Two-stage ADMM-based distributed optimal reactive power control method for wind farms considering wake effects

Since the connection of small-scale wind farms to distribution networks,power grid voltage stability has been reduced with increasing wind penetration in recent years,owing to the variable reactive power consumption of wind generators.In this study,a two-stage reactive power optimization method based on the alternating direction method of multipliers(ADMM)algorithm is proposed for achieving optimal reactive power dispatch in wind farm-integrated distribution systems.Unlike existing optimal reactive power control methods,the proposed method enables distributed reactive power flow optimization with a two-stage optimization structure.Furthermore,under the partition concept,the consensus protocol is not needed to solve the optimization problems.In this method,the influence of the wake effect of each wind turbine is also considered in the control design.Simulation results for a mid-voltage distribution system based on MATLAB verified the effectiveness of the proposed method.

Power Flow Model for Medium-voltage Distribution Systems Considering Measurement and Structure Characteristics

Medium-voltage distribution systems(MVDSs)mainly consist of a feeder head,lines,distribution transformers,and the equivalent load or power supply interfaced with the distribution transformers.The information of such load or power supply can be measured via the three-wattmeter method(THM)and the two-wattmeter method(TWM).The measurements can be used to perform the control of the power supply and simulate the characteristics of the load,so the models of the load and the power supply need to consider the measurement characteristics.Existing research works on three-phase power flow(PF)just consider the measurement characteristics of THM.Hence,the PF equation of the bus measured via TWM is firstly built.Based on conventional measurements,an accurate and general model of the grounded and ungrounded slack bus is proposed.Furthermore,the influence arising from the connection type and angle shift of distribution transformers on the admittance matrix is considered,and thus a general three-phase transformer model is summarized,which is applicable for all the transformers mentioned herein.Finally,Newton's method is adopted to solve the PF calculation,and the performance of the proposed PF model is demonstrated through designed tests.

±1100kV特高压直流换流站绝缘配合关键问题研究

换流站的绝缘配合设计是±1 100kV特高压直流输电工程实施中的关键技术之一,对换流站设备设计、选型、制造和试验均具有重要的指导作用。为合理确定±1 100kV换流站设备的绝缘水平,降低工程的技术难度并提高经济性,基于准东-成都±1 100kV特高压直流输电工程,对±1 100kV特高压换流站的避雷器布置方案和避雷器选型这两个关键问题进行了研究。研究结果表明:对于1 100kV特高压换流站,推荐以±800kV换流站避雷器布置方案为基础,在高端换流变压器阀侧增加A2避雷器,可以将高端换流变压器阀侧套管绝缘水平从2 250kV降低至2 100kV,高压直流1 100kV穿墙套管的绝缘水平从2 250kV降低至2 150kV;采用特高压交流1 000kV高性能4柱并联避雷器电阻片,可进一步将高端换流变压器阀侧套管绝缘水平降至2 000kV,高压直流1 100kV穿墙套管的绝缘水平降低至2 000kV。

A novel two-degree-of-freedom spherical ultrasonic motor using three travelling-wave type annular stators

In order to promote the tolerance and controllability of the multi-degree-of-freedom(M-DOF) ultrasonic motor, a novel two-degree-of-freedom(2-DOF) spherical ultrasonic motor using three traveling-wave type annular stators was put forward. Firstly,the structure and working principle of this motor were introduced, especially a spiral spring as the preload applied component was designed for adaptive adjustment. Then, the friction drive model of 2-DOF spherical motor was built up from spatial geometric relation between three annular stators and the spherical rotor which was used to analyze the mechanical characteristics of the motor.The optimal control strategy for minimum norm solution of three stators' angular velocity was proposed, using Moore-Penrose generalized inverse matrix. Finally, a 2-DOF prototype was fabricated and tested, which ran stably and controllably. The maximum no-load velocity and stall torque are 92 r/min and 90 m N·m, respectively. The 2-DOF spherical ultrasonic motor has compact structure, easy assembly, good performance and stable operation.

Comparison of Formation Mechanism Between Helium and Argon Atmospheric Pressure Plasma Jets

To compare the formation mechanisms of He and Ar atmospheric pressure plasma jets(APPJs),an intensified charge-coupled device(ICCD)are utilized to observe the dynamic process of APPJ.The experimental results show that,He APPJ is first ignited,which is independent of the dielectric barrier discharge(DBD)between the two wrapped electrodes when the high voltage placed at the downstream.The intensity and APPJ length under positive discharge pulses are bigger than that under negative discharge pulses due to the space charge effect.The He APPJ is formed by the DBD development when the high-voltage electrode placed at the upstream side of tube.However,the plasma plume in Ar APPJ is formed by the propagation of DBD whatever the high-voltage electrode is arranged on upstream or downstream side of ground electrode.The difference in formation mechanism between He and Ar APPJs is mainly caused by the gas properties.Moreover,during the discharges,Ar tends to lead to thermal instability and electron Maxwellian instability.

