Analysis of Additional Damping Control Strategy and Parameter Optimization for Improving Small Signal Stability of VSC-HVDC System

The voltage source converter based high voltage direct current(VSC-HVDC)system is based on voltage source converter,and its control system is more complex.Also affected by the fast control of power electronics,oscillation phenomenon in wide frequency domain may occur.To address the problem of small signal stability of the VSCHVDC system,a converter control strategy is designed to improve its small signal stability,and the risk of system oscillation is reduced by attaching a damping controller and optimizing the control parameters.Based on the modeling of the VSC-HVDC system,the general architecture of the inner and outer loop control of the VSCHVDC converter is established;and the damping controllers for DC control and AC control are designed in the phase-locked loop and the inner and outer loop control parts respectively;the state-space statemodel of the control system is established to analyze its performance.And the electromagnetic transient simulation model is built on the PSCAD/EMTDC simulation platform to verify the accuracy of the small signal model.The influence of the parameters of each control part on the stability of the system is summarized.The main control parts affecting stability are optimized for the phenomenon of oscillation due to changes in operation mode occurring on the AC side due to faults and other reasons,which effectively eliminates system oscillation and improves system small signal stability,providing a certain reference for engineering design.

Harmonic State-Space Based AC-Side Impedance Model of MMC and High Frequency Oscillation Characteristics Analysis

Recently,high-frequency oscillation of themodularmultilevel converter(MMC)based high-voltage direct current(HVDC)projects has attracted great attentions.In order to analyze the small-signal stability,this paper uses the harmonic state-space(HSS)method to establish a detailed frequency domain impedance model of the AC-side of the HVDC transmission system,which considers the internal dynamic characteristics.In addition,the suggested model is also used to assess the system’s high-frequency oscillationmechanism,and the effects of the MMC current inner loop control,feedforward voltage links,and control delay on the high-frequency impedance characteristics and the effect of higher harmonic components.Finally,three oscillation suppression schemes are analyzed for the oscillation problems occurring in actual engineering,and a simplified impedance model considering only the highfrequency impedance characteristics is established to compare the suppression effect with the detailed impedance model to prove its reliability.

Study on the Three Dimension Attenuated Model and the Algorithm of Environmental Noise in Substations

为了准确计算变电站噪声对周边环境影响的大小，通过对噪声衰减理论和变电站环境特点的分析，建立了变电站仿真数学模型，并基于该模型提出了变电站三维空间噪声预测算法。噪声衰减计算中，最复杂的是求解几何衰减中菲涅耳数，而求解菲涅耳数的关键是求解绕射声的声程差，利用凸包算法求解变电站内多声源、多障碍等复杂场景的声程差问题。仿真计算结果与实测结果对比显示，该模型和算法能够准确预测变电站周边三维空间中任意位置的噪声大小，为开展变电站环境噪声预评价、新建变电站规划设计中噪声控制优化以及运行变电站噪声的工程治理方案优化等提供技术支持和理论依据。

Inhibiting Ice Accumulation on Conductors by Using Sheath Treated with Super-hydrophobic Surfaces

Ice accumulation on transmission lines often leads to great damage to power systems.Super-hydrophobic surfaces are proposed to inhibit ice accumulation on electrical power equipment.A novel anti-icing method was presented for conductors with super-hydrophobic sleeves.The super-hydrophobic sleeves were prepared by applying polydimethylsiloxane(PDMS) and nano-silica hybrid coating on the outer surface of polyethylen terephthalate(PET) sleeves.Hydrophobicity and ice adhesion strength of the super-hydrophobic surface were investigated.Ice accumulation experiments were carried out on ordinary conductors,super-hydrophobic coated conductors,PET sleeve-covered conductors,and super-hydrophobic PET sleeve-covered conductors.Ice accumulation morphology,accumulated ice weight,and the icicle length of these four types of conductors were studied and analyzed.At the end of the 3 h ice accumulation experiment,the ice weight and icicle length on the conductor with super-hydrophobic PET sleeve was only approximately one tenth and one seventh of that on the untreated conductor respectively.Furthermore,the water contact angles of super-hydrophobic coated aluminum surface and super-hydrophobic coated PET sleeve were about 163o in average.The results indicate that the super-hydrophobic coating is effective in inhibiting ice accumulation on conductors.However,the use of a super-hydrophobic PET sleeve is a significantly more effective method,comparing with the application of a super-hydrophobic coating directly on the conductor.

