yahoo.com makkawy_06@ hotmail.com. Abstract The effect of different Mechanical contact devices on the DC Joint resistance of overlapping bolted connection ...
13 th Middle East Power Systems Conference, MEPCON' 2009, Assiut University, Egypt, December 2023, 2009
Joint Resistance of Bolted Copper BusBar Connections as influenced by Mechanical Contact Devices Material and Configuration. Ghareeb Moustafa
Steffen Grossmann
TU Dresden – Germany ghareebmoustafa@ yahoo.com
grossmann@ ieeh.et.tudresden.de
Mazen AbdelSalam
S. S. Dessouky Samir M. ElMakkawy
Assiut University Egypt
Suez Canal University Egypt
mazen2000as@ yahoo.com
Abstract The effect of different Mechanical contact devices on the DC Joint resistance of overlapping bolted connection was investigated experimentally, A thermal network calculations are done and the result power and temperature are compared with that obtained experimentally the agreement between measured and calculated values are quite good. Furthermore, the behavior of the joint resistance of bolted copper joint when loaded by AC current was measured with changing mechanical contact devices shape and material, It was found that use of steel plate significantly increase the AC contact resistance especially if the direction of the plate is normal to current direction. In addition, the temperature rise and power dissipated through the joint with different mechanical contact device was measured.
I. INTRODUCTION The main function of the substation is to provide a point in the electrical system where energy can be tapped from the transmission lines, transformed to lower voltage and directed via busses to switches and circuit breakers for the purpose of either protecting the various circuits in emergencies, or switching circuits according to load, need for maintenance, etc. Furthermore, of the many types of connections used in substations components, overlapping bolted joints are the most widely used. They are versatile, dependable, and economical. There are several factors influencing the performance of an electrical contact. Such important factors are: design, environmental conditions, mechanical and electrical loads, materials and assembling procedures. As the performance demands on electrical networks and plants in general, and electrical contacts in particular, constantly increases, the need for more reliable rules for design and testing of electrical contacts becomes necessary. As for the design factor, a number of more or less important design parameters can be identified, including: contact force, specific contact pressure (i.e. pressure distribution on the contact surface) , contact surface topography (preparation), etc (1).
ssdesouky@ yahoo.com
makkawy_06@ hotmail.com
A welldesigned contact device should have an adequate mechanical strength to maintain the mechanical integrity of a connector under normal and overload conditions of conductor operation. It should also establish and maintain a low contact resistance, thus preventing or minimizing the excessive heating of the joint under overload conditions. In fact, the temperature rise of the joint should not exceed that of the conductor under normal or emergency conditions (2). The simplest type of connection is made using a hand tool and a mechanical connector, most of which use mechanical contact devices such as washers, bolts, screws, etc. These connectors are inexpensive and easily installed but give rise to doubts about their reliability, primarily because of their performance under operating conditions. It is well established that the electrical contact resistance of clean contacts, depends on such parameters as: normal force, contact microhardness, electrical resistivity and surface texture (roughness) Traditionally, surface texture parameters are defined by the variance of the height and the slope of the surface (3). The most significant quantity to assess is the joint resistance to guarantee reliable operation of the joint. The joint resistance should be no greater than limiting value. Such limiting values do not exist at present. Furthermore, it is known that the life time of an electrical joint will be determined significantly by the joint resistance R0 after assembly (before aging) (4). The quickness of ageing depends on the performance factor k which equal the joint resistance divided by the resistance of an unjointed bar of the same length, it has proven that maximum initial performance factor must be equal or less than 1.5 to have a life time 25…30 year for bolted joint (5) (6). Previous studies on the effectiveness of different mechanical contact devices (2) have shown that the use of
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discspring (Belleville) washers combined with thick flat washers assures the most satisfactory mechanical stability of a bolted joint under stressrelaxation and currentcycling conditions. The same combination was also found to be the most effective in reducing the deleterious effects of thermoelastic ratcheting on the mechanical integrity of bolted aluminumtoaluminum connections (7). Recently the effect of Shape Memory Alloy (SMA) Belleville Washer on the contact resistance was studied (8). Distributions of temperature in electrotechnical devices and plant as well as thermal transfer between the parts of device and from device to the environment can be calculated by means of thermal networks (9) (10).
