Mean Uptime Research Articles

OPTIMUM UNIT COMMITMENT SOLUTION OF A POWER SYSTEM BASED ON SALP SWARM ALGORITHM

This paper presents an algorithm to solve the unit commitment problem in a power system. The proposed algorithm employs the Salp swarm algorithm technique to search the optimum unit schedule for a particular daily demand pattern and specific time horizon. Different constraints are taken into consideration, transition cost (start-up and shut down )cost, mean-up time, mean-down time, spinning reserve, and power balance. The proposed algorithm is applied to 10-units and 26-unit. The obtained results are compared with other methods. It reveals the robustness of the proposed algorithm in terms of minimizing overall running costs.

Optimization and Reliability Evaluation of Hybrid Solar-Wind Energy Systems for Remote Areas

T his paper presents optimization and reliability analysis of renewable energy system for farm house electricity and irrigation load demand located at remote area. Optimum sizing of solar PV, wind turbine and battery energy system has been obtained to fully utilize the capacity of the system. The economic analysis have been carried out for different configuration of hybrid energy system in terms of net present cost (NPC), operating cost (OC), levelized cost of energy (LCOE). This paper proposes a recent evolutionary technique based on meta-heuristic optimization algorithm called grasshopper optimization algorithm (GOA), to be used for selecting optimal energy system configuration. Markov method has been employed for evaluating the reliability indices i.e. system failure rate, repair rate, availability, unavailability, mean up time (MUT), mean down time (MDT), loss of load probability (LOLP) and expected energy not supplied (EENS). Economic analysis, GOA and Markov method-based result shows that hybrid PV/wind/battery energy system is more reliable and cost effective system as compared to other configuration.Â

Real-Time Intraoperative Determination and Reporting of Cerebral Autoregulation State Using Near-Infrared Spectroscopy.

BACKGROUND:Cerebral blood flow (CBF) is maintained over a range of blood pressures through cerebral autoregulation (CA). Blood pressure outside the range of CA, or impaired autoregulation, is associated with adverse patient outcomes. Regional oxygen saturation (rSo2) derived from near-infrared spectroscopy (NIRS) can be used as a surrogate CBF for determining CA, but existing methods require a long period of time to calculate CA metrics. We have developed a novel method to determine CA using cotrending of mean arterial pressure (MAP) with rSo2that aims to provide an indication of CA state within 1 minute. We sought to determine the performance of the cotrending method by comparing its CA metrics to data derived from transcranial Doppler (TCD) methods.METHODS:Retrospective data collected from 69 patients undergoing cardiac surgery with cardiopulmonary bypass were used to develop a reference lower limit of CA. TCD-MAP data were plotted to determine the reference lower limit of CA. The investigated method to evaluate CA state is based on the assessment of the instantaneous cotrending relationship between MAP and rSo2 signals. The lower limit of autoregulation (LLA) from the cotrending method was compared to the manual reference derived from TCD. Reliability of the cotrending method was assessed as uptime (defined as the percentage of time that the state of autoregulation could be measured) and time to first post.RESULTS:The proposed method demonstrated minimal mean bias (0.22 mmHg) when compared to the TCD reference. The corresponding limits of agreement were found to be 10.79 mmHg (95% confidence interval [CI], 10.09–11.49) and −10.35 mmHg (95% CI, −9.65 to −11.05). Mean uptime was 99.40% (95% CI, 99.34–99.46) and the mean time to first post was 63 seconds (95% CI, 58–71).CONCLUSIONS:The reported cotrending method rapidly provides metrics associated with CA state for patients undergoing cardiac surgery. A major strength of the proposed method is its near real-time feedback on patient CA state, thus allowing for prompt corrective action to be taken by the clinician.

The Reliability Analysis of the Parallel Mechanical Leg (2UPS+UP) Based on a Repairable Markov Model

The reliability of the parallel mechanical leg (2ups+up) based on the repairable Markov model was studied. First the repairable Markov model was built up, and the matrix of the state transition rate was got. Then by solving the steady-state equation the steady state indexes such as the availability, the failure frequency, mean up-time, mean down-time, mean cycle time were obtained. Finally we gave the reliability and mean time to first failure by solving the differential equation group.

A Study on Joint Availability for k out of n and Consecutive k out of n Points and Intervals

The performance of maintenance systems can be described by many indexes such as availability, mean up-time and mean down-time and so forth. The availability is the most important measure among them. Availability has many types, including instantaneous, steady-state and average availability, etc. In this paper, the joint availabilities for k out of n intervals and consecutive k out of n intervals under the Markov assumption are presented in recursive equations. When n = k , the k out of n and consecutive k out of n systems reduce to a series system, and in this situation, the corresponding results have been given by Csenki [9] and Cui et al. [17] using different methods. In contrast, the major contributions of the paper are the extensions of the known results for series systems; that is, the situations when k < n have not been covered in the previous studies and will be presented in the present paper. Some numerical examples and discussions are given to illustrate the results obtained in the paper. Finally, the conclusions are summarized

Activity levels after pulmonary rehabilitation – what really happens?

