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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 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 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 Auction mechanism analysis provides favourable economic outcomes for key stakeholders involved in the restructured power market. Real power pricing based on locational marginal pricing has been implemented in the electricity market worldwide. In this study, the optimal power flow is considered to minimise the operating cost of the active power generation in the ex‐ante energy market and an augmented optimal power flow in the ex‐ante reserve market. The double‐sided auction mechanism has better control over the energy and reserve markets, enhancing social welfare in the restructured power markets. Single‐ and double‐sided auction mechanisms are considered to analyse the allocation and pricing economics in the ex‐ante day‐ahead energy and ex‐ante day‐ahead reserve markets. Locational marginal pricing is calculated and analysed for both the on‐and off‐peak demand periods. The proposed auction model was validated using an IEEE 30‐bus power system. The benefits of the double‐sided auction are assessed from technical and economic perspectives.

Abstract An equal‐weighted cost function‐based weighting factor tuning method for model predictive control (MPC) in power converters is presented here. In conventional MPC methods developed for power converters, the use of weighting factors is a necessity to determine the importance of sub‐terms in multi‐objective cost functions. The proposed method is based on equalizing the importance of cost terms by using normalized values rather than the actual values. As a consequence of normalized values, all sub‐terms are transformed into a similar range independent from their actual values. Hence, the weights of cost terms are equalized, leading to the elimination of weighting factor tuning necessity. The proposed equal‐weighted cost function not only offers simplicity in the MPC design but also guarantees the desired controller performance with the dynamically weighted sub‐terms. The proposed control method is validated on a grid‐connected single‐phase three‐level T‐type inverter. Experimental results are carried out to demonstrate the effectiveness of the proposed method under steady‐state and dynamic conditions. Moreover, the comparative results with the conventional MPC are presented. The results reveal that the proposed approach exhibits excellent performance against the variations in the operating point of the inverter.

Abstract In applications where DC current is superimposed, such as inductors for non‐isolated DC‐DC converters, powder core can be considered as an attractive solution owing to its superior features such as: high saturation flux density, high Curie temperature, and soft‐saturation property. However, in powder cores, a drop of inductance appears prominently due to DC superimposition characteristics. This feature is not a desirable characteristic because it leads to an increase in the inductor ripple current under heavy load, which complicates the thermal design of power semiconductors and capacitors. This paper verifies the improvement of DC superimposition characteristics of powder cores by applying coupled inductors. First, the effect of coupled inductors on the improvement of DC superimposition characteristics is experimentally verified. Second, a measurement method for the DC superimposition characteristics of coupled inductor is proposed. Then, a magnetic circuit model is built using the measurement results. Finally, a coupled analysis of magnetic and electric circuits is performed, and it is confirmed that lower magnetic field operation can be realized by coupled inductors even under heavy load conditions. It has been proven that coupled inductors are effective in improving the DC superposition characteristics of the powder cores.

Abstract A fault current compensation technique is proposed in this paper for resonant grounded power distribution systems in bushfire prone areas. Arc suppression devices with residual current compensation inverters are used to compensate fault currents due to single line‐to‐ground faults in order to mitigate powerline bushfires. The main contribution of this paper is the design of a compensation technique for the T‐type residual current compensation inverter using a non‐singular terminal sliding mode control scheme. The main objective of the proposed scheme is to reduce the fault current and bring its value to a level so that it cannot ignite fires. The proposed controller is designed based on the selection of a sliding surface in a way the singularity problem can be avoided and chattering effects in existing sliding mode controllers can be eliminated. The desired current injection through the residual current compensation inverter is ensured by enforcing the control law into the terminal sliding surface where the control law is determined by satisfying the Lyapunov stability criteria. The performance of the non‐singular terminal sliding mode controller is compared with an integral sliding mode controller by considering different values of fault currents where these values are varied by changing fault resistances. Results for simulation in the software and processor‐in‐loop simulations are verified against operational standards which are essential for mitigating powerline bushfires. This work focuses to design a non‐singular terminal sliding mode controller for the residual current compensation inverter which is used in an arc suppression device to compensate both active and reactive components of the fault current and keeps its value below 0.5 A within 2 s after activating the residual current compensation inverter which is a requirement as per the operational standard. This controller is designed based on the selection of a terminal sliding surface while satisfying the condition for avoiding the singularity problem.

