Abstract:In order to meet the power flow regulation requirements of distribution network and improve the power flow regulation ability as well as response speed,a novel multi-line hybrid unified power flow controller for distribution network (D-MHUPFC) is proposed. Based on Sen transformer,unified power flow controller (UPFC),and hybrid on-load tap changer,D-MHUPFC can quickly adjust the multi-line power flow of distribution network. Compared with the traditional regulation mode,D-MHUPFC has the advantages of compact structure,fast response,good economy,and high reliability. Then,the multi-line power flow equation considering D-MHUPFC is derived with the ZIP load model,and its cooperative control strategy is optimized. Finally,the effectiveness of proposed D-MHUPFC is proved by a 10 kV distribution network simulation platform. The results show that D-MHUPFC and its control strategy can quickly adjust the multi-line power flow within 0.15 s,and transfer the overload power as well as improve the section transmission limit. Moreover,D-MHUPFC can realize decoupling control,and the compensation errors are less than 1%. It is verified that D-MHUPFC has the same power flow regulation ability as UPFC.

Abstract:A hybrid direct current transformer (DCT) with modular multilevel structure and series-connected switches experiences current spike in sub-module branch when using original control method. The existence of current spike causes extra current stress on components. Based on the operating principle and current waveforms,an optimization modulation method is proposed,which is based on the phase shift of medium-voltage side full-bridge. The current stress of series-connected switches branch is analyzed,combined with limitation of phase shift angle for operation safety,the algorithm for the optimum phase shift angle is determined,and the optimized control strategy is established. This strategy is independent of the original power control loop,so it does not change the power transmission status and interfere with power control,which makes it easy to be put into practical application. The effectiveness of current stress reduction is verified by simulation and prototype test,and a certain improvement of efficiency is shown by the results of prototype test. Based on the analysis of operation principle and simulation and test results,the optimal control strategy is proved to have positive significance for the safe operation of equipment and selection of components.

Abstract:The unreasonable planning of the access location and capacity of distributed generation (DG) affects the distribution network. The flexible power distribution unit has the advantages of flexible power flow distribution and voltage improvement. The steady-state calculation model and control strategy model of the flexible distribution unit are put forward. In order to locate and fix the capacity of DGs more reasonably,the comprehensive acceptance standard model of three indexes,namely life cycle cost (LCC),network loss and voltage deviation are established firstly. Then,the improved sparrow search algorithm (ISSA) is proposed to solve the model. Gaussian variation and Tent chaos are introduced to improve the global searching ability and convergence speed of the algorithm. Finally,the IEEE 33-node distribution system is taken as an example for simulation analysis. Considering the two cases of unconnected and connected flexible distribution unit respectively,particle swarm optimization (PSO) algorithm,sparrow search algorithm (SSA) and ISSA are used to solve the problem. The results show that the power flow of the system and the system operating status after the DGs are configured can be improved by taking into account the flexible distribution unit. And the optimization result of the ISSA is more efficient and accurate than the optimization results of other algorithms.

Abstract:In hybrid AC/DC microgrid,interlinking converter (ILC) can balance power flow between AC and DC microgrid,thus stabilizing AC frequency and DC voltage in islanding mode. As a topology of ILC,modular multilevel interlinking converter (MMILC) has advantages of good harmonic characteristics,high quality waveforms and low switching loss. A two-stage droop control strategy for MMILC is proposed. Firstly,based on the energy fluctuation characteristics of the sub-module capacitance,a new unified droop characteristic of AC and DC side for MMILC is established. Then,a mathematical relation between DC voltage and equivalent sub-module capacitor voltage is given to adjust power reference according to deviation of dc voltage. Also the dead zone and an allowable minimum operation range of the droop characteristic are set. Finally,the control performance of MMILC in different load profiles and overload conditions are simulated. The simulation results show that the proposed two-stage droop control strategy of MMILC can automatically compensate the power to both AC/DC sides in different load profiles. Under overload conditions,MMILC can instantaneous block the active power transmission,which validates the correctness and availability of the control strategy.

