Research On Intermittent Subsynchronous Oscillation And Subsynchronous Control Interaction

Subsynchronous oscillation is one of the power system stability problems. With the development of power system in the world, especially the continuous construction of high voltage, large capacity and long distant ac/dc transmission and the scale of renewable energies connected to the grid for generating in China have caused new subsynchronous oscillation problems. New subsynchronous oscillation phenomena emerged in succession, which challenge the research on power system stability. The thesis focuses on two types of new emerging subsynchronous oscillation problems, intermittent subsynchronous oscillation (ISSO) and subsynchronous control interaction (SSCI) and carries out the research as follows:When large scales of thermal power plants deliver their power by HVDC and ac hybrid series compensated transmission lines, the converging subsynchronous oscillation may happen, which frequently exceeds fatigue accumulation threshold value of the generator shafts. It is called intermittent subsynchronous oscillation in the thesis. Compared with the characteristics of conventional subsynchronous torsional interaction caused by HVDC, the thesis analyzes the root cause leading to ISSO and the ISSO path in detail. Based on single-machine infinite bus system connected with HVDC, a method analyzing electrical damping sensitivity with respect of electrical parameters in ISSO path is proposed, by which the most sensitive position can be determined. According to the actual ISSO problem existing in Hulunbuir League power system in the east of Inner Mongolia, the equivalent electromagnetic transient simulation model is built, which has5power plants in total including10generators. The test signal method is used to analyze the electrical damping characteristics of the system. According to the most sensitive position by analysis above, a disturbance simulation method is proposed which can stimulate similar ISSO with the actual case and provide feasible simulation method for damping characteristics simulation of mitigation device and experimental verification.The research in recent years indicates that subsynchronous control interaction may happen when a wind power plant including DFIG-based wind-power generators deliver its power by fixed series compensated long distant transmission lines. It may lead to the damage of wind-power generators. In the thesis, the mechanism and interaction path of SSCI is analyzed by math modeling based on a single machine infinite bus system where a DFIG-based wind-power generator is connected to the fixed series compensated transmission lines. And an approximate condition used to determine the existence of SSCI is given. A method used to distinguish induction generator effect (IGE) and SSCI of DFIG is proposed. The effects of wind speeds, series compensation levels and the parameters of rotor side converter controller on the characteristics of SSCI are analyzed by time domain simulation method. According to the analysis of SSCI path and the influence of the controller parameters on SSCI, the interdicting method and the supplementary damping method are proposed to mitigate SSCI and their effectiveness is verified by time domain simulation.Based on the topology of voltage sourced converter in parallel, the method using subsynchronous oscillation-dynamic stabilizer (SSO-DS) to mitigate SSO is proposed in the thesis. And based on SSO-DS, the control strategy of reactive current modulated by subsynchronous signals is proposed. The concrete control process of this control strategy is introduced in detail. And the damping provided by the control strategy is deduced by complex torque coefficient approach and the related influence factors on the direction and size of the damping are analyzed. Based on IEEE first benchmark model where there may be a subsynchronous resonance caused by fixed series compensation, the availability of this new mitigation device called SSO-DS with proposed control strategy is verified by test signal and time domain simulation methods.According to ISSO which exists in Hulunbuir League power system with ac and dc transmission, especially in Hulunbuir power plant, a wide band-pass SSDC damping method and a narrow band-pass SSDC damping method are proposed and their feasibilities for ISSO mitigation are analyzed. By math deduction and simulation, SEDC and SSO-DS are compared in the differences of response time and the ability of providing damping torque. SSO-DS is also compared with subsynchronous resonance-dynamic stabilizer (SSR-DS) based on the topology of SVC in damping ability by analysis and simulation. And time domain simulation method is used to compare the ISSO damping effect among SEDC, SSR-DS and SSO-DS.Based on the theories and methods above and considering response speed, mitigation effect, the floor space limit of the power plant and so on, SSO-DS is recommended to be used in Hulunbuir power plant for ISSO mitigation. In order to test its controller, the real-time digital and physical closed-loop simulation platform is built in this thesis. The structure of the platform and the function of every part are introduced. A method is proposed to interface the real-time digital simulator with the rotating speed acquisition panel by simulating the real rotating speed sensor. By means of the real-time simulating environment, input and output characteristics of SSO-DS test is done to verify the correction of its controller design. A testing method is proposed to measure effective damping area in multi operating conditions for ISSO mitigation and as a result, the parameters of SSO-DS controller can be optimized. Finally, time domain simulation is used to verify the effectiveness for SSO-DS to solve the problems of Hulunbuir power plant. Now SSO-DS device developed by cooperation has been used to solve the problem which has been bothering Hulunbuir power plant for many years, and have had good mitigation effect.