clipping off-mpp operating point calculation

[mikofski]

Feature request
An algorithm that determines the off-mpp operating point for inverters that are clipping

Describe the solution you'd like
Assuming that if the converted AC out is greater than rated AC (at the ambient temperature), then the inverter will increase the voltage until the AC output, what is the DC output, DC voltage, and inverter efficiency loss

Describe alternatives you've considered
I scripted something really, REALLY basic, I don't suggest something as simplistic as this:

p_ac = pvlib.inverter.sandia(v_mpp, p_mpp, inverter)  # use MPP first time
voltage_test_range = (v_oc - v_mpp)/100.0  # divide excess voltage into 100 steps
v_dc = v_mpp
while p_ac > ac_rated:
    v_dc += voltage_test_range  # add a little
    i_dc = np.interp(v_dc, v, i)  # v and i are the full IV curve
    # NOTE: v must be monotonically increasing for np.interp to work!
    p_dc = v_dc * i_dc
    p_ac = pvlib.inverter.sandia(v_dc, p_dc, inverter)

But it would be better to redo this with a scalar root finder from scipy.optimize like brentq.

Also, it the AC rated power should be adjusted depending on the ambient temperature, but this could be separate.

Additional context
Sorry, I couldn't find any related issues/prs?
ICYMI: @abhisheksparikh

[cwhanse]

I agree, would be a nice feature. Somehow it should accept any of the inverter models, or, be some kind of extension within the existing functions so that a user doesn't have to choose to call an inverter function or this new function.

We could also open issues for:

• inverter temperature derating (would be easy if there is a source for the %AC power / degree C relationship)
• non-unity power factor operations (I think this is difficult, as it would require knowledge of how efficiency changes with power factor, which is not measured AFAIK).

[mikofski]

may be related to #1199 ?

[kurt-rhee]

I'd be happy to work on this in the medium term future. Is there a reference that I should work off of? Is a reference necessary for what would essentially be a point on a curve finder?

[cwhanse]

@kurt-rhee I think the request is to find the point on the IV curve where the AC output of the inverter matches a desired value. pvsystem.i_from_v (or its v_from_i counterpart) seems relevant, for the DC input. Some thought is needed to define how this capability faces the user - additional inputs for a standard inverter function? Or some new function that lives in pvsystem? I don't have an opinion here.

Also, if the inverter has multiple Arrays as inputs, and the Arrays have different conditions, there are likely to be multiple solutions. For now, I think we should assume a single Array and worry about the multiple MPPT case later.

[mikofski]

Hi Kurt, I’m happy to jump on a call to discuss. I’m not sure if there are any references, but if not we could put this in the example gallery. I think the description already explains it well, but to clarify, the system must operate off-MPP when clipping, but finding this operating point is not trivial. It’s subject to two constraints:

1. Ideally AC out from the inverter must equal the rated power (possibly temperature derated)
2. The dc voltage and current must be on the system IV curve

So we probably need to iterate or use a bounded root finder like brentq. Perhaps we can just call this function?
ideal_offmpp_clipped_operation(system, ac_rating): -> (v_dc, i_dc)

[kandersolar]

Another consideration: maximum/minimum voltage/current limitations of the inverter. If there is no suitable point to the right of the MPP, should the algorithm "fall back" to looking for a suitable point on the left (low voltage, high current) side?

Whether the algorithm interpolates a pre-computed I-V curve, or is given SDM parameters and calls i_from_v internally, must also be decided.

The list of considerations here is already long enough to make me wonder if we are "inventing" too much. Has anyone looked for a reference we could follow? I haven't.

[kurt-rhee]

I also haven't been able to find a reference. Both PlantPredict and SolarFarmer have some notes about keeping the inverter input voltage and current within some operating window, but neither have references.

https://mysoftware.dnv.com/download/public/renewables/solarfarmer/manuals/latest/CalcRef/Inverter/Inverter.html

https://terabase.atlassian.net/servicedesk/customer/portal/3/article/1292009483

[mikofski]

Good points about minimum voltage and other constraints. PVsyst also has help page with nice diagram.

I propose one of two options:

1. Without a reference this is called the “ideal” operation. IMHO opinion this is sufficient and no reference is required because we are simply solving a set of constraints.
2. put this in the example gallery

IMHO the reference situation is getting out of hand. Solving a set of equations is math. Only implementations of algorithms should require a reference. We are not describing a sky model here, the two situations are not comparable. There are generally available references for math and other basic solar facts.

[adriesse]

Good references serve different useful purposes, but perhaps there's no need to get hung up on that right now if the fall-back is example gallery? Once there is a solution, we'll see how straight-forward it is.

[markcampanelli]

This is interesting: Essentially we are putting a (temperature-dependent) upper bound on the quantity that we are trying to maximize. I don’t think anything a-priori prevents one from defining this like any other nonlinear constraint https://docs.scipy.org/doc/scipy/reference/generated/scipy.optimize.NonlinearConstraint.html#scipy.optimize.NonlinearConstraint, and then solving for the maximum power as before, but under this additional constraint. I might try to add this feature to the MPPT-under-mismatch algorithm that I put together in #1923. Of course, the devil might be in the details!

[kurt-rhee]

Okay finally getting around to this.

My initial thought is to create a function to solve for maximum power at the given ambient temperature #1199 and then I like @mikofski 's brentq solution for calculating the operating point given the constraint.

In a utility scale photovoltaic site with a central inverter, a given inverter will be connected to a set of combiners. Each of these combiners will then be connected to a set of modules which are not necessarily unique. For example, even intra-combiner there can be strings that have different bin classes.

This means that we will need to combine all of the IV curves together into a single IV curve before solving for the point that satisfies the maximum power constraint. This would then necessitate that a user provide a given IV curve in a 2D array of voltages and their corresponding current values.

Perhaps I am overthinking this though, looking forward to hearing everyone's thoughts.

[mikofski]

Regarding…

This means that we will need to combine all of the IV curves together into a single IV curve before solving for the point that satisfies the maximum power constraint.

While this is true, if we only consider the “ideal” case, the situation when not all irradiance, module, & string conditions are known, could we first test the general case where the entire system is uniform? Then afterward we could provide particular solutions for more detailed scenarios? Would you be open to this approach first?

[cwhanse]

combine all of the IV curves together into a single IV curve before solving for the point that satisfies the maximum power constraint.

That's how a single MPPT works, but I don't think that's how clipping works for the whole inverter. I'm no expert by any means, but my understanding is that each MPPT applies the DC current and power limits. In contrast, the temperature limit is applied to the entire device, and I don't know how a temperature curtailment is apportioned among the MPPTs.