• Journal of Power Electronics
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• v.13 no.2
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• pp.296-303
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• 2013

This paper proposes a high efficiency three-phase cascaded phase shifted H-bridge multi-level inverter without DC/DC converters for grid-tied multi string photovoltaic (PV) applications. The cascaded H-bridge topology is suitable for PV applications since each PV module can act as a separate DC source for each cascaded H-bridge module. The proposed phase shifted H-bridge multi-level topology offers advantages such as operation at a lower switching frequency and a lower current ripple when compared to conventional two level topologies. It is also shown that low ripple sinusoidal current waveforms are generated with a unity power factor. The control algorithm permits the independent control of each DC link voltage with a maximum power point for each string of PV modules. The use of the controller area network (CAN) communication protocol for H-bridge multi-level inverters, along with localized PWM generation and PV voltage regulation are implemented. It is also shown that the expansion and modularization capabilities of the H-bridge modules are improved since the individual inverter modules operate more independently. The proposed topology is implemented for a three phase 240kW multi-level PV power conditioning system (PCS) which has 40kW H-bridge modules. The experimental results show that the proposed topology has good performance.

• The Transactions of the Korean Institute of Power Electronics
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• v.24 no.6
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• pp.419-427
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• 2019

Several multilevel converter topologies have been proposed and compared. The three-level (3L) neutral-point-clamped (NPC) topology is promising and widely accepted. However, this topology suffers from uneven loss distribution among switches due to its fixed switching strategy. The 3L active NPC (ANPC) topology, which exhibits improved loss distribution profile, was proposed to address this disadvantage. The 3L T-NPC topology, a hybrid configuration of 2L and 3L NPC topologies, was introduced to address not only the loss distribution problem but also the reduction in the number of switches. In the present research, the application of these three topologies in PMSG-based medium-voltage wind turbines was investigated. The power devices considered were 10 kV IGCTs. Performance was evaluated in terms of a power loss of 10 kV IGCT for each NPC topology, which is a crucial indicator of thermal behavior, reliability, cost, and lifetime of any converter. The comparison was performed using ABB make 10 kV IGCT 5SHY17L9000 and the simulation tool PLECS.

• Journal of Power Electronics
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• v.12 no.4
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• pp.567-577
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• 2012

This paper introduces a new scheme to balance the DC bus voltages of a cascaded H-bridge converter which is used as a Distribution Static Synchronous Series Compensator (D-SSSC) in electrical distribution network. The aim of D-SSSC is to control the power flow between two feeders from different substations. As a result of different cell losses and capacitors tolerance the cells DC bus voltage can deviate from their reference values. In the proposed scheme, by individually modifying the reference PWM signal for each cell, an effective balancing procedure is derived. The new balancing procedure needs only the line current sign and is independent of the main control strategy, which controls the total DC bus voltages of cascaded H-bridge. The effect of modulation index variation on the capacitor voltage is analytically derived for the proposed strategy. The proposed method takes advantages of phase shift carrier based modulation and can be applied for a cascaded H-bridge with any number of cells. Also the system is immune to loss of one cell and the presented procedure can keep balancing between the remaining cells. Simulation studies and experimental results validate the effectiveness of the proposed method in the balancing of DC bus voltages.

• Journal of Power Electronics
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• v.17 no.1
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• pp.242-252
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• 2017

This paper deals with the blocking of DC-fault current during DC cable short-circuit conditions in HVDC (High-Voltage DC) transmission systems utilizing Modular Multilevel Converters (MMCs), where a new SubModule (SM) topology circuit for the MMC is proposed. In this SM circuit, an additional Insulated-Gate Bipolar Translator (IGBT) is required to be connected at the output terminal of a conventional SM with a half-bridge structure, hereafter referred to as HBSM, where the anti-parallel diodes of additional IGBTs are used to block current from the grid to the DC-link side. Compared with the existing MMCs based on full-bridge (FB) SMs, the hybrid topologies of HBSM and FBSM, and the clamp-double SMs, the proposed topology offers a lower cost and lower power loss while the fault current blocking capability in the DC short-circuit conditions is still provided. The effectiveness of the proposed topology has been validated by simulation results obtained from a 300-kV 300-MW HVDC transmission system and experimental results from a down-scaled HVDC system in the laboratory.

• Journal of Power Electronics
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• v.16 no.5
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• pp.1725-1734
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• 2016

This paper presents a novel seven-level (7L) voltage source converter for high-power medium-voltage applications. The proposed topology is an H-bridge connection of two nested neutral-point clamped (NNPC) converters and is referred to as an HNNPC converter. This converter exhibits advantageous features, such as operating over a wide range of output voltages, particularly for 10-15 kV applications, without the need to connect power semiconductors in series; high-quality output voltage; and fewer components relative to other classic seven-level topologies. A novel sinusoidal pulse width modulation technique is also developed for the proposed 7L-HNNPC converter to control flying capacitor voltages. One of the main features of the control strategy is the independent application of control to each arm of the converter to significantly reduce the complexity of the controller. The performance of the proposed converter is studied under different operating conditions via MATLAB/Simulink simulation, and its feasibility is evaluated experimentally on a scaled-down prototype converter.

• Journal of Electrical Engineering and Technology
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• v.13 no.1
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• pp.240-249
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• 2018

Individual DC voltage balance problem is an inherent issue for cascaded H-bridge (CHB) based converter. When the CHB-based static synchronous compensator (STATCOM) is operating at zero current mode, the software-based individual DC voltage balancing control techniques may not work because of the infinitesimal output current. However, the different power losses of each cell would lead to the individual DC voltages unbalance. The uneven power losses on the local supplied cell-controllers (including the control circuit and drive circuit) would especially cause the divergence of individual DC voltages, due to their characteristic as constant power loads. To solve this problem, this paper proposes an adaptive voltage balancing module which is designed in the cell-controller board with small size and low cost circuits. It is controlled to make the power loss of the cell a constant resistance load, thus the DC voltages are balanced in zero current mode. Field test in a 10kV STATCOM confirms the performance of the proposed method.

• Journal of Power Electronics
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• v.17 no.4
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• pp.933-941
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• 2017

In this paper an improved low frequency selective harmonic elimination-PWM (SHE-PWM) technique for Cascaded H-bridge (CHB) converters is proposed. The proposed method is able to eliminate low order harmonics from the output voltage of the converter for a wide range of modulation indices. To solve SHE-PWM equations, especially for low modulation indices, a modified method is used which employs either the positive or negative voltage polarities of H-bridge cells to increase the freedom degrees of each cell. Freedom degrees of the switching angles are also used to increase the range of available solutions for non-linear SHE equations. The proposed SHE methods can successfully eliminate up to $25^{th}$ harmonic from a 7-level output voltage by using just nine switching transitions or a 150 Hz switching frequency. To confirm the validity of the proposed method, simulation and experimental results have been presented.

• Journal of Energy Engineering
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• v.14 no.1
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• pp.37-45
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• 2005

The fuel cell system is one of very useful energy sources. The system has advantages as renew-able and environmental sources. To obtain AC electricity from fuel cells, inverters are necessary. A multilevel converter is used as an inverter for a high power fuel cell system. Through harmonic analysis, it is shown that the harmonic components and THD increase while fundamental component decreases as voltage sag increases. To solve the voltage sag problems, three different approaches are investigated in this paper; installation of a boost converter at the fuel cell output, control of pulse widths, and use of ultracapacitors. The proposed three approaches are analyzed and compared using simulation and experimental results.