor. Inverter phase tracking is relatively slow so it cannot follow a fast varying AC input frequency/phase. Inverter will just open relay and release from generator when this happens.

Block diagram looks like this. This diagram happens to be for low freq inverter but HF inverter is similar for AC synchronization.

View attachment 78665

Great work RC!
In my experience working with Generac customer service, they want the generator to supply the home on its own. They say to configure the ATS to switch out all power supplies such as the utility grid, solar inverters & battery inverters. Your note about the frequency stability wobbles because of unstable governor gives us a good reason why the service engineer makes that recommendation.