HVDC grids with heterogeneous configuration stations under DC asymmetrical operation

•HVDC grid with monopolar and bipolar converter station configurations is analyzed.•DC asymmetric operation depends on the grounding system and the control strategy.•HVDC mesh grids can result unstable after a pole contingency.•Grounding systems impact on over voltages and over currents after contingencies.•Droop gain in distributed voltage control influences in the system stability.