Design and Implementation of Photovoltaic Power Conditioning System using a Current-based Maximum Power Point Tracking

Transcription

1 606 Journal of Electrical Engineering & Technology Vol. 5, No. 4, pp. 606~613, 2010 DOI: /JEET Design and Implementation of Photovoltaic Power Conditioning System using a Current-based Maximum Power Point Tracking Sanghoey Lee*, Jae-Eon Kim** and Hanju Cha Abstract This paper proposes a novel current-based maximum power point tracking (CMPPT) method for a single-phase photovoltaic power conditioning system (PV PCS) by using a modified incremental conductance method. The CMPPT method simplifies the entire control structure of the power conditioning system and uses an inherent current source characteristic of solar cell arrays. Therefore, it exhibits robust and fast response under a rapidly changing environmental condition. Digital phase locked loop technique using an all-pass filter is also introduced to detect the phase of grid voltage, as well as the peak voltage. Controllers of dc/dc boost converter, dc-link voltage, and dc/ac inverter are designed for coordinated operation. Furthermore, a current control using a pseudo synchronous d-q transformation is employed for grid current control with unity power factor. A 3 kw prototype PV PCS is built, and its experimental results are given to verify the effectiveness of the proposed control schemes. Keywords: Single-phase photovoltaic power conditioning system, Digital phase locked loop, dc/dc boost converter, dc/ac inverter, CMPPT, dp/di 1. Introduction Photovoltaic (PV) energy is currently considered as one of the most useful renewable natural energy sources in the world because it is clean, free, abundant, pollution-free, and inexhaustible. Due to the rapid growth in solar cells and power electronics technology, PV energy has received increasing interest in electrical power applications. However, present energy-conversion efficiency of PV array is still low. It requires maximum power point tracking (MPPT) control techniques to extract the maximum possible power from PV arrays in order to achieve maximum operating efficiency [1]. A PV array currently exhibits an extremely nonlinear voltage that generally varies with array temperature and solar isolation, making the maximum power point (MPP) difficult to locate. To overcome this problem, various methods, such as the perturbation and observation method [2], [3] and incremental conductance methods [4]-[6], have been proposed for the MPPT algorithm of PV arrays. In the perturbation and observation method, the operating voltage of PV array changes the duty ratio in order to locate variations in directions for maximizing PV array current. If power increases, the operating voltage is further perturbed in the same direction; if it decreases, the direction of the perturbation is reversed. This method does not Corresponding Author: Department of Electrical Engineering Chungnam National University Daejeon, Korea. (hjcha@cnu.ac.kr) * Department of Electrical Engineering Chungnam National University Daejeon, Korea. (lee24044@yahoo.co.kr) ** Department of Electrical Engineering Chungbuk National University Cheongju, Korea. (jekim@cbnu.ac.kr) Received: January 28, 2010; Accepted: July 21, 2010 require solar panel characteristics, but it remains unsuitable for applications under rapidly changing atmospheric conditions. The disadvantage of the perturbation and observation method can be minimized by comparing the incremental and instantaneous conductance of PV arrays. This method is more accurate and can provide good performance under rapidly changing conditions. In this paper, a current-based incremental conductance method that produces smooth transition and fast response to the MPP is proposed. The proposed current-based maximum power point tracking (CMPPT) method adjusts a reference current proportional to the power slope with respect to PV array current, and provides advantages of smooth and rapid transition to the MPP. In addition, digital phase locked loop (DPLL), which detects the phase of grid voltage as well as its peak voltage, is addressed. A PI controller using a pseudo synchronous d-q transformation is employed for grid current control in the single-phase dc/ac inverter. Results from analysis, simulation, and hardware implementation of the power conditioning system are detailed. Experimental results are also obtained by using a 3 kw prototype PV PCS, which then verifies the feasibility of the proposed control schemes. 2. Photovoltaic Power Conditioning System Fig. 1 shows the circuit configuration of a 3 kw transformer-less PCS with grid connection. The transformer-less PCS is composed of a PV array, dc/dc boost converter, dc-link, and dc/ac inverter. L-C filter PV voltage V is set in wide range variation ( V). The dc/dc boost converter

