Abstract:Micro-energy networks integrate distributed energy systems,multiple loads and control devices in parks or communities. Micro-energy networks are capable of realizing multi-energy cogeneration and co-supply functions,which are conducive to enhancing the operational flexibility and economy of multi-energy networks. To effectively cope with the uncertainty of load and new energy output within the micro-energy networks,a polyhedral uncertainty set is established to portray the fluctuation of load and photovoltaic output. An energy hub (EH) coupling matrix describing the topology of micro-energy networks and the coupling relationship between the energy flows is constructed. Then,a two-stage robust optimization economic dispatch model is established for micro-energy networks in both grid-connected and islanded operation modes to minimize the system operation cost. Linear decision rule (LDR) and duality theory are used to develop model-solving strategy. Finally,a case is carried out using micro-energy networks based on improved IEEE 33-node distribution network. The results show that LDR can effectively approximate the relationship between decision variables and uncertain variables through linear affine function within a certain accuracy range,which reduces the difficulty of solving the two-stage robust optimization economic dispatch model.

Abstract:In order to solve the problems of system peaking and consumption caused by the integration of large-scale new energy sources into the grid,this paper analyzes the source-load-storage peaking capacity and its complementarities,so as to tap the system's peaking capacity and promote new energy consumption. Firstly,taking into account the peaking pricing and compensation for ancillary services for thermal power,energy storage and electric vehicles,the peaking cost sharing and compensation mechanism is analyzed and a deep peaking model is established. Secondly,considering the uncertainty and correlation of wind power,a typical wind power sequence based on kernel density estimation and Frank Copula function is generated and a step-type demand response model is established to realize the graded compensation of the response amount. Then,it is possible to improve the response enthusiasm of demand-side users. Finally,with the objective of minimizing the total operating cost,a joint source-load-storage peak-peaking optimization model considering uncertainty and pricing compensation is constructed,and the improved IEEE 30-node system is used as an example for analysis. The results show that the proposed model can increase the system's peak-shaving capability and renewable energy consumption level by utilizing the flexible peak-shaving potential of resources on the three sides of source,load,and storage.

Abstract:At present,new energy is introduced into the power system in large quantities. In order to make it replace the traditional hydropower and gas units as the black start power supply,segmentation restoration strategy of power grid based on the combined optical storage system is studied. Firstly,according to the structure and control mode of the optical storage system,the energy storage system establishes a stable grid-connected bus voltage is proposed,and the photovoltaic system is connected to the grid step by step. Secondly,in order to enhance the damping inertial support of photovoltaic power generation system,virtual synchronous control is proposed to be added to the inverter side of photovoltaic system. Finally,in the process of grid connection between the units to be started and the system load,the energy storage system is configured with additional damping controller to suppress the sub-synchronous oscillation generated by the grid black start,so as to ensure the stable recovery of the grid. In PSCAD/EMTDC,the electromagnetic transient simulation model of black start of the combined optical storage system is built and the simulation analysis is carried out. The results show that the busbar voltage deviation and system frequency deviation in the recovery process of the system are less than 0.65% and less than 1%,which meets the requirements of black start and verifies the feasibility of the start-up strategy proposed in the paper.

Abstract:Under the background of the new power system,it is important to make full use of the distributed generation in the distribution network to achieve carbon peaking and carbon neutrality goals. With the wide access of distributed power sources,there is bi-directional power flow between the transmission and distribution networks. If the transmission and distribution networks still follow the traditional equivalence method for optimal power calculation,accurate results will not be obtained. Therefore,a transmission-distribution-network-coordinated optimal power flow model is established. The node tearing method is used to decouple the coupling variables of the transmission network and distribution networks. On this basis,the iterative form of the model with immobile points is obtained by mathematical derivation. The traditional heterogeneous decomposition algorithm only uses the latest calculation results for iterative computation. Hence,combining with the Anderson acceleration idea in mathematics,the heterogeneous decomposition algorithm based on Anderson acceleration is proposed by using the historical iteration values to optimize the iterations of immobile points. It is demonstrated by numerical experiments that,the established model can fully utilize the initiative of the distribution networks and reduce carbon emissions. The proposed algorithm has high accuracy. In addition,compared with the heterogeneous decomposition algorithm,its convergence performance is significantly improved.

