Multi-terminal DC distribution system has become one of the important critical technologies of the power grid system due to its many advantages. The study of overvoltage and overcurrent and their protection methods in typical faults is one of the key technologies. In this paper, the simulation analysis of overvoltage and overcurrent under typical faults are carried out for ±10 kV three-terminal DC distribution system and the protection scheme of overcurrent and overvoltage is proposed. And the influence of system overcurrent protection scheme on overvoltage and overvoltage protection scheme on overcurrent are analyzed. Firstly, based on PSCAD/EMTDC, the simulation analysis model of three-terminal DC distribution system is established. Secondly, the causes of fault current in pole-to-pole fault are analyzed, and two overcurrent protection schemes and parameters are proposed. On this basis, the influences of overcurrent protection schemes on overvoltage are simulated and analyzed. Finally, based on the simulation results, the configuration scheme of arrester is proposed, and the switching impulse withstand voltage of key equipment under different current limiting schemes is simulated and analyzed, as well as the overcurrent of typical faults is simulated and analyzed when the system is unconfigured and configured the arrester. At last, the coordinated protection method of overcurrent and overvoltage is proposed. The research results can provide reference for the selection of overcurrent and overvoltage protection schemes for multi-terminal DC distribution system.

Ice coating caused by ice and snow disasters is extremely harmful to DC power grids. Therefore, the research and application of ice melting technology are of great significance for DC grids to combat with ice disasters. In normal operation, the current of metal return is zero and does not have an anti-icing effect, so it is especially important to discuss it. The paper uses the thermal effect of current to melt the metallic return. Firstly, the critical ice-melting current of the metal return in the four-terminal flexible DC grid and the ice-melting time corresponding to different melting currents are calculated, and the power flow calculation model of DC grid with metallic return is established. The paper innovatively uses the bipolar converter station power reversed method to melt the metallic return, selects different ice-melting conditions, and analyzes the ice-melting of the four-terminal power grid to obtain the most suitable ice-melting conditions. The simulation analysis of the ice-melting control process is carried out. Finally, it is concluded that the bipolar reversed power mode is more suitable for melting the metallic return, which can realize ice-melting without power failure and has less power requirement for the converter station.

DC solid state transformer is one of the core components in DC power grid, and its efficiency is an important index. The efficiency of a DC solid state transformer cascaded with resonant DC-DC converters as the basic unit is mainly determined by the resonant DC-DC converters. This paper analyzes the working stages of series resonant converter and discusses the main factors that affect the turn-off loss. The zero voltage switching-on (ZVS-on) condition of the circuit is derived. The scheme of improving the efficiency of the converter by optimizing the phase lag angle and resonant frequency is discussed. An experimental prototype of series resonant converter is built, and the experimental results preliminary verify the effectiveness of the efficiency optimization scheme proposed in this paper.

The mainstream protection scheme of DC power grid is using DC breakers to remove faults at present. However, HVDC breaker is expensive and it is costly for each DC line equipped with DC breakers. Firstly, a partition protection strategy suitable for future large-scale DC grid is proposed, which aims to minimize the number of DC breakers and ensure the reliability of power supply. Secondly, the influence of partial power interruption which caused by fault DC area on the system is qualitatively analyzed. Then the basic partition principle of DC grid is formulated, considering unbalanced power, the mode of system-level control and the type of converters. Besides, according to the characteristics of DC breakers, AC/DC converters, DC/DC converters, AC breakers, high speed switches and other fault removal components, the timing coordination strategy is formulated in detail. Finally, a seven-terminal DC power grid is modeled in PSCAD/EMTDC simulation platform. The simulation results verify the correctness of the proposed partition protection strategy and the corresponding timing coordination strategy of DC grid.

In order to solve the problem that there are many criterion in the traditional passive isolated island detection method based on frequency and phase and the switching process is long in the actual flexible HVDC system, based on the actual situation of flexible DC transmission system, a d-axis reference current islanding detection method for AC/DC hybrid power grid is proposed. In this method, the output d-axis reference current of the outer loop controller is used as the criterion, and the threshold setting method is given according to the characteristics of the outer loop controller, which changes obviously under the condition of isolated island and can effectively avoid misjudgment. Finally, an AC/DC hybrid connecting simulation model is built to verify the effectiveness, accuracy and convenience of the isolated island detection method proposed in this paper.

For the demands for power transmission in long-distance, high voltage, high power, ± 800 kV VSC-UHVDC transmission engineering applications are imminent. The electrical connection of DC side of the converter station is the most important part of the project engineering. It is the core part of secure and reliable operation and the general technical solution. Based on the research of the single line diagram of the converter station, the comparison of DC fault clearance solutions and the requirements of the system operation, this paper provids the research methods of the DC electrical equipment configuration of the converter station, the pole line electrical wiring and the neutral busbar electrical wiring. The DC electrial connection purposed by the research uses a bipolar MMC structure with high-low valve series in series with "half+full-bridge" hybrid topology to clear DC faults, and the detailed DC equipment configuration and wiring scheme are also proposed. The research results are highly feasible and economical, which is of great significance for improving the overall design level of ±800 kV flexible DC converter station.

