Frequency Regulation And Post-Fault Active Power Support With Doubly-Fed Induction Generators

With the increasing wind power integrated to the power grid,issues concerned to the frequency stability of the power grid are more significant than ever before.Under the practical operations,the wind turbine generators(WTGs)are required to participate in the frequency regulation of the power grid and provide post-fault active and reactive power support.In this paper,the control strategies of the doubly-fed induction generator(DFIG)to participate in the inertia and primary frequency regulations under the active power shortage are studied,as well as the electromagnetic transients of the DFIG under the fault and the control strategies to provide the post-fault active power support.The main contents of the research are listed below.(1)The control strategies of inertia and primary frequency regulation are proposed considering the rotor speed constraint of the DFIG The active power reserve is realized by the over-speed operation and the variation of the pitch angle at low and medium wind speed.The reference of the output power of the DFIG is determined based on the signals of the frequency deviation,frequency derivative and rotor speed.The proportional coefficient of the frequency signal is tuned considering the rotor speed constraint.The output power is adjusted according to the grid frequency variations in order to guarantee effecti-ve support to the grid frequency.(2)The electromagnetic transients of the DFIG under the fault and the control strategies to provide the post-fault active power support are studies.Based on the voltage,flux equations of the stator and rotor and the control equations of the rotor-side converter(RSC),the accurate analytical expressions of the.stator current,rotor current are derived.The impacts of the PI parameters.on the decay time constant and the angular frequency of the DC decay component are analyzed.The fault current limiter(FCL)is inserted in series with the terminal of the DFIG after the faults.The control strategy to the RSC is modified by incorporating the stator flux transients in the control equation of the rotor voltage,thus the rotor current is regulated to control the steady-state component of the stator power,in this way,the active power of the DFIG is controlled to provide active power support after the faults.The feasibility of the control strategy is validated by the simulation results.