Venkataramana Ajjarapu

Definition and classification of power system stability IEEE/CIGRE joint task force on stability terms and definitions

The problem of defining and classifying power system stability has been addressed by several previous CIGRE and IEEE Task Force reports. These earlier efforts, however, do not completely reflect current industry needs, experiences and understanding. In particular, the definitions are not precise and the classifications do not encompass all practical instability scenarios. This report developed by a Task Force, set up jointly by the CIGRE Study Committee 38 and the IEEE Power System Dynamic Performance Committee, addresses the issue of stability definition and classification in power systems from a fundamental viewpoint and closely examines the practical ramifications. The report aims to define power system stability more precisely, provide a systematic basis for its classification, and discuss linkages to related issues such as power system reliability and security.

The continuation power flow: a tool for steady state voltage stability analysis

The authors present a method of finding a continuum of power flow solutions starting at some base load and leading to the steady-state voltage stability limit (critical point) of the system. A salient feature of the so-called continuation power flow is that it remains well-conditioned at and around the critical point. As a consequence, divergence due to ill-conditioning is not encountered at the critical point, even when single-precision computation is used. Intermediate results of the process are used to develop a voltage stability index and identify areas of the system most prone to voltage collapse. Examples are given where the voltage stability of a system is analyzed using several different scenarios of load increase. >

Bifurcation theory and its application to nonlinear dynamical phenomena in an electrical power system

A tutorial introduction in bifurcation theory is given, and the applicability of this theory to study nonlinear dynamical phenomena in a power system network is explored. The predicted behavior is verified through time simulation. Systematic application of the theory revealed the existence of stable and unstable periodic solutions as well as voltage collapse. A particular response depends on the value of the parameter under consideration. It is shown that voltage collapse is a subset of the overall bifurcation phenomena that a system may experience under the influence of system parameters. A low-dimensional center manifold reduction is applied to capture the relevant dynamics involved in the voltage collapse process. The need for the consideration of nonlinearity, especially when the system is highly stressed, is emphasized. >

Bibliography on voltage stability

This paper provides a comprehensive list of books, reports, workshops and technical papers related to voltage stability and security.

An optimal reactive power planning strategy against voltage collapse

This paper introduces a method of determining the minimum amount of shunt reactive power (VAr) support which indirectly maximizes the real power transfer before voltage collapse is encountered. Using a relaxation strategy that operates with a predictor-corrector/optimization scheme, a voltage stability index that serves as an indirect measure to the closeness of reaching the steady state voltage stability limits is obtained. Sensitivity information that identifies weak buses is also available for locating effective VAr injection sites. >

Extended Reactive Capability of DFIG Wind Parks for Enhanced System Performance

This paper discusses the impact of utilizing the capability curve of a doubly fed induction generator (DFIG) based wind park on steady state and dynamic power system operation. The interconnection requirements set forth by FERC in order 661-A mandate the operation of wind parks within a power factor range of 0.95 leading and lagging. This operation drastically underutilized the reactive output of the machine. The results presented demonstrate that committing the full reactive capability of a DFIG park for generation dispatch produce a significant reduction in system losses. This additional reactive support was also found to improve post-fault voltage profiles by damping oscillations and preventing overshoots immediately after being subjected to a disturbance. This utilization of extended reactive limits in voltage control may prevent system collapse.

A practical minimum load shedding strategy to mitigate voltage collapse

This paper presents a new approach for determining the minimum load shedding to restore the solvability of a power system described by differential-algebraic equations. Through parameterization of a given control strategy (i.e. control direction in parameter space), the continuation method is applied to find the equilibrium point associated with the system post-contingency boundary. Then, the invariant subspace parametric sensitivity (ISPS) is used to determine the most effective control strategy so that a practical minimum load shedding can be derived. The system adjustments that could further reduce the minimum load shedding, such as rescheduling of real power generations and changing of generator terminal voltages are also investigated The approach is illustrated with the New-England 39-bus system and the reduced Iowa 162-bus system.

A Novel Parameter Identification Approach via Hybrid Learning for Aggregate Load Modeling

Parameter identification is the key technology in measurement-based load modeling. A hybrid learning algorithm is proposed to identify parameters for the aggregate load model (ZIP augmented with induction motor). The hybrid learning algorithm combines the genetic algorithm (GA) and the nonlinear Levenberg-Marquardt (L-M) algorithm. It takes advantages of the global search ability of GA and the local search ability of L-M algorithm, which is a more powerful search technique. The proposed algorithm is tested for load parameter identifications using both simulation data and field measurement data. Numerical results illustrate that the hybrid learning algorithm can improve the accuracy and reduce the computation time for load model parameter identifications.

Identification of voltage collapse through direct equilibrium tracing

This paper describes an approach for identifying power system voltage collapse. Unlike the conventional two-step procedure, it simultaneously solves the system differential and algebraic equations (at steady state) to obtain the equilibrium points. Combined with a parameterized continuation technique, the methodology identifies voltage collapse during the direct equilibrium tracing, without rebuilding system dynamic Jacobian and checking its singularity. This significantly reduces the computational cost. All the assumptions of slack and PV buses are removed. The generator field and armature current limits are also accurately implemented. Numerical test results with the New England 39-bus system are presented. Three more systems with up to 8267 buses and 1112 machines are employed to demonstrate the capability of the approach.

A Speed-Adaptive Reduced-Order Observer for Sensorless Vector Control of Doubly Fed Induction Generator-Based Variable-Speed Wind Turbines

In this paper, a speed-adaptive reduced-order observer for sensorless vector control of doubly fed induction generators (DFIGs) is proposed. The observer is a simulation of the rotor current dynamic model with feedback of the estimation error and a speed-adaptation loop. Feedback and adaptation gains are designed based on the closed-loop observer model. A parameter sensitivity analysis reveals that this observer is robust against machine parameter variations in the normal operating regions. Simulation results demonstrate desired steady-state and dynamic performance of this sensorless control approach for DFIG-based variable-speed wind turbines.