Research On The Online Detection Methods Of The Cascade Converter Stability Based On The Equivalent Impedance

At present, the DC Distributed Power System (DC DPS) has been widely used invarious fields of marine, space and so on, and cascaded converters are the most basic andcommonly used component units in DC DPS. So it is very meaningful to study thestability control and detection methods of cascaded converters. A much more perfectconclusion has been made on the cascade converter stability control methods with manystudies at home and abroad. Nevertheless, for the online detection of cascaded convertersstability, there are few researches on non-injection online detection, except for the earlyperiod small signal disturbance injection method. Therefore, in this paper, the non-injectedonline detection methods on the stability are theoretically analyzed and summarized fromthe advanced mathematics and the circuit principle perspective.Firstly, this paper analyzes the stability theory for evaluating the overall performanceof the standard sub-module cascaded system in detail. Combined with the definition ofsystem stability in the principles of automatic control, the forbidden region theory whichis based on the equivalent impedance method is proposed as the criteria for stability.Secondly, reference to the ideas of online detection in the photovoltaic MPPT, themathematical model of the photovoltaic MPPT is analyzed in detail according to theadvanced mathematics and circuit principle. Through mathematical derivation andcomputer simulation, the general conditions for realizing MPPT in nonlinear system isobtained and then the implied mathematical premise is further excavated. Therefore,mathematical and physical conditions are obtained for achieving MPPT online detection,in addition, the specific implementation method is given in this paper by analyzing thedefinitions of impedance, dynamic resistance and instantaneous resistance.Finally, reference to the online detection method of photovoltaic cells-convertercascade system，it is inferred that independent excitation source is essential for thedetection of system structural parameters. Moreover, according to the requirements ofindependent excitation source， and at the same time combined with the analysis to thecascade converter small signal model and to the signal characteristics on the DC bus， the implementation method for cascade converter stability online detection is obtained,namely, the blind source separation theory is applied to decoupling excitation and itsresponse signal.