Information is the reflection of the attributes of objective things and people’s thoughts, and information acquisition ability and information processing level are the vital symbols of the process of human society. In recent years, with the rapid development of the new generation digital technology, information processing has entered the Big Machine Era, the information system of which greatly promotes the development of the economy and society, drives profound changes in various trades and industries, and changes people’s lives. The wide application of digital technology in the power industry has facilitated the transformation of the power system from the physical system, cyber-physical system to the social-cyber-physical system. In this paper, the development course of information technology is reviewed and the evolution process of power system digitalization is systematically expounded. The development trend of digitization in three dimensions, including the physical world, information space and society, is analyzed. In the end, the digitization technology of power system is prospected.

Coherency-based dynamic equivalence is a commonly used method to establish the equivalent model of a wind farm, in which it is considered that a group of wind turbine generators (WTGs) of the same type and similar operating states could exhibit coherent dynamic performances to the external power system and its total dynamic output characteristics can be reflected via a single WTG. In this paper, theoretical basis and tenable conditions for this dynamic equivalent method is mainly studied based on small-signal model. Therefore, full-order linearized state-space model of a group of WTGs (GW) is established at first. Secondly, drawing on modal decomposition in modal analysis, the equivalent decomposition of a GW dynamic system is derived. Based on the results of the derivation, in analogy with mode shape, controllability and observability, reasons and tenable conditions which WTGs in a GW could exhibit coherent dynamics to the external AC grid and the complete dynamic performance of the GW can be reflected by a single WTGs are discussed. Finally, the proposed methods and conclusions are validated through study cases. The methods and conclusions provide theoretical basis for coherency-based dynamic equivalent of wind farm (CDEWF) and are of certain significance to be guidance or reference in researches on WTGs clustering in CDEWF.

Wind power converter (WPC) is a key component of wind energy conversion system (WECS). It is responsible for the important functions of wind power conversion and grid connection. Its failure will cause the shutdown of the entire WECS so that its reliability has a great impact on the reliability of the entire WECS and even the entire wind farm. Due to the harsh operating environment of offshore wind farms and the difficulty in repairing and replacing faulty components, WPC has a more serious impact on the overall reliability of offshore wind farms. Based on the mechanism of the WPC failure, a reliability model of the WECS in offshore wind farms is established, which considers the WPC failures. Based on the established reliability model, this paper further considers the wind turbine wake effect and the failure of the wind farm’s collection system to evaluate the reliability of the offshore wind farms. This paper studies the impact of WPC failure on the overall reliability of offshore wind farms and conducts a sensitivity analysis of the main factors affecting the reliability of WPC and offshore wind farm. The results of case studies show that the WPC failure has a great impact on the overall offshore wind farm’s reliability. After considering the impact of WPC failures, the capacity factor of offshore wind farms will be significantly reduced.

This paper proposes a deadbeat current control strategy for wind power generation systems based on hexagonal modular multilevel converter (H-MMC). First, the system topology and the mathematical model of H-MMC are introduced, and the expressions of bridge arm voltage and bridge arm current are obtained. Then the machine side and grid side are controlled respectively, the machine side, grid side current reference value, circulating current and neutral point voltage reference value are calculated, thus the bridge arm current reference value is calculated, and then no difference in the bridge arm current Beat control is carried out, the reference value of the bridge arm voltage is calculated, combined with nearest level modulation (NLM), the control signal of each sub-module is obtained. The control method has the advantages of simple control, low switching frequency, and is not affected by frequency fluctuations on the machine and grid sides. Finally, a direct-drive permanent magnet wind power generation system model based on H-MMC is built in Matlab/Simulink to verify the effectiveness of the proposed control method.

In view of the rapid increase in the scale of wind power integration and the difficulty of accurately predicting wind power, an online transient stability risk assessment method considering the uncertainty of wind power output is proposed in this paper. Based on the transient stability participation factors, the dominant wind farms are extracted and merged; the weighted wind farm output Euclidean distance is used to cluster the scenarios; the maximum risk operating scenarios are determined for each type of scenario subset according to the impact of uncertain variables on safety and stability; scene screening is adopted for safety and stability assessment calculations. Through the above steps, a safe and stable operation risk considering the uncertainty of wind power output can be obtained. The proposed method helps dispatch operators to accurately grasp the safe and stable operating boundary of the system in real time. An example of an actual power grid verifies the effectiveness of the method.

The sub-& super-synchronous control interaction (SSCI) between direct-driven wind turbine generators in offshore windfarm and AC power grid probably cause an unusual stability problem, which seriously restricts the consumption and utilization of offshore wind power. The chracteristics of SSCI are varying with the operating conditions and the number of wind turbine generators, which have brought a great challenge for the design of SSCI suppression method. In this paper, a grid-side supplementary damping control (SDC) scheme based on static var generator (SVG) is proposed to stabilize SSCI in offshore wind farms. Firstly, the impedance-based analysis reveals the mechanism for SDC to suppress the SSCI. Then, the structure of SDC and the parameter optimization method based on damping maximization are introduced. Finally, the oscillation suppression effect of SDC is verified by electromagnetic transient simulation, which shows that the proposed SDC can provide positive damping in a wide range of oscillation frequencies, and the performance demonstrates that SDC can improve the impedance characteristics of the wind farm in the sub-& super-synchronous frequency, and effectively suppress the SSCI caused by offshore direct drive wind turbines.

