Dynamic phasor-based hybrid simulation for multi-inverter grid-connected system

To realize the efficient transient simulation of a grid-connected power generation system based on multiple inverters, this paper proposes a hybrid simulation method integrating the models of electromagnetic transient and dynamic phasors. Based on a demonstration of the concepts and properties of dynamic phasors, the models of single-phase and three-phase inverters described by dynamic phasors are established first. Considering the numerical compatibility problem between dynamic phasors and instantaneous values, an interface scheme between dynamic phasors and instantaneous values is designed, and the efficiency and precision differences of various transformation methods are compared in detail.Finally, by utilizing MATLAB/Simulink, a hybrid simulation platform of a multi-inverter grid-connected system is built, and the efficiency and accuracy of the hybrid simulation are validated via comparison with the full electromagnetic transient simulation.

Water vapor effects on the oxidation resistance of modified potassium silicate coating at high temperature

An inorganic potassium silicate coating with pigments of alumina,aluminum phosphate,NiCrAlY and copper chromite black was prepared on CB2 stainless steel.Oxidation behavior in either ambient air or O_(2)+H_(2) O mixture at 630℃ for 2000 h was comparatively studied,and the coating exhibited excellent resistance under both test conditions.The water vapor considerably accelerated the oxidation of the uncoated CB2 steel,as the hydroxide,the main constituent of the coating,had a negligible evaporation rate at test temperature,while it had a limited effect on the coated sample.Meanwhile,the existence of coating may prolong or eliminate the incubation period in the O_(2)+H_(2) O mixture at 630℃.After oxidation,the coating matrix is in an amorphous state and fillers as alumina and copper chromite black are stable in the coating.Leucite(KAlSi_(2) O_(6))formed by Al from NiCrAlY and potassium silicate in the coatings was detected after tests either in O_(2) or O_(2)+H_(2) O mixture.

Hybrid Multi-Infeed Interaction Factor Calculation Method Considering Voltage Regulation Control Characteristics of Voltage Source Converter

Voltage source converter based high voltage direct current(VSC-HVDC)can participate in voltage regulation by flexible control to help maintain the voltage stability of the power grid.In order to quantitatively evaluate its influence on the voltage interaction between VSC-HVDC and line commutated converter based high voltage direct current(LCC-HVDC),this paper proposes a hybrid multi-infeed interaction factor(HMIIF)calculation method considering the voltage regulation control characteristics of VSC-HVDC.Firstly,for a hybrid multi-infeed high voltage direct current system,an additional equivalent operating admittance matrix is constructed to characterize HVDC equipment characteristics under small disturbance.Secondly,based on the characteristic curve between the reactive power and the voltage of a certain VSC-HVDC project,the additional equivalent operating admittance of VSC-HVDC is derived.The additional equivalent operating admittance matrix calculation method is proposed.Thirdly,the equivalent bus impedance matrix is obtained by modifying the alternating current(AC)system admittance matrix with the additional equivalent operating admittance matrix.On this basis,the HMIIF calculation method based on the equivalent bus impedance ratio is proposed.Finally,the effectiveness of the proposed method is verified in a hybrid dual-infeed high voltage direct current system constructed in Power Systems Computer Aided Design(PSCAD),and the influence of voltage regulation control on HMIIF is analyzed.

Design of a graphene oxide@melamine foam/polyaniline@erythritol composite phase change material for thermal energy storage

At present,only a single modification method is adopted to improve the shortcomings of erythritol(ET)as a phase change material(PCM).Compared with a single modification method,the synergistic effect of multiple modification methods can endow ET with comprehensive performance to meet the purpose of package,supercooling reduction,and enhancement of thermal conductivity.In this work,we innovatively combine graphene oxide(GO)nanosheet modified melamine foam(MF)and polyaniline(PANI)to construct a novel ET-based PCM by blending and porous material adsorption modification.PANI as the nucleation center can enhance the crystallization rate,thereby reducing the supercooling of ET.Meanwhile,GO@MF foam can not only be used as a porous support material to encapsulate ET but also as a heat conduction reinforcement to improve heat storage and release rate.As a result,the supercooling of GO@MF/PANI@ET(GMPET)composite PCM decreases from 91.2℃ of pure ET to 57.9℃ and its thermal conductivity(1.58 W·m^(-1)·K^(-1))is about three times higher than that of pure ET(0.57 W·m^(-1)·K^(-1)).Moreover,after being placed at 140℃ for 2 h,there is almost no ET leakage in the GMPET composite PCM,and the mass loss ratio is less than 0.75%.In addition,the GMPET composite PCM displays a high melting enthalpy of about 259 J·g^(-1) and a high initial mass loss temperature of about 198℃.Even after the 200th cycling test,the phase transition temperature and the latent heat storage capacity of the GMPET PCM all remain stable.This work offers an effective and promising strategy to design ET-based composite PCM for the field of energy storage.

