Low Cost DC to AC Converter for Photovoltaic Power Conversion in Residential Applications

Transcription

1 ~ Low Cost DC to AC Converter for Photovoltaic Power Conversion in Residential Applications Ulrich Hemnann; Hans Georg Langer Department of Power Electronics and Electrical Drives Aachen University of Technology Jiigerstr. 17/19, D-5100 Aachen, Germany Heinz van der Broeck Philips Research Laboratories Aachen WeiBhausstraBe, D 5100 Aachen, Germany Abstract - The paper describes the development and experimental results of a low cost 500 W DC to AC converter for photovoltaic power conversion in residential applications. The converter uses low cost technology usually applied in consumer products. The DC to AC converter is specially designed for operation at a wide DC input voltage range (30 V V) in order to allow optimal power conversion with an arbitrary number of series connected solar arrays. A step up chopper is used for MPP tracking and provides a constant 200 V DC link for the following push pull converter. This galvanic isolating converter operates at 100 lchz and controls the current in the mains sinusoidally. A thyristor bridge alternates the current after each half line period. The required auxiliary power is kept below 7 W and is taken from the choke of the step up chopper. I. Introduction Generation of electrical energy faces many problems today. In a world of growing environmental awareness nuclear power plants find less and less acceptance, and conventional power plants are criticized because of their CO, exhaust. Thus, regenerative energy systems are becoming more important than ever. One natural way of direct voltage generation is given by the photovoltaic. From an ecological point of view the production of photovoltaic panels and their use within decentralized power systems makes already sense today, not only in southern countries. Such systems are, however, still too expensive to compete with the electrical power delivered by the utilities. This is mainly given by the fact that photovoltaic panels are only produced at low quantities. panels for their own residential use. The idea is to install photovoltaic panels on the roof of a building and connect them via a DC to AC converter to the mains. Sun power thus contributes to the supply of appliances, and if less power is required than delivered from the panels, the difference is fed to the grid of the utility. In Germany some communities and utility companies support this alternative energy source by paying 2 German Mark (US $ 1.5) for each kwh delivered to the mains. Under special conditions the German government supports financially the purchase of photovoltaic panels and DC to AC converters additionally [ 13. Nevertheless, it is expensive to install photovoltaic systems privately due to the high cost of the converters which are only available in the kw range and which are produced under the same conditions as converters for industrial applications. Hence, this paper presents the development of a low cost 500W DC to AC converter for photovoltaic power conversion in residential applications. The converter uses low cost technology normally found in appliances and consumer products, e.g. SMPS of Tvs or PCs and phase control of electrical drives. The DC to AC converter is specially designed for operation at a wide DC input voltage range in order to allow optimal photovoltaic power conversion with an arbitrary number of 2 to 10 series connected solar arrays each rated 20V/2.5A peak power. Hence, photovoltaic power conversion may be applied in residential applications at relatively low cost at the beginning, allowing further power boosting by connection of additional solar panels in series. Many people are aware of this situation and are willing to promote the application of photovoltaic by buying solar /93$ IEEE 588 l i _.

2 Solar Boost DC-M DC to DC Converter Inverter Generator Converter 18 I,, L1 D1 Tr 1, L2 Fig. 1: Circuit configuration of the converter II. Converter Topology Figure 1 shows the power stages of the DC to AC converter. A step-up chopper is used to boost the solar panel voltage to a constant 200 V DC link and to ensure operation at the maximum power point (MPP). A push pull converter with a centre tapped transformer winding provides galvanic isolation and controls the current in the mains sinusoidally. In addition, a thyristor bridge is required to alternate the converter output current after each half period of the line voltage [2]. If a wide input voltage range has to be covered a boost converter is required. Without a boost converter the push pull converter has to be designed for the lowest input voltage V,. This means that the secondary voltage of the push pull converter becomes extremely high for operation with the maximum number of solar panels. It will be difficult to find adequate diodes and the filter choke in the link has to be larger. I I I ["I rl For the chosen voltage level power MOS FETs with a low on resistance %, can be applied to minimize the losses. A loolchz operation of the step up chopper and of the push pull converter leads to a small transformer and reduces the filter size. The transformer is specially designed for high efficiency. Fig. 2: Step up chopper with auxiliary supply In. Auxiliary supply In some DC to AC converters for photovoltaic application the auxiliary power is taken from the mains. This is not favorable because it causes unnecessary losses when no sunpower is available. Our converter takes all auxiliary energy from the solar panels, avoiding any losses when it does not operate

