The voltage-source-converter-based railway power conditioner（RPC） has the ability to suppress the negative-sequence current under balanced power system; however

it may deteriorate the performance under unbalanced power systems. This paper aims to reveal how the negative-sequence voltage affects the operation characteristics of RPC and the mechanism process of negative-sequence current. Firstly

the principle of RPC is analyzed and the compensation scheme of power-controlled mode is established. Secondly

the effects of negative-sequence voltage on the voltage and currentof V/V transformer’s secondary side are analyzed. Thirdly

the mechanism process of how negative sequence voltage affects the compensation characteristics of RPC is revealed step by step. Then

the analytical expression of current unbalance coefficient is derived using a symmetric component method according to the RPC’s control process as well as the key influencing factors are also analyzed. Finally

the correctness of the analytical expression is verified from multiple aspects by using the electromagnetic transient simulation model.