Zero-sequence voltage compensation can improve sensitivity of the zero-sequence voltage in single-circuit transmission systems while it has adverse effect on the zero-sequence directional component of the healthy line in double-circuit transmission systems. The effect is analyzed in different types of double-circuit transmission systems through comparing the relation of the zero-sequence voltage and current before and after voltage compensation. It is found that in the weak magnetic and strong current double-circuit transmission system with electricity connection on one side, the zero-sequence directional component will mal-operate on the side where opposite direction is identified before compensation. In the other situations, the zero-sequence directional component still recognizes forward direction on the side where forward direction is identified before compensation, while it will not operate on the side where opposite direction is identified before compensation. PSCAD simulation results verify the correctness of the theoretical analysis.