Abstract:按照“碳达峰”“碳中和”目标，预计到2060年，中国非化石能源消费比重将达到83%，电能消费比重达到70%，全社会用电量超过16万亿千瓦时，新能源发电装机达到50亿千瓦，新能源发电量占比由目前的8%提高到60%以上。由于大规模新能源发电具有间歇性、随机性和波动性，无疑将给电力系统平衡调节和灵活运行带来重大挑战。因此在构建新型电力系统过程中，亟须增强电力系统的灵活性和柔性，提高资源优化配置能力，实现多能互补、源网荷储高效协同，有效平衡新能源的波动性、随机性和不确定性。虚拟电厂作为聚合灵活性资源的有效手段，借助先进的通信、计量、控制等技术，无须改变灵活性资源各自的并网方式和地理位置即可实现聚能、储能、供能与用能，有效连接了灵活性资源与电力系统，实现资源整合与分配，是新型电力系统在能源供需侧实现互动化、智能化的重要途径。通过虚拟电厂相关技术的应用，将有效提升电力系统的运行灵活性，大幅降低可再生能源并网风险，提升能源生产和消费效益，助力“双碳”目标的实现。

Abstract:There may exist system operational risks and the risks of losing the revenue because of the generating uncertainty of the stochastic units during the operational process of the virtual power plant (VPP),which contains multiple large scale stochastic units. To solve these problems existing in VPP,a two-stage bi-level decentralized planning (BLDP) model incorporating the conditional value-at-risk (CVaR) based on the characteristics of hierarchical control in the multi-agent system is built. Multi objective programming (MOP) model is selected and compared with the BLDP model constructed in this paper to verify the effectiveness of the BLDP model. It indicates that when VPP participates in the market independently and trades with other VPPs under the BLDP scenario,the system will achieve higher profits and be affected less by the risk levels. Meanwhile,the system can get higher yield rate under BLDP scenario than that under MOP scenario under different trading mechanisms,when VPP participates the power market independently without mutual dealing,results obtained under the BLDP scenario are 103% higher than that under the MOP scenario,while when it takes part in both the market and mutual dealing,the results under the BLDP scenario are 147% higher than that under the MOP scenario. The results have demonstrated the availability of the proposed model under different risk levels.

Abstract:With the support of advanced measurement technologies and regulation terminals,demand-side flexible resources such as electric vehicles and air-conditioning loads in smart buildings have good power response potential in real time and can be aggregated to form virtual power plants to participate in frequency regulation auxiliary service of the system. To protect the needs of users when virtual power plants participate in frequency regulation auxiliary services,a control strategy for smart building-based virtual power plants is proposed in this paper. Firstly,the frequency regulation operation mode of the virtual power plant is analyzed,and a typical demand-side resource model and virtual energy storage model of the smart building-based virtual power plants are established. Secondly,considering the uncertainty of frequency regulation signal and user side,a day-ahead optimal scheduling model based on opportunity constrained planning is established. Then the real-time power allocation is realized based on the principle of cost optimization. Finally,the effectiveness and feasibility of the frequency regulation strategy of the virtual power plants proposed in this paper are verified by simulation calculations,and the joint scheduling of demand-side resources can improve the revenue of the virtual power plants while safeguarding the needs of users.

Abstract:Under the background of dual carbon strategy and coal shortage,the existing demand response mechanism of evaluation and incentive is relatively extensive,causing challenges such as excessive redundancy of invitation and difficulty in mining high-quality users,so the accurate evaluation and dynamic incentive decision method of aggregators' demand response considering contribution degree is proposed. Firstly,the accurate evaluation system of demand response considering static and dynamic process is developed and the static evaluation follows the existing demand response evaluation system at the grid side,while the dynamic evaluation focuses on the consistency between the actual load and the target load in each period. Then,the user contribution index is described from the economic and load dimensions,and a two-part incentive settlement scheme with static and dynamic evaluation is designed. Based on that,the dynamic incentive decision model of aggregator's demand response with the contribution degree is got. Finally,the extensive numerical study confirms that the proposed method can achieve the directional incentive of high-quality users,thus improving the response quality and reducing the invitation redundancy without affecting the existing demand response system.