Key stress extraction and equivalent test method for hybrid DC circuit breaker

Firstly, relevant stress properties of millisecond level breaking process and microsecond level commutation process of hybrid HVDC circuit breaker are studied in detail on the basis of the analysis for the application environment and topological structure and operating principles of hybrid circuit breakers, and key stress parameters in transient state process of two time dimensions are extracted. The established digital simulation circuit for PSCAD/EMTDC device-level operation of the circuit breaker has verified the stress properties of millisecond level breaking process and microsecond level commutation process. Then, equivalent test method, circuits and parameters based on LC power supply are proposed on the basis of stress extraction. Finally, the results of implemented breaking tests for complete 200 kV circuit breaker, 100 kV and 50 kV circuit breaker units, as well as single power electronic module have verified the accuracy of the simulation circuit and mathematical analysis. The result of this paper can be a guide to electrical structure and test system design of hybrid HVDC circuit breaker.

Edge Computing-Based Tasks Offloading and Block Caching for Mobile Blockchain

Internet of Things(IoT)technology is rapidly evolving,but there is no trusted platform to protect user privacy,protect information between different IoT domains,and promote edge processing.Therefore,we integrate the blockchain technology into constructing trusted IoT platforms.However,the application of blockchain in IoT is hampered by the challenges posed by heavy computing processes.To solve the problem,we put forward a blockchain framework based on mobile edge computing,in which the blockchain mining tasks can be offloaded to nearby nodes or the edge computing service providers and the encrypted hashes of blocks can be cached in the edge computing service providers.Moreover,we model the process of offloading and caching to ensure that both edge nodes and edge computing service providers obtain the maximum profit based on game theory and auction theory.Finally,the proposed mechanism is compared with the centralized mode,mode A(all the miners offload their tasks to the edge computing service providers),and mode B(all the miners offload their tasks to a group of neighbor devices).Simulation results show that under our mechanism,mining networks obtain more profits and consume less time on average.

Mathematical methods for main tenance and operation cost prediction based on transfer learning in State Grid

The electric power enterprise is an important basic energy industry for national development,and it is also the first basic industry of the national economy.With the continuous expansion of State Grid,the progressively complex operating conditions,and the increasing scope and frequency of data collection,how to make reasonable use of electrical big data,improve utilization,and provide a theoretical basis for the reliability of State Grid operation,has become a new research hot spot.Since electrical data has the characteristics of large volume,multiple types,low-value density,and fast processing speed,it is a challenge to mine and analyze it deeply,extract valuable information efficiently,and serve for the actual problem.According to the features of these data,this paper uses artificial intelligence methods such as time series and support vector regression to establish a data mining network model for standard cost prediction through transfer learning.The experimental results show that the model in this paper obtains better prediction results on a small sample data set,which verifies the feasibility of the deep transfer model.Compared with activity-based costing and the traditional prediction method,the average absolute error of the proposed method is reduced by 10%,which is effective and superior.

Overvoltage and Insulation Coordination for ±1 100 kV UHVDC Converter Station

In order to specify the overvoltage and insulation level of equipments in ±1 100 kV UHVDC converter stations,the arrester configuration scheme and some basic parameters for ±1 100 kV Zhundong converter station are proposed.Overvoltage of equipments in AC system,valve hall,DC busbar,and neutral busbar are also calculated in accordance with the fundamental principles of arrester configuration for UHV converter stations and the existing experiences in insulation coordination of ±800 kV UHVDC converter stations.The work is done also for specifing insulation levels for ±1 100 kV UHVDC converter stations of the ±1 100 kV UHVDC power transmission project from Zhundong to Sichuan in China.Thus,the recommended insulation margins that determine the insulation levels of converter station equipments are proposed: the insulation margins for thyristor valves are 10%/10%/15% for switching impulse/ lightning impulse/ steer front impulse; the insulation levels of lighting and swtiching impulses are recommended as 2 600 kV and 2 150 kV respectively for 1 100 kV DC buses,and as 2 250 kV and 2 100 kV respectively for equipments at the valve side of high-voltage Y/Y converter transformers.