2 Sanghoey Lee, Jae-Eon Kim and Hanju Cha 607 Fig. 1. Circuit configuration of PV PCS. controls the PV current I. These two actions perform the proposed CMPPT function. The dc/ac inverter controls the grid current with unity power factor. 2.1 Proposed CMPPT The PV array is nonlinear with existing operating points, and the PV array produces maximum power, as illustrated in Fig. 2. The perturbation and observation method measures voltage and current, allowing for the evaluation of the momentary operating region. In accordance with the region, the reference voltage is either increased or decreased, such that the system operates close to MPP [2], [3]. Implementing the perturbation and observation method is simple because it only increases or decreases reference voltage. However, this method cannot readily track any immediate and rapid change in the environment. One alternative is the incremental conductance method; it can track MPP accurately by comparing the incremental conductance and instantaneous conductance of a PV array. [4], [5]. In this paper, the proposed CMPPT is improved from the perspective of the incremental conductance method. The P-I and V-I curves for CMPPT is presented in Fig. 2. The flowchart shown in Fig.3 presents the systematic progress of CMPPT, where V (n) and I (n) are the present voltage and current of the PV array, and V (n-1) and I (n-1) are their previous values, respectively. When V (n) /I (n) + dv / di < 0 or di = 0 and dv < 0, decreasing reference current I * forces V (n) /I (n) + dv / di to approach zero. When V (n) /I (n) + dv / di < 0 or di = 0 and dv > 0, increasing reference current I * forces V (n) /I (n) + dv / di to approach zero. When V (n) /I (n) + dv / di = 0 or Fig. 2. P-I curve and V-I curve for CMPPT. Fig. 3. Flowchart of CMPPT. di = 0 and dv = 0, as the PV array is at the MPP, reference current I * is kept at a constant value, and thus, oscillation is reduced. The low-frequency ac ripple mitigation method [12] is used to reduce PV array current ripple. 2.2 Current and Voltage MPPs in PV Array Characteristics Fig. 4 shows the MPP changes of voltage and current due to the irradiation characteristics from 0 to 1 at the fixed temperature of 25 C. To show the relative values between the voltage and current MPP, the unit is shown in per unit (p.u.). At the midpoint of irradiation change duration, voltage MPP changes slowly while current MPP exhibits a big slope, as shown in Fig. 4. The current curve is increased linearly by PV array irradiation characteristics, indicating that the current MPP value always changes at any irradiation. The voltage curve has a small variation in VMPPT; therefore, the controller is not always needed to control MPP. However, VMPPT cannot reach an accurate MPP at any irradiation and might encounter MPP confusion at low irradiation because of the PV array characteristics, as shown in Fig 4. CMPPT, on the other hand, can always control the current; it can reach MPP easily and accurately because of the short circuit current characteristics in the PV array. Therefore, CMPPT is desired if the aim is to increase system efficiency. Fig. 5 shows the MPP changes of both voltage and current due to temperature characteristics (0-45 ) at a fixed irradiation of 1. To show the relative values between voltage and current MPP, the unit is shown in per unit (p.u.). At 250V, voltage MPP changes slowly while current MPP exhibits a big slope. Fig. 5 shows that the current curve decreases linearly with PV array temperature, indicating that CMPPT always changes current MPP values at