Abstract:In the distribution network,the participation of flexible resources such as distributed new energy and controllable load in demand response has become an important measure of load adjusting in the new power system. How to consider the uncertainty of user responsiveness and balance the interests of multiple participants is very critical. In this paper,the fuzzy control model of user responsiveness is established firstly,and the benefit function model of multi-agent demand response participants such as power supplier,load aggregator and user is given considering the fuzziness of user responsiveness. Furthermore,with the objectives of minimizing the deviation of daily load curve and minimizing the system cost,the upper optimization adopts the optimal demand response scheme of the power supplier. The lower optimization obtains the best task allocation between the power supplier and the load aggregator,so as to establish the two-layer model of multi-agent collaborative demand response of the power supplier,the load aggregator and the user. A solution method based on Stackelberg game theory and k-means clustering algorithm is proposed. Finally,the simulation results of historical data in an area show that the model can effectively screen high-quality demand response resources to suppress load fluctuation under the consideration of user responsiveness and collaborative of multi-agent interests.

Abstract:The virtual power plant (VPP) aggregates distributed energy resources (DERs) from the distribution network,which not only improves the response capability of aggregated resources,but also participates in electrical market to improve resource schedulability and obtain higher operation profits. The recent studies about the evaluation of VPP are not comprehensive,and there is a lack of analysis on the index weight for the VPP. In this paper,firstly,the evaluation model of response capability of the VPP is built with considering the response time,duration and so on. Then,a method for evaluating the overall responsiveness of VPP is proposed. Taking the independent responsiveness of distributed resources as a constraint,the multi-dimensional quantitative indicators of responsiveness are optimized to obtain the overall responsiveness of aggregated resources. At the same time,considering the instability of aggregated responsiveness,the indicator weight is calculated and the responsiveness is scored. Finally,the response capability curve and score of VPP within 24 hours are obtained through an example,and the effectiveness of the method in this paper is verified through analysis. The overall response capability information calculated by the method in this paper can be reported as the basis for power grid regulation,and the response capability score obtained can intuitively evaluate the aggregate response capability,which is helpful for the analysis and control of VPP response capability.

Abstract:With the rapid development of renewable energy,the role of energy storage has become increasingly prominent. In terms of sizing problems of shared energy storage to provide primary frequency regulation for multiple renewable energy stations,an optimal energy storage configuration method is proposed aiming to minimize the total cost of shared energy storage investors. Firstly,the empirical distribution of historical frequency data is fitted,and the result is used to generate frequency data to further configure the energy storage. Then,based on the frequency data,an optimal configuration model of energy storage is developed which meets the requirements of primary frequency regulation. The primary frequency regulation constraints,energy storage rate characteristics constraints,and primary frequency regulation participation rate constraints and so on are considered in the optimal configuration model. The model is a mixed-integer linear programming model which can be solved by mature solvers. Finally,the proposed method is simulated and analyzed according to the actual frequency data,and the configuration results of the lithium-ion battery and flywheel are compared. The results show that the total cost can be reduced by installing shared energy storage compared with independent energy storage. Although the capacity of the lithium-ion battery energy storage systems is significantly greater than that of flywheel energy storage systems,the total cost of lithium-ion battery energy storage systems is lower.

Abstract:For the purpose of investigating the air-cooling heat dissipation system of lithium-ion battery cabin,based on the simulation model of the actual battery cabin,the optimization scheme of air-cooling heat dissipation is designed. Due to the thermal characteristics of lithium-ion batteries,safety accidents like fire and explosion happen under extreme conditions. Effective thermal management can inhibit the accumulation and spread of battery heat. However,the existing battery cabin air cooling system is relatively simple and the heat dissipation efficiency is low. In order to optimize the heat dissipation system,deflectors are installed in the battery cabin to change the temperature field and flow field in the cabin. When the environment temperature is 25 ℃ and the wind speed is 4 m/s,the air-cooling heat dissipation system of 1 C charging battery cabin is conducted. The results show that by adding one deflector can reduce the average and the maximum temperature in the battery cabin by 2.9 ℃ and 4.5 ℃ respectively. Adding two deflectors can reduce the average and the maximum temperature in the battery cabin by 5.5 ℃ and 8.6 ℃ respectively. Reasonable deflector arrangement can optimize the air-cooling heat dissipation system,improving the heat dissipation efficiency and the safety of battery cabin operation.