3 I I In order to simplify the auxiliary power supply it is incorporated in the step up chopper. Only one additional winding is required on the chopper choke as well as two diodes and two capacitors (see also fig. 2). Under steady state conditions the voltage ikvz of the auxiliary supply is automatically stabilized if the DC link voltage is kept constant. This is independed on whether the chopper operates in the discontinuous or in the continuous mode. The operation of the auxiliary supply is illustrated in figure 2.The output voltage is directly proportional to the voltage swing at the choke which is equal with the constant DC link voltage. The auxiliary power needed for the whole converter is less than 7 W which guarantees a sufficient efficiency even at low power conversion. IV. Maximum Power Point (MMP) Tracking The solar panels are the most expensive components of a photovoltaic power source. Thus, their output power has to be maximized, which will be controlled by the boost converter as already mentioned. pbt t In - "8 Fig.3: Typical voltage-current and power-voltage relationships of a photovoltaic panel Figure 3 shows the typical current voltage relationship of a solar panel as well as the corresponding output power. The nonlinear voltage current characteristic is influenced by the light intensity and the temperature. As the change of these parameters happens slowly there is no need for a high dynamic performance of the boost converter. The operation point of the chopper can be simply be set by the duty cycle of a standard PWM chip. It is advantageous that the maximum current, which can be drawn from a panel, is not higher than 1.5 times the current for the maximum output power. Thus, no special short circuit protection is required for the transistor of the step up chopper. Different control schemes can be considered for operating the photovoltaic panels at the maximum power point MPP. The highest flexibility can be achieved by applying a micro controller (pc). In this case both, the current and the voltage of the panel, have to be measured continuously via analog to digital converters. Using these values the power can be calculated in the pc. Then the duty cycle has to be changed slightly before the next measurement and the power calculation are camed out. The new result has to be compared to the old one. Depending on the difference it can be decided whether the duty cycle has to be further increased or decreased to track the point of maximum output power. It has to be taken into account that the sampling and the analog to digital data conversion are disturbed by the chopper operation. Thus, some measurements have to be taken and averaged for each duty cycle. It is also necessary to move the operation point over a certain range before a positive or negative gradient of the power can be observed. The strategy of adjusting the point of maximum power may be modified in different ways. However, the operation point of the photovoltaic panel has always to oscillate around the maximum power point. If the power flow of the photovoltaic panel has to be monitored inside the converter system continuously, some kind of pc is required anyway so that it will also be used as a MPP controller as described before. In most applications it is, however, sufficient to measure the energy delivered to the mains. For ths purpose a kwh meter can be installed between the converter and the line. In this case the questions arises, whether the maximum power point tracking can also be performed by an analog circuit. This is of special interest for low cost circuits because the digital controller requires some components of high complexity (e.g. 8bit micro controller, EPROM, crystal, A/D converter, interfaces, digital PWM IC,..) [2]. Although some of these components may be integrated in the basic pc the cost for these components are still high compared to standard analog circuits. In addition, an auxiliary voltage of 5V is required for the pc and the EPROM, and the current consumption of the digital ICs is always high. 590

