An active damping scheme of the voltage-source converter driving the induction machine damping unit for mitigating subsynchronous oscillation in power systems

An induction machine damping unit (IMDU), installing on the main shaft of the turbine-generator unit, is capable of damping subsynchronous oscillation (SSO) of the shaft. The electrical damping of IMDU in the subsynchronous frequency range is the key index of measuring its damping effectiveness. As the electrical damping can be increased by raising its rated frequency, the IMDU should be driven by a voltage-source converter (VSC). In this paper, an active damping scheme, namely, adding a feedback path from the speed of IMDU to the output voltage waveform of the VSC driving it, is proposed. The relationship of the electrical damping of IMDU and the d_q components, V_d and V_q, of its terminal voltage, is analysed and it is demonstrated that by introducing a proportion regulator in the d-axis and an integrator in the q-axis, the electrical damping of IMDU in the subsynchronous frequency range can be increased further. It is verified through eigenvalue analysis and time-domain simulation that with the active damping scheme applied, the required capacity of IMDU can be reduced and the SSO will decay in a faster rate.