Grids

How are transformers with a manual tap changer modelled?

If a transformer is operated with off-nominal operational voltage, this is reflected in the primary turns ratio $r^{HV}$ and secondary turns ratio $r^{LV}$, which are defined as

$r^{HV}=\frac{V_{nom}^{HV}}{V_R^{HV}}$

$r^{LV}=\frac{V_{nom}^{LV}}{V_R^{LV}}$

for the high and low voltage side of the transformer respectively. $V_{nom}$ is the nominal voltage of the grid and $V_R$ is the rated voltage of the transformer.

A primary turns ratio and secondary turns ratio different than 1 indicates the off-nominal operation.

Changing $r^{HV}$ or $r^{LV}$ has the same effect as changing the physical tap position at the transformer on the high or low voltage side.

Learn more about running static calculations with off-nominal voltages.

What is an ideal tap changer?

An ideal tap changer is a tap changing transformer that has in infinite number of taps. As such, the tap position can be continuously varied as opposed to discrete values as in a real tap changer. The ideal tap changer can control the voltage at the reference node continuously and therefore always maintains the setpoint voltage.