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Piotr BORKOWSKI ARC EROSION OF CONTACTS IN ELECTRICAL SWITCHES
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Electrical switches controlled by electrical energy flow are indispensable and especially important for successful performance of their assumed functions (supplying electrical energy by switching and breaking current) in electrical circuits used in power engineering and industrial automatics, mining and metallurgy, in the railroad and ship industry, as well as in communal flats. Among these switches, the most widely used ones are disconnectors and especially electrical contactors in which the functions of breaking and closing an electrical circuit are performed by switching contacts – i.e. two or more mating contacts of which at least one is movable.
One of the main elements of a disconnector are switching contacts. The reliability of operation of power systems and broadly conceived automatics depends on switching contacts quality and proper functioning in a switch in different operating conditions – especially in circuit overload conditions.
The operation of contact switches, their switching capacity, as well as contact life and operational reliability are mainly determined by a switching arc which appears in contactors during switching operations and by one of its damaging effects – contact erosion. Contact erosion results from many thermal phenomena that take place in a contactor system during the processes of current switching in an electrical circuit. It already starts immediately before the opening of contacts (bridge erosion) and during the arc stage (arc erosion). In some cases, erosion processes still occur after the switching arc is extinguished (“post-arc” erosion). In the case of switches used in power engineering, contact life is mainly determined by arc erosion which is the subject of the monograph.
During the operation of a switch in steady and transient states, various physical, chemical and electromechanical phenomena occur in contacts and determine its rated parameters. Contacts have to be resistant to such phenomena as heating and temperature increases leading to the ageing of isolation (switch life limitation), contact welding, the already mentioned contact wearout due to an electric arc action (arc erosion), mechanical abrasion, oxidation, electromechanical corrosion and others. Therefore, the quality of material which is used to produce contacts or only their operating elements (contact tips and rivets) is very important.
Rated parameters of electrical contacts defined by a manufacturer relate to predicted operating conditions of a given switch. However, circuit and environmental conditions that often occur in the process of switch operation differ (sometimes significantly) from those whose parameters given in technical information (the catalogue) are defined as rated. An electrical circuit designer is forced to make his or her own decision on what switch to use in order to obtain its high reliability in the conditions in which it will operate. It is not an easy decision to make – it requires a lot of knowledge and professional experience.
Satisfying various demands placed by electrical switch designers is practically impossible when basic contact materials, such as copper or silver are used. Silver is characterized by the highest electrical conductivity, thermal conductivity and heat capacity. However, its low melting and boiling points result in low resistance to electric arc operation. Other metals, such as tungsten and molybdenum have high melting and boiling points, are resistant to electric arc operation, but are also characterized by low electrical conductivity. Therefore, Ag-Me silver-based composite materials such as AgW, AgMo, AgNi, CuW and CuMo have been used in practice for many years, both with and without small percentage additions of other metals. Also Ag-MeO silver-metal oxide composites (AgCdO, AgSnO2, AgFe2O3) have been widely used. In the case of switches used in circuits in which high short-circuit currents occur, silver-tungsten (AgW) and silver-tungsten carbide (AgWC) composites as well as their derivatives obtained due to modifications with other metals or carbon are of greatest technical significance. For certain types of switches, AgC composites are of special importance due to their high resistance to welding. A characteristic feature of silver-tungsten composites is high dependence of their electrical properties on their manufacturing method. It is possible to use many different technological solutions here and research has continuously been conducted in the field by leading contact manufacturers.
In low-voltage switching apparatus used for basic applications (shunting), where high resistance to arc erosion during breaking high currents is required, there are now commonly used silver-metal and silver-metal oxide composite materials, containing 90% of silver. In low-voltage industrial and installation switches, tungsten-silver composite materials having a skeleton structure and containing 25-60% of silver prove to be most suitable.
Demands placed by switch users on contact materials manufacturers are varied and constantly increasing, which makes it necessary to conduct ongoing research and development in the field of materials engineering. Since the problems presented above are interdisciplinary, it is very important to cooperate with research and development centers dealing with electrical switches (apparatus) – especially with experienced ones where it is possible to conduct research in the area of determining electrical properties of contact materials. The Institute (at present: Department) of Electrical Apparatus of Lodz University of Technology has been conducting research in the field of contact properties for many years. Due to this fact, there exist test stands (only such stands in Poland) for comparative tests and the assessment of switching properties of contact materials in model conditions.
The problem of qualitative and quantitative determination of erosion in contacts during breaking work currents and short-circuit currents, typical for low-voltage switches, has not yet been sufficiently examined and presented theoretically. The present state of knowledge of these problems makes it difficult to fully determine the quantitative and qualitative influence of all physical quantities and factors that determine contact erosion in switches. That is why, apart from the presentation and analysis of research results obtained by different authors, this monograph presents the author’s own research providing a basis for better and more conscious actions in the areas of switch construction, development of new contact materials, as well as research in the field.
