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This paper deals with the innovative “variable-voltage” bus concept for future more-electric aircraft platforms. Using new functionalities and opportunities offered by innovative sources actively controlled by power electronics, there is an opportunity for a significant increase of their output power (within powertrain capabilities, machine thermal limits, and power converter maximum current) to supply increased load demands under certain conditions. This paper investigates applications of this concept to satisfy power demands for the wing ice-protection system for business jet. The study includes a detailed study into the control challenges of the proposed approach and suggests corresponding theoretical solutions. Furthermore, the controller design aspects are explored with considerations to achieve stable operation. The concept is tested in simulation environment and validated through experimental studies. This paper might be of interest for the researchers and engineers focused on innovative solutions for future aircraft platforms. [ABSTRACT FROM AUTHOR]

This paper develops a comprehensive dynamic-phasor-based small-signal model for the modular multilevel converter (MMC) system under unbalanced grid conditions, which takes into consideration the dynamics of the MMC system under a balanced grid condition as well as the other emerging internal harmonics produced by the unbalanced grid components and more complex controllers. The small-signal model is validated by comparing the time-domain responses from a detailed electromagnetic transient (EMT) simulation in PSCAD/EMTDC. Based on the eigen-analysis and PSCAD-based time-domain simulation studies, this paper reveals the poorly damped modes that cause unstable oscillations in MMC system under the unbalanced grid conditions. From the results, more poorly damped modes involved in the internal harmonics are emerging under unbalanced grid conditions compared with the modes under the balanced grid conditions, and can highly impact the system dynamic behaviors by the selection of the control parameters. It is also shown that, under the unbalanced grid conditions, the feasible region of the control parameters of the vector current control gets narrowed indicating the reduction of the stability margin, smaller (larger) value of proportional (integral) gain of the circulating current suppressing controller can enhance the system stability, and larger values of the proportional and integral gains of the negative-sequence current suppressing control can result in the harmonic instability of the MMC system. Besides, the parameters of multiple complex-coefficient filters for positive- and negative-sequence components extraction also have a big impact on the small-signal stability of the MMC system and are required to be properly selected. [ABSTRACT FROM AUTHOR]

An introduction is presented in which editor discusses various papers within the issue including analysis and design issues of industrial application of networked control systems (NCSs), self-triggered sampling scheme (STS) to reduce data losses, and load frequency control (LFC) for power systems.

This paper studies the optimal control problem for large-scale systems with unknown parameters and dynamics. By using robust adaptive dynamic programming (RADP) method, a decentralized optimal control design is given for large-scale systems with unmatched uncertainties. The convergence of the proposed RADP algorithm and the asymptotic stability of the closed-loop large-scale system are studied rigorously. Finally, a numerical example of a large-scale power system is adopted to illustrate the effectiveness of the obtained algorithm. [ABSTRACT FROM PUBLISHER]

More electric aircrafts (MEAs) comprise a vast amount of power electronic loads, which usually behave as constant power loads (CPLs). The incremental negative impedance of CPLs threatens system stability. To ensure an effective control of power flow in MEAs, eliminating the undesired behavior of CPLs is a necessity. This aim requires spontaneous power estimation of the time-varying uncertain loads. In this paper, an adaptive backstepping controller, which is interconnected to a third-degree cubature Kalman filter, is developed for a dc microgrid (MG) feeding nonideal CPLs. At first, the load power is considered as an artificial state and augmented into the system states, which enables estimation of not only the dc MG states but also the unknown value of the load power. The estimated load power is then forwarded to a backstepping controller. The systematic approach of this controller allows obtaining the control signal, which is the duty ratio of the switch, to not only system stabilizing but also tracking a desired voltage of the dc bus under the load power variations. The proposed adaptive controller is tested on a dc MG that has one CPL. The conducted experimental results verify the proposed nonlinear control in tracking the desired voltage of the dc bus under slow and fast variations of the load power. [ABSTRACT FROM AUTHOR]

