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

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

Abstract 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 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 Wireless power transfer (WPT) has been widely used in recent years due to its convenient access to power and security in some specific cases. However, the inherent non‐linearity of the WPT system can result in unexpected stability problems. In this paper, the stability issue of voltage‐mode controlled inductor‐capacitor‐capacitor and series (LCC/S) compensated WPT system is studied for the first time. The discrete‐time model is first introduced to establish the voltage‐mode proportional (P) controlled and proportional integral (PI) controlled LCC/S compensated WPT system. Meanwhile, a modelling method of PI controller without introducing new state variables is proposed in this paper. Then, with the bifurcation diagram and eigenvalues of the Jacobian matrix, the stability of the LCC/S compensated WPT system is analysed with respect to the proportional coefficient and integral coefficient of the controller. Furthermore, by studying the eigenvalues of the Jacobian matrix, the stability boundary can be obtained. In addition, how the load and mutual inductance affect the stability boundary is investigated compressively, the law of influence provided by the operating condition upon the stability can be concluded. Finally, simulation and experimental measurements are provided to verify the accuracy of the discrete‐time model and the effectiveness of the theoretical stability analysis.

Abstract Multi‐tap and multi‐capacitors type arc suppression coils are commonly used in distribution systems. Due to their low compensation accuracy and limited arc suppression capability, it is hardly to avoid personal safety accidents and wildfires absolutely. In this paper, a novel single‐phase grounding fault voltage full compensation topology based on antiphase transformer is proposed, which is composed by an antiphase transformer, a phase‐selection switch and a multi‐tap single‐phase voltage regulator. The proposed topology can control the neutral point voltage, thus cause the fault voltage being zero. By this way, the lack of arc suppression coil is overcome. The novel compensation topology features low costs, high precision of fault voltage compensation. The principle of the novel compensation topology is explained. The mathematical expressions of residual voltage and current are derived in detail. Finally, the simulation model and experimental platform were built. The results show that the proposed novel compensation topology can effectively make up for the shortcomings of traditional arc suppression coils when it is used with them.

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 Single‐stage switched boost inverter (SBI) with buck‐boost capability finds wide applications in renewable energy systems (RES). This paper aims at a comprehensive topological review of various single stage SBI circuits. The boosting techniques of SBI include switched inductor, switched capacitor, and transformer assisted switches. Each SBI derived structure has its own merits and demerits depending on the applications. The performance evaluation of SBIs, based on various perspectives like boost factor, voltage stress across active devices, passive elements count, DC link voltage, switching losses, voltage gain, and efficiency, are also discussed. A suitable modulation strategy is used to obtain the desired boosted and inverted ac output voltage in one stage SBI. Different modulation strategies adopted for SBIs are also included at the end of the topological review. This topological review's contribution enlightens the researchers in selecting appropriate SBI with effective modulation strategies suitable for renewable energy applications. Besides, the benefits and shortcomings of these converters are discussed in detail.

Abstract This paper presents a sparse quaternion‐valued minimization (SQVM) based control technique of a two‐stage grid supportive photovoltaic (PV) power system with power conditioning capabilities. The grid side converter (GSC) is controlled by utilizing an SQVM control technique. The proposed algorithm enables the GSC to mitigate the load harmonics current and provides reactive power compensation at the point of common coupling (PCC). The proposed control is tuned to mitigate the DC offset error, harmonics current and to improve the frequency response of the proposed system. A DC link pre‐predictive is incorporated to improve the dynamic response by reducing the burden on the outer Proportional‐Integral (PI) voltage controller loop. Further, it is used to extract the fundamental component of load currents for generating the reference currents. An adjustable DC link voltage loop is incorporated in the proposed control technique to adapt the PCC voltage variation, which helps in minimizing the losses of the PV system. For real‐time execution of the adjustable DC link voltage concept, the DC link voltage is adapted with a variation in the PCC voltage. The operation and control of the system topology are validated experimentally under various scenarios.

