Computationally efficient long-horizon direct model predictive control for transient operation

In this paper we present modifications in the sphere decoder initially introduced in [1] and modified in [2] that allow for its implementation in transient operation. By investigating the geometry of the integer problem underlying direct model predictive control (MPC), a new sphere that guarantees feasibility and includes a significant smaller number of candidate solutions is computed. In a first analysis, the computational complexity can be reduced by up to 99.7% when a variable speed drive system consisting of a three-level neutral point clamped (NPC) voltage source inverter and a medium-voltage induction machine is examined. As also shown, optimality is sacrificed only to a limited extent, thus maintaining the very fast transient response inherent to direct MPC.