Three-Phase to Single-Phase Direct Connection for Rural Co-Generation Systems Ricardo Q. Machado 1 , Simone Buso 2 , José A. Pomilio 1 and Fernando P. Marafão 1 1 State University of Campinas School of Electrical and computer Engineering P.O. Box 6101 13081-970 Campinas Brazil ricardom@dsce.fee.unicamp.br 2 University of Padova Department of Information Engineering Via Gradenigo 6/B 35131 Padova Italy simone.buso@dei.unipd.it Abstract – This paper proposes a solution for the direct connection of a three-phase induction generator to a single-phase feeder. This high quality power system is intended to be used in micro-hydro power plants, without control of the turbine. The generated power that is not consumed by the local load is driven to the single-phase feeder. The power flow control is provided by a three- phase PWM inverter, which guarantees the local power quality and controls the power flow through the single- phase feeder. This converter allows balancing the induction generator currents, voltages and frequency. The paper describes the inverter control strategy, presents design criteria of the controllers and shows experimental results. Keywords - Induction generator, single-phase to three-phase conversion, digital control, rural electrification, PWM inverter. I. INTRODUCTION The advantages of the three-phase induction machine have encouraged significant efforts in seeking approaches to overcome the Induction Generator (IG) poor voltage regulation and frequency variation [1-4]. Some advantages of the induction machines are its robustness, simple construction, little maintenance requirements, wide availability, low cost, and higher power-weight ratio than other electrical machines. Customers in rural areas, that usually have a single-phase feeder, may request a three-phase system from the utility and find that it is uneconomical to meet a relatively small three- phase need [5]. On the other hand, there might be energy sources available in some of these areas to produce electric power. For this situation, some authors have proposed the direct connection of the induction generator to the single- phase feeder to obtain three-phase balanced voltages [6-9]. Some of these alternatives, based on the ‘Steinmetz’ connection, operate only under strict conditions and are affected by the AC load variations. An alternative technique is based on current control [9], but this is not able to guarantee the AC voltage controllability. II. THE PROPOSED SYSTEM The proposed system comprises an IG directly connected to the single-phase feeder where an AC capacitor bank (C AC ) provides the IG magnetization. As the direct connection of an IG to the single-phase utility feeder causes strongly unbalanced voltages and currents at the IG terminals, a three- phase inverter is connected to balance the IG voltages and, as a consequence, its currents (Fig. 1). The inductors (L conv ) are used to do the connection between the inverter and the Point of Common Coupling (PCC). The interaction between L conv and the AC capacitors provide the necessary low frequency filtering and the resulting AC voltage is, in practice, free from switching noise. One important feature of the proposed system is that it does not comprise any speed-governor for the IG, so that the generated power depends on the prime-mover power availability. All the power-flow control is based on the local load demand. If the local load demand is lower than the generated power, the excess is driven to the grid. Otherwise, it is possible to absorb additional power from the electric system. If the feeder is not connected, some strategy must be adopted to consume the excess of power, or to reduce the generated power. It is also necessary to guarantee the synchronism before reconnecting the feeder when it is energized. Since the inverter does not manage real power, it is not necessary to have a DC source in the DC link. III. CONTROL STRATEGY The control strategy imposes sinusoidal symmetric and balanced voltages on the AC bus. The control of these voltages provides the local load reactive power compensation, assuring that the IG will continue to run normally without losing its magnetization. In addition, the load current harmonics can be partially compensated, but the effectiveness depends on the inverter output impedance compared with the other system impedances (AC capacitors, IG and single-phase feeder impedances). Additionally, the current through the single-phase feeder should have unity power factor (PF). 0-7803-8269-2/04/$17.00 (C) 2004 IEEE. 1547