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Abstract The advantages, such as the mature control method, less volume of the converter and generator, make the doubly fed induction generator system prevalent in the wind power industry. Nevertheless, the doubly fed induction generator is more susceptible to grid faults and disturbances. The instantaneous high voltage fault may occur due to the excessive local reactive power after the doubly fed induction generator system achieving low voltage ride through, and then the wind turbine might be disconnected again. Hence, not only the low voltage ride through but also high voltage ride through capability should be required for the doubly fed induction generator system to meet the grid code requirements. A comprehensive review of the state of the art low voltage ride through and high voltage ride through technologies for the doubly fed induction generator system is presented. Firstly, different types of common low voltage ride through and high voltage ride through techniques are classified according to their features, i.e. auxiliary hardware, linear or nonlinear control strategies etc. The pros and cons of different low voltage ride through and high voltage ride through techniques are given. Furthermore, the latest developments of low voltage ride through and high voltage ride through technologies are introduced. Finally, the future trends of both the low voltage ride through and high voltage ride through technologies are discussed.

Abstract Four‐switch three‐phase inverters (FSTPI) are the cost‐optimised version of two‐level voltage source inverters (TL‐VSI), promoted for low‐power/low‐voltage machine drive systems. Adding impedance‐based networks to the input of conventional VSIs provides both step‐up and step‐down features; however, the shoot‐through (ST) modulation index limits the maximum attainable modulation index. Furthermore, in FSTPI, one switching leg has two series capacitors instead of power switches. Therefore, they are exposed to the voltage imbalance problem. This paper proposes a novel step‐up FSTPI with independent shoot‐through and non‐shoot‐through (NST) modulation indexes, improved boost factor, continuous input current, and high efficiency. This topology incorporates a quasi‐z‐source switched‐boost network with independent power switches and three duty ratios that affords two active states and one shoot‐through event. These features enhance the performance of this topology if compared with former FSTPI and quasi‐z‐source FSTPI. For evaluating the performance of this topology, simulation in MATLAB software and experiment on a prototype have been done. Simulation and experimentation results confirm the proposed topology.

Abstract In this work, an average current‐mode control strategy based on a generalised predictive control formulation for passive front‐end three‐phase boost‐type converters is proposed. A novel design procedure for the generalised predictive control strategy is introduced which considers both the cost function and disturbance model as design parameters to set the controller's dynamic response and robustness against component variations. A maximum robustness criterion was used for achieving stability up to a 70% inductance reduction with maximum possible bandwidth. The proposed strategy was compared against both a PI and a predictive deadbeat average current‐mode control using both simulations and experimental results on a 2-kW converter. The generalised predictive control presented less performance variations between different operating points than the PI controller. Also, the proposed strategy is more robust than the predictive deadbeat strategy, showing a better transient response with a 50% inductance reduction and remained stable for a 71% inductance reduction, while the predictive deadbeat could not. Finally, the proposed strategy achieved a 1.4% output voltage load transient response for a 595W load power step, and a 2.8% output voltage line transient response for a 100V input voltage step, outperforming existing state‐of‐the‐art strategies.

Abstract An active power filter (APF), which operates under the power‐balance‐based scheme (PB scheme), suffers heavier DC‐link voltage fluctuation when the loads change suddenly, so that it is apt to be interfered and degraded or even out of stable operation. By analyzing the cause of DC‐link voltage fluctuation, this study proposes an improved PB scheme which feeds forward the increment of the fundamental‐frequency positive sequence component of the source current. The feed‐forward channel is designed by use of the cascaded delayed signal cancellation algorithm and integrated into the conventional PB scheme APF without adding any extra hardware circuit. The current increment fed forward is added into the output of the voltage controller to timely amend the reference current of the current control loop, and thus the active power flow, which caused by loads change, between the APF and its external circuitry can be suppressed. As a result, the DC‐link voltage fluctuation is mitigated to allow the APF to better accommodate the abrupt load changes. The configuration of the improved control system is introduced and the stability is analysed. The experiment results demonstrate the effectiveness of the proposed control scheme.

Abstract This paper examines a multilevel transformer‐less grid‐tied inverter topology is proposed for PV applications. Nowadays, transformer‐less inverters are more desirous due to their small filter size, cheaper and higher efficiency. However, the removal of transformer results in the existence of leakage current due to absence of galvanic isolation. The flow of leakage current is the cause of personal safety and deteriorating power quality. A transformer‐less inverter topology is proposed which is able to solve the leakage current issue. Besides, it has the ability to generate the boosted voltage without the need of magnetic component or boosting circuit. It works on the principle of flying/switched capacitor which are able to naturally balance the voltage. Hence, it does not have the limitation of modulation index and no need of complex control scheme to balance or regulate the voltage across an auxiliary capacitor. The performance of the proposed topology is verified by implementing a laboratory prototype which validates the operation and performance of the proposed topology. Results depicting the output voltage, load current and voltage across the auxiliary capacitor are presented for various operating conditions. Moreover, a comprehensive comparison is done, in terms of component count, level and boosting ability of output voltage with leakage current condition.

