Andreas Giannakis

Renewable energy implementation in residential-scale grid-connected systems is already popular, and the growth of such systems is increasing every year. Grid-connected solar systems are relatively simple to install and operate; in addition, such power plants are relatively inexpensive compared to other renewable sources and grid-connection options. However, most residential grid-connected system owners believe that they will become independent from a distribution system operator (DSO) once a renewable energy source (RES) becomes available. As a rule, after installation, without DSOs, grid-connected systems are typically shut down due to a faulty grid, despite RES availability; therefore, such RES grid-connected system owners at later phases aim to become at least partially independent of DSOs. After such a change, the importance of developing a grid-tied inverter connection to an off-grid system will increase. While developing such a system connection the main issue analyzed in this paper is to control the excess power without affecting the accumulated energy levels in a system. As a result, a solution for such a system structure is presented with its main advantage to control excess power without affecting the accumulated energy level.

Redundant and backup energy source is crucial in both household and industry. Energy storage systems are already used extensively to provide solutions to various issues such as grid frequency stabilization and electrification of remote areas. In addition to that, they can cover potential excessive energy demand when required. One of the most important parts of battery energy storage systems is power electronic converter that is used for the power transmission from the batteries to the grid and vice-versa. A typical power electronic converter topology is the multilevel converter with independent sources. This topology has several advantages compared to other topologies utilized in battery energy storage systems, such as, lower harmonic distortion of the output waveforms and battery balancing which can be obtained in natural way. This paper investigated and presents a modified multilevel converter with independent sources - multilevel converter with unfolding stage and voltage regulators.

The main obstacle for the further development and commercialization of solid-state DC circuit breakers is the high ON-state power losses caused by the active power semiconductor devices. This paper presents an experimental evaluation of the electrical ON-state performance among several commercial high-power semiconductor device technologies rated at 1200 V and 1700 V at elevated temperatures. In addition, the potential of reducing ON-state losses by applying the maximum gate voltage, namely overdriving, has been assessed. It is shown that under nominal gate voltages, the normally-ON silicon carbide junction-field-effect transistor exhibits the lowest ON-state losses for both voltage classes, as well as at both temperatures. By using the overdriving concept, the ON-state voltage of silicon insulated-gate bipolar transistors has been minimized up to 10%. In addition to that, both the silicon carbide metal-oxide-semiconductor field effect transistors and normally-ON junction-field-effect transistors experience voltage reduction up to 16% and 33% respectively when overdriving, at elevated junction temperatures.

In this study, an innovative approach to matrix-converter-based AC-–AC variable frequency drives (VFDs) is introduced. The possibility of using AC–AC matrix VFDs for reactive power compensation in conventional AC–DC–AC VFD-loaded power distribution lines is investigated. It is found that the interaction of a large number of conventional AC–DC–AC VFDs with a conventional capacitor-based local compensation device leads to overcompensation in 0.4 kV power distribution lines. This is due to the fact that the conventional compensation device is designed to compensate the lagging reactive power produced by inductive loads, such as AC motors. This highlights the demand for the compensation of leading reactive power that is not predicted by the designer. To solve this problem, the modification of a certain number of previously installed VFDs by replacing their conventional AC–DC–AC converters with AC–AC matrix converters is proposed. This can lead to improvements in the power factor in 0.4 kV power distribution lines. In this study, the range of reactive power produced by conventional AC–DC–AC VFDs was determined mathematically, by simulation, and experimentally. The range of reactive power produced by the novel AC–AC matrix VFD was also determined. On that basis, the number of VFDs to be modified is defined to keep the power factor close to unity.

Solid-state DC breakers is the ultimate technology for protecting medium-voltage DC grids having equipment vulnerable to excessive fault-current stress, due to the very short fault-clearing times. A typical implementation of such breakers is based on insulated gate bipolar transistors (IGBTs). However, as the rated voltage of the grid increases, series-connection of IGBTs is inevitable and impose challenges on transient and steady-state blocking voltage distribution. This paper presents a novel voltage-balancing scheme that combines passive resistive-capacitive-diode (RCD) snubber circuits and a gate driver containing a gate-current balancing magnetic-core. The proposed concept aims at minimizing the required snubber capacitances, which may be high. The impact of the proposed configuration on the breaker design and its performance are studied under different fault scenarios. Using the proposed breaker configuration, the snubber capacitance has been reduced by up to 60%. Besides, both simulations and experimental results reveal the mitigation of high peak voltage stress across the early turned-OFF IGBTs under gate signal propagation delays.

This paper presents a design and performance evaluation of three different overvoltage suppression circuits employed in a solid-state circuit breaker for medium- to high-power and low- to medium-voltage DC grids. The evaluated criteria are the requirements for passive components, as well as the breaker performance in terms of peak switch overvoltage, peak short-circuit current, fault clearance time and rate of voltage rise during the fault clearing process. Additionally, the impact of the stray inductance in the solid-state breaker design on the anticipated overvoltage is also investigated. Design and operating limitations of each configuration from an application perspective are also discussed. From simulations of a 1.8kV and 500A breaker for high-power medium-voltage DC applications it is shown that the most effective overvoltage suppression is achieved by means of using a metal-oxide varistor and a snubber capacitor connected in parallel with the main switch at a cost of high fault current. A medium-power solid-state DC breaker prototype rated at 1.5kV and 50A was also designed and constructed. The experimental results reveal that using only a metal-oxide varistor can be sufficient when the breaker employs high-voltage discrete semiconductors with long current falling time and is used in medium-power medium-voltage grids.

Today, the advanced technology is a part of the everyday’s life. As a result, most of the applications used require a more complex system in order to achieve a better performance. These systems have a mathematic background indicating the need of a better mathematical tool to increase the reliability of them. One of the most significant problems coming up against these systems is undoubtedly the non-linearity of the equations governing them. Herein, a linearization method is proposed and studied through intelligent control. The transformation of a non-linear system into a linear is based on fuzzy logic and more specifically on Takagi-Sugeno technique. Firstly, it is analyzed in a theoretical level followed by two examples. The fuzzy model was developed through Matlab program. Finally, the efficiency of the above method was investigated setting up various values for the under study variables and comparing the results of them with the “actual” ones. The square error method was used for a better evaluation indicating that this method is a useful technique except from the applications where the high accuracy is mandatory.

The present study deals with the combined operation of a doubly-fed induction generator (DFIG) with sensorless based control, which is common to wind energy conversion systems (WECSs) and an energy storage system (ESS). The scope of this paper is to present a coordinative control system to enhance the DFIG ability to recover from a fault condition. Despite the numerous benefits of the sensorless control, this approach suffers from high sensitivity to the dynamics effects. Herein, a three-phase short-circuit is considered as a significant and most common dynamic effect. The newly protection method - combined with the classical protection methods, i.e. crowbar, breakers - refers to a battery based energy storage system connected to the DC link of the back to back converter through a fuzzy-controlled bidirectional DC/DC converter. The aforementioned configuration aims to mitigate the DC bus voltage fluctuations during grid faults and subsequently to ensure the seamless operation of the overall system. The set-up was modeled via Matlab/Simulink and the simulation results obtained, validated the effectiveness of the proposed control system.