Optimal Scheduling for Flexible Regional Integrated Energy Systemwith Soft Open Point

The Regional Integrated Energy System(RIES)has brought new modes of development,utilization,conversion,storage of energy.The introduction of Soft Open Point(SOP)and the application of Power to Gas(P2G)technology will greatly deepen the coupling of the electricity-gas integrated energy system,improve the flexibility and safety of the operation of the power system,and bring a deal of benefits to the power system.On this background,an optimal dispatch model of RIES combined cold,heat,gas and electricity with SOP is proposed.Firstly,RIES architecture with SOP and P2G is designed and its mathematical model also is built.Secondly,on the basis of considering the optimal scheduling of combined cold,heat,gas and electricity,the optimal scheduling model for RIES was established.After that,the original model is transformed into a mixed-integer second-order cone programming model by using linearization and second-order cone relaxation techniques,and the CPLEX solver is invoked to solve the optimization problem.Finally,the modified IEEE 33-bus systemis used to analyze the benefits of SOP,P2G technology and lithium bromide absorption chillers in reducing systemnetwork loss and cost,as well as improving the system’s ability to absorb wind and solar and operating safety.

The Performance of Lightning Rod Arrester and Its Effect on the Top Potential of Tower for 500 kV Transmission System

When the tower overhead is struck by the lightning impulse, the lightning current flows into the earth through the impedance of tower and the grounding resistance, which heightens potential of the tower overhead and possibly induces insulator flashover. Lighting rod arrester is used to shield the tower and provide another routine for lightning current, decreasing the potential of tower overhead. In this paper, the performance of a 500 kV lighting rod arrester is tested used in AC transmission system under the current impulse. Besides, the influence of the lightning rod arrester performance on the top potential of tower is also studied. The results show that, when the rod arrester is connected to the tower, the top potential of tower can be obviously limited under the lightning strike.

Optimal operation of electricity, natural gas and heat systems considering integrated demand responses and diversified storage devices

In recent years, the increasing penetration level of renewable generation and combined heat and power(CHP) technology in power systems is leading to significant changes in energy production and consumption patterns. As a result, the integrated planning and optimal operation of a multi-carrier energy(MCE) system have aroused widespread concern for reasonable utilization of multiple energy resources and efficient accommodation of renewable energy sources. In this context, an integrated demand response(IDR) scheme is designed to coordinate the operation of power to gas(P2 G) devices, heat pumps,diversified storage devices and flexible loads within an extended modeling framework of energy hubs. Subsequently, the optimal dispatch of interconnected electricity,natural gas and heat systems is implemented considering the interactions among multiple energy carriers by utilizing the bi-level optimization method. Finally, the proposed method is demonstrated with a 4-bus multi-energy systemand a larger test case comprised of a revised IEEE 118-bus power system and a 20-bus Belgian natural gas system.

Generation dispatch method based on maximum entropy principle for power systems with high penetration of wind power

This paper proposes a generation dispatch model based on the maximum entropy principle. The objective is to find an optimal generation dispatch strategy that minimizes the generation cost and satisfies the security constraints of power systems, while taking into account the uncertainty of wind power. Since in many situations, only partial information of the probabilistic variables can be obtained, the maximum entropy principle is introduced to find the most likely realized probability distributions of the power flow, thus providing an accurate probabilistic circumstance to solve the generation dispatch model. The proposed method is tested on the IEEE 39-bus system, and is compared with the methodologies based on Monte Carlo simulation and Gram-Charlier expansions.

Low frequency oscillation characteristics of East China Power Grid after commissioning of Huai-Hu ultra-high voltage alternating current project

The low frequency oscillation characteristics of East China Power Grid after commissioning of the first ultra-high voltage alternating current(UHVAC)project-the Huai-Hu UHVAC project are studied.Several low frequency oscillation cases occurred in East China Power Grid in the past few years are reviewed and summarized.Based on the analysis of the different typical operation modes,the main low frequency oscillation modes in East China Power Grid in the early stages of development of ultra-high voltage(UHV)are summarized,and the impacts of the significant power grid maintenance on low frequency oscillation characteristics are analyzed.Besides,the oscillation mode of UHV generators to East China Power Grid is researched,and the importance of the power system stabilizator(PSS)is emphasized.Furthermore,the comparative analysis between the time domain and the frequency domain is carried out,and the influences of the governing system on low frequency oscillation characteristics are revealed.Finally,both the focus and the direction of low frequency oscillation research are presented.