Abstract:As a new type of power electronic device to replace traditional mechanical switches,the soft open point (SOP) has the advantages of fast response speed and controllable power flow. In this paper,a novel cascaded H-bridge SOP with shared modules is proposed,which simplifies its structure and saves some modules. However,the traditional control method is no longer applicable,so a new square-wave-like modulation method suitable for common modules is proposed in this paper. Firstly,the modulation wave of the shared module is calculated according to the specific working conditions,and then the modulation wave of the non shared module is calculated in coordination to achieve the modulation of all modules of the new topology. Square-wave-like modulation adds zero level time on the basis of square wave modulation to reduce the instantaneous change value of shared module voltage during zero crossing. At the same time,the phase of the modulated wave can be changed by coordinating the zero level time and phase relationship to avoid the occurrence of over modulation of non shared modules. Finally,a three-phase experimental platform with common module SOP is built to verify the correctness and effectiveness of square-wave-like modulation strategy.

Abstract:The rapid development of new energy in the distribution network increases its flexibility while making it more complicated to operate and manage the network. For this reason,a novel flexible switch,which is composed of integrated gate- commutated thyristors (IGCT) and parallel voltage converters,is proposed in this paper. Applying virtual synchronous generator (VSG) control on the parallel voltage converterensures smoothness and steadiness while switching from one feeder to another. Firstly,the proposed flexible switch is compared with the flexible multi-state switch and the mechanical switch on their main characteristics. Then,the operating features of the flexible switch before,during,and after feeder fault are analyzed,as well as its switching logic. Finally,the switching process between different feeders is simulated on a control in-the-loop experiment based on RT-Lab. The voltage and current waveforms are obtained under different operating modes and during flexible switching. The experiment resultsshow that the flexible switch has short-term voltage support capability and does not generate voltage and current impulses during the switching process.

Abstract:Unified power flow controller (UPFC), static synchronous series compensator (SSSC),thyristor controlled series compensation (TCSC) and phase shifter (PST) can be able to adjust line parameters and flexibly control power flow. They have been applied in domestic and foreign power grids,and have played an important role in balancing power flow in transmission channels and improving power supply capacity. The theoretical research and engineering practice of UPFC,SSSC,TCSC and PST are summarized. Firstly,the basic structure and power flow control principle of four kinds of power flow control devices are analyzed,and projections of power flow control devices are introduced. Then,the key technologies of power flow control devices are summarized from five aspects:topology,converter technology,location and capacity,control strategy and fault protection. Finally,the typical application scenarios of power flow control devices in the future power grid are prospected,and the research direction of power flow control technology in the future is analyzed.

Abstract:To explore the factors affecting the arc extinction performance of the self-disengaging lightning protection device,a numerical simulation model of arc extinction process of the device based on magneto-hydrodynamic theory is established in this paper. The effect of the initial phase angle of the current and the peak airflow velocity of the device on the arc extinction performance of the device is investigated,and the validity of the model is verified by high-current arc-burning tests. The arc extinction time of the device is related to the initial phase of the current,and it decreases with the increase of the initial phase of the current in the interval of 0°~180° electrical phase. The peak airflow velocity of the device is critical to arc extinction. When the peak airflow velocity of the arc extinction is higher than 243 m/s,the arc can be extinguished in half a period and the re-ignition is suppressed. When the peak airflow velocity of the arc extinction is lower than 243 m/s,the 'arc blockage' appears at the outlet of the gas arc extinction cylinder of the device,leading to the arc re-ignition. The findings of this paper provide a theoretical basis for optimizing the arc extinction performance of air-blown lightning protection devices.

Abstract:In order to meet the grid connection requirements of new energy power generation and ensure the stable operation of the power system,in view of the security problems brought by the uncertainty and volatility of intermittent new energy power generation to the stable operation of power grid equipment,a bilevel programming model aimed at compensating the output error and stabilizing the grid connection power fluctuation is proposed. First of all,the capacity and error satisfaction rate characteristic curve is constructed to obtain the optimal energy storage capacity to improve the economy of the energy storage system. Then,the upper planning model takes the minimum output error as the goal. The charging and discharging power distribution strategy of the energy storage system is established,the cycle life of the energy storage battery is considered and unnecessary compensation values is set to avoid its overcharge and overdischarge. The lower planning model takes the minimum grid connection fluctuation rate as the objective function,and the model predictive control algorithm is adopted to carry out the advanced rolling optimization control for the compensated photovoltaic output,so as to achieve the smooth fluctuation of photovoltaic output. Finally,based on the above model,the model evaluation index function is established,and a 21 MW photovoltaic power station in Xinjiang is taken as an example to verify the feasibility of the strategy.

