Abstract:With the rapid development of communication and control technology in power distribution system,it has gradually shown the typical characteristics of cyber-physical systems,which not only brings new opportunities for development,but also brings multiple types of risks in the information system to the power system. As a result,cyber-physical cascading failures in the power system have occurred frequently,leading to serious accidents. Therefore,analyzing and studying the evolution process and failure consequences of cyber-physical cascading failures and exploring the internal mechanism of cyber-physical cascading failures have important theoretical and practical significance. Based on the event-driven model,an architecture of power distribution cyber-physical system is proposed to analyze the interaction mechanism between physical and information systems,and a research framework for the evolution mechanism of cyber-physical cascading failures is proposed. Then,the importance of the internal nodes of the information system is studied,the total risk value of the system and the defense resources are comprehensively considered,and the relevant parameters such as the probabilities of the information nodes being successfully attacked are calculated and the correlation matrices are established. Finally,a simulation of the evolution mechanism of cyber-physical cascading failures is carried out on a case,which verifies the rationality of the proposed mechanism and its validity in the deduction of the failure process and the calculation of the consequences.

Abstract:The security and stability control system is an important defense line to ensure the reliable operation of the power grid. Serious physical consequences can be caused by cyber attacks against the security and stability control system. In order to quantitatively evaluate the impact of cyber attacks on security and stability control system and solve the problem that the existing risk assessment methods do not fully consider the susceptibility of cyber attacks,a risk assessment method of security and stability control system considering the impact of cyber attacks is proposed. Firstly,the hierarchical structure of the security and stability control system is analyzed. Then,the risk points of cyber attacks on the stability control device body and device's inter-station communication from the three perspectives of attack object,attack methods,and attack consequences are analyzed. Secondly,the susceptibility of cyber attacks is quantified based on the fuzzy analytic hierarchy process,and a successful probability model of cyber attacks for security and stability control system is established combined with the defense unit model of cyber attacks built by Petri nets. Finally,the risk assessment is carried out on the standard and actual systems combined with the physical consequences and the probability of successful attacks. The risk values under two conditions of normal operation and cyber attack are calculated to verify the validity of the proposed model.

Abstract:As a key link to ensure the quality of power grid monitoring data,the state estimation provides a reliable data basis for the energy management system. Considering the problems of the large measurement errors and the vulnerability to the cyberattacks in active distribution network,a distributed state estimation method for active distribution network with the false data injection attacks (FDIAs) is studied in this paper. Firstly,the internal state of the subsystem is estimated according to its own measurement,and it is corrected with the global information obtained by the average consensus algorithm,thus the fully-distributed state estimation is realized. Secondly,the weight function is introduced into the subsystem state estimation to modify the weight matrix of the objective extremum function dynamically for improving the robustness of the state estimation. Subsequently,the phasor measurement unit is configured on the boundary nodes and the nodes which are vulnerable to the FDIAs for enhancing the ability to identify the attacks. Finally,the simulations are performed on the IEEE 118-bus distribution network system for verification. The results show that the proposed state estimation method can not only reduce the estimation error effectively,but also identify the FDIAs accurately,which verifies that the accuracy of state estimation and the identification ability of FDIAs are improved.

Abstract:The flat design of security and stability control system (SSCS) can realize the information interaction between each control sub-station and each sub-grid,which is conducive to reducing the number of SSCS layers and improving its reliability,thus increasing the difficulty of searching the control paths and differentiating the vulnerable components. The bidirectional/loop communication channels are added to the tree-shape SSCS to reduce the intermediate stations and find all the paths from the top to bottom stations based on the adjacent matrix. Considering availability of components and controllable capacity of underlying stations,the control capability of the flat SSCS is newly quantified. Average control ability and average associated path number are defined to evaluate the influence of flattening on the control ability of each station. Analytical sensitivity model of the control capability of flat SSCS with respect to the components' availability is proposed to determine the vulnerable components. The numerical analysis shows that the flat design reduces levels and intermediate stations,and improves the controllability of the SSCS. The impact on the controllability of the SSCS increases by the number of paths and controllable power,but decreases by the number of layers.

