• Journal of Power Electronics
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• v.18 no.2
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• pp.407-417
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• 2018

A novel family of Buck mode three-level direct ac converters with a high frequency link is proposed. These converters can transfer an unsteady high ac voltage with distortion into a regulated sinusoidal voltage with a low THD at the same frequency. The circuit configuration is constituted of a three-level converter, high frequency transformer, cycloconverter, as well as input and output filters. The topological family includes forward, push-pull, half-bridge, and full-bridge modes. In order to achieve a reliable three-level ac-ac conversion, and to overcome the surge voltage and surge current of the cycloconverter, a phase-shifted control strategy is introduced in this paper. A prototype is presented with experimental results to demonstrate that the proposed converters have five advantages including high frequency electrical isolation, lower voltage stress of the power switches, bi-directional power flow, low THD of the output voltage, and a higher input power factor.

• The Transactions of the Korean Institute of Power Electronics
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• v.26 no.3
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• pp.205-213
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• 2021

Due to the advantage of reducing the voltage applied to the switch semiconductor, the input series and output parallel combination is widely used in systems with high input voltage and large output current. On the other hand, the LLC converter is widely used as a high-efficiency power converter, and when connected by ISOP combination, there is a possibility that input voltage imbalance may occur due to a mismatch of passive devices. To avoid damaging the switching device, this study analyzed the DC-link voltage imbalance of a high-capacity supply using an ISOP LLC converter. In addition, the case where DC-link unbalance control was applied and the case not applied was analyzed respectively. Based on this analysis, an initial start-up algorithm was proposed to prevent input power semiconductor device damage due to DC-link over-voltage. The effectiveness of the proposed algorithm has been verified through simulations and experiments.

• Journal of Power Electronics
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• v.9 no.5
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• pp.679-690
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• 2009

The paper presents a new photovoltaic inverter for stand-alone induction motor application. The proposed system is composed of two stages. First stage is for the photovoltaic dc power feeding and second stage is dedicated to the motor-inverter subsystem and control technique. A direct torque control (DTC) with a novel switching strategy for motor torque ripple minimization is introduced. The novel DTC strategy is based on selecting a suitable voltage vector group for motor torque ripple minimization. A three-level voltage source inverter (VSI) is used instead of a two level inverter because the first has more available vectors and lower ripples in the output current and flux than the second, thus it has lower torque ripples. The photovoltaic array and battery bank are sized and the configuration is indicated based on sun-hour methodology. Simulation results show a comparison between three systems; two level VSI with conventional DTC strategy, three level VSI with conventional DTC, and the proposed system that has a novel DTC switching strategy applied to three level VSI. The results show that the proposed system has lower ripples in the current, flux and torque of the motor.

• The Transactions of the Korean Institute of Power Electronics
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• v.27 no.5
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• pp.446-454
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• 2022

This study proposes a carrier-based pulse width modulation (PWM) method for leakage current reduction and neutral point (NP) current control in a three-phase three-level converter, which is a carrier-based PWM version of the previously proposed low-frequency common mode voltage PWM. Three groups of space vectors with the same common mode voltage are used. When the averaged NP current needs to be positive or negative, the specific groups are employed to produce low-frequency common mode voltages. The validity of the proposed PWM method is verified through experiments.

• Journal of Power Electronics
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• v.13 no.5
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• pp.909-918
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• 2013

This paper proposes a coordinated control of the reactive power between the STATCOMs (static synchronous compensators) and the grid-side converters (GSC) of wind farms equipped with PMSGs (permanent-magnet synchronous generators), by which the voltage fluctuations at the PCC (point of common coupling) are mitigated in the steady state. In addition, the level of voltage sags is reduced during grid faults. To do this, the GSC and the STATCOM supply reactive power to the grid coordinately, where the GSCs are fully utilized to provide the reactive power for the grid prior to the STATCOM operation. For this, the GSC capability of delivering active and reactive power under variable wind speed conditions is analyzed in detail. In addition, the PCC voltage regulation of the power systems integrated with large wind farms are analyzed for short-term and long-term operations. With this coordinated control scheme, the low power capacity of STATCOMs can be used to achieve the low-voltage ride-through (LVRT) capability of the wind farms during grid faults. The effectiveness of the proposed strategy has been verified by PSCAD/EMTDC simulation results.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.13 no.4
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• pp.134-141
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• 1999

