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This paper presents the design and development of a three-phase 10-kV/400-V 500-kVA electronic power transformer (EPT). The power circuit is designed in a modular fashion, i.e., the main circuit consists of many identical ac–dc–dc–ac modules (abbreviated as power modules). Each power module consists of a high-voltage power cell, a low-voltage power cell (LVPC), a medium-frequency isolation transformer, and a filter. The corresponding control and protection system is developed. A special three-stage startup strategy is designed to shorten the startup time and reduce the startup inrush current. The negative-sequence current compensation is introduced in the input stage to handle the unbalanced loads. To keep the dc-link voltages balanced, an individual dc voltage balancing controller based on regulating the output power of each parallel LVPC is proposed. The detailed control hardware design and software implementation are discussed. The functions of this 10-kV EPT prototype are verified through the laboratory and field tests. The results are shown in this paper. Currently, the prototype is operating in the industrial power grid. [ABSTRACT FROM AUTHOR]

Three-dimensional porous composite microspheres of LiFePO4 and nitrogen-doped graphene have been synthesized by a solvothermal process coupled with subsequent calcination. The morphology and microstructure of the composites were investigated by scanning electron microscopy, X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. The electrochemical properties were evaluated by constant current charge/discharge tests, cyclic voltammetry and electrochemical impedance spectroscopy. The unique porous structure of the microspheres was constructed by the assembly of nitrogen-doped graphene and LiFePO4 nanoplates and can remarkably enlarge the electrode/electrolyte interface area, facilitate the electron transfer process, shorten the ionic diffusion path and accelerate the ionic transport throughout the electrode. In electrochemical measurements of the specific charge capacity, rate capability and cycling stability, the obtained porous composite microsphere materials offer remarkably promising results for applications in high-performance lithium-ion power batteries. [ABSTRACT FROM AUTHOR]

This paper presents the design of the voltage source converter (VSC) excitation system and an integrated prototype using digital signal processor (DSP) as the core of the excitation controller is developed. Compared with the static excitation systems with thyristor, the VSC excitation system owns one more channel offering positive damping to power system by controlling reactive power of VSC on its ac side, and this approach is approved much faster than field voltage regulation. To design the prototype, the VSC excitation system has to meet the practical condition of the selected synchronous generator (SG) set such as generator capacity, terminal voltage, field voltage and field current. The designed VSC excitation system consists of converters containing a front-end three-phase VSC and a back-end H-bridge chopper, a step-down transformer, and a coordinate excitation controller. Through a series of experiments, it is demonstrated that the VSC excitation system is able to improve both transient stability and steady-state stability limit of power systems. It is a prospective substitute to the conventional thyristor-based static excitation system and an attractive solution to strengthen power system stability as well as increase the capability of long-distance transmission systems. [ABSTRACT FROM PUBLISHER]

This study presents a hybrid compensator based on distribution static synchronous compensator (DSTATCOM) and thyristor switched capacitor (TSC), which is intended to continuously compensate reactive power with higher quality and lower cost. The proposed coordination control strategies, in essence, are to continuously assign the reactive power between the DSTATCOM and the TSC. Usually, the fast response characteristic of the DSTATCOM will be lost for that the output of the DSTATCOM is limited by the conventional coordination control strategy. In order to improve the transient performance of hybrid compensator, the fast voltage control strategy is proposed to fast respond to the demand, and reactive power reserve control strategy is proposed to restore the capacity. The operation principle, control algorithm, and design procedure of a prototype are described. Laboratory tests and field tests have validated that the performance of the compensator is satisfactory, and the cost reduction of hybrid compensator is significant. [ABSTRACT FROM AUTHOR]

Control strategy of a cascaded multilevel converter based electrical power transformer (EPT) in a distribution system with capabilities of low voltage ride-through and unbalanced load current management is investigated in this study. The mathematical model and decoupled control schemes of the system, including a high-voltage side control scheme, an isolation-stage control scheme, and a low-voltage side control scheme, are presented in detail. A dual current control scheme is introduced to control both positive and negative sequence currents for enhancing the low voltage ride-through capability of the high-voltage side cascaded H-bridge converter. Positive, negative, and zero-sequence voltages are controlled for the low voltage side three-phase four-wire converter in the decoupled control scheme, respectively, for unbalanced load current management. A proportional resonant controller (PRC) is utilized to control the zero-sequence voltage, while the root locus method is applied in the PRC design. Three-dimensional space vector pulse width modulation (PWM) switching strategy is then used for the low voltage side converter. Simulation studies were conducted with MATLAB/Simulink to validate the coordinated control strategy. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

