UC Irvine UC Irvine Previously Published Works

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1 UC Irvine UC Irvine Previously Published Works Title A cost-effective three-phase rid-connected inverter with maximum power point trackin Permalink Authors Chen, Y Smedley, K Brouwer, J Publication Date DOI /IAS License CC BY 4.0 Peer reviewed escholarship.or Powered by the California Diital Library University of California

2 A Cost-effective Three-phase Grid-connected Inverter with Maximum Power Point Trackin Yan Chen 1, Keyue Smedley 1, and Jack Brouwer 1. Dept. of Electrical Enineerin and Computer Science. National Fuel Cell Center University of California, Irvine Irvine, CA 9697, USA Abstract Solar enery is desirable due to its renewable and pollution-free properties. In order to utilize the present utility rid infrastructure for power transmission and distribution, rid connected dc-to-ac inverters are needed for solar power eneration. However, previously proposed voltae source inverters with a two-power-stae structure or a cascaded structure increase the circuit complexity, power losses, and system cost. In addition, conventional maximum power point trackin (MPPT) methods usually need power calculation and complex loic judment, so expensive multipliers and/or microprocessors are needed. This leads to hih inverter capital cost, which becomes a major barrier for the wide use of solar power eneration. A cost-effective MPPT method interated within the One-Cycle Control (OCC) core is proposed in this paper. When interated with a three-phase boost type inverter, the proposed method tracks MPP with ood precision, and solar power is converted into three-phase ac power with a sinle power stae. There is no power calculation in the controller, which yields a simple and cost-effective solution. Experiments have been carried out with a photovoltaic source to verify ood performance. therefore dc-ac rid-connected inverters are necessary for power conversion. To avoid introducin additional distortions to the power rid, the enerated currents from these inverters should have low harmonics. Furthermore, when the output currents are in phase with the rid voltaes, the maximum active output power is achieved by minimizin the reactive output power. Thus inverters that have hih power quality, hih efficiency, hih reliability, low cost, and simple circuitry are desired. As far as the alternative source is concerned, each photovoltaic (PV) module or fuel cell stack usually supplies a dc voltae lower than the peak value of the rid voltae, and their output voltaes vary in a wide rane accordin to various operatin conditions [1]. Series connection of several modules or cells can be a simple way to increase output voltae so as to employ a buck type rid connected inverter [] for power conversion. However, this method may reduce the overall efficiency [3] because no power will be collected throuh the inverter when its input dc voltae drops below the peak value of the ac output voltae. There are many publications dedicated to this problem. In article [4] and [5] a two-stae topoloy that first boosts the dc voltae by a dc-dc Keywords-rid-connected; boost type inverter; One-Cycle Control; maximum power point trackin (MPPT) converter and then inverts it into ac is used. However, this method increases the complexity of the circuit and losses. In I. INTRODUCTION articles [3] and [6] a cascade structure is employed to increase the dc voltae, which needs more power switches and still In recent years, many concerns have been raised reardin results in a complex circuit. In article [7], a current source fossil fuel-electricity power eneration, since it pollutes the inverter (CSI) employin a lare inductor is used for power environment and depletes limited enery supplies. On the conversion. However, this type of circuit could be bulky, other hand, alternative power sources, such as solar heavy and expensive for a hih power application. In paper photo-voltaic enery, have ained a lot of attention because [8], a One-Cycle Controlled (OCC) boost-type three-phase they are renewable, friendly to the environment, and flexible rid connected inverter was proposed. It has a sinle power for installation. However, these types of sources supply dc stae, a low value dc side inductor and its input dc voltae power while the present power rid accepts 60Hz ac power, /06/$0.00 (c) 006 IEEE

