Abstract:In this paper,several principle questions of half-wavelength transmission are studied,including power frequency overvoltage in steady-state operation,power frequency overvoltage caused by faults,static power angle synchronization stability and transient power angle synchronization stability.The circuit model of the long-distance transmission system is established at first for steady-state and transient analysis.The sending-end system and the receiving-end system are both considered in the model.A test system based on an actual transmission line is given to facilitate the description of system characteristics.Based on the circuit model,the resonant transmission distance of the system is found and calculated.Theoretical analysis and numerical calculations are carried out to determine the feasibility transmission distance.It is demonstrated that the transmission distance should be in a certain range,which is larger than the resonant transmission distance,to satisfy the steady-state overvoltage and the small signal synchronization stability as well as the frequency deviation constraints.For transmission distances in the feasible range,the three-phase short circuit fault at a certain point of the transmission line will cause the most serious transient power-frequency overvoltage,and the system is very likely to lose synchronization stability.Considering the transient power-frequency overvoltage and the transient synchronization stability,the half-wavelength transmission system is technically impossible to operate.

Abstract:In order to solve the problems of power supply capacity improvement and UHVDC power dissipation in Southern Suzhou 500 kV power grid,the first unified power flow controller (UPFC) project applied in 500 kV power gird is put into operation.Based on the application of Southern Suzhou 500 kV power grid UPFC project and the analysis of power flow regulation demand,this paper sorts out the key control performance indexes and control function requirements of UPFC system.Finally,through system commissioning,the control functions and performances of the UPFC are tested,it mainly includes UPFC basic control performance indexes such as steady-state control precision and dynamic response,N-1 emergency power control function,UPFC locking and automatic restarting control function,which are used to test control ability and operation effect of Southern Suzhou 500 kV power grid UPFC project,it accumulates important practice experience and data for the construction of the follow-up engineering and standard establishment.

Abstract:Fast and accurate fault location is the basis of fast self-healing of power grid and the basic key technology to realize intelligent power distribution network.In recent years,many scholars at home and abroad have conducted in-depth research on fault location and achieved fruitful results.The existing research on fault location of distribution network is reviewed.The fault zone positioning method using transient electrical characteristics and unsteady electrical characteristics,the fault location method based on traveling wave method,impedance method and other related parameter identification.The superiority and existing problems of all kinds of methods are reviewed.Combined with the research status at home and abroad,the development trend and the research on the future of distribution network fault location is prospected.

Abstract:Load forecasting is the basis for planning of the grid.The accuracy of the forecast is related to the safe,reliable and economic operation of the grid.In order to solve the problem of low prediction accuracy due to the unstable original data,this paper proposes an ARIMA prediction model based on the ensemble empirical mode decomposition (EEMD),which performs the noise processing on the monthly load of a certain area,and then performs empirical mode decomposition to make the components smoothed.Each component is predicted by ARIMA model.Finally,the component prediction results are added to obtain the final predicted value.The example shows that the prediction accuracy of the regional monthly load of the EEMD-ARIMA model is higher than that of the ARIMA model.

Abstract:The accuracy of power costumer baseline load (CBL) prediction will greatly affect the evaluation of demand response.In this paper,taking multidimensional electricity behavior and its influencing factors into consideration,a user differentiated baseline load prediction method based on load subdivision is proposed by elaborating user behavior characteristics.This method uses Ward-FCM cluster analysis and combines the load characteristic index to improve the effect of user load curve cluster analysis.Based on the analysis of multi-dimensional influencing factors such as meteorology and time,a refined analysis method of electricity consumption behavior is put forward considering urban micro meteorological factors such as temperature,humidity and temperature cumulative effect and social behavior factors during holidays.The temperature-sensitive,holiday-sensitive and both-insensitive differential power behaviors are obtained.On this basis the customer baseline load forecasting analysis of differentiated power consumption is performed and a comprehensive evaluation model is established to analyze the prediction accuracy.The analysis results show that the proposed algorithm can further improve the accuracy of baseline load forecasting.

Abstract:As the strategy of electric energy substitution continues to advance,the scale of electric energy substitution will continue to expand,and it will have a great impact on the constitution of the medium and long term power load.The existing medium and long term load forecasting method has not fully considered the electric energy substitution,and it is necessary to carry out a separate analysis.The important impact of the policy on the future development of electric energy substitution is analyzed,and the calculation method of electric energy substitution is given.On the basis of above,based on the Logistic model,the total amount of the terminal energy consumption and the proportion of the electricity in the terminal energy consumption are predicted respectively.In the process of forecasting,the existing research results are fully integrated and the credibility of the forecast results is improved.Finally,the forecast result of electric energy substitution can be obtained.This model can be used as a supplement to traditional medium and long term load forecasting and to guide future power,grid and associated energy planning.

