R&D Creatively appling the 1.0 rule in winter

Question

question

jbakuwel asked Apr 11, '19

R&D Creatively appling the 1.0 rule in winter

Hi all,

We're getting questions from our off grid customers who are driving an electric car if it is possible to increase their generating capacity in winter. So far, all these clients have an AC coupled 5KW SMA Sunny Boy (SB) with a 5KW Multi. 5KW of generating capacity is sufficient for these clients but to reach that in winter more panels are required. The existing SB is already oversized, more panels would push it over it's limits, so an additional SB would be required.

This would however violate the 1.0 rule. While an obvious way forward would be to use a 10KW Quattro instead of the 5KW Multi, this would likely also require a bigger battery bank and result in a system that can safely be considered an overkill, not to mention the additional costs.

So, I've been thinking if it's possible to add another SB and manage this safely in software. The SB's have a user/installed configurable parameter that limits their maximum power (ie. not via frequency shifting which they of course also do):

When the batteries are below a certain SoC (often in the morning), the SB's could go beyond the limit stated in the 1.0 rule to provide power to charge the car's battery as well as the domestic battery. Often in winter we have a few hours of sun, and then rain again (4 seasons in one day :-) ) so using those few hours to the max would be great.

Once the charging amps for the domestic battery drop below a certain level (say 10A) and the SoC goes above another certain level (say 90%), Venus would limit the total SB's generating capacity to the capacity of the Multi (ie. in this case 5KW).

This would likely be implemented as a custom script running on Venus. It would be great if Venus would be able to set that paramater in the connected SB's. My preference would be to use a writeable ModbusTCP register but MQTT would be fine too of course.

I'd imagine that a failsafe function in the Multi would also be handy. This watchdog (that can be enabled and disabled) would monitor the health of Venus and if Venus for one reason or another would stop responding, it would cut the power as the Multi would rely on Venus to prevent the issue described in the 1.0 rule article from occuring.

Does anyone see any issues with this approach? Is this something that would be feasible?

Imagine the newspaper articles describing how Victron enables us to drive an electric car only using solar power! :-)

Jan

Venus OS

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Modifications Space

Node Red Space

Answer 1 · 2019-04-11T21:25:07Z

boekel answered · Apr 11, 2019 at 09:25 PM

All feasible, but why not just connect the PV inverter at the input side of the Victron inverter?

2

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Phil Gavin commented · Apr 12, 2019 at 09:15 AM

He said they are off grid customers.

0 ·

boekel ♦ Phil Gavin commented · Apr 12, 2019 at 05:49 PM

Ah I missed that.

Well than just use DC-coupled PV for a better solution.

0 ·

Answer 2 · 2019-04-11T23:41:56Z

Guy Stewart (Victron Community Manager) answered · Apr 11, 2019 at 11:41 PM

R&D indeed, it's risky and unsupported, but it sounds like you already know that.

I presume you have another reason for not wanting to use DC bus solar on an MPPT?

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Answer 3 · 2019-04-12T09:22:25Z

Phil Gavin answered · Apr 12, 2019 at 09:22 AM

This issue being where does the power go when there is a sudden loss of load.

I would consider a Voltage/Frequency relay and a dump load.

But in full knowledge, that you are in unsupported territory.

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Answer 4 · 2019-04-12T09:46:24Z

rainerb answered · Apr 12, 2019 at 09:46 AM

Probably not the solution but maybe an idea in which direction you could go.

For my ESS system with Fronius Galvo inverter and MPPT, I wrote a bash-script that is running every 10 minutes on the Venus GX.

This script reads out the battery SoC, determines the actual solar power and according to this I change a few ESS settings (Max. inverter power, Grid setpoint) AND the script also switches my Fronius Inverter to Standby if its output power is below 15 Watt. I need this because I get strange humming and other resonance noise if the pv output power is only a few watts.

I simply used one of the Venus GX relais and connected it directly to an input of the Fronius inverter.

Additionally, in your case I would definitely think about a few fail-safe features too, because I would not rely on cron-jobs and scripts with a few dbus commands.

Best regards

Rainer

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Answer 5 · 2019-04-14T05:26:14Z

jbakuwel answered · Apr 14, 2019 at 05:26 AM

Hi all,

Thanks for the replies. This is indeed for off grid customers, so ESS does not apply. I've wondered about having one PV inverter on the output side of the Multi and one on the input side of the Multi. We'd somehow need to make that PV inverter see a "grid" though... not sure how to do that.

The controller that controls the relay/contactor for a dump needs to respond very fast... and maybe it can't be fast enough.

Going DC coupled would indeed be a straight forward solution (and in fact for one customer we've already done that). For another customer though the additional array will be on a different building (and far from the batteries) so for that customer we're considering using an AC coupled system (since there's already AC between the buildings) where the combined power of the (two) PV inverters is limited to the rated power of the Multi.