Optimal dispatch approach for rural multi-energy supply systems considering virtual energy storage

In response to the underutilization of energy and insufficient flexible operation capability of rural energy supply systems in China,this study proposes an optimal dispatch approach for a rural multi-energy supply system(RMESS)considering virtual energy storage(VES).First,to enable the flexible utilization of rural biomass resources and the thermal inertia of residential building envelopes,this study constructed VES-I and VES-II models that describe electrical-thermal and electrical-gas coupling from an electrical viewpoint.Subsequently,an RMESS model encompassing these two types of VES was formulated.This model delineates the intricate interplay of multi-energy components within the RMESS framework and facilitates the precise assessment of the adjustable potential for optimizing RMESS operations.Based on the above models,a day-ahead optimal dispatch model for an RMESS considering a VES is proposed to achieve optimal economic performance while ensuring efficient energy allocation.Comparative simulations validated the effectiveness of the VES modeling and the day-ahead optimal dispatch approach for the RMESS.

Effective Ultra-High-Speed Identification Scheme of Lightning Strikes Suitable for VSC-Based DC Grids

To ensure their sound and continuous operation to the greatest extent,VSC-based DC girds have extremely stringent requirements for transmission line relay protection.In terms of guaranteeing their reliability,accurate identification of lightning strikes on DC transmission lines is one of the urgent key problems to be solved.An effective ultra-high-speed identification scheme of lightning strikes suitable for the VSC-based DC grid is proposed in this paper.First,an 1-mode reverse voltage traveling wave(RVTW)is constructed applying the pole-mode transformation theory.Next,fault traveling wave propagation characteristics along the DC transmission line are analyzed in depth utilizing Peterson's law.Then,differences of time-frequency electromagnetic transient characteristics of 1-mode RVTWs between disturbances and faults caused by lightning strikes are distinguished in detail by means of the classical wavelet transformation multi-resolution analysis theory.Finally,extensive simulations are carried out to evaluate the performance of the proposed identification scheme,and by which its excellent rapidity,reliability and robustness are validated.Index Terms-Lightning-strike identification,Multi-resolution analysis,Relay protection,Traveling-wave protection,VsC-based DC grid,Wavelet transformation.

Impedance Analysis of Grid Forming Control Based Modular Multilevel Converters

Grid-forming control(GFC)is promising for power electronics based power systems with high renewable energy penetration.Naturally,the impedance modeling for GFC is necessary and has gained significant attention recently.However,most of the impedance analyses for GFC are based on a twolevel converter(TLC)rather than a modular multilevel converter(MMC).MMC differs from TLC with respect to its dominant multi-frequency response.It is necessary to analyze the impedance of GFC-based MMC owing to its superiority in highvoltage direct current(HVDC)transmission to interlink two weak AC systems with high renewable energy penetration.As the main contribution,this paper presents the AC-and DC-side impedance analyses for the GFC-based MMC with both power and DC voltage control using the harmonic transfer function(HTF),and compares the impedances of GFC-based MMC and TLC.It is inferred that although the impedance is mainly influenced within 200 Hz,the instability still could occur owing to negative resistance triggered by relatively larger parameters.The difference in AC-side impedance with power and DC voltage control is not apparent with proper parameters,while the DC-side impedance differs significantly.The generalized Nyquist criterion is necessary for AC-side stability owing to the relatively large coupling terms under GFC.Moreover,the coupling between AC-and DC-side impedances is noneligible,especially considering the DC-side resonance around the system resonant peak.The effects of parameters,system strength,and virtual impedance on the impedance shaping are analyzed and verified through simulations.

