Ricardo Lizana

Circuit Topologies, Modeling, Control Schemes, and Applications of Modular Multilevel Converters

Modular multilevel converters have several attractive features such as a modular structure, the capability of transformer-less operation, easy scalability in terms of voltage and current, low expense for redundancy and fault tolerant operation, high availability, utilization of standard components, and excellent quality of the output waveforms. These features have increased the interest of industry and research in this topology, resulting in the development of new circuit configurations, converter models, control schemes, and modulation strategies. This paper presents a review of the latest achievements of modular multilevel converters regarding the mentioned research topics, new applications, and future trends.

Model Predictive Control of an AFE Rectifier With Dynamic References

In this paper, a finite control set model predictive controller for closed-loop control of an active front-end rectifier is presented. The proposed method operates in discrete time and does not require additional modulators. The key novelty of the control algorithm presented lies in the way dynamic references are handled. The control strategy is capable of providing suitable references for the source active power and the rectified voltage, without requiring additional control loops. Experimental results show that fast and accurate tracking of dynamic dc voltage and reactive power references can be achieved, while respecting the restrictions on maximum power levels of the rectifier.

Modular multilevel cascaded converter based on current source H-bridges cells

Modular multilevel cascaded converter emerges as an important alternative in high voltage DC transmission systems, due to its high reliability and power quality. This converter is composed by several identical cells whose are, in turn, composed by a chopper or an H-bridge and a floating DC capacitor. This paper proposes a modular multilevel converter with a novel cell topology based on a current source H-bridge. The use of this cell could improve the input and output power quality and, due to the IGCT current and voltage ratings, reduce the number of cells in series to achieve the required DC voltage and at the same time increase the transmitted power. Analysis and simulation of the proposed topology and its control are given.

Decoupled Current Model and Control of Modular Multilevel Converters

Modular multilevel converters offer several benefits, such as high scalability and power quality, which are particularly advantageous for high-voltage dc transmission systems. However, the control of such converters is very challenging due to the number of control objectives to be achieved simultaneously. The input and output currents must be controlled at their own references, and these references must be properly generated in order to keep the average capacitor voltage constant. The circulating current can be controlled either to minimize losses or to reduce capacitor voltage ripple. Additionally, the capacitor voltage must be kept balanced among cells during the converter operation. In this paper, a model with four independent dynamical components of the arm currents, which also considers the effect of ac and dc systems, is proposed. The proposed model facilitates the dynamical analysis of currents and simplifies the design and implementation of current controllers. Analysis of the proposed current model and experimental results, which confirm the performance of the designed controllers, is shown.

Control of Arm Capacitor Voltages in Modular Multilevel Converters

Modular multilevel converters have several attractive features such as a modular structure, high voltage capability and scalability, excellent quality of the output voltage waveforms, as well as, low expense for redundancy. However, the main drawback of this converter is the complexity of the control system due to the multiple control objectives which include the input, output, circulating current and the capacitor voltages. Several control schemes for the capacitor voltages have been proposed, being usually divided into the control of the average voltage and the balancing of the capacitor voltages among the submodules and among the arms. These control schemes can effectively control the capacitor voltage when the reference is the same for all arms, but they cannot manage arbitrary voltage references when each arm capacitor voltage must be controlled independently as required, for example, to achieve maximum power point tracking in photovoltaic applications. This paper proposes a control strategy for a modular multilevel converter capable to manage the average capacitor voltage of each arm independently. The proposed scheme is based on a decomposition of the energy of the arms in different components, according to theirs symmetries. Results validating the performance of the proposed control scheme are shown and the impact on the output current is also discussed.

Capacitor voltage balance of MMC converters in bidirectional power flow operation

Modular multilevel cascaded converter offers several advantages for high voltage DC transmission systems such as high modularity and power quality. However, they also present an interesting challenge from the control point of view, due to its coupled current dynamics and the voltage balance of floating capacitors. This work presents a voltage balance scheme which does not affect the input nor the output voltages and currents, and its dynamics is independent on the power operating point. Simulations results show a good dynamic of the voltage balance under changes in the DC voltage reference and reversion on the power flow.

Modular multilevel converter for large-scale multistring photovoltaic energy conversion system

This paper presents a new grid connected photovoltaic energy conversion system configuration for large scale power plants. The grid-tied converter is based on a modular multilevel converter using voltage source H-bridge cells. The proposed converter is capable of concentrating a multimegawatt PV plant with distributed string MPPT tracking capability, high power quality and increased efficiency compared to the classic two-level voltage source converters. The main challenge is to handle the inherent power unbalances which may occur, not only between the different cells of one phase of the converter, but also between the three phases. The control strategy to deal with these unbalances is analyzed in this paper. Simulation results for a downsized 7 level MMC composed of 18 H-bridge cells and PV strings are presented to validate the proposed topology and control method.

Control of MMC-HVDC transmission system operating with local variables

The search for new energy sources has lead to the installation of renewable energy plants located far away from the centers of consumption. In this scenario, the transmission of electrical energy over long distances has become a key issue and high voltage DC (HVDC) transmission systems have demonstrated to be the best cost effective solution. Among the different HVDC technologies, modular multilevel converters (MMC) provide the highest power quality, modularity, reliability and availability. In this paper, the control of two HVDC stations based on MMC is proposed. A set of power and voltage limits are defined for each station and the control scheme is designed to work inside these limits. Due to this operation mode, the controller in one station does not require information from the other station. The different operating modes of the MMC-HVDC transmission system are analized and simulation results are obtained, in order to validate the proposed control strategy.

Modular Multilevel Converter Machine Drive using current source H-bridges

This paper proposes a Machine Drive based on a Modular Multilevel Converter. The proposed converter uses current source cells with asymmetrical IGCTs as switching devices. A control scheme to manage the input and output current and, additionally, reduce the oscillations in the inductors currents is also proposed. The resulting machine drive will exhibit high power and high dynamical performance as shown in the simulation results.

Modular Multilevel Converter based on 5-level submodule with DC fault blocking capability

The use of conventional half-bridge submodules in the Modular Multilevel Converters brings important benefits such as the reduction in the power losses and semiconductor devices. However, the system with half-bridge submodule lacks the capability of blocking DC-faults currents. In this paper, a 5-level submodule with DC-fault blocking capability is presented. The proposed submodule allows to operate with different internal capacitances in order to reduce the cost of the system. Furthermore, a control and balance strategy for the novel 5-level submodule is proposed.