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 By relying on the input–output feedback linearization approach, a novel adaptive controller for flexible joint robots is proposed in this work. First, a model‐based controller is developed to get a structure that is useful in the development of the adaptive controller. The adaptive version is developed by using two techniques. To stabilize the output function, an adaptive neural network controller is used, which approximates the non‐linear function that contains the uncertainties. The desired rotor position required by the input–output feedback linearization controller is defined with the structure of a link dynamics adaptive regressor‐based controller. The main reason to adopt the mentioned structure in the definition of the desired rotor link position is to guarantee its differentiability. Real‐time experiment comparisons among the model‐based controller, a model‐based controller with desired compensation, an adaptive controller based on joint torque feedback, and an adaptive neural network‐based controller are carried out. Experimental results support the theory reported in this document and the accuracy of the proposed approach.

Abstract In droop‐controlled DC microgrids, parasitic resistances of long conductive lines introduce additional terms for the power calculation and impact the power sharing accuracy. This paper proposed a comprehensive local control design for enhancing power sharing accuracy and restoring DC bus voltage while increasing stability performance in DC microgrids. A passive controller is used in the primary control to ensure the sufficient bandwidth of controller in case of frequent operation modes alteration and voltage deviation in the DC microgrid. A concept of Virtual Negative Line Resistance (VNLR) is used in the secondary control layer to compensate the real line resistance such that line resistance no longer degrades power sharing accuracy. The common DC bus voltage needs to be monitored in the proposed secondary controller. Simultaneously, the common DC bus voltage can be restored as the designed value. The monitored DC bus voltage signal is filtered by a designed low‐pass filter such that mid‐high frequency dynamics can be decoupled between secondary controls and primary controls. Then the entire local control scheme relaxes three Degrees of Freedom (DoF) which can be used for upper layer controls. Finally, the proposed control method has been experimentally validated in a 50 V DC microgrid laboratory testing system.

Abstract This study suggests a dynamic current cut‐off frequency‐based pole‐zero cancellation speed controller for permanent magnet synchronous motors (PMSMs). The proposed self‐tuning algorithm automatically increases the current cut‐off frequency during only the transient periods and restores it as approaching the steady‐state operation. The outer loop control injects the active damping effect, resulting in a closed‐loop order reduction by pole‐zero cancellation from the particularly structured feedback gain. These two benefits contribute to the following advantages: (a) lowering the steady‐state current cut‐off frequency to improve the relative stability margin and (b) securing the capability of assigning the desired cut‐off frequency to both the inner and outer loops in the first‐order low‐pass filter form. A 500‐W PMSM experimental prototype platform confirms the effectiveness of the proposed controller.

Abstract This paper focuses on design of a control system for full bridge DC‐DC converter based on Lyapunov stability theorem which guarantees system stability. To achieve this goal, a full bridge DC‐DC converter is used as a Li‐ion battery charger and sinusoidal ripple current (SRC) is implemented as the charging algorithm. In initial step, state space equation of the system including DC‐DC converter and battery are obtained. After that, switching function is extracted by applying Lyapunov stability theorem to state space equation. Also in order to achieve unity power factor and allowable current Total Harmonic Distortion (THD) range based on standards, an Active Front End (AFE) PWM Rectifier is used. To evaluate the effectiveness of the proposed controller scheme, a laboratory prototype is implemented. Experimental result shows appropriate performance of proposed controller in tracking of reference values in comparison with fixed PI controller. In addition, results of AFE rectifier performance is evaluated by showing the unity power factor and low THD value.

