Leakage current reduction is one of the important issues for transformerless photovoltaic (PV) systems. Many interesting solutions have been reported to reduce the leakage current for three-phase PV ...inverters. However, most of them are limited to two-level inverters. Moreover, there is a potential risk of the overcurrent phenomenon. In order to solve the problem, a Z -source three-level four-leg inverter with a new modulation strategy is proposed in this paper. First, the mathematical modeling of the three-level four-leg Z -source inverter for leakage current reduction is established for the first time. Second, a new carrier-based modulation strategy is proposed by utilizing the effective large, medium, small, and zero vectors, instead of the invalid vectors, to achieve the constant common-mode voltage, as well as the leakage current suppression. Finally, the proposed solution is carried out on the Texas Instruments TMS320F28335 DSP + Xilinx XC3400 field-programmable gate array digital control hardware platform. The experimental results verify the effectiveness of the proposed solution.
This article presents a reduction method of circulating current in parallel three-level inverters using modified discontinuous pulse-width modulation (DPWM) based on an interleaving scheme. The ...harmonics and current ripple are the same as that of a single inverter with the same current capacity as the parallel system with DPWM. An interleaved DPWM improves the output current quality. However, a circulating current is generated by the asynchronous phase carriers. The circulating current limits the power rating. To alleviate these problems, the proposed method reduces the high-frequency circulating current with switching frequency by 79% even at a high modulation index. The switching sequence and high-frequency circulating current are analyzed to prove the performance of the proposed method. The effectiveness and reliability of the proposed reduction method are compared to the conventional SVM. The validity of the proposed method is verified through simulations and experimental results.
This paper introduces a novel bidirectional switched-capacitor converter topology with buck-boost capabilities. Unlike traditional T-type inverters that use two capacitors, the proposed topology ...utilizes only a single capacitor, reducing the number of capacitors by 50%. Normal, buck, and boost modes make the topology versatile for various applications. It can be used as a three-phase two-level or three-level inverter in all modes. A switched capacitor unit boosts the input voltage without requiring boost inductors or filter capacitors. This not only increases the DC link voltage but also boosts the power throughput without an increase in current which is a significant advantage in power electronics. The proposed topology controls switched capacitor unit inrush current with a soft-starting circuit. The buck mode operation is achieved by connecting a capacitor whose voltage is maintained at half the DC link voltage, resulting in a superior output voltage quality with reduced total harmonic distortion. In addition, a novel switching strategy maintains capacitor voltage balance during buckand boost modes along with the suppression of inrush current during steady state and improves DC link voltage stability. The proposed topology is compared to conventional and recent works in terms of device count. Finally, the proposed concept is validated experimentally and the results are found to be promising.
This article proposes an improved integrated modulation strategy of dual-parallel three-level inverters to suppress leakage current. Dual-parallel three-level inverters are widely used in several ...applications, such as in photovoltaic systems, wind power generation, and motor drive systems to increase the power capacity. The current ripple of each inverter appears greater on the load side due to the parallel operation of inverters. Thus, asynchronous modulation methods, such as interleaved or integrated modulation, have been proposed to improve the output current quality of three-level inverters. These asynchronous modulations generate a significant leakage current due to limited common-mode voltage (CMV) reduction. In this article, the proposed five-level reduced leakage current space vector modulation (RLC SVM) strategy adopts a switching sequence that lowers the CMV frequency. The proposed modulation solves problems such as insulation breakdown and electromagnetic interference by significantly suppressing the leakage current of the existing integrated modulation. Thus, the five-level RLC SVM method reduces the CMV, ZSCC, and output current distortion, as well as the leakage current. The feasibility and effectiveness of the proposed modulation method are verified based on simulations and experiments.
For neutral point (NP)-clamped three-level inverter-fed permanent-magnet synchronous motor system, the tuning process of weighting factors in the cost function of conventional model-predictive torque ...control (MPTC) is complex. Also, large imbalance of NP potential can cause the deteriorated performance of flux and torque. To solve these problems, an improved MPTC is proposed. This algorithm first uses the deadbeat principle to unify the dimension of torque and flux. Then, the NP potential term in the conventional cost function can be eliminated by introducing the partition control of NP potential imbalance, which also eliminates the whole process of tuning weighting factors. After that, under the large NP potential imbalance, sectors are dynamically divided by modifying the magnitude and phase angle of medium and small vectors to improve the performance of torque and flux linkage. The experimental results show that compared with conventional MPTC, the improved MPTC can achieve fast suppression of NP potential imbalance with reducing the fluctuation of torque, flux linkage and current when NP potential imbalance is large.
