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Abstract The efficiency of the power‐to‐hydrogen (P2H) alkaline electrolyzer (AE) is the primary factor constraint on its large‐scale application. Decreasing the specific energy consumption (SEC) of AE is positive to enhancing the efficiency of P2H system. However, the factors influencing the SEC of AE are yet to be fully revealed. In this paper, the influence of power quality and stack temperature on the SEC of AE is explored. Firstly, the models of double star rectifier and PWM current‐type rectifier are established. And the mathematical model on the SEC of rectifiers is proposed. The influence of power quality on the SEC is analyzed by comparing the current and voltage waveforms under four types of rectifiers. Secondly, the model of AE that considers the stack temperature is proposed. Unlike the existing simplified models, the proposed model can assess the SEC considering changes in stack temperature. Finally, the results show that the SEC with the 6‐pulse rectifier is 10.99% and 11.98% higher than that with the double‐star rectifier and PWM current‐type rectifier. The stack temperature is inversely related to the SEC of AE. This study provides a reference for reducing the SEC and enhancing the efficiency of the P2H system.

Abstract Recently, there has been a push to integrate renewable energy system (RES) into grid‐connected load system in enhancing reliability and reducing losses. However, integrating these systems introduces power quality (PQ) issues, especially with non‐linear, critical, and imbalanced loads. Addressing this, a hybrid mantis search‐reptile search algorithm (HMS‐RSA) combined with a unified power quality conditioner (UPQC) to mitigate PQ problems related to current and voltages in RES systems. In other words, the UPQC, enhanced by fractional order proportional integral derivative controller parameters tuned using the proposed HMS‐RSA assists in enhancing the power quality. The approach has been validated by connecting a non‐linear load to the system, which typically creates PQ issues. The proposed method is implemented in MATLAB/Simulink and their performance is analysed in three scenarios, such as sag, swell, and disturbance, and the total harmonic distortion is evaluated to quantify improvements in PQ. Finally, the proposed method is compared with existing approaches, such as ant colony optimization (ACO), artificial bee colony optimization (ABC), and bacterial foraging optimization (BFO). The method also outperforms ACO, ABC, and BFO in terms of convergence speed and effectiveness in mitigating PQ issues.

Abstract To solve the problem of negative‐sequence and reactive power in electrified railway, this paper proposes a comprehensive compensation method based on V/v transformer and electromagnetic single‐phase var compensator (ESVC) for co‐phase power supply. First, based on the principles of static var generator and single‐phase rotary phase shifting transformer, the topology and model of ESVC have been established, and the compensation mechanism has been analysed. Second, the topological structure and mathematical model of co‐phase power comprehensive compensation device (CPCD) are put forward based on the principles of V/v transformer and ESVC, and the operation mode of CPCD in multiple scenarios is designed. Then the strategy of CPCD with double closed loop control is analysed: corresponding compensation mode is selected according to negative‐sequence unbalance degree and expected power factor value. In this strategy, the compensated current output by ESVC is taken as the quantity of external loop control, and the rotation angle of ESVC is taken as the quantity of internal loop control to realize double‐closed loop control of CPCD. Finally, the comprehensive CPCD compensation model is built on the simulation platform and validated the accuracy of the mathematical model and the effectiveness of the control strategy.

Abstract The presented work operated a PIPO Current fed front end converter‐based solid‐state transformer (SST) for maximum power point tracking (MPPT) and voltage boost operation in medium power range PV applications. The proposed system employed the perturb and observe (P&O)‐based MPPT technique and upheld the ZVS and ZCS operations of all primary side switches and secondary side rectifier diodes, respectively. The proposed system introduced a new fundamental extractor titled ‘Pre‐filter double fundamental signal extractor (DFSE) based multi‐layered DFSE (p‐DFSE‐MDFSE)’ and utilised it in the grid controller for synchronisation and harmonics mitigation operation. The primary goal was to achieve a compact size, reduced weight and cheaper magnetic component‐based front‐end converter of SST on the PV array side and a fast, secure and trustworthy mitigation control of current harmonics on the utility grid side. The proposed system was inspected on the OPAL‐RT (RT‐LABv2021.3.2.307) platform for various grid anomalies, and its conduct was found to be well within the IEEE 519 norms.

Abstract During the operation of the ITER machine, hundreds of MW/Var of active and reactive power will be exchanged with the grid. The E‐STATCOM scheme composed of the Modular Multilevel Converter (MMC) and split supercapacitor energy storage has been proposed to improve the power compensation performance of the existing reactive power compensation system in the previous study. However, one of the main technical challenges which is lack of research is to precharge all submodule capacitors and supercapacitors from zero to their nominal voltage values efficiently during a startup process. As the capacitance and operating voltage of supercapacitors are much different from capacitors in each submodule, the startup of E‐STATCOM is a more complicated process. To coordinate the energy exchange between submodule capacitors and supercapacitors, submodule capacitors and the grid, this paper presents an optimized four‐stage AC side startup strategy for the E‐STATCOM. The proposed method minimizes the use of current‐limiting resistors while suppressing the surge current in the zero‐voltage startup process of supercapacitors, in addition to optimizing the energy consumption. The pulse current charging, constant current charging and constant power charging strategies of supercapacitors are adopted in different charging stages, and the detailed coordinated control scheme between submodule capacitors and supercapacitors are described and analyzed. The effectiveness and performance of the proposed method are verified by simulation results and hardware‐in‐the‐loop (HIL) experiments.

