A multi-machine equivalence scheme is proposed for the electromagnetic transient numerical simulation of large-scale DFIG-based wind farm.Since the short circuit current is related to the real-time rotor speed after the rotor Crowbar protection is put into operation during the short circuit fault of DFIG based wind generator,it is proposed that,the rotor speed just before the short circuit fault is taken as the classification index of DFIG-based wind generators.With the minimum value of the criterion function E after convergence as the criterion,the K-means clustering algorithm is applied to classify the generators with similar index.As there may be several types of DFIG-based wind generator in a group,the capacityweighted aggregation is applied to calculate the parameters of the equivalent wind power generators,i.e.the parameters of rotor motion equation,impedance,converter and its controller,and box-type transformer.The detailed model of a DFIG-based wind farm and its equivalent model are established on PSCAD/EMTDC platform and the simulative results show that,under the same fault condition,the fault current of equivalent model fits well that of the detailed model.

The wind turbine with low-voltage ride-through capability is required not to disconnect from grid under certain fault conditions,for which the relevant relay protections should cooperate with it.The cooperation between the relay protection of transmission system beyond the wind farm outgoing line and the relay protection of wind farm is analyzed for different faults of transmission system components,wind farm outgoing line,wind farm internal grid and wind turbine.The current configuration of relay protection for the wind farm centrally connected to power grid is analyzed and it is pointed out that,the selectivity of wind turbine protection and converter protection is not enough,which leads to the disconnection of wind farm from grid before the fault is removed by wind farm internal relay protection or system relay protection,and the current configuration regards the wind power as a distribution network with single source,which neglects the contribution of wind turbine to short-circuit fault.Improvements are proposed for the relay protections of wind turbine,main transformer and outgoing line to ensure the successful low-voltage ride-through,and their problems are pointed out.

The electromagnetic transient models are developed with PSCAD/ EMTDC for three kinds of wind turbine:squirrel-cage wind turbine,DFIG wind turbine and D-PMSG wind turbine.The validity of the electromagnetic transient model for D-PMSG wind turbine is verified by comparing its simulative results with the results of digital-analog hybrid experiments under different fault conditions and control strategies.The validity of the electromagnetic transient model for DFIG wind turbine is verified by comparing its simulative results with the results of short-circuits experiment for real wind farm.Based on these models,the electromagnetic transient characteristics of wind turbines under different fault conditions are analyzed,which show that,the type,working status and adopted control strategy of fan,the fault type and the characteristics of weak power source are the main factors affecting on the fault current of grid-connected wind farm.The research lays the foundation for improving existing protection relay and developing new protection principles.

A DFIG-based wind generator electromagnetic transient model is established on the RTDS platform in this paper. Stator flux oriented vector control scheme is adopted to achieve power decoupling control in the DFIG based wind model. In order to research the impact of the wind farm on the existing protection relay of the network, this paper establishes single machine electromagnetic dynamic equivalent model of DFIG-based wind farm. To test the feasibility and the accuracy of the equivalent model, we establish a detail model of wind farm. From the result of simulation, we know that the instantaneous value of power, voltage and current of the two models. The validity of proposed equivalent model is verified. In order to study the impact of large wind farm on the existing protection relay, a power grid model integrated with wind farm equivalent model is built based on the RTDS platform. A closed-loop protection relay testing experiment is accomplished to study the impact of the wind farm on the existing protection relay. Performance under different short circuit fault conditions is tested.

With the increasing development of wind power, the capacity of wind farms is larger and its impact on grid operation has become more prominent. The fault current that wind farms provide is much different from the traditional synchronous machines. Wind farms in China are mostly centrally connected to power system. The central integration enlarges the impacts on power system. The correct action of the protection system is greatly affected. This paper analyzes the phenomenon that the wind farm side protection relay detects almost entirely zero sequence current and not correctly acts in case of ground fault, taking the outgoing line's fault of a real wind farm for example. Furthermore, a real wind farm model is established in PSCAD/EMTDC to reproduce this phenomenon and analyze its influencing factors.

