Güneş pili santralları

Abstract

Extensive research and development activities are currently being conducted ali över the world in the area of utilizing renewable energy resources. The interest of the utilization of local renewable energy resources for devoloping countries has been enhanced by the dramatic increase of oil prices in the early seventies. Öne of the most promising renewable resources is the solar energy. The recent developments in the solid state industry accompanied by a parallel increase in energy prices and the environmental restrictions as well as the need for reliable sources of energy lead to the consideration and assessment of new sources of energy which can secure the needs of public with a minimum impact on the environment. An important candidate is the photovoltaic (PV) source of energy, where the solar radiation is directly converted into electricity that can either be residentially used as a local self-sufficient source such as telecommunication, vaccine refrigeration, lighting, battery charging and water pumping applications ör interconnected to available AÇ public grid. in this work about the photovoltaic systems that are connected to the grid, the voltage obtained from the array is converted to three phase sinusoidal voltage by a static inverter vvhich supplies the public grid and the system is simulated. A utility - interactive PV system (Figüre 1) consists of a variety of subsystems: a PV array subsystem, a power conditioning subsystem, a utility interconnection subsystem, and control subsystem. The PV array subsystem converts solar energy into direct current (DC) electrical povver and delivers it to the power conditioning subsystem (PCS) through the DC interface. The array subsystem also provides protection and necessary electrical isolation between the PCS and the array, and may include experimental instrumentation for monitoring the performance of the array. The utility interconnection subsystem, through the alternating current (AÇ) interface with the PCS, provides synchronization with the utility and, if necessary, acts to electrically isolate the PV system from the utility. The control subsystem, operating through the PCS, oversees the performance of the entire PV system. it also enables overall coordination of the system protection, communicates status information to the utility dispatch center, and, if desired, provides an information and tracking VII feedback loop with the PV array. in central PV stations, the PCS may also processes operational commands from the utility dispatch center. in operation, the PCS converts DC power from the array into AÇ povver, provides optimum amount of power to be extracted from the PV array for any given insolation and environmental conditions, matches frequency and phase of the voltage desired by the utility, and provides. protection not only for its internal components but also for the equipment external to the PCS.. i , UTILITYCONTROL AND rı^MTi,T INFORMATION SIGNALS CONTKOL ^ SUBSYSTEM rJ^IS PV POWER CON- UTILITY ^- ARRAY -*- DITIONING -+. INTERCON. *~ SOLAR l SUBSYSTEM | | SUBSYSTEM | | SUBSYSTEM | UTILITY ENERGY| |4 4 Figüre 1. Block Diagram of a Utility-Interactive Photovoltaic System To achieve a compatible integration of the PV system with the utility, it is essential that the design of the PCS accommodate the dynamic range of interactions between the PV system and the utility grid. These arise from changes in both grid conditions and the output of the PV array. The proper and safe interconnections of PV subsystems require not only the identifications of their mutual functional constraints, but also a knovvledge of how to select ör design the PV subsystems vvithin such constraints. These constraints, therefore, are important in the selection ör evaluation of a PCS that is suitable for central station PV systems. A solar celi (SC) generator possesses a line of maximum povver, and it is most desirable that the operation of the load line should be close to the maximum povver line of the generator. in such a case, good matching exists between the generator and the load for the best performance of the system and maximum utilization of the solar cells. The VIII-T operating points of the photovoltaic system can generally be accomplished by either carefully selecting the I-V characteristics of the load to be connected to the SC generator, ör incorporating an electronic control device (a maximum-power- point-tracker (MPPT)), which provides the necessary impedance matching the SC generator and the inclusion of a MPPT in PV systems depends on several factors: load type and profile; climatic conditions; the fractional cost of the MPPT and its efficiency; and the gain in energy. An electrical circuit design can be simulated before it is actually built, and necessary changes may be done without touching any hardvvare. Any design that is thought to be complete can be checked easily. Building an electrical circuit is the most practical way to check it, but it is expensive and time consuming. it is useful to simulate the design carefully by using a computer program. Figüre 2 shows the PV system simulated by using PSpice which is a member of the Spice (Simulation Program with Integrated Circuit Emphasis) family of circuit simulators. The solar celi is a semiconductor device that converts the solar radiation directly to electrical energy. The celi is a nonlinear device and can be represented by the I-V terminal characteristics, Figüre 3, ör by an approximate electrical equivalent circuit as shown in Figüre 4. The solar celi is an electrical celi of low level voltage and power, therefore the cells are in series and in parallel combinations in order to form an array of the desired voltage and power levels. The I-V equation of a single celi is given by: I = İL - Is [exp[(q/nkT). (V + RsI) - l] ] where İL is the light generated current, Is is the saturation current, kT/q is the thermal voltage, n is the perfection factor, Rs is the series resistance of the celi. For each characteristic curve there is an optimum operating point with respect to the power. Proper load selection allows the maximum power to be transferred. in this work, to convert the DC voltage into three phase sinusoidal voltage, a three-phase bridge inverter is used. The power circuit of a three-phase bridge inverter using thyristors is shown in Figüre 5, where commutation and snubber circuits are omitted for simplicity. The inverter consists of three half bridge units where the upper and lower thyristors of each unit are switched on and off alternately for 180° intervals. The three half-bridges are phase-shifted by 120°. The inverter output voltage wave shapes are determined by the circuit configuration and switching pattern. These waves are rich in harmonics.