Abstract:As the typical provincial transmission power grid,Jiangsu power grid is faced with the pressures and challenges induced from system-integration of rapid developments of renewable energy during the energy transition period,which needs to be upgraded in "source-network-load-storage" and other aspects. Through the analysis of energy development from load,power supply,grid structure in Jiangsu,challenges faced by Jiangsu power grid such as insufficient transfer abilities of important transmission sections are put forward. Considering the large-scale integrated renewable energy and structure characteristics of Jiangsu transmission power grid,transmission-capacity demands of the Yangtze-river-crossing section during the "14th Five-Year Plan" and future are emphatically analyzed. Afterwards embedded DC transmission technology that both the transmission-and receiving-ends are located in Jiangsu AC network and distributed on both sides of the Yangtze River are proposed. Comparing with the traditional long-distance and high-capacity DC transmission technology constructed under the interconnection mode,the embedded DC transmission technology proposed in this paper usually uses the existing overhead transmission lines or existing/reserved AC cable channels to construct the AC-DC hybrid power grid in Jiangsu,greatly improving transmission capacities of important sections with limited transmission corridor spaces and power supply potential abilities of the existing power system to satisfy the development demands of Jiangsu power system.

Abstract:The voltage-source converter based multi-terminal direct current (VSC-MTDC) transmission system is mainly applied in grid-connection and long-distance transmission of new energy power generation. New energy power generation has volatility and uncertainty. When using traditional droop control strategy,VSC will be overloaded due to uneven power distribution,protection malfunction will be caused by excessive DC voltage deviation,and oscillation will occur due to communication delay. Therefore,a coordinated control strategy based on model predictive control (MPC) is proposed in this paper. The strategy optimizes parameters according to system state and sends VSC power reference,eliminating the influence of droop coefficient,line resistance and system topology on power allocation. The robustness of MPC also improves the stability of the system in the case of communication delay. The four terminal VSC-MTDC model is built in Simulink,and different operating conditions are set for time-domain simulation. The simulation results show that the control strategy can quickly adjust the converter station power and control the DC voltage when the system is disturbed,and ensure the system stability when the communication delay occurs,so as to improve the adaptability of the power system to the fluctuation and uncertainty of new energy generation.

Abstract:The amplitude of AC line current at the inverter side is affected by DC commutation failure and fluctuates for a short time after single-phase trip.The fluctuation range increases with the increase of the number of DC commutation failures,resulting in the misjudgment of the traditional single-phase fault nature criterion based on the amplitude of the recovery voltage. To solve this problem,a transmission line fault equivalence model is built firstly to evaluate the impact of DC commutation failure on the sound phase voltage. Then the time domain expressions are derived for the disconnected phase voltage during the recovery voltage stage for different fault nature. After that,a single-phase fault nature identification criterion based on time domain voltage integration is proposed,and its effectiveness is verified by PSCAD/EMTDC simulation. The results show that the proposed method has good resistance to transition resistance and DC interference,and the fault nature identification criterion is suitable for DC commutation failure.

Abstract:The purpose of this paper is to obtain the temperature variation characteristics of the optical phase conductor, the criteria of whether the transmission line is covered by ice or not and the methods for predicting the thickness of ice for the case which is more close to the actual ambient temperature. In this paper,the 3D model of the temperature field of icing optical phase conductor is established when the ambient temperature conforms to the sine function,and the finite elements method is used to solve it. The amplitude and lagging phase characteristics of the temperature of the icing part and the uncoated part of the transmission line are analyzed. The relationship between the lagging phase and the fluctuating amplitude of ambient temperature is investigated. The results reveal that the effect of the fluctuating amplitude of ambient temperature on the lagging phase can be ignored. The relationship between the lagging phase and the thickness of ice is studied. Then the criteria of whether the transmission line is covered by ice or not and the prediction formula of the thickness of ice based on the lagging phase feature are proposed. Finally the variations of fiber temperature are modeled and obtained when the ambient temperature varies according to 4 nonsinusoidal functions,and the error of the prediction formula of ice thickness is analyzed based on this. The proposed icing criterion and the prediction formula of ice thickness based on lagging phase are validated by simulation.

