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IEICE Electronics Express, Vol.7, No.7, 506–512

Design of a low cost, high performance PV inverter
T. T. Maa)
Department of Electrical Engineering, CEECS, National United University, 1 Lien-Da, Kung-Ching Li, Miaoli36003, Taiwan R.O.C. a) tonyma@nuu.edu.tw

Abstract: The conventional grid-connected photovoltaic (PV) inverter that steps up low DC voltage to high DC voltage and cascades with the high frequency inverter is complicated in control and of low e?ciency due to two stages. This paper presents a novel PV inverter system formed by a hybrid DC-DC converter and a full-bridge DCAC converter. The hybrid DC-DC converter combines the boost and ?yback topologies to produce a semi-sinusoidal output current and to achieve the high step-up objective. A full-bridge DC-AC converter controlled with low-frequency switching techniques is then used to convert the current into sinusoidal form and to feed power to the grid with unity power factor. The overall e?ciency of the designed system is high due to the losses of both stages are reduced. In this paper, the circuit operating theory of the proposed PV inverter is ?rstly addressed then an 80 W prototype system is designed and built. The feasibility and effectiveness of the proposed circuit are con?rmed with some simulation and experimental results. Keywords: photovoltaic module, ?yback converter, full-bridge DCAC converter, grid connected PV inverter system Classi?cation: Electron devices, circuits, and systems
References
[1] Energy and Power, “IEEE Aerospace and Electronic Systems Magazine,” Jubilee Issue, pp. 19–26, 2000. [2] A. Nasiri, S. A. Zabalawi, and G. Mandic, “Indoor Power Harvesting Using Photovoltaic Cells for Low-Power Applications,” IEEE Trans. Ind. Electron., vol. 56, no. 11, pp. 4502–4509, Nov. 2009. [3] C. Rodriguez and J. D. K. Bishop, “Organic Architecture for Small- to Large-Scale Photovoltaic Power Stations,” IEEE Trans. Ind. Electron., vol. 56, no. 11, pp. 4332–4343, Nov. 2009. [4] F. Giraud and Z. M. Salameh, “Steady-State Performance of a GridConnected Rooftop Hybrid Wind-Photovoltaic Power System with Battery Storage,” IEEE Power Eng. Rev., vol. 21, no. 2, p. 54, Feb. 2001. [5] T. Shimizu, K. Wada, and N. Nakamura, “Flyback-Type Single-Phase Utility Interactive Inverter With Power Pulsation Decoupling on the DC Input for an AC Photovoltaic Module System,” IEEE Trans. Power Electron., vol. 21, no. 5, pp. 1264–1272, Sept. 2006.
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IEICE 2010

DOI: 10.1587/elex.7.506 Received February 19, 2010 Accepted February 27, 2010 Published April 10, 2010

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IEICE Electronics Express, Vol.7, No.7, 506–512

1

Introduction

With the ever growing concern about global warming, environmental pollution, and the rising cost of fossil fuels like oil and coal, there is a greater interest in developing distributed generations (DG) with renewable energy sources (RES) to meet the growing energy demand. It has been well accepted that DG and RES technologies o?er the promise of ?exible, clean, abundant energy gathered from self-renewing resources. Of these renewable energy sources, photovoltaic (PV) technologies are becoming cost-e?ective today in an increasing number of markets, and are making important steps to broader commercialization [1]. In addition to central PV inverter systems which are normally designed with relatively high power ratings, the application of small dispersed PV complementary energy systems is another rapidly growing area [2, 3] and is developed toward architectures consisting of a number of PV inverters with relatively small power ratings and which can be incorporated into roo?ng materials or dispersedly installed at any location in the building near an outlet. The number of PV module in the applications of this kind is around 1–5 per inverter, and there will be as many inverters as the application needs [4, 5]. However, the key to the success of applying small dispersed PV energy systems is a simple, low cost and high performance inverter. In this paper, a novel PV inverter of this kind is proposed and experimentally tested.