Abstract To improve the power density of series‐connected multi‐pulse rectifiers (MPRs), the harmonic generation principle is studied and the optimization design method for main transformers is proposed. According to the topology of series‐connected 18‐pulse rectifier, its operation waveforms are analysed, and the Fourier series of the waveforms is obtained. Based on the operation waveforms and its Fourier analysis, the principle for harmonic generation of the rectifier is researched, and the phenomenon of autotransformer cannot be applied to series‐connected MPRs is researched. According to the characteristics of the transformer, its vector graphics are analysed. Based on the vector graphics, the optimal structure of the transformer with the highest power density is obtained. The results show that the premise of the normal operation of series‐connected MPRs is the potential difference between the neutral points of different secondary windings, and only isolation/semi‐isolation transformers meet the requirement; the kilovolt‐ampere (kVA) rating of the optimal isolation type transformer is 104.6% pu, and the kVA rating of the optimal semi‐isolation type transformer is about 71.23% pu.

Abstract A trilateral control for LCL filter‐based system is introduced by the authors with a single grid current sensor in weak grid conditions. The LCL filter increases the complexity when the uncertain nature of the grid comes into the picture. Moreover, the traditional three‐loop control technique requires three current sensors on the inverter side, three voltage sensors to sense voltage across the capacitor, and three current sensors on the grid side combined for sensing. A novel trilateral control technique utilising a single sensor is implemented to sense the grid current. This technique has reduced a considerable number of current sensors and voltage sensors. The α axis of grid current is proportional to sensed ‘a’ phase grid current. The β current in the utility grid is acquired by employing the controller reference quantities of the grid current. The computation of another variable, that is, the current in the inverter side inductor and the voltage across the capacitor, is executed by an estimation algorithm. The proposed technique provides the feature of reducing implementation financial value and weight that reduces the complexity and size of hardware. The synchronisation technique is executed by a modified dual second‐order generalised integrator digital phase‐locked loop for the grid‐connected converter. The implemented system offers the advantage of ease of implementation, good performance, and high stability. The validity of the proposed scheme in the implemented system is demonstrated by the simulated waveform obtained on the MATLAB/Simulink platform. Finally, the effectiveness of the proposed system is further justified by the experimental waveform procured from a prototype developed in the laboratory.

Abstract The characteristics of traction network (TN) of the three‐phase 3‐kV traction power supply system (TPSS) are: three‐phase conductors are flat layout (layout‐1, layout‐2); the material and cross‐section of conductors are ferromagnetic and irregular, respectively. So the phase impedance matrix (PIM) is unbalanced and the closed‐form formula for conductor impedance is hard to get. To address these issues, firstly, the PIM is derived, and the cause of unbalance is revealed. From the perspective of structure optimization, a new flat layout (layout‐3) and an equilateral triangle layout (layout‐4) is proposed. Then, to calculate the wideband frequency self‐ and mutual impedances of the special phase conductors, a frequency‐ and current‐dependent closed‐form formula based on 4‐parameters equivalent tubular conductor (ETC) model is proposed. Furthermore, an equivalent TN circuit model at 50 Hz is established to calculate the receiving‐end voltage and its negative sequence voltage unbalance factor (NSVUF). Finally, using these two indexes, the performances of the layout‐1 to 4 are evaluated and compared. The results show that the conductor spacing and power supply frequency affect the two indexes much while the dimension of the power supply rail (PSR) not; layout‐4 can completely balance the receiving‐end voltage and improve its magnitude which can enhance the power supply capability.

Abstract This paper introduces a discrete Fourier transform (DFT)‐based method for estimating the amplitudes and frequencies of interharmonics, which arise from the interaction between wind power generators and electric power systems. This method exploits the synergy of choosing signal enhancement, mains frequency estimation, and re‐sampling techniques. Consequently, it accomplishes a cost‐effective estimation process. The performance results show that signal enhancement and re‐sampling techniques are necessary to establish suitable conditions for performing amplitude and frequency estimation. Furthermore, numerical results show that the proposed method, relying on low‐cost digital signal processing techniques, offers competitive performance and significant computational complexity reduction compared to a previous DFT‐based method.

Abstract A three‐phase to single‐phase direct AC power generation system based on the open‐winding permanent magnet synchronous generator (PMSG) with the dual‐inverter is proposed in this study for the standalone power system. The negative DC terminals of the dual‐inverter are commonly connected, and the positive DC terminals are reconstructed as the output port of the single‐phase AC power; the three‐phase AC power generated by PMSG could be converted to be the high power quality single‐phase AC power directly without any LC filters. Different from the traditional OW‐PMSG system, zero‐sequence current is employed to achieve adjustable frequency and voltage of the single‐phase AC power, and the corresponding dual voltage‐loop control strategy with the separate positive‐sequence and zero‐sequence currents regulation of PMSG is given to achieve high output performance. The effectiveness of the proposed control strategy and the output characteristics have been verified by simulation and the experiment of the prototype, and the results show that the high‐quality single‐phase AC power can be obtained at a wide speed range and at different load operating conditions.

