Title : Feasible Performance Evaluations of Digitally-Controlled Auxiliary Resonant Commutation Snubber-Assisted Three Phase Vo

International Journal of Industrial Electronics, Control and Optimization . © 2020 IECO. 366 converter, and the voltage gain of the converter can be adjusted applying different modulation techniques to reach the desired result [8]. The interesting feature of Z-source inverters in tolerating short circuit has drawn a lot of attention. Numerous structures have been proposed based on Z-source inverters aiming to widen voltage gain range. Impedance source converters are interesting choice in grid-connected PV system applications [9], [10]. The Y-source converter is an impedance source converter with a high voltage gain. The voltage gain of this converter can flexibly alter by varying the shoot-through period [9]. The discontinuous input current of Y-Source converter is an obstacle in renewable power generation applications. To address this issue, the structure of the Y-Source converter has faced modifications, and several converters such as Modified-Y-Source converter [11] and Quasi-Y-Source converter [12] have been proposed. Choosing a suitable converter for the power conversion stage requires considering the voltage gain, continuous or discontinuous input current besides the number of converter elements. In terms of the number of elements, the converter proposed in [11], is better than the Quasi-Y-Source converter [12] utilizing one less element with the same performance [11]. As shown in Fig. 1, the output of the inverter, together with output filter, forms the injected power to the grid. In a grid-tied PV system, there are special standards which have been set to define the quality of the injected power to the grid. According to IEEE std. 1547, the total harmonic distortion of the injected current should be limited by 5% [13]. To fulfill the grid codes, the system must inject a sinusoidal current to the grid. This is realized by applying different output filter configurations. Among all filter types, LCL filters are the most suitable filter types due to their performance in mitigating current harmonics [5]. Designing the LCL filter is an issue that is depended on the power conditioning method. Different control schemes utilizing such filters have been proposed in the literature. Grid Current Feedback (GCF) and Converter Current Feedback (CCF) are the main methods that are adopted to ensure high-quality power [14]. In [15], an active damping approach has been proposed to suppress harmonics. The proposed method in [15] utilizes grid current and capacitor voltage feedback. Capacitor current feedback active damping along with grid current feedback have also been employed in [16]-[18]. In [19], resonance damping is realized by applying a CCF control scheme, which requires one less current sensor in comparison with GCF with capacitor current feedback control. CCF method has an innate capability of suppressing LCL filter resonance frequency [20]. In this method, only one current sensor is required. The current sensor in the control scheme provides versatile goals, including injected current control and overcurrent protection. Therefore, the device will be protected against fault consequences. Reduced number of current sensors along with less complexity in control and, of course, its innate resonance damping are the factors that made the CCF method more interesting [20], [21]. Sampling frequency (fs) value should be considered in the design procedure of the LCL filters [22]. In the GCF method, the resonance frequency ( fres) should be in the range of fs 6 < fres < fs 2 to perform in the stable zone while the CCF method requires an LCL filter satisfying fres < fs 6 [22]. Different power conditioning systems for grid-tied inverters have been proposed, including the control scheme design, tuning of controller parameters, and active damping of resonance frequency [20], [23], [24]. As the LCL parameters play an important role in system stability, the first step should be the proper design of the LCL filter for any method. Parameter design of the LCL filter for the GCF method based on stable region and considering delay effect is scrutinized in [25]. In [26], a design procedure has been proposed for LCL with multi-tuned traps to deal with grid inductance variations. Since distributed power generation is usually used in remote places where there is not a stiff grid, the grid oscillations should be taken into account. Grid inductance variation is a common issue in such grids. The control part of the grid-connected inverter system should be able to maintain system stability against variations in grid inductance. In [27], GCF control has been adopted in a multi-parallel grid-connected inverter system, and the LCL filter parameters are design based on resonance frequency limit defined by the Bode diagram to deal with uncertain grid inductance. The main contributions of the current paper are as follows. I. Adequate tuning of LCL filter for Converter Current Control of a grid-tied PV inverter system. The tuning of the LCL filter parameters for the GCF control method has been investigated in [25]. CCF control method imposes a different stability region for resonance frequency. A tuning procedure is missing for LCL-type grid-connected inverter with the CCF control method. This study proposes a tuning method for the CCF control method considering limitations imposed by the delay effect. II. Mathematical tuning of the Proportional Resonant (PR) controller parameters for CCF control based on the tuned values of LCL parameters. PR controller performs well in tracking a sinusoidal reference. The proportional gain (KP) and resonant gain (Kr) of the PR controller should be tuned according to the intended phase margin (PM) and gain margin (GM) of the system [28], [29]. A step-by-step tuning of the PR controller for GCF control with capacitor current active damping has been proposed in [17], [18]. In this paper, the tuning of the PR controller parameters has been exclusively provided for the CCF control method. III. A power conditioning system considering for high-quality power injection of grid-connected Modified-Y-Source PV inverter system. Different power conditioning systems have been proposed to provide high-quality power in the grid-connected PV systems. International Journal of Industrial Electronics, Control and Optimization . © 2020 IECO. 367 In [10], the power conditioning system is designed for gridconnected trans-Z-source PV inverter. In [17], the power conditioning system has been improved by utilizing the LCL filter with GCF control. In this paper, the power conditioning system is designed based on incremental conductance maximum power point tracking (MPPT) algorithm and CCF control method. In the rest of the paper section 2 is devoted to the tuning of the LCL filter. Principles of Modified-Y-Source converter is investigated in section 3. The tuning of the PR controller parameters is discussed in section 4. The other remaining parts include design examples and simulation results. And the last section concludes the paper. II. LCL FILTER IN CONVERTER CURRENT CONTROL METHOD A. LCL Filter in the Grid Tied Inverter Systems A grid-connected PV inverter system is shown in Fig. 1, which includes PV arrays, an inverter, LCL filter, and the grid. The solar energy is absorbed by PV arrays, and the power is converted to AC form by the inverter. The LCL output filter conditions uinv to be qualified enough to be injected into the grid. The resonance frequency in the LCL filter is calculated by the following equation: