In this paper，the analytic hierarchy process（AHP） is used to analyse the influencing factors of power cable fire risk from four aspects： power cable operation parameters，cable fire safety，cable type and cable laying. Moreover，the evaluation set of power cable fire risk early warning signals is put forward，and the power cable fire risk assessment model is established. The results show that the early warning signal obtained by the power cable fire risk assessment model could scientifically and accurately reflect the fire early warning state of the power cable and provide guidance for the operation and maintenance of the power cable.

The wildfire is one of the main causes leading transmission line tripping，which endangers the operation of power grid seriously. In order to improve the wildfire prevention level of overhead transmission lines，a fire risk level prediction model of overhead transmission lines is proposed. This model is established based on the weather index of forest fire model as well as the impact of surface environment and historical fire behavior. Then a graph-based optimization theory is introduced to determine the weight of indexes. Finally，the forecast precipitation data of 3 km×3 km resolution is used to modify the risk level of the wildfire. This model has been successfully applied to Wildfire Monitoring and Early Warning Center of China Southern Power Grid. Based on the risk prediction，the transmission operation and maintenance staffs can inspect the high fire-risk transmission lines in advance. It reduces the probability of the overhead transmission line tripping caused by wildfires，which improves the safety and stability performance of the power grid.

To study the temporal and spatial characteristics of near-ground wind field during typhoon period and accumulate the field wind speed data around the coastal transmission line tower during typhoon，a domestic 3D scanning Doppler lidar is deployed in an in situ wind field observation campaign during the landing of No. 1604 Typhoon "Nida". Data from Doppler beam swing and plain position indicator scan modes are retrieved and analyzed，verifying the capabilities of the observation method based on Doppler lidar. The results indicate that the eye zone of typhoon "Nida" shows a typical "U" shape in air pressure while its eyewall zone presents a relatively typical "double peak type" shape in wind speed. The variation of wind speed，wind direction and gust coefficient with height is obvious between the strong wind area of eyewall zone and the eye zone of typhoon. Wind speed profile above 300 m height obviously deviates from the form of exponential law or logarithmic law profile. Wind direction deflects at a certain degree to the right side under the influence of geostrophic effect from near surface to upper air，and gust coefficient decrease with the increase of height and mean wind speed. The influence of terrain on local wind field can be distinguished in radar radial wind velocity distribution map of low elevation horizontal azimuth scanning，and the wind speed around transmission towers under different terrain is obviously different.

The transmission line breaking across high-speed railway is one of the important factors affecting the safety of power system. In this paper，the finite element model of transmission lines across high-speed railway sections is established to analyze the flow field disturbance in the process of high-speed train crossing the lines； wind load and dynamic characteristics of transmission lines are calculated. The vibration response characteristics of transmission lines in time domain are studied by numerical simulation method，and the influences of speed and length of train are considered. The vibration response characteristics of transmission lines in time domain are verified by field measurement. The results show that the flow field disturbance caused by high-speed train crossing will cause line vibration for more than 10 s，and the vibration frequency is close to the line modal natural vibration frequency，which is easy to cause line resonance. The vibration of the line can be reduced by increasing the height of the line and decreasing the span length of the line. The results of this paper are of great significance to the in-depth study of the structural design theory of transmission lines across high-speed railways.

Taking ±800 kV DC composite arrester of ultra-high voltage （UHV） Xinsong converter station as the research object，finite element models of arresters with additional guy cable are established. The analysis of the dynamic characteristics and the influence of initial prestress are carried out.The response of acceleration，displacement and stress of the arrester before and after setting guy cable are calculated and analyzed. The results show that the horizontal stiffness of the arrester can be improved by setting guy cable，and the natural frequency of the arrester can be increased. The larger the initial prestress of the guy cable is，the more obvious the fundamental frequency of the arrester will be. Increasing the initial prestress of guy cable can effectively reduce the seismic response of arrester. When the initial prestress of the guy cable is 100~150 MPa，the seismic response of arrester decreases most obviously； When the initial prestress of the guy cable reaches 200 MPa，the stress response of the root of bushings of the arrester can be greatly reduced by setting guy cable. The safety factor of bushing can be increased from 3.76 to 6.15，and the top displacement response of arrester under earthquake can be reduced by 50.2%. The effect of seismic reinforcement is significant，and keeping on increasing the initial prestress of the guy cable to higher than 200 MPa has little effect on the seismic capacity of arrester.

The transmission lines in southeast coastal areas of South China Power Grid are affected by typhoon and other gale weather all the year round. It is difficult to determine the actual anti-wind capacity of a large number of old transmission lines. The wind prevention and transformation of old lines has become an important task for disaster prevention and mitigation. In this paper，the evaluation method of wind energy reliability for transmission lines suitable for China is proposed by comparing with the evaluation regulations of wind energy reliability abroad，and the quantitative evaluation indexes are as follows： the anti-wind reliability of old transmissions lines can be characterized based on the reproduction period of maximum wind load that the structure can withstand，the maximum wind load that the structure can withstand is the largest wind load to satisfy the wind standard combination，different reproduction periods of the checking wind wind load can be calculated based on the 50-year baseline reproducible wind load multiplied by the reproduction period corresponding load factor.The paper gives the evaluation method of anti-wind capability based on reliability and puts forward the quantitative index，which make the evaluation and transformation of the transmission old lines in coastal areas more scientific and feasible.