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 Frequency droop control is a simple and effective frequency control method. However, it is not appropriate as a primary frequency control for microgrids with energy storage systems and voltage source converters (VSCs) owing to their slow response speed. In addition, it is difficult to determine the grid balance only with frequency deviation. In this paper, a new converter‐fed microgrid primary frequency control method based on the estimated active power imbalance for fast response and less variability in the transmission system is proposed. The active power imbalance is estimated via frequency deviation, frequency drop speed and the modified swing equation. The estimated imbalance indicates the status of the power imbalance more quickly and accurately than the frequency deviation. The proposed method does not require communication devices, responds quickly to frequency drops, and prevents unnecessary operation of the grid‐connected VSC because it uses the estimated imbalance instead of the frequency or angle for droop control. Simulation results show that the proposed method is significantly faster than frequency droop control methods, improves the rate of change of frequency and frequency nadir and reduces variability in the transmission system.

Abstract This paper studies the electromechanical transient modelling techniques of the modified modular multilevel converter (MMC), named active MMC, which is equipped with embedded energy storage in submodules. Firstly, the mathematical model of the active MMC and its equivalent circuits at the AC and DC sides are established. Then, the control scheme of active MMC that focus on the cooperation of the MMC converter and the energy storage submodules is illustrated. The proposed electromechanical transient model are implemented on PSS/E and compared with the electromagnetic transient model on PSCAD in a two terminal active MMC sytem; the results of the active MMC system under AC and DC fault prove the validity of the proposed model. Lastly, stability studies of the practical system are carried out, and the simulation results prove the improvement by the application of the active MMC on the rotor angle stability and frequency stability.

Abstract An islanded microgrid is cyber‐physical system, and the control relies on the communication system significantly. Improper parameters of the cyber system can result in instability of a microgrid system. To evaluate the impact of a networked control system on control performance, a cyber model is developed to represent data acquisition periods and communication delays. Simplification of the networked control system model is proposed to enhance the computational performance, making the analytical method applicable for large‐scale systems. Based on the analysis, a two‐dimensional stability region of a microgrid in the space of cyber parameters can be obtained. To validate the proposed method, a microgrid control scheme is proposed for power dispatch and regulation based on the droop and proportional‐integral (PI) feedback control. The analytical method is compared to the time‐domain simulation, and it is shown that the stability regions are nearly identical. The critical values of cyber parameters are determined based on the analytical results. The proposed control strategy with the given cyber parameters is validated for transient stability following dynamic events. Simulation results indicate that the design of a microgrid as a cyber‐physical system needs to be guided by critical values for cyber parameters to prevent system instability.

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 Non‐isolated inverters have the disadvantage of large leakage current, which will damage the equipment and personal safety. In order to solve this problem, this paper proposes two H10 three‐phase inverter topologies named forward‐clamped and reverse‐clamped H10 inverters. Each of the topologies is composed of two DC bus switches, a clamping circuit and a traditional three‐phase inverter, but the clamping method of these two topologies is different. A general control strategy combining carrier modulation and logic operation for these two topologies are proposed. The control strategy allows the common‐mode voltage of these two topologies to vary between one third and two thirds of the DC bus voltage, but the frequency of the common‐mode voltage of the reverse‐clamped inverter is three times that of the forward‐clamped inverter. The loop impedance of the reverse‐clamped inverter increases significantly at high frequency, so the leakage current suppression capability is better. Simulation and experiment platforms of the inverter systems are built. The obtained results verify the correctness of the theoretical analysis.

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 It is known that constant power load (CPL) may cause a negative impedance, which seriously affects the stability of power system. In this paper, a new control algorithm for DC–DC buck converter feeding unknown CPL is proposed. First, under the assumption of known extracted power load, the standard passivity–based control (PBC) is presented to reshape the system energy and compensate for the negative impedance and a proportion‐integration (PI) action around passive output is added to improve disturbance rejection performance, which forms the PBC plus PI (PBC+PI). Then, a parameter estimation algorithm is developed, based on immersion and invariance (I&I) technique, in order to online estimate the extracted power load. In the next step, the online estimation scheme is adopted to construct an adaptive strategy. Finally, the stability analysis of the cascaded system containing a closed‐loop control system and observer error dynamics is conducted. Simulation and experimental results are demonstrated to validate the performance of the proposed controller.

