Research On Parallel Control Strategy Of Off-grid Three-phase Inverters

With the rapid development of micro-grid power supply system and higherrequirements of the power quality for the load, the off-grid technology for powerinverter is widely concerned. In off-grid mode, the parallel techniques of inverter canachieve modularization and high-capacity with good system stability and easymaintenance, so they have been widely used. However off-grid inverters in parallelwould induce issues such as the circulation between modules, output waveform with avariety of harmonic components, slow dynamic response, and so on, this paper studieson solving these problems.The three-phase inverter’s modeling is made. Combined with this model, theperformance of the PR controller is analyzed from two aspects: the principle analysis ofgeneralized integral and the derivation of the PR control transfer function. A controlstrategy for the single off-grid inverter is proposed, which uses PR controller andimproved repetitive controller in parallel as voltage loop and strings into the innercurrent loop of the P part to compose the two closed-loop control structure. The rootlocus design method for the PR control parameters and inner current loop parameters isadvanced, and the repetitive controller design method also be advanced in this paper.Then based on the study of the single inverter, the droop control principle analysisis advanced by comparing PQ droop characteristics in different output impedance. Withthe analysis of circulation in parallel system, this paper gets the conclusion thatincreasing the inductive value of the equivalent output impedance of the system andmaking it mainly inductive is helpful for the power sharing and circulation suppression.Then combined with study on the control algorithm’s influence on the output impedanceof single inverter, the design method of the virtual impedance is proposed, the virtualimpedance’s design and implementation of a two-phase stationary coordinate system isalso proposed. Because of the virtual impedance’s influence on the output power of theinverter, a novel calculation method for the given voltage reference and the design andimplementation method for the PQ droop Control strategy based on the virtual impedance are proposed.Finally, the simulation experiments are made to verify the conclusions derived inthis paper and the effectiveness of control strategies. The results show that the proposeddual-loop control and droop control strategy based on the virtual inductance can wellrealize the off-grid inverters’ parallel, meanwhile effectively inhibit the circulation ofparallel inverters and improve the quality of output Waveform...