Abstract:The fault characteristics of direct current (DC) grids can be analyzed by addressing the grid damping characteristics. However,the frequency-dependent characteristics of the transmission line parameters are usually ignored in the conventional grid impedance modeling methods,which cannot accurately reflect the grid damping characteristics. To compare and analyze the fault currents characteristics of the DC grids with cables and overhead lines,an impedance model of DC grid considering the frequency-dependent characteristics of the transmission line parameters based on vector fitting is proposed in this paper. Then,the proposed model is applied to compare and analyze the fault current features including time delays,initial rising rates and amplitudes of the DC grids with cables and overhead lines. Meanwhile,the impacts of the DC grids key parameters on the fault current characteristics of the two DC grids are investigated. Compared with the sweeping results of the frequency-dependent line model,the proposed model performs well in reflecting the damping characteristic of the DC grid,where the root square error is less than 0.6,informing that the proposed model is much more accurate than the simplified model. Finally,the simulation is conducted in a symmetrical monopolar two-terminal DC grid with pole-to-pole fault. The simulation results show that the fault current rising rate of the DC grid with cables is 24.96% higher than that of the DC grid with overhead lines when the arm inductance is increased,which validates that the fault currents of the DC grids with cables are more sensitive to the inductive key parameters than the DC grids with overhead lines.

Abstract:Fault location is of great significance for fault handling and power restoration of distribution network. In the fault location method for active distribution network,the calculation speed of matrix algorithm is high,but this method has a poor fault tolerance. The fault tolerance of intelligent optimization algorithm is strong,but this method has local convergence problems in large-scale distribution network,and it has a low speed. To solve the problems above,a comprehensive approach for the fault location in distribution network which combines the advantages of both methods is proposed. Firstly,the matrix algorithm is used to locate the fault section rapidly after fault occurs in active distribution network,which uses the alarm information uploaded by the feeder terminal unit (FTU). Then,in order to avoid outputting wrong positioning results caused by poor fault tolerance of matrix algorithm,the positioning results of matrix algorithm is verified by switching function. The location results that fail the verification is included in the suspicious fault set whose dimension is much lower than that of the distribution network. To output the final positioning results,the grey wolf optimization (GWO) algorithm is used to optimize the set above. Finally,the simulation tested by Matlab shows that the proposed method can achieve fault location rapidly with strong fault tolerance in active distribution network.

Abstract:The implementation of residents' demand response (DR) is of great importance in achieving energy conservation and emission reduction. In order to further promote residents' DR and investigate the influence of various factors on DR participation,an approach for analyzing residents' DR behavior that considers multiple influencing factors is proposed in this paper. Firstly,a multi-factor influence model is developed based on the main influencing factors,namely social relationships,attribute influence and their own willingness. Secondly,based on the interaction among the three levels of factors and the Markov property of decision-making process,Markov decision equations are established for residents' DR,which are applied to analyze the characteristics of DR behavior at different stages. Finally,the influence of each factor on DR behavior is quantified through case study. The results show that the participation of residents in DR considering multiple influencing factors is lower than the participation influenced by one factor. Moreover,the interaction among different factors can also lead to the changes in residential participation.

Abstract:In order to meet the development needs of economic and efficient in oilfield distribution network,and to realize the goal of low carbon more quickly,the investment decision-making method of energy-saving measures optimization is proposed. Firstly,the loading behaviors of pumps is analyzed,an calculation model for special line loss of oilfield distribution network based on load superposition characteristics is proposed,which is corrected according to the actual load of oilfield. Next, according to different energy-saving models and the GM(1,1)gray prediction model,the optimization investment decision model of oilfield distribution network is established. Then,based on the characteristics of oilfield energy-saving measures,the direct comparison objective method and the taboo search method are respectively selected to solve the solution of the different oilfield scale. Finally,a 30-node 6 kV distribution network line in an oilfield is taken as an example for simulation verification. The results show that the energy-saving and loss reduction scheme of oilfield distribution network obtained by the investment decision-making method in this paper has high cost performance and can effectively improve the voltage at the end of the line.