Abstract:XLPE cables have become an important part of urban transmission and distribution networks in recent years.As cable systems become more complex,simple withstand voltage test can not meet the needs of cable fault detection.However,the traditional frequency-tuned resonant system has a large amount of pulse interference,which makes it impossible to use for partial discharge (PD) measurement.It introduces an improved frequency-tuned resonant system,using pulse width modulation and time domain elimination to achieve PD measurement and withstand test at the same time.The effectiveness of the system is verified by experiments.

Abstract:To scientifically and reasonably evaluate the severity of voltage sags,this paper presents a multi-level evaluation method of voltage sag based on combination of improved analytic hierarchy process and entropy weight method.This method calculates the relative importance of the duration and sag amplitude of each sag event by constructing comparison matrix,judgment matrix and quasi-optimal consistent matrix,and gives the objective weight of these two eigenvalues.From the perspective of single voltage sag event and monitoring station,this method evaluates the severity of voltage sag in different levels.The calculation results obtained by this evaluation method well correspond to actual severity of voltage sag.

Abstract:The existing power system planning and operation mainly considers the uncertainty of the single renewable energy output,and there are few studies considering the correlation between renewable energy outputs.In this paper,a multi Markov Chain Monte Carlo method is proposed to predict the PV power time series.This method establishes several Markov chains that obey the complete conditional distribution to each other to simulate the random variation of the atmosphere over the PV plants.These Markov chains can fully retain the correlation between the PV plants.This paper simulates the time series of three sets of PV plants with different correlation levels using multi MCMC method.It is proved that this method can inherit the general statistical characteristics of historical sequences more accurately than the original MCMC method and can more effectively reflect the characteristics of the interaction between PV power plants.The multi MCMC method is more suitable for future power electrical system planning and operation design.

Abstract:High-voltage DC circuit breaker is one of the core equipment in multi-terminal DC transmission and DC grid technology.Existing DC circuit breakers based on capacitors are compared and analyzed in this paper,and advantages,disadvantages and applications of them are also pointed out.Then,an assembly DC circuit breaker based on pre-charged capacitors is proposed.The topology,working principle and control strategy of the proposed scheme are described in detail.Besides,the feasibility and effectiveness are validated by building a three-terminal ring DC model in PLECS.Finally,future research directions and problems to be solved for DC circuit breaker based on capacitors are pointed out.

Abstract:Reactive power optimization in distributed network with distributed generation is a complex nonlinear optimization problem.In this paper,the improved particle swarm optimization algorithm is used for the reactive power optimization of distribution network.The optimization model is established with the minimum system loss and average deviation of voltage as the objective function and the node voltage and capacitor switching capacity as the constraints.The position variance is introduced into the particle swarm optimization to prevent the particle swarm algorithm from falling into the local optimal solution.The particle is mutated according to the fitness value of the particle in the population,and the performance of algorithm is improved on the basis of guaranteeing the convergence speed of the algorithm.IEEE14 node distribution system with distributed generation as an example is simulated,the results demonstrate that distributed generation can enhance the stability of the power grid and the algorithm has better optimization performance.

Abstract:In order to eliminate the risk on the safe operation of power grid caused by the internal defects of composite insulators.A water capsule flexible coupling method based on ultrasonic phased array is proposed for small diameter composite insulator.This method uses the silicone rubber film and the coupling device to achieve good ultrasonic coupling.The defects detection experiments of the plane silicone rubber,composite insulator sheath and core were carried out respectively.Experimental results show that the linear scanning imaging is better than the sector scan and defects of the diameter of 0.8 mm can be detected,which provides a new method for the ultrasonic defects detection of composite insulator umbrella group.For the composite insulator sheath and core,the method of water capsule coupling can detect defects with a diameter of 0.8 mm and 1.0 mm through the phased array sector scanning.It provides the possibility for the on-line detection of composite insulator.

Abstract:Controlling air conditioning load to participate in the power grid load shifting has great potential for maintaining stable operation of the system,reducing the reserve of generators,and improving the efficiency of the grid.Based on the principle of the household decentralized air conditioner,a simplified air conditioning room temperature adjustment model was built in the MATLAB/Simulink environment.Based on the model,the characteristics of the fixed/inverter air conditioner temperature adjustment and influence of adjusting frequency conversion interval were studied in turn.Considering characteristics of air conditioners,a strategy is proposed to conduct an adaptively complementary operation of the air conditioner according to grid load.Finally,the model simulation test based on RT-LAB real-time simulation platform is carried out,which verifies the correctness of the model and effectiveness of the strategy to participate air conditioners in peak shaving control.

