Active Thermal Sharing Control for a Cascaded Converter in Medium-Voltage Applications

The cascaded three-level neutral-point-clamped (C3L-NPC) converter has been adopted in practical projects for power transmission in medium-voltage (MV) distribution networks. However, this type of converter comprises multiple submodules (SMs), in which thermal imbalance may occur due to a mismatch between component parameters of the SMs. This may lead to decreased system reliability. To address this shortcoming, an active thermal sharing control strategy for C3L-NPC converters is presented in this article. A thermal control loop is incorporated into the inner current controller within each SM. Active and reactive power regulation is conducted based on the individual junction temperature of each SM. A high-level controller is used to regulate the total power and to calculate the temperature reference. The control strategy enables decoupled thermal and power regulation, and each control loop can be independently designed. The effectiveness of the approach has been experimentally validated using a testbed down-scaled from the ANGLE-DC project—the first operational MV direct-current (MVdc) link in Europe. It is shown that the junction temperature of the SMs is effectively balanced without affecting the output power under cooling system failures.