0093-9994 (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2018.2845893, IEEE Transactions on Industry Applications IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 00, NO. 00, 201X 1 Low-Voltage-Ride-Through Control of a Modular Multilevel Single-Delta Bridge-Cell (SDBC) Inverter for Utility-Scale Photovoltaic Systems Paul Sochor, Member, IEEE,, Nadia M. L. Tan, Senior Member, IEEE, and Hirofumi Akagi, Fellow, IEEE Abstract—This paper presents theoretical and experimental discussions on low-voltage-ride-through (LVRT) operation of a modular multilevel single-delta bridge-cell (SDBC) inverter intended for utility-scale photovoltaic (PV) systems. Modern grid codes require grid-tied inverters to provide dynamic grid support during grid-fault events by injecting reactive current. This paper discusses decoupled positive- and negative-sequence reactive-current control, focusing on asymmetric voltage sags with imbalanced magnitude and phase relationships. The main objective is to present a feedforward control method based on calculation of the zero-sequence current required for achieving power balance during normal and grid-fault conditions. More- over, this paper demonstrates a practical method that minimizes overcurrent stress in the three inverter clusters by adjusting active power drawn from PV arrays. Experimental results on a three-phase 12.6-kVA system prove that the SDBC inverter is capable of seamlessly operating through asymmetric voltage sags. Index Terms—Low-voltage-ride-through (LVRT), modular multilevel cascade inverters, photovoltaic systems. I. I NTRODUCTION The next-generation utility-scale photovoltaic (PV) systems aim at employing high-power PV inverters rated beyond 10 MW, which are connected directly to medium voltage (MV), thus resulting in higher system efficiency, easy scalabil- ity through modularity, and lower system costs. Different types of modular multilevel cascaded H-bridge inverters combined with solid-state transformers have drawn considerable atten- tion to utility-scale PV applications [1]–[6]. Among them, a modular multilevel single-delta bridge-cell (SDBC) converter, referred to as the SDBC converter or just the SDBC [7], [8], is a prominent circuit topology that has been applied to various grid-tied applications such as static synchronous compensators (STATCOMs) [7]–[14] and PV inverters [3]–[6]. As the penetration of distributed generation from renewable energy sources in power systems advances, modern grid codes have been demanding higher efforts to maintain the reliability Manuscript received November 08, 2017; revised March 08, 2018; accepted May 14, 2018. Paper 2017-IPCC-1309.R1, presented at the 2017 IEEE Energy Conversion Congress and Exposition (ECCE), Cincinnati, OH, USA, Oct. 1–5. This work was supported by the Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Next-generation power electronics.” P. Sochor, H. Akagi are with the Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, Tokyo 152-8552, Japan (e-mail: paul.sochor@akg.ee.titech.ac.jp; akagi@ee.titech.ac.jp). N. M. L. Tan is with the Department of Electrical Power Engineer- ing, Universiti Tenaga Nasional, Selangor 43000, Malaysia (e-mail: na- dia@uniten.edu.my). Digital Object Identifier 10.1109/TIA.2017.2704539 and availability of the grid. The latest gird codes require grid-tied inverters to stay connected and support the grid voltage by injection of reactive current. This requirement is referred to as low-voltage-ride-through (LVRT) with dynamic grid support [15]–[17]. SDBC inverters have been discussed less frequently in technical literature than single-star bridge-cell (SSBC) in- verters. For SSBC inverters, various LVRT control methods have been proposed, mostly for STATCOM applications [12], [18]–[20]. A well-known LVRT control method for SSBC- based STATCOMs relies on feedback control to determine the zero-sequence voltage that makes all the bridge-cell capacitor voltages balanced during normal and grid fault conditions [18]. Other methods inject both positive- and negative-sequence grid currents on the basis of feedback control with the aim of balancing the bridge-cell capacitor voltages [19]. For both SSBC and SDBC inverters, it is important to ensure that additional power components generated by the positive- and negative-sequence voltage and current bring nei- ther overvoltage to each bridge-cell capacitor nor overcurrent to each inverter cluster. It has been shown that both SSBC and SDBC-based STATCOMs cannot maintain stability in certain asymmetric grid conditions [12]. The authors of [6] have analyzed that an SDBC converter based on decoupled positive- sequence and negative-sequence control has the LVRT capabil- ity, and have addressed the overcurrent flowing into each clus- ter during certain grid fault situations. However, the authors of [6] have focused only on the positive-sequence reactive- current requirement for grid codes. As a result, they have presented a feedback approach, in which negative-sequence reactive-current and zero-sequence-current components have been applied to capacitor-voltage balancing, considering the two current components as two degrees of freedom. On the other hand, this paper presents a feedforward approach, paying attention to both positive-sequence and negative-sequence reactive-current requirements set by grid codes. Thus, the negative-sequence reactive current is no longer any degree of freedom for achieving capacitor-voltage balancing. The explicitly-calculated zero-sequence current, along with the active power available from the dc-power sources connected to the dc side of each bridge cell, is respon- sible for achieving capacitor-voltage balancing, and permits overcurrent mitigation during asymmetric voltage sags. To the best knowledge of the authors, no technical paper has been presented on a general analytic solution of the zero- sequence current that works under asymmetric grid voltages