3. Solvent cleaned to remove grease and abrade the surface with a steel wire brush.
Figure (1a) plate and twin nut direction perpendicular to current direction assembly number 1, 2, 5, 6, 7 and 8 table (1).
This paper is discussing whether special type of mechanical contact device has any influence on a contact resistance of a bolted joint copper busbar. At the same time, a comparative study between the different type of the joint cleaning and its effect on contact resistance during tightening and releasing was made. Moreover, the power dissipated through the joints with different mechanical contact devices is measured. The experiments were carried out in Institute of Electrical Power Systems and High Voltage Engineering of the Dresden University of Technology Germany
Figure (1b) plate and twin nut direction parallel to current direction assembly number 9 table (1). Figure 1: copper busbar and test object assembly.
. Plate stainless steel
II. EXPERIMENTAL DETALS
Plate steel S52
A. Copper Busbars All the tests in this study were performed using copper busbars from SE CuF25 ( Figure 1) measuring 80 mm by 10 mm by 180 mm long, delivered from SIEMENS to TUD Dresden –Germany; the two busbares are connected by four bolt arranged in rectangular shape in an overlap distance of 90 mm Figure 1 .
B. Figure 2) 1.Stainless steel Plate, KLTPlate 76X38x2,5; X5CrNi1810 2. Normal steel Plate, 76X38x2,5 3.Washer: A12, A270, ISO 7019 4.Single nut: Hexagonal ,A270, ISO 4017 5.Stainless steel Twin Nut, M12/62x25x14; X5CrNi1810 6.Normal steel Twin Nut , M12/62x25x14 7.Screw: M12x55 Hex, A270, ISO 4017 8.Screw: M12x55 Hex, Normal steel , ISO 4017 The contact surfaces of the connectors intended for the contact resistance measurements were cleaned used three different methods for cleaning before measuring the joint resistance with increasing and decreasing torque 1.Solvent cleaned by alcohol. 2.Solvent cleaned and polished the surface by fleece sheet.
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Washer
Twin nut stainless steel
Single nut
Twin nut steel S52
Figure 2: Photograph for the Mechanical Contact Device.
Table 1: test number, D.C and first ac Joint resistance reading.
C. DC Joint resistance measurement The DC contact resistance was measured by a micro ohmmeter MO2 50 (manufacturer: Rasmus, Germany). D. AC Joint Resistance Measurement The joint was connected as shown in Figure 3, for different joint assemblies in Table 1 operating under the same environmental and loading condition of 50 Nm. The applied current was 2300 A to 2350 A. The temperature of the busbars was measured by ALMEMO 2590 (manufacturer: AHLBORN), until it reaches the steady state temperature. Moreover, the joints resistance and power dissipated were also measured by a SinglePhase Precision Power Analyzer LMG95 (manufacturer: HOTEK TECHNOLOGY) to eliminate the effect of magnetic field produce by high current on the measuring value, Measuring terminal of power analyzer must be crossed as shown in Figure 3. Before connected the circuit, all contacting surfaces of the copper busbars were prepared following the same procedure. III. RESULTS Figure 4 shows a comparison between DC joints resistance and Initial AC Joint resistance at the same contact force. It can be seen that the initial AC contact resistance of the copper joint with Plate and twin nut from normal steel is higher than the AC value for other connection assemblies.
Figure 4: comparison between DC joint resistance and initial AC joint resistance.
B. Effect of alternating current on the joint resistance Figure 6 shows the relation between joint resistance and joint temperature for all tests. It is clear from the figure that using normal steel plate has a great effect on joint resistance especially when used with normal steel nut, this can be ascribed to the eddy current produced on normal steel plate when subjected to AC current. This cause increasing in joint resistance and increasing in power dissipated as shown in Figure 7 . If we compare the result obtained from assembly 3, 4 and 5, 6 and 7, 8 we can observe that changing screw material has no effect in AC or DC joint resistance. Figure 3: Circuit diagram for AC test circuit.
