Mechanism Analysis And Control Strategy On High Voltage Ride Through Of DFIG For Wind Power Generation

Doubly-fed wind generator is the most common used, but also the most sensitive generator to grid faults because its stator side connect to grid directly. The grids connected to the wind farm are usually located at the remote location where is always far away from the center of the load area,and it belongs to the weak grid region which is prone to grid faults. Once grid fault occurs at the wind farm which does not have the fault ride through ability, lots of wind turbines will disconnect from the grid.Once wind turbines can’t provide the frequency and voltage support to power grid, this will lead to a series of chain reaction, and the safe and stable operation of power grid and wind field can’t be guaranteed.In recent years, low voltage ride through has attracted widely concern both at home and abroad, and mounts of research results has been obtained. However,High voltage ride through did not get enough attention. The difference between HVRT and LVRT is not enough obviously.further more,study on HVRT currently is mainly focus on the transient process.Researching during the fault has not been concerned. So corresponding theoretical analysis and control strategies has been made in this paper. The concrete research content is as follows:1)During high voltage fault,the reactive power need to be compensate the DFIG to support the grid. Reactive power compensation fast and smoothness have strict requirements in grid code.The power of inverter is gradually increased,Power devices’ switching loss has seriously affected the efficiency of the system.Switching frequency of MW-level inverters are offen set at about 1kHz,some even low than 1kHz.For conventional proportional-integral (PI) control,Reducing the switching frequency has greatly influence on the dynamic performance of the system.The low bandwidth has influenced the stability of the system.Therefore, the transient and steady-state performance of the system during grid fault depends largely on the PI parameters.Based on the above shortcomings of conventional vector control.Rotor current predictive control(RCPC) has adopted in this paper.Has greatly improved the speed and smoothly of the reactive power compensation and accelerated flux attenuation.2)The transient process of HVRT and LVRT are very similar.They both oscillation at 50Hz in stator flux.But the difference during the fault is greate.The rotor voltage may be increase since the stator voltage swell during the fault.Rotor side will not be controlled if the rotor voltage swells so large that enter the modulation area. Another,the large voltage in rotor side will make it charge the DC side.It seriously affected the stability of the system.In this paper, the modulation closed-loop control was used to improve the stability of the system.3)The biggest difference between LVRT and HVRT is the source.The Line impedance in votage sag is inductance characteristics.But the Line impedance in votage swell is capacitance characteristics.At this point, the stability of system will be affected. The stator current feedback impedance compensation was used in this paper.The stability of system during voltage swells has been improved.