ObjectivesTo assess the changes in physical activity in subjects with chronic obstructive pulmonary disease over 6months after pulmonary rehabilitation. DesignProspective, observational study. Activity was measured over 2-day periods at the end of rehabilitation, and repeated every 6weeks for 6months using the ActivPAL uni-axial accelerometer. These results were compared with the shuttle walking test (SWT) and the St. George's Respiratory Disease Questionnaire (SGRDQ). SettingUK community hospital. ParticipantsAdults completing a community rehabilitation programme. Main outcome measureTime spent standing and mobilising (‘uptime’). ResultsOf 34 subjects recruited, 28 completed the 6-month study period (mean age 69years, mean forced expiratory volume in 1second 1.3l). Participants wore the monitor for 13.8 to 14.2hours/day. At baseline (post-rehabilitation), participants spent 1.7 [standard deviation (SD) 1.3]hours/day walking and 3.5 (SD 2.6)hours/day standing. Taking the group as a whole, mean uptime decreased marginally by 13.6minutes after 24weeks compared with baseline, with significant individual variability. In all but one subject who showed decreased activity, this was apparent after 6weeks. There were no significant changes in the mean SWT or SGRDQ. Significant associations between total uptime and the SWT were found, but coefficients were weak. It was not possible to predict individual responses from baseline data. ConclusionThe accelerometer provides useful supplementary data in patients completing rehabilitation programmes, and the results reveal wide variation. The weak associations between activity data and the SWT suggest that monitors provide additional information. More work is required to determine the factors associated with early deterioration in activity in order to design appropriate interventions.

Evaluation of reliability indices accounting omission of random repair time for distribution systems using Monte Carlo simulation

This paper describes a methodology for reliability assessment of electrical distribution system accounting random repair time omission for each section. A distributor segment in a distribution system has failure rate and repair time which decide mean up time (MUT) and mean down time (MDT) at load points. Usually the reliability analysis is based on exponential failure and repair laws. The repair time is a random variable. Certain values of repair times are smaller which may be neglected based on the tolerance time of consumers at each load point. Complete reliability analysis with repair time omission has been presented. Modified mean up time, mean down time and unavailability (h/year) have been obtained at each load point using state transition sampling technique. These have been obtained accounting outage due to random repair time omission. The methodology has been implemented on two distribution systems.

Probabilistic analysis of a series–parallel repairable system with three units and vacation

This paper studies the steady-state availability and the mean up-time of a series–parallel repairable system consisting of one master control unit, two slave units and a single repairman who operates single vacation. Under the assumption that each unit has a constant failure rate and arbitrary repair time distribution, by using the supplementary variable method and the vector Markov process theory, we obtain the explicit expressions for the steady-state probabilities of the system, the steady-state availability and the mean up-time. A special case without vacation is given. Numerical results are provided to investigate the effects of various system parameters on the steady-state availability and the mean up-time.

MUT of a one out of two system with preventive maintenance

We are interested in the MUT (Mean Up Time) of a one out of two system in cold standby with preventive maintenance: a preventive maintenance occurs when the working unit reaches a given age. We study in details the stationary distribution of the Markov chain describing the state of the system at the beginning of its working periods. We give exact analytical formulas from which we derive a way to compute the MUT and we compare the results with those of Smith and Decker [M.A.J. Smith, R. Decker, Preventive maintenance in a 1 out of n system: The uptime, downtime and costs, European Journal of Operational Research 99 (1997) 565–583] which are based on approximations. We also investigate some discontinuities problems.

Nonparametric estimation of some important indicators in reliability for semi-Markov processes

In this article we present nonparametric estimators of the mean time to failure (MTTF), the mean up time (MUT), and the mean down time (MDT) of an arbitrary finite semi-Markov process. Strong consistency and weak convergence is proved for the estimators of MTTF, MUT, and MDT, as the time interval of observation becomes large.

Uptime as a measure of recovery in children postappendectomy

Purpose: The aim of this study was to objectively measure recovery time after open and laparoscopic appendectomy using an activity monitor (PAL 1). Methods: The PAL 1 records the amount of time that the subject is upright (uptime). Children wore the PAL 1 for 10 days continuously, beginning within the first 48 hours postoperatively. Results: Uptime data were collected for 5 or more days for 42 children (open, n = 16; laparoscopic, n = 26). All children had low levels of uptime in the initial postoperative period. There was weak evidence of a quicker recovery rate for children in the laparoscopic group ( P = .09). The difference in mean uptime between groups was of statistical significance by day 7 postoperatively with children in the laparoscopic group having a higher mean uptime than those in the open group (difference of 0.7 hours; 95% confidence intervals 0.0 to 1.4 hours in a 24-hour period). Conclusions: The results of this study show that recovery postappendectomy can be quantified by the measurement of uptime and that children undergoing a laparoscopic procedure may recover marginally more quickly than those undergoing an open procedure.

