Design of Cuk Derived TransformerlessCommon Grounded PV Micro-inverter in CCM
Abstract
Photovoltaic micro-inverters dispense withthe line frequency transformer, however at the cost ofsystem grounding and ground leakage current problems.These have been erstwhile resolved by the topologies derivedfrom buck, boost, buck-boost, Zeta, Watkins-Johnsonand´Cuk converters, or combinations of these. Thederived inverters, employing second order input and outputfilters, offer the most efficient, lightweight and economicalsolution in the class. This paper presents designand detailed operation of a´Cuk derived, common-groundPV micro-inverter in continuous conduction mode (CCM)operation. The inverter is shown to be compatible withboth linear and non linear loads, in stand-alone and gridconnected modes of operation. Optimal design rules ofpassive components are rigorously derived to ensure attenuationof input voltage ripples arising from the twineffects of switching and double-frequency output poweroscillation. Additionally, the design rules also incorporateconsiderations of efficiency maximization and some aspectsof easing control complexity. Inverter performanceis experimentally validated with a 300 VA, 110 V, 50/60 Hzlaboratory prototype.
EXISTING SYSTEM:
Several common-grounded, transformerless topologies havebeen reported in literature, which implicitly combinebuck-boost and inversion functions. Of these, a buckboostderived topology comprises five active switches andtwo diodes. In every half cycle, the inductor connectionswith respect to the output capacitor are reversed by a switchnetwork. A variant , based on the same principle, usesan extra diode. However, both are unable to transact reactivepower because the series diodes prevent current reversal. Usinga pair of coupled inductors,proposes a combination of´Cuk and Watkins-Johnson topologies. However, this resultsin higher currents in the coupled windings, which increasesboth inductor size and losses. In, the circuit topologyalternates between a´Cuk converter, during the negative half-cycle of the output ac voltage, and Zeta converter, for thepositive half. Since the plant model changes every half-cycle,controller design becomes exceedingly complex.An interesting´Cuk derived topology achieves cyclicreversal of the connection between the intermediate dc linkcapacitor and the output filter, during each half cycle of theoutput ac voltage. Plant models in the two modes, correspondingto positive and negative output voltages, are similar .Since the inverter retains all the advantages of a0278-0046 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.´Cuk converter,in terms of efficiency, weight and cost , it is the bettersolution in the class.
PROPOSED SYSTEM:
This paper presents a modification in the topology of, which allows bi-directional power flow, hence enablingapplications involving reactive loads, in addition toUPF loads. It thus includes a thoroughly different switchingstrategy, apart from justified circuit design rules.Circuit diagrams of the inverter in grid connection (GC)and stand-alone (SA) modes are shown in Fig. 2. The circuitdoes not require any common-mode noise filter. The negativeconductor and grid neutral (N) are shorted to ensure comrounding hence reduced or no common mode ground current.In GC mode of operation earth connection is provided throughgrid neutral, whereas in SA mode the negative rail is earthedto avoid runaway potential at the circuit nodes.
CONCLUSION
This paper presents design, operation and performance ofa cost effective, compact, non-isolated, single-stage, singlephasedc-ac PV interface. The topology is specifically designedto practically eliminate common-mode ground leakage current,which has been validated experimentally. All the operatingmodes, including dead-time operation, are detailed. Optimaldesign of passive components is described, which considerripple components due to both switching and double-frequencypower oscillations. Additionally, design of all inductors isaimed at maximizing overall efficiency. Output filter designensures improvement in resonance damping for easing controlcomplexity, while minimally affecting power losses. Inverterperformance, with linear. Scale: i_non linear loads in stand-alone modeand UPF operation in grid connected mode, is verified byexperiment. Output voltage THD in stand-alone operation andoutput current in grid-connected mode are both shown to13satisfy IEEE 519-2014 stipulations. Comparison with recenttopologies, of comparable rating, is carried out on the basisof efficiency and switch utilization which shows that thetopology returns the most competitive efficiency figures. Thusthis micro-inverter fulfills all the demands of stand-alone andgrid-connected operation and is a strong candidate for reliablecommercialization.ACKNOWLEDGMENTThe authors would like to thank Amit K. Basu, StyendraKumar and Nandkishore for their support during developmentof the experimental hardware.
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