Flexible DC power grid can realize large-scale renewable energy wide-area coordinated complementarity and reliable transmission, and is an important development direction for high-voltage large-capacity flexible DC power transmission in the future. DC line short-circuit current is one of the important basis for selecting critical equipment parameters such as converter valves, DC circuit breakers and reactors in flexible DC grid. In this paper, a DC discharge equivalent circuit mathematical model considering AC infeed is established, and an analytical method is proposed to analyze the influence of the injected power of the converter station on the DC fault current. On this basis, the equivalent circuit model of the four-terminal bipolar flexible DC power grid is established based on the actual engineering, and the sensitivity analysis method is proposed to evaluate the influence of the DC power flow fluctuation on the fault current of the DC line. Finally, the validity and accuracy of the method are verified by PSCAD simulation. The method proposed in this paper can provide the necessary basis and reference for the equipment selection of flexible DC power grid.

As the basis of HVDC power transmission control system, the phase-locked loop (PLL) has a direct influence on the regulation effect of HVDC control system because of its phase-tracking ability to commutation voltage. In view of the current situation that many new PLLs appear recently but lack of horizontal analysis, this paper analyzes the working principle, advantages and disadvantages of traditional αβ PLL, single-phase PLL based on second-order generalized integrator-quadrature signal generator (SOGI-QSG), and new three-phase PLL based on multiple delayed signal cancellation (MDSC). Common faults in actual power grid are simulated test in the RTDS hardware closed-loop test platform of traditional HVDC transmission system, response characteristics of each PLL under various fault conditions are longitudinal compared, the advantages and disadvantages of each PLL are analyzed, which provides theoretical and experimental basis for selecting suitable PLL for future HVDC engineerings.

The deblocking and starting of HVDC converter is a common and basic operating mode. Different applications of SCR converter lead to various unique deblocking methods. Therefore, it is necessary to study the principle of deblocking and starting. Combining the equipment parameters of the actual engineering and using the electrical circuit theories and power electronic technologies, the triggering of the converter single valve is approximated as the transient process of the second-order RLC circuit response. The process of capacitor charging of RC circuit of other valves is analyzed when a single valve is turned on, and the law of bias voltage change is deduced. The formation mechanism of the full bridge conduction of converter is studied, the criterion of full bridge conduction of the12-pulse bridge and the the expression of output pulse voltage are given to explain the process of continuous current generated by the high-voltage DC system to successfully deblock. The electrical recording data obtained during deblocking the inverter during an actual project commissioning verify the correctness of theoretical derivation of this paper.

To solve the problem of inaccurate acquisition of component reliability parameters in practical HVDC transmission projects, a solution model of HVDC system component reliability parameters based on interval algorithm is proposed. Firstly, the reliability evaluation model of the HVDC subsystem is established by using an analytic method, and the reliability index of the HVDC system is obtained by using the time series Monte Carlo method. Secondly, the analytical expression of the reliability index of the HVDC system on component reliability parameters is derived by using the full probability formula. Then, the reliability index of the components is established, and the interval algorithm for combining interval bisection and Krawczyk-Hansen is used to solve the reliability parameters of main components. Finally, a double twelve pulse HVDC system is used to verify the correctness and effectiveness of the proposed model and method. The results of the case study show that the model and method can find the exact interval solution of the component reliability parameters in a large initial parameter range.

Based on the state equation method, an analytical calculation method for the DC loop harmonic impedance of the HVDC converter is established. MATLAB and PSCAD /EMTDC software are used to build model and simulate the DC harmonic impedance to verify the correctness of the proposed method. The typical frequency spectrum of the DC harmonic impedance of the converter is obtained. The coupling relationship between the DC harmonic impedance spectrum and the control parameters is analyzed, and the key control parameters that affect the DC harmonic impedance are extracted, and a DC loop harmonic impedance adjustment and DC resonance suppression method based on control parameter modulation is formed. The efficiency and flexibility of DC resonance suppression of the converter have been improved.

The calculation of AC fault current plays an important role in the selection of circuit breakers and DC protection, especially in the case of asymmetric faults. However, the calculation of AC fault current for DC protection mostly depends on electromagnetic transient simulation. The method has several drawbacks such as time-consuming simulation, lack of physical mechanism analysis and parameter influence factor analysis. In order to solve the problems above, this paper derives rising rate of the bridge arm current and AC line current before the response of control system, and the analytical models of the d-axis and q-axis positive sequence components of AC line short-circuit current and three-phase short-circuit current after the response of control system for the AC asymmetric fault scenario in the symmetric single pole MMC-HVDC system. The analytical model is compared with the simulation results of an HVDC engineering project to verify the correctness of the fault analysis model. The significant value of this study is that the dynamic process of the short-circuit current under AC asymmetric fault can be obtained by theoretical calculation without multiple electromagnetic transient time domain simulation results, and the final calculation results can provide reference for the design of control and protection strategy.

