A buck-boost ac-ac converter with inverting and non-inverting operations is proposed. It compensates both the voltage sag and swell when used as a dynamic voltage restorer. Its basic switching cell is a unidirectional buck circuit, owing to which it has no shoot-through concerns. It achieves safe commutation without using RC snubbers or soft commutation strategies. Further, it can be implemented with power MOSFETs without their body diodes conducting, and for current freewheeling external diodes of good reverse recovery features can be used to minimize the reverse recovery issues and relevant loss. The detailed theoretical analysis and experimental results of a 300-W prototype converter are provided.

A modular multilevel converter (MMC) is regarded as a promising topology in high-voltage direct current (HVDC) systems. However, the MMC consists of numerous sub-modules (SMs) and switching devices, which lead to a considerable switching loss, and increased cost and size of the heat sink. To mitigate these issues, this paper presents a novel modulation method composed of fundamental frequency modulation (FFM) and multi-carrier-based sinusoidal pulse-width modulation (MSPWM) schemes. The main purpose of this modulation method is the reduction of switching loss while maintaining good harmonic performance. However, the FFM scheme leads to an unbalancing problem of each SM in the MMC. Accordingly, this paper additionally proposes the rotation method and selective voltage balancing control for SMs to ensure effective performance of the method. Simulation and experimental results verify the effectiveness and performance of the proposed modulation scheme through switching loss and spectral analyses.

Low-voltage direct current (LVDC) distribution has attracted attention due to increased DC loads, the popularization of electric vehicles, energy storage systems (ESS), and renewable energy sources such as photovoltaic (PV). This paper studies a ±750 V bipolar DC distribution system and applies a 3-level neutral-point clamped (NPC) AC/DC converter for LVDC distribution. However, the 3-level NPC converter is fundamental in the neutral-point (NP) imbalance problem. This paper discusses the NP balance control method using zero-sequence voltage among various solutions to solve NP imbalance. However, since the zero-sequence voltage for NP balance control is limited, the NP voltage cannot be controlled to be balanced when extreme load differences occur. To maintain microgrid stability with bipolar LVDC distribution, it is necessary to control the NP voltage balance, even in an imbalance of extreme load. In addition, due to the bipolar LVDC distribution, the pole where a short-circuit condition occurs limits the short current until the circuit breaker operates, and a pole without a short-circuit condition must supply a stable voltage. Since the conventional 3-level NPC AC/DC converter alone cannot satisfy both functions, an additional DC/DC converter is proposed, analyzed, and verified. This paper is about a 3-level NPC AC/DC converter system for LVDC distribution. It can be used for the imbalance and short-circuit condition in bipolar LVDC distribution through the prototype of the 300 kW 3-level NPC AC/DC converter system and experimented and verified in various conditions.

Low-voltage direct current (LVDC) distribution systems have been evolving into interesting ways of integrating distributed energy resources (DERs) and power electronics loads to local distribution networks. In LVDC distribution systems, voltage regulation is one of the most important issues, whereas AC systems have concerns such as frequency, power factor, reactive power, harmonic distortion and so on. This paper focuses on a voltage control method for a LVDC distribution system based on the concept of multi-agent system (MAS), which can deploy intelligence and decision-making abilities to local areas. This paper proposes a distributed power flow analysis method using local information refined by local agents and communication between agents based on MAS. This paper also proposes a voltage control method by coordinating the main AC/DC converter and multiple DERs. By using the proposed method, we can effectively maintain the line voltages in a pre-defined normal range. The performance of the proposed voltage control method is evaluated by case studies and compared to conventional methods.

As a proportion of generation using renewable energy increases in a power system, the need for facility investment is increasing in not only the distribution system but also the transmission system. However, it is inefficient to invest in new equipment to deal with the short peak output of renewable energy generation. For this reason, this paper proposes a method for operating the grid more flexibly, through an optimal operating strategy of the direct current (DC) line at medium-voltage (MV) level, thereby delaying the facility investment in the transmission line. In order to determine the optimal operating point of the converter connected with a DC line, the swarm intelligence-based optimization technique, which can minimize multi-objective function, is used. In addition, in order to overcome the communication dependency of the centralized control, which needs to receive information about the optimal operating point computed by the system operator, this paper proposes a decentralized emergency control (DEC) method in case of a communication fault.

With the rapid expansion of renewable energy and digital devices, there is a need for direct current (DC) distribution technology that can increase energy efficiency. As a result, DC distribution research is actively underway to cope with the sudden digitization and decentralization of load environment and power supply. To verify the possibility of DC distribution, Korea Electric Power Corporation (KEPCO) Research Institute made a DC distribution system connected with a real power system in Gwangju. The construction of the demonstration area mainly includes design of protection and grounding systems, operating procedures of insulation monitoring device (IMD), and construction of power converters. Furthermore, this paper goes beyond the simulation and the lab testing to apply DC distribution to a real system operation in advance. It is designed as a long-distance low-loaded customer for rural areas and operated by the DC distribution. In addition, safety and reliability are confirmed through field tests of DC distribution elements such as power conversion devices, protection and grounding systems. In particular, to improve the reliability of non-grounding system, the insulation monitoring device was installed and the algorithms of its operational procedures are proposed. Finally, this paper analyzes the problems caused by operating the actual DC distribution and suggests solutions accordingly.