Research On Remote Testing Of Offshore Wind Turbine Harmonic Adaptability

Wind farms are characterized by extensive use of cables and power electronic equipment, leading to a more serious harmonic resonance problem, especially for the offshore wind farms. Wind turbine grid harmonic adaptability should be carried on in order to avoid the large scale break down under some harmonic in the grid. For offshore wind turbines and onshore wind turbines in special environment, the test equipment can not reach the wind turbine to carry out the harmonic adaptability test, requiring some connecting cable. Affected by the cable, the test conditions at the distant test point should be modified. Some related research is done in this paper in order to eliminate the influence of the cable on the harmonic adaptability test of the wind turbine, and to figure out the harmonic and resonance characteristics of the wind farm. The main research contents and results are as follows:(1) The typical model of DFIG-based wind power system is built. And the characteristics of different models of different components were discussed respectively. The method for the remote detection on harmonic adaptability of the wind power generator is proposed. This paper establishes the distributed parameter model of the cable, using the numerical method to calculate changes of each order harmonic voltage on both ends of the cable with the cable length and the equivalent element parameters. The remote test corresponding harmonic voltage value is worked out with the proximal wind turbine test voltage and change of each order voltage amplitude. Considering the performance of the harmonic disturbances generator and the test efficiency, this paper puts forward the solution of harmonic test in groups. The simulation results show the effectiveness of the proposed method.(2) The influences of different lengths, capacitance and inductance parameters of the cable on the resonant characteristics of the wind turbine is studied in the condition of wind power grid with load by SIMULINK. After that, harmonic and resonance problems of the wind farm are focused on. The effects of the reactive power compensation capacity and the resonant characteristics of the wind farm on the system PCC are also analyzed. The greater the reactive power compensation capacity of PCC, the lower the resonant frequency and the resonant impedance. For the same topological structure of the wind farm, with the increase of the wind turbine capacity, the resonant frequency and the resonant impedance are shifted to the lower direction.(3) In order to solve the wind farms harmonic and potential resonance problem, wind turbine and wind farm composite filtering method is proposed to avoid resonance from the source. For wind turbine, LCL active filter is designed to eliminate the high frequency harmonics of DFIG rectifier, without energy consumption of source of passive damping filter and the oscillation of the undamped filter. For wind farm, based on the harmonic resonant frequency figured out by the simulation results, the method of resonances and harmonics mitigation with a designed C-type filter at the PCC is proposed, to provide reactive power as well. The simulation results show that with LCL active damping filter and C type filter, the harmonic content at grid side is significantly reduced, and resonant harmonic content at PCC decreased obviously. Some other methods of mitigating harmonic and avoiding resonance are also put forward, comprehensive responses to wind power harmonic and resonance problem.