Proportional Resonant Control For Triple Line-Voltage Cascaded Power Converter

With the unceasing enhancement of the power level in new energy powergenerations, electric drives, static power compensations etc., the traditional powerdevices and topological structures have been difficult to meet the current requirements.The converters that consist of the conventional power devices based on the multipletechniques can enhance the voltage and power level, and improve the performance ofthe whole system. Among them, the triple line-voltage cascaded power converter withthe advantages such as higher equivalent switching frequency, good power quality,high reliability, convenient modularization and extension, has been widely applied insome medium and high voltage occasions. However, the converter has morecomplicated control algorithm and structure that is difficult to be realized in aclosed-loop method due to its structural characteristic. Therefore, this paper mainlyconducts some researches around the relevant operation and control issues of tripleline-voltage cascaded voltage-source converter(LVC-VSC).This paper firstly detailed analyzes the working principle and operationalcondition of triple LVC-VSC when its transmission power is balanced and unbalanced.According to the external circuit characteristic of the converter, the paper respectivelyestablishes the equivalent switching circuit model under the balanced and unbalancedtransmission power, and deduces the equivalent mathematical models in three-phaseand two-phase stationary frame, which provide the design and realization of thecontrol system with the theoretical foundation.For the operation control of triple LVC-VSC, a carrier phase-shifting spacevector pulsewidth modulation suitable for its structural characteristics is proposed,which offers better utilization of dc-link voltages and simplifies the modulationalgorithm effectively. When the transmission power of triple LVC-VSC is balanced,the paper designs a proportional resonant (PR) control strategy for the equivalentswitch circuit model based on instantaneous power theory, which achieve balancedstable control of the converter power in a two-phase stationary frame. And by addinga voltage compensation control in the double closed-loop control structure, the doublefrequency fluctuation of each dc-link voltage is effectively suppressed. However, thisstrategy cannot guarantee the each dc-capacitor voltage stabilizing in the reference when the transmission power of triple LVC-VSC is unbalanced. Thus, the paper putsforward an improved PR control strategy adding a voltage-sharing compensationcontrol, which not only can ensure stable transmission of the converter power but alsocan independently adjust the transmitted power of each cascaded unit to make threedc-capacitor voltages equal. Besides, the improved method can also solve theasymmetrical problem of line currents caused by the unbalanced transmission power.The two proposed control strategy in the condition of different transmission powerboth has such advantages as less hardware and computation, simpler control structure,which is suitable for the high power converters with multiple cascaded structures.At last, the paper builds an experiment platform of triple LVC-VSC to make anexperimental research on the proposed PR control strategy under the balanced andunbalanced transmission power. The experimental results show that the proposedequivalent switch circuit model and its PR control method can effectively realize thestable operation of triple LVC-VSC, and has better dynamic and static performanceand robustness.