Resonance Mechanism And Suppression Strategy Study Of Centralized Large-scale PV Grid-connected System

With the scale of PV plant expanding more and more inverters are connected to the point of common coupling(PCC).As a result the output impedance decreases and the influence of grid impedance on PV plant becomes worse.The interaction between PV plant and power grid may lead to resonance even instabilities which threathen the security of large scale PV(LSPV)plant.The aim of the thesis is to enhance the stability of LSPV plant in a weak grid and improve the power quality.On the basis of the deep study on the resonance mechanism of the LSPV system approaches to suppress the resonance are investigated,which is of value in theory and of significance in practice.In view of the complexity of the existing related stability analysis methods and their limitations applicable to the system containing different types of inverters,the thesis is focused on LSPV plant containing different types of inverters and brings in the admittance network into the stability analysis of PV system for the first time.And in the stability analysis,an inverter set((37)-set)with several same unstable poles is defined.Then stability criteria of the interaction between different inverters and the interaction between the inverter and the power grid are derived.Finally the stability criterion of LSPV plant is obtained as:(a)the admittance network is stable;(b)any two inverters contains no common unstable poles.Given that the existing stability analysis of grid-connected PV system mostly ends with studying on the influence of grid impedance on phase margin or root locus the thesis reveals the resonance mechanism by studying on the influence of grid impedance on the harmonic amplification for the first time.With the equivalent grid impedance increasing the stability margin of the loop gain corresponding to the admittance network decreases,resulting in that the harmonic amplification ability of admittance network near the cutoff frequency enlarges.When the stability margin is reduced to zero,the harmonic amplification ability will be infinite.At this time,the admittance of the PV plant and the grid admittance are conjugated at their intersection frequency,and resonance is induced.For a system where all inverters are equal,when the stability margin of the stability equivalent inverter is reduced to zero,the harmonic amplification ability of the inverter at the cutoff frequency is infinite.At this time,the inverter admittance and the grid admittance are conjugated at their intersection frequency,and resonance is induced.Based on the resonance mechanism,a resonance suppression strategy based on the inverter admittance reconstruction is proposed.The phase-frequency characteristic of the inverter admittance is corrected by introducing a phase lag network in the PCC voltage feed-forward to make it far away from the capacitive,thereby ensuring that the equivalent loop gain of the admittance network always has enough phase margin.At the same time,a second-order trap with phase compensation is proposed to eliminate the influence of the lag network on fundamental tracking performance.The method can suppress system resonance and harmonic amplification without real-time detection of grid impedance or resonant frequency.For systems where all inverters are equal,the interaction between different inverters can be neglected.Then,a resonance suppression strategy based on self-adaptive compensation of phase margin is proposed.In the strategy the phase of the open-loop transfer function is compensated by a phase-lead network,and the phase compensation amount and the cut-off frequency are adjusted in real time according to the detected grid impedance,so as to ensure that the inverter always has sufficient stability margin and bandwidth during the change of equivalent grid impedance,thereby realizing the suppression of resonance and harmonic amplification.Compared with the admittance reconstruction strategy,the proposed method needs less compensation parameters and can ensure that the inverter has enough bandwidth.Therefore,this method is superior to the admittance reconstruction strategy for systems that can ignore the differences between inverters.In order to avoid the use of an additional compensation network to suppress resonance,this thesis proposes an inverter parameter design method that enhances the ability of a LSPV plant to adapt to grid impedance from the perspective of resonance prevention.In order to seek the optimal matching of inverter filtering and control parameters,the thesis proposes a unified design of the filter and control parameters for the first time.And in order to ensure the stability of the inverter itself and the robustness to the grid impedance at the same time the inverter open-loop transfer function and output impedance(admittance)are taken into account simultaneously to design the inverter parameters for the first time.In the design process,a method for normalizing the parameters that affect the stability and robustness of the inverter is proposed.This method can simplify the analysis on the inverter stability and robustness to grid impedance.Then the relevant parameters can be designed simply according to the stability and robustness requirements,and combing with the limiting conditions of the switching frequency ripple.The design method can ensure the stability of the inverter itself as well as the robustness to the grid impedance without the complicated iterative calculation in the traditional method.This method involves the design of LCL parameters and controller parameters at the same time,so it should be used for the resonance prevention of the PV plants to be built soon.The thesis verified the correctness of the stability criteria and the resonance mechanism of LSPV grid-connected system,the effectiveness of the resonance suppression technology,and the practicality of the inverter design method through simulation and experiments.The stability analysis method and the solutions to resolving the resonance problem formed in the paper can provide theoretical guidance for the stability study,resonance suppression and prevention of a large number of renewable energy grid-connected systems in practice.