The Alternate Arm Converter: A New Hybrid Multilevel Converter With DC-Fault Blocking Capability

This paper explains the working principles, supported by simulation results, of a new converter topology intended for HVDC applications, called the alternate arm converter (AAC). It is a hybrid between the modular multilevel converter, because of the presence of H-bridge cells, and the two-level converter, in the form of director switches in each arm. This converter is able to generate a multilevel ac voltage and since its stacks of cells consist of H-bridge cells instead of half-bridge cells, they are able to generate higher ac voltage than the dc terminal voltage. This allows the AAC to operate at an optimal point, called the “sweet spot,” where the ac and dc energy flows equal. The director switches in the AAC are responsible for alternating the conduction period of each arm, leading to a significant reduction in the number of cells in the stacks. Furthermore, the AAC can keep control of the current in the phase reactor even in case of a dc-side fault and support the ac grid, through a STATCOM mode. Simulation results and loss calculations are presented in this paper in order to support the claimed features of the AAC

Energy-based control of a DC Modular Multilevel Converter for HVDC grids

© 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other worksDC-DC converters are expected to interconnect High Voltage Direct Current (HVDC) grids with different configurations or voltage levels, which will increase reliability and operational flexibility. The DC Modular Multilevel Converter (DC-MMC) is suggested as a potential non-isolated topology for HVDC applications. This paper presents an energy-based closed-loop control for the DC-MMC. Such control structure provides a full control of the current components, while ensuring energy balancing in normal and fault operation. First, the converter configuration is described and the fundamental equations are presented. Then, the normal operation of the converter is explained from the steady state conditions and the energy and current components. A closed-loop control is designed to ensure a proper dynamic response, balance the internal energies of the converter and minimize the AC circulating current. Also, the control structure is modified to ensure DC fault ride through capability. Simulation results in Matlab Simulink are used to validate the converter control under normal operation and DC faults.Postprint (author's final draft

DC side and AC side cascaded multilevel inverter topologies: A comparative study due to variation in design features

This paper presents a comparative study between DC side and AC side cascaded topologies for the hybrid modular multilevel converter (MMC) which are becoming popular in recent years. A multilevel converter with half or full bridge sub modules connected across DC link is another alternative for high-voltage applications as it has the same number of sub modules and footprint as AC side cascaded topology with the same DC link voltage and AC side voltage. The compared AC side cascaded structure offers a two-level converter as the high voltage stage and cascaded H-bridge (which is full bridge) sub modules with electrically isolated DC sources or capacitors for the low voltage stages which has number of features suitable for HVDC application. The comparison aspects are investigated against 6 different converter sub module number and configuration options for losses, harmonic profile of the output voltage, and the DC fault current characteristics (before blocking the IGBT gate signals during the DC fault) with the same input DC voltage and the same load for both (DC and AC side) topologies. The major results and findings of this investigation are presented, compared and discussed

DC/DC converters for high voltage direct current transmission

High Voltage Direct Current (HVDC) transmission has to date mostly been used for point-to-point projects, with only a few select projects being designed from the outset to incorporate multiple terminals. Any future HVDC network is therefore likely to evolve out of this pool of HVDC connections. As technology improves, the voltage rating, at the point of commission, of the these connections increases. Interconnection therefore requires the DC equivalent of the transformer, to bridge the voltage levels and create a multi-terminal network. This thesis investigates new potential DC/DC converter topologies, which may be used for a range of HVDC applications. Simple interconnections of new and legacy HVDC links is unlikely to require a large voltage-step, but will be required to transfer a large amount of power. As the HVDC network develops it may become feasible for wind-farms and load-centres to directly connect to the DC network, rather than requiring new and dedicated links. Such a connection is called an HVDC tap and is typically rated at only a small fraction of the link's peak capacity (around 10\%). Such taps would connect a distribution voltage level to the HVDC network. DC/DC converters suitable for large-step ratios (>5:1) may find their application here. In this work DC/DC converters for both small and large step-ratios are investigated. Two approaches are taken to design such converters: first, an approach utilising existing converter topologies is investigated. As each project comes with a huge price-tag, their reliability is paramount. Naturally, technology that has already proven itself in the field can be modified more readily and quickly for deployment. Using two modular multilevel converters in a front-to-front arrangement has been found to work efficiently for large power transfers and low step-ratios. Such a system can be operated at higher than 50 Hz frequencies to reduce the volume of a number of passive components, making the set-up suitable for compact off-shore applications. This does however incur a significant penalty in losses reducing the overall converter efficiency. In the second approach DC/DC converter designs are presented, that are more experimental and would require significantly more development work before deployment. Such designs do not look to adapt existing converter topologies but rather are designed from scratch, purely for DC/DC applications. An evolution of the front-to-front arrangement is investigated in further detail. This circuit utilises medium frequency (>50 Hz) square current and voltage waveforms. The DC/DC step-ratio is achieved through a combination of the stacks of cells and a transformer. This split approach allows for high-step ratios to be achieved at similar system efficiencies as for the front-to-front arrangement. The topology has been found to be much more suitable for higher than 50 Hz operation from a losses perspective, allowing for a compact and efficient design.Open Acces

