Robust Position Observer for Sensorless Direct Voltage Control of Stand-Alone Ship Shaft Brushless Doubly-Fed Induction Generators

The aim of this paper is to investigate an adaptive sensorless direct voltage control(DVC)strategy for the stand-alone ship shaft brushless doubly-fed induction generators(BDFIGs).The proposed new rotor position observer using the space vector flux relations of BDFIG may achieve the desired voltage control of the power winding(PW)in terms of magnitude and frequency,without any speed/position sensors.The proposed algorithm does not require any additional observers for obtaining the generator speed.The proposed technique can directly achieve the desired DVC based on the estimated rotor position,which may reduce the overall system cost.The stability analysis of the proposed observer is investigated and confirmed with the concept of quadratic Lyapunov function and using the multi-model representation.In addition,the sensitivity analysis of the presented method is confirmed under different issues of parameter uncertainties.Comprehensive results from both simulation and experiments are realized with a prototype wound-rotor BDFIG,which demonstrate the capability and efficacy of the proposed sensorless DVC strategy with good transient behavior under different operating conditions.Furthermore,the analysis confirms the robustness of the proposed observer via the machine parameter changes.

Low Voltage Direct Current Supply and Utilization System:Definition,Key Technologies and Development

With rapid increase of distributed solar power generation and direct current(DC)based loads such as data centers,electric vehicles(EVs),and DC household appliances,the development trend of the power system is changed from conventional alternate current(AC)to DC.Traditional AC power systems can scarcely meet the development demand of new DC trends,especially since both the generation side and load side are comprised of DC-based electronic power components.With this background,low voltage direct current supply and utilization system(LVDCSUS)has attracted more and more attention for its great advantages over an AC grid to overcome challenges in operation,reliability,and energy loss in renewable energy connection,DC load power utilization and a number of other aspects.However,the definition of the LVDCSUS is still not clear even though many demonstration projects have been put into planning and operation.In order to provide a clear description of LVDCSUS,first,the characteristics of LVDCSUS are illustrated in this paper to show the advance of the LVDCSUS.Second,the potential application scenarios of LVDCSUS are presented in this paper.Third,application of LVDCSUS technologies and some demonstration projects in China are introduced.Besides the development of the LVDCSUS,key technologies,including but not limited to planning and design,voltage levels,control strategies,and key equipment of LVDCSUS,are discussed in this paper.Finally,future application areas and the research orientations of LVDCSUS are analyzed.

A Study on the Load Modeling of Railway Vehicles Using PSCAD/EMTDC Based on MVDC

This paper,the kinetic equation,traction force,and braking force for railway trains are reviewed.In addition,the driving characteristics are interpreted as to how the power of the electric vehicle relates to the weight,speed,track curve,and track gradient of the electric vehicle.The driving characteristics of these trains are analyzed through PSCAD/EMTDC(power systems computer aided design/electromagnetic transients including DC)modeling.

R&D and application of voltage sourced converter based high voltage direct current engineering technology in China

As a new generation of direct current(DC)transmission technology,voltage sourced converter(VSC)based high voltage direct current(HVDC)has been widely developed and applied all over the world.China has also carried out a deep technical research and engineering application in this area,and at present,it has been stepped into a fast growing period.This paper gives a general review over China’s VSC based HVDC in terms of engineering technology,application and future development.It comprehensively analyzes the technical difficulties and future development orientation on the aspects of the main configurations of VSC based HVDC system,topological structures of converters,control and protection technologies,flexible DC cables,converter valve tests,etc.It introduces the applicable fields and current status of China’s VSC based HVDC projects,and analyzes the application trends of VSC based HVDC projects both in China and all over the world according to the development characteristics and demands of future power grids.

高压直流换流站穿墙套管的局部放电在线测量(英文)

An online partial discharge(PD) measurement performed on a high voltage direct current(HVDC) wall bushing successfully identified the presence of internal discharges.The wall bushing is a sulfur hexafluoride gas-insulated bushing,rated for 500 kV dc and terminated on a thyristor-controlled HVDC converter bridge.The measurement of PD within the HVDC station environment is particularly challenging due to the high levels of electromagnetic noise caused by thyristor switching events and external air-corona from the neighboring high-voltage equipment.An additional challenge is the""mixed"voltage stress on the bushing insulation,which has both ac and dc high-voltage components.There are also fast transients during the firing of thyristors in the HVDC conversion process that cause added stress to the insulation.As a result,the analysis and interpretation of PD data for HVDC equipment is more complex;PD pulses may occur in response to the ac,dc,or switching transient voltage stresses.In this paper,an online PD measurement strategy for noise filtering and isolation of PD sources within the bushing are discussed.The PD measurement data is plotted on a phase-resolved diagram where the line supply power cord voltage was used as a reference. The phase-resolved diagram appears to suggest that the fast transients,caused during switching,trigger some PD events.Measurements were also performed with the aid of a modern PD measurement instrument having noise separation capabilities.The findings from the online PD measurements are verified with physical evidence,found after the bushing was removed from service,suggested internal PD had occurred inside the bushing.

