Michael M.C. Merlin

University of Edinburgh | UoE · School of Engineering

Unleashing the potential of distributed renewable energy sources (RESs), intelligent and autonomous microgrids are becoming pivotal in attaining net‐zero carbon emission goals. Hybrid AC/DC microgrids rise as cutting‐edge microgrid topologies, capitalising on the best of both AC and DC systems. However, the integration of intermittent renewables an...

The micro-modular multilevel converter ( $\mu $ MMC) concept proposes a benchtop-scale, low-voltage, open-source, and affordable hardware prototype of a modular multilevel converter (MMC) intended for research and teaching applications. The $\mu $ MMC ( $\mu $ originates from its rating being approximately one millionth of usual transmission-sc...

This paper presents a delta-configured, modular multilevel, STATCOM with integrated partially-rated energy storage (ES) and submodules (SM) based on the Multi-Busbar Sub-module (MBSM) topology. The soft-paralleling of SM capacitors using the nature of diodes leads to lower SM voltage deviation and lower circulating current to be used for SM balanci...

In the line of research on inclusion of Energy Storage Systems (ESS) into HVDC stations, this paper presents the integration of partially rated ESS into Modular Multilevel Converter (MMC) substations using parallel branches to the arm inductors. The proposed ESS-MMC topology keeps the design of the original MMC unmodified, as it just adds partially...

The use of power converters as solid state transformer is an attractive solution to modernize the power network, but this solution has not been fully addressed for MV and HV substations. This paper presents a customized and simple control for the Modular Multilevel Matrix Converter (M3C), specially conceived for its operation on synchronous ports,...

A fundamental strategy for utilizing green energy from renewable sources to tackle global warming is the microgrid (MG). Due to the predominance of AC microgrids in the existing power system and the substantial increase in DC power generation and DC load demand, the development of AC/DC hybrid microgrids (HMG) is inevitable. Despite increased theor...

This paper discusses blackstart provision enabled by Modular Multilevel Converters (MMC) with integrated partially-rated Energy Storage (ES) systems. It is demonstrated that ES units integrated in an MMC for ancillary service provision can be utilised to energise the auxiliary power supply electronics of the submodules (SMs), fully charge the conve...

Energy management in modular type convertersconstitutes a key aspect of their operational stability. This paper introduces a full energy management structure for the Extended Overlap-Alternate Arm Converter (EO-AAC) ensuring both equal energy distribution across all six stacks and the maintaining of ripple-free DC current during steady state. The p...

The Alternate Arm Converter (AAC) is a 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...

The half-bridge Modular Multilevel Converter (MMC) is a Voltage Source Converter (VSC) with high efficiency, controllability and modularity. The topology is weak to DC side faults unless bipolar sub-modules are used, but this results in decreased efficiency. Power-Groups (PGs), a thyristor augmented multilevel structure, have been proposed as a way...

In multi-terminal dc networks or future dc grids, there is an important role for high step-ratio dc-dc conversion to interface a high voltage network to lower voltage infeeds or offtakes. The efficiency and controllability of dc-dc conversion will be expected to be similar to modular multi-level ac-dc converters. This paper presents a modular multi...

The Hybrid MMC, comprising a mixture of full-bridge and half-bridge sub-modules, provides tolerance to DC faults within compromising the efficiency of the converter to a large extent. The inclusion of full-bridges creates a new freedom over the choice of ratio of AC to DC voltage at which the converter is operated, with resulting impact on the conv...

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 a short overlap period) resulted in (i) substantial 6-pulse ripples in the DC current waveform, (ii) l...

Achieving DC fault tolerance in modular multilevel converters requires the use of a significant number of Sub-Modules (SMs) which are capable of generating a negative voltage. This results in an increase in the number of semiconductor devices in the current path, increasing converter conduction losses. This paper introduces a thyristor augmented mu...

The rapid development of HVDC in last decade is proceeding the DC technologies from point-to point connection to multi-terminal networks, and the family of resonant modular multilevel DC/DC converters (RMMC) has been proposed as a promising and competitive candidate for HVDC tap application in future multi-terminal connections. The main obstacle ex...

During the recent years, the number of High Voltage Direct Current (HVDC) links, used for accommodating renewable energy sources and for interconnecting different power systems, has increased significantly. Apart from enhancing system stability, HVDC links can be used to provide ancillary services, such as frequency support. This paper investigates...

Low frequency AC (LFAC) has been proposed as a means to avoid some of the large converter station costs of high voltage DC (HVDC) while delivering some of the benefits in terms of better cable utilization and its technical feasibility has been established. It is said to offer lower costs than HVDC or conventional high voltage AC (HVAC) for a range...

It is likely that any HVDC grids will evolve over time and will likely be multi-vendor and use several different technologies. This paper studies a four terminal network where the interconnecting link is operated as a normally open point. This allows the network to be reconfigured in the event of DC side fault, without the need for DC circuit break...