3 can vary over a wide rane. These key features of OCC are all desirable for low cost hih efficiency PV power eneration. As far as maximum power point trackin (MPPT) methods are concerned, many methods have been addressed previously. The Perturb and Observe (P&O) method needs to calculate dp/dv to determine the maximum power point (MPP) [9][10]. Thouh the method is relatively simple, it can t track the MPP when the irradiance chanes rapidly; and it oscillates around the MPP instead of directly trackin it. The Incremental Conductance method tracks MPP rapidly but it has hih alorithm complexity, which also employs the calculation of di/dv [11]. Thouh this method can accurately determine MPPs, a diital sinal processor (DSP) or a microprocessor is usually needed for these complex calculations. The Constant Voltae method [1], which uses 76% open circuit voltae as the MPP voltae, and the Short-Circuit Current method [13] are simple, but they do not always accurately track the real MPP. In this paper, a cost-effective MPPT method interated within the OCC controller is proposed. It features the followin advantaes: i) A sinle power stae: MPPT and dc-to-ac power conversion can be achieved within a sinle power stae; ii) Simple control circuit: only the PV output voltae is sensed and used to achieve MPPT; the control circuit preserves the simplicity of OCC method; iii) No power calculation and no need for microprocessors or DSPs: the complexity of the control circuit is reatly reduced; iv) Good MPPT capability with acceptable precision: well tracks the real MPP. II. REVIEW OF THREE-PHASE BOOST-TYPE GRID-CONNECTED IVERTER [8] Fiure 1 shows the power stae when a PV source is connected to the input side of a boost-type inverter [8], where V is the output voltae of the PV array, and meanwhile the input voltae of the inverter; I is the output current from the PV and C in is the input dc capacitor of the inverter; L is the dc side inductor. S ap, S an, S bp, S bn, S cp and S cn are six switches in the bride. Each switch is realized by an IGBT in series with a diode as shown in the dashed line box; and L fa, L fb, L fc and C a, C b, C c are output inductors and capacitors that form an output filter for the inverter. v a, v b and v c are three-phase rid voltaes. Fi.1 Power stae with PV array connection Each line cycle can be divided into six reions as shown in Fi.. For example, in Reion I (0~60º), v a >0, v c >0 and v b <0. Additionally, their differential voltaes are: v ab 6 6 V, vcb V (1) where, V is the value of the line-to-neutral voltae. Fi. Six reions in a line cycle Similarly, in other reions two independent phase voltaes can always be selected so as to obtain the two differential voltaes, which are referred to the third phase voltae, always reater than 6 V. Therefore, a boost converter operation is uaranteed as lon as the dc voltae V satisfies: 6 V V. At any iven instant, one of the upper switches (S ap, S bp, S cp ) and one of the lower switches (S an, S bn, S cn ) are turned on. For example, in Reion I, S an and S cn are kept off and S bn is on for the entire reion. S ap, S bp and S cp are controlled at the switchin frequency. There are three staes for different switchin patterns: 1) Stae I (Fi.3(a)): S bp is turned on and S ap, S cp are off. The inductor current I L increases, and output currents are supplied by C a, C b, C c ; ) Stae II (Fi.3(b)): S ap is turned on and S bp, S cp are off. I L decreases throuh C a, C b and v a, v b. i c is supplied by C c, C b; 3) Stae III (Fi.3(c)): S cp is turned on and S ap, S bp are off. I L decreases throuh C c, C b 996

4 and v c, v b. i a is supplied by C a, C b. I L 1 3k = ( Vref V ) () R V s Simultaneously, three sinusoidal ac currents can be injected into rids. (a) Stae I Fi.4 Diaram of the OCC core for the boost type inverter III. CONTROL PRINCIPLE OF MPPT (b) Stae II (c) Stae III Fi.3 Three staes for different switchin patterns Fiure 4 shows the diaram of OCC core for the boost type inverter. It comprises an interator with reset, two comparators, two flip-flops and other linear and loic components, where V ref is an adjustable constant and related to output power. R 1 and C 1 are interation components, and T s ι = R 1C1 =. V p and V n are selected from v a, v b and v c in each reion respectively; k is the voltae sensin ratio; I L is the dc side current; and R s is the current sensin resistance. PWM sinal Q p, Q n and Q t are distributed to the correspondin switches for drivin IGBTs. In a balanced three-phase system, the dc side current can be derived as: For a eneric PV array that is comprised of M modules in parallel and N cells in series in each module, the output power is: P = V I = V M q( V+ IRs )/ NAKT { I I [ e 1] } LG where, I LG -- liht-enerated current of each module; I os -- reverse saturation current of each module; q -- electronic chare; R s -- series resistance; A -- ideality factor; K -- Boltzmann s constant; T -- temperature in o C. Fiure 5 shows the typical output voltae V vs. output current I curves of a PV source. It can be seen that the MPPs vary with solar irradiance and the cell temperature. When the PV array is connected to the boost inverter, the actual operation point of the PV source is determined by the external circuit as well as the solar irradiance and cell temperature at I 5ºC 1000W/m ºC * MPPs 600W/m 00W/m os Fi. 5 V ~I curves of a PV source V (3) 997