Abstract:The multi-uncertainty of source and load poses significant challenges to the optimal scheduling of 'source-load-storage' integrated microgrid. However,a limitation of the traditional optimization model is its one-sidedness and use of a single time scale,which can result in suboptimal scheduling outcomes. Striking a balance between reliability and economy presents a considerable obstacle,as does coordinating the relationship between uncertainty analysis methods and varying time scales. Based on the data-driven multi-discrete scene distribution robust method,a two-stage distributed robust day-ahead optimal scheduling model of microgrid is proposed,which is solved by column and constraint generation algorithm. By combining the improved distributed robust optimization uncertainty method with a multi-time scale scheduling strategy and model predictive control theory,the accuracy of the scheduling can be enhanced. This is achieved through the gradual refinement of the scheduling time scale and reduction of the prediction period length. The day-ahead-day multi-time scale rolling optimization scheduling model is established to minimize the generation cost and adjustment cost,while also exhibiting a high degree of resilience to system uncertainties. Combined with the simulation analysis of the example,the proposed model has demonstrated advantages in incorporating new energy sources,reducing operating costs,and balancing considerations of safety and economy.

Abstract:The capacity of distributed photovoltaic impacts the safe and reliable operation of the distribution network. Energy storage in distribution feeders has emerged as a pivotal solution for consuming distributed photovoltaic. However,it is challenged to balance the economic viability of energy storage investing and grid reliability. In response,a method of optimizing the configuration of energy storage in distribution feeders based on the new model of counter configuration of the renewables from energy storage is proposed,which takes into account both the economy and reliability aspects. Firstly,an economic model for optimizing energy storage in distribution feeders is established. Secondly,a grid reliability simulation is performed based on the results of the economic optimization of energy storage in distribution feeders. Finally,considering the comprehensive indexes of economy and reliability,the different nodes are evaluated for site selection using technique for order preference by similarity to ideal solution (TOPSIS),and the energy storage configuration scheme is determined by the cumulative distribution curve. The simulation results show that the consuming capacity of distributed photovoltaic in the distribution feeders is effectively improved,and better balance the economic viability of energy storage investing with grid reliability,which can be used as a reference for the reasonable determination of ratios of the energy storage for renewables.

Abstract:The rational utilization of the complementarity between new energy sources can smooth out the fluctuation of their power output,and the consideration of complementarity in new energy planning can make the new energy planning layout more rational. Based on this,the paper considers the complementarity of new energy outputs and investigates the optimal planning method of new energy system taking into account the source-load matching. The positive correlation between new energy output and load is utilized to maintain the dynamic balance between new energy output and load. Firstly,based on the idea of source-load matching,an equalization index describing the degree of closeness between the new energy output and the load,and a consistency index describing the degree of positive correlation between the new energy output and the baseline load output are proposed. Then,a multi-region new energy optimization planning model considering the source-load matching is established,and a constraint on the equilibrium degree of the new energy output and the load matching is introduced to ensure the consistency of the change characteristics of the two. Finally,a case study is conducted to analyze the power grids of four regions in the northern part of China. The results show that the new energy optimization planning considering the source-load matching can consider the new energy consumption rate and penetration rate comprehensively,and provide a solution for the new energy planning layout considering the complementarity of new energy output.

Abstract:When applying the fault enumeration method to evaluate the reliability of the whole transmission and distribution network composed of transmission network and flexible interconnected distribution network,apart from analyzing the influence of internal faults of the transmission network and distribution network on their respective power supply reliability,the interaction between the transmission network and distribution network should also be considered under the fault scenario. Based on existing independent reliability evaluation algorithms for the transmission network and distribution network,the reliability evaluation method for the whole transmission and distribution network is proposed in this paper. The proposed method in this paper can account for the interrelation between the operation states of the transmission network and the distribution network under the fault condition. The node load reduction after the failure of the transmission network is refined into the optimized load cutting scheme of the distribution network. While improving the accuracy of the reliability evaluation results of the transmission network,the load loss of the distribution network caused by the failure of the transmission network is included in the reliability evaluation of the distribution network. In addition,the load loss caused by internal faults of the distribution network and its influence on the reliability of the transmission network are included in the reliability evaluation of the transmission network. For the whole transmission and distribution network,the reliability index of the transmission and distribution network is calculated by the analytical method based on fault enumeration. The validity of the reliability evaluation model is verified by the results of the numerical analysis.

