When HVDC transmission system operates in mono-polar condition or is struck by lightning current in the converter station, it will lead to unbalanced distribution of neutral current between YY and YD converter transformers. If the current difference is too large, it will cause damage to converter transformer and even HVDC block. In view of this technical problem, this paper reveals the mechanism of neutral point current difference in converter transformer based on transformer magnetic potential balance theory, and proposes a new method to restrain the neutral point current difference based on the construction of YY+d converter transformer with the third winding which connects to triangle.The PSCAD model based on the converter transformer parameters of actual project is built up for verification simulation. This paper provides a technical basis for the solution scheme for engineering problems caused by the current difference of converter neutral point, and has a reference and guiding significance for the design, operation, maintenance and protection configuration of converter transformers in practical DC projects.

Basin insulator damage has a direct impact on the operation of the gas insulation combination switch equipment. Therefore, it is an effective means to evaluate its working condition by nondestructive testing. This article adopts the method of finite element simulation of the finite element model is established, and then according to the dispersion to calculate the corresponding equation theory knowledge, finally draws the corresponding dispersion curves, thus the ultrasonic guided wave propagation and attenuation in the basin insulator and the phenomenon such as modal transformation of the corresponding research, and in view of the deficiency in the basin insulator adopts the method of ultrasonic guided wave testing accordingly. The results show that the method can accurately obtain the location and size of the defect, greatly improve the accuracy of the basin insulator detection, and have a good application prospect for the basin insulator nondestructive detection.

In order to improve the accuracy of transformer fault diagnosis, a transformer fault diagnosis method based on adaptive deep learning model is proposed. The method uses dissolved gas in oil as fault diagnosis feature, and builds diagnostic model based on deep learning theory. To solve the shortcomings of slow convergence speed and low convergence precision in the training process of traditional fixed learning rate based deep learning model, an adaptive deep learning model construction method is proposed. This method adaptively adjusts the learning rate according to the changing characteristics of the iterative process, and effectively improves the training accuracy and speed of the deep learning model. Parameters of adaptive deep learning model for transformer fault diagnosis such as hidden layer number, learning rate adjustment coefficient are proposed. The experimental results show that the proposed method has a strong ability of feature extraction and analysis and has better convergence speed and convergence precision, which can effectively improve the accuracy of transformer fault diagnosis.

Voltage stability analysis has become an important work for actual power grid planning and formulation of operation mode, the existing off-line software can not achieve voltage stability analysis and computation comprehensively. Based on the VSASP software, a new voltage stability assessment and control software is developed in this paper which can achieve the computation and visualization of fast contingency screening, determinstic assessment, probabilistic assessment, and preventive control based on continuous power flow and sensitivity analysis. The design of overall function modules framework, the realization theory of each function module, and the engineering application of the software are given. Application results of the software show that it is an effective tool for voltage stability analysis, and it can provide powerful technical support for voltage stability assessment and control of actual power grids.

In most of the existing literature, the unit recovery schemes for black start aim at the shortest system recovery time and the maximum power generation. The damage to the line before recovery and the probability of line fault in the recovery process are rarely considered. Therefore, a unit recovery path optimization considering the windage yaw rate after typhoon is considered in this paper. First, the minimum air gap between the wire and the steel tower under different wind speeds can be obtained through simulating the wind speed during the typhoon, and the windage yaw rate model can be then established. Taking the starting characteristics of the unit, the recovery time and the operation constraints into consideration and taking the windage yaw rate as the line weight value, the Dijkstra algorithm is applied to get the shortest windage yaw rate of path each unit. The relationship between the path windage yaw rate and the recovery time is fully considered in order to minimize the unit recovery time under the premise of the lowest windage yaw rate. Finally, the proposed method is proved to be able to bypass the fault line and improve the success rate of the system recovery by using the IEEE 39 bus system and the actual power system in Guangdong province, which means the method is scientific and effective.

Because of insufficient sensitivity of the transformer differential protection in protecting the single-phase ground fault, the necessity of the zero-sequence differential protection of the transformer is discussed. However, due to human factors, such as zero-sequence current transformer wiring error, protection setting error, zero-sequence current transformer selecting error, and so on, zero-sequence differential protection incorrect action accidents often occur. These errors of zero-sequence differential protection cannot be verified by the normal load current, which limit the application of zero-sequence differential protection. In this paper, the zero-sequence differential protection is configured with self-checking function, which uses the magnetizing inrush current to verify the correctness of zero-sequence protection circuit, and is proved to be correct by dynamic simulation and experiment test. The self-checking function does not require manual participation, is reliable and practical, and can effectively prevent incorrect action of zero sequence protection caused by human error.

Because of its simplicity, economy and reliability, current protection is widely used in distribution network of 35 kV and below. However, due to the small short-circuit current of the inverter-interfaced distributed generation (IIDG), current protection on the IIDG side fails to operate correctly and the distance protection also faces great challenges. The fault characteristics of the IIDG and its impact on the distance protection is analyzed in this paper. Based on this, a protection network configuration method based on the combination of delay distance protection and current protection is proposed. The method is simple and easy to implement, and is able to endure fault resistance and noise. A distribution network model with IIDG is built in PSCAD/EMTDC, and the simulation results verify fault characteristics and protection configuration method.

At present, the resistance box and the electronic load used in the on-site detection of the EV off-board charger are large in size and heavy in weight which results in the limited use in some detection occasions. If the power battery pack of the mobile detection vehicle is directly used as the charging load, the charging mode is single and the electromotive force cannot be flexibly set, which results in long charging time and low efficiency. Aiming at these problems, this paper proposes a topological scheme that the lightweight controllable charging load consists of a vehicle-mounted power battery pack with a cascaded DC/DC converter,researches the double closed loop control methods such as constant input voltage, constant input current, constant input power and constant resistance of load, then achieves the typical working modes including constant input voltage,constant input current, constant input power and constant resistance of load. The operating modes has the ability to continuously adjust in the rated range. In a word, the proposed scheme can achieve self-test of the off-board charger while reducing the weight of the charging load. Finally, the simulation model in Matlab/Simulink is established to verify the feasibility of the proposed topology and the effectiveness of the control methods.

In order to solve the problems of low efficiency, weak experience and poor interaction in traditional operation and maintenance training of intelligent substation, a novel substation operation and maintenance training system based on virtual reality technology is designed. Firstly, the three-dimensional operation scenario of substation is constructed by virtual display technique, and mapped to reduce memory and improve the authenticity of the model. Considering the possible power failure and signal processing of substation, a standardized and functional virtual reality interactive functional equipment module library is designed to improve the modeling efficiency of the 3D substation. Secondly, the whole operation and maintenance training system of virtual substation is constructed by using human-computer interaction engine and virtual reality equipment. Finally, operations like equipment operation, equipment maintenance, protection setting and fault analysis can be simulated in a realistic virtual scenario. The training results demonstrate that the training process is vivid and interactive, which can avoid the dangers and consequences of non-standard operation in various operating scenarios, and greatly improve the training effect.