Research On High Voltage DC Converter For Offshore Doubly-fed Wind Power Generation System

With the continuous expansion of offshore wind power generation,the use of flexible HVDC will become an inevitable development trend.Doubly-fed induction generator(DFIG)holds dominant position in wind turbine market due to its advantages of low cost and strong controllability.However,many problems of traditional DFIG flexible DC grid topology were found,such as high maintenance costs,low energy transfer efficiency and complex control strategy etc.In this paper,a new type topology of doubly fed wind power generation system is proposed.First,this paper presents a new type topology of doubly fed wind power generation,which includes 6 parts,i.e.a rotor side converter(RSC),a stator side converter(SSC),low-voltage DC bus,a grid side converter(GSC),high-voltage DC transmission bus and the converter in the receiving end station.Comparison results with the traditional topology are given in terms of energy transfer efficiency,converter capacity,system operation and so on.In the new topology,the stator and rotor of DFIG are connected with low-voltage DC bus through the RSC and the SSC respectively.The RSC plays a major control role.The SSC isolates the DFIG stator from the power grid,which plays a major role in response to the power grid voltage drop.The stator voltage-oriented control strategy is employed by both the RSC and SSC.The RSC adopts double loop control strategy,and the outer loop is a power loop,the active power part realizes maximum power tracking as the target,the reactive power part controls the reactive power of the stator being zero as the target.The inner loop is a current loop,and its goal is to improve the dynamic response ability of DFIG.The goal of the SSC is to realize the voltage stability of the stator side.The linkage between low voltage DC bus and high voltage DC bus is realized by GSC which completes the bi-directional flow of the energy.In the start-up phase,the GSC operates in the voltage step-down mode to supply the excitation power of the DFIG,and in the steady state,it transmit the energy to the network and meanwhile keep the voltage of the LV DC bus stabile.Second,the design of the converter in the receiving end station is given.In view of the flexible HVDC technology,this paper presents an active power allocation strategy to stabilize the voltage of the high-voltage DC transmission bus.On the one hand,it keeps the voltage of the high-voltage DC transmission bus stabile as the wind power changes.On the other hand,it adjusts the allocation of the output active and reactive power of the converter in the receiving end station according to the grid's demand.Considering the grid connection of the converter in the receiving end station in the case of three-phase unbalanced grid,this paper analyzes the positive sequence and negative sequence of the voltage as well as harmonics in the unbalanced grid.The three-phase unbalance of grid will lead to generate 2nd harmonic in the DC bus side,which results in generating the 3rd or higher harmonics in the AC side of the converter in the receiving end station.The second-order generalized integrator(SOGI)is a lowpass filter that can filter most of the harmonics in the grid.It extracts the positive and negative sequence component of the voltage in the unbalanced grid.Meanwhile,the SOGI is used as the phase-locked loop(PLL)in the converter.Compared with the traditional PLL which includes decoupling and PI control strategy,the orthogonal principle of the deviation and negative sequence component is adopted in this paper to realize the real-time angle tracking,which simplifies the control strategy.The expressions of the active and reactive power concerning the positive and negative sequence components are presented according to the pq theory based on the analysis of the output power at the public connection point.Additionally,the negative sequence power is set to be 0 in order to restrain the 2nd harmonic in the DC side,therefore the efficient connection into the unbalanced grid is realized.Finally,a 3kW flexible HVDC test platform for the DFIG wind power generation is built and the proposed topological structure and the control strategy are verified.Experiments show that the converters can operate normally when the DFIG runs at the whole range of the wind speed.