Resilience Enhancement of Urban Energy Systems via Coordinated Vehicle-to-grid Control Strategies

Coordinated vehicle-to-grid(V2G)control strategies can sustain essential loads of an energy system during islanding,thereby increasing resilience.In this context,this paper investigates the resilience enhancement benefits of smart V2G control,the value of electric vehicle(EV)owner cooperation on system resilience,as well as the complementary effects of PV and EV interaction in an urban multi-energy microgrid(MEMG).By using a rolling horizon approach to optimize day-ahead operation of the MEMG and subsequently dispatching EVs,uncertainties in outage start time,EV arrival/departure times,and initial state of charge(SOC)are mitigated.Results show that smart V2G control can provide a substantial essential load curtailment reduction compared to a non-EV scenario,meanwhile,non-coordinated grid-to-vehicle(G2V)operation was shown to slightly burden the system with a slight increase in non-essential load curtailment.Investigations into the influence of EV cooperation on resilience showed that a high percentage of system-prioritized(SP)EVs could help greatly further reduce essential load curtailment compared to individual-prioritized(IP)EVs.Finally,the complementary benefits of smart V2G control and PV were demonstrated,showing a reduction in both PV and essential load curtailments with increasing numbers of EVs.Overall,the application of smart V2G control,especially with cooperation of EV owners,can drive significant resilience enhancement during islanding,while further benefits can be obtained through having a sufficient number of EVs to utilize high PV penetration.

A DC Current Flow Controller for Meshed Modular Multilevel Converter Multiterminal HVDC Grids

This paper proposes the design of a novel DC current flow controller(CFC)and evaluates the control performance of balancing and regulating the DC branch currents using the DC CFC in a meshed multi-terminal HVDC(MTDC)grid.The DC CFC consists of two identical full bridge DC-DC converters with the capacitors of the two converters being connected in parallel.The scalability of the DC CFC is easily achievable due to the identical bridge converter topology;the cost of this DC CFC is also relatively low due to its simple physical structure and low voltage ratings.The control performance of the DC CFC is tested on a meshed 3-terminal(3-T)HVDC grid,which is based on modular multilevel converters(MMC).The DC branch current control in the meshed MTDC grid is achieved using the proposed control strategy of the DC CFC,and is verified through case studies on the real-time digital simulator(RTDS).

Fully Decoupled Branch Energy Balancing Control Method for Modular Multilevel Matrix Converter Based on Sequence Circulating Components

The modular multilevel matrix converter(M3C)is a potential frequency converter for low-frequency AC transmission.However,capacitor voltage control of high-voltage and largecapacity M3C is more difficult,especially for voltage balancing between branches.To solve this problem,this paper defines sequence circulating components and theoretically analyzes the influence mechanism of different sequence circulating components on branch capacitor voltage.A fully decoupled branch energy balancing control method based on four groups of sequence circulating components is proposed.This method can control capacitor voltages of nine branches in horizontal,vertical and diagonal directions.Considering influences of both circulating current and voltage,a cross decoupled control is designed to improve control precision.Simulation results are taken from a low-frequency transmission system based on PSCAD/EMTDC,and effectiveness and precision of the proposed branch energy balancing control method are verified in the case of nonuniform parameters and an unbalanced power system.

Simplified Cluster Voltage Balancing Control Based on Zero-sequence Voltage Injection for Star-connected Cascaded H-bridge STATCOM

The cluster DC voltage balancing control adopting zero-sequence voltage injection is appropriate for the starconnected cascaded H-bridge STATCOM because no zerosequence currents are generated in the three-phase three-wire system.However,as the zero-sequence voltage is expressed in trigonometric form,traditional control methods involve many complicated operations,such as the square-root,trigonometric operations,and inverse tangent operations.To simplify cluster voltage balancing control,this paper converts the zero-sequence voltage to the dq frame in a DC representation by introducing a virtually orthogonal variable,and the DC components of the zero-sequence voltage in the dq frame are regulated linearly by proportional integral regulators,rather than being calculated from uneven active powers in traditional controls.This removes all complicated operations.Finally,this paper presents simulation and experimental results for a 400 V±7.5 kvar star-connected STATCOM,in balanced and unbalanced scenarios,thereby verifying the effectiveness of the proposed control.

