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Abstract This article proposes a deep learning (DL) model made of Long Short Term Memory (LSTM) and Adaptive Neuro Fuzzy Inference System (ANFIS) to detect fault in smart distribution grid assisted by communication systems using smart meter data. In smart grid, data analysis for fault identification and detection is crucial for grid monitoring. Nowadays, there are several DL techniques developed for smart grid data analysis applications. To solve this problem, a novel data analysis model based on deep learning and Neuro‐fuzzy algorithm is developed for fault location in a smart power grid. First, the LSTM is applied for training the data samples extracted from the smart meters. Then, an ANFIS algorithm is implemented for fault detection and identification from the trained data. Finally, faults are located with the higher accuracy. With this intelligent method proposed, single‐phase, two‐phase and three‐phase faults can be identified using a restricted amount of data. The novelty of the proposed method compared with other methods is the capability of fast training and testing even with large amount of data. To verify the effectiveness of our methodology, an intelligent model of the IEEE 13‐node network is used. The effectiveness and robustness of the proposed model are evaluated using several parameters such as accuracy, precision‐recall, F1‐score, Receiver Operating Characteristic (ROC) curve and complexity time. The obtained results indicate that the proposed deep learning model outperforms existing deep learning methods in the literature for fault detection and classification with 99.99% accuracy.

Switched-boost inverter (SBI) is a single-stage power converter suitable for interfacing between photovoltaic (PV) arrays and loads. Although, it has many advantages such as it's not requiring an expensive complex dead-time circuitry, yet its duty ratio is limited between 0 and 0.5. This means that it has some difficulties in increasing both the DC-link output voltage and its AC-output voltage. So, most researchers proposed it for low-voltage power applications or using with step-up transformers. Accordingly, this study contributes towards improving the performance of the SBI for being used in the modified nanogrid within an open energy system. It introduces a novel approach closed-loop control technique to overcome most of the inverter drawbacks. Also, it enhances both the DC-link and the transformer-less rated AC output voltages to a sufficient value suitable for feeding domestic loads. And in the same time, the proposed robust controller utilizes a hysteresis band for balancing between voltage stability and PV-maximum power achievement. This research also includes a more sophisticated fast-charging slow-discharging control strategy for the battery energy management system that depends on a bidirectional buck-boost converter. Test results ensure the robustness of the proposed controller against sensitivity, nonlinearity, and time-variant parameters of the overall system.

This study investigates a small film DC-link capacitor inverter for a permanent magnet synchronous motor drive system. Two control methods, including a d–q-axis current control and a positive torque region current control, are originally proposed to improve the torque dynamics of the drive system. In addition, a damping compensation method is used to improve the input power factor and reduce the input current harmonics. A current-loop predictive controller and a speed-loop predictive controller are first proposed to improve the dynamic responses of the interior permanent magnet synchronous motor drive system. To implement the drive system, a digital signal processor, TMS 320LF2808, is used as a control centre to execute the control algorithms. Experimental results can validate the correctness and feasibility of the proposed methods.

Wind energy conversion systems (WECSs) require a suitable control to maximise the power generated by wind turbines independently on the wind conditions. Variable-speed fixed-pitch wind turbines with doubly-fed induction generators (DGIG) are used in WECSs for their higher reliability and efficiency compared to variable-pitch wind turbine systems. This study proposes an effective control algorithm to maximise the efficiency of fixed-pitch wind turbines with DFIGs using particle swarm optimisation control to compensate for the errors in the estimation of the circuit parameters of the generator. The proposed control algorithm generates an optimal speed reference to optimise the mechanical power extracted from the wind and the optimal d-axis rotor current through stator reactive power management to minimise the electrical losses of the doubly-fed generator. The optimal speed reference is provided by a maximum power point tracking control below the rated wind speed and a soft-stalling control above the rated wind speed, while the optimal d-axis rotor current is searched by a particle swarm optimisation algorithm. The proposed control system has been verified by numerical simulations and it has been demonstrated that the energy generated for typical wind speed profiles is greater than that of a traditional control based on a model-based loss minimisation.

This study proposes a fault-tolerant matrix-converter-controlled IPMSM-drive system, in which a predictive speed-loop controller and a predictive current-loop controller are implemented to improve its dynamic responses, including transient responses and load disturbance responses of the IPMSM-drive system in both normal and faulty conditions. In addition, a sliding-mode estimator is used to estimate the rotor position, rotor speed, and stator currents of the IPMSM when the encoder or current sensor in the IPMSM-drive system is faulty. As a result, the IPMSM-drive system allows for either an encoder fault or a Hall-effect sensor fault. The detailed fault detection, diagnosis, and control are discussed here. In case of an encoder fault or a Hall-effect current sensor fault, the IPMSM-drive system still functions. A digital signal processor, TMS 320LF2407A, is used as a control centre to execute the fault-tolerant algorithm, state-estimating algorithm, and control algorithm. Experimental results can validate the correctness and feasibility of the proposed fault-tolerant and predictive control methods.

