Energy Internet is the key of meeting the ever-increasing energy demand and building a sustainable energy eco-system in the future. As the fundamental physical realization of the Energy Internet, integrated energy systems (IES) need to be properly planned to ensure the safe, efficient and low-carbon operation in later stages. Firstly the basic planning methods for various individual energy systems and the general framework of IES planning are introduced, and the models of coupling elements among different subsystems are combed. Literatures in the field of joint planning of electricity-gas coupling systems and electricity-gas-heating/cooling coupling systems are classified according to the types of subsystems involved and are analyzed in detail. Finally, the limits of current work as well as potential research topics in this field are proposed.

With the gradual progress of the energy system reform, the integrated energy market (IEM) has become a research hotspot, and the optimal competition strategy of the integrated energy service provider (IESP) is one of the key technologies in this field. Most energy markets use pool-based market operating models, therefore, the optimal competition strategy of IESP under the pool-based market is studied and a two-level optimization model for the competition strategy of IESP is established in this paper. In the two-level model, the upper model decision variables are energy price of users and bid strategy of IESP, and its goal is its own benefit, the lower models are the user response model based on the price elasticity of demand and electrical and natural gas pool-based market model. To solve the problem, the two-level model is transformed into a single-layer quadratic programming problem with linear constraints. Finally, a case analysis is given to demonstrate the effectiveness and feasibility of the proposed method.

The influence of enhanced governor of hydropower generator unit on ultra-low frequency oscillation stability of power grid is studied. Based on the characteristic that the enhanced governor mainly modifies the dead zone of the primary frequency control, and by the usage of the governor model which considers the dead zone of primary frequency control, the describing function of primary frequency control’s dead zone of enhanced governor is deduced in which the offset value of the steady state frequency of power grid is considered. The theory analysis results of the influence of enhanced governor on ultra-low frequency oscillation stability of power grid are proposed based on the describing function, and are simulated and testified in the single generator with load system and Yunnan power grid. The results show that the influence of enhanced governor on the stability of ultra-low frequency oscillation of power grid is related to the factors including the stability of governor system without dead zone of primary frequency control, the offset value of the steady state frequency of power grid and the disturbance size and so on. When the offset value of the steady state frequency of power grid is equal to the setting value of the dead zone, the ultra-low frequency oscillation of the hydropower generator unit with enhanced governor will develop into a self-excited oscillation.

Data-driven method is limited to be applied in fault location of distribution network because of the relatively small number of fault samples. In order to solve the problem of poor learning effect with small samples in deep learning, a new fault area location method based on deep convolution neural network (CNN) with transfer learning is proposed in this paper. Firstly, the characteristics of transfer learning and CNN are analyzed, the feasibility and advantages of their applied in fault area location of distribution network are also discussed. Then, a deep CNN model based on transfer learning is constructed by using ResNet50 network. The validation of the IEEE 33 bus system shows that the proposed method can locate the fault area location accurately even with limited amount of samples by using the voltage and current information of only two measurement points, and is not easily affected by the factors of transition resistance, fault type and noise.

The multi-objective fuzzy collaborative optimization model of multi-energy flow system under different operating conditions is established in this paper to solve the (frequently occurred) non-linear working characteristics problem of the devices in that system, where this non-linear problem causes the offset of operation characteristics and design points of those devices. Firstly, a high-order nonlinear model of devices under off-design performance conditions is constructed on the basis of considering renewable energy sources and energy storage systems including electricity storage (ES) and heat storage (HS). Secondly, according to the needs of different engineering application modes, a collaborative optimization model of multi-energy flow system with economic, environment-friendly and energy saving as optimization objectives is proposed, and a multi-objective fuzzy collaborative optimization model is developed based on fuzzy theory. Then, the physical constraints, such as: equipment load rate, start-up time, working hours and off-design performance under consideration, are taken into consideration. Finally, the model is applied and verified by a chosen case study of a multi-energy flow system. The results demonstrate the availability of regarding the proposed multi-objective fuzzy collaborative optimization of multi-energy flow system under off-design performance conditions as the basis of optimal scheduling, collaborative operation, the design of different operation modes and engineering calculation analysis of multi-energy flow system.

Firstly, the dynamic characteristics of the high-end valve hall and valve group of UHV Xinsong converter station are tested and analyzed on the spot. The dynamic response of the valve hall structure under different seismic excitations with a peak value of 0.4 g is calculated. The acceleration, displacement and stress responses of the key parts of the valve hall are obtained, and the seismic performance of the valve hall under strong earthquake is analyzed. The results of dynamic characteristics test and modal analysis are consistent. The first three frequencies of the high-end valve hall are between 2.0 and 5.0 Hz. The vibration modes are X and Y translations and X-Y plane torsion. The main vibration frequency of the valve hall structure is close to the predominant earthquake frequency, and it is easy to resonate under earthquake. Seismic response analysis of the valve hall under strong earthquake shows that the dynamic amplification factor of acceleration at the top of the steel column and the installation position of the through-wall sleeve are larger; and the structural response has a certain torsional effect. However, the maximum component stress of the valve hall structure is far from the yield stress of the material, and the overall seismic performance of the steel structure valve hall is better.

