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Abstract In recent times, multilevel inverters (MLIs) are gaining popularity for grid integration of distributed power generation sources. In this paper, a proportional‐resonant (PR) controller based on current control logic is proposed for a single‐phase eleven‐level inverter topology, enabling the integration of distributed power generation sources into the grid. The presented topology utilizes eight two‐quadrant switches and three dc sources to produce an eleven‐level output voltage. The level‐shifted pulse width modulation (LS‐PWM) scheme is used for providing switching signals to the inverter under grid‐connected operation. Simulation and real‐time results are presented for the eleven‐level inverter to validate the presented current control logic under the grid‐connected mode of operation. The hardware‐in‐loop emulator (Typhoon HIL‐402) is used for taking real‐time results of the presented grid‐connected topology. The total harmonic distortion (THD) of the eleven‐level inverter voltage and grid current is 12.10% and 0.46%, respectively. The efficiency of the inverter while supplying 2 kW active power to the grid is 98.4%.

Abstract DC microgrid is a high‐efficiency plan for exploiting and controlling distributed generation (DG) resources that can supply local loads in grid‐connected or islanded mode. Conventionally, the droop control method is utilized to attain proper load sharing among the DGs as a decentralized approach in DC microgrids. However, accurate current sharing among DG units with different line resistances is not achievable by the conventional droop control, whereas this method also increases the DC bus voltage deviation. In this paper, a new droop‐based distributed control method is proposed in the secondary control level to overcome the limitations of droop control. To implement the proposed method, only the amount of precalculated virtual voltage drop of DG units is shared with adjacent DGs through a low‐bandwidth communication, and other calculations are local. So, there is no need to define and share new parameters, and get feedback from the DC bus voltage. By employing the proposed method, not only accurate current sharing among the DG units is guaranteed but also DC bus voltage is restored to the nominal value. To ensure the stability of the proposed method, the robust stability and performance of the closed‐loop system are analyzed by structured singular value (µ) under resistive load and communication delay changes. Finally, a DC microgrid is evaluated using MATLAB/Simulink as well as experimental tests to illustrate the validity and efficiency of the proposed method.

Abstract The increase in penetration of large‐scale renewable generation significantly affects power system characteristics. Thus, it is necessary to assess the steady state and dynamic performance of a power system for different installing scenarios of renewable power plants such as photovoltaic (PV) ones. The paper specifically investigates the impacts of very large‐scale photovoltaic (VLS‐PV) generation on the power system dynamic performance including rotor‐angle and frequency stabilities. For this purpose, a detailed equivalent dynamic model for a multi‐unit VLS‐PV system and corresponding controllers are developed. The paper also proposes a comprehensive set of replacement scenarios comprising different PV penetration, location, and voltage control strategies as well as simulating various large disturbances. To investigate the impacts of VLS‐PV generation on dynamic system performance, a study framework for frequency and rotor‐angle stability involving the modal and transient analysis is presented as well. By using the VLS‐PV detailed model along with other dynamic components of a power system such as synchronous generators, automatic voltage regulators (AVRs), and governors, the stabilities studying instructions are employed for the IEEE 39‐bus and 118‐bus test systems.

Abstract Being a renewable influenced utility grid, the wind integrated hydrothermal system (WHTS) is considered as a future‐proof and contemporary energy system to reduce the dependency on depleting fossil fuels. However, due to intermittent nature of wind energy and complex operational constraints, offering the day‐ahead operation schedule of WHTS with joint awareness of fuel economy and emissions is classified as a complicated multi‐objective optimization problem. The major challenge is to maximize the utilization of clean hydro power to accommodate uncertain wind energy together with reduced dependency on fossils based thermal power. In this context, the paper devises an efficient control and multi‐criteria energy scheduling method, essentially comprised of adaptive repair and volume restriction to maximize the utilization of available hydro power. The pareto‐metaheuristic optimizer based on non‐dominated sorting is exploited for the day‐ahead scheduling of WHTS based utility grid. The most appropriate solution from pareto‐optimal archive is selected using fuzzy‐ranking index. The expected reductions in fuel cost and emissions are reflected using a significant probability scheme. The effectiveness of the proposed approach based on pareto‐metaheuristic is appraised by various case studies. The simulation results provide a suitable day‐ahead generation schedule of WHTS based grid, considering both economics and environmental aspects.

Abstract Inertial support for hybrid AC/DC microgrid systems is provided by the virtual synchronous generator (VSG). However, the fast performance of the system is neglected while enhancing the stability of the system. To address this problem, an adaptive virtual inertia control strategy based on VSG technology is proposed. This control approach adaptively adjusts the system virtual inertia when the system operation deviates from the nominal value, by slowing down the rate of change of AC frequency and DC voltage and improving the deviation of AC frequency and DC voltage. Meanwhile, when the AC frequency and DC voltage restore back to the rated value, the rate of change of AC frequency is accelerated and the DC voltage fluctuation is reduced by dynamically adjusting the virtual inertia of the system. The proposed adaptive virtual inertia control method combines the advantages of large and small inertia to effectively improve the dynamic response of the system voltage and frequency in both rectifier and inverter modes. Finally, the simulation and experimental results verify the effectiveness of the proposed control algorithm.

Abstract Wind farms are increasingly interested in participating in the secondary frequency control, especially in power systems where AGC is organized by regulation zones comprising generation units of different technologies. In order to estimate the wind farms behaviour and overall impact on automatic generation control (AGC), a reasonably simple wind farm model is needed. First‐order and second‐order linear models of wind farms are proposed in the literature in AGC‐related studies. However, these simple models neglect key dynamic features for AGC integration of wind farms, such as artificial ramp limitations, dead bands, communication delays and start‐up delays caused by turbine orientation. This paper presents a still simple wind farm model, however representing all relevant dynamics and availability constraints at power system control level. Applying an illustrative case study, model parameters are tuned and validated by means of field measurements, recorded during a response trail run of a real wind farm of 30 MW installed power operating within the Spanish power system.

