Network Resistance as a Function of Frequency (Freq. Sweep)

The second line is the derivative of the first line. Does that help?

It just looks like a parallel resonant circuit with some resistance (tank circuit) to me. Resonant frequency looks like about 1400 hz. Other than the Q of the circuit, I don't see how anything else can be deduced without more information.

Every Grid has resistance and inductance.

At power frequency, the load resistance is dominant, but the inductance of motors, generators and transformers is significant & many situations can be modelled with resistance & inductance.

There is also capacitance, in open power lines, buried cables and equipment. In small systems without solid insulated cables it can be ignored, but long power runs make it significant (particularly since generators are limited in their capacity to feed capacitance VA compared to inductance VA).

The frequency you have highlighted is where the impedance changes from inductive-resistive to capacitative-resistive. At resonance there is only resistance.

As has been pointed out, this is a characteristic of L-C-R parallel resonant circuits.

I suggest you check what parallel capacitance & inductance is in the model system & see if the resonant frequency matches the frequency - impedance plot.

It may have a low band pass filter somewhere in the circuit.