At present, VSC-HVDC transmission is a common choice for offshore wind power grid connection. In the whole wind power transmission system, DC submarine cable, as the connection unit of VSC-HVDC converter stations, plays a crucial role in the safety and operation reliability of the whole system. Based on the DC submarine cable in a planning offshore wind power VSC-HVDC transmission project, a simulation model of the whole offshore wind power VSC-HVDC system is established in PSCAD/EMTDC. A total of 17 kinds of fault types may happen in the system are considered. The fault zones include coupling transformer grid side, coupling transformer valve side, bridge arm reactor valve side and DC side. As a conclusion, the highest core-ground overvoltage of ±250 kV submarine cable is 506.4 kV and the corresponding fault type is the grounding fault at the start point of DC submarine cable. Considering 30% insulation margin, the insulation level of the DC submarine cable with rated voltage of ±250 kV is determined to be 660 kV. The calculated results can be an important reference for improving the safety of submarine cable operation.

Offshore VSC-HVDC converter stations are necessary facilities for large-capacity and long-distance offshore wind power VSC-HVDC transmission project. Based on the electrical wiring of the offshore VSC-HVDC converter station and the type of electrical equipment of valve hall, an advanced electrical equipment layout scheme of valve hall for the offshore VSC-HVDC converter station is proposed in this paper. The adjacent arrangement of three-phase bridge arms in the same pole and the positive and negative bridge arms are arranged in different valve hall respectively, which can better meet the requirements of offshore platform structure design and construction. On this basis, the valve hall size calculation method of typical offshore VSC-HVDC converter station is put forward in this paper. The electrical equipment layout scheme of valve hall and the valve hall size calculation method proposed in this paper can greatly reduce the valve hall size of offshore VSC-HVDC converter station and reduce the project cost. It is significance to promote the large-scale intensive development and grid connection technology of offshore wind power in China.

Lugaozhao DC project is the first high voltage, large capacity, multi-terminal LCC based DC system in China. It is reconstructed based on a two-terminal DC system, and has various operation modes to realize the mutual support of Yunnan hydropower resources and Guizhou thermal power resources and to relieve the provincial power balance pressure. According to the multi-terminal operation characteristics and fault behavior of Lugaozhao DC project, this work mainly analyzes the system stability and control measures of the DC polar faults, DC line faults, bipolar zone faults under three-terminal operation mode. Furthermore, to handle the key problem of multi-terminal DC power loss calculation method, converter station polarity reversal phenomenon, and adaptability of control strategy, security and stability control strategy and system architecture are proposed for multi-terminal DC systems. The presented security and stability control system provides a strong support for the mature application and promotion of multi-terminal DC technology in China.

An increasing number of distribution generations (DGs) are being integrated in distribution network (DN), which may cause voltage deviation (VD) and network loss. By considering the distributed energy storage system (DESS) as an effective solution, this paper proposes an optimal scheduling model, the objectives include main network electricity purchasing cost, network loss cost, and voltage deviation penalty combined with the power four-quadrant output control strategy of the battery energy storage system (BESS). Besides, interval robust optimization (RO) model is applied to deal with the uncertainties of wind power output and load demand. The second-order cone relaxation (SOC) technique is used to convexify the power flow equations in the optimization model. Consequently, the original optimal scheduling model is converted to a second-order cone programming (SOCP) problem. It solves the problem which is difficult to consider the voltage in DN. Meanwhile, Big-M method is used to deal with the nonlinear terms with absolute value in the objective function. Therefore, the entire DES optimal scheduling model of the DN is transformed into a mixed integer second-order cone programming problem (MISOCP); and a mature CPLEX solver is invoked to solve the optimal scheduling model. Finally, comparative studies based on IEEE 33 are performed to validate the effectiveness of the proposed methods.

In order to further promote the implementation of the renewable energy consumption guarantee mechanism and respond to the provincial renewable energy consumption guarantee work scheme, this paper proposes a target decomposition method of renewable energy consumption responsibility weight at prefecture level based on power flow tracking. The method takes into account the network topology structure, line power flow and the power characteristics of generators and loads in the process of system operation constraints, which are applicable to the actual power system, provides a new realization way and more accurate calculation basis for provincial regions to achieve the national renewable energy consumption and non-water rights heavy target value. Based on an IEEE 30 bus system and a numerical example of provincial power grid, the effectiveness of the proposed method is verified.

Taking the T-shaped LW15A-363/Y high-voltage circuit breaker structure as the prototype, based on the similarity theory, an experimental model with 1:4.5 reduced-scale is designed and constructed. El Centro wave, Lanzhou wave and Chi-Chi wave are adapted as the ground motion input, respectively along the X and Y axial to the model, the simulated earthquake shaking table test is carried out under different peak ground acceleration (PGA). The seismic response law of the model is studied in the weak parts such as the root of the insulating porcelain column, the top and root of the equipment. In addition, C3D8R element is used for porcelain column and S8R element for flange. ABAQUS finite element model of model structure is established by sweeping mesh division method, and the dynamic response at El Centro wave under X direction excitation is analyzed. Results show that there is a large stiffness difference between inside plane and outside plane of T-shaped high-voltage circuit breaker for single axial symmetry structure, so the dynamic response of structure model under seismic excitation in the Y direction is greater than the X direction; and the seismic response at the end of the insulated porcelain column in the Y direction is the largest and the acceleration amplification coefficient is from 1.1 to 2.2. Between the finite element analysis and the test, the maximum error of natural vibration frequency is 13.2%, the maximum error of acceleration response peak is 9.2% and the maximum error of the displacement response peak is 14.3%, so the two fit well. The research can provide reference for seismic design and engineering application of such circuit breaker structures.