Studies On Wind Farm-Power Grid Coordination Oriented Active Control Strategies Of Wind Farm

Wind power has become a kind of mature new energy power generation technology of relatively low cost in recent years,of which penetration rate in power grid is increasing with a high speed.However,it is indicated by the practical operating experience that the adverse impact resulting from the disturbance on power grid due to the wind farm trip-off accident has been worse as the proportion of installed wind capacity increases.Besides,wind turbine has lower controllability compared with fossil fuel generation units,which is a potential adverse factor in terms of safe and stable operation of power grids.Therefore,for the power grid with high wind power penetration,there exist two major problems to be solved:one is the secure and stable operation of the wind farm itself,and the other is the control strategy of wind farm for the purpose of participating frequency/voltage regulation of regional power grid.In recent years,many scholars have studied the related problems and put forward various valuable strategies and schemes in this field.However,there still exist some aspects to be further analyzed,namely the deeper exploration on the regulation ability of doubly-fed induction generator(DFIG),together with the wind farm active control strategies concerning spatial distribution and electric topology of wind farms.Thus,taking the grid-connected wind farms of medium and large size as the object to be studied,the wind farm active control problem for the purpose of coping with extreme wind condition and increasing the ability of friendly integration are deeply studied in this dissertation,and the corresponding strategies are proposed,which are proved to be effective and advantageous by means of theoretical derivation and simulation tests.In the aspect of the secure operation and stability control of wind farm,the following work have been fulfilled:Aiming at solving steep down-ramping of wind power resulting from wind turbine cascading trip-off due to extreme wind condition,based on the principle of fluid mechanics and taking the constraints of equivalent damage load on blades and tower into account,the maximum limitation of theoretical active power output value under different average wind speeds and turbulence intensity is analyzed,and a fast active power output regulation strategy of wind turbine in the condition of extreme wind speed put forward.Thus,the load bearing capacity of wind turbines is fully explored to maximally maintain the output power of wind farms and the impact of large-scale wind turbines trip-off on the power system can be reduced to a great extent;On the basis of DFIG reactive power characteristics,the limits of active power and reactive power output is analyzed.To deal with the problem of insufficient reactive power margin under abnormal operation condition of wind farm,an emergency power control strategy taking reactive power capacity margin into account is proposed,by which the active power of wind turbines will be initiatively reduced to increase the reactive power capacity margin attainable.By this means,the risk of large-scale wind turbine trip-off caused by additional voltage disturbance in this scenario can be mitigated;To improve the low voltage ride-through(LVRT)ability of wind farms,the transient characteristics and the LVRT ability of DFIG under fault scenarios are analyzed firstly from the perspective of the whole wind farm.Based on the curve fitting of the rotor current transient process of the DFIG with crowbar quitting,the adaptive crowbar switch-in and switch-off scheme,together with initiative wind-turbine trip-off strategy,is designed,which further ensures the ability of reactive power supporting and LVRT of wind farms under fault scenarios,and therefore improves the operation level of wind farms.In the aspect of wind farm participating in regional power grid regulation,the following work has been accomplished in this dissertation:To cope with the challenge that small signal stability of power systems is weaken by high penetration wind power,a kind of supplementary damping control strategy of DFIG is proposed.By appropriately regulating the active power injected to the power grid after a disturbance occurs,the power oscillation can be lowered remarkably.Furthermore,Multiple-scenario analysis is carried out to highlight the potential of DFIG supplementary damping control in the power system with PSS.The above research can deeply explore the potential of wind farms to take part in the regulation in power grid side,improve the operation level of wind farm integration,and fulfill the coordinated optimized operation between wind farm and power grid.