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

Zero phase sequence voltage injection for the alternate arm converter

The Alternate Arm Converter (AAC) is a voltage source converter being developed as an alternative to the Modular Multilevel Converter (MMC) for HVDC power transmission and reactive power compensation. Each Arm of the converter contains high voltage series IGBT Director Switches and full-bridge cells, which enables the VSC to ride through AC and DC network faults. This paper describes how the AAC can be optimised by modulating the converter terminal voltages with zerophase sequence triplen harmonic components. The optimisation reduces the ratio of the number of the full-bridge cells compared to the simpler Director Switches which offers a valuable improvement in footprint and efficiency

The Extended Overlap Alternate Arm Converter:A Voltage Source Converter with DC Fault Ride-Through Capability and a Compact Design

The Alternate Arm Converter (AAC) was one of the first modular converter topologies to feature DC-side fault ride-through capability with only a small penalty in power efficiency. However, the simple alternation of its arm conduction periods (with an additional short overlap period) resulted in (i) substantial 6-pulse ripples in the DC current waveform, (ii) large DC-side filter requirements, and (iii) limited operating area close to an energy sweet-spot. This paper presents a new mode of operation called Extended Overlap (EO) based on the extension of the overlap period to 60 ◦ which facilitates a fundamental redefinition of the working principles of the AAC. The EO-AAC has its DC current path decoupled from the AC current paths, a fact allowing (i) smooth DC current waveforms, (ii) elimination of DC filters, and (iii) restriction lifting on the feasible operating point. Analysis of this new mode and EO- AAC design criteria are presented and subsequently verified with tests on an experimental prototype. Finally, a comparison with other modular converters demonstrates that the EO-AAC is at least as power efficient as a hybrid MMC (i.e. a DC fault ride-through capable MMC) while offering a smaller converter footprint because of a reduced requirement for energy storage in the submodules and a reduced inductor volume

Assessment of HVDC technologies for an offshore MTDC grid

This thesis examines various HVDC converter technologies that could be used in offshore Multi-Terminal DC (MTDC) grids. MTDC grids rely on AC/DC converters to interface with AC systems and also for control services. Two AC/DC topologies were compared, the half-bridge Modular Multi-level Converter (MMC) and the Alternate Arm Converter (AAC). As new DC system voltages emerge the DC/DC converter could be an enabling technology for interconnection and future MTDC networks. As yet there is no consensus on DC/DC converter topology and a critical comparison of several potential designs was conducted. An MMC based DC/DC converter had distinct advantages compared with other designs. Several average value converter models of the converters were developed to allow efficient simulation of MTDC networks, while maintaining a high level of accuracy of the converter characteristics. These models were verified with full switching models for steady state and fault conditions. Two o shore MTDC networks were studied; a four-terminal network, and a MTDC network. The four-terminal network used a normally open point to connect two existing point-to-point links, allowing reconfiguration in the event of a DC fault. The MTDC network uses a DC/DC converter to interconnect a bipole HVDC link with the previously studied four-terminal network. Several simulation studies show how new converters can improve the operation of a MTDC and provide additional capabilities such as DC fault blocking.Open Acces

Fault blocking converters for HVDC transmission : a transient behaviour comparison

A thorough comparison of the transient behaviours of two state-of-the-art converters suitable for HVDC transmission is presented. The Alternate Arm and Mixed-Cell Modular Multilevel Converter topologies both have DC fault blocking capability and are selected for the comparison. Converter performance is evaluated and compared under various transient conditions including charging sequence, unbalanced operation, and DC fault recovery. The study is conducted using high-fidelity converter simulation models, integrating detailed controllers that reflect real-scale projects. The main findings of the study assist in the selection of the most suitable converter, given specific performance specifications such as capacitor voltage ripple, cell capacitor requirements, and response during transient operation

The Alternate Arm Converter (AAC) - "short-overlap" mode operation - analysis and design parameter selection

