1

Compression Connectors for Stranded Aluminum Power Conductors

Magne Runde, Harald Jensvold and Mario Jochim

Abstract--12 different commercial compression connectors for

240 mm2 stranded aluminum cable conductors have been examined in order to identify and clarify the correlations between the quality of a connector and its design characteristics, assembly procedures and other relevant parameters. The quality of the joints is assessed with basis in resistance measurements during short-circuit tests and thermal cycling, as specified by the IEC 61238-1 standard. Visual inspections of cross-sectioned connections, hardness measurements and other examinations revealed that large mechanical deformations in connector-conductor interface significantly improve the quality of a joint. Joining soft (annealed) conductors is considerably more difficult than joining hard-drawn conductors, but excellent results can be obtained if the work hardening during compression increases the conductor hardness to a level approaching that of hard conductors. The sequence in which the compression indents is made can be decisive for the quality of a joint. Unless the connector barrel or sleeve contains excessive amounts of contacting compound, it is advantageous to set the first indents at the ends of the barrel and the last ones in the middle.

Index Terms-- Connectors, power cable connecting, aluminum power conductors.

I. INTRODUCTION

Insulated cables for high voltage power transmission and distribution often have stranded aluminum conductors. For

splicing and terminating the conductors a large number of connectors are available. Among the most popular ones are the so-called compression connectors or compression joints.

In a typical compression joint the conductors to be joined are fed into a hollow cylinder or tube, usually referred to as the barrel or sleeve. Then a mechanical or hydraulic tool is used to make a number of indents or compressions that secure the conductors and at the same time establish a good electrical connection across the barrel-conductor interface. Systems where the conductor is secured by one or more bolts screwed through the walls of the barrel have for long been widely used for low voltage applications, and are becoming increasingly

popular also for higher voltages.

M. Runde is with SINTEF Energy Research, NO-7465 Trondheim, Norway and with the Department of Electrical Power Engineering, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway (e-mail: magne.runde@sintef.no).

H. Jensvold is with SINTEF Energy Research, NO-7465 Trondheim, Norway (e-mail: harald.jensvold@sintef.no).

M. Jochim is with SINTEF Energy Research, NO-7465 Trondheim, Norway, on leave from Dresden University of Technology D-01062 Dresden, Germany (e-mail: mjochim@gmx.de).

Due to the well-known difficulties with aluminum as a contact material [1], aluminum connectors have to be carefully designed and correctly applied to make a reliable and low-resistance connection. A connector failure in a high voltage power cable is always accompanied by a dielectric failure of the insulation system. Furthermore, replacement may involve time consuming and expensive excavations. Thus, the consequences of having power cable connections susceptible to overheating and failure may become rather severe.

Nevertheless, experience from utilities has indicated that the quality of the various available aluminum compression connectors varies substantially [2]-[4]. This impression has been supported through laboratory testing. Accelerated tests on four different connector systems using a current-cycling procedure revealed large differences in the connector performance [5]. European tests on several 150 mm2 aluminum cable connectors also disclosed major problems with many of the investigated systems [4].

The compression joints in use for a given conductor cross section comprise barrels with different lengths and diameters, a variety of compression tools and dies, different recommendations with regard to the compression sequence, etc. The detailed relationships between all these factors and the long-term behavior of the connector under different operating and environmental conditions are indeed elusive. However, knowing some of the clues can be of great value, so the objective with the present paper is to attempt to identify at least some the major parameters determining the quality of such connectors.

The approach is largely empirical. 23 different connections for 240 mm2 stranded aluminum cables have been tested according to the IEC 61238-1 standard [6], and then ranked in quality according to the test results. Then, through careful visual inspections of cross-sectioned connections, hardness measurements and other examinations, it is searched for correlations between the test results and design features, material properties, assembly procedures and other matters that are likely to influence the electric properties of the joint.

Portions of this work have been presented earlier in two conference publications [7], [8].

II. CONNECTIONS INVESTIGATED Fig. 1 shows a photo and Table I gives key data of the 12

connectors examined. A connector here means a certain

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Fig. 1. The tested connectors.

combination of barrel and compression tool. Ten of the connectors (A – J) require specially designed mechanical or hydraulic tools for assembly, while two (K and L) are bolted connectors mounted simply by using a wrench. The manufacturer of the barrel is the same as the manufacturer of the compression tool in all cases, except for A and D.