The monograph presents a complex elaboration of the phenomenon of arc erosion of contacts in electrical switches, for a wide range of currents, both based on the analysis of literature data and the author’s own research.
An analysis of chosen phenomena in an electric arc foot has been conducted, as well as theoretical thermal models of an electric arc, methods of estimating energy voltage losses on contacts and methods of determining the characteristics of short arc voltage have been presented. There have also been presented physical and mathematical models of contact erosion, as well as contact erosion in different physical states of the material used.
On the basis of physical, thermal and mathematical models, there has been proposed a mathematical model of a thermal short arc. The model makes use of geometric solids having concentrated heat sources and is described by equations relating to power values supplied to the anode and cathode. The proposed geometric thermal models of a short arc burning between contacts make it possible to conduct a computer simulation of thermal power (variable in time) supplied to contacts and to determine the relationship between thermal power supplied to the anode and to the cathode. It is the first such mathematical elaboration in the field.
The monograph also proposes modeling contact erosion with the use of computer simulation and an analysis of the process of heating, melting and evaporation of contact material. There has been proposed a new approach to calculating contact mass loss, making use of two calculation methods:
One of the main elements of a disconnector are switching contacts. The reliability of operation of power systems and broadly conceived automatics depends on switching contacts quality and proper functioning in a switch in different operating conditions – especially in circuit overload conditions.
The operation of contact switches, their switching capacity, as well as contact life and operational reliability are mainly determined by a switching arc which appears in contactors during switching operations and by one of its damaging effects – contact erosion. Contact erosion results from many thermal phenomena that take place in a contactor system during the processes of current switching in an electrical circuit. It already starts immediately before the opening of contacts (bridge erosion) and during the arc stage (arc erosion). In some cases, erosion processes still occur after the switching arc is extinguished (“post-arc” erosion). In the case of switches used in power engineering, contact life is mainly determined by arc erosion which is the subject of the monograph.
During the operation of a switch in steady and transient states, various physical, chemical and electromechanical phenomena occur in contacts and determine its rated parameters. Contacts have to be resistant to such phenomena as heating and temperature increases leading to the ageing of isolation (switch life limitation), contact welding, the already mentioned contact wearout due to an electric arc action (arc erosion), mechanical abrasion, oxidation, electromechanical corrosion and others. Therefore, the quality of material which is used to produce contacts or only their operating elements (contact tips and rivets) is very important.
Rated parameters of electrical contacts defined by a manufacturer relate to predicted operating conditions of a given switch. However, circuit and environmental conditions that often occur in the process of switch operation differ (sometimes significantly) from those whose parameters given in technical information (the catalogue) are defined as rated. An electrical circuit designer is forced to make his or her own decision on what switch to use in order to obtain its high reliability in the conditions in which it will operate. It is not an easy decision to make – it requires a lot of knowledge and professional experience.
Satisfying various demands placed by electrical switch designers is practically impossible when basic contact materials, such as copper or silver are used. Silver is characterized by the highest electrical conductivity, thermal conductivity and heat capacity. However, its low melting and boiling points result in low resistance to electric arc operation. Other metals, such as tungsten and molybdenum have high melting and boiling points, are resistant to electric arc operation, but are also characterized by low electrical conductivity. Therefore, Ag-Me silver-based composite materials such as AgW, AgMo, AgNi, CuW and CuMo have been used in practice for many years, both with and without small percentage additions of other metals. Also Ag-MeO silver-metal oxide composites (AgCdO, AgSnO2, AgFe2O3) have been widely used. In the case of switches used in circuits in which high short-circuit currents occur, silver-tungsten (AgW) and silver-tungsten carbide (AgWC) composites as well as their derivatives obtained due to modifications with other metals or carbon are of greatest technical significance. For certain types of switches, AgC composites are of special importance due to their high resistance to welding. A characteristic feature of silver-tungsten composites is high dependence of their electrical properties on their manufacturing method. It is possible to use many different technological solutions here and research has continuously been conducted in the field by leading contact manufacturers.
In low-voltage switching apparatus used for basic applications (shunting), where high resistance to arc erosion during breaking high currents is required, there are now commonly used silver-metal and silver-metal oxide composite materials, containing 90% of silver. In low-voltage industrial and installation switches, tungsten-silver composite materials having a skeleton structure and containing 25-60% of silver prove to be most suitable.
Demands placed by switch users on contact materials manufacturers are varied and constantly increasing, which makes it necessary to conduct ongoing research and development in the field of materials engineering. Since the problems presented above are interdisciplinary, it is very important to cooperate with research and development centers dealing with electrical switches (apparatus) – especially with experienced ones where it is possible to conduct research in the area of determining electrical properties of contact materials. The Institute (at present: Department) of Electrical Apparatus of Lodz University of Technology has been conducting research in the field of contact properties for many years. Due to this fact, there exist test stands (only such stands in Poland) for comparative tests and the assessment of switching properties of contact materials in model conditions.