Abstract In the non‐effectively grounding distribution system, residual current under single‐line‐to‐ground (SLG) fault threatens the safety of human being and power supply equipment. Active arc suppression device has been proved to be effective for SLG fault arc suppression when the line impedance is ignored. However, in practice, line impedance varies with the fault location and the load current flowing through the impedance brings about additional voltage drop, which increases the fault current and is not dealt with by the conventional methods. To achieve accurate SLG fault arc suppression with the existence of line impedance, the neutral‐to‐ground voltage reference for full ground‐fault current compensation is firstly derived and a detection method is then proposed. The pre‐fault and post‐fault line currents are used to eliminate the influence of load current on the line impedance voltage drop. A dual‐loop voltage and current controller is then designed. The prototype of active arc suppression device was developed. The results of simulation and prototype experiment validate the effectiveness of the proposed method.

Abstract This paper presents a novel voltage self‐balancing converter (VBC) applied to DC solid‐state transformer (DCSST), analyzes the operating principle of the proposed VBC, and proves the voltage balance performance. Based on this converter, a novel DCSST topology is proposed, which can be used in medium and low voltage DC distribution application, microgrid and medium and low voltage energy storage system. Compared with the previous work, under the same voltage level, it can effectively reduce the number of transformers and switches, reduce the voltage level of switches, and realize the soft switching of the VBC switches, so as to improve the system efficiency and power density, and then improve the system reliability. Meanwhile, the fault handling ability and fault tolerance ability of DCSST are improved, which makes DCSST more reliable when multiple isolated bi‐directional DC/DC Converters (IBDC) are connected in series. In addition, with this topology, the input voltage self‐balancing control strategy of the IBDC is not needed because of the voltage self‐balancing characteristic of VBC, which simplifies the control complexity and greatly improves the control accuracy of the control system. Experimental results agree well with the accuracy and correctness of the proposed topology.

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 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 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 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 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 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 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 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 One of the crucial challenges of the distribution network is the unintentionally isolated section of electricity from the power network, called unintentional islanding. Unintentional islanding detection is severed when the local generation is equal to or closely matches the load requirement. In this paper, both ensemble learning and canonical methods are implemented for the islanding detection technique of synchronous machine‐based distributed generation. The ensemble learning models for this study are random forest (RF) and Ada boost, while the canonical methods are multi‐layer perceptron (MLP), decision tree (DT), and support vector machine (SVM). The training and testing parameters for this technique are the total harmonic distortion (THD) of both current and voltage signals. THD is the most important parameter of power quality monitoring under islanding scenarios. The parameter and data extraction from the test system is executed in a MATLAB/Simulink environment, whereas the training and testing of the presented techniques are implemented in Python. Performance indices such as accuracy, precision, recall, and F 1 score are used for evaluation, and both ensemble learning models and canonical models demonstrate good performance. Ada‐boost shows the highest accuracy among all the five models with original data, while RF is robust and gives the best results with noisy data (20 and 30 dB) because of its ensemble nature.

Abstract Fault analysis is an essential prerequisite for fault detection, location and isolation. A detailed fault analysis method on the steady state of the pole‐to‐pole fault for voltage source converter (VSC) based DC systems is presented in this paper. According to fault characteristics under different transition resistances (TRs), the steady‐state response is divided into three scenarios: low transition resistance with blocked converter (LTRBC), medium transition resistance with blocked converter (MTRBC) and high transition resistance with unblocked converter (HTRUC). When TR is relatively low, fault currents through insulated gate bipolar transistors (IGBTs) are large and the converter is blocked. After an in‐depth study of freewheel diodes’ conduction states, the iterative method is utilized to solve the steady state under blocked converter. When TR is relatively high, fault currents through IGBTs are not large enough to block the converter. With the unblocked converter, control strategy and pulse width modulation (PWM) are investigated to obtain steady‐state fault currents. Finally, a reasonable simulation model was built in PSCAD/EMTDC software. The simulation results verified the fault analysis accuracy, with much less calculation error than that of the conventional method. Besides, the proposed analysis method was proved applicable to fault scenarios under different TRs.

