两级式光伏发电系统低电压穿越控制策略研究

随着中功率两级式光伏逆变器在大中型发电系统中的大规模应用,基于两级式光伏逆变器的低电压穿越控制技术得到越来越多的研究。与单级式光伏逆变器相比,两级式光伏逆变器存在前级DC/DC变换器和后级DC/AC逆变器,控制更复杂,低电压穿越难度更大。文中首先进行了系统建模,然后提出了一种基于控制模式无缝切换的低电压穿越控制策略,DC/DC变换器在稳态时作为MPPT控制器进行最大功率点跟踪,DC/AC逆变器作为恒压源稳定直流母线电压。在低电压穿越时,DC/DC变换器以恒直流母线电压方式运行,DC/AC变换器以有功无功模式运行。此方法可以解决低电压穿越过程中有功不匹配而导致的直流母线过压的问题。最后,通过在一台40k W的两级式光伏并网逆变器样机上进行实验,验证了理论分析的正确性及可行性。

LVRT Control Strategy of Two-stage PV Power System

With the application of two-stage PV grid-tie inverter in the large-scale and medium power systems, it is widely researched that the LVRT control strategy of photovoltaic inverter based on the two power stages topology. Compared with single-stage PV inverter, it is a combination of the first power stage of DC/DC converter and the second power stage of DC/AC inverter for two-stage PV inverter. The control is more complex and it is more difficult in the low voltage ride through. Firstly, the system modeling is carried on and then the control strategy is proposed based on control mode seamless switching in LVRT. At the steady state the DC/DC conversion unit tracks the maximum power point and the DC/AC inverter unit works as a constant voltage source. In LVRT, the DC/DC conversion unit runs in the constant busbar voltage mode and DC/AC inverter unit runs in the p-q mode. The problem is resolved that the DC bus over-voltage caused by unbalanced energy between the DC/DC unit and DC/AC unit. The experiments in a 40 kW PV grid-tie inverter of two power stage verify the correctness and feasibility of the theoretical analysis finally.