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.

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.

Power system blackout after power flow transfers caused by overloaded line tripping often occurs.To avoid cascading tripping,real-time emergency control is necessary.The key issue is to predict whether there are new overloaded lines in the system effectively and accurately.The paper firstly forms the system's minimal basic circuit set using graph theory on-line based on network topology analysis,together with component parameters and system operation state.Secondly,after a line is overloaded,it obtains circuit containing the overloaded line,searches for the lines which have the opposite power flow direction to the overloaded line in the circuit,and extends the search range using ring sum operation.Finally,it calculates in series power flow of a line after the overloaded line trips using distribution factors to confirm the lines to whom most of the power flow of the tripped line has been transferred.As a result,the computational needs for further analysis and system security assessment can be significantly reduced.

Voltage control of power systems is one of the key links in maintaining the power system voltage level.Based on the model predictive control(MPC) theory and method,a distributed control model and structure are obtained by employing the decomposition-coordination technology and the auxiliary problem principle to realize the decoupling of the mathematical model and structure of voltage predictive control.Thus the global centralized optimization problem is decomposed into some sub-problems that are only related to their local model and information and can be solved in a distributed manner.It is beneficial to reducing the dimensions of the optimization problem and improving the flexibility and security of control and calculation.Some simulations conducted on a typical power network indicate that the proposed method can effectively control the power system voltage level and prevent voltage instability,as well as obtain a control performance similar to the centralized MPC.

It is possible to prevent the total collapse of the unstable post-fault power system, by determining the splitting surfaces to calm down the system oscillation. How to determine the proper splitting surfaces in a short time is a key problem for power system controlled islanding. A novel and fast splitting surfaces searching method was presented. The aim of this method is to make the generation load imbalance in each island as small as possible. The method contains three phases: firstly, domains of each generator are formed according to the power flow tracing algorithm; secondly, initial splitting surfaces are determined based on the generators’ grouping information; thirdly, the final splitting surfaces are improved based on the initial splitting surfaces. A C++ program was developed based on the method. Simulations on the IEEE 118-bus power system show the proposed method is effective and fast.

This paper mainly study on the coordinated voltage control scheme with model predictive control (MPC). As the decomposition-coordination technology and the auxiliary problem principle is employed into the MPC based voltage control problem, a distributed voltage control model and structure can be obtained, and the global centralized optimization problem is decomposed into some sub-problems. All sub-problems are only related to their local model and data, and can be solved in a distributed manner, which is benefit to reduce the dimension of the optimization problem and improve the flexibility of control and calculation. Simulations conducted on typical power networks indicate that this voltage control scheme can prevent the voltage instability effectively and the distributed MPC scheme can obtain similar control performance to the centralized MPC scheme.

If the trip of overload branch may cause cascading tripping in corresponding section, emergency overload control should be executed to ensure the security of the transmission section and maintain its integrality and transmission capacity. A series of improvements against the conventional sensitivity algorithm are proposed, which include the advance of the integrative sensitivity, the use of equal and opposite quantities in pairs analysis method, and the introduction of heuristic rules. The choice of control node and its quantity ensures that the power flow of overload branch won't arise and normal branch won't overload, so the control strategy can availably and quickly eliminate overload. The rapidity and validity of proposed algorithm is verified by a lot of calculation results of a certain real system.

The long - range distribution features of ESP(Earth Surface Potential) of HVDC operation under monopole ground return mode are analyzed by finite element method,and the distributions of both shallow and deep soil structure models are calculated with ANSYS. The influences of substation outlet,substation earthing resistance,river and lake are taken into account. Results show that,soil structure has notable effect on surface potential and the computed result with the deep soil structure model is more accurate; the shorter the distance between substation overhead outline and grounding electrode is,or the longer the line is,or the fewer lines there are,the greater the potential difference between two line ends is. The river affects the ESP weakly while the edge of lake does comparatively strongly.

A Doubly-Fed Induction (DFIG) wind generator dynamic model is developed in PSCAD. Then a novel active power control strategy based on the indirect speed control is introduced to implement the power control tasks and the limits of these controls are also discussed. The novel strategy ensures that wind power generators without wind speed measurement can achieve the balance control and maximum power tracking to perform active power control tasks constrained by the available wind. In the simulation study, a single-machine infinite bus system with a wind turbine rated at 2 MW is conducted to demonstrate the behaviors of the generator in both steady state and transient state. The simulation results show a good performance of the dynamic model and the effectiveness of the control strategies.

Self-adaptive system islanding refers to the separation of an interconnected power system into electrically isolated islands automatically based on the identification of unstable mode. A fast searching algorithm for self-adaptive power system islanding, which is called multilevel reduced graph partitioning (MLRGP) algorithm, is presented in this paper. The aim of this algorithm is that the generation load imbalance in each island is as small as possible. The algorithm computes a reasonable islanding strategy of power system G=(V, E) in O(|E|) time. A C++ program is developed based on the algorithm. The verification of the islanding program is proven with simulations on a practical 212-bus power system. It costs only about 20 ms to get a reasonable islanding strategy on an ordinary PC, which satisfies the speed demand of self-adaptive system islanding.

