Welding and Switching
The research programme Welding and Switching is focused on studies of thermal plasmas, their technological applications and on research of the discharge phenomena in the high-voltage apparatus.
Arc plasmas are the essential component in switching devices in power networks, and in applications of joining technology, like arc welding, plasma and hybrid welding. Despite the long-term research work, the detailed knowledge about the physical mechanisms of the arc interaction with surrounding material is still missing.
Combination of various diagnostic methods is used to obtain the space- and time-dependent parameters, like e.g. temperature, composition, pressure, velocity. Adoption and development of optical diagnostic methods for characterization of plasma itself and surrounding material belongs to core task of the research programme. Pyrometry, emission and absorption spectroscopy, high-speed camera techniques are used for the quantitative analysis of the arc plasmas. Detailed knowledge of the arc properties and dynamics and their control mechanisms deliver new advanced diagnostic and control methods and allows for the derivation of new approaches for process improvement.
Study of degradation of high-voltage apparatus due to occurrence of partial discharges is important for ageing assessment and lifetime prediction of high-voltage components. For understanding of basic physical phenomena wide band electrical diagnostics (> 1GHz) and sensitive optical diagnostics, like e.g. iCCD cameras and photomultiplier is applied. Obtained knowledge is used for conception of advanced diagnostics tool for status monitoring of high-voltage components.
Detailed analysis and deeper understanding of the basic plasma processes is the basis for further development and optimization of arc-based switching devices. Switching performance at high electrical currents at all voltage levels requires the safe operation of power system components. The development of switching devices at up-to-date technical level taking into account environmental aspects is no longer conceivable without physical modelling of the functional component plasma and accompanying experimental studies.
For the analysis of complex arc dynamics combinations of optical emission spectroscopy, optical absorption spectroscopy and high-speed camera techniques are used. For the determination of plasma parameters, like e.g. temperature and composition, combination of complementary methods, like e.g. analysis of atomic and ionic line radiation and video spectroscopy have been developed as a general solution approach, allowing for the study of broader spatial regions and extended temporal ranges.
The novel diagnostic methods of arc plasma lead to a deeper understanding of the physical processes and as a consequence to development of improved switching concepts and operation principles. The use of such methods for parameter and design optimization saves significantly time and costs in the development of switchgears.
Conventional welding processes require noticeable innovations in order to remain competitive as a cost-effective joining technology considering new materials and increased quality requirements. Furthermore, the process safety and efficiency are the necessary features for modern technologies. Such innovations can be achieved only on the basis of a detailed understanding of the mechanisms of action in the arc that previously did not exist sufficiently.
Qualification of optical diagnostics methods for the measurements on non-symmetric objects and development of corresponding data processing methods is necessary. Here, the tomographic reconstruction based on the use of several high-speed cameras complementary to OES diagnostics in combination of iterative data evaluation procedure is the right solution.
For optical investigations on submerged arc welding process adapted setup for time and space resolved optical analysis by high-speed imaging and high-speed video spectroscopy has been developed. The process becomes optical accessible through a tiny metallic tube which is filled with non-reactive gas at specified pressure.
The results from the research on the fundamentals have been used for further development of innovative process concepts such as laser-assisted arc welding and submerged arc welding. Increase the stability and process reliability, as well as the efficiency of the welding process are added values through research.
Partial discharges inevitable occurs in all applications of high-voltage engineering leading to degradation and failure of high-voltage apparatus, such as power transformers, underground power cables, insulation of power electronics, components of power transmission lines. Understanding of basic physical phenomena is a key for ageing assessment and lifetime prediction of high-voltage components.
Development of reliable lifetime prediction and PD diagnostic tools is highly desired for operating control of high-voltage apparatus and construction of sustainable power networks.
For the analyses of partial discharges wide band electrical diagnostics (> 1GHz) and sensitive optical diagnostics, e.g. iCCD cameras, photomultiplier and setup for optical emission spectroscopy, is applied. The correlation between electrical and optical signals for typical defects in power apparatus is currently under investigations.
The light emitted by plasma contains valuable information about its composition, temperature and pressure. The diagnostics of spatial and temporal evolutions of these parameters provides the knowledge about the plasma itself, its interaction with surrounding materials as well as the impact on the environment and on human health. In-situ analysis of this data allows for precise process control leading to higher process efficiency, operation safety and diminution of environmental impact. An example are the stronger requirements for reduction of harmful emissions in arc welding and switching applications. Appropriate improvement can be achieved by application of optical sensors. Also the studies in the field of light sources devoted to the aspects of health and human well-being that play, for instance, a special role in the case of night work, have been performed.
