Electrodynamic Forces in a High Voltage Circuit Breakers With Tulip Contact System—FEM Simulations

Paper concerns the effects of electrodynamic forces that act on the contacts of the tulip contact system that is often implemented in high voltage circuit breakers. The high voltage circuit breaker often consists of two such systems. One of the systems is treated as an arcing one - made of tungsten coated elements. Capable of implementing the phenomenon of thermal-expansion. The second is made of one or two crown laces. The first system consists of a single piece of large mass, cut in such a way as to obtain the effect of increasing the contact surface. The second is a system, often of several dozen contacts, so as to increase the contact area and reduce the transition resistance. The main problem of actual validation through dynamic measurements (electrodynamic forces) is the specificity of the circuit breaker operation. The contact system is located directly in the switch chamber filled with CO2 or SF6 gas. Hence, tests under normal working conditions are very difficult - even impossible. Therefore, the authors proposed employment of FEM (Finite Element Method) in order to obtain values of electrodynamic forces acting on the contact system by executing the detailed 3D coupled simulation. The analysis of the results brought interesting conclusions that concerned operation of such contact layouts in short circuit conditions.

When performing analytical calculations concerning the 65 determination of electrodynamic contact bouncing forces that 66 result in contacts abrupt repulsion, the main difficulty is 67 determining the actual points of conductors contact. It is often 68 the case that there are many zones of contact, which results 69 in the formation of several parallel densities of the current 70 lines as in the Figure 1 above. As a result, the electrodynamic 71 contact bounce force is reduced. In the case of a single point 72 of actual contact between the conductors surfaces, the force 73 value can be determined from the dependency: where: i is instantaneous current value; D is outer diameter of 76 the contact and a is radius of the actual contact surface.

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In the case of contacts with a cross-section other than 78 cylindrical, e.g. rectangular, of the equivalent diameter (D) 79 the circular cross-section is determined. An example may be 80 rectangular contact strips for which in practice the equivalent 81 diameter D is determined. In the case of frontal loop contacts, 82 apart from the F ez force, there is also a loop electrodynamic 83 force F ep resulting from the loop shape of the contact, which 84 additionally increases the effect of electrodynamic bounce. 85 In some constructions it is used to accelerate the opening 86 velocity of moving contacts during short-circuit currents. 87 An example of this type of solution with a contact system for 88 a low voltage circuit breaker is shown in the Figure 3 below. 89   109 In order to determine the value of the electrodynamic force 110 for the contact system, a 3D model was made for numerical 111 analysis. The model was made for one actual point of sur-112 face contact. Initial stage of model geometry was shown in 113 Figure 4 below.

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After preparing the model and loading it into the Ansys 115 Maxwell 3D environment, it was possible to assign boundary 116 conditions, assign material properties and impose a current 117 of 6 kA on the analyzed contact, in this case without a non-118 periodic component. Waveform of the short-circuit current in 119 a single-point contact used during this analysis was shown in 120 Figure 5 below.

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The computational simulation of the short-circuit current 122 flow through the contacts allowed to determine the current 123 density at the actual point of contact and it was shown below 124 in Figure 6.

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As can be seen from the waveform in Figure 7 below, the 126 interaction of the components in the Y direction is responsible 127 for the repulsion of one contact from the other. In order to 128 illustrate this phenomenon even better, the Y components of 129 the two contacts are summarized on the waveform below. 130 The presented forces in this case can lead to electrodynamic 131 bounces in the case when the spring pressing the moving con-132 tact has a lower value than the sum of the forces presented as 133 in equation (3) above and in the waveform in Figure 8 below. 134 In order to determine the vectors of the generated elec-135 trodynamic forces in the analyzed single-point contact, the 136 results of calculations from Maxwell 3D were imported to 137 the Ansys Transient Structural calculation module. On the 138 basis of the performed analyses, it was possible to generate 139 the values of the electrodynamic force vectors, which reached 140 the highest value in the contraction of the contact, with the 141 highest short-circuit current density.

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Two components of electrodynamic forces can be dis-143 tinguished from the analysis of electrodynamic forces in a 144 contact system layout presented above:   The occurrence of electrodynamic forces in the con-154 tacts of electrical devices is a generally unfavorable phe-155 nomenon, which significantly reduces the contact pressure 156 force, or at high values of the short-circuit current it leads 157 to their abrupt opening. In order to prevent this phenomenon, 158 electrodynamic force compensation systems are used. This 159 is done by increasing the contact pressure force by applying 160 the special cuts in the current paths or appropriate shaping of 161 the current path inside an electrical apparatus, e.g. a circuit 162 breaker. While current is flowing, forces are pressing the 163 movable contacts or any contact elements to the fixed con-164 tact, compensating the electrodynamic bounce force. In tulip 165 contacts, due to the uniform distribution of the contacts in 166 relation to each other, with unidirectional current flow, the 167 contacts are pressed against the pin, which in this case is a 168 favorable phenomenon.

