Research On The Parallel Single-phase Inverters Without Control Interconnection

The Distributed Power System (DPS) has attracted more and more attentions because it has many desirable features such as modularity, expandability, maintainability, redundancy and increased reliability. The kernel of the AC DPS is the control technique of parallel inverters. Compared to other methods of parallel inverters, the scheme without control interconnection has a lot of merits such as the convenient of placement, fewer interconnection, the stronger anti-interference and the better redundancy. However, this control method also has some problems which need to be solved. So, this paper focus on the parallel inverters, which controlled by droop method, as the investigated subject.The conventional droop method used in parallel inverters is introduced and a method for calculating the droop coefficient is proposed. In order to decrease the loop current of the system in the paralleling moment, a digital phase lock loop (DPLL) is designed, which can improve the precision of the phase difference between the inverter module and the AC bus. The stability of the parallel system is related to calculation interval of the active and reactive power according to the analysis result of small-signal model. Basing on this result, a novel calculation method is brought forward, which can shorten the time as 1/40 as the conventional computing time and improve the stability of the parallel system. In the parallel system controlled by conventional droop method, the power can't be shared properly because of the parameters difference between the inverter modules. Furthermore, the difference between the line impedance and output impedance of inverters is neglected, which can result in the oscillation of loop current. For eliminate these problem a new decoupling droop control method is presented. Two 500VA prototypes are developed. The simulation and experiment results show that paralleled inverter modules have a good performance.