Identical Parallel Units Research Articles

Suppression of the Second Harmonic Subgroup Injected by an AC EAF: Design Considerations and Performance Estimation of a Shunt APF

This paper proposes a design methodology for an active power filter (APF) system to suppress the second harmonic subgroup injected by an AC electric arc furnace (EAF) to the utility grid. The APF system is composed of identical parallel units connected to the utility grid via a specially-designed coupling transformer. Each APF converter is a three-phase three-wire two-level voltage source converter (VSC). The number of parallel APF units, coupling transformer MVA rating, and turns ratio are optimized in the view of the ratings of commercially-available high voltage (HV) IGBTs. In this research work, line current waveforms sampled at 25.6-kS/s on the medium voltage (MV) side of a 65-MVA EAF transformer are then used to extract the second harmonic subgroup, 95-, 100-, and 105-Hz current components, by multiple synchronous reference frame (MSRF) analysis, which was previously proposed to decompose EAF current interharmonics and harmonics in real-time. By summing up this digital data of the second harmonic subgroup, the reference current signal for the APF system is produced in real-time. A detailed model of the APF system is then run on EMTDC/PSCAD to follow the produced reference current signal according to hysteresis band control philosophy. The simulation results show that the proposed APF system can successfully suppress the second harmonic subgroup of an AC EAF.

Stochastic analysis of parallel system with two discrete failures

In this paper two identical parallel units have been analyzed by using discrete distribution and regenerative point technique. The distribution for repair time and inspection policy for detecting the kind of failures (minor or major) are taken as geometric distribution. Various reliability measures of system effectiveness such as mean time to system failure, steady state availability and busy period for both inspection and repairman were obtained. Graphs has been plotted for the system, that concluded the behaviour of profit function which increase along with increase in rate of repair mechanism and decreases with increase in failure rate.

Stochastic behaviour analysis of a Markov model under multi-state failures

To solve a real problem, how to calculate the reliability of a system with time-varying failure rates in an industry system. This is a study of a system consisting of three identical units in parallel, each of which can be in two states: operational or non-operational. Under the assumptions that each unit has a constant failure and repair time rate. The system is regarded as operational if at least one of its elements is working. The system analyzed incorporating three types of failures namely partial failure, human failure and catastrophic failure, which also leads to the system failure. With the help of probability analysis, supplementary variable technique, Laplace transformations, and the Markov process theory, the transition state probabilities, availability, reliability, mean time to failure, sensitivity analysis and cost effectiveness of the system have been evaluated. Finally, some particular cases, numerical examples and graphical illustrations have also been taken to describe the model.

Approximately Optimal Testing Policy for Two-Unit Parallel Standby Systems

ABSTRACT Due to the impact of time-dependent unit unavailability, the testing strategies (e.g., simultaneous or staggered testing) have significant effect on the system unavailability for complex standby systems. Several testing strategies have been studied to minimize average system unavailability for certain systems. For example, uniformly staggered testing has been shown to be the best for two identical units in parallel with and without common cause failure between units. However, this result may not be suitable for parallel systems with nonidentical units. This study provides a discussion to obtain the optimal testing strategy for parallel systems for the case in which units are not necessary to be identical. Moreover, a maintenance manager who is aware of the importance of each system in the plant should establish a testing schedule based on plant-level, not on system- or component-level. That is, the manager should set up testing schedule for each system based on its contribution to the plant. Therefore, a good testing schedule should consider the balance between the maintenance cost and risk (or unavailability) coming with the maintenance task for each system. This study provides a cost-effective model taking both cost and risk into account to establish a good testing policy (including testing strategy and test interval) for two-unit parallel systems.

Scheduling of Multistage Multiproduct Batch Plants Operating in a Campaign-Mode

In this work, mixed integer linear programming models for scheduling multistage multiproduct batch plants operating under campaign mode are proposed. It is assumed that each plant stage includes identical parallel units operating out of phase. Given the plant topology and the number of batches of each product to be processed in the campaign, the objective is assigning batches to units in each stage in order to minimize the cycle time of the campaign. An asynchronous slot-based continuous-time representation for modeling the assignment of batches to units is used. These formulations require postulating a priori a suitable number of production slots for each unit that integrates the plant, which severely affects the model computational performance. Then, to reduce the computational effort, a solution strategy is proposed where a simplified model, which includes preordering constraints, is first solved. Finally, a detailed scheduling model is posed where the optimal cycle time of simplified model is used as ...