Table 1 : test number, D.C and first ac Joint resistance reading. A. Relation between joint resistance and joint force Effect of increasing and decreasing torque on the joint resistance is assessed as shown in Figure (5) where the cleaning method was changed as explained previously. This figure shows that at the same joint force the joint resistance is always higher during tightening than during releasing this behavior is known as hysteresis of the joint resistance. Also it is clear from Figure 5 that the Joint resistance does not depend on joint torque at torque greater than 20 Nm for polished and abraded joint.
If the direction of the steel plate was changed (assembly number 9 figure (1b)) to be in the same direction of the passed current the joint resistance and power dissipated will decreased when compared with that when the plate direction is perpendicular to the current direction. C. Power Dissipated through the joint The power dissipated through the special (normal steel) and normal copper joints was measured by Power analyzer. Experiment was carried out two times the first one is to measure the power dissipated at 20 o C ( Figure 8 ) and the second one is to measure the power dissipated when current flows for about 2 hours ( Figure 10). From this figure, it can be seen that the power dissipated and the temperature rise
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through the normal steel contact devices copper joint is higher in comparison to the other type.
Figure 5: Relation between joint resistance and joint torque at assembly No.3.
Figure 8 : Power dissipation versus AC current for different joints at T= 20 o C.
IV. THERMAL N ETWORK
Figure 6: Relation between joint resistance and temperature raise at different Mechanical contact Devices
A. Simulation concept The Thermal Network Method (TNM) is based on a substitution of an arbitrary 3D geometry by a circuit consisting of thermal resistances, capacitances and heat sources. For such a network the currents correspond to heat flow and the nodal potentials to temperatures. Due to similarity of mathematical formulations the electrical circuit programs can be used to obtain a solution. The basic advantage of the thermal network analysis is the fast computation time as steady state computations of large models can be performed within a few seconds. Therefore, the TNM is very suitable for parameter studies and become popular as a tool supporting the industrial design (11) (12) . The thermal network is modeled by an equivalent electrical and thermal variables and calculation equations. Where are reported in Table 2 Table 3 respectively. Table 2: relation between electrical and thermal flow field Electrical domain Voltage V [V] Current I [A] Electrical Resistance R [Ω] Electrical Capacitance
C [F]
Thermal domain Temperature T [°C] Heat flow P [W] Thermal resistance Rth [K/W] Thermal Cth [J/K] Capacitance
By using the network simulation program PSPICE with the corresponding thermal model libraries and calculation equations Table 3 for thermal power losses and thermal resistances for busbar the thermal networks were built Figure 9.
Figure 7: Power Dissipated vs. Temperature with different twin material and shape.
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Table 3: Calculation Equations For Thermal Power Losses And Thermal Resistances
Thermal Network Element
Equation
Power Losses
Convection resistance
Figure (10a)
Radiation resistance
Losses Figure (10b)
Convectio
Radiation
Conducting along conductor
Figure 10: power dissipated versus AC current for different joints after being heated.
I. CONCLUSIONS
Figure 9: Thermal Network model of busbar conductor
B. Thermal network results The thermal network results of power and temperature at different applied currents are shown in Figure 10. The figure shows a good agreement of the calculated values with those measured experimentally. The different between measured and computed value for temperature may be attributed to instrument error or the constant room temperature 20 o C in the calculation procedure.
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1. Results of contact resistance measurements with changing applied torque show clearly that the joint resistance of bolted copper power connectors can be significantly decreased by surface preparation, such as abraded or polished. 2. From the available data and the experimental tests, the DC joint resistance of cleaning copper basbar joint is not affected by changing mechanical contact devices or increasing applied torque. 3. The Joint resistance of busbar joints is affected by mechanical contact devices material when subjected to AC current. 4. The detrimental effect of normal steel devices resulting in the form of high AC joint resistance and high power dissipated. 5. Changing screw material (stainless steel – normal steel) does not effect on DC or AC joint resistance.
