Integrated treatment of adequacy and security in bulk power systemreliability evaluations

Traditionally, bulk power system reliability evaluations have concentrated on the analysis of system adequacy, the ability to supply all loads within performance requirements. The authors focus on extending the scope of reliability evaluations to include a probabilistic assessment of system security, involving considerations of transient stability and the effects of cascading sequences. For each disturbance, first-impact states are defined which depend on the primary effect of the disturbance (static or dynamic problems, or no problem at all). Based on this analysis, a set of probabilistic indices is proposed to assess the security level of the predisturbance state. The possibility and consequences of secondary effects such as cascading are considered, leading to another set of indices to measure load losses due to static and dynamic phenomena. The approach is illustrated in a numerical example     

Aging, maintenance, and reliability - approaches to preserving equipment health and extending equipment life

A survey is offered for the various maintenance methods available for operators to preserve equipment health and extend equipment life. The methods range from the simplest, "follow-the-manual" types to detailed probabilistic approaches. To get the most out of maintenance, one has to select a mathematical model where optimization is possible. Another choice, of which operators are becoming more and more aware, is to apply a maintenance policy. This article concentrates on maintenance policies and their applications. Maintenance policy is based not on a rigid schedule but on the "as needed" principle. This can be implemented with or without mathematical models.     

Amobarbital--a probe of hepatic drug oxidation in man

Some aspects of the fate of amobarbital were investigated since this drug is being used as a probe to gauge drug oxidation in man. The mean ratio of orally available over intravenously injected amobarbital was established as 0.99 +/- 0.11 (SD), by comparing integrated concentration-time curves, indicating complete absorption and absence of a first-pass effect. One subject ingested 200 mg of amobarbital sodium, and amobarbital concentrations in serum were monitored for 5 days thereafter. Elimination of amobarbital under these conditions followed first-order kinetics. One subject ingested amobarbital 7 times over a period of 3 yr; plasma clearances (32.1 +/- 1.8 [SD]ml/min) exhibited remarkable constancy, while biologic half-lives (26.5 +/- 3.1 hr) and distribution volumes (73.6 +/- 8.0 L) showed some fluctuation. The distribution of parameters of amobarbital elimination was investigated in 36 unrelated subjects. Amobarbital half-lives (23.8 +/- 6.7 hr) appeared to be normally distributed, while the clearances (36.7 +/- 10.0 ml/min) might not follow a normal distribution.     

The adenosine triphosphate (ATP) depletion test: comparison with the caffeine contracture test as a method of diagnosing malignant hyperthermia susceptibility

The adenosine triphosphate (ATP depletion ratio, which is the ratio [ATP] in skeletal muscle equilibrated with carbogen and 4% halothane for 30 minutes divided by [ATP] in skeletal muscle equilibrated with carbogen alone for 30 minutes is less than normal in most but not in all rigid MHS patients. The ratio is normal in non-rigid MHS patients. This diagnostic tool is, therefore, useful in the diagnosis of rigid MH. It is not, however, such a sensitive diagnostic parameter as the caffeine contracture test.     

A failure-repair model with minimal and major maintenance

A Markov model for a continuously operating service device whose condition deteriorates with time in service is proposed. The model incorporates deterioration and Poisson failures, minimal repair, periodic minimal maintenance, and major maintenance after a given number of minimal maintenances. An exact recursive algorithm computes the steady-state probabilities of the device. A cost function is defined using different cost rates for the different types of outages. Based on minimal unavailability or minimal costs, optimal solutions of the model are derived. Major maintenance is seldom beneficial if optimal maintenance intervals are used. If a maintenance policy is based on nonoptical intervals between maintenances, periodic major maintenance can reduce costs     

Algorithms for Solving Reliability Models of Systems Exposed to a 2-State Environment

An algorithm is described for solving the reliability model of a repairable system of n identical components placed in a fluctuating 2-state environment. An important application is where the effects of alternating normal and severe weather conditions are to be evaluated. A modified version of the algorithm can accommodate the case where faults are followed by switching operations, as in power transmission networks. Whereas the standard Markov method to solve such models is cumbersome and other available methods are often limited in scope, the proposed algorithm is fast, reasonably accurate, and does not require a computer.     

