0885-8969 (c) 2019 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/TEC.2019.2944994, IEEE Transactions on Energy Conversion > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract—Grid connected voltage source converters are inevitable means now for expanding and supporting power systems. Nevertheless, their harmonic injection and high frequency switching may cause degradation of power quality and detraction of the equipment reliability. Therefore, the converters injected harmonics must observe the corresponding standards, while lower switching frequencies and rapid transient responses are desirable. In this paper, a new combined control method with a flexible switching table is presented for the converters to enhance the power quality and transient response. The method employs a virtual flux in connection with a combined control for the first time. In comparison with the conventional virtual flux direct power control, the proposed method provides better power quality, reduced THD and faster transient response, in addition to less grid parameters dependency. Compared to the conventional combined control method, it determines more accurately the converter voltage vectors, while avoiding grid voltage measurement and cumbersome controller tuning. Index Terms—Converters, harmonics, hysteresis controllers, power grids, power quality, reactive power, reliability, renewable energy, switching frequency, THD, transient response, virtual flux. INTRODUCTION HREE-PHASE grid connected voltage source converters (VSCs) have been widely employed in different industrial applications such as active power filtering, flexible ac transmission systems (FACTS), and converter-based renewable energy sources to exchange power between ac grids, loads and/or sources [1]-[4]. However, utilizing VSCs causes harmonics in power lines, which may get close to the THD limits set by IEEE Standard 519 [5], [6]. In addition, the harmonic injection threatens the power system reliability Manuscript submitted March 18, 2019. Support of INSF through the Chair of Wireless and Contactless Power Transfer is acknowledged. A. Jabbarnejad is with the School of Electrical and Computer Engineering, University of Tehran, Tehran, Iran (e-mail: jabbarnejad.a@ut.ac.ir). S. Vaez-Zadeh is with the Advanced Motion Systems Research Lab and the Center of Excellence on Applied Electromagnetic Systems at the School of Electrical and Computer Engineering, College of Engineering, University of Tehran, Tehran, Iran (e-mail: vaezs@ut.ac.ir ). M. Jahanpour-Dehkordi is with the Advanced Motion Systems Research Lab, School of Electrical and Computer Engineering, College of Engineering, University of Tehran, Tehran, Iran (e-mail: m_jahanpour@alumni.ut.ac.ir ). through causing problems like the aging and failure of isolators of grid equipment, leading to the malfunction of power breakers, and decreasing the longevity of the equipment [7]. The above problems can partly be overcome by exploiting the potential of VSC control systems. The common control method for VSCs is vector control that provides merits such as excellent steady state system performance, low active and reactive power ripples, and a constant switching frequency [8]. Nevertheless, it provides moderate transient response due to PI current controllers and uses PWM subsystem [9], [10]. In addition, it needs a decoupling circuit and adaptation of the controllers to the variations in the system parameters and operating point [9]. In order to overcome these drawbacks, various non-linear control methods including direct power control (DPC) are proposed in the literature [11]. Compared to vector control, the main advantages of DPC are simpler control structure [10], faster current/power dynamic response [12], [13], and lack of the current decupling circuit [14]. However, it suffers from high and variable switching frequency [12], high power ripples and current THD [15], non-accurate switching table [10], and required voltage sensors [14]. The virtual flux (VF) concept is thus used in VF-DPC to overcome some of the difficulties associated with DPC including power quality and current THD problems [14]. However, it may hardly satisfy IEEE Standard 519. Alternatively, several modified control strategies exist for VSCs to overcome the mentioned problems. In particular, a predictive DPC using SVM [16], an adaptive robust predictive DPC [17], a dead-beat predictive DPC [18], and a VF-based predictive DPC [19] appear in the literature to obtain both fast transient response and constant switching frequency. Nevertheless, they suffer from drawbacks such as complex online computations, needing a precise modelling of the filter, high sensitivity to parameter variations of the controlled system, and extensive hardware implementation. Recently, a VF-DPC with a new switching table is introduced, which is voltage sensorless and improves power quality and current THD in comparison with the methods based on a conventional switching table [20], [21]. However, its produced power ripples are not desirable and its current THD is close to the limit of IEEE Standard 519. Furthermore, a vector current control derived from DPC is introduced [22]. The method avoids any phase locked loop (PLL) and enjoys high power quality and low current THD Combined Control of Grid Connected Converters Based on a Flexible Switching Table for Fast Dynamic and Reduced Harmonics Alireza Jabbarnejad, Sadegh Vaez-Zadeh, Senior member, IEEE, Mohammad Jahanpour-Dehkordi T