Abstract The poor damping of bidirectional dc (bi‐dc) converter caused by constant power load makes power system prone to oscillation, and non‐minimum phase characteristic also jeopardises voltage stability. To solve these challenges, the interconnection and damping assignment passivity‐based control (IDA‐PBC) is utilised to improve transient response. The influences of the right‐half‐plane (RHP) zero on the stability margin and controller design are illustrated by zero dynamics analysis. Then the port‐controlled Hamiltonian modelling is used to obtain the IDA‐PBC control law, which is suitable to the bi‐dc converter and independent of the operation mode. The system dissipation property is modified, and thus the desired damping is injected to smooth the transient voltage. To remove the voltage error caused by RHP zero and adjust the damping ratio, an energy controller with an adjustment factor is introduced. Besides, a virtual circuit is established to explain the physical meaning of the control parameter, and the parameter design method is given. Passivity analysis assesses the controller performance. Simulation results are analysed and compared with other control strategies to test the proposed IDA‐PBC strategy.

Abstract In the islanded microgrid, it is necessary to distribute the harmonic powers correctly among generators and mitigate the voltage harmonics at the point of common coupling (PCC) for effective microgrid operation. These control objectives are hard to satisfy at the same time with only one virtual impedance component. To simultaneously improve the microgrid voltage quality and share the distortion powers, a consensus‐based distributed power‐sharing scheme is proposed along with PCC harmonics mitigation. In the proposed control scheme, the controllable parts of output impedance at the fundamental and harmonic frequencies are separately tuned by simple integral controllers without depending on feeder impedance parameters. Compared to conventional control schemes, the proposed method is less complicated while ensuring a high voltage quality despite sudden changes in load power. A small‐signal state‐space model of the microgrid is analysed to validate the stability of the proposed control scheme, and its effectiveness is verified by experiments with a scaled‐down microgrid system.

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

Abstract A microgrid with a solar photovoltaic (SPV) array, wind generator, battery energy storage (BES), and a bidirectional DC–DC converter with seamless transition capability from on‐grid mode (OGM) to off‐grid mode (FGM) and grid reconnected mode is presented. This microgrid, based on renewable sources, increases the authenticity and sustainability of the supply and has the ability to feed the load in OGM and FGM as well as during mode shifting. The undetermined disturbances and unpredictability of the microgrid in OGM are managed by a sixth‐order complex filter (6thOCF)‐based control. With this control, the load current quadrature fundamental component (LCFC‐Q) is obtained without a DC offset even during voltage imbalances. This control provides effective mitigation of harmonics and monitors the flow of active power, which improves the grid current quality. The control approach based on a resonant plus proportional resonant (R+PR) controller along with the harmonics compensation ability is used to control the microgrid in FGM. A dual 6thOCF (D6thOCF) is cascaded in series with the phase‐locked loop for effortless synchronization of the grid. This enhances the system performance without affecting the stability. The microgrid performance is validated through simulated and experimental results under dynamic and steady‐state conditions.

Abstract Multilevel converters have seen rising demands in the past decades, due to their increased power ratings, enhanced power quality, low switching losses and reduced electromagnetic interference. Prominent among them are the three‐level (3L) neutral point clamped and the flying capacitor inverter topologies along with their derivatives. Nevertheless, the main drawback of these topologies is the requirement of a front‐end boost DC–DC converter to compensate the high dc‐link voltage demand, which is usually twice the grid peak voltage. This multi‐stage power conversion further pulls down the overall system efficiency. A single‐stage dc–ac power converter with boost capability offer an interesting alternative compared to the two stage approach. Considering this aspect, a novel three‐level three‐phase boost type inverter is introduced in this paper for general‐purpose applications (prominently grid‐connected renewable energy). The proposed inverter would reduce the DC‐link voltage requirement to half using the same or even less number of active and passive components, compared to the conventional three‐level neutral point clamped and flying capacitor family. The principle of operation and theoretical analysis are discussed in detail. The design methodology along with simulation and experimental waveforms for a 5 kVA inverter are presented to prove the concept of the proposed inverter topology for practical applications.

Abstract This paper presents a single‐switch zero‐voltage switching (ZVS) power‐factor correction converter based on the class‐E2 converter at 1 MHz switching frequency. A design method for ensuring the ZVS for the entire line‐voltage period is proposed. By visualising the ZVS region in the parameter space, circuit parameters can be easily obtained to achieve the ZVS for the entire line‐voltage period. Additionally, a closed‐loop controller is applied for achieving a high power factor, low total harmonic distortion of the input current and output voltage regulation. The experimental circuit achieved the ZVS in the entire line‐voltage period against load variations. As a result, the implemented converter achieved the same level of power‐conversion efficiency as the 100‐kHz power‐factor correction converters and a high power factor with low total harmonic distortion, which denoted the effectiveness of the proposed design method.

Abstract The design of a cascade controller is demonstrated for a buck–boost converter that is combined with two control loops consisting of inner and outer controllers. The outer loop is implemented by a fractional‐order proportional‐integrated‐derivative (FO‐PID) controller that works as a voltage controller and generates a reference current for the inner control loop. To provide faster dynamic performance for inner loop, a self‐tuning regulator adaptive controller, which tries to regulates the current with the help of a novel improved exponential regressive least square identification in an online technique, is designed. Moreover, in the outer loop, to tune the gains of the FO‐PID controller, a novel algorithm of antlion optimizer algorithm is used that offers many benefits in comparison with other algorithms. The system provided by the boost mode is a non‐minimum phase system, which creates challenges for designing a stable controller. In addition, a single loop controller is proposed based on a PID controller tuned by a particle swarm optimization algorithm to be compared with the cascade controller. Cascade loop can present significant benefits to the controller such as better disturbance rejection. Finally, the strength of the presented cascade control scheme is verified in different performing situations by real‐time experiments.

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 Voltage sags and momentary interruptions of power are the most severe voltage disturbances. Here, a tansformerless active voltage quality regulator with the novel three‐leg circuit is proposed to mitigate long duration and deep voltage sags. The shunt rectifier and series inverter of this novel three‐leg circuit are all full‐bridge structures, so it has lower DC bus voltage and smaller filter parameters than the conventional half‐bridge topologies. The totem‐pole bridgeless boost circuit is formed among the three bridge arms and the DC bus voltage will be charged to exceed the peak value of the supply voltage. It can compensate for deep voltage sags (that may be more than 50% of the nominal voltage). Compared to the conventional transformerless series‐connected topology, the proposed topology has the merits of higher efficiency, smaller volume and lower cost. The validity and feasibility of the proposed topology are verified by theoretical analysis, simulation and experiment.

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

Abstract Classic droop control ensures the synchronization of distributed generation (DG) units inside a microgrid without requiring any deployment of communication links, however it causes unwanted frequency fluctuations and degrades system dynamics. Angle droop control and V−I control have been developed as two major global positioning system (GPS)‐based control methods, both of which realize a fixed‐frequency operation of the microgrid through synchronizing DG units with GPS timing technology and brings improvement to the dynamic response of the overall system. This paper reveals the similarities and the correlations between these two methods that they can both be regarded as different forms of virtual impedance control. A novel adaptive virtual impedance control method is proposed accordingly to generalize GPS‐based control strategies into a unified control frame. An impedance and inner controller design approach considering both stability constraints and power quality requirements based on the small‐signal model of GPS‐based microgrids is presented for practical implementation. An adaptive transient resistance concept is adopted to enhance the system stability during large disturbances and grid faults. Case studies are presented to validate the system performance and fault ride‐through abilities of the proposed control scheme.

Abstract This paper proposes a common‐mode voltage injection‐based pulse width modulation strategy to optimize the AC current ripple of parallel interleaved converters. In general, modulation methods entail a trade‐off between switching times and voltage error. Given the redundancy in the available vector sequences, we sequentially minimize the switching times and the voltage error. Specifically, we propose eight candidate vector sequences with minimized switching times for each 60° sector. Then, we quantitate the current ripple introduced by the eight vector sequences, and according to the calculations, we split each 60° sector into eight subsectors, each employs the respectively optimal vector sequence with the minimized current ripple. For implementation, the candidate vector sequences are further unified by a common‐mode voltage injection scheme. The injection depends on the momentary subsector in which the reference lies. Despite the complex geometry of the subsectors, we propose a simple decision procedure that can be easily implemented in mainstream microcontrollers. Compared to the conventional methods, the proposed common‐mode voltage injection‐based pulse width modulation has a smaller AC current ripple at the same switching loss. The experimental results verify the theoretical analysis and the effectiveness of the proposed common‐mode voltage injection‐based pulse width modulation strategy.

Abstract The motivation of this paper is to introduce a new Hebbian least mean square control algorithm for two level self‐supported voltage source converter based distributed static compensator. The Hebbian least mean square controller is employed on the various aspects such as, weight updating, direct and quadrature unit voltage template, active and reactive part of reference supply currents and switching signal generation. Firstly, a set up equations using Hebbian least mean square are premeditated to compute both active and reactive weight updating. Secondly, DC and AC proportional integral controller outputs are involved with the updating weight to generate the reference supply current. The error signal obtained from both reference and actual source current is processed through the hysteresis current controller for the gate signal firing. The MATLAB/Simulink results as well as experimental results prove that the proposed algorithm is competent to afford the better voltage regulation, voltage balancing, source current harmonic reduction and power factor correction. Finally, performance obtained from the proposed controller reveals the applicability for various loading condition in the distribution grid.

Abstract The paper presents the performance analysis‐based reliability estimation of a self‐excited induction generator (SEIG) using the Monte‐Carlo simulation (MCS) method with data obtained from a self‐excited induction motor operating as a generator. The global acceptance of a SEIG depends on its capability to improve the system's poor voltage regulation and frequency regulation. In the grid‐connected induction generator, the magnetizing current is drawn from the grid, making the grid weak. In contrast, in the SEIG stand‐alone operation, an external capacitor arrangement is implemented to render the reactive power support. This capacitor arrangement is connected across the stator terminals during the stand‐alone configuration of SEIG. The capacitor serves two purposes, which include voltage build‐up and power factor improvement. Therefore, the paper deals with obtaining the minimum capacitor value required for SEIG excitation in isolated mode applications, including stand‐alone wind power generation. The SEIG performance characteristics have been evaluated for different SEIG parameters. The simulation and experimental results are then compared and found satisfactory. Then, SEIG reliability is estimated considering the MCS method utilizing SEIG excitation's failure and success rates during experimental work in the laboratory. Finally, the SEIG reliability evaluation is performed considering different wind speeds.

Abstract Voltage‐controlled voltage‐source inverters (VVSIs) have been widely used in microgrids. Typically, LC filters are adopted by the VVSIs to improve the quality of the output voltage. Resonances of the LC filters that may cause oscillations to the VVSIs can be damped by well‐designed voltage controllers. However, the virtual inductance (VI) that is widely applied to improve power sharing between VVSIs will reduce the stability margin of the voltage control loop and introduce new oscillations on the LC filters even if the well‐designed voltage controllers have been used, which has not been reported in the literature. Furthermore, since the VI will introduce a cross‐coupling into the system, the conventional methods for analyzing the resonances of the LC‐filtered VVSIs cannot be used for analyzing the oscillations caused by the VI. Therefore, the stability of the LC‐filtered VVSI with the VI is investigated based on a new model proposed in this paper. Besides, to enhance the stability of the VVSI, the application of a low‐pass filter or a band‐pass filter in the VI is studied. Moreover, when connecting the VVSIs, the effects of the line impedance on the stability of the VVSIs while using the VI are investigated. Finally, simulation and experimental results have been provided, which verifies the correctness of the analysis in this paper.

Abstract Given the urgent need for photovoltaic (PV) energy to participate in grid frequency regulation, a control strategy of the DC/DC converter of a two‐stage PV system based on the virtual governor (PV‐VG) is proposed in this paper. First, a one‐step PV maximum power point estimation (MPPE) method is proposed, which can obtain the reference voltage corresponding to the maximum power point and the power deload point, and some complex iterative calculations and sampling processes are circumvented. Then, based on the constant DC bus voltage of the two‐stage PV system, the coupling mathematical model of boost circuit duty cycle (D) and PV output power is established. According to the power setting and frequency deviation, the D is taken as the direct control target to reflect the direct coupling characteristics of D and grid frequency (f‐D) so that the PV system can adjust the output power reference independently according to the grid frequency deviation. Finally, a simulation model is built in Matlab/Simulink and RT‐LAB, and the results show that the accuracy of the proposed one‐step MPPE method is over 99.8%, and the correctness and efficiency of the PV‐VG control strategy are verified in some case study.

Abstract A considerable amount of rotating synchronous machines are being replaced by the increasing capacity of converter‐based resources, which has stimulated a demand for grid‐forming converters (GFCs) for grid supporting. The inertia and frequency responses of GFC are remarkably similar to those of synchronous generator (SG), but the inadequate capability of sourcing short‐circuit current of GFCs during grid fault can lead to several serious consequences. The low short‐circuit current level fails to trigger protection relays and cannot provide sustained voltage support during the fault or more effectively support the recovery of voltage after fault clearance. The short‐term overcurrent capability of GFC can be promoted either from the point of view of the power devices or from the topology of it. The overcurrent capability of converter is hard‐earned, so full use of it should be made. This paper proposes an optimal control strategy of short‐circuit current of GFC, which can enable the GFC to source the short‐circuit current with an optimal angle within the allowable magnitude. A detailed simulation model of the proposed method here is set up in PSCAD/EMTDC. The simulation results show that the proposed method can achieve better voltage supporting results compared with the traditional reactive support control.

Abstract In this paper, a battery‐supported unified power quality controller (UPQC) for a small hydro‐based isolated power generation is investigated as a voltage and frequency controller. The self‐excited squirrel cage induction generator driven by constant power prime mover (small hydro turbine) has unacceptable voltage, frequency and power quality under non‐linear loads. The battery‐supported UPQC‐based voltage controlled‐voltage source converter is designed, modelled and simulated in MATLAB environment. Here UPQC has series and shunt converters with a battery and it is used to feed the local loads. This small distributed power generation system is capable to feed unbalanced non‐linear loads because a shunt converter is used to take care the compensation of unbalanced loads and it always maintains the source currents balanced and sinusoidal. Moreover, a series converter is used to inject the series voltages in the self‐excited squirrel cage induction generator terminals to regulate the common coupling point voltage during unbalanced non‐linear loads. Performance of a battery‐supported UPQC is also validated experimentally using a laboratory prototype of a 3.7 kW, 230 V, 50 Hz capacitor excited induction generator.

Abstract This paper presents an adaptive power sharing control method of parallel‐connected hybrid inverters in microgrid. Normally the AC microgrid is composed of hybrid inverters, other power generation equipment and some loads in parallel. Droop control or virtual synchronous generator is usually adopted to control parallel inverters, so the output power ratio between each inverter is fixed by droop parameters. The input of hybrid inverter is PV panel and battery, so the power output ability of each hybrid inverter is different because of the different PV panel capacity, illumination condition, battery capacity and battery state of charge. Hence, the output power of some hybrid inverters is short or redundant sometimes, which makes microgrid unstable or causes power generation restraints. It is necessary to recognize the actual power capacity of each inverter and control the output power reasonably. This paper analyzes the characteristics of parallel‐connected hybrid inverters with droop control in microgrid. An adaptive power sharing method is developed to identify the unbalanced between the each inverter on DC bus. By adjusting the droop parameters, the output power of each inverter is regulated where the controlled parameters are defined in system transfer function with an optimal target. Experimental tests are executed following the numerical simulation. The results show that the output power of each inverter can be adjusted adaptively according to the practical needs. The excessive power from one inverter can be absorbed by the others. Therefore, the stability and the power capability in the system are both ensured by adopting the proposed methods. The outcomes of the study provide a reference to the development of the adaptive inverter in microgrid.