Abstract:The preventive control strategy can be solved quickly and effectively through the linear programming model based on active power sensitivity. However,the active power sensitivity obtained by the DC power flow method ignores the influence of voltage and network loss,which makes the control strategy checked by AC power flow difficult to meet the accuracy requirement. For this reason,an online decision-making method for overload preventive control of transmission equipment based on the control objects relaxation is proposed. Firstly,the key equipment is determined by overload security margin,and the active power sensitivity of adjusting measures to key equipment under different post-fault operating conditions is calculated. Secondly,the active power adjusting measures are clustered. Thirdly,a linear programming model for overload preventive control online decision-making is established. Finally,the active power limit of key equipment is relaxed according to the active power accuracy of preventive control,and the search direction of the optimal solution is determined by AC power flow calculation. The correctness and effectiveness of the proposed method are verified through the analysis of the actual power grid.

Abstract:Electric vehicles (EVs) can be used as energy storage devices to provide auxiliary frequency regulation service. However,the lack of inertia and damping caused by vehicle-to-grid (V2G) technology will affect the grid's stability. In addition, the users' charging plans are difficult to be completed on time by the current frequency regulation strategies. Therefore, the technology of virtual synchronous generator is utilized to make EVs have similar inertial and damping characteristics to synchronous generator. Then,based on the T-S fuzzy controller,an auxiliary frequency regulation control algorithm for EVs considering user's charging plans is proposed, and the V2G power is jointly determined by the charging states of EVs and the grid frequency. Simulation results demonstrate that the proposed strategy can intelligently control the power flow between the grid and EVs under the conditions of different charging plans, battery states and grid status. As a result,the proposed strategy can effectively improve the frequency stability of the power grid and avoid the risk of overcharge and overdischarge of the batteries while meeting the charging demand of users.

Abstract:Aiming at the transient overvoltage problem of high proportion new energy multi direct current (DC) transmission system caused by DC blocking fault,the interaction mechanism between multiple DC lines is analyzed firstly and the multi-outfeed voltage infection factor is introduced,which reveals the fundamental reason of transient overvoltage of other sound DC near area alternating current (AC) systems caused by DC blocking. Secondly,based on the quantitative relationship between reactive power output of the synchronous condenser in different time scales and rectifier side commutation bus voltage and improved rectifier side control of DC system,the capacity of the synchronous condenser installed on the rectifier side bus is configured and the required investment capacity is reduced. Thirdly,based on the above strategy,integrating the reactive power regulation capability of the synchronous condenser and rectifier,a transient overvoltage suppression strategy with coordinated cooperation of multi DC and multi reactive power equipment is proposed. Finally,a double-circuit DC transmission system model is built in PSCAD simulation platform to verify the suppression effect of the proposed coordination strategy on transient overvoltage and the configuration effect of condenser capacity.

Abstract:Fault trip of overload line transfers the power flow to other lines in the associated transmission section, which results in interlocking tripping accident. The key to prevent cascading tripping accidents is to quickly formulate a reasonable and effective power flow control strategy to eliminate line overload. In view of the defects of low accuracy of sensitivity method and too much calculation of optimization method, a double-layer optimal power flow control strategy of transmission section based on positive and negative comprehensive sensitivity is proposed. Considering the overall regulation effect of nodes on overload and heavy load line power flow, the positive and negative comprehensive sensitivity index is proposed, and then the power flow control strategy is quickly formulated by double-layer optimization method. In order to prevent the surge of computation caused by too large search space, a multi-objective selecting optimization model of the first layer scheduling nodes is established to optimize the scheduling nodes involved in the adjustment. On this basis, the second layer power flow regulation optimization model is constructed to obtain the accurate output adjustment of the scheduling nodes. The above model realizes the accurate dispatching of power flow out of limit on the transmission section. The simulation results of IEEE 39-bus system and a provincial power grid verify the rapidity and accuracy of the proposed method.

Abstract:The modular multilevel converter (MMC) is widely used in the field of high-voltage and high-power transmission due to its advantage of modular structure. As a large number of submodules (SM) are contained in the MMC,the switch failure of SM is one of the key issues affecting the reliability of MMC. The switch open-circuit fault is usually hard to be detected immediately,which threatens the normal operation of the system. A SM open-circuit fault localization method based on Pauta criterion is proposed in this paper. Firstly,the open-circuit fault characteristics of SM are analyzed in this paper. The SM open-circuit fault causes the abnormal changes of the SM capacitor voltage,which can be judged by the Pauta criterion. Then,a confidence interval is constructed by the average SM capacitor voltage and three times of the standard deviation of SM capacitor voltages in an arm. The SM open-circuit fault can be located through judging whether the SM capacitor voltage exceeds the confidence interval or not for a certain time. The proposed method does not require additional sensors,creation of accurate mathematical models and manual setting of empirical thresholds,and its algorithm is quite simple. Finally,a three-phase MMC is simulated with PSCAD/EMTDC and a single-phase MMC prototype is built in the laboratory to verify the proposed method. The simulation and experimental results both confirm that the proposed Pauta criterion based SM open-circuit fault localization method can locate the SM open-circuit faults effectively and quickly.

