MODULATION TECHNIQUE FOR SINGLE-PHASE TRANSFORMERLESS PHOTOVOLTAIC INVERTERS WITH = REACTIVE POWER CAPABILITY
Abstract
This paper underpins the principles for generating reactive power in single-phase transformerlessphotovoltaic (PV) inverters. Two mainstream and widelyadopted PV inverters are explored, i.e., H5 and HERIC. Withconventional modulation techniques, reactive powercannot be realized in H5 and HERIC due to the absence offreewheeling path in negative power region. Based on thestudy, modulation techniques are proposed to providebidirectional current path during freewheeling period. Withproposed modulation technique, reactive power control isachieved in H5 and HERIC inverters, without anymodification on the converter structures. Theperformances of the proposed modulation techniques arestudied via MATLAB simulation and further validated withexperimental results.
EXISTING SYSTEM:
Various topologies and modulation techniques have beenintroduced and published . H5 and HERIC aretwo mainstream transformerless PV inverters due to the simplestructure and high efficiency. The leakage current has beensuccessfully reduced to adhere to standard requirement. Eventhough transformerless inverters in literature are capable ofsuppressing the leakage current, most of which are designed forunity power factor operation only. In fact, reactive powersupport is required for next-generation PV inverter in order toallow high penetration of PV system in the utility grid .To achieve this target, many international standards have beenrevised. According to VDE-AR-N4105 , reactive powercapability is essential for grid-connected PV inverters. In light of this, conventional bipolar modulation is reportedas a potential candidate for next-generation PV inverter.Besides leakage current elimination, bipolar modulationtechnique is able to provide reactive power support. However,reactive power capability comes at a cost of high switching lossdue to two-level modulation. In every switching transition, thevoltage changes across the inductor by twice of input voltage.Thus, efficiency is low for bipolar modulation. As a result, several transformerless PV inverter topologieswith reactive power capability have been proposed via three-level modulation (unipolar modulation). In order toprovide reactive power control in conventional H5 topology,combined unipolar and bipolar pulse width modulation (PWM)was proposed in. The PWM is switched from unipolar tobipolar during the negative power region. Although reactivepower control is attained, the implementation is complicated.The current ripple and switching loss are high due to theadoption of bipolar PWM.
PROPOSED SYSTEM:
The proposed PWM is studied in terms of reactive powercapability, CMV, leakage current and efficiency. Theperformances of which are validated via MATLAB simulationand experimental results. It is worth noting that H5 and HERICtopologies are explored in this paper due to their simplestructure, high efficiency and the wide adoption in the industry.The proposed PWM allows them to maintain the existingstructure while achieving reactive power control. While the leakage current suppression method has been mature, the reactive power capability of the transformerlessinverters are not properly investigated. Hence, the reactivepower capability of single-phase transformerless PV inverter isexplored in this section. Two commercial PV inverters areexplored here, H5 and HERIC topologies. The principles ofreactive power generation in single-phase PV inverter are thenunderlined.
CONCLUSION
In this paper, the general principles of reactive power generation for single-phase transformerless PV inverter areunderpinned. In order to generate reactive power, a new currentpath is required in order to achieve zero-voltage state duringnegative power region. Based on the analysis, modulationtechniques are proposed which provides bidirectional currentpath during freewheeling period. As a result, reactive powercontrol is realized in H5 and HERIC inverters, without anymodification on the converter structures. Furthermore, thecommon-mode behavior of which is not compromised. TheCMV is maintained at constant which helps to suppress theleakage current. The overall performances were verified viasimulation and experimental investigation. The results provethat with the proposed modulation method, H5 and HERICinverters are suitable for transformerless and reactive powerapplications. The similar principles can be extended othersingle-phase transformerless PV inverter topologies. In short,the proposed working principles extend the conventionalinverters applications to next-generation PV systems whichrequire reactive power capability.
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