Conductors arrangement of high voltage DC overhead transmission line has an important influence on the electromagnetic environment, corridor width and lightning protection performance of the line, which largely determines the technical economy of the line. Based on the different arrangements of ± 500 kV three-circuit flexible DC three-circuit conductor (including three groups of metal return lines) on the same tower, the finite element method, traveling wave method and strike distance method are used to calculate the lightning resistance. After technical and economic analysis, the vertical layout of ± 500 kV flexible DC three-circuit pole conductor and inverted triangle layout of metal circuit are proposed, which provides reference for the engineering design of ±500 kV triple-loop VSC-DC conductor.

The series current limiting reactor can effectively limit the short circuit current of the 500 kV system, but it will increase the transient recovery voltage steepness when the circuit breaker breaks, which will affect the normal breaking of the circuit breaker. In this paper, based on the demonstration project of 500 kV/90 kA current limiter in Guangnan station, the electromagnetic transient simulation software EMTP is used to study the influence of the fault current limiter on the transient recovery voltage of the circuit breaker. The results show that after the reactor is connected, when the short-circuit fault occurs in the area within 20 km from Guangnan station, the transient recovery voltage of the circuit breaker will exceed the standard value; when 600 nF capacitors with with higher capacitance are connected in parallel to the current limiting reactor, the transient recovery voltage of the circuit breaker can be limited to the standard range.

With the rapid development of DC asynchronous interconnection of power grids, a large number of intermittent renewable energy sources are connected and the load fluctuation is reinforced. In order to ensure the frequency quality of the power grid, higher requirements are imposed on the primary frequency regulation performance of the hydropower units. At present, the governor of hydropower units can meet the primary frequency regulation requirement of the power grid by adjusting the PID parameters, but these para-meters after being adjusted may affect the dynamic stability of the system and cause the system to experience ultra-low frequency oscillation. In order to solve the above problems, this paper proposes a PID parameter switching method based on Prony analysis for the hydropower unit governor. Different control parameters are used to realize sectional control, so that the governor can simultaneously take into account the primary frequency regulation performance and the dynamic stability damping level, thus realize suppression of ultra-low frequency oscillation of the power grid from the power plant side. The effectiveness and robustness of the proposed method are verified by the time domain simulations of the actual system of Yunnan Power Grid after asynchronous interconnection.

Intermittent arc grounding fault frequently occurs in non-solidly earthed networks. Establishing an accurate arc model is the basis for analysis of fault characteristics. Firstly, an improved cybernetic arc model is introduced in this paper, which can effectively demonstrate arc process in different situations through controlling the arc length. Then, it is applied to simulation analysis of intermittent arc grounding fault. Arc extinction and reignition criteria are proposed by comparing stable ignition voltage, air withstand voltage and system recovery voltage in diverse conditions. Finally, a typical simulation model of 10 kV distribution network is built via PSCAD. Impacts of different arc model parameters on fault characteristics are analyzed. Simulation and measurement results indicate the validity of the arc model and the feasibility of the proposed criteria.

The temperature rise design and test under total loss equivalent direct current and alternating current of large capacity dry-type air core reactors based on isothermal isobaric design method are investigated using a combination way of theory, simulation and test, according to the differences on temperature rise distribution under direct current and alternating current of dry-type air-core reactor based on Northwest Yunnan to Guangdong ±800 kV UHVDC transmission project of China Southern Power Grid Company Limited. The test results show that: Firstly, the reactor could meet the requirement of temperature rise under total loss equivalent direct current and alternating current through reasonable conductor selection and coil structure design. Secondly, the coil temperature rise under total loss equivalent direct current is apparently higher than that under total loss equivalent alternating current. Lastly, the metal part adopts a special structure to ensure that the heating temperature is controllable. The research results could provide a basis for the design of AC/DC composite dry-type air core reactor in the later stage.

Infrared detection of substation equipment can detect overheating defects effectively in time, which can prevent the occurrence of equipment faults. It is difficult to analyze and process massive infrared images from autonomous inspection of substation robots and unmanned aerial vehicles(UAVs) with traditional manual diagnosis method. However, at present the intelligent diagnosis based on infrared image is mostly based on traditional machine learning algorithm, which has low recognition accuracy and poor generalization ability. Firstly, object detection of transformer, bushing, circuit breaker and other seven substation equipments is carried out based on improved Faster RCNN method, which realizes accurate positioning and identification of equipment. Then, defect recognition is carried out based on temperature discrimination method. The infrared image collected from the scene is used for testing, and the recognition accuracy of the seven types of equipments reaches more than 90.61%, and the accuracy rate of defect recognition accuracy is 81.33%. The experimental results show the effectiveness and accuracy of the proposed method.