Observation of V-type electromagnetically induced transparency and optical switch in cold Cs atoms by using nanofiber optical lattice

Optical nanofiber(ONF)is a special tool to achieve the interaction between light and matter with ultralow power.In this paper,we demonstrate V-type electromagnetically induced transparency(EIT)in cold atoms trapped by an ONFbased two-color optical lattice.At an optical depth of 7.35,90%transmission can be achieved by only 7.7 pW coupling power.The EIT peak and linewidth are investigated as a function of the coupling optical power.By modulating the pWlevel control beam of the ONF-EIT system in sequence,we further achieve efficient and high contrast control of the probe transmission,as well as its potential application in the field of quantum communication and quantum information science by using one-dimensional atomic chains.

Field experiment using transient energy method to locate a single-phase to ground fault

Distribution networks in China and several other countries are predominantly neutral inefficiently grounding systems(NIGSs),and more than 80%of the faults in distribution networks are single-phase-to-ground(SPG)faults.Because of the weak fault current and imperfect monitoring equipment configurations,methods used to determine the faulty line secti ons with SPG faults in NIGSs are in effective.The developme nt and application of distributi on-level phasor measurement units(PMUs)provide further comprehensive fault information for fault diagnosis in a distribution network.When an SPG fault occurs,the transient energy of the faulted line section tends to be higher than the sum of the transient energies of other line sections.In this regard,transient energy-based fault location algorithms appear to be a promising resolution.In this study,a field test plan was designed and implemented for a 10 kV distribution network.The test results dem on strate the effective ness of the transient en ergy-based SPG locati on method in practical distributi on networks.

Research on the adsorption of environmentally friendly insulating gas C_(4)F_(7)N decomposed components on the surface ofγ-Al_(2)O_(3)

C_(4)F_(7)N is considered to be the most potential alternative gas for SF_(6).However,harmful decomposition by-products generated by the discharge or overheating faults will pose threat to safe operation and service life of gas insulated equipment(GIE).Herein,we investigated the adsorption performance ofγ-Al_(2)O_(3)on C_(4)F_(7)N and its decomposition products to evaluate the possibility of using it as by-products adsorbent or exhaust treatment material.It is found thatγ-Al_(2)O_(3)demonstrates superior adsorption capacity for CF_(4),C_(2)F_(6),C_(3)F_(8),C_(3)F_(6),C_(2)F_(4),and keeps stable after the interaction.The first-principles calculation infers thatγ-Al_(2)O_(3)has stronger interaction with C_(4)F_(7)N,C_(3)F_(6)and C_(3)F_(8)than that of C_(2)F_(4),C_(2)F_(6)and CF_(4).There exists obvious charge transfer between C_(4)F_(7)N andγ-Al_(2)O_(3),which belongs to chemisorption.The interaction between F 2p orbital and C=Cπorbital determines the gas-solid adsorption performance.The desorption properties were also explored and the recovery time was in order of CF_(4)<C_(2)F_(6)<C_(2)F_(4)<C_(3)F_(8)<C_(4)F_(7)N<C_(3)F_(6).Overall,theγ-Al_(2)O_(3)is inappropriate to be used as the adsorbent for decomposition by-products of C_(4)F_(7)N gas mixture-based GIE,while it can be used as exhaust treatment material for C_(4)F_(7)N wasted gas.

Transverse Propagation Characteristics and Coherent Effect of Gaussian Beams

As an important electromagnetic Held in experiment,Gaussian beams have non-vanishing longitudinal electric and magnetic components that generate significant energy fluxes on transverse directions.We focus on the transverse energy flux and derive the theoretical propagation properties.Unlike the longitudinal energy flux,the transverse energy flux has many unique physical behaviors,such as the odd symmetry on propagation,slower decay rate on resonant condition.By means of the characteristics of transverse energy Bux,it is feasible to find the suitable regions where the information of coherent lights could be extracted exactly.With the typical laser parameters,we simulate the energy fluxes on receiver surface and analyze the corresponding distribution for the coherent light beams.Especially for coherent lights,the transverse energy Bux on the y-z plane with x=0 and x-z plane with y=0,contains pure coherent information.Meanwhile,in the transverse distance|y|<2 W0(W0 is the waist radius)and|x|<W0/3 the coherent information could also be extracted appropriately.

