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2. ENGINEERING AND TECHNOLOGY
Solar Energy or more precisely photovoltaic energy is one of the most promising sources of electricity for the future and it can be used as a distributed generator (DG) to play its role in ‘smart grids of the future’. Distributed PV (photovoltaic) generators can provide numerous potential benefits such as augmenting the capacity of distribution systems, deferring capital investments on distribution and transmission (T&D) systems and improving power quality and system reliability. The PV energy which possesses very special I-V and P-V characteristics has to be conditioned by a PV inverter before it can be consumed by an ac load and/or the grid. Technical improvements in maximum power point tracking (MPPT) and islanding detection are proposed for a three-phase photovoltaic grid tied inverter (GTI) keeping in mind the requirements of the international standards for connecting a DG to the utility grid. This PhD thesis will contain four major sections which are briefed below. A three phase GTI has been simulated using Matlab/Simulink to test the various control blocks and algorithms involved in the building of the power conditioning unit. A DS1104 dSpace DSP controlled, 5.625 kW three-phase GTI laboratory prototype has then been built. Various hardware components, including inverter switches, gate drivers, LCL filter, rectified dc source, boost circuit, transformer, 16A current protection circuit, additional sensing interface circuits and PWM level shifter have been designed and built within the laboratory. The software algorithm created in Simulink communicates directly with the built hardware via the graphical user interface that has been designed with dSPace Control Desk. Algorithms have been developed for the inverter in order to protect it from operating out of nominal frequency and voltage ranges. An algorithm has been developed iii to ensure the boost dc link voltage is controlled to 300V when dc voltage source varies between 150V and 265V. The Z-Source inverter (ZSI), with nine operating states that employs an extra shoot through (ST) state compared to the eight states (6 active and 2 zero states) in traditional VSI is one of the most recent boost topologies that has been proposed in the literature. A step by step design procedure of a ZSI has been developed. A topology comparison between Z-Source inverter and dc-dc boost with VSI is done using literature and simulations. Merits and demerits of the two topologies are summarised and the choice of the topology is justified. MPPT is a process by which maximum power from a PV panel or array is tracked and absorbed during a particular weather condition (insolation level and temperature). There are various MPPT techniques in the literature which are reviewed and a new MPPT approach based on the P&O (Perturb and Observe) method is proposed. The proposed technique is tested on the three phase GTI simulation, it is analysed and compared to the conventionally reviewed P&O MPPT approach. The issue of islanding of GTI’s has raised concerns of equipment and personal safety, for which reason the inverter has to detect and stop the inverter during loss of grid. Passive techniques can detect the grid failure quite well when there is a large power mismatch between the DG and the load but not when the mismatch is small. Active techniques can work well with lower levels of power mismatch but they degrade power quality by introducing disturbances into the power system. A novel wavelet based antiislanding technique is proposed and incorporated into the running hardware protection. This uses physical measurements to reduce the non-detection zone close to zero and keep the power quality of the inverter output unchanged. The developed algorithms have been validated in the laboratory prototype and yield very satisfactory performance.
Hanif, M.: Investigation to Improve the Control and Operation of a Three-phase Photovoltaic Grid-tie Inverter. Doctoral Thesis. Dublin Institute of Technology, 2011.