Abstract:Overhead transmission lines are the significant component of electric power system,where the over current can be coupled with the transient electromagnetic field or excitations (such as high-altitude electromagnetic pulse). As the coupling path of strong electro-magnetic interference,overhead transmission lines cause serious interference to the power system. Among the existing modelling methods,the classical transmission line theory may generate large error when dealing with the high-frequency coupling problem,where the cross dimension of the transmission line is not electrically small. Numerical full-wave method (such as moment of method) which relies on the grid subdivision with low efficiency when dealing with long transmission lines. Moreover,the number of cables is usually large in electric power system,and the ground are considered as the lossy ground. Therefore,to address the above mentioned problems,an asymptotic method is proposed to calculate the high-frequency coupling current along overhead transmission lines in electric power system. Based on the asymptotic theory with high calculation efficiency,the scattering and reflection process are introduced to quantify higher-order model components. In addition,the arbitrary number of wires,arbitrary parameters of the ground and different excitations are considered to derivate the current expression. Finally,the validity and reliability of the proposed method are tested using the full-wave simulation and antenna irradiation experiment. The proposed method can quickly calculate high-frequency coupling current,which can provide theoretical basis and data support for protection and electromagnetic effect study of overhead transmission lines.

Abstract:Pulse injection to simulate partial discharge (PD) is the main method for functional verification of gas insulated switchgear (GIS) ultra high frequency (UHF) PD monitoring device. Due to the different coupling modes of pulse injection in field verification,the equivalence law between simulated PD and actual PD is not clear,and the effectiveness of functional verification of monitoring device cannot be guaranteed. In this paper,a typical PD defect (tip,suspension,insulator bubble) and internal/external pulse injection UHF PD detection platform for 126 kV GIS are established,and the effective pulses of UHF signals are normalized and extracted. Then a signal processing method of weighted intrinsic mode functions (IMF) based on empirical mode decomposition is proposed. The signal equivalence is characterized by calculating the average and maximum values of the Euclidean distance of the signal,and finally compared with the conventional signal deviation method. The research shows that compared with the conventional signal equivalence analysis method,the weighted IMF method can effectively solve the problem of large local difference of UHF signal waveform. The internal sensor pulse injection analog PD and suspended PD have the highest equivalence. The average value M_e and the maximum value M_a of Euclidean distance are 3.82% and 10.28%,respectively. Therefore,the function verification of UHF monitoring device can use constant parameter pulse injection instead of suspension defect,and the internal UHF sensor injection can be preferred for analog PD. The research in this paper can provide reference for the pulse injection method for the function verification of UHF PD monitoring device.

Abstract:Permanent magnet synchronous motors (PMSMs) are widely utilized in the field of manufacturing and production due to their notable advantages such as simple structure,stable operation,high efficiency,and diverse configurations. Operating under conditions with variable speeds and load fluctuations,PMSMs inevitably experience various faults,including bearing faults,eccentricity faults,demagnetization faults,and interturn short-circuit faults. An overview of common fault types in PMSMs is provided in this paper and the existing researches on signal-based condition monitoring methods in the field of automation for the detection and diagnosis of electrical and mechanical faults are summarized. Subsequently,diagnostic methods are summarized,and their strengths and limitations are analyzed,discussing the pros and cons of different signal processing approaches in application. Finally,considering the current state of research,the challenges and future directions in the monitoring and diagnosis of PMSM faults are discussed.

Abstract:The transmission tower and tower line coupling system is an important carrier of the power system transmission process. Strong winds in coastal areas have a significant impact on the transmission tower and tower line system. To study the dynamic response characteristics of tower line system under the action of random fluctuating wind fields,the 1W2C9 drum-type self-supporting transmission tower in Guangdong is taken as the research object. Finite element models for the single tower and one tower two-line tower line coupling system are established. By combining the Davenport wind speed spectrum with the fourth order autoregressive method,the fluctuating wind fieldswith spatial correlation are simulated for a single tower with 9 nodes and tower line system with 49 nodes. The dynamic response of a single tower and tower line system under this fluctuating wind field is studied,and the influence of different wind direction angles on theirdynamic response is explored. The results indicate that the maximum dynamic response of both the single tower and tower line system is greater than their pseudo-static response. The existence of transmission lines enhances the stability of the transmission tower line system,reducing its dynamic response. A wind direction angle of 90° proves to be the most unfavorable for the dynamic response of the single tower and tower line system.

