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Ohmic losses

Ohmic loss definition

For a given conductor, the ohmic loss is always calculated by

\(Pw [W] = Rw [Ohm] * I^2\)

The resistance of the wire Rw is the basic parameter defining the wiring loss.

The resistance of a wire is calculated by
\(Rw [mohm] = Rho [mOhm * mm^2 / m] * Length [m] / Section [mm^2]\)

The resistivity Rho is a function of the metal and the temperature.

Loss fraction

People usually define the wiring loss by the Loss Fraction, i.e. the ratio of the Power loss Pw to the considered power (usually expressed as a percentage). This is practical as this is supposed to correspond to the amount of energy loss that you would accept. This also corresponds to the percentage of voltage drop.

Unfortunately this is not a reliable parameter, as the Loss Fraction is proportional to the actual Power. This is quite easy to understand: \(Loss Factor = Rw * I^2 / P = Rw * I^2 / (U * I) = Rw * I / U = Rw * P/U^2\) The Loss Factor is proportional to the current, and if we consider U as a constant, to the power.

Therefore for defining a Nominal Loss Factor, we have to specify at which reference power. In PVsyst, we consider 2 possible reference powers: the nominal power of the PV array at STC, or the nominal PNom of the inverters. This choice is available in the Project's settings.

By the way, even if you define the wiring losses in terms of percentage, the relevant parameter in PVsyst will always be the corresponding resistance.

Warning: during the simulation, the energy loss has to be evaluated (and accumulated) at each time step according to the present power or current (Pwloss = Rw * I²). Therefore the wiring loss over a given operating period is not the specified value. In the annual results, the wiring energy loss is usually around 60% of the specified nominal loss fraction, depending on the irradiance distribution.

DC Wiring Losses (in the subarrays)

The DC losses are defined in each subarray, based on a global resistance for the whole subarray; the wiring loss is calculated from the global current Imp of the whole subarray. This may be defined in a generic way by a Nominal Loss Fraction at STC power, or by evaluating this resistance from the wires of your real PV array.

For an array of PV modules, the resistance is accounted as the network of all wires, as "seen" by all MPPT inputs of the subarray in parallel. For each MPPT input, you may have connexions to one (or several) combiner boxes; you should account for all cables (positive and negative in series) from inverter to these combiner boxes. Then each combiner box receives N strings, you should account all string resistances (positive and negative wires) in parallel for each combiner box.

There is a detailed procedure for this calculation, as well as a simplified tool for helping these evaluations.

Why defining a voltage drop is not possible ?

With the I/V characteristics of an array of PV modules, defining a voltage drop doesn't make sense. See the explanation here.

AC wiring losses (inverter to injection point)

The wiring losses in the AC part are evaluated by the loss in each wire, calculated by the wire length and section, and the metal. Most often you have tri-phased systems (rarely 2 wires for little mono-phased systems), so that the loss of one wire should be multiplied by 3.

NB: Some installations have "4 wires*. The 4th wire is indeed the neutral conductor. When the phases are balanced, you don't have any current in this wire, and therefore no loss. PVsyst doesn't consider systems with unbalanced phases.

Losses from inverter to the injection point

The sold energy E_Grid is accounted at the injection point. Up to the injection point:

  • You always have an AC circuit at the output of the inverter.
  • In big installations, you may have one or several MV transformers.
  • In very big installations, the power of several MV transformers may be gathered in a HV transformer.

The required wire sections and wiring loss definition are defined in a dialog located in the Detailed losses. The procedure is explained here

MV and HV Transformers

The transformers have also windings which undergo a wiring loss. As a simplification, we consider that the losses in th secondary winding are gathered with the losses of the primary. The wiring losses in a transformer are named "copper losses", and the nominal loss (at the transformenr nominal power) is usually mentioned on the datasheets. Ser the External transformer losses page for details.

Power factor

When you have a Power Factor, the current in wires will increase: \(Pactive = U * I * cos(phi) => I = Pactive / (U * cos(phi)\) Therefore in all calculations the current should be multiplied by 1/Cos(phi).

In the present time, PVsyst doesn't evaluate the losses of Reactive "power" due to the Reactance. The reactive power is not a real power, and therefore the reactive loss is not an energy loss.