Connector K covers conductor sizes in the range 150 – 300 mm2, while all others are for use with 240 mm2 conductors only. The inner cross-section of the barrel of connector J is designed to fit sector-shaped conductors, while the rest of the barrels have cylindrical openings. Barrel lengths are in the range 129 – 153 mm.

Six of the connectors have barrels with a partition in the

middle, the rest are through connectors. The inner surface of the barrel of connectors E, I and K has grooves, the rest have smooth contact surfaces.

Connector L is a terminal lug, where the electrical connection to both the conductor and the bus-bar is established by tightening one single bolt. Only this connector is delivered completely dry. The barrels of all the others are supplied with contacting compound (“grease”); the amount varying from almost nothing to 40 g per barrel, as shown in Table I.

TABLE I

THE EXAMINED CONNECTORS Connector Inner

diameter of barrel [mm]

Vicker’s hardness of

barrel

Barrel partition

Grooves in barrel

Amount of contacting

compound [g]

Compression tool and procedure

A 19.0 – 19.9 25 Yes None 8 5 mm wide hexagonal compressions made with a mechanical hand-operated tool

B 19.4 40 Yes None 2 Deep, stepped indent compressions made with a hydraulic tool in a 360o die

C 20.8 24 No None 5 10 mm wide circular compressions made with a hydraulic tool

D 21.4 39 No None 40 Deep indent compressions made with a hydraulic tool in a V-shaped die

E 19.8 26 Yes Circular 0.6 Deep indent compressions made with a hydraulic tool in a 360o die

F 20.8 24 No None 5 Deep, stepped indent compressions made with a hydraulic tool in a 360o die

G 19.5 23 Yes None 11 Deep, stepped indent compressions made with a hydraulic tool in an 180o die

H 21.4 39 No None 40 Deep stepped indent compressions made with a hydraulic tool in a V-shaped die

I 19.8 26 Yes Circular 0.6 Deep indent conical shaped compressions made with a hydraulic tool in a 360o die

J 25.6 a 103 No None 35 5 mm wide hexagonal compressions made with a mechanical hand-operated tool

K 26.0 106 Yes Axial 0.2 Compressions made by tightening bolts until shear-head breaks at a torque of 60 Nm

L 22.0 – 24.0 107 None 0 Compression made as the inner part rotates when the bolt is tightened

a) The barrel is designed to fit sector shaped conductor. Value for inner radius is taken as the radius of a circle giving the same cross-sectional area.

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By applying these 12 connectors on two different 37 strand 240 mm2 aluminum conductors 23 different connections were made. Thus the terms connection and joint here refer to a certain combination of connector (i.e. barrel and tool), conductor and compression procedure. The 23 combinations included in the test series are described in detail in the result section.

The two conductor types used will be referred to as soft and hard. The soft conductor has been annealed at 375 oC for at least 1 h after it was spun, compacted and wound on drums. Table II compares a few important properties of the conductors. As can be seen, there is a considerably difference in hardness and mechanical strength.

TABLE II CONDUCTOR PROPERTIES

Hard Soft

Nominal diameter [mm] 18.2 18.2

Tensile strength (σ0.2) [N/mm2] 140 - 160 51 - 60

Vickers hardness 40 - 46 22 – 25

Power cables with such so-called soft or annealed conductors have become increasingly popular among electric utilities due to their greater mechanical flexibility compared to cables with conventional, hard-drawn conductors. Space is limited in modern substations, and terminating a stiff cable to a compact switchgear unit can be a rather arduous undertaking.

III. EXPERIMENTAL

A. Electrical Test The IEC 61238-1 [6] standard used in the present context

to assess the quality of a connection is widely accepted in Europe. It prescribes an accelerated ageing procedure of 1000 thermal cycles and six short-circuit tests. Six specimens of the connection to be tested are put in series with a reference conductor, and the temperature of this reference conductor is cycled between 35 oC and 120 – 140 oC using ac. The short-circuit tests are carried out after 200 cycles, by applying a current that increases the temperature of the reference conductor from 35 oC to between 250 and 270 oC within nominally 1 s.

