A. Soom

Event timing and shape analysis of vibration bursts from power circuit breakers

Noninvasive vibration diagnostic techniques are implemented to assess the mechanical condition of power circuit breakers. A diagnostic system, the prototype commercial portable diagnostic system (PCPDS) has been developed Hardware of the PCPDS includes a portable computer, and a data acquisition unit and computer communication cards. Signal processing techniques include the discrete energy statistics envelope, short-time power spectrum, timing extraction algorithm and chi-square based shape test. Decision-making is carried out via a voter program, to which individual results from the timing and shape analysis programs are passed. Statistical and empirical thresholds have been established, that classify the circuit breaker as being in normal-transitional-abnormal (green-yellow-red) condition.

Mechanical failure detection of circuit breakers

The mechanical condition of a circuit breaker, i.e., whether the unit is in normal or abnormal condition, has been evaluated using noninvasive diagnostics, together with signal processing and decision-making techniques. Test procedures involved obtaining vibration signals from transducers placed along the external surfaces of (and, under certain conditions, within the interior of) a circuit breaker. Using the method, quantitative assessment of a circuit breaker can be carried out for any operating condition. Tests of oil and sulfur hexafluoride circuit breakers, under both normal and abnormal conditions, were carried out. In all cases, the abnormalities was detected. >

A noninvasive diagnostic instrument for power circuit breakers

Noninvasive diagnostics, involving short-time spectra, short-time energy, and automatic timing techniques, have been applied to assess the mechanical condition of oil and puffer circuit breakers and to determine the timing of events occurring during operation. A portable instrument that employs these techniques has been developed and tested at various utilities. The capabilities of this instrument and the results of the field trials are presented. >

Noninvasive condition assessment and event timing for power circuit breakers

Noninvasive diagnostics, involving short-time spectra, short-time energy, and automatic timing techniques, were used to assess the mechanical condition of oil and puffer circuit breakers and to determine the timing of events occurring during operation. Analysis of the vibration signatures leads to (1) the characterization of a circuit breaker through a single index, the resolution ratio, and (2) automatic timing algorithms. The accuracy of the automatic timing algorithms permits both the timing of events and the detection of shifts in the timing associated with changes in the condition of a unit. Application of these methods to several oil and SF/sub 6/ puffer circuit breakers is described. Results are reported for circuit breaker operation under unenergized and energized-loaded conditions. >

Unsteady Friction in the Presence of Vibrations

In this paper, results from the authors’ recent work on the vibrations of sliding contacts in the presence of dry friction are summarized. We examine some idealized models of smooth and rough contacts, in which the assumed sliding conditions, the kinematic constraints and the mechanism of friction, i.e., the adhesion theory of friction, in this case, are well-defined. Instantaneous and average normal and frictional forces are computed. The results are compared with experiments. It appears that when contacts are in continuous sliding, quasi-static friction models can be used to describe friction behavior, even during large, high-frequency fluctuations in the normal load. However, the dynamics of typical sliding contacts, with their inherently nonlinear stiffness characteristics, can be quite rich and complex, even when the sliding system is very simple. A three degree-of-freedom vibratory model of a rough block in sliding contact with a planar moving countersurface, from which some preliminary results have been obtained, is used to illustrate some of these complexities.

Measurements for noninvasive mechanical diagnostics of power circuit breakers

Abstract The authors have developed techniques for assessing the mechanical condition of deenergized power circuit breakers based on noninvasive acceleration measurements during trip and close operations. An important element of the research has been the selection and implementation of measurement techniques that are capable of providing vibration data that are repeatable and sufficiently rich in information content to be useful for diagnostic purposes. The most critical aspects of the measurement methods, transducer location and attachment, as well as circuit breaker vibrational response and signature repeatability are addressed in this paper.

High frequency noise generation from components in sliding contact: flutter instabilities including the role of surface roughness and friction

Abstract One type of troublesome friction-induced noise, common in brakes, clutches and mechanical seals, is high frequency chirp or squeal. Chirp frequencies typically range from around 1 kHz to more than 10 kHz. We have found that the essential physical ingredients needed to model this problem are two finite distributed mass elastic systems coupled by friction and an interfacial contact stiffness, transverse to the direction of sliding. The contact stiffness is associated with the roughness of the sliding surfaces and, sometimes, with the presence of wear particles within the contact. Lumped parameter models are not adequate to capture the dynamics. Our approach is to perform an eigenvalue analysis, using finite elements, of pairs of coupled sliding elastic rings. Due to the presence of friction, the stiffness matrix is asymmetric and mode coupling or mode splitting can occur. Typically, around ten per cent of the first forty or so vibratory modes appear prone to instability. Generally one or two of these show up as instabilities in the actual physical system. No stick-slip action needs to be invoked and these instabilities can occur with a single constant coefficient of friction.

Thermomechanics of Sliding Contact

When relatively hard surfaces that make asperity contact slide against one another at speeds on the order of a m/s or more very high local temperatures can be generated. If the sliding conditions are of sufficient severity and duration, thermal distortions at local or component levels can occur. Overall thermal deformations are primarily determined by thermal gradients. We have found that during the early parts of a sliding interval, all heat input is confined to small volumes at individual asperities which form “hot mounds” surrounded by much larger cool regions where there is little or no temperature change. Thermal distortions are essentially non-existent and overall component level interactions, including thermal softening, are not much different from isothermal. Eventually, more of the surface and near surface regions are heated and the possibility of component level thermal deformation increases.

In search of dynamic effects in dry sliding friction

Abstract There are many instances in mechanical systems when sliding contacts are subject to unsteady loads. Of interest for system modeling is the behavior of friction processes under such conditions. If quasi-static models can be applied one can focus on other, possibly more important aspects of the problem. In this paper, we suggest that a distinction between a constant (scalar) and non-constant friction coefficient be made. When unsteady friction and normal force measurements are made with care, we find that the average (or DC) friction coefficient can still be found to apply during contact vibrations at thousands of Hz. One only needs to invoke simple concepts (e.g., the Greenwood-Williamson theory of rough surface contact) to explain the relation between instantaneous normal and friction forces. When additional materials, e.g., wear particles or a solid lubricant are introduced at the interface, part of the dynamic normal contact load will be carried via damping mechanisms which do not, in general, contribute to tangential friction. It is unclear whether this effect should be considered a dynamic friction behavior or just a reflection of system and contact dynamics. The results of a series of concurrent average and time-varying friction measurements are used to illustrate these ideas.

Machine diagnostics using the concepts of resolution ratio and automatic timing

Noninvasive diagnostics have been developed and applied to determine the condition of circuit breakers. Vibration data are obtained from transducers placed at appropriate locations on a unit. Analysis of the vibration signatures leads to the characterization of a circuit breaker via a single index, the resolution ratio (RR). The analysis of the vibration signatures also includes the automatic timing (AT) of events occurring during operations of a unit. Both RR and AT are being integrated into a multifactor decision making process that will evaluate diagnostic inputs from several sources.