Ancheng Xue

Power System Dynamic Security Region and Its Approximations

Dynamic security region (DSR) for security assessment and preventive control is defined in terms of variables that are under the control of the dispatcher prior to a fault. The boundary of DSR can be expressed using the function describing the stable manifold of the controlling unstable equilibrium point. The function is shown to be the solution of a partial differential equation. Quadratic and linear approximations to the function characterizing the boundary of DSR are derived. Numerical tests of the approximations are conducted. Potential applications of the method are discussed

Blackout Model Based on OPF and its Self-organized Criticality

Based on the essence of self-organized criticality and optimal power flow (OPF), this paper develops a model to capture the cascading failures and blackouts in power systems, which avoids some shortcomings of existent blackout models. The proposed model contains two dynamics, one is the fast dynamics which simulates the serial blackouts in power system, and the other is the slow dynamics which reflects the growth of the power system. Simulations in IEEE-30 bus system with the proposed model show that, the processing of the cascading and blackout can be captured by the fast dynamics, together with the self-organized criticality property of fast dynamic respect to the micro-scale. Besides, the macro-scale of self-organized criticality of power system can be revealed from the viewpoint of total load demand vs. the total network transfer capability. Furthermore, improving the transmission ability of network could effectively prevent blackout and reduce its risk.

A new transient stability index of power systems based on theory of stability region and its applications

This paper presents a new transient stability index based on the implicit expression of the stability region of the power system related to the controlling unstable equilibrium point, and then gives out an estimation of the index with the quadratic approximation of stability region. Two applications of the proposed index, calculating the critical clearing time (CCT) incorporating the curve fitting approach and contingency ranking are discussed. Simulations in IEEE 3-machine 9-bus system and 10-machine 39-bus New England system show the effectiveness of the proposed index

A comprehensive method to compute the controlling unstable equilibrium point

The accuracy of the transient stability assessment provided by transient energy function method depends on the determination of the controlling unstable equilibrium point (CUEP). However, it is still an uncompleted problem to determine the CUEP fastly and accurately. This paper discusses the CUEP calculation methods, such as the BCU method, the dynamic PEBS method and the shadowing method, and then proposes an algorithm for CUEP determination in the light of the reflected gradient system. Furthermore, it proposes a comprehensive method, which could take advantage of the BCU method, the dynamic PEBS method, the shadowing method and the reflected gradient system method, and improve the robustness of the CUEP determination. Simulation results in the IEEE 3-generator 9-bus and New England system show the effectiveness of the proposed method.

Comparison of the linear approximations for the dynamic security region of network-reduction power systems

The dynamic security region (DSR) of bulk power system has been accepted more and more in recent years for providing plenty of security information and good prospect in online application. This paper compares three linear approximations for the dynamic security region of network-reduction power systems. The three linear approximations are the Q-linear approximation based on the quadratic approximation of stability region, the L-linear approximation based on the linear approximation of stability region and the L0-linear approximation based on the invariant assumption of the normal vector for the boundary of the stability region corresponding to different control variable. The three linear approximations are all obtained with a same critical point lying just on the boundary of dynamic security region. The critical point is searched with numerical simulation. The accuracy of the three linear approximations is compared, using the linear approximation obtained with the curve fitting approach or the actual boundary of DSR searched as the benchmark. Simulation results in IEEE 3-machine 9-bus system and 10-machine 39-bus New England system show that all the three linear approximations display fairly accurate estimation. Furthermore, from the computational viewpoint, the L-linear and the L0-linear method are two alternative choices to approximate the dynamic security region

Analysis of Transient Voltage Stability via Quadratic Approximation Method

Based on the quadratic approximation of the stability region boundary, this paper proposes a transient voltage stability margin index, and analyzes the transient voltage stability of a one-generator-one-load system including an induction motor with electro-mechanical transient model. Simulations show that the proposed index has the advantages of high accuracy, high engineering practicability and facility of computer-implementing. And it can be applied to obtain the margin of the transient voltage stability, estimate the critical clearing time in transient voltage emergency and rank the severity of the contingencies. Simulations also show that, for the system in this paper, heavier load and larger portion of constant mechanical torque in the load are more inclined to transient voltage instability.

Stability-Constrained Optimal Power Flow Based on a Novel Transient Stability Margin

A new approach to solve stability-constrained optimal power flow (SCOPF) based on a novel transient stability margin and its sensitivities is proposed. The transient stability constraints originally represented by the differential-algebraic equations are converted to the inequality constraints of the transient stability margin and its sensitivities with respect to the control variables in this approach. Furthermore the transient stability margin is based on the implicit expression for the stability region boundary related to the controlling unstable equilibrium point (CUEP), and this boundary can be approximated by the quadratic approximation of the stability region. The converted SCOPF problem can be easily solved by any standard nonlinear programming technique as the conventional OPF with the similar computation complexity. Simulation results in the New England 10-generator 39-bus system show the effectiveness of the proposed approach.

Power System Transient Stability Assessment Based on Quadratic Approximation of Stability Region

This paper presents an approach to estimate the critical clearing time (CCT) of the multi-machine power systems based on the quadratic surface which approximates the boundary of stability region relating to the controlling unstable equilibrium point. A decomposition method is developed to obtain the coefficients of the quadratic approximation surface. The CCT is determined by the crossing point of the quadratic surface and the continuous faulted trajectory. Simulations in IEEE 9-bus and New England system show the effectiveness of the proposed approach