The resistance across each of the six specimens is measured before initiating the thermal cycling, after 200 cycles (both before and after the short-circuit test), after 250 cycles and then for every 75 cycles for the rest of the test.

The subsequent evaluation is carried out with basis in k-values. The k-value is the ratio between the resistance of the connection and the resistance of a corresponding length of the reference conductor. Hence, joining two conductors with a connection with a k-value equal to unity yields the same resistance as a conductor without the joint.

The k-values obtained from the six specimens are used to determine five evaluation parameters. Details on the procedures can be found in the standard [6]. Table III lists these parameters, together with a brief description about what

TABLE III EVALUATION PARAMETERS USED IN THE IEC 61238-1 STANDARD AND THE ASSOCIATED ACCEPTANCE CRITERIA FOR BOTH 1ST AND 2ND EDITION OF THE

STANDARD. Maximum acceptable

values Evaluation parameter Significance

1st ed. 2nd ed.

δ A measure of the scatter in the k-values among the six specimens before the thermal cycling

0.15a 0.3

β A measure of the scatter over the last 11 measurements, i.e., after the short-circuit tests

0.15a 0.3

D A measure of the change in k-value for each of the specimens over the last 11 measurements

0.15 0.15

λ Ratio between the maximum and the initial k-value for a specimen

1.5 2

θmax/θref Ratio between maximum temperature of the connection and the reference conductor during the test

1 1

a) For bolted connectors a maximum value of 0.3 is accepted. type of information each of them convey. For all five parameters increasing values mean a poorer test result. If one or more of the evaluation parameters exceed the values given in Table III the connection fails the test.

Two versions of the IEC 61238-1 exist. The original version, in Table III referred to as 1st edition, has considerably stricter pass criteria than the recently proposed 2nd edition. The test procedures are unaltered, but a significantly larger scatter among the six specimens is tolerated (higher δ and β values are accepted), and secondly, a doubling instead of a 50% increase of the contact resistance of a specimen is tolerated, see Table III.

B. Cross-Sectioning and Hardness Measurements Samples from all the connectors have been embedded in

epoxy resin and cross-sectioned through the deepest part of one or more of the indents. The cross-sectional surfaces were polished down to a mirror finish using silicon carbide paper and diamond paste. This clearly reveals the interior of the joint, including how the conductor strands are distributed and deformed.

Vicker’s hardness number was measured on various locations in these cross-sections using a standard hardness probe. The examinations include both connections that had been subjected to the electrical test and virgin (un-tested) samples.

IV. RESULTS

A. Electrical Test In Fig. 2 the measured k-values obtained throughout the

IEC 61238-1 test are plotted for the six specimens of all the 23 tested connections. Recall that the short-circuit tests were carried out between the second and third resistance measurement. Table IV describes the tested connections and lists the calculated values of the evaluation parameters.

Each connection type is identified by a capital letter and in

4

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

A1

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

A2

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

A3

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

A4

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

A5

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

B

0 200 200 1000Cycle no.

02468

10

K-v

alue

C

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

D1

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

D2

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

D3

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

E1

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

E2

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

E3

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

E4

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

F

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

G

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

H1

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

H2

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

I

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

J

0 200 200 1000Cycle no.

0

1

2

3

K-v

alue

K

0 200 200 1000Cycle no.

0

10

20

30

K-v

alue

L1

0 200 200 1000Cycle no.

02468

10

K-v

alue

L2

Fig. 2. Measured k-values during the tests. Note the different vertical scaling for connections C, L1 and L2. These tests were stopped after 200 cycles, because the k-values had then become unacceptably high.

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TABLE IV RESULTS OF IEC 61238-1 TEST

Con-nection

Con- ductor

No. of indents

Order of indents

Evaluation parametersa δ β D λ Θmax /Θref

Test resultb 1st

ed./2nd ed. Q

(see text)