This study utilizes the electronic power transformer (EPT) to enhance the damping of interarea oscillations in a multimachine power system based on active power modulation. The dynamic model of EPT is established, and decoupled vector control schemes of inner current control loops and outer active and reactive power control loops are modeled and designed. A power oscillation damper (POD) applied to the active power control loop of EPT is introduced to improve the stability of the power system. The POD controller parameters are designed based on the frequency response method. A study system based on the classical two-area four-machine system is built for damping controller design and time domain simulation, and the design procedures are presented in detail. The effectiveness of the designed POD controller is verified through eigenvalue analysis and time domain simulations. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The active power adjustment capabilities of zero‐sequence and negative‐sequence voltages are mathematically compared in this study. Based on the comparison, the input currents and operating range of the electronic power transformer (EPT) using negative‐sequence voltages under unbalanced grid voltage conditions have been analysed, which can provide theoretical guidance on the design and control of the EPT. A novel reference current in the input stage is proposed for balancing three‐phase dc voltages. The proposed reference current can be rapidly adjusted to improve the dynamic response of the system and reduce the dc‐link voltage deviations when unbalanced grid voltage happens. These analysis and control schemes are supported by both simulation and experimental validation. [ABSTRACT FROM AUTHOR]

The increasing wind power penetration poses significant technical problems to the development of electric power systems. In this article, an integrated control strategy is proposed based on the characteristics of the directly driven permanent magnet synchronous generator wind turbine. The directly driven permanent magnet synchronous generator wind turbine is incorporated with superconducting magnetic energy storage, which can smooth the wind power fluctuations, enhance the low-voltage ride-through capability of the wind generation system, and achieve an uninterruptible power supply to local loads under isolated grid operation. According to the MATLAB/SIMULINK-based (The MathWorks, Natick, Massachusetts, USA) digital computer simulation model, the proposed control strategy is confirmed to be effective in power control of grid operation. [ABSTRACT FROM PUBLISHER]

Abstract—Single-phase microgrids are widely used in residential houses. The energy storage system with grid-interactive inverters in these microgrids should operate in both grid-tied and standalone modes. In this article, proportional resonant controller for the single-phase grid-interactive inverter with LCL-filter is designed and implimented in the both operation modes. Also, a novel seamless transfer strategy based on hysteresis current control is proposed when off-grid transfer operation is needed. With the proposed method, the inverter is able to provide voltage support to the local critical loads during the whole transfer process when the grid fails. For the grid-tied transfer process, a phase difference eliminating method based on inverter frequency regulation is introduced in this article to eliminate the phase difference between the grid voltage and the inverter voltage when the grid recovers. Simulation and experiments are carried out and the results verify the effectiveness of the proposed control method. [ABSTRACT FROM PUBLISHER]

Improving energy efficiency is very important in solving the problem of energy crisis and environmental problems. Beam pumping units consume significant amount of electricity because they are widely used but usually have relative low efficiency particularly of the induction motor in petroleum exploitation. This paper first analyzes the beam-pumping-unit motion law in petroleum exploitation and derives the up-slowly-and-down-fast operation principle of the suspension point to improve its efficiency. For the practical application, a dual pulsewidth-modulation frequency converter for a beam pumping unit with a 45-kW induction motor is developed and field tested. The unit is capable of starting the motor softly, operating at near unity power factor, providing near sinusoid current, and emitting low total harmonic distortion in grid side, making power flows in dual direction, and capable of adjusting frequency smoothly in the range of 5–75 Hz in the motor side. Thorough consideration on the protection schemes and special designs in framework make it working reliably in atrocious environment. Field operation results show significant energy savings of the proposed design. [ABSTRACT FROM AUTHOR]

The electronic power transformer (EPT) is a novel transmission and transformation device, which consists of a series connection of two voltage‐source H‐bridge converters and a DC–DC converter with high‐frequency isolation transformer. When applied to microgrids, EPT will not only deliver energy to loads from the utility grid but also inject some amount of excess power into the utility grid. Hence, the capability of bidirectional power flow is important for EPT, which depends on the DC‐link stage. The traditional synchronous chopper control for a dual active bridge (DAB) converter has some limitations in the application of bidirectional power flow area. This study proposes a novel synchronous chopper control strategy for the DAB converter to implement bi‐direction power flow, details the basic principle and steady‐state operation and presents the mathematical derivations. A three‐phase three‐stage circuit configuration of 10 kV/400 V bi‐direction EPT system based on the novel control DAB converter is designed, and corresponding control schemes for the system are discussed. The performance of this EPT system is validated by the MATLAB/Simulink‐based simulations and the laboratory prototype experiments. [ABSTRACT FROM AUTHOR]

