Operation of HVDC converters controlled as virtual synchronous machines under unbalanced conditions

This thesis presents a comprehensive evaluation of control structures for HVDC converters operaterating as Virtual Synchronous Machines (VSM) under unbalanced conditions in both grid-connected and islanded mode. The converters taken into considerations are the two-level VSC and MMC. The obtained control for the converters is a Current Controlled VSM where a quasi-stationary virtual impedance emulates a simplified positive sequence electrical model of a synchronous machine. A Negative Sequence Current Generator is defined, which depends on the control objective for the unbalanced conditions, and for the MMC an additional energy control and circulating current controller are used. It is shown how three general strategies can be selected for controlling the negative sequence currents: i) calculation of the negative sequence current references according to the desired active and reactive power flow characteristics, ii) applying a negative sequence virtual impedance resulting in unbalanced currents as in the steady-state response of a synchronous machine, or iii) operation with a negative sequence voltage controller for eliminating unbalances in the locally measured voltages. For the first approach, four objectives for shaping the power flow characteristics can be selected: i) balanced three-phase currents, ii) constant instantaneous active power flow, iii) constant instantaneous reactive power flow, or iv) reduction of dc side voltage fluctuations. Comprehensive simulation results are presented to evaluate the performance and applicability under grid connected and islanded operation. The results demonstrate how utilization of the negative sequence currents to control the power flow characteristics is only applicable for grid connected operation, while the other two approaches can be utilized in both islanded and grid-connected modes. However, closed loop control for balancing the local voltages depends on a high equivalent grid impedance and is not suitable for operation in strong grid conditions.

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