Inverter Efficiency Curve

[MarcoMalagnino]

Dear Mermoud,

I am trying to understand the modelling of the inverter effciency in PVsyst. I have some problems to undestand the way PVsyst uses in order to build the efficency curve starting from the max and Euro efficiency.

In the Help "Grid inverters, Efficiency curve" you write:

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Interpretation of the efficiency curve:

- Up to the power threshold (often named "Starting operating at ..." in datasheets), the input power corresponds to internal consumption for the internal needs of the device, and doesn't produce power.

- From this threshold, the device is supposed to produce an AC power proportionally to (Pin - PThresh).

- This production is penalized by an ohmic loss of the internal components (transformer and transistors), which increases quadratically with power (like R · I²). When translated into efficiency, this gives a maximum efficiency usually around 50-60% of the nominal power.

PVsyst model for automatic construction of the efficiency profile

In most of the specifications, the manufacturers or databases give the so-called European Efficiency (or CEC efficiency in the US) which is intended to provide an average efficiency over yearly operating conditions.

From these two values specified by the user - Maximum efficiency and European efficiency - PVsyst constructs a default profile, with the following hypothesis:

- The AC production is proportional to the DC available energy, minus the Pthresh "internal consumption",

- The efficiency is penalized by a resistive loss of the output circuitry (transistors and transformer), in a quadratic way as function of the power (R * I²),

- This transfer curve has to match the specified maximum power value, at a DC power arbitrarily fixed at a value of 60% of Pnom,

- The Pthresh "effective" value (=> slope of the curve) is adjusted in order to match the Euro (or CEC) average efficiency.

The contribution of the Resistive loss is fixed according to a normalized resistance factor, proportionally to the difference between the given Max- and Euro- efficiency. The normalized resistance factor commands the losses at high powers (above the 60% point), with respect to the losses due to Pin_threshold. We choose a usual value of 3.0 (arbitrary units) as default.

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Could you please explain in an easier way this modelling method and why, if the threshold is 8000W (imposed by the manufacturer), do you calculate a Pthresh "effective" value? How do you calculate this "effective" value?

Thanks in advance