Abstract:There are stability risks after PV plants drop into island operation,especially in casa of unintentional islanding caused by grid faults and the islanding-state detection device fails.In order to estimate those risks,firstly,the paper theoretically analyzes the influence of three factors (power difference,PV permeability and motor inertia) on the stability characteristics of the island system in a short time after PV unintentionally dropped into island operation.Then the electromagnetic transient model of the PV station is established.According to that,the modified Dali Grid integrated with a PV station is modeled in PSCAD/EMTDC,verifying the theoretical analysis conclusions by simulation.Finally,quantitative stability risk index and method about PV station dropping into islanding are proposed according to the simulation results,which provide the theoretical basis for the stability risk estimation.

Abstract:The auxiliary material is often used to reduce the resistance of transmission line tower grounding grid for the limitation of the extension and vertical grounding resistance reduction measures.For the lack of unified standard guidance when grounding modules are used in the actual tower grounding project,some problems occurred in application such as blind construction and low reduction efficiency.In this paper,the common lightning protection software CDEGS is used to simulate and calculate the influencing factors when grounding module is used to reduce the resistance of transmission line tower grounding grid.First of all,simulation is carried out for the laying position and density of the grounding module in the"-"shaped grounding grid.Secondly,the efficiency of grounding module in the grounding grid is analyzed through the simulation.Then,the influence rule of soil resistivity and soil structure on the grounding module reduction efficiency is analyzed.Finally,the corresponding construction suggestions are given for the actual transmission line using grounding module to reduce the resistance.The conclusions of this paper can provide references for the lightning protection of transmission lines and the construction of the grounding grid of towers.

Abstract:With large-scale injection of new energy into power system and the development of demand side enabling technology the scheduling technology with flexible loads becomes a hot issue on automatic dispatching field.Therefore,this paper proposes a day-ahead scheduling technology covering 3 kinds of typical flexible loads,which provides technical support for the coordinated scheduling of resources on all of generating,grid and demand sides.Based on the existing automatic dispatching system,a day-head scheduling framework is designed,considering flexible loads.Flexible loads are classified according to the users'energy consumption characteristics,and their optimization models are built.At last,based on the provincial power system data,day-ahead scheduling simulations are conducted.With these simulation results,the influences of flexible loads on system are analyzed and the mentioned scheduling technology is verified.

Abstract:Modular multilevel converter (MMC) has been widely applied in the VSC-HVDC (voltage source converter-high voltage direct current transmission) field.In MMC application,it is very difficult to upgrade the program of the sub-modules of MMC.The structure of VSC-HVDC control system and sub-module of MMC is described,the program function of sub-module controller is analyzed.A method of program remote upgrade for MMC is introduced,the design of main hardware modules and software flowchart of the program upgrading system is presented.Safety and reliability evaluation of program remote upgrade for MMC is also described.This work is applied and verified based on dynamic simulation experiment platform for MMC research.

Abstract:In order to satisfy the requirements of energy saving and process adjustment in oil field,the frequency converters and other non-linear power electronic equipments are widely used,and the harmonic pollution of oil field distribution network is raising serious.The harmonic on-line monitoring system lays a foundation for measuring and analyzing the harmonic distribution characteristics of oil field power distribution network.However,due to the economic constraints of application,the installation quantity of harmonic measuring terminal is limited,so it is necessary to optimize its installation position.Considering both the observability and the redundancy of harmonic state estimation,a method for the optimization of measuring terminal addressing is proposed.The observability and redundancy of the system under different harmonic measurement terminal configurations are calculated by using node correlation matrix and observability logic judgment method of harmonic state estimation.On the premise of ensuring the maximum observability of the system,the installation position of the measuring terminal is optimized with the maximum redundancy.In the case of limited number of terminals,the considerable measure and precision of harmonic state estimation of oilfield power distribution network can be improved.Simulation results show that the method is effective.

Abstract:In order to meet the rapid adjustment needs of power grid operation after accidents,a new model of transient stability constrained optimal power flow (TSCOPF) is established.The model is to achieve the objective of minimum power adjustment.Furthermore,variable step strategy is adopted to adjust the steps of before and after clearing fault separately.The objective of the model is closely related to the degree of system instability.It solves the problem that the power adjustment of the traditional TSCOPF is too large.To ensure that the fault cleaning time and simulation end time can be simulated accurately,the two stages of the variable step strategy makes the numerical method reach the key time with integer steps.Primal-dual interior point method is used to solve the problem.Numerical simulations on the three test systems,IEEE-9,NE-39 and IEEE-118,have shown that the proposed method is helpful for dispatchers to deal with accidents quickly which improves the practical level of TSCOPF.