Uptime normative values in children aged 8 to 15 years

The‘Uptimer’is a custom‐made lightweight battery‐operated remote activity monitor that records the amount of time an individual spends in the upright position, which is also known as‘uptime’. The aims of this study were to determine levels of uptime over 24 hours and the relation between uptime and a child's age, sex, height, weight, and body mass index (BMI). Uptime was recorded in 529 normally developing children (318 females, 211 males), aged between 8 and 15 years. All children wore an Uptimer continuously for a 24‐hour period that included a typical school day. Mean uptime for children in this study was 5.4 hours (SD 1.3; range 1.5 to 10.3 hours), over a 24‐hour period. Uptime followed approximately a normal distribution in this population and did not have a linear relation to age, height, weight, nor BMI. Results of this study may be used as a normative database for the evaluation of uptime in children with physical disabilities.

Cost-benefit analysis of a one out of n : G system with repair and preventive maintenance

This paper deals with the cost-benefit analysis of a 1 out of n:G system. The system consists of n identical units. Initially, one unit is placed in operation and the remaining (n−1) in cold standby. The operating unit is taken off when it either fails or is due for preventive maintenance (PM). A standby, if available, takes its place. If no standby is available when an operating unit is taken off, the system goes down. There is only one server who attends to all repair and PM actions on a first-come-first-served basis. Under suitable assumptions, the expressions are obtained for the mean up-time of the system in [0,t), the mean time the server spends on repair in [0,t) and the mean time the server spends on PM in [0,t). Under the assumption that revenue is earned and repair and PM costs are incurred linearly with time, an expression for the expected net revenue in [0,t) is obtained. The special case of age replacement is taken up and the steady-state expected net revenue per unit time is computed for various values of the system parameters.

Comparison of mean time to first failure and mean up time

Two unit standby redundant repairable system, parallel systems and series systems are studied. The time between failures and the repair time are assumed exponentially distributed. Expressions for mean time to first failure (MTTFF) and mean up time (MUT) have been obtained. Comparisons have been made between MTTFF and MUT and the difference illustrated by numerical examples for different systems.

Availability Analysis of 1-Server n-Unit System with Slow Switch Subject to Maintenance

This paper deals with the availability of two 1-server n-unit systems with slow switch subject to maintenance. Explicit expressions for the Laplace transform of the availability and the mean up-time of the system are obtained. The steady-state availability of the system is discussed. Details are given in a Supplement.

Availability analysis of 1-server 2-unit system with slow switch. subject to maintenance

This paper deals with the availability of 1-server 2-unit system with slow switch. The switch is taken up for maintenance after completing r (fixed number) switching operations. Explicit expression for the Laplace transform of the mean up-time of the system during (0t) is obtained. The steady-state availability to the system is discussed, xne case when the switch-over is instantaneous is also considered.

Availability of 1-Server 2-Dissimilar Unit System with Slow Switch

The paper describes a 1-server 2-dissimilar unit cold standby redundant system with slow switch. Explicit expressions for the Laplace transform of mean up-time of the system and the steadystate availability of the system are also obtained in the Supplement.

Stochastic behaviour of a 2-unit system with 1-server subject to delayed maintenance

This paper considers a 2-unit redundant system with arbitrary failure and repair time distributions and the repair facility is subject to preventive maintenance. The pdfs of time to preventive maintenance (i.e. the waiting time) and the time for preventive maintenance of the repair facility are arbitrary. The following characteristics of the system are discussed: (i) the availability of the system in a given interval, (ii) the probability of first passage time to system failure and (iii) the mean up-time of the system in a given interval.

A Note on Kabak's “Some Aspects of Optimal Design”

Abstract Kabak in (1) considers a system's steady-state availability with mean uptime of U and mean downtime of D. He then formulates a model of total system cost of: as an approximation to the actual model which may be written as: The approximation is used because it is assumed that D << U. In the above formulas K 1 U a is the cost of equipment that has a mean uptime of U; and K 2 D −b is the cost of equipment that has a mean downtime of D. The downtime costs are assumed to be proportional to the downtime and are therefore given by the last term in the above equations. Kabak solves the first equation, (the approximation), with geometric programming because, he points out that the exact model “does not lend itself to solution by geometric programming.” However, this author asserts that the “necessary” conditions for the solution of any geometric program can show that the exact model's geometric programming solution is tightly bounded and gives considerable information as to the effect of approximating the...

A Theoretical Analysis of System Quality

Specification of the rationale for developing a system quality measure entails considerable analysis and a careful evaluation of certain fundamental and often cherished reliability measures, e.g., mean time between failures or mean uptime. From such an analysis has come the following conclusions. In order to adequately assess the effects upon system performance of differing kinds and amounts of redundancy and to predict more accurately system availability, any nonclassical measure should quantify both length and quality of performance. A measure has been developed that meets such requirements, is distribution free, and can be evaluated in terms of system history. Two evaluational models with their peculiar difficulties are discussed.