Multilevel converters, such as the modular multilevel converter (MMC) or the alternate arm converter (AAC), rely on charged capacitors in their cells to generate their AC voltage waveform. Since the cell capacitors are physically large and occupy approximately half the cell volume, their capacitance must be kept minimal while limiting the voltage f...

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...

A power electronic converter for use in high voltage direct current power transmission and reactive power compensation comprises three converter limbs, each converter limb including first and second DC terminals for connection in use to a DC network and an AC terminal for connection in use to a respective phase of a three-phase AC network, each con...

A power electronic converter for use in high voltage direct current power transmission and reactive power compensation includes at least one converter limb, which includes first and second DC terminals and an AC terminal. Each converter limb defines first and second limb portions connected in series between the AC terminal and a respective one of t...

DC/DC connections between DC voltage levels in High Voltage Direct Current (HVDC) schemes are useful for interconnection between links and as taps to inject/extract relatively small amounts of power. Taps with high voltage transformation ratios (such as 500 kV to 50 kV) are particularly challenging to achieve with reasonable use of semiconductor ra...

This paper presents a DC/DC converter suitable for HVDC applications requiring a high step-ratio, such as HVDC taps. A circuit and its method of operation are introduced and demonstrated through a full scale, 500 kV and 200 MW, simulation model. The paper also includes an analysis of the power losses in all major circuit components for the simulati...

The paper proposes a power electronic converter to solve the problem of voltage constrained Low Voltage (LV) distribution cables where spare thermal capacity is available. The converter utilises a high frequency AC link inverter as part of a shunt/series topology similar to that of a Unified Power Flow Controller (UPFC). The design of the converter...

A new operating mode of the Modular Multilevel Converter (MMC) using modified arm current waveforms inspired from the working principle of the Alternate Arm Converter (AAC) is presented in this paper. A reduction in the cell voltage deviation is observed at power factors close to unity at the cost of an increase in power losses, especially when rea...

Voltage ratings for HVdc point-to-point connections are not standardized and tend to depend on the latest available cable technology. DC/DC conversion at HV is required for interconnection of such HVdc schemes as well as to interface dc wind farms. Modular multilevel voltage source converters (VSCs), such as the modular multilevel converter (MMC) o...

Operation of HVDC converters under HVDC pole voltage imbalances is analysed. Asymmetrical HVDC pole current injection is achieved by directing current to the ground return path through a device installed in the AC side of the converter. Several operation modes, including asymmetric monopole, are presented and their sizing requirements are discussed...

This paper presents a method to calculate the minimal size of cell capacitors in multilevel VSCs which meets a maximum voltage deviation criterion under ideal conditions. This method is applied to the Modular Multilevel Converter (MMC), the Alternate Arm Converter (AAC) and the hybrid multilevel converter with ac-side cascaded H-bridge cells (AC-CH...

The Alternate Arm Converter (AAC) is a hybrid HVDC converter that maintains low power losses while protecting against DC-side faults. This paper shows the development of a Reduced Dynamic Model (RDM) of the AAC which enables time efficient system level modelling of multi-converter HVDC systems. The methodology and equations to develop the RDM are d...

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 a...

Power losses in high power HVDC converters are dominated by those that occur within the power electronic devices. This power loss is dissipated as heat at the junction of semiconductor devices. The cooling system ensures that the generated heat is evacuated outside the converter station but temperature management remains critical for the lifetime o...

Demand for DC/DC conversion in HV applications is expected to rise because of the increasing number of HVDC links using different DC voltage levels. This paper presents a DC/AC/DC system consisting of two VSCs connected through an inductor. The two VSCs are Alternate Arm Converters (AAC). Since the two AACs share the same AC voltage level, they can...

The large number of proposals for wind farm developments across northern Europe and the North Sea have lead to the suggestion that a European Supergrid using High Voltage DC (HVDC) would be beneficial. Grid formats of HVDC will use Voltage Source Converters (VSCs) with a fixed voltage polarity and power flow controlled through current magnitude and...

This paper looks at DC to DC conversion for high power applications. After having briefly reviewed the domain, a resonant bidirectional DC/DC converter already suggested in the literature is closely studied using a novel mode of operation, dubbed Alternate Discontinuous Mode. This operating mode alternates the two halves of the converter in such a...

With increasing offshore wind generation, there is a strong argument for implementing a multi-terminal DC grid offshore by the interconnection of individual HVDC links. The point of intersection of three or more lines can be used to interconnect projects with different voltage levels and to control power distribution. It is being proposed that thes...

Interconnection of High Voltage Direct Current (HVDC) links into networks is key to a future HVDC grid spanning large regions. Various factors including the evolution of cable technology will lead to a variety of DC voltages and a need to provide a DC/DC step up/down at high power ratings (beyond 500 MW). This paper analyses a High Voltage (HV) DC/...

Voltage-Source Converters have brought numerous advantages to HVDC transmission. However, they suffer from high losses and are usually weak against faults on the DC-side. In this paper, a new topology which brings together some concepts from traditional Current Source Converters and multi-level converters, is presented. Two stacks of H-bridge cells...