Research On Active Support Technology Of Direct Drive Wind Motor Under Low/High Voltage Ride Through

Wind power is mainly concentrated in the wind power base in the Three-North Region of China,but the central and eastern regions with the fastest growth in energy demand are areas with low energy distribution in China.Therefore,large-scale wind power bases sent out to load centers through UHVDC has become the main way to achieve cross-regional wind power transmission in China.At the same time,the characteristics of large scale,high penetration rate,high voltage and long distance transmission also make China’s wind power grid stability issues face serious challenges.In this paper,based on the scenario that direct-drive wind farms are concentrated for outgoing transmission via UHVDC,research is carried out to improve the low and high voltage fault ride-through capability of direct-drive wind turbines and the active support capability of the delivery system,for which the following aspects are carried out:(1)This paper analyzes the transient voltage characteristics of the DC system after commutation failure and the power support demand,and establishes an expression for the relationship between the parallel network voltage of the direct-drive wind farm and the reactive power demand of the DC feeder system.Secondly,the expression of the transient reactive current response of the parallel network is established by the correlation equation between the grid-side converter and the filter of the direct-drive wind turbine,and the closed-loop transfer function is optimized with the target of rise time and overshoot,and the current loop parameters with better dynamic performance are obtained.(2)This paper proposes a capacity utilization evaluation method for the GSC output current to find the active and reactive currents that maximize capacity utilization,and selects the most suitable reference value of active and reactive currents to support commutation failure faults based on a comprehensive consideration of capacity utilization and voltage fluctuation levels.(3)An active support control strategy for direct-drive wind turbines in different voltage intervals is proposed.Relying on virtual synchronous machine control,the active and reactive loops are improved,a virtual synchronous machine control strategy based on transient current compensation is proposed,and the virtual parameters are optimized so that direct-drive wind farms have active support capability for voltage and power.After considering the GSC current and the DC voltage constraint together,the magnitude of the DC voltage to be adjusted after the parallel network voltage exceeds the critical value is calculated.Finally,the improved control strategy is verified by PSCAD simulation to provide some active support to the transient voltage and power of the delivery system while effectively avoiding the large-scale off-grid of wind turbines in case of commutation failure faults in the DC system.