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Abstract This paper proposes a coordinated control strategy to enhance the low/high‐voltage ride‐through (L/HVRT) capability of grid‐tied micro‐grids (MGs). The novel control scheme, which is based on grid‐following concepts, contains two layers for each power source of MG. The former, known as the primary layer, has reverse‐droop and non‐linear robust sliding mode control. Furthermore, the secondary layer detects voltage perturbations based on L/HVRT curves. Then, the reactive power required for supporting the main grid is determined through the dynamic‐voltage‐support requirement of the grid codes. Finally, coordinated reactive power‐sharing among the sources is performed based on the presented method and free capacity of the power sources. Several case studies are carried out to assess the effectiveness of the proposed control scheme, which points out that the aims of this study, such as compliance with the grid codes, sharing reactive power among the sources, tracking the reactive power precisely and feeding local loads, are achieved. The superiority of the presented strategy against its grid‐forming counterparts is demonstrated in comparison tests for the studied grid‐tied MG. Simulations are performed in an offline MATLAB/Simulink environment and then validated by experimental real‐time results.

Abstract The constant growth of distributed energy resource (DER) in the electric power system (EPS) is a reality on account of some aspects, for example, technical, financial, environmental. The insertion of DERs can be positive, providing ancillary services, for example, harmonic mitigation, or negative, causing overvoltage, depending on the proper control and the characteristics of the grid. This paper proposes a pq‐based control strategy for distributed generation, wherein the studied system is composed by a photovoltaic DER and a three‐phase converter for processing the generated power and providing ancillary services (ASs) to a three‐wire EPS. The power control is based on the power balance between the grid and the converter. Besides supplying active and reactive power, the proposed strategy can provide ASs by controlling the grid currents from a pq transformation. Some advantages over other solution can be mentioned, such as reduced sensor number and instantaneous control response. In addition, neither additional harmonic‐extraction method nor a phase‐locked loop is required. To validate this study and theoretical considerations, simulation results are presented and evaluated. From tests in a downscaled laboratory setup, experimental results are also presented to verify the effectiveness of the proposed control strategy.

Abstract When traditional finite‐set predictive power control is applied to Vienna rectifier, the input current will produce a large number of third harmonics under unbalanced power network, which will seriously affect the normal operation of Vienna rectifier. This paper proposes a compound control strategy for three‐phase Vienna rectifier based on grid imbalance. And the new finite control set‐model predictive direct power control (N‐FCS‐MPDPC) is used in the inner current loop. By introducing a redefined reactive power, this method can effectively control the second harmonic of instantaneous active power and the second harmonic of instantaneous reactive power at the same time, and realizes the normal operation of Vienna rectifier under the condition of unbalanced power grid. Linear auto disturbance rejection control (LADRC) is adopted in the outer voltage loop, which overcomes the problem of voltage overshoot and slow dynamic response in PI control. Furthermore, in order to solve the difficulty of grid‐side voltage acquisition when the grid is unbalanced, a virtual flux based no‐grid voltage sensor is introduced to measure the grid voltage. Simulation, along with experimental results, is presented to show that the proposed control strategy has good robustness and fast tracking performance under the condition of power grid imbalance.

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 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 Capability to deliver maximum power of a converter is essential for increasing power density that is limiting in many applications. Therefore, control algorithm should guarantee operation within converter thermal limits. This requirement can be formalized by a hard constraint in the cost function of model predictive control (MPC). However, the temperatures of the converter elements are not constant in the steady state, which complicates evaluation of the cost on long prediction horizon. Therefore, the evaluation is simplified utilizing the analysis of steady state behaviour of the model and derived current derating laws calculated in off‐line manner. The derating law is used as the terminal set in MPC which allows using one‐step‐ahead evaluation for efficient real‐time implementation. The steady state analysis also provides coefficients for power loss balancing. The proposed approach is applied to control of dual converter, which has high redundancy of switching elements and, thus, wide space for optimization. It is shown in simulation that the proposed approach has better performance than previously published algorithms, at lower computational cost. Experimental evaluation of the algorithm performed on a converter prototype of rated power of 10 kW shows that the proposed controller is able to safely operate the converter near the thermal limit.