Recent blackouts have fully presented the damage of the cascading overload trip. The basic reason for cascading overload trip is poor global superiority of the back-up protection and existing auto-mechanisms which only focus on action behavior of themselves to cut component, but don't consider the damage of power transferring and the change of network. This paper first illustrates the meaning of transmission and point that the objective of security protection of transmission is to maintain its integrality and transmission capacity to avoid cascading overload trip. Then, implement condition and key technology are discussed, which are online searching for transmission section, real-time prediction of cascading overload and real-time control to avoid cascading trip on transmission section. This paper has brought forward the primary scheme. Its validity is verified by calculation result of a certain real system.

Self-adaptive system islanding refers to the separation of an interconnected power system into electrically isolated islands automatically based on the identification of unstable mode.A fast searching algorithm for self-adaptive power system islanding,called multilevel reduced graph partitioning algorithm(MLRGPA),is presented to make the generation load imbalance in each island as small as possible.A reasonable islanding strategy of power system is proposed,and a C++ program is developed to realize the algorithm.The islanding program is verified with the simulations for a practical 212-bus power system.It takes only 20 ms to get a reasonable islanding strategy on an ordinary PC to satisfy the rate requirement of self-adaptive system islanding.

An integrative sensitivity of transmission section power flow adjustment is proposed,according to which the operation sequence of control nodes is determined;the adjustment method of reverse and equivalent match is used to avoid the involvement of balance machine;a series of heuristic rules is presented to ensure that,the normal branches will not be overloaded during the determination of control nodes and corresponding control quantity and the overload degree of section branches may mitigate as soon as possible,which quickly results in the control strategy without the precise power flow calculation to ensures the rapid and valid elimination of overload.Simulative calculation of a provincial system proves it.

To improve the stability of the power system integrated of wind farm, the wind power system energy function is proposed in this paper based on the energy conservation law. Then a stability control strategy based on the energy function is introduced. The impacts of DFIG operation limits on the control effect are also discussed. By variable speed operation, the control strategy restrains the wind power system oscillations by eliminating the system unbalancing energy during the disturbances. In the simulation study, a testing system including a DFIG based wind farm which is implemented in DIgSILENT/Power Factory is conducted to demonstrate the effectiveness of this strategy. The simulation results show that, in different system operation statuses, the control strategy is effective in improving the power system synchronous stability without deteriorating the system voltage stability.

Cascading overload trip due to lack of transmission section protection is an important reason of large-scale blackout. This paper illustrates the meaning of transmission section and points out that the objective of transmission security protection is to maintain its integrality and transmission capacity, and to avoid cascading overload trip. In addition, the implement condition and key technology are discussed, which are online capture of transmission section, real-time prediction of cascading overload and real-time emergency control to avoid cascading trip. This paper has brought forward the primary scheme. Its validity is verified by the calculation result of CEPRI 36 test system and a province system.

In this paper, a method, which is essential in initiating emergency control measures, is proposed for fast and accurate online detection of transient instability based on real-time measurement. Under a certain disturbance, the candidate critical cluster of power system is predicted and refreshed at each sampling step. The method identifies the loss of synchronism at each time step by observing the geometric characteristic of the post-fault trajectory of the generalized one-machine equivalent system. A new structure of the emergency control scheme relies on SIME (for Single Machine Equivalent). It is unnecessary to calculate the unstable equilibrium point and some limitations caused by the time-invariant assumption are also eliminated. Simulations are carried out on the IEEE 50 generator test system and a real power system. The results indicate that the proposed method has the features of reliability and effectiveness in preventing the system from the loss of synchronism.

In this paper, a software package of finite element (FE) analysis for induction machines is presented. The back-EMF in windings is expressed by magnetic vector potential, which is used to couple with transient finite element analysis. The equations of drive circuit and rotor bars are described by loop circuit equations. The pre-processing and post-processing of this package are developed based on AutoCAD. The FE model and some machine parameters can be input by friendly graphical user interfaces. The calculated magnetic field distribution and transient results can be shown in AutoCAD or graphical interfaces.

Introducing the concept of morphology into the ultra-high-speed protection by traveling wave current polarity comparison,a highly efficient algorithm based on the morphological gradient(MG) is proposed.Undergoing the MG transform,the aerial mode with larger maximum is chosen to ensure the sensitivity for any fault type without dead zone.The ATP simulations show that the scheme for ultra-high-speed protection enables to correctly distinguish the internal and external faults,and is insensitive to fault locations,fault path resistances,fault inception angles and fault types.Furthermore,the protection scheme by traveling wave current similitude degree comparison,which is independent on the polarity of certain points of two ends,is presented to improve the discrimination reliability.

On the basis of the characteristics of line boundary, this paper analyzes the different features of internal and external faults. The difference is that the ratio of high frequency to low frequency components in case of one external fault is less than that in case of the same internal fault, in which high frequency is in the band of line trap and low frequency is within 1~10kHz. Based on the different features, the paper presents a new non-unit transient-based protection unit using backward traveling-wave, called boundary unit (BU). The practical BU algorithm is designed using wavelet transform. A number of ATP simulation tests show that the new algorithm is feasible, and ultra-high-speed, and that the BU performance is superior to that of the former non-unit transient-based protection using current traveling-wave. The paper also indicates that the non-unit transient based protection is difficult to reliably protect the whole line, therefore the non-unit transient based protection may be used as an ultra-high-speed unit of the whole-line protection.