Abstract High penetration of renewable rotary generations like wind turbine to utility system causes contribution in power oscillations due to occurrence of different phenomena. Ferroresonance and subsynchronous resonance (SSR) in utility system are able to lead in power oscillation, instability of the system and devastating impact on rotary generators. Hence, investigation on impact of such oscillations on operation of nearby renewable rotary generations, like DFIG (doubly fed induction generator) in Wind Park seems to be beneficial to attain insightful guidelines for future of research in this field. SSR causes increasing the voltage, thus, it brings about saturation of transformer core and results in occurrence of ferroresonance. Ferroresonance causes frequency difference between two points in the network and consequently power oscillation, which is superimposed on torsional oscillation in SSR. Therefore, as SSR and ferroresonance encounter each other, they are investigated under one work to scrutinize impact of such phenomena on behaviour of DFIG. The results show deviation of electrical waveforms and devastating stress on mechanical parts of DFIG rotary components due to huge magnitude of electrical and mechanical quantities in effect of SSR and ferroresonance.

Abstract There exist the problems of the low utilization rate of DC voltage, power imbalance and current backflow in the modulation process of traditional unsymmetrical cascaded units. In view of these, a pulse width modulation strategy with an improved trapezoidal wave as the modulation wave and its corresponding power balance method are proposed. First, through double Fourier analysis of the output line voltage, an appropriate sine coefficient δ is obtained. Second, the waveform of the modulation wave is determined according to δ. Third, through the simple logic operation of the initial driving signals of the switch transistors, the required switch driving signals are obtained, and the power balance of each unit with the same voltage level is realized. In order to verify the effectiveness of the strategy and the method, a simulation and experiment platform are built, and a large number of simulations and experiments are carried out. The results show that compared with the traditional sinusoidal modulation method, the utilization rate of DC voltage obtained by the method can be improved by 22.28% and compared with the trapezoidal wave modulation method, the waveform distortion coefficient of the output voltage can be significantly reduced, and the utilization rate of DC voltage can be further improved. Meanwhile, the power balance can be achieved within one voltage cycle and the phenomenon of current backflow is eliminated.

Abstract The considerable expansion of non‐linear loads with interharmonic currents characteristic such as induction furnace, cycloconverter and variable‐speed drive is one of the new and important challenges in the control of industrial microgrids. For this reason, this paper presents an enhanced control scheme that tries to share power and especially interharmonic currents through parallel inverters accurately, which is not addressed in previous literature. In this regard, assuming the inherent delays of digital control, the proposed control strategy firstly suppresses the inherent impedances of inverters, especially in interharmonic frequencies. Then, the enhanced impedance‐power (I‐P) droop scheme is employed based on virtual inductors to compensate for the mismatch of line impedances and to achieve proper power‐sharing. Besides, novel techniques are introduced in the proposed structure to limit the values of virtual inductances and even to benefit from the negative virtual impedance method. The notable feature of the presented scheme is the sharing of interharmonic currents with high accuracy and low computations. Finally, the effectiveness of the proposed method is evaluated through simulation and experimental results.

Abstract This paper presents a new harmonic compensation system using filter compensator devices in a hybrid micro‐grid (MG). In order to reduce the amplitude of current and voltage harmonics, two separate systems, including a switched power filter compensator (SPFC) and a switched active power filter (SAPF) have been used in the AC side of the MG. The gain coefficients of both SAPF and SPFC modules (with different controllers) using meta‐heuristic algorithms are optimized and applied for three harmonic loops of voltage, current and controller error to obtain an optimal response for reducing the harmonic amplitude of current and voltage. The objective function, containing controller error, current and voltage harmonics is defined and followed by the optimized coefficients for the PID controller of each of the SAPF and SPFC modules. To find the best answer to this problem, the related gains using four meta‐heuristic algorithms, including artificial bee colony (ABC), particle swarm optimization (PSO), harmonic search (HS), and differential evolution (DE) are optimized and compared. The simulation results show that using the proposed method, the total harmonic distortion of the system can be significantly reduced.

Abstract Due to the diverse features of source‐network‐load‐storage for a microgrid, the operation mode of grid‐tied inverter is complicated. The grid net model would be cumbersome and difficult to carry out effective analysis based on uniform model of inverter in accordance with the traditional method. To meet the new IEC/TS 62989 microgrid standard, considering charging and discharging modes of storage unit, a benchmark equivalent model is proposed here by the simplification principle of cluster analysis. Then the microgrid can be divided into multiple source‐network‐load‐storage subsystems based on the model, that helps to carry out the energy independence and modular control of the distributed power generation systems. The simplification of the microgrid module is designed according to the principle of cluster analysis. The simulation and experimental results are given to verify the validity of the model.

Abstract Concerning sub‐synchronous oscillation in power system connected with virtual synchronous DFIGs, an oscillatory stability analysis method based on the dissipation rate of dynamic energy is proposed. First, the dynamic energy function that reflects the sub‐synchronous oscillation characteristics of virtual synchronous DFIG is built, and the expression of dynamic energy considering virtual synchronous control parameters is analysed. Then, according to Lyapunov's stability law, the variation rate of dynamic energy function is defined as the energy dissipation rate. The energy dissipation rate reflects the variation trend of system dynamic energy during oscillation, thus it is used as the stability criterion of sub‐synchronous oscillation. On this basis, the effects of different virtual synchronous control parameters on sub‐synchronous oscillation are analysed for a comprehensive consideration of system stability. Finally, simulation tests are conducted on RT‐LAB platform. The results verify that, when the energy dissipation rate is positive, system oscillation diverges; when the energy dissipation rate is negative, system oscillation converges. Meanwhile, the larger the virtual resistance is, the worse the stability of system is; the larger PI control parameters of reactive power control loop are, the worse the stability of system is.

