Analysis Of Transient Fault Characteristics And Research On Protection Strategies For Doubly Fed Wind Farms

At present,high-voltage centralized transmission of wind power units,after being aggregated at collection stations,has become an important means of clean energy utilization in China.However.DFIGs have transient fault characteristics that differ from traditional synchronous generators,and their impact on the safe and stable operation of the power grid is becoming increasingly evident,particularly when large-scale wind power systems are connected to the grid.This presents significant challenges to the reliable operation of the existing line protection.Therefore,this study focuses on the transient fault characteristics and short-circuit current analysis of doubly fed wind farm(DFWF)and proposes protection strategies for transmission lines and collecting lines of DFWF.We have developed a protection scheme suitable for large-scale doubly fed wind power systems,which is of great significance in ensuring the safe and stable operation of wind power.The main achievements of our study are as follows:To address the issue of complex transient fault characteristics and difficult analysis of short-circuit currents in doubly fed wind farms,a calculation model is proposed for the analysis of short-circuit currents in DFWF.Firstly,a single-machine short-circuit current analysis calculation model for DFIGs that takes into account the low voltage ride-through control strategy of wind turbines is developed.Secondly,the influence of the internal chained structure and dynamic response of the phase-locked loop in DFWF on short-circuit currents is carefully considered,and a short-circuit current analysis calculation model for doubly-fed wind farms is established.To address the issue of decreased performance of existing protection actuations for transmission line in DFWF.a series-compensated protection scheme based on frequency-domain model matching is proposed.Firstly,a frequency-domain equivalent model for a multi-machine grid-connected doubly fed wind farm is established using the short circuit current analytical formula.Secondly,through the analysis of the fault network of the transmission line,both internal and external faults are identified and distinguished based on the differences in their frequency-domain models,by defining a frequency domain model matching criterion.Finally,simulation results demonstrate that the proposed method can accurately identify internal and external faults of the transmission line in a fast and effective manner.To address the issue of risk of misoperation in current protection for collecting lines in DFWF.a protection scheme based on frequency-domain admittance model is proposed.Firstly,the short-circuit current of the collecting line is derived by using the short-circuit current analytical formula of DFIGs that includes the low voltage ride-through feature,along with the typical network architecture of collecting lines in DFWF.Secondly,a frequency-domain admittance model of the collecting lines is constructed based on the high-frequency component of the short-circuit current in DFWF.Thirdly,the protection criterion is devised by analyzing the differences between the frequency-domain admittance models for internal and external faults.Finally,simulation results show that the proposed method is effective in identifying and protecting against different types of faults.