The UHVDC sending-end power grid with large amount of renewable energy has serious transient overvoltage problems. Firstly, the structure of renewable power systems and the overvoltage protection logic of the convertor-based renewable generators are introduced. Then, the transient overvoltage characteristic of the high renewable power-penetrated UHVDC sending-end power grid under the DC blocking fault and the commutation failure is analyzed. And the mechanism of cascading overvoltage tripping of the renewable generators is analyzed in the meantime. On this basis, the local and global transient voltage rise severity indexes based on the multiple-two-element table are established under single fault and the multiple-fault set, respectively. Finally, the effectiveness of the proposed indexes is shown by some case studies of the BPA simulation system of Qinghai Power Grid in China.

With the expansions of the scale of HVDC transmission from west to east in the Southern China Power Grid (CSG) and the increase of the transmission power in Yunnan Power Grid, the coupling between the AC and DC stability control systems becomes deeper and the ability of regulation and control turns to be weakened, which further worsen the high-frequency problem of Yunnan Power Grid. Hence, a multi-DC coordinated stability control system is designed and implemented. It reduces the risk of water abandonment and supports the safe and stable operation of CSG by sharing the associated power sources amang HVDCs. In addition, in order to verify the multi-DC coordination stability control system, Xindong HVDC bipolar block field test is planned and implemented. The field test results verify that the stability control strategy is correct, and the devices acted reliably. At last, further improvements of the proposed system are also prospected.

The adjustment of HVDC tie-line power can promote the consumption of wind power. However, the reactive power control equipment will act frequently when tie-line power changes, which will reduce the service life of it and increase the probability of fault. The optimization model of HVDC tie-line power considering reactive power control equipment action times is established, which describes reactive power characteristics by the static operation model of converters, and considers the constraints for AC power flow, stepwise operation of tie lines and action times of reactive power control equipment. The mixed integer nonlinear programming problem is decomposed based on benders decomposition method, and the reactive power deviation penalty and the link constraints are established, which realize the unified values of reactive power control equipment between master problem and sub-problems. A case study is carried out based on the wind power export equivalent system, verifying the model can improve the benefits of coordinated optimization.

VSC-HVDC transmission is the main transmission mode for large-scale offshore wind power far from the coast transmission to the onshore power grid. In this paper, several implementation methods of bipolar transmission are proposed, and symmetrical monopolar connection is recommended based on the comparison between the symmetrical monopolar and the bipolar connection. In addition, DC voltage optimization scheme is given. The electrical characteristics differences between VSC-HVDC transmission system for the offshore wind farm integration and for other applications are analyzed. Considering the high reliability and compact design requirements of offshore converter stations, as well as the different characteristics of offshore and onshore converter stations, the conclusion of main connection of converter station at both ends is studied and put forward, the configuration of the VSC-HVDC transformers, pre-insertion circuit, bridge-arm reactors, DC energy dissipation devices and the grounding method are analyzed, and the converter station connection schemes of offshore and onshore converter stations are put forward which can simplify equipment requirements, save space and reduce investment on the premise of ensuring reliability.

The reliability model of VSC-HVDC system combined with offshore wind farm is established by using the sequential Monte Carlo method, considering the correlation between the wind speed of offshore wind farm and VSC-HVDC component failure. Firstly, the time-varying model of component reliability parameters considering extreme weather is proposed. Based on this, the continuous Markov chain Monte Carlo method is used to simulate the wind speed, and Copula is used to represent the correlation between the wind speeds to modify the reliability parameters of components. Secondly, according to the characteristics of numerous components and various failure modes of offshore wind farm and VSC-HVDC connected system, the wind farm and the system are divided into several subsystems, and reliability evaluation models for each subsystem are established respectively. Finally, all models are combined to establish the reliability evaluation model of the whole offshore wind farm and VSC-HVDC connected system is established. An example system consisting of two offshore wind farms and one three terminal VSC-HVDC system is analyzed, and the accurate and effectiveness of the proposed model and method are proved.

The heat generating mechanism and dissipating feature research are carried out on the welded IGBT device in high voltage flexible DC converter valve. Based on the insulated gate biopolar transistor (IGBT) heat characteristics, the total thermal resistance is tested by transient hyperbolic method. The total thermal resistance corresponds with caculated results according to structural parameters and heat conductivity coefficients, which are derived from each layer of IGBT. This validates the heat conductivity coefficients, which are used for the calculation model by finite element method. Then, the integrated three-dimensional finite element model consisting of chip, substrate, bottom plate, silica GEL and shell is built. Steady-state thermal field, transient thermal field, heat and force coupling field are calculated successively. The steady-state thermal field shows that the selection of convection heat transfer coefficient of the bottom plate is the key to heat dissipation, and changing heat conductivity coefficient of silica gel has no obvious effect on the heat dissipation. From the transient thermal filed, the temperature reaches steady state after 28 seconds, and the highest temperature is 80.2 ℃. The thermal uniformity of each chip is good. The result of steady-state thermal filed is used as load to carry out the heat and force coupling calculation. The warp and residual stress results show that IGBT operates reliably. The heat dissipation of IGBT device is analyzed in detail by means of test and simulation, which provides technical support for the optimal design and reliable operation subsequently.

