Stability Research And Harmonic Suppression For Parallel-connected Inverters

In the dire situation of increasing energy crisis and environmental pollution, Microgrid, which is composed of distributed generation, energy storage units, energy conversion devices, loads, monitoring and protective equipments and so on, has attracted worldwide attention and study in the last few years. However, harmonic interaction among equivalent source and load sub-systems of the microgrid may adversely influence the overall instability or even lead to system crash even if each sub-system is stable. Based on the impedance/admittance matching theory, the parallel-connected inverters are studied in the dissertation, and theoretical study and practical experiment of mathematical modeling to inverters, stability analysis of paralleled inverters, adverse effect of constant power load(CPL) and harmonic suppression are carried out to study the harmonic resonance caused by mismatch between the distributed generation units and loads.By calculating the relation between output voltage and output current of a inverter, the complete coupled ac admittance matrix of one inverter in the d-q reference frame is established and the relationship between parameters of inverter and the admittance is analyzed. To reduce the computation, the simplified dd channel admittance is derived by handling the complete admittance model. Then the relevant Thevenin equivalent circuit of parallel-connected inverters is established and the analysis of stability in different line impedances, different number of inverters and the system including constant power load by Nyquist criterion is introduced, respectively.To eliminate the harmonic resonances of the above three conditions, an active damping control strategy is proposed. The basic principle is the output current is fed to reference current part with the aid of proportional coefficient. This approach can reduce the equivalent output impedance of inverter and improve the stability margin. Besides, CPL output admittance has a resonant peak which violates Nyquist criterion. Hence a virtual resistance control scheme is employed to eliminate the resonant peak. The PSCAD/EMTDC simulations and actual tests are carried out to verify the correctness of theoretical analysis.Finally, an experiment platform of parallel-connected inverters is built based on TMS320F2812 DSP and the software and hardware of system is designed, respectively. At last, the experimental results prove the correction of theoretical analysis.