The coordination strategy and operation condition of the C1 capacitor protection of AC filter used in HVDC systems of CSG are analyzed. The advantages and disadvantages of the three unbalance protection principles are studied. The paper points out the defects in the sensitivity and reliability of the present unblanced protection principle using the counting method. Combining the ratio method, bridge differential method and pulse method, the paper proposes to improve the fixed value of pulse unbalance counting protection and first puts forward the multi-grade counting method. The coordination with the bridge differential protection is proposed based on the special operation status.