When there is fault in submodule of modular multilevel converter (MMC), MMC will run in the state of asymmetric bridge arm,which can cause circular current increasing of fundamental frequency, further affects the performance of MMC. Therefore, this paper considers the comprehensive effect of redundant sub-modules and bridge-arm asymmetric operation, and analyzes the mathematical model of MMC and sub-module (SM) switch function and circular current in detail. On this basis, this paper proposes an optimized redundant protection strategy. Unlike previous strategies, the sub-modules in healthy arms need not be bypassed when SM faults occur and asymmetry degree will be reduced by regulating the base voltage of SM. A model of two-terminal MMC-HVDC system is built in PSCAD/EMTDC, the simulation result proves that the proposed strategy can ensure the system operate well under arm-asymmetric conditions and the ability of the converter to resist the sub module fault is enhanced.

In order to analyze the air gap flashover characteristics of switching impulse voltage of fittings in Xinsong converter station at the altitude of 2 400 m, according to the fittings structure size of valve hall in Chuxiong converter station, this paper selects Φ750 mm/Φ880 mm cap, Φ1 200×300 mm/Φ1 600×300 mm/Φ1 900×300 mm grading ring and Φ1 100 mm/ Φ1 600 mm/ Φ2 000 mm equalizing ball, all in typical size, and carries out an experiment in Kunming UHV base at the altitude of 2 100 m. On this basis, this paper checks the air clearance of transmission side valve hall in Xinsong station and presents the discharge voltage curves for different altitudes after altitude correction. The study of air gap discharge characterstics and altitude correction provides a reference for practical engineering.

To study the seismic response of UHVDC insulators interconnection system under different actions of earthquake, the slipping fitting at the top of insulator is simulated as nonlinear spring, and the finite element method (FEM) is applied to analyze the seismic response. The data of acceleration, displacement and stress response at the critical parts of single and interconnected insulator system are obtained under multiple earthquake wave. The influence rules of tubular busbar on composite insulator are obtained from the comparison of dynamic responses of single and interconnected system. Comparing with equivalent single post insulator, results of the interconnected system show that the frequencies of interconnected system are increased; accelerations at the top of the insulator are much higher, and the displacement responses show much randomness; stresses at the bottom of the insulator are small and far less than the ultimate strength of composite insulator; the slippage distance of the fittings has great influence on the acceleration at the top of the insulator, and the effects of slippage on equipment must be taken into consideration in seismic design when the tubular busbar is used.

Solid state transformer is one of the key technologies of energy internet, which is closely related to the acceptance of renewable energy and flexible power conversion. In order to solve the problem of low efficiency in high voltage intelligent solid state transformer, focusing on the input stage of the intelligent solid state transformer, the synchronous modulation usually used by the cascade input stage of solid state transformer is improved. The hybrid modulation is adapted to improve the efficiency of the input stage. Finally the efficiency of two modulations is compared based on MATLAB/Simulink simulation, thus the efficiency of hybrid modulation is verified.

With increased penetration of power electronics interfaced renewable generation such as photovoltaics and wind turbines, the power gird will face huge challenges. The virtual synchronous generator(VSG) technologies, which learn from the operation of the traditional power system to control the inverter to mimic a synchronous generator, can overcome the shortages of power electronic equipment, such as low inertia and damping. This is of great significance to improve the integration of distributed generation and to build the smart grid. This paper presents the principles and basic characteristics of virutal synchronous generator. The current research results are reviewed from the aspects of methematical model, control strategies and stability analysis. The inertia, frequency and voltage regulation functions of VSG have been realized, and the operation and stability characteristics of sigle-machine system has been widely discussed. However, the high-frequeny characteristics, storage systems, transient characteristics and interaction mechanism of massive VSG still need to be researched. Finally, the application senarios in power system and practical engineering applications of virtual synchronous generator are introduced from the aspects of connection of renewable energy, microgird and VSC-HVDC.