Abstract:In the true bipolar DC distribution network,due to the difference in cable impedance,the traditional bottom droop control has inherent contradictions between reducing voltage deviation and improving shunt accuracy,which affect the stability of the system and the economy of operation. Based on the underlying control,a voltage-current secondary control strategy based on DC/DC transformer (DCT) suitable for true bipolar topology is proposed. The control strategy is based on communication to collect the current of parallel DCT and the voltage of series DCT with different polarity. The consistency algorithm is adopted to achieve convergence of current control and voltage control, thereby achieving the optimal load distribution and effectively suppressing voltage imbalance. A true bipolar DC distribution network model is built by PSCAD/EMTDC. The simulation and evaluation results verify the effectiveness in diverse working conditions,adaptability to different topologies and reliability when a fault occurs in a distribution network of the proposed control strategy.

Abstract:The high-precision time synchronization of each node in the power distribution field network is an important technical support for realizing real-time and refined business. For the power distribution field network based on high-speed power line broadband carrier communication,a step-by-step precision time synchronization strategy is designed. Firstly,the clock of the central node is calibrated in real-time by the communication master station through the long-distance communication network and used as the master clock of the field network. Secondly,relying on the low latency,the broadcast mechanism,and the network time base of the field network,the time and frequency of the clocks of each node in the network are accurately monitored and their deviations are obtained. Thirdly,the clock frequency is adjusted by re-calibrating the temperature compensation coefficient of the node clock,so that the node time is gradually synchronized within a timing cycle. Finally,the time synchronization performances of the field network are tested in the laboratory and job site. The test results show that the time synchronization accuracy of the field network is within ±0.23 s during a day,which is 28% higher than that of the power distribution field network based on narrowband power line carrier,and these phenomena of time gap or time repetition are avoided in the time synchronization process.

Abstract:At present,the fluctuation of large-scale wind-photovoltaic power output and load increases the demand for operational flexibility in the distribution network. Soft open point integrated with energy storage system (ESOP) contains both the flexible power regulation ability of soft open point and the power storage ability of energy storage system,which can improve the operational flexibility and economy of the distribution network. Firstly,the principle and mathematical model of ESOP are elaborated. Then,two distribution network flexibility evaluation indexes including net load regulation and branch load margin are proposed from the perspective of node and grid flexibility resources respectively,and the distribution network operating costs and ESOP planning and construction costs are proposed as the economic indicators. After that,an optimal planning model with the flexible and economic operation of the distribution network with ESOP as an objective function is established. Finally,the model is solved by the second-order cone programming algorithm. Through the simulation of the modified IEEE 33-bus system,the results show that the proposed flexibility evaluation indicators can accurately quantify the operational flexibility of the distribution network,and the proposed optimal planning model can realize the operational flexibility and economy distribution network.

Abstract:Most serious breakdown faults of cross linked polyethylene(XLPE) cables in distribution network occur at cable joints,which are mainly caused by process defects in the manufacturing and installation of cable joints. In this paper,the charge simulation method is used to study the characteristics of electric field distribution near four typical defects of cable joints,namely,air gap,water film,metal debris and metal breakout. The electric field calculation and random walk theory are combined to analyze the distribution of electrical tree caused by several defects. At the same time,the actual electric field distribution around the sample of XLPE cable joints in the distribution network with defects is measured,and the measured results are compared with the calculated results. The results show that the electric field measurement method can directly and effectively identify the types of internal defects of cable joints,and the electric field distortion caused by conductive defects is more significant than that caused by air gap and water film defects. When the electric field distortion caused by defects is greater than the critical electric field value,the electric tree will develop. The track length of the electric tree development is positively related to the field strength. The electrical tree caused by conductive defects is more likely to develop towards the cable core,which is more likely to cause the breakdown of XLPE insulation.

