Alireza Nami

Modular Multilevel Converters for HVDC Applications: Review on Converter Cells and Functionalities

In this paper, the principle of modularity is used to derive the different multilevel voltage and current source converter topologies. The paper is primarily focused on high-power applications and specifically on high-voltage dc systems. The derived converter cells are treated as building blocks and are contributing to the modularity of the system. By combining the different building blocks, i.e., the converter cells, a variety of voltage and current source modular multilevel converter topologies are derived and thoroughly discussed. Furthermore, by applying the modularity principle at the system level, various types of high-power converters are introduced. The modularity of the multilevel converters is studied in depth, and the challenges as well as the opportunities for high-power applications are illustrated.

A Hybrid Cascade Converter Topology With Series-Connected Symmetrical and Asymmetrical Diode-Clamped H-Bridge Cells

A novel H-bridge multilevel pulsewidth modulation converter topology based on a series connection of a high-voltage diode-clamped inverter and a low-voltage conventional inverter is proposed in this paper. A dc link voltage arrangement for the new hybrid and asymmetric solution is presented to have a maximum number of output voltage levels by preserving the adjacent switching vectors between voltage levels. Hence, a 15-level hybrid converter can be attained with a minimum number of power components. A comparative study has been carried out to present high performance of the proposed configuration to approach a very low total harmonic distortion of voltage and current, which leads to the possible elimination of the output filter. Regarding the proposed configuration, a new cascade inverter is verified by cascading an asymmetrical diode-clamped inverter, in which 19 levels can be synthesized in output voltage with the same number of components. To balance the dc link capacitor voltages for the maximum output voltage resolution as well as synthesize asymmetrical dc link combination, a new multi-output boost converter is utilized at the dc link voltage of a seven-level H-bridge diode-clamped inverter. Simulation and hardware results based on different modulations are presented to confirm the validity of the proposed approach to achieve a high-quality output voltage.

Voltage-sharing converter to supply single-phase asymmetrical four-level diode-clamped inverter with high power factor loads

The output voltage quality of some of the single-phase multilevel inverters can be improved when their dc-link voltages are regulated asymmetrically. Symmetrical and asymmetrical multilevel diode-clamped inverters have the problem of dc-link capacitor voltage balancing, especially when power factor of the load is close to unity. In this paper, a new single-inductor multi-output dc/dc converter is proposed that can control the dc-link voltages of a single-phase diode-clamped inverter asymmetrically to achieve voltage quality enhancement. The circuit of the presented converter is explained and the main equations are developed. A control strategy is proposed and explained in details. To validate the versatility of the proposed combination of the suggested dc-dc converter and the asymmetrical four-level diode-clamped inverter (ADCI), simulations and experiments have been directed. It is concluded that the proposed combination of introduced multioutput dc-dc converter and single-phase ADCI is a good candidate for power conversion in residential photovoltaic (PV) utilization.

Five level cross connected cell for cascaded converters

Proposed here is an alternate Five-level four-quadrant cascaded multilevel converter cell configuration that compared to the other cell configurations, for dc fault current limitation, will be more compact and avoid the external dc breaker. Loss comparison on cells with dc fault blocking capability for the cascaded converter is also presented.

Comparison between symmetrical and asymmetrical single phase multilevel inverter with diode-clamped topology

In this paper, a different configuration based on different DC bus voltage for a diode-clamped multilevel inverter has been presented. Two different symmetrical and asymmetrical arrangements of a four-level diode clamped inverters have been compared, in order to find an optimum arrangement with lower switching losses and optimised output voltage quality. The optimised asymmetrical arrangement has been compared with a conventional four- level inverter. The comparison results show that an asymmetrical configuration can obtain more voltage levels in output voltage with same number of component compared with the conventional four-level inverter and this will lead to the reduction of harmonic content of output voltage. A predictive current control technique has been carried out to verify the viability of new configuration. The advantages of this control method are simplicity and applicability for n-level multilevel inverters, without a significant change in the control circuit.

