Photovoltaic panel integrated power conditioning system using a high efficiency step-up DC–DC converter

Abstract

This paper presents a high efficiency photovoltaic (PV) panel integrated power conditioning system (PCS) by proposing a high efficiency step-up DC–DC converter. The suggested PCS consists of a high efficiency DC–DC converter and a single-phase DC–AC inverter. Each PV panel has its own DC–AC converter, performing the maximum power point tracking (MPPT) function and increasing its flexibility and expandability. Moreover, the proposed DC–DC converter converts the low PV panel voltage into a high DC-link voltage with a high step-up voltage conversion ratio. It can reduce the switching power losses, increasing the power conversion efficiency. The performance of the suggested PCS has been verified through a 180 W prototype of the PCS for a 60 Hz/120 V_ac power grid. The proposed DC–DC converter achieves a high efficiency of 96.0%. The PCS including the DC–DC converter and DC–AC inverter achieves an efficiency of 93.1% with an almost unity power factor.

Introduction

The photovoltaic (PV) power conversion has been an active research topic for renewable energy conversion applications [1], [2], [3]. Among the various kinds of PV power conditioning systems (PCSs), the PV panel integrated PCS [2] has been studied for “plug and play” concept [3] to optimize the energy yield from the PV panel. As shown in Fig. 1(a), a conventional system uses a PV array where many PV modules are connected in series to obtain sufficient DC input voltage for generating AC utility line voltage from the DC/AC inverter. However, difficulty is encountered in avoiding shadows created by neighboring buildings, utility poles, trees, and other obstacles that may partially cover some of the PV modules in the array. As a result, the total output power generated from the PV array decreases remarkably [4]. To overcome this defect, as an AC module strategy, a low-power AC utility interactive inverter is mounted on each individual PV module in the PV panel integrated PCS, as shown in Fig. 1(b). The PV panel integrated PCS operates in order to generate the maximum power from the corresponding PV module even under the partially shaded conditions [5]. Another advantage of this system is that the number of the parallel connected inverters, which is equal to the number of PV modules, can be selected in consideration of the dimensions of the roof on which PV modules are installed. This greatly improves the flexibility of the PV generation system.

The typical system configuration of the PV panel integrated PCS is shown in Fig. 1(c). The PV panel has a low voltage characteristic where its output voltage typically ranges from 20 V to 50 V. In order to generate 60 Hz, 240 V_ac output from the PV panel, the output voltage of the DC–DC converter should be high enough to generate the DC-link voltage around 350 V. A DC–DC converter with a high voltage conversion gain is necessary for converting the low PV panel voltage into the high DC-link voltage. Up to now, various DC–DC converters have been investigated for high step-up applications. In [6] and [7], the flyback converter has been used for the PV power conversion. The flyback converter in [6], however, suffers from high power losses due to its hard-switching operation. Especially, for the high step-up applications like PV panel integrated PCS, the hard-switching operation causes high power losses. Considering its operating conditions for the harshest environment, the life time of the PCS is reduced. In [8] and [9], the asymmetrical half-bridge flyback converter has been utilized to reduce the switching power losses for the PV power conversion. It reduces the switching power losses by the soft-switching operation for the switching power devices. The asymmetrical half-bridge flyback converter in [8], however, requires a large turns ratio of the transformer for generating a high DC-link voltage from the low PV panel voltage. The large transformer turns ratio causes high voltage stresses on the switching power devices. In [10] and [11], the full-bridge converter with a phase-shift control has been used for high step-up applications. All power switches operates under soft-switching condition without any increased voltage stresses. Compared to the previous converters in [6], [7], [8], [9], however, the phase-shifted full-bridge converters [10], [11] require lots of power switching devices and the associated control circuits. Eventually, the manufacturing cost of the system increases, which limit the practical utilization of the PV panel integrated PCS [12].

To address the above-mentioned problem, this paper proposes a high efficiency step-up DC–DC converter for the PV panel integrated PCS. Fig. 2 shows the circuit diagram of the proposed PCS. It consists of the proposed step-up DC–DC converter and a single-phase half-bridge DC–AC inverter. The proposed DC–DC converter has a high step-up voltage conversion ratio, which is desirable for a low PV panel voltage. By adding one diode and one capacitor to the conventional flyback converter, the output diode can operate under soft-switching condition. The series resonance between the transformer leakage inductor L_lk and the resonant capacitor C_r enables the output diode D_o1 to operate at zero-current switching (ZCS) condition. It can reduce the switching power losses and increase the power efficiency. The operation principle of the proposed DC–DC converter is presented. A DC-link voltage controller is described for a constant DC-link voltage regulation. An effective maximum power point tracking (MPPT) method is presented to extract the maximum electrical power from the PV panel [13], [14], [15]. A grid current controller is also suggested to provide the PV electrical power into the power grid with a unity power factor [16]. Experimental results are obtained from a 180 W prototype of the PCS for a 60 Hz/120 V_ac power grid. The proposed DC–DC converter achieves a high efficiency of 96.0%. The PCS including the DC–DC converter and DC–AC inverter achieves an efficiency of 93.1% with an almost unity power factor.