3 608 Design and Implementation of Photovoltaic Power Conditioning System using a Current-based Maximum~ Fig. 4. CMPP and VMPP under irradiation-changing conditions [p.u. / 0.5 Irradiation MPP]. Fig. 6. P-V curve and dp/dv slope. Fig. 7. P-I curve and dp/di slope. Fig. 5. CMPP and VMPP under temperature-changing conditions [p.u. / 25 Temperature MPP]. any temperature. Therefore, CMPPT controller can always operate to track MPP. The reverse is true for VMPPT because of the small change in voltage MPP value. Figs. 4 and 5 show the comparison of the inherent current source characteristics of solar cell array and voltage. Comparative results show that CMPPT has a robust and fast response under rapidly changing environmental conditions compared with VMPPT. Fig. 6 shows a VMPPT P-V curve and slope calculated by dp/dv. As shown in this figure, the slope is 0 when P-V curve is in MPP. The blue color refers to the value of the operation region slope. Fig 7 shows a CMPPT P-I curve and the slope calculated by dp/di. Results using different methods show that both curve powers are the same, but the slopes differ. In VMPPT, slope operation region value changes from 0 to 30, while it changes from 0 to 350 in CMPPT. This result indicates that the CMPPT method has more comparative points compared with VMPPT. Furthermore, by using the same sampling period, results reveal that VMPPT has smaller steps, hence causing longer comparative time compared with CMPPT. In effect, CMPPT can track MPP more quickly and accurately, Hence, CMPPT can track quickly and accurately MPP by using slope as modified MPPT method. Implementing the CMPPT method is more convenient due to the independent of slope value on array temperature and irradiation., as opposed to the VMPPT approach [10], [11]. 2.3 Dc/dc Boost Converter Fig. 8 shows the block diagram of the dc/dc boost converter controller. This controller is composed of a PI compensator, voltage limiter, and PWM generator, where I * is calculated from CMPPT method and V a is a dccompensating variable for voltage across switch Sb. Fig. 8. Block diagram of dc-dc boost converter. The transfer function of the dc/dc boost converter can be derived as I I L1 * 2 L1s K 2.4 Dc-link Voltage Control K s K i s K Fig. 9 shows the block diagram of the dc-link voltage control, where V dc * is the reference dc-link voltage; P in is the input power to the dc-link capacitor C d from solar cell array, such as (2); V peak is the peak value of grid voltage; and P out is the output power from the dc-link capacitor C d to the grid, as shown (3). i (1)

4 Sanghoey Lee, Jae-Eon Kim and Hanju Cha 609 P P in out V I (2) V 2 peak I Band stop filter is used to mitigate the effect of the 2nd harmonic voltage resulting in performance degradation. I L2 * is a command value of the grid current. To balance P in and P out, the dc-link voltage controller (Fig. 9) is employed. Its transfer function can be derived as (4) [13]: V V dc * dc peak L 2 K dcs Kidc * 2CdcVdc 2 s K dcs K V idc (3) (4) Tc 2 c (8) T 2 Fig. 10 shows the block diagram of digital PLL, where V grid is the grid voltage, after which it transforms to V ds ; V qs is a virtual voltage through the aforementioned all-pass filter; and V ds and V qs are converted to V de and V qe in the synchronous frame, where V de denotes a difference between grid voltage phase and estimated phase [7] [8]. In addition to phase estimation, digital PLL can calculate the instantaneous peak value of the grid voltage. V peak is the same as V qe., and V peak is used to detect voltage sag and swell in the utility. c Fig. 9. Block diagram of dc-link voltage control. 2.5 Digital Phase Locked Loop In the utility system, PLL control is needed to synchronize inverter output voltage to the interconnected utility. Generally, a PLL control system in single-phase is constructed with zero crossing detection. However, DPLL is used in this paper because the phase detection time of its method is faster compared with the conventional zero crossing detection method. The single-phase DPLL is implemented in virtual phase; that is, it is delayed by 90 from the measured grid voltage and is generated by passing through an all-pass filter, such as Fig. 10. Block diagram of digital PLL. 2.6 Ac/dc Inverter for Unity Power Factor Fig. 11 shows a block diagram of the current controller in the dc/ac inverter, where current control using a pseudo d-q transformation is employed, and which suggests stable control performance. The proposed current control scheme is verified through simulation, as shown in Fig. 12, where I de is measured grid current I grid and I qe is the virtual current through all-pass filter. All voltages are in a synchronous frame. In this figure, I qe and I de are the dc values, while V grid is in phase with I grid. s H ( s) (5) s The all-pass filter in s domain is transferred into z domain by using a bilinear transformation, such as H T 2 ( T 2) z 1 c c ( z) 1 Tc 2 ( Tc 2) z (6) Therefore, output y (k) (i.e., in virtual phase) from the measure grid voltage V grid can be obtained as follows: y( k ) cy ( k 1) cv ( k ) v ( k 1) grid grid (7) where Fig. 11. Block diagram of dc-ac inverter current control.