Abstract Bidirectional DC‐DC converters are widely used in electric vehicle charger, where excellent steady‐state and dynamic performances are crucial. However, those two performances are easily affected by external disturbances and parameter uncertainties. To solve this problem, this paper proposes a disturbance observer‐based second‐order sliding mode control (SOSMC) for the buck/boost converter. With the super‐twisting algorithm (STA) second‐order sliding mode control method, satisfactory steady‐state and dynamic performance is achieved. In addition, to further improve the dynamic performance and reduce the control burden, a high‐order sliding mode disturbance observer (HOSMDO) is employed to estimate the lumped disturbances. Thus, the mismatched disturbances caused by input voltage fluctuation, load variation, output voltage regulation, power flow change, and model uncertainties can be effectively suppressed. Moreover, the state variable containing inductor current is also observed, which removes one current sensor, reducing system cost. The design methods of the second‐order sliding mode control and the high‐order sliding mode disturbance observer are presented as well as the stability analysed through the Lyapunov theory. Simulation and experimental results verify the effectiveness of the proposed control strategy.

Abstract Here, a general state‐space average model of the half‐wave zero‐voltage switching quasi‐resonant Cuk converter (HW‐ZVS‐QRCC) is derived and validated. The development of an extended Lyapunov‐function‐based robust passivity‐based control strategy for HW‐ZVS‐QRCC is carried out using the derived model. The first feature of the proposed controller is that that the resulted closed‐loop HW‐ZVS‐QRCC is globally asymptotically stabilized even if the physical constraint on the magnitude of the control signal is taken into account. Also, to deal with the uncertainties in the conventional passivity‐based control (PBC) design, an integral function of the output voltage error is incorporated in PBC structure such that zero‐voltage regulation error is assured. Lastly, it shows which variables are most suitable for feedback purposes over a wide range of controller gains. Simulation and experimental results are also provided to validate the theoretical conclusions.

Abstract It is generally known that, with conventional PI control, the stability and dynamic response performances of a power converter cannot be optimized simultaneously. To overcome this limitation, an uncertainty and disturbance estimator assisted sensorless load current feedforward control for dc‐bus voltage regulation is presented to achieve fast dynamic response to load variations while stability is guaranteed by tuning PI controller. The proposed control scheme is verified on a cascaded converters system comprising a front end dc‐dc DAB converter followed by a single‐phase inverter as an illustrative case. Load‐current feedforward is used to enhance the transient response of the front end DAB converter while an uncertainty and disturbance estimator is used to compensate for any non‐idealities arising from model uncertainties and error in load current estimation. To validate the proposed control scheme, experimental results obtained from a 250‐W cascaded converter system are presented and compared rigorously to the performance of conventional PI control.

Abstract This paper proposes the implementation of Affine Projection Sign Algorithm (APSA) based control for DSTATCOM to mitigate distribution side power quality issues like harmonic elimination, reactive power compensation, load balancing, power factor compensation, voltage regulation, etc. Affine projection algorithms have already finds application in power quality mitigation, but they suffer from high complexity and degraded performance due to impulsive interference. Whereas, sign algorithms are known to be robust against the impulsive noise and converges faster. Hence, sign algorithm with affine projection is proposed for mitigation of power quality problems in this work. APSA is used to estimate the reference active and reactive power component of polluted load current, which is further used to generate the reference signal for DSTATCOM. The performance of the algorithm is investigated for some real operating conditions of power system network in MATLAB/SIMULINK environment. The algorithm is tested in real‐time simulator for validation. The result shows that the control algorithm is able to resolve distribution side power quality problems with fast convergence.

Abstract Various disturbances in voltage source converter based high‐voltage, multi‐terminal DC grids will significantly cause DC voltage deviations when the classical voltage droop control is adopted. The unsatisfactory voltage deviations weaken the voltage regulation ability and increase the risk of instability. Two flexible controls at the top of the classical voltage droop control to reduce voltage deviations are proposed, and the power–voltage (P–V) curves of the classical voltage droop control are translated co‐ordinately. Firstly, a global DC voltage oscillation damping control designed based on the Lyapunov theory is proposed for transient voltage deviations. Secondly, a constant weighted voltage control is designed for reducing steady weighted voltage deviations. The steady active power injections keep almost unchanged under proposed controls, consistent with those under only the classical voltage droop control. The stability influence and the power flow impact are carried out to evaluate the proposed controls theoretically. Tests are conducted on a multi‐terminal DC grid with five‐terminal voltage source converters. Simulation results show the proposed controls' effectiveness and superiority under various fault conditions, including both the outage of a constant power voltage source converter and a droop voltage source converter.

