ELECTRIC GENERATOR BASED ON SOLAR CELLS

Transcription

1 ELECTRIC GENERATOR BASED ON SOLAR CELLS 1. INTRODUCTION Nowadays, the photovoltaic systems (PHVS) are can be a definitive solution to the rural electrification problem, with clear advantage respect other alternatives [1]. The panels lack movable parts and they are practically unalterable; they do not contaminate nor produce any noise, they don t consume fuel and do not need maintenance. In this paper a co-generation system connected to the AC mains is presented. It is structured by an independent power electronics generator and it has a connection to the electric system [2]. When AC mains and the PHVS feed the load and the local demand is minor that the generator capacity, it is allowed that the excess of energy can be injected in the electrical system and consumed by other loads. When the injection of energy is allowed, it says that the system is interactive with the AC mains. A system of cogeneration of electrical energy formed by a photovoltaic arrangement must have the following generic operation characteristics [3]: Operation in the maxim efficiency point. Simple and robust. High quality waveform with low distortion. A complete analysis and experimental results are included in the final version. 2. SYSTEM DESCRIPTION In the electrical generation systems energy based on solar cells, the most common strategy is that they are independent systems of the AC mains with capacity to feed independent electrical loads. On the other hand, the cogeneration of electrical energy supposes the interconnection of independent equipments to the electric AC system to inject active power to the system. The block diagram in figure 1 shows a photovoltaic system for generation with direct connection to the AC mains. Photovoltaic Cell Inverter Connection inductor Inverter control Figure 1. - Photovoltaic system The principle of operation is that a single-phase IGBT inverter is fed from a DC voltage obtained from solar cells and it is driving by the control circuit. The generation of the control signals is obtained by the comparison of two signals: the current reference and the feedback of the output current in the link inductor [6]. 2.1 Photovoltaic System In this application the photovoltaic system is formed by the series/parallel connection of a matrix array cells (15 rows by 7 columns) resulting a maximum voltage of 290 without load; the instantaneous maximum power is 1800 watts. In figure 2 the power profile with load is shown; the reading corresponds to the March month in the city of San Luis Potosí.

2 Hour Figure 2. Graphic of variation power in the photovoltaic system with the half of the load. 2.2 Inverter control One of the objectives of the PHVS is to provide active power to the electric system; therefore it is important that the current generated by the system is in phase with the network voltage, and it should be necessary to have a low harmonic distortion. In this way the reactive power injection is avoided, and the distortion is reduced. The switching frequency (f sw ) is a parameter in the current control, and it is necessary to obtain the commutation pattern. In order to obtain this pattern, it is necessary to incorporate a comparison circuit for two signals (figure 3): the reference signal (I ref ) and the feedback current signal (Ii nv ), which is taken from the link inductor. Current Sensor Dead time circuit. To IGBTs IGBT drivers ON OFF filter reference current. Figure 3. Control circuit structure: dead time generation, hysteresis comparator. The hysteresis comparator has the objective of generate the switching pattern. 2.3 Transformer and link inductor. The objective of insert a link inductor between the inverter output and the AC mains is to obtain a sinuosoidal current. The inductor reduces the harmonic content of the current. The PWM voltage in the inverter has the commutation frequency selected. Considering that the inductor limits the di/ of the current circulating, the current generated by the inverter follows the sinusoidal waveform of the reference (figures 5 and 6).