Abstract:AC submarine cable is an important component in AC transmission system of offshore wind farms. However,the issue of charging current limits its application in offshore wind power with long-distance and large-capacity. Taking HYJQF41-F290/500 kV single-core AC submarine cable as an example,the overvoltage distribution along the cable in different working conditions is analyzed,considering the long-distance and high-voltage transmission. The simulation model of the cable is established and its parameters are corrected firstly. According to the different compensation degrees of the shunt reactor,the capacity of the shunt reactor under the configuration scheme of single-end compensation and two-end compensation calculated out. Finally,based on the single-phase grounding fault,the closing and opening conditions in the high-voltage and long-distance wind farm models,and the distribution law of overvoltage along the submarine cable under different reactive power compensation schemes is tested by simulation. The results show that the maximum overvoltage of submarine cables appears in different positions with different compensation schemes. Based on this,the selection methods for high-voltage submarine cable and overvoltage in insulation level design of substation equipment are given out under different compensation schemes,which have certain guiding significance for engineering practice.

Abstract:In order to improve the flicker suppression performance of static var compensator (SVC) in response to impact loads such as DC electric arc furnace,a SVC rolling predictive control method based on the improved Takagi-Sugeno (TS) fuzzy algorithm is proposed. Firstly,the electrical model of DC electric arc furnace is established and its reactive power characteristics are simulated and analyzed. Then,an improved method of range adaptive correction is proposed to address the issue of abnormal output set to 0 when the classical TS fuzzy prediction algorithm is applied to fluctuating loads. This method can eliminate the outliers caused by the application mechanism of a class of algorithms,so as to improve the reliability and accuracy of TS fuzzy algorithm for reactive power prediction of fluctuating loads. Finally,based on the model training time constraint,a reactive power half-cycle rolling predictive control model is established to predict the reactive power 10 ms in advance. It is competent to improve the lag characteristics of the traditional SVC control system response. The simulation results show that the average flicker improvement rate of the proposed method increases by 54.17% compared with the traditional SVC control method,and the suppression effect of flicker phenomenon is significantly improved.

Abstract:In order to explore the economic and carbon reduction benefits of photovoltaic-storage-charging integrated stations and achieve reasonable configuration of internal components,a multi-objective optimization configuration method for stations that takes into account economic and low-carbon aspects is proposed. Firstly,based on the functions and requirements of each module in the charging station,the sources of carbon emissions generated by station is explored,and a mathematical model for the cost and carbon emissions of each module in the station is established. Then,with the goal of minimizing the annual investment and operating cost of the system and carbon emissions,a multi-objective particle swarm optimization algorithm based on three-black-hole capturing strategy is used to optimize the configuration of various modules of stations in different load scenarios,and the optimal configuration plan for each component module of the station under three scenarios is obtained. The comparative results show that the method proposed in this article can effectively reduce the cost and carbon emissions during planning and operation,and improve the economic and environmental benefits of stations. Finally,the technique for order preference by similarity to ideal solution is used to provide a compromise optimization plan for the optimal scenario,which can provide reference for the current investment and construction of photovoltaic-storage-charging integrated stations.

Abstract:In order to coordinate the interests between electric vehicle owners and the active distribution network,a multi-objective hierarchical optimization method for the optimal scheduling of the active distribution network with electric vehicles is proposed. This method takes into account both the comprehensive satisfaction of electric vehicle charging and the operational benefits of the active distribution network. The upper layer model focuses on maximizing the charging benefits of electric vehicle owners,and the normalized normal constraint method is used to solve the optimal charging and discharging plan of electric vehicle,which is input to the lower layer optimization model. The lower layer model aims to maximize the operational efficiency of the active distribution network by adjusting the output power of the controllable distributed generation according to the charging and discharging plan of electric vehicle. The second-order cone program and weighted minimal modular ideal point method are used to solve the nonlinear multi-objective problem. Simulation results show that the proposed multi-objective hierarchical optimization method can promote the comprehensive satisfaction of electric vehicle charging exceeding 0.9 while reducing active power loss by 94.12% and operating costs by 30.90%,which achieves a win-win situation for both electric vehicle owners and the active distribution network.