6. The joint resistance of bolted copper busbar with normal steel contact devices decreases when the direction of used plate and twin nut was changed, this ascribed to changing of the amount of magnetic field which cutting plate and twin nut. 7. The agreement between the theoretical and experimental result of temperature and power dissipated at different joint assembly obtained from this study is quite good.
Contacts and the 22nd International Conference on Electrical Contacts,2023 Sept. 2004, PP.111 117. 11. Christoph Gramsch, Andreas Blaszczyk, Helmut Löbl1 and Steffen Grossmann. "Thermal Network Method in the Design of Electric Power Equipment". Scientific Computing in Electrical Engineering SCEE 2006. 12. Ina Berg, Helmut LöbL, Steffen Grossmann and Frank Golletz. "Thermal Behavior Of Network Components Depending On Outdoor Weather Conditions". 20th International Conference on Electricity Distribution,Prague, 811 June 2009.
References
1. K.E. Olsson. “Influence Of Mechanical Design Parameters On Electrical Contact Performance”. Proceedings of the Thirty Fifth Meeting of the IEEE Holm Conference on Electrical Contacts, 1820 Sept. 1989, PP.133 140. 2. M. Braunovic. “Effect of different types of mechanical contact devices on the performance of bolted aluminum to aluminum joints under current cycling and stress relaxation”. Proc. of 32nd IEEE Holm conference on Electrical Contacts,1986.PP.133141. 3. Michael T. Singer and Kristopher Kshonze. "Electrical Resistance Of Random Rough Contacting Surfaces Using Fractal Surface Modeling". Proceedings of the Thirty Seventh IEEE Holm Conference on Electrical Contacts, 69 Oct 1991,PP. 7382. 4. Ralf Bergmann, Helmut Löble, Helmut Böhme and Steffen Grossmann. "Model To Assess The Reliability of Electrical Joints". 18th international Conference on Electrical Contacts, chicago 1620.9.19996, PP.173179. 5. Steffen Grossmann, Habil Helmut Löbl and Habil Helmut Böhme. "Contact Lifetime of Connections in Electrical Power System". 16th International Conference on Electrical Contacts ,Loughborough University of Technology, 1992. 6. H.Böhme and H.Löbl. "Zur Theorie des Langzeitverhaltens von Aluminium Schraubverbindungen". ELEKTRIE, Berlin 41(1987),PP.179183. 7. Milenko Braunovic and Milutin Marjanov. "Thermoelastic Ratcheting Effect in Bolted AluminumtoAluminum Connections". IEEE Transactions on Components, Hybrids, and Manufacturing Technology, VOL. 11, NO. I, March 1988, PP.5463. 8. C. Labrecque, M. Brauinovic, P. Terriault, F. Trochu and M. Schetky. "Experimental and Theoretical Evaluation of the Behavior of a Shape Memory Alloy Belleville Washer under Different Operating Conditions". Proceedings of the Forty Second IEEE Holm Conference on Electrical Contacts,1620 Sept. 1996,PP.195 204. 9. H. Löbl. " Basis of Thermal Networks" (unpuplished). Dresden University of Technology, 1999. 10. Thomas Schoenemann, Mario Schenk,Helmut Löbl, Marianne Pleines, Tomasz Magier. "Optimal Design of Generator Circuit Breakers up to a Capacity of 2000 MVA using Thermal Models under Consideration of Electrical and Thermal Contact Resistances". Electrical Contacts, 2004. Proceedings of the 50th IEEE Holm Conference on Electrical
List of symbol K : Skin effect αconv : Convection coefficient αrad : radiation coefficient Aconv : surface area for convection Arad : surface area for radiation Nu : Nusselt number Gr : Grasshof number C1 : Factor n1 : Exponents ε12 : Emissivity between the radiating and absorbing surface T1,T 2 : Absolute temperatures of both surface λfluid : Thermal conductivity of fluid ρ20 : Specific electrical resistance at 20 o C : Temperature Pr : Prandtl number Ich : Characteristic lenghth Rconv : Convection resistance Rrad : Radiation resistance σ : Stefan Boltzmann constant 5.67*10 8 W/(m 2 K 4 ) : Electrical resistance R( )
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