Investigation of the interaction characteristics revealed that the destabilizing influence on rotor torsionals is due primarily to the constant current or constant power control method employed in virtually all HVDC systems. Thus, the research effort was directed towards modifications of the HVDC converter's current control loop;in concept by adding a new control function, a subsynchronous damping controller (SSDC). This was accomplished, and the developed SSDC consists of a wide-bandwidth controller sensitive to the frequency of an ac signal synthesized from voltage and current measured at the HVDC converter terminals. With such a design, the SSDC can be made to provide positive damping over the entire range of subsynchronous torsional frequencies.
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