Abstract An efficient power sharing between the solar PV array and the utility is the core aim of this work. A mechanical sensorless direct torque controlled (DTC) induction motor (IM) is implemented for the water pumping application. The PV array is operated at maximum power point (MPP), ensuring full utilization. Therefore, the integrated system ensures uninterrupted power for water pump operation regardless of the irradiance level. For serving the purpose of grid integration and IM control, two voltage source converters (VSCs) are used. The two VSCs share a common DC link. An artificial neural network (ANN) based direct power control is utilized to maintain the power transfer in both directions between the PV array‐grid integration and an induction motor drive with a water pump. Moreover, an observer based current adaptive approach is used for motor current estimation from a single current sensor used for DC link current sensing. The overall system comprising of the three‐phase grid in conjugation with PV array operating induction motor drive used for water pumping, is simulated and verified through test results. MATLAB/Simulink environment with Sim‐power system toolbox is used to perform the software simulation. In the laboratory, a test bed of grid connected water pumping system is developed.

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 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 Solid‐state transformers (SSTs) have flexible capabilities to improve the power quality issues. These improvements impose disturbances into the SST converters that require proper control algorithms. In this paper, the problems of the unequal active power flow in different phases of the SST due to the grid voltage unbalance conditions and the negative sequence current injection are discussed, and an effective method is proposed to equally sharing the active powers between the SST clusters by creating an optimum zero sequence current. Based on the power dependencies, an analytical vector for the zero‐sequence current is proposed, which can equally share the power among the SST clusters and balance the HVDC‐link voltages. Also, the proposed control strategy by using PIR controllers can achieve the zero‐sequence current based on a direct current control method. The provided equal power‐sharing among the clusters is kept in the considerable unbalanced voltage of the grid and prevents the SST outage during faults. The simulation and experimental results confirm the proposed control method to equalise the power‐sharing among the SST clusters and voltage equalisation of the HVDC‐links while the SST tries to improve the grid voltage unbalance and in the fault conditions of the grid.

Abstract The beat phenomenon can be a critical issue in railway traction applications. This paper proposes a dual‐frequency compensation (DFC) method for the beat‐less algorithm to shift the spectrum of the beat voltages to a higher‐frequency region. Two variables for frequency compensation are derived by mathematical calculations based on the single‐frequency compensation (SFC) method. The applicability and superiority of the DFC method are analyzed. It is proposed that the DFC method can be a transition between the modulation index compensation (MIC) method and the SFC method, so the beat phenomenon can be greatly suppressed in the full speed range. The implementation of the DFC method is proposed to guide how to generate the two variables. The experimental verification is carried out on the StarSim hardware‐in‐loop platform, and the results demonstrate that the DFC method is more effective compared with the SFC method for beat phenomenon suppression. It also shows the modulation index with the DFC method is higher than that with the MIC method.

Abstract One of the control problems drawing engineers' and researchers' attention is the tracking control with predicted reference signals. The applications of this control problem are found in electric power control systems, automatic driving systems, and so on. The difficulty of the tracking control with the predicted signal is that the prediction errors lead to performance degradation of control systems. This paper aims to shape the predicted signals based on past information of reliable reference signal and system dynamics information to reduce the performance degradation caused by prediction errors. First, a signal shaping mechanism called prediction governor was proposed, and its optimal design problem was formulated. Then, an optimal solution for the design problem was analytically derived and the performance limitation of the prediction governor was clarified. The proposed optimal prediction governor is tailor‐made for a given control system. By adding it to the control system, the influence of the prediction error generated by a given prediction tool can be minimised. Finally, the usefulness of the proposed optimal prediction governor was illustrated through an electric power balancing control problem with a microgrid system.