Abstract Online experimental systems that include remote and virtual experiments based on the Internet has become a new form of experiment in engineering education. This paper proposes a virtual experiment for the complex process control system. It is based on the networked control system laboratory (NCSLab), which provides a reliable framework for virtual thermal experiment. Taking the combustion system of 1000 MW ultra‐supercritical coal‐fired once‐through boiler‐turbine units as an example, the framework and the implementation process of the online system are given in this paper. A new presentation form for complex processes control experiment is provided, which could be a solution for simulating physical experimental equipment in virtual environments. The potential danger in practical experiments could be avoided. Additionally, the real‐time operation for complex control systems could be presented in a 3D scene. Meanwhile, the flexibility and efficiency could be increased in engineering experimental education. Moreover, using the experiment evaluation method, the references and feedback for teaching and assessment are provided in this paper.

Abstract This paper proposes a vehicle‐to‐grid (V2G) operation methodology of electric vehicles (EVs) for frequency regulation, EV charging, and state‐of‐charge (SOC) management. This V2G operation methodology exhibits multifunctionality in regard to instantaneous power adjustment of EVs for primary frequency control during charging. The proposed method can stabilize grid frequency under the high penetration of renewable energy by simultaneously managing the desired state‐of‐charge (SOC) of the batteries. The results indicate that both charging and charged EVs can assist in frequency regulation so that V2G services can help embracing renewable energy sources and stabilize a power system. The feasibility of this method is verified by a practical power grid model to demonstrate the efficacy of EV charging control for SOC management using DIgSILENT/PowerFactory.

Abstract Development of advanced grid‐synchronization technique for unbalanced and distorted grid is considered in this paper. In this context, self‐tuning filter (STF) is considered to extract the fundamental component from the measured unbalanced and distorted voltages. Standard STF considers balanced grid voltages which is not always possible to ensure in the actual power grid. To mitigate this issue, an extended STF is proposed by analyzing the standard STF in the state‐space framework. To make the proposed ESTF grid‐following, a robust open‐loop frequency estimator is also applied. The closed‐loop system enjoys excellent filtering benefit from the extended STF side while taking advantage of the fast convergence speed property of the open‐loop frequency estimation technique. Numerical simulation and experimental results with double second‐order generalized integrator phase‐locked loop are provided to validate the theoretical developments.

Abstract In this paper, a multi‐objective optimization technique is proposed for the planning of a networked microgrid based on peer‐to‐grid (P2G) and peer‐to‐peer (P2P) energy trading schemes. Two different criteria's including annual profit and energy index of reliability are taken into consideration to form a multi‐objective function. The networked microgrid consists of three individual microgrids containing their own combinations of generation resources, batteries and residential loads. All microgrids are connected together and also to the main grid to meet the energy exchange requirements of P2P energy trading. A cooperative game theory technique based on a particle swarm optimization algorithm is used to model the networked microgrid, and to find the suitable sizes of the players that simultaneously maximize the payoff values of both objective functions. Besides, a comparative analysis is carried out for both P2G and P2P energy trading schemes. The results show that the outcomes are maximum when both criteria are considered in the optimization and P2P energy trading is carried out. The sensitivity analysis is performed on the selected parameters and verified the right change 0.003% and 4.5% in discount rate and electricity prices, respectively.