H2 c Soft 6 1-5-6-4-3-2 d 0.043 0.090 0.039 1.16 0.87 P/P 0.48

A5 e Soft 16 9←3-1-2-10→16 0.034 0.090 0.005 1.17 0.83 P/P 0.49

E4 Hard 1-4-3-2 d 0.035 0.078 0.001 1.28 0.86 P/P 0.49

B Soft 4 1-4-3-2 0.072 0.062 0.005 1.09 0.84 P/P 0.50

G Soft 4 1-4-3-2 0.061 0.051 0.006 1.10 0.87 P/P 0.51

A4 e Soft 16 9←3-1-2-10→16 0.055 0.082 0.004 1.17 0.84 P/P 0.51

A3 e Soft 16 9←3-1-2-10→16 0.054 0.074 0.007 1.19 0.87 P/P 0.51

D3 Hard 6 1-3-4-6-5-2 0.074 0.065 0.020 1.20 0.79 P/P 0.53

H1 Soft 6 6-5-1-2-3-4 0.047 0.085 0.003 1.28 0.79 P/P 0.54

F Soft 4 1-4-3-2 0.083 0.126 0.015 1.15 0.86 P/P 0.62

E2 Soft 4 1-4-3-2 d 0.102 0.144 0.005 1.32 0,92 P/P 0.69

I Hard 4 4-1-2-3 0.095 0.161 0.009 1.46 0.79 F/P 0.71

A1e Soft 16 9←3-1-2-10→16 0.051 0.208 0.009 1.49 0.90 F/P 0.74

E3 Hard 4 4-1-2-3 0.099 0.169 0.014 1.47 0.85 F/P 0.74

A2e Hard 16 9←3-1-2-10→16 0.092 0.181 0.111 1.32 0.88 F/P 0.86

J Hard, sector 12 12←8-1-2→7 0.109 0.151 0.088 1.67 0.90 F/P 0.87

D2 Soft 8 1-3-4-5-8-7-6-2 0.085 0.374 0.036 2.27 1.00 F/F 1.16

K Hard 4 bolts, alternately tightened 0.074 0.179 0.285 3.43 1.00 F/F 1.21

D1 Soft 6 1-3-4-6-5-2 0.065 0.384 0.046 3.54 1.07 F/F 1.35

E1 Soft 4 4-1-2-3 0.099 0.534 0.026 2.23 0.97 F/F 1.37

L2 Hard 1 bolt 0.561 - - 6.23 0.79 F/F -

C Soft 8 5-4-3-1-2-6-7-8 0.182 - - > 5.6 > 4 F/F -

L1 Soft 1 bolt 0.674 - - 11.9 0.98 F/F - a) Values exceeding the acceptance criteria of IEC 61238-1 1st ed. (both 1st and 2nd ed.) are underscored with broken (solid) lines. b) Left (right) character shows result according to the IEC 61238-1 1st ed. (2nd ed.) requirements, see Table III. P: pass; F: fail. c) Contacting compound in barrel was removed before assembly. d) Order of indents is not as specified by the manufacturer. e) The inner diameter of the barrels in the A connectors is slightly different: A1 and A2: 19.9 mm, A3: 19,5 mm, A4: 19.2 mm, A5: 19.0 mm.

most cases also a number. The capital letter refers to the connector type as presented in Fig. 1 and Table I, whereas the number is used to distinguish between connections using the same connector, but different conductors and/or compression procedures.

The number of compression indents made during assembly of the connection and their sequence are also given Table IV. For example, 1-4-3-2 means that the first two indents are made at the ends of the barrel, while the third is next to the second, and the fourth is set between the first and the third, see Fig. 3. With the exception of connections E2, E4 and H2 indents were made as specified by the manufacturer.

As shown in Table IV only 11 and 16 connections passed the 1st and 2nd edition of the IEC 61238-1 standard, respectively. Thus seven of the investigated connections had evaluation parameter values exceeding the acceptance criteria for both editions.

As a mean for classifying the quality of the connections somewhat more accurately than simply by considering the pass/fail results, a “quality parameter” Q is defined as

++++=

ref

DQθθδβδ max

5.115.015.015.051 . (1)

Hence, Q is determined by scaling each evaluation

parameter to its acceptance value as set by the 1st edition of the standard (see Table I) and then taking the average of these five ratios. For connections employing mechanical connectors, δ and β are divided by 0.3 instead of 0.15. Q is not found in the IEC standard, but created solely for the purpose of the present investigation. In Table IV the connections are

1 4 3 2

Fig. 3. Order of compression indents for connection E2.