Abstract This study presents a strategy involving a reduction in the stator current sensor count for control of power converters, in a doubly fed induction generator (DFIG)‐based wind energy conversion system (WECS). The reduced sensor vector control scheme is effectively used in lieu of the conventional vector control algorithm during sensor failure without any change in the overall control structure. This keeps the system operational in case of sensor failure. The reduced sensor scheme necessitates precise estimation of stator flux amplitude and an orientation angle, for control of power converters under all operating conditions. A composite generalised integrator phase‐locked loop is presented to estimate accurately the stator flux amplitude and orientation angle, even at operating system irregularities such as unbalanced voltages, lower‐order harmonics in the grid voltage, DC bias in sensing circuitry, and so forth, which otherwise lead to distortions in flux and orientation angle. The grid‐side converter control is based on a twin frequency improved second‐order generalised integrator. It enables fast and accurate power quality improvement in DFIG‐based WECS during load and grid abnormalities. A laboratory prototype is developed to validate the control of the system.

Abstract The classical switching‐table‐based direct power control usually employs one active voltage vector during the whole sample period, which will bring distinct current harmonics and large power ripple. This paper proposes an improved deadbeat direct power control for a dual three‐phase permanent magnet synchronous generator. The deadbeat control of both active power and reactive power is employed to calculate the duty cycle of the active vector. By this way, the active and reactive power can follow the references as much as possible during each sample period. Then a weight coefficient is introduced and its effects on the steady‐state performances of active and reactive power are analysed. With this proposed method, the current harmonics and power ripple are suppressed considerably. Meanwhile, the simple structure and fast dynamic response are preserved. The experimental results validate the effectiveness of the proposed strategy.

Abstract In view of the growing load demand in developing countries, overloading in existing transmission lines is a major concern. In addition to the voltage drop due to overloading, the proliferation of non‐linear loads in recent years distort the shape of sinusoidal voltage. Stable transmission of power through a transmission line under such condition is a challenge. In the presence of harmonics, control of transmittable power using static synchronous series compensator needs additional efforts, where increased power demand can be fulfilled by taking into consideration various other parameters. If an attempt is made to increase transmittable power through a transmission line using SSSC, power loss, the magnitude of voltage and its shape deteriorate in the presence of non‐linear loads. Thus, the trade‐off between transmittable power, transmission loss and voltage quality is tackled by optimal control of the compensator. In this study, a novel optimal control of SSSC is proposed with the objective of minimising transmission loss by maintaining the voltage within the limits while enhancing the transmittable power under non‐sinusoidal conditions. A two‐machine model of power system with 400 kV transmission line with parameters as per standards issued by the Central Electricity Authority is considered for validation of the algorithm using MATLAB platform and results are presented.

Abstract This paper proposes a power sharing control scheme for power electronics interfaced distributed generation to attain accurate harmonic power dispatch as well as unbalanced power sharing. The proposed controller is composed of two main blocks: 1) a harmonic power sharing unit, which leads to an accurate harmonic power sharing, and 2) individual triple‐droop control units separately applied to the control unit of each phase, which properly provide unbalanced power sharing. For this purpose, a virtual harmonic conductance concept is developed to control the inherent impedance of the inverter under non‐linear load conditions. As a result, the fundamental power sharing performance as well as the harmonic power dispatch accuracy is ensured. Simulation results from an islanded microgrid prototype using the CIGRE low‐voltage distribution network are provided to validate the effectiveness of the proposed method.

Abstract To facilitate the application of various renewable generations as well as diversified loads, hybrid DC/AC microgrids have aroused widespread interest; however, direct access of different types of sources and loads to the utility through multiple converters may be inefficient. In addition, with the complexity growth of the system, the stable and efficient operation puts forward new requirements for the isolation protection and more intelligent control. Facing these problems, the isolated two‐stage converter topology based on solid state transformer (SST) is designed and the automatic seamless switching power sharing control strategy is proposed. The proposed topology replaces the conventional passive transformer with dual‐active‐bridge converter (DABC) to realize voltage matching and galvanic isolation for higher controllability and less volume occupation. Constant ratio control method is adopted for DABC to operate as an ideal DC transformer. Further, AC frequency and DC voltage are measured as characterizations of power level to adjust the transferred power of the converter. Through the non‐directional strategy, bidirectional power support and decentralized cooperation are realized, and the capacity and sensitivity of microgrids are considered. The stability of the proposed control strategy is analyzed and the feasibility is verified by several real‐time hardware‐in‐loop experiments.

