In many recent blackouts and large system disturbances, the performance of protection systems was found to have played an important role in both exacerbating the disturbance and limiting the spread of the disturbance. In the past, protection systems were not widely modelled in transmission planning studies. Instead, the response of protection relays to faults was assumed to follow fixed behaviour based on conservative values for primary and backup protection operating times. For these reasons, the incorporation of more accurate representations of protection systems into planning studies may help identify potential protection misoperations and ensure that protection relay performance is tuned to yield optimum responses to system disturbances. This study begins by discussing how protection relays may participate and respond to cascading tripping, power swings, and other steady-state and transient power system phenomena studied in power system planning. It will then examine how the settings for relays may be populated with conservative ranges where actual settings are not known or difficult to transfer to the planning simulation tool. The paper concludes with an overview of a tool which has been developed to automatically create relay models in planning simulation based on high-level user guidance.