To determine the corrosion wastage of the inner and outer walls of the old steel poles of the transmission lines, this paper explores the feasibility of ultrasonic guided wave B-scan detection technology. Firstly, by simulation analysis, the paper discovers that when the loss of corrosion thickness reaches 20%, there is an obvious stress concentration. Secondly, the model and theory of ultrasonic guided wave B-scan technology based on magnetostriction are introduced. By comparing the frequency dispersion curve of steel tube and polygonal steel tube, the propagation characteristics of guided wave in round steel tube and polygonal steel tube are analyzed, and the applicability of ultrasonic guided wave B-scan technology in steel tube detection is summarized. At last, the simulation examination shows that when using the highest sensitivity, the defects with the loss ratio greater than 0.86% of cross-section can be found, after through one weld, the defects with the loss ratio around 2% of cross-section can still be clearly identified. And it also proves the B-scan detection technology can meet the needs of inspection by field application.

The suspension span frame is connected with the transmission tower through the temporary beams on both sides of the transmission line crossing span and the hanging flexible sealing device is used to protect the safety of the crossing objects below. The impact load of the conductor is transmitted to the transmission tower through the suspension span frame under the condition of conductor breaking accident. At present, the condition of conductor breaking accident is not considered in the design of the spanning tower, in view of this, influence of suspension span frame on transmission tower under accident condition and reinforcement measures are analyzed through the 5E1-SZKK tower. A simulation method of conductor impact based on nonlinear dynamics of flexible body is proposed. The simulation results show that the maximum value of the impact load on the tower is 107.5 kN and the loading direction of each point is complex. The 790-820 and 900-952 diagonal members of the tower exceed the limits by 0.438 and 0.473 respectively. After the reinforcement of 5E1-SZKK tower by increasing the section technology, the utilization ratio of the members meets the requirements.

The nondestructive detection of power cable body can be realized by X-ray digital imaging, but there is no depth processing and defect recognition method for X-ray digital images at present. The cable body and defects can not be detected and identified directly from the original digital image. Therefore, this paper studies the advanced processing method of power cable X-ray digital image and the intelligent identification technology of buffer layer defect, and puts forward the full convolution neural network (FCN) method. By using gray level processing technology, the original image gray scale range is compressed to the human eye recognizable range, then the defect identification is carried out, and then the image data is trained by the traditional convolution neural network (CNN) and the proposed method. The intelligent recognition of power cable buffer layer defects is realized. Compared with the traditional CNN, the proposed FCN has more clear and intuitive recognition effect.

The random fluctuation of load and new energy output affects the active power balance and frequency stability of power grid. The conventional synchronous generator with large capacity participates in frequency regulation, but the response speed is slow. As a new power source, fuel cell(FC) has a small capacity and is not suitable for balancing active power, but it is flexible in operation and fast in response, so it is possible to be used for power grid frequency regulation. Firstly, this paper establishes a power flow model with FC. Then after introducing the droop control of active power, a dynamic power flow algorithm with fuel cell participating in primary frequency regulation is proposed. In order to simulate the response delay of FC, the inertia link is considered in the dynamic power flow. The frequency regulation effect is improved by introducing a rapid discharge of DC capacitance. The example analysis gives the results of dynamic power flow, analyzes the effect of FC output delay and DC capacitor discharge on the frequency regulation results, and verifies the effectiveness of the proposed algorithm.

Artificial neural network and machine learning are gradually used in the identification of power system’s self-organized criticality (SOC). Currently, traditional methods on identification of SOC such as OPA model take time or just give opinion on the trending of SOC’s evolution. And most of the artificial neural network methods are based on labeled samples regardless of information provided by unlabeled samples. Identification method of power system’s self-organized critical state based on semi supervised learning-radial basis function neural network is proposed in this article. This method can both use unlabeled samples to improve learning performance and has optimal approximation and global optimization of traditional RBF neural network. Consequently it takes less computation time and get higher rate of correctness, which are concerned most in online identification. This method is verified correct and superior by simulation analysis on gird of Chinese western area. And it can provide theoretical and technical basis for real time prevention of large-scale blackouts.

In order to deal with large-scale of candidate measures for reliability improvement of medium-voltage distribution network, a cost-effectiveness based two-stage optimal selection methodology for reliability cross-domain improvement strategy is proposed. Firstly, based on multi-source data such as asset information management system and reliability management system of power grid company, the refined reliability evaluation of distribution network is carried out. Then, the feeder is used as the implementing unit to list all the reliability improvement measures, which are divided into three dimensions of outage frequency, outage time, and outage impact user range. The empirical logic function is used to screen the improvement measures. Finally, the boundary conditions of the refined modeling measures are determined, and the reliability improvement measures are further sorted and screened based on the reliability marginal benefit. The cost-benefit optimization under the designated capital investment or reliability target is achieved. The practicability and effectiveness of the proposed method are verified by an example of medium voltage distribution network in a county-level power supply bureau in western China.