This paper presents converter operation principles and theoretical analyses for “short-overlap” mode operation of the Alternate Arm Converter (AAC), which is a type of modular multilevel Voltage Source Converter (VSC) that has been proposed for HVDC transmission applications. Fourier series expressions for the ideal arm current and reference voltage are derived, for the first time, in order to develop an expression for the sub-module capacitance required to give a selected peak-peak voltage ripple of the summed sub-module capacitor voltages in an arm. The DC converter current contains non-negligible low order even harmonics; this is verified by deriving, for the first time, a Fourier series expression for this current. As the DC converter current needs to be filtered to form a smooth DC grid current, a novel DC filter arrangement is proposed, which uses the characteristics of a simplified DC cable model, as well as the capacitance of the DC link and additional DC link damping resistance, in order to form a passive low pass filter. Results obtained from a simulation model, which is based on an industrial HVDC demonstrator, are used in order to verify the presented converter operation principles and theoretical analyses

Modular Multilevel Converters in Hybrid Multi-Terminal HVDC Systems

High-voltage direct current (HVDC) systems are becoming commonplace in modern power systems. Line commutated converters (LCCs) are suitable for bulk power and ultra-HVDC (UHVDC) transmission, while with inflexible power reversal capability and possible commutation failures. However, voltage source converters (VSCs) possess flexible power reversal capability and provide immunity to commutation failures. Modular VSC topologies offer improved performance compared to conventional 2 level/3 level VSC-based HVDC. The family of modular VSCs includes the well-established modular multilevel converter (MMC) and other emerging modular VSC topologies such as the DC-fault tolerant alternate arm converter (AAC) that share topological and operational similarities with the MMC. It is noteworthy that the integration of LCC and modular VSCs leads to unique benefits despite the challenges of different HVDC configurations. Hence, it is necessary to explore the system performance of different HVDC converter topologies, especially more complex hybrid multiterminal HVDC (MTDC) systems and DC-grids combining different converters. This thesis focuses on the combination of the LCC, MMC and AAC to constitute different hybrid HVDC transmission systems. It is of significance to provide a common platform where the proper comparison and evaluation of different HVDC systems and control methods can be completed and independently validated. Therefore, this thesis also provides an overview of current HVDC benchmark models available in the existing literature. In addition, the detailed modeling methods of HVDC systems are discussed in this thesis. For ensuring the static security of HVDC systems especially the future DC-grids, this thesis proposes a generalized expression of DC power flow under mixed power/voltage (P/V) and current/voltage (I/V) droop control, considering the DC power flow for normal operation and after converter outage. Detailed simulation models are established in PLECS-Blockset and Simulink to study the hybrid HVDC/MTDC systems and DC grid combining the LCC with the MMC and (or) AAC. The detailed sets of results demonstrate the functionalities of developed hybrid HVDC systems and validate the performance of systems complying with widely accepted HVDC operating standards. The developed LCC/AAC-based HVDC/MTDC systems and LCC/MMC/AAC-based DC grid in this thesis are prime steps towards the study of more complex MTDC systems and a key element in the development of future DC super grids

Energy and director switches commutation controls for the alternate arm converter

The Alternate Arm Converter (AAC) is promising multilevel Voltage Source Converter (VSC) suitable for High Voltage Direct Current (HVDC) transmission systems. This converter exhibits interesting features such as a DC Fault Ride Through capability thanks to the use of Full-Bridge Sub-Modules (SM) and a smaller footprint than an equivalent Modular Multilevel Converter (MMC). After an analysis of the converter operating modes called Non-overlap and Overlap mode, a sequential representation of the AAC operation is proposed. The main originality of this paper is the use of the Petri Net to describe all the phases and to highlight their sequencing. According to the phases identified thanks to the sequential approach, models and control structures for the grid currents, the internal energy and the Zero Current Switching (ZCS) are detailed. Furthermore, the step-by-step approach proposed in this paper allows a clear and rigorous modelling of this complex converter

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