Abstract With the large‐scale penetration of wind and solar energies in the power system, the randomness of this renewable energy increases the non‐linear characteristics and uncertainty of the system, which causes a mismatch between renewable energy generation and load demand and it will badly affect the bus voltage control of distribution network. In this context, this study applies pumped storage hydroelectric (PSH) which tracks the load variation rapidly, operate flexibly and reliably to balance the power of the system to minimize the bus voltage deviation. Moreover, to obtain the optimal control policy of PSH, a deep‐reinforcement‐learning algorithm, that is, deep deterministic policy gradient, is utilized to train the agent to address the continuous transformation of the pumped storage hydro‐wind‐solar (PSHWS) system. The performance of a well‐trained agent was evaluated on the IEEE 30‐bus power system. Simulation results show that the proposed method achieves an improvement of 21.8% in cumulative deviation per month, which implies that it can keep the system operating in a safe voltage range more effectively.

Abstract Direct voltage fluctuations due to the presence of relatively large DC reactors (as an essential part of HVDC breakers), lack of inertia, and unwanted frequency fluctuations in the AC side of HVDC grids, have major consequences on the stability of HVAC/HVDC grids. The use of the DC Power System Stabilizer (DC‐PSS) can damp and eliminate voltage oscillations caused by the presence of the DC reactors. However, DC‐PSS cannot address the issues of inertia and unwanted frequency fluctuations. A method to improve inertia is proposed here that can operate well with the droop controller, and DC‐PSS does not interfere with power‐sharing and does not interact with any of these elements. Since the presence of a droop controller in HVAC/HVDC grids associates with power and direct voltage, the method proposed here can improve direct voltage fluctuations by eliminating severe power peaks. Moreover, this method does not change the voltage level of the entire system, so there is no need to change the set‐points of controllers. In addition, all parameters of the controllers are tuned by an intelligent algorithm, and the Participation factor (PF) scheme is used to find the proper placement of the proposed controller.

Abstract DC/DC converters are ubiquitous in renewable energies such as photovoltaic power systems. A novel and general approach is proposed that consists of three matching principles, which enables one to assign a best set of energy storage elements to a DC/DC converter to meet both desirable transients and small ripples, facilitating the design of a controlled DC/DC system. First, the principle of specification matching introduces a ‘virtual’ lower order model to meet the prescribed specifications. Second, the rule of order matching augments the ‘virtual’ model to a higher order, the same as that of the actual DC/DC circuit if necessary. Third, the principle of parameter matching helps derive desirable element parameters through solving an optimization nonlinear function with constraints. These principles are realized through a specific design method for boost DC/DC converters of a photovoltaic power system. Given some specifications, a numerical optimal design is executed to verify the effectiveness of the specific design tool as well as the proposed theory. A photovoltaic power system with the designed boost DC/DC converter is implemented in both the simulated environment and the physical environment.The experimental results show that both transient responses and steady state responses (or ripples) meet the given specifications. The proposed theory can either work alone in simple designs or assist in advanced control designs for a better performance.

Abstract This paper presents an in‐depth analysis for the two‐variable impedance model of a grid‐connected system. The two‐variable model describes the impedance using the frequency as well as the operating point variable. The impedance‐based stability analysis method cannot be directly applied because the conventional models are derived for a specific steady‐state operating point and the operating point varies during operation. Thus, this paper develops a new analysis method to shed light on the role the two‐variable model plays instability analysis. The proposed method not only analyzes the system stability at different operating points, but also enables the controller design to ensure system stability. Experimental results are presented to demonstrate the effectiveness of the proposed method.

Abstract Due to the negative impact of a highly stochastic wind power fluctuation on the power quality and stability during high penetration of wind power in power systems, there is growing interest in power smoothing and energy redistribution in wind power systems by using large‐scale energy storage technologies. The aim of this work is to use a vanadium redox flow battery as an energy storage system (ESS) to smooth wind power fluctuation with two system configurations and corresponding control strategies. As the first step, a vanadium redox flow battery (VRFB) mathematical model, underlain by electrochemical theories, is built to describe the battery behaviours in charge/discharge cycles. Accordingly, the aforementioned system configuration and power fluctuation smoothing schemes are proposed. At the end of this work, simulations are conducted on a 3.6 MW doubly‐fed induction generator (DFIG) at a diversity of wind speeds for an assessment of feasibility that this work can be applied to a high penetration of wind power in practice. Simulation results reveal that power fluctuations can be improved by power smoothing. The root mean square deviation (RMSD) percentage can be reduced to less than 1%.

Abstract Asynchronous interconnection will reduce the inertia of each grid, and it is difficult for a small inertia grid to maintain frequency stability under the fluctuation of new energy or load. In order to improve the frequency stability of asynchronous power grids connected by a voltage source converters based multi‐terminal direct current (VSC‐MTDC) system, a joint primary frequency regulation (JPFR) strategy is proposed in this paper. The main principle of the proposed JPFR strategy is firstly to formulate frequency regulation protocols (FRPs) between asynchronous power grids. Next, an improved analytical steady‐state model of the VSC‐MTDC system is adopted. The improved model can calculate the power reference of each VSC according to the FRP, avoiding the impact on the grid not participating in the protocol. Then, based on the sequential search method, the results of the improved model are modified to prevent overload. Finally, simulations of a four‐terminal VSC‐MTDC system are carried out for validating the proposed JPFR strategy.

Abstract This paper presents an approach to the power management of nanogrid clusters assisted by a novel form of peer‐to‐peer (P2P) electricity trading. DC nanogrids have lower power loss in real time and are suitable for P2P trading. Here, cost of electricity for clusters involving different kinds of photovoltaic (PV) power production (as a secondary source) is reduced by a newly proposed P2P trading scheme. For power management of individual clusters, multi‐objective optimization is applied to minimize simultaneously total power consumption, grid power consumption, and total delay incurred locally by scheduling. The temporal surplus of self‐supplied PV power of a cluster can be sold through P2P trading to another cluster subjected to a temporal power shortage. In P2P trading, a cooperative game model is used for buyers and sellers to maximize their welfare. To increase P2P trading efficiency, prediction of load demand and PV power production is considered for power management of each cluster to resolve instantaneous imbalances between load demand and PV power production. A gated recurrent unit network is used to forecast future load demand and PV power production in nanogrid cluster could reduce electricity cost by 29.2%.

