The paper discusses two wide-scope soft-switching multi-level DC-DC Converters (MMDCs), and a comparative evaluation of these FB MMDCs is also presented. The discussed topologies have all merits belonging to conventional FB MMDCs, for example, smaller voltage stress on the primary switches, and no added primary clamping devices and modular primary structure. In addition, the primary switches of each converter can obtain zero voltage switch (ZVS) or zero current switch (ZCS) in a wide load range. The proposed FB MMDCs are assessed and evaluated based on their performance, components and topology structure indices, such as soft switching characteristics, current stress, power loss distribution, number of added devices and complexity of structure. Experimental results verify the feasibility and advantages of the new topologies.

A modular multilevel converter (MMC) parallel system based on half-bridge sub-module (HBSM) is applied in order to build the integrated test equipment for large-capacity offshore wind turbine generator.The operation principle of the device and its primary parameter selection method are elaborated in detail. Power electronics technology is adopted to send low/high voltage signals. The voltage signal is synthesized from the reference voltage, the fundamental frequency voltage difference, and the harmonic command voltage, which are given by split-phase V/F control, and consequently the amplitude, phase and frequency of the output AC voltage can be adjusted. PSCAD /EMTDC simulation results show that the system designed can meet the grid-interconnection requirements for both fault ride-through and grid adaptability tests, including high/low voltage ride-through, voltage imbalance, frequency deviation, harmonic output, voltage fluctuation tests for large-capacity offshore wind turbines.

Due to its solderless, wireless, and double-sided heat dissipation, the press-pack IGBT (insulated gate bipolar transistor) is widely used in large-capacity inverters. Its reliability and life prediction have also attracted the attention of academics and industry. In this paper, a finite element simulation method for temperature field of press-pack IGBT module based on equivalent conductivity is proposed. It directly reflects the characteristics that the power dissipation of the internal chip of the press-pack IGBT module changes with temperature. This method also further improves the accuracy of temperature field simulation, and establishes the basis of simulation calculation for module reliability analysis and life prediction. In addition, the temperature field simulation of a certain type of crimping IGBT module under MMC conditions is obtained, and the internal temperature distribution of the module under this condition is obtained.

In order to promote the development of urban Energy Internet and meet the new needs of urban energy reform, the construction of urban Energy Internet based on flexible DC distribution network is proposed. Then, the key technology analysis on the core network of the physical infrastructure layer of the urban Energy Internet, namely the flexible DC distribution network, has been carried out. Then a complete set of technical system including planning and design, key equipment development, operation control, fault protection and other of the multi-terminal flexible DC distribution network is established, which provides strong technical support for the planning and construction of flexible DC distribution network. Finally, the design and analysis of the flexible DC distribution network demonstration project have been carried out, the overall construction plan of the project has been proposed. And Zhuhai Tangjiawan three-terminal flexible DC distribution network project with the largest capacity and highest voltage level in the world has been successfully put into operation, which lays a solid foundation for advancing the construction and operation of flexible DC power distribution technology in China, providing support for the development of the urban Energy Internet and building an international energy transformation city.

In order to solve the problem of voltage stability of DC distribution network bus, a variable gain linear extended state observer (VGLESO) is designed, which effectively solves the problem of peak output of the traditional high gain linear extended state observer (LESO) in the initial stage of operation. Based on this, a variable gain sliding mode active disturbance rejection control (ADRC) strategy is proposed, which is applied to the voltage outer loop of AC-DC bidirectional converter control system in DC distribution network. The variable gain sliding mode active disturbance rejection control strategy can track and compensate the total disturbance of the system without adding additional current transformer, effectively restrain the voltage fluctuation of the bus and improve the dynamic response of the system. The stability of VGLESO and variable gain sliding mode ADRC is proved theoretically. Finally, the feasibility and correctness of the control strategy are verified by MATLAB /Simulink.

Polypropylene has been used in radio-frequency capacitors and power cables as composite insulating material. While being mixed with natural clay nano-particles, it can be used as a nanocomposite insulating material in practical engineering. The objective of this study is to evaluate the electrical properties of polypropylene filled with natural clay as nano-material. A comprehensive test platform for testing electrical breakdown characteristics of material samples is setup. Polypropylene samples having 0%, 2% and 6% of natural clay by weight are exposed to voltages at two different rates-of-rise while measuring the RMS current. Breakdown voltages of each samples are also determined at two different ramp-rates. The important findings of this study are:1) The breakdown strength of the natural nanoclay filled polypropylene is higher than the unfilled polypropylene and the optimum concentration of nanoclay appeared to be 2% by weight; 2) Within the scope of the experimental samples in this article, the addition of nanoclay particles at 2% will significantly improve the electrical properties of the material. The results of this paper can provide reference for the application of polypropylene composite insulation materials.

