Abstract—Nowadays in applications of renewable energy sources it is important to develop powerful and energy-saving photovoltaic converters and to keep the prescriptions of the standards. In grid connected PV converters the obvious solution to increase the efficiency is to reduce the switching losses. Our new developed control method reduces the switching losses and keeps the limitations of the harmonic distortion standards. The base idea of the method is the utilization of 3-state control causing discontinuous current mode at low input power. In the following sections the control theory, the realizations and the simulation results are presented. Keywords—Discontinuous current, high efficiency, PV converter, control method. I. INTRODUCTION HE photovoltaic arrays are pretty expensive, so it is needed to use them with the highest efficiency. The most frequent main circuit arrangement is the three phase bridge. The main advantage of our method is, that it is unnecessary to change the aforementioned topology of the converter, only the modification of the control is needed. In our previous work we successfully developed and used this control method in single phase photovoltaic converters [1]. In this paper we will introduce an algorithm developed for three phase converters. The presented method joins the benefits of Flat-top modulation – used in continuous mode control for decreasing switching losses – with the discontinuous mode control developed earlier for single phase converters. Manuscript received March 26, 2008 Attila Balogh is with the Budapest University of Technology and Economics, Department of Automation and Applied Informatics, Budapest, Hungary (phone: +36 (1) 463 1552, fax: +36 (1) 463 2871, e-mail: balogh@aut.bme.hu). Eszter Varga is with the Budapest University of Technology and Economics, Department of Automation and Applied Informatics, Budapest, Hungary (phone: +36 (1) 463 1552, fax: +36 (1) 463 2871, e-mail: hukk01@gmail.com ). Istvan Varjasi, PhD, is with the Budapest University of Technology and Economics, Department of Automation and Applied Informatics, Budapest, Hungary (phone: +36 (1) 463 1552, fax: +36 (1) 463 2871, e-mail: varjasi@aut.bme.hu). II. THE TOPOLOGY OF OUR CONVERTER The main parts of the converter are: the PV array, the Boost DC/DC converter and the three phase inverter with filters (see Fig. 1). The PV array converts the solar energy to DC power. An optional step-up converter may raise the voltage level of the PV array to the voltage level of the three phase bridge. Fig. 1 Topology of the converter The control of the boost chopper algorithm includes the maximal power point tracking (MPPT). The boost chopper is not necessary when the voltage of the array at MPP is greater than the peak line voltage of the grid. In that case the reference of the dc voltage is derived from the MPPT algorithm. The three-phase bridge converts the dc voltage to nearly sinusoidal ac currents. The high order harmonics of the output current are filtered out by passive low-pass filter. III. THE TRADITIONAL CONTROL There are several control methods of the grid connected PV converter. Most of them consist of an outer control loop for the DC voltage and inner current control loop. From these we use the grid voltage oriented current control. This control structure is very similar to the field oriented control (FOC) of ac machines. The controllers work here also in a rotating frame, but this rotating frame is connected to the grid voltage vector, so we may named this control system as grid voltage oriented control. The current component in the direction of the grid voltage vector is named as current “d” and is proportional to active power, while the orthogonal current component is named as current “q” and is proportional to reactive power. In this arrangement there is no zero order current, so it is enough to measure only two currents. The phase currents are transformed to the rotating d-q coordinate-system [2]. For the current control the currents are sampled at symmetry point(s) of PWM cycle, so the sampled current is close to the average value for a given switching period. 3 State Current Mode of a Grid Connected PV Converter Attila Balogh, Eszter Varga, and István Varjasi T World Academy of Science, Engineering and Technology International Journal of Electronics and Communication Engineering Vol:2, No:4, 2008 559 International Scholarly and Scientific Research & Innovation 2(4) 2008 scholar.waset.org/1307-6892/13078 International Science Index, Electronics and Communication Engineering Vol:2, No:4, 2008 waset.org/Publication/13078