Research On Key Technologies Of Distributed Generation System Low Voltage Ride Though

With the increase of global environmental pollution and serious energy crisis, Distributed Generation(DG) technologies using renewable resources like wind energy and photovoltaic (PV) energy are developed quickly around the world. As we all know, the DG unit capacity is small and they are easily interrupted by the grid fluctuation. When the grid fault happens, DG may disconnect from the grid, so some countries presented the grid codes for DG. Low Voltage Ride Through(LVRT) requirement is one of the most important research topics. In this paper some key DG LVRT technologies, such as variable speed constant frequency wind power generation of doubly-fed induction generator (DFIG) flexible cutting-in and steady state operation control methods, rotor side converter(RSC) LVRT control strategy, grid side converter(GSC) LVRT control strategy, three phase PV full power converter LVRT control strategy are studied deeply, the contributions of this dissertation are as follows:According to characteristic of DFIG, the gird voltage orientated Vector control(VC) strategy is studied. Idle-load cutting-in and steady state operation are studied by experiments. Flexible cutting-in, decoupling control of active power and reactive power are obtained when the rotor speed changes.To improve the dynamic response, a DFIG Direct Power Control(DPC) is studied. The cutting-in and steady state operation DPC modes are established and studied by experiments. The experimental results show that using DPC achieves the same purpose with VC and DPC improves the LVRT ability during grid fault.Based on analysis of DFIG stator flux transient period, how the DFIG rotor excited converter is affected by the voltage sag moment and percent during the grid fault is researched. The cause of stator and rotor overcurrent and the relationship of DFIG physical values are found. This paper proposed a DFIG RSC LVRT control method by reducing the DC flux and negative flux. The experimental results prove that the mentioned control method prevents the stator and rotor overcurrent and improves the LVRT ability.GSC dynamic process of different running state during grid fault is analyzed. A GSC LVRT control method is proposed based on GSC response. The method modifies the modulation voltage signal during grid fault to control AC current and stabilize the DC-link voltage. The experimental results prove that the GSC LVRT control method make DC-link voltage stable and transfer rotor power to the grid quickly.Based on analysis of transient period of DFIG rotor voltage and current, the LVRT limiting condition using RSC control strategy to achieve LVRT is found. A DFIG LVRT method combining the improved control stragety with hardware protect is studied when the limiting condition is not met. The experimental results prove that the method limit the stator and rotor overcurrent, protect the rotor converter and provide power to the grid quickly during serious grid fault.The full power converter is studied. The steady state operation and LVRT control of a 3-phase PV converter is studied. A LVRT control strategy is proposed by regulating the forward compensation voltage during symmetric grid fault. The positive and negative controllers are used to control positive and negative current to achieve LVRT when asymmetric grid fault happened.A double PWM converter experimental system is established based on the TMS320F2407A. The DFIG VC and DPC steady state control, cutting-in control, the RSC and GSC LVRT control, RSC control method combined with Crowbar circute protect control experiments are made based on the system. The PV converter steady state control and LVRT control experiments are made based on the system also.