Abstract:With the development of active distribution network and Internet of Things technology,the access of reactive power equipment is becoming more complicated and marginalized,and the related computing of voltage control is also developing towards edge computing. Due to the limited computing power,the pure software calculation of edge nodes takes a long time,which cannot meet the requirements of real-time control. Aiming at this problem,a distribution network voltage control strategy based on system on chip field programmable gate array (SoC FPGA) hardware parallel computing is proposed. Firstly,the software and hardware computing framework based on SoC FPGA is designed. Then,targeted improvements are made to the distribution network voltage control model and genetic algorithm solution method for FPGA computing. Finally,the FPGA hardware solution structure is designed in modules. The verification of example scenarios shows that,compared with the pure software solution method of edge nodes,the average solution efficiency of the proposed strategy in scenarios where voltage is lower than lower limit and where voltage exceeds upper limit is increased by 2.41 times and 2.15 times respectively,which can effectively improve the real-time performance of voltage control.

Abstract:Aiming at the requirements of data storage and sharing between side devices and among marketing distribution dispatching systems in cloud master station platform of power Internet of Things,a power data storage and sharing method based on blockchain and data lake is proposed. Firstly,the distributed power data storage architecture on edge layer is designed. Through ring signature and CryptoNote protocol,the data interaction between side device storage nodes is encrypted to realize the identity authentication of both parties,and the data sharing between nodes in power data security storage system is realized based on intelligent contract of blockchain. Then,the data sharing and access control model between the operation and dispatching platforms is constructed based on the data lake and the smart contract,in which the hash value of the data is stored in the blockchain,and the data is stored in the data lake,so as to realize the cross platform and cross domain secure sharing and access of data. Finally,an experimental platform is built to demonstrate the proposed method. The results show that the maximum storage delay time of the proposed method is short,and the throughput and security are also high,so it has a good application prospect.

Abstract:To solve the direct current (DC) bus overvoltage problem existing in low voltage ride through (LVRT) control of the photovoltaic (PV) grid-connected systems and improve the fault recovery capacity of the grid-connected system,a cost-effective LVRT control strategy based on flexible power point tracking (FPPT) is proposed in this paper. To begin with,the graphical approximation method is used to quickly estimate the operating voltage of the power curtailment point,and the position of the PV reference operating point during the fault process is deduced,based on which the rapid active power curtailment control strategy is proposed. Subsequently,the voltage against current curve of PV is fitted to estimate the PV short-circuit current and the voltage of the maximum power point is calculated by using the Lambert-W function,based on which the fast recovery control for faults is proposed. Finally,simulations and experiments are analyzed by Matlab and hardware-in-the-loop testing platform,respectively. The results prove that the proposed control strategy has good fault ride-through capability and fault recovery effect,which contributes to promote the application of grid-connected PV power generation technology.

Abstract:It is difficult to identify and locate the defects quickly when the long cable line cross-bonded box has defects such as grounding,water inflow and three-phase sequence error. Aiming at three types of typical defects of cross-bonded box,a cross-bonded model of long cable lines is established to analyze the amplitude and phase change rule of circulation under the defects. Then,the curve fitting of the ratio of circulation at both ends and the scatter data of defect box number are carried out to obtain the defect location formulas. Finally,it is verified by an example. The results show that the circulation amplitude of the defective phase increases and the non-defective phase is normal when the ground is grounded inside the box. When the phase sequence is wrong,the amplitude of two-phase circulation increases obviously,and the amplitude of the other phase circulation is smaller. When the tank is filled with water,the circulation amplitude of each phase increases and is basically equal. The fitting degree of the positioning formula is about 0.96,and the range of defect box number can be determined through the example verification. The proposed method is not affected by the change of wire core current and ground resistance,and can provide reference for the defect identification and location of long cable line cross-bonded box.

Abstract:The access to droop-controlled components makes the operation of the power system more complicated,and also brings new challenges to the power system analysis. Power flow calculation is the most basic and important type of calculation in power system analysis. When performing power flow calculations for power systems with droop control components,it is necessary to consider new problems that are not considered in conventional power flow calculations. The basic theory of droop control is introduced and the research on power flow calculation of power systems with droop control is summarized. Firstly,the principle of droop control,the types of main components,and the application scenarios of power flow calculation with droop control are briefly introduced. Secondly,the processing methods of droop-controlled components in power flow calculation are listed. Then,the questions of alternating current (AC) system power flow calculation with droop control are considered. In addition,the domestic and foreign solutions are also presented. Finally,combined with the new requirements of power flow calculation in power system,it is proposed that the power flow calculation considering droop control needs to establish a unified calculation model in modeling,and the mathematical programming method can be used for the algorithm,but it needs to be further improved according to the actual scene.