Abstract The fluctuating outputs from renewable energy sources (RESs) and variable load power demand deteriorate the current quality, thus an improved generalised integrator with a frequency‐locked loop (IGI‐FLL) control strategy is used, which is not only easy but simple to attain a quick and precise fundamental extraction. IGI‐FLL control method is utilised with all‐pass filter and is capable to accomplish enhanced harmonics, noise and DC offset rejection ability while estimating fundamental component of the load current. In addition, the FLL structure is used to increase the frequency immunity against voltage variations. It provides improved steady‐state and dynamic performances with flexibility in parameter tuning in comparison to SOGI‐FLL algorithm. It also presents a cost‐effective and robust genetic algorithm (GA)‐based optimisation to provide optimum battery charging and discharging power while satisfying the state‐of‐charge (SOC) limits under RES uncertainty. In this islanded microgrid, a Synchronous Reluctance Generator for pico‐hydro and photovoltaic array is used so that this hybrid system utilises maximum solar PV array power, matches the load, or else charges the battery energy storage while maintaining power balance in the microgrid. Comparative outcomes authenticate the efficacy of the algorithm to handle the issue of RES uncertainty and load unbalance in an islanded microgrid.

Abstract This study deals with a new control for a double‐stage grid‐integrated solar photovoltaic array (SPVA)‐based energy generation system, which is based on adjoint least mean square (ALMS) control. This ALMS control algorithm has some new features: (1) it provides a fast rate of convergence, (2) harmonics mitigation ability, and (3) ease of implementation. The solar PV energy generating system includes a DC–DC boost converter, a voltage source converter (VSC), a ripple filter, a three‐phase grid, and local non‐linear loads. The VSC is controlled for feeding the active power from the solar array to the grid and three‐phase non‐linear loads. Moreover, VSC control is used to improve the grid current quality. A DC–DC boost converter is coupled between the photovoltaic array and the DC‐link capacitor, which tracks the maximum SPVA power from the SPV array by using perturb & observe algorithm. Here, the main goals of SPVA generating system are to eradicate power quality problems caused by unbalanced and non‐linear loads and to provide sinusoidal grid currents at solar PV array irradiance variation. Moreover, a comparison of ALMS control is made with other control algorithms. Performance of this ALMS control is studied at varying non‐linear loads and under different environmental conditions on a developed prototype in the laboratory. The simulated results of the system are validated with test results. Detailed behaviour of the grid‐integrated SPV system and harmonics spectrum of grid currents are given here while meeting the IEEE 519 Standard.

Abstract This study presents a mode adaptive multifunctional control strategy that avoids the collapse of the microgrid due to the subsequent failure of the generation units and faulty grid. Its control allows the distributed generation‐based AC microgrid consisting of a PV panel, wind generation unit and battery energy storage to operate seamlessly during utility connected and outage operating modes. On the occurrence/clearance of the faults, the isolation/reconnection of the microgrid to the utility is ensured via static transfer switches (STSs). The change of mode occurs between the current to voltage control or vice‐versa. To ensure high‐quality power in both the modes, bias compensated normalised least mean square (BC‐NLMS) algorithm is implemented to estimate the total active current component of loads and to improve the power quality of the grid indices. Extended second order generalised integrator‐based frequency locked loop (eSOGI‐FLL) control avoids frequency and phase oscillations and provides the effective frequency and phase synchronisation. In an islanded operating mode, the enhanced proportional resonant (PR) controller along with eSOGI‐FLL regulates the load voltages. The battery energy storage facilitates uninterruptable supply to the critical loads. The efficacy of the microgrid control is illustrated by the simulated as well as experimental results.

For the characteristics of a random distribution and a large number of buses in the power system, the authors introduce distributed compressed sensing to compress and reconstruct the power quality data. They built a distributed IEEE14 bus system in PSCAD. This model was used to analyse the correlation and sparsity of power quality data and to obtain the four types of data that should be used in the latter simulation, the power quality data used in this study are the voltage amplitude of each bus in a power system. To make the signal sparser, they constructed a distributed compressed sensing learning dictionary for power quality data. The simulation results show that the performance of the distributed compressed sensing learning dictionary constructed in this study is more suitable for power quality data. The application of distributed compressed sensing in a power system can ensure the accuracy of reconstructed data when the quantity of data is reduced by 1/3, which greatly reduces the system storage space. Additionally, the speed of reconstruction also increases by 3/5.

Voltage sag disturbances are the most important type of disturbance in power quality analysis and the extraction to voltage compound disturbances is more difficult. Here, four composite voltage sag disturbance models are constructed and the feature extraction method by modified generalised S-transform (GST) is given. By optimising the introduced r-parameters, it can obtain better time resolution and frequency resolution by the modified GST. Results of simulation experiments and the application in a steel plant main line show that both time domain disturbance and frequency domain disturbance information can be extracted with higher resolution.

The DC distribution network is becoming the basic supporting component of the urban distribution network. A variety of power supplies, loads, and energy storage devices can be directly integrated into a DC distribution network, which can improve the economical and energy efficiency of the power system. This study proposes two charging strategies, two-stage charging and one-by-one charging, to make the converter start up safely or quickly return to the normal operation state after the fault clearance. Owing to the existence of numerous DC capacitors in voltage source converters, the reactive power consumption in the start-up process needs to be explicitly calculated. Mathematical expressions of reactive power requirements for different charging strategies are investigated in detail, from where it can be seen that the current-limiting resistance can significantly affect the reactive power and the entire charging time. Simulation results show that the AC voltage can be seriously affected by the oscillation of the reactive power, which will lead to voltage drop and waveform distortion in mode switching.

