Chance Failure Research Articles

A Guideline for Reliability Prediction in Power Electronic Converters

Reliability prediction in power electronic converters is of paramount importance for converter manufacturers and operators. Conventional approaches employ generic data provided in handbooks for random chance failure probability prediction within useful lifetime. However, the wear-out failures affect the long-term performance of the converters. Therefore, this article proposes a comprehensive approach for estimating the converter reliability within useful lifetime and wear-out period. Moreover, this article proposes a wear-out failure prediction approach based on a structural reliability concept. The proposed approach can quickly predict the converter wear-out behavior unlike conventional Monte Carlo-based techniques. Hence, it facilitates reliability modeling and evaluation in large-scale power electronic-based power systems with huge number of components. The proposed comprehensive failure function over the useful lifetime and wear-out phase can be used for optimal design and manufacturing by identifying the failure prone components and end-of-life prediction. Moreover, the proposed reliability model can be used for optimal decision-making in design, planning, operation, and maintenance of modern power electronic-based power systems. The proposed methodology is exemplified for a photovoltaic inverter by predicting its failure characteristics.

Validation of Process Performance through Reliability Measurement

This study employed excels graphic tools to model and evaluate the reliability of production system using Eleme Petrochemical in Nigeria as a case study. The mean time between failures (MTBF) is the reliability parameter used. The exponential distribution function model was used to evaluate the consistency of ten production components and five cooling tower fans in series arrangement as an individual component in a production system. The Eleme Petrochemical production probability distribution function has exponential reliability distribution model with a reliability of 0.9999 within five hundred hours of operation while the reliabilities of critical components ranges from 0.63 to 0.89 for five hundred hours of operation. For 1000 hours of operation the reliability of components ranges from 0.40 to 0.63. Most of the system components have reliability of 0.37 at their mean lives. The hazard rate has exponential distribution described as chance failure phase in reliability analysis. This report recommends scheduled maintenance of 500hours and provision of redundancy to ensure maintainability and serviceability. Original Research Article Ihueze and Ebisike; BJAST, 15(6): 1-13, 2016; Article no.BJAST. 20813 2

Probabilistic model for planning keeping of power transformer spare components with general repair time distribution

The paper suggests a method to optimize the quantity of spare power transformer components. The presented sparing policy is conceived to provide minimum annual cost consisting of expected capital cost for spares, failures renewal and load curtailment costs. The method identifies minor and major failures. Power transformer is a complex system, consisting of six components (functional parts). It is assumed that each component has two independent, competing failure modes: wear-out failure mode, modeled by two-parameter Weibull distribution, and a chance failure mode, characterized by an exponential distribution. Duration of failure renewal is not a deterministic variable. It is assumed that failure state residence time of each power transformer component is Weibull distributed (renewal times of power transformer components may follow any probability distribution, Weibull distribution being only a special case).The application of the method suggested and the benefits it provides are demonstrated for one transformer station (TS) 110/xkV/kV with 2×31.5MVA transformers in case of radial supplying of customers and when outage of one power transformer does not affect customers supplying. In addition, the influence of power transformer refurbishment on expected total cost has also been analyzed.

Method for determining optimal power transformers exploitation strategy

The paper suggests a procedure for determining an optimal combination of activities and measures to minimize the expected total costs during the planned exploitation period of power transformers. Combinations of different types of preventive maintenance, spare parts keeping and installation of condition monitoring systems of power transformer individual components are taken into consideration. Power transformer is a complex system, consisting of six components (functional parts). It is assumed that each component has two independent, competing failure modes: a wear-out failure mode, modeled by two-parameter Weibull distribution, and a chance failure mode, characterized by an exponential distribution.The superposition of failures and planned maintenance are taken into account in the analysis of some forms of preventive maintenance. The application of the method suggested and the benefits it provides are demonstrated for one transformer station (TS) 110/10 kV/kV with 2 × 31 500 kVA-110 kV Star-10 kV Delta transformers in case of radial supplying of customers as well as in case when outage of one power transformer does not affect the power supply to customers.