A new method for optimal FTU placement in distribution network under consideration of power service reliability

Modern distribution network with high penetration of intermittent renewable sources and the so-called prosumers requires more reliable distribution automation(DA) system for safe operation and control. The design of optimal feeder terminal units(FTU)placement is critical and economical for the effective DA application. Previously proposed solutions of optimal FTU placement aiming to ensure the accuracy of state estimation(SE), typically include the following two main shortcomings: 1) only to obtain the optimal FTU placement in quantity, and the analysis of FTU location is not considered yet; 2) few consider the uncertainty of intermittent power injections in the analysis of state estimation. In this paper, a modified methodology of FTU placement is proposed not only aiming to ensure the accuracy of state estimation with the minimum number of meters, but also finding those specific FTU locations to guarantee the power service reliability. Moreover, the uncertainty models of those intermittent power injections are also considered by using probability density function(PDF). The resultant optimization problem is addressed by using the covariance matrix adaptation evolution strategy(CMA-ES). Case studies demonstrate the feasibility and effectiveness of the proposed methodology.

Evaluation method of node importance for power grid considering inflow and outflow power

The effective identifications of important nodes in power grid are foundations of differentiated management and stable operation of power grid. However, the current studies on this field are not thorough enough. This paper applies the model based on co-citation(MBCC)-hypertext induced topic selection(HITS) algorithm used in web page ranking to power grid and proposes an index to determine node importance of power grid from the perspectives of inflow and outflow power. MBCC-HITS algorithm is modified considering load, power source, and line flow comprehensively for the differences between Internet and power grid in this paper. Then a method for evaluating node importance is proposed using the modified algorithm. Lastly, the rationality and validity of the proposed method are verified by comparisons with other methods in case studies of IEEE 14-bus and 118-bus systems.

Probabilistic production simulation of a power system with wind power penetration based on improved UGF techniques

Universal Generating Function(UGF)techniques have been applied to Multi-State System(MSS)reliability analysis,such as long term reserve expansion of power systems with high wind power penetration.However,using simple steady-state distribution models for wind power and large generating units in reliability assessment can yield pessimistic appraisals.To more accurately assess the power system reliability,UGF techniques are extended to dynamic probabilistic simulation analysis on two aspects of modelling improvement.Firstly,a principal component analysis(PCA)combined with a hierarchal clustering algorithm is used to achieve the salient and time-varying patterns of wind power,then a sequential UGF equivalent model of wind power output is established by an apportioning method.Secondly,other than the traditional two-state models,the conventional generator UGF equivalent model is established as a four discrete-state continuous-time Markov model by Lztransform.In the construction process of such a UGF model,the state values are transformed into the integral multiples of one common factor by choosing proper common factors,thus effectively restraining the exponential growth of its state number and alleviating the explosion thereof.The method is suitable for reliability assessment with dynamic probabilistic distributed random variables.In addition,by acquiring the clustering information of wind power,the system reliability indices,such as fuel cost and CO_(2) emissions through different seasons and on different workdays,are calculated.Finally,the effectiveness of the method is verified by a modified IEEE-RTS 79 system integrated with several wind farms of historical hourly wind power data of Zhangbei wind farm in North China.

A distributed normalized Nash equilibrium seeking algorithm for power allocation among micro-grids

In this paper, a power allocation problem based on the Cournot game and generalized Nash game is proposed. After integrating dynamic average consensus algorithm and distributed projection neural network through singular perturbation systems, a normalized Nash equilibrium seeking algorithm is presented to solve the proposed power allocation problem in a distributed way.Combine Lyapunov stability with the singular perturbation analysis, the convergence of the proposed algorithm is analyzed. A simulation on IEEE 118-bus confirms that the proposed distributed algorithm can adjust the power allocation according to different situations, while keeping the optimal solution within the feasible set.

Method to Quantifying the Logical Node Importance for IEC 61850 Based Substation Automation Systems

The problem of logical node(LN)importance quantification in an IEC 61850 based substation automation system(SAS)is investigated in this paper.First,a weighted and directed static complex network model is established by analyzing the characteristics of SAS,according to IEC 61850.Then,we propose a method,which combines topology value and information adjunction value by introducing a first-order linear feedback controller to quantify the value of LNs.On this basis,some definitions for equivalent network conversion are proposed to greatly reduce the complexity of the original network topology.Also,the absolute value and relative value are introduced to quantify LN importance from the perspective of the node’s necessity and influence,respectively.Finally,simulation results of the case study demonstrate that the proposed method is effective and provides a broader and clearer perspective for viewing the logical node importance for IEC61850 based SAS.