Abstract:Target recognition and temperature extraction of the typical component of gas insulated switchgear (GIS) are the key to realizing the infrared intelligent detection of equipment heating state. In this paper,an improved YOLOv4 algorithm based on convolutional block attention module (CBAM) is proposed to achieve rapid target detection and hot spot temperature extraction of GIS bus,disconnector and other components. Firstly,the original infrared images are acquired at a substation site,and an infrared dataset containing typical GIS components is constructed by sharpening the images and marking components. Then,the deep separable convolutional network is used to reduce the amount of model parameters,and the CBAM is integrated to optimize the recognition ability of the model,on the basis of which a GIS infrared component target rapid detection algorithm with improved YOLOv4 is constructed. Finally,the gray-scale difference method is used to extract the temperature value of the hot area for the detected typical target components of GIS. The results show that the proposed algorithm can achieve a recognition speed of 31.5 frame per second and an recognition accuracy of 82.3% on the GIS infrared feature dataset,which is significantly better than other target algorithms. The error between the calculated value and the measured value of temperature rise of GIS components is within ±1℃. The algorithm proposed in this paper can be deployed in edge intelligent terminals such as unmanned aerial vehicles and inspection trolleys to achieve refined identification and rapid diagnosis of the temperature rise status of on-site GIS equipment,thus improving the digitalization and intelligence level of health management of GIS.

Abstract:Soil direct burial is widely used for laying power cables because of its advantages of short construction period and good heat dissipation performance. During the burial process,various laying conditions have an important impact on the temperature rise and load capacity of the cable. In this paper,in order to investigate the influences of various laying conditions such as backfilling sand, adding concrete protection plates and installing sleeves on the temperature field and load capacity of direct buried cables,Comsol finite element simulation software is applied and electromagnetic field,thermal field and fluid field are modeled on 110 kV cross-linked polyethylene (XLPE) power cables. The results show that backfilling with sand increases the cable load capacity,and the increase of the cable load capacity slows down with the increase of the backfilling thickness. The adding concrete protection plate has less effect on the load capacity,and the increase of the load capacity is less than 0.5% compared with that without the cover. The filling high thermal conductivity material in the drainage tube increases the cable load capacity,and the increase of the load capacity is higher than 50% when all three phases are filled with high thermal conductivity material and the middle phase is filled with high thermal conductivity material compared with the case without the filling. Compared with the case of no filling,the increase of load capacity when all three phases are filled with high thermal conductivity material,the middle phase is filled with high thermal conductivity material,and the other two phases are filled with low thermal conductivity material is higher than 50%. When the above factors are considered at the same time,it is favorable to the increase of cable load capacity.

Abstract:In order to comprehend the ageing characteristics profoundly and evaluate the ageing degree of cross-linked polyethylene (XLPE) cable insulation,the XLPE cable insulation is subjected to accelerated ageing experiment at 135 oC. The variation of elongation at break is characterized by the tensile test. The carbonyl concentration and antioxidant contents in XLPE are measured by Fourier transform infrared spectroscopy. Results show that the ageing of XLPE cable insulation exhibits critical time characteristic,namely elongation at break of XLPE decreases slowly and then decreases dramatically with ageing time. Meanwhile the carbonyl concentrations also increase slowly at first and then increases rapidly. The critical time is 2016h when aged at 135 oC. While the content of antioxidant gradually decreases without inflection point. Based on the chain free radical theory,the thermal ageing kinetics model considering the reaction of antioxidant is established,and the variation of antioxidant content and carbonyl concentration during the thermal ageing of XLPE cable insulation are furtherly calculated by simulation. Simulation results are in good agreement with the experimental results. The results indicate that the antioxidant content could reveal the ageing degree of XLPE insulation.

Abstract:There are problems such as building occlusion and electromagnetic interference in substations,which lead to a rapid decline in the accuracy of traditional personnel control methods based on electromagnetic wave positioning. In order to avoid the reduction of power safety management and control efficiency due to the degradation of single sensor positioning accuracy,it is very important to study the position estimation technology of patrol personnel based on multi-source information fusion. However,most of the existing fusion localization algorithms are difficult to robustly select sensor fusion strategies under the condition of unknown map information. A fusion positioning method based on satellite and near-ultrasonic signal feature analysis is proposed in this paper,which only relies on signal statistical features to realize environmental information discrimination and adaptively select fusion strategies. Firstly,the fingerprint database is constructed by using the multi-sensor signal feature statistical model,and the fingerprint library data is processed based on the t-distributed stochastic neighbor embedding (t-SNE) dimensionality reduction algorithm and the density peaks clustering (DPC) algorithm fingerprint library data. Secondly,a back propagation (BP) neural network is built based on the clustering results,and the signal environment features are mapped to the parameters of the Kalman filter. Finally,the neural network output is used to optimize the Kalman filter-based multi-source positioning switching model to form an adaptive fusion positioning algorithm. Experiments are carried out using data collected in a real substation semi-occluded environment. The results show that,compared with the fusion positioning method of unknown environmental information and known environmental information,the proposed method saves map marking information when the map information is unknown and realizes highly robust location estimation.