Abstract This paper presents a novel approach to adapt the conventional direct power control (DPC) methods that are applied to ac/dc converters with an inductive capacitive inductive (LCL) filter. These new solutions improved the conventional DPC with an active damping algorithm via multi‐variable constraint adjustment, which is based on an augmented look‐up table and multi‐variable comparators. The novel look‐up table proposed for the active damping algorithm of the LCL filter allows an optimal voltage vector assignment to be chosen using differences in converter‐side current and capacitor voltage. This method is also compared with the DPC method with active damping via the addition of a virtual damping resistor and a harmonic rejection loop. The steady‐state (total harmonic distortion of the grid current) and dynamic performances (response time on step changes of the set current), as well as the immunity under distorted grid voltage conditions of the proposed system, are verified through simulation results and experimental measurements.

Abstract In the isolated wind‐diesel hybrid power system, the system voltage and frequency may be affected simultaneously by the fluctuated wind turbine output power and variable load, which can make the system crash in extreme cases without control strategy. Therefore, in order to reduce the voltage and frequency deviation caused by system power fluctuation, both the battery and super twisting algorithm sliding mode state observer (STATCOM) are equipped to adjust the active power and reactive power for load power demand in the isolated small‐scale power system. Then, the system mathematical model is established which considers the cross‐coupling effect exist in the small‐scale power system. In order to improve the control effect of battery and STATCOM, the unified control strategy is designed for battery and STATCOM by using the super twisting sliding mode state observer and the adaptive sliding mode controller according to the constructed cross‐coupling effect model. Furthermore, the parameters of controller are further optimized by using the fuzzy method. Finally, the simulation experiments are performed by the real‐time digital simulator to validate the effectiveness of the proposed control strategy under different operation points.

Abstract Traditional permanent magnet synchronous generator (PMSG)‐based wind turbine (WT) normally utilises phase lock loop (PLL) for obtaining accurate phase angle of AC grid to make sure of maximum power point tracking (MPPT). However, a weak AC system with a low short circuit ratio (SCR) and high control bandwidth of PLL might have impaired effects on the stability of the system. To overcome these, a novel power synchronous control of PMSG‐based WT with grid forming ability of PMSG for integrating very weak AC system is first proposed in this study. Grid‐side converter (GSC) of PMSG emulates the inertia response of synchronous generator (SG) by artificially coupling the DC‐link voltage with the grid frequency based on the certain droop property, while the converter voltage magnitude is regulated according to DC‐link voltage deviation by controlling the modulation index of GSC. This novel scheme can realise the grid synchronisation without PLL by mimicking the natural characteristic of SG. Small‐signal analysis of the proposed scheme indicates the DC‐link capacitor energy can be exerted for system inertia support during system disturbances, while the system damping can be provided by spontaneously alternating converter voltage magnitude. More importantly, MPPT property of PMSG is not compromised while providing inertia and damping for the system, and the related inertia and damping coefficient of WT with the proposed control can be easily tuned for satisfying the system requirement. Non‐linear simulations of one PMSG connected with one large SG considering a sudden change of active power reference and variable wind speeds have been studied to verify the effectiveness of the proposed grid‐forming control.

Abstract The increasing number of distributed energy resources (DERs) leads to the need for DER that can provide ancillary service, such as low‐voltage ride‐through (LVRT) voltage support. The support is made to improve the short‐term voltage stability in power systems. Recently, DER LVRT voltage support through reactive power regulation are becoming common. Designing LVRT voltage support is highly influenced by the accuracy of the modelling and the data needed. However, in some cases, the required modelling can challenge the computational complexity, effort, and required data. Nevertheless, modelling the systems through an oversimplification may result in inaccuracies and hypothetical solution. Thus, the best compromise between model accuracy and simplicity will alleviate the problem. The study presents a methodology for estimating the effectiveness of the support. Unlike the commonly computer‐assisted approach, such as the dynamic RMS simulation, the benefit of the methodology is on the computing process, which is much simpler since the dynamic DER modelling work is not needed, and the evaluation of the effectiveness can be achieved even when the grid information is incomplete. Its accuracy is validated against typical RMS simulation results obtained using PowerFactory DIgsilent software for well‐characterised networks.