Abstract:Integrated energy systems enable the complementary utilization of various forms of energy. With the rapid increase in installed capacity of distributed generations,their intermittency and randomness have posed significant challenges to the operational efficiency and safety of the system. To address the impact of uncertainties in both energy supply and demand on the economic scheduling of building integrated energy systems,this study first models the operational characteristics of each subsystem,including the power grid,natural gas network,and various coupled equipments. Next,a thermal cell model of the building users is constructed based on the quantitative relationship among temperature,thermal radiation,and thermal load. Then,a day-ahead optimization scheduling model for the building integrated energy systems is established with the objective of minimizing the operating cost. A chance-constrained programming approach is employed to convert the non-linear scheduling model into a mixed-integer second-order cone programming problem easy to solve. Finally,simulation analysis is conducted in the Python environment using the CPLEX solver. The results demonstrate that the proposed model and solution method are capable of effectively characterizing and addressing uncertainty risks in the system,facilitating the consumption of renewable energy,and improving the economic efficiency of system operation.

Abstract:Time-frequency domain reflection (TFDR) method has the advantages of maturity,convenience and high resolution,so it is widely applied to locate defects in various cables. To ensure the fracture defect of the instrumentation and control (I&C) cable can be effectively discriminated and precisely located in nuclear power plant (NPP),an improved TFDR method is proposed by intervening thermal stress in this paper . Subsequently,different degrees of fracture defects are simulated in a 50-meter multi-core I&C cable,and traditional TFDR and improved TFDR methods are used for detection and verification. Comparing the detection results of both,the peak localization of improved TFDR method for weak defect fracture is improved to 0.2,which is 2.5 times of the traditional TFDR method,and the localization error rate is less than 2.5%. Compared to the traditional TFDR method,improved TFDR method utilizes the thermal stress fluctuation variance curve as the basis for defect identification,the identification power of weak fracture defects can be substantially improved. The original localization distance can be corrected to improves the accuracy of weak fracture damage in improved TFDR method. So the effective identification and localization of weak defects can be further achieved in improved TFDR method.

Abstract:Strong corrosive active sulfur in the mineral insulating oil has been considered to be the main culprit to induce the insulation failures of oil-immersed power equipment. In order to protect the oil-paper insulation from sulfur corrosion,active sulfur is cleared in the refining process of crude oil. Some inactive sulfur with high antioxidant is still retained in the mineral oil to improve the oxidation stability of oil. However,the activation of inactive sulfur under the operating condition of oil-immersed power equipment is not gained attention. This paper focuses on the reaction mechanism of winding corrosion induced by non-active thiophene sulfide in mineral insulating oil. The thermal pyrolysis products of thiophene sulfide are investigated by the material phase analysis method,and the activation energy changes of thiophene sulfide at different heating rates are analyzed in combination with the pyrolysis kinetics analysis method,and the experimental study of inactive thiophene sulfide under the action of oil paper insulation thermal field is carried out. The results of gas chromatography,mass spectrometry and Fourier transform infrared spectroscopy show that the main activation product of thiophene sulfide (thiophene,benzothiophene,dibenzothiophene) during pyrolysis is H₂S,which is highly corrosive and volatile. The thermogravimetric and differential thermogravimetric curves of thiophene sulfides are basically the same at different pyrolysis rates,and thiophene is the most prone to pyrolysis,followed by benzothiophene and dibenzothiophene. Under the condition of low temperature superheating of oil paper insulation,due to the continuous accumulation of system energy,the inactive thiophene sulfide will be activated and then generate low molecular highly corrosive sulfide,which intensifies the corrosivity of oil products and leads to sulfur corrosion of oil paper insulation.

Abstract:The zero-value piece of insulator string can be detected by measuring its electric field strength. The traditional way is always by measuring the electric field distribution curve of the whole string to judge out the zero-value piece position,which is inefficient and difficult to operate in field. In this paper,an improved method of identifying zero-value insulator by just measuring the partial electric field of insulator unit is proposed. Firstly,the simulation model of insulator string zero-value unit detection is established,and the partial electric field distortion characteristic caused by zero-value piece is analysed. Then the partial electric field variation criterion for zero-value identifying is obtained basing on the simulation result. Finally,the identification method is verified by designing the detection device and carrying out the point to point zero-value piece evaluating experiments. The results show that the point to point zero-value insulator detection can be realized using the three-array-probe electric field measuring method. The length of the array should be larger than the length of the electric field distortion interval,which can be taken as 130 mm for the research object in this paper. The identification criteria is that the deviation between probe 1 and 3 is less than 10%,and the deviation between probe 2 and the average is less than 50% at the same time. When detecting zero-value piece at the high-voltage and medium-voltage end,the best position for the sensor device identification is directly facing the steel foot,while for the low-voltage end,the best position is facing the shed edge. The results of the study can provide theoretical support for the intelligent operation and maintenance of external insulation in power transmission and distribution.

