电网电压不平衡时全功率风电并网变流器的控制策略

电网电压不平衡条件下,电压负序分量将导致直驱式永磁同步风电机组(permanent magnet synchronous generator,PMSG)全功率并网变流器的直流侧电压出现2倍电网频率的谐波,长期处于此工况下将显著影响直流侧电容的使用寿命,危及机组的稳定运行。建立了电网侧变流器的数学模型,在计及并网阻抗对有功功率影响的基础上,提出了一种以增强直流侧电压稳定性为目标的控制策略。其中,电网侧变流器正负序电流指令通过保证直流侧电容及并网电抗器之间无2倍频振荡的有功功率流得到,指令的计算无需求解复杂的高阶矩阵,也没有引入更多的变量。由直驱式永磁同步风电机组的仿真结果验证了所提出的电压控制策略的有效性。与传统的平衡控制策略相比,这种控制策略能在实现直流侧电压稳定控制的同时,使输出至电网的有功功率2倍频波动得到有效抑制,提高了该型机组不平衡电网电压条件下不间断安全稳定运行能力。

A Control Strategy for Grid-Connecting Full Power Converter of Wind Power Generation System Under Unbalanced Grid Voltage

When grid voltage is unbalanced,the negative sequence component of grid voltage leads to harmonics with twice as much as power frequency in DC side voltage of full power grid-connecting converters for direct drive permanent magnet synchronous generators(PMSG),and it will evidently affect the service life of capacitors at DC side of the converter and jeopardize stable operation of wind power generating units while such an operating condition is persistent.A mathematical model of the full power grid-connecting converter at grid side is built,and on the basis of taking the influence of grid-connecting impedance on active power into account,a control strategy,which takes the enhancement of DC side voltage stability as the object,is proposed.In the proposed control strategy,by means of ensuring no double-frequency oscillation in active power flow between DC side capacitor and grid-connecting converter the positive and negative current instructions are obtained,thus it is unnecessary to solve complex high-order matrix for the calculation of the instructions and there are not more variables to be led in as well.The effectiveness of the proposed voltage control strategy is verified by simulation results of direct drive PMSG.Comparing with traditional balanced control strategy,the proposed control strategy can effectively suppress the double-frequency oscillation in the active power transmitted to power grid while the DC side voltage stability control is implemented,thus the ability of uninterrupted secure and stable operation of direct drive PMSG under unbalanced grid voltage is improved.