I would like model a grid connected system without reverse power flow
That is already possible with HOMER simply by setting the "maximum grid sale" to zero kW. That means HOMER will not allow any power to flow from the system to the utility grid. If the grid-connected system produces more power than is required to meet the electric load, the excess electricity will not be sold to the grid. Instead, it will serve any deferrable or optional load you have defined (such as an electrolyzer) or be recorded as excess electricity. The "maximum grid sale" input is in the Grid Inputs window.