Scheduling multistage batch plants with parallel units and no interstage storage

Scheduling production optimally in multistage multiproduct plants with parallel units is a very difficult but routine problem. While most batch plants avoid interstage storage due to minimize contamination, cleaning, waste, and so forth, the scheduling of plants with no interstage storage and mixed wait policies have received little attention compared to those with unlimited storage/wait. In this paper, we develop and evaluate several different mixed-integer linear programming formulations for scheduling plants with identical and nonidentical parallel processing units and unlimited and zero-wait interstage policies. Because the best approach for handling identical parallel units seems to be sequence-based and that for handling nonidentical units seems to be slot-based, we employ judicious mixes of these approaches to address real plants with mixes of stages with identical and nonidentical units. Our models also allow mixes of unlimited and zero interstage wait policies and scheduling objectives of makespan, tardiness, earliness, and weighted just-in-time. The weighted just-in-time scheduling seems to be more difficult than even the makespan scheduling, and more importantly, a modeling approach that does well for the former does not necessarily suit the latter. While the models presented in this paper do address successfully the scheduling needs of realistic batch plants, considerable future work is warranted for models that can solve large scheduling problems of this type.

Novel continuous-time formulations for scheduling multi-stage batch plants with identical parallel units

Scheduling production optimally in multi-stage multi-product plants is a very difficult problem that has received limited attention. While the case of non-identical parallel units has been addressed, the case of identical parallel units is equally worthy of attention, as many plants are or can be approximated as such. In this paper, we construct and compare several novel MILP formulations for the latter. In contrast to the existing work, we increase solution efficiency by considering each stage as a block of multiple identical units, thereby eliminating numerous binary variables for assigning batches to specific units. Interestingly, a novel formulation using an adjacent pair-wise sequencing approach proves superior to slot-based formulations. Furthermore, we develop heuristic variations of our proposed formulations to address moderate-size problems. A novel heuristic strategy inspired from list scheduling algorithms seems to be efficient for moderate-size problems and scales well with problem size.

Bioreactors for hybrid liver support: historical aspects and novel designs.

A novel bioreactor construction has been designed for the utilization of hepatocytes and sinusoidal endothelial cells. The reactor is based on capillaries for hepatocyte aggregate immobilization. Three separate capillary membrane systems, each permitting a different function are woven in order to create a three dimensional network. Cells are perfused via independent capillary membrane compartments. Decentralized oxygen supply and carbon dioxide removal with low gradients are possible. The use of identical parallel units to supply hepatocytes facilitates scale up. In vitro studies demonstrate long-term external metabolic function in primary isolated hepatocytes within bioreactors. These systems are capable of supporting essential liver functions. Animal experiments have verified the possibility of scaling-up the bioreactors for clinical treatment. However, since there is no reliable animal model for investigation of the treatment of acute liver failure, the promising results obtained from these studies have limited relevance. The small number of clinical studies performed so far is not sufficient to reach conclusions about improvements in the therapy of acute liver failure. Although important progress has been made in the development of these systems, various hepatocyte culture models and bioreactor constructions are being discussed in the literature, which indicates competition in this field of medical research. An overview, which emphasizes the development of hepatocyte culture models for bioreactors, subsequent in vitro studies, animal studies, and clinical application, is also provided.

Long-term liver cell cultures in bioreactors and possible application for liver support.

Hybrid artificial liver systems are being developed as a temporary extracorporeal liver support therapy. A short overview is given which emphasizes the development of hepatocyte culture models for bioreactors, subsequent in vitro studies, animal studies and the clinical application of hybrid liver support systems. An own bioreactor construction has been designed for the utilization of hepatocytes and sinusoidal endothelial cells. The reactor is based on capillaries for hepatocyte aggregate immobilization, coated with biomatrix. Four separate capillary membrane systems, each permitting a different function, are woven in order to create a three-dimensional network. Cells are perfused via independent capillary membrane compartments. Decentralized oxygen supply and carbon dioxide removal with low gradients is possible. There is a decentralized co-culture compartment for nonparenchymal liver cells. The use of identical parallel units to supply a few hepatocytes facilitates scale-up.

Optimizing the income of a repairable reliability system with early standby activation

This paper presents optimization analysis concerned with the income of a repairable reliability system having two identical units in parallel and one standby with common-cause failures. Expressions to obtain optimun net income of the system with respect to repair are developed. Several plots of resulting expressions are shown.

Using spatial information to analyze correlations between test structure data : John K. Kibarian and Andrzej J. Stojwas. IEEE Trans. Semicond. Mfg4(3), 219 (1991)

This paper deals with a parallel load-sharing reliability system with cold standby redundancy and ample repair facilities. That is, we have n identical parallel units, of which at most k units are operating simultaneously. If less than k units are available, the system operates at a proportionally reduced level. For this system, an approximate method is given for the calculation of the probability distribution of that proportion of the system capacity that cannot be used in a given time period. The method is based on an approximation of the k-out-of-n multistate system by a two-state single component. Validation of the approximation using Monte-Carlo simulation shows satisfactory performance. Also, sensitivity results are given, showing in particular a decreasing sensitivity of the measures of performance to the distributional form of the unit lifetimes and repair times as the size of the system increases. Furthermore, it is found that the effect of the distributional form of the unit lifetimes dominates that of the unit repair times.