In this paper，DC deicing technologies based on both full bridge modular multi-level converter （MMC） and thyristor rectifier are analyzed and studied. The basic operating principles and control strategies of these two technologies are described in detail. The simulation models are established to simulate the operation conditions of the two DC deicing technologies，and verify the conclusions drawn. These two technologies are compared in detail and their advantages and disadvantages are pointed out. Finally，an engineering application case of the full bridge MMC converter based deicer is introduced.

DC ice melting device is an indispensable device for ensuring the safe and stable operation of the power grid during the freezing period. This paper focuses on the special scene of ice melting device design in the HVDC converter station. The influence of the ice melting device on the power quality of ice melting AC bus，HVDC converter station AC bus，and AC filter equipment of converter station are studied. The design principles and methods of AC filter for the ice melting device in converter station are proposed，and it is verified by the design example and simulation. The proposed method can effectively reduce the influence of the ice melting device on power quality and equipment of the HVDC converter station，and ensure the safe and stable operation of HVDC transmission system and the ice melting device.

As a typical high-rise structure sensitive to wind load，transmission tower has certain risk of collapse in extreme weather such as typhoon. In order to study the wind-induced response of transmission tower and the mechanism of tower collapse，an equivalent scale model of the transmission tower is designed and tested in a wind tunnel. The dynamic response and local deformation of the tower model and its collapse process under strong transverse wind are recorded and analyzed in the wind tunnel test. Further，the structural weaknesses of the model and the change of vertical forces on the bottom main materials are analyzed and compared with the results of finite element method. The results show that the main materials of the transmission tower are susceptible to uneven vertical forces，which increases the probability of metal fatigue under single direction wind load and induces extreme strain in lower middle part of main material of the transmission tower. Therefore，reinforcement of specific parts of main material can effectively enhance the wind resistance of the tower.

In order to study the mechanical characteristics of the dry-type air-core reactor inclined supported by composite insulators under seismic，the seismic simulation shaking table test is carried out on dry-type air-core reactor and the responses of acceleration，strain，displacement of each part are analyzed based on Yuheng-Weifang 1 000 kV UHV AC transmission project. The test results indicate three points. Firstly，during the seismic simulation shaking table test，the acceleration amplification factor at the root of upper bracket is 2.80 which is the maximum value among each part. Secondly，after load effect combination，the maximum calculated stress of insulators is 35.73 MPa，the safety coefficient is 2.18 which meets the requirement of greater than 1.67 specified in the standard and the insulators are the weakest links of the whole support system. Thirdly，the reactor has no drop of insulation struts，no break of binding band，no drop of top cover，no deformation of spider arms，no crack and damage on composite insulators and no crack in its gluing position after the seismic test，which means that the reactor pass the test. This seismic simulation shaking table test provides the reference for the research on seismic performance of dry-type air-core reactor.

The ubiquitous Internet of Things provides more control methods and means for the distribution system to cope with the influence of extreme weather，enabling the distribution system to integrate all resources to achieve optimal scheduling. In this context，this paper proposes an emergency control method of distribution system to deal with extreme weather based on the ubiquitous Internet of Things. The proposed method couples the natural gas system with the power system and establishes an emergency control model for the distribution system under extreme weather. Moreover，the Monte Carlo simulation method is used to study the load loss of the distribution network under extreme weather，which is used as an indicator to measure the advantages and disadvantages of this method. Finally，the standard distribution system model is taken as an example to verify the validity of the proposed model.

The light-trigged thyristor valve monitoring system of EHVDC returns the valve-side threshold voltage signal to the valve control system in real time. The valve control system integrates this signal and the grid-side dispatching information for dispatching control and protection. Several abnormal events of return of thyristor voltage monitoring unit （TVM） occur in a certain station in China. In order to investigate the cause，the valve monitoring system is analyzed，several shortcomings are found in the TVM pulse width control circuit. This paper introduces the architecture of TVM and pulse width control circuit，analyzes the principles of three kinds of pulse width control circuits，which are positive zero-crossing voltage monitoring （POS），negative zero-crossing voltage monitoring （NEG） and BOD over-voltage monitoring （BOD），and gives the optimization scheme. In the experimental verification，the experimental results of the original circuit and the optimized circuit are compared under the condition of the effective value of 380 V and the pulse voltage of 7 kV，respectively，which fully verifies the effectiveness of the circuit optimization. It provides technical support and safety guarantee for the later upgrade and transformation of the valve monitoring system of light-trigged thyristor converter station at home and abroad.