ANALYSIS AND DESIGN OF PHOTOVOLTAIC AND FUEL CELL BASED CONVERTER SYSTEMS

Abstract

The steadily increasing energy consumption, the soaring cost, the exhaustible nature of fossil fuels, and the intensifying concerns over the global environment have created much interest in alternative energy sources such as solar, wind, fuel cell etc. as future energy solution. Alternative energy sources integrated at distribution level is termed as Distributed Generation (DG). Photovoltaic (PV) and Fuel Cells (FC) are the best environmental friendly technologies for DGs and hence receiving increased attention. The two-stage Power Conditioning Unit (PCU) including the DC-DC boost converter with the Pulse Width Modulated Voltage Source Inverter (VSI) is the state-of-art technology used nowadays worldwide for grid interfacing of the FC. However, it has reduced power conversion efficiency because of the two-stage configuration. Further, it has reduced reliability because of more no of components used in this configuration. The author has proposed a FC based DG system with a single-stage power conditioning unit. The system is analysed by modelling various units of FCDG system, performing mathematical analysis and simulation studies. The Proton Exchange Membrane Fuel Cell (PEMFC) model used for the simulation studies is based on physical processes inside the PEMFC stack and is modeled using the experimental data obtained from Avista Labs SR-12 0.5 kW PEMFC stack. The inverter controller is designed to control the active power fed to the grid, the reactive power transfer between the inverter and the grid, the DC-link voltage, the quality of the injected power and grid synchronization. The designed control scheme of the inverter consists of a cascade of two independent controllers, where the external voltage controller generates the reference current that is tracked by the inner current controller which generates the pulses for the inverter switches. Another facet of work done is with respect to solar PV as source of electricity. The conventional grid-interfaced PV systems use a DC-DC converter with Maximum Power Point Tracking (MPPT) control and a DC-AC inverter for grid interfacing. The proposed PV system uses one power conversion stage, thus simplifying the system topology. The single-diode PV circuit model used in the PV system simulation studies is modelled using the experimental values of Kyocera KC200GT 0.2 kW PV module. The MPPT, grid synchronisation, reactive power compensation, output current harmonic reduction is included simultaneously in the control circuit of the VSI connecting the PV to the grid. Due to the utilization of only one energy conversion stage, the single-stage grid-connected PV system proves to be simpler, more efficient and economical than its two-stage counterpart. However, the complexity of the control scheme is somewhat increased. The increased use of power electronic devices in various loads results in many power quality problems in the power system network. Shunt Active Power Filters (SAPFs) are extensively used to compensate the load current harmonics, reactive power and load unbalance at distribution level. The ii principle of operation of the SAPF is to supply the undesired harmonics and reactive power to the load, so that the mains current is of improved quality. However, its implementation results in additional hardware cost. With the objective of reducing the cost and increasing the efficiency, grid-interactive FC system have been proposed which includes the functionality of SAPF. A control algorithm is developed such that the features of SAPF have been incorporated in the conventional inverter interfacing the FC to the grid without any additional hardware cost. Thus the grid-interfacing inverter is effectively utilised to perform the following functions: control of active power from the FC source to the grid, load reactive power demand support, current harmonic compensation and current imbalance compensation at PCC. With appropriate control of grid-interfacing inverter all the four objectives can be accomplished either individually or simultaneously. This concept, thus, reduces the overall design and cost of the system. The motivation for the last part of the work is given below. The cost of FC is too high to justify its widespread use. The PV power generation has large variations in its output power during day and night and during varying weather conditions. Hence a PV-FC hybrid system can prove to be better to provide a reliable power source for grid-connected applications than a system comprising any of these single resources. The last part of the work proposes a utility-interactive hybrid DG system consisting of PV and FC to realize a reliable power supply for a grid connected critical load. These sources can be operated independently or in conjunction as per the requirement. The proposed system ensures maximum utilization of the PV array, and necessary utilisation of FC stack resulting in optimum operational costs. The power circuit topology consists of two DC-DC boost converters, where one of them is fed by a PV array and the other by an FC stack. The Incremental Conductance MPPT control algorithm is implemented in the DC-DC converter connecting the PV array to the DC-link. This ensures extraction of maximum power from the PV array under all conditions. The difference of required power and the PV power is provided by the FC and governed through proper control of DC-DC converter connecting the FC stack to the DC-link. The outputs of the two DC-DC converters are connected to a common DC-link. The power is fed into the grid through an inverter for which the common DC-link acts as the energy source. The inverter control is so designed that apart from feeding active power into the grid, the system can also provide reactive power and harmonic compensation for PCC load.