The fast and reliable identification of DC line faults is one of the key technologies for the development of flexible multi-terminal direct current (MTDC)systems based on modular multilevel converters (MMC). a DC grid fault identification scheme based on single terminal current modulus analysis is proposed by analyzing the transient characteristics of short-circuit faults in the line of flexible MTDC grids. The scheme realizes the rapid positioning of the DC line fault by detecting the polarity and magnitude of the dynamic deviation value of the current-mode fault component, and uses the difference of the current zero-mode fault component after the fault to realize the rapid determination of the fault pole. The four-terminal flexible DC grid model of the modular multilevel converter is built on the PSCAD simulation platform. The simulation example shows that the protection can detect the DC line fault quickly and reliably under different fault locations and transition resistances and accurately identify fault poles.

For common grounding electrode, if one of the lines is being maintained and another is under mono-polar earth operation, operating current will flow into the ground and be picked up partly by tower at overhaul side, which will finally flow into the grounding grid of converter station through lines and affect the equipments and personal safety in converter station. In this paper, personal safety risks such as stepping voltage and touch voltage caused by the current flowing into the grid of converter station during the line maintenance of Yulongling common grounding electrode are calculated and analyzed, and DC bias of transformers in converter station is evaluated with the power system grouding analysis software CDEGS under monopole grounding operating mode with rated current. Results show that the current flowing into the grounding grid during the line maintenance are unable to cause personal safety risks but may result in the excess of transformers bias current in Suidong converter station.

In recent years, quite a few breakdowns of cable joints occurred due to closing of circuit breakers after tripping in the 110 kV cable line. To study the reason why cable joints break down, the transient behavior of cable joint under the condition of breakers opening and closing is studied in this paper. Firstly, a RLC equivalent model of cable joint is given and an 110 kV cable line is simulated by PSCAD, then the effect of closing or opening angle, the length of line and the number of joints on overvoltage of joint are analyzed, to find out the joints with the most serious over-voltage. Additionally, the internal electric field distribution of joint with serious overvoltage is simulated by ANSYS. The results show that the overvoltages of the joint at 60%~75% of B phase and C phase line are more serious than other joints; meanwhile there are more than 40 kHz high frequency components of overvoltage in joint shielding. What’s more, the maximum electric field strength on the interface between silicone rubber and cross-linked polyethylene and the maximum field strength at the end of the high-voltage shielding tube are doubled than when there is no overvoltage, and the maximum interfacial strength exceeds the security threshold. In the process of breakers closing after tripping, the joints at 60%~75% of the line repeatedly suffer overvoltage, in which the increase of electric field strength at the end of high-voltage shielding tube and the high frequency oscillation in joint shielding both easily cause breakdown of joints.

In the process of tension release of EHV transmission lines, the breaking of the traction line will lead to serious accident of whipping of both the wire and the moving plate, and cause great loss to the important facilities in the transmission line. In order to prevent similar accidents from happening, by analyzing the motion characteristics of the line upon breaking, an anti-slip-line device is designed, which can lock the traction rope immediately after it’s broken. Through calculation analysis and practical engineering application, the device can meet the needs of field engineering application. Multiple tests of the anti-slip-line device show that the average slip-away distance of the traction rope is 0.89 m, and the average reaction time of the device is 0.426 s, which means that the device can effectively avoid accidents and can be used as an effective measure to prevent accidents by traction rope breaking.

Magnetoresistance sensing technology provides a new technical means for current measurement in power system. For the copper bar of distribution cabinet, the magnetic field distribution characteristics of copper bar are analyzed. A current measurement method without fastening the copper bar and sensor position is proposed. The measuring principle is discussed in detail, and a method of resisting space magnetic field interference is proposed. The simulation model is established in the MATLAB environment, and the measurement principle is simulated. The simulation results verify the correctness of the measurement principle. The factors affecting the measurement accuracy such as sensor position deviation, copper bar thickness, space interference magnetic field and so on, are simulated and analyzed. The simulation results verify the effectiveness of the proposed anti-interference method and the practicability of the measurement method.

Saturated iron-core type superconducting fault current limiter(SICS-FCL) has non-linear impedance characteristics. The electromagnetic transient process after connecting to 500 kV power grid has an important impact on the safe and stable operation of power grid. According to the single-phase three-column core structure of the 500 kV SICS-FCL, the coupling relationship between the main electromagnetic parameters is analyzed by magnetic circuit decomposition method. The complex magnetic circuit analysis model is transformed into equivalent circuit model. The electromagnetic transient simulation model is built on the PSCAD/EMTDC simulation platform. Finally, combined with the parameters of 500 kV power grid and the design scheme of SICS-FCL prototype, the transient impedance characteristics under steady-state operation and different short-circuit faults are analyzed through specific examples. The results are consistent with the theoretical analysis.