High-frequency Transformer Design for Modular Power Conversion from Medium-voltage ac to 400 vdc
As a result of the increasing use of cloud computing and big data, the power consumption of the data center alone, will reach 10 % of the total electrical power consumption in the world by the year 2020. Considering such a booming data center load development, high copper cost and conduction loss, due to low voltage (480 Vac) power distribution outside the server hall, needs to be solved. In the proposed system, medium-voltage ac is used as the distribution voltage in the data center, and the proposed power conditioning system block (PCSB), is used to convert medium-voltage ac directly to 400 Vdc inside each server hall. This system can eliminate line frequency bulky transformers, reduce front end conduction cable costs, and save conduction loss. The proposed high-frequency modular medium-voltage ac, (4160 Vac and 13.8 kVac), to low-voltage dc, (400 Vdc) PCSB, is shown in Fig. 1. Each system block consists of ac-dc H-bridge converters, together with high-frequency isolated dc-dc converters. A total of five cascaded PCSBs are employed to convert 4160 Vac directly into 400 Vdc. The inputs of the ac-dc H-bridges are in series, and the outputs of the dc-dc stages are connected in parallel. In this work, a high-frequency isolated CLLC resonant converter is proposed for a next generation dc data center. The medium-voltage high-frequency transformer is the most crucial component in terms of insulation and power density. A novel UU core with a sectionalized winding structure is chosen to enhance insulation capability, restrict leakage inductance, and reduce magnetic loss. Transformer insulation parameters are calculated based on the IEEE standard requirements. The transformer turn number is determined based on the transformer loss and volume tradeoff. Different working frequency impacts on transformer design are also analyzed based on a similar method; 400 kHz is found to be the best working frequency for 3F36 material. Different materials optimal working frequency is also different. A 15 kW/500 kHz transformer prototype is developed, as shown in Fig. 2. The transformer is tested to pass three insulation tests according to the IEEE Std. C57.12.01 standard. Based on the transformer prototype, a 15 kW 500 kHz CLLC resonant converter is built. Experiment results of 98 % peak efficiency and 2.9 kW/L power density are presented.