Abstract:Temperature plays a crucial role in influencing the mechanical properties of materials. Accurately measuring the temperature of devices is essential for understanding the evolution of their mechanical properties under stress and evaluating their health and lifespan. However,traditional methods encounter challenges in measuring transient temperatures and lack sufficient time-resolution capability,particularly when it comes to the rapid temperature changes at the solder interface during the switching process of power devices. In this paper,based on the close correlation between the intensities of the characteristic spectral lines of the laser-induced elements and the temperatures,a method of measuring the surface temperatures with the time-resolved capability of the order of microsecond is proposed,and a quantitative relationship between the surface temperatures of the sample and the spectral characteristics is established. The findings demonstrate that an increase in the surface temperature of the material results in enhanced intensity and signal-to-noise ratio of laser-induced plasma spectra. This enhancement is influenced by the spectral acquisition delay and gate width. To establish a quantitative relationship between surface temperature and spectral properties,back propagation-artificial neural network (BP-ANN) and partial least squares (PLS) are employed for fitting and calibration. The fitted models can achieve linear correlation coefficient indexes exceeding 0.99. Notably,the BP-ANN fitted model exhibites a small fitting bias,with a root mean squared error (RMSE) of 2.582 and a correctness rate of 98.3%. The method provides an effective means for transient temperature measurement of objects and gives a strong support for the assessment of the health status of the soldering interface of power devices.

Abstract:In order to analyze the loss,temperature rise and the mechanical structural response under the combined magnetic tension and thermal stress of the stator core,the electromagnetic-temperature-stress coupling calculation for the stator core of a synchronous generator is carried out in this paper. Firstly,the core loss and the magnetic pull per unit area are theoretically deduced,followed by an analysis of the core's temperature rise characteristics. On this basis,the mechanical structure response of the core under the coupling excitation of magnetic pull and non-uniform thermal load is obtained. Then,a three-dimensional finite element model of the CS-5 synchronous generator is established. This model calculates the magnetic pull per unit area,loss curve and temperature distribution of the stator core. Furthermore,the deformation,strain and stress of the stator core under the simultaneous action of magnetic tension and thermal load are obtained. The results show that the stator slot temperature is highest when the generator runs stably. The deformation at the groove is largest and the stress at the bottom of the groove is higher. Finally,the temperature rises of the end face,inside slot and outside circle of the stator core are monitored in real-time by thermocouples and a temperature monitor. The measured temperature distribution of the stator core is in good agreement with the finite element simulation results,verifying the effectiveness of the electromagnetic-temperature-stress coupling method. In this paper,the temperature distribution of the stator core and the mechanical structural response distribution of the stator core under magnetic and thermal coupling excitation are obtained,providing a technical reference for the reverse optimization design of the generator structure and the prevention of stator core deformation.

Abstract:Aiming to address the problems of high cost,large size and complex structure of the traditional back-to-back three-level converter,this paper proposes a new dual-port three-level converter topology. This new topology reduces the number of switching devices and optimizes the system size through device multiplexing. Firstly,the operating principle of the proposed converter topology is analyzed in detail,outlining the six effective switching states,current flow paths,and corresponding dual-port output levels of the single leg,while also discussing and the voltage stresses of each switching device. Secondly,a level-shifting pulse width modulation for the proposed converter topology is investigated and designed,and the effect of modulating wave overlap is avoided by adding DC offsets. Thirdly,the key problems of the modulation scheme under both the common frequency and different frequency operation modes,such as the modulation regime range and phase angle difference constraints are analyzed. The principles for selecting DC offsets and constraint range of two-port modulation system in each operating mode are introduced. Finally,the different operating modes are verified using the DSP-FPGA-Typhoon HIL 402 experimental platform. The experimental results show that level-shifting pulse width modulation with DC offsets makes the output power quality of the converter better and the harmonic content lower on the premise of achieving stable output of voltage and current for the proposed converter topology.