5 any instant. In order to extract the maximum power from the PV array, the boost inverter should: i) allow input voltae to vary in a lare rane; ii) make input power track the MPP for different operatin circumstances (e.., temperature, irradiance). From the analysis in [8], the boost inverter allows input voltae to chane as lon as 6 V V is satisfied. The input power of the inverter can then be derived from (): P in 1 3k = V I L = ( V Vref V ) (4) R S In order to make P in approach MPPs automatically, a cost-effective MPPT method is proposed in this paper. Fiure 6 shows the diaram of the controller with the MPPT function interated in the OCC core. Only V ref (in Fi.4) is replaced by versus V in Fi.7. At any time, when the input power of the inverter equals the output power of the PV, a temporary steady state of the system is achieved. These steady state operatin conditions are represented by a series of intersection points between P in and P in Fi.7. These intersection points are the desired operatin points for the circuit proposed in this paper. When solar irradiance or temperature chanes, the operation point can move up or down alon the P in curve to satisfy the condition P in =P. By this means, the inverter can adjust its input power automatically accordin to the variation of P caused by operatin circumstances. For any particular application case in which the output properties of the PV source are known, parameters R s, k and k in (7) can be tuned to make the P in curve closely approach the MPPs. Thus, the MPPT function is achieved with an acceptable precision. k V in order to achieve a better MPPT trackin capability. k is the voltae sensin ratio and V is the output voltae of the PV array. Thus: V = k V (5) ref Fi.7 Simulation results for P and P in versus V Fi.6 OCC controller with MPPT function Substitutin (5) into (4), the followin can be obtained: P in 1 3k = ( kv V) (7) R S Since R s, k and k are constant for any particular circuit desin and V is fixed, the input power of the inverter P in is only related to its input voltae V. With the help of Matlab simulation of the proposed circuit desin, the output power P of the PV and the input power P in of the inverter can be drawn as shown in Fi.7, where the solid lines indicate P vs. V at three different solar irradiance levels (as shown in Fi.5) at 5ºC, and the dashed lines are the correspondin curves at 50ºC. The P in curve is superimposed onto the same raph IV. EXPERIMENTAL VERIFICATION Experiments have been conducted with the proposed MPPT method usin a PV array consistin of 8 modules that are divided into two roups. Each roup comprises four series-connected modules and then these two roups are parallel-connected. The PV array was installed on the rooftop of the Enineerin Laboratory Facility at University of California, Irvine with a 60º anle of incidence and facin rouhly 45 o east of due south. The specifications of each panel are as follows: Model type: Shell SP75; Peak power: 75W (the peak power is achieved with direct irradiance levels of 1000W/m of spectrum AM 1.5 when the cells are at 5 o C); Short circuit current: 4.8A; 998

6 Open circuit voltae: 1.7V; Other key parameters of the system are: CLK1 and CLK frequency: f s = 40 khz; DC side capacitor: C = 3mF; DC side inductor L = 0.6mH; k = ; R s = 0.33Ω; k = ; Fiure 8 presents the experimental observations. The upper two curves show the output power P and the actual maximum power P max of the PV durin the daytime (All the data of Fi.8 are based on the experiments conducted on Au. 6 th, 005, in Irvine CA. Weather conditions were sunny, o F, humidity of ~50%, and winds enerally SSW at 6 mph.). The bottom bar chart shows the relative error between P and P max. The achieved power curve closely matches maximum power throuhout the period. When P max hits its peak around 13:00 the relative error is only 3.8%. However, before 10:00 and after 16:00 when the temperature drops sinificantly from that of midday, the error increases up to almost 0%, because the proposed MPPT method does not account for temperature variations. Nonetheless, the proposed the MPPT method closely tracks MPPs (especially durin the period of peak output) with an acceptable precision. In addition, the experimental results well match those predicted by the Matlab simulation. Power(W) P max 400 o--- P :00 9:00 10:00 11:00 1:00 13:00 14:00 15:00 16:00 17:00 17:40 Time Fi.8 The extracted power P, the maximum P max and relative error vs. time Relative Error(%) 0 Fiures 9 and 10 present typical voltae and current wavefo acquired durin the experiments. Since the total output power of the PV is small, the output voltae of the inverter is reduced to half of the nominal phase voltae in order to et better three-phase current wavefo. Fiure 9 shows the input voltae and current of the inverter. The dc side current is kept almost constant by the proposed control method. The current ripple in the line cycle frequency rane is caused by the unbalanced situation of the three-phase system. Fiure 10 shows phase A voltae and three output currents of the inverter. The currents are approximately sinusoidal and follow the correspondin phase voltaes respectively. Thus, a near unity power factor can be achieved. The THD of the current is approximately 3.5%. V. CONCLUSIONS In this paper, a cost-effective MPPT method is proposed for the three-phase boost-type rid-connected inverter. The control method is simple and can be interated within the OCC core by addin a few simple components. Complex power calculation is not needed, and multipliers or 999

