A Bidirectional AC/AC Series Resonant Converter with High Frequency Link Leonardo Freire Pacheco and Ivo Barbi IBEPE – Brazilian Institute of Power Electronics and Renewable Energies UFSC – Federal University of Santa Catarina Florianópolis, Brasil leofpacheco2@gmail.com, ivobarbi@gmail.com Abstract— The study and use of solid state transformers (SST) has grown exponentially, and new technologies are being developed every day. The application of a series resonant converter in a SST allows improvement of its efficiency, through soft switching of the power switches. The present paper proposes a bidirectional topology capable of receiving an AC voltage in its input, and delivers the same AC voltage on its output through a series resonant converter and a push-pull AC-AC converter, with all switches operating with soft switching. The converter has three different switching elements and is capable of adjusting the output voltage level by changing the switching frequency. In this paper, the description of the converter’s operation and simulation results are presented. A prototype is being built to be tested in laboratory. Keywords—Solid-State Transformers, Series Resonant Converter, Soft Switching. I. INTRODUCTION With the exception of renewable energies, today’s electric power system can be divided in three stages: Generation, Transmission and Distribution [1]. The generation converts some other power source (usually mechanical) into electric power. Meanwhile, the transmission are in charge to take this energy to long distances. At the end, distribution delivers this energy to houses, industry, commerce, among others. Each stage is connected to another through step-up or step-down substations. This type of architecture has been around for more than a hundred years [2]. Looking closer at the distribution system, is easy to notice that it has some primary agents in distribute the energy, and they are: medium and low voltage conductors, poles and distributions transformers. A Solid-State Transformer (SST), also known as electronic transformer, is an equipment that contemplates all functionalities of the conventional transformer, but it does so with reduced volume and weight [3]. That occurs because it operates with high frequency, due to the switching of the semiconductors elements inside it. Other important features of a SST is his controllability, capability of interaction with solar or wind power generation systems, storage systems and current limitation [4]. SST was first proposed by [5]-[7], when the author suggested the use of four switching elements, to process a sinusoidal voltage through an intermediary high frequency stage. This switching, although effective, was not a soft switching, which made the converter efficiency undesirable. While [7] didn’t propose any type of adjustment of the voltage level, [8] proposes a topology capable of controlling the output AC voltage level through phase-shifting from a DC input. Nowadays, several projects contemplate the features of a SST, besides incorporating new concepts and functionalities [9]-[13]. Knowing that the higher the switching frequency is, the lower will be the SST’s volume and weight [4], a viable alternative to increasing the frequency is the use of series resonant converters. Proposals [14]-[16] were among the first on the resonant converters field. Among them, is the series resonant converter (SRC), that uses a LC pair in series to create a resonance, in which the switches must switch in synchrony, aiming for commutation at times where current or voltage is zero. This type of commutation, known as soft switching, dates back to the 1980s [17], [18]. With the use of the resonance, is possible to elevate the switching frequency, since the switches will not be dissipating as much power as they would be if operating with hard switching. Another vantage with the use of SRC is the possibility to use a transformer in its output, which brings galvanic isolation and adjust of the voltage level through the relation of the primary and secondary winding. II. SERIES RESONANT CONVERTER CHARACTERISTICS A. Operation Modes Regarding the switching frequency, the SRC can operate in three different modes: with switching frequency above, below, or equal to the resonant frequency. When operating with switching frequency above the resonant frequency, the switches commutate under zero voltage, or Zero Voltage Switching (ZVS) [19]. If it operates with switching frequency below the resonant frequency, the switches commutate under zero current, or Zero Current Switching (ZCS) [19]. If it operates at the resonant frequency, the capacitive impedance annuls the inductive impedance, and the resonant current is in phase with the voltage. It can be demonstrated that the static gain of the converter, for first harmonic approximation, is given by equation (1), 2 4 2 1 1 64 o o o q I              (1) This work is supported by Companhia Energética de Brasília (CEB), as part of R&D Program ANEEL 001/2016. 2018 13th IEEE International Conference on Industry Applications Mo9TrackA.5 978-1-5386-7995-1/18/$31.00 ©2018 IEEE 305 ISBN 978-1-5386-7995-1