The large current generated by a direct start of the super-large-capacity high-voltage induction motor would have a huge impact on the power grid as well as the motor itself. Traditional soft starters have the shortcomings of discontinuous adjustment, voltage sags, sudden torque mutation, secondary current impact and high cost. To resolve this issue, the authors propose a novel coupled autotransformer and magnetic-control (CATMC) soft-start method. The structure of the new CATMC soft starter combines the functions of the autotransformer and magnetic control reactor via an innovative electric and magnetic circuit design. In this study, the authors analyse the magnetic circuit structure and working principles of the CATMC soft starter. Then, to validate its principle and performance, the authors conduct a simulation study using ANSYS software and design and test an 18 MW/10 kV CATMC soft starter prototype. The simulation results demonstrate that the CATMC soft starter effectively avoids secondary current impact and constrains the motor starting current to

One of the smartest evolution in the electrical power system is the integration of various renewable sources. It is an effective way to cater consumers growing power demands. The intermittency issues in renewable sources may lead to generation and demand imbalance. This imbalance creates various issues like frequency fluctuations, voltage variation, power quality problem and so on. This study proposes a method to reduce the frequency deviation. More the electric vehicles (EVs) in this system, then more the mobile storage devices for vehicle to grid (V2G). The charging/discharging rates of EVs are controlled as per the regulation signal coming from the system operator. The proposed scheme has been simulated using the frequency regulation data acquired from PJM. The proposed algorithm controls the total charging/discharging power based on the regulation signal and this is used to modify the charging/discharging rates of EVs. The result shows a better performance during grid to vehicle (G2V), at time 20:00 the regulation signal is −650 kW and it fully met by EVs. However, during V2G the performance is quite good, at time 08:00 the regulation signal is 800 kW and EVs meet only 180 kW due to less number of EVs for discharging.

This study presents a step toward the grid connection of a wave-energy park through an electric power conversion system (EPCS) developed and installed for the wave-energy harvesting in Lysekil, Sweden. The EPCS comprises a rectifier, a DC bus, and an inverter followed by a harmonic filter (HF). The higher- and lower-order harmonics injected by the inverter in a power quality context are investigated. The lower-order voltage harmonics partially distort the voltage-source inverter output grid current. A phase-locked loop-based (PLL) grid-phase tracking is used to attenuate the lower-order harmonics by reflecting the grid harmonics in the inverter output. An expression for the grid-current harmonics as a function of the grid-voltage harmonics has been derived and implemented. A mathematical model is derived to obtain a transfer function for the PLL, and finally, proportional–integral gains are tuned for stable system operation. An HF for mitigating the higher-order harmonics has been implemented. The total harmonic distortion is evaluated experimentally, and the results fulfil the grid-code requirements at various frequencies and harmonic orders.

Power quality and current limitation are the most important aspects of the grid-connected power converters under fault. Since the distributed energy resources are widely used, fault management strategy is important for micro-grids applications. This paper presents a new control strategy for low-voltage ride-through for 3-phase grid-connected photovoltaic systems. The proposed method, which is designed in a synchronous frame using positive and negative sequence components, can protect the inverter from overcurrent failure under both symmetrical and unsymmetrical faults and provides reactive power support. The method does not require a hard switch to switch from MPPT to a non-MPPT algorithm, which ensures a smooth transition.

The purpose of this paper is twofold: (i) to propose a methodology to accurately evaluate power quality indices, and (ii) to highlight the importance of power quality monitoring services at different voltage levels of three-phase power systems. The proposed methodology is based on wavelet packet transform (WPT) and is able to perform power quality analysis in three-phase power systems operating under stationary and non-stationary conditions, in the presence of both harmonics and unbalance. Several test cases are examined: synthetic voltage and current waveforms including noise, simulated test systems, a modified IEEE 13-bus test feeder, as well as experimental tests. The obtained results in all cases demonstrate the accuracy and the effectiveness of the proposed WPT method and its superiority over fast Fourier transform to accurately evaluate power quality indices in case of non-stationary disturbances.

The idea of ‘energy router’ (ER) is put forward as a critical application of the energy Internet (EI), especially in the new generation of electric power system to fulfil the power management and to deal with the integration of massive distributed generations. Furthermore, AC/DC hybrid network becomes a kind of development trend since it can improve the power quality and efficiency compared with adding converters for DC loads. This article proposes a decentralised control method for multi-port converter (MPC)-based ERs to achieve flexible power regulation and ‘plug and play’ operation. With the proposed schemes, DC bus voltage can be maintained together by multi-ER through appropriately adjusting their transmitted power according to the pre-set power-sharing ratio. Furthermore, the sharing ratio can be online changed for flexible power regulation. The stability of the system is ensured by Lyapunov method, detailed simulation results are presented to validate the feasibility of the proposed scheme under different operating conditions.