It means that we won't be able to use all available power to charge both the domestic and the car battery but it will provide the customer with the needed power in the winter. Both PV inverters will have their max power set to combined 5KW (as they use a 5KW Multi) and the ratio will be determined by the panel layout.

Yet another way to do this would be to increase the capacity of the domestic battery but that too is not always feasible.

Jan

1

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Phil Gavin commented · Apr 14, 2019 at 06:12 PM

FYI

https://www.youtube.com/watch?v=XA-4NkE6lBw

0 ·

Answer 6 · 2019-04-14T05:27:39Z

jbakuwel answered · Apr 14, 2019 at 05:27 AM

Hi Rainer,

I think I'm going to turn the SB off too when it's output power drops below a certain level... lots of relays clicking inside too often for my liking. The question remains though how to decide when to turn the PV inverter (Fronius in your case, SB in my case) back on. How are you doing that?

cheers,
Jan

2

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

rainerb commented · Apr 14, 2019 at 06:36 AM

Hi,

In my case I programmed the script in such a way that it has a duration of 5 minutes. During this time I calculate an average solar output power. The script is automatically started every 10 minutes (by cron-job).

If the program decides to switch the fronius off, then it is done for at least 5 minutes. After that, the relay is released and the fronius has 5 minutes time to come up again. In the evening the internal fronius DC voltage settings (startup voltage) will keep him usually off itself. In the morning it can happen that I switch the fronius 1x to 2x to off when it has not reached my 15W setpoint (I can tolerate that).

If the fronius output power is >15W or if it is zero or if it is offline (night), my script does not do any relay action. Usually I have about 5 Venus GX relay clicks per day - I hope that is no lifetime reduction issue.

Regards

Rainer

0 ·

rainerb commented · Apr 14, 2019 at 06:43 AM

I do not know your SB, but the fronius has other inputs that can be assigned eg. to perform a power reduction. Usually this is used by grid companies to reduce the output power of the inverters by an external signal.

Maybe there are modbus commands to do the same (eg. full power if the batteries are empty and reduced power if they are nearly full or so).

0 ·

Answer 7 · 2019-04-14T05:38:55Z

jbakuwel answered · Apr 14, 2019 at 05:38 AM

Hi Guy,

Is having a general purpose watchdog "assistant" that can be enabled and disabled in the Multi something feasible to implement? The enabling and disabling could be implemented in VE.Configure or perhaps even via a writeable ModbusTCP register or MQTT.

It would be very useful for any custom activity/functions in Venus that the Multi would (need to) rely on, including but not solely for the one discussed in this thread.

cheers,
Jan

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Answer 8 · 2019-04-15T20:02:33Z

Anil Ghatikar answered · Apr 15, 2019 at 08:02 PM

I was using an unsupported PV inverter ( APS 1000 Watt) on AC out 2 of Quattro and programmed the Quattro AC out 2 to switch on and off based on battery Voltage. through assistants

As an additional safety relay added a contractor on the AC inverter switched by BMV NO contact ( Contactor on only when Battery is below 90 % SOC)

4

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

boekel ♦ commented · Apr 15, 2019 at 08:12 PM

That won't help you against the 1:1 rule, when the PV generates more then the Multi/Quattro can push into the batteries (and not enough AC is used at the same time), there is not enough time for the frequency shift and bad things can happen.

0 ·

jbakuwel boekel ♦ commented · Apr 15, 2019 at 08:50 PM

Hi Boekel,

Either I don't understand the 1:1 rule or you don't :-)

The Multi will frequency shift limit the PV inverters so that should be fine. Besides, the charger in the 5KW multi is only capable of 70A; at 48V that's just under 3.5KW. There only is not enough time to regulate if the batteries are full and the load suddenly disappears. At least that is how I understand it.

Jan

0 ·

boekel ♦ jbakuwel commented · Apr 15, 2019 at 10:10 PM

simplified: if you put 10kWp of PV on the output, it cannot absorb all the power if there is more available than it can put into the batteries, whether the batteries are full or not.
How it manages to regulate 1:1 I don't know, besides conversion losses maybe the charger can handle a short overshoot? (and usually some power is used also).

0 ·

Anil Ghatikar boekel ♦ commented · Apr 16, 2019 at 02:22 PM

The PV generators are diconnected before the battries are full so there is nothing to push independent of the load. The system behaves as a normal system. The PV generators are connected back only when 1. SOC is below 90 % and load is above a threshhold.

In case the Load drops below a threshhold OR if the SOC crosses 90 % the PV generator is disconnected using AC out 2 so the Multi does not have to use frequency control

an additional power contactor works as fail safe based on battery terminal voltage through BMV independent of MUlti and venus

0 ·

question