Abstract:In order to adapt to the transmission mode with digital sampling value,to solve the problems that the phase detection is not easy to be implemented by the lack of professional phase detection equipment for intelligent substation,this paper proposes a distributed phase detection system for intelligent substation which based on WIFI wireless transmission network.Introduces the system architecture and implementation scheme of the phase detection system,researches the key technology of distributed digital phase detection,such as adaptive transmission of sampling value,area synchronization with wireless and phase measurement.The experiments show that the phase acquisition precision of the system based on wireless communication is high,the result of the phase detection is accurate and the operation is convenient,it can meet the functional requirements of the whole substation phase detection in the intelligent substation.

Abstract:In practical engineering,the cross and crossing of AC and DC lines are more common at present.The communication lines are complex,especially in the South.With many DC projects,the cross over phenomenon of AC and DC in the China Southern Power Grid is more obvious.This paper is based on the practical PSCAD simulation model of Jin-Zhong converter station,built a simulation model of AC and DC concomitant systems,and selecte 220 kV and 500 kV communication systems as typical scenarios for analysis by simulation of metal touch faults outside the AC transformer protection range.According to the simulation waveform,the characteristic analysis results of transformer differential protection are given in the end.

Abstract:The co-phase power supply system can provide the compensation of negative-sequence current,reactive current and harmonic current and solve the problem of electrical separation in the traction network.This paper introduces the working principle of single-phase combined co-phase power supply system and train regenerative braking.By establishing a simulation model,the influence of three-phase unbalance,power factor and harmonics on co-phase power supply system and three-phase power network is analyzed in regenerative braking condition.And the effectiveness of the co-phase power supply system in solving the power quality problems without change the control strategy in regenerative braking is verified.

Abstract:In terms of large scale wind power integrated into the power system.Based on the framework of regional energy system,a heat storage control method for thermal system is proposed in this paper.By introducing heat storage device and distributed heat pump this method can improve the operational flexibility of cogeneration unit and the peak regulation demand of wind power is met.At the same time,the distributed heat pump can further improve the peak regulation capacity of the system and improve the wind power consumption.Finally,considering the characteristics of building heat storage and human thermal comfort,it can improve the flexibility of heat storage control and realize the effective tracking of wind power fluctuation.The simulation results show the effect of heat storage control method on the peak regulation,and also analyze the influence on peak regulation of wind power with different heat load characteristics.

Abstract:The entrance/exiting of converter is one of the key techniques of UHVDC project.When telecommunication between stations is lost,the exiting of one converter at rectifier side due to fault sometimes would cause overvoltages of the rest converter in series.In this paper,the overvoltages are analyzed from the perspective of control strategy and an optimized strategy,which adjusts the voltage reference at rectifier side and current margin at inverter side respectively,is purposed.The real time digital simulator (RTDS) simulation results prove that the optimized strategy can effectively suppress the overvoltages under the circumstances mentioned above and can be adopted in future UHVDC projects.

Abstract:In order to analyze the deep cause of fault for surface fitting state between stress cone and cable body,this paper carries out cable disintegration,X-ray inspection,material inspection and simulation work for a 220 kV fault cable terminal.It is find that the interference fit on joint stress cone is higher than the installation process requirement,and the lap joint is not flat.When the stress cone and cable body surface are well bonded,the maximum electric field strength of the stress cone and the main insulation of the cable under operating voltage are much smaller than the breakdown field strength of the corresponding insulating material.When there is a small air gap between the stress cone and the cable surface,the internal electric field strength of the air gap is greater than the air breakdown field strength under the operating voltage,indicating that there is a discharge phenomenon inside the air gap under the operating voltage.Therefore,when the cable terminal stress cone,the cable surface is poorly attached,and there is a slight air gap at the interface,the main insulation damage caused by long-term discharge inside the air gap is the cause of the cable failure under the operating voltage.

Abstract:The series reactor can limit the short circuit current of the power grid,but it also brings challenges to the relay protection.An improved setting method for the distance protection of transmission lines with series reactor is presented.The compensation coefficient of zero sequence current is adjusted according to the series reactor's impedance and line's impedance.The distance protection settings are calculated according to the series reactor's impedance and the original distance protection impedance.The improved setting method can guarantees the reliability and sensitivity of distance protection just by modifying the settings and without changing the code.The simulation results demonstrate the effectiveness of this method.