Abstract Recently, the offshore wind farms (OWFs) have excellent potential in global power networks. So, the demand for high power and high‐quality transmission system is becoming more pressing with the fast growth of OWFs. A promising solution is provided by the voltage source converter‐high voltage direct current (VSC‐HVDC) system. This paper proposes a control method for controlling the VSC‐HVDC system to integrate OWFs into power grids based on model predictive control (MPC). To minimise the distortion and filtering of the harmonics injected into the utility grid, the receiving end converter of the VSC‐HVDC system is connected with LCL filter. The proposed control strategy maintains active/reactive power‐sharing with efficient regulation of AC voltage among the different OWFs operation conditions. The detailed system components modelling and simulation are executed in the MATLAB/Simulink environment, which is used to perform both dynamics and transient evaluation of the suggested control strategy. The acquired simulation results emphasize the capability of the proposed control strategy to maintain system stability under different operation conditions compared to the synchronous reference frame‐based current limiting droop control method (SRF‐based CLD).

Abstract In this paper, a flexible power dispatch strategy based on the elliptical trajectory is proposed to maximize the operating region for dynamic voltage restorer (DVR) during voltage sags. This control scheme introduces two individual parameters to accomplish continuous power delivery within the voltage security boundaries required by the loads. These two key parameters are presented in the exact mathematical equations of instantaneous power dispatch, which allows the individual control of active power and reactive power of DVR. Under the provisions of the given control strategy, the load voltage regulation and flexible power dispatch are combined together by the voltage sag compensation of the elliptical trajectory. Thus, the voltage source converters output current can be limited within the maximum rated value. A smooth transition can be achieved as well during changes in the operation scheme when fulfilling the presented power delivery. Eight scenarios of different active and reactive power delivery are presented to validate the feasibility of the proposed strategy. Finally, laboratory results are used to reveal the effectiveness of the given control schemes.

Abstract Solid‐state transformer (SST) is an attractive concept, which provides galvanic isolation and voltage scaling by means of a medium‐frequency link. In this paper, a single‐stage alternative current (AC)‐AC SST solution without bulky energy storage elements is presented. The front‐end rectifier (FER) and rear‐end inverter (REI) of the SST works at line‐frequency, while the intermediate LLC series resonant converter (SRC) fulfils the tasks of galvanic isolating and regulating the output voltage. To achieve high efficiency and bidirectional power flow capability of the SST, the zero voltage switching (ZVS) condition of the LLC SRC is comprehensively analysed, and the parameters design method of the LLC SRC is developed, where the resonant currents are constructed by applying the same gating signals to the primary‐side and secondary‐side bridges and the dead‐time is properly selected. Consequently, the presented SST has the merits of high conversion efficiency, naturally bidirectional power flow capability, potentially high power density and high reliability. The experimental results on a 1.5 kW SST prototype show that sinusoidal input and output current, ZVS operation over the full load range are achieved, and the peak efficiency is 97.63%. The experimental results verify the functionality and effectiveness of the developed methods.

Abstract A new control strategy for a wind‐BES (battery energy storage) microgrid is presented, which controls the power transfer and the mode of operation seamlessly from an islanded mode (IM) to the grid connected mode (GCM) and vice versa. In GCM, the DC link voltage regulation, the load reactive power compensation and harmonics current mitigation are performed by the load side voltage source converter (LVSC) using a double sinusoidal signal integrator (DSSI) based control algorithm. The maximum power from the wind generation is extracted by operating the generator side voltage source converter (GSC). At the grid fault or grid disturbances, this microgrid operates in IM and LVSC is controlled in the voltage control mode. A synchronizing controller is utilized to control the operation of synchronization. The grid current and point of common coupling (PCC) voltage distortions are kept within the limit as per the IEEE‐519‐2014 and the IEEE‐1547 standards. The system operation is demonstrated through test results at several conditions such as, changing wind speeds, varying loads and seamless transition.