Abstract In modern power systems, the proliferation of power electronics converters, and distributed generation raises important issues concerning inter‐connected switching units in terms of performance, stability and robustness. Such phenomenon are more prominent in micro‐grids, such as modern local low voltage distribution systems, more electric aircraft power systems etc., where many power converters are connected to the same non‐stiff low power ac grid, and strongly interact with each other. Locally designed converter control systems on the same electrical bus exhibit interactive behaviour. If not taken properly into account, external disturbances to the system at given operating conditions may result in the degradation of performance, failure to meet operating conditions and, in some cases, instability. This paper presents a new approach to synthesise converter controllers in more electric aircraft ac distribution grids (which is, however, applicable to all power electronics embedded systems), keeping in consideration dynamic interactions among subsystems. An optimal control design approach based on H2 optimisation is therefore proposed. Phase‐locked loops have also been considered, and their tuning has been included in the general tuning procedure of the whole system. Simulation, and experimental results show good improvements in terms of dynamic performance, and interaction mitigation.

Abstract Spike voltage generated during the turn‐off transient is one of the key threats to IGBT. When the traditional RCD snubber is applied in the five‐level active neutral point clamped (5L‐ANPC) inverter to clamp the spike voltage, a transient large current caused by the charging of the snubber capacitor will increase power losses and current stresses of IGBTs. This study proposes a LRCD snubber for the 5L‐ANPC inverter. It can effectively suppress the spike voltages and the transient large currents of IGBTs, thus reducing the power loss. For the 5L‐ANPC inverter, the traditional commutation method also poses a threat to IGBTs. Near the zero‐crossing point of the output voltage, the traditional commutation method will cause the IGBTs that operate at the carrier frequency to withstand three times of the rated voltage. This study proposes a safe commutation strategy based on the LRCD snubber. When the output voltage passes through zero‐crossing point, the proposed commutation strategy uses four transition switching states to avoid the breakdown of IGBTs induced by overvoltage. It can ensure that all the IGBTs operate in the safe operating area (SOA). Experimental results are presented to verify the validity of the proposed method.

Abstract This study proposes a non‐isolated dc–dc converter based on a coupled inductor and switch‐capacitor–boosting techniques. The goal of the proposed topology is to increase the voltage gain with low‐duty cycle, so voltage stresses are decreased across power switches and diodes. The power switches are controlled by pulse width modulation. The common ground of input and output of the proposed converter makes it suitable for many applications such as photovoltaic systems. The proposed converter topology is discussed in continuous and discontinuous operation modes. A dynamic analysis of the proposed converter is also provided. Simulation and experimental verifications are presented to prove the effectiveness of the proposed topology.

Abstract Inverters‐based islanded microgrids are operated by local digital signal processors, driven by their own local clocks. These local clocks have drifts affecting the clock signals which are used to control the inverters in real time. In this scenario, a steady‐state deviation is observed in active power sharing, degrading the microgrid performance. This paper presents a new control scheme that improves the microgrid performance by cancelling the active power sharing error due to clock drifts. The study includes a theoretical stability analysis and a design procedure of the control parameters based on the dynamic characteristics. Finally, selected experimental results are obtained in a laboratory microgrid with four inverters. In this setup, each inverter was equipped with its digital signal processor with drifting local clock.

Abstract In order to concentrate the frequency spectrum of the output voltage and improve the quality of grid currents for the three‐level neutral point clamped inverter with the model predictive control, this paper proposes an optimal switching sequence model predictive control algorithm. Based on the increments of grid currents and the neutral‐point voltage, the predictive model of the inverter is established in αβ frame. Moreover, including grid currents and the neutral point voltage tracking, the cost function is designed and simplified by arranging the voltage vector sequence in a sampling cycle appropriately. Meanwhile, the calculation of the optimal dwell time for each voltage vector sequence is derived by Lagrange multiplier method, and its solving process is simplified to reduce online computations. Furthermore, according to different voltage vector sectors, a voltage vector sequence preselection principle is introduced in this paper, besides, considering redundant small vectors in a pair have opposite affections on the neutral point voltage, the amount of voltage vector sequence that needs to be verified is further cut down, for obtaining the optimal voltage vector sequence. Finally, the experimental results show that the proposed method can concentrate the output frequency spectrums, which maintains a good quality of grid currents with a small neutral‐point voltage fluctuation.

Abstract Flying capacitor multi‐level converters use charged capacitors as a critical element. For proper operation, the capacitor voltages must be known and regulated. Direct measurement is straightforward, but when the number of required measurements is high, this is a complicated approach. This paper introduces an estimation method for n capacitor voltages. The scheme is based on a system of (n+1) equations, which is defined in a way that it incorporates information of the actual capacitor voltages and open‐loop estimates of the capacitor voltages. The solution of the system gives the best estimates of the capacitor voltages in the sense of least square errors. The method requires only output voltage and output current sensors and its computational burden is low, saving cost and design efforts, and simplifying the hardware system. The performance of the proposed scheme is evaluated experimentally in a 9‐level flying capacitor chopper, where the estimates are used by the voltage balancing strategy and by the output current controller.

Abstract This work demonstrates a maiden application of a fractional‐order based three‐degree‐of‐freedom tilt‐integral‐derivative controller for escalating the load frequency control performance of power system having wind power generator integrated. The disturbance observer is housed with the proposed fractional‐order‐three‐degree‐of‐freedom‐tilt‐integral‐derivative controller to efficiently estimate the wind velocity's uncertain profile and subsequently enrich the control law. The mastery of the proposed control algorithm has been tested on multi‐area interconnected power systems by performing an extensive comparative study with other prevalent techniques reported in the state‐of‐art. Harris' Hawks optimisation is applied to explore the proposed controllers' optimum gains, exercising an integral error‐based criterion. Time response measurements of the studied test systems in the wake of load fluctuation and intermittent output of wind power generator explicitly establish the efficacy of the proposed Harris' Hawks optimisation tuned disturbance observer‐based fractional‐order‐three‐degree‐of‐freedom‐tilt‐integral‐derivative controller over its other counterparts concerning damping of system oscillations. Furthermore, the developed control system's robust stability is affirmed using Kharitonov's stability theorem, considering ±25% variation in system parameters.

