GRID/OFF-GRID MULTILEVEL SPLIT VOLTAGE CONVERTER FOR PHOTOVOLTAIC SYSTEM FEEDING VARIETY OF LOADS

Abstract

With growing pressure on depleting fossil fuels reserves, the focus has been shifted for harnessing more and more energy from renewable sources for sustainable growth. Photovoltaic system has emerged as a most appropriate solution due to proximity to the load centers and more or less predictable nature of intermittency. Conventional centralized photovoltaic (PV) grid tie inverters suffer from the problem of lower efficiency, high filter size, and limited depth of operation for remaining in connection with the grid, particularly under lower insolation/partial shading condition. The effect is reported to be more pronounced for single stage inverters integrating with weak distribution systems. Further with the advancement of technology the residential loads have witnessed a paradigm shift from linear loads to nonlinear, dynamic and constant power loads which are customizable and configurable to cater specific application. To avoid multiplicity of AC – DC through DC-DC or DC-AC conversions, the concept of DC nanogrids have also evolved to cater the need of a variety of such loads. Investigation of a multirole bidirectional cascaded multilevel converter (CMC) configuration in which a single phase supply is split into 3 different DC links enabling staggered PV connections and possibility of feeding variety of loads have been carried out. The considered configuration is also utilized to feed isolated single phase loads in an off-grid mode using appropriate modulation technique. Considered loads include dynamic, constant power and passive loads in addition to open-end winding induction motor drive (OEIM), battery charging/discharging etc. The connectivity of the load is considered on DC buses while in off-grid/grid connected mode and on AC side too while in off-grid modes. The proposed system configuration and the considered control algorithm suits to the majority of the residential loads while enabling it to act smartly for stabilizing the grid in case of the need. Further investigation on algorithm involved the DC bus balancing embedded in the control to ensure balanced voltage operation or operation at different voltages dictated by individual MPPT controller across the DC links. The control scheme is further explored for bidirectional power transfer with smartly charging/discharging control of the split battery stacks at customized rates depending on the SoC’s of battery stacks and on feeder loading conditions, without disturbing the DC bus voltages. The exploration has been extended for operation during under-voltage grid condition where the customization in proposed algorithm enables the rotating charge control algorithm which helps the grid to stabilize its voltage in conjunction with maintaining life cycle of the battery. The proposed configuration and control enjoys the advantage of 3 separate DC buses having both voltage and power level 1/3rd of the total DC voltage and power, which enables the reduction in the voltage rating of capacitor; making system more modular and compact and deriving power from AC is with reduced voltage THD and providing immunity against unbalanced DC link voltages across the H-bridges. The complete model of the CMC with a variety of loads and their embedded control is analytically derived and simulated in MATLAB Simulink environment before testing on hardware prototype for its validation. A detailed stability analysis is also presented in the d-q frame for the control design to access the feasibility of operation with a variety of loads. The effectiveness of the control algorithm under low grid frequency and dip in voltage conditions are clearly demonstrated through results. The results clearly show derived current from the grid at unity power factor ensuring improved power quality operation. Further, keeping the entire voltages on the DC buses constant or at voltage dictated by MPPT controller ensures immunity against disturbance both from AC or DC side. A comparative analysis is also done for the operation of PV under partial shading condition for a conventional 2 –level PV inverter vis-à-vis proposed CMC-based approach. Further PV-CMC system for enhanced performance under voltage sag is studied to demonstrate the LVRT capability. The d-q based control provides efficient independent and smartly control with active/reactive or both power support depending on the voltage sag and PV panel power condition. The thesis also proposes control techniques for off-grid mode, which will match the utilization and storage of power provided by the PV panels of the same capacity and same size of the battery connected at each level. The control method utilizes rotation policy for the operation of each bridge at each level in three fundamental cycles, to enhance both the lifetime of the battery, the operation of H bridges and PV panels used. Same scale hardware prototype using open end induction motor, passive loads and battery loads (charging/discharging) on different DC links is developed and experimentally validated using requisite hardware and DSP controllers (dSPACE 1104 and dspic33FJ16GS502). The development of hardware including fabrication of various control cards, interface card, voltage and current measurement cards etc. have been indigenously done. Both simulation and experimental results are presented which always show good agreement with theoretical analysis.