Abstract In order to improve the efficiency of battery electric vehicles, the utilization of reconfigurable batteries and multilevel inverters based on low‐voltage Si MOSFETs gains growing attention. Focusing on the half‐bridge as basic switching unit, this paper examines voltage and current waveforms during switching. It is shown that the current commutation speed depends on various factors such as sign and amplitude of the current as well as the particular switching scenario. For instance, turning off a positive cell current takes 0.21 μs while turning it on takes 0.9 μs. In addition, it is revealed that depending on the switching scenario and current direction, the lithium‐ion cell supports or opposes the remagnetization of the parasitic inductances and therefore the switching losses of the MOSFETs. Switching a positive load current of 125 A from active to bypass converts 276 μJ into heat while switching from bypass to active converts merely 32.5 μJ into heat.

Abstract Nowadays, most of the converters used in high‐power high‐voltage (HV) applications are the conventional modular multilevel converters (MMC). However, in the case of DC‐DC conversion, an imbalance of the capacitor voltages occurs and the conventional MMC fails to operate correctly. This paper introduces an arm current reversal‐based modular multilevel DC‐DC converter, which successfully provides balance among the capacitor voltages while operating in DC‐DC conversion. The proposed configuration is used in medium voltage DC grids to feed DC loads or to interconnect between two DC grids of different voltage levels. The proposed converter is a two‐stage DC‐DC modular converter, which consists of a single‐phase half‐bridge MMC with half‐bridge submodules (HBMMC) followed by a single‐phase H‐bridge MMC with half‐bridge submodules (SMs). The operational concept of the proposed converter is based on reversing the arm current direction and reversing the output terminals with the help of the H‐bridge MMC stage, which ensures the same direction of the voltage at the load terminals. The proposed converter provides a high conversion ratio, bidirectional power flow, simple architecture, and a simple control scheme. Detailed illustrations, analysis, and design of the proposed converter are presented. Besides, MATLAB‐based and Opal RT‐based simulation results and experimental results are presented to validate the proposed configuration claims.

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 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 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 Remarkable progress in distributed power generation invigorated research into DC microgrids using a controllable DC–DC converter contingent to the functionalities therein. The study explores the circulating current issue in a parallel‐connected DC–DC converter and its associated current‐sharing capability. Despite, the conventional droop being a promising solution with less control complexity, the current sharing is achieved at an expense of voltage drop. The prime objective of this research work is to design a control strategy that guarantees a minimized circulating current with proper current sharing for a parallel‐operated DC–DC converter. This paper puts forth a dual reference‐based control to alleviate the droop and load effects of the system. The primary reference is generated by considering the effect of a marginal change in the input power of the converters. The virtual resistance‐based droop technique utilizes the prime reference for secondary droop reference generation. A current sharing algorithm is designed to assimilate adaptability to the scheme running under a variable line and load parameters. The comprehensive approach to the proposed control strategy has the advantages of improved current sharing and voltage regulation. A parallel DC–DC converter with the proposed control mechanism was investigated using MATLAB/Simulink and validated experimentally.

Abstract Cascaded H‐bridge inverter (CHBI) is widely used in the industry application for its outstanding features. However, CHBI needs a large number of isolated DC sources. If some of the DC sources are out of work, the DC voltage cannot maintain the desired value and even decreases to zero, thus degrading the quality of the output voltage of the CHBI. Thus, a soft‐switching modulation strategy based on the carrier phase shift SPWM is proposed in this paper to cope with the DC source failure. The strategy is proposed to achieve two goals: 1) one is to adjust the DC voltage of faulty cells back to the normal; 2) the other one is to make the pulse width modulation (PWM) pulses of each cell change without cell voltage level‐skip. The correction and efficiency are verified by a prototype of three‐cell CHBI built in the laboratory.