The electronic power transformer has many advantages, such as reactive power compensation, voltage regulation, harmonic suppression, and power factor correction. When a three-phase electronic power transformer with a star configuration is applied in the distribution network, three-phase load unbalance will cause three-phase DC voltage unbalance if the electronic power transformer controller generates improper asymmetric voltage signals, which may trigger over-voltage protection. The effects of three-phase load unbalance to the work condition of the electronic power transformer are analyzed, and a control algorithm is proposed to balance three-phase DC voltages in this article. Meanwhile, input stage control, isolation stage control, output stage control, and individual DC voltage balancing control are presented. Simulation and lab experimental results show that the electronic power transformer still works properly even under serious unbalanced load conditions. [ABSTRACT FROM AUTHOR]

The optimal control of state‐of‐charge (SOC) for superconducting magnetic energy storage (SMES), which is used to smooth power fluctuations from wind turbine, is essential to improve its technical and economical performance. Without an efficient control strategy, the SMES may go to the state of over‐charge or deep‐discharge, which will pose a significant effect on its service life and its technical performance. In this study, combined with wind forecasting technology and real‐time monitoring of SOC for the SMES, a double fuzzy logic control strategy is proposed that is applied to regulate SOC of SMES with the purpose of not only effectively smoothing the power fluctuations of wind turbine, but also preventing the SMES from occurring of the state of over‐charge/deep‐discharge and adjusting it to the appropriate SOC. The effectiveness of the proposed control strategy is verified by the simulation results. [ABSTRACT FROM AUTHOR]

Integrating a battery energy storage system (BESS) with a wind farm can smooth power fluctuations from the wind farm. Battery storage capacity (C), maximum charge/discharge power of battery (P) and smoothing time constant (T) for the control system are three most important parameters that influence the level of smoothing (LOS) of output power transmitted to the grid. The economic cost (EC) of a BESS should also be taken into consideration when determining the characteristic parameters of BESS (C, P). In this study, an artificial neural network‐based long‐term model of evaluated BESS technical performance and EC is established to reflect the relationship between the three parameters (C, P, T) and LOS of output power transmitted to the grid, the EC of BESS. After that, genetic algorithm is used to find optimal parameter combination of C, P and T by optimising the objective function derived from the mathematical model constructed. The simulation results of the example indicate that the parameter combination of C, P and T obtained by the proposed method can better not only meet the technical demand but also achieve maximum economic profit. [ABSTRACT FROM AUTHOR]

A lot of large-scale industrial critical loads suffer from voltage interruptions and sags which can cause a significant financial loss. The thyristor-based solid-state transfer switch (STS) and mechanical-based automatic transfer switch (ATS) are used to protect loads against these power quality problems. However, the STS has a great deal of loss consumption because of the forward voltage drop of the thyristors, and ATS can hardly satisfy the voltage-sensitive load due to the long transfer time. This paper proposes a 400 V/1000 kVA hybrid ATS (thyristor switch in parallel with mechanical switch) (HATS) to realize low power consumption under normal situation and transfer fast when required. The proposed HATS has four transfer strategies which can be chosen according to different applications, it realizes the overlapping transfer of the neutral wire, and it can make a decision to transfer or not according to the different fault locations (transfer on the source side and not transfer on the load side). The design details of the prototype are presented, and laboratory tests and field experiments show the prototype good performance. [ABSTRACT FROM PUBLISHER]

The on‐load tap changing transformers have been widely used for their ability to maintain output voltages at desired levels despite the load or voltage changes in the ac mains. However, they present several disadvantages such as slow response, high‐maintenance cost and discontinuous voltage regulation. This study proposes a novel on‐load voltage regulator based on electronic power transformer (OLVR‐EPT) to eliminate these drawbacks. By injecting an appropriate voltage component to traditional power transformer windings through EPT output stage, the resultant load voltage can be maintained at the desired value. There are no tap‐changers anymore, so the voltage regulation is fast and continuous. Two sample schemes of OLVR‐EPT are given and corresponding control method is developed in this study. The simulation and experimental results demonstrate that OLVR‐EPT is able to maintain load voltages at the desired value effectively. [ABSTRACT FROM AUTHOR]