Abstract Having a robust and accurate grid synchronization unit for distributed generators (DGs) has been accentuated due to rapidly growing integration of inverter‐based DGs to the autonomous microgrids. Although most of the synchronization methods are based on the phased locked loops (PLL) technique, digital filters could result in more salient dynamic performance in non‐ideal sinusoidal situations. This paper presents a novel digital filter as a grid synchronization method that overcomes the flaws of conventional PLLs under harmonic distorted and unbalanced cases as well as large delay of window‐based digital filters. For the sake of computational advantages, the proposed method is executed in two distinct offline and online stages. In the offline stage, the second‐order Taylor series expansion approximation of the bandpass oscillatory signal calculates the state‐transition matrices and state vectors. Then, by applying the concept of Kalman filter, the state vectors are updated in the online calculation section much easier and faster compared to all‐online approach. Compared to the existing methods, the proposed method generates a dynamic response with less settling time, more accuracy, and immunity against harmonic distortion, imbalances, and noise.

Abstract This paper proposes a distributed rule‐based power management strategy for dynamic power balancing and power smoothing in a photovoltaic (PV)/battery‐supercapacitor hybrid energy storage system. The system contains a PV system, a battery‐supercapacitor hybrid energy storage system (HESS), and a group of loads. Firstly, an active compensation technique is proposed which improves the efficiency of the power smoothing filter. Then, a distributed supervisory control technique is employed that prevents the BESS and SC from SOC violation while maintaining the balance between generation and load. To this end, the system components are divided into three different reactive agents including an HESS agent, a PV agent, and a load agent. These agents react to the system changes by switching their operational mode upon satisfying a predefined rule. To analyse the hybrid dynamical behaviour of the agents and design the supervisory controllers, the agents are modelled in hybrid automata frameworks. It is shown that the proposed distributed approach reduces the complexity of the supervisory control system and increases its scalability compared to its equivalent centralized method. Finally, the performance of the proposed approach is validated using a test system simulated in MATLAB/Simulink.

Abstract Wave energy emulators provide an option to extend studies of wave energy converters (WECs) in an electric power laboratory facility and to evaluate the performance of controllers in an experimental set‐up. In addition, the integration of control strategies in different steps of the energy conversion process is an important step to improve the WEC overall behaviour that has not yet been widely discussed. This paper proposes a real‐time emulator and a hierarchical control scheme for a hyperbaric WEC consisting of floating bodies, hydropneumatic storage system (HSS), hydraulic turbine, and doubly fed induction generator (DFIG). The proposed emulator is based on a typical electric power laboratory facility, where a numerical model reproduces the dynamics of the wet subsystem and a hardware is the WEC electrical subsystem. Thereby, a squirrel‐cage induction motor (SCIM) is coupled shaft‐to‐shaft with the DFIG. The SCIM reproduces the characteristics of torque and speed that would be observed in the real system, and then, the DFIG is subjected to a primary drive with same dynamic characteristics of the real plant. The hierarchical control integrates the generator and HSS controllers considering the optimal operating pressure for local sea state conditions. Experimental results illustrate the performance of the emulator and controllers.

Abstract This article demonstrates the benefits of optimising the drivetrain to improve the level and quality of electrical power output from a wave energy converter. The study considers a spherical buoy connected to a permanent magnet synchronous generator through a mechanical drive. The wave energy converter is equipped with a model predictive control system that maximises electrical power from the generator. Three different scenarios are compared: (i) when the drivetrain is not optimised, (ii) when only the gear ratio is optimised, (iii) and when both gear ratio and flywheel inertia are optimised. The performance of all three configurations is compared in terms of their effect on the generator operating range, the natural frequency of the system, the amount of generated electrical power, and control forces. The results demonstrate that the drivetrain optimisation leads to a significant increase in the electrical power output while shifting the generator's operating range to areas with the highest efficiency. Moreover, drivetrain designs that utilise a flywheel reduce the power take‐off loads and facilitate smoother power production.

Abstract Wave energy converters (WECs) exhibit higher energy capture capacity when the wave frequency is close to the nature frequency of the device. A possible way to enhance energy capture efficiency is to modify the nature frequency of a WEC to match the incoming wave frequency. This paper presents an inertia self‐tuning phase control strategy which can optimize the wave energy capture efficiency of a floating multi‐body (FMB) WEC by regulating automatically the total mass of the capture energy part according to the incoming wave conditions. A variable‐frequency inertia self‐tuning control system is designed to conduct the proposed control strategy. The mathematical model of rotational motion of the FMB WEC is built and the dynamic performance of the control system is analysed in detail. Simulation results illustrate the proposed control strategy can achieve a phase optimization and hence significantly improve wave energy extraction of the FMB WEC in both regular and irregular waves. For regular waves, the maximum capture width ratio)CWR( under the control strategy is 7.3 times higher than that no control strategy is used. For irregular waves, the maximum CWR under the control strategy is 4.4 times that without the control strategy at the same wave frequency.

Abstract Mooring systems are a significant capital cost of a floating wave energy converter and their premature failure negatively impacts operational costs. Excessive peak loads and accumulated fatigue damage can lead to failure, so these factors are cost drivers in wave energy converter design. Here, the potential to reduce platform motion and mooring loads through modification of the power take‐off (PTO) control are investigated. An approximate velocity tracking control strategy is implemented with a linear quadratic regulator design method using differential weighting of system states. It is demonstrated that the controller can be tuned to capture similar mean power to an optimally tuned, passively damped system while significantly reducing mooring line cyclic loading. The relative accumulated fatigue damage in the mooring lines in a high energy sea‐state is found to be reduced by between 43% and 92% as a result of using the approximate velocity tracking control strategy.

