Research On Analysis Of Subsynchronous Oscillation In AC-DC Power Systems And Its Countermeasures

In this paper, subsynchronous oscillations in AC-DC power systems and its countermeasures are focused, and the mechanisms of subsynchronous resonance in series capacitor compensated transmission system and subsynchronous oscillation in HVDC transmission system are analyzed detailedly. The oscillation modes are determined by resonance frequency of series compensated system or by negative damping range of HVDC transmission system. From the electrical damping of power system, the damping characteristics of power are studied wholly through complex torque coefficient approach realized by time domain simulation. Based on those, mechanisms and control system design of static var compensator used to damp subsynchronous resonance in series capacitor compensated system and supplementary subsynchronous damping controller used to damp HVDC transmission system are studied relatively deeply. And the correctness of the mechanisms is proved through time domain electromagnetic simulations.The transfer and conversion principles of subsynchronous frequencies between segments are derived in the electromechanical torsional interaction by establishing the mathematical model of AC-DC system. The frequency response characteristics of subsynchronous oscillations are analyzed in frequency domain, which can be used as theoretical instructions to design the countermeasures. In consideration of nominal multi-machines power plant, the mathematical model of torsional interaction among multi-shafts is established, and the in-phase modes and anti-phase modes torsional frequencies and mode shapes as well as the method to polymerization of multi-units are provided. Meanwhile, the influences of the excitation system and power system stabilizer on the subsynchronous oscillation characteristics are analyzed. The PSS with rotation speed deviation as input and the generator shaft have obvious torsional interactions. It is recommended that the PSS should filter out the subsynchronous frequency signals if the transmission lines are compensated by series capacitor.A comprehensive research is developed on the operation principles of static var compensator to damp subsynchronous resonance and the subsynchronous frequency modulation of fundamental wave susceptance of SVC to generate the complementary subsynchronous frequency current. It is proved that the optimum control strategy is to modulate the generated current anti-phase with the speed deviation of generator. Upon its fast response, the supplementary subsynchronous damping controller is added into the control system of HVDC to damp subsynchronous oscillations caused by HVDC, while it has no impact on the characteristic harmonics of AC and DC sides. Both of the tow countermeasures are realized in certain power plants or convertor stations, which exert suppression functions to guarantee the power system from subsynchronous oscillatins.