Smart Transformer-Based Single Phase-To-Neutral Fault Management

The smart transformer (ST), with its high control capability, offers new features for optimizing the management of the distribution grid. The ST, by controlling the voltage in each phase independently, is able to manage the operations during grid faults, in particular in the single-phase ones. The ST, when a single-phase-to-ground fault occurs, can rapidly reduce the voltage in the phase under fault. Thus, it guarantees system safety and operates with the remaining two healthy phases to avoid unnecessary interruption in the healthy lines. However, the two-phase asymmetrical operation challenges the system performances due to the high power 2nd harmonic ripple in the dc voltage, that reduces the capacitors, lifespan and the high current flowing in the neutral conductor. These issues would force the grid operator to increase the ST maintenance, in order to avoid unplanned faults in the ST hardware, due to failure of aged capacitors. This paper presents a flexible-control strategy, based on the phase-shift angle control between two healthy-phase voltages, that reduces the impact of the power 2nd harmonic oscillation and thus can delay the maintenance intervention. The proposed strategy aims to improve system performances by avoiding neutral-line current overload and attenuating ac side active power (low voltage dc-link voltage) oscillation. Depending on the grid conditions (small dc link capacitance or low ampacity of neutral cable), the voltage angle can be adapted, minimizing the impact of the two-phase operation system in the ST-fed grid. The effectiveness and feasibility of the proposed approach have been validated experimentally with a simplified microgrid setup.