Abstract:To avoid possible conflicts between the maintenance scheduling and the unit commitment for hydro-thermal power systems under the traditional non-collaborative optimization mode,a collaborative maintenance scheduling and unit commitment model for hydro-thermal systems is proposed,in which the total cost of the system and the adjustment cost of maintenance scheduling as an objective are minimized with considering the coupling characteristics of cascade hydropower. Furthermore,the model is transformed into a mixed integer linear programming (MILP) model. To improve the solution of the MILP model,the objective scaling ensemble approach is proposed,including two stages. In the first stage,the coefficients of integer variables in the objective are scaled and the integer variables that may take 0 are probed. In the second stage,the reduced MILP model is solved after determining and fixing the integer variables with a value of 0. Finally,a 6-unit hydro-thermal power system and an actual provincial 86-unit hydro-thermal power system are taken as test systems. The results show that the proposed model can obtain a more economical and reasonable maintenance scheduling and unit commitment scheme compared with the non-collaborative optimization mode,thus effectively improving the solution speed by the objective scaling ensemble approach.

Abstract:There are some disadvantages in existing inverter-interfaced distributed generators (IIDG) low voltage ride-through control methods,such as unsatisfactory voltage support effects,single control objectives,and the risk of over-limiting the output current. In this paper,a low voltage ride-through control considering multiple control objectives is proposed. Firstly,the preset phase voltage is used as the constraint condition,and the voltage support equation is constructed to realize the voltage support control. Secondly,by restricting the magnitude of the negative sequence reactive current injected by the IIDG into the grid,the active output oscillation suppression control is realized. At the same time,to make full use of the remaining capacity of IIDG,the maximum active power output and current-limiting strategy are used to calculate the positive sequence active current reference value of the IIDG to ensure that the maximum active power output control is achieved under the premise that the output current of IIDG does not exceed the limit. The simulation results show that compared with the existing positive-sequence reactive current injection control method,the proposed low-voltage ride-through control method in this paper has a better fault control effect.

Abstract:Ground fault tests of ultra-high voltage direct current (UHVDC) transmission line indicate that voltage differential protection of the other pole may be false tripped when the ground fault happens on the line of the pole with a larger number of operating valve groups. In order to solve this problem,it is pointed out that the transmission velocity of common-mode of the fault traveling wave is less than that of differential-mode,and there is a time difference between common-mode and differential-mode to reach the protection measurements. Then,it is analyzed that the cause of the voltage differential protection false tripping is that the time difference between common-mode and differential-mode to reach the protection measurements is longer than the setting time of the protection. In addition,the polarity of common-mode component is used as the criterion to identify the fault pole and the protection of voltage mutation is improved accordingly. Finally,a real-time digital simulation model of UHVDC transmission is established to verify the correctness of the analysis results for the impoved method.

Abstract:Large scale new energy access to weak alternating current (AC) power grid is prone to transient overvoltage,phase-locked synchronous stability and other problems,which bring problems for the full use of new energy. In this paper,the influencing factors of the stability level of the new energy access system are explored. The short-circuit ratio index of new energy is used as the quantitative evaluation basis of the system voltage intensity,and the sensitivity of the short-circuit ratio of new energy generation power is derived. The comprehensive sensitivity index of the short-circuit ratio of new energy is constructed by comprehensively considering the sensitivity of the average short-circuit ratio and the lowest short-circuit ratio of the system,and an optimal power allocation control scheme for new energy generation based on equal comprehensive sensitivity criterion is proposed. Based on this method,the optimal control of new energy power generation is carried out,and the full utilization of renewable energy is realized on the basis of ensuring the security and stability of the system. Finally,the feasibility and effectiveness of the proposed method are verified by taking a new energy centralized transmission system in Northwest China as an example.