To ease the heavy burden of inverter control, including line current regulation, harmonic current suppression, filter resonance suppression, grid distortion suppression, phase locked loop etc., the study proposes a master–slave current regulation method for an LCL-filter-based grid-connected inverter operating under variable grid condition. The proposed system is composed of a grid-connected voltage-source inverter (GC-VSI) in series with a bidirectional voltage-source converter (B-VSC) at the inverter output. The GC-VSI is responsible for line current regulation, while the B-VSC takes charge of harmonic current suppression and grid impedance cancellation. Owing to low volt-ampere of the B-VSC, the efficiency of the inverter system is not sacrificed. To confirm its validity, a simulation platform with the proposed inverter system has been built. The simulation results demonstrate the performance enhancement of the proposed inverter system operating under different grid conditions.

This article presents an extended backstepping control method to reduce the grid-side current harmonics caused by grid voltage distortion. A dynamic model considering the grid voltage as an external disturbance is derived in the stationary reference frame. The extended back-stepping design procedure based on pole displacement guarantees that the system is stable without damping. The proposed method has a general low-order harmonics suppression instead of selective harmonics elimination. Zero steady-state error can be obtained tracking the grid currents. Stability, as well as good transient and steady-state performance are verified through numerical simulation. A comparison with the state-of-the-art proportional resonant control with active damping is also conducted. The results show that a good low-order harmonics reduction in the grid current is achieved by the proposed control method.

In this work, a dual-loop control strategy is applied to a highly distributed architecture of photovoltaic (PV)/battery-based DC microgrid, suitable for swarm electrification of developing regions. Typically, in such schemes, resource sharing among the spatially dispersed PV generation and battery storage resources is enabled via communication-based control methodologies, which adds cost and complexity to the overall system. Alternately, a communication-less and decentralised control methodology is proposed which utilises inner loop current control and outer loop voltage droop (V–I droop) control for the coordinated resource sharing among the distributed resources. Various scenarios of power sharing among the contributing households are evaluated and the efficacy of the proposed control scheme is validated through simulations on MATLAB/Simulink. Results show that the proposed decentralised control strategy is capable of ensuring stable and coordinated operation without any dedicated layer of communication among the dispersed generation/storage resources.

This study deals with a novel control strategy for permanent magnet synchronous machines (PMSMs) to incorporate disturbance compensation features in the existing current controller maintaining at the same time the integrity of the reference tracking performances. The torque produced by a PMSM arises from the interaction of stator currents and rotor magnetic flux, therefore an intrinsic disturbance is propagated to the output, in case the flux is not perfectly sinusoidal. In particular, the authors focus is to propose the effective correction of undesired harmonic effects generated on the torque from the non-ideal machine rotor. With the knowledge of the back electromagnetic force waveform, it is possible to determine the current waveforms necessary to produce a smooth torque in normal operation and through model-base considerations, the required voltage harmonic waveforms are also computed. The method is also extended to the flux weakening operation of the machine. The analytical solutions found are used in place of the classical αβ/dq transformations, without modifying the field-oriented control performance of the main controller. The effectiveness of the proposed control scheme is verified by means of test-bench measurement.

This paper presents a cascaded closed-loop control of a five-phase induction motor (IM) drive system with common-mode voltage reduction capability. The outer loop uses standard PI regulators for speed reference tracking, while the inner control loop is implemented using a reduced order finite-state model-based current predictive approach. Establishing certain constraints, different combinations of switching state vectors are obtained, leading to the generation of different magnitudes of common-mode voltages. As a result, precise speed and current control is achieved in addition to the reduction of common-mode voltage and computational burden. Experimental results are presented to validate the concept.

For a non-linear anisotropic permanent magnet synchronous machine (PMSM), a prediction model for model predictive control (MPC) considering effects like cross-coupling and saturation is developed in a straight forward procedure. The objective of the designed MPC is either tracking of reference currents or torque tracking. Both approaches use the projected fast gradient method (PFGM) as optimisation algorithm. The latter approach makes look-up-tables for current references obsolete and additionally minimises winding losses. This two approaches are compared in a simulation study with a state of the art PI controller.