Reducing energy consumption is very important in solving the problem of energy crisis and environmental problem. Usually, an elevator with a diode rectifier wastes regenerated power from its electric motor as heat on breaking resistors. This paper proposes an energy feedback digital system used in an elevator of 18.5 kW which is capable of recycling the regenerated power: obtaining near-unity power factor, sinusoid output current, and low total harmonic distortion in generator-operation state. It has reliable protection circuits and cute man–machine interface, and the feedback point of the dc voltage can be set arbitrarily in a certain extent, meeting the large-range variation of power system voltage. Field operation results showed good performance and energy-saving effect. [ABSTRACT FROM AUTHOR]

Based on the grey theory and the theory of grey elimination of redundancy controlling (ERDC), the grey excitation controller for synchronous generator is studied. The excitation controllers, involving both the grey predicting controller (GPC) and the ERDC controller. The simulation results show that both controllers have better performance than the general AVR (automatic voltage regulator) with PSS (power system stabilizer). Moreover, the grey predicting controller provides better performances than ERDC controller for the generator, in appropriate predicting step size being chosen. In general terms, the GPC can improve the static and transient stability of power system effectively. [ABSTRACT FROM AUTHOR]

Objective: To develop a novel chronic intermittent hypoxia chamber for rat models. Design: The intermittent hypoxia chamber included two animal cabins (28 × 20 × 15 cm) and a hypoxia chamber (80 × 80 × 60 cm) between them, as well as the actuating device. Pure nitrogen was fed into the hypoxia chamber continuously in order to keep the low O concentration. Each animal cabin could move in and out of the hypoxia chamber with precise timing function. As a result, the animal cabins could be covered by the hypoxia chamber and got the same low O concentration as in the hypoxia chamber and normal O concentration as out of the hypoxia chamber. Twelve healthy, male Sprague-Dawley rats (200 ~ 250 g) were selected to test the effect of the intermittent hypoxia chamber. The O concentration in the hypoxia chamber was 10 ± 0.5%, and the cycle time of intermittent hypoxia was 180 s (the hypoxia and normoxic time was 90 s, respectively). The hypoxia chamber ran 8 h per day. The arterial blood gas analysis (ABSA) of rats was conducted when the animal cabin was located inside and outside the hypoxia chamber. Results: The chronic intermittent hypoxia chamber ran safely and reliably. The ABSA showed that the lowest PaO was 35.75 ± 4.02 mmHg and the lowest SaO was 68.62 ± 8.36% when the animal cabin was covered by the hypoxia chamber. Conclusions: The chronic intermittent hypoxia chamber designed by us was suitable to establish a chronic intermittent hypoxia model for rats. [ABSTRACT FROM AUTHOR]

Many industrial and commercial sensitive loads are subject to the voltage sags and interruptions. The static transfer switch (STS) based on the thyristors is applied to improve the power quality and reliability. However, the transfer will result in severe inrush current in the load transformer, because of the DC component in the magnetic flux generated in the transfer process. The inrush current which is always 2 ~ 30 p.u. can cause the disoperation of relay protective devices and bring potential damage to the transformer. The way to eliminate the DC component is to transfer the related phases when the residual flux linkage of the load transformer and the prospective flux linkage of the alternate source are equal. This paper analyzes how the flux linkage of each winding in the load transformer changes in the transfer process. Based on the residual flux linkage when the preferred source is completely disconnected, the method to calculate the proper time point to close each phase of the alternate source is developed. Simulation and laboratory experiments results are presented to show the effectiveness of the transfer method. [ABSTRACT FROM AUTHOR]

In order to control the cascaded H-bridges (CHB) converter with staircase modulation strategy in a real-time manner, a real-time and closed-loop control algorithm based on artificial neural network (ANN) for three-phase CHB converter is proposed in this paper. It costs little computation time and memory. It has two steps. In the first step, hierarchical particle swarm optimizer with time-varying acceleration coefficient (HPSO-TVAC) algorithm is employed to minimize the total harmonic distortion (THD) and generate the optimal switching angles offline. In the second step, part of optimal switching angles are used to train an ANN and the well-designed ANN can generate optimal switching angles in a real-time manner. Compared with previous real-time algorithm, the proposed algorithm is suitable for a wider range of modulation index and results in a smaller THD and a lower calculation time. Furthermore, the well-designed ANN is embedded into a closed-loop control algorithm for CHB converter with variable direct voltage (DC) sources. Simulation results demonstrate that the proposed closed-loop control algorithm is able to quickly stabilize load voltage and minimize the line current's THD (<5%) when subjecting the DC sources disturbance or load disturbance. In real design stage, a switching angle pulse generation scheme is proposed and experiment results verify its correctness. [ABSTRACT FROM AUTHOR]