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Due to the flow of short-circuit currents, electrodynamic 172 forces influence in the contacts may result in bouncing and 173 erosive changes on the contact surfaces. During the opening 174 of the contacts, bridges of the molten metal may form, which 175 may lead to permanent grafting of the connector contacts 176 while the contacts are closing.

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During the closing of the electrical device contacts, the 178 movable contact collides with the fixed contact. The mov-179 able contact of the apparatus is influenced by the forces 180 from the pressure spring of the connector, electrodynamic 181 forces and forces from the generated intra-arcuate pres-182 sure (directed against the forces coming from the connec-183 tor drive). After the contacts collide, the kinetic energy is 184 converted into potential energy in the form of elastic stress 185 in the movable contact of the switch, and then the poten-186 tial energy is again converted into kinetic energy of the 187 FIGURE 7. Determined X , Y and Z components of electrodynamic forces in a contact with one actual point of contact.     Mechanical models for studying the phenomena of electro-223 dynamic bounce as shown in Figure 11. Below.

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Those can be used to study impact phenomena or to deter-225 mine the coefficient of impact for a certain material.
In the designed contact, the clamping force value of the 227 movable contact should be selected in such a way that in the 228 case of the peak current of the connector, the spring pressure 229 forces are greater than the expected electrodynamic force.

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In order to analyze the kinematic system that checks the time 231 and amplitude of electrodynamic bounces, various contact 232 models with specific degrees of contacts freedom are used, 233 FIGURE 11. Shaw's contact bouncing control mechanism [5].
a model with two degrees of freedom will be reflected and 234 will vibrate, causing bounces of smaller amplitude in turn, 235 until those stop.  adynamic shaping shown in Figure 12: FIGURE 12. Setting the contact surfaces at an angle and electrodynamic compensation by means of adynamic shaping of the contact. Figure 13:

Contact sharing shown in
3. Electrodynamic compensation by dividing one con-251 tact into more parallel contacts (tulip contact) shown 252 in Figure 14: shown in Figure 15:  That was shown in Figure 16: In circuit-breakers with high current-carrying capacity of 265 contacts, the system shown in Figure 16 is often used, favorable than in a typical contact. The circuit shown in 271 Figure 12 has additional, more advantageous features due to 272 the adynamic shape of the contact allowing for additional 273 electrodynamic compensation of the forces that push the 274 contacts away from each other. For contactors and circuit 275 breakers, the example shown in Figure 13 is used, which is 276 employed to separate the flowing current by using several 277 parallel contacts. In this case, the arising electrodynamic 278 forces in the contact have smaller values because the cur-279 rent value in a given contact is lower. Example shown in 280 Figure 16 is found in medium voltage circuit breakers which 281 are equipped with tulip contacts. Evenly spaced contacts are 282 pressed against the pin by compression springs. In this case, 283 during the flow of rated or short-circuit currents, the contact 284 pressure force through the pressure springs is increased by the 285 additional action of electrodynamic forces F ek compensating 286 the electrodynamic forces F ez acting at the contact point of 287 the contact system.

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The above-mentioned theoretical methods of the compen-289 sation of electrodynamic forces in the contacts of electric 290 devices were used by the authors in order to perform simu-291 lation calculations to verify the presented methods.

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For this purpose, a 3D model presenting the geometry of 293 the system 5 ( Figure 16) was made, where the working sur-294 faces of the contacts were positioned at an angle α in relation 295 to the direction of interaction of contact forces, in accordance 296 with the theoretical assumptions. After importing the model 297 to the software, the boundary conditions were assigned and a 298 simulation for a short-circuit current of 40 kA was performed. 299 After the computational simulation was performed, the 300 current density distribution in the analyzed contact system 301 was derived. The highest values of the short-circuit cur-302 rent density were found at the contact points, as shown 303 in Figure 17 below.

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The determined values of the electrodynamic forces in the 305 executed simulation reached the average of 10 -40 N/m 3 . 306 The maximum value achieved was witnessed in the right con-307 tact of the current path shown and it amounted to 50 N/m 3 . 308 This was shown in Figure 18 below.