Abstract The main goal of an interconnected power system is to transfer power from one area to another while the network frequency and tie‐line flow remain within the prescribed limits. However, both of these quantities may violate their desired values during this transfer due to disturbances in the network. This paper proposes a stratagem for choosing the right feedback path for an interconnected power system to maintain the system frequency and tie‐line flows within the prescribed limits while external disturbances exist. Area control error (ACE), a combination of frequency error and tie‐flow deviations, is used as the performance indicator. In the proposed approach, feedback control is designed using active disturbance rejection controller (ADRC) based load frequency control to tackle ACE. It is observed that the individual load change monitoring is sufficient for selecting the right feedback paths rather than the consideration of simultaneous load changes of all load centres. The effectiveness of the proposed controller for selecting the feedback paths has been tested by conducting several case studies. The results demonstrate that the proposed controller can reduce transient magnitude around 57% for ACE, 55% for frequency error and 72% for tie‐line error as compared to the PID controller.

Abstract This paper considers the problem of false data injection attacks (FDIAs) on load frequency control of interconnected smart grids (ISGs) with delayed electric vehicles (EVs) and renewable energies. By intruding incorrect information, unauthorised users can corrupt the system information leading to degradation in the performance and disruptions of ISGs. In this paper, a model of ISGs subject to FDIAs in aggregator of EVs and power plants is first presented. This mathematical representation comprises dynamic interactions of power plants, delayed EVs, renewable energies and FDIAs on both system states and outputs. Based on recent advanced techniques on functional observers and matrix inequalities for time‐delay systems, then a new distributed functional observers based scheme is developed to realise the tasks of detecting and isolating FDIAs. Also, an effective procedure presented in tractable linear matrix inequalitiesis build with an optimisation process for the synthesis of the detector. The proposed detector is distributed, of reduced order, avoids the risk of centralised malicious incidents, therefore easy for implementation and monitoring tasks. The stability of ISGs and contribution of EVs subject to FDIAs are also discussed. Comprehensive simulations are given to demonstrate the effectiveness of our proposed method by using three‐area ISGs.

Abstract An efficient control strategy for two connected microgrids (MGs) is proposed to ensure stable and economic operation. One of the most important means of improving energy efficiency is to achieve the best response for sudden and stochastic disturbances to which the MGs are subjected. Traditionally, MGs are controlled using a linear controller, such as conventional proportional‐integral (PI) controller. Fuzzy PI (FPI) controller‐based model reference adaptive control that can adapt to a wide range of operating conditions for regulating the voltage is investigated and its performance is compared with the conventional linear PI controller that is not able to mitigate these disturbances efficiently. Parameters of the proposed controller are optimised using an advanced optimisation technique called global porcellio scaber algorithm (GPSA). Performance of the controllers is demonstrated on two connected microgrids for a number of scenarios such as load variations, weather fluctuations and faults. Simulation results verify that the proposed control strategy is effective and feasible under various operating conditions for this system. The results also show that the dynamic performance of the system with the model reference adaptive fuzzy PI (MRAFPI) controller is better than that with the most common controller used for this application, the conventional PI controller, for different operating conditions.

Abstract This study proposes a hierarchical control structure for load power‐sharing among distributed generations (DG) in a multi‐bus microgrid (MG). The proposed control structure uses an average consensus‐based distributed protocol (CDP) for the secondary control layer and virtual oscillator control (VOC) as a primary controller. The proposed method can nullify the power‐sharing inaccuracy caused by the mismatches of the equivalent impedances seen from the outputs of DGs. The eigenvalue‐based small‐signal analysis is used to analyse the secondary controller and communication time‐delay on system stability. The results of the stability analysis are used to design secondary controller and time‐delay compensator. The validity of the proposed method is verified by both offline simulations in Matlab/Simulink and control hardware‐in‐the‐loop (CHiL) using a real‐time Opal‐RT platform.

Abstract Generating torque and suspension force with one set of windings at the same time can increase the copper space factor and improve the torque output capacity. This structure has a unique application advantage for bearingless motors used in onboard flywheel energy storage. A consequent‐pole bearingless permanent magnet synchronous motor (PMSM) with integrated winding is designed and researched. Regulating suspension current has minimal effect on the torque performance of the motor in this design. The effectiveness and feasibility of this design are verified through a finite element calculation. Its advantages and application scope are discussed.

Abstract With the advantages of low cost, small size and high resolution, the magnetic encoder is suitable for permanent magnet synchronous motor. However, due to the installation accuracy and the motor vibration, the rotor position error from magnetic encoder can seriously impact the performance of the motor. Therefore, it is necessary to reduce the detection error of the rotor position from the encoder. In this letter, a new rotor position detection method is proposed based on neural network. The characteristic of the position error from magnetic encoder is analysed, and the backpropagation neural network is presented to obtain the error compensation function. On the basis of the compensation function, the rotor position error can be corrected and thus the precise rotor position can be detected. The experimental results show that not only the rotor position error but also the related total harmonic distortion of the phase current and the torque ripple can be reduced effectively.

