In view of the lack of flexibility in the input and exit of the current metal sheath loop suppression device for high voltage cables，the problems such as excessive induced voltage of the sheath and excessive power loss were not considered when determining the suppression impedance value，a loop suppression device was designed which could dynamically input or exit the loop suppression impedance of the sheath according to the field conditions. Firstly，the structure and operation flow of the suppression device was described. Then，the variation characteristics of sheath circulation，induced voltage and sheath loss when the suppression impedance was pure resistance，pure inductance and combined resistance inductance were analysed. On this basis，an optimization model was established to solve the optimal impedance value which can minimize the power loss when meeting the requirements of sheath circulation and induced voltage. Based on the optimal impedance value，the parameters of thyristors and other components were configured. The results show that the proposed method can obtain the optimal value of cable suppression impedance that meets the engineering requirements，and achieve a good circulation suppression effect.

The flexible DC interconnection of medium and low voltage can improve the flexibility and reliability of the distribution network，and the dual active bridge converter is a key link in achieving voltage conversion and electrical isolation in flexible interconnection equipment.The current source resonant dual active bridge utilizes DC capacitors to participate in resonance and soft switching design，which can significantly improve the power density and efficiency of the dual active bridge. However，there are significant differences in the characteristics between the current source resonant dual active bridge and the traditional voltage source resonant dual active bridge. The resonant cavity is composed of a DC resonant capacitor and an AC side resonant inductor，which leads to complex time-domain modeling of high-frequency current and affects the frequency design of zero current switching （ZCS），making it difficult to obtain analytical expressions for the ZCS switching frequency and resonant frequency. An equivalent circuit modeling method for high-frequency isolation links based on fundamental components from a frequency domain perspective was proposed，established an analytical relationship between ZCS switch frequency and resonant frequency，and verified the effectiveness and accuracy of the proposed method through simulation and experimental results.

Up to now，the distribution network planning project optimization method rarely considered the impact of the speed of load growth on the power supply efficiency of the project and the year when the project is selected，and rarely considered the urgency，technical benefit and economic benefit of the project at the same time. Therefore，the optimal model for medium and low voltage distribution network planning projects was established with the goal of maximizing technical and economic benefits. According to the urgency of the project，the technical benefits of the project were rewarded and punished. In order to measure the annual change rate of the power supply benefits of the project，the differential weight method was used to obtain the five-year comprehensive level of power supply benefits after the project was put into operation. An improved multi-objective particle swarm algorithm solution model was proposed，and a series of project combination schemes with mutually advantageous objectives were obtained，and the optimal project combination scheme was obtained by multi-level screening of the project combination scheme set according to the target preference of the power supply enterprise. Taking the distribution network planning project database in a certain region as an example，it show that the proposed model can better consider the change of power supply efficiency and the urgency of the project，and achieve multi-regional and multi-objective comprehensive improvement.

DC high current comprehensive characteristic test device is used to detect the released characteristics of DC low-voltage electrical overcurrent protection electrical device，which is used in low-voltage electrical industry for DC products for long time delay，short time delay，instantaneous tripping test and short circuit test. With the wide application of DC low-voltage electrical products in non-ferrous metallurgy，chlor-alkali chemical industry，low-voltage power distribution，DC transmission and distribution，new energy and railway applications，the DC high current comprehensive characteristic test device used for DC electrical product testing needs to be traced regularly to ensure that it can meet the accuracy requirements of the test detection capability. In the field of measurement calibration，the present situation of relevant calibration methods was analyzed，and the research direction of calibration methods of this test device was reviewed. According to the corresponding product of the test device and the national standard，a new calibration parameter was proposed. Aiming at the calibration problem of the DC high-current device，the measurement method of optical fiber current sensor was proposed，so as to evaluate the uncertainty of the data generated.

A dual active bridge （DAB） DC-DC converter voltage control strategy based on double-integral sliding mode control was proposed，which can obtain good control effect under the condition that the model accuracy requirements are low and the controller design process is simple. Firstly，the reduced-order model of the converter was established based on the single-phase shift modulation mode. Then，the output voltage controller of the converter was designed by using the double-integral sliding mode control theory. The design process introduced the time domain analysis method to analyze the sliding surface coefficients. Finally，simulation and experimental results show the effectiveness of the proposed control strategy.

