Zero-sequence injection technique for capacitor lifetime extension on the low-voltage converter of a smart transformer

The converter on the low voltage side of a Smart Transformer often has to supply unbalanced loads. The consequent neutral current flows through the DC link electrolytic capackors in a DC-split configuration and hence represents the main cause of their overheating and of their premature failure. Since this kind of unbalanced operation is a very frequent condition to be faced, one way to extend the capacitor lifetime and the system reliability is to limit the neutral current through a zero-sequence current control. Nevertheless, this technique produces a large deviation of the PCC voltages, which no longer meet the limits set by the standards. To this purpose, the present paper proposes a zero-sequence injection technique that can achieve the best trade-off between the capacitor lifetime extension and the quality of the voltages at the PCC.