Inverter model: efficiency

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Inverter model: efficiency

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The efficiency is the ratio of the output power with respect to the input power. It depends mainly on the power and can also be a function of the input voltage.
In PVSyst there are 4 ways of  defining the efficiency of inverters :
 - from a single efficiency curve eff = f(Input power),  specified by up to 8 points
 - from a single efficiency curve eff = f(Input power),  automatically built from the Maximum, EURO or CEC efficiencies and the Power Threshold (Pthresh)
 - from a set of 3 efficiency curves eff = f(Power, input voltage), all of them specified by up to 8 points
 - from a set of 3 efficiency curves eff = f(Power, input voltage), all of them automatically built from the Maximum, EURO or CEC efficiencies and Pthresh.

Single efficiency profiles: this is the most common definition: we define the efficiency as a function of the output (or the input) power. This profile has usually a power threshold, a knee towards low powers, a maximum around the middle of the power range, and a decrease at high powers due to ohmic losses in the output circuits (transistors and transformer).  It is used during the simulation, according to the operating power.
When specified by up to 9 points, the transfer function (Pout = f(Pin)) is linearly interpolated.    

Set of 3 efficiency profiles: this is a more accurate definition: we define an efficiency profile as above for 3 different input voltages. At each step,  the simulation will perform a quadratic interpolation between these 3 curves, as function of the real input voltage.

Automatic efficiency profiles: this option is used when only the Maximum and EURO or CEC efficiencies efficiencies are available. PVsyst then construct default profiles, with the following hypothesis:

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

2.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).

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

4.The Pthresh must exceed 0.5% of Pnom: PVSyst requires this condition in order to build comparable curves between inverters.

5.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.

The accuracy of this model is probably largely sufficient for representing most inverters. The less difference between Max- and Euro-efficiency, the less possible discrepancies of the model. With modern high-efficiency devices, there is about no room left for uncertainties...

The EURO or CEC efficiencies are operating averages over a full year, defined in another page.


MPP Tracking loss

The accuracy of the Maximum Power Point Tracking is sometimes mentioned.  

The losses due to this technical inaccuracy (i.e. the difference between the operating point and the true MPP) is namely related to the MPP algorithm and the variability of the irradiance. This variablilty is of course not represented when using hourly data.  

PVsyst doesn't take explicitly the MPP tracking loss into account: these should be included in the efficiency profile proposed by the manufacturer.