摘要
Abstract
With the implementation of the"carbon peaking and carbon neutrality goals",the wind-thermal bundled system has become the main form of wind power transmission in recent years.However,when the doubly-fed wind farm is integrated into power system with series compensated transmission lines,it may have an impact on the subsynchronous resonance(SSR)of the thermal power unit and threaten the safe and stable operation of the system.Therefore,the oscillation problem cannot be ignored.The influence of sub-synchronous control interaction(SSCI)caused by doubly-fed wind farms on SSR of thermal power units is not clear.
Firstly,the'modular'modeling of each component is carried out for the doubly-fed wind farm and the thermal power unit bundled series compensated transmission system.By extending the complex frequency domain model of the thermal power unit to the doubly fed induction generator(DFIG),a complex torque coefficient method with decentralized elimination of variables suitable for the system with doubly-fed wind farm is put forward.Secondly,the complex torque coefficients of the thermal power unit and DFIG in classical example are obtained by using the proposed method.The mechanism of multi-mode SSR caused by the system is explained from the perspective of damping characteristics,and the SSR mechanism is verified by time-domain simulation.Then,the conclusions and methods are further verified by the actual system examples in North China.Finally,the influence of dominant oscillation factors including series compensation degree,SSCI,series compensation transmission line resistance and wind farm grid-connected capacity on SSR is analyzed.
The mechanism analysis of SSR generation in the system shows that,after the system is subjected to small disturbances,if the electrical resonance frequency caused by series compensation is close to the SSR modal frequency,the electrical damping of the thermal power unit will show a large negative value,resulting in SSR instability.At the same time,when the system has a weak damping SSR mode,the generated small SSR component will enter the control link of the DFIG through the line.If the DFIG shows a large negative damping in this mode,it will help to increase the subsynchronous component current corresponding to the weak damping mode,and then stimulate each other with the shaft of the thermal power unit,resulting in the aggravation of the SSR component in this mode,thus causing SSR instability.
The main conclusions of this paper are as follows:(1)Compared with the traditional complex torque coefficient method,the proposed method can avoid the dimension disaster problem to a certain extent,realize the modularization of each component,and is easy to model and versatile.Compared with the time domain simulation method,it can provide more oscillation information of the system,explain the oscillation mechanism and characteristics,and calculate quickly.It is suitable for multi-machine systems.(2)The inappropriate series compensation degree is the dominant factor causing SSR instability of the system,and different series compensation degrees may lead to different modes of SSR.At the same time,DFIG grid connection may introduce SSCI.If the DFIG presents a large negative damping in a SSR weak damping mode,it will help to increase the subsynchronous component current of the mode,and then stimulate each other with the shaft of the thermal power unit,causing new SSR mode instability.When the two act on different modes,it may lead to multi-mode SSR instability in the system.(3)The series compensation degree has a great influence on the damping characteristics.Within a certain range of series compensation degree,reducing the series compensation degree is beneficial to the stability of SSR.When the resistance of the series compensated transmission line changes,the change trend of different mode damping is different.Selecting a reasonable series compensated transmission line resistance is beneficial to reduce the risk of SSR.With the increase of the capacity of the doubly-fed wind farm,the damping of the thermal power unit increases and the stability of the SSR increases.关键词
双馈风电场/复转矩系数法/阻尼特性/次同步谐振/次同步控制相互作用Key words
Doubly-fed wind farm/complex torque coefficient method/damping characteristic/sub-synchronous resonance(SSR)/sub-synchronous control interaction(SSCI)分类
动力与电气工程
