This thesis introduces a medium voltage direct-current (MVDC) system for large spacecrafts with megawatt (MW)-scale power and propulsion systems for interplanetary transport. The proposed MVDC system comprises a nuclear electric propulsion (NEP) powered permanent magnet generator, a solar photovoltaic (PV), a backup battery energy storage system (BESS), an electric thruster, AC/DC loads, and the spacecraft low-voltage DC (LVDC) power system. The NEP is the main power source for the thruster, while the solar PV and BESS support payload and low-voltage systems. The energy sources are connected to the MVDC bus through power electronics converters. Unidirectional boost DC-DC converter and bidirectional boost DC-DC converters are adopted on the PV and BESS sources, respectively, while an isolated DC-DC converter is used to provide power to the thruster. To attenuate external disturbance and enhance the robustness of the proposed MVDC system, uncertainty and disturbance estimator (UDE) based controllers are designed for the power electronics converters in the MVDC system.