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H2 A5 E4 B

D3A4G

H1

A1 J

D2 K D1 E1

A2E3

F E2 I

A3

L2 L1C

Fig. 4. Cross-sections through the deepest part of an indent of one specimen from all the investigated connections, sorted according to the IEC 61238-1 test results (see Table I)

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ranked according to their Q-value, with the joint that came out best in the test on top.

B. Cross-Sections of Connections Fig. 4 shows photos of cross-sectioned joints, again starting

with the connections that came out best in the test. As can be seen, there are great differences as to how the strands distribute, deform and interact with the barrel walls.

C. Hardness Measurements on Compressed Conductors The hardness of the conductor is, as could be expected,

closely related to how much the conductor is deformed during the compression process. This is clearly seen when comparing the cross sections (Fig. 4) with some typical results from hardness measurements of compressed conductors given in Fig. 5.

Fig. 5. Vicker’s hardness number measured in the conductor part of the cross-sections of connections B, D1 and H1 (upper curves) and A1, A5 and C (lower curves). All these connectors are with soft conductors. Measuring points are 2 mm apart, and 3 – 6 points are obtained on each sample, see the schematic drawings to the right. The Vicker’s hardness number for (uncompressed) soft and hard conductor is typically 22 – 25 and 40 – 46, respectively.

When connectors employing heavy deformations (B, D, E, F, G, H and I) are applied on soft conductors, the average value of the Vickers’s hardness number over the conductor cross section increases from 22 – 25 and up to 31 – 37 when making the joint. The maximum values are usually found close to the deepest indent and are in these cases typically in the range 33 – 42.

For connectors that only produce small deformations in the conductor when making the joint (A, C, J, K and L) far less changes in hardness are observed. Only near the conductor-barrel interface, a possible hardness increase in the soft conductor is sometimes measured; see typical examples in the lower curves of Fig. 5.

When joining hard conductors (Vicker’s hardness of 40 – 46) no overall and significant change in conductor hardness from the compression process is observed.

Furthermore, despite very careful examinations no observable differences in hardness between virgin (assembled, but un-tested) and tested joints have been found.

V. CORRELATION BETWEEN TEST RESULTS AND DESIGN AND ASSEMBLY PARAMETERS

A. Hardness and Deformation Hardening It is evident from the test results that it is significantly more

difficult to make a reliable joint between soft than between hard conductors. Consider for example connector D. Used on soft conductors (D1 and D2) it fails, whereas used to join hard conductor (D3) this connector passes the test by a wide margin.

The Vicker’s hardness number of the various connector barrels is different by a factor more than four. However, no clear correlation between the hardness of the connector barrel and the quality of the joint emerges from the test results.

It is obvious that creating large mechanical deformations in the conductor during the compression is advantageous. Especially connectors applying deep and stepped indents (i.e. with a second indent at the center) such as H, B, G and F, give high quality joints, also with soft conductors. In contrast, connectors L, C, K and J, which have considerable less deformed compression zones, turn out to be of significantly poorer quality. These results are in agreement with findings in other, less comprehensive investigations [5].

2 4 6 8Location in z-direction [mm]

20

25

30

35

40

Vick

er's

Har

dnes

s N

umbe

r

A5

A1C

D1

B

H1

z

The large mechanical deformations during the compression cause significant work hardening and thereby a considerable increase in hardness of soft conductors, see Fig. 5. In connections showing the best test results on soft conductors, the hardness in the mostly deformed regions of the conductor increases to a value almost comparable to what is seen in hard conductors. Hence, when designing connectors for soft conductors, such a hardness increase should be among the principal design objectives.

z

B. Barrel and Conductor Dimensions Connector A does at first sight not fit into this pattern. As

can be seen from Figs. 2 and 5 the conductor is only little deformed during compression, and the hardness is virtually unaltered, at least in the major part of the cross section. Still A5 comes out with excellent test results. This somewhat unexpected result can be explained by considering the results with the other connections using A connectors.

Joints A1, A3 and A4 are identical to A5 except that the inner diameter of the barrels is slightly larger. It turns out that minor increases in inner diameter, from 19.0 mm (A5), to 19.2 mm (A4), to 19.5 mm (A3), and finally to 19.9 mm (A1) have a very substantial effect on the test results. In the quality ranking in Table IV, connector A5 comes out second, while A1 is number 13, and fails when using the 1st edition of the IEC standard.