Abstract:In order to enhance the flexibility and reliability of multi-energy microgrids with ice storage systems,integrate with distributed generations and energy storage systems effectively,an autonomous cooperative control is proposed in this paper,which consists of control of alternating current (AC) and direct current (DC) microgrids and interlinking converters (ICs). Slack terminals in AC and DC microgrids adopt P-f and P-V droop control respectively. The control of the IC with AC microgrid consists of interlinking power control loop,virtual synchronous control and inner voltage control. The control of the IC with DC microgrid includes interlinking power control and phase shift control. With the proposed method,it is possible to achieve followings. When electric energy is abundant,ice storage,storage battery and other devices absorb electric energy adaptively according to their rated capacity and bear the energy storage energy reasonably. At peak load,the ice storage device is put into operation preferentially,and other energy storage devices is used for other cooling loads,equivalent electrical loads and electrical loads,which is reasonably borne according to the rated capacity ratio. The effectiveness of the proposed control is verified by simulation. It should be noted that the proposed method improves the stability and controllability of the whole system,which provides a novel idea for system-level controller design for multi-energy microgrid.

Abstract:Traditional strategy for multi-fault repair of distribution relies on decision-maker to make the subjective judgment,which is apparently lack of scientific basis. It is easy to make mistakes in judgment. The emergency repair resources cannot be reasonably allocated and the power supply cannot be restored at the first time. In order to solve the problem,an optimization model for multi-fault repair in distribution system is established,which has comprehensively considered multi-group collaboration and rush-repair order. The non dominated sorting genetic algorithm (NSGA)-Ⅱ algorithm with adaptive parameters is designed. After the Pareto front is obtained,the knee solution algorithm based on angle selection can directly solve a relatively ideal scheme without the participation of decision maker. Simulation results of the distribution network in a town show that the adaptive parameter adjustment strategy can improve the global search ability in the early stage of population evolution and the local search ability in the later stage of evolution. The final repairing scheme obtained by the knee solution algorithm based on angle selection can be selected directly from multiple feasible schemes,which can reduce the burden of decision-makers and it is suitable for practical emergency repair work.

Abstract:Digital physical hybrid simulation is an effective means of active distribution network simulation and analysis,and the interface algorithm is the key to ensuring the stability and accuracy of the system. Aiming at the problem that the existing interface algorithm cannot be applied to the hybrid simulation of active distribution network,an improved power interface algorithm based on virtual line compensation is proposed. Firstly,based on the analysis of the structure and stability conditions of the ideal transformer model (ITM) method,it is proposed to add a virtual line between the digital side and the physical side. Then,the stability of the system is compensated based on the virtual current on the line,and a range of values for the line impedance is given. At the same time,for the errors introduced by the inherent delay and stability compensation of the interface,it is proposed to compensate the phase difference between the two sides of the interface according to the virtual power flowing on the virtual line to ensure the accuracy. Finally,the proposed method is compared with the ITM method,damping impedance method and virtual impedance method by simulation and experiment. The results show that the proposed method is effective and superior in improving the stability and accuracy of the hybrid simulation system of active distribution network.

Abstract:Compared with the traditional three-section current protection,the current differential protection can improve the protection performance as a whole in the active distribution network,but reduce the wide coverage of fibre channel . The 5G,with its advantages of high reliability,low latency and wide coverage,is expected to become a new communication scheme for differential protection in active distribution network. However,the 5G communication may still be affected by environmental interference and other factors,resulting in the missing of sampling data,which may affect the correct operation of the protection under the normal operation and external fault scenarios. To solve the above problems,firstly,combined with the ρ plane method,the influences of various sampling data missing scenarios on the protection operation are analyzed qualitatively. Then,a strategy to reconstruct the missing sampling data by using order combination Neville interpolation method is proposed. Finally,an active distribution network simulation model is built in PSCAD,and the interpolation precisions of various interpolation methods are compared under a variety of missing sampling data scenarios,which verifies the effectiveness of the strategy proposed in this paper for improving the protection operation characteristics. Through theoretical calculation and analysis,it is concluded that the proposed strategy does not affect the real-time performance of protection in high-performance central processing unit (CPU).