This paper provides the concept of design, implementation, and system integration of energy efficient power management solution for a fault-tolerant more electric engine/aircraft (MEE/MEA). The power management solution consists of three components, i.e., the power management system (PMS), the stability analysis module, and the condition-based control (CBC) module. The PMS is to optimize the electrical power system (EPS) level efficiency based on constraints with consideration of power limits of the dc system in the MEE/MEA architecture, which are calculated by the stability analysis module, and system reconfiguration, which is provided by the CBC module. This paper focuses on the design and implementation of PMS algorithm for EPS of a fault-tolerant MEE/MEA architecture. A PMS solution to optimize EPS level efficiency optimization is proposed, which is based on multiple objective optimization problem (MOOP). The proposed MOOP is solved with a modified nondominated genetic algorithm with local search mechanisms. The proposed power management solution are implemented in the MEE/MEA simulation model as well as hardware-in-loop virtual testbed and results show the effectiveness of the proposed algorithm. [ABSTRACT FROM AUTHOR]

In previous research, the variable conductance of a parallel active power filter has been used for antiparallel resonance with parameter variations in power systems. Its disadvantages are many sensors, complicated harmonic detection, and complexity of the algorithm for successful accomplishment in antiparallel resonance. In order to eliminate such drawbacks, this paper presents the parallel active power filter based on source current detection for antiparallel resonance with robustness to parameter variations for power factor correction using capacitor banks. The proposed method employs only single harmonic detector and an open-loop shaping design for robustness to variations of source impedance and a sudden change of capacitor bank values for power factor correction. The proposed antiparallel resonance controller can be readily applied to both large-scale and laboratory-scale power systems without any change. The large-scale simulation and the laboratory-scale experiment results confirm the validity, the accuracy, and good performance for antiparallel resonance, harmonic suppression, and robustness to parameter variations in power systems. [ABSTRACT FROM AUTHOR]

This paper presents a compendious summary of power hardware-in-the-loop (PHIL) simulations that are used for designing, analyzing, and testing of electrical power system components. PHIL simulations are an advanced application of real-time simulations that represent novel methods, which conjoin software and hardware testing. This contribution outlines necessary requirements for the implementation of PHIL simulations, which are defined by the nature of the digital real-time simulator, the power amplifier, and the power interface (PI). Fundamental characteristics, such as the input/output systems, PI, interface algorithm, and system stability considerations, are discussed for PHIL setups, in order to illustrate both flexibility and complexity of this compound simulation method. The objective of this work is to elaborate an understandable overview of PHIL simulation for electrical power systems and to constitute a contemporary state-of-the-art status of this research area. [ABSTRACT FROM AUTHOR]

A stability study of distribution networks with a high penetration of distributed generators (DGs) actively supporting the network is presented in this paper. A possible way of mitigating the voltage rise caused by DGs is the local control of reactive power. Among the different possible options, the Q(U) control (reactive power as a function of the voltage) is one of the commonly suggested solutions. However, the Q(U) control can, under certain conditions, lead to instability. This paper summarizes the results of a stability study conducted on a single-inverter system and on a multiinverter system. It shows that the requirements for reaching a stable operation can easily be met for integrated systems but could be a significant constraint for systems relying on communication. [ABSTRACT FROM PUBLISHER]

In this paper, a framework for stability analyses of a typical inverter-based islanded microgrid with two types of nonlinear loads is presented, namely ideal constant power loads (CPLs), which are the loads supplied by tightly regulated power electronics converters, and dynamic CPLs, which are used to represent motor-drive systems with large time constants. The comprehensive dynamic model of the considered microgrid is first developed, based on which a bunch of small-signal models are deduced using Taylor expansion made at different stable operating points. Afterward, eigenvalue-theorem-based stability analysis and parametric sensitivity analysis are successively performed on the obtained small-signal models to verify the stability of the system, predict the system's unstable regions, and identify the effects of parameters on the stability boundaries. In the meantime, the impacts of different kinds of nonlinear loads on the system stability are studied. Hardware-in-the-loop (HIL) real-time simulation platform of a 30-kVA microgrid, which is mainly formed by a 10-kVA photovoltaic (PV) system, a 10-kVA wind energy conversion system, a 10-kVA lithium-ion battery energy storage system, and two CPLs, is established in Typhoon HIL 602 device. The validity of the theoretical results is verified by real-time simulation results. [ABSTRACT FROM AUTHOR]

