Passivity-Based Control Strategy for SMES Under an Unbalanced Voltage Condition

This paper presents a novel nonlinear control strategy for a superconducting magnetic energy storage (SMES) system during network unbalance. Grid voltage unbalances tend to substantially degrade the operation of a SMES system; conventional control methods cannot completely address this issue. In this paper, three selectable control targets for the SMES are identified according to the working principle of the SMES under an unbalanced voltage condition to reduce the impacts of second harmonics in output power, output reactive power, and grid-side current. Next, the port-controlled Hamiltonian models of the SMES are established, and the positive- and negative-sequence interconnection and damping assignment passivity-based control (PBC) strategies are proposed accordingly. Simulation results show that the PBC has stronger robustness in both steady and dynamic states compared with the conventional proportional integral method, which effectively suppresses the oscillations caused by the unbalanced voltage.