2

Operating principles of the proposed PV inverter

c

IEICE 2010

DOI: 10.1587/elex.7.506 Received February 19, 2010 Accepted February 27, 2010 Published April 10, 2010

A. Circuitry and control arrangements As shown in Fig. 1 (A), in addition to a conventional boost-type DCDC converter and a full bridge DC-AC converter, the proposed PV inverter system includes an extra secondary winding, Ns , a diode, D2 , a ?ltering capacitor, C , to form a boost type ?yback con?guration and to increase the e?ciency and output voltage gain with a coupled inductor. A low pass ?lter is constituted by a clamping capacitor, Cclamp , connecting in series with, C , and the output inductor, Ls . By cascading the output voltage, VCc , of the boost converter and the output voltage, Vc , of the ?yback converter, a high output voltage, Vd , is easily obtained. In addition, there is a low voltage stress imposed on the power switch and diode as well as on the output capacitors compared to that of conventional boost converters. The low side capacitor, Cclamp , functions as an output capacitor and a snubber capacitor to suppress the voltage spike on Q1 during the turn-o? transient period, which also recycles the leakage energy in the coupled inductor. In this design, a power switch, Q1 , with low voltage rating is used to reduce conduction loss, and as a result the overall e?ciency can be signi?cantly improved. For the switching control of the proposed PV inverter system, a semi sinusoidal pulse width modulation (PWM) method is utilized. The duty cycle of the switching device, Q1 , is obtained from comparing a saw-tooth voltage waveform, Vtri , and a half-cycle sinusoidal voltage waveform. By setting the peak value of the current ?owing through Q1 to be proportional to the grid voltage and

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IEICE Electronics Express, Vol.7, No.7, 506–512

controlling the converter under discontinue current mode (DCM), a half-cycle sinusoidal current waveform can be produced for the output current of the ?yback converter, Id , which is also proportional to its primary current, i.e., the peak current of Q1 , IQ1 . In order to convert this half-cycle sinusoidal current waveform into an alternative current waveform, the switching signal for the full-bridge DC-AC converter is made synchronous to the grid voltage, Vs . The switching devices, (TA+, TB ?), of the inverter are turned on during the positive half cycle and the other pair of switches, (TA+, TB ?), are turned on during the followed negative half cycle. The output of the full-bridge DCAC converter is then connected to the grid (Vs ) via a low pass ?lter formed by VCc and Cc and Ls to achieve feeding unity power factor power to the grid. It should be noted that the overall e?ciency of the proposed inverter can be further increased due to the fact that the discharging path of the clamping capacitor, Cclamp , does not include the diode. B. Operating principles Before analyzing the proposed PV inverter the following assumptions are made: 1) The main switching device and the two diodes are ideal. 2) The magnetizing current of the transformer is always positive. 3) The leakage inductances Lk1 and Lk2 are much less than the magnetizing inductance, Lm . 4) The voltages and currents in the circuit are all periodic under steady state operation. 5) The duty of the main switch is de?ned as D and the switching period is de?ned as T s. To analyze the circuit of the proposed PV inverter shown in Fig. 1 (A), the

c

IEICE 2010

DOI: 10.1587/elex.7.506 Received February 19, 2010 Accepted February 27, 2010 Published April 10, 2010

Fig. 1. (A) The circuit of the proposed PV inverter; (B) Typical current waveforms of the converter operated in DCM.

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IEICE Electronics Express, Vol.7, No.7, 506–512

transformer T1 is modeled as a magnetizing inductance, Lm , and two leakage inductances Lk1 and Lk2 , and an ideal transformer which consists of a turns ratio of Np /Ns . The typical current waveforms of the converter operated in DCM are illustrated in Fig. 1 (B). Fig. 2 (A) to (G) illustrate all operating modes within a positive half-cycle of the grid voltage (TA+, TB ? are on). In Mode (A):t0-t1, Q1 on, the clamping capacitor and C supply energy to the grid; In Mode (B):t1-t2, Q1 o?, Cds is charging; In Mode (C):t2-t3, the two diodes are turned on. In Mode (D):t3-t4, ID1 is charging the clamping capacitor and C is discharging; In Mode (E):t4-t5, ID2 is charging the ?yback capacitor, C ; In Mode (F):t5-t6, the currents of two diodes are approaching to zero; In Mode (G):t6-t0, the two diodes are turned o?. In the negative half-cycle of the grid voltage the TA? and TB + are turned on.

c

IEICE 2010

DOI: 10.1587/elex.7.506 Received February 19, 2010 Accepted February 27, 2010 Published April 10, 2010

Fig. 2. All possible operating modes (A to G) within one switching cycle of the converter.