In order to increase the anti-interference ability and fault location accuracy of DC resistance fault location method, the error causes of the DC resistance method using in high voltage cable jacket fault location is analyzed, the measures of reducing errors of cable jacket fault location are given. According to the principle of DC resistance method, fault location error sources are analyzed. Functional expressions of voltage measurement and current measurement as well as result of cable jacket fault location are derived. On this basis, ground stray current interference effect for voltage measurement and current measurement is analyzed, the law of propagation and combination of the cable fault location error in DC resistance method are also analyzed. An error factor of cable fault location relative to circuit components parameters, frequency of interference signal, counting time of voltage-to-frequency converter, as well as injection DC current is also analyzed. Some measures aiming at reducing errors of cable jacket fault location are proposed. Calculation results and experiment data verify the proposed measures.

Due to the construction of urban underground corridors, transmission lines are often parallel to gas pipelines, which may lead to the loss of adjacent natural gas pipelines when the lightning striked transmission lines. In order to study the impact of lightning transmission lines on natural gas pipelines, this paper establishes the scattered flow model of the adjacent pole tower auxiliary grounding network by COMSOL Multiphysics simulation software to analyzes the factors, such as the external extension, area, laying direction, the number of connecting lines and soil resistivity on the pole tower ground resistance, how to change the voltage peak of the natural gas pipeline. Herein, a pipe overvoltage protection scheme based on the auxiliary ground network of the pole tower is proposed. The simulation results show that reducing soil resistivity, increasing the extension, area and number of wires will reduce the voltage peak of the natural gas pipeline. The 135° laying auxiliary grounding grid has the best effect on the peak protection of the induction voltage of natural gas pipeline. The conclusion of this paper can provide reference for the construction of urban comprehensive energy corridor.

Transient recording fault indicator is often used in conjunction with the distribution automation master station to diagnose distribution network single-phase grounding fault in practical engineering application. In view of the problem of reverse, or wrong phase during fault indicator installation process,waveform preprocessing method based on steady state characteristics is proposed. In view of the problem of asynchronous three-phase recording, total least square-estimation of signal parameters via rotation invariance techniques(TLS-ESPRIT) waveform synchronization correction is proposed to improve the credibility of the recorded waveform. Aiming at the problem such as low sampling frequency and large transient waveform characteristic error, a fault location algorithm based on the cor-relation coefficient method is proposed to improve the accuracy of single-phase grounding fault location in the master station. Finally,field actual cases verify that the method proposed in this article can effectively correct the recorded waveforms,accurately locate the grounding fault,and improve the grounding fault handling capability of the distribution network.

In order to avoid the problem that the short circuit current control measures made by off-line extreme operation condition are too conservative, an on-line decision-making method for limiting short circuit current and meeting the requirement of security and stability is proposed in this paper. After determining on-line short circuit current calculation method by referring to relevant calculation standards, the objective function is established, which satisfies the short circuit current security and the minimum influence on the security and stability of the system. Based on the cluster parallel computing platform, a two-stage optimization method is used to search the control measures. Firstly, by solving the integer programming problem with the least number of control measures, some security and stability check schemes which can meet the requirement of short-circuit current security are obtained. Then, based on the security and stability check results, the control measure of limiting short-circuit current is selected according to the objective function. The effectiveness of the proposed method is verified by actual cases.

The high temperature superconducting (HTS) dynamic synchronous condenser (DSC) attracts close attentions because of excellent reactive compensation characteristics such as fast response time and small synchronous reactance. In this paper, the overall electromagnetic design of a 10 Mvar-class HTS DSC is designed and analysed. The rated terminal voltage and armature current of the air-core water-cooled stator are 11 kV and 525 A, respectively. The excitation winding wound by 10 mm wide REBCO high temperature superconducting material coated conductors, is cooled to below 30 K through helium gas. At rated gap field 1.4 T and no-load excitation current 375 A, the maximum flux density on the REBCO coils is 3.05 T. The analysis of the screening and damping layer shows that it has some conflicted features and needs more accurate design. A simple circuit simulation is carried out to calculate the reactive compensation performance. At the full-load excitation current 428 A, this prototype can output 10 Mvar reactive power.

