| The development of renewable energy generation grid-connected systems promotes the application of inverters in the power grid.Since a single inverter cannot meet the requirement of mass power transmission,paralleled inverters are widely used in the renewable energy generation grid-connected systems.In addition,paralleled inverters are widely used in energy storage and large-scale harmonics suppression.However,inverters are composed of switching devices,which inevitably causes switching frequency harmonics due to pulse width modulation(PWM).High frequency harmonics will interfere with the normal working conditions of protection and control equipment,trigger system resonance,radiate electromagnetic energy into the space,form electromagnetic interference,cause electromagnetic compatibility and so on.The superposition of ripple could happen in paralleled inverters system,which will aggravate the above-mentioned problems.Therefore,suppression of switching frequency harmonics in paralleled inverters system is particularly important.In this paper,an optimization strategy is studied for suppressing switching frequency harmonics in paralleled inverters.Firstly,the component of output voltage of SPWM and SVPWM controlled inverters is analyzed through the Fourier expression,which verifies switching frequency harmonics do exist in the inverter system.The passive filtering is used to suppress switching frequency harmonics.The resonance phenomenon of filter is analyzed,and the method of resonance suppression is given.Then,the design of filter parameters and simulation analysis are given.In addition,the electromagnetic compatibility of inverter is analyzed in this paper.Secondly,three passive filtering configurations for paralleled inverters system are presented.Besides the traditional configuration,two modified filter configurations are proposed and discussed.One has common grid-side inductor while both filter capacitor and grid-side inductor are shared in the other one.The equivalent circuit is used to model the system,the equivalent impedance and the expression of the equivalent power supply are derived respectively.The resonance phenomenon of paralleled inverters system is analyzed.In addition,the parameter design of the filter structure and simulation analysis of three structures are given.Thirdly,the carrier phase shift(CPS)technology is introduced.The CPS technology can offset ripple and increase the equivalent switching frequency.Thus,switching frequency harmonics could be suppressed.The Fourier expression of the output voltage of paralleled inverters under the CPS-SPWM modulation strategy is analyzed.The effectiveness of the strategy is verified by simulation and the factors that influence the suppression effect are studied.In addition,the realization of CPS technology is studied,and the global coordinated controller is put forward to control each inverter.At last,the passive filtering and CPS technology are combined and two optimization switching frequency harmonics suppression schemes for paralleled inverters are put forward.In scheme one,switching frequency harmonics are suppressed by passive filtering before CPS technology.In scheme two,switching frequency harmonics are suppressed by passive filtering after CPS technology.In order to highlight the effect of suppressing switching frequency harmonics by combining passive filtering and CPS technology,this paper also sets up a control group where switching frequency harmonics are suppressed only by passive filtering with carrier waves not changed.Through simulation,these schemes are compared in terms of the filtering effect,physical size,cost and current control precision.Simulation results verify the optimization effect on switching frequency harmonics suppression of the strategies which combine passive filtering and CPS technology on ripple suppression.Furthermore,a better suppression strategy is obtained by comparing two optimal schemes.Therefore,the scheme which combines filter structure with grid-side inductor shared and CPS technology is advised for paralleled inverters system in this paper.In addition,the application of the proposed optimization switching frequency harmonics suppression strategy is discussed based on the practical situation,example is also given. |
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