All SiC Grid-Connected PV Supply with HF Link MPPT Converter: System Design Methodology and Development of a 20 kHz, 25 kVA Prototype

Serkan Öztürk,Işık Çadırcı,Muammer Ermiş,Mehmet Canver

doi:10.3390/electronics7060085

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https://doi.org/10.3390/electronics7060085

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Abstract

Design methodology and implementation of an all SiC power semiconductor-based, grid-connected multi-string photovoltaic (PV) supply with an isolated high frequency (HF) link maximum power point tracker (MPPT) have been described. This system configuration makes possible the use of a simple and reliable two-level voltage source inverter (VSI) topology for grid connection, owing to the galvanic isolation provided by the HF transformer. This topology provides a viable alternative to the commonly used non-isolated PV supplies equipped with Si-based boost MPPT converters cascaded with relatively more complex inverter topologies, at competitive efficiency figures and a higher power density. A 20 kHz, 25 kVA prototype system was designed based on the dynamic model of the multi-string PV panels obtained from field tests. Design parameters such as input DC link capacitance, switching frequencies of MPPT converter and voltage source inverter, size and performance of HF transformer with nanocrystalline core, DC link voltage, and LCL filter of the VSI were optimized in view of the site dependent parameters such as the variation ranges of solar insolation, module surface temperature, and grid voltage. A modified synchronous reference frame control was implemented in the VSI by applying the grid voltage feedforward to the reference voltages in abc axes directly, so that zero-sequence components of grid voltages are taken into account in the case of an unbalanced grid. The system was implemented and the proposed design methodology verified satisfactorily in the field on a roof-mounted 23.7 kW multi-string PV system.

Highlights

Various grid-connected photovoltaic system concepts and topologies have been summarized previously [1,2,3,4]: (i) micro inverters [5,6]; (ii) residential systems supplied from a PV string [7,8]; (iii) commercial/residential systems supplied from multiple PV strings having their own maximum power point tracking (MPPT) converters and a central inverter [9,10]; and (iv) commercial/utility-scale
Variations in ac components of vpv and ipv while the MPPT converter is supplied from the multi-string PV system in Figure 2 are given in Figure 19, for operation at two different switching frequencies
Since the experimental results are the same with the theoretical ones based on the dynamic model of the PV array, the mathematical model and system design methodology can be successfully used in the design of the SiC power MOSFET-based high frequency (HF) link MPPT converter

Summary

Introduction

Various grid-connected photovoltaic system concepts and topologies have been summarized previously [1,2,3,4]: (i) micro inverters [5,6]; (ii) residential systems supplied from a PV string [7,8];(iii) commercial/residential systems supplied from multiple PV strings having their own maximum power point tracking (MPPT) converters and a central inverter [9,10]; and (iv) commercial/utility-scalePV plants supplied from a common DC link with a central inverter [11]. Various grid-connected photovoltaic system concepts and topologies have been summarized previously [1,2,3,4]: (i) micro inverters [5,6]; (ii) residential systems supplied from a PV string [7,8];. (iii) commercial/residential systems supplied from multiple PV strings having their own maximum power point tracking (MPPT) converters and a central inverter [9,10]; and (iv) commercial/utility-scale. In the PV systems performing direct conversion of PV power to AC, the voltage of the common DC link is varied by the central inverter against the changes in solar insolation and panel temperature [13]. Transformerless PV supplies require more complex and expensive inverter topologies as compared to PV supplies having transformers for galvanic isolation to prevent the flow of common-mode currents [18,19,20,21,22]

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Results

Conclusion