The Research On Suppressed Technology Of The Resonant Problem Of Grid-Connected Inverter Based On LTLCL Filter

Grid-connected inverter is a channel of energy injection from the distributed power generation equipment to the power grid,but it will produce a certain amount of switching harmonics in the energy exchange,resulting in distortion of the grid voltage,so it's necessary to access switching ripple filter on the AC side.The performance of the filter will directly affect the grid-connected inverter in size and cost,and its inherent resonance characteristics will reduce the stability of the grid-connected inverter and lead to resonance.In this thesis,three-phase three-wire grid-connected inverter is analyzed to enhance its performance of accessing the network form three aspects: structure and parameter design of switching ripple filter,topology and parameter design of passive damping,control structure and parameter design of active damping.LCL filter can be a good way to reduce the filter size and enhance attenuation rate of the high-frequency switching harmonic compared to the traditional L filter.On the basis of the LCL filter,an LTLCL filter topology is proposed,which can not only maintain the-60dB/dec harmonic attenuation equaling with LCL filter,but also effectively suppress the harmonics because of its impedance tending to be 0 at switching frequency and switching frequency multiplier.Besides,a parameter design method for LTLCL filter which includes design criteria and design steps is presented to ensure that it can be applied to grid-connected inverters of different power and voltage levels.The passive damping topology of the single resistor form is discussed.It is found that the topology can not take into account the damping performance and damping loss.So based on the analysis of the existing passive damping topology,a parallel RC passive damping for LTLCL filter is proposed.Under the premise of maintaining the damping performance,it can reduce damping loss greatly.In order to make the damping parameters easy to be designed,a downgrade method is proposed to simplify the LTLCL model as an equivalent LCL filter which has the same resonant peak performance as the orignal LTLCL filter and it ensures the accuracy of the parameters.Based on this method,the design method and steps of passive damping parameters are given.Finally,the influence of the parasitic parameters on the stability of the grid-connected inverter is analyzed from the aspect of damping.The S-domain mathematic model of grid-connected inverter based on LTLCL filter is established which keeps accurate at nyquist frequency.The control structure of active damping for LTLCL filter is studied form the perspective of nonminimum-phase response,and the conclusion can be used to guide the selection of damping strategy of different grid-connected inverter.In addition,an iterative method of damping parameters for different active damping is proposed,which maximizes the performance of damping by optimizing the position of closed-loop poles accurately.The concept of the critical grid impedance included in the design method is important for analying the stability of the inverter in different grid environments.The influence of the impedance change on the harmonic controller in the current loop is analyzed and it is found that the increase of the grid impedance will reduce the bandwidth of the current loop and cause the resonance of the harmonic controller.Then,an optimized parameter design method of current controller is proposed which can effectively avoid the controller resonance.The results of the experiment verify the correctness of the theoretical analysis,finally,not omly the size and cost of the filter is reduced,but also the stable opration of the grid-connected inverter in the condition of different grid impedance is achieved.