The emulation of synchronous machines is becoming an attractive solution to provide virtual inertia and voltage support to weak grids and microgrids, representing an alternative to conventional current-injection control strategies. However, synchronverters operate as voltage sources and this makes it difficult to limit the current when an event like voltage sag takes place. Here, a method to limit the output current of synchronverters in parallel connection is proposed. First of all, a fictitious reference current is generated, and its value is then followed by an internal current controller. A method to avoid the loss of synchronism when the current saturates is proposed and studied in detail. Also, the implementation of the controller is addressed carefully. All the control system improvements are tested on a prototype consisting of two 15 kVA synchronverters connected in parallel to a configurable weak grid.

In a DC microgrid, because the output of renewable energy such as photovoltaic is intermittent, hybrid energy storage system (HESS) combining ultracapacitors and batteries is usually used to solve this issue. According to the characteristics of ultracapacitors and batteries, the low-frequency power fluctuations are assigned to batteries, while the high-frequency power fluctuations are assigned to ultracapacitors. When considering an ultracapacitor group, state-of-charge (SoC) of the ultracapacitors will be unbalanced between each other, on account of the differences of initial states or attributes. A novel distributed control method is proposed in this study. In general, ultracapacitors control the DC bus voltage and batteries control SoC of the ultracapacitors. Additionally, the distributed control adds the difference between one ultracapacitor's SoC and the average SoC of the ultracapacitor group to its current control loop. As a result, the dynamic stability of SoC among the ultracapacitors can be achieved, and the issues of overcharging and/or over discharging of the ultracapacitors can be avoided. At last, the effectiveness of the proposed control strategy is validated by the simulation model built in MATLAB/Simulink.

This paper presents a novel control scheme for a modular multilevel cascaded converter (MMCC) functioning as an active power conditioner (APC) to control the reactive power, eliminate the current harmonics, and compensate unbalanced load current simultaneously. This combines a modified predictive current controller with the inter-cluster and intra-cluster voltage balance control for MMCC sub-module capacitors. Simulation studies of this MMCC-APC for a power network containing both an unbalanced thyristor controlled rectifier and a reactive load are performed and results verifying its performance under varying degrees of load current distortion measured by THD levels are presented.

In recent years, with the increasing of wind power development, the demand for low voltage ride-through (LVRT) technology was proposed to ensure the safe and reliable operation of the power system. The key issue of LVRT strategy for doubly-fed induction generator (DFIG) varies according to the network synchronisation modes. Firstly, the existed LVRT technologies when DFIG is connected to AC network are surveyed. Then relevant literatures about LVRT are summarised and analysed in regard to power surplus of two-terminal system and DC voltage control of multi-terminal system, respectively, when the wind farm is integrated with VSC-high-voltage direct current (HVDC). In the end, the coordinated control strategy of multi-terminal VSC–HVDC system and LVRT technology for islanded system are prospected in this study.

This paper presents the analysis of the DC-link voltage switching ripple in five-phase PWM voltage source inverters with balanced load. The analysis is particularly important for the design and selection of a DC-link capacitor making it possible to meet desired electrical performance of the inverter system. Simple and practical equation for designing the DC-link capacitor is proposed relating the value of capacitance to the maximum value (peak-to-peak) of the DC-link voltage ripple. The amplitude of the DC-link voltage switching ripple is analytically derived as a function of modulation index, and the amplitude of the output current and phase angle. In order to show peak-to-peak voltage ripple distribution, different diagrams are introduced. Reference is made to continuous symmetric centred PWM modulation (i.e., space vector modulation SVM). In order to verify proposed developments, simulations have been carried out by Matlab/Simulink considering full range of modulation index and output phase angle.

It can be challenging to guarantee the stability of grids with many converters with LCL filters connected due to the presence of multiple resonances within the system. This paper presents an active damping technique to mitigate multiple resonance effects and harmonics in power converters connected to weak grids. The proposed technique employs grid current and capacitor voltage feedback to achieve active damping for a wide range of multiple resonance frequencies. The effectiveness of the proposed wide frequency active damping and improved controller stability is demonstrated through frequency domain analysis and experimental results for single and parallel grid-connected converters.

The modular multilevel converter (MMC) has emerged as a suitable topology for high power drive applications. However, the voltage fluctuations of its floating capacitors increase the control complexity of the converter. In this study, the MMC dc-port voltage is manipulated to regulate the amplitude of these fluctuations to a constant value during the whole frequency range. The proposed approach has several advantages when compared with the conventional ones since it minimises the voltage fluctuation in the capacitor cells. Additionally, it decreases the common-mode voltage at low frequencies and the capacitor rms current, increasing their expected lifespan and reducing the winding insulation damages and the leakage currents in the bearing of the machine. The effectiveness of the proposed control strategy is validated with a laboratory-based prototype composed of 18 power cells, feeding a vector-controlled induction machine.