3 V l I inv V PWM V ac VLoad I ref Figure 4. System connection. V CD V lmin V PWM V lmax -V PWM V lmax V lmin Figure 5. Differential voltage in the link inductor when the photovoltaic system is connected to the electric system. The condition for the inductor calculation is that V pwm (t) > V ac (t) for all t, to guarantee the total control of the power injected to the line; therefore it is necessary to find the maximum and minimum rank of inductor values that full fills this condition (figure 5). The voltage in an inductor is given for: Ldi V L = (1) An important consideration is that the di/ of the output current inverter must be bigger than the reason di/ reference current for all the cycle, this means: di inv diref >, t (2) Considering the maximum commutation frequency f swmax I > TI pω cos( ωt) (3) 1 T = (4) f swmax Therefore the maximum and minimum values of the voltage in the inductor, with voltage variations in the CD bus and considering the worst case in line voltage are: V Lmin = V V (5) PWM peak

4 V V + V Lmax = (6) PWM peak Replacing values in (3) L max I = VLmax (7) T L V I T min = Lmin (8) Figure 6. Current signal 2.4 Thermal design. The objective of study the R qda of the sink for the inverter is to estimate the system efficiency and to guarantee that the maximum power transference between the solar panels and the electric AC system is obtained. The final version will include a more complete analysis of the thermal design. 3. EXPERIMENTAL RESULTS. The experimental results obtained in a laboratory prototype are shown. DC voltage feeding the inverter system is obtained from a photovoltaic array. The maximum switching frequency of the is 10 khz, and the link inductor is 13mH. In figure 7 is shown the current waveform generated, the reference current signal and the AC mains voltage are shown. The reference signal is obtained directly from the AC mains voltage, and how it can be seen, the system and the voltage are synchronized. The power injected by the prototype is 1KW. Figure 7. Upper trace: I inv, 2.0 A/div, 2.0 ms/div (I Ref superposed). Lower trace: ac mains voltage, 100 V/div. Figure 8. Upper trace: inverter current: 2.0 A/div, 2.0 ms/div. Lower trace: voltage of network, 250 V/div.

5 In Figure 8 the PWM commutation pattern in the inverter is shown; the current sensor signal with the reference current are shown too. In Figure 9 the DC bus voltage, current reference, and the output signals are shown. It is observed that the DC bus current has an approximated value of 2.8 A, indicating that there is active power flow towards the load. Figure 9. Upper trace: voltage of the DC bus, 250 V/div, 2,0 ms/div. Central trace: current of reference and current of the investor, 2 A/div. Lower trace: current of the DC bus 1 A/div. Figure10. Upper trace: Current injected to the line 20 V/div. Lower trace: current of the DC bus 1 A/div The voltage in the DC bus is 290 V. In figure 10, it can be seen that the current delivered by the photovoltaic generator is I = 8 A rms with 127 V rms mains voltage, therefore the output power of 1Kw. 4. CONCLUSIONS In this paper the operation principle and development of a CD/CA converter for applications in photovoltaic cogeneration systems with direct connection to the electric AC system has been presented. A brief analysis and the design of key elements in the system are including. Experimental results that validate the proposed design are shown. In the final version a more extensive analysis with complete results will be present. A performance system curve will be included. REFERENCES [1] [2 Willian Sweet., Power & Energy, Technology 1999 Analysis & Forecast, IEEE SPECTRUM, January [3] Juan H. Almazán Covarrubias. Convertidor CD-CA basado en el convertidor reductor elevador, para aplicación en Sistemas de Alimentación Ininterrumpibles, Tesis de Maestría, Centro Nacional de Investigación y Desarrollo Tecnológico, CENIDET, Cuernavaca Morelos, México, Junio [4] J.A. Gualda., S. Martínez, P.M. Martínez., Electrónica Industrial: Técnicas de Potencia, Segunda Edición, Editorial Alfaomega Marcombo. [5] Muhammad H. Rashid, Electrónica de Potencia; circuitos, dispositivos y aplicaciones, Segunda Edición, Editorial Prentice Hall. [6] Alan J. Ibañez Mtz., Generador fotovoltaico conectado a red, tesis de Maestría, Cenidet, Cuernavaca Morelos, Septiembre [7] Arturo Romero, Robert Foster, Bombeo de agua con sistemas fotovoltaicos Asociación Nacional de Energía Solar, A.C., Octubre de 2001.