Abstract:For investigation of the temperature variation characteristics of the overhead transmission line under different meteorological conditions and the influence of wind speed on the effectiveness of the overhead transmission line icing monitoring based on the distributed optical fiber sensing technology,a 3D temperature field model for the ice-covered optical phase conductor is established. The variation characteristics of optical fiber temperature under different ambient temperature,wind speed and icing-thickness are analyzed and obtained using the finite element analysis. From this,the results reveal that the temperature difference between the icing part and the non-icing part is independent of the initial value of the ambient temperature,and the critical wind speed expression separately considering the rate of ambient temperature change or icing-thickness is provided. In addition,the critical wind speed value under the ambient temperature change rate and icing-thickness within the common range is presented. Based on the wind speed value and interpolation,a more accurate critical wind speed value can be given under the ambient temperature change rate and icing-thickness within the common range. It provides a reference for researching the temperature variation characteristics of ice-covered overhead transmission lines under different meteorological conditions and the influence of wind speed on the effectiveness of overhead transmission line icing monitoring based on distributed optical fiber sensing technology.

Abstract:Pollution flashover is one of the main factors threatening the safety of power system. In order to prevent pollution flashover accident,it is necessary to regularly analyze the contamination of insulator surface. However,the traditional power outage sampling method has a long detection period and consumes a lot of manpower and material resources. In this paper,a strategy and a model for the surface pollution analysis of insulators using laser-induced breakdown spectroscopy are proposed. 10 glass insulators with different equivalent salt deposit density (ESDD) and non-soluble deposit density (NSDD) are taken as research object,and the glass insulator's surface can be divided into inner,middle and outer ring. The calibration of coefficient of correlation of the model under different test strategy is studied,using Na 589.592 nm,Al 396.192 nm as characteristic spectral lines. The result shows that for ESDD,the coefficient of correlation can reach 0.948 1 when inner and middle rings are selected as test objects. And the relative error of detection is within 5%. For NSDD,the coefficient of correlation can reach 0.938 3 when middle ring is selected as test object. And the relative error of detection is within 15%. The analysis method can realize the quick on-site analysis of insulator surface contamination and improve the operation ability of transmission line operation safety maintenance,which has important engineering application value.

Abstract:Obtaining the cable partial discharge fault location quickly and accurately is of great significance to the safe and stable operation of the cable system. In order to resolve the problem of the traditional traveling wave positioning method on time synchronization,a double-ended positioning of partial discharge for cable based on variational mode decomposition-Wigner Ville distribution (VMD-WVD) phase method is proposed to reduce the influence of reflected waves identification in long-cable. It is marked the wave head of the double-ended partial discharge signal accurately to achieve the synchronization of the double-ended partial discharge signal by the proposed method. Through phase positioning,the advantages that synchronizing the incident signal of double-ended partial discharge in the long cable are realized. It is built a PSCAD model of a long line to analyze the positioning accuracy of the VMD-WVD phase method under the influence of fault location,cable length and sampling rate in long cables. The results show that the average positioning accuracy by the VMD-WVD phase method for the above three influencing factors is 0.54%,0.85% and 0.69% respectively,which is higher than the value by the traditional traveling wave positioning method. The research results provide a new idea for the precise positioning of partial discharge in long cables.

Abstract:In order to obtain the characteristic parameters of dampness status of resin impregnated paper (RIP),the time domain dielectric response technique is performed on the dampness status evaluation,and the RIP moisture contents are intensively studied based on polarization and depolarization current (PDC) method. Firstly,RIP insulation samples with different moisture contents are prepared in the laboratory and the PDC curves are measured. Then,the optimization algorithm combining genetic algorithm and interior point method is introduced,and the parameters of extended Debye model are obtained by Matlab simulation. Finally,two characteristic parameters of dampness status are extracted,which are the relaxation contribution coefficient of the maximum time constant branch A₉ and the stable depolarization charge quantity Q_d-5000. Conclusions from the research results are as follows. The PDC curve moves upward as the moisture content increases. The dampness of the RIP insulation results in the increase of its conductivity and the decrease of its insulation resistance，absorption ratio andpolarization index. The parameters of the 9-branch extended Debye model are sensitive to the RIP insulation dampness status,and the A₉ shows a linear relationship with moisture content. The depolarization charge quantity is sensitive to moisture content,and the Q_d-5000 shows an exponential relationship with moisture content.

