Experimental Investigation of Contact Resistance of Slotted and Perforated Bolted Busbar Connections

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1 Experimental Investigation of Contact Resistance of Slotted and Perforated Bolted Busbar Connections RAINA TZENEVA 1, YANKO SLAVTCHEV 2, NIKOS MASTORAKIS 3, VALERI MLADENOV 4 1 Department of Electrical Apparatus, Faculty of Electrical Engineering, Technical University of Sofia 8, Kliment Ohridski St, Sofia-1000, BULGARIA; tzeneva@tu-sofia.bg 2 Department of Logistics and Materials Handling, Technical University of Sofia 8, Kliment Ohridski St, Sofia-1000, BULGARIA; blamail@tu-abv.bg 3 Military Institutions of University Education, Hellenic Naval Academy, Terma Matzikyriakou, 18539, Piraeus, GREECE; mastor@wseas.org 4 Department of Theoretical Electrical Engineering, Faculty of Automatics, Technical University of Sofia 8, Kliment Ohridski St, Sofia-1000, BULGARIA; valerim@tu-sofia.bg Abstract: - The paper reported discusses how introducing a slotted hole shape or groups of small holes around the bolt holes in high power bolted busbar connections increases significantly the true contact area and therefore reduces contact resistance. The new designs, formally called S and G type are compared with the classical one of bolted busbar connections by measuring the contact resistance of representative connections. It has been estimated that the new cases lead to a considerable reduction of contact resistance and improvement of the reliability and the life of the connection. Key-Words: - Bolted busbar high power connections, Contact resistance, Experimental investigation, New connection design, statistical processing, t-criterion of Student 1. Introduction In electrical power transmission and distribution systems we find a lot of contacts between conductors. These contacts have to carry currents of hundreds or thousands of Amperes for many years without any revision. The process of designing flat busbur contacts is complex. A great many design parameters may be varied independently, and the balance between cost and quality is often a matter of trial and error. It is necessary that a conductor joint be mechanically strong and have a relatively low resistance which must remain substantially constant throughout the life of the joint. Efficient joints in copper and aluminium busbar conductors can be made very simply by bolting, clamping, riveting, soldering or welding, the first two being used extensively. Bolted joints are compact, reliable and versatile but have a somewhat more uneven contact pressure compared to clamped plates joints. There are two major factors that affect the reliability of a power joint. The first one is the design of the connection and the material from which it is fabricated. The second one is the environmental influence to the joint. The fundamental requirements for the design of reliable high-power connections used in bare overhead lines are given in [1]. The fundamental design criteria for power connectors are: maximization of electric contact true area, optimization of frictional forces with conductors (buses), minimization of creep and stress relaxation, minimization of fretting and galvanic corrosion, minimization of differential thermal expansion along and normal to interfaces. Summarizing the major connection design criteria, mentioned above it is worthwhile noting that all the criteria can be met simultaneously by working out an outline that achieves a sufficiently large contact load, a large area of metal-tometal contact and sufficient elastic energy storage in the connection to maintain an acceptable connector s contact load throughout the service life of the connection. 2. Theoretical background All joint surfaces are rough and their surface topography shows summits and valleys. Thus under the joint force F two joint surfaces get into mechanical contact only at their surface summits. Electrical current lines are highly constricted at the contact spots when passing through, as presented schematically in Fig. 1a. This constriction amplifies the electric flow resistance and hence the power loss. Obviously, the more the contact spots, the smaller the power loss at the interface of the conductors. Power connections with superior performance are designed to maximize both the number and the life of the contact spots. For this reason, it is essential to keep in mind that the load bearing area in an electric joint is only a fraction of the overlapping, known as apparent area. Metal surfaces, e. g. those of copper conductors are often covered with oxide or other insulating layers. ISBN: ISSN:

