254 https://doi.org/10.6113/JPE.2019.19.1.254 ISSN(Print): 1598-2092 / ISSN(Online): 2093-4718 JPE 19-1-24 Journal of Power Electronics, Vol. 19, No. 1, pp. 254-264, January 2019 Seamless Mode Transfer of Utility Interactive Inverters Based on Indirect Current Control Kyungbae Lim * , Injong Song * , Jaeho Choi † , Hyeong-Jun Yoo ** , and Hak-Man Kim ** †,* School of Electrical Engineering, Chungbuk National University, Cheongju, Korea ** Department of Electrical Engineering, Incheon National University, Incheon, Korea Abstract This paper proposes an indirect current control technique based on a proportional resonant (PR) approach for the seamless mode transfer of utility interactive inverters. Direct-current and voltage hybrid control methods have been used for inverter control under grid-connected and islanded modes. A large bandwidth can be selected due to the structure of single-loop control. However, this results in poor dynamic transients due to sudden changes of the controller during mode changes. Therefore, inverter control based on indirect current is proposed to improve the dynamic transients by consistently controlling the output voltage under all of the operation modes. A PR-based indirect current control topology is used in this study to maintain the load voltage quality under all of the modes. The design processes of the PR-based triple loop are analyzed in detail while considering the system stability and dynamic transients. The mode transfer techniques are described in detail for both sudden unintentional islanding and islanded mode voltage quality improvements. In addition, they are described using the proposed indirect control structure. The proposed method is verified by the PSiM simulations and laboratory-scale VDER-HILS experiments. Key words: Indirect current control, PR control, Seamless transfer, Unintentional islanding, Utility interactive inverter I. INTRODUCTION Microgrids integrate renewable energy sources (RESs) such as photovoltaic generators, wind power generators, fuel cells and other power sources with engine generators and batteries. Microgrids have been widely applied because of their flexibility and capability to supply power reliably [1]-[6]. Among them, inverter based distributed generation (DG) has been one of the main types of equipment in microgrids due to its accurate power conversion. Therefore, utility interactive inverter control has recently become one of the most popular issues in the research of microgrids. An inverter in the grid-connected mode must supply power to a grid or local loads with the acceptable level of power quality [7]. Therefore, a DG-based inverter can be defined as a current source or a sub-power source [8]. However, an inverter needs to be operated as a voltage source when strategic intentional islanding operation is needed or the recloser switch is opened at the point of common coupling (PCC) due to a sudden grid fault or grid quality degradation. This is necessary because the inverter has to supply the full local load demand in place of the main grid under the islanded mode [9]. On the other hand, the inverter needs to change the mode from the islanded mode to the grid-connected mode when the grid has recovered from a pre-fault state or when the intentional islanding is no longer necessary. During these transitions, the inverter has to keep the power quality within an allowable range to avoid load damage or decreases in the load span due to an unrated local load voltage during mode transitions. There have been a number of studies on mode transition techniques to maintain power quality during mode transfers. Current control and voltage control have been used in both the grid-connected mode and the islanded mode [10]-[12]. However, the load voltage quality was fairly degraded during the transient state of the mode transfer process, and the regulation performance of the load voltage was quite sensitive to changes of the operation process, especially when islanding occurs. A droop-based voltage control scheme has also been proposed [13]. The droop-based inverter was defined as a © 2019 KIPE Manuscript received Sep. 13, 2018; accepted Oct. 18, 2018 Recommended for publication by Associate Editor Kyo-Beum Lee. † Corresponding Author: choi@cbnu.ac.kr Tel: +82-43-261-2425, Fax:+82-43-276-7217,Chungbuk Nat'l Univ. * School of Electrical Eng., Chungbuk National University, Korea ** Dept. of Electrical Eng., Incheon National University, Korea