Why do manufacturers recommend some PV DC input on hybrid inverter systems?

[godawgs]

The general consensus in talking with hybrid battery manufacturers (sp. Sol-Ark, EG4) is that even though your battery pack capacity is sufficiently large enough to carry through large AC-coupled power fluctuations, there is an ample amount of DC-coupled solar input as well on top of that. Apparently, this results in less complete black-outs if the hybrid inverter loses all AC-coupled power, for any reason.

Does anybody know the technical reason why this would be the case? The battery and PV DC current eventually get processed through the same DC bus, so what does adding in the solar DC add to the stability?

Thanks for your insight!

 

[RCinFLA]

Does anybody know the technical reason why this would be the case? The battery and PV DC current eventually get processed through the same DC bus, so what does adding in the solar DC add to the stability?

That is not the issue. Internal PV SCC can cut back near instantly. The issue is having to swallow a sudden excess in AC-coupled power.

There are two primary ways this situation arises. AC coupling injected power is relatively large and much the AC coupled power is either going to large house loads or back fed to grid.

1) When off grid, large house load is suddenly switched off leaving a lot of excess AC coupled power to instantly deal with.
2) When on grid, grid suddenly drops again leaving a lot of excess AC coupled power to instantly deal with.

Battery to HVDC bus converter in HF inverters require a finite amount of time to make a power flow direction mode change. It takes several milliseconds to make the switchover during which time the HVDC capacitor bank must absorb any excess AC-coupled power without allowing the HVDC bus voltage to rise too much.

 

[godawgs]

That is not the issue. Internal PV SCC can cut back near instantly. The issue is having to swallow a sudden excess in AC-coupled power.

There are two primary ways this situation arises. AC coupling injected power is relatively large and much the AC coupled power is either going to large house loads or back fed to grid.

1) When off grid, large house load is suddenly switched off leaving a lot of excess AC coupled power to instantly deal with.
2) When on grid, grid suddenly drops again leaving a lot of excess AC coupled power to instantly deal with.

Battery to HVDC bus converter in HF inverters require a finite amount of time to make a power flow direction mode change. It takes several milliseconds to make the switchover during which time the HVDC capacitor bank must absorb any excess AC-coupled power without allowing the HVDC bus voltage to rise too much.

Thank you! I understand about having to manage large changes in power requirements/availability by the inverter. I'm still unclear how having a large amount of PV DC would help that situation?

 

[RCinFLA]

Thank you! I understand about having to manage large changes in power requirements/availability by the inverter. I'm still unclear how having a large amount of PV DC would help that situation?

I am assuming when you say PV DC you are referring to AIO internal PV SCC controller.

If you have AIO internal PV SCC supplying significant power on top of AC coupling incoming power to satisfy AC loads or grid back feed and AC load suddenly drops out, the internal PV SCC can cut back quickly to help reduce the net excess overproduction.,

For AC coupled external PV GT inverters, frequency shifting by hybrid inverter to cut back external PV GT inverters can take up to two seconds to react. This is not fast enough for a sudden overproduction situation. SolArk, Deye, and likely EG 18k, can redefine the function of generator input port so it becomes the AC input for external AC coupling. If hybrid inverter gets into trouble with sudden AC coupled PV overproduction it can open the generator port pass-through relay, instantly pulling the plug on AC coupled PV GT inverter to save the hybrid inverter from possible damage.

 

[godawgs]

I am assuming when you say PV DC you are referring to AIO internal PV SCC controller.

If you have AIO internal PV SCC supplying significant power on top of AC coupling incoming power to satisfy AC loads or grid back feed and AC load suddenly drops out, the internal PV SCC can cut back quickly to help reduce the net excess overproduction.,

For AC coupled external PV GT inverters, frequency shifting by hybrid inverter to cut back external PV GT inverters can take up to two seconds to react. This is not fast enough for a sudden overproduction situation. SolArk, Deye, and likely EG 18k, can redefine the function of generator input port so it becomes the AC input for external AC coupling. If hybrid inverter gets into trouble with sudden AC coupled PV overproduction it can open the generator port pass-through relay, instantly pulling the plug on AC coupled PV GT inverter to save the hybrid inverter from possible damage.