Abstract This study proposes a new three‐port multidirectional DC‐DC converter (TMC) to integrate an energy storage system (ESS) or main grid to a bipolar DC microgrid (BDCMG). This converter provides a voltage balancing function and controls the input and output power of the BDCMG so that during overload conditions injects power to the BDCMG and in low‐load conditions saves the available power to an ESS or transfers to the main grid. Given this integration of activities, the total number of converters in the BDCMG can be decreased. This feature enables that the converter can be used in portable systems where the overall efficiency and weight of the system are important. In the study, the performance of the proposed converter is analysed. Simulation studies are performed and a BDCMG laboratory setup is implemented in order to evaluate the performance of the proposed TMC.

Abstract The dynamic operation of the conventional boost converters is limited in the continuous conduction mode, due to the presence of at least one right‐half plane (RHP) zero in their control transfer function which can limit the open‐loop bandwidth of the converter. This problem complicates the control design for the output voltage regulation and conversely influences the closed‐loop system stability. To cancel this problem, a new step‐up boost converter using the magnetic coupling and the switched capacitor (SC) is presented. In the proposed converter the effect of the RHP zero can be successfully eliminated, the voltage transfer ratio can be enhanced and the circumstances are provided to reach a small operating duty cycle as well as stress reduction of the diodes and power switch. Using the averaging technique, the state‐space model of the converter is analytically studied and the essential conditions are given for achieving a minimum‐phase structure. Based on the mentioned advantage, a simple classical PI controller for the output voltage regulation is designed. To confirm the theoretical analysis of the proposed converter, simulations and practical results are presented. Also, the performance of the proposed converter is compared with the other boost converters without RHP zero.

Abstract In order to control dual active bridge conventionally, control scheme such as single phase shift is used which reduces the system efficiency particularly when voltage conversion ratio differs from unity. This causes more circulating power and high current stress in switches. Moreover, voltage obtained at primary side of high frequency transformer is of two‐level only, which increases voltage stress across it. Furthermore, it has only one degree of freedom which confines its ability to regulate power flow. In this paper, a transistor clamped dual active bridge dc–dc converter is proposed with three degrees of freedom which improves its ability to regulate power flow, and because of incorporation of five‐level voltage on primary side of high frequency transformer, current stress in switches and voltage stress across high frequency transformer reduces. Mathematical modelling for the proposed converter is done for obtaining power flow equation. In addition to the phase shift control, proposed converter has two more control parameter which enhances the power flow controllability of the converter. Comparative analysis with conventional dual active bridge shows that the proposed converter is having more efficiency. Performance of the proposed converter is verified via simulation and hardware implementation.

Abstract In this study, multi‐harmonic oscillation behaviour and stability problem in double‐input buck/buck–boost inverter are investigated both theoretically and experimentally. First, the observer‐pattern model is obtained by the local orthogonal and autonomous transformation in order to eliminate the time‐variance effect from both fundamental component and the ‘hidden’ second‐harmonic in the double‐input cascade system. Based on the proposed model, bifurcation analysis method is employed to reveal the underlying mechanism of the multi‐harmonic oscillation behaviour and identify the dominant harmonics. It is shown that the occurrence of Hopf bifurcation results in the double‐adding harmonic oscillation, and the interaction between the double‐adding harmonic components and the ‘hidden’ second harmonic component is also responsible for the multi‐harmonic oscillation behaviour. Meanwhile, the occurrence of the multi‐harmonic oscillations leads to the decrease of the system efficiency. For stability enhancement, the harmonic‐stability sensitivity is presented to evaluate the influence of the key circuit parameters on the system stability quantitatively. In addition, the stability boundaries of the double‐input inverter for these key parameters are provided to guide the system optimal design. Finally, the effectiveness of the above analysis is verified by those experimental results.

Abstract A novel single‐phase five‐level rectifier based on two active switches is presented. The proposed novel circuit topology, with two active switches and a split dc‐link, can produce a distinct five‐level voltage with a reduced number of active switches. This circuit operates in unitary power factor mode with imposing a sinusoidal grid‐side current with low total harmonic distortion (THD) and controlled dc‐link voltage, which can be used as a low‐cost active rectifier to improve the efficiency of ac‐to‐dc rectification. This paper describes the circuit topology, the principle of operation, the feature of the circuit topology and the double‐loop control strategy in detail. The operation efficiency of the proposed topology is analysed in detail to show its benefits compared to the traditional five‐level PFC typologies. An experimental prototype with 220 V AC input, 400 V output, 500 W rated power, and 97.3% output efficiency is built to validate the proposed rectifier under real conditions of operation. The simulation and experimental results show the correctness and good performance (unitary power factor operation, sinusoidal current with low THD and dc‐link voltage regulation) of the proposed single‐phase five‐level rectifier.

Abstract This study presents a single‐phase single‐stage step‐up inverter (SSSI), which can be used as an interface converter in micro‐source generation systems such as photovoltaic (PV) and fuel cells. The proposed inverter has several appealing features. (1) It can provide an AC output voltage higher than its DC input because of the built‐in boost circuit. (2) It has characteristics of a simple structure and reduces the component counts and cost. (3) The shoot‐through problem for the power devices can be avoided in the proposed SSSI. (4) metal‐oxide‐semiconductor field‐effect transistors (MOSFETs) can be used without reverse recovery issues of body diodes. Therefore, high system reliability and efficiency can be obtained. This study describes the operating principles in discontinuous conduction mode (DCM), pulse‐width modulation strategy, and parameter design guidelines. In order to verify the feasibility and performance of the converter, experimental results are obtained from a lab prototype.

