Single-Stage Low Cost Grid Connected Inverter in Photovoltaic Energy Applications D. Petreuş 1 , Ş. Dărăban 2 , I. Ciocan 3 , T. Pătărau 4 , C. Morel 5 1 Technical University of Cluj-Napoca, Cluj-Napoca, Romania, dorin.petreus@ael.utcluj.ro 2 Technical University of Cluj-Napoca, Cluj-Napoca, Romania, stefan.daraban@ael.utcluj.ro 3 Technical University of Cluj-Napoca, Cluj-Napoca, Romania, ionut.ciocan@ael.utcluj.ro 4 Technical University of Cluj-Napoca, Cluj-Napoca, Romania, toma.patarau@ael.utcluj.ro 5 ESEO, Angers, France, Cristina.Morel@eseo.fr Abstract — Due to the rising fuel costs and growing worldwide demand for electricity, renewable energy sources become a necessity rather than a luxury. This paper presents an original control strategy for a buck-boost based converter used as a low cost inverter in a photovoltaic (PV) system. The main objective of the inverter is to harvest maximum power from the PV module and to inject it into the grid. The design of the converter is detailed and, together with the control method, a macro-model for the inverter is proposed. The macro-model is developed in order to have shorter simulation times and test various maximum power point tracking (MPPT) algorithms. Simulations and experimental results validate the proposed PV system. Keywords — single-stage inverter, maximum power point (MPP), power factor (PF). I. INTRODUCTION Nowadays global energy demand is growing and renewable energy sources begin to play an important role in energy co-generation and distribution. In this context, photovoltaic energy remains a good alternative in both, stand alone (SA) and grid connected (GC) systems. Even if the SA systems are independent of the mains, they may require additional backup electrical-generating systems for reliability or peak demand, and they are constrained by the dimensions, capacity or aging of the energy storage devices. GC systems have been developed for more than 20 years, as an alternative energy source to the electrical grid, especially in developing countries where the utility is not stable or in remote areas where a low/medium power backup energy source is needed [1]. In this paper a single-phase inverter is built to connect a solar energy source to the mains. The system has no galvanic isolation with the grid, and is being developed for low cost, high efficiency and high power factor (PF). In order to implement a simple and small inverter, the DC-DC stage (between solar panels and inverter) is left out, and there is only one DC-AC stage (between the PV module and the grid), which reduces the total losses. In this case, the inverter is responsible with both, MPP tracking, to maximize the energy harvesting, and convert the generated DC power into a suitable AC current source for the grid. Both tasks must be made with high efficiency, over a wide power range, due to the variable weather conditions. The current injected into the mains must obey the regulations, such as the EN61000-3-2 [2] and the IEEE std. 1547 [3], which state the maximum allowable current harmonics. A typical solar-based inverter has two stages cascaded, with simpler controllers but lower efficiency. Usually, the first stage assures voltage boosting or MPPT function and high frequency link galvanic isolation, and the second stage inverts the rectified sinusoidal current into a sinusoidal waveform, synchronized with the mains. A single-stage inverter reduces components used, increases system efficiency and has lower costs than a conventional multiple-stage inverter. As new control techniques and topologies are developed, a single-stage inverter will become more and more popular [4]. II. SINGLE-STAGE INVERTER TOPOLOGY Single or multiple-stage GC inverters can be developed around different converter topologies, such as: full bridge inverters, as in [5-8], push-pull or boost converters, as it is presented in [9], but most of them are flyback or buck- boost based converters, as in [10-15]. Zero voltage switching or zero current switching mode controls are typically used in these converters in order to reduce the switching losses, according to [4], [13]. The proposed control technique is implementing a hysteretic current mode control (HCMC) applied to a flying inductor converter [16]. The topology from Fig. 1, was chosen to provide the AC output voltage (230V/50Hz) even if the PV module voltage is above or below it, without using transformers or voltage inversion [17]. Variable frequency HCMC has the main advantage of a good robustness and stability control, without needing a compensation ramp. Furthermore, the absorbed current from the PV module has reduced distortions compared to other current control techniques, which makes the input power decoupling easier. Also, the implementation of this control strategy is relatively simple and low cost [18]. The proposed control strategy will be detailed in the next sections. Fig. 1. The proposed single-stage inverter topology. 15th International Power Electronics and Motion Control Conference, EPE-PEMC 2012 ECCE Europe, Novi Sad, Serbia 978-1-4673-1972-0/12/$31.00 ©2012 IEEE DS3d.5-1