2 a) b) Fig.1 a) Contact surface and current lines; b) Contact area with α spots As a consequence, the load bearing area may have regions that do not contribute to the current flow since only a fraction of it may have metallic or quasi-metallic contact and the real area of electric contact, i.e. the conducting area, could be smaller than the load bearing area (Fig. 1b) [2]. A conducting area is referred to as quasi-metallic when it is covered with a thin (< 20Å) film that can be tunneled through by electrons. This quasi-metallic electric contact results in a relatively small film resistance R f. The summits of the two electric joint surfaces, being in metallic or quasi-metallic contact, form the so called α- spots where the current lines bundle together causing the constriction resistance R c. The number n, the shape and the area of the α-spots are generally stochastic and depend on material parameters of the conductor material, the topography of the joint surfaces and the joint force. For simplicity it is often assumed that the α- spots are circular. Looking at one single circular α-spot its constriction resistance R c depends on its radius a and the resistivity ρ of the conductor material. Under the assumption that the bulk material above and under the α- spot is infinite in volume, the value of the constriction resistance can be calculated by means of the Holm s ellipsoid model. ρ R c = (1) 2a If a single α-spot is completely covered with a thin film of resistivity ρ f and thickness s, its film resistance R f is given by ρ f s σ f R f = = (2) 2 2 πa πa where σ f is the tunnel resistivity i.e. the resistance of the film across one cm². The total resistance R 1 of a α-spot, referred to as contact resistance, results in the sum of the constriction resistance R c and the film resistance R f R 1 = R c + R f (3) The new slotted hole shape arises from [3]. Boychenko and Dzektser have shown that changing the connection design can equally be effective in increasing the contact area. In other words, cutting slots in the busbar in a manner as shown in Fig. 2, the actual surface area of a joint can be increased by 1.5 to 1.7 times of that without slots. The contact resistance of joint configuration with slots (b) is 30-40% lower than that of (a) and is mechanically and electrically more stable when subjected to current cycling test [4], [5]. The beneficial effect of sectioning the busbar is attributed to a uniform contact pressure distribution under the bolt, which in turn, creates a larger contact area. This case is investigated in [6]. Fig. 2 Effect of making a slot in the overlapping busbar joint This idea is developed by M. Braunovic [4]. Fig. 3 depicts typical high voltage 700 kv power connectors used for connections of stranded conductors and for connecting a variety of power equipment at the substation site. These connectors are made of two parts: keeper and current-carrying member comprised of a grooved section and flat end (pad, tongue). The conductor is secured in the groves by either highstrength aluminum (7075 grade) or steel bolts. Keeper and current-carrying part are made of cast or wrought aluminum. ISBN: ISSN:

In the case of an old connector design, the keeper is made of a solid block of cast aluminum while in the new connector design; the keeper is sectioned into two - Fig. 3a or three Fig.3b segments.

the joint as a whole. This was associated with the inability of a relatively large and rigid keeper to maintain a good contact between the conductor and current-carrying part of a connector.

Beneficial effect of sectioning was associated with a more uniform stress distribution between the keeper and conductor that assured a larger contact area at the conductor-connector current carrying