Abstract:Currently,there are issues such as insufficient research on the physical characteristics of windings in grounding transformers under outlet short circuit conditions and a high number of damage accidents. Given this issues,a dry-type grounding transformer is chosen as the research subject. A three-dimensional multi-physics field coupling model is constructed using finite element simulation software. The aim is to analyze changes and distribution patterns of multiple physical parameters such as winding electromagnetic fields,winding temperature rise,and winding stress deformation under the outlet short circuit condition of the low voltage side of the grounding transformer. The key factors influencing grounding transformer's ability to resist outlet short circuit are also explored. Results indicate that after an outlet short circuit occurs in the grounding transformer,the magnetic flux and equivalent stress on its winding side,influenced by its yoke core and adjacent pillar cores,are greater than those on the front side,resulting in extremely uneven overall deformation. Due to the grounding transformer's special winding structure and operating mode,the transient temperature of its low voltage winding during outlet short circuit faults can reach 140.9 ℃,which is much higher than that of the high voltage winding. Finally,measures are proposed to improve the grounding transformer's ability to resist outlet short-circuit based on the dual constraints of dynamic and thermal stability,and structural optimization is carried out for the weak areas of grounding transformer to resist outlet short-circuit.

Abstract:If a subsequent commutation failure occurs in the line commutated converter-based high voltage direct current (LCC-HVDC) transmission system,it will have a seriously negative impact on the safe and stable operation of the AC-DC hybrid power grid. Aiming at the problem of commutation failure during the current deviation control stage in the fault recovery process of the LCC-HVDC system,the relationship between current deviation control parameter and commutation failure is analyzed theoretically. It is found that to avoid commutation failure at this stage,the DC voltage and AC commutation voltage at the inverter side of the LCC need to meet certain constraints. These constraints are directly affected by the current deviation control parameter. Based on the theoretical analysis results,a current deviation control parameter tuning method is proposed,which can improve the control requirements for DC voltage recovery speed and degree during the fault recovery process of the system. The method makes it easier for the system to meet the stable operation constraints between DC voltage and AC commutation voltage,thereby reducing the probability of subsequent commutation failure. Finally,the CIGRE standard test model in the PSCAD/EMTDC simulation platform is used to verify the correctness of the theoretical analysis and the effectiveness of the parameter tuning method.

Abstract:Partial discharge at the oil-paper interface is one of the main reasons for the failure of oil-paper insulation in oil-immersed transformers. In this article,based on the numerical model of multi-branching streamer discharge,a multi-branching streamer discharge model in oil-paper insulation system is constructed by further considering the charge transport characteristics of oil-paper interface. The surface discharge characteristics under the influence of the dielectric constant ratio between natural ester insulating oil and insulating paper,and the spacing between needle electrode and paperboard are studied by finite element method,respectively. The results show that the distance between the needle electrode and the paperboard,and the dielectric constant ratio of insulating oil and insulating paper both significantly affect the streamer branching. The smaller the clearance between the needle electrode and paper-board is, the more serious the inhibition of streamer branches at the oil-paper interface is,which resulting in the streamers near the oil-paper interface are more significant effect on the propagation of the z-axis streamer. When the dielectric constant of the insulating oil is larger than that of the paperboard,the development and branching of the streamer in the oil will be more significant. On the contrary,the streamer is more likely to develop along the surface of the paperboard and accumulate charge at the oil-paper interface.

Abstract:When the earth faults occur in different phases of two feeders connected to the same bus in the resonant grounding system,the conventional fault line selection devices are inaccurate. To address the difficulties of fault line selection in the earth faults that occurred in different phases of two feeders connected to the same busbar for resonant grounding system,the line zero-sequence admittance is calculated by the zero-sequence current at the first end of the line and the zero-sequence voltage of the bus. The analysis of the fault characteristics indicates that,the zero-sequence admittance amplitude in faulty lines is larger than that in the non-faulty line,and this characteristic also appears in the high-impedance grounding faults. Based on the ratio of zero-sequence admittance amplitude,a fault line selection method for earth faults occurring in different phases of two feeders connected to the same bus in a resonant grounding system is proposed. The difference in zero-sequence admittance amplitude between fault and non-fault lines is used to establish a line selection criterion,which allows all fault lines to be selected. The results simulated by PSCAD confirmed that the proposed method works effectively,with high accurary. This method is not affected by the fault location and the compensation degree of the arc suppression coil,and it can also withstand high fault resistances.