A novel soft-switched three-level T-type inverter is proposed in this article. The proposed inverter provides turning- on by zero-voltage transition and turning- off by zero-voltage switching (ZVS) ...for main switches without any voltage or current stresses. The switching energies are transferred to the snubber capacitors, so the turning- off process of the main switches is improved by these snubber capacitors. Besides, the auxiliary switches are turned on by zero-current switching and turned off by ZVS. Furthermore, all snubber diodes are operated by soft-switching (SS), and the proposed SS snubber cell decreases the electromagnetic interference noises. The novel snubber cell has a simple structure, ease of application, common-ground switches, and low-cost features. The theoretical analysis of the inverter is clarified, and the operating modes for steady-state analysis are presented in detail. The experimental results rated 1-kW output power and 100-kHz switching frequency are provided to justify the theoretical analysis. The proposed unique snubber cell can be easily implemented in other three-level inverters, and the practice is demonstrated in the article.
In this article, a duty-cycle correction-based model predictive current control (DC-MPCC) is proposed for permanent magnet synchronous motor (PMSM) supplied by a neutral-point clamped three-level ...voltage source inverter (NPC-3LVSI). Unlike the conventional MPCC, which evaluates the impact of basic voltage vectors on the concerned state variables, the proposed DC-MPCC modifies the output voltage levels with optimized duty-cycle corrections. First, the last three-phase voltage levels are assumed to be kept during the next control period. Then, the current tracking error and neutral-point potential are predicted. After that, the voltage levels are modified with zero, one, or two state changes, which formulate seven candidate solutions. Subsequently, the duty-cycle corrections of the modified voltage levels are computed based on the principle of minimizing current tracking error and neutral-point voltage drift. Finally, the optimal switch sequence is generated by evaluating and sorting a cost function with a penalty on switch actions. The proposed DC-MPCC features variable switching instants, relatively lower sampling frequency, and satisfactory performance under low switching frequency. Experimental tests carried out on an NPC-3LVSI fed PMSM drive, with 100 Hz fundamental frequency and 400 Hz switching frequency, accompanied by a video demonstration, validate the effectiveness of the proposed method.
Transformerless topologies of many topologies are widely used in photovoltaic (PV) systems because these topologies have many advantages in terms of the weight, size, and efficiency. A three-level ...inverter has an outstanding performance and is advantageous in the switching device selection than a two-level inverter. In the transformerless PV systems using the three-level inverter, the PV systems should suffer from the leakage current and generate the neutral-point voltage unbalance. To solve two problems, this paper proposes modulation techniques to reduce the leakage current and balance the dc-link voltages. The cause of the leakage current in the three-level inverter is analyzed. The proposed technique LMZVM using the large, medium, and zero vectors reduces the common mode voltage that causes the leakage current than that of the conventional PWM. Moreover, the proposed technique LMSVM using the large, medium, and small vectors balances the dc-link voltages with reduced CMV as the same in LMZVM. The effectiveness of the proposed techniques is verified by comparing its results with those of the convectional PWM. The results are obtained through simulations and experiments.
This paper proposes two direct torque control (DTC) strategies to reduce the common-mode voltage (CMV) in the fivephase induction motor driven by three-level five-phase inverter. In each technique, ...31 voltage vectors consisting of 30 nonzero voltage vectors, and a zero voltage vector are selected out of available 243 voltage vectors. The selection of these voltage vectors are based on their capabilities of reducing the CMV, eliminating the x-y stator flux and maintaining the torque ripple under control. The ten large, ten small, and a zero voltage vector are same in both schemes, however only difference in designing the control strategy for the two schemes is the utilization of different ten medium voltage vectors. First proposal (DTC-I) reduces the CMV to 3V dc /10 and second proposal (DTC-II) reduces to V dc /10. These schemes are compared with DTC technique of five-phase induction motor fed by two-level five-phase inverter in order to judge the capabilities of proposed schemes in context of reducing the CMV. The simulation and experimental results validate the proposed DTC techniques.
Neutral-point voltage unbalancing is a common problem in three-level inverters. The original neutral-point voltage balancing circuit is the buck-boost converter. Since the inductor carries low ...frequency components, the power density is limited. At the same time, the efficiency is low because the soft switching cannot be realized. In this paper, a neutral-point voltage balance circuit based on resonant switched-capacitor converter (RSCC) is proposed. The proposed converter uses a series resonant inductor and resonant capacitor to transfer the unbalanced energy between two dc-link capacitors. Based on phase shift modulation, the RSCC circuit controls the output current to compensate the neutral-point current of the inverter to achieve the suppression of neutral-point voltage ripple. To achieve the minimum loss, a step-by-step optimal design is presented and the optimal switching frequency, resonant inductance and resonant capacitance can be derived. The proposed circuit has the advantages that all switches of RSCC can realize the zero voltage switching (ZVS), and the magnetic volume can be greatly reduced because only high frequency components flow through the inductor. A 400V-10A prototype of RSCC and the inverter is built, and main experimental results are provided to verify the time-domain analysis and the effectiveness of the proposed RSCC voltage balance circuit.
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