Abstract The virtual synchronous machine technology is considered as an important technique to effectively control the shortcomings of renewable energy‐based power electronics interfaces, providing backup inertia and regulating grid stability. Conventionally, the virtual synchronous machine with a large capacity is responsible for controlling the entire grid stability against renewable penetration. It is usually operated as a centralised control system. But what if virtual synchronous machines with small capacities are independently operated by their additional droop control schemes, and will they present better performance than the single virtual synchronous machine? This study proposes the multiple virtual synchronous machine system with different active power‐frequency (P‐f) droop characteristics to improve inertia support regarding frequency stability improvement. The comprehensive small‐signal modelling of the multiple virtual synchronous machine unit is designed to include the additional P‐f droop characteristics. Then, the dynamic characteristics (steady‐state and transient responses) and static stability of the multiple virtual synchronous machines are compared with the single virtual synchronous machine at the same rated capacity in both eigenvalue/sensitivity‐domain and time‐domain analysis. The obtained results reveal that the system with the presence of several virtual synchronous machines is more stable than the system with single virtual synchronous machine, maintaining stable and secure system operation during the contingency.

Abstract The inaccessibility of a utility grid is the challenge for rural and remote areas. This work presents the application of solar photovoltaic (PV) integrated battery energy storage (BES) for rural area electrification. The addition of a BES at DC link, is realised by means of a DC–DC bidirectional converter. The BES is discharged/charged in accordance with the solar PV generation and load variations. This converter control also maintains the voltage for the maximum power point tracking (MPPT) with perturb and observe (P & O) control at the DC link. The voltage source converter (VSC) works by means of voltage control algorithm in a solar PV‐BES system. The system manages the power for the load network with frequency and voltage regulation by the non‐ideal discrete proportional and resonant controller (PR). The fundamental component from the load current, is acquired using an adaptive digital filter, which improves the power quality. The utilization of buck‐boost converter with an optimum BES rating as related to the system, when it is linked directly at the VSC DC link. The system's operations at steady state and dynamic circumstances i.e. solar insolation change and load variation, load disconnection, are authenticated with test results on a developed prototype.

Abstract This study discusses a control strategy applied to the switched‐inductor single‐ended primary‐inductance converter (SEPIC) converter suitable for applications where the voltage delivered by a DC source varies from above or below the nominal value. The SEPIC converter can provide a non‐isolated positive output voltage for the above applications; moreover, it exhibits better characteristics when combined with a switched‐inductor cell. A design‐oriented procedure is given that appropriately selects the elements of the converter for a given output voltage and power delivery requirements. A control scheme with two loops is chosen. A high‐gain compensator is used in the inner loop such that the average inductor current follows a current reference. A proportional integral (PI) controller is used in the outer loop for output voltage regulation. The controller was designed using loop‐shaping techniques and applied to regulate an output voltage of 21 V. The design was verified with experimental results in a switching regulator where step changes were applied to the load. The performance of the switching regulator was also tested for variations of the input voltage.

Abstract As one of the most representative series active compensators, dynamic voltage restorer (DVR) can solve voltage‐related power quality uses effectively. The in‐phase voltage control and energy‐optimised control are the two typical control strategies of DVR. However, the in‐phase voltage control results in a large amount of active power exchange between the main grid and the dc link of DVR, and hence high capacity of storage system is needed in the dc link while the energy‐optimised strategy requires high injected voltage, resulting in oversizing of the DVR inverter. Thus, in order to avoid the aforementioned drawbacks, this study proposes a modified DVR, which connects an external capacitor in series with the LC filter (LCC) of DVR (LCC‐DVR). Based on the operational principles of the LCC‐DVR, an updated procedure of reference voltage is proposed to avoid overmodulation of DVR caused by load variations, ensuring the continuous operation capability of the LCC‐DVR. The overall control scheme of the LCC‐DVR and the comparative analysis among the proposed strategy and traditional strategies are also presented. The effectiveness of the proposed topology and method is clearly validated by the simulation and experimental results.