3 In the case of an old connector design, the keeper is made of a solid block of cast aluminum while in the new connector design; the keeper is sectioned into two - Fig. 3a or three Fig.3b segments. Numerous report from the field showed that in the old connector design, contact resistance between the current-carrying part and conductor was unstable that often lead to unacceptable overheating of the joint as a whole. This was associated with the inability of a relatively large and rigid keeper to maintain a good contact between the conductor and current-carrying part of a connector. Sectioning the keeper mitigated the problem and significantly improved both mechanical and electrical stabilities of a joint. Beneficial effect of sectioning was associated with a more uniform stress distribution between the keeper and conductor that assured a larger contact area at the conductor-connector current carrying interface. b) Fig.3 New connector design; a) the keeper is sectioned into two or b) three segments by longitudinal slots The next elaboration of this idea for the influence of the longitudinal slots on the true area of high power bolted connections leads to 3 new designs of high power bolted busbar connections (slotted bolt holes - design S, slotted bolt holes, ending with small holes design SH and groups of small holes around the bolt holes design G. they are investigated in details in [7], [8], [9] and [10]. The mechanical changes, associated with the contact penetration depth and the contact pressure, in the contact a) area between two busbars in a bolted busbar connection are studied by the help of the finite elements simulation tool ANSYS Workbench and significant rise of these two parameters is proved. If a higher contact penetration increases α-spots both in numbers and dimensions, which in turn expands the true contact area and decreases contact resistance, then a new hole-shape could be introduced for this connection. 3. Experimental Investigation of the Contact Resistance Series of contact resistance measurements were carried out on the overlapping connections, described as follows: Copper busbars (Young s modulus Е = Pa, Poisson s ratio µ = 0.34, width 60mm, height 10mm, length 160mm, busbars overlap 60mm with 2 holes of Ø10.5mm; Aluminium busbars (Young s modulus Е = Pa, Poisson s ratio µ = 0.35, width 60mm, height 10mm, length 160mm, busbars overlap 60mm with 2 holes of Ø10.5mm; Fasteners: bolts Hex Bolt GradeB_ISO 4015 M10 x 40 x 40 N, steel E = Pa, µ = 0.3; nuts Hex Nut Style1 GradeAB_ISO 4032 M10 W N, steel E = Pa, µ = 0.3; washers Plain Washer Small Grade A_ISO , steel E = Pa, µ = 0.3. Applied tightening torques of 30Nm. The measurements follow exactly the procedure, given in [4].The contact resistance was derived from the voltage drop measured between the potential leads positioned on the buses of the connection as shown in Fig 4. The same arrangement was used for the connections without slots and perforations, i.e. classical case of bolted busbar connections. The current applied across the connection joint was 200A d.c. The voltage drop was measured using digital voltmeter IZOT 9204D. The voltage drop was measured every 20 seconds over a period of 1 hour. Fig. 4 Positioning of the voltage probe leads for the contact resistance measurements ISBN: ISSN:

4 The voltage drop leads consisted of multi-strand wire cords welded to silver-plated pins. The pins were inserted into small holes drilled into the buses about 5mm from the edges of the connections. Hence, the total spacing between the potential leads at all measuring locations was about 70mm. The resistance derived from the voltage drop measurement across this spacing can be considered as a measure of the true contact resistance between the two buses since the bulk contributions arising from the busbars are essentially insignificant. An estimate of the bulk contributions to the overall resistance measured across the connection was approximately 0.1µΩ. The resolution of the voltage measuring system was about 1µV, leading to a resistance resolution of µω. This resolution was sufficient to detect small resistance changes associated with the connection resistance arising from different connection configuration slotted and perforated buses design. The contact surfaces of the buses have not any surface treatment (as received). They were cleaned by wiping them with cotton swabs soaked in 50/50 freon/methanol mixture and then brought into contact and joint assembled. First, the contact resistance of the classical case of copper connection is compared with the contact resistance of case 8, design S (the busbar holes have 4 slots, 3mm long, 1mm width, arranged in such a way that the pairs of slots are on mutually perpendicular axes, rotated at 45 degrees about the busbar axes) is compared. The results are presented in Fig E E E E E E E-06 Rs8 Fig. 5 Contact resistances of the classical case R C and case 8, design S-R S8, copper connections as a function of the time (1 hour) Fig. 6 shows the contact resistances of the classical case and case 2, design G (two horizontal groups of two holes of diameter Ø1mm and distance of 0.9mm between the holes, parallel to the busbar axis) copper connections as a function of the time. 3.4E E E E E E E E-06 Rg2 Fig. 6 Contact resistances of the classical case R C and case2, design G- R G2, copper connections as a function of the time (1 hour) The next 3 investigated connections are made from aluminium. The contact resistance of case 9, design S (the 4 slots of length 3mm and with of 1mm are not rotated) and of case 8, design G (four groups (two horizontal and two vertical) of three holes of diameter Ø0.8mm and distance of 0.2mm between the holes) is compared with the contact resistance of the classical case. The results obtained are illustrated in Fig E E E E E E-06 Rs9 RG8 Fig. 7 Contact resistances of the classical case R C, case 9, design S - R S9, and case 8, design G - R 8G, aluminium connections as a function of the time (1 hour) 5. Discussion and conclusions The measured data were statistically processed using t- criterion of Student that compares the mean values of two data massifs [11]. xa xb t= 1 1 sσ + n n a x a - mean value of data in massif a; x - mean value of data in massif b; b b (4) ISBN: ISSN:

5 n a - number of data in massif a; n b - number of data in massif b; s - estimator of the massifs variance Σ 2 ( x x ) Σ( x x ) 1/ 2 2 Σ ia a jb b s Σ = (5) n + 2 a nb Summary of the t-test values, probability p and the average of the contact resistance for each of the four new cases, as a percentage of the classical case, are presented in Table 1. Table 1 R contact t p % of R c R S < R G < R S < R G < The t-test p-values for all of the investigated cases are less than 0.001, so the results are significant at a 5% level of significance. The null hypothesis is rejected in favor of the alternative that the massifs compared contain data that do differ. 2. The newly incorporated geometry designs with slots and groups of small holes have proven to exert a substantial influence on the contact resistance of bolted busbar connections. 3. All of the investigated new bolted busbar connections, i.e. S and G designs, reveal a decrease in the contact resistance values when compared with the classical case. The major decrease is for the aluminium joints (approximately 50%). References: [1] R. S. Timsit, The Technology of High Power Connections: A Review, 20-th International Conference on Electrical Contacts, Zurich, Switzerland, p. 526, [2] R. Holm, Electric Contacts, Theory and Application, Berlin, Germany, Springer-Verlag, [3] V. I. Boychenko, N. N. Dzektser, Busbar Connections (in Russian), Energia, [4] M. Braunovic, Effect of Connection Design on the Contact Resistance of High Power Overlapping Bolted Joints, IEEE Transactions on Components, Packaging and Manufacturing Technology, vol. 25, Issue 4, pp , Dec [5] M. Braunovich, Effect of Connection Design on the Performance of Service Entrance Power Connectors, IEEE Transactions on Components, Packaging and Manufacturing Technology, vol. 27, Issue 1, pp.72-78, March [6] R. Tzeneva, P. Dineff and Y. Slavtchev, Bolted Busbar connections, XIV-th International Symposium on Electrical Apparatus and Technologies SIELA 2005, 2-4 June2005, Proceedings of papers, vol. I, pp , Plovdiv, Bulgaria, [7] R. Tzeneva, Y. Slavtchev, N. Mastorakis and V. Mladenov, Bolted Busbar connections with Slotted Bolt Holes, WSEAS Transactions on Circuits and Systems, Issue 7, vol. 5, pp , July [8] R. Tzeneva, Y. Slavtchev and V. Mladenov, Bolted Busbar Connections with Slotted Bolt Holes, Proceedings of the 10-th WSEAS Conference on Circuits, Vouligmani Beach, Athens, pp , Greece, July [9] R. Tzeneva, P. Dineff, Bolted Busbar Connections with Particularly Slotted Bolt Holes, Proceedings of the XLI International Conference on Information, Communication and Energy Systems and Technologies ICEST 06, pp , 29-th June-01-st July, Sofia, Bulgaria, [10] R. Tzeneva, Electric Field Distribution in Perforated Bolted Busbar Connections, XV-th International Sympozium on electrical Apparatus and Technologies SIELA2007, vol. I, pp , 31 May- 1June 2007, Plovdiv, Bulgaria, 2007 [11] R. Tzeneva, Y. Slavtchev and V. Mladenov, New Connection Design of High Power Bolted Busbar Connections, Volume 1 of the Proceedings of the 11-th WSEAS International Multiconference CSCC (Circuits, Systems, Communications, Computers), Agios Nikolaos, Crete Island, Greece, July 23-28, 2007, pp [12] H. Schenck, Theories of engineering experimentation, Mir, Moskow, 1972 (in Russian) ISBN: ISSN:

New Connection Design of High Power Bolted Busbar Connections

Proceedings of the 11th WSEAS International Conference on CIRCUITS, Agios Nikolaos, Crete Island, Greece, July 23-25, 27 228 New Connection Design of High Power Bolted Busbar Connections RAINA TZENEVA