Digital signal processing (DSP) methods have been used by many researchers for detection and classification of transient disturbances because of their fast and powerful abilities to recognise waveform distortions. However, some DSP methods such as the wavelet transformation (WT) show less accuracy when applied to noisy real data. In this study, disturbance features are extracted in the wavelet domain based on the WT levels. Moreover, a new feature extraction algorithm namely normalised Renyi entropy with the signal energy is applied. This algorithm has been proven to be effective and robust for noisy signals. However, their application in power systems has not yet been tested. Using a laboratory setup of an islanded micro-grid, experimental results validate the efficacy of the wavelet-based normalised Renyi entropy in the detection and classification of four disturbance types (voltage sag, interruption, harmonics, mixture of harmonics and sag).

Photovoltaic (PV) systems are weather-dependent. A solar eclipse causes significant changes in these parameters, thereby impacting PV generation profile, performance, and power quality of larger grid, where they connect to. This study presents a case study to evaluate the impacts of the solar eclipse of 21 August 2017, on two real-world grid-tied PV systems (1.4 MW and 355 kW) in Miami and Daytona, Florida, the feeders they are connected to, and the management areas they belong to. Four types of analyses are conducted to obtain a comprehensive picture of the impacts using 1 min PV generation data, hourly weather data, real feeder parameters, and daily reliability data. These analyses include: individual PV system performance measurement using power performance index; power quality analysis at the point of interconnection; a study on the operation of voltage regulating devices on the feeders during eclipse peak using an IEEE 8500 test case distribution feeder; and reliability study involving a multilayer perceptron framework for forecasting system reliability of the management areas. Results from this study provide a unique insight into how solar eclipses impact the behaviour of PV systems and the grid, which would be of concern to electric utilities in future high penetration scenarios.

Integration of solar PV as a distributed generator (DG) require efficient and coordinated control measures for the proper synchronization. In this paper, a hybrid control algorithm for single stage solar photovoltaic (PV) system integrated with low voltage (LV)/medium voltage (MV) grid is proposed. The hybrid algorithm utilizes I cos ϕ technique and quasi-Newton back-propagation (QNBP) neural network (NN). The main contributions of the present work are: i) support the utility grid by feeding power and connected loads, ii) provide harmonics elimination, reactive power compensation, load balancing, iii) also works in power factor correction (PFC) and zero voltage regulation modes, iv) provides power quality improvement. The proposed control of grid tied PV system provides very fast response during static and dynamic conditions. The obtained results are compared with other well-established algorithms available in the literature. The comparison of the proposed algorithm is done on the basis of various parameters such as DC voltage undershoot and overshoot, settling time and THD in grid currents. The developed system is demonstrated in MATLAB/SIMULINK platform. Using the proposed algorithms there is significant improvement of 1.1% in total harmonic distortion (THD) of source current. The results of proposed controllers are experimentally validated on a developed laboratory protype.

Here, a coordinating harmonic suppression strategy of a DC microgrid was proposed. The harmonics introduced from the connected grid and those generated by local non-linear loads were both considered and evaluated. Then, a coordinating harmonic suppression strategy was presented for two targets. One is to suppress the voltage pulsating in the DC bus capacitor, which aims to mitigate the adverse impact of the grid voltage harmonics. The other is to obtain sinusoidal output voltage waveforms, that is, to improve the power quality of the microgrid system by properly eliminating the harmonics introduced by the non-linear loads. Simulation studies indicated that the proposed coordinating control can enhance the system stability and ameliorate the power quality of the load side inverters as well.

Electrified railways contain DC devices and cause power quality (PQ) issues such as harmonics, unbalance, and low-power factor, which should be solved by the most effective compensation technology to reduce the cost. In addition, it is common that more than one issue is caused at a supply bus, with either some issue affecting more than the others or all issues equally affecting, depending on different buses. Different PQ compensation technologies are used in the past work for the improvement of PQ at the network bus side. However, few of them have provided theoretical justification on which technologies should be selected for different network buses with different issues. This study investigates a subjective–objective methodology to provide suggestions on which compensation technologies should be selected for network buses, why and how. The importance of different PQ issues will be obtained and shown as overall weighting factors. The overall weighting factors are derived from the subjective weighting factor, obtained from existing industry standards, and the objective weighting factor, obtained from the entropy of information. The methodology will be applied to a real 124-bus AC–DC network with electrified railways. Based on the overall weighting factor obtained, the most effective compensation technology can be determined.

The automatic transfer switch (ATS) is a medium-voltage switchgear, which can improve the reliability and quality of the power supply. The mechanical switch is an important part of the ATS based on the LC resonance, and its breaking time directly affects the overall switching time of the ATS. In order to reduce the breaking time and improve the breaking performance, the optimal design of the mechanical switch is made. Firstly, the working principle of the ATS based on the LC resonance is introduced, the general technical scheme of the magnetically controlled mechanical switch is given, and then, two aspects of increasing the voltage of the driving power supply and the size of the discharge capacitance are considered to reduce the breaking time. Experimental results show that the breaking time of the magnetically controlled mechanical switch is significantly reduced from 15 down to 6.39 ms or so by optimising the design, and the breaking performance is effectively improved, which is conducive to the design of the ATS based on the LC resonance.

When connecting a large number of generating sets into power grid, there will be the harmonic wave generated in power grid causing voltage and current distortion, which affects power quality seriously. Based on the instantaneous reactive power theory, the active current, reactive current and harmonic current can be real-time monitored. The P–Q method can be adopted to control reactive power compensation. By building the converter control model, the variation of reactive power of power grid between photovoltaic grid-connected inverter working under stand-alone mode and grid-connected mode can be analysed via a simulation system. The experimental analysis of reactive power compensation and harmonic suppression, the availability of the control strategy of the photovoltaic grid-connected inverter system can be proved. Using this novel strategy, the photovoltaic grid-connected power system can not only provide active power but also compensate reactive power and suppress harmonic, which will improve power quality.