The problem of qualitative and quantitative determination of erosion in contacts during breaking work currents and short-circuit currents, typical for low-voltage switches, has not yet been sufficiently examined and presented theoretically. The present state of knowledge of these problems makes it difficult to fully determine the quantitative and qualitative influence of all physical quantities and factors that determine contact erosion in switches. That is why, apart from the presentation and analysis of research results obtained by different authors, this monograph presents the author’s own research providing a basis for better and more conscious actions in the areas of switch construction, development of new contact materials, as well as research in the field.
The monograph presents a complex elaboration of the phenomenon of arc erosion of contacts in electrical switches, for a wide range of currents, both based on the analysis of literature data and the author’s own research.
An analysis of chosen phenomena in an electric arc foot has been conducted, as well as theoretical thermal models of an electric arc, methods of estimating energy voltage losses on contacts and methods of determining the characteristics of short arc voltage have been presented. There have also been presented physical and mathematical models of contact erosion, as well as contact erosion in different physical states of the material used.
On the basis of physical, thermal and mathematical models, there has been proposed a mathematical model of a thermal short arc. The model makes use of geometric solids having concentrated heat sources and is described by equations relating to power values supplied to the anode and cathode. The proposed geometric thermal models of a short arc burning between contacts make it possible to conduct a computer simulation of thermal power (variable in time) supplied to contacts and to determine the relationship between thermal power supplied to the anode and to the cathode. It is the first such mathematical elaboration in the field.
The monograph also proposes modeling contact erosion with the use of computer simulation and an analysis of the process of heating, melting and evaporation of contact material. There has been proposed a new approach to calculating contact mass loss, making use of two calculation methods:
- calculating contact mass loss with the use of the ANSYS package, based on the analysis of temperature field distribution variable in time and space, created by heat input of an arc in contacts;
- calculating on the basis of the UMS (Ubytek Masy Styku – Eng. Contact Mass Loss, CML) computer program, which makes it possible to conduct a quantitative analysis of electrode mass loss. A deduction module integrated into the UMS program presents solutions to be evaluated by a user with reference to their usability. The user may reject a solution proposed by the program and demand another one, suitable for the given input conditions.
The monograph presents numerous instances of the author’s own research, as well as research conducted by other authors on the influence of current, time, arc voltage and energy, contacts shape
and dimensions, contact material and other parameters on contact erosion in electrical switches. There is also presented broad research (so far not described in the literature) on transport and migration of eroded contact material. Contact erosion and degradation after one breaking operation are qualitatively different from erosion and degradation after more breaking operations. Among others, this results from the fact that products obtained as a result of erosion after one such operation do not overlay, unlike in the latter case.
The mechanism of erosion of contacts made of silver-tungsten composite materials described in the last chapter makes it possible for contact materials designers to make more conscious and purposeful choices on composite composition and structure. Consequently, it also allows them to make such choices on the manufacturing technologies of composites functioning as contact materials for switches that will be used for specific applications and in specific operating conditions. Such contact materials give the possibility of constructing electrical apparatus characterized by better functional properties, as well as increasing continuous and short-circuit current values, and extending contact life. The obtained analysis and research results make it possible to understand complex erosion phenomena better.
The monograph can be used by research workers, students and engineers dealing with the problems of analysis, design and operation of electrical switches equipped with a contact system, as well as conducting research in the field. The subject of the monograph is both of high scientific significance, as well as application significance for switch designers and contact materials manufacturers. It is also important for the process of making operation of switches more rational and, as a result, increasing the reliability of circuits in which such switches are used. The monograph will also prove useful for the purposes of university teaching.
and dimensions, contact material and other parameters on contact erosion in electrical switches. There is also presented broad research (so far not described in the literature) on transport and migration of eroded contact material. Contact erosion and degradation after one breaking operation are qualitatively different from erosion and degradation after more breaking operations. Among others, this results from the fact that products obtained as a result of erosion after one such operation do not overlay, unlike in the latter case.
The mechanism of erosion of contacts made of silver-tungsten composite materials described in the last chapter makes it possible for contact materials designers to make more conscious and purposeful choices on composite composition and structure. Consequently, it also allows them to make such choices on the manufacturing technologies of composites functioning as contact materials for switches that will be used for specific applications and in specific operating conditions. Such contact materials give the possibility of constructing electrical apparatus characterized by better functional properties, as well as increasing continuous and short-circuit current values, and extending contact life. The obtained analysis and research results make it possible to understand complex erosion phenomena better.
The monograph can be used by research workers, students and engineers dealing with the problems of analysis, design and operation of electrical switches equipped with a contact system, as well as conducting research in the field. The subject of the monograph is both of high scientific significance, as well as application significance for switch designers and contact materials manufacturers. It is also important for the process of making operation of switches more rational and, as a result, increasing the reliability of circuits in which such switches are used. The monograph will also prove useful for the purposes of university teaching.