The objective of this paper is to propose a modularized bidirectional single-phase grid-connected inverter (BGI) with constant-frequency asynchronous sigma–delta modulation (CF-ASDM) control for the renewable energy (RE)-supplied dc power system. The proposed BGI can be operated either in the inverter mode (IM) or rectified mode (RM); both are necessary features for the RE-supplied dc power system. By comparing to the conventional ASDM with variable switching frequency, the proposed CF-ASDM can simplify the filter design and can reduce the computational burden of the digital signal processor while maintaining the merit of quick dynamic response. Moreover, the proposed BGI has a redundant central controller which can increase the system reliability. The major contribution of this paper includes developing the modularized BGI with CF-ASDM control scheme for IM and RM operations thoroughly and applying it to the RE-supplied dc power system successfully. In this paper, the mathematical equations to achieve the constant switching frequency operation for the CF-ASDM will be derived. Computer simulations and hardware experiments are shown to verify the performance of the proposed modularized BGI with CF-ASDM control. [ABSTRACT FROM PUBLISHER]

This paper proposes a wavelet-based power system stabilizer (WPSS) by using both the scaling coefficient energy and the wavelet coefficient energy with border distortions of a one-cycle sliding window. The boundary scaling coefficient energy is used for real-time extraction and compensation of electromechanical oscillations in the synchronous generator, whereas the boundary wavelet coefficient energy is used for the real-time detection of electrical oscillations and for tripping the WPSS at the fault inception time. The performance of the proposed WPSS was assessed in an experimental power system laboratory setup and compared with a conventional power system stabilizer. [ABSTRACT FROM PUBLISHER]

AC cascaded system is one of the basic components of the power conversion system in the modern more-electric aircraft (MEA). However, the ac cascaded system suffers from an instability problem, i.e., even if each subsystem is stable individually, the formed cascaded system may become unstable. Most of the existing stabilization methods are a linear control method, which is based on a small signal model and can only ensure small signal stability near the operation point in theory. Therefore, these existing linear stabilization methods may be ineffective to fix large-signal stability issues. In this paper, a large signal stability constraining dichotomy solution (SCDS)-based model predictive control (MPC) is proposed to improve the stability of the ac cascaded system in the MEA by the Lyapunov criteria. This approach first theoretically presents how to add the system stability item to the MPC cost function in an analytic way instead of using the traditional “experience-based” weight method. Besides, the SCDS-MPC itself is also an improvement to the existing finite control set MPC. It can realize a constant switching frequency and achieve better steady-state/dynamic current control performance. Finally, a 1.5 kW ac cascaded system is fabricated in the laboratory to verify the proposed SCDS-MPC method. [ABSTRACT FROM AUTHOR]

In this paper, an adaptive method for tuning a proportional-resonance controller for synchronization of the grid-connected inverters is presented. In the proposed approach, the grid frequency is obtained by minimizing the error signal using a frequency-locked loop mechanism that consists of a resonant adaptive filter and a perturbation-based extremum seeking algorithm. Simulations and experimental studies are presented to demonstrate performance of the proposed controller in face of the frequency variations of an emulated grid voltage waveform. The results are compared with conventional nonadaptive methods, which indicate that performance characteristics for voltage/frequency tracking and power factor can be achieved based on the IEEE 1547 standard. [ABSTRACT FROM AUTHOR]

The eleven papers in this special section focus on power electronics-enabled autonomous systems. Power systems are going through a paradigm change from centralized generation to distributed generation and further onto smart grid. Millions of relatively small distributed energy resources (DER), including wind turbines, solar panels, electric vehicles and energy storage systems, and flexible loads are being integrated into power systems through power electronic converters. This imposes great challenges to the stability, scalability, reliability, security, and resiliency of future power systems. This section joins the forces of the communities of control/systems theory, power electronics, and power systems to address various emerging issues of power-electronics-enabled autonomous power systems, paving the way for large-scale deployment of DERs and flexible loads. [ABSTRACT FROM PUBLISHER]

