A HYBRID ZVZCS DUAL-TRANSFORMER-BASED FULL-BRIDGE CONVERTEROPERATING IN DCM FOR MVDC GRIDS
High-power dc-dc converter is the essentialcomponent for connecting the renewable energy sourcesto medium-voltage dc (MVDC) grids. In this paper, anovel ZVZCS converter with two full-bridge cells sharinga bridge leg and connecting the secondary windings oftwo transformers in series is proposed for MVDCapplications. One big feature of the proposed converter isthat the required inductance used for energy transmissionis reduced remarkably and even can be embedded in thetransformer in some cases. Special but simple controlstrategy adopted by the converter makes it work indiscontinuous current mode (DCM), which can realizezero-current switching (ZCS) for the main switches andrectifier diodes in the whole load range. Meanwhile, theauxiliary switches with small current rating can realizezero-voltage switching (ZVS) naturally. Hence, theswitching loss is reduced, which is very important forhigh-power applications. The effects of the turns ratio ofauxiliary transformer on the total loss and neededinductance are comprehensively analyzed and proved.With detailed parameters design procedure, a simulationmodel is established in the software PLECS and theoperation principle of the converter is verified. A 120V1200V/1kWprototypewasbuiltto validate the operationprinciple of the proposed converter.
The high-gain high-power dc-dc converters are the keycomponents in the pure dc grid and therefore, recently,different kinds of non-isolated and isolated convertertopologies have been proposed and studied .Generally, the high gain can be obtained by switchedcapacitor, LC resonance, voltagemultiplier , and so on for the non-isolated dc-dcconverters, avoiding using a high-power high-voltageisolation transformer, but suffering from switches withhigh-voltage stress. The transformers can be used toachieve high gain with galvanic isolation in isolated dcdcconverters, allowing for low-voltage switches at theirlow-voltage terminals and reducing the insulation level ofrenewable energy sources. Full-bridge (FB) converters, LCC resonantconverters and dual active bridge (DAB)converters have been widely studied forhigh-gain high-power applications. A 1.5/15 kV 1.6 MWthree-phase DAB converter is designed to collect thepower from PV and step up to a MVDC bus bar.However, the DAB converter suffers from switches withhigh-voltage stress at its high-voltage terminal andactually, a bidirectional converter is not necessary in theapplication due to the unidirectional power flow of PV.Soft-switching techniques are widely adopted for FBconverters to reduce switching loss, which dominates theconversion efficiency especially in high-voltage highpowerapplications.Thesoft-switchingtechniquesforFBconverterscan be classified into two types one iszero-voltage switching (ZVS) and the other iszero-voltage and zero-current switching (ZVZCS). Since IGBTs have much higher voltage/currentratings compared with MOSFETs, IGBTs are preferredfor high-power applications and ZCS is helpful forIGBTs to reduce switching loss due to its current taileffect. In conventional ZVZCS PWM FB converters, onlyone leg achieves ZCS and the other leg achieves ZVS,moreover, there is serious voltage oscillation across therectifier diodes caused by the reverse recovery no matterif ZVS or ZCS is realized for the switches.
In this paper, a novel hybrid ZVZCS FB converter withvoltage-doubler rectifier is proposed for high-gain highpowerdc grid systems, which can significantly reduceswitching loss of active switches. Compared with theconverter, the proposed converter has followingfeatures: 1) two switches are saved by sharing a switchingleg between the two FB converters; 2) the high-voltagefilter inductor is replaced by a low-voltage inductorwithout high voltage insulation issue and the requiredinductance is further reduced significantly, even theleakage inductance of transformer can meet the demandby proper design, resulting in a compact size of theproposed converter; 3) the output current waveform isoptimized for the improved efficiency; 4) no voltageoscillation across the rectifier diodes with reduced reverserecovery loss.
This paper proposes a hybrid ZVZCS FB dc-dc converter suitable for high-gain high-power dc gridsystems, which contains four main switches, twoauxiliary switches, one main transformer and oneauxiliary transformer. The current stress of auxiliaryswitches is much lower than that of main switches andthe processed power by auxiliary transformer is muchsmaller than that of main transformer, and the powerdistribution between the two transformers is onlydetermined by the turns ratio of main transformer. Theturns ratio of auxiliary transformer affects the requiredinductance of L, current stress of two main switches andtwo auxiliary switches, the switching losses andconduction losses of switches, and it should be designedto be as small as possible to improve the converterperformance. By introducing an auxiliary circuit, theturn-off losses of switches can be significantly reducedcompared with the conventional ZVZCS FB converters,which dominate the all power loss in high powerapplications. Simulation and experimental results verifythe operation principle of the converter and parametersdesign.
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