Mario Lopez

MMC based SST

The Modular Multilevel Converter (MMC) is a type of DC-AC electronic power converter suitable for HVDC applications, thanks to its modularity and to the symmetrical design of the cells. Cells in conventional MMCs use a capacitor in the DC link, meaning that the net power balance of the cell needs to be equal to zero. It is possible however to modify the cells of the MMC to provide the capability to transfer (inject or drain) power. The use of such cells opens several new functionalities and uses for the MMC. Cells with power transfer capability can be used to connect the medium/high voltage DC and AC ports intrinsic to the MMC, with low voltage DC/AC ports, the power transfer among ports being realized at the cell level. This results in multiport power converters, potential applications including solid state transformers (SST). This paper analyzes the design and control of multiport power converters based on MMC topologies, including their use as a SST. Topologies, control strategies and implementation issues will be covered.

Design and implementation of the control of an MMC-based solid state transformer

Implementation of the control of a Solid State Transformer (SST) is described in this paper. The SST topology considered is derived from a Modular Multilevel Converter (MMC), in which the cells have the capability to transfer (inject or drain) power. The MMC is combined with an isolation stage formed by Dual Active Bridges (DABs) and a DC/AC power converter. The resulting modular multiport power converter can connect both high voltage and low voltage AC and DC ports, providing isolation between the high voltage and the low voltage terminals, and with full control of the power flow. Implementation of the control of this power converter is not trivial, due to the large amount of power devices and sensors involved, and to the complexity of the control algorithms. Furthermore, the need to provide isolation among the different stages adds further concerns mainly related with cost. This paper discusses the configuration, selection of the required hardware, as well as implementation aspects for the control of the proposed SST topology.

Modular Power Electronic Transformers: Modular Multilevel Converter Versus Cascaded H-Bridge Solutions

This article analyzes two modular power converter concepts, cascaded H-bridges (CHB) and modular multilevel converter (MMC) topologies, with special attention to the latter design. Both concepts have some characteristics in common and can provide the required functionalities for power electronic transformers (PETs). This analysis will cover aspects like the number of required cells, characteristics of the power devices, functionalities, and potential uses.

Operation and control of MMCs using cells with power transfer capability

Cells in conventional Modular Multilevel Converters (MMC) designs use a capacitor for energy storage. This means that the net power balance for each cell (neglecting losses) needs to be equal to zero, the MCC realizing therefore a power transfer between its DC and AC sides. This paper analyzes the design, operation and control of MMCs in which the cells have the capability to transfer (inject or drain) power. The use of such cells opens several new functionalities and uses for the MMC. On one hand, it would allow integrating elements like distributed energy storage (e.g. batteries), low-voltage/low power sources (e.g. PV) and loads at the cell level. Cells with power transfer capability can also be used connect the medium/high voltage DC and AC ports intrinsic to the MMC, with low voltage DC/AC ports at the cell level. This would result in multiport power converters, potential applications of this topology including solid state transformers (SST).

Control strategies for MMC using cells with power transfer capability

Conventional MMCs use cells which typically consist of a half-bridge and a capacitor. Due to their limited energy storage capability, the net power balance of the cells is zero (neglecting losses), the MMC therefore realizing a bidirectional power transfer between its DC and AC ports. It is possible however to provide the MMC with the capability to transfer power at the cell level. The use of such cells opens new functionalities and uses for the MMC, including integration at the cell level of distributed energy storage (e.g. batteries), low-voltage/low power sources/loads, and its operation as a multiport power converter, combining high and low voltage AC and DC ports. Existing control strategies for MMCs assume that all the cells have an identical design and operate identically. However, use of cells with power transfer capability can result in imbalances in their operation, provided that not all the cells transfer power, or that they do not transfer the same amount of power. This paper addresses the design and control of MMCs using cells with power transfer capability, with special focus on the design of suitable control strategies and on the definition of their limits of operation.