5 610 Design and Implementation of Photovoltaic Power Conditioning System using a Current-based Maximum~ 3. Experimental Results The overall system of the 3 kw PV PCS with line connection, as shown in Fig. 13, is implemented fully in a software that adopts digital signal processing, TMS320F2812. The switching times of each converter are implemented fully in the software, and PWM pulses are generated through an internal pulse generator of the DSP. Voltage and current signals are measured by using an internal 12-bitresolution analog-to-digital converter in the DSP. Furthermore, a 4-channel 12-bit digital-to-analog converter is used to display all of the waveforms. The switching frequency of the inverter is 10 khz while dead time is 3 μsec. Hence, output command PWM is compensated by the dead time effect. To verify the proposed CMPPT, the PV PCS is examined by a test bed composed of a simulated utility source (3P4S/1P2S; 12 kva; model: NIS ), simulated distribution line (12 kva; model: NIS31477), and dc power supply for PV simulation (Vmax = 600 V, Imax = 30 A, 15 kw) from NF (Japan). This test bed is used to certify the Korean license-to-sell protocol for the PV PCS. The adopted PV array model is based on sol_dow_181u1f_ss PV module with the following electrical characteristics during standard testing: maximum delivered power, PM = W; short-circuit current, ISC = 7.9A; and open circuit voltage, VOC = 32.4 V. Experimental results are presented in Figs All experiments are conducted on the test bed. In the experimental test of PV PCS using CMPPT, current reference I* is either increased or decreased by a dp/di slope value every 1 ms. Therefore, CMPPT can reach MPP more quickly and accurately because the dp/di slope is bigger than the dp/dv slope. Moreover, oscillation is reduced by maintaining reference current I* at constant value. The experimental results are given to confirm the good performance of CMPPT. Fig. 12. Proposed PV PCS simulation by using PSIM. Fig. 13 Prototype of the 3 kw PV PCS. Fig. 14 shows the operation of DPLL, while traces (a) and (b) represent the measured dq-transformation voltage Vds and Vqs, where Vqs is generated from the all-pass filter. As shown in Fig. 15(a), programmable power supply ES2000S (NF in Japan) generates voltage sag of 30% with respect to utility grid (220 V, 60 Hz). Fig. 15(b) shows negative peak voltage Vpeck, which is same as Vqe,. This peak voltage can be used to detect immediately the voltage sag. Figs. 16 and 17 show the experimental results by using CMPPT when irradiation is at 0.5. Fig. 16(a) shows the utility current that is increased to MPP and then maintained at MPP with perturbation. The PV array voltage, which is decreased because of PV array characteristics, is shown in Fig. 16(b). The PV array current increased to MPP current by CMPPT is shown in Fig. 16(c). Fig. 17 shows MPP in V-I and P-V curves when irradiation is at 0.5 and MPPT efficiency is around 99%. Figs. 18 and 19 show the experimental results of irradiation change from 0.5 to 0.7 by using CMPPT. It exhibits smooth transition and fast response for MPP. Fig. 18(a) shows the utility current with changes in irradiation ranging from 0.5 to 0.7. The utility current is increased rapidly by the CMPPT controller, thereby generating maximum power. Fig. 18(b) shows the decreased PV Fig. 14. Experimental results of DPLL: (a) utility voltage Vgrid; (b) all-pass filter Vqs; (c) synchronized transformation Vde; and (d) utility phase ˆ [4 ms/div].

