Multilevel Converter with Nearest Level Control for Integrating Solar Photovoltaic System

 Abstract

This paper presents a new Scott-ternary solar multilevel converter (ST-MLC) based solar photovoltaic system for medium voltage applications. This system employs a single solar photovoltaic (PV) array as an input source and two voltage source converters (VSCs) to feed a three-phase grid. Solar power is fed from a two-phase to a three-phase medium voltage grid with balanced grid currents. The legs of VSCs are cascaded to form a two-phase system. The multi-winding transformers are connected in a Scott manner to develop a nine-level three-phase power conditioning system. The turn ratios of the transformers have ternary progression for the generation of higher levels. The modeling of the system is presented, and a closed-loop nearest level control (NLC) strategy is incorporated to have minimum losses. Simulation results show the performance of the solar photovoltaic system under steady-state and dynamic irradiance. Varying solar profile is considered on basis of the daily average global solar irradiance (GSI) from the geographical information system (GIS) for New Delhi, India. The harmonic performance is analyzed for the converter voltage and grid current. Moreover, a detailed comparative analysis is covered for the ST-MLC to evaluate its superiority over its existing counterparts. Performance validation is done in a real-time test bench to validate the control and operation of the large-scale solar photovoltaic system.

Index Terms

Photovoltaic Array, Power Quality, Multilevel Converters, Fundamental Switching

Block Diagram:

Fig. 1. Medium voltage grid-tied solar photovoltaic systems

Expected Simulation Results:

Fig. 2 Simulation results set I (a) modulating signal, teaser and main primary winding voltages (b) transformer secondary side grid currents and transformer primary

currents (c) steady-state performance at 1000 W/m2 (d) step reduction in irradiance

Fig. 3 Simulation results set II (a) Solar power and converter voltage dynamics with step reduction in irradiance (b) Solar power and converter voltage dynamics

                                                              with step rise in irradiance.

Fig. 4. Simulation results set III (a) step increase in irradiance (b) average daily global solar irradiance (GSI) variation (c) forenoon GSI variation (d) afternoon

                                                                         GSI variation

Fig. 5. Simulated harmonics spectrum of (a) nine-level converter voltage (b)

                                                                        grid current

Conclusion:

A new Scott ternary nine-level converter-based solar photovoltaic system is presented for medium voltage integration. It has used a single PV source to feed solar power, which has reduced the demand for the current and voltage sensors at the PV side. The design and modeling of Scottternary configuration have given the understanding for the level generation through cascading of transformers. The inherent galvanic isolation has provided safety to the converters from undesirable conditions. The leakage current issues are minimized as a single PV array is utilized. The loss equation and thermal model are presented, and selection of switching frequency is fundamental to have minimum switching losses. The advantages of minimum losses in NLMT also take care of thermal heating issues. This system also uses fewer current rating switches than other existing multilevel topologies, which do not use Scott connection. The NLMT employed here has quadrature displaced modulating signals to test the system performance in a closed-loop. The results are obtained from simulation in MATLAB/Simulink under different environmental conditions. The validation in real-time is done for a steady-state, step-change in irradiance, and variation in global solar irradiance as per latitudes and longitudes of the Indian environment (forenoon and afternoon GSI). As discussed in the comparative analysis, the presented system has given a better performance and new contribution in renewable energy integration.

REFERENCES

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