Multilevel Converter Topologies for STATCOMs

This paper presents a new converter topology for voltage source converter based high voltage dc application, named modular embedded multilevel converter (MEMC). MEMC is based on a three level structure and consists of series-connected IGBT bridge stacks, like the popular modular multilevel converter (MMC) and series-connected thyristor stacks. The IGBT bridge stacks are leveraged to generate the output the voltage waveforms and to commute the thyristors. The basic operation principle and the methods to commutate the thyristors are explained in detail. Due to the three level structure, the total number of IGBT stacks are reduced by half compared with MMC, leading to much lower energy storage, weight, volume and system complexity. Also by replacing part of the IGBT stack in MMC with thyristor stack, both of the system conduction loss and switching loss can be further reduced. The simulation results proved the analysis and verified the feasibility and benefits of the proposed converter topology.

This paper discusses a novel isolated three-phase AC-DC converter using Modular Multi-level Converter (MMC) topology for a medium voltage application. The MMC at a primary stage of an isolated AC-DC converter can directly convert a three-phase AC into a high-frequency AC voltage of 1 kHz. In this paper, the features of the proposed circuit are discussed according to the voltage ratings of the switching devices. Moreover, the total harmonic distortion (THD) of the input current, the output voltage, the number of cells and the capacitance of the DC capacitor are evaluated on each switching devices in the proposed system. Finally, the proposed system performance with SiC MOSFET of 3.3 kV is demonstrated by the simulation. As a result, the proposed converter achieves the low THD of the input current waveform which is approximately 2.64%.

In this paper, a high gain modular multilevel dc-dc converter topology is proposed. As compared with the conventional modular multilevel step-up converters, the proposed converter offers an additional degree of control-freedom and higher steady-state voltage gain. The proposed converter operates in continuous conduction mode. A detailed small-signal modeling for the proposed converter based on state-space average modeling is presented. In addition, this study covers both the small-signal and steady-state analysis for the high gain modular multilevel dc-dc converter operating in continuous conduction mode. The performance of the proposed converter is compared with conventional modular multilevel dc-dc converter. To confirm this design, simulations have been carried out on a 1 kW, 48 V to 288 V converter system. Finally, the validity of the converter is verified by simulation results.

Multilevel converter topologies have shown greal potential in high power converters with advantages including high efficiency and low distortion, and could provide the high power density necessary in new space and weight-constrained applications. A comparison of the passive component requirements, in terms of stored energy, of both flying capacitor multilevel converters (FCMCs) and modular multilevel converters (MMCs) is presented. The two topologies have different advantages, and their respective power densities. For low switching/modulation frequency ratios the MMC is favorable whereas the FCMC can yield higher power density in applications where the ratio can be large, e.g. in dc-dc, line-frequency converters and variable speed drives.

The Modular Multilevel Converter (MMC) topology is getting popularized for the medium to high voltage applications after its inception. This paper presents a modified topology of the conventional MMC, named as Hybrid MMC (HMMC). This topology has reduced number of switch counts than the conventional MMC. Therefore, it is more efficient and has less complex circuitry than other existing multi-level topologies. In this paper a modified phase-shifted carrier pulse width modulation scheme (PS-PWM) is used for the switching of HMMC. The three-phase circuit is simulated in the Matlab/Simulink for different load conditions and the results are provided to validate the concepts behind the topology.

The Modular Multilevel Converter (MMC) is a consolidated attractive technology taking great importance in medium / high power applications compared to other topologies of multilevel converters. One of the main lines of research is on voltage modulation techniques and its influence on the converter performance. Specifically for a relatively high number of submodules (SMs) that may depend on the specific application type. In this paper, a brief overview of the MMC basic operation is presented and the modulation techniques based on switching frequency as the main parameter are studied. Also, three different modulation techniques are studied to later be performed in Matlab/Simulink, in simulation is considering two cases of MMC systems with 5 and 20 SMs/arm, in order to comparing the impact of modulation technique on MMC performance.

This paper presents a modular multilevel cascade converter (MMCC) topology based on a resonant high-frequency link, which is applicable for transformerless medium-voltage motor drives requiring a wide range of operation frequency. The today's most practical topology for medium-voltage motor drives proposed in 1990s, contains a bulky and costly phase-shifted multi-winding transformer. On the other hand, existing MMCC topologies based on double or triple star-configured cascade voltage-source converters can incarnate transformerless motor drives, however being subject to limitation of the operation frequency due to the interference between the inter DC-voltage balancing control and the output-voltage control. This paper proposes an MMCC topology based on a resonant high-frequency link, which is characterized by controlling high-frequency zero-sequence voltage and current via the resonant link for the inner DC-voltage balancing control between the supply- and motor-side converters. Simulation results obtained from a 6.6-kV 800-kVA model confirm the validity and effectiveness of the proposed MMCC topology.

