Oversized DC PV on Victron ESS – How is it curtailed?

DIY
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1

Hi everyone,

I have a question about how a Victron ESS curtails output when the DC-coupled PV system is significantly oversized.
I’m planning to build my first Victron ESS in the near future, and this question has been going round in my head. I haven’t been able to resolve it so far.

For example, consider the following setup:

  • 48 V ESS

  • Cerbo GX

  • Several SmartSolar MPPTs

  • LiFePO₄ battery

  • Later, 3× MultiPlus-II 48/6500

  • Around 30 kWp of DC-coupled PV in the longer term

  • Distributed across three different orientations, so the full rated PV output is only available simultaneously very rarely

Of course, I realise that having to curtail PV output is not particularly sensible economically or in terms of energy yield. For now, I’m purely interested in the technical theory and how the system behaves in this borderline case.

In my mind, the scenario would look something like this:

Due to the three orientations, the nearly 30 kWp PV system would produce significantly less than its rated output for most of the time. The 3× 6.5 kW units could handle a large proportion of this on the AC side. However, if, for example, 20–25 kW of DC PV were available for a few hours, the battery were full and the Multis could only deliver part of it to the house or grid, perhaps the final 20% of the PV output would have to be curtailed — in other words, the peaks would be smoothed out.

The actual question is: Is this possible at all, and if so, how does the curtailment happen technically?

Is there active power control whereby the Cerbo/MultiPlus units provide the SmartSolar MPPTs with a specific power setpoint or limit?

My current understanding is roughly as follows:

  1. The battery reaches the configured CVL, e.g. 55.2 V.

  2. With DC surplus feed-in enabled, the DC voltage is allowed to rise slightly above the CVL.

  3. The MultiPlus units try to draw the additional DC power on the AC side and feed it into the house or grid.

  4. Once the Multis reach their maximum DC→AC output, they can no longer draw any more power from the DC bus.

  5. The DC voltage continues to rise towards the permitted limit.

  6. The SmartSolar MPPTs then leave the MPP and reduce their PV output.

Is that correct?

Or is there also some kind of direct, coordinated power limitation between the GX, MultiPlus and SmartSolar MPPTs?

In short: Can 3× MultiPlus-II 48/6500 technically be operated without problems with nearly 30 kWp of DC PV if, due to the different orientations, for example 80% of the generation falls within the available AC power and you consciously accept that the remaining peaks will be curtailed when the battery is full?

Thanks everyone!
Moritz

2

Treat the PV array like a battery: you can draw as much as you need, but no more than its rated power. If you have a 50kW setup, you can pull just 1kW (or whatever your charger’s max capacity is) and the rest simply goes to waste. This is called oversizing. The MPPT will only draw as much current as it can handle.

The absolute most critical thing is that you never exceed the maximum MPPT voltage (Open-Circuit Voltage – Voc). You must calculate the maximum string voltage based on the coldest winter temperatures. If the voltage is too high, you will fry the charger. If the current (Amps) is too high, the charger will simply limit it, and nothing bad will happen.

3

Thanks @magnum_pl. I don’t think I fully understood your answer, so I’ll try to describe it again:

Thanks for the feedback.

My question is really about the control mechanism:

Let’s assume that the SmartSolar MPPTs could currently deliver, for example, 20 kW, the battery is full, and the 3× MultiPlus-II 48/6500 can only convert or supply, say, 12 kW DC-to-AC on the AC side.

What specifically ensures that the MPPTs then only deliver this remaining power instead of supplying 20 kW?

Are the MPPTs given a specific power or current setpoint via GX/DVCC? That’s precisely the technical process I’d like to understand.

So my question is specifically whether a system with, for example, just under 30 kWp of DC PV and 3×6k5 can be operated stably if you deliberately accept that the highest power peaks will be curtailed when the battery is full.

Thanks,
Moritz

5

You can over-panel an mppt.
But..
You need to stay within Isc and Voc limits, the latter varies with temp.
Use the mppt calculator on the main site and punch in your panel specs, it will confirm if it works or not.

6

Venus OS, e.g. on a Cerbo GX, tells the MPPT via Ve.Direct or VE.CAN, depending on the MPPT, what charging voltage or current is permitted.
The control variable is derived from the BMS’s CVL and CCL, as well as any optional manual DVCC settings.
The limitation can therefore occur for the following reasons:

  • Battery full
  • Battery CCL reached
  • MPPTs/RS at 100%

It would actually be helpful if someone could comment on what happens when DC feed-in is enabled and the grid is available. In this case, the MPPTs ignore the CVL.
I can’t reproduce this with my system. Perhaps setting a grid limit is sufficient for the test.

7

The system always tries to obey the BMS limits. If the battery is full it will reduce (ideally) both CVL and CCL, though most just limit CCL. Solar will throttle.
Even if you export, the system will try adhere to the CCL from the BMS, but not the manual limit set in DVCC. The system will always try protect the battery. If you have a DIY battery there are more variables at play that could affect how the system behaves.

8

From a practical POV, this is what happens in my system:

  • 10kWp AC-coupled solar, 8kW inverter
  • 8 kWp of panels in two strings on a MPPT RS450/100, so max 100A on the DC/battery side
  • Grid feed-in limit of 8kW

a) when there’s more sun on the DC strings than the MPPT can handle, it just doesn’t – it outputs 100A x Battery Voltage and that’s it, so max about 5600W

b) when the battery is full and all PV systems produce at their maximum, the MPPT throttles production as I have configured my feed-in limit (the MPPT throttles since the AC inverter is not controlled by the Victron system)

I don’t know from experience what happens if you have more MPPT power than (victron) inverter power, but I strongly assume it’s the same – the batteries get fully charged and then their voltage limits from the BMS are followed, so the MPPTs would throttle.

As I understand it, the GX gives the MPPT a voltage setpoint – in fact, if you configure CCL in DVCC, it is ignored. So what you can do, e.g. with dbus-serialbattery, is reduce the CVL when the battery is nearly full and one cell reaches the cell voltage limit. This works well and regulation is very quick.