4 If the controller operates analogously, the 15V auxiliary voltage already needed for switching the MOS FETs can be used. Two different analog control circuits for MPP tracking have been studied. IS Fig. 4: Boost converter with an integrated simplified MPP controller [3] The simplest one can easily be integrated in the chopper control. It is based on the fact that the point of maximum power output is nearly always met if the panel voltage is set to about 78% of its value at no load [3]. The chopper operation has thus to be interrupted regularly for about 511x3 so that 78% of the no load voltage can be sampled. For the next seconds the chopper controls the panel voltage by using the sampled voltage as a reference. (see also fig. 4) Another control scheme operates similarly to the digital one. It samples a part of the voltage and the current of the panel alternately (V2i = k*v2;-, ; 12i+l = k *I2i ; e.g. k=0.8) and varies the duty cycle linearly so that the voltage (or the current) of the panel moves towards the value sampled previously. If this point is reached, a part of the current (or voltage) is sampled and the duty cycle moves slowly backwards to meet the next point before a part of the voltage (or current) is sampled again. If this procedure is repeated continuously it will lead to a self-adaptive, self-perturbing control of the photovoltaic solar panel [4]. After some cycles the operation point always oscillates around the MPP between the voltages v, = kvb 5 v 5 vb and the CUffentS 1, , = k.1, where Pa = Pb < PMpp. This control scheme can also be incorporated easily in the chopper control as illustrated in figure 5. The additional circuitry consists of only four standard ICs which contain analog switches, opamps, comparators and exor gates. In order to show how accurate this control scheme meets the MPP a brief calculation can be shown. The assumption is made that the characteristic of the panel is described by the function u3 + i3 = 1. Here, the normalized values Fig. 5: Circuit diagram of the used MPP controller U = U/Uo and i = I/& are used where U, is the panel voltage at no load and Io is the short circuit current of the panel. The maximum power point can be determined to impp = umpp = 0.5(1'3) and %pp = 0.5(2'3). If the variation of both, voltage and current, is k = 0.8 the minimum output power is still 96.3% of hpp. If the duty cycle varies linearly between va < v < v b the average value of the power can be determined to pave = hpp. The 1.2% power reduction can be accepted. A digitally controlled system can hardly achieve a better conversion efficiency. V. Inverter The inverter of the photovoltaic line coupler is accomplished by four thyristors. In principle it is a conventional line frequency phase controlled inverter. The power is flowing to the line for a conducting period of the thyristors between 7r < ut < 27r. In order to avoid a commutation failure the thyristor current has to become zero before the end of the period 27r. In conventional phase controlled inverters this is guaranteed by a fire angle a < 180". In addition, the commutation angle U as well as the turn off time tq of the thyristor have to be taken into account. Thus, the current is normally commutated at a = 7r - U - y where y is the extinction angle y > w tq. Figure 6 shows the equivalent circuit and the most important voltages and currents for this operation. The DC link current is almost constant because of the large choke. However, it is desirable not to use a bulky choke and to feed the line by a sinusoidal current instead of a rectangular one. Therefore, a small choke with a low

5 l l 21 Z< 4 21 Zf -- h -- vncos(oc) because of the small choke and because the opposite line voltage can not be limited by the push pull converter. When the line voltage becomes zero, the current is at its maximum point. In the following time the line voltage changes its sign and the current can again be controlled by the push pull converter. Thus, it declines rapidly to the reference curve. A A IN= VN period. If the fire angle setting as well as the current \ control is optimized, an almost sinusoidal current with a power factor one can be fed in the line as can be seen in the final measurement (figure 12). / oc VI. Realization Fig.7: Measured line voltage and current when operating the thyristor inverter with a small choke and fire angle of a = 172" The very Small choke requires other fire angles for the thyristors as demonstrated by the measurement presented in figure 7. Here, the fire angle a = 172" is applied and the link 1s Composed by the 1OOkHz Push Pull COnverter VII. Experimental results The realized converter has been tested intensively. Most measurements were carried out by supplying the converter from an adjustable DC power source and by connecting it to the mains. Figure 8 shows the discontinuous operation of the boost 592

