Carlos J. Tavora

Equilibrium Analysis of Power Systems

The flow function of a power system is the vector of node powers expressed in terms of the node angles. The Jacobian is the matrix of partial derivatives of the flow vector with respect to the angle vector. The ratio of the determinant of the Jacobian to the value which it has when the node angles are set to zero is a dimensionless stability margin which lies between one and zero (steady-state in- stability) and is easily computed from a load flow. The margin of torque is the least change in power flows that will cause instability. Maximizing the torque margin or maximizing the stability margin with a load constraint yields the optimum dispatching change to maximize security.

Characterization of Equilibrium and Stability in Power Systems

The evaluation of the stability of a power system during a transient requires that the dynamics of the subsystems be decomposed into relative and collective motions. This decomposition must establish a distinction between synchronous and frequency equilibria. Such a decomposition is made possible by specifying a system center of angle which is defined as the inertia weighted average of all rotor angles. The angular velocity of the center of angle accurately describes the frequency of the system. An angular coordinate of each rotating element in the system can be specified relative to the center of angle. In terms of these coordinates a simple expression is obtained for the exact transient kinetic energy of the system. A transformation relating center of angle referenced variables to the usual one machine reference allows the simultaneous use of both references each where best suited.

Stability Analysis of Power Systems

A procedure for locating directly the lowest saddle point of the potential energy function associated with a power transmission network is presented. This procedure is used to compute a margin of stability that specifies the maximum asynchronous transient energy that can be retained by the system while in synchronism. The potential function is shown to be convex in the principal region of the angle space. A unique solution is shown to exist for the load flow problem if the algorithm of solution is initiated at the origin.

The Remote Link Unit---An Advanced Remote Terminal Concept

Remote terminal units are extensively used in digital automation systems to interface a central processing unit (CPU) to remotely located sensors and actuators. This paper presents a conceptual description of the remote link unit (RLU) which overcomes inherent limitations of remote terminal units. The RLU utilizes a single type of interface card to support most process I/O interfaces. The RLU identifies sensors, actuators, and subsystems through the use of electronic nameplates. Each nameplate provides information regarding device function, location, interface parameters, and calibration status. In addition it stores programs for device handling, engineering unit conversion, and device diagnosis. The RLU allows sensors and actuators to be relocated or substituted without requiring changes in the CPU software to correct device addresses or conversion constants. The electronic nameplate may also be used for inventory control, automatic calibration, and maintenance of devices.

A Design for the Remote Link Unit---A Multiprocessor Remote Terminal

The Remote Link Unit (RLU) is an advanced remote terminal which provides a complete interface between a CPU and remote subsystems. The RLU consists of several intelligent Link Modules (LM) which provide the direct interfaces to the subsystems, and an intelligent Link Manager (LMG) which maintains control over the LMs and supervises communications with the CPU. By implementing the RLU with a hierarchy of specialized processors, it is possible to achieve with the RLU highly efficient, reliable, and flexible operation.

A Universal Process Control Interface

The Interface Configuration Adapter (ICA) implements most process I/O interfaces through four independently configured ports. Each port is configured under software control to interface ac, dc, or digital signals. An ICA based interface board can replace a multiplicity of signal-oriented interface boards. The ICA is inherently testable.