With the popularization of transmission channel visualization and intelligent identification technology, effective management and control of hidden dangers in power transmission channels has become more and more important. The clearance distance of a hidden hazard is an important criterion for judging the degree of hazard of a hidden hazard. However, the currently applied ranging technology has a relatively large error. This paper proposes a technology suitable for feature extraction and feature matching of transmission channels. And image set is obtained by feature matching. We generate a depth map of the three-dimensional space scene of the transmission channel. Based on the reconstructed depth map, the hidden danger in the transmission channel is measured. It can improve the efficiency of daily inspections and operation and maintenance management of transmission line hidden dangers and channel scenarios. It can help operation and maintenance personnel find hidden dangers in time. Thereby, the safe and stable operation of the line can be guaranteed.

Transient angle instability and transient voltage instability are usually mixed and coupled in transient instability scenarios in complex grid. In this context, this paper provides a quantitative analysis method of the identification of dominant mode of transient instability in complex grid based on the ratio of generator transmission power limitation to its spinning capacity (TPLR). Differently from most existing work that analyzes the mechanism on the dominant mode of transient instability through direct method such as characteristic root which has low applicability for analyzing, an assessment method is conducted here through time-series simulation that specifically considers the limitation of transmission power of key generator under different installed scale. The transient instability mode can be identified based on the curve of TPLR, i.e. the ratio of transmission power limitation to the spinning capacity. The method is demonstrated on a complex transmission network with transient voltage instability and transient angle instability scenarios. The simulation results help verify the accuracy and applicability of the provided transient instability mode identification method.

Grid frequency supports, e.g., the virtual inertia control (VIC) and the frequency damping control (FDC), of photovoltaic (PV) systems, are more demanded than ever before. To improve the frequency quality in terms of important features, i.e., the rate of change of frequency (RoCoF) and the frequency nadir, the dual-mode control has been proposed in the literature. The VIC and FDC are respectively implemented in different stages of the control process. For such a control method, the switching threshold between the control modes should be properly designed, where the grid operation characteristic and actual frequency dynamic should be considered. In this context, a novel dual-mode frequency control of PV systems is proposed in this paper. The virtual inertia mode (VIM) and frequency damping mode (FDM) of the dual-mode control are introduced at first. Then, a historical-data-based large disturbance identification method is realized to identify the switching threshold between two modes, which is obtained by the confidence interval. With the proposed method, the frequent switching between the control modes can be avoided, and the frequency support performance in the grids with different inertia levels is guaranteed. Simulation results validate the proposed dual-mode control method with large disturbance identification.

In response to the needs of high-power electric propulsion systems for power processing units that can adapt to a wide range of voltages and can achieve high-efficiency conversion throughout the range, this thesis plans to research on the technology of high-power electric propulsion wide-voltage power processing unit. This thesis proposes an isolated widevoltage flexible mainstream combined converter and its control method. By optimizing the number of series series on the secondary side of the dual-bridge topology, the stress of the device is reduced. By embedding a lossless spike absorption diode on the secondary side, the resonant voltage spike of the rectifier diode is effectively reduced. Through the optimized design of the smooth switching control strategy, the adjustment range of the converter is broadened. Through the control strategy of the in-phase shift switch of the leading bridge, the current sharing of the converter is realized and the power density of the converter is improved. The prototype developed in this thesis has an input voltage of 80-120V, an output voltage of 500-1500V, a rated power of 5kW, and a conversion efficiency of up to 95%, which verifies the correctness and effectiveness of the related technology.

In order to solve the shortcomings of the frequency domain reflection(FDR) method which can only locate cable defects offline, this paper proposes a cable defect online location method based on FDR and electromagnetic coupling technology. Firstly, a cable containing local defects is modelled based on a parametric model of cable distribution, and a Kaiser window function is used to add a window to the original localisation spectrum to improve the sensitivity of the localisation of cable defects. This was verified on 105 m and 500 m power cables in the laboratory by varying the load resistance at the end of the 105 m cable under offline testing to simulate the line conditions under different cable loads and to analyse the effect on the reflection intensity at local defects and at the end of the cable. In the powered test case, the 500 m cable was energised by a series resonant platform and tested using the proposed online location method in order to test the effectiveness of the proposed method. The simulation and experimental results show that the method is able to locate the defective points online when the cable is energised, compared to the offline location test method.

