The longitudinal distance protection of series compensated line may appear external-area fault misoperation situations in certain special operating conditions. With an external-area actual fault investigation， the influence of potential transformer installed in different locations on protection is analyzed. It shows that for smaller capacity power system， the conventional forward and reverse direction quadrilateral distance relay as weak feed discriminant components should not be used for double-ended series compensated lines. Then， an improvement scheme for the longitudinal distance protection is given. Finally， the choice of protective level current of series compensation capacitance and protection configuration of series compensated line are discussed.