Comparative analysis of modular multiport power electronic transformer topologies

Conventional line-frequency transformers are a key element in the current power transmission system. Although they are a relatively cheap and well established technology, they are not able to provide new functionalities demanded by the power system operator. Solid State Transformers (SSTs), also called Power Electronic Transformers (PETs) are power electronics-based arrangements able to provide, in addition to the basic functions of a conventional transformer, new functionalities like harmonics, reactive power and imbalance compensation, and power flow control. This paper addresses a comparative analysis of modular PET topologies, including the popular CHB-based approach and MMC-based topologies. Criteria for the evaluation will include aspects like the number of cells required, ratings of the power devices, number and type of the PET ports and design requirements for passive elements.

Auxiliary power supply based on a modular ISOP flyback configuration with very high input voltage

This paper proposes a Flyback-based Input-Series Output-Parallel (ISOP) Auxiliary Power Supply (APS), intended to feed the control system of the cells of a Solid-State Transformer (SST). The SST topology is based on a modular Multiport Multilevel Converter (MMC). Energization of the cells auxiliary circuitry is not trivial due to the high voltages involved (tens of kV for the electric power distribution system), most of the commercially available control and driving circuitry not being usable due to the isolation requirements. It is possible to energize the control circuitry from an APS, connected to the cell capacitor voltage. However, in the SST under consideration, cells target DC voltage is in the range of 1.5kV to 2.5kV. Design of an APS capable of feeding the auxiliary circuitry from such high voltage and the required isolation is not trivial. A modular APS using autonomous Flyback converters in Continuous Conduction Mode (CCM) and based on commercial AC adapters is proposed in this paper. The solution is scalable and therefore applicable to cells with larger DC voltages.

Fault tolerant cell design for MMC-based multiport power converters

The Modular Multilevel Converter (MMC) is a promising technology for medium-high voltage DC/AC converters, being adequate for HVDC transmission systems. Among the appealing characteristics of the MMC are their modularity, and consequently their scalability, as well as the fact that there is no bulk storage element. One key aspect for the operation of the MMC is the response in the event of a short circuit in the DC link. Conventional MMC cells consist of a half-bridge and a capacitor, and have no capability to block the short circuit in the DC side, meaning that expensive and bulky circuit breakers might be needed in this case. Several fault tolerant cell designs have been proposed. However, these desings always bring an increase in the number of power devices and losses. Conventional MMC design can be enhanced to provide added functionalities, e.g. multiport power converters and solid state transformers (SST). A mean to achieve this is by providing the cells the capability to transfer power. This enhancements will imply an increase in the number of power devices and passives, as well as further complexity of the control. However, the resulting cells structures can offer new opportunities regarding fault tolerance. This paper revises the fault tolerance capability of MMCs, and analyzes the behavior of MMC-based multiport power converters in the event of faults. A new cell structure will be proposed capable of blocking the DC short circuit current, therefore protecting the power converter with reduced extra elements.1

Operation of modular multilevel converters under voltage constraints

MMCs are normally designed to operate in the linear region of the PWM. This limits the peak-to-peak phase voltage in the AC port to be lower than the DC port voltage. It is possible to increase the AC voltage beyond this limit by the use of overmodulation strategies. However, this is at the price of an increase in the harmonic content (THD) of the voltages and currents, and consequently, of a decrease of the power quality. While this type of operation is not desired in normal conditions, there are exceptional circumstances in which the MMC could be forced to operate in this mode. These would include transient anomalies, e.g. a temporary decrease of the DC port voltage or a temporary increase of the AC port voltage, or quasi-permanent conditions, e.g. the failure (and subsequent disconnection) of one or more cells in one or more arms of the MMC. Under this circumstances, the voltage margin between the DC and the AC port voltages required for the normal operation of the MMC might be lost. Consequently, the MMC should operate in the overmodulation region, or turned-off otherwise. This paper addresses the use of overmodulation techniques in MMC under voltage constraints. Under these circumstances, the MMC control should guarantee stable operation, (i.e. a controlled power transfer between the DC and AC ports with the cell voltages maintained at their target values) and minimize the distortion of the currents, and consequently the adverse effect on the power quality.