Power grid was considered as a directed weighted network to identify the vulnerability of the network structure with the given operating modes.The directed electric betweenness was presented to identify vulnerable lines according to their power components,which can more practically make quantification of the used-ratio of each line by each plant-load pair in power system,while the differences of line-impedance under different voltage-levels were also considered by introducing the generator output coefficient.All the transmission paths between generators and loads were considered in the introduced directed global admittance instead of only the shortest path,thus the physical background of the presented model is more reasonable in power system.And the validity of the proposed algorithm is verified by the calculation result of IEEE-39 bus test system.

Large-scale wind farm usually contains hundreds of wind turbines, which is directly integrated to the high voltage transmission network through long distance transmission line. Because of the different control strategy, the electrical information between the wind generator and conventional synchronous machine is very different during the external shortcircuit fault conditions. And due to the excessive number of wind turbines, it's scarcely possible to simulate the electromagnetic transient process of a complete wind farm model because of the limit of the hardware source. In order to study the impact of the wind farm on the existing protection relay of the network, it is an important issue to establish a suitable electromagnetic dynamic equivalent model of wind farm to describe the electromagnetic transient process of the wind farm not just a single wind generator. Doubly-fed induction generator (DFIG) wind turbine is established in this paper on the RTDS platform, and a scheme is proposed to establish a suitable electromagnetic dynamic equivalent model of wind farm based on the model above. Finally the equivalent model is established and verified. A power grid model integrated with wind farm equivalent model is built on the RTDS platform. The electrical information of the high voltage feeder of the wind farm is input to really protection relays. A closed-loop protection relay-testing experiment is accomplished on wind farm outgoing line to study the impact of the wind farm on the existing protection relay. Performance under different short circuit fault condition is tested, and finally typical misstrip and maloperation cases are analyzed.

If the trip of an overload line causes cascading trip in the corresponding transmission section,emergency overload control should be executed to ensure the security of the transmission section.The primal-dual interior point method is used to achieve the control scheme for eliminating line overload by solving non-linear programming problem.The equal and opposite adjustment in pairs is introduced to avoid the out-of-limit of the balancing machines.In order to minimise adjustment and avoid load shedding under serious conditions,the integrative sensitivity is calculated according to the combined influence on the overload lines and close-to-limit lines,and the control nodes are separated into positive and negative output nodes.For possibly avoiding new overloads and reducing corrective times,the range of key transmission lines is expanded from including soly the close-to-limit lines to the transmission section that including the overload line.The efficiency and validity of the algorithm is verified by the calculation results of CEPRI 36-node test system.

This paper proposes an algorithm fast searching for key transmission section based on power component of line. Firstly, graph theory is used to calculate the real power transfer among individual generators and loads, then classify the lines based on the Ward's cluster analysis method to search the set of lines which have the similar power component to overload line, finally determine the parallel transmission section of the line according to the direction of the overload line and the set lines. As a result, the works of further calculation and security analysis can be reduced greatly. And fast security protection of key sections to prevent cascading tripping can be achieved accordingly. At last, the validity of proposed algorithm is verified by the calculation result of IEEE 14 test system.

Major features of IEC 61850 include SCL-based engineering, uniform data model and communication-based information transfer. Owing to XML-based characteristics of SCL, a substation engineering can be done easily with programming. There are four kinds of information file in the SCL-based engineering - SSD (System Specification Description), ICD (IED Capability Description), CID (Configured IED Description) and SCD (Substation Configuration Description). The SCD file has all information about substation configuration. From SCD file, data or information of interest can be easily extracted using SCL parser and they can be stored in the database automatically.

As a renewable, clean energy, wind energy has attract more and more attention. It has become a common understanding of the world to vigorously develop wind energy. In order to guarantee the safety and reliability for wind power integration operation, it is of great significance to establish an appropriate wind power generator system model and analyze its electromagnetic transient characteristics. This paper analyzes the control strategy of doubly-fed induction generator (DFIG) based wind generator. Vector decoupling control technology has been adopted to establish the mathematical model of DFIG based wind generator. Furthermore, an electromagnetic dynamic simulation model of DFIG based wind generator is established based on the platform of PSCAD/EMTDC. A short circuit fault is simulated to study the dynamic response of DFIG based wind generator. Finally, transient characteristic of DFIG is analyzed by this paper.