Recently the issue of voltage stability has become a major challenge for the safe operation of the power grid. The new generation synchronous condenser and STATCOM are receiving more and more attention due to their excellent voltage support capabilities. This paper first briefly introduces the basic principles and characteristics of the new generation synchronous condenser and STATCOM. Then, these two types of reactive power compensation equipment are compared technically from three aspects:coping with system voltage sag, voltage surge, and impact on short-circuit current. Based on the comparison results, the reactive power compensation equipment choice suggestion in different application scenarios are given from a technical perspective. Finally, the paper compares the economics of the new generation synchronous condenser and the chained STATCOM using the isochronic method, and gives suggestions for the selection of reactive power compensation equipment in different application scenarios from the perspective of economics.

Micro-energy network realizes the complementary of distributed energy production according to the characteristics of users’ energy demand and local resource conditions. In order to realize the efficient operation of micro-energy network group, this paper proposes an optimal energy transfer model based on multi-energy collaboration in micro-energy network group. The concept of micro-energy network group is given and the basic structure of micro-energy network group based on the interconnection of external energy hubs is first established. Then, the micro-energy network model of internal energy hub and optimal energy transfer model are established. The optimal energy transfer model of micro-energy network group is subdivided into two operation phases, the multi-energy transfer optimization within micro-energy network and between micro-energy network. Phase 1 is to address the problem of energy shortage at the micro-energy network, while the phase 2 is to solve the problem that phase 1 failed to address through the coordination of multiple energy hubs in the micro-energy network group. Finally, the model is validated by a system including electricity, gas, heat /cooling load and external energy hub.

Aiming at a variety of network attacks on smart meters (SM), which are widely used in smart grid, may be subjected to in the process of measuring and monitoring power consumption, a new abnormal pattern detection framework is proposed to prevent energy fraud in smart meters. The proposed method first sends the user’s power characteristic data to the aggregator based on smart meter. And the distributed data model is used to aggregate the data to better solve the problem of user privacy protection. Then deep belief network (DBN) is used to compare the obtained data with the expected data to better obtain the data features and optimize the top-down features of the training results. Finally, the aggregator marks the SMs in clusters from 1 to N, and transmits the execution data to the meter data management system (MDMS), check through the deep belief network extraction features and replaces the faulty or damaged SM, for more accurate non-technical losses detection analysis. The experimental results show that the proposed method has higher detection rate and applicability than the traditional smart meter data anomaly detection.

Lightning trajectory is determined by movement of thundercloud. As a meteorological phenomenon, movement of thunderstorm is governed by the physical mechanism of the weather system and could be of a specific probability distribution. Based on the lightning data of Dawan District of Pearl River Delta, the time-domain distribution characteristics and trajectory statistics law of lightning are analyzed. The number of lightning days obeys Burr distribution and highly uneven distribution in time domain. The number of lightning in the top 50 days with most lightning accounts for more than 50% of all lightning throughout the investigated 10 years. In the top 40 days with most lightning, about 50% of the lightning storm move from Northeast to Southwest and 40% move from Northwest to Southeast. Meteorological investigation indicates that the moving statistical law of lightning storm is caused by the Northeast and Northwest Shihu wind of Dawan District. Potential application of the statistics of lightning movement is analyzed in the end.

Aiming at the problems appearing at the scene of the unmanned aerial vehicle (UAV) checking transmission lines, such as high requirements for hardware, long running time of the image recognition algorithm and the shortage in the number of training pictures, this paper proposes a new method for the daily inspection of transmission lines based on improved YOLOv3 pruning algorithm. The improved algorithm uses the basic YOLOv3 framework, adding the SPP module, extracting multi-scale features in the same convolutional layer and carrying out pruning treatments (reducing the channel numbers and sliming the framework) on the basis of the original YOLOv3. The improved YOLOv3 pruning algorithm is validated by the pictures of actual site inspection of transmission lines. The test result shows that this method has the advantages of less running time, diversity of input images size and lower hardware requirements for the algorithm. And also the accuracy of fault detection is almost unchanged, with the average iteration time of each step reduced by 0.81 s and overall performance improved by 25%.