The nominal conductor diameter is 18.2 mm, so the conductor-barrel air gap before compression varies within a factor two among the A connections, from 0.4 to 0.8 mm. Moreover, the inner diameter of the barrels of the other low-deformation connectors, e.g. C, J and K is much greater, yielding air gaps in the range 1.3 – 3.9 mm. Consequently, it is reasonable to attribute the good results of A5 to the very small air gap. Then only modest barrel compression is

8

necessary to bridge the gap and provide sufficiently deformations and thus good contact between barrel and conductor.

However, this rather extreme sensitivity with regard to barrel and conductor dimensions makes the A connector very vulnerable, and this is undoubtedly a disadvantage. Connectors employing deep indents seem to be far less susceptible to minor deviations in barrel and conductor dimensions.

The underlying metallurgical or physical mechanisms responsible for the apparent correlation between conductor hardness and test results are not easily disclosed. As can be seen from Fig. 2, phenomena occurring during the short-circuit tests in many cases have a very detrimental effect on the contact resistance. The short-circuit current tests increase the bulk temperature of the connection to almost 200 oC, and the local temperature in the contact spot even more, and the associated thermal expansion imposes considerably mechanical tensions in these areas. It is reasonable to assume that increasing strength and hardness in the barrel-conductor interface make the minute contact spots mechanical stronger and more resistant to such strains and stresses, and thus less affected by the short-circuit currents.

C. Number and Sequence of Compression Indents Connectors E and H are tested on soft conductors both with

the first indents set in the middle of the barrels (E1 and H1) as well as with the first indents set at the ends of the barrels (E2 and H2). As can be seen from Table IV, starting at the ends yields significantly better results.

A possible explanation is that applying the end indents first to a certain extent “locks” the conductor in the barrel. It cannot dodge or partly slide out of the barrel when the remaining two or four indents are set in the middle. In the opposite case, the conductor is to some extent gradually squeezed out of the barrel. Somewhat less material is left inside the barrel, and this reduction in “fill factor” may be assumed to result in smaller conductor deformations, less hardness increase in the barrel-conductor interface, and thus a poorer contact. Although detailed examinations on cross-sections of connectors E and H reveal no notable relationship between indentation order and conductor fill factor and hardness, this explanation cannot be ruled out. As shown in the case with connector A, effects that are not observable from such cross-sections may still largely influence the test result.

Consequently, when designing compression joints and specifying the assembly procedures, great attention should be paid to the sequence in which the indents are made. Furthermore, the amount of compacting compound in the barrel should, as discussed in the next section, also be taken into account when assessing the assembly procedure.

Connector D was tested with both six and eight indents, but according to Table IV with only marginal improvements. Both connections failed. This indicates that the number of indents is of less importance than the order in which they are applied.

The effect of having a partition or “wall” in the middle of

the barrel is also rather subtle. Connectors B and F apply the same compression tool and procedure, and are also in several other respects quite similar. The main difference is that B has a barrel partition, while F does not. When tested on soft conductors both passed, but B came out somewhat better than F (ranked as number four and ten, respectively). Whether the partition accounts for the better result for B is not clear, but it may be reasonably to suspect that the partition to some extent prevents the conductor from being squeezed away during the compression.

D. Grooves and Contacting Compounds In general, the main motivation for having grooves in

contact surfaces is that this is assumed to increase the oxide film penetration ability, and secondly, that grooves drain away excessive contacting compound. Contacting compounds are applied primarily to slow down contact degradation by preventing dust, corrosive gases, humidity and other damaging agents to reach the small contact spots.

From Table I it is clear that there is very little consensus among the present connector manufacturers on the use of contacting compounds and grooves; only a few connectors have grooves and the amount of contacting compound varies with orders of magnitude.

As long as tests using the same connections with and without contacting compound and/or grooves not are carried out, no firm conclusions on the effect of these design parameters can be drawn from the results in Table IV.

However, one simple additional experiment using connector H clearly demonstrates that the assembly procedure is far from indifferent to the result if the barrel contains much contacting compound. This connector is supplied with 40 g, and as can be seen from Fig. 6, when setting the indents at the ends of the barrel first, large quantities of contacting compound remain between the conductor strands and severely reduce the areas of metal-to-metal contact. Although this joint was not subjected to the electrical test, it is left with no chances of passing.