7 Sept.-4 Oct. 001 Pae(s): vol.4 V [3] G.R. Walker, P.C. Sernia, Cascaded DC-DC converter connection of photovoltaic modules, Power Electronics, IEEE Transactions on, Volume 19, Issue 4, July 004 Pae(s): [4] F. Antunes, A.M. Torres, A three-phase rid-connected PV system, I L Confjerence of the IEEE Volume 1, -8 Oct. 000 pp vol.1 Industrial Electronics Society, 000. IECON th Annual [5] J.C. Lima, J.M. Corleta, A. Medeiros, V.M. Canalli, F. Antunes, F.B. Libano, F.S. Dos Reis, A PIC controller for rid connected PV system usin a FPGA based inverter, Industrial Electronics, 000. ISIE 000. Proceedins of the 000 IEEE International Symposium on Volume 1, 4-8 Dec. 000 Pae(s): vol.1 Fi.9 Input of the inverter V (50V/div), I L (5A/div) [6] M. Calais, V.G. Aelidis, L.J. Borle, M.S. Dymond, A transformerless five level cascaded inverter based sinle phase photovoltaic system, v a Power Electronics Specialists Conference, 000. PESC IEEE 31st Annual Volume 3, 18-3 June 000 Pae(s): vol.3 i a for rid connected photovoltaic applications, Power Electronics and [7] J.S. Siva Prasad, B.G. Fernandes, Active commutated thyristor CSI i b Volume 3, Au. 004 Pae(s): Vol.3 Motion Control Conference, 004. IPEMC 004. The 4th International, i c Inverter, Applied Power Electronics Conference and Exposition, 006, [8] Y. Chen, K. Smedley, Three-Phase Boost Type Grid Connected Mar [9] O. Wasynczuk, Dynamic behavior of a class of photovoltaic power systems, IEEE Trans. Power App. Syst., vol. PAS-10, pp , Fi.10 Output of the inverter: v a (100V/div), i a,i b,i c (5A/div) microprocessors are not necessary. The proposed circuit requires only one power stae to achieve the MPPT function and dc-to-ac power conversion, which makes the whole system simple and cost-effective. Experiments were conducted usin one embodiment of the proposed desin as applied to a photovoltaic source. Data shows that the proposed method has ood MPPT capability and hih quality output performance, and it is a ood candidate for use in solar power eneration. REFERENCES [1] G.K. Andersen, C. Klumpner, S.B. Kjaer, F. Blaabjer, A new reen power inverter for fuel cells, Power Electronics Specialists Conference, 00. pesc IEEE 33rd Annual, Volume, 3-7 June 00 Pae(s): vol. [] C. Qiao, K.M. Smedley, Three-phase rid-connected inverters interface for alternative enery sources with unified constant-frequency interation control, Industry Applications Conference, 001. Thirty-Sixth Sept [10] E. Koutroulis, K. Kalaitzakis, N.C. Voularis, Development of a Microcontroller-Based, Photovoltaic Maximum Power Point Trackin Control System, IEEE Transactions on Power Electronics, Vol.16, No.1, pp.46-54, January 001 [11] K.H. Hussein, I. Muta, Maximum Photovoltaic Power Trackin: An Alorithm for Rapidly Chanin Atmospheric Conditions, IEEE Proceedins on Generation, Transmission, and Distribution., Vol.14, No.1, pp.59-64, January 1995 [1] T. Nouchi, S. Toashi, R.Nakamoto, Short-Current Pulse-Based Adaptive Maximum-Power-Point Trackin for a Photovoltaic Power Generation System, Electrical Enineerin in Japan, Vol.139, No.1, pp65-7, 00[Denki Gakkai Ronbunshi, Vol.11-D, No.1, pp.78-83, January 001] [13] J.H.R. Enslin, M.S. Wolf, D.B. Snyman, W. Swieers, Interated Photovoltaic Maximum Power Point Trackin Converter, IEEE Transactions on Industrial Electronics, Vol.44, No.6, pp , December 1997 [14] Y. Chen, K. Smedley, Three-Phase Boost Type Grid-Connected IAS Annual Meetin. Conference Record of the 001 IEEE Volume 4, Inverter, US and International Patent Application.