Optimization Research On Extreme Capacity Limitations And Active Power-Frequency Control Of Doubly Fed Induction Generator

HVDC transmission is increasingly becoming the preferred method for trans-regional and long-distance transmission,because of its fast and reliable control,low loss,high economy and other advantages.Asynchronous interconnected power system is an important form of interconnected grids between regions.It is only HVDC transmission lines between sending system and receiving system.It can avoid the large-scale transfer of power flow through AC lines under HVDC blocking,reduce the risk of power angle instability between grids,and improve the stability of the system.However,under the asynchronous interconnection mode,the sending system and the receiving system are isolated and the mutual power support capability between them is weak,causing the frequency stability problem to be prominent.In particular,the sending system is more sensitive to frequency problems because of the phenomenon of small power system containing large capacity generator and relatively weak network structure.Moreover,the sending system needs to transmit a large amount of energy to the receiving system while satisfying the electricity demand in the region.So its frequency stability problem involves the entire system.Therefore,effective frequency control measures of the asynchronous network sending system are proposed to enhance the system stability,based on the analysis of the frequency response in this paper.The main contents and achievements of this paper are as follows:The dynamic response mechanism of the frequency of the asynchronous networked sending system is analyzed,and the conclusion that short-term frequency control measures are important for the stability of the system is proposed based on the review of the research status at home and abroad.Therefore,methods and control strategies to reduce Transient maximum frequency deviation and the quasi-steady-state frequency deviation are proposed in this paper based on the analysis of frequency stability,and validates the effectiveness of the proposed method and strategy based on the proposed evaluation index.At last,the effectiveness of these methods and strategies is verified based on the proposed evaluation indicators.(1)A primary frequency reserve optimization method considering frequency deviation is proposed based on the analysis of power system primary frequency control principle,the effect of primary frequency reserve configuration on frequency deviation and the asynchronous networked sending system’s requirements for reserve capacity configuration.The mathematical model is established with the objective of minimizing the sum of the difference between the actual standby capacity and the optimal reserve capacity of each unit,and a sequential quadratic method is used to solve the problem.Finally,the validity of the method is verified by practical examples.(2)A primary frequency regulation strategy for the hydropower unit that suppresses the water hammer effect is proposed based on the establishment of the hydropower unit model and the analysis of the factors affecting the water hammer effect.Delayed operation time is added to the hydropower unit governor to eliminate system power imbalance due to water hammer effect.The delay time is the sum of the occurrence time of transient maximum frequency deviation and margin time.The occurrence time of transient maximum frequency deviation is adjusted in real time,and it is predicted according to the system disturbance.The margin time is a comprehensive value according to the actual system.Finally,the validity of this strategy is verified by practical examples.(3)HVDC emergency power control can enhance the mutual power support capability between sending and receiving system.However,when the HVDC modulation power is large,the corresponding emergency control measures must be configured on the AC side.Therefore,the emergency cutting strategy coordinated with HVDC power control is proposed.On the basis of analyzing the factors affecting the transient frequency deviation,the B-spline surface fitting method is used to realize the prediction of the maximum deviation of transient frequency in the early stage of HVDC power reduction,and this is used as the starting criterion of emergency cutting machine.The total amount of cutting machines is determined based on the principle of power balance and prediction deviation.The sequence of cutting off the unit depends on the electrical distance between the unit and the HVDC transmission line.Finally,the validity of this strategy is verified by practical examples.