Security Risk Assessment of Cyber Physical Power System Based on Rough Set and Gene Expression Programming

Risk assessment is essential for the safe and reliable operation of cyber physical power system. Traditional security risk assessment methods do not take integration of cyber system and physical system of power grid into account. In order to solve this problem, security risk assessment algorithm of cyber physical power system based on rough set and gene expression programming is proposed. Firstly, fast attribution reduction based on binary search algorithm is presented. Secondly, security risk assessment function for cyber physical power system is mined based on gene expression programming. Lastly, security risk levels of cyber physical power system are predicted and analyzed by the above function model. Experimental results show that security risk assessment function model based on the proposed algorithm has high efficiency of function mining, accuracy of security risk level prediction and strong practicality. © 2014 Chinese Association of Automation.

High Frequency Stability Constraints Based MMC Controller Design Using NSGA-III Algorithm

The occurrence of high frequency resonances(HFRs)has been frequently observed in several MMC-HVDC projects.To avoid these HFRs,the controller design of an MMC must satisfy two requirements:1)The controller should remain stable while in the high frequency range,and 2)MMC impedance should not possess a negative real part in the high frequency range.So far,majority of the related studies on MMC controller design have been unable to address these requirements precisely.This paper first describes the simplified high frequency MMC impedance model developed indigenously by the authors.Subsequently,the driving mechanism of two kinds of HFRs is revealed using the said developed model,including:1)MMC controller instability,and 2)Interaction instability between MMC and AC cables.Furthermore,the mathematical expressions outlining the controller stability constraint and positive damping constraint are proposed.Additionally,NSGA-III based multi-objective optimization algorithm is adopted,to identify the region most suitable for satisfying the proposed stability constraints under the MMC controller parameters.The proposed controller design method is capable of effectively evading the HFRs triggered by the incompatible MMC.The detailed time-domain simulations generated using PSCAD/EMTDC software validate the proposed designed method and endorse the improved results.

Energy-band alignment of atomic layer deposited(HfO_2)_x(Al_2O_3)_(1-x) gate dielectrics on 4H-SiC

We study a series of（HfO2）x（Al2O3）1-x /4H-SiC MOS capacitors. It is shown that the conduction band offset of HfO2 is 0.5 e V and the conduction band offset of Hf AlO is 1.11–1.72 e V. The conduction band offsets of（Hf O2）x（Al2O3）1-x are increased with the increase of the Al composition, and the（HfO2）x（Al2O3）1-x offer acceptable barrier heights（〉 1 e V）for both electrons and holes. With a higher conduction band offset,（Hf O2）x（Al2O3）1-x/4H-SiC MOS capacitors result in a ～ 3 orders of magnitude lower gate leakage current at an effective electric field of 15 MV/cm and roughly the same effective breakdown field of ～ 25 MV/cm compared to HfO2. Considering the tradeoff among the band gap, the band offset, and the dielectric constant, we conclude that the optimum Al2O3 concentration is about 30% for an alternative gate dielectric in 4H-Si C power MOS-based transistors.

Characterization of Obtuse Triangular Defects on 4H-SiC 4° off-Axis Epitaxial Wafers

We investigate the triangular defects with different structural features on 4H-SiC epilayers by a Nomarski micro-scope,a Candela optical surface analyzer and ultraviolet photoluminescence(UV-PL)imaging.Both the foreign particles and the substrate scratches can cause the formation of the obtuse triangular defects.The central area of some obtuse triangular defects can have the spatially confined core,in which the in-grown stacking faults can be observed under the UV-PL imaging.In contrast,the obtuse triangular defects induced by the scratches appear in the form of band-like defects,of which the width depends on the scratch direction and reaches the maximum when the scratch direction is parallel to the step flow direction.The formation mechanisms of these obtuse triangular defects are discussed.