This paper solves the load frequency control problem and proposes a new event-triggered sliding-mode control (SMC) algorithm in multi-area interconnected power systems. First, we consider a three-area interconnected power system, and construct its corresponding mathematical model. The prescribed \mathcal H\infty , or energy-to-energy, performance is utilized to test the effectiveness of the corresponding disturbance attenuation. Then, an appropriate discrete-time integral switching surface is designed for each subsystem, and the discontinuous controller is designed to guarantee the resulting subsystem asymptotically stable and robust. Using time-delay system analysis technique, the event-triggered scheme, sliding mode dynamics, and the network-induced delays are converted into a network dynamic system with time-varying delays. Thus, corresponding sufficient conditions are derived and an event-triggered SMC law for reaching motion is designed to ensure the reachability of the sliding mode surface in a finite time. Finally, examples and simulations are provided to illustrate the feasibility and the effectiveness of the developed event-triggered SMC scheme. [ABSTRACT FROM AUTHOR]

An artificial immune system (AIS)-based control of generator excitation systems for the U.S. Navy's electric ship is presented in this paper to solve power quality problems caused by high-energy loads such as direct energy weapons. The coordinated development of the AIS controllers mainly consists of two parts—innate immunity (optimal) and adaptive immunity. The parameters of the controllers for the former, to provide optimal performance, are determined simultaneously using particle swarm optimization. For dramatic changes in the ship's power system, adaptive control based on the immune system feedback law is developed. The feedback law adapts the controllers' parameters only during transient disturbances. After the disturbance, the controllers' parameters are restored to their innate values. A ship's real-time power system and the proposed AIS control of all excitation systems have been implemented on a real-time digital simulator and a digital signal processor, respectively. Results from the hardware-in-the-loop studies show that the AIS controllers can provide effective control of all generators' terminal voltages during pulsed loads, restoring and stabilizing them quickly. [ABSTRACT FROM PUBLISHER]

This paper deals with the design of robust regulation for both angular speed stability enhancement and voltage in synchronous generator machines with an excitation control system. The designed controller is based on the block control technique combined with a sliding-mode (SM) control approach. The block control is used to design a nonlinear sliding manifold with the desired stability property. In order to attenuate the chattering effect which can appear in the presence of the exciter's unmodeled dynamics, a high-order SM algorithm and a robust exact differentiator are employed. The efficiency of the proposed controller, i.e., robustness and high accuracy, is verified via simulations. [ABSTRACT FROM AUTHOR]

This paper concentrates on the design, control, and implementation of an LCL-filter-based shunt active power filter (SAPF), which can effectively compensate for harmonic currents produced by nonlinear loads in a three-phase three-wire power system. With an LCL filter added at its output, the proposed SAPF offers superior switching harmonic suppression using much reduced passive filtering elements. Its output currents thus have high slew rate for tracking the targeted reference closely. Smaller inductance of the LCL filter also means smaller harmonic voltage drop across the passive output filter, which in turn minimizes the possibility of overmodulation, particularly for cases where high modulation index is desired. These advantages, together with overall system stability, are guaranteed only through proper consideration of critical design and control issues, like the selection of LCL parameters, interactions between resonance damping and harmonic compensation, bandwidth design of the closed-loop system, and active damping implementation with fewer current sensors. These described design concerns, together with their generalized design procedure, are applied to an analytical example, and eventually verified by both simulation and experimental results. [ABSTRACT FROM PUBLISHER]

Load frequency control (LFC) is a very important method to keep the power systems stable and secure. However, due to the introduction of communication networks in multi-area power systems, the traditional LFC method is not effective again. This motivates us to investigate an adaptive event-triggering H\infty LFC scheme for multi-area power systems. Compared with the existing time-invariant event-triggering communication scheme, an adaptive event-triggering communication scheme is presented, where the event-triggering threshold can be dynamically adjusted to save more limited network resources, while preserving the desired control performance. Compared with the existing emulation-based method, where the controller must be known a priori, the stability and stabilization criteria derived in this work can provide a tradeoff to balance the required communication resources and the desired control performance. The effectiveness of the proposed method is verified by two numerical examples. [ABSTRACT FROM PUBLISHER]