Ths paper describes on the stability improvement of output voltage control on the power suwJy for railway. On the transient states such as input voltage sudden change, the inpJt and output voltage beccxre unstable by L-C resonance occurred due to constant output voltage control. In this paper, the new clamping circuit for system stability is proposed, and control method using band attenuated filter and feed-forward terms is introduced. The propoesd damping circuit is composed with sma1l size R-L. Also, the 3 level PWM method is adopted to decrease distortion of output voltage. The output voltage is controlledl with variation under 10% without oscillation at transient states and have total hanmnic distortion under 3%.der 3%.

• Proceedings of the KIPE Conference
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• 2019.07a
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• pp.315-316
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• 2019

This paper proposes the operation scheme and control method for a four-level hybrid half-bridge flying-capacitor inverter (4L-HHBFCI). With in-phase disposition level-shifted modulation (IPD), the flying capacitor voltage ripple is less than 1% of the reference value, while the line-to-line voltage total harmonic distortion is 23.27% at unity modulation index. The performance and effectiveness of the proposed inverter operation have been verified by simulation results.

• The Transactions of the Korean Institute of Power Electronics
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• v.26 no.3
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• pp.227-236
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• 2021

This research proposes a modeling and controller design of a three-level boost (TLB) converter for the implementation of maximum power point tracking (MPPT) in the photovoltaic power conditioning system (PCS). Contrary to the output voltage control of the conventional controller, the Photovoltaic PCS requires an input voltage controller for MPPT operation. A TLB converter has the advantage of decreasing the inductor size and increasing efficiency compared with the existing booster converter. However, an optimal controller is difficult to design due to the complexity of the TLB operations, which have two operational modes on the duty ratio boundary of 0.5. Therefore, the unified linear model equations of the TLB converters, which can be applicable to both operational modes, are derived using linearized solar cell expressions. Furthermore, the transfer functions are obtained for the controller design. The MPPT voltage controller is designed using MATLAB SISOTOOL. In addition, a controller for capacitor voltage unbalancing is described and designed. The simulations and experimental verifications are conducted to verify the effectiveness of the small-signal analysis and control system design.

• Journal of Power Electronics
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• v.18 no.5
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• pp.1458-1469
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• 2018

This paper proposes a novel common-mode voltage (CMV) elimination (CMV-EL) method based on model predictive control (MPC) to eliminate CMV for three-level T-type inverters (3LT2Is). In the proposed MPC method, only six medium and one zero voltage vectors (VVs) (6MV1Z) that generate zero CMV are considered as candidates to perform the MPC. Moreover, the influence of dead-time effects on the CMV of the MPC-based 6MV1Z method is investigated, and the candidate VVs are redesigned by pre-excluding the VVs that will cause CMV fluctuations during the dead time from 6MV1Z. Only three or five VVs are included to perform optimization in every control period, which can significantly reduce the computational complexity. Thus, a small control period can be implemented in the practical applications to achieve improved grid current performance. With the proposed CMV-EL method, the CMV of the $3LT^2Is$ can be effectively eliminated. In addition, the proposed CMV-EL method can balance the neutral point potentials (NPPs) and yield satisfactory performance for grid current tracking in steady and dynamic states. Simulation and experimental results are presented to verify the effectiveness of the proposed method.

• Journal of Power Electronics
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• v.14 no.5
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• pp.946-956
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• 2014

Cascaded H-Bridge (CHB) converters can be directly connected to medium-voltage grids without using transformers and they possess the advantages of large capacity and low harmonics. They are significant tools for providing grid connections in large-capacity renewable energy systems. However, the reliability of a grid-connected CHB converter can be seriously influenced by the number of power switching devices that exist in the structure. This paper proposes a fault-tolerant control strategy based on double zero-sequence voltage injection and DC voltage optimization to improve the reliability of star-connected CHB converters after one or more power units have been bypassed. By injecting double zero-sequence voltages into each phase cluster, the DC voltages of the healthy units can be rapidly balanced after the faulty units are bypassed. In addition, optimizing the DC voltage increases the number of faulty units that can be tolerated and improves the reliability of the converter. Simulations and experimental results are shown for a seven-level three-phase CHB converter to validate the efficiency and feasibility of this strategy.