Special Issue on Electric Power Applications

The continuing trend toward greater electrification in consumer, commercial, industrial, and transportation applications promises a dynamic and increasingly important role for power electronics. The term ‘power electronics’ refers to electric and electronic circuits whose primary function is to process energy. The growing penetration of power electronics in energy systems requires attention, with its principal challenges being cost reduction and reliability. Power electronic systems are indispensable parts of modern engineering applications, covering a wide range of engineering branches.

This Special Issue of Applied Sciences entitled “Electric Power Applications” includes 16 published papers, confirming the scientific community’s interest in this area. The topics of interest for the call included modern applications of electricity, power electronics converters, electric motor drives, renewable energy systems, power filters, distributed power, and power grid equipment. A summary of the contributions in this Special Issue can be found below.

In [1], a current closed-loop control strategy based on an improved QPIR (quasi-proportional integral resonant) controller is presented for a three-phase LCL grid-connected inverter. The proposed controller enables the independent control of active power and reactive power without coupling between the α axis and β axis in a two-phase static coordinate system. Compared with traditional controllers, the improved QPIR control strategy has higher grid-connected current control accuracy under the condition of stable and fluctuating grid frequencies while also showing a good dynamic response.

Since the number of distribution terminal units (DTUs) integrated in power systems is gradually increasing, it is necessary to reduce the fault incidence of DTU devices and improve the efficiency of fault elimination. This was the aim in [2], where the authors propose a DTU fault analysis method using an association-rule-mining algorithm. The practicality of the method is demonstrated by experiments using a realistic DTU fault database, allowing for the conclusion that DTU fault incidence can be reduced, and fault elimination ability can be enhanced.

The authors of [3] explored the simultaneous operation of the imbalance-netting process (INP) and the cross-border activation of regulating reserves (CBRR), proposing a function for correction power adjustment to prevent undesirable simultaneous activation. Extensive dynamic simulations were carried out in a testing system with three control areas to evaluate the impact on the frequency quality, load-frequency control, and performance. The results confirm that the simultaneous operation of the INP and CBRR reduces the balancing energy and decreases the unintended exchange of energy, thereby improving load frequency and performance.

The authors of [4] developed an adaptive and scalable protection coordination (ASPC) system for overcurrent relays (OCRs) in distributed-generator-integrated distribution networks consisting of two performance stages. The first stage determines the min–max confidence interval of the fault current for different fault types, while in the second stage, three common algorithms (Particle Swarm Optimization, the Gravitational Search Algorithm, and a genetic algorithm) are used to find the optimal setting parameters of the OCRs that satisfy all the coordination constraint conditions. A real 22 kV distributed-generator-integrated distribution network was used as a testbed to validate the relay coordination results obtained by the ASPC system.

The influence of an inverter-interfaced distributed generation (IIDG) unit’s response during transmission network faults is analyzed in [5]. An improved power-based fast fault current (FFC) concept is proposed to adjust fault current injection according to the required active and reactive power. This proposal is compared with the current-based FFC concept in different 110 kV networks with loop and radial topologies and for different short-circuit capacities of the aggregated network supply.

In the field of the maintenance of electric power system devices and networks, time-based maintenance (TBM) is still widely used. Recently, many companies have gradually been introducing condition-based maintenance and its upgraded version, called reliability-centered maintenance (RCM), which considers the reliability of the entire system and not only individual elements. An RCM model is developed and tested for a transmission substation in [6], where the planning and actual performance of maintenance are carried out using an optimization algorithm. The novelty of this paper consists in its integrated treatment of all maintenance processes that are included in the pre-processing phase and used in the optimization process for reliability-centered maintenance. The proposed RCM optimization algorithm is tested and compared with a previous TBM in an existing Slovenian substation, resulting in the conclusion that RCM allows for the maintenance of the reliability and technical condition of the equipment and components while reducing maintenance costs.

The extension of the Direct Torque Control (DTC) strategy to symmetrical five-phase induction machines with distributed windings in normal and faulty operation is described in [7]. Virtual voltage vectors were used to nullify the stator voltage and current components in the x–y plane. The experimental results obtained under healthy and faulty conditions (one and two open phases) show that the speed, torque, and flux references were successfully tracked in all cases. This led the authors to conclude that DTC is viable and can be extended to the case of multiphase drives, particularly when the required control goals are robustness, low computational cost, and a natural fault-tolerant capacity.

