Stability Research For Parallel-connected Inverters In Meshed System

With the aggravation of energy crisis and deterioration of environment,the distributed generation units are becoming more and more popular.However,harmonic instability issues have great influence on the system operation.The current study of microgrid stability mainly concentrate in the fundamental harmonic and low-frequency oscillation,high-frequency harmonic suppression and shaping the impedance of the system are relatively scarce.Aiming at this questions,a new way using the impedance based stability criterion is proposed to analyze the high-frequency stability of the individual bus circuit configuration.In practice,nevertheless,several inverters can be connected to a meshed structure through the line impedance between two nodes.Using the delta-wye conversion,the equivalent individual bus circuit with virtual point of common coupling(VPCC)can be obtained to analyzing the stability of the meshed system.Furthermore,a modified Thevenin equivalent circuit which contains only one source and one load is got so as to reduce computational burden and analyze system stability with impedance-based stability criterion only once.But this method is restricted when the node number is more.Hence the impedance based closed-loop transfer function matrix of the microgrid which contains many meshed inverters is obtained.Then the stability of the system is assessed by the Matrix-Fraction Description(MFD)stability criterion.The analysis of the system has smaller amount of computations and simple character because of the impedance-based analytical approach.In the research process of high-frequency oscillation,the low-frequency oscillation is often accompanied by high-frequency harmonic in the system.In order to explaining impacts on low-frequency oscillation of the high-frequency harmonics,the correlation between the two issues are studied.Firstly,the instantaneous power calculation is affected by the high-frequency harmonic components because the low-pass filter can not inhibit harmonic waves.Furthermore,the low-frequency stability margin is decreased due to the higher instantaneous power.Moreover,an inverter current feedforward control scheme is proposed to mitigate the high-frequency oscillations and improves the low-frequency stability.Lastly,the simulation and experiment are performed to verify theoretical analysis and validate the effectiveness of the scheme.Finally,an experiment platform of parallel-connected inverters is build and the hardware and software of the system is designed.The experimental results prove the correction of the theoretical analysis.