Abstract:The defects of poor metal shielding connection of medium and low voltage cable lines are tested for a long time,which requires the lines are out of service. A method to measure the resistance of cable metal shield circuit by using different frequency coupling is presented. Combined with electromagnetic induction law and Ohm's law,the circuit resistance or branch resistance is calculated by constructed simultaneous equations，which is used for judging the connection state of metal shielding. Through laboratory simulation test,it is found that the test cable metal shielding resistance parameters are consistent with the actual values when the original grounding state is not changed. The connection state of cable metal shielding with resistance below 10 Ω can not be detected and judged for single-circuit cable lines. While double-circuit and above cable lines are not easy to be affected by the grounding grid,and the connection state of cable metal shielding can be judged by comparing the circuit resistance and branch resistance with theoretical value,or by comparing the branch resistance with each other. The detection technology is verified by the engineering application,which is effective and feasible to judge the metal shielding connection state for medium and low voltage cables.

Abstract:SF₆ gas mixture is one of the popular SF₆ alternatives. For the purpose of quantitatively evaluating the arc extinguishing performance of SF₆ mixtures,one-dimensional arc decaying model and Boltzmann equation are combined to divide the whole arc extinguishing process into thermal recovery stage,pre-dielectric recovery stage and post-dielectric recovery stage,which are evaluated by three parameters,namely thermal recovery rate,pre-dielectric recovery rate and post-dielectric recovery rate,respectively. The harmonic mean of the above three parameters is calculated as the harmonic average recovery rate to evaluate the arc extinguishing performance of SF₆-N₂,SF₆-CO₂,SF₆-CF₄ and SF₆-Air mixtures. Based on the proposed method,the effects of SF₆ proportion,buffer gas type and gas pressure on the interrupting performance of the SF₆ mixtures are preliminarily investigated. The results show that the overall arc quenching ability of the gas mixtures presents a decreasing trend as the lowering of SF₆ proportion. Among the four gas mixtures with SF₆ proportion ranging from 10% to 50%,the SF₆-N₂ gas mixture has the best interrupting performance,followed by SF₆-Air,SF₆-CO₂,and SF₆-CF₄ successively.

Abstract:In recent years,high voltage cross linked polyethylen (XLPE) cable buffer layer ablation defects occur frequently,which has a serious impact on the safe operation of the cable. It is extremely urgent to study how to reduce the impact of the buffer layer defects. Therefore,how to reduce the severity of the buffer layer ablation defects by improving the aluminum sheath structure is studied in this paper. Firstly,the reason of ablative defect of buffer layer is analyzed theoretically. Secondly,the buffer layer voltage calculation model is established. Finally,a defect cable is taken as the simulation object to calculate the suspension potential and the electric field intensity of the buffer layer after reducing the minimum inner diameter and the curvature at the trough of the aluminum sheath respectively with the white spot defect of the buffer layer. Taking the defective cable as an example,the simulation quantitative analysis shows that when the minimum inner diameter decreases,the suspension potential of insulation shielding decreases by 30%,and the electric field intensity between buffer layers decreases by 30.9%. When the curvature at the trough of the aluminum sheath decreases,the suspension potential of insulating shield decreases by 13.7%,and the electric field intensity between buffer layers decreases by 13.3%. When the buffer layer white spot appears,the decrease of the minimum inner diameter and the curvature at the trough of the aluminum sheath lead to the suspension potential of the insulating shield layer and the electric field intensity between the buffer layer decrease,thus reducing the possibility of partial discharge and the impact caused by the buffer layer defect.