Abstract Control systems play a critical role in improving the economic viability of wave energy converters (WEC). This paper presents a framework for a causal power‐maximising feedback controller inspired by Phi method. Quadratic damper control and PID control are combined to collectively achieve the optimal power absorption conditions for the peak wave periods, which results in a nonlinear controller being suboptimal in irregular waves. The proposed controller is tested on a fully submerged heaving spherical point absorber in regular and irregular wave conditions in time domain. The tuning of the PID gains are investigated in frequency‐domain from a stability and control bandwidth perspective. The modelled WEC dynamics includes linear hydrodynamic forces (excitation and radiation) and the nonlinear viscous drag force. The main benefits of the proposed controller are (i) it does not require any time‐consuming optimisation either online or offline; (ii) has a wider bandwidth and better power absorption performance as compared to the conventional spring‐damper (PI) controller; and (iii) easy to implement in practice like standard PID control.

Abstract LCL‐type resonant dual‐active‐bridge (LCL‐DAB) DC–DC converters have more significant advantages in eliminating backflow power and achieving higher voltage gain compared with traditional dual active bridge (DAB) DC–DC converters. Therefore, LCL‐DAB DC–DC converters have a broad application prospect in DC micro‐grids, electric vehicles, uninterruptible power supplies etc. Since at this stage only a few researches on LCL‐DAB DC–DC converters have been done, here, the specific system operation region with triple‐phase‐shift (TPS) modulation scheme designed under no backflow power is presented. Furthermore, a dynamic compensation based feedforward control method under no backflow power is proposed, which enables the LCL‐DAB DC–DC converter to operate under no backflow power in wide region and achieve outstanding dynamic performance against output/input disturbance. Finally, the experimental results verify the feasibility and effectiveness of the proposed control method.

Abstract Dynamic voltage restorer (DVR) plays an essential role in achieving high‐quality voltage restoration in the distribution system. This paper presents an elliptical‐trajectory‐based approach to guarantee continuous power delivery by considering the active and reactive power limits during voltage sags. Besides, two parameters are proposed in the accurate mathematical analysis of current references. Thus, the control objective can be fulfilled by the flexible current injection strategy that combines a precise balance between the positive and negative sequences. The proposed control method can determine the required power injections by selecting the two proper values to support that the converter output current is smaller than the maximum rated value. Under the presented control, the maximum power limit and flexible power dispatch are combined together to develop four operation schemes. The main advantage is the smooth transition process during changes in the operation schemes and the disappearance of active and reactive power oscillations. Also, these four control states are applied to reveal the flexibility of the proposed designs. Finally, the hardware tests in a laboratory prototype are implemented to validate the feasibility of the proposed method.

Abstract This paper proposes a frequency‐control scheme for Doubly Fed Induction Generator (DFIG)‐based (Type‐3) wind turbines to improve the primary‐frequency‐control when the grid power balance is disturbed. The increasing penetration level of renewable energy sources, like wind power plants, reduces the total available inertia of modern grids, which deteriorate the frequency response in case of sudden power‐mismatches. The proposed closed‐loop participation of wind power plant interacts with the thermal units to reduce the frequency nadir and frequency settling‐time, during the inertial and primary stages. The designed disturbance observer decreases the uncertainties in the estimation of grid parameters, which results in robust PI performance in adjusting the ancillary power provided by wind turbines. Certain measures considered within the control loop to limit the rate‐of‐change‐of‐frequency within the permissive range to avoid the protective relays tripping. Comparative simulations studies on modified IEEE 9‐bus and 68‐bus test systems verify the effectiveness and advantages of the proposed method.

Abstract The sustained development of wave energy in the past two decades makes it one of the most promising renewable energy resources to be added to the diverse mixture of supply systems. The inherent difficulty of grid integration of wave energy involves various aspects such as suitable control of power converters and power conditioning processes, allowing for the extraction of the best quality power. This paper presents a comprehensive review of different aspects of grid integration of wave energy devices, including classification of wave energy devices based on their impacts on grid integration, grid requirements imposed by the grid codes and storage technologies used for the grid integration of wave energy converters (WECs). This study also analyses various grid integration studies on wave energy converters, with particular emphasis on power converter technology and control. Furthermore, specific attention is given to the combinational studies that use wave energy combined with other renewable resources due to their positive synergies in lowering the costs of energy produced and that hold an opportunity for future research. An economic case is presented for wave energy devices based on value on the grid.

Abstract Extracting energy from ocean waves has become a heated topic since the energy crisis of the 2000s. Among all the different concepts and designs of Wave Energy Converter (WEC), point absorber is a widely adopted type with great potential, and various configurations and constraints are applicable to it. Here, the point absorber WECs with four different set‐up configurations are explored: single body heaving WEC, two‐body heaving WEC, two‐body WEC with a flat plate (Reference Model 3), and a two‐body WEC with a cylinder‐shaped second body. Dynamic models are established for each case and wave tank tests are conducted for verification. The results show that the power capture of a point absorber can benefit from several aspects: the two‐body WEC with a streamlined shape can double the wave capture width ratio (up to 66.5%) over the single‐body WEC or Reference Model 3, while coupling other motion or mooring dynamics can further improve the capture width ratio by 12% by increasing the relative motion stroke.