A variety of structures are used in the design of the inner cooling systems for HV converter valves, while the problem of deposition on the pin-type grading electrodes is still frequently observed. In this paper, the deposition status in the cooling water ways of the quadruple valve towers, the double valve towers and water ways inside the valve modules are measured and counted on site. The effects of environmental factors including electric field parameters and fluid field parameters are analyzed. The distribution of electric field and electric current on the pin-type grading electrodes is numerically calculated. By means of simulating calculation, the effects of electric polarity and electric current on the deposition distribution are verified. The results show that the relative electric polarity of the grading electrodes is the dominant factor affecting the deposition degree. The deposition tends to occur on the grading electrodes located at the high electric potentials. A positive correlation between the electric current and the deposition thickness on the grading electrode is obtained. The morphology of the deposition is in significant consistence with the distribution of electric current on the pin-type grading electrode.

The deposition problem on grading electrodes in the inner cooling system of HVDC converter valves will cause water pipe blockage, leakage and heat loss failure, even DC blocking accident, endangering the safe and reliable operation of HVDC system. In this paper, simulated corrosion and deposition experiments are conducted to determine the decisive factor which influence the charged property of sediment-forming ions. The existence form of sediment-forming ions in cooling water under different conditions is calculated based on thermodynamic theory. The results show that the corrosion of the metal heat sink is the root of sediment-forming ions, but the existence form of the sediment-forming ions in the cooling water is determined by the pH of the cooling water during the migration, instead of being determined by the original corrosion reaction. In weakly acidic environment, most of the sediment-forming ions exist in the form of positive aluminum complex ions, while in weakly alkaline environment, the sediment-forming ions almost all exist in the form of negative aluminum complex ions.

In the upgrading of line commutated converter-high voltage direct current (LCC-HVDC) projects, the flexible reconstruction scheme of receiving conventional converter station is safe and feasible. Based on ±800 kV UHVDC project, a series hybrid UHVDC system is formed by replacing the double low-end converters on the inverter side with double VSC converters. In this paper, a voltage distribution strategy for high-end and low-end converters considering reactive power compensation is proposed to solve the voltage allocation problem of series converters in hybrid UHVDC system. This strategy considers the scheme that VSC and AC filter provide reactive power together. By establishing the functional relationship between rated capacity of VSC and rated power of LCC, the voltage distribution results of LCC and VSC are determined after optimization. Based on the electromagnetic transient model, coordinated control strategy is designed and compared with the transient response characteristics of conventional UHVDC system before reconstruction. The results show that the flexible transformation scheme improves the resistance ability to commutation failures of UHVDC system, and UHVDC system under voltage allocation strategy performs good operation characteristics.

In order to solve the problem of the existing calibration device of DC total electric field strength measuring instrument, such as poor controllability, low accuracy, and unable to meet the standard requirements to carry out the calibration work of DC total electric field strength measuring instrument, the optimization researches of calibration system of DC total electric field strength and ion current density measuring instrument are carried out, the technical advantages of the optimized calibration system are analyzed, and the development is carried out test analysis and verification. The results show that the optimized calibration system can not only meet the requirements of the standard, but also have the comprehensive function of calibrating the ion current density meter, which improves the convenience of calibration. Moreover, the calibration system uses a circular flat plate design to make the electric field distribution more uniform and isotropic, which effectively improves the calibration accuracy.

Good frequency response characteristics of high-voltage DC voltage divider is the basis of the reliable operation of the DC control and protection system. However, due to the lack of a portable high-voltage broadband test voltage source, the current DC voltage divider frequency response test can only be carried out during the delivery test, and generally only the power frequency voltage test is carried out, the actual on-site transient characteristics of the DC voltage divider cannot be effectively acquired in the field. Based on the high-voltage power amplifier and arbitrary waveform signal generator, this paper proposes a portable high-voltage broadband power supply, and improves the ability of the high-voltage broadband power supply to drive capacitive loads through inductance compensation measures. It can generate test voltages with a frequency range of 50~3 000 Hz and amplitude of 10 kV for DC voltage dividers of different voltage levels, so as to meet the frequency response test requirements of various DC voltage dividers. The frequency response test power supply and method designed in the article are light and easy to use, which is helpful to promote the on-site DC voltage divider frequency response characteristic test.