Fault Ride Through Strategy of DFIG Using Rotor Voltage Direct Compensation Control Under Voltage Phase Angle Jump

Wind power has developed rapidly in recent years,and large-scale wind power facilities connected to power grids will bring many new challenges.Some new operation charac-teristics of power grids with doubly-fed induction generator(DFIG)may exhibit,for example voltage phase angle jumps(VPAJ).VPAJ can negatively impact the fault ride through(FRT)performance of DFIG.This paper firstly investigates the physical mechanism and the operation characteristics of DFIG with VPAJ.It is noted that the current control strategies designed for voltage amplitude changes are not suitable for VPAJ.Secondly,the paper develops an FRT optimization control strategy under VPAJ which optimizes the DFIG operation characteristics.Finally,simulations of a 250 MW wind farm are presented which validate the proposed FRT strategy.

Research and comprehensive evaluation on delivery schemes of the Grand Inga hydropower station

Congo River has abundant hydropower resources,and large-scale cascade power stations,such as the Grand Inga,can be constructed in downstream locations.However,the fragile economic foundations of the Democratic Republic of the Congo and neighbor!ng Central African countries,and the small-scale regional power consumption market prohibit the implementation of large-scale hydro power projects.As the high-voltage,I on g-dista nee power transmission tech no logy matures,hydropower from the Grand Inga can be delivered to load centers in other regions of Africa.This study establishes a 6 dime nsional comprehe nsive assessment model using the best-worst method to evaluate large-scale,I on g-distance,cross-border power intercon necti on projects.The model is applied to evaluate all the can didate in ter-regional power delivery schemes of the Inga III hydropower station,and the evaluation results can effectively help investment institutions and policy makers in policy making and potential market targeting.

Analysis of rail potential and stray current in MVDC railway electrification system

In contrast to the conventional direct current railway electrification system(DC-RES),the medium voltage direct current(MVDC)-RES is considered promising for long-distance high-speed corridors.In the MVDC-RES,traction substations(TSSs)are placed much farther and train loads are much heavier than in the conventional DC-RES.Hence,the MVDC-RES brings a drastic change in catenary voltage,TSS spacing,and train loading,which affects rail potential and stray current.In this connection,this work performs some significant quantitative analysis of rail potential and stray current in the MVDC-RES environment.An MVDC simulation model is proposed and different grounding schemes are analyzed for a single-train and two TSSs scenario as well as for a multi-train multi-TSS scenario.According to the simulation and analysis,the maximum values of rail potential and stray current at MVDC-RES distances and the maximum safe distance between adjacent TSSs are determined.

A Fault Current Limiting Hybrid DC Circuit Breaker

Due to the low impedance characteristic of the high voltage direct current(HVDC)grid,the fault current rises extremely fast after a DC-side fault occurs,and this phenomenon seriously endangers the safety of the HVDC grid.In order to suppress the rising speed of the fault current and reduce the current interruption requirements of the main breaker(MB),a fault current limiting hybrid DC circuit breaker(FCL-HCB)has been proposed in this paper,and it has the capability of bidirectional fault current limiting and fault current interruption.After the occurrence of the overcurrent in the HVDC grid,the current limiting circuit(CLC)of FCL-HCB is put into operation immediately,and whether the protected line is cut off or resumed to normal operation is decided according to the fault detection result.Compared with the traditional hybrid DC circuit breaker(HCB),the required number of semiconductor switches and the peak value of fault current after fault occurs are greatly reduced by adopting the proposed device.Extensive simulations also verify the effectiveness of the proposed FCL-HCB.

Facile carbon cloth activation strategy to boost oxygen reduction reaction performance for flexible zinc-air battery application