Abstract:At present,the original drawing information of the substation is stored in a single way,and the degree of digital analysis is not high. In order to reduce the huge workload of manually analyzing the original drawing of the substation,a method is proposed to intelligently extract the information of the equipment and connection relationship of the substation based on the portable document format (PDF) drawing for building a physical circuit model,thus implementing the method according to the smart substation physical configuration description (SPCD) file. Firstly,the primitive information is extracted and processed from drawings. Then electrical symbol recognition is realized through string similarity matching running Karp-Rabin greedy string tiling (RKR-GST) algorithm,and substation images are classified by gradient boosting decision tree-logistic regression (GBDT-LR) hybrid algorithm based on features. Finally,the digital description from the original drawing to the physical circuit model is completed according to the SPCD file. Experiments show that the correct rate of electrical symbol matching is 93%,and the correct rate of physical circuit identification is more than 90% when there are errors in primitives.

Abstract:Spatial load forecasting faces the problems of multiple characteristic factors and data shortage. A spatial load forecasting method based on double-layer extreme gradient boosting (XGBoost) and data enhancement is proposed. Firstly,the area to be predicted is divided into several sub regions according to the supply range of feeder power. Secondly,a feature selection model based on double-layer XGBoost is constructed. The first layer XGBoost scores and sorts the features. The combined features are loaded into the second layer XGBoost for sub regional load forecasting. The best feature variables of each sub region are selected according to the load forecasting results. Then,the training set samples of each sub region are enhanced by the generative adversarial network (GAN),and the load of sub regions is forecasted through the extreme learning machine (ELM). Finally,the predicted values of sub regions are added to obtain the load of the region to be predicted. Taking local areas of Shanghai as an example,the simulated experiment and comparative analysis are carried out. The results show that the proposed method can solve the problems of characteristic variable selection and data shortage at the same time,and has high prediction accuracy.

Abstract:The flexible direct current (DC) grid based on modular multilevel converter (MMC) has a high current peak value and a very fast rising speed during a DC short-circuit fault. In order to limit the impact of fault current on the system in a short period of time,a fault current-limiting combined control strategy suitable for half-bridge MMC was proposed. By using the high controllability of MMC itself,the fault limit can be achieved without external current limiting device,and it can reduce the technical requirements for DC circuit breakers. Firstly,two different current-limiting links and their basic principles in the current-limiting control strategy are explained. Secondly,the influences of the two current-limiting links on the DC fault current,AC current and bridge arm current are analyzed respectively,and the calculation formula of DC fault current under current-limiting control is deduced. Finally,a half-bridge MMC four-terminal DC grid model is built on the PSCAD/EMTDC platform for simulation analysis. The results show that the current limiting combined control strategy can effectively limit the DC fault current and reduce the power and voltage fluctuations of the near-end converter at the fault point. It can also reduce the peak value of AC current and bridge arm current.

Abstract:With the access of high-density distributed power generation,the inertia level of power grid continues to decline. Especially under the condition of independent power supply of new energy,the inertia level of power grid is far lower than the minimum inertia demand. By applying the virtual DC motor control strategy to the port converter,the system inertia can be flexibly adjusted by using the control parameters to reduce the inertia difference of the power grid and enhance the stability of the power grid. However,the magnitude of the virtual equivalent inertia of the control method is not clear,and the influence of the virtual inertia on the equivalent inertia needs to be further explored. Therefore, based on the virtual DC motor model,the small signal model is used to analyze the influence of virtual inertia and equivalent inertia on the dynamic characteristics of the system,and the equivalent relationship between them is pointed out. Secondly,based on the corresponding relationship between the virtual inertia and the equivalent inertia,an equivalent inertia calculation method which uses the voltage curve similarity measured by Frecher algorithm is proposed to determine the equivalent inertia of the virtual DC motor control. Then,the correlation constraint and curve feature constraint are introduced,and an improved Frecher algorithm is proposed to ensure the correctness of the equivalent inertia calculation. Finally,the optimal fitting method is used to realize the function of the equivalent inertia relationship,and the quantitative relationship between the virtual inertia and the equivalent inertia is given. The simulation results verify the effectiveness of the proposed equivalent inertia calculation method.