This study proposes a technique for arbitrary current sharing between winding sets of any multiple three-phase machine by using a decoupled control in two-dimensional orthogonal vector space decomposition subspaces. With ability to arbitrarily set the d–q currents in all winding sets, both active and reactive power of each winding set can be arbitrarily set as well. Normally, this behaviour requires machine model in multi-stator variables where control implementation may be challenging due to the cross-coupling between winding set variables. On the contrary, in this study, the correlation between multi-stator and the vector space decomposition modelling has been used so that benefits of both modelling approaches are fully utilised: a decoupled control feature thanks to vector space decomposition and an ability to individually control winding set currents, a feature of the multi-stator modelling approach.

The increasing of large-scale wind power integration capacity brings great challenges to the safe operation and management of power grid. Modern grid codes put forward the requirements of fault ride through (FRT) capability: (i) to remain continuous operation at a given time; (ii) to contribute to support the voltage stability during FRT by providing reactive current. Aiming at the FRT problem of doubly fed induction generator (DFIG), critical elements of the rotor side converter (RSC) and the grid side converter (GSC) to FRT are analysed, and the improving RSC control scheme, adding extra hardware equipment, and improving reactive current control scheme are studied. Matlab/Simulink simulation studies are conducted to verify the effectiveness of the proposed strategies. The results show that when the grid fault is light, the improving RSC control strategy can effectively control the rotor winding overcurrent, when the grid fault is serious, it needs additional hardware equipment; the coordinated control strategy of the RSC and the GSC providing reactive current support is identified as the most advantageous approach because it not only provides enhanced reactive support for the grid, but also avoids over-speed risk of DFIG based wind turbine shaft.

This study proposes a dedicated modulation scheme for a bidirectional quasi-Z-source (qZS) modular multi-level converter. The operation principle and a suitable pulse-width modulation method are proposed. The relation between the modulation index and shoot-through duty ratio is derived. A formula for calculating the required value of qZS capacitance is given. The simulation results presented in the study validate the operation and the performance of the proposed topology.

This paper presents a novel converter topology for six-phase switched reluctance motor (SRM) drives, which reduces the number of switches and diodes by half, compared with the conventional asymmetric half-bridge (AHB) converter, and needs no additional energy storage component. A dynamic model of a six-phase SRM is developed in the MATLAB/SIMULINK environment and conventional control technique is applied to the proposed converter, demonstrating successful operation with modified conventional current control technique, lower converter losses, and higher system efficiency compared with the AHB converter. Furthermore, a direct torque control (DTC) method has been proposed for the novel converter to further reduce the torque ripple. Experimental tests comparing the proposed converter with the AHB are described and verify the predictions of the simulations.

Currently, virtual synchronous generator (VSG) technology is proposed to be a solution to stability controlling of micro-source inverter. However, the deviation from phase locked loop (PLL) influences the reference frequency calculating which is important to the VSG control system. This paper focuses on the virtual inertia controller in the VSG control system, an enhanced controller is achieved for avoiding the effect of PLL on the VSG dynamics. For analysing dynamic characteristics of the enhanced VSG control system, the linearised small-signal model of VSG in isolated operation is well established, including electrical system, enhanced virtual inertia controller, voltage and current controller, virtual impedance, and droop controller. By small-signal stability analysis, the influence rules of the main parameters (load, line, virtual inertia and proportional-Integral gain) on VSG system impacts are provided, which helps identifying possible feedback signals for controller design to improve the system stability. Results obtained from time-domain small-signal model are compared with those from the electrical simulation to verify its accuracy.

In this study, the integration of multi-terminal high voltage direct current (HVDC) systems to weak power grids is analysed under different grid strengths and control methods. In this study, a modular multi-level converter based MTDC network is considered. The analysis showed the importance of voltage control of DC network. The voltage control of DC network needs to be performed from the converter station connected to the strongest AC network. Furthermore, the analysis shows that either droop or proportional-integral control can be used for controlling the DC voltage of the MTDC network connected to one or more weak grids. However, the droop control of DC voltage provides faster response compared to PI with larger steady state error. Finally, the analysis showed that the modular multi-level converter (MMC)-based MTDC is capable of ride-through unbalanced AC side faults even if the faults occur in a weak grid.

The interest in hybrid electric aircraft has invigorated research into superconducting power networks and superconducting electrical machines. Underpinning this is the ability to control the flow of electrical current at cryogenic temperatures, using power electronic devices. The authors have, for the first time, directly compared the performance of technologically relevant semiconductor materials for the realisation of high-performance cryogenic power devices using a bulk resistivity model. By validating the model using both computational and experimental results, the performance of technologically relevant semiconductors has been calculated down to a temperature of 20 K where the freeze out of dopants is shown to be the major limiting factor in determining the performance of power electronic devices. Both Ge and GaAs are predicted to have a superior conductivity in comparison to the industrial standards Si and 4H-SiC due to greater carrier mobilities and lower dopant ionisation energies.