In [8], the authors solve the issues of optimal supply and the purchase of reactive power in the IEEE 30-bus power system when considering voltage stability and reducing total generation and operational costs. A modified version of the artificial bee colony (MABC) algorithm is used to solve optimization problems. Additionally, its results are compared with those of an artificial bee colony algorithm, the particle swarm optimization algorithm, and a genetic algorithm. The optimization results prove that the proposed MABC algorithm has lower active power loss and reactive power cost and a better voltage profile than the other algorithms.

In [9], the impact of installing photovoltaic solar panels on diesel–electric trains is analyzed in order to improve energy efficiency and reduce greenhouse gas emissions. The energy produced by the solar panels is quantified and compared with that produced by an auxiliary diesel generator during six different journeys worldwide under various solar resource potentials and meteorological weather conditions. The results reveal that the minimum annual fuel reduction of auxiliary generators provided by the solar panels was above 50% in all cases, demonstrating the feasibility of the proposal.

A new type of converter for power factor correction applications, named Independent Double-Boost Interleaved Converter (IDBIC), is analyzed in [10]. It is based on three voltage levels at the output combined with interleaved operation at the input and high voltage gain. The proposed converter was tested through simulation and experimentation to highlight the overall impact of the solution, with the authors concluding that the total harmonic distortion and power factor improve near the converter-rated power. The authors also observed that the converter has better performance in terms of power quality at higher voltage gain, while its efficiency is lower at a low supply voltage. According to the authors, the proposed solution can be successfully applied to electric vehicles, high-power electrical traction, rapid-battery-charging applications, and chemical electrolysis processes.

The research in [11] provides a theoretical design for transformer winding resistance under short-circuit conditions. The damping, stiffness, and mass parameters between windings are comprehensively considered, and a classical “mass-spring-damping” axial vibration mathematical model of transformer windings is established, yielding a more detailed and refined mode shape structure compared with the previous V-type and M-type mode shapes. In addition, the authors analyze the displacement and acceleration distributions of the model when a transformer with different damping ratios is short circuited at different frequencies.

In [12], the authors propose a fault diagnosis method based on a gradient-boosting decision tree (GBDT) to improve the efficiency of the protection systems in an intelligent substation. This method presents a higher fault diagnosis accuracy compared with the existing methods based on recurrent neural networks and random forest algorithms. Additionally, the GBDT algorithm also performs very well when faced with inadequate data (e.g., false alarms of fault information and multiple faults), allowing the authors to conclude that it can play a better role in practical applications.

In [13], a novel method is introduced to facilitate the selection process of the generators in a ship power plant. This method uses different parameters related to the life-cycle costs of engines and the operating routines of ships along with other cost factors such as flat annual cost, maintenance, personnel, or the acquisition and installation costs. The results provide valuable insight into the total cost from every aspect and present the optimum generator for minimal expenditure and a maximum return of investment.

The state-of-the-art technology for energy harvesting (EH) used in wireless sensors that acquire real-time electrical data in transmission lines is reviewed and analyzed in [14]. Although EH systems for transmission lines are reviewed therein, many other applications could potentially benefit from introducing wireless sensors with an EH capacity, such as power transformers, distribution switches, or low- and medium-voltage power lines, among others. The authors conclude that, among the different existing energy harvesting methods, there is not a universal solution since the most suitable EH system depends on the characteristics of each application, such as the geographical location, the intensity of the current flowing in the line, or the line’s voltage.

Since renewable generators are usually connected to an electrical power system through power electronic converters lacking natural responses to frequency variations, [15] focuses on proposing a solution based on advanced controls that allow such generators to participate in frequency control. Particularly, this paper studies the benefits of introducing power reserve control in photovoltaic generators and extended optimal power-point-tracking control in wind generators to provide frequency control in low-inertia power systems and for the interactions between them. The results show that these control systems help in stabilizing system frequency thanks to the cooperative action of both types of renewable generators.

In [16], a novel triple phase shift (TPS) closed-loop control scheme of a dual active bridge (DAB) LCC resonant DC/DC converter is presented to perform the unity power factor operation of wireless charging at an optimized rectifier AC load resistance. The primary-side inverter phase shift angle regulates the battery charging current/voltage, while the secondary side rectifier phase shift angle controls the rectifier AC load resistance to match its optimized settings. Finally, the inverter-to-rectifier phase shift angle is set to achieve the unity power factor operation of a DAB rectifier and inverter. Simulation analyses and experimental tests were carried out in a small power laboratory experimental setup to verify the proposed TPS closed-loop control scheme.