Optimal spares availability strategy for power transformer components

The paper suggests a method to optimize the spare amount of power transformer components. The proposed strategy is conceived to provide minimum annual cost consisting of expected failures renewal cost, capital cost for spares and load curtailment cost. The method identifies minor and major failures. Minor failures are repairable, while major failures can be repairable or unrepairable. Power transformer is a complex system, consisting of six components (functional parts). It is assumed that each component has two independent, competing failure modes: wear-out failure mode, modelled by two-parameter Weibull distribution, and a chance failure mode, characterized by an exponential distribution. The application of the method suggested and the benefits it provides are demonstrated for one transformer station (TS) 110/ x kV/kV with 2 × 31.5 MVA transformers. In addition, the influence of performing power transformer refurbishment on expected total cost has also been analyzed.

Method for effects evaluation of some forms of power transformers preventive maintenance

The paper suggests a method for effects evaluation of a few activities and measures, undertaken as a part of preventive maintenance of power transformers. The method enables calculation of expected failures repair cost and load curtailment cost. The method identifies minor and major failures. Power transformer is a complex system, consisting of five components (functional parts). It is assumed that each component has two independent, competing failure modes: wear-out failure mode, modeled by two-parameter Weibull distribution, and a chance failure mode, characterized by an exponential distribution. The application of the method is demonstrated for one transformer station (TS) 110/ x kV/kV with 2 × 31.5 MVA transformers. Also, by applying the method, influence of system for condition monitoring of transformer windings and oil on failures repair cost and load curtailment cost is evaluated.

Probabilistic method for planning of maintenance activities of substation components

This paper suggests a probabilistic method for the calculation of operation cost throughout a planning period and availability evaluation of substation components, in connection with planned maintenance activities. This method is conceived to provide lower operation cost, consisting of failure repair cost and maintenance cost, and higher availability of substation components. The method identifies minor and major failures. It is assumed that each component has two independent, competing failure modes: wear-out failure mode, modelled by a two-parameter Weibull distribution, and a chance failure mode, characterized by an exponential distribution. The application of the method suggested is demonstrated for a 145 kV air-blast circuit-breaker. By applying the method, influence of condition monitoring systems application on operation cost is evaluated. Also, because of comparison, expected operation cost in case of time-based maintenance performing is calculated.

Effect of gravity on onset of microexplosion for an oil-in-water emulsion droplet

Microgravity combustion experiments have been carried out for an emulsion droplet suspended at a quartz fiber by free-fall method. Attention was mainly paid to the effect of gravity on the occurrence of microexplosion that may be caused by the bubble nucleation at temperatures below the superheat limit. The oil-in-water emulsion consisted of the base fuel and water was employed after degasification. The base fuel employed was n -dodecane and n -tetradecane. The water content was 0.2 in volume. The waiting time for the onset of microexplosion was measured for about 30 runs under normal gravity and microgravity. The onset probability of microexplosion was discussed from the statistical point of view by using the weakest link destruction model. The results showed that the distribution function of the waiting time correlated with the mixed Weibull distribution for both gravity conditions. That is, the distribution function is classified to the wear-out type at the initial heating period, and it is classified to the chance failure type at constanttemperature period. The change in the type of Weibull distribution occurs slightly earlier under normal gravity than microgravity for both base fuels. The Weibull distribution of the waiting time at constant-temperature period for normal gravity is almost the same as that for microgravity. The dependence of the onset rate of microexplosion on the superheating of water is independent of the gravity condition.

Open repair technologies for MCM-D : THOMAS A. WASSICK and LAERTIS ECONOMIKOS. IEEE Transactions on Components, Packaging, and Manufacturing Technology, Part B, 18(1), 154 (February 1995)

A dynamic model is developed for a system element reliability distribution over a generalized strength space. A differential equation is obtained describing the time-dependence of the reliability distribution function (RDF). The equation covers a wide class of power reactor system components which perform under intense stress conditions where a standard subdivision into a “burn-in” period, a “chance failures” range and a “wear-our” period is inapplicable.The hazard distribution function (HDF) over strength is introduced within the model and it is shown that a standard hazard rate is a strength-averaged failure intensity parameter with the RDF as a weighting function.It is shown that a well-known “bathtub” form of the hazard rate function corresponds to an analytical solution of the principal RDF transfer equation under some simplifying assumptions.

