5. A means of contractualizing voltage regulation: the UQ diagram

From the previous paragraph, it is clear that the PQ diagram incorporating the voltage regulator's limitations provides an indication of the alternator's actual tuning possibilities. However, this diagram is only valid for a given stator voltage. What's more, in most cases, the alternator is fed into the transmission grid via a step-up transformer, known as a group transformer. As a result, for the grid operator, it is not immediately clear from a PQ diagram what the voltage and reactive power produced by the generator on the HV side behind the group transformer are. In the following, we will refer to these last two quantities as U _r and Q _r .

This is why the U _r Q _r diagram, commonly...