Research On Grid Voltage Support Strategy After Large-scale Wind Farms Are Connected

With the continuous increase of the grid-connected capacity of wind power,voltage problem of wind power grid-connected operation has caused researchers to pay attention to the voltage regulation capability of wind farms.The current reactive voltage control methods of wind farms mainly include: installing reactive power compensation devices in the wind farms and using the reactive power voltage regulation capabilities of the wind turbines to perform voltage control.This paper takes wind farm conmposed of the most widely used double-fed wind generator actively supporting the grid voltage as the theme,and conducts in-depth and detailed research from the two aspects of theoretical analysis and simulation verification.Some research results have been obtained.(1)Based on the relationship between DFIG stator output active power and reactive power,the reactive power output capability of DFIG is studied.In order to fully tap the reactive power output capability of DFIG,the DFIG active power control mode is improved.When the reactive power output capacity of DFIG is sufficient than the grid demand,DFIG works in MPPT control mode,and the reactive power output of DFIG is zero.When the reactive power output capacity of DFIG is insufficient compared to the grid demand,DFIG works in power-limited operation mode to limit the DFIG stator output active power to enhance its reactive output capability.The principle of reactive power compensation,mathematical model,and control strategy of SVG were studied.SVG was installed in the wind farm to support PCC’s voltage of the wind farm.With the goal of supporting the voltage at the PCC of the wind farm,a coordinated voltage control strategy of DFIG and SVG based on limited power operation is designed.Set the PCC’s voltage threshold.When the PCC’s voltage does not exceed the limit,DFIG works in MPPT mode,and SVG supports the PCC’s voltage.When the PCC’s voltage exceeds the limit,DFIG enters the power limit operation mode,and DFIG supports the PCC’s voltage.Priority is given to the reactive voltage regulation capability of DFIG,and SVG assists DFIG to support the PCC’s voltage.By constructing a simulation model containing DFIG and SVG,it is verified that the proposed coordinated control strategy can make full use of the reactive power voltage regulation capability of DFIG,increase the reactive power margin of the wind farm,and improve the voltage recovery speed.(2)When the TPSS and the wind farm are connected to the same PCC,under the influence of the unbalanced and distorted power grid caused by the TPSS,the mathematical model and operation of DFIG under unbalanced and harmonic power grids are studied.Taking DFIG stator output sinusoidal current as the goal,the current control strategy of RSC is improved.The vector oriented control strategy based on the combination of PI and RC is adopted.The PI regulator is used to track the constant component and control the average power output by the DFIG.The RC regulator is used to track the periodic component caused by the negative sequence and harmonics and control the DFIG to output the sinusoidal stator current.By constructing a simulation model for connecting wind farm to the traction power supply system,the simulation results show that the use of a vector control strategy based on the combination of PI and RC can suppress the negative sequence and harmonics of the DFIG stator output current,and the total harmonic distortion(THD)of the DFIG stator current is lower than traditional PI control.(3)In order to improve the voltage of TPSS,the voltage support strategy of the wind farm is designed.Reasonably use the reactive power output capacity of DFIG and capacitors to realize the active support of the wind farm to the grid voltage,increase the voltage level of TPSS and reduce the investment in the reactive power compensation equipment of the wind farm.The simulation results show that the proposed voltage support strategy can maintain the outlet voltage of the wind farm at about 1 pu under normal conditions and after a sudden increase in traction load,and at the same time increase the voltage level of the traction station.