Single-phase traction loads generate negative sequence currents, reactive current and harmonics to the power grid. Modular multilevel converter (MMC) is a better alternative to implement railway power conditioner (RPC), which is connected to 25 kV supply system. A MMC with delta configuration is fit for controlling negative-sequence current because circulating currents across delta-connected clusters provide extra flexibility in unbalance control. A RPC based on delta-connected MMC is developed. Steinmetz theory is used to calculate fundamental frequency phase reference current. Since the obtained fundamental frequency phase reference current always has a 90 degree phase difference with the line voltage, the dc voltage balancing of the MMC is easy to achieve. In addition, harmonic reference current among the three phases are balanced to reduce the total rating of the converter. The design and control of the RPC based on delta-connected MMC are validated by simulation results.