Current source modular multilevel converter for HVDC and FACTS

A current source modular multilevel converter (MMC) is proposed for high voltage AC/DC power conversion applications, such as HVDC and FACTS. Current source converters possess the advantage of short-circuit fault tolerance, which is a pivotal feature for grid applications. By partially following the circuit duality transformations, the proposed converter is derived from the well-known voltage source MMC. Inductor-based current source cells are connected in parallel and form a current source arm that can synthesize a desired current waveform. By adding a reduced-energy capacitor in parallel to each current source arm, these arms can be further connected in series, thereby allowing voltage scaling. By using fully controllable switches, the converter is capable of providing full control on its active and reactive power. Protection schemes against open-circuit failures inside the inductor cells are also proposed. Simulation results show the operation of the current source MMC and its capability of DC fault tolerance.

Analysis of modular multilevel converters with DC short circuit fault blocking capability in bipolar HVDC transmission systems

This paper analyzes the station-internal phase-to-ground fault in bipolar HVDC transmission systems. An overvoltage problem due to the existence of the bipolar cells in the modular multilevel converter (MMC) arms closer to the grounding pole are presented. Consequently, a new hybrid arm MMC is proposed to overcome the overvoltage problem while providing the benefits of: a lower number of cells, fewer switching devices and lower conduction losses. Guidelines are developed and confirmed by simulation results to determine the required number of cells to block the DC side fault.

Tolerance Band Adaptation Method for Dynamic Operation of Grid-Connected Modular Multilevel Converters

The use of modular multilevel converters (MMC) in high-voltage direct current (HVdc) transmission systems has grown significantly in the past decade. The efficiency, cell capacitor voltage ripple, and dynamic performance are three contradictory aspects of the MMC which are related to the converter switching scheme. Previously introduced tolerance band (TB)-based schemes enable efficient and simple control for grid-connected MMCs. This paper addresses the dynamic operation of TB switching schemes by proposing a dynamic boundary setting technique for steady-state operation and a switching scheme scheduling controller for transient fault handling. The performance of proposed methods are validated in a realistic point-to-point HVdc link, modeled in real-time digital simulator where two converters with 512 cells per arm are implemented. Utilizing the proposed methods will enable efficient implementation of TB-based schemes for different operating points, and also a robust transient fault handling.

Multilevel Converters in Renewable Energy Systems

In photovoltaic, fuel cells and storage batteries, the low output DC voltage should be boosted. Therefore, a step-up converter is necessary to boost the low DC voltage for the DC link voltage of the inverter. The main contribution of this chapter is to electrical energy conversion in renewable energy systems based on multilevel inverters. Different configuration of renewable energy systems based on power converters will be discussed in detail. Finally, a new single inductor Multi-Output Boost (MOB) converter is proposed, which is compatible with the diode-clamped configuration. Steady state and dynamic analyses have been carried out in order to show the validity of the proposed topology. Then the joint circuit of the proposed DC-DC converter with a three-level diode-clamped converter is presented in order to have a series regulated voltage at the DC link voltage of the diode-clamped inverter. MOB converter can boost the low input DC voltage of the renewable energy sources and at the same time adjust the voltage across each capacitor to the desired voltage levels, thereby solving the main problem associated with capacitor voltage imbalance in this type of multilevel converter.

A Novel Random Hysteresis Current Control for a Single-phase Inverter

Abstract Although a hysteresis current control technique is a simple and a reliable modulation scheme in switching control systems, its harmonic spectrum around the switching side band may not distribute properly. Random pulse width modulation technique is an effective scheme that distributes the harmonic spectrum of the load current, and it can reduce the acoustic noise and the mechanical vibration of inverter-fed motor drives. This paper presents a new method for hysteresis current control based on random band control for a better spreading of the harmonic spectrum of the load current. In this paper, the effect of the random-band hysteresis current control is analysed to determine the best hysteresis band variation and the results have been compared with the traditional hysteresis current control. Theoretical analysis and simulations have been performed to describe the method.