Abstract The free‐piston Stirling linear generation system has tremendous applications in space power supply, biomass generation, solar thermal generation, factory waste heat generation, and other industrial fields. However, the free‐piston Stirling linear generation system suffered from the disadvantages of fluctuation of output electric energy and self‐starting. To solve the aforesaid problems, a two‐level electric energy conversion control mode which combines three‐phase AC/DC and bidirectional buck/boost DC to DC converter is proposed in this paper to make the generation system with stable output DC power. Then, both the linear and non‐linear mathematical model of the free‐piston Stirling engine are established. Additionally, to improve the performance of frequency and displacement tracking, the traditional proportional‐integral (PI) in the position loop is replaced by Proportional Resonance (PR) controller. Moreover, the Hysteresis‐SVPWM current control strategy with high robustness and dynamic response is applied to the rectifier to realize the unit power factor control of the AC side of the generator and the stability and controllability of the DC side voltage. Finally, the three‐phase voltage Source Rectifier (VSR) and Bi‐directional DC/DC with an appropriate inductance and capacitance for energy storage are modelled. The effectiveness of the proposed system is validated using theoretical analysis and simulations in Matlab.

Abstract This article presents an approach to implement a Nonlinear Model Predictive Controller (NMPC) in real‐time with a non‐standard cost index. The proposed technique's applications are presented to maximize the energy produced by a Wave Energy Converter (WEC) when the cost index is a non‐quadratic piecewise discontinuous functional of some design variables. The presented framework is based on pseudo‐quadratisation and weight scheduling, which is implemented using the ACADO toolkit for MATLAB/Simulink. The proposed strategy features code generation and deployment on the real‐time target machines for industrial applications. The simulations and experiments confirm the success of the proposed approach in achieving the feasible operation of the NMPC and an optimal power capture by the wave energy converters.

Abstract The high variability and unpredictability of renewable energy resources require optimization of the energy extraction, by operating at the best efficiency point, which can be achieved through optimal control strategies. In particular, wave forecasting models can be valuable for control strategies in wave energy converter devices. This work intends to exploit the short‐term wave forecasting potential on an oscillating water column equipped with the innovative biradial turbine. A Least Squares Support Vector Machine (LS‐SVM) algorithm was developed to predict the air chamber pressure and compare it to the real signal. Regressive linear algorithms were executed for reference. The experimental data was obtained at the Mutriku wave power plant in the Basque Country, Spain. Results have shown LS‐SVM prediction errors varying from 9% to 25%, for horizons ranging from 1 to 3 s in the future. There is no need for extensive training data sets for which computational effort is higher. However, best results were obtained for models with a relatively small number of LS‐SVM features. Regressive models have shown slightly better performance (8–22%) at a significantly lower computational cost. Ultimately, these research findings may play an essential role in model predictive control strategies for the wave power plant.

Abstract As a clean and renewable energy, wave energy has abundant reserves, and its reasonable development and utilization can effectively alleviate the problems of energy shortage and environmental pollution. This paper proposes a mechanical rectification wave energy converter. The reciprocating linear motion of the buoy is directly converted into the rotary motion through the ball screw. The magnetic coupling force transmission system is adopted to ensure the sealing of the device. Using mechanical rectification mechanism to ensure the service life of the generator. In addition, a dynamic model of the mechanical rectification wave energy converter is established. Through simulation analysis, the curve of power with load resistance is obtained. And there is an optimal load resistance value corresponding to the maximum power in different wave conditions. In order to improve the power generation efficiency, the maximum power point tracking control algorithm is designed based on the admittance differentiation method. By controlling the duty cycle of the switch tube in the Buck‐Boost chopper circuit, the variable load is realized, and then the maximum power point tracking control is realized. Finally, simulation analysis verifies the feasibility of the algorithm under different wave conditions.

Abstract A flume experimental study of a two‐body wave energy converter (WEC) with a hydraulic power take‐off (PTO) was conducted under the conditions of both regular and irregular waves. The experimental results were analysed using the control variable method, and the effects of the wave elements and external load on the relative heave motion response and capture width ratio of the device are discussed. For the regular waves, three modes were found when analysing the variation of the relative heave motion response with the incident wave height, and the initial moving line of the WEC model was obtained. For the irregular waves, the experiment displayed two motion states. One was a following motion with a similar value to the incident wave period, and the other was a delayed motion with a larger value than the wave period. Based on the calculation of the capture width ratio of the system both under the regular and irregular wave conditions, the maximum value occurred at a frequency ratio of ω/ωn=0.59 and a hydraulic load of p = 0.20. The capture width ratio in the irregular waves was approximately 10%‐40% higher than that of the regular waves for the same wave parameters.

Abstract Power electronics–based converters for photovoltaic (PV) systems are susceptible to overcurrents; it is important to design their controllers to reduce the transient current for all viable operating conditions. To design a current controller and find the maximum transient current via simulation‐based techniques, the exact values of the system parameters, initial states, and inputs are required. However, they are not precisely known in practice, some system parameters such as inductances may change over time, and output power and load are variable. The uncertainty in the parameter (filter inductance) and input of the system (injected power) should be considered in the analysis of a PV system controllers as it can degrade their performance, which are designed for the system nominal parameters. This paper employs reachability analysis for a grid‐connected PV system to (1) find the maximum transient current, (2) devise an improved PI current controller and (3) compare the maximum transient current in PI‐ and internal model control (IMC)‐based controllers with uncertain‐but‐bounded input power and inductance error. Simulation and experimental studies showcase the results.

Abstract Microgrids suitable operation depends on the system's proper design, based on accurate component models. A complete model considering entire system dynamics is a vital necessity for a successful design. Using dynamic load in microgrid small‐signal model results in a model that shows transient and steady‐state dynamics, since designing a low‐inertia system like microgrid need extra accuracy. In this paper, an inverter‐based microgrid's small‐signal model in islanded operation mode containing a dynamic load model has been achieved. The Exponential Recovery Load (ERL) model is presented here to study the dynamic behavior of load. Microgrid stability analysis has been carried out for both static and dynamic load models. To attain this goal, the entire component equations are obtained and linearized around an operating point, then the state‐space model is formed and eigenvalues are derived. System stability condition has been investigated by plotting eigenvalues and root locus method. In the final step, a simulation on Matlab/Simulink is performed to show the microgrids operation in the presence of a static and dynamic load.