This paper develops a new field weakening method for an integrated permanent magnet synchronous machine vehicle drive. This method adopts both the DC-link and the rotor speed as a feedback signals to generate the demagnetising current reference. This eliminates the effect of the rotor speed on the FW controller dynamics and ensures stable operation across the whole range of speeds. Mathematical analysis of the system is used to design the controller gains to achieve the desired dynamic response. A maximum torque per ampere (MTPA) strategy is included in the control design, with a linearised function to reduce the computational demand. Experimental results show stable and satisfactory operation at different speeds and load conditions. They also illustrate smooth transition between the MTPA operation and the FW operation when the rotor speed crosses base speed.

With the distributed new energy power generation increased, the power grid system becomes more and more complex, leading to its stability more likely to be affected. Among them, the resonance caused by impedance mismatch is particularly critical. The small-signal model of phase-locked loop and current control in the inverter control system have been established by using harmonic linearisation method. Combining the average model of inverter main circuit, the inverter output impedance model has been obtained. Using grid impedance and the inverter output impedance model, the stability analysis method based on impedance is used to analyse the influence of grid impedance on the stability of grid-connected inverters. Finally, the simulation proves the correctness of the analysis method.

This paper deals with a novel robust current control scheme for PMSM drives, based on the model predictive control (MPC) theory. It has been designed a simple quadratic formulation of the reference tracking problem in order to compute the optimal commanded voltage. Considering only equality constraints allows the efficient computation of a closed-form solution which solves the minimisation problem. Authors' proposed objective function is extended to include the inequality constraints, such as current and voltage limitations, therefore, the problem indirectly penalises the violation of boundaries through a weighting strategy. However, the controller does not require any calibration effort, maintaining the application suitable for different drive systems. The increased robustness is achieved through the integration of a moving horizon estimation (MHE), which exploits the same explicit formulation obtained for the MPC, but with the objective of estimating the voltage disturbance affecting the machine. The dual estimation problem is directly integrated within the prediction step of the MPC, improving drastically the local accuracy of the nominal model prediction with further advantages in terms of precise reference tracking and disturbance rejection capabilities. The effectiveness of the proposed control is verified by mean of test-bed experiments.

The LLCL filter is widely used in grid-tied inverter systems due to its small size, low cost and good attenuation for high-frequency harmonics. However, the LLCL filter resonance phenomenon and the wide range of impedance may compromise the system stability. In order to solve these problems, a coordination control strategy for LLCL-filter-based grid-tied inverter with indirect sliding mode power control and virtual impedance was presented. The LLCL filter design parameter has been carefully adjusted to ensure the system stable operation under a wide range of grid impedance variations. Comparing with the traditional proportional integral control system through simulation experiment, the output current harmonics of grid-tied inverter are analysed. The proposed strategy is feasible with good control accuracy and tracking performance.

Single-phase FACTS device has a bulky and short-life electrolytic capacitor to absorb the ripple pulsating at twice of the line frequency in DC side, resulting in lower power density. This paper introduced a structure of three-phase bridge which added an additional leg connected to an AC capacitor based on single-phase H-bridge. The 2-ripple energy of the electrolytic capacitor in single-phase H-bridge could be transformed to the film capacitor of the AC side in three-phase H bridge. The size of single-phase FACTS is reduced by ten times compared to the single-phase H-bridge. A simple control strategy had been studied, through two kinds of controllers: the digital quais-PR controller had been used to control gird current and AC capacitor voltage and current; the two cosine controller had been used to eliminate the rest of two-ripple harmonic in the DC side, there was no controller to maintain DC voltage. The experiments verified the feasibility of the control strategy.

The modular multilevel converter (MMC) has the advantages of high efficiency, low harmonic, modular design, and easy cascade, which has been widely used in the field of high voltage and large capacity energy conversion. In order to improve the dynamic response speed of the MMC-HVDC based on the modular multi-level converter, a novel method which combines a model predictive control (MPC) of MMC-HVDC system with improved sub-module voltage balanced control strategy is proposed. The method, which utilises the prediction model, feedback correction and rolling optimisation to obtain the optimal voltage control, overcomes the difficulties in the traditional way of setting PI parameters of the internal loop current controller and the outer loop controller and tackles the problem of low dynamic response. Finally, a 21-level MMC-HVDC system simulation model is built on PSCAD-EMTDC software platform. The simulation results indicate the effectiveness and feasibility of the control strategy.

