| China is vast in territory,the energy base and load center show an inverse distribution.To improve the unbalanced situation of energy demand and power supply,there is a long-term development strategy named " power transmission from west to East,mutual supply from north in China’s power grid.At the same time,with the transformation of global energy and the goal of “carbon neutrality” and “peak carbon dioxide emissions”,the technology of Ultra-high voltge AC(UHVAC)transmission has become an important networking option for global energy Internet and integration of renewable energy.Secondly,due to the outstanding advantages in strong controllability,large transmission capacity,small transmission loss,high-voltage DC transmission(HVDC)technology is widely applied in various scenarios such as longdistance transmission,the integration of renewable energy and as well as asynchronous networking.Based on the background of imperious demand of cross-regional and longdistance power transmission,this dissertation studies the oscillation problems of the long-distance transmission of large-scale power in the form of UHVAC transmission or HVDC asynchronous interconnection.The mechanism and damping characteristics as well as suppression method of low-frequency oscillation(LFO)and ultra-lowfrequency oscillation(ULFO)issues are investigated integrally.The main work and contributions are included.(1)To clarify the risk of low-frequency oscillation in large-scale power transmission system,the key factors that limit the transmission capacity of UHVAC system are found out and the mechanism of low-frequency oscillation are studied.Firstly,the Philips-Heffron model of synchronous generator is established for the analysis of low frequency oscillation,and the electromagnetic damping torque coefficient is derived to evaluate the stability of low frequency oscillation by complex torque coefficient analysis method.Then,by analyzing the influence of voltage level,operation condition,transmission power and electrical distance on damping characteristics of the system,the sensitive factors limit the power transmission capacity of AC transmission system are defined.The results show that the damping level of 1000 k V UHVAC transmission system will be weaken with the increase of transmission capacity and electrical distance,which also increases the risk of low frequency oscillation.Compared with the conventional 500 k V AC system,the stable operation margin is seriously reduced and the dynamic stability is worse.Finally,the simulation results in multi-machine system verify the correctness of analysis of the key factors and conclusions of the low frequency oscillation stability analysis.In addition,some planning and operation suggestions of AC systems are given by quantitative analysis.(2)In regard of the problem of ultra-low frequency oscillation in hydro-dominant power system,the mechanism and influencing factors of ultra-low frequency oscillation are studied,and the optimization method of governor parameters is proposed to suppress ultra-low frequency oscillation.First,the transfer functions of mechanical power of different types of generator units are derived.Then,the comparisons on the small signal stability of the prime mover and governing system of hydropower units and thermal power units are studied.It is concluded that the time constant of water hammer effect of hydropower units will introduce a lag phase in the ultra-low frequency oscillation frequency band and provide negative damping torque,thus causing ultra-low frequency oscillation.Secondly,the mode analysis method is used to calculate the participation factors of each part.The results show that the governor is the key part to ultra-low frequency oscillation.Then,in order to further clarify the influence of the governor on the damping characteristics of the system,according to the complex torque coefficient analysis method,the mechanical damping torque coefficient of the hydropower unit under three control modes including opening mode,power mode and islanding mode is derived,and the influence of governor parameters on damping level is presented.It is determined that the proportional coefficient and integral coefficient of the governor are the important factors affecting the damping characteristics in ultra-low frequency band.The unreasonable setting of turbine governor parameters will worsen the damping of system and increase the risk of ultra-low frequency oscillation.Moreover,to suppress the ultra-low frequency oscillation,the optimization method of governor parameter is proposed,which considers the frequency modulation capability and damping requirements.Finally,the correctness of the mechanism analysis and the effectiveness of the suppression method are verified by the simulations.(3)Aiming to make it clear that the relationship and difference between low frequency oscillation and ultra-low frequency oscillation,the key factors and evolution mechanism that affect different types of oscillations in power system are investigated.First of all,an electromagnetic/mechanical torque model is established for the unified analysis of both low frequency oscillation and ultra-low frequency oscillation.Then,the mode analysis method is applied to research the influence of different parameters on the oscillation frequency.The results show that the electrical distance is the key factor to determine the oscillation frequency.Then,by changing the electrical distance and observing the change situation of the participation factors of various state variables,it is found that with the increase of the electrical distance,the participation degree of the prime mover and the governor in the dominant oscillation mode will increase significantly,then the mechanism of the evolution from low frequency oscillation to ultra-low frequency oscillation in single machine system is revealed.Secondly,a modified Philips-Heffron model is proposed for the analysis of ultra-low frequency oscillation by taking the load voltage as the state variable.The small signal analysis results show that the modified model can correctly identify the ultra-low frequency oscillation modes and is consistent with the simulation results.Finally,the influence of energy structure on the characteristics of ultra-low frequency oscillation is analyzed.It is found that the damping level of ultra-low frequency oscillation can be improved by adjusting the proportion of hydropower and thermal power.The simulation results in a multi-machine system verify the analysis and conclusions.(4)In order to improve the dynamic damping level of large-scale energy transmission system,a multi-objective robust control method is presented to suppress different types of oscillations.In view of the problem that when the hydro-dominant power system operates from synchronous interconnection to asynchronous interconnection,the oscillation of hydropower system is changed from low-frequency oscillation to ultralow-frequency oscillation.Based on this,taking different oscillation modes as control objectives,the principle of additional robust controller to suppress system oscillation is analyzed.Then,based on the H2/H ∞ robust control theory,the feedback controllers with optimal robustness and controllability are designed,including HVDC additional damping controller for low frequency oscillation and governor additional damping controller for ultra-low frequency oscillation,so as to improve the dynamic damping level of the system as a whole.Lastly,the effectiveness and robustness of the designed controller under different oscillation modes are verified by simulations,which greatly guarantees the successful transmission of large-scale power units and the stability of power system. |
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