The development of wireless power transfer (WPT) technology has showed great application prospects in many areas. Although there have been deeply studies on the key parameters like transfer distance, efficiency and transfer power of WPT, the research on electromagnetic environment has not been paid enough attention to. In this paper, researches on electromagnetic environment progress of magnetic coupling WPT worldwide are summarized, and typical research cases on magnetic coupling WPT are introduced, which include animal experiments and 3-dimensional numerical simulation method. Based on the research results, it shows that the WPT related products in the areas of implantable devices, electric vehicles and consumer electronics can meet the regulation of related standards. Meanwhile, it is the general method to study electromagnetic environment of WPT by 3-dimensional numerical simulation method, but researches should be focused more on the long-term medical experiment or other technology solutions in the future.

To effectively detect the hidden fault of transformer winding, a new vibration detection method of transformer winding is proposed, which combines grey correlation analysis and entropy weight theory, and is based on the inherent relations of vibration frequency response (VFR) curves at several measuring points. The grey correlation degree and grey area correlation degree of VFR curves at each measuring point are computed based on grey theory, then entropy weight correlation degree is defined and the state of transformer winding is tested. The VFR experimental results of some 220 kV transformer winding under normal and lossened condition are analyzed. The results show that the computed results based on proposed method agree well with the preset condition of transformer winding, which well illustrates the loosened degree of transformer winding, and has better application value.

According to the structural characteristics of optical fiber composite overhead ground wire(OPGW), a method of OPGW icing load monitoring based on strain analysis is proposed. A theoretical model of OPGW icing loading monitoring is established which based on the relationship between monitoring temperature, strain of optical fiber and icing thickness. The relationship between the stress and temperature of OPGW and the temperature and strain of the optical fiber is analyzed. An icing monitoring system based on Brillouin optical time domain reflectometer is developed and simulation platforms are built. The accuracy and applicability of the theoretical model of icing monitoring are verified by simulation. This method is applied to the actual transmission line, it proves the feasibility and practicability of OPGW icing load monitoring method which bases on strain analysis by comparing the data with other icing monitoring data.

The communication mechanism of 4-in-1 metering system based on concentrator mode is studied, single point fault and communication bottleneck in 4-in-1 metering system are analyzed. A large scale meter management architecture based on Ad Hoc networks is put forward, in which the communication network of the smart grid is used as a virtual private network of many kinds of metering devices, and a wireless Mesh network is formed by a plurality for metering devices nearby. Based on the extended SCEP protocol and lightweight certificate, a meter authentication scheme is proposed to solve the identity authentication and security problem of multiple meter network. A data acquisition protocol based on IPv6 is designed, and prototype system tests are conducted. Results show that the key functions such as network topology discovery, data collection and so on are realized in this system. The new mechanism can significantly reduce the repetitive construction of communication network, and support the new business of " Internet plus" mode.

Starting from the history of Guangdong power industry, the power market reform in Guangdong province is retrospected in this paper. It is suggested that Guangdong should further construct the generation (consumption) contract trade market. Based on an illustration of the main principles of constructing the generation (consumption) contract trade market, a market model with two specific layers is proposed: basic generation contract trade market and bilateral generation (consumption) contract trade market. Based on this proposal, institution of each market is designed including market definition, implementation area, business mode, transaction method, trade period, settlement, etc. Finally, the pros and cons of the two markets are analyzed.

It is of great practical significance to establish the evaluation system of power equipment utilization efficiency aiming at power grid capacity optimization for collaboration of power grid planning and construction. An evaluation system of power equipment utilization efficiency aiming at power grid capacity optimization is introduced. Four major evaluation indices, including capacity-load ratio, maximum load rate, annual equivalent mean load rate and annual mean utilization time, and power grid capacity optimization limits are given. Considering the key factors influencing utilization efficiency, and a capacity optimization method is adopted to enhance the overall utilization efficiency. Case study in Yunnan power grid verifies the reasonability and effectiveness of the evaluation system and the optimization method proposed in this paper.

The investment proportion of distribution network projects in power grid corporation is increasing in recent years. However, due to the large number and complex types of distribution network projects, and the heavy workload of cost data processing and analysis, the cost management and cost level analysis of distribution network projects have been short boards of power grid corporation. From the characteristics of distribution network projects, fully considering the item segregation of construction budget and the depth of data, the cost level analysis index system for distribution network projects is constructed, and the pretreatment method for basic cost data is proposed. Combining with project examples, three kinds of analysis methods of distribution network projects, i.e., the comparative analysis method, the cost composition analysis method, the influence factor analysis method are discussed. These methods have achieved good results in the application of China Southern Power Grid.