drcoolzic avatar image
drcoolzic asked

Can anyone explain why and how the 1.0 factor rule solves the AC coupling problem?

I have read the document "AC-coupling and the Factor 1.0 rule" carefully and I can understand the potential problem posed by AC-coupling but I have difficulty understanding how the "Factor 1.0" rule helps in this situation. This rule is simple: “The max PV power must be equal or less than the VA rating of the inverter/charger”. But I do not understand why this rule fix the problem described in section 2.2 “example and background” of the above document.

Let's assume that our installation has a MultiPlus II 5000VA connected to a battery and an AC coupled Fronius Primo 3.6 kW. Now let's apply the problem described in section 2.2 to this configuration. Let’s assume that on a sunny day you need to draw 4000 W from your system: 3.6 kW comes directly from the Fronius and 400 W from the battery through the MP2. Now let's assume that the battery is full and the load is suddenly cut off. The Fronius still delivers full power and the question is how fast the system regulates itself and where the power goes in the meantime.

I can understand that shutting down the Fronius takes time because it involves communication between the two systems: the MP2 has to change the frequency in response to the overload situation and the Fronius has to interpret it before shutting down. However, I find it hard to understand that the MP2 cannot cope with the overload situation because it is internal to the device and the electronics are supposed to react very quickly.

But suppose the MP2 is not fast enough to handle this situation properly, what does the Factor 1.0 rule change? There is no explanation in section 2.2 (or elsewhere in the document) as to why the PV power must be lower than the inverter power and how it fixes the problem.

I would appreciate if someone has an explanation as to why this rule solves the problem and why this factor needs to be 1.0 instead of 0.5 or 2.0?

AC PV Couplingpower factor
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4 Answers
marekp avatar image
marekp answered ·

Internal electronics of MP is rated at the maximum VA.

If system has no way to feed the grid or supply loads it will charge the battery using power from AC coupled inverter. It cannot do it with power grater than its maximum VA rating. This is why AC coupled inverter cannot produce more that maximum rating of MP.

There is no problem charging already charged battery for short time needed to throttle down AC coupled inverter.

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pedaaa avatar image pedaaa commented ·
and you really think there is no chance, that the Multi can handle a overload for those few seconds?!

I assume there will also be no problem, if you use a 1,1 or 1,2 rule...

But i might be wrong?!

Nevertheless Victron needs to state approved and safe limits in their manuals, thats clear.

it would not be clever to allow people to spec above limits.

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Alexandra avatar image
Alexandra answered ·


The factor also applies to the battery as well. And it mostly so the systen can handle the production overshoot. Or pick up the load if AC PV drops off suddenly.

This mostly because the overshoot power will have to be absorbed by something. And if that something is not big enough bad things will happen.

You can go 0.5 pv to 1 inverter not a problem.

If you are wanting to add PV power say for battery charging it is best to use DC for this now. So if you have 5kva inverter 5kva AC PV and run all day at 4000W loads, that DC the more efficient way to charge the system of you are offgrid or have bad grid.

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davidrenaud avatar image
davidrenaud answered ·


When talking about factor 1.0 rule, it would be good to mention PV INVERTER power and not only PV power : For example, running a Fronuis 4.0 overloaded with 6000Wc PV behind a MP2 5000VA is in respect of the factor 1.0 rule.

I have tested this config at 100% SOC, no grid (ESS mode): As Marekp said, In case a heavy load switch OFF, depending of battery pack and max cell voltage, MP2 charger overshoot the "charged voltage" with 0.1 - 0.2V. After Fronius throttling, inverter draw power from battery to bring back DC voltage to target value.

If One go over this 1.0 rule : in the particular case of no grid / WHATEVER SOC /PV inverter at full blast / heavy load switch off,/ low to no remaining loads: He take the risk to see voltage to raise suddenly on the load side to unknown levels.

PV inverter delivers at a voltage a bit higher than the lines so than current can flow. If there is no way or "too small" ways for current to flow, we may excpect voltage to go higher and higher until a security is triggered on inverter side or something break on MP side resulting in a "loss of main" for the PV inverter).

the' rule 1.0 is a simplification done for anyone, made in the worst case situation of an inverter behind MP2, with... ZERO LOAD.

True equation should be:

MP2 power >= PV inverter power - UNERRING loads.

But this doesn't match warranty contracts ;o)

Still, there is no rule 1.03 : all power produced by PV inverter should be able to be absorbed.

Anyway, this equation make sense for specific cases with heavy continuous day loads and light night loads.

For example : we can imagine a small business with a fronuis 6.0 behind a MP2 3000 and an uninterruptable load of 3000W (during the PV production period).

This case enter in the factor 1.0 "full equation" because MP2 is able to draw 100% of power excess (or remaining power).

but if you setup this case, you're on your own and it would be wise to double check the load power, ,COS FI and make a risk analysis of all potential unwanted load disconnection and immediate actions to be taken to derate PV inverter power.

Best regards.

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ludo avatar image ludo commented ·
In your "small business" example, what happens on Sunday when business is closed?
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tuky avatar image
tuky answered ·

So if I understood it correctly, the main problem is that MP has to deal with excess power when you switch of the load. For example: You have 2kW MP and 5kW Fronius. Multiple loads are drawing 4kW power, let's say of them is 2kW water heater. The battery is full. Fronius is producing 4kW from PV, and 2kW MP controls the situation keeping the balance between loads and production. The 2kW heater turns off, and other loads are still drawing 2kW. Now MP has to deal only with 2kW of the excess power which is within its limits. After that, you can switch off the other 2kW loads and MP has to deal the excess power again within its range. So the correct statement should be that you can not use and disconnect loads greater then the rated power of the MP.

Is this correct? Probably it is, but since Victron has a broad customer base, with various degrees of technical understanding (or misunderstanding), I presume there would be a lot of burned inverters and warranty claims. So the factor 1.0 is a much safer bet and easier to understand for the average user.

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ commented ·

If you can 100% ensure that the loads are switched off one by one this could work like you described it.

But there is also another scenario that you can't control:
You are connected to the grid, only small loads are running (lets say 500W) and the PV inverter is running at 5kW and the battery is full.
500W are going directly to the loads and 4,5kW are feed-back into the grid.
Now the grid fails and the MultiPlus is disconnecting the grid and now you have 4,5kW going through the MultiPlus to the battery and that will kill the MultiPlus within a few seconds.

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tuky avatar image tuky Matthias Lange - DE ♦ commented ·

You are right, I didn't think of it that way.

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