To ground capacitance of a power line modelling
I have never seen it modeled as a current source, if you know the distances and sizes of your cables and the distance to ground it is possible to calculate the capacitance itself and consider it an impedance in your power flow.

$$
Cn = \frac{2\pi\epsilon}{ln\frac{D}{r}}\frac Fm
$$

Is the most basic formula, there are more advanced ones, but this one is a decent approximation.

So I would just calculate, get the impedance depending on your frequency and just add it to your matrix. This will just affect the elements on the main diagonal.

source for the formula: 

https://circuitglobe.com/capacitance-of-transmission-line.html