Improve System Dynamics Research Articles

Closing the loop: promoting synergies with other theory building approaches to improve system dynamics practice

AbstractThis paper contributes to the development of better methods for the conceptualization of system dynamics models. It does so by looking at the model building process as a theory building process and by reviewing theory building approaches in other social sciences, such as grounded theory and case study research. Concepts of these modes of qualitative research constitute a tool set to build relevant system dynamics models, grounded in data, and with higher potential to provide rigorous and relevant generic structures. The deductive capability of simulation models, on the other hand, helps to close the loop in the theory building process, putting together the inductive and deductive stages in theory building. Copyright © 2008 John Wiley & Sons, Ltd.

Dynamic performance of a hybrid inventory system with a Kanban policy in remanufacturing process

In this paper we study a hybrid system with both manufacturing and remanufacturing. The inventory control strategy we use in the manufacturing loop is an automatic pipeline, inventory and order based production control system (APIOBPCS). In the remanufacturing loop we employ a Kanban policy to represent a typical pull system. The methodology adopted uses control theory and simulation. The aim of the research is to analyse the dynamic (as distinct from the static) performance of the specified hybrid system. Dynamics have implications on total costs in terms of inventory holding, capacity utilisation and customer service failures. We analyse the parameter settings to find preferred “nominal”, “fast” and “slow” values in terms of system dynamics performance criteria such as rise time, settling time and overshoot. Based on these parameter settings, we investigate the robustness of the system to changes in return yield and the manufacturing/remanufacturing lead time. Our results clearly show that the system is robust with respect to the system dynamics performance and the remanufacturing process can help to improve system dynamics performance. Thus, the perceived benefits of remanufacturing of products, both environmentally and economically, as quoted in the literature are found not to be detrimental to system dynamics performance when a Kanban policy is used to control the remanufacturing process.

Transformers-a successful application of high temperature superconductors

With the exception of core steel and improvements in insulation, power transformers have changed little in the last fifty years. In general, modern power transformers larger than 10 MVA have full load efficiencies exceeding 99% and are routinely subjected to a form of life-cycle cost evaluation as a part of the purchase process. However, resistive losses in the windings constitute a significant proportion of losses at full load and the desire to further economically reduce these losses using superconducting windings has been an elusive goal for greater than a quarter century. Using high temperature superconducting (HTS) windings this goal is now within reach both technically and economically. Major HTS transformer projects are underway in Europe, Japan and the United States with all projects reporting success in their initial phases. With the desire for improved efficiency and lower losses within technical reach, additional benefits such as reduced size and weight, improved fire safety, overload without loss of transformer life and improved system dynamics through lower impedance and controlled fault current levels have also been identified. These additional benefits may well prove to be significantly more important to the applications of HTS transformers than improved efficiency through loss reduction in the windings. This paper summarizes the status of current HTS transformer projects and the remaining economic and technical challenges that must be met.

The Influence of Volts/Hertz Control on Filter Impedance Effects in VSI-Fed Induction Machines

The use of a transient volts/hertz controller can produce great improvement in the dynamic behavior of a voltage source inverter (VSI)-fed induction machine with a dc link filter. The effects of transient volts/hertz control are investigated and methods for selecting the proper controller gain GW for a wide range of systems are given. The information is presented in terms of 1) the upper and lower critical gains (GW <L xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CU</L> and GW <L xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CL</L> ) needed to ensure stability, 2) a form of optimum gain (GW <L xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</L> ) which gives the best dynamic performance for a specific machine and operating point, and 3) an acceptable gain (GW <L xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</L> ) which gives acceptable dynamic performance over a range of operating conditions. Utilizing nondimensional parameters, eigen-values, and root locus plots, the values of the gains of the volts/hertz control are studied for a wide range of parameters. Graphs of the critical gains and methods of choosing GW <L xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</L> and GW <L xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</L> are presented. The results indicate that volts/hertz control can always be used to improve system dynamics, and with properly chosen controller gain the dynamic performance of the inverter-filter-motor system is typically better than the motor alone for a wide range of motor sizes and operating conditions.