Spectrally sensitive optical sensors are used for optimization of welding process in sense of weld quality and material consumption. The monitoring of harmful emissions during welding or switching operations can be performed for improvement of safety issues. Implementation of spectral control of illumination minimize the biological impact on the soil and marine fauna, and provides an accelerated physiological adjustment to shift work during the night, contributing to human health and safety by influencing the human factor.
The experimental diagnostics is supported by numerical simulation and modelling. Despite established technologies and long-time research work, the detailed knowledge about the specific properties and physical mechanisms in the arc plasmas is still missing. This hinders technological innovations. Detailed models, which include precise description of all relevant processes, give the missing information and corresponding knowledge. Continuous improvement of non-equilibrium models, particularly by taking into account departures from thermal, chemical and ionization equilibrium leads to close agreement between theory and experiment and, therefore, to the development of efficient and realistic simulation tools which can be used for design and parameter optimization.
Numerical simulations help to accelerate the development of devices and reduces the corresponding costs.
An improved modelling and new diagnostic methods of arc plasmas and adjacent areas (electrodes, walls, cold gas) leads to a deeper understanding of the physical processes and as a consequence to development of improved switching and welding concepts and operation principles. Increase of the stability and process reliability, as well as the efficiency of the welding process are added values through research.
For further improvement of process efficiency and reliability in arc based technologies, such as, for example arc welding, gas and vacuum circuit breakers detailed knowledge about the arc plasma properties and dynamics is necessary. Special attention must be paid to the regions adjacent to the arc column, like e.g. electrodes and walls, since the interaction of the arc plasma with surrounding material is an important part of every arc based process. The analysis of the physical processes inside the arc column and in the regions where the arc interacts with surrounding materials by means of optical diagnostics and numerical modelling is in the focus of core-funded project.
For the determination of plasma parameters, like e.g. temperature and composition, combination of complementary OES methods, like e.g. analysis of atomic and ionic line radiation is applied as a general solution approach, allowing for the study of broader spatial regions and extended temporal ranges. High-speed camera based tomographic reconstruction is applied for the measurements on non-symmetric arcs. Absorption spectroscopy using appropriate light sources is a suitable method for study of colder plasma regions and arc surrounding gas.
Dr. Sergey Gortschakow
Phone: +49 3834 - 554 3820
In the frames of joint research projects "MOMOS" ("Multiphysics Online/Offline Monitoring System - MOMOS") supported by German Federal Government (BMBF), the partial discharge phenomena in various power network components are studied by means of combined physical and engineering diagnostic methods. The long-term experience in the research of technologically relevant gas discharge plasma forms the basis for deeper understanding of partial discharge phenomena and allows for the development of new diagnostics. Study of synergetic action of various stress mechanisms on the parameters of a single partial discharge is one of the project aims. In particular, the influence of ageing phenomena on the discharge type, properties and activity is under investiagtion. This knowledge will be finally used for improvement of maintenance and increase of safety operation of modern power network.
As the result, a concept of novel monitoring system for status evaluation and lifetime estimation of various power network components during their operation has to be developed. Such a system can be used, for example, for monitoring of offshore parks components via remote data transfer and analysis.
Dr. Sergey Gortschakow
Phone: +49 3834 - 554 3820
The project is aimed at the description of the interaction of arcs and electrodes in arc welding processes. In particular, the establishment of the arc roots and the sheath voltages as well as the resulting energy transfer to the electrodes should be determined by a model for the examples of a tungsten inert gas (TIG) process and a gas metal arc welding (GMAW) process. Main emphasis will be put on the impact and the proper description of the metal evaporation at the electrodes.
The project work is focused on the development of a non-equilibrium model for argon-iron vapour mixtures which enables a continuous description of the arc plasma from the centre where it is near to the local thermodynamic equilibrium (LTE) state to the regions in front of the electrodes where distinct deviations occur from the thermal, chemical and ionisation equilibrium. Based on a magneto-hydrodynamic multi-fluid simulation of the plasma and the heat and current balances of the electrodes including the emission mechanisms at the electrode surfaces the the arc root structure and the metal evaporation is determined in a self-consistent description way.
The results of the simulations should provide a significantly improved understanding of the arc root structures in TIG and GMAW processes as well as improved relations of arc length, current and total voltage which can be used for further studies among others for the welding process control.