This study describes transient electromagnetic phenomena in electrical conductors that are connected to modern power electronic circuits and therefore subjected to current controlled pulses with fast rise times and high peak current ratings. Electromagnetic field diffusion is discussed and a brief review of alternating current skin effect phenomena is presented before it is generalised to transient regimes of operation. It is shown that the magnetic field diffuses into the conductor from the outside on the initiation of a current pulse and therefore the axial current density distribution inside the conductor changes with time under a transient current due to the well-known electromagnetic relations – the current begins at the conductor surface and diffuses inwards. The implications of such behaviour are briefly discussed in the context of modern power electronics and its applications.

A control topology input-parallel–output-series converter system is investigated, which is suitable for high-voltage and high-output power applications. In this control method input current sharing (ICS) and output current sharing (OVS) taken into account. As observed, ICS is automatically attained as OVS sharing is controlled. In this design, two modules are connected in series, Module 1 provided variable voltage but in Module 2 by using three-level neutral-point-clamped converter and designing a comprehensive control feedback system, which regulates the DC voltages at the constant magnitude and constant frequency, helps to provide constant output voltages in the event of uncertain disturbance and load variation, A feedback double closed-loop control topology (inner current loop control and outer voltage loop control) with the combination of sliding mode control and a proportional–integral control method. A simulation model is built, by using MATLAB/Simulink, and the simulation results show excellent performance and feasibility of this system. This will be beneficial for research and improvement for the grid-connected converter.

In this study, a three-phase motor inverter with sinusoidal output voltages based on the application of gallium nitride transistors and advanced control is analysed. In comparison to standard silicon-insulated gate bipolar transistors much higher feasible switching frequencies of 100 kHz and above are possible and reduce the output sine filter component size such that the filter can be directly included into the inverter. This considerably improves the electromagnetic interference (EMI) behaviour of the drive system as well as the acoustic noise, eases the inverter-to-motor wiring and protects the motor isolation against high du/dt rates. The study describes the dimensioning and design of the used two-stage LC filter including motor current control based on proportional-integral-type phase current controllers. The LC filter damping is performed actively by capacitor current feedback. Using this active damping scheme avoids additional losses of conventional sine filters and guarantees high system efficiency up to 98%. Finally, experimental results of a laboratory prototype verify the proper behaviour of the proposed concept.

As a new type of HVDC technology based on voltage source converter, VSC-MTDC has many technical advantages and broad application prospects. The paper describes a mathematical model of VSC-HVDC system in dq synchronous rotating coordinate system, but the currents of active and reactive are coupling. Here, the use of internal control principle to achieve a decoupling of the current inner loop, independence control of active and reactive power, the inner current loop controller is designed and the basis of the controller parameter setting indicators is given. Moreover, the proposed strategy was simulated by MATLAB. The results show that the method can achieve decoupling control of active and reactive power and have good dynamic quality and robustness, which verify the correctness and validity of the control strategy in a three-terminal VSC-HVDC.

The current source converter (CSC)-HVDC system is the key technology for long distance large power transmission. However, the traditional CSC-HVDC has two serious shortcomings: (i) vulnerable to commutation failures; and (ii) not being able to control its reactive power. This study will propose a Flexible CSC topology, which is based on the dc current reinjection conversion concept. The Flexible CSC reduces the converter current waveform to zero during the commutations of valves and, thus, removes the converter dependence on line commutation. The Flexible CSC is also free to control the amplitude and phase of its output ac current. So, it can control freely active power and reactive power. Use this converter to construct a new CSC-HVDC system. This new CSC-HVDC system can operate in four quadrants, so as to control reactive power in wide range. In particular, it can export leading reactive power to the ac grids. This study describes the structure and control of the Flexible CSC and builds an experimental prototype. The simulation and experimental results will be shown to demonstrate the feasibility of the converter topology, the effectiveness of the active reactive power controllability, and the commutation failure elimination method.

Modular multilevel converter (MMC) has a very good application prospect in the flexible high-voltage direct current (HVDC) transmission, but higher self-loss will affect the efficiency and restrict the development. Here, the MMC in voltage source converter-based HVDC as the research object, the control and modulation algorithms are designed on the basis of analysing its working principle. The capacitor voltage imbalance problem is existed in the converter because that the capacitor of each module is independent. The factors of switching actions in balancing control technology based on the sorting method of sub-module capacitor voltages are analysed in detail. Then, an optimised balancing control algorithm by setting two balancing control thresholds is proposed to reduce the average switching frequency. Finally, a two ends HVDC transmission system was built in MATLAB/SIMULINK to verify the optimisation. The simulation research results show that the optimised balancing control can ensure sub-module capacitor voltage is in balanced and does not increase the AC output voltage harmonic content, and obviously reduce the average switching frequency of switching devices, and with the increase of number of level, the more obvious optimisation effect.