6 Sanghoey Lee, Jae-Eon Kim and Hanju Cha Fig. 15. Experimental results of peak voltage: (a) utility voltage Vgrid; (b) negative peak voltage Vpeck,Vqe; (c) DA conversion Vgrid; and (d) utility phase ˆ [10 ms/div]. Fig. 16. Experimental results of CMPPT (irradiation: 0.5): (a) utility current Igrid [2.5 A/div]; (b) PV array voltage V [30 V/div]; and (c) PV array current I [1 A/div][2.5 s/div]. Fig. 17. Experimental results of CMPPT (0.5 irradiation). array voltage used for tracking until MPP voltage responds to irradiation change. Fig. 18(c) shows the PV array current that is increased rapidly until MPP current change occurs as a response to CMPPT. Fig. 19 shows the MPP in P-V curve used to verify CMPPT operation maintained at around 99% MPPT efficiency. Fig. 20 shows the CMPPT operation when irradiation is influenced by a sudden change (i.e., from 0.7 to 0.2). With abrupt change, irradiation PV voltage is increased and PV current is decreased by 611 Fig. 18. Experimental results of CMPPT (irradiation change: ): (a) utility current Igrid [2.5 A/div]; (b) PV array voltage V [30 V/div]; and (c) PV array current I [1 A/div][2.5 s/div]. Fig. 19. Experimental results of CMPPT (irradiation change: ). Fig. 20. Experimental results of CMPPT (irradiation change: ): (a) utility voltage Vgrid [100 V/div]; (b) PV array voltage V [60 V/div]; (c) PV array current I [2.5 A/div]; and (d) utility current Igrid [8 A/div] [10 ms/div]. the CMPPT controller. At any condition, the CMPPT algorithm operates continuously to locate MPP. Fig. 21 shows the manual soft start from standstill to 3 kw power generation. The utility voltage is shown in Fig. 21(a), while Fig. 21 (b) shows the utility current that is quickly increased. The PV array voltage is reduced as characteristics in Fig. 21(c). Fig. 21(d) shows the PV array current that is increased in order to track MPP current by CMPPT

7 612 Design and Implementation of Photovoltaic Power Conditioning System using a Current-based Maximum~ The output grid current and voltage in 3 kw power generation are shown in Fig. 22. It is noted that the grid current is exactly in phase with the grid voltage and is sinusoidal current with low distortion.. The power efficiency of PV PCS is around 96%. Acknowledgements This work is the outcome of the Manpower Development Program for Energy and Resources supported by the Ministry of Knowledge and Economy (MKE) References Fig. 21. Experimental results of manual soft start from standstill to 3 kw power generation: (a) utility voltage V grid [50 V/div]; (b) utility current I grid [2.5A/div]; (c) PV array voltage [20 V/div]; and (d) PV array current [1.6 A/div][0.25 s/div]. Fig. 22. Experimental results of utility: (a) utility voltage V grid [50 V/div] and (b) utility current I grid [2.5 A/div][5 ms/div]. 4. Conclusion In this paper, the proposed CMPPT method has generated a solar cell array current command directly by using a modified incremental conductance method. In addition, controllers of dc/dc boost converter, dc-link voltage, and dc-ac inverter are designed and realized for the coordinated operation of these elements. The DPLL using an all-pass filter, which can detect phase and peak voltage, is presented. PI control using a pseudo synchronous d-q transformation is proposed for grid current control. A 3k W prototype PV PCS is built and tested to verify the proposed schemes. Experimental results of the PV PCS have revealed the advantages of CMPPT with respect to VMPPT. [1] J. Kwon, K. Nam, B. Kwon, Photovoltaic Power Conditioning System With Line Connection, IEEE Transactions On Industrial Electronics, Vol. 53, No. 4, p , August [2] J. Gow and C. Manning, Controller arrangement for boost converter systems sourced from solar photovoltaic arrays or other maximum power sources, Proc. Inst. Electr. Eng. Electr. Power Appl., Vol. 147, No. 1, pp , Jan pp , Jan [3] J. Enslin,M.Wolf, D. Snyman, and W. Sweigers, Integrated photovoltaic maximum power point tracking converter, IEEE Trans. Ind. Electron., Vol. 44, No. 6, pp , Dec [4] O. Wasynzczuk, Dynamic behavior