Research On The Performance Improvement For Gridconnected Inverters In DG Systems

Grid-connected inverters are widely used for interfacing renewable energy resources with the utility grid in distributed generation(DG)systems.The performance of the grid-connected inverter has a great influence on the efficiency and stability of the DG system.Based on the analyses of a number of key control strategies for the single-phase and three-phase gridconnected inverters,this paper proposes some techniques to improve the performance of the grid-connected inverter in DG systems,which is validated by using simulations and experiments.This paper is organized as follows.(1)Typically,the inverter is connected to the grid through an inductor.The inductor brings in the cross-coupling between the dq axes,which degrades the dynamic performance.For threephase systems,the dq axes are usually decoupled via state feedback(SF).However,the SF technique is greatly affected by the parameters mismatch.The complex vector notation is used to simplify the model of the three-phase system into a single-input-single-output(SISO)model.The complex vector proportional-integrator(CVPI)controller is designed to decouple the system with robustness against parameter mismatch,which improves the performance of the inverter under weak grid condition.(2)To design the dq-frame current regulator for single-phase voltage-source inverters,an orthogonal signal generation(OSG)block is required to generate the virtual orthogonal signal.However,the OSG block makes the control system complex and introduces an extra transient disturbance,which deteriorates the dynamic performance.In this paper,the quasi-vector control(QVC)scheme,which uses the reference-current-based OSG method,is analyzed thoroughly.Based on this structure,the dq-axes decoupling control,which is usually neglected for singlephase systems,is studied.Two decoupling techniques,i.e.the reference current feed-forward control(RCF)and the quasi complex vector proportional-integrator control(QCVPI)are implemented and analyzed.Results show that the QVC+RCF controller presents best dynamic performance in normal cases.The QCVPI controller gains robust ability against parameter mismatch but needs the active resistor to suppress transient caused by the grid voltage disturbance.(3)Due to the presence of a second harmonic ripple across the dc bus voltage,it is very challenging to design the dc bus voltage control scheme in single-phase two-stage gridconnected inverters.This paper presents a fast dc bus voltage controller,which uses a second order digital finite impulse response(FIR)notch filter in conjunction with input power feedforward scheme to improve the steady-state and dynamic performance.To gain the input power without extra hardware,the Kalman filter is incorporated to estimate the dc bus input current in real time.At the same time,a modulation compensation strategy is implemented to eliminate the nonlinearity of the grid current control loop,which is caused by the dc bus voltage ripple.(4)For grid-connected transformerless photovoltaic(PV)inverters,serious leakage currents(common-mode currents)can be caused by the stray capacitance between the PV array and the ground.The leakage-current-suppression ability of the single-phase and three-phase grid-connected inverters is analyzed by comparing topological structures and modulation methods.For single-phase inverters,the H bridge topology can suppress the leakage current by using bipolar modulation strategy,which deteriorates the output current quality at the same time.The modified topologies,i.e.H5,H6,and HERIC,can almost eliminate the leakage current while remaining high-quality output current.For three-phase two-level inverters,the space vector pulse width modulation(SVPWM)leads to serious common-mode currents.The near state PWM(NSPWM)and active zero state PWM(AZSPWM)strategies can reduce the common-mode currents,but the total harmonic distortion(THD)of the output current is increased.Three-phase three-level inverters have a native advantage in the suppression of the leakage current.PWM with two medium and one zero vectors(2MV1ZPWM)can eliminate the leakage current,but gain some disadvantages meanwhile,such as poor output current quality,lacking the neutral-point balancing ability and low dc voltage utilization ratio.As a compromise solution,PWM with large vectors,medium vectors,and one zero vector(LMZPWM),results in lower leakage current than SVPWM,but the neutral-point balancing ability is still missing.This paper proposes the multi-mode hybrid PWM(MMHPWM)strategy,which takes the leakage current suppression,neutral-point balancing,output current quality and the dc voltage utilization rate into consideration.