| Wind generators grouping tripped off(GTO) from grid occasionally occurred recently, which brings serious power impacts to power system. Wind generators GTO from grid not only affects the operation efficiency of wind farms, but also becomes the potential safety hazard to the operation of power system. Therefore, revealing mechanisms of wind generators GTO from grid and constructing preventive and control measures have important theoretical significances and practical values for enhancing the operation level of wind power and promoting large-scale utilizations of wind energy.In this paper, firstly, considering influences of crowbar protection and overspeed protection, the mathematical model of wind turbines with doubly-fed induction generator(DFIG), which is the mainstream wind power generating set at present, is established to lay a foundation for analyzing influences of load conditons of DFIG, degrees of voltage disturbances and protection strategies on operations of wind turbines with DFIG.Based on superposition principles, the analytic model to calculate rotor currents stimulated by voltage disturbances is established. The influences of degrees of voltage disturbances and load conditons of DFIG on rotor currents after voltage disturbances are analyzed.According to grouping of DFIGs equipped with passive crowbar protection, the mechanism of DFIGs simultaneously tripped off (STO) from grid caused by action of crowbar protection induced by serious voltage disturbances is explained. The simulation models of wind turbines with a DFIG connected to grid is developed on PSCACD/EMTDC, using which the critical terminal voltages to trigger crowbar protection after voltage dip are obtained. Based on this, the simulation model of the wind farm located at Northeast China is developed on PSAT, using which DFIGs STO from grid is analyzed based on the measured operation datas of the wind farm.For grouping of DFIGs equipped with passive crowbar protection, according to a sequence of a single DFIG tripped off from grid caused by rotor overcurrent, the phenomenon that DFIGs absorb reactive power from grid due to a short-time state of the squirrel cage induction motor before stator windings disconnected from grid, which causes voltages of collecting systems in wind farms to drop further and may lead to other DFIGs to trip off from grid, are explained. On this basis, the mechanism of DFIGs cascading tripped off (CTO) from grid near full load conditions is revealed and a calculating and analyzing method of DFIGs CTO from grid based on section calculations of power flow is bult. The simulation model of the wind farm located at Northeast China for analyses of DFIGs CTO from grid is developed based on PSCACD/EMTDC and PSAT, using which a real case of DFIGs CTO from grid happened in the wind farm is analyzed. The analysis results verify the rationality of the revealed mechanism of DFIGs CTO from grid and the effectiveness of the established calculating and analyzing method of DFIGs CTO from grid.For grouping of DFIGs equipped with active crowbar protection, according to different strategies of crowbar protection, the electromechanical transient process of a DFIG induced by voltage disturbances is analyzed. The influences of different factors on torque equilibrium of DFIGs during the electromechanical transient process, such as degrees of voltage disturbances, resistance values of crowbar protection and the reference power of DFIGs after reset of crowbar protection, are investigated. On this basis, the mechanism of DFIGs overspeed tripped off (OTO) from grid caused by action of crowbar protection induced by rotor overspeed is revealed. The criterion for checking DFIGs OTO from grid is deduced and a calculating and analyzing method of DFIGs OTO from grid is bult. The simulation model of the wind farm located at Northeast China for analyses of DFIGs OTO from grid is developed based on PSCACD/EMTDC and PSAT, using which a real case of DFIGs OTO from grid happened in the wind farm is analyzed. The analysis results verify the rationality of the revealed mechanism of DFIGs OTO from grid and the effectiveness of the established calculating and analyzing method of DFIGs OTO from grid.In respect of prevention and control of DFIGs GTO from grid, for DFIGs STO from grid, the preventive measures based on raising setting values of passive crowbar protection and increasing converter capacity are proposed. For DFIGs CTO from grid, the prevention and control strategy based on an optimal setting of reference power of DFIGs after reset of active crowbar protection is proposed and setting formulas for reference power of DFIG is deduced. For DFIGs OTO from grid, the preventive measures based on an optimal setting of resistance values of active crowbar protection is proposed and setting formulas for resistance values of crowbar protection is deduced. On this basis, the effectiveness of the proposed prevention and control measures is verified. |