This article develops a novel decentralized controller for multi-area secondary load frequency control (LFC) of power systems. The proposed robust-adaptive approach estimates the external disturbance input and uncertainties, which are assumed to be matched with the control input. In this regard, a disturbance-observer state-feedback controller is designed. To offer a systematic approach, controller design conditions are derived in terms of linear matrix inequality constraints. Thereby, for any given representation of the LFC area, the controller gains can be straightforwardly obtained by using numerical solvers. Moreover, in order to enhance the steady-state performance, the controller is modified through the heuristic genetic algorithm. The controller design procedure is wholly offline and can be simply implemented. The developed decentralized approach does not need the information of the other areas, which reduces the cost and the number of measuring units. It is also robust against the power fluctuations of the load and renewable energy sources. To show the superiorities of the developed controller, four scenarios are considered. These scenarios comprise the aggregation of the photovoltaic, wind turbines, and electric vehicle to the power system. OPAL-RT experiments are given to verify the transient and steady-state performance and robustness of the proposed controller. [ABSTRACT FROM AUTHOR]

Power hardware-in-the-loop (PHIL) technology allows for the testing of physical equipment including power electronic converters in a simulation environment that closely mirrors the reality of the electric power grid. A challenge involved in PHIL systems is the design of the interface between the digital network model and physical equipment as this has a significant influence on stability of the real-time simulation. While a small time step supports stability and accuracy, its lower limit is given by the real-time constraint. The proposed multirate partitioning (MRP) interface addresses this issue by using a rather small time step at the hardware–software interconnection and then employing a staged adaptation of the time step within the digital model in accordance with the real-time constraint. Thus, multiple rates are used. The Nyquist stability criterion confirms enhanced stability and bandwidth by the MRP compared with single-rate (SR) counterparts. Moreover, a PHIL test of two parallel photovoltaic converters feeding a low-voltage network reveals different behaviors depending on the interface. Using the MRP, the waveforms more closely track the real-world curves of active and reactive power with the relative accuracy increasing with the speed of the transients. Informed decision making regarding the integration of renewables in the grid is so supported. [ABSTRACT FROM AUTHOR]

Power system simulation is an important means to study the dynamic behavior, to ensure the safety and stability, and to optimize the operation of a power grid. Pure physical simulation and digital simulation have their own advantages and disadvantages. A three-phase four-wire hybrid simulation platform integrating the advantages of both the digital simulation and physical simulation is developed by combining the physical simulation system and real-time digital simulator. The platform is rated at 400 V and 50 kVA with the short-circuit capacity of 500 kVA and can supply ten times the rated current to support the simulation of various short-circuit faults. An improved interface algorithm based on an ideal transformer model is proposed to extend the stability region of a hybrid simulation. Hybrid simulation experiments are conducted under two cases, and the comparison with the digital simulation demonstrates the performance of this platform and the potential applied to a modern power system. This platform has been equipped in a key laboratory of smart power grid technology. [ABSTRACT FROM PUBLISHER]

As the penetration of complex grid-connected devices, such as power electronic inverters, increases, so too does the complexity of analyzing system stability. A graphical application of the Nyquist stability criterion is presented that indicates how an individual load and source each contribute to the closed-loop system grid eigenvalues. The case study is not limited to particular impedance forms or scenarios like the more common complex torque coefficient method or passivity theory method. From the individual frequency responses of the load and source impedances, the graphical technique indicates how each impedance contributes to the system stability. Examples are provided that successfully indicate the cause of instability for a digitally controlled voltage source inverter (VSI) operating as a microgrid, with a current source inverter as a load. A second example is provided that identifies the potential instability of a VSI running an induction machine. A 42-kW inverter system is used to confirm the findings, showing a close correlation with the theoretical analysis. [ABSTRACT FROM PUBLISHER]