Abstract In this study, an induction heating (IH) application is carried out by transferring the maximum power extracted from photovoltaic (PV) panels to the stainless steel with the designed regular and irregular 32 pulse density modulation (PDM) controlled series resonant inverter. The main objective of this study is to analyse the changes in the system due to regular and irregular PDM patterns. Maximum power point tracker (MPPT) is used to control the output power at different solar irradiation values with varying PDM patterns. Regular and irregular PDM control methods are compared in terms of MPPT efficiency, cost, algorithm complexity, logic control structure and current/voltage stresses of the power switches. Zero current switching (ZCS) conditions are provided by using phase locked loop (PLL) technique at all power points of the PV system. The appropriate switching conditions are tracked continuously at resonant frequency, and therefore soft switching is realised. Perturb and observe (P&O) algorithm is used with the aim of tracking the maximum power in PV panels and high tracking efficiency is obtained with PDM‐controlled P&O MPPT algorithm. ATMEL328P‐AU microcontroller is used to control the inverter in the system.

Abstract Current controllers for voltage source inverters give better control of current waveform and also provide compensation for load transients and non‐linearities. Despite the increasing employment of multilevel inverters in high‐power medium voltage applications and its superior performance compared to two‐level inverters, the extension of hysteresis current controllers to multilevel voltage source inverters is much less established. In this study, a current error space vector‐based hysteresis controller is implemented on a three‐phase cascaded half‐bridge voltage source inverter. The vector‐based method measures the current error in a space vector form and keeps the error within the assigned tolerance region. It has been observed from the simulation tests and experimental verifications that the implemented current regulator maintains the load current as close as possible to the set reference current with a significantly minimised error. Another attractive feature in the implemented closed‐loop current controller is that the number of voltage levels is maintained as the output of the inverter with a fast dynamic response.

Abstract Energy storage systems are recognised as the potential solution to alleviate the impacts of reduced inertia and intermittency in power systems due to the integration of renewable energy sources. Several energy storage technologies are available in the market with diverse power and energy characteristics, operational limitations, and costs. Besides, frequency regulations in power systems have different requirements, for example, inertial response requires high power for a short period while primary frequency regulation requires steady power for a longer time. Thus, it is crucial to find out the optimum sizes and types of storage technologies for these services. In this paper, a methodology for sizing fast responsive energy storage technologies for inertial response, primary frequency regulation, and both inertial response and primary frequency regulation is developed. The sizing of storage systems for inertial response, primary frequency regulation, and both inertial response and primary frequency regulation is done separately. The sizing of storage for inertial response is done in two steps. A region reduction iterative algorithm is proposed to estimate the storage size for inertial response. The sizing of the storage system for primary frequency regulation is done analytically. The sizing methodology incorporates the frequency dynamics of storage, converters, and other associated controls that affect the frequency response. Moreover, an economic analysis is carried out to find the optimum combination of storage technologies for inertial response, primary frequency regulation, and both inertial response and primary frequency regulation services. The accuracy of the proposed sizing method has been compared with the metaheuristic algorithm based technique. The effectiveness of the proposed method is also compared with those in the literature. Simulation results show that the proposed method outperforms the existing methods in the literature. Finally, the non‐linear simulations revealed the validity of the optimal solutions.

Abstract This paper suggests an improved step‐up step‐down DC‐DC system along with three‐input and four‐output for smart home applications. In this configuration, two unidirectional power ports have been identified as an Input power supply and one bidirectional power port for the power‐saving element, which can be used as a bidirectional converter for the hybrid vehicle to discharge in a dependent structure. This system can be used to combine renewable energy sources like photovoltaic (PV), fuel cell, battery and hybrid vehicle (HV) to prepare power for remote smart homes. By using this system, serving different loads with different voltage range from high voltage to ultra‐low voltage is possible, also battery charge and discharge with the energy‐saving method can be achieved. In this system, the condition of all possible low‐voltage load and high‐voltage load conditions has been assumed. In this structure, nine power switches have been used, in which all of these switches are in control with independent and dependent duty cycles. By using these cycles, maximum power can be earned from PV sources, bus‐bar voltage regulation, and battery power control is possible too. In this topology, depending on environmental conditions, five scenarios have been identified. To prove the capability of the system before the build, some valid simulations are needed. In this study, the suggested system has been simulated with power system computer aided design/electromagnetic transients including DC (PSCAD/EMTDC).

Abstract This study proposes a high step‐up galvanically isolated dc‐dc converter based on the quasi‐Z‐source (qZS) network. The voltage gain of the converter can change in a wide range. This feature makes the converter suitable for applications with a wide input voltage range. The range of the dc gain is increased by the implementation of the combined energy transfer principle. A reconfigurable structure is used to combine the energy transfer by the isolation transformer and the coupled inductor of the impedance‐source network. The adaptive mode change activates the energy transfer via an additional winding in the qZS network at lower voltages, which results in considerable efficiency improvement. Consequently, the input voltage range of the converter is extended to the values useful in photovoltaic applications impacted by partial shading. The proposed approach is verified in a photovoltaic (PV) microconverter, where the energy harvest from a PV module is possible during shading by enabling the global maximum power point tracking (GMPPT). It is demonstrated that the dual‐mode operation of the converter helps to achieve a flat efficiency curve along with outstanding maximum power point tracking (MPPT) performance.

