Medium voltage transfo loss

What this step does

The loss of the medium-voltage transformers: the no-load loss drawn while a transformer is connected and the load loss that grows with the square of the power through it. pvperf models one medium-voltage transformer per inverter. The row acts on the inverter output.

The math

1. Running total
2. Loss at a timestep

$$E_{after} = E_{before} - E_{row},\qquad E_{row} = \frac{\Delta t}{1000\,P_{DC}}\sum_i L_i \tag{1}$$ $$L_i = N_{inv}\left(f_{Fe}\,P_{ref}\,[\text{connected}] + \frac{f_{Cu}}{P_{ref}\,\text{PF}^2}\,P_{c,i}^2\right) \tag{2}$$

where

Symbol	Meaning	Unit
$E_{before}$	the running total of the loss diagram before this row	kWh/kWp
$E_{after}$	the running total after this row	kWh/kWp
$E_{row}$	the row taken as a loss	kWh/kWp
$\Delta t$	the length of one timestep	hours
$P_{DC}$	the DC nameplate of the system	kWp
$L_i$	the power lost at timestep $i$ over the whole system	W
$N_{inv}$	the number of inverters	—
$f_{Fe}$	the no-load loss, as a fraction of the reference power	—
$P_{ref}$	the AC power reference of one inverter: its AC nameplate, or its share of the array's STC reference power, the DC nameplate reduced by the module and string mismatch derates (the Model tab's AC power reference step sets which)	W
$[\text{connected}]$	1 while the transformer is connected; 0 on a timestep without export when the night disconnect is on	—
$f_{Cu}$	the load loss at the reference power and unit power factor, as a fraction of the reference power	—
$\text{PF}$	the power factor	—
$P_{c,i}$	the power the load loss is computed on: the output of one inverter; when the Model tab's AC resistive reference step is set to Cascade, the power after its AC wiring	W

Reading the number

A loss, negative. About 1 % over a year for the usual fractions on a utility system; zero on a rooftop system without its own transformer. The no-load part weighs more at low load and at night.

References

1. IEC 60076-1:2011, Power transformers, Part 1: general (no-load and load losses)

API

loss.mv_transformer

Symbols follow the symbol registry.