Basin insulator is an important insulation component in gas insulated switchgear (GIS), and the accurate identification of its different defective partial discharge (PD) signals is of great significance to ensure the long-term safe and stable operation of GIS. In this paper, a classification and identification method for PD of basin insulators based on local mean decomposition (LMD) and long short-term memory (LSTM) neural network is proposed. Firstly, the PD signal is decomposed using the LMD method assisted by paired Gaussian white noise, and then the components obtained from the decomposition are segmented to extract the energy share, Renyi entropy and Hurst index of each segment to form feature matrices, and finally the feature matrices are sent to LSTM for training and classification. A basin insulator PD experimental platform is established in the laboratory to simulate the actual working conditions, and the PD signals of four different defects are collected for analysis and processing. The results show that the proposed method can effectively identify PD signals of different defects of basin insulators, and the feature parameters extracted by LMD decomposition can effectively characterize the characteristics of the PD signals in different frequency bands, and the recognition accuracy is significantly higher than that without LMD decomposition.

In order to solve the problem of excessive transfer potential of the metal fence on Qingkeyang ground electrode site of the Hainan converter station in Qinghai, based on the measured soil resistivity of Qingkeyang ground electrode and the design of the pole ring, a simulation model is established to study the influence of the surface potential distribution of the pole site on the transfer potential of the metal fence under different working conditions. The research results show that the transfer potential of the metal fence is not only related to the current into the ground, but also closely related to the frozen soil. According to the research results, two different schemes of the transfer potential treatment plan are proposed, which may be referenced for design and construction in the subsequent project.

Large-current switchgears are important basic components of power system. In order to reduce the temperature rise and energy consumption of the large-current switchgear, the thermal-fluid coupling model is established based on the thermal convection and radiation. Then, three structure factors (vertical spacing, deflection angle and inlet speed of three-phase busbars) are carried out through the response surface methodology (the average surface temperature of B-phase busbar as the response value). The results show that the thermal-fluid coupling model, which is put forward for Large-current switchgear in this paper, is more accurate than traditional engineering algorithm. The deflection angle has little effect on the average surface temperature of B-phase busbar, while vertical spacing and inlet speed have significant impact on the temperature rise and energy consumption of B-phase busbar. The recommended optimal switchgear structure is the vertical spacing of 200 mm, the deflection angle of 73.5 °, and the inlet wind speed of 2.24 m/s. Compared with the initial experimental model, the temperature rise is reduced by 5.1%, and the energy saving rate is increased by 5.89%. The research provides an important basis for predicting the temperature rise and structural optimization for large-current switchgear.

In wildfire risk assessment, numerous factors are contributed to wildfire occurrence. And the physical model construction is complex. This paper develops a method to evaluate the wildfire risk of power transmission line corridors based on Naive Bayes Network. First, the data of 14 wildfire-related factors including anthropogenic, physiographic and meteorologic factors, are collected and analyzed. Then, the relief algorithm is used to rank the importance of factors according to their impacts on wildfire occurrence. After eliminating the least important factors in turn, an optimal wildfire risk assessment model for power transmission line corridors is constructed based on the Naive Bayes Network. Finally, this model is carried out and visualized in Guangdong Province in southern China. Then a cost function is proposed to further verify the applicability of the wildfire risk distribution map. Then a cost function is proposed to further verify the applicability of the wildfire risk distribution map. The fire-spots monitored by satellites during the first season in 2020 shows that 80.9% of fire-spots fall in higher and high risk area. This method can guide operation and maintenance personnel to carry out differentiated wildfire management work.

The line tree distance refers to the shortest distance between the transmission line and the tree, and this distance directly reflects the risk probability of a tree barrier accident on the power transmission line. This paper relies on the principle of photogrammetry to match and obtain point cloud information along the power transmission line from the stereo pairs of satellite remote sensing images, thereby constructing a three-dimensional model along the transmission line. On this basis, the supervised classification method is used to identify and extract the spatial parameters of the canopy area, and the catenary equation of the power transmission line is used to obtain the spatial parameters of the transmission line, thereby realizing the measurement of the distance between the line and the tree. According to the actual measurement of ±800 kV Fufeng and Jinsu line, the maximum measurement error of the proposed method for tree crown elevation is 10.69%, and the maximum measurement error for line tree distance is 15.80%. This method pinpoints 20 key concerned intervals, and can realize regular, wide-range, high-precision, risk-free and economical measurement of line-tree distance and timely warning of tree obstruction risk.