Resonance Problem Analysis And Detection Method Of Distributed Clustered Grid-connected PV

Distributed clustered grid-connected PV system is a complex higher order system where a large number of photovoltaic inverters are at the junction of the PCC.Distributed photovoltaic power generation is incorporated into the distribution network through photovoltaic inverters.It will lead to the grid has multiple power quality problems,including voltage swells,voltage sag,voltage fluctuation and flicker,voltage interruption,voltage and current harmonics,voltage and current unbalance component,resonance and so on.It has seriously affected the power quality of the grid.Therefore,it is imperative to effectively restrain the resonance problem of distributed clustered grid-connected PV system.Accurate and effective detection of the resonant disturbances is an important precondition for the resonance control.In this paper,a mathematical model for the resonance of distributed clustered grid-connected PV system is established,which is based on the typical structure of distributed clustered grid-connected PV system.Based on the principle of series and parallel resonance,the resonance generation reason of distributed clustered grid-connected PV is analyzed.The introduction of a large number of inverters in distributed clustered photovoltaic(PV)makes the distribution network become a complex high order electrical network which has many inherent resonance points.Besides,the grid contains a rich spectrum of disturbances.When the frequency of these disturbances is matched with the natural resonant frequency of the system,the resonance phenomenon will occur.Secondly,the influencing factors of resonant frequency distribution of the distributed clustered grid-connected PV are studied.Influencing factors of distributed clustered photovoltaic resonant frequency are analyzed from the LC parameters,parallel inverter units,control parameters,structure change of the system and PV power fluctuation.Then the resonance detection principle based on wavelet transform(WT)and Hilbert-Huang transform(HHT)are analyzed separately in this paper.The performance of resonance is that the amplitude of the waveform increases gradually.Over voltage and over current protection action even lead to inverter shutdown.On this basis,a resonance detection method of the distributed clustered PV considering singulari ty is proposed in this paper.Firstly,the resonance voltage or current of the clustered grid-connected PV is analyzed through WT.The frequency band range with obvious singularity can be obtained.Then the above frequency band is analyzed by HHT.In order to improve the detection speed,only harmonics of the frequency band range containing the mutation point are concerned.The frequency and amplitude of each harmonic component in the above frequency band are obtained.Finally,the analysis results of HHT are observed.In view of the magnitude of the harmonics in the above frequency band,the frequency of the harmonic signal whose amplitude showed positive growth is the resonant frequency.Simulation and experiment results show that the proposed method can accurately and effectively detect the resonant disturbance s of distributed clustered grid-connected PV system.