A probabilistic model for evaluating the remaining life ofevaluating the remaining life of electrical insulation in rotatingmachines

A Markov model is introduced for estimating the remaining life of electrical insulation in large motors and generators. The model incorporates various levels of insulation deterioration and periodic (minor) maintenance. The model permits computation of the average time before insulation failure occurs. A mathematical formulation of the expected transition time from any deterioration state to the failure state (expected remaining life) is given. Data requirements are discussed, and numerical examples are presented. This model would also allow assessment through sensitivity studies of the relative effects of various other parameters in the process which are controllable, e.g. maintenance policy and stress level. Even though it is theoretically possible to apply the Markov models to any particular insulation deterioration mechanism, this approach is best applied to an overall insulation determination problem which combines the effects of all individual processes. The models are fairly flexible. They allow, for example, representation of changing stresses acting on the machine insulation as the utility operating environment changes     

Asymmetry of the mean-variability tradeoff raises questions about the model in investigations of individual bioequivalence

Tradeoff between changes of intraindividual variations of 2 drug formulations and of the difference between their means is a characteristic of a procedure suggested for the determination of individual bioequivalence [Schall and Luus 1993] and to be proposed by the Food and Drug Administration for adoption. Hauck et al. [1996] investigated properties of the tradeoff. Their procedure was applied and extended in the present study. The tradeoff was shown to be asymmetric. Notably, a small change in intrasubject variations can elicit, under various conditions, a comparatively large change in the allowable difference between means which can still be compatible with the declaration of bioequivalence. For instance, when the intraindividual coefficients of variations are 40% and 38% for the reference and test formulations, respectively, the allowable difference between means may increase, as a benefit, by 12.3%. A penalty by 11.2% is elicited if the intrasubject variations of the reference and test products are 40 and 42%, respectively. In addition, 4-period crossover trials were simulated. Ratios of estimated variances of the 2 formulations followed an F-distribution. Distributions of changes in allowable deviations between means were calculated from the tradeoff relationships; generally substantial changes were noted with high probabilities. For example, with an intraindividual variation of 30% there is an estimated 37% probability that a benefit of 10% increase, or larger, is gained by chance in the allowable difference between means, and an additional 36% probability that a penalty of a 10%, or larger, decrease in the allowable difference is suffered. With an intrasubject variation of 40%, the estimated probabilities are 42% and an additional 42% for a 10% expansion and contraction, respectively, of the allowable difference between means. Consequently, the strong asymmetry of the tradeoff could result in very large probabilities for benefits and penalties. Therefore, the investigated model assessing individual bioequivalence does not appear to be suitable for implementation.     

The present status of maintenance strategies and the impact ofmaintenance on reliability

In this paper, the most frequently used power system maintenance strategies are reviewed. Distinction is made between strategies where maintenance consists of replacement by a new (or “good as new”) component and where it is represented by a less costly activity resulting in a limited improvement of the component's condition. Methods are also divided into categories where maintenance is performed at fixed intervals and where it is carried out as needed. A further distinction is made between heuristic methods and those based on mathematical models; the models themselves can be deterministic or probabilistic. From a review of present maintenance policies in electric utilities, it is concluded that maintenance at fixed intervals is the most frequently used approach, often augmented by additional corrections. Newer “as needed”-type methods, such as reliability-centered maintenance (RCM), are increasingly considered for application in North America, but methods based on mathematical models are hardly ever used or even considered. Yet only mathematical approaches where component deterioration and condition improvement by maintenance are quantitatively linked can determine the effect of maintenance on reliability. Although more complex, probabilistic models have advantages over deterministic ones: they are capable of describing actual processes more realistically, and also facilitate optimization for maximal reliability or minimal costs