Abstract:The layout environments of the extra-high voltage (EHV) submarine cable are complex and changeable,and the ampacity of the EHV submarine cable varies under different layout environments. Therefore,it is essential to analyze the ampacity of the EHV submarine cable in typical layout environments. In this paper,a steady-state thermal circuit model for the alternating current (AC) 500 kV cross linked polyethylene (XLPE) EHV submarine cable based on IEC 60287 standard is established,and the influences of layout sections,layout methods,environment temperatures,and layout depths on the ampacity of EHV submarine cable are studied. The results of the steady-state thermal circuit model are verified by a multi-physical field finite element model. It is found that the landing section of the submarine cable is the bottleneck section of ampacity of the whole line. When the submarine cable in the landing section is laid by pipeline method,the ampacity is about 150 A lower than that by the direct burial layout method.The ampacity of submarine cable decreases with the environment temperature and the layout depth. The finite element calculation results verify the accuracy of the results of the steady-state thermal circuit model.

Abstract:Ablation of the buffer layer in high voltage cross-linked polyethylene (XLPE) cables has been the attractive type of cable failure. The ablation of buffer layer commonly takes place under the pressure and moisture. Therefore,it is important to investigate the effect mechanism of pressure and moisture on the ablation of buffer layer. In this study,the simulated experiments of dry and wet water-blocking buffer tape under different pressures are carried out,whereby dynamic variations of current density,voltage and alternating current (AC) volume resistivity in the processes of ablation are obtained. The micromorphology of buffer tape specimens is obtained through the scanning electronic spectroscopy. It is shown that the AC volume resistivity of dry buffer tape samples gradually increases with time in the process of ablation,accompanied with the transient surge of current and the plunge of voltage. While the AC volume resistivity of wet buffer tape samples gradually decreases with time. Moreover,a decrease of AC volume resistivity occurs to the buffer tape samples after the water absorption. With the increase of pressure from 1.09 kPa to 5.45 kPa,a decrease of volume resistivity occurs to both the dry and wet buffer tape samples. Combined with the results of stimulated ablation experiments and the micromorphology of buffer tape,the effect mechanisms of the pressure and moisture on the conductive property of buffer tape are explained.

Abstract:Efficient and accurate state estimation is the basis for the safety and stability of the integrated energy system (IES). Particle filter has high precision and strong adaptability to nonlinear systems,and it has been applied to state estimation of power systems. To improve the precision of state estimation in IES,a forecasting-aided state estimation method based on improved particle filter is proposed. Firstly,a regional IES model including an electricity-heat-gas network is constructed. Secondly,the particle filter algorithm is applied to the electricity-heat-gas network. The prediction step of the particle filter is improved because of the tracking error problem of traditional particle filtering algorithm,which is based on particle filter theory. Finally,the improved particle filter algorithm is verified by using the classical IES example. The results show that this method can effectively solve the tracking error problem of the traditional particle filter algorithm,which can improve the precision of state estimation in IES.

Abstract:Aiming at the efficiency and credibility of data feed in power blockchain application system,the trusted data feed technology based on 5G and Oracle mechanism is studied. Firstly,the data feed method in the blockchain system is discussed,and the Oracle data feed method applicable to the power 5G blockchain is analyzed. Secondly,the trusted data feed technology of the power 5G business system based on distributed Oracle is proposed,and the overall system architecture based on cloud-edge-terminal integration is designed,as well as the workflow of data source registration,evaluation and on chaining. Among them,the distributed data collection and sharing are realized through the blockchain system,and the data source evaluation is realized based on threshold signature algorithm and verifiable random function. The evaluation algorithm is run by the edge Internet of Things (IoT) agent node to ensure the security and availability of the system. Through the division and optimization of 5G slice tasks,the efficient transmission of authentication data and business data is achieved. Finally,the proposed scheme is deployed in the electricity 5G blockchain based power information acquisition system,and experimental verification is carried out from the aspects of communication performance,business performance,resource utilization,etc. The results show that the average data transmission delay of the proposed scheme under different load stress tests is about 10 ms,and the bit error rate and packet loss rate are less than 0.9%. In the case of 100-level concurrent service requests,the performance of the scheme is improved by more than 80% compared with the existing schemes,so it has good feasibility and promotion value.