Prof. Dr. Dirk Uhrlandt
Phone: +49 3834 - 554 461
The regulation of the arc length in gas metal arc welding is an important part of control concepts in many welding power sources to adjust a stable welding process. Up to now, estimations and statistical values from the current-voltage characteristics are used as input for the arc length determination. Here, often for simplicity a one-dimensional arc model is assumed where mainly the arc column length can be vary whereas variations in the arc sheath regions, wire electrode and contact tube are neglected. However, in particular the arc sheath regions must be expected to contribute considerably to the power input and to cause additional voltage fluctuations.
The detailed experimental study of the current path from the contact tube over wire, droplet, arc including sheathes up to the work piece is aimed in the research project. The different parts of this path are analysed separately by specific experimental techniques as well as by determining and using the different time scales of the underlying physical mechanisms. A description of the current path in form of a process model will be available as a result of the basic study which describes the dynamic and interdependent behaviour of the subsystems. Herewith the prerequisites for a considerable improvement of the arc length control in gas metal arc welding processes in contrast to existing solutions will be provided.
Prof. Dr. Dirk Uhrlandt
Phone: +49 3834 - 554 461
High voltage circuit breakers are essential safety elements in power grids. Self-blast circuit breakers represent the state of the art. Here the pressure build-up in a heating volume, necessary for arc quenching, is attained by the ablation of nozzle material due to arc radiation. One focus of circuit breaker development is the substitution of the strong greenhouse gas sulphur hexafluoride by carbon dioxide. The time interval of the current zero crossing and the period immediately after current interruption are of high importance for the interruption performance of the circuit breaker. Physical effects such as flow reversal in the heating channel, translation from an ablation-controlled to an axially blown arc, the extinction of the arc and a continued evaporation of nozzle material after current zero cannot be quantitatively determined by existing models.
The deeper understanding of these effects, their transient behavior and their effects on the dielectric recovery of the gap between the contacts after current zero, as well as the complete modeling based on circuit breaker simulations are the main intention of this work. The working approach comprises the experimental investigation of a circuit breaker model with peak currents in the range of some kilo amperes measuring the spatially resolved distribution of the arc resistance over the complete time range, analyzing the radiation of the electrical arc by optical emission spectroscopy and analyzing the nozzle evaporation after current zero by absorption. In addition, the gap between the contacts is simulated including the quantification of the nozzle ablation. The simulation results (temperature, density, gas composition) finally serve as input data for the modelling of the dielectric strength and the spatially resolved characteristics of the breakdown path.
Prof. Dr. Dirk Uhrlandt
Tel.: +49 3834 - 554 461
Electrical contact opening discharges are a potential ignition source for flammable gas mixtures. Electrical equipment operated in potentially explosive atmospheres, which may generate discharges on contact interruption, must therefore comply with safety requirements, such as those for "intrinsic safety" set out in international standard IEC 60079-11. An assessment for the safe use of electrical equipment in explosive atmospheres can be performed with the so-called "Spark-Test-Apparatus". In this device electrical discharges are generated in a hydrogen-air gas mixture, but the results are scattered and poorly reproducible. For the development of an alternative to the "Spak-Test-Apparatus" with better reproducible results, the physical mechanisms of the electrical contact opening discharges are analyzed and a multiphysical model is developed.
The project is carried out as a cooperation of PTB Braunschweig, TU Ilmenau and INP. INP is involved with its expertise in the diagnostics and modeling of plasmas.
Dr. Steffen Franke
Tel.: +49 3834 - 554 3839
Fast DC hybrid circuit breaker for use in industrial and on-board networks for system integration of renewable energies and for energy recovery from electrical drives
The current restructuring of the German energy supply is dependent on the increasing use of renewable energy sources. This is currently leading to an increasing use of combined AC / DC network structures. In addition to the integration of decentralized, time-fluctuating plants and systems, multidirectional power flows must be efficiently fed into industrial networks and distributed in such networks. The aim of the "AutoHybridS" research project is to secure the demand-oriented distribution of energy on bus systems with nominal voltages of up to 850 V DC. For this purpose, extremely quick-release, inexpensive, autonomous switching devices for maximum system stability and security of supply are developed and tested. These should not only ensure a safe and selective connection and disconnection of the electrical consumers, but also enable the integration of renewable energy sources and storage easily and flexibly.
Dr.- Ing. Diego Gonzalez
Tel.: +49 3834 - 554 3859