Another problem here, problem #21. We’re at loss with the solution, as this is the first time we’re seeing a relation between generator curves, to R-X diagrams, and to a distance relay. Does anyone understand the solution or is able to provide more clarity?

If you ever look at a loss of excitation relay it is sometimes plotted on the R-X diagram like this to show protection. Usually there is a primary (LOE2) and a secondary (LOE1). :

These RX circles of the LOE relays can be converted to the P-Q plan and shown as a relation to the Generator cap curve (GCC) as shown below:

As you begin to under-excite the generator (i.e. reduce field current), you begin to move your operating point on the P-Q plan downward and approach the GCC iron end stator limit. Just below the GCC you have the LOE relay that will trip the unit for protection.

But the whole point of the question is easier. The only way the relay can “know” where it is on the PQ plan or plot a RX diagram is to have both current and voltage inputs. Only answer A has both current and voltage inputs.