Abstract:The flexibility of the power grid can be significantly promoted by the participation of power customers in dispatch. However, as the uncertainty of customer behavior,the development of demand response services is limited. To solve this problem,the framework of incentive-based demand response is constructed in this paper. The way that load aggregators integrate demand-side resources to participate in the power market is elaborated. And the behavior of power customers responding to incentive policies is transformed into demand elasticity. Then,a data-driven demand elasticity prediction method based on the integrated long short-term memory (LSTM) is proposed. Meanwhile,to improve the performance of the prediction model,the original data is smoothed and scaled,and the weight coefficients of the loss function are added. The simulation results show that,compared with the traditional LSTM algorithm and the k-proximity prediction method,the average forecasting error with the proposed model for the demand elasticity is reduced by 5.33% and 28.8%,and mean absolute percentage error (MAPE) for the total load prediction is reduced by 2.06% and 3.09%. Additionally,based on integrated LSTM,the influence of smoothing and scaling data preprocessing on prediction accuracy is analyzed. The results show that the prediction accuracy can be significantly promoted by data preprocessing.

Abstract:The fiber optical current transformer (FOCT) has distinguishing advantages of small size,simple insulation,no magnetic saturation. It can be better compatible with modern digital control and protection systems,which has become an important development direction of current transformer. However,the reliability of FOCT in long-term operation is low at preasent,so its popularization and application in digital substations is seriously restricted. In this paper,the fault tree analysis of FOCT is comprehensively carried out from both the circuit and the optical path,and a fault mode analysis system covering the overall structure of FOCT is established. Firstly,the structure and operating principle of FOCT are analyzed,and the fault is divided according to its circuit composition. Secondly,the main fault modes occurring in the long-term operation of FOCT are extracted,and the listed fault modes are investigated one by one. Then,the reliability improvement measures are proposed from the aspects of scheme design,component selection and construction technology. Finally,the fault tree is used to analyze the cause of a light source fault combining with an actual case,and the reliability improvement measures are proposed to verify the effectiveness of the proposed method. The research can provide a theoretical basis for FOCT fault analysis and operation reliability.

Abstract:The surge of negative sequence component of current affects the waveform quality of grid side current and endangers the normal operation of the system when the three-phase voltage of solid state transformer (SST) in the distribution network is unbalanced and harmonic pollution occurs. In order to solve this problem,a new dual-sequence control strategy is proposed to effectively suppress the negative sequence component of current at the input level of SST. In this paper,the operation principle and input level mathematical model of modular multilevel converter-solid-state transformer (MMC-SST) are introduced firstly. The positive and negative sequence components of voltage and current signals at grid side are separated and extracted by improved second-order generalized integral-quadrature signal generator (SOGI-QSG). The mathematical model of proportional complex integral (PCI) controller is established by introducing orthogonal intermediate variables. A new dual-sequence control strategy based on PCI is proposed under the positive and negative sequence model in the two-phase static coordinate system (αβ coordinate system) to control the positive and negative sequence components of the current at grid side. Finally,the simulation system of MMC-SST is built in Matlab/Simulink. Compared with traditional proportional integral (PI) control and improved PI dual-sequence control,the results verify the feasibility and superiority of the proposed dual-sequence control strategy.

Abstract:To make sure the half-bridge three-level dual active bridge (HBTL-DAB) circuit works correctly,the problem of capacitor voltage equalization between the upper and lower direct current (DC) buses on the three-level side must be solved. In the traditional symmetrical control mode,the capacitor voltage equalization control under load can be achieved by fine-tuning the duty cycle. However,in the case of no-load,the symmetrical control mode is difficult to realize the balance adjustment of the upper and lower DC bus capacitor voltages from the control strategy. Therefore,in view of the above problem,an asymmetric control method for the HBTL-DAB circuit is proposed. Firstly,the reason why the symmetrical mode cannot be equalized is theoretically analyzed. And then according to the asymmetric method,energy deviation on the upper and lower DC bus capacitor between positive and negative half cycles under no-load conditions is calculated. The capacitor voltage equalization control is achieved by adjusting the energy deviation,and the main factors affecting the effect of voltage balance control are also pointed out. Finally,the asymmetric control method is verified under various working conditions by simulation. The results show that no matter in condition of existing external continuous unbalance factors,or external initial unbalance factors,or internal pulse error factors,voltage balance adjustment is achieved by the asymmetric control method under no-load and loaded conditions.

