Performance of synchronous motors loss-of-synchronism protection
Synchronous motors are rarely designed to operate asynchronously for extended periods of time. When a synchronous motor experiences a mechanical load exceeding its pull out torque, it enters asynchronous operation, under which the interaction between the stator and rotor magnetic fields causes damaging mechanical stress and vibrations. Also, induced currents in the rotor cause extra heating that quickly damages the rotor windings. Asynchronous operation must be detected and dealt with by allowing time for resynchronization or by tripping the motor to avoid thermal overload and damage. Two protection elements are often used to detect asynchronous motor operation: the power factor element and the out-of-step element. This paper compares the performance of the two elements to protect a 20 MW synchronous motor subjected to pull out conditions.
Dynamic simulations help improve generator protection
This paper describes a digital simulation study of a set of two 160 MW generating units operating in the Juan de Dios Batiz Paredes thermal power station, in Topolobampo, Sinaloa, Mexico. This plant belongs to Comision Federal de Electricidad, the national Mexican utility. We first discuss the factors that limit the active and reactive power delivered by a generating unit, such as thermal and voltage limits, power-system imposed limits, and the minimum excitation limiter. We then describe generator protection functions related to the capability curve. Later, we propose a P-Q plane-based scheme that provides generator loss-of-field protection and capability-curve violation alarming. Finally, we present the simulation results of loss-of-field and loss-of-synchronism conditions of one of the two generating units for several cases, including different initial load conditions, different loss-of-field modes, and different numbers of units on line.
The nature of A. C. machine torques
The generation of torque in A.C. machines is not well understood by engineers. National and international standards define only average steady state values and do not cover transient or oscillatory components. The only transient data routinely provided by the manufacturer is short circuit torques. This situation leaves the application engineer in a poor position to properly apply motors or generators. The user needs the complete torque picture. It is the purpose of this paper to present easily usable formulas and a physical picture of torque generation by A.C. machines and thereby fill this long standing need. This paper should eliminate the need for new standards as has been proposed since the data for torque calculation is normally available. Practical and interesting cases will be explored to develop a clear understanding of the nature of A.C. machine torques and their calculation. Torque is a basic physical quantity that can be treated independently and not as a dependent function of power and energy. Each element of a winding under transient as well as steady state conditions contributes a value of torque that is the triple vector product of the radius, the current, and the flux density T-= R-r×(I-×B-). This wholly empirical formula is firmly established as Langsdorf explains it is the basis for the work that follows.