of a class of photovoltaic power systems, IEEE Trans. Power App. Syst., Vol. PAS-102, No. 1, pp , Sep [5] K. Hussein, I. Muta, T. Hoshino, and M. Osakada, Maximum photovoltaic power tracking: An algorithm for rapidly changing atmosphere conditions, Proc. Inst. Electr. Eng., Vol. 142, pt. G, No. 1, pp , Jan [6] Y. Kuo, T. Liang, J. Chen,"Novel maximum-powerpoint-tracking controller for photovoltaic energy conversion system," IEEE Transactions On Industrial Electronics, Vol. 48, No. 3, p , June [7] S. Sakamoto, T.Izumi, T Yokoyama, T Haneyoshi, A New Method for Digital PLL control Using Estimated Quadrature Two Phase Frequency Detection, IEEE CNF PCC(Power Conversion Conference), Vol 2, p , April [8] V.Blasko, V.Kaura, Operation of a Phase Locked Loop System under Distorted Utility Conditions, IEEE Transactions On Industrial Electronics, Vol.33, No.1, p , January/February [9] I. Hwang, K. Ahn, H. Lim, S. Kim, Design, development and performance of a 50 kw grid connected PV system with three phase current-controlled inverter, Proc. Photovoltaic. Specialist, 2000, p , Sept [10] M.A.S. Masoum, M. Sarvi, Voltage and current based MPPT of solar arrays under variable insulation and temperature conditions, Proc. UPEC 2008, p. 1-5, 1-4 Sept [11] M.A.S. Masoum, H. Dehbonei, Theoretical and experimental analyses of photovoltaic systems with voltageand current-based maximum power-point tracking, IEEE Trans. Energy conversion Vol. 17, No. 4, pp , 2002.

8 Sanghoey Lee, Jae-Eon Kim and Hanju Cha 613 [12] S. Lee, T An, H Cha, Mitigation of Low Frequency AC Ripple in Single-Phase Photovoltaic Power Conditioning Systems, Journal of Power Electronics Vol. 10, No. 3, pp , [13] Y. K, Digital Control of inverter for grid-connected PV system, master thesis Kyungpook National University, [14] H. Cha, S. Lee, Design and Implementation of Photovoltaic Power Conditioning System Using a Current Based Maximum Power Point Tracking, IEEE-IAS Annual Meeting, October 2008, pp.1-5. Sanghoey Lee received his B.S. degree in Instrumentation Control Engineering from Konyang University, Korea in 2002, and M.S. degree in Electrical Engineering from Chungnam National University, Korea in From 2005 to 2007, he worked at the Institute for Advanced Engineering in Yong-in, Korea. He is currently pursuing his PhD degree in Electrical Engineering in Chungnam National University, Daejeon, Korea. His research interests are power quality, advanced converter and control for renewable energy systems, and micro-grids. Hanju Cha received his B.S. degree in Electrical Engineering from Seoul National University, Korea, and M.S degree in the same field from Pohang Institute of Science and Technology, Korea in 1988 and 1990, respectively. He obtained his PhD in Electrical Engineering from Texas A&M University, College Station, Texas in From 1990 to 2001, he was with LG Industrial Systems in Anyang, Korea where he was engaged in the development of power electronics and adjustable speed drives. In 2005, he joined the Department of Electrical Engineering, Chungnam National University, Daejeon, Korea. He worked as a visiting professor in United Technology Research Center, Hartford CT, USA in His research interests are high power dc-dc converter; ac/dc, dc/ac, and ac/ac converter topologies; power quality and utility interface issues for distributed energy system; and micro-grids. Jae-Eon Kim received his B.S. and M.S. degrees from the University of Hanyang in 1982 and 1984, respectively. He was affiliated with KERI as a researcher from 1984 to 1989; a senior researcher form 1989 to 1996; and a team leader of advanced distribution systems and custom power lab from 1997 to He received his PhD from Kyoto University, Japan in He has been an assistant professor from 1998 to 2004; an associate professor from 2004 to 2009; and currently, a professor at Chungbuk National University. His current interests are analysis of power quality; operation and design of power distribution systems with distributed generation; and advanced distribution systems, such as micro-grids or smart grids.