Apart from the basic renewable generation function, the multi-functional grid-connected inverter (MFGCI) derived from traditional grid-connected inverter could provide ancillary power quality enhancement functions for the interfaced grid. However, with multiple functions integrated into one control system, the control scheme of MFGCI would become inevitably more complicated and vulnerable to the disturbance in distribution networks. Here, a control scheme of MFGCI for rejecting both the steady-state and dynamic-state disturbance is designed. The proposed control scheme includes an inner loop controller and an improved reference current generation algorithm, which constitutes an overall MFGCI control scheme. To reject the steady-state disturbance which would distort the output current of MFGCI, the inner loop controller by combining the linear adaptive disturbance rejection controller (LADRC) and repetitive controller (RC) is designed. While for the dynamic-state disturbance which would amplify the output current of MFGCI or even destroy the stability of the system, an improved reference current generation algorithm with enhanced dynamic disturbance rejection capability is proposed. The proposed MFGCI could provide overall improvements in both dynamic- and steady-state disturbance rejection capability. Finally, some experiments are conducted to verify the superiority of the proposed MFGCI.

With the increasing number of electric vehicles, the vehicle-to-grid (V2G) technology has become a hotspot for the present research because it can cause peak-load shifting; it is also a practical technical means to improve the power grid capacity of new energy absorption in the field of new energy access to grid. In the interaction between electric vehicle and power grid, its bidirectional AC/DC transform control is mostly pulse width modulation control; large amplitude fluctuations will cause damage to AC/DC converters and electric vehicle batteries when electric vehicles are connected to these intermittent new energy sources (such as wind, tidal energy etc.). In order to solve these problems, from the system point of view, the authors combine the theory of instantaneous reactive power and introduce a virtual coordinate system, reduced the impact of volatile energy and improve the power quality of electric vehicle charging; finally, the feasibility of the method is verified by simulation.

In this study, a cascaded single-phase converter topology is proposed for single-phase microgrid integrated into high-voltage power distribution systems. In this system, the lower single-phase converter works in voltage controlled mode to reduce the harmonic distortions of supply voltage to local critical load. At the same time, the upper single-phase converter is placed in between the main grid and the output of the lower single-phase converter. Alternatively, the upper power cell works in current controlled mode to block the harmonic current pollutions from the local non-linear load. Through collaborative control of the cascaded upper and lower converter, the proposed system can provide active harmonic filtering to both single-phase supply voltage and single-phase grid current, but without any harmonic extractions in the entire process.

In recent years, the urban rail transit has a great development in many cities of China. It is important to keep the DC transit grid strong and the power transmitted/power transmission high quality. During the process of rail operation, braking energy leads to a wide range of load oscillation for the traction and city grid, which cannot be absorbed inside the rail transit system. In this study, a novel architecture of urban rail transit power supply based on power electronics transformer braking energy feedback is put forward. Based on the circle grid, the topology adds an inverter to LLC series resonant converter to carry the power from 1500 V DC traction grid to 1180 V AC. Analysis together with simulation on the topology and control strategy is made on the basis of MATLAB/Simulink. Process of braking energy feedback and transmission of energy are illustrated with simulation. By paralleling the three-phase H bridge and braking energy feedback device, double direction of the power flow is ensured. As a consequence, the power quality increases and harmonics reduce. The energy can be used more efficiently to achieve better effects on energy conservation.

The ever-increasing integration of photovoltaic (PV) energy has led to the fast development of utility-scale PV power plants worldwide. A novel grid connection interface for utility-scale PV power plants based on the modular multi-level converter (MMC) is explored. The grid connection interface is a DC boost interface by nature. It adopts the multistring topology, employs DC/DC boost converters, utilises a centralised MMC, and integrates an energy storage system. Meanwhile, the two-level control system for the DC boost interface is also designed. Compared with the conventional AC boost interfaces, the proposed DC boost interface avoids problems regarding reactive power transmission, harmonic resonance, and synchronous stabilisation. It also improves the power quality, efficiency, and the integration capacity of the PV generation. A case study of a utility-scale PV power plant with the DC boost interface is carried out in PSCAD/EMTDC. The simulation results validate the effectiveness and the merits of the DC boost interface under different operation conditions. The proposed DC boost interface proves more suitable for utility-scale PV power plants in areas with high solar irradiance. It also lays a solid foundation for the future research regarding the control and protection of utility-scale PV power plants.

The increasing penetration of renewable generation and large implementation of the DC loads are challenges that stress the current distribution systems by causing uncertain voltage variations. The AC−DC hybrid distribution network is seen as an innovative solution to address the problem of accommodating highly dispersed sustainable energy. Specifically, the solid-state transformer (SST), which is capable of enhancing power-quality performance and controllability of the power distribution, has attracted increasing focus in this decade as a promising advanced converter. It is of great interest to investigate how to operate the multiple SSTs with different modes in a coordinated manner to optimise the distribution network performance. This paper formulates the coordinated operation model for multiple SSTs to enable fast voltage regulation for the hybrid active distribution network equipped with large-scale penetration of invertor-based solar power and DC load. A hybrid optimal power flow describing the controlling modes of SSTs and operating interaction between AC−DC subsystems is proposed in this paper. The case studies on a modified IEEE 33-AC−DC hybrid power distribution system show that the voltage profile can be optimised by using the flexible controlling resource of SSTs, facilitating the timely performance adjustment in the presence of fast fluctuating distributed generator's output.

