Study On Stability Criterion Of A Grid-Connected System Of Direct-Driven Wind Farm And Interaction Of Sub-Synchronous Oscillations Among Wind Turbine Clusters

China’s CO2 emissions are aiming to peak by 2030 and to become carbon neutral by 2060.A new power system based on renewable energy is a fundamental measure for China’s low-carbon transition.Unlike traditional rotating generators,most new energy units are grid-connected with three-phase inverters,and the interaction between inverters and the grid may trigger sub-synchronous oscillation problems,endangering the safe and stable operation of the grid.In 2015,a large direct-drive wind farm in the Hami region of Xinjiang experienced persistent SSO.To prevent and control SSO,there is an urgent need to construct an online stability domain for a multiple-converter grid-connected system,however,the generalised Nai However,the computational complexity of the generalized Nyquist criterion makes it difficult to analyze and warn of SSO online,and new stability criteria with low computational effort and low conservativeness are urgently needed.Moreover,there are a large number of different types of machine groups in new energy generation systems,and most of the previous studies are based on the single machine equivalence model of wind farms,lacking the study of the interaction of different machine groups within wind farms.Therefore,the main research content and results of this paper are as follows:(1)This paper presents a minimum forbidden stability criterion based on Geier’s principle.Its fast estimation of the range of the system eigenvalues rather than the specific eigenvalue calculation can improve the efficiency of stability domain construction,and the conservativeness of the criterion is reduced by using the reduced forbidden zone range and probabilistic principles.The minimum forbidden stability criterion is less conservative than the norm criterion and the left half-plane forbidden criterion,resulting in more accurate stability analysis results.Compared to the generalised Nyquist criterion,the minimum no-zone stability criterion has a shorter computation time,providing an important tool for fast and accurate online analysis and early warning of SSO.(2)The stability of the grid-connected system of direct-drive wind farms is analyzed based on the minimum forbidden stability criterion of Geier’s principle.The minimum forbidden stability criterion is applied to the analysis of the grid-connected stability of direct-drive wind farms,providing a new online analysis tool for the effective prediction of SSOs.(3)The interaction of SSO between different clusters within a wind farm is analysed by applying the minimum forbidden stability criterion and the generalised Nyquist criterion.The results show that the "oscillating clusters" can cause the"stable clusters" within the wind farm to oscillate at the same frequency.An increase in the proportionality and integration coefficients of the phase-locked loops of the"oscillating fleet" will lead to an increase in the frequency range of the SSO within the wind farm.This paper also verifies the validity and practicality of the new criterion for the stability of the minimum forbidden zone based on Geier’s principle.