That makes sense (again!). So I'm wondering now - in a hybrid system, what would the best/ideal ratio of AC-coupled to DC-coupled PV production? I'm guessing many would say 100% DC. But I already have 22 kW of AC coupled production. I can add a max of 10kW of DC coupled PV to the hybrid inverters, so wondering if that would be sufficient to provide the flexibility you are referring to.

Appreciate you for the guidance!

 

[RCinFLA]

That makes sense (again!). So I'm wondering now - in a hybrid system, what would the best/ideal ratio of AC-coupled to DC-coupled PV production? I'm guessing many would say 100% DC. But I already have 22 kW of AC coupled production. I can add a max of 10kW of DC coupled PV to the hybrid inverters, so wondering if that would be sufficient to provide the flexibility you are referring to.

Appreciate you for the guidance!

Only internal PV SCC power is safest.

Internal PV SCC is instantly hardware controlled making it easy to control overproduction situation.

Should not AC couple PV power beyond inverter's HVDC to battery converter maximum (charging) power capability. Lower is safer.

When AC coupling is used, hybrid inverters usually do not allow battery to be fully charged to hold a reserve to dump excess power to battery while frequency shifting is being setup to reduce external AC coupled PV power.

If overproduction dumped to batteries is too strong, or last too long in time, it can force DC input voltage to hybrid inverter to exceed it maximum limit. Hybrid inverter has no other choice but to shut down to save itself when this happens. You lose backup power.

 

[godawgs]

Only internal PV SCC power is safest.

Internal PV SCC is instantly hardware controlled making it easy to control overproduction situation.

Should not AC couple PV power beyond inverter's HVDC to battery converter maximum (charging) power capability. Lower is safer.

When AC coupling is used, hybrid inverters usually do not allow battery to be fully charged to hold a reserve to dump excess power to battery while frequency shifting is being setup to reduce external AC coupled PV power.

If overproduction dumped to batteries is too strong, or last too long in time, it can force DC input voltage to hybrid inverter to exceed it maximum limit. Hybrid inverter has no other choice but to shut down to save itself when this happens. You lose backup power.

Ok. Got it. So 22kW AC coupled ~ 92A. I'm planning on getting (2) 11.4kW inverters, each with ~47A max AC output. Each can also charge at 50A. So keep max battery at 90%ish SOC, and it seems like I can keep my 22kW AC coupling. I also figured that I can add in about 13kW of DC coupled PV. From your analysis, I think all of this will work?

 

[godawgs]

Only internal PV SCC power is safest.

Internal PV SCC is instantly hardware controlled making it easy to control overproduction situation.

Should not AC couple PV power beyond inverter's HVDC to battery converter maximum (charging) power capability. Lower is safer.

When AC coupling is used, hybrid inverters usually do not allow battery to be fully charged to hold a reserve to dump excess power to battery while frequency shifting is being setup to reduce external AC coupled PV power.

If overproduction dumped to batteries is too strong, or last too long in time, it can force DC input voltage to hybrid inverter to exceed it maximum limit. Hybrid inverter has no other choice but to shut down to save itself when this happens. You lose backup power.

@RCinFLA - You are a solar whiz so thought I'd ask an off-topic question: The reason I went with AC-coupled systems to start with is because I was concerned about shading. So optimizers and microinverters were the best solution. I haven't kept up completely with dc-coupled solar technology, but I still don't see that string-level DC setups can adequately (safely) account for and protect shady situations. Is that still the case? Other than bypass diodes to protect panels, it's still best to use panel-level control for shady situations, correct? Otherwise, strings are so much easier to install.

 

[RCinFLA]

I am not a fan of DC optimizers unless they are coordinated by a central inverter like SolarEdge systems.

If you cannot avoid shading issues, microinverters are a good choice to optimize PV output. As far as AC coupling use, I don't believe microinverters are any more or less difficult to safely control than a central PV GT inverter.

 

[godawgs]

I am not a fan of DC optimizers unless they are coordinated by a central inverter like SolarEdge systems.

If you cannot avoid shading issues, microinverters are a good choice to optimize PV output. As far as AC coupling use, I don't believe microinverters are any more or less difficult to safely control than a central PV GT inverter.

Thanks. I have SEDG inverters. Just wanted to confirm I didn't miss some tech that made shading a non-issue for string inverters or panels.

Really appreciate your guidance! Cheers.