Vaithianathan Venkatasubramanian

WACS-Wide-Area Stability and Voltage Control System: R&D and Online Demonstration

As background, we describe frequently used feedforward wide-area discontinuous power system stability controls. Then we describe online demonstration of a new response-based (feedback) Wide-Area stability and voltage Control System (WACS). The control system uses powerful discontinuous actions for power system stabilization. The control system comprises phasor measurements at many substations, fiber-optic communications, real-time deterministic computers, and transfer trip output signals to circuit breakers at many other substations and power plants. Finally, we describe future development of WACS. WACS is developed as a flexible platform to prevent blackouts and facilitate electrical commerce.

Designing the Next Generation of Real-Time Control, Communication, and Computations for Large Power Systems

The power grid is not only a network interconnecting generators and loads through a transmission and distribution system, but is overlaid with a communication and control system that enables economic and secure operation. This multilayered infrastructure has evolved over many decades utilizing new technologies as they have appeared. This evolution has been slow and incremental, as the operation of the power system consisting of vertically integrated utilities has, until recently, changed very little. The monitoring of the grid is still done by a hierarchical design with polling for data at scanning rates in seconds that reflects the conceptual design of the 1960s. This design was adequate for vertically integrated utilities with limited feedback and wide-area controls; however, the thesis of this paper is that the changing environment, in both policy and technology, requires a new look at the operation of the power grid and a complete redesign of the control, communication and computation infrastructure. We provide several example novel control and communication regimes for such a new infrastructure.

Local bifurcations and feasibility regions in differential-algebraic systems

The dynamics of a large class of physical systems such as the general power system can be represented by parameter-dependent differential-algebraic models of the form x/spl dot/=f and 0=g. Typically, such constrained models have singularities. This paper analyzes the generic local bifurcations including those which are directly related to the singularity. The notion of a feasibility region is introduced and analyzed. It consists of all equilibrium states that can be reached quasistatically from the current operating point without loss of local stability. It is shown that generically loss of stability at the feasibility boundary is caused by one of three different local bifurcations, namely the saddle-node and Hopf bifurcations and a new bifurcation called the singularity induced bifurcation which is analyzed precisely here for the first time. The latter results when an equilibrium point is at the singular surface. Under certain transversality conditions, the change in the eigenstructure of the system Jacobian at the equilibrium is established and the local dynamical structure of the trajectories near this bifurcation point is analyzed.

Dynamics of large constrained nonlinear systems-a taxonomy theory [power system stability]

This paper provides an overview of the taxonomy theory which has been proposed as a fundamental platform for solving practical stability related problems in large constrained nonlinear systems such as the electric power system. The theory reveals a two-level intertwined cellular nature of the constrained system dynamics which serves as a unifying structure, a taxonomy, for analyzing nonlinear phenomena in large system models. These broadly divide into the state space aspects (related to dynamic stability issues among others) and the parameter space aspects (connected with bifurcation phenomena among others). In the state-space formulation, the boundary of the region of attraction for the operating point is shown (under certain Morse-Smale like assumptions) to be composed of stable manifolds of certain anchors and portions of the singularity surface. Such boundary characterization provides the foundation for rigorous Lyapunov theoretic transient stability methods. In the parameter space analysis, the feasibility region which is bounded by the feasibility boundary provides a safe opening region for guaranteeing local stability at the equilibrium under slow parametric variations. The feasibility boundary where the operating point undergoes loss of local stability is characterized in the form of three principal bifurcations including a new bifurcation called the singularity induced bifurcation.

Voltage dynamics: study of a generator with voltage control, transmission, and matched MW load

A comprehensive analysis of the dynamic behavior for a rudimentary but representative model of the power system is carried out for the case when control gain and load are varied as parameters. The voltage dynamics model is subject to algebraic constraints in the form of load flow equations and is studied as a differential-algebraic system in state and parameter spaces. Singularities in the state-space (noncausal points) and bifurcations in the parameter space are the principal and interacting structural elements. A rich structure of bifurcations emerges which is analyzed. The mathematical analysis is facilitated by singularly rescaling time, which transforms the differential algebraic system into a smooth dynamic system. In the state space, the characteristics of stability boundaries are observed and a description of the regions of attraction of all equilibria are given. The loosely understood term of voltage collapse is classified into well-defined types on both the dynamic and parameteric sides. >

Oscillation monitoring system based on wide area synchrophasors in power systems

With the growing implementation of synchrophasors or PMUs across the power grid, it is now possible to observe and analyze system wide dynamic phenomena in real-time. This paper presents the framework of an automatic real-time oscillation monitoring expert system for extracting the modal information and mode shape of electromagnetic oscillations while they are still emerging in the real power system. The algorithms use three signal processing engines, namely, Pronys method, matrix pencil method and Hankel total least squares (HTLS) method. Results from these engines are processed using a custom developed set of rules for handling the complexities of modal analysis from real-time PMU measurements. The results are illustrated on several actual PMU recordings from power systems in North America. A simple powerful control design is also explored to initiate a supplementary HVDC modulation scheme to damp out inter-area oscillations when the oscillation monitoring system detects poorly damped oscillations.

Oscillation monitoring from ambient PMU measurements by Frequency Domain Decomposition

This paper extends the method of frequency domain decomposition (FDD) towards real-time analysis of ambient PMU measurements in power systems for the purpose of oscillation monitoring. The main idea of FDD is to apply singular value decomposition (SVD) to the power density spectrum matrix. The resulting singular values correspond to individual modes under specific conditions. The damping ratio, modal frequency and the mode shape of poorly damped (with less than 5% damping ratios) oscillatory modes can be directly determined from ambient PMU measurements. The theoretical background is presented in the paper. The technique is tested on small linear systems to show its effectiveness and weaknesses. Finally, the application of FDD in power system is shown.

Fast time-varying phasor analysis in the balanced three-phase large electric power system

Traditional phasor representation of sinusoidal signals, the standard analytical tool for power system stability analysis, is limited by the quasistationary assumption on the speeds of the phasor states. This paper provides a rigorous formulation of a time-varying phasor representation for the balanced three-phase large power system with no restrictions on the speeds. Power balance equations become a set of differential equations in the phasor dynamic states and singularly perturbed behavior of the resulting dynamics is explored. >

A fast voltage security assessment method using adaptive bounding

This paper presents a fast method for use in power system online voltage security assessment. The objective here is to quickly assess the effects of large numbers of contingencies (line outages) to determine the worst case. The methodology is intuitively focused on the relationship between a node voltage and the availability of reactive power in the local neighborhood, and it also takes advantage of other information from the power flow solution. The concept of electrical distance is introduced for choosing the corresponding voltage control area locally around the contingency event. For the calculation of post-contingency electrical distance and power flow, the computation efficiency can be achieved by utilizing many mature algorithms from static security analysis (SSA).

Hard-limit induced chaos in a fundamental power system model

Abstract The paper investigates complex nonlinear phenomena in a fundamental power system model represented in a single-machine infinite-bus formulation. The generator electro-magnetics, electromechanics and its excitation control are modelled together by fourth-order differential equations. It is shown that when excitation control gains are set high (as in common industry. practice) and when the excitation hard-limits are taken into account, this representative power system model undergoes global bifurcations including period-doubling cascades which lead to sustained chaotic behaviour. Specifically sustained complex oscillations result from the interaction of hard-limits and the system transients over a large range of realistic parameter values. The emergence of strange attractors is demonstrated in the paper by detailed numerical simulations and preliminary analysis.