RT&A 2016, # 4(43) Vol.11

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Gertsbakh I., Vaisman R.

 

Resilience of Finite Networks Against Simple and Combined Attack on Their Nodes

We compare the reliability of two finite networks with the same vertex degree and the same number of nodes; a regular 16x16 grid and a Poisson network. Both networks are subject to random removal of their nodes, and the network failure is defined as the reduction of the maximal component beyond some critical level α. The main tool for comparing the network resilience are the marginal cumulative D-spectra (signatures) of the net- works. It was demonstrated that the regular grid for small α is less reliable than the Poisson network. We study also the situation when multiple hits of the same node are allowed. We demonstrate that finite networks behave similar to infinite random network with regard to the fraction of nodes to be removed to create “similar” giant components containing the same fraction of network nodes. Finally, we consider a combined attack on net- work nodes by two-type of “shells” where the node fails only if it is hit by “shells” of both types. For this case, we derive a formula for determin- ing the minimal number of “shells” which destroy the network with given probability.

 

Vladimirov A.

 

Asymptotics of mean-field closed networks

We establish the convergence of equilibria of finite symmetric closed networks with the FIFO service discipline and a general service time with bounded second moment to the unique equilibrium of the non-linear Markov process.

 

Ibrahim Yusuf, Ramatu Idris Gatawa

 

Probabilistic Models for Reliability Analysis of a System with Three Consecutive Stages of Deterioration

In this paper we present availability and mean time to failure estimation of a system where the deterioration rates follow the Weibull distribution. The paper presents modeling and evaluation of availability and mean time to system failure (MTSF) of a consecutive three stage deteriorating system. The system has three possible modes: working with full capacity, deterioration and failure mode. The three stages of deterioration are minor, medium and major deteriorations. Minor and major maintenance are allowed at minor and medium deterioration states and replacement at system failure. Explicit expressions for the availability and mean time to failure of the system are obtained analytically. Graphs have been plotted to determine the behavior of availability and mean time to system failure with respect to time for different values of deterioration, maintenance and replacement rates. Also, high values of the shape parameter decreases mean time to system failure and availability. The system is analyzed using differential difference equations. 

 

Mustafayev R., Hasanova L.

 

Study Of Starting Duty Of Wind Power Plant With Asynchronous Generators

Presently, the park of wind power plants (WPP) consists mostly of frequency controlled asynchronous generators. As the generators the squirrel-cage asynchronous machines and generators made on the basis of double fed asynchronous machines (DFAM) are used. When WPPs locate far from the powerful sources of energy generation of power system and they are connected with the power system by "weak" power grids, i.e. by grids, which are not equipped with reactive power sources, then the unwanted voltage dips may occur when connecting the WPPs to the power system in the places of their connection to the power system. The comparative analysis on the developed three-coordinated mathematical models of asynchronous machines of start by underfrequency relay and connection of WPPs with the above asynchronous generators to the power system has been carried out. It has been found, that in terms of impact of starting duties on electric power networks the most preferable are the systems of WPPs with squirrel-cage asynchronous generators. The values of starting currents when start by underfrequency relay of WPPs with squirrel-cage asynchronous generators are almost 48% lower than in the system of WPPs with DFAM eactive power compensation of asynchronous generators wind power and small hydroelectric power stations increases the reliability of connecting them to the so-called "weak" power grids of power systems. The methods of reactive power compensation for asynchronous generators of various designs.

 

Chovanec A., Breznická A.

 

Simulation Modelling Of A Sporadic Demand Applying A Bootstraping

This technique bootraping has been successfully used in various applied statistical problems, although not many applications have been reported in the area of time series. In this paper we present a new application of Bootstrap to time series. A fundamental aspect of supply chain management is accurate demand forecasting. We address the problem of forecasting intermittent (or irregular) demand, i.e. random demand with a large proportion of zero values. Items of spare parts with sporadic consumption can make a significant, up to 60% portion of the value of supplies in service and workshop inventory areas of many industrial segments. An understanding of key features of demand data is important when developing computer systems for forecasting and inventory control.

 

Chacko V.M.

 

X-Exponential Bathtub Failure Rate Model 

The properties of x-Exponantial Bathtub shaped failure rate model are discussed. Estimation process and failure rate behavior is explained.    

 

Baccelli F., Rybko A., Shlosman S., Vladimirov A.
 

Metastability of Large Networks with Mobile Servers

 We study symmetric queueing networks with moving servers and FIFO service discipline. The mean field limit dynamics demonstrates unexpected behavior which we attribute to the metastability phenomenon. Large enough finite symmetric networks on regular graphs such as cycles are proved to be transient for arbitrary small inow rates. However, the limiting non-linear Markov process possesses at least two stationary solutions. The proof of transience is based on martingale technique.

 

Pandey V., Kadekodi P.

 

Effective Way of Conducting Highly Accelerated Life Testing - Linking the Failure Mode Effects Analysis and Finite Element Analysis

In today’s competitive marketplace, the design phase presents a perfect opportunity to test a product to find its maximum limitations and weak links. On the same context HALT (Highly Accelerated Life Test) has been adopted by many industries. HALT is a destructive stress testing methodology for accelerating product reliability during the engineering development process. It is a great process used for precipitating failure mechanisms in an electronics hardware design and product which may occur into the field. The traditional HALT process which is followed by most of the industries, deals with destructive stress testing and subjective approach to fix the design weaknesses based on experience, followed by iterative HALT to check the robustness against the design fixes done which may not be relevant fixes. This paper summarizes the effective way of conducting HALT by emphasizes on the “Analysis First” approach, the FMEA (Failure Mode Effect Analysis) and FEA (Finite Element Analysis) which will help identifying the critical functions along with associated components to be monitored during HALT and reduces the iteration of HALT by analyzing the board robustness against the stresses i.e. temperature and vibration prior to HALT respectively. And also presents the specification limits derived based on the product specification and chamber standard deviation, up to which the root cause and design fixes needs to be done, eliminating the subjectivity around it.

 

Artyukhova M., Polesskiy S., Linetskiy B., Ivanov I. 

 

Simulation Of Reliability For Electronic Means With Regard To Temperature Fields

The paper considers the technique of modeling of electronic reliability based on modeling electrical components environment temperature. As experience of the simulation and exploitation of electronic shows, one of the main factors that significantly affect the reliability characteristics is the thermal effect. This is confirmed by the statistics of a number of companies. In the paper for the simulation were used systems ASONIKA-K and ASONIKA-TM. On the example of a real electronic mean proved the need for a point temperature estimate for each electrical component and the account of these temperatures, instead of the average values in predicting the reliability indices. Such approach will significantly improve (20% - 40%) the accuracy of estimates of the mean time to failure. Developed engineering method to predict reliability, built on the "downward" hierarchical circuit simulation.

 

Ahmadur Rahman, Showkat Ahmad Lone, Arif-Ul-Islam

 

Parameter Estimation of Mukherjee-Islam Model under Step Stress Partially Accelerated Life Tests with Failure Constraint  

In this paper, we have studied the estimation of parameters under failure censored data using step stress partially accelerated life testing. The lifetimes of test items are assumed to follow Mukherjee-Islam distribution. The estimation of different parameters and acceleration factor are obtained by Maximum Likelihood Method. Relative absolute bias (RAB), mean squared error (MSE), relative error (RE), standard deviation and confidence intervals are also obtained. Asymptotic variance-covariance matrix and also test method are given. Simulation studies have been introduced to illustrate the performance of all the statistical assumptions.