Abstract:Power battery performance plays a pivotal role in the comprehensive performance of electric vehicles,and thus accurate identification of the parameters of the lithium-ion battery model is crucial for subsequent state-of-charge estimation and state-of-health prediction of the battery system. In order to improve the accuracy of parameter identification algorithm of lithium-ion battery model,a second-order RC equivalent circuit model of the battery is established,and a least-squares algorithm based on variable forgetting factor is used to identify the parameters of the lithium-ion battery model online. By building a test platform for charge and discharge experiments,based on the experimental data of two different operating conditions,the proposed algorithm,recursive least squares algorithm and traditional forgetting factor least squares algorithm are used to identify the parameters,and the accuracy of the proposed algorithm is described based on the comparison of the error between the estimated port voltage and the actual value obtained from the experimental test. The experimental results show that the recursive least squares algorithm based on the variable forgetting factor shows fast convergence and high estimation accuracy in the identification of lithium-ion battery parameters.

Abstract:In order to improve the detection efficiency of zero value insulator of transmission line,the unmanned aerial vehicles (UAV) equipped with an electric field detection device can realize live detection without climbing the tower. Firstly,the 1:1 simulation model of 220 kV porcelain insulator string is established. Through Comsol finite element numerical simulation,the influence of string type,path jitter,UAV and electric field probe on the spatial electric field distribution of insulator string is analyzed. Then,the identification model and background detection system of zero value insulator are established. Finally,the laboratory and field measurements are carried out. The results show that when double insulator strings have zero value insulators at the same time,their spatial electric field distribution does not affect each other. The offset distance of straight line detection path should be less than 50 mm during UAV flight. The space electric field distribution of insulator string is not affected by UAV and electric field probe. Using the zero-value insulator detection algorithm proposed in this paper,the field detection of zero-value insulators is realized by the UAV equipped with an electric field detection device.

Abstract:Currently,thermal power units need to undergo deep peak regulation transformation to improve the power grid's ability of large scale new energy consumption. In this paper,the deep peak regulation optimal scheduling of thermal power units under large-scale new energy grid connection is studied. Firstly,the cost of deep peak regulation of thermal units is studied and the different cost mathematical models of deep peak regulation are proposed including energy consumption,operating compensation and spinning reserve capacity. Then,taking the lowest comprehensive operating cost as the objective function,a deep peak regulation scheduling model considering the operating constraints of thermal power units is established,and the branch and bound method is used to solve the scheduling model. Finally,the test system is composed of eight thermal power units,one wind farm and one photovoltaic power station. The deep peak regulation optimal dispatching of thermal power units under large scale new energy grid connection is analyzed from the aspects of peak regulation depth,new energy consumption and thermal power enterprise income. The analysis results show that the new energy consumption of thermal power units can be significantly increased by peak regulation of thermal power units,but the benefit of thermal power units decreases significantly with the further increase of peak regulation depth. Under the current compensation standard of deep peak regulation,the benefit of deep peak regulation of thermal power units is lower than that of basic peak regulation.

Abstract:With the development of multi-source AC/DC distribution networks,DC fault detection technology has become the key to DC protection. Aiming at the characteristics of large fault current with a rapid rise and difficulty to extract fault features when distribution line faults occur in the DC part,a fault detection technology of AC/DC distribution network based on deep belief network (DBN) is proposed combining feature extraction in the time domain and frequency domain. By analyzing the characteristics of the fault equivalent circuit,Fourier transform and phase-mode transform are used to extract the frequency domain and time domain characteristics of the fault current and voltage signals as the input of DBN,and the Softmax classifier is used to output the fault pole selection and fault area identification results. An AC/DC distribution network model is built on PSCAD to test the algorithm. The simulation result shows that the proposed detection method still has high accuracy and the strong ability to tolerate noise under the influence of line-distributed capacitance and control strategy. The further algorithm comparison experiment shows that the necessary combination of fault feature extraction and effective deep learning model training can complete the complex fault detection.