Dynamic Behavior of a 21-Level (Line-to-Line) BTB System Based on Series Connection of Sixteen Converter-Cells under a Single-Line-to-Ground Fault Condition: Experimental Verification by a 200-V, 20-kW Laboratory System

This paper deals with dynamic behavior of a 21-level (line-to-line) BTB system based on series connection of sixteen converter-cells under a single-line-to-ground (SLG) fault condition. When the SLG fault happens, an imbalance in the supply voltage brings an amount of negative-sequence current to the BTB system. The negative-sequence current should be mitigated because it would result in an increased supply current and a dc-capacitor voltage fluctuation. This paper proposes a new control method intended for reducing both negative-sequence current and dc-voltage fluctuation to acceptable levels. A laboratory system rated at 200 V and 20 kW confirms the validity of the control method. In addition, this paper addresses the issue of magnetic saturation in the 16 three-phase transformers