With the growing number of grid connected solar farms, management of power quality has become an important task for the proponents of such solar farms and the network owners and operators. One major concern is the harmonic compliance at their Point of Connection, particularly at low order harmonics up to the 50th considering relatively low switching frequencies (e.g. 3 kHz) of the large inverters used in these solar farms. Norton/Thevenin models are commonly used as inverter harmonic models to determine the harmonics injected into the grid. Employing software models of inverters and their control systems, harmonic injection method is widely used to determine the parameters of these Norton/Thevenin models at specific harmonic frequencies of interest, as an alternative to experimental determination of such parameters. This paper gives a methodology for the determination of these parameters mathematically using the details of the inverters and the control system thus making the approach attractive in comparison to harmonic injection method. It has been found that the Norton/Thevenin model parameters determined using the described theoretical approach is comparable to those determined using the harmonic injection method.