Abstract:Due to the diversity and randomness of users' electricity consumption behaviors,the imbalance of load data classes is increasingly obvious. Traditional load curve classification technologies have become ineffective to deal with the im-balanced class problem of data. Therefore,an algorithm combing improved K-means with long short term memory (LSTM) neural network and convolutional neural network (CNN) classification model is proposed. Firstly,to improve the classifica-tion accuracy of the K-means on imbalanced data,a method of relative k-nearest neighbor density peaks (RKDP) based on the density peak clustering algorithm (DPC) is proposed to select the initial clustering centre of K-means. Secondly,in order to improve the performance of RKDP-K-means in processing high-dimensional load data,an au-to-encoder based on LSTM is used to extract load characteristics from high dimensional data,and com-bined with RKDP-K-means to obtain accurate load profiles labels. Finally,based on LSTM neural network and CNN,load characteristics were extracted to construct load curve classification model to realize the classification of large-scale load curve. Different algorithms are employed to classify Ireland smart meter data set and London load data set. The results show the proposed algorithm is more effective and practicable in large-scale load curve classification.

Abstract:One of the components to realize the mutual benefit and win-win between electric vehicle (EV) and power grid is to effectively predict the charging load of EVs while the difficulty of charging load prediction is increased because of the randomness of temporal and spatial transfer of EV and a variety of coupling factors in the transfer process. In this paper,a method for predicting the spatial and temporal distribution of EV charging load considering dynamic transfer planning and coupling factors is proposed. Firstly,an individual travel mathematical model with multiple types of EVs is established based on travel chain technology. On this basis,considering the traffic flux,road conditions and temperature,the mathematical model of energy consumption per mileage of EV is constructed. Secondly,based on Markov decision process theory,considering the residual path and road network congestion information,the road network information is dynamically updated and the temporal and spatial transfer path of EVs is randomly planned. Finally,based on an example,the temporal and spatial distribution of EV and its charging load are compared and analyzed under different strategies,functional areas and travel days. The results show that the proposed method can fully reflect the travel decision of EV owners,and the prediction results can truly reflect the differences in the amplitude and distribution of charging load due to EV types and functional areas.

Abstract:In view of the low calculation accuracy of physical analytical model of harmonic loss,a support vector regression (SVR) model based on crisscross optimization (CSO) algorithm is proposed to evaluate the harmonic loss of overhead lines. Firstly,the SVR model designed to minimize structural risk is used to fit the relationship between line characteristics and harmonic losses. Then,the SVR hyperparameters are globally searched by the CSO algorithm. The optimal hyperparameter group is obtained by dynamic optimization,and the CSO-SVR harmonic loss evaluation model is established. Based on a large power quality test platform,the harmonic test of low voltage overhead lines is carried out. And the proposed model is verified by the measured data of this test. The results show that using CSO algorithm to optimize hyperparameters of SVR can effectively improve the line loss evaluation performance of SVR model. Compared with other models,the proposed model presents higher accuracy.

Abstract:The isolated DC/DC converter connects the low-voltage direct current (LVDC) distribution network and the user-side DC load serving as an important equipment in LVDC distribution network. In this paper,LLC resonant converter with zero voltage switching on primary side and zero current switching on secondary side is adopted. The operation principles of LLC resonant converter are analyzed and the resonant parameters are presented. The gallium nitride device with lower on state resistance and effective output capacitance is used as the primary side switches to increase switching frequency and efficiency,thus decreasing magnetic component volume. Then,the driver of GaN devices,synchronous rectification and magnetic components are optimized and designed. Finally,a 375 V/48 V/500 W LLC resonant converter prototype is applied to verify the correctness of the design. The peak efficiency of the prototype is 97.6%,which is about 1% higher than that of the converter using traditional Si devices.