Abstract Fast‐acting smart inverters that utilize preset operating conditions to determine real and reactive power injection/consumption can create voltage instabilities (over‐voltage, voltage oscillations and more) in an electrical distribution network if set‐points are not properly configured. In this work, linear distribution power flow equations and droop‐based Volt–Var and Volt–Watt control curves are used to analytically derive a stability criterion using Lyapunov analysis that includes the network operating condition. The methodology is generally applicable for control curves that can be represented as Lipschitz functions. The derived Lipschitz constants account for smart inverter hardware limitations for reactive power generation. A local policy is derived from the stability criterion that allows inverters to adapt their control curves by monitoring only local voltage, thus avoiding centralized control or information sharing with other inverters. The criterion is independent of the internal time‐delays of smart inverters. Simulation results for inverters with and without the proposed stabilization technique demonstrate how smart inverters can mitigate voltage oscillations locally and mitigate real and reactive power flow disturbances at the substation under multiple scenarios. The study concludes with illustrations of how the control policy can dampen oscillations caused by solar intermittency and cyberattacks.

Abstract In recent years, renewable energy sources have been integrated on a large scale in power systems all around the world to address the environmental sustainability concerns. With conventional thermal generators being phased out, large offshore wind power plants present a viable alternative to provide blackstart services for power system restoration. By means of simulations, grid‐forming wind turbines are shown to successfully energize the offshore transformer and the high‐voltage direct current (HVDC) export link in a controlled manner, to ultimately supply onshore block load. Two methods for energizing the offshore network have been compared: the prevalent hard‐switching approach and the more complex soft‐start method. Additionally, control has been implemented to mitigate the significant transients in the export link associated with pre‐charging of the onshore converter. It is shown that soft‐start can provide faster energization with smaller transients compared to hard‐switching. Moreover, the sensitivity analyses performed in this study illustrate the impact of pre‐insertion resistor and voltage ramp‐up rates on transients during hard‐switching and soft‐start, respectively. The results presented also show that grid‐forming wind power plants can deal with controlled pre‐charging of the onshore converter from its DC terminals that is essential for the safe energization and operation of the export link.

Abstract A three‐phase soft‐switched DC–DC resonant converter with variable frequency to perform the output voltage control is proposed. The open delta‐wye connection provides a double output voltage reducing the primary current stress, turns ratio, weight and volume of high‐frequency transformer. To achieve high efficiency, an LLC resonant tank is designed so that all switches can perform zero‐voltage switching for the entire power range. In order to verify the operating principle of the converter, a design example is described, with the following converter specifications: input voltage 96 V, output voltage 380 V and a switching frequency up to 120 kHz. The feasibility and practicality of the proposed converter are confirmed by experimental results with a 1.5 kW prototype.

Abstract A comprehensive energy system state estimation is of great significance to promote efficient, stable and reliable operation of the system. Based on coupling characteristics of electricity‐gas‐heat multi‐energy flow and bilinear transformation method, this paper proposes a robust state estimation method suitable for integrated energy systems. First, the state estimation models of electricity, gas and heat energy subsystems and coupling components are constructed. Then, by introducing pseudo state variables and intermediate variables, the nonlinear state estimation process is divided into four stages. Among them, the first and third stages are solved by WLAV method and WLS method respectively, and the second and fourth stages are simple nonlinear transformation processes. Finally, the robust state estimation method is verified on a 40‐node simulation example. The bilinear robust state estimation method proposed not only solves the difficulty of selecting initial values of heterogeneous networks and the problem of non‐convergence of algorithms, but also can search and identify various types of bad data, and has high computational efficiency and good robustness.

Abstract Sparse least mean square algorithm is proposed for the DSTATCOM as an optimal current harmonic extractor to cope with the intermittent nature of loadings. Sparse least mean square is the improved version of adaptive least mean square learning mechanism with regards to sparsity. This innovative approach is utilized for better parameter estimation due to its algorithmic simplicity and parallel computing nature. Hence, sparse least mean square is expected to reduce the computation and storage requirements significantly. This suggested controller consists of six subnets. Three subnets for active and another three for reactive weight component are used to extract the fundamental component of the load current. Several factors like previous weight, normalizing weight and learning rate are involved in the sparse least mean square based weight‐updating rule to have better dynamic performance, reduced computational burden and better estimation speed etc. The detailed control algorithm is formulated using MATLAB/Simulink, and validated using experimental analysis. Among these two algorithms, the sparse least mean square offers better voltage regulation, voltage balancing, source current harmonic reduction and power factor correction under various loading scenarios.

Abstract In recent years, methods for loadability determination based on real‐time measurements have been proposed for long‐term voltage stability scenarios. One of these methods corresponds to the reduction of the complete system into a basic Thevenin equivalent at a load bus. This paper proposes a method to determine the Thevenin equivalent at a load bus in power systems based on short circuit power (SSC) and its application to real records. The proposed Thevenin equivalent evaluation is performed by processing two consecutive sets of voltage magnitude and power measurements, both recorded at the same bus of a power system. This method establishes a Thevenin equivalent based on the determination of system's equivalent voltage (VS) which is obtained from SSC. This condition properly accounts for variable power factor load conditions and constant power factor alike in contrast with existing methodologies which assume constant power factor loads. The performance of this method is evaluated with real records from the Ecuadorian electric power system and contrasted by simulation.