Statistical analysis of onset of microexplosion for an emulsion droplet

A experimental study has been carried out on the burning behavior of an emulsion droplet suspended on a quartz fiber. Attention was paid mainly to the coccurrence of microexplosion, which may be caused by the bubble nucleation at temperatures below the superheat limit. The oil-in-water emulsion consisting of the base fuel and water was employed after degasification. The base fuels used were n-tetradecane and n-hevadecane. The water content varied from 0.1 to 0.3 by volume. The waiting time for the onset of microexplosion was measured for about 30 runs. The microexplosion is assumed to be a random process, and the onset probability of microexplosion was discussed from the statistical point of view. The Weibull distribution, which is derived mathematically from the weakest link destruction model, was adopted for this purpose. The results showed that the distribution function of the waiting time correlated with the Weibull distribution. The distribution function is classified the wearout type at the initial heating period and as the chance failure type at the constant-temperature period. Thus, the type of Weibull distribution of the waiting time is dependent on the variation of the droplet temperature with time. The onset rate of microexplosion increases exponentially as the superheating of water increases. The reliance of the onset rate on the superheating of water is independent of the base fuel.

Microexplosion of an Emulsified Fuel Droplet under Microgravity. Statistical Analysis of Onset of Microexplosion.

An experimental study was carried out on the microexplosion behavior of an emulsified fuel droplet under normal gravity and microgravity conditions. The oil-in-water emulsion consisted of n-dodecane, and water was used after degasification. The waiting time, which was defined as the time period between ignition and the onset of microexplosion, was measured. The onset probability of microexplosion was discussed from the statistical point of view. The Weibull distribution, which was derived mathematically from the weakest link destruction model, was adopted for this purpose. The results show that the distribution function of the waiting time correlated with the Weibull distribution, and that it is classified as the wear-out type at the initial heating period of the droplet. In contrast, the onset of microexplosion when the droplet temperature remains constant can be considered as the chance failure type. The Weibull distribution of the waiting time under the constant temperature condition for the normal gravity is almost the same as that for the micro gravity case.

Estimation of failure probability under a bivariate normal stress-strength distribution

The probability of a chance failure is a parameter of great importance for design reliability and is often examined through stress-strength models. The uniform minimum variance unbiased estimator for the same under a bivariate normal stress-strength distribution has been obtained.

Effect of CaCl<sub>2</sub> Concentration on the Probability Distribution of Stress Corrosion Cracking Failure Time of Type 304 Stainless Steel

In order to compare the failure time distribution of a laboratory accelerated test with the field failure time distribution of stress corrosion cracking of Type 304 stainless steel, the effect of CaCl2 concentration on the probability distribution of the failure time Tf, crack initiation time Ti and crack propagation time Tp for smooth plate specimens with or without crevice under a constant condition of 373K and 200MPa has been analyzed. Two parameter Weibull distribution fitted approximately to all the distributions obtained. The apparent shape parameter mi' for Ti of both type of the specimens with or without crevice was found unity, indicating that a chance failure mode was operating. On the other hand, mf' and mp' for Tf and Tp show a maximum value of 4 or 6 at an intermediate concentration range of 30 or 40mass% CaCl2, corresponding to a wearout failure mode, and decrease to unity with a decrease of CaCl2 concentration. This fact means that SCC fracture and propagation at a low concentration range take place by a change failure mode. Since the same chance failure mode had been reported for the field failure time distributions of Type 304 and 316 stainless steel heat exchanger tubes, an approximate acceleration factor of the laboratory test conducted under the condition of 25 mass% CaCl2, 373K, 200MPa with crevice was estimated to be ca. 104 by comparing the mean failure time of both distributions of the field and the accelerated test.

