In recent years, different protection strategies for high-voltage direct current (HVDC) grids have been developed by the industry and academia for the reliable and safe operation of direct current grids. The proposed strategies have different impacts on the AC networks where HVDC grids are connected. In the case of embedded HVDC grids embedded in a synchronous AC network, a partial loss of the HVDC grid might cause the desynchronisation of the AC grid. Therefore, it is of utmost importance to assess the impact of those protection strategies into the stability of the whole network. The objective of this study is to propose a methodology for the transient stability assessment of a simple but representative AC grid in case of DC fault. After validation of the methodology, some HVDC link protection criteria are defined in terms of the Critical Time to Return to Operation. These criteria will be helpful for the design of HVDC protection systems or for the sizing of future HVDC links in order to respect the constraints of the existing protection strategies.