Methods Of Reducing Second Harmonic Current In The Front-End DC-DC Converter In Two-Stage Inverter

With the alarming problem of air pollution and green house effect, power generationsystem based on clean energy, including solar, wind power and fuel cell, is becoming moreand more widely used. Since two-stage inverter can achieve voltage matching and galvanicisolation without the bulky line frequency transformer, it is commonly used as the interfacebetween the input power source and the grid or load. On the other hand, in the clean energybased power generation system oriented to resident application, the utility power line isusually single-phase, so the inverter used is also single-phase. Since the instantaneousoutput power of the single-phase inverter pulsates at twice the output frequency, there willbe current also pulsating at twice the output frequency, i.e., second harmonic current (SHC),in the front-end dc-dc converter and the input power source.This paper firstly analyzes the generation mechanism of SHC, and then points out thatthe SHC will not only do harm to the input power source, but also decrease the conversionefficiency of the converter. Afterwards, the existing methods of reducing the SHC in thefront-end dc-dc converter are discussed and compared.Based on the control loop, the propagation mechanism of the SHC is analyzed fromthe viewpoint of output impedance, and the key point of reducing SHC in the front-enddc-dc converter is to increase the magnitude of the inductor branch closed-loop impedance.It is revealed that the dual-loop control can increase the magnitude of the inductor branchclosed-loop impedance much more compared with the single voltage loop control, thus itsability of suppressing SHC in the front-end dc-dc converter is superior to the single voltageloop control. Nevertheless, the increase of the inductor branch closed-loop impedance willlead to large voltage fluctuation of the intermediate bus during load transient, which mayleads to malfunction of the converter.To achieve a low SHC in the front-end dc-dc converter and good load transientresponse simultaneously, this paper puts forward two different control strategies based onthe dual-loop control, namely, dual-loop control with notch filter inserted load currentfeed-forward, and notch filter inserted dual-loop control with load current feed-forward. These two control strategies are compared with the existing notch filter inserted dual-loopcontrol, and the mechanism of reducing SHC in the filter inductor and speeding up dynamicperformance during load transient is analyzed from the viewpoint of output impedance.Finally, a2kW prototype of two-stage single-phase inverter has been built and testedin the lab. Experimental results are presented to show the effectiveness of the proposedcontrol strategies, i.e., the SHC in the front-end dc-dc converter is low and the loadtransient response is very good.