Research On Analysis Method Of Subsynchronous Oscillation In Power System And Its Countermeasures

With the unit capacity and total installed capacity gradually expanding, and the FACTS equipments and HVDC transmission widely using in power system, the electric network of China has entered a period with the characteristic of large capacity, extra voltage, long distance and AC-DC hybrid transmission. Due to the complicated structure and complex operation condition of the grid, more and more attention is paid on the phenomena of subsynchronous oscillation (SSO), about which the research will encounter many new challenges. Considered the practical network, this dissertation focuses on the analysis of SSO and its countermeasures. The main work is as followings:(1) From the view of frequency response of linear system, the basic formula of the complex torque coefficient approach is further derived, and then the complex frequency domain expression between electromagnetic torque and the rotor angular increment is obtained. From the view of system stability, the applicability of complex torque coefficient method is analyzed. The results show that the complex torque coefficient method can be used in the multi-machine system with asymptotic stability.Based on IEEE Second Benchmark model for SSO, the impact of the calculation method, the shaft model of non-research unit and equivalent model of non-research model on the analysis results are analyzed. The results show that the electrical damping of non-research unit shaft with a single rigid body model and that with multi-mass models agree with each other well, except near its natural tensional vibration frequency.When non-research unit is equivalent to the power model, it will bring some errors, and analysis accuracy will be influenced. The results further improve the theoretical system of the complex torque coefficient method.(2) In terms of interaction between the tensional oscillation and HVDC system, the mechanism of SSO caused by HVDC is analyzed in detail which provides the theoretic basis for SSO and its countermeasures. Furthermore, this dissertation proposes a design of SSDC for mitigating SSO caused by HVDC and develops particular software for parameter setting and characteristic analysis. Finally, SSDC is used in HU-LIAO HVDC system and its effectiveness is proved.(3) Focus on the first AC-DC parallel transmission system with TCSC in China, the impact of operation mode and compensation mode on the SSO characteristic is analyzed, using the complex torque coefficient and time domain simulation method. The results show grid connection mode is more stable than separate grid mode and TCSC can effectively enhance the stability of AC line and HVDC system, thus improve the total power grid stability.(4) The mechanism of SSSC on active damping SSO is analyzed theoretically. A new method based on single-phase SSSC for mitigating SSO is proposed following the principle that the energy converting between the electrical and mechanical side of generator will be reduced in unbalanced three-phase system. The conclusions provide that the single-phase SSSC can inhibit SSO problem quickly and effectively in the tolerance of three-phase unbalance factor of the network.(5) The damping characteristic of VSC-HVDC is analyzed in this dissertation. Then the complex torque coefficient method is adopted to investigate the impact of control strategy and operation condition on the damping characteristic. A method of using VSC-HVDC equipped with hybrid SSDC (H-SSDC) is proposed to damp SSO caused by the fixed series compensation, and its effectiveness is verified in a typical system. The results show that the VSC-HVDC can largely improve the electrical damping on the adjacent unites, and with H-SSDC, VSC-HVDC can effectively mitigate SSO resulting from the fixed series compensation.