Reliability evaluation of tidal and wind power generation system with battery energy storage

This paper presents a method for reliability evaluation of a hybrid generation system of wind and tidal powers with battery energy storage.Such a system may widely exist in coastal areas and islands in the future.A chronological multiple state probability model of tidal power generation system(TPGS)considering both forced outage rate(FOR)of the TPGS and random nature of tidal current speed is developed.In the evaluation of FORs of TPGS and WPGS(wind power generation system),the delivered power related failure rates of power electronic converters for TPGS and WPGS are considered.A chronological power output model of battery energy storage system(BESS)is derived.A hybrid system of tidal and wind generation powers with a BESS is used to demonstrate the effectiveness of the presented method.In case studies,the effects of various parameters on the system reliability are investigated.

High‐altitude tunnel‐catenary gap impulse discharge characteristics and breakdown voltage correction

Air gap impulse discharge characteristics of railway tunnel‐catenary at high‐altitude are of great significance to the electrical insulation design.The existing altitude correction methods for non‐uniform air gaps are not applicable to tunnel‐catenary.To investigate the impulse discharge characteristics of the tunnel‐catenary,a typical structural model was built in an artificial climate chamber.Different altitudes were simulated by adjusting the atmospheric pressure.The 50%breakdown voltages of the 300–700 mm tunnel‐catenary gap were tested at 243–4000 m.The test found that the altitude/atmospheric pressure and gap length have an evident effect on the 50%impulse breakdown voltage,and the influence of humidity is relatively small.The polarity effect was analysed by streamer theory.The{d,P,h}parameter method was employed to obtain the fitting formulas for the impulse breakdown voltage.A high‐altitude correction method for the tunnel‐catenary breakdown voltage based on 1 km altitude was proposed.Field natural tests were conducted to verify the accuracy of the proposed method.The results show that the breakdown voltage obtained by the correction method has an average error of 3.66%in the results of the field tests,indicating that the method has engineering application prospects.

Mechanical defect identification for gas‐insulated switchgear equipment based on time‐frequency vibration signal analysis

Mechanical defect is an important reason for the failure of gas‐insulated switchgear(GIS)equipment.Based on the time‐frequency characteristic vibration signal analysis on five kinds of mechanical defects,a novel intelligent algorithm model combining complementary ensemble empirical mode decomposition(CEEMD)and genetic al-gorithm improved kernel fuzzy mean clustering(GAKFCM)was proposed to identify the mechanical defect type.First,the mechanical defect platform and detection sys-tem were built.Then CEEMD and IMF sensitivity factors were used to analyse the time‐frequency signal of five kinds of vibration defects,and the feature extraction was performed on the processed vibration signals.Finally,the mechanical vibration defect recognition model was established based on the GAKFCM algorithm and its validity was verified.Results show that the developed detection system can detect mechanical vibration signals sensitively.Singular values,frequency band lines and entropy can reflect the energy attenuation and distribution differences for different type of me-chanical defect vibration signals.The proposed GAKFCM clustering model combining the above vibration feature parameters can effectively find and diagnose the mechanical defect of GIS equipment.Its recognition accuracy reaches 96.74%,especially for the loose contact seat bolts and poor contact failures of the disconnector.

Single-ended Time Domain Fault Location Based on Transient Signal Measurements of Transmission Lines

Precise fault location plays an important role in the reliability of modern power systems.With the in-creasing penetration of renewable energy sources,the power system experiences a decrease in system inertia and alterations in steady-state characteristics following a fault occurrence.Most existing single-ended phasor domain methods assume a certain impedance of the remote-end system or consistent current phases at both ends.These problems present challenges to the applicability of con-ventional phasor-domain location methods.This paper presents a novel single-ended time domain fault location method for single-phase-to-ground faults,one which fully considers the distributed parameters of the line model.The fitting of transient signals in the time domain is real-ized to extract the instantaneous amplitude and phase.Then,to eliminate the error caused by assumptions of lumped series resistance in the Bergeron model,an im-proved numerical derivation is presented for the distrib-uted parameter line model.The instantaneous symmet-rical components are extracted for decoupling and inverse transformation of three-phase recording data.Based on the above,the equation of instantaneous phase constraint is established to effectively identify the fault location.The proposed location method reduces the negative effects of fault resistance and the uncertainty of remote end pa-rameters when relying on one-terminal data for localiza-tion.Additionally,the proposed fault analysis methods have the ability to adapt to transient processes in power systems.Through comparisons with existing methods in three different systems,the fault position is correctly identified within an error of 1%.Also,the results are not affected by sampling rates,data windows,fault inception angles,and load conditions. Index Terms—Fault location,distributed parameter line model,transient signal,renewable energy,instantaneous phase.