Abstract:Ampacity is an important parameter in power cable operation. In order to provide a reference for the operation of ultra-high voltage cables laid in tunnel,a three-dimensional geometric model is established to simulate the thermal-fluid coupling by COMSOL Multiphysics according to the actual cable tunnel structure and internal cable arrangement. The distribution of temperature and wind velocity under different operating modes and environmental conditions is analyzed based on finite element method. The ampacity of tunnel ultra-high voltage cables is calculated. It is observed that the highest temperature appears at the conductor. The temperature decreases gradually along the radial direction of cable. Temperature and wind velocity at the exit are increased compared to the entrance. With the increase of current,the influence of cable heating on ambient temperature also increases. The steady-state load capacity of cable for double-loop or four-loop laying is higher than that for eight-loop laying. Cable surface temperature decreases with increasing ventilation rate.

Abstract:In the new energy power generation system,the inverter is connected to the grid in parallel,which expands the grid capacity. In addition,due to the existence of grid impedance,the stability of the inverter is reduced and the difficulty of power quality governance in the load increases. The problem of power quality governance of parallel inverter in weak current network is studied. Firstly,H∞ repetitive control combined with voltage feedforward is used as the current inner loop control strategy and the stability performance of single and multiple parallel inverters in weak current network is analyzed. The composite strategy of feedforward channel series complex filter and forward channel series lead correction link is adopted to improve the stability of inverter. Secondly,the current detection algorithm is used to separate the unbalanced,harmonic and reactive currents in the load,which solves the governance of power quality problems caused by unbalanced,nonlinear and reactive loadd. Finally,the simulation experiment is carried out in Simulink. The simulation results show that the power quality problems of multiple loads under the condition of weak power grid can be solved by parallel inverter.

Abstract:In the process of wireless charging,the fluctuation of output power is caused by the change of battery resistance,which shortens the battery life and affects the safety of charging. For the magnetic coupling resonance type wireless power transmission variable load system,the control circuit is usually added at the receiving end to stabilize the output power,but it expands the volume of the electrical equipment. Therefore,two constant power control strategies based on the principle of reflected impedance are proposed in the article,namely Sepic circuit constant power output control strategy and phase shift control constant power output control strategy. Firstly,the influence of load resistance on the output power is analyzed. Secondly,the relationship between the duty cycle of the Sepic circuit drive signal and the output power,and the relationship between the phase shift angle of the drive signal and the output power in the phase shift control are deduced respectively. The principle of the constant power control strategy in the article is given. Finally,a simulation model is built through Matlab/Simulink to verify the correctness of the theoretical analysis and the proposed control strategies. The simulation results show that both control strategies can effectively achieve constant power output.

Abstract:The two-terminal fault location theory based on the lumped parameter model is widely used in transmission line protection. For short and medium distance transmission lines,the model error is negligible. However,there are still other factors that affect the ranging results. It is easy to produce large error if the fault is removed in a very short time. In order to improve the accuracy and universality of ranging,the absolute error of the two-terminal fault location algorithm based on the lumped parameter model under the common influencing factors is analyzed. By using orthogonal experiment design and simulation calculation,the influence degree of different influencing factors on ranging error is compared. It is found that data window selection is the most influential factor. Besides,the ranging error of the proximal fault of the line,compared to other places,is greatly affected by the influencing factors. According to the two features,a practical algorithm of adaptive optimal data window based on segmented processing is proposed. By verification,the influence of ranging error calculated by the improved algorithm is greatly reduced.

Abstract:SF₆/N₂ gas mixture is considered as one of the most promising gases to replace SF₆ because of its good insulation performance and environmental benefits. However,there is a lack of research on the mixing ratio of SF₆/N₂ gas mixture in different conditions. Under the premise of ensuring the insulation level of gas-insulated transmission lines (GIL),a multi-physical field coupling calculation model is established. From the perspective of heat characteristics of gas mixture,the relationship between SF₆/N₂ mixing ratio and GIL temperature rise under different insulating gas pressure,load current and ambient temperature is analyzed to determine the appropriate SF₆/N₂ mixture ratio under different conditions. The results show that there is a negative correlation between the insulation gas pressure (or the SF₆ content) and GIL temperature rise at the same pressure. The SF₆ content is suggested to be in the range of 40%~60% when the load current exceeds 3 kA for a long period of time,40%~70% when the equipment is running in the middle and low latitudes,and 30%~40% when the equipment is in the high latitudes. In addition,considering the safety margin,the insulation and temperature rise requirements of GIL can be met when SF₆ content is not less than 70% of the recommended value.