A novel high-conversion-ratio bidirectional three-phase DC–DC converter is proposed in this paper, which features not only low switch voltage and current stresses, but also automatic voltage balancing and current sharing ability without adding extra circuit or control method. Both total current ripple of the inductors and total voltage ripple of the capacitors are reduced by using interleaving technology. The proposed converter is suitable for fields of renewable energy, electric vehicles, energy storage systems and so on. The proposed topology can achieve three times the conversion ratio of the traditional three-phase DC–DC converter by only adding two capacitors. High voltage conversion ratio can be achieved with a moderate duty cycle which can avoid the extreme working condition existed in the traditional boost converter. Control method, operating principle, voltage conversion ratio, characteristics of automatic voltage balancing and current sharing, voltage and current stresses current and voltage ripples are analyzed in more detail. Finally, feasibility and superiority of the proposed converter are verified by software simulation and prototype experiment.

The modular multilevel converter (MMC) with large-scale sub-modules has the advantage of simpler modulation, lower switching frequency, and lower harmonic component, thus would be very promising in voltage source converter (VSC)–high voltage direct current transmission systems. Conventional capacitor voltage balancing algorithm suffers from insufficient grouping and sorting techniques. Therefore, it might result in excessive computation and high switching frequency of power electronic devices. To address the problem, a modified capacitor voltage balancing sorting algorithm is proposed in this paper. The proposed algorithm could avoid sorting all the module capacitor voltages by selecting only a certain number of the largest or smallest module capacitor voltages, and thus reduces time complexity greatly without losing control precision. Furthermore, the proposed algorithm focuses on the sub-modules whose capacitor voltage exceeds the limits, while the switching states of the other sub-modules are maintained to some degrees by employing the maintaining factor. Therefore, the switching frequency of the power electronic devices is further reduced. The performance of the proposed algorithm is evaluated through a time-domain MMC–HVDC simulation in PSCAD/EMTDC. Results show that the proposed algorithm is able to balance the module voltage with lower computation and reduce the switching frequency of power devices significantly, without noticeably increasing the capacitor voltage ripples.

In order to solve the deterioration of power quality caused by power electronic devices in active distribution networks, it is necessary to improve the three-phase four-wire active power filters (TFAPF) control strategy. This study adopts the non-harmonic detection and proposes an improved control strategy of TFAPF based on the multi-model principle. Firstly, the mathematical model of TFAPF in the stationary coordinate system is established by replacing the network side current and the DC bus voltage directly as the state variables. Secondly, TFAPF in active distribution networks requires the power fluctuation and fast dynamic response, the proposed control strategy of TFAPF in the stationary coordinate system is applied, namely proportional integral, vector resonant and repetitive control composite controller as the current loop controller of TFAPF with a fast dynamic response, high accuracy and small amount of calculation, provide guarantee for the power quality in active distribution networks. Finally, the feasibility of the proposed control strategy is verified by simulation.

In order to protect the power electronic equipment and make the output current amplitude of distributed generation (DG) maintain a stable value for different short-circuit faults, this paper proposed a current-limiting strategy based on proportional-resonant controller (PR), filtering inductance reference current (FIRC), and shunt active power filter (APF) for droop controlled microgrid. First, the relationship is discussed in detail among the output current of DG, the output voltage of DG and FIRC, theoretical derivation demonstrates that the error between output current of DG and FIRC can be decreased through adjusting the value of PR parameters, filtering capacitance and the compensation current obtained by APF. Moreover, some advantages of proposed strategy for microgrid protection are discussed. Then, when the fault is detected, the current-limiting strategy is implemented by setting the FIRC value. Finally, the experiments prove that the proposed strategy can precisely limit the maximum phase current to the given value which cannot be influenced by the short-circuit fault type, grounding resistance, fault initial phase, non-linear load, and so on.

This study presents an independent phase-current-control architecture for a three-phase voltage source converter. It comprises three independent single-phase phase locked loops (PLLs), and a dual-loop scheme including an outer loop (responsible for active power and ac voltage control) and three similar inner current control loops. To obtain reliable phase-tracking, an enhanced second-order generalised integrator-based PLL is developed. The phase-tracking angle is strategically selected from the three PLLs to minimise the impact of imbalance-induced distortions. To validate the proposed control scheme, extensive simulations are conducted for a 101-level, 500-MW, 500-kV (dc) half-bridge modular multi-level converter in PSCAD/EMTDC. The simulation results prove that the proposed scheme enables the system to ride through unbalanced conditions effectively and reliably.