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The performed calculations confirmed the presented return 310 of the electrodynamic forces F ez acting in the contact, 311 which at the same time confirms the thesis that the angular 312 arrangement of the contacts allows for the reduction of the 313 VOLUME 10, 2022 contacts of the contact crown was prepared. In this case, 328 the generated force pressing the tulip lamellas against the 329 arcing contact and the forces pushing the contacts at the 330 point of contact with the pin were analyzed. In order to 331 determine the interaction of electrodynamic forces in the tulip 332 contact, a short-circuit current of 40 kA was passed through 333 the contact along with the non-periodic component of the 334 current raising it to 50 kA in the initial phase of its duration. 335 The obtained waveform is shown below in Figure 20. The 336 voltage level value for those simulations was set at 72,5 kV. 337 For this voltage, the device can be filled with CO 2 or SF 6 gas. 338 Of course, the extinguishing medium determines the current 339 carrying capacity. And so for CO 2 gas this load capacity is, 340 for example, 2750 A, and for the same cross-sections when 341 filled with SF6 gas it is equal to 4000 A. Undoubtedly, it is 342 related to the ability to receive energy from the electric arc 343 and deionizing the contact gap. The key decision related to 344 the selection of the contact system for research and analysis 345 was the trend:

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• restrictions on the use of SF 6 gas,

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• developing the process and mechanism of thermal 348 exposure to the next generations,

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• research on the use of CO 2 gas and its mixtures and also 350 other gases.

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Current flowing through the tulip contact was analyzed, the 352 highest current density was demonstrated at the contact points 353 between the lamellas and the contact pin inserted into the 354 crown formed by the system. Additionally, the distribution of 355 the short-circuit current density flowing through the contact 356 is presented in the Figure 21 above.     of the designed device is defined by the peak value of the 395 current switched off, for example, by a high-voltage switch 396 without depriving it of its ability to be used further (it is not 397 causing its damage).

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One of the most important high voltage apparatuses are 399 circuit breakers. Those allow for switching off fault currents, 400 e.g. short-circuit currents in a protected circuit after receiving 401 a signal from the protection automatics in order to start the 402 circuit breaker drive. An example of the design of a circuit-403 breaker with three poles mounted on a common supporting 404 beam with a common operating mechanism is shown below 405 in Figure 24.

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Development of the structure and production of a high-407 voltage circuit breaker requires from the manufacturer not 408 only constructional knowledge and ''know-how'', but also 409 a large technical base and machinery park. Global manu-410 facturers such as ABB, SIEMENS or ALSTOM produce 411 overhead circuit breakers for voltages up to 300 kV and 412 for currents up to 50 kA in a single-break version, i.e. 413 with one set of connection chambers for a particular pole 414 of the HV switch. In the case of higher voltages, sys-415 tems with a double set of switching chambers (420 kV) 416 and with a four set of switching chambers (800 kV) are 417 used.

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There are two typical designs of SF 6 gas overhead switches 419 in the circuit breaker design. The first type are the live-420 tank circuit breakers, the connection chamber of which is 421 built into a porcelain or composite insulator at high potential 422 VOLUME 10, 2022   The arcing contacts built into the switching chamber are 449 most exposed to the effects of an electric arc during switching 450 operations. The arcing contact system and the main con-451 tact system in the switching chamber operate in a specific 452 switching sequence. When switching off e.g. fault currents, 453 first the main contact of the switch opens without arcing, 454 and then the arcing contact opens, taking over the erosive 455 processes related to the action of the electric arc for which 456 it is adapted (made of an alloy of tungsten or molybde-457 num with copper). When closing the circuit-breaker, the arc 458 sequence of the arcing contact and the main contact are 459 reversed.

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To create a 3D simulation model of a live tank high volt-  The circuit breaker components have been accurately mea-493 sured and dimensioned. This allowed for the preparation of 494 3D models of individual elements of the high-voltage circuit 495 breaker connection chamber, including: main contacts, arcing 496 contact, nozzle, cylinder and other elements. The prepared 497 3D assembly of modeled elements of the high-voltage switch 498 structure is shown in Figure 28 above and in 499 The 3D modeled simplified structure of the high-voltage 500 circuit breaker chamber made it possible to perform simula-501 tion analyses of electrodynamic forces for the short-circuit 502 FIGURE 29. Modeled 3D assembly of the HV switch according to the executed measurements-cross section.   Table 1 below.

527
After setting the material properties, the model was given 528 current forces in the form of specific short-circuit currents.

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The initial direction of current flow for calculations was   switch, an example of the density distribution in the contact 563 system is shown below in Figure 31. In the case of the 564 simulation analysis performed, particular attention was paid 565 to the forces occurring in the contacts of the main and arcing 566 contacts.       which confirms the lack of information on the study of 675 electrodynamic forces in high voltage contact systems. In the 676 R&D manufacturers departments of contact systems and 677 static breakers, static tests can be found [x]. The works 678 concern only the contact system made of one element, cut 679 in an appropriate manner. Then, outside the circuit breaker 680 chamber, it is possible to conduct tests by pulling out one of 681 the contacts and determining the force statically. It is not a 682 force related to the flow of current. It is a mechanical force 683 obtained through the elasticity of the material, sometimes also 684 by pressure springs.