Flexible and all-solid-state zinc-air batteries(ZABs)are highly useful and also in demand due to their theoretical high energy densities and special applications.The limitation in their performance arises due to their catalyst-coated cathode electrodes in terms of catalytic activity and stability as well as cost.In this paper,a novel and environmentally friendly activation strategy is developed to activate the carbon cloth(CC)for the electrodes.The activated CC serves as a catalyst-free air cathode with high conductivity,excellent mechanical strength,and flexibility,in addition to low cost.The strategy is performed by simply electro-oxidizing and electroreducing CC under ultrahigh direct current(DC)voltage in a diluted NH4Cl aqueous electrolyte.It is found that the electro-oxidation not only results in the formation of a graphene-like exfoliated carbon layer on the surface of CC but also induces the incorporation of oxygen-containing groups and doping of nitrogen and chloride atoms.After the electroreduction,theπ-conjugated carbon network of CC is partially restored,leading to the recovery of electroconductivity.Such an electroactivated CC shows excellent oxygen reduction reaction activity.The aqueous flexibility and all-solid-state ZABs are assembled using such an electroactivated CC cathode without any catalyst loading.Both ZABs can achieve good durability and deliver high peak power density and an energy density as high as 690 Wh kg^(−1),demonstrating the excellent potential of this electroactivated CC in practical devices.

Hybrid Simulation of ±500 kV HVDC Power Transmission Project Based on Advanced Digital Power System Simulator

In order to effectively imitate the dynamic operation characteristics of the HVDC （high voltage direct current） power transmission system at a real ±500kV HVDC transmission project, the electromechanical-electromagnetic transient hybrid simulation was carried out based on advanced digital power system simulator （ADPSS）. In the simulation analysis, the built hybrid model＇s dynamic response outputs under three different fault conditions are considered, and by comparing with the selected fault recording waveforms, the validities of the simulation waveforms are estimated qualitatively. It can be ascertained that the hybrid simulation model has the ability to describe the HVDC system＇s dynamic change trends well under some special fault conditions.

Frequency Regulation of VSC-MTDC System with Offshore Wind Farms

Frequency regulation of voltage source converter-based multi-terminal high-voltage direct current(VSC-MTDC)system with offshore wind farms enhances the frequency stability by compensating the power for a disturbed AC system.However,it is difficult to reasonably allocate frequency-regulation resources due to a lack of coordination mechanisms between wind farms and the MTDC system.Moreover,it is difficult for the frequency control of the wind farms to manage changes in wind speed;and the risk of wind-turbine stalls is high.Thus,based on the kinetic energy of wind turbines and the power margin of the converters,the frequency-regulation capability of wind turbines is evaluated,and a dynamic frequency-support scheme considering the real-time frequency-support capability of the wind turbines and system frequency evolution is proposed to improve the frequency-support performance.A power adaptation technique at variable wind speeds is developed;the active power in the frequency-support stage and restoration stage is switched according to the wind speed.A hierarchical zoning frequency-regulation scheme is designed to use the frequency-regulation resources of different links in the MTDC system with wind farms.The simulation results show that the novel frequency-regulation strategy maintains frequency stability with wind-speed changes and avoids multiple frequency dips.

Deep Learning-Based Robust DC Fault Protection Scheme for Meshed HVDC Grids

Fast and reliable detection of faults is a significant technical challenge in transient-based protection for a modular multi-level converter(MMC)based high voltage direct current(HVDC)system.This is because existing protection schemes rely heavily upon setting a complicated protective threshold,the failure of which causes high DC-fault currents in HVDC grids,and MMC is prone to such strong transient currents.In this context,this paper proposes a DC-line fault diagnosis technique based on a tuned long-short-term memory(LSTM)algorithm to improve the response and accuracy of transient-based protection.The discrete wavelet transform(DWT)extracts the transient features of DC-line voltages in the frequency-time domain.Many healthy and faulty samples are incorporated during training even by considering the noise influence.After training,numerous test samples are run to evaluate the proposed algorithm’s robustness under various fault conditions.Test results show the proposed algorithm can detect DC faults and has a high recognition accuracy of 98.6%.Compared to contemporary techniques,it can perform well to identify DC-line faults because of the efficient training of characteristic features.

Development and advances in modular multilevel converter based HVDC projects in China

1 Introduction To achieve the goal of carbon neutrality,promoting renewable energy integration has become an urgent priority for the green energy transition.Due to the geographical distribution of renewable energy bases,bulk electrical power is transmitted to load centers through long-distance transmission lines.The line-commutated converter(LCC)based high voltage direct current(HVDC)technology,emerging in the 1970s,is the most mature solution.

Circuit breakers in HVDC systems: state-of-the-art review and future trends

High voltage direct current(HVDC)systems are efficient solutions for the integration of large-scale renewable energy sources with the main power grids.The rapid development of the HVDC grid has resulted in a growing interest in DC circuit breakers(DCCBs).A fast and reliable circuit breaker is a necessary requirement in the development of large scale HVDC grids.This paper provides a comprehensive review and survey of the HVDC CBs and discusses potential research directions.Operational principles and the main features of various DCCBs are described and their merits and shortcomings are also highlighted.