Modified half-bridge modular multilevel converter for HVDC systems with DC fault ride-through capability

One of the main challenges of voltage source converter based HVDC systems is DC faults. In this paper, two different modified half-bridge modular multilevel converter topologies are proposed. The proposed converters offer a fault tolerant against the most severe pole-to-pole DC faults. The converter comprises three switches or two switches and 4 diodes in each cell, which can result in less cost and losses compared to the full-bridge modular multilevel converter. Converter structure and controls are presented including the converter modulation and capacitors balancing. MATLAB/SIMULINK simulations are carried out to verify converter operation in normal and faulty conditions

Modular multilevel converter with modified half-bridge submodule and arm filter for dc transmission systems with DC fault blocking capability

Although a modular multilevel converter (MMC) is universally accepted as a suitable converter topology for the high voltage dc transmission systems, its dc fault ride performance requires substantial improvement in order to be used in critical infrastructures such as transnational multi-terminal dc (MTDC) networks. Therefore, this paper proposes a modified submodule circuit for modular multilevel converter that offers an improved dc fault ride through performance with reduced semiconductor losses and enhanced control flexibility compared to that achievable with full-bridge submodules. The use of the proposed submodules allows MMC to retain its modularity; with semiconductor loss similar to that of the mixed submodules MMC, but higher than that of the half-bridge submodules. Besides dc fault blocking, the proposed submodule offers the possibility of controlling ac current in-feed during pole-to-pole dc short circuit fault, and this makes such submodule increasingly attractive and useful for continued operation of MTDC networks during dc faults. The aforesaid attributes are validated using simulations performed in MATLAB/SIMULINK, and substantiated experimentally using the proposed submodule topology on a 4-level small-scale MMC prototype

Full Bridge MMC Converter Optimal Design to HVDC Operational Requirements

This project is funded by RTE, Paris, FrancePeer reviewedPostprin

Design and implementation of 30kW 200/900V LCL modular multilevel based DC/DC converter for high power applications

This paper presents the design, development and testing of a 30kW, 200V/900V modular multilevel converter (MMC) based DC/DC converter prototype. An internal LCL circuit is used to provide voltage stepping and fault tolerance property. The converter comprises two five level MMC based on insulated gate bipolar transistors (IGBTs) and metal oxide semiconductor field effect transistor (MOSFET). Due to low number of levels, selective harmonic elimination modulation (SHE) is used, which determines the switching angles in such a way that third harmonic is minimized whereas the fundamental component is a linear function of the modulation index. In addition, instead of using an expensive control board, three commercial control boards are embedded. This is required to implement the sophisticated DC/DC converter control algorithm. Simulation and experimental results are presented to demonstrate the converter performance in step up and down modes

Series Chain-link Modular Multilevel AC/DC Converter (SCC) for HVDC Applications

Introduction of the Modular Multilevel Converter (MMC) has enabled the exploitation of Voltage Source Converters (VSCs) in an increasing number of High Voltage Direct Current (HVdc) applications. Subsequently, some new topologies and solutions have been presented to tailor the MMC concept to specific uses. Particular attention has been paid to reduction of the converter footprint for applications where plant size is a critical economic aspect, for example, in off-shore installations. This paper introduces a new series connected modular multilevel AC/DC converter, the Series Chain-link Converter (SCC), which gives a significant reduction in the required number of submodules (SMs) and a more compact distribution of the energy storage, compared to an MMC. In the paper, the operating principle of the converter and its design are discussed in detail; the sub-module count and energy storage requirement are also given. The basic control loops required for the practical operation of the converter are presented and designed. The SCC concept has been experimentally validated on a small-scale 450V DC, 415V ac, 4.5kVA laboratory prototype, confirming the practical viability of the topology

Modular multilevel converter based LCL DC/DC converter for high power DC transmission grids

This paper presents a modular multilevel converter (MMC) based DC/DC converter with LCL inner circuit for HVDC transmission and DC grids. Three main design challenges are addressed. The first challenge is the use of MMCs with higher operating frequency compared to common transformer-based DC/DC converters where MMC operating frequency is limited to a few hundred hertz due to core losses. The second issue is the DC fault response. With the LCL circuit, the steady state fault current is limited to a low magnitude which is tolerable by MMC semiconductors. Mechanical DC circuit breakers can therefore be used to interrupt fault current for permanent faults and extra sub-module bypass thyristors are not necessary to protect antiparallel diodes. Thirdly, a novel controller structure is introduced with multiple coordinate frames ensuring zero local reactive power at both bridges in the whole load range. The proposed controller structure is also expandable to a DC hub with multiple ports. Detailed simulations using PSCAD/EMTDC are performed to verify the aforementioned design solutions in normal and fault conditions