Thus it is evident that when assembling connectors containing large quantities of contacting compound by using

Fig. 6. Cross section of connector H assembled by applying the first indents at the ends of the barrels and the last in the middle, i.e., in opposite order of the manufacturer’s specification.

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tools that deform and compact the conductor as much as in Fig. 6, it is compellingly necessary to allow excessive compound to escape. In practice this means setting the indents from the middle of barrel and towards the ends, as specified by the manufacturer of connector H.

Connector D uses the same barrel as H and with the same amount of compacting compound, but the compression tool applied deforms the conductor strands to a lesser extent. As can be seen from Fig. 2, there is still some space between the conductor strands, and even if the intents are applied from the ends and towards the middle, the compacting compound does not act as a barrier between the strands, as is the case in Fig. 6.

It is interesting to note that the indent sequence prescribed for connector H is the opposite sequence of what is found to give the best result on connector E. However, when considering the effect of contacting compounds, connectors H and E are not comparable as the amount in connector E is too small (< 1 g) to cause problems of the type seen in Fig. 6.

Most of the joints made with connectors having grooves (E, I and K) come out with rather poor test results. It is from this however, not possible to draw any other conclusion than that it is unnecessary to groove the inside of the barrel to make a high quality connector.

E. Indent Types The bolted joints that are included in the tests (K, L1 and

L2) all failed, leaving the impression that screwed connectors are inferior to connectors where both barrel and conductor are deformed by a hydraulic or manual compression tool.

A fundamental difference between a bolted connector such as K and the more common compression connectors such as A – J, is that in the former one a certain force is applied on the conductor, while in the others a certain deformation is created. This may have some important implications with regard to how the connectors function on soft and hard conductors.

When the shear-head bolts of connector K break at a predetermined torque, the extent of the resulting conductor deformations depends on the mechanical properties of the conductor. A soft conductor will become more heavily deformed than a hard conductor. Recalling the importance of deformations to the quality of such joints, this may have a considerable impact. Although the IEC test not has been carried out on connector K for soft conductors, it is suspected that it will come out better than when joining hard conductors. This is opposite to what appears to be the case in connectors A – J. The amplitude of the deformation is in these cases independent on whether the conductor is soft or hard, and as discussed earlier, these connectors come out with better test results when joining hard conductors.

The connector-conductor interface of the mechanical terminal lug (connector L) is different from the rest of the tested connectors. Contact is established when the sharp edges of the inner rotating part penetrate the outer strands of the conductor. The area of mechanical contact between conductor and connector is limited, and only small deformations are created. Redesigning the connector in such a way that the

sharp edges penetrate further into the conductor may lower the resistance across this interface, but will also increase the risk of cutting strands, which is clearly not desirable.

Although the very poor test results for connector L at least partly may originate in insufficient electric contact between the fixed and the rotating part of the connector as well as between connector and bus bar, it seems that such an edge contact is inferior to the compression indents used in the other tested connectors. Applying several large and deep indents apparently results in a much greater area of metal-to-metal contact between barrel and conductor. In particular, the deep, stepped indent compression turns out to yield stable and good connections.

VI. DISCUSSION A comprehensive discussion of to what degree the IEC

61238-1 standard reflects the strains and stresses cable joints can be subjected to during service is clearly beyond the scope of this work. However, the authors feel that the test results, in qualitative terms, are fairly consistent with their experience built up through many years of damage analyses of connections failed in service. Thus, this standard is assumed to be a reasonable method for checking the quality of a connection. Viewed in this light, the overall result of the test is indeed discouraging.

However, it is interesting to note that some of the connections underwent the entire test with almost no observable increase in the contact resistance. This finding clearly demonstrates that it is possible to make reliable joints between stranded aluminum conductors; the important matters are correct design and usage.

From the plots of Fig. 2 it appears that most of the connections had an initial k-value well below one, with some of those coming out best showing the lowest values. However, the initial resistance of a connection is far from decisive for the final outcome of the test. For example, the six specimens of both connections F and D1 have initial k-values in the range 0.7 – 0.8, but connection F passed while D1 failed the test.