Abstract Unexpected imbalances between supply and demand during real‐time operation in the wake of high penetration of renewable energy resources necessitate system operators to improve flexibility of the system. Moreover, increasing growth in flexible distributed resources installation in distribution networks with multiple MicroGrids (MGs) such as fuel‐based distributed generators, storage units, and controllable loads makes distribution system operators be able to manage energy imbalances locally through exploiting potentials of aforementioned resources in flexibility enhancement. This paper proposes a receding horizon data‐driven chance‐constrained energy flexibility trading framework for active distribution networks with multiple MGs. Specifically, a distance‐based confidence set is constructed in a data‐driven manner to apply net load uncertainty into this modelling. According to this proposed data‐driven approach, the energy flexibility is modelled using flexibility envelope notion to demonstrate its dynamics in different scenarios in an imminent horizon. Furthermore, MGs' flexibility trading problem is modelled as an exact potential game that possesses a pure strategy Nash Equilibrium (NE). An iterative decentralized algorithm based on alternating direction method of multipliers is developed to achieve the NE strategy of MGs. Numerical results are also represented to show the performance efficiency of the proposed energy flexibility trading framework.

Abstract Back‐to‐back (BTB) converters, which consist of an AC/DC rectifier cascaded with a DC/AC inverter, have been widely adopted in the interconnection of power systems. The DC link, being the interface between the rectifier and inverter terminals, is susceptible to disturbances, particularly from power fluctuations. Regardless of the magnitude of power or changes in the power flow direction, stability degradation will occur. To lessen the influence of power, a DC‐link current compensation control scheme is proposed and applied to the rectifier terminal. The proposed scheme reduces the DC link impedance gain of the rectifier terminal to balance the impedance at both ends, and thus enhance stability and controllability. The scheme is applied to a BTB converter to improve the system's resistance to disturbance, regardless of variations in power magnitude or power flow direction. The accuracy of the theoretical analysis and the effectiveness of the proposed scheme are verified using simulations and a scaled‐down experimental setup.

Abstract The cost of energy should be minimised to increase market penetration of wind energy conversion systems. The existing wind farm control strategies are not helpful since they speed up the damage in upstream wind turbines. Therefore, a control strategy is proposed to maximise the energy yield while equalising lifetime of one of the most fragile sub‐system of the wind turbines. The DC‐link capacitor bank of the back‐to‐back converters is considered to this end. The proposed control strategy can be directly applied in systems based on type 4 wind turbines with full power converters. This exposition reveals the formulation of the control strategy by analysing the strengths and weaknesses of the existing strategies based on the maximum power output. The lifetime‐based control strategy is extended to formulate a multi‐stage reactive power dispatch method with the help of a total energy‐based strategy. The effectiveness of all control strategies is validated using the model‐based simulations.

Abstract The work presented here deals with a comparison between integral, classical and multiscalar backstepping controllers applied for a PV water pumping system. The solar pumping system is controlled in its first part by a backstepping maximum power point tracking approach in order to extract a maximum power from solar panels. The second part, composed mainly by induction motor–pump is controlled by backstepping structures. The integral backstepping control structure gives interesting features in terms of stability using recursive Lyapunov design, increases robustness despite parameters variation, and provides good tracking and optimization performance. In order to validate the study with a real solar data, a measured irradiance profile is used to fed the PV system, based on solar measurements in tropical insular context. The measurements are collected at Sonapi site located in Haiti country. The fast variability of the tropical irradiance profile will allow to test the robustness of the used control algorithms and determine their limits. Simulation of the proposed solution is validated under Matlab/Simulink. Results demonstrate clearly that integral backstepping provides the best solution with a good tracking, and optimization performance: fast dynamic response and stable static power output, even when weather conditions (irradiation) are rapidly changing.

Abstract Due to the intermittent nature of the renewable energy systems (RESs), more specifically, solar panels and wind turbines, their sole use does not lead to a smooth and reliable power. To overcome this issue, the concurrent grid‐integration of RESs to form a microgrid is reported. In the DC‐bus microgrid, the produced power by RES is initially given to the shared DC‐bus through an individual source‐side converter and then transmitted to the utility via a common grid‐side converter. By increasing the number of RESs, the number of required power converters, and therefore, the investment cost also increase. Using the cost‐effective multi‐input low‐switch converters is a promising alternative to alleviate this significant need for individual converters. Recently, a nine‐switch‐based unified expandable power converter (UEPC) has been presented for concurrent integration of AC and DC sources with a tangible fewer switch count. This unified structure has been utilized in two configurations named AC‐AC‐AC and AC‐AC‐DC. In this paper, both configurations are evaluated and compared in terms of current stress and switching loss. Considering the current stress analysis, the best port for interfacing with the grid to lower the total current rating of power switching devices is also determined. The high‐performance capability of both configurations is finally verified using MATLAB/Simulink.

Abstract This paper proposes a novel distributed bilayer coordinated control scheme (BCCS) to realize the autonomous economic operation of islanded hybrid AC/DC microgrids, which only requires a sparse communication link. In the lower layer, the optimal power reference iterative algorithm (OPRIA)‐based droop control is proposed to realize the economic dispatch of each subgrid individually, while the frequency of AC subgrid and the bus voltage of DC subgrid are regulated to their rated values. Then, the upper‐layer control for the interlinking converter is designed to achieve the global economic operation of the hybrid AC/DC microgrid through optimizing the exchanging power between AC and DC subgrids. The proposed BCCS has the advantage of reaching the economic operation and frequency/voltage regulation simultaneously for the hybrid AC/DC microgrid. Besides, the proposed BCCS realizes the stable and economic operation with less communication, flexible controllability, and good compatibility of renewable energy sources. Numerical simulation results based on Matlab/Simulink verify the effectiveness and excellent performance of the proposed BCCS even with the integration of renewable energy sources, and the test based on RT‐LAB illustrates the good real‐time performance of the proposed control scheme.

