Analysis And Monitoring Of The Influence Of Generation And Transmission Equipment's Control Systems On Power System Oscillations

Power system oscillation is one of the critical problems threatening the secure opera-tion and stability of a power system.With the development of the power system,more and more new generation and transmission equipments are installed in the power grid.System stability characteristic is significantly changed with the implementation of renewable ener-gy and flexible AC/DC transmission systems,and new oscillations,which are different from conventional low frequency oscillations,are also discovered.Control systems of gen-eration and transmission equipments are found to have strong relationship with these oscil-lations.These control systems will introduce new control mode oscillations to the power grid,and may also influence the damping of low frequency oscillations,thus to further in-fluence the oscillation characteristics of the power system.Therefore,analysis and moni-toring of the influence of these control systems on power system oscillations is important for ensuring the stability of power system.In this dissertation,literature review of the analysis and control of power system os-cillation is firstly introduced.The main idea of this dissertation is proposed basing on the limitation of the existing researches.Then,analysis and monitoring of the influence of gen-erator excitation system,turbine governor system,flexible AC transmission system(FACTS)based damping controllers,and rotor side converter of double fed induction gen-erator(DFIG)on the oscillation characteristics of power system are investigated.Main contents and contributions are summarized as follows:(1)The problem that the gain of power system stabilizer(PSS)may not simultaneous-ly satisfy both the damping ratio and the principle that it must be lower than 1/3 of its criti-cal gain in Ximeng power plant is investigated.Mechanism analysis of this problem is un-dertaken using Heffron-Phillips model and a novel PSS is proposed by adding a parallel component on the conventional PSS.The novel PSS satisfactorily solves the problem.Sim-ulation results verify the correctness of the analysis and the effectiveness of the novel PSS.(2)The influence of turbine governor system on low frequency oscillation is investi-gated by torque analysis.The characteristics of frequency modes introduced by the gover-nor system are also investigated by common frequency model of the power system.Fur-thermore,the influence of frequency mode and dead band on low frequency oscillations is also simulated in the 4-machine-2-area system.Validations of the analysis results are un-dertaken in a practical interconnected power system.(3)Through analyzing the relationship between deviation of rotor speed,electrical power and mechanical power with and without the governor system,a method for monitor-ing the influence of turbine governor system on low frequency oscillation is proposed.Case studies are respectively undertaken in the 4-machine-2-area system and a practical inter-connected power system to verify the effectiveness of the proposed method.(4)The problem that multiple FACTS based damping controllers(FACTS-DCs)may cause control mode oscillations due to destabilizing interactions between them is investi-gated.Mechanism of the oscillation of a single FACTS-DC is firstly analyzed.Then mech-anism of the oscillation between multiple FACTS-DCs is carried out.A feedback controller is introduced on each FACTS-DC to mitigate this oscillation.The effectiveness of the feedback controller is validated by simulation studies in the 10-machine 39-bus system.(5)The problem that DFIG may cause control mode oscillations near the frequency of 10Hz when connecting to weak AC grid is investigated.A simplified model for mechanism analysis of the control mode is proposed.Mechanism of the instability and characteristics of this mode is carried out based on the proposed model.Simulations results verify the cor-rectness of the analysis.