From the same plots it is clear that passing short-circuit currents is far more detrimental to the contact resistance than thermal cycling. All connections that failed experienced a significant increase in their k-values during the short-circuit tests carried out after 200 temperature cycles. Thus, short-circuit testing with resistance measurements before and after, seems to be a viable tool for quick and inexpensive quality screening of connectors. The time and cost spent on the 800 thermal cycles carried out after the short-circuit tests seem to provide very little additional information about the quality of the joint.

VII. CONCLUSIONS Electrical testing and subsequent analyses of connectors for

stranded 240 mm2 aluminum conductors show that when designing aluminum connectors great attention should be paid to the following matters:

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- Increasing mechanical deformations in a connector-

conductor interface significantly improve the ability of a joint to withstand stressed imposed by short-circuit currents and thermal cycling. In particular, deep stepped indent compressions appear to be suitable for this purpose.

- Joining soft (annealed) conductors is considerably more difficult than joining hard-drawn conductors. However, excellent results can be obtained also on soft conductors if the work hardening during compression increases the conductor hardness to a level approaching that of hard conductors.

- The sequence in which the compression indents is made can be decisive for the quality of a joint. Unless the barrel contains excessive amounts of contacting compound, it is advantageous to set the first indents at the ends of the barrel and the last ones in the middle.

VIII. REFERENCES

[1] M. Braunovic, “Power Connections” in Electrical Contacts : Principles and Applications, P. G. Slade, Ed. New York: Marcel Dekker, 1999, pp. 155-277.

[2] C. Dang and M. Braunovic, “Metallurgic and contact resistance studies of sleeve connectors in aluminum cable splices,” IEEE Trans. Comp. Hybrids, Manuf. Technol., vol. 13, pp. 74-80, Mar. 1990.

[3] D. Fournier, “Aging of defective electrical joints in underground power distribution systems,” in Proc. IEEE Holm Conf. Electrical Contacts, 1998, pp. 179-192.

[4] J. Neyens, D. Meurice and Y. Tits, “Connectors for aluminium cable : A review of the different technologies and their interaction with cable accessories,” in Proc. CIRED 1997, IEE Conf. Publ. no. 438, pp. 3.17.1-3.17.5.

[5] M. Braunovic and C. Dang, “A comparative assessment of different current-cycling procedures for testing power connections,” in Proc. IEEE Holm Conf. Electrical Contacts, 1997, pp. 67-90.

[6] “Compression and mechanical connectors for power cables with copper or aluminium conductors,” IEC International Standard 61238-1, 1st ed., 1993.

[7] H. Jensvold, M. Runde and G. I. Tveite, “The effect of conductor hardness on aluminium compression joints,” in Proc. Int. Conf. Electrical Contacts, 1994, pp. 497-503.

[8] H. Jensvold and M. Runde, ”A comparative evaluation of different aluminium joints,” in Proc. Int. Conf. Electrical Contacts, 2000, pp. 349-354.

Magne Runde was born in Skien, Norway in 1958. He received a MSc degree in physics and a PhD degree in electrical power engineering from the Norwegian University of Science and Technology (NTNU) in Trondheim, Norway in 1984 and 1987, respectively. He has been with SINTEF Energy Research in Trondheim, Norway since 1988. From 1996 on he is a professor in high-voltage technology at NTNU. He fields of interest include circuit-breakers, electrical contacts, diagnostic testing of power apparatus, and power applications of superconductors. Dr. Runde is presently a member of CIGRÉ SC 13 “Switching Equipment” and the International Advisory Group for the International Conference on Electrical Contacts.

Harald Jensvold was born in Orkanger, Norway in 1941. He received a BE degree in electric power engineering from Trondheim College of Engineering, Trondheim, Norway in 1965. He has been with the Norwegian Electric Power Research Institute (EFI) later to become SINTEF Energy Research in Trondheim, Norway since 1965. His professional interests cover HV and LV distribution network technology, especially the effects of electrical, mechanical and environmental stresses on components and apparatus.

Mario Jochim was born in Hoyerswerda, Germany in 1977. He received a MSc degree in electric power engineering from Dresden University of Technology in 2001. He is presently working towards a doctorate degree at the same university.