Abstract This paper proposes a robust investment and operation model to attend the power and heat needs of a microgrid (MG) connected to the distribution system. The optimization algorithm decides on the best investment and operation of combined heat and power (CHP), boilers, PV power generation and battery energy storage systems (BESS). For the BESS, the algorithm estimates the optimal energy storage capacity (MWh) as well as the maximum hourly delivery capacity (MW). The non‐linear and non‐concave heat rate chart is recast by a mix‐integer linear model to have a tractable and precise model. The model considers the uncertain in some parameters using probability density function (pdf) to portrait its behavior. Thus, the problem has been modeled using stochastic programming approach, and its objective function is the expected value of the annual operational cost. The model is tested using a real case where two adjacent consumers share the power and heat facilities to minimize the overall up to 17% depending on the gas price scenario. The results demonstrate the benefits of employing different technologies and the synergies of all technologies operating together.

Abstract Virtual synchronous generator technology can effectively improve the anti‐interference characteristics of the system frequency and bus voltage in the microgrid, and solve the problems of insufficient damping and low inertia of the system. However, in an islanded microgrid with multiple distributed generation, the difference in line impedance will cause local voltage deviations, which in turn leads to a series of problems, such as reducing power distribution accuracy and increasing bus voltage drop. Therefore, for the island‐type microgrid multi‐inverter distributed power generation parallel system, in order to solve the problem of low power distribution accuracy and large frequency oscillation caused by system parameters in virtual synchronous generator control, an improved virtual synchronous generator control algorithm based on adaptive droop coefficient is proposed in this paper, which not only eliminates the resistance component of the line impedance, makes the system impedance characteristic present a purely inductive nature, but also realises real‐time adjustment of active and reactive power. While maintaining the stability of the bus voltage and system frequency, it maintains high power distribution accuracy and improves the dynamic performance and operational stability of the power grid system.

Abstract In contemporary grid infrastructure, grid frequency is still predominantly controlled by conventional power plants. However, the increasing installed power of distributed energy resources and the declining amount of synchronous generators will necessitate major participation of wind farms as well as photovoltaic systems in frequency control. From a technical point of view, PV inverters are able to provide not only manual but also automatic frequency restoration reserves as well as frequency containment reserves. However, market and regulatory conditions are not designed for them to participate yet. As power electronic‐based systems, photovoltaic inverters are able to react even faster to frequency deviations than conventional power plants. This characteristic is leveraged to analyse the contribution of photovoltaic systems to stabilise the grid frequency. Therefore, a new control reserve product, called fast frequency containment reserves is developed. It is shown that the frequency stability can be considerably improved by integrating PV systems into frequency control.

Abstract Voltage regulation and power management are necessary to maintain balance of supply and demand in DC microgrids. In such systems, power sharing is normally done through parallel operation of distributed energy resources equipped with droop controllers. However, low power sharing accuracy and not allowing the microgrid to maximise the available power from the renewable sources are two main problems associated with conventional droop control methods. In addition, the 2D droop method is a parameter‐dependent method for extracting maximum available energy from renewable sources. In this paper, a novel power management strategy based on the optimal 3D droop coefficients is developed for a DC microgrid. Optimal state estimation is attained using a combination of extended Kalman filter and adaptive recursive least square method. Reference currents of renewable energy sources (wind and solar) and battery energy storage system are estimated using the proposed prediction based model. The proposed strategy not only increases the power sharing accuracy but also remains the bus voltage around a nominal value. The performance of the proposed method is evaluated for the considered DC microgrid in two different scenarios. Results show the high effectiveness and robustness of the proposed method. It has been concluded that precise estimation of the sources reference currents and 3D droop coefficients are critical for optimal power management and bus voltage regulation in DC microgrids.

Abstract The interconnection of microgrids to form a network known as the multi‐microgrid (MMG) brings higher resilience and support in providing power to the loads. The dynamic response during interconnection is usually bypassed with the addition of synchro‐checkers installed on all the microgrids and the exchange of critical setting points between the microgrids prior to interconnection via a low‐bandwidth communication. This paper introduces a novel decentralized framework for maintaining the voltage and frequency of the microgrids within permissible operating range during the interconnection. The proposed control strategy is triggered when the microgrids violate the defined states of operation and the fast control action from each type of Distributed Generation (DG) in the microgrid is designed to tackle the transients in the voltage and frequency in order to regulate them within the IEEE standards. Moreover, the study is extended to evaluate the performance of interconnecting single microgrids to form MMG using the decentralized proposed control strategy and then sectionalizing the MMG back into separate autonomous microgrids. The results have been verified using PSCAD/EMTDC. The proposed control strategy demonstrates superior performance and simplicity for carrying out the energy management for interconnecting and sectionalizing of MMG without any kind of communication and synchro‐check infrastructures.

Abstract Pitch control is a basic function of large‐scale wind turbines to improve energy capture efficiency. Pitch system is also a component system with a high failure rate due to the complex pitch behaviour. It is helpful to the practical design and control if a clear pitch behaviour is obtained. However, pitch behaviour is affected by the structure of wind turbines, control strategies, wind resource characteristics, and so on. So, there is still a lack of sufficient understanding. In this study, the pitch behaviour of a 2 MW wind turbine is investigated based on SCADA data. Four key evaluation indicators describing pitch behaviour are proposed, which include pitch status, pitch time consumption, pitch angle peak, and mean pitch angle. Moreover, the SCADA data extraction algorithm for pitch behaviour investigation is also given. Then, the pitch behaviour is investigated based on statistical evaluation. Some valuable conclusions have been given. For example, for the investigated wind turbine, the time proportions for the four states (non‐pitch, increasing pitch, pausing pitch, and decreasing pitch) are 85.80%, 5.97%, 1.45%, and 6.78%, respectively. The pitch cycles, pitch angle peak, and mean pitch angle increase with the increase of the standard deviation of wind speed.