Interval uneffectiveness distribution for a k-out-of- n multistate reliability system with repair

This paper deals with a parallel load-sharing reliability system with cold standby redundancy and ample repair facilities. That is, we have n identical parallel units, of which at most k units are operating simultaneously. If less than k units are available, the system operates at a proportionally reduced level. For this system, an approximate method is given for the calculation of the probability distribution of that proportion of the system capacity that cannot be used in a given time period. The method is based on an approximation of the k-out-of- n multistate system by a two-state single component. Validation of the approximation using Monte-Carlo simulation shows satisfactory performance. Also, sensitivity results are given, showing in particular a decreasing sensitivity of the measures of performance to the distributional form of the unit lifetimes and repair times as the size of the system increases. Furthermore, it is found that the effect of the distributional form of the unit lifetimes dominates that of the unit repair times.

Cost—reliability optimal release policies for software systems : Shigeru Yamada and Shunji Osaki. IEEE Trans. Reliab.R-34(5), 422 (1985)

This paper deals with a parallel load-sharing reliability system with cold standby redundancy and ample repair facilities. That is, we have n identical parallel units, of which at most k units are operating simultaneously. If less than k units are available, the system operates at a proportionally reduced level. For this system, an approximate method is given for the calculation of the probability distribution of that proportion of the system capacity that cannot be used in a given time period. The method is based on an approximation of the k-out-of-n multistate system by a two-state single component. Validation of the approximation using Monte-Carlo simulation shows satisfactory performance. Also, sensitivity results are given, showing in particular a decreasing sensitivity of the measures of performance to the distributional form of the unit lifetimes and repair times as the size of the system increases. Furthermore, it is found that the effect of the distributional form of the unit lifetimes dominates that of the unit repair times.

Intensification of Activated Sludge Waste Water Treatment

The efficiency of two possible intensification methods of activated sludge waste water treatment (pure oxygen activation and activated carbon addition) were compared. Experiments were carried out in laboratory scale equipment with variable reactor volume (maximum capacity 20,0 dm3, settling tank vol. 15 dm3). In order to detect even minor differences, we operated completely identical units in parallel runs, with one representing a traditional system as reference. A model sewage of industrial character, diluted milk, was used as influent and in the course of activated carbon intensification experiments an anion-active detergent was added as a poorly biodegradable model material to be removed mainly by adsorption. Reactor loads were gradually increased - from low values to overloading - by decreasing the dilution of milk or by reducing reactor volume. The effects of different intensification methods on the effluent pollution level (COD value and anion-active detergent content) and on the settling properties of sludge were examined as a function of loading. The efficiency of both intensification methods was found to be increasing with increasing loading. Comparison of the two systems showed activated carbon intensification to be more efficient in the removal of soluble COD and extremely useful in the elimination of a poorly biodegradable material.

Enumeration of all minimal cut-sets for a node pair in a graph : S. Arunkumar and S. H. Lee. IEEE Trans. Reliab.R-28, (1) 51 (1979)

This paper deals with a parallel load-sharing reliability system with cold standby redundancy and ample repair facilities. That is, we have n identical parallel units, of which at most k units are operating simultaneously. If less than k units are available, the system operates at a proportionally reduced level. For this system, an approximate method is given for the calculation of the probability distribution of that proportion of the system capacity that cannot be used in a given time period. The method is based on an approximation of the k-out-of-n multistate system by a two-state single component. Validation of the approximation using Monte-Carlo simulation shows satisfactory performance. Also, sensitivity results are given, showing in particular a decreasing sensitivity of the measures of performance to the distributional form of the unit lifetimes and repair times as the size of the system increases. Furthermore, it is found that the effect of the distributional form of the unit lifetimes dominates that of the unit repair times.

Stochastic Behaviour of a Special Complex System

The system has three subsystems: A, B, C. A has several non-identical units in series. B has 2 identical units in parallel. C has 2 identical units in standby (cold) redundancy. Switching is perfect. Unit failure rates are constant, repair rates need not be constant. The system fails if A or B fails; C is useful, but not essential. The supplementary variable technique is used to find Laplace transforms of system state probabilities.

An Optimal Maintenance Policy for a 2-Unit Parallel System with Degraded States

The system consists of two identical units in parallel with good, degraded, and failed states. We discuss a maintenance policy that maximizes the expected net profit-rate from the system over an infinite time span. General equations which describe the system behavior under a certain maintenance policy are derived. The optimal maintenance policy can be determined by solving those equations.