The shunt active filters are based on power converters driven to generate a current waveform injected in parallel to the system in order to compensate harmonics or to supply reactive power. Several topologies have been studied based either on voltage-source converters (VSC) or currentsource converters (CSC), from the standpoint of power topologies and from the point of view of control strategies. The topology-based on a CSC has advantages especially in variable load operations, but the losses appear as a disadvantage. This work presents a shunt active filter based on a multilevel current source topology where the total DC link current is divided into each power module. The proposed control scheme solves the problem of the DC link current equalization typical of the current source multilevel topologies while minimizes the DC link conducting losses.

This paper proposes a modified topology of modular multilevel converter (MMC) to accelerate the DC fault ride-through speed. The modified topology mainly consists of a half bridge sub-module (HBSM) and a modified series double submodule (MSDSM). The difference between the series double submodule (SDSM) and the MSDSM is a damping resistance which is used to increase the capability of the energy dissipation when DC fault occurs. This improvement can disperse the energy to dissipate. Moreover, the selection of resistance is analyzed. In addition, an improved pre-charging strategy based on this modified topology is presented to shorten the start-up time of MMC. At the same time, the value of additional limiting resistance can be lowered because the damping resistances of the MSDSMs are more fully utilized. Simulation results demonstrate the validity of the proposed topology and pre-charging strategy.

Multilevel converters have gained popularity in both medium voltage and low voltage applications. To find out the connections between various multilevel voltage source converter topologies and reveal how to obtain new topologies, this paper has presented four methods to derive multilevel converter topologies. Many existing topologies as well as new topologies can be derived with the methods presented in this paper. The fundamental characteristics of the multilevel converters which determine their usability such as dc-link neutral point voltage balancing and flying capacitor voltage control are also investigated in the paper. It is expected that further topologies will be invented based on the work in the paper for emerging applications.

In this paper a generalized control approach for modular multilevel converters is applied to a converter connecting a three-phase and a five-phase system directly without any central DC link. This new 3 × 5 modular multilevel matrix converter offers an alternative to other multilevel converters applied to five-phase machines, but foremost acts as example to prove the generality of the control. The cascaded control structure consists of an inner current control and an outer branch energy balancing control. Both are systematically derived and verified by simulation.

This paper proposes a modular multilevel converter (MMC) topology, designed particularly for load balancing in 50 Hz railway applications. The asymmetrical character of the traction load requires higher device rating for a Double-star or Delta-connected MMC compared to a symmetrical static compensator. This is due to the symmetrizing components to rebalance the capacitor voltages in the submodules. In this paper a reduced chain-link MMC topology, consisting of a two phase-leg converter, is proposed in order to avoid overrating the semiconductors in the main current path. In addition, the main circuit is equipped with dc-dc dual half-bridge (DHB) converters that connect the submodule capacitors of the two legs. The DHBs are operating as “Power Channels,” enabling energy exchange to restore the power imbalance among the capacitors. Simulation results show that the proposed converter is capable of achieving load balancing under any operating conditions, while the balance of the capacitor voltage is ensured.

This paper presents the modular multilevel converter (MMC), which is a new topology of voltage source converter (VSC). The converter is characterized by a modular arm structure consisting of cascade connection of half-bridge sub-module (SM) with floating dc capacitors. The topology and modulation strategy are analyzed. The applications of MMC are HVDC, FACTS and so on, which have higher and higher voltage. More and more SMs are connected in the bridge, so nearest-voltage-level (NVL) modulation strategy is taken in this paper. The transient mathematical model for MMC in dq0 reference frame is developed, and the corresponding decoupled controllers are designed to control the active and reactive powers respectively. In order to testify the proposed controllers, simulations are implemented with the PSCAD/EMTDC.

This paper presents a study on capabilities of High Voltage Direct Current connection (HVDC) based on Voltage Source Converter (VSC) technology in Modular Multilevel Converter (MMC) topology to enhance transient stability of a large power system in fast and large voltage fluctuation events. A fast voltage support (FVS) control is equipped to the standard control of the MMC-VSC station. The use of FVS control together with active/reactive power (P&Q) coordination control to maximize the capability of MMC-VSC-HVDC in supporting system voltage is analyzed. Furthermore, the performance of MMC-VSC station control operated under a special operational mode, STATCOM mode, during short circuit in DC grid is as well discussed and highlighted.

This paper demonstrates the interoperability of an emerging alternate arm converter (AAC) with the state-of-the-art modular multilevel converter (MMC) in high-voltage direct current (HVDC) systems based on a hybrid VSC-HVDC system. The paper also showcases the parameter derivation of the hybrid HVDC system and its detailed control structure. The study provides preliminary steps towards detailed analysis of AAC interoperability in complex hybrid dc grid configurations. A detailed set of results based on the 800 MVA hybrid voltage source converter (VSC)-HVDC system showcases the interoperability performance of the AAC under different operating scenarios and verifies its associated control functions.