Abstract The global increase in the use of solar photovoltaics and wind turbines has led to a rapid increase in the penetration level of inverter‐based distributed generators (IBDGs). Until recently, the operation modes of IBDGs under abnormal power system conditions were not a major concern owing to the small number of IBDGs used in power systems. However, several events have indicated that the momentary cessation (MC) mode of an IBDG can eventually deteriorate the stability of power systems. Among the various parameters that define the MC mode, the direct impact of the MC recovery ramp rate on the transient stability of the power system has been addressed by computing the stability margin based on a single machine equivalent method. Furthermore, this paper introduces the critical momentary cessation recovery ramp rate for a specific system, which can be used as an important index in the planning of an IBDG installation. Case studies on modified IEEE 39 bus system and Korean power system were conducted to validate the effectiveness of the proposed method.

Abstract Due to the low impedance of DC (Direct Current) power transmission line, short‐circuit current has a very high increment speed, which causes problems as difficulty in current limitation method. This paper proposes a new method to realize the current‐limiting control of short‐circuit current by using active control of power electronic transformer. The method improves the topology of dual active bridge modules that contribute the DC transformer, and installs an IGBT (Insulated Gate Bipolar Transistor) switch in reverse direction on supporting capacitor branch of each module to control the capacitor on or off, which can guarantee the storage energy within the capacitor will not release in initial stage in case of DC short circuit fault happen. Therefore, there is no large inrush current flow out. Furthermore, combined with the requirements of current tracking control during low‐voltage ride‐through, the transformer output current under short‐circuit condition is actively controlled in order to meet the needs of renewable energy power generation equipment. Finally, the simulation and hardware‐in‐the‐loop experiments verify the correctness of this method.

Abstract Smart grid comprises utility system and dispersed renewable generations. It establishes bidirectional power transmission and communication among utility system, distributed generations and consumers. Sub‐synchronous resonance (SSR) is a major concern in transmission line. Furthermore, impedance swing during SSR leads in operation of out of step blocking characteristic of distance relay. SSR brings about a significant rise in the magnitudes of voltage and current. It also raises probability of occurrence of ferroresonance. Here, the impact of SSR in utility system is analysed on operation of doubly‐fed induction generator and distance relay with out of step blocking element is given here. Smart grid urge to establish a self‐healing protection, hence, an adaptive algorithm based on sub harmonic and ferroresonance detection is proposed for distance relay. The algorithm discriminates ferroresonance from other nonlinearities by wavelet transform and neural network and utilises time domain analysis to distinguish between different types of ferroresonance. The algorithm is able to observe smart grid protection strategy. It is capable of receiving the commands according to protection strategy to take a correct decision in such conditions to enhance reliability of power electricity in smart grid. Finally, the proposed algorithm is examined in SSR condition to certify the correct operation.

Abstract Adaptive full order observer based control algorithm for unified power quality conditioner is presented. The full order adaptive observer based algorithm is capable of estimating fundamental positive sequence components parameters, including frequency, amplitude, and phase form power supply systems. Due to its adaptive response, monitoring accuracy, and faster detection of fundamental positive sequence components under all different loads and grid conditions, it is used for power quality improvement. In addition to full order adaptive observer based control algorithm, a biogeography‐based optimization method is employed here for tuning the Kp and Ki gains of a proportional integral controller on unified power quality conditioner system. The proposed method minimizes the dc‐link voltage oscillations and enhances its step response by optimally tuning the proportional integral controller. This is done by reducing the objective function of the integral time absolute error for different power quality perturbations. The numerical values obtained by biogeography based optimization are selected as proportional integral gains and it also improves the controller's performance. The full order adaptive observer control integrated with biogeography based optimization algorithm is tested interface with MATLAB/Simulink on developed unified power quality conditioner under different power quality problems at different intervals. The performance results demonstrate the quick convergence rate with effective performance.

Abstract This work presents a novel strategy, designed from the distribution system operator viewpoint, aimed at estimating the hosting capacity in electric distribution systems when controllable plug‐in electric vehicles are in place. The strategy seeks to determine the maximum wind‐based distributed generation penetration by coordinating, on a forecast basis, the dispatch of electric vehicle aggregators, the operation of voltage regulation devices, and the active and reactive distributed generation power injections. Different from previous works, the proposed approach leverages controllable features of electric vehicles taking into account technical electric vehicle characteristics, driving behaviour of electric vehicle owners, and electric vehicle energy requirements to accomplish their primary purpose. The presented strategy is formulated as a two‐stage stochastic mixed‐integer linear programming problem. The first stage maximises the distributed generation installed capacity, while the second stage minimises the energy losses during the planning horizon. Probability density functions are used to describe the uncertainties associated with renewable distributed generation, conventional demand, and electric vehicle driving patterns. Obtained results show that controlling the power dispatched to electric vehicle aggregators can increase the distributed generation hosting capacity by up to 15% (given a 40% electric vehicle penetration), when compared to an uncontrolled electric vehicle approach.

