There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

The modular multilevel converter (MMC) has been a subject of increasing importance for medium/high-power energy conversion systems. Over the past few years, significant research has been done to address the technical challenges associated with the operation and control of the MMC. In this paper, a general overview of the basics of operation of the MMC along with its control challenges are discussed, and a review of state-of-the-art control strategies and trends is presented. Finally, the applications of the MMC and their challenges are highlighted.

Operation, Control, and Applications of the Modular Multilevel Converter: A Review

Model predictive control (MPC) is a very attractive solution for controlling power electronic converters. The aim of this paper is to present and discuss the latest developments in MPC for power converters and drives, describing the current state of this control strategy and analyzing the new trends and challenges it presents when applied to power electronic systems. The paper revisits the operating principle of MPC and identifies three key elements in the MPC strategies, namely the prediction model, the cost function, and the optimization algorithm. This paper summarizes the most recent research concerning these elements, providing details about the different solutions proposed by the academic and industrial communities.

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Model Predictive Control for Power Converters and Drives: Advances and Trends

Modular multilevel converter (MMC) is one of the most promising topologies for medium to high-voltage high-power applications. The main features of MMC are modularity, voltage and power scalability, fault tolerant and transformer-less operation, and high-quality output waveforms. Over the past few years, several research studies are conducted to address the technical challenges associated with the operation and control of the MMC. This paper presents the development of MMC circuit topologies and their mathematical models over the years. Also, the evolution and technical challenges of the classical and model predictive control methods are discussed. Finally, the MMC applications and their future trends are presented.

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

The evolution of power electronics and its control has been mainly driven by industry applications and influenced by the development achieved in several technologies, such as power semiconductors, converter topologies, automatic control, and analog and digital electronics. Digital signal processors (DSPs), in particular, have experienced an exponential development in processing power, which until now has not been fully exploited for control purposes in power converters and drive applications. Presently, the control system technology finds itself in a paradigm-changing tipping point, in which more demanding control goals, system flexibility, and functionalities required by emerging applications are driving the control system technology development, in addition to stabilization and robustness, which was the main focus in the past. This article walks briefly through the history of the mainstream power converter control scene, with an emphasis on the more recent introduction of predictive control, and gives a glimpse on the challenges and possibilities ahead. Special attention is given to finite control set (FCS)-model predictive control (MPC), because of its simplicity, flexibility, inherent adaptation to power electronic circuits and their discrete nature, both in the finite amount of switching states and the digital implementation with microprocessors.

Model Predictive Control: MPC's Role in the Evolution of Power Electronics

This paper proposes a control method for circulating currents in modular multilevel converters (MMCs) under unbalanced voltage conditions. Under balanced voltage conditions, only negative-sequence circulating currents exist. Consequently, the conventional method has considered only negative-sequence circulating currents in MMC. However, under unbalanced voltage conditions, there are positive-sequence circulating currents, zero-sequence circulating currents, and negative-sequence circulating currents in MMC. Thus, under unbalanced voltage conditions, a control method should consider all of these components. This study adopts a dual vector current controller as an ac-side current controller to reduce the ac-side active power ripple under unbalanced voltage. It analyzes the effect of the unbalanced voltage on circulating currents in MMC and then proposes a control method considering each component of circulating currents under the unbalanced voltage. The effectiveness of the proposed controlling method is verified through simulation results using PSCAD/EMTDC.

Circulating Current Control in MMC Under the Unbalanced Voltage

This paper proposes a model predictive control (MPC) method for modular-multilevel-converter (MMC) high-voltage direct current (HVDC). To control the MMC-HVDC system properly, the ac current, circulating current, and submodule (SM) capacitor voltage are taken into consideration. The existing MPC methods for the MMC-HVDC system utilize weighting factors to configure the cost function in combinations of the SM capacitor voltage balancing algorithm, ac current control, and circulating current control. Because all combinations of the switch states are considered in order to minimize the cost function, their possible combinations increase geometrically according to the increase of the level of the MMC, which is a significant disadvantage. This paper proposes a new MPC method with a reduced number of states for ac current control, circulating current control, and the SM capacitor voltage-balancing algorithm. The proposed cost functions are divided into three types according to their control purposes. Each cost function determines the minimum number of states for controlling the ac current, circulating current, and SM capacitor voltage. The efficacy of the proposed controlling method is verified through simulation results using PSCAD/EMTDC.

Model Predictive Control With a Reduced Number of Considered States in a Modular Multilevel Converter for HVDC System

This paper proposes an enhanced control method for a high-voltage direct-current (HVDC) modular multilevel converter (MMC) To control an MMC-HVDC system properly, the ac current, circulating current, and submodule (SM) capacitor voltage are considered The ac-side current is a fundamental frequency component, and the circulating current is a double-line frequency component Existing control methods control the ac current and circulating current by separating each component However, the existing methods have a disadvantage in that the ac-side current must be separated into the positive and negative sequences for control under an unbalanced voltage condition The circulating current consists of not only negative-sequence components but also positive- and zero-sequence components under an unbalanced voltage condition Therefore, an additional control method is necessary to consider the positive- and zero-sequence components of the circulating current The proposed control method has the advantage of controlling not only the ac-side current of the MMC but also the circulating current without separating each of the current components to control each arm current of the MMC In addition, it can stably control the positive- and zero-sequence components of the circulating current under the unbalanced voltage condition

A Control Method of HVDC-Modular Multilevel Converter Based on Arm Current Under the Unbalanced Voltage Condition

Provided is a method for suppressing a circulating current in a modular multi-level converter for a high voltage direction-current (HVDC) transmission system. The HVDC transmission system converts an alternating current (AC) into a direct current (DC) and vice versa, transmits energy using a DC cable, and including a modular multilevel converter generating a high voltage source by stacking a plurality of sub-modules in series. In the circulating current suppression method, a circulating current (idiffj; j=a,b,c) of a,b,c phase in an abc 3-phase stationary reference frame, a DC current (idc) flowing in a DC cable, a current reference value (i*dc) of a DC component that needs to flow in the DC cable are inputted. The circulating current (idiffj) of the a,b,c phase is controlled to become zero. A compensation value (V*diffj) for suppressing a harmonic component of the circulating current is outputted.

Method for Suppressing Circulating Current in Modular Multilevel Converter for High Voltage Direct-Current Transmission

The present invention relates to a control device for a doubly-fed induction generator in which a feedback linearization method is enabled and further provides a control device for a doubly-fed induction generator in which a feedback linearization method is embedded, characterized in that the control device divides and measures positive sequency components and negative sequency components from stator voltage and current, rotor voltage and current, and signals of stator magnetic flux and rotor magnetic flux of the doubly-fed induction generator.

Jung-Woo Park

Papers

Circulating Current Control in MMC Under the Unbalanced Voltage

Model Predictive Control With a Reduced Number of Considered States in a Modular Multilevel Converter for HVDC System

A Control Method of HVDC-Modular Multilevel Converter Based on Arm Current Under the Unbalanced Voltage Condition

Method for Suppressing Circulating Current in Modular Multilevel Converter for High Voltage Direct-Current Transmission

Control device for doubly-fed induction generator in which feedback linearization method is embedded