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IEICE Electronics Express, Vol.7, No.7, 506–512

3

Speci?cations of key devices

Referred to Fig. 1 (B) and the equivalent circuit of Mode (C) as shown in Fig. 2, at the time instance t2 when the two diodes, D1 and D2 , are turned on due to forward biased the voltage imposed on the main switch, VDSS , equals to the voltage on the output capacitor. As depicted in Fig. 2 (C), when the diode, D1 , is o? and D2 is on the voltage imposed on the main switch, VDSS , is given by: VDSS = Vp + Vc · Np . Ns (1)

It follows that VDSS = VCc . When the converter is operated in Mode (G), the two diodes, D1 and D2 , are o? and the voltage imposed on the main switch, VDSS , equals to Vp , and then the value of VCc can be calculated as follows. VCc + Vc = VCc + Ns · (VCc ? Vp ) = Vd . Np (2)

In Eq. (2), the voltage, Vd , is equivalent to the peak valued of the grid voltage. Rearranging Eq. (2), the voltage on the output capacitor can be mathematically expressed as follows. VCc = N p · Vd + N s · Vp . Np + Ns (3)

4

Experimental system and measured results

In this paper, an experimental 80 W PV system is constructed for test purposes. The details of the experimental system are given below. ? Input voltage: 10–40 VDC ? Grid voltage/frequency: 110 V/60 Hz ? Switching frequency: 20 kHz/5 V ramp ? PV module: 80 W (VMPPT = 15 V) In hardware implementation of the proposed PV system, there exists two control loops. The outer control loop is a maximum power point tracking (MPPT) controller which is in charge of calculating the operating voltage command for achieving the maximum power operation of the PV module. The inner loop is a voltage regulation loop designed for regulating the voltage of PV module, Vp , to approach the voltage command. To construct the control voltage, vcon , for the converter the output of the voltage controller should be multiplied by a half cycle sinusoidal signal. As can be seen in Fig. 1 (A), the trigging signal for the main switching device of the ?yback converter is obtained from comparing the control voltage, vcon , with a sawtooth voltage waveform, Vtri .

c

IEICE 2010

DOI: 10.1587/elex.7.506 Received February 19, 2010 Accepted February 27, 2010 Published April 10, 2010

4.1 Experimental results To verify the feasibility of the proposed circuits and design concepts, an 80 W grid connected PV inverter system is practically constructed. In the hardware system, the PWM switching signals are controlled by the UC3524.

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IEICE Electronics Express, Vol.7, No.7, 506–512

Fig. 3. (A) The experimentally constructed 80 W PV inverter system (B) The measured results of the proposed PV inverter (Vp ? = 15 V, the injected power is controlled at 60 W). The MPPT controller and the voltage regulator are implemented by using MATLAB real-time control interface. To have a clear picture of the hardware arrangement for the PV inverter system, Fig. 3 (A) shows the photograph of the related circuits. A set of measured experimental results with the output power controlled at 60 W shown in Fig. 3 (B) veri?es the design performance presented above. As can be seen in Fig. 3 (B), Vp is regulated at the VMPPT = 15 V and the unity power factor (PF = 1.0) is achieved.

5

Conclusion

c

IEICE 2010

DOI: 10.1587/elex.7.506 Received February 19, 2010 Accepted February 27, 2010 Published April 10, 2010

In this paper, the concepts of topology design, operating principles, controllers, and experimental results of a novel boost-?yback PV inverter has been presented. In the proposed boost-?yback converter, the leakage energy can be recovered to the output terminal and the voltage stress on the power switch can be signi?cantly reduced. It is important to note that the coupled inductor concept applied to achieve the high step-up voltage gain has been investigated and veri?ed. Based on the measured results, the conversion e?ciency is signi?cantly improved (about 95.5% at 0.75 p.u. output power). The

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IEICE Electronics Express, Vol.7, No.7, 506–512

features of the proposed PV inverter can be summarized as high e?ciency, high voltage gain, low voltage stress on switching elements, cost e?ective and simple in hardware implementation. These features are especially important in the low power applications of PV modules.

c

IEICE 2010

DOI: 10.1587/elex.7.506 Received February 19, 2010 Accepted February 27, 2010 Published April 10, 2010

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