３０４ステンレス鋼の応力腐食割れ破断寿命の確率分布に及ぼすＣａＣｌ２濃度の影響

In order to compare the failure time distribution of a laboratory accelerated test with the field failure time distribution of stress corrosion cracking of Type 304 stainless steel, the effect of CaCl2 concentration on the probability distribution of the failure time Tf, crack initiation time Ti and crack propagation time Tp for smooth plate specimens with or without crevice under a constant condition of 373K and 200MPa has been analyzed. Two parameter Weibull distribution fitted approximately to all the distributions obtained. The apparent shape parameter mi' for Ti of both type of the specimens with or without crevice was found unity, indicating that a chance failure mode was operating. On the other hand, mf' and mp' for Tf and Tp show a maximum value of 4 or 6 at an intermediate concentration range of 30 or 40mass% CaCl2, corresponding to a wearout failure mode, and decrease to unity with a decrease of CaCl2 concentration. This fact means that SCC fracture and propagation at a low concentration range take place by a change failure mode. Since the same chance failure mode had been reported for the field failure time distributions of Type 304 and 316 stainless steel heat exchanger tubes, an approximate acceleration factor of the laboratory test conducted under the condition of 25 mass% CaCl2, 373K, 200MPa with crevice was estimated to be ca. 104 by comparing the mean failure time of both distributions of the field and the accelerated test.

Modelling early, chance and wearout failures by a single failure rate equation

An attempt has been made to model early, chance and wearout failures by a single failure rate equation. Failure rate has been assumed to exponentially decrease in an early failure region, remain constant in a chance failure region and exponentially increase in a wearout failure region. A six-parameter physical model and a reduced five-parameter model are suggested. Equations are given for estimation of model parameters from experimental data.

A dynamic model for element reliability

A dynamic model is developed for a system element reliability distribution over a generalized strength space. A differential equation is obtained describing the time-dependence of the reliability distribution function (RDF). The equation covers a wide class of power reactor system components which perform under intense stress conditions where a standard subdivision into a “burn-in” period, a “chance failures” range and a “wear-our” period is inapplicable. The hazard distribution function (HDF) over strength is introduced within the model and it is shown that a standard hazard rate is a strength-averaged failure intensity parameter with the RDF as a weighting function. It is shown that a well-known “bathtub” form of the hazard rate function corresponds to an analytical solution of the principal RDF transfer equation under some simplifying assumptions.

Hot redundant versus cold redundant systems

Unforeseen breakdowns of spaceborne systems and ground-based systems, caused by chance failure of parts, are minimized by incorporating redundant units in them. The redundancy can be active or stand-by. In this paper both types of redundant systems have been discussed and analysed.

Bounds on the probability of failure of composite materials

Bounds are obtained for the cumulative distribution function of the strength of composite materials. The analysis is based on the chain-of-bundles probability model for composite strength. The bounds are computed from probabilities for the chance failure of two or more adjacent fibers in a bundle. A Weibull distribution is assumed for fiber strength, and fibers adjacent to failed fibers are subjected to overloading according to a specified load sharing rule. As the composite grows in size, the bound approaches a Weibull distribution. The Weibull shape parameter for the composite is double that for the fiber, but the scale parameter is somewhat less. Features of the limiting Weibull bound are shown to be consistent with experimental observation, though numerically the bound is quite conservative.

Average Life of Equipment Subject to Compound Failures

When systems are subjected to both chance and wear-out failures, the mean of the failure distribution—which would be the average life of the system—does not exist in closed form. In this article, such a distribution is developed from basic engineering phenomena; its general mean is developed in nonclosed form; a particular solution easier to evaluate is derived, and the attention of the researchers in “equipment life”, “reliability”, and “maintenance” as well as in statistics and mathematics is drawn toward developing the relevant integral transform for this function. Some related distributions depicting various combinations of chance failures, wear-out failures, and aging and standby processes are also discussed.

Fatigue Behavior of Composite Laminate

The paper discusses a characterization of fatigue behavior of [0/±45/90] s glass/epoxy laminate in terms of the following parameters: static properties; S-N relationship; reliability; effect of thickness variation; damage initiation and growth; temperature increase; secant modulus change; effect of preloading on residual modulus and strength; and effect of ply failure on compression buckling strength. Some of the findings are the following. The primary failure process responsible for up to about 106 cycles of fatigue life is the wear-out followed by the chance failure. Change of secant modulus can be used as a measure of damage extent. Preloading to a high level has negligible effect on the residual tensile strength when the fatigue stress is low. However, ply failure and partial delamination result in a moderate loss of compression buckling strength.