When a large number of energy storage devices are integrated into the power grid through a virtual synchronous generator （VSG），improper selection of control parameters in traditional fixed inertia and damping control strategies can lead to long adjustment times or large overshoot，and fail to fully leverage the flexible advantages of VSG control. To address this issue，an adaptive control strategy for energy storage system based on the crisscross optimization （CSO） algorithm was proposed. Firstly，the VSG model of the energy storage system was established，and the minimum value of the sum of the frequency error of the VSG system and the total harmonic distortion of the voltage was taken as the objective function of CSO. The battery state of energy （SOE） constraint was introduced to solve the optimal inertia and damping. This algorithm has a faster convergence speed and effectively avoids parameter local solutions. On this basis，an improved inertia and damping adaptive control strategy was designed to effectively improve the dynamic performance of VSG. Finally，the effectiveness of the proposed strategy was verified by building a simulation model using Matlab/Simulink.

Considering that the problem of voltage fluctuations on the DC side caused by the three-phase imbalance of the AC grid of the flexible interconnection device，a DC voltage fluctuation suppression strategy for flexible interconnection device based on the Torricelli point was proposed. Firstly，by defining the Torricelli point of the triangle formed by the voltage vector in the three-phase unbalanced fault system as the "new neutral point"，the voltage amplitude asymmetry and phase angle drift faults in the traditional three-phase coordinate system were transformed into voltage amplitude faults in the Torricelli point coordinate system； secondly，the relationship between the active power oscillation of the three-phase power system and DC fluctuations was analyzed，a new control strategy was established to suppress the voltage fluctuations of the DC bus voltage by suppressing the fluctuation of double frequency component of active power effectively. Finally，the proposed method was proved to be effective by the Matlab/Simulink simulation model.

The telecommunication power supply applied in 5G base stations can save energy consumption by reducing the output voltage when the equipment works in standby state. Nevertheless，frequent and rapid output voltage regulating of the converters is challenging. To solve this problem，the concept of partial power conversion was adopted in the design of power supply converters. A partial voltage regulating circuit was added at the output side of the converter，and fast output voltage regulating was realized with control of outer voltage loop，inner current loop，load current feedforward and output voltage feedforward loop. At the same time，higher energy conversion efficiency was guaranteed. A 1 kW DC/DC converter was built and verified the effectiveness of the proposed topology and control strategy.

Inverter products have been developing towards higher power density and more compact structure. To enhance the power density of inverter systems within a limited structural space， several prototypes need to be made and a large number of experiments conducted during the research and development process. To shorten the development cycle and reduce the number of prototypes and experiments， a thermal simulation method was proposed for the power unit of the inverter. Through this method， the system steady-state temperature rise of each scheme and the temperature rise of core components can be simulated during the research and development stage. It is possible to compare the advantages and disadvantages of different schemes without making prototypes or conducting experiments. Experimental results verify that the thermal simulation method has relatively accurate prediction of the cooling capacity of the thermal system. This method can effectively improve engineers' ability to control system thermal characteristics during the power unit design stage， quickly compare the advantages and disadvantages of different cooling solutions. It can also reduce the number of prototypes and experimental times， increase the first-time design yield， and reduce research and development costs.

In order to intervale evaluate saddle node bifurcation points（SNB） in a power system during load growth quickly and accurately， a quadratic approximation method to directly calculate the power system voltage collapse point of was proposed. Based on the approximate quadratic PV-curve of output PQ nodes in the system， load parameters were introduced into the node power balance equation， by using the complex function derivative rules to differentiate power equation twice， the first and second derivatives of node voltage to load parameters were derived theoretically， and the expression of PV curve function was further determined， then vertex coordinates were used to determine the initial position of saddle junction bifurcation point in the power system and the voltage collapse point was approximated by multiple iterations of convergence. The proposed method avoids multiple power flow calculations using the continuous power flow method，significantly reducing computational complexity. Simulation results on IEEE 14 and IEEE 118 bus systems demonstrate the effectiveness of this method. Compared with the supplemented P'Q node method and Thevenin's equivalent method， the quadratic form iterative approximation method has higher computational efficiency and robustness.