Control of Smart Transformer under Single-Phase to Ground Fault Condition

In low voltage (LV) electric distribution grid, the continuity of power supply service is degraded by various grid faults. Even under single phase to ground faults, the continuous operation capability of healthy phases is also limited. The smart transformer (ST) can establish each phase voltage independently and thus can switch the operation mode from three-phase to two-phase operation during single phase faults, ensuring continuous power supply in two healthy phases and minimizing outage area. However, different from the three-phase operation mode, both high 2nd-order oscillation in dc-link and high current in neutral conductor challenge system performances and reliability under the two-phase operation. Thus, an improved two-phase voltage control strategy is needed either to limit the neutral current amplitude avoiding over ampacity (reducing the power losses and suppressing neutral potential variations), or to suppress dc power oscillation (preventing the lifetime reduction of dc-link capacitors). This paper focuses on the control system design of ST voltage control for the two-phase operation to improve the grid voltage quality. The simulation and experimental results clearly verify the effectiveness and correctness of the proposed strategy.