Conventional automatic voltage regulator (AVR) schemes use thyristor-based rectifiers in which the generator field voltage is varied by changing the thyristor's firing angle. The conventional AVR has been known to cause voltage sags and extended under-voltages, especially under fault conditions. Several control schemes such as digital proportional–integral–derivative (PID) controllers, adaptive control schemes, and intelligent control schemes have been proposed in the literature. However, these schemes increase the complexity of the system. This work proposes a DC−DC converter-based AVR as an alternative to the rectifier-controlled AVR. The DC−DC converter is arranged in step-down (buck-type) configuration. The DC−DC converter uses metal-oxide-semiconductor field-effect transistor/insulated-gate bipolar transistors and has a faster response than the thyristors, thereby resulting in faster and precise control of the generator field voltage. The DC−DC converter is connected to the field circuit of the generator, and the terminal voltage of the generator is regulated by a PI controller by varying the converter duty cycle to control the generator field voltage. Simulation and experimental investigation has shown that the proposed scheme exhibits a better transient response and is robust under DC input voltage variation and generator load changes.

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.

Low-frequency oscillations in power system degrade power quality and may trigger blackouts. This study identifies the source location of these oscillations using measurements from phasor measurement unit (PMU), offline credibility estimation and classification models. The performance of these classification models is ranked for each reported feature to use highly ranked models during the online stage. This proposed framework named as credibility search ensemble learning was tested and validated with promising results using western interconnection power system in North America (WECC-179). The reliability and robustness of the proposed framework were checked against measurement errors in PMUs as well as for practical topology change scenarios. Experimental results and performance comparison with average weight-based approach proved that the proposed approach is capable enough to predict the source location of oscillations with good accuracy. An interfacing tool, for MATLAB-WEKA, was developed and employed in this work for validation and testing of the proposed approach.

In recent years, energy storage technology is widely used in distribution networks. It can significantly improve stability and power quality of the grid. An improved droop control strategy for energy power storage converter is proposed here, which based on complex filter, dividing the voltage into positive and negative sequence components in the αβ-axis. This method has small amount of calculation and easy realisation. This strategy can improve the output power quality of energy power storage converter. Finally, it proved that the improved voltage control strategy can effectively shorten the time of voltage recovery in the case of the power fluctuation of the contact line, and provide reference for other access storage power distribution network operation control through MATLAB/Simulink simulation.

The automatic transfer switch can solve the problem of voltage drop and instantaneous power interruption, which is one of the most reliable and economical ways to improve the reliability and quality of the power supply. In view of the application demand of the important electric field, this paper puts forward the technology scheme of a compound automatic transfer switch based on forced current commutation, which is characterised by high reliability, fast switching and active and passive combined protection. First, the general technical scheme and working principle of the compound automatic transfer switch are introduced. Then, the simulation model of power distribution system with one master and one standby automatic transfer switch is established, and the system simulation is carried out to analyse the influence of the fault type on switching effect. Finally, the feasibility of the compound automatic transfer switch is verified by experiments, and its operational characteristics are obtained. It provides theoretical and technical basis for the product development and engineering application of the compound automatic transfer switch.

Extensive deployment of interoperable distributed energy resources (DER) is increasing the power system cyber security attack surface. National and jurisdictional interconnection standards require DER to include a range of autonomous and commanded grid-support functions, which can drastically influence power quality, voltage, and bulk system frequency. Here, the authors investigate the impact to the cyber-physical power system in scenarios where communications and operations of DER are controlled by an adversary. The findings show that each grid-support function exposes the power system to distinct types and magnitudes of risk. The physical impact from cyber actions was analysed in cases of DER providing distribution system voltage regulation and transmission system support. Finally, recommendations are presented for minimising the risk using engineered parameter limits and segmenting the control network to minimise common-mode vulnerabilities.

The network reconfiguration of a vessel power system (VPS) is one of the most important approaches to restore its power supply and improve its survivability. The current research on the reconfiguration mainly focuses on the selection and optimisation of recovery paths, with little consideration of the influence of recovery time series. The limited capacity of the VPS, many loads of the motor load and large starting current have a great impact on the electric-energy quality of the power system. Therefore, the different recovery time series will affect the system performance and the recovery rate. The effect of the different recovery time series on the network reconfiguration and fault control has been analysed through the simulation of target network to determine the corresponding relationship between the residual power-supply capacity, the load recovery time series and the recovery rate, which can provide the basis of studying an optimal recovery strategy.

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.