Fast Homoepitaxial Growth of 4H-SiC Films on 4° off-Axis Substrates in a SiH4-C2H4-H2 System

Homoepitaxial growth of 4H-SiC epilayers is conducted in a SiH_(4)-C_(2)H_(4)-H_(2) system by low pressure hot-wall vertical chemical vapor deposition(CVD).Thick epilayers of 45μm are achieved at a high growth rate up to 26μm/h under an optimized growth condition,and are characterized by using a Normaski optical microscope,a scanning electronic microscope(SEM),an atomic force microscope(AFM)and an x-ray diffractometer(XRD),indicating good crystalline quality with mirror-like smooth surfaces and an rms roughness of 0.9 nm in a 5μm×5μm area.The dependence of the 4H-SiC growth rate on growth conditions on 4°off-axis 4H-SiC substrates and its mechanism are investigated.It is found that the H_(2) flow rate could influence the surface roughness,while good surface morphologies without Si droplets and epitaxial defects such as triangular defects could be obtained by increasing temperature.

Research and Modeling on the Characteristic Changes of EMI Filter Passive Components under the Influence of Aging

At present,the power density of power electronic devices in data centers and electric vehicles is constantly increasing,and numerous electronic components are concentrated in a tight,high-temperature environment,which aggravates the performance degradation of electronic components.Consequently,X and Y capacitors,common-mode inductors,and differential-mode inductors used for electromagnetic interference(EMI)suppression suffer from aging effects,and their performance continues to decline.However,the electromagnetic compatibility test is often conducted immediately after the power electronic equipment leaves the factory.The electromagnetic compatibility of power electronic equipment is affected by aging,which is not assessed in current industrial testing.This study conducts aging experiments on passive electronic components in EMI filters and measures the impedance in the frequency range from 150 kHz to 30 MHz.Subsequently,a multi-element aging model based on electromagnetic field analysis is established.The proposed model is suitable for electromagnetic compatibility analysis considering aging.Finally,the aging performance of a commercial two-stage EMI filter is predicted to verify the model proposed in the study.The proposed model explains the degeneration of the EMI filter with aging in the frequency range of 150 kHz to 1 MHz,with a maximum amplitude error of 0.58 dB and phase error of 1.0°.

Design of Transformer Magnetic Bias Protection Device in High Power Electronic Equipments

As the power electronics technology is widely used in the power system, it may also bring the DC component to the transformer operation, resulting in DC bias and may cause great harm to the transformer. In this article, the device to protect transformer from DC magnetic bias is designed. On the basis of load DC current, a magnetic bias protection device is developed by combination of current sensor, electric information collection circuit, signal filtering circuit, signal modulating circuits, fault feature judging circuit, automatic range tracking circuit, intelligent logic synthesis unit and implementation output circuit. By operating in temperature-rise test equipment in the high power electronic lab, the device is proved with reliability, high sensitivity and worthy of promotion and application.

Effect of Counterparts and Applied Load on the Tribological Behavior of the Graphene–Nickel Matrix Self-Lubricating Composite

Graphene-oxide (GO) has been recognized as an excellent lubrication material owing to its two-dimensional structure and weak interlayer interactions. However, the functional groups of GO that can contribute to anti-friction, anti-wear, and superlubricity are yet to be elucidated. Hence, further improvement in GO-family materials in tribology and superlubricity fields is impeded. In this study, macroscale superlubricity with a coefficient of friction of less than 0.01 is achieved by exploiting the high adhesive force between amino groups within aminated GO (GO–NH_(2)) nanosheets and SiO_(2). It was observed that GO–NH_(2) nanosheets form a robust adsorption layer on the worn surfaces owing to the high adsorption of amino groups. This robust GO–NH_(2) adsorption layer not only protects the contact surfaces and contributes to low wear, but also causes the shearing plane to transform constantly from solid asperities (high friction) into GO–NH_(2) interlayers (weak interlayer interactions), resulting in superlubricity. A SiO_(2)-containing boundary layer formed by tribochemical reactions and a liquid film are conducive to low friction. Such macroscale liquid superlubricity provides further insights into the effect of functional groups within functionalized GO materials and a basis for designing functionalized GO materials with excellent tribological performances.