The present trend of inserting increasingly more solar photovoltaic (PV) sources into the electricity grid leads to a significant reduction in mechanical inertia. Inertia represents energy reserve in the grid, that inherently and instantaneously supports frequency stability. Therefore, recent studies have re-examined the frequency stability of the grid. Most of these consider scenarios where the participation from grid-following inverters remains relatively low. More importantly, these studies include an infinite bus in their analyses. The infinite bus acts as a very stiff voltage source which has a significantly stabilising influence, the more so as all control refers back to the infinite bus. Here, the frequency stability of a grid with significant generation from grid-tied PV inverters is considered without reference to an infinite bus. In the proposed simplified grid model, all the synchronous generators (SGs) are lumped as one large SG complete with classically operating controls. Similarly, all the PV generators are lumped as one large, quasi-instantaneous, non-linear power source. In this simplified network, the feasible operating region is identified using bifurcation techniques. It transpires that the stable operating region is bounded by a locus of Hopf bifurcations linked (inter-alia) to the fraction of power generated by grid-tied PV inverters.