Photovoltaic cell

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1 Introduction

In the past years, the energy generation was only been focussed on with fossil fuels and the energy requirement was met for satisfying inhabitants life. With the growth of human population, energy generation turned up as a potential problem. Nowadays electrical power generation is geared up using different renewable energy sources. Evident increase in cost, availability and economic impact of oil, gas paved the way to opt other power generation technologies. Thus a new technology based on sun's energy called photovoltaics(PV) came in to practice which converts sun's energy to electricity. Photovoltaic (solar cell) power systems are designed using photovoltaic cells and this renewable energy is considered as reliable future energy generation technique.

A typical photovoltaic cell is capable of producing 3 MW per 0.5 volt dc. Each cell can provide high solar conversion efficiency, permanent power with reduced operating cost and are free of pollution. Depending on size, sunlight intensity and efficiency of PV cell, current generated varies. Photovoltaic cells are arranged to form photovoltaic modules and photovoltaic modules are clubbed to form photovoltaic arrays and in turn photovoltaic systems. Photovoltaic systems are used mainly for two types of applications, Stand-Alone and Grid-Connected systems. As the study progresses with grid-connected systems, we are not dealing much about stand-alone systems.

A Grid-Connected photovoltaic system consists of PV modules (arrays), inverter, protection device for auto-shutdown during grid failure, load and electricity meter. These systems vary in size and are installed within buildings and houses for domestic purposes. When sunlight is captured by PV modules, it is fed to an inverter which converts DC power to AC power and supply AC power to loads connected. After load power consumption, excess-generated power is fed back to distributed grid system and can feed loads up to 10kW. Even though the cost of producing photovoltaic power is expensive compared to utility power, the demand for grid-connected systems are increasing. Electricity bill will be reduced by using power produced by grid-connected systems. Also it allows consumer to retail excess-generated power to area power supplier. There are some regulations before installing PV systems with grid power supplier and have to agree terms and conditions with local power supplier. Two major technical issues associated with grid-connected PV systems are power quality and harmonic distortion produced by inverters. Harmonic distortion can be reduced by using good quality inverters.

Grid-connected systems are highly efficient if they are capturing maximum amount of sunlight and energy conversion taking place in inverters. For serving that purpose, maximum power point tracking (MPPT) methods are used. MPPT is an electronic systematic method by which it helps all PV arrays to produce maximum accessible power within their capacity. This study proposes a highly efficient grid-connected PV system using precise MPPT method with high power quality and less harmonic distortion.

The proposed grid-connected PV system is coupled with a power quality controller to enhance power quality and MPPT. In order to achieve minimal harmonic distortion and unity power factor, a single stage current control inverter (SSCCI) is used. The main components of the proposed system are SSCCI (current source), DSP control module, PV array, transistors, load, transformer, MPPT block, DC capacitor and interfacing-sensing modules. The SSCCI input is linked to DC capacitor and its output is coupled with grid through transformer. The operation principle works on two stages; sunny day time and night time. During day time, the designed system delivers power at maximum power point to load or grid and some energy is utilised to improve power factor and reduce harmonic distortion. Hence maximum power can be achieved from PV arrays keeping at a operating point. At night, the grid supplies active power to load and SSCCI supplies reactive power to load. SSCCI produces current which should be controlled in order to balance grid reactive power and minimise active power produced by PV modules. For current control, the proposed system is controlled using phase-locked loop scheme with PV current-voltage feedbacks which provides reference current to detect high or low current. This scheme is commonly called as polarized current-control scheme (PRT).

The proposed system can be simulated using PSpice software which is available in lab and MPPT can be achieved for the designed system. The grid power can be assumed from a real electrical system and PV power reading can be obtained from Northumberland Building for simulation purpose where PV arrays are installed. This proposed system is expected to be used in large scale applications for future generation.