Abstract Operating distribution grids is increasingly challenging due to the increasing penetration of photovoltaic systems. To address these challenges, modern photovoltaic inverters include features for local control, which sometimes lead to suboptimal results. Improved communication infrastructure and photovoltaic inverters favour global control strategies, which receive information from all the systems in the grid. An estimation of distribution algorithm is used to optimize a global control strategy that minimizes active power curtailment and use of reactive power of the photovoltaic inverters, while maintaining voltage stability. Optimized global control outperforms every other local control evaluated in terms of apparent energy used for control (9.9% less usage compared to the second best alternative in all scenarios studied) and ranks second in terms of voltage stability (with a 0.14% of total time outside the voltage limits). Two new indicators to compare control strategies are proposed, and optimized global control strategy ranks best for both efficiency index (0.98) and average apparent power use (0.48 kVA).

Abstract This study focuses on the improvement of the power quality in interconnected power distribution systems using connected microgrids. The power quality issues at the secondary distribution level are addressed using a unified power quality conditioner, which aids in controlling voltage imbalances and current harmonics. Further, a power control method, adaptive network‐based fuzzy inference system, is proposed as a global solution by installing independent compensating devices at the point of common coupling. The proposed strategy can improve the power quality in traditional installations because the proposed unified power quality conditioner method maximizes power utilization and improves the efficiency of the system. The microgrids integrate renewable energy sources, which generate green energy with low loss. Case studies, a prototype, and simulations via MATLAB/Simulink are used to validate the proposed method, and the results prove the enhancement in power quality control. The proposed control strategy had a response time of 0.3 s, reduced peak voltage distortions, and restored the voltage at the interconnecting point to a normal level. The total harmonic distortion of currents decreased from 21.13% to 14.74% when a PI technique was used to control the UPQC‐P, and the proposed adaptive network‐based fuzzy inference system‐based UPQC‐P reduced the harmonic content to 2.43% from 21.13%.

Abstract In order to solve the problems of power quality reduction and power fluctuation caused by large‐scale wind power grid‐connected, an advanced control strategy to smooth the power fluctuation and allocation of hybrid energy storage system is proposed. Based on theoretical researches, the mathematical model of hybrid energy storage system is adopted to well analyse the fluctuation and smoothing strategy of wind power. Compared with the traditional filtering algorithms, the study proposes a method combined optimal exponential smoothing with complete ensemble empirical mode decomposition with adaptive noise and normalized energy entropy to improve the accuracy of grid‐connected output power and power allocation. Furthermore, the fuzzy control theory is used to improve the reliability of the algorithm after obtaining the smoothed power output and initial power allocation instructions. To prove the validity of the algorithm, case studies are constructed to demonstrate the performance in this paper. Experiments and example simulations show that the proposed method can effectively realize adaptive power allocation and improve the accuracy of identification. After effectively improving the efficiency and service life of the energy storage system, it provides a basis for large‐scale grid operation.

Abstract During summer 2018, the Nordic system's kinetic energy dropped below a critical level. As a consequence, Svenska kraftnät, the Swedish transmission system operator, requested the largest production unit to reduce its power output to guarantee system's security. This action resulted in a deviation from the generation dispatch determined by the market and in high costs for the Nordic transmission system operators. In this regard, this paper presents a tool for comparing mitigation strategies from an economic point of view and evaluates potential economic benefits of utilizing the emergency power control functionality of high‐voltage direct current lines for the provision of fast reserves as a compliment to frequency containment reserves. Moreover, the analysis is extended to the years 2020 and 2025 using inertia estimations from the Nordic transmission system operators. The findings of the paper suggest that the frequency of redispatching actions will increase in the future and that the cost of security for Nordic transmission system operators could be reduced by 70% if high‐voltage direct current links are used for frequency support.

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 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 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 Covert communications hide the very existence of wireless transmissions and thus can address privacy and security issues in non‐orthogonal multiple access (NOMA) systems. In this paper, we investigate covert communications in uplink NOMA systems with channel inversion power control (CIPC), where a public user's transmission is exploited to confuse a warden's detection on a covert user's communication behaviour. More precisely, the public link with CIPC causes the channel uncertainty of received signals at the warden. Specifically, the warden's detection error probability is derived and a guideline to make the optimal choice of CIPC parameters to obtain the maximum covert rate is proposed. The analysis and simulation results show that the covert rate is restricted by the interference between two users when the public user's CIPC parameter is relatively large.