Abstract A series of DC power supply converters can be described as linear affine systems with finite inputs and constrained switching frequency. In this study, a practical‐stability‐oriented general control framework with duty‐cycle‐like characteristics is deduced for such converters. First, a concept called End Point Equivalence Modulation (EPEM) is introduced to release constraints. Following this, a Control Lyapunov Function (CLF) is employed to design the switching law and the relationship between the boundary of the convergence zone and the switching frequency is derived. In addition, an optimized switching strategy is presented to halve the switching frequency without degrading the control performance, which is also helpful for reducing switching loss. Finally, a single‐phase inverter and buck converter are considered as case studies and analysis show that Sinusoidal Pulse Width Modulation (SPWM) and volt‐second balance guarantee their practical stability, respectively, which can be theoretically derived and proved under EPEM. All the proposed methods and controllers are validated both by simulations on MATLAB/Simulink and experiments in a rapid control prototype platform based on a FPGA module.

Abstract An inverter‐based microgrid is a small‐scale power network governed by a distributed control system. In this system, the nodes are the digital controllers of the power inverters, normally located at separate points within the microgrid. A relevant issue is that these controllers operate at different frequencies due to inherent clock deviations in the local hardware oscillators. This paper evaluates the effects of these clock deviations on the performance of microgrids equipped with inverters that emulate the operation of synchronous machines. A systematic procedure is presented to derive steady‐state expressions of the inverter active power and microgrid frequency as a function of clock drift rates. This procedure is applied to swing and governor equations of the virtual synchronous generators, revealing the mechanism that allows clock drifts to be absorbed, making their presence negligible. In addition, it allows recognising the controllers that should never be implemented in a distributed control system, since they cause an unsatisfactory behaviour that can even lead to a blackout in the microgrid. Therefore, the relevance of this study is the identification of the control schemes that are most sensitive to clock drifts, which makes it easier to choose the most suitable control implementation for a particular application. Furthermore, technical guidelines are reported to help researchers on developing control solutions more robust to clock drifts. In this study, the theoretical results are validated by experimental tests in a laboratory microgrid.

Abstract This paper proposes an equivalent temperature estimation‐based finite control set model predictive control (ETE‐FCS‐MPC) method to balance the thermal stress among transistors in three level neutral point clamp (3L‐NPC) traction inverters, which can be used to prolong the whole service life and improve the reliability of the traction inverter. Compared with other methods, the proposed ETE‐FCS‐MPC method requires lesser calculation without additional sensor. Firstly, an energy loss‐based equivalent temperature estimation method is proposed to avoid the complexity of junction temperature prediction. Secondly, an energy variance‐based thermal stress balance control objective is introduced. Then, a normalized evaluation function is established to select the optimal switching state. Finally, the effectiveness of this method is verified on hardware‐in‐the‐loop tests.

Abstract Three‐level inverter systems based on the conventional Finite Control Set‐Model Predictive Control (FCS‐MPC) principle are characterized by large current ripples, difficult‐to‐design weighting factors and variable switching frequencies. In order to solve these problems, in this study a two‐stage model predictive control is proposed that consists of two objective functions: the current prediction and neutral point potential prediction objective functions. The first objective function was based on the mathematical model of a three‐level T‐type, and the second was based on the zero‐sequence injection principle and intended to modify the modulated signals. Then, a multi‐carrier modulation method was adopted to attain the switching signal for the inverter system. In addition, this paper analyses the sensitivity of the inductance and load resistance parameters for the predictive model. Finally, the performance of the proposed strategy is evaluated based on the time‐domain simulation and experiment study.

Abstract To ensure the safety and effectiveness of charging a battery, constant current and constant voltage (CC‐CV) charging is widely used in inductive power transfer (IPT) systems. A double‐channel T‐circuit (DT) is proposed to obtain CC output or CV output by selecting different work modes with a simple feedback control for various power demands. With the merit of the proposed circuit, fewer components are employed compared to previous work with zero phase angle (ZPA) of inverter in the whole charging process. Besides, an IPT system using DT cascaded with LCC‐S (DTLS) is proposed to improve design freedom of the charge current and charge voltage with the constraints of the magnetic coupler (MC) parameters for CC‐CV charging. Finally, a prototype with 60 V/2 A output was built to verify the theoretical analysis. The experimental results show that the fluctuation of the charging current is less than 2% in CC mode, and that of charging voltage in CV mode is within 4% with the maximum efficiency up to 93.36% during the whole charging process.

