Development of 125kW Grid-Tied PV Inverter and Research on Efficiency vs. Transformer Turn Ratio Bong-Joo Choi 1 , Yong-Seung Oh 1 , Kang-Soon Ahn 1 , and Hee-Jun Kim 2 1 R&D Center., Willings Co., Ltd., Korea 2 Division of Electrical Engineering and Computer Science, Hanyang University, Korea E-mail: hjkim@hanyang.ac.kr Abstract — This paper reviewed the development of a 3-phase 125kW grid connected PV inverter and changes in the conversion performance of the inverter based on the gain of the low frequency isolation transformer for isolation between solar and the grid. The 125kW PV inverter consists of a transformer with the grid frequency for isolation between input and output (galvanic isolation) and it has the output power of 60Hz, 3P 3W 380Vac. P&O technique was used for MPPT (Maximum Power Point Tracking) and Active Phase Shift technique was used through changes in reactive power to prevent a islanding operation. Inverter control was achieved based on 16-bit DSP. This paper discussed about the conversion performance of the 125kW inverter in relation to the system and the result of development of THD, MPPT etc and mentioned about the result of comparison of the conversion performance of the inverter based on the efficiency of the low frequency transformer. I. INTRODUCTION Currently, installations of PV power plants are remarkably increasing backed by increased interests in PV inverters and the government policy to grant subsidies. In the case of these MW capacity PV power plants, central type large capacity inverters with 100kW or higher capacity are applied in most cases and the inverter mentioned in this paper has been developed mainly for such MW capacity PV power plants. The developed inverter is a 125 kW 3-phase grid-connected PV inverter with no DC-DC converter which can maintain insulation from boost and grid using LF transformer. In this case, one of the greatest influencing factors to the efficiency of this inverter is the LF transformer. This paper analyzed the performance of the inverter developed as such and summarized the result and reviewed changes in the conversion performance based on changes in the turn-ratio of the low frequency transformer that accounts for a large part of the conversion performance of the inverter. II. SYSTEM CONFIGURATION AND SPECIFICATION Fig. 1. gives an overview of the developed 125kW PV inverter. It consists of IGBT stack, L-C filter, LF transformer, grid, and controller parts. The part marked TR is the low frequency transformer. TR is a part that greatly affects the overall efficiency of the inverter, and this study measured its efficiency according to the voltage ratio of the transformer. And, Fig. 2. is a photo of the inverter produced and Table 1. shows the electric specification of the inverter. Fig. 1. 125kW inverter block diagram. Fig. 2. Figure of the developed inverter TABLE I. Electrical Specification of 125kW PV Inverter Control Method PWM to Control Inductor Current MPPT Range 450 ~ 850 V DC P PVmax 137 kW P OUTmax 125 kW Operating Grid Voltage 323 ~ 427 V AC (380V AC ±15%) Operating Grid Freq. 59 ~ 61 Hz Output Current THD < 3 % at nominal power Inverter Efficiency > 95 % at nominal power Power Factor > 0.98 Operating Temperature 0 ~ 40 ℃ Anti-Islanding Detecting Time 1-4244-2491-7/09/$20.00 ©2009 IEEE