Güneş pillerinden şebekeye enerji aktarılmasının analizi ve tasarımı

Abstract

Bu çalışmada, güneş pilleri dizisinden elde edilen de gücün ulusal şebekeye aktarılması için mikrokontrolör tabanlı kontrolü tasvir edilmiştir. Güneş pilleri dizi akım-gerilim karakteristiğinin lineer olmayan, ışık şiddeti ve sıcaklığa bağlı olarak değişen modeli ile güç dönüştürücü yapısının genel modeli oluşturularak akım kont rolü yapılmıştır. Elde edilen sonuçlar ışığında güç dönüştürücü yapısının fiziksel olarak gerçeklenmesine gidilerek güneş pilleri dizisinden ulusal şebekeye bir faz olarak enerji aktarılması için yazılım ve donanım geliştirilmektedir.

The conversion of sunlight directly into electricity using photovoltaic properties of suitable materials is the most elegant energy conversion process. A laboratory curiosity for more than a hundred years, solar cell technology has seen enourmous development during the last three decades, initially in providing electrical power for spacecraft, more recently for terrestrial systems. The driving force for technological development is the realization that the traditional fossil energy resources, coil, oil, and gas, are not only limited, but they also contribute to unpre dictable and possibly irreversible climate changes in the near future through the emission of carbon dioxide. The increasing concern for environmental pollution problem in industrialized countries has also discredited nuclear power as a longterm alternative energy concept. From this perspective, the use of sunlight offers a conceivable alternative to the worldwide energy problems. Solar cells use the photovoltaic effect for their operation, which was discovered in 1839 by Becquerel, who studied the behavior of solids in electrolyte solutions. Technological development began with the development of a diffused silicon pn-junction in 1954, a forerunner of the present silicon solar cell, which con verted light into electricity with reasonable effiency. The initial applications were on a small scale, and the first real impact of solar cells was only realized with the advent of space exploration. The demand for a reliable, longlasting power source was the major reason for the application of solar cells, and by 1958 the first silicon solar cells were used in spacecraft. Interest arose in solar cells as an alternative energy source for terrestrial appli cations in the mid-1970s after the political crisis in the Middle East and the oil embar go, and the realization that fossil sources were limited. Increased research efforts resulted not only in futher improvement of the effiency of silicon solar cells and a considerable reduction of energy costs. The cost target for electricity from a photo voltaic plant operatig for 30 years was established in 1986 to be equal to about 0.06US$/kWh. Photovoltaic power can be generated in houses, intermediate size commercial installations, or large central power stations. In a residential photovoltaic system (typically few kilowatts in size) the array is mounted on the roof. The available dc power, which varies with solar insolation and temperature, is converted to single- phase 50 Hz ac and fed to the utility line. The consumer's load is connected at the ac line terminal. In daytime, the solar power supplies to the consumer and the surplus is fed to the utility ; in cloudy weather or after dusk, the utility line feeds the load. The power conversion scheme used in the system is shown in Fig. 1. Basically the dc array power power is converted to 50 Hz ac line through an isolated high frequecy transformer link. The dc voltage of the photovoltaic array is first converted to high-frequency ac by an inverter, which is then transformer-coupled and rectified to dc line voltage and then then converted to 50 Hz ac line current through a polarity-reversing inverter. Compared to the conventional 50 Hz line commutation scheme with transformar isolation, the high-frequency link scheme used here permits considerable weight reduction of the power converter and the smooth fabication of the sinusoidal output current wave in phase with the line voltage. DC LINK CURRENT A A TRANSFORMER VOLTAGE JpL RECTIFIED VOLTAGE PV ARRAY DC-DC CONVERTER OUTPUT OJRREHT Fig.l. High-frequency link power conversion! scheme The solar array characteristics profoundly influence the converter and the control system. The array cell characteristics, as a ruction of light intensity and temperature are given by the equation I - İl - n". Io. exp i(v+i-Jts) AKTn, -1 Io= Io' T T. exp î£g_oJj__±_ BK L Tr T II ==",. [isc+KXTc -28°)].% 100 (1) (2) (3) X : cell illumination (mW/cm2) T : cell temperature in K lor : saturation current at Tr = 25 °C I : cell output current V : cell output voltage K/q : Boltzmann's constant divided by elctronic charge = 8,62. IO"5 eV/K Ki : short circuit current temperature coefficient at Isc Isc : cell short circuit current at 28 °C and 1000 mW/cm2 II : light-generated current Ego : band gap for silicon = 1. 1 1 eV B=A : ideality factors Rs : series resistances The converter which is connected at the array terminal can be represented by an equivalent resistive load at static condition:.o >İ ARRAY VOLTAGE VQ (V) Fig.2. Array volt-ampere curves showing maximum power and load line hyperbolas The intersection of the load line with slope G and the array Vg -Ig curve defines the operating point and the dc power absorbed by the converter. A constant power is a hyperbola and maximum power point touch the hyperbola at one point only. If the converter is assumed lossless and utility line voltage remains constant,from power balance equation : P = Vl.Il = Vg.Ig IL = %^ = k.PG (4) (5) the line current amplitude may be obtained. The power conditioner has a microcomputer based control system, the micro computer is responsible for control of the output ac power in accordance with the generated array dc power, maintaining a unity power factor condition at the ac line terminal, and has the functions of power maximum tracking, safe voltage and current current zone control and diagnostics. The microcomputer makes the feedback control of current to maintain a balance between array power and the power fed to the utility assuring stability under all operating conditions. PV ARRAY IVIaac. Power Tracki ri|g Control Safe Zone Control I G Line Current Referans «p^HlZJ-*^^ m Unit Amplitude Ref.Current Wave Actual ac Link Current MICROCONTROLLER Fig.3. Block diagram of control system. The current i" is generated by the phase-locked loop to be cophasal with the utility voltage. The ac link current is forced to track the command 1^ by the histe- rezis band on-off control principle of the high frequency insulated gate bipolar transistor inverter.