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CU-Boulder ECEN 4517 - A Simple, High-Quality Output PV System

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CIEP A Simple, High-Quality Output PV System Aimed at Peak Demand Reduction Alh IbSez Martinez and Hugo Calleja Acapulco, MEXICO October I5 - 19 Cenidet - Departamento de Electr6nica Interior Internado Palmira sln 62490 Cuernavaca, Morelos email: [email protected] Abstract A grid connected photovoltaic system aimed at peak demand reduction in areas with hot weather is presented. It uses the "Perturbe and observe" technique to track the maximum power point of the cell array, and provides a high quality current waveform. The islanding of the system is avoided by injecting a small ripple in the output current. The system was tested with a 500 W cell array, and its performance was satisfactoiy. I. THE PEAK DEMAND PROBLEM The term demand usually refers to the power required by a user. Utilities must have enough installed capacity to satisfy this requirement; however, the value required by a user exhibits severe fluctuation. Figure 1 shows the energy consumption patterns followed by a group of residential users in areas with warm weather. As can be seen, energy consumption is much higher during the summer than during the winter. The increment is due to air conditioning equipment [ 11. Thus, a utility having the required capacity to efficiently satisfy the summer demand, will operate with much less efficiency during winter. Figure 2 shows a typical demand for two different days; one during summer and the second during winter; again, if the utility has the capacity to satisfy the peak demand at noon, its operation will inefficient during the early hours of the day Comumos mensuales durante un aiio. 3000 -c D-411 4-C-401 *C-410 +k403 *E409 + D-408 2500 ___..-..-...----.-_.. 01 1' ' I' " I' ' EFMAMJJASOND be Mes Figure 1. Energy consumption patterns Camparadon de mnsumos verano - invierno KWh 0 2 4 6 8 10 12 14 16 18 20 22 24 Hora Figure 2. Individual energy consumption patterns during summer and winter. Thus, from the utility point of view, the average demand should be as close as possible to the maximum demand. The consumption of out-of-peak energy is encouraged by the rate structure; peak energy might cost as much as five times the cost of the out-of peak energy. On the other hand, the Mexican utility applies a rather complex rate structure. The cost of energy, for residential users, is lowered during the summer months; the reduction being directly proportional to the average temperature during this season. This reduction represents an economic burden for the utility, and residential users are effectively subsidized: It should be noted, however, that according with the graph shown in figure 2, summer peak demand occurs around 14:OO hours. At this time of day the irradiance from the sunlight is also at a maximum. Thus, a logical conclusion is that a photovoltaic system can help in reducing the peak demand [2]. 11. DESIGN OBJECTIVES The idea of using a PV system to lower the peak demand is not new. In fact, it has already been tested with mixed results. The main difficulties were related to poor quality current waveforms, in systems improperly scaled down from 220 V designs, or to reliability issues. With this in mind, the following design objectives are established [3][4]: 0-7803-6489-9100/$10.000 2000 IEEE 301The system must provide high quality current waveforms 0 The system must be capable of transferring the maximum power supplied by the solar cells, to the grid. 0 In order to increase the reliability of the system, it is desired to have a simple design. 0 The system will inject only active power into the grid, without provisions for any other fimction such as power factor correction. 111. INTERCONNECTION ISSUES The PV system, modeled as the voltage source Vpv in figure 3, is connected to the grid through an impedance z. In order to have a power flow from the system to the grid, its output voltage of the system must be higher than the grid voltage VCA. The output current delivered by the system is: VPV Figure 3. Power transfer to the grid PL and QL are the apparent, active and reactive power demanded by the user, and SCA, PCA and QCA are the corresponding power delivered by the grid. Nevertheless, since the line current is reduced, the losses in the distribution grid are also reduced. Q Iv. THE PV SYSTEM k diagram of the photovoltaic generator is shown 'k connected to the grid, it does not include a I CONTROL CIRCUIT I The output voltage of the system is then: Figure 4. Vector diagram VFV- VCA Z IFV = The impedance can be resistive or reactive; a inductive impedance is usually used because it filters out the current harmonics. Let the impedance be given by z = IZI 14, then: According with the last equation, the output voltage must include a phase shift such that th injected current is in phase with the grid voltage. The analysis performed assumes that the grid voltage and the output of the system are at the same frequency. It is a well known fact that the frequency in the electric grid is not constant, although it does not change noticeably; thus, the control circuit in the PV system must include a frequency-tracking device. It should be noted that, since the PV system will inject only active power into the grid, the power factor seen by the mains will be lower than that obtained without the system. This can be seen in the vector diagram of figure 4, where SL, ................ ................ CELL ARRAY INVERTER Figure 5. Photovoltaic system diagram The power stage corresponds to a single-phase bridge, voltage fed inverter, whose output current is regulated as to inject into the AC grid the maximum power available from the solar cells array. The output of the inverter is modulated with the hysteresis band technique, and the circuit used to implement this function is shown in figure 6. With this technique the frequency-tracking feature is automatically provided, thus simplifying the hardware. A sample of the AC voltage is taken through the digital potentiometer RDI~; this sample is applied to one of the inputs of a comparator, while the other is fed by the output of a Hall effect current sensor. In turn, the output of the comparator is used turn on and off the transistors in the inverter stage. In normal operation, the modulation will produce a current waveform that follows the


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CU-Boulder ECEN 4517 - A Simple, High-Quality Output PV System

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