Signal Detection Method Of Grid Synchronization Without Phase-Locked Loop Based On Hilbert Transform And Its Application

With the increasing global environmental problems and the attention given by the international community to the development of new energy sources,the penetration rate of new energy distributed generation systems in the power grid is getting higher and higher,and the trend of power electronics in the power system is also gradually strengthened.Generally,the new energy distributed generation system is connected to the distribution network through a grid-connected inverter.The grid-connected power quality is directly determined by the grid-connected control strategy.In the grid-connected control strategy,the grid voltage synchronization signal needs to be obtained.Therefore,the performance of the grid synchronization signal detection technology is the key to determine whether the grid-connected control strategy can function normally.Because the distribution network is prone to complicated conditions such as power grid imbalance,distortion,frequency fluctuation and frequency offset,it is particularly important to study a grid synchronization signal detection method that can accurately and quickly extracts the grid synchronization signals under complex grid conditions.Therefore,this paper mainly studies the detection method of the grid synchronous signal that is suitable for the condition of the complicated power grid.Several typical synchronous signal detection methods based on synchronous rotating coordinate system phase-locked loops are analyzed,which are SSRF-PLL,DSOGI-PLL and modified DSOGI-PLL.It can be analyzed that SSRF-PLL is suitable for the ideal working condition of power grid voltage;DSOGI-PLL is suitable for working conditions with unbalanced grid voltage and lowly harmonic content;and the modified DSOGI-PLL is suitable for unbalanced power grid and serious distortion conditions.At the same time,the technology of synchronous signal detection based without phase-locked loop is also analyzed.It does not require the design of phase-locked loop,and it has better performance when the grid frequency fluctuates or shifts,however,it needs to add multiple Low-pass filter generally.In order to meet the detection requirements of grid-connected synchronization signals under complex grid conditions.A method without phase-locked loop that extracts reference sinusoidal signal of gird synchronization is proposed in this paper based on the Hilbert transform and pre-set rotation frequency.First,this method transforms the positive and negative frequency components at specific time with presetting synchronous rotation frequency.Then it eliminates the positive sequence,fundamental negative sequence and negative sequence harmonic components using the amplitude and phase characteristics of Hilbert transform and certain operations.The proposed method does not need phase-locked loop structure,and can accurately extract the positive sequence component of the fundamental wave of the grid synchronization under complex grid conditions.This paper also analyzes the deficiencies of the traditional grid-connected control strategy under imbalanced grid conditions.In order to achieving the unity power factor operation which no negative sequence current is injected into the grid,the grid imbalance control strategy the based on this proposed method is researched.The proposed method can accurately and quickly extract the grid synchronous signal under the complex conditions of power grid imbalance,distortion,frequency fluctuation,and frequency offset,and provides calculation reference value for the grid-connected control strategy.Finally,the effectiveness and superiority of the proposed method are validated by building a simulation and physical platform,at a same time,a grid-connected control system simulation is needed.