Abstract For the dual three‐phase permanent magnet synchronous motor(dual‐PMSM), the deadtime can introduce fifth and seventh current harmonics, and they are only constrained by the small stator leakage impedance, which means even if there are only small fifth and seventh voltage harmonics, the resulting fifth and seventh current harmonics will be very large. The harmonic current will make an increment of the system loss, thus lead to an reduce of the efficiency of the speed control system. This paper proposes a harmonic current suppression strategy for dual‐PMSM based on least mean square (LMS) adaptive notch filter. This strategy suppresses the fifth and seventh harmonic currents by introducing four LMS adaptive notch filters in the subspace. The proposed algorithm has good dynamic performance and is insensitive to motor parameters that are offset within a certain range. The effectiveness of proposed method is verified by a set of comparative simulations and experiments.

Abstract In this article, a single‐stage bridgeless LLC resonant converter with a constant frequency control method is proposed for a high‐brightness light‐emitting diode (LED) driver. LED strings require drivers with high power density and high efficiency. This article presents a novel LED driver circuit topology based on switch integration and the switching controlled capacitor (SCC) technique. By combining the merits of a totem‐pole bridgeless boost PFC unit and a high frequency SCC‐LLC half‐bridge resonant converter, the proposed topology can achieve high efficiency. Due to the SCC‐LLC circuit topology structure, constant frequency control of the LLC resonant converter is achieved by the regulation of the equivalent resonant capacitance. The proposed novel LED driver topology can automatically achieve soft‐switching characteristics and a high power factor. The operating principle of different operation modes and the characteristics of the proposed LED driver are analysed in detail. Finally, to demonstrate the feature of the proposed topology, a 100 W/70 V laboratory prototype is built. The satisfactory experimental results correspond with the theoretical analysis and parameter design.

Abstract For the LCL‐type grid‐connected inverter, when the capacitor voltage feedforward is applied, the delay in the digital control system could change the phase characteristics of capacitor voltage feedback and affect the stability of the system. To solve the problem, a delay compensation method based on lead compensator is proposed in this paper, which can reduce the impacts of control delay on the capacitor voltage full feedforward control system. In this study, a lead compensator is introduced into the feedforward channel to compensate the phase lag, and the compensation function is designed to compensate for the phase margin but not change the system amplitude gain. The proposed strategy can keep the phase margin of the system above 25° when the grid impedance changes from 0 to 2.6 mH (0.1 p.u.), which enhances the robustness of the grid‐connected inverter under weak condition. The grid current distortion caused by grid background harmonics is greatly reduced and is kept within 4%. Moreover, the proposed strategy has fast dynamics while ensuring the system stability. Finally, the simulation and experimental results are presented to verify the effectiveness and robustness of the proposed control strategy.

Abstract Enlarging the speed range of wind turbines is an effective way to increase power production in low‐frequency band wind which contains more energy and is very challenging for DFIG‐based wind energy conversion system (WECS). The analysis shows that current controllability of the converter is the main factor limiting bidirectional speed range extension (BSRE). This problem can be solved by increasing dc‐bus voltage, but a constant and high dc‐bus voltage will increase the electrical stress and switching losses of the converter. This paper proposed a dynamic minimum dc‐bus voltage (DMDCV) technique in which the dc‐bus voltage can automatically adjust to enable the back‐to‐back voltage source converter to work in linear modulation mode at its minimum value, thus supporting BSRE while also achieving multiple goals including low electrical stress and low switching losses. Two implementation methods of DMDCV with and without modulation depth loop are proposed and discussed. The co‐simulation results verified the feasibility of DMDCV and the advantages of BSRE in improving power generation.

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 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%.