When the surface area of the wall bushing in the UHV DC converter station is polluted, there is a risk of wet snow flashover in the snow climate. The snow removal in time of the wall bushing can reduce the risk of outdoor insulation. For the horizontal insulator, we designed snow removal tests under the influence of various factors. The effect of various snow removal factors on the discharge characteristics were studied on the short insulator (4 m), while the UHV full-size wall bushing insulator (10.87 m) was studied by withstand test. The energized snow removal test results showed that under the same snow removal ratio, a position with a lower electric field value along the string should be selected as the starting snow removal position. Simultaneous snow removal on multiple sections was easier to cause surface discharge than the continuous long gap snow removal. The ratio and the degree of discharge appear to decrease first and then increase in a “spoon” shape. It was recommended to remove snow from the middle of the bushing firstly, and the first snow removal ratio in a continuous area was from 13% to 20% of the insulation distance. The research results of this paper can effectively guide the energized snow removal operation of ±800 kV DC wall bushings, and provide a reference for the energized snow removal operation of outdoor horizontal insulation equipment of other voltage levels.

DC chopper is one of the key technologies of offshore wind power voltage source converter based high voltage direct current (HVDC) systems, used to consume the system surplus power in case of onshore power grid failure. In this paper, a novel DC chopper topology structure is proposed. It combines the corresponding advantages of concentrated DC chopper and modular DC chopper. It can effectively solve the problem of direct series voltage balance of large-scale power devices and has the advantages of no water cooling system and low manufacturing cost. Then the detailed design and development of the modular solid-state switch based on integrated gate commutated thyristor (IGCT) and the concentrated dissipation resistor are presented, followed by the corresponding functional verification tests. Furthermore, the overall 120 kV DC chopper equipment is developed based on IGCT and finally achieved the ability to break 1.5 kA current in 500 Hz to dissipate 180 MW power at different duty ratios, which has reached the worlds leading level. And it is especially appropriate for offshore wind power HVDC system engineering application.

Heretofore, the comprehensive fault analysis approach for AC/DC hybrid grids has not been sufficiently investigated. Inspired by the conventional symmetrical components method, a novel “sequence component network” model based on the mapping relation is proposed to study the characteristics of AC (DC) side after DC-side (AC-side) faults in AC/DC hybrid grids. Subsequently, the AC-side electrical characteristic after DC-side faults was discussed employing the “sequence component network” model. Finally, taking into account the influence of different grounding modes, a DC-type integration system model of PV stations was established in PSCAD/EMTDC to verify the proposed approach.

Positive and negative voltages/currents may present long-term unbalance after single-pole breakage faults in ring flexible DC distribution networks, which endangers the safe and stable operation of the system, whereas little research on it has been implemented. Therefore, characteristics of the single-pole breakage fault are studied. First, by establishing the equivalent model of the system before and after the fault, the voltage and current characteristics of the single-pole breakage fault on the DC line, the tie line between the sending-end converter and the DC bus, and that between the receiving-end converter and the DC bus were analyzed. Then, the influence of system grounding mode on the characteristics of the single-pole breakage fault was discussed. In addition, the impact of sudden load change on the fault characteristics was studied. Finally, the model of a ring flexible DC distribution system was built in PSCAD/EMTDC to verify the theoretical analysis.

Fault location is of great significance for fault handling and power restoration of distribution network. In the traditional fault location method of active distribution network with new energy access, the matrix algorithm has poor fault tolerance and the optimization algorithm has slow location speed. A section location algorithm is proposed to combine the advantages of the matrix algorithm and the optimization algorithm. Firstly, matrix algorithm is used to quickly locate fault sections after the active distribution network fails. Then, the positioning result of matrix algorithm is verified by switching function. According to the verification results, it is decided whether to use the optimization algorithm to optimize the fault sections, and then output the final results. Finally, the simulation test by MATLAB shows that this method can achieve fault location rapidly with strong fault tolerance in active distribution network.