6 causes losses and because the line current shows a power factor near one H- 2P t Fig.8: Voltage at the switch and current in the choke of the step up chopper converter. The measured losses of this chopper can be approximately described by the following formula. pbss = (5 + ( Us/V) Is/A) W The losses are mainly depending on the current and increase slightly as a function of the input voltage. Figure 9 shows the corresponding efficiency of the boost converter versus output power with the input voltage as a parameter V,=1OoV. P-3mW V,=46V. P-=lsOW x v,*v, P~lOoW I I I I I I I I, I I p/p I e n u Fig. 10: Total efficiency of the converter The overall efficiency of the converter is presented in figure 10. The power loss versus output power shown in figure 11 demonstrates the influence of the step up chopper on the efficiency. e , I I I I I, I 1 I I I i Y x 100v; 330w x 45V; 150W + 3ov l00w 0.31 I I I I f I P, 1 p, mar Fig. 9: Efficiency of the boost converter versus input power for different number of solar panels. The efficiency of the push pull converter is also high as can be seen later. The efficiency of the inverter is always better than 99% as only the voltage drop of the thyristors Figure 12 presents the line current and line voltage when feeding 360 W to the 220V/50Hz mains. From this measurement it becomes obvious that there are nearly no low frequency harmonics so that the IEC 555 standards are fulfilled. All high frequency harmonics will be suppressed by an additional small hf-filter. Finally, the converter was connected to the 230V mains and supplied by six photovoltaic panels, each rated 17V/5OW. Although it was cloudy, the test was successful and the converter showed the expected performance. In the 593

7 converter are very small because of the high frequency operation (1OOkHz). Thus, the whole circuit could be built at small size and at low cost. The maximum power point is always adapted by the boost converter automatically. The corresponding control unit employs a simple analog circuit. The use of the boost converter also allows a very wide input voltage range (30V.. 170V) Without additional losses or restrictions for the push pull converter. +I- 5ms t Fig. 12: Line voltage and line current of the test set-up (2OOV/div; 2A/div; 5ms/div) special operating point 190W were drawn from the panels and 160W were fed in the line. Figure 13 presents the measured voltages and currents of the panels in the V-I plane for a few seconds after start up. It confirms that the analog MPP controller operates properly. PS 3 OW div - IS 9,5A div 0 2ovldiv Fig.13: Power and current versus voltage of the solar generator measured for a few seconds after start up. WI. Conclusion A DC to AC converter has been presented which consists of three power stages: a step up chopper, a push pull converter and a thyristor inverter. It can be supplied by photovoltaic panels and it feeds a sinusoidal current into the mains with a power factor near one. Only standard components are applied for both, power and control. The transformer, required for galvanic isolation, and the filter chokes used in the DC link and for the boost The use of three power stages instead of two without boost converter allows an optimized design of each subsystem. Thus, the overall efficiency is high, although the boost converter causes about one third of the total losses. For 170V input voltage and 350W output power 40W losses occur leading to an efficiency of about 90%. All power required for the auxiliary supplies are already included. The power conversion of the system starts automatically if the input voltage is above 30V and if a minimum power of 20W is available from the solar panels. The converter is protected against a short circuit at the output. If the mains voltage disappears the converter operation stops and starts again if the voltage returns. Acknowledgment The authors would like to thank the "Solarenergie Forderverein e.v., Aachen" who provided solar panels for testing the DC to AC converter under working conditions. References Solarenergie-Forderverein e. V. "Solar Brief zum 1000-DHcher-ProgrammN, Aachen 1990 Steigerwald,R.L.; Ferraro A. ; Turnbull F.G. "Application of power transistors to residential and intermediate rating photovoltaic array power conditioners", IEEE Trans. on Ind. Applications, Vol.IA-19, No.2, MarchIApril 1983, pp Schoeman,JJ ;van Wyk,JD :"A Simplified Maximal Power Controller for Terrestrial Photovoltaic Panel Arrays"; Conference Record of PESC 1982, pp Boehringer,A : "Die selbsttiitige Einstellung der Extremwerte von Funktionen der Form z = g(x".y) nach einem Kennlinienverfahren"; Regelungstechnik No. 12, 1969, pp