A Similarity Comparison Based Pilot Protection Scheme for VSC-HVDC Grids Considering Fault Current Limiting Strategy

In the voltage source converter based high-voltage direct current(VSC-HVDC)grids,fast and reliable protections are the key technologies.The traditional protection schemes are easily affected by fault resistance,line distributed capacitance,etc.Meanwhile,the influence of fault current limiting strategy(FCLS)has not been fully considered.In this paper,the fault characteristics under FCLS and the feasibility of traditional travelling wave protections are analyzed.To improve the reliability and sensibility,a similarity comparison based pilot protection scheme is proposed,which focuses on the relationship between the fault characteristics and the state of the protected transmission line,with the establishment of a precise frequencydependent transmission line model.The criteria based on the similarity comparison calculated by cross-wavelet can identify the fault effectively.Meanwhile,the protection scheme can also endure the influence of error synchronization.Finally,the protection performance is verified in the PSCAD/EMTDC under different fault conditions.

Current trajectory image-based protection algorithm for transmission lines connected to MMC-HVDC stations using CA-CNN

In the presence of an MMC-HVDC system,current differential protection(CDP)has the risk of failure in operation under an internal fault.In addition,CDP may also incur security issues in the presence of current transformer(CT)saturation and outliers.In this paper,a current trajectory image-based protection algorithm is proposed for AC lines connected to MMC-HVDC stations using a convolution neural network improved by a channel attention mechanism(CA-CNN).Taking the dual differential currents as two-dimensional coordinates of the moving point,the moving-point trajectories formed by differential currents have significant differences under internal and external faults.Therefore,internal faults can be identified using image recognition based on CA-CNN.This is improved by a channel attention mechanism,data augmentation,and adaptive learning rate.In comparison with other machine learning algorithms,the feature extraction ability and accuracy of CA-CNN are greatly improved.Various fault conditions like different net-work structures,operation modes,fault resistances,outliers,and current transformer saturation,are fully considered to verify the superiority of the proposed protection algorithm.The results confirm that the proposed current trajectory image-based protection algorithm has strong learning and generalizability,and can identify internal faults reliably.

Small-signal Stability Analysis and Improvement with Phase-shift Phase-locked Loop Based on Back Electromotive Force Observer for VSC-HVDC in Weak Grids

Voltage source converter based high-voltage direct current(VSC-HVDC)transmission technology has been extensively employed in power systems with a high penetration of renewable energy resources.However,connecting a voltage source converter(VSC)to an AC weak grid may cause the converter system to become unstable.In this paper,a phase-shift phaselocked loop(PS-PLL)is proposed wherein a back electromotive force(BEMF)observer is added to the conventional phaselocked loop(PLL).The BEMF observer is used to observe the voltage of the infinite grid in the stationaryαβframe,which avoids the problem of inaccurate observations of the grid voltage in the dq frame that are caused by the output phase angle errors of the PLL.The VSC using the PS-PLL can operate as if it is facing a strong grid,thus enhancing the stability of the VSC-HVDC system.The proposed PS-PLL only needs to be properly modified on the basis of a traditional PLL,which makes it easy to implement.In addition,because it is difficult to obtain the exact impedance of the grid,the influence of shortcircuit ratio(SCR)estimation errors on the performance of the PS-PLL is also studied.The effectiveness of the proposed PSPLL is verified by the small-signal stability analysis and timedomain simulation.

Wide Speed Range Permanent Magnet Synchronous Generator Design for a DC Power System

A wide speed range permanent magnet synchronous generator(PMSG)system is studied in this paper,including the PMSG design and comparative study on control strategies with a pulse width modulation(PWM)rectifier,the purpose of which is to regulate the DC-link voltage.It is of great importance to study the foregoing DC power system based on the PMSG and PWM rectifier,where vector control(VC)can be implemented and the corresponding field-weakening strategy can be realized by injecting a field-weakening current component without any auxiliary devices.Large machine inductance is desired in order to limit the short-circuit current and the loaded voltage drop.Different control strategies including VC,direct torque control(DTC)and direct voltage control(DVC)are studied and compared with both simulations and experiments.

Wind turbines output power smoothing using embedded energy storage systems

The ability of an energy storage system to improve the performance of a wind turbine(WT)with a fully rated converter was evaluated,where the energy storage device is embedded in the direct current(dc)link with a bidirectional dc/dc converter.Coordinated dc voltage control design of the line-side converter and the energy storage dc/dc converters was proposed using a common dc voltage measurement for smoothing the output power.A transfer function and Bode diagram were introduced to analyze the system performance with different control parameters.MATLAB/Simulink simulations are presented to demonstrate the effectiveness of the proposed methods.It was found that the proposed methods smooth the power output from the WT to the grid and thus improve the quality of the generated power.