Contrasting the requirements on MMC control for grid code compliance and distance protection

The reduction in synchronous generation and the increase in power-electronic interfaced renewable resources and HVDC links are causing significant challenges to distance protection schemes, including incorrect zone selection, phase selection, or functional directional elements. At the same time, current existing grid codes require converters to inject reactive current during AC-side faults to support the grid voltage. In existing literature, these two topics are investigated separately. However, the contrast between these two requirements has not been studied. In this paper, we contrast the requirements originating from the grid code and distance protection on converter control. First, the critical control parameters that affect the two sets of requirements are identified using sensitivity analysis. Second, the impacts of the identified critical parameters on the two sets of requirements are examined for time domain simulations with the aim of investigating whether there are conflicts between these two control objectives. The research indicates that increasing fault current injection droop and the ratio of AC-side power used for total energy control leads to a greater margin for grid code compliance while yielding adverse consequences for distance protection.