Neutral Current Optimization Control for Smart Transformer-Fed Distribution System under Unbalanced Loads

In a three-phase four-wire low voltage (LV) distribution system, unbalanced loads lead to neutral current (NC) looping resulting in increase of power losses and variation of neutral potential. Compared to the conventional power transformer, smart transformer (ST) has strict current limitations to avoid overcurrent. However, its advantages on the downstream LV grid voltage regulation can provide the capability to regulate excessive NC. This article proposes a closed-loop NC optimization control in order to, on the one hand, minimize the NC current in the normal operation satisfying the standard EN 50160 requirement, on the other hand, suppress the NC current in extreme cases to avoid the overcurrent damage of the ST. The proposed control strategies are validated by experimental tests via the hardware-in-the-loop setup and a case study based on a 350-kVA, 10-kV/400-V, ST-fed distribution network under unbalanced loading profile according to the three-phase four-wire distribution grid in the Manchester area. The results clearly prove the effectiveness and flexibility of the proposed NC optimization control strategies on the NC suppression and minimization.