Micro-grid can be used as a supplement to power supply of large power grids. Combined with smart park, users can easily manage the micro-grid. In the tourist scenic area, the local distributed new energy is mainly used to provide energy for the system. The micro-grid control system as the core of the system controls the optimal operation of the entire smart park. In order to ensure the efficient operation of the entire system, the energy management system is needed for smart control and automatic scheduling. In the grid-connected mode, the main-grid complements the system energy and charges the park energy storage device. In the island mode, the distributed energy is fully used to provide energy for the smart park system. The energy power fluctuates greatly, and the energy management system needs to schedule the micro-grid energy, energy storage system, electric vehicle load and so on, and power quality needs to be monitored. Use of micro-grid control system in the smart parking deploying photovoltaic power generation, wind power generation, charging and exchanging devices and other devices, real-time monitoring of the park of cars, charging cars automatically complete charging, automatic processing of visitor cars into and out of smart park, and improve work efficiency.

This study proposes a two-layer hierarchical control to actualize optimal total harmonic distortion (THD) compensation in different buses of parallel-connected inverters in islanded microgrids which had not been studied so far. The proposed secondary layer is used to realize THD compensation of sensitive load bus (SLB) and make distributed generators (DGs) distribute the compensating efforts between them according to their rated capacity. It is noteworthy that improving THD at the SLB can lead to an increase in THD at local buses and/or DG terminals. Although the THD limitations of these buses are not as strict as the THD limitation of SLB, it is necessary to control them within their allowed range. This important problem is not well studied in the literature. A novel complementary part is designed and added to the secondary control to tune the compensation portion of each DG while the THD limitations in DG terminals and local buses are considered. The proposed method actualizes a multi-level voltage quality control in multi-bus islanded microgrids with parallel DGs through a simple yet effective solution. Furthermore, considering the DGs peak current limitation is added to the controller and a method for calculating this peak value is proposed.

Electrical and electronic devices, when exposed to one or more power quality problems, are prone to failure. This study aims to enhance the quality of power in three-phase four-wire distribution grid using fuel cell integrated unified power quality conditioner (FCI-UPQC). The proposed FCI-UPQC has a four-leg converter on the shunt side and three-leg converter on the series side. A combination of a synchronous reference frame and instantaneous reactive power theories is utilised to generate reference signals of the FCI-UPQC. Also, this study proposes an adaptive neuro-fuzzy inference system (ANFIS) controller to maintain the DC-link voltage in the FCI-UPQC. The ANFIS controller is designed like a Sugeno fuzzy architecture and trained offline using data from the proportional–integral controller. The obtained results proved that the proposed FCI-UPQC compensated power quality problems such as voltage sag, swell, harmonics, neutral current, source current imbalance in the three-phase four-wire distribution grid. The presence of fuel cell in this work makes more effectiveness of the proposed system by providing real power support during supply interruption on the grid side.

The unified power flow controller (UPFC) is an excellent flexible alternating current transmission device, which has a wide range application in the power system. It can be expected that more UPFC engineering applications will be introduced to the power system in the future, and they will have an unignorable impact on the power grid. It is of great significance to analyse the influence of UPFC on power system energy conservation. First of all, the basic characteristics of UPFC are briefly introduced. Secondly, the influence of voltage quality, reactive power index, and energy saving after UPFC is introduced to the power system is analysed in detail from reactive power compensation and power flow regulation. Finally, comparison simulation of a three-machine nine-node system is simulated, the simulation results show that the power system with UPFC introduced can make the power flow distribution more reasonable, reduce the voltage sag, improve the voltage quality, and power transmission capacity, thus reducing the loss of the power system.

With the increasing penetration renewable energy, some changes are transforming the power system dynamic characteristics, resulting in new stability issues. One of the issues is the lack of inertia and damping. One is sub-synchronous oscillation (SSO) caused by inter-harmonics associated with inverter-based renewable power generators (IPRG). A novel technology is proposed to transmit stable and high-quality power for power system with high penetration of renewable energy. Thus, the object of this paper is synchronous motor-generator pair (MGP) system, a new gird-connection device, which is proposed to solve above problems. First, the two mass block shaft model of MGP is established, which verifies the small signal stability of this system, and it has high damping that can be controlled flexibly. Then, the harmonic response of MGP is analysed through the internal electromagnetic relationship. Result shows that comparing with the input power from renewable energy sources to M side, the harmonic content of output power on G side is greatly reduced, especially the even harmonic. These characteristics are verified by experiments. So transmitted by MGP, the power quality and stability are greatly improved.

Since the concept that inverters can be controlled to behave like a synchronous generator has been proposed, the virtual synchronous generator (VSG) started to become the mainstream control of distributed energy resources connected distributed system. This concept simulates the frequency and voltage regulation of synchronous generator to improve the overall stability of the medium- and low-voltage power system. Although significant research works have considered the application of VSGs in active and reactive power sharing, it seems to be that the issue of the proportional power sharing of VSGs of different capacities has not considered much in the previous works, especially when the distribution lines have increased resistive components. Such resistive disturbs the power flow and causes the power coupling. When focusing eyes on the parallel operation, VSGs need to coordinate parallel operation to increase the microgrid power quality, reliability, and for easy maintenance. Based on an improved droop control and virtual complex impedance, this study adjusts PI controller parameters to implement the proportional power sharing and decoupling for VSGs of different capacities. Simulation results are shown to verify the feasibility and validity of the control method.

Plug-in electric vehicles (PEVs) have become a key factor driving towards smart cities, which allow for higher energy efficiency and lower environmental impact across urban sectors. Industry vision for future PEV includes the ability to recharge a vehicle at a speed comparable to traditional gas refuelling, i.e.