Transient Optimal Control Strategy For Doubly-fed Wind Power Generation System To Improve Low Voltage Ride-through

With increasing penetration of wind generation in power grid, the capability of maintaining grid connection for wind generation system during grid voltage sags is attained intensively attention at high-penetration countries. In order to prevent over voltage and current from destroying sensitive excitation converter and to reduce stress impact on mechanical components, disconnecting wind-power generator from grid is taken in a condition of low wind power penetration during more serious grid voltage sags occurring. However, in a condition of high penetration of wind power to grid, a large-scale disconnection from grid is bound to aggravate grid fault, even leading to grid collapse. Therefore, western European countries superior in wind-power technology have developed strict wind-power grid codes for connection. With unprecedented develop of wind power in China, it is urgent need to learn superior low voltage ride-through (LVRT) technology.A bypass resistor-set known as crowbar is used to damp excessive rotor current in order to protect excitation converter and to improve LVRT for wind-power doubly-fed induction generator (DFIG).This LVRT strategy has following deficiencies: Rotor circuit of DFIG shorted by crowbar consumes reactive power during the fault, which will lead to bad influence on the recovery. DC link capacitor may be exposed to an over-voltage by crowbar resistance and an over voltage may threaten safe operation of converter. Excessive energy stored by DC link capacitor is difficult to relieve. Crowbar removal may cause rotor current spike and torque fluctuation.In order to solve the deficiencies of conventional crowbar technology, a novel fault ride-through (FRT) technology is proposed. In terms of generally three ways to improve LVRT for DFIG during grid fault:(1) enhancing the amount of reactive current injected to grid;(2) optimizing converter control strategy;(3) protecting converter from over current by using crowbar set, the novel FRT strategy is developed.The proposal transient control mode contains four parts: crowbar set, parallel converter, reference reactive current and reactive current priority. By comparative study to conventional crowbar technology, the novel FRT technology has capacity to promote wind farm terminal voltage, to enhance active and reactive power of output, to reduce torque fluctuation, to support the voltage at the point of common coupling (PCC) and to resume terminal voltage immediately during the recovery. The design of transient control mode is guided by E.ON grid code in this paper, which has practical significance for China to develop and improve wind-power grid code for connection.