Hybrid distribution transformer (HDT) is an integrated system of the conventional distribution transformer and the PWM converters, which can greatly enhance the controllability of the conventional distribution transformer and is important for the intelligence of the future distribution network, since HDT can support the AC–DC power network and control the voltage and current in real time. However, the impact current, over voltage, and saturation will lead to the start-up failure when an incorrect start-up scheme is adopted in HDT. Therefore, it is significant to design a reliable start-up scheme to guarantee a safe operation for HDT. Here, a three-phase circuit scheme and the control system of HDT are proposed. Then, a soft start-up scheme that can realise the constant-current charging for the DC bus capacitor is proposed. Specifically, the soft start-up scheme is based on the following measures, bypassing the DC-bus current-limiting resistors when the voltage of DC-bus capacitor exceeds the threshold, limiting the output of PI controllers during the start-up, separating the integral coefficient according to the DC-bus voltage error. Finally, the proposed start-up scheme is verified in MATLAB, which lays the foundation for the optimal design of the start-up scheme of HDT.

The PFC Boost converter controlled by the mode of CCM has strong non-linearity due to the use of non-linear elements such as power switch and multipliers. Here, a globally optimised control strategy of dynamic slope compensation is proposed to eliminate bifurcation, chaos behaviours. Time-varying adjustment of control efforts can achieve the optimal compensation within each switching cycle. First, the model and the system transfer function of the circuit controlled by the global dynamic slope compensation control mode have been derived and the stability of the system is analysed from the frequency domain at the same time. The voltage outer loop is used to compensate the worst-stability zero-crossing time, which ensures that the circuit meets the frequency domain stability requirements throughout the whole cycle. Finally, the experimental circuit has been built to verify the theory, it can be found that the theory has good prospects in practical application and promotion through comparison between the simulation and the measured circuit waveforms.

Here, a distributed hierarchical control strategy is proposed for the parallel operating of voltage source inverters (VSIs) in AC microgrids (MGs). For the output voltage control of a VSI, conventional double-loop proportional-integral (PI) control method has deteriorative performance under disturbance parameters conditions and has a narrow range of stable operation. Here, the sliding mode control (SMC)-based inner-loop current control (level0) and mixed H(2)/H(∞)-based outer-loop voltage control (level1) are proposed for the output voltage control of a VSI, which combines the advantages of robustness and fast transient response. The droop control (level2) is applied to avoid circulating currents among the VSIs without using any critical communication between them. The multi-agent system (MAS)-based distributed economic automatic generation control (EAGC) and distributed automatic voltage control (AVC) (level3) are proposed to restore the voltage magnitude and frequency deviations caused by droop controller and achieve optimal active power dispatch at the same time.

Hybrid distribution transformer (HDT) has powerful control ability, which is significant for the intelligentisation of the distribution network. A kind of HDT is presented here, the grid current control system of which is emphatic researched. Based on the mathematical model of ideal HDTs, a grid current control system is designed with quasi proportional-resonant controller (QPR). Using the characteristics that infinite gain and large bandwidth at the resonant frequency of QPR controller, the grid current system achieve the effects of unit power factor and sinusoidal symmetrical waveform. Tracking control with no static error of grid current was achieved in MATLAB. Compared with the traditional PI control, QPR controller can modulate AC signals in the static coordinate system, rather than rotary coordinate system. So the QPR controller has advantages of lower steady-state error, stronger anti-interference ability, and simpler calculation. Simulation results show that it has good dynamic and steady performance.

With the growth of DC electricity load, the difficulty to improve the power quality, and the increasing penetration of renewable power, flexible DC distribution network, which contains complex renewable energy generation equipment and a large number of power electronic converters, become an important research direction. The modelling of the DC distribution network is an essential step for the research on the coordination control. However, the traditional modelling will be a burden for simulation software. This article models all electrical devices as equivalent power sources based on the proportional–integral control and the power–voltage curve, ignoring the complex internal control processes. Based on the simplified models of power, load, DC/DC converters, and AC/DC converters considering droop control, a simplified model of a multi-terminal flexible DC distribution network is derived. Taking the impact of droop control into account, the simulation of DC distribution power flow under the different distribution of generation and load is done. Results show that the simplified model of DC distribution network with droop control can accurately and rapidly simulate the transient process and steady states of the testing network.

A fractional-order current-controlled memristor pinched hysteresis loop area is calculated in this study. The area is divided into two parts: one equals to the half of instantaneous power and the other is the part memory of the memristor. Moreover, two parts of the area are affected not only by the cosine components, but also by the sine components. The voltage of the fractional-order current-controlled memristor is no longer an odd function with respect to time and the coefficient of cos(ωt) in its Fourier series is zero. In a closed loop, the average power and the memory rely only on sine harmonics of the voltage. Meanwhile, the power and the memory are related to the order of the fractional-order derivative.