Abstract:The winter in southern China is very cold,along with high humidity. Regional ice coating problem often appears in the long distance high voltage transmission lines,which may lead to serious power accidents such as tower collapse. A new direct current (DC) ice-melting device with series-parallel switching of rectifier bridge is studied to avoid widespread power outages. Based on the conventional 12-pulse DC ice-melting device,a new DC ice-melting topology is proposed through principle analysis,parameter design and simulation verification,and it simultaneously meet the high current demand for wire melting and the high voltage demand for ground wire melting. Compared with the conventional 12-pulse DC ice-melting device,the new DC ice-melting topology is able to select half-capacity transformer and half-current thyristor,which significantly reduces the equipment costs. Meanwhile,the harmonic current injected into the system decreases effectively when operating in high current output mode,so as to reduce the voltage distortion of the connected system. The new DC ice-melting device with the function of series-parallel switching of rectifier bridge is able to reduce the equipment costs and improve the system voltage quality when the DC ice-melting device is put into operation.

Abstract:Silicone rubber is widely used in composite insulators and anti-pollution flashover coatings in the power industry. However,after long-term operation,it is prone to aging chalking and dirt deposition,which reduces the surface hydrophobicity. Hundreds of millions of silicone rubber insulators are in service for many years in the power grid,and it is urgent to study a new type of cleaning agent that can remove surface contamination and repair the aging layer of silicone rubber. The effects of different non-ionic surfactant content and repair agent content on cleaning efficiency and hydrophobicity were investigated. Experiments show that when the content of non-ionic surfactant is 3%,the cleaning efficiency is up to 95%. When the content of the repairing agent is 11%,the hydrophobicity is the best,and its hydrophobic migration can reach the HC1 level. Through scanning electron microscopy analysis,it is found that the micropores and cracks on the surface of the silicone rubber disappear after cleaning,and the surface is smooth,indicating that the cleaning agent has the ability of removing the aging layer and contamination,and repairing the aging insulators. This provides a theoretical and practical basis for continuing to develop silicone rubber clean-repairing agents and extending the service life of insulators.

Abstract:The uncertainty of haze weather makes it difficult to carry out research on its impact on power equipment. However,whether the research under simulated haze environment is equivalent to that under actual haze weather is related to the guiding value of the research conclusion to the actual work. For this reason,in this paper,the samples are contaminated under simulated haze environment and natural haze environment respectively. By testing the surface contamination layer resistance and leakage current of the contaminated samples under the two environments,the equivalence of the effects of the two environments on insulators is analyzed and discussed. It is found that from the measured data of salt density,ash density and surface pollution layer resistance,the insulators under the two haze environments have an equivalent corresponding relationship,that is,the insulators with pollution accumulation under natural haze for one year are equivalent to those with pollution accumulation under simulated haze for 10-20 days. However,from the perspective of leakage current parameters,composite insulators under the two haze environments are not equivalent,so the conclusion of leakage current under the simulated haze cannot be directly used to guide the actual work.

Abstract:Relay protection device is an important part to ensure the safe and stable operation of power system. With the rapid increase of the number of substations and relay protection devices,the daily inspection workload of the operation and maintenance personnel has become saturated,which can not guarantee the high quality and no dead angle inspection every time and brings hidden dangers to the reliable operation of the protection devices. In this paper,an intelligent inspection technology of protection device based on convolution neural network image recognition algorithm is proposed. With the help of the cameras installed in the front and back of the cabinet,the unmanned or few people inspection of the protection device can be realized. Firstly,the intelligent inspection system of the protection device is introduced,and the intelligent inspection items that can be realized is analyzed. The conclusion that convolution neural network can be used for image recognition is drawn. Secondly,taking the platen state recognition as an example,the training sample set and test sample set required by the inspection items are introduced,and the convolution neural network level of the inspection items is given. Then the training sample set is used to train the convolution neural network of different inspection items,and finally each network is tested. The test results show that the neural network image recognition rate of each inspection item is above 96%,even 98%,and the recognition effect is good.