Research On Some Kev Issues Of Current Sharing And Flexible Power Distribution For Paralleled Multi-Inverter System

As one of the key technologies of the UPS(Uninterruptible Power Supply)system,distributed generation,new energy,and micro-grid,the subject research on parallel operation of multi-inverter has great practical significance.It is a primary task for multi-inverter system to distribute the load currents or powers among parallel inverters as required.On the basis of summarizing current multi-inverter parallel technologies at home and abroad,this paper intensively studies wireless equal current sharing and communication integrated flexible power distribution technology,and correlated issues for multi-inverter system based on droop control and virtual impedance method.Some applications have phase locking requirement.For instance,the output voltage of the online UPS system needs to be in phase with the utility voltage under effective current sharing condition.There exists a contradiction between the droop method and phase locking since the two regulations both adjust the frequency of output voltage.It will result in unstable current sharing of the parallel system.In this paper,the output impedance of UPS inverter is designed to be resistive,and the R-type droop control is employed.Moreover,an adaptive virtual resistor is added to regulate the magnitude of the output impedance according to the requirement of instantaneous reactive power sharing,which makes the power sharing less sensitive to frequency change and eases the burden of frequency droop control.Besides,the root-mean-square(RMS)loop is adopted to improve the output voltage quality.And a secondary P-V droop mechanism is put forward to decrease the amplitude deviation and improve the dynamic performance of current sharing.Simulation and experimental results show the parallel system obtains effective and fast current sharing under phase locking condition with the proposed scheme.From the thevenin equivalent model of inverter,it can be seen the characteristic of inverter contains equivalent output impedance and no-load voltage gain.Most of the existing research only considers the effect of output impedance on current sharing with the implied condition of the same no-load voltage gain.However,different hardware parameters and control strategies will lead to different voltage gain,which is also an important factor that impacts the current sharing performance.This paper considers the effect of the mismatch of both the equivalent output impedance and no-load voltage gain on current sharing.It is pointed out through analysis that RMS loop can improve the current sharing accuracy,but weaken the effectiveness of current sharing improvement of the virtual impedance.This paper proposes a wireless current sharing scheme which is compatible with inverters of different characteristics.By placing the virtual impedance loop in front of the RMS loop,the accuracy of current sharing of parallel inverters is effectively improved.Simulation and experiment prove the validity of the theoretical analysis and the effectiveness of the proposed scheme.For a parallel inverters system based on droop control,the distribution of frequency controlled power(FCP)is accurate due to the global frequency in steady state,while power distribution errors exist in voltage controlled power(VCP)sharing owing to the mismatch of no-load voltage gains,equivalent output impedances and feeder impedances.The existing virtual impedance methods only consider improvement of impedance matching for VCP distribution.This paper points out that a weak impedance matching for FCP distribution may result in FCP overshoots and even oscillations during load transients,and then cause VCP oscillations due to the strong coupling.This paper derives the impedance conditions for accurate power distribution,puts forward the concept of matched impedances set(MIS),and defines the optimal matched impedances.And on this basis a 2-DOF(degree of freedom)tunable adaptive virtual impedance method considering impedance matching for both real and reactive power is proposed.The dynamic response is promoted while accurate power,sharing is realized.Besides,the power sharing controller obtains a larger parameter adaptation.The stability and dynamic performances are analyzed with the small-signal state-space model.Simulation and experimental results are presented to investigate the validity of the proposed scheme.Finally,an improved RL-type droop control strategy for a meshed parallel inverters system with complex impedances is proposed in this paper to realize flexible power distribution control.Compared with the common ac bus structure,inverters and loads can freely connect to any node in a parallel system with meshed structure,which means the line impedance between two nodes and the electric network structure can change.Thus,it is difficult to extract feeder impedances effectively.Besides,the powers injected into loads need go through more than one non-generating node,which make the coupling between active and reactive power more complicated.An adaptive virtual resistance,tuned by the integral of the linear combination of output active and reactive powers,is added to eliminate load sharing errors.In addition,the RL-type droop method is modified through injecting the power droop information of all other inverters,which can completely compensate the voltage amplitude and frequency deviations caused by droop equations.Through the per-unit process of droop coefficients,the central controller broadcasts the control variables to all inverters,which improves the simultaneity of control data,minimizes the mismatch of control delay and enhances the system stability.The proposed scheme is extended to L-type and R-type droop.And the three modified methods are compared with the small signal analysis and simulation.Experimental results are presented to investigate the power distribution performance of the proposed control strategy.