Research On Control Strategy Of Three- Phase Grid-connected Converter Under Grid Faults

Vigorously promoting new energy technology and application had been an important task of the energy strategies in countries around the world,in order to alleviate the contradiction between the increasing energy demand and the environmental damage caused by traditional fossil energy.The operation mode and control strategy of grid-connected converter,which serves as the interface connecting new energy power source with grid,have always been the key research contents in the field of DG grid-connected technology.As the penetration of new energy generation systems in power system continues to increase,the control of grid-connected converters has been extended some specific performance or functional requirements,such as the capability of low-voltage ride-through,the function of island protection,etc.Particularly,the safe operation of grid will be seriously threatened if a large number of grid-connected converters are disconnected while a transient fault occurs in the grid.Therefore,it is of great significance to research the operating mode and control strategy of maintaining the converter grid-connecting under short-term grid fault for improving power supply security and supporting the fault-restoration of grid.While a transient fault occurs in grid,the grid voltage would drop instantaneously,and no longer be symmetrical three-phase voltage.So the grid-connected converter would bear the violent fluctuations of negative sequence voltage,negative sequence current,or DC-bus voltage,and may even be damaged by over-voltage or over-current.Making the three-phase grid-connected converter capable of riding through the short-term grid fault is taken as the research objective in this paper.The control of grid-connected converter under grid fault condition has been researched in four aspects: the extraction method of synchronous signal,the control method of grid-connected current,the power control strategy,and the stability analysis.Effective and accurate synchronization is the guarantee to grid-connected converter for reliable grid-connecting.In view of the fact that the grid voltage contains negative sequence or harmonic components in the case of a fault,which affects the accurate extraction of the information such as amplitude,phase and frequency of grid voltage,a multi-channel synchronization signal extraction method based on extended feedback structure is proposed in this paper.Compared with conventional methods,the structure of proposed method is more simple.Also,proposed extended feedback structure enables information such as positive,negative sequence and harmonics of voltage to be output simultaneously in synchronous and stationary coordinate systems.Therefore,the information such as the magnitude,phase and frequency of the voltage can be accurately and quickly obtained using this method,even in the case where the grid voltage contains negative sequence or harmonic components,and the dynamic performance of the system is good during the grid transient process.In order to make the distributed generation system have the ability to compensate the voltage sag of the fault grid,the grid-connected converter needs to be able to control the output of positive sequence reactive power and negative sequence reactive power independently,that is,the ability to separately control the positive sequence and negative sequence components of the output current.In this paper,a direct PI control method for the positive sequence and negative sequence currents in dual synchronization frame is proposed.The proposed method does not require complicated decoupling network and filter to eliminate the double frequency oscillation in dq frame,and does not need to reconstruct the reference value.So the controller structure is simplified while maintaining good dynamic and static performance.When the grid is faulty,the grid-connected converter will face harsh operation conditions such as DC bus voltage fluctuation or output over-current.In order to realize the optimal utilization of the power capacity of the grid-connected converter under the guarantee of the safe operation,the demarcation principle of the safe reference value for the power control is explored.Based on the analysis of the safe operation constraint and safe operation capability of grid-connected converter,a novel power control strategy for the grid-connected converter under grid fault and safety constraint is proposed.By reasonably distributing the proportion of positive sequence and negative sequence components in the power reference value,the proposed control strategy can give priority to reactive power output to compensate the voltage sag of power grid,and make full use of the residual capacity of grid-connected converter to output active power to alleviate the unbalanced pressure of DC power,thus realizing the support of grid voltage under the guarantee of the safe operation of grid-connected converter.Finally,the stability operation region of grid-connected converter with the capability of operating under grid faults is discussed.Small signal of grid-connected converter with the capability of operating under grid faults is established.Based on the model analysis,the stability of the grid-connected converter system with the capability of operating under grid faults is discussed in depth,and the stability margin of the system under different PLL band widths and short circuit ratios is derived.The application advantage of the scalable feedback structure based synchronous signal extraction method in the non-ideal power grid is further demonstrated by the stability analysis results.