Abstract The hybrid permanent magnet (PM) variable flux memory machine employs high‐ and low‐coercive‐force hybrid PMs to simultaneously obtain a high torque density and regulable PM flux property. The operating trajectories and steady working‐points of this type of machine switch back and forth in multiple regions. To identify operating modes under various PM magnetisation states and speed ranges, this paper proposes a mode recognition and coordinated control method based on probabilistic neural networks (PNNs). A PNN algorithm is trained offline and executed in real‐time to obtain a certain operating mode with the maximum probability of current state. According to the mode recognition result, a coordinated control scheme for applying short‐time pulses or continuous flux‐weakening current in a reasonable manner is presented. The proposed method exhibits an excellent capability in real‐time operating modes identification, hence achieving magnetisation current control of multiple modes easily. Its feasibility and effectiveness are validated by experimental results on a hybrid PM VFMM prototype.

Abstract A novel dual‐phase‐conducted type bearingless switched reluctance flywheel motor is proposed, which has a hybrid outer rotor comprising circular segment and scalloped lamination segment, and only one set of winding is placed on each stator pole. In order to drive the flywheel to accelerate or slow down, two phases of windings in this motor conduct simultaneously. One phase provides radial suspension force, and the other phase provides electromagnetic torque. There exits satisfying decoupling between torque and suspension force. With the circular segment of the rotor, the continuity and stability of radial force can be enhanced. Finite element analysis results confirm the feasibility of the theoretical scheme.

Abstract The hybrid high‐voltage direct current (HVDC) with cascaded multi‐infeed MMC inverters is proposed in recent years, and it will be put into reality in near future. But the commutation failure of the line‐commutated converter (LCC) inverter will make the whole HVDC system to be blocked because the DC fault current could reach beyond the safety region of the MMC inverter. To solve such problem, this letter proposes a novel fault current limiting method based on the LCC control strategy, namely, the virtual impedance method. Such method can limit the fault current effectively and need no extra devices, which can avoid the block of the whole HVDC system on one side, and save the cost of the construction on the other side. The theoretical analysis shows that the proposed method can improve the high‐frequency characteristic of the DC system and realise the fault current limitation effect. The simulations validate the analysis finally.

This work presents a novel strategy, designed from the distribution system operator viewpoint, aimed at estimating the hosting capacity in electric distribution systems when controllable plug‐in electric vehicles are in place. The strategy seeks to determine the maximum wind‐based distributed generation penetration by coordinating, on a forecast basis, the dispatch of electric vehicle aggregators, the operation of voltage regulation devices, and the active and reactive distributed generation power injections. Different from previous works, the proposed approach leverages controllable features of electric vehicles taking into account technical electric vehicle characteristics, driving behaviour of electric vehicle owners, and electric vehicle energy requirements to accomplish their primary purpose. The presented strategy is formulated as a two‐stage stochastic mixed‐integer linear programming problem. The first stage maximises the distributed generation installed capacity, while the second stage minimises the energy losses during the planning horizon. Probability density functions are used to describe the uncertainties associated with renewable distributed generation, conventional demand, and electric vehicle driving patterns. Obtained results show that controlling the power dispatched to electric vehicle aggregators can increase the distributed generation hosting capacity by up to 15% (given a 40% electric vehicle penetration), when compared to an uncontrolled electric vehicle approach. [ABSTRACT FROM AUTHOR]

Smart grid comprises utility system and dispersed renewable generations. It establishes bidirectional power transmission and communication among utility system, distributed generations and consumers. Sub‐synchronous resonance (SSR) is a major concern in transmission line. Furthermore, impedance swing during SSR leads in operation of out of step blocking characteristic of distance relay. SSR brings about a significant rise in the magnitudes of voltage and current. It also raises probability of occurrence of ferroresonance. Here, the impact of SSR in utility system is analysed on operation of doubly‐fed induction generator and distance relay with out of step blocking element is given here. Smart grid urge to establish a self‐healing protection, hence, an adaptive algorithm based on sub harmonic and ferroresonance detection is proposed for distance relay. The algorithm discriminates ferroresonance from other nonlinearities by wavelet transform and neural network and utilises time domain analysis to distinguish between different types of ferroresonance. The algorithm is able to observe smart grid protection strategy. It is capable of receiving the commands according to protection strategy to take a correct decision in such conditions to enhance reliability of power electricity in smart grid. Finally, the proposed algorithm is examined in SSR condition to certify the correct operation. [ABSTRACT FROM AUTHOR]

Adaptive full order observer based control algorithm for unified power quality conditioner is presented. The full order adaptive observer based algorithm is capable of estimating fundamental positive sequence components parameters, including frequency, amplitude, and phase form power supply systems. Due to its adaptive response, monitoring accuracy, and faster detection of fundamental positive sequence components under all different loads and grid conditions, it is used for power quality improvement. In addition to full order adaptive observer based control algorithm, a biogeography‐based optimization method is employed here for tuning the Kp and Ki gains of a proportional integral controller on unified power quality conditioner system. The proposed method minimizes the dc‐link voltage oscillations and enhances its step response by optimally tuning the proportional integral controller. This is done by reducing the objective function of the integral time absolute error for different power quality perturbations. The numerical values obtained by biogeography based optimization are selected as proportional integral gains and it also improves the controller's performance. The full order adaptive observer control integrated with biogeography based optimization algorithm is tested interface with MATLAB/Simulink on developed unified power quality conditioner under different power quality problems at different intervals. The performance results demonstrate the quick convergence rate with effective performance. [ABSTRACT FROM AUTHOR]