DC cable losses

[Abdel]

Hi,

How to calculate DC cable losses in PVsyst ?

[Lazare Fesnien]

Hi,

In the detailed losses window, you must click on "Ohmic losses"

The ohmic resistance of the wiring circuit induces losses ( ELoss = Rw · I² ) between the power available from the modules and the power at the terminals of the sub-array.

The relevant parameter for the simulation is the Rw value, which is an equivalent resistance of the wires, as "seen" from the input of the global sub-array (i.e. the set of MPPT inputs defined in this sub-array). You should define one Rw value for each sub-array in your system.

First stage evaluation: percentage of STC

The Rw value evaluation will highly depend on the sub-array structure.

However PVsyst provides a convenient way of defining a default value to be used during the first stages of the study of a PV system.

We specify a Power loss ratio with respect to the STC power.

We can consider the equivalent "resistance" of the STC operating point: RarraySTC = Vmp / Imp (at STC) [ohm].

Then the Wiring loss fraction = Rw / RarraySTC (ratio, or percentage).

In PVsyst we have chosen a default initial value of 1.5% (at STC) for this usual loss. This default may be modified in the Hidden parameters ("Default Wiring Resistance loss ratio at STC") for the initialization of any new project/variant.

Loss in the simulation

The ohmic losses behave in a quadratic way with the array current: Ploss = Rw · Iarray².

Now if the array is not operating at STC, the wiring loss fraction will become:

Ploss / Parray = Rw * Iarray² / (Varray * Iarray) = Rw * Iarray / Varray.

i.e. proportional to Iarray. This means that at half the irradiance (half the current), the wiring loss fraction will be half, etc. Therefore the Wiring energy loss has to be evaluated at each hour of the simulation, and accumulated in terms of energy.

The final result of the Ohmic wiring loss (in terms of percentage) as shown on the loss diagram will be from this hourly energy balance. It is usually of the order of 60% of the Loss fraction specified a STC.

Final calculation: evaluating the resistance

In the latest stages of the project, the equivalent wiring resistance should normally be calculated according to the real lengths and sections of the installed wires.

The program offers a special tool intended to optimise the wire diameters at each stage of the array.