Modal control of an HV DC system for the damping of subsynchronous oscillations

A novel control scheme is presented for the design of a supplementary subsynchronous damping controller (SSDC) equipped at the rectifier end of an HV DC link that is connected in parallel with an existing resonating AC line. To stabilise the unstable subsynchronous resonance (SSR) modes, an approach based on modal control theory is developed for the design of the SSDC. Since the subsynchronous oscillations resulting from high levels of compensation on the AC line can be effectively damped out, the power transfer capability of the AC line can be enhanced by the proposed SSDC. In addition, the SSDC can also be used to increase the damping of torsional interaction between the HV DC link and turbine generator, thereby qualifying the HV DC link equipped with the proposed SSDC as an alternative to a new, second AC line where long distances of transmission may economically justify the use of an HV DC line.