At the Pacific Gas and Electric Company (PG&E), located in San Francisco, California, thistemperature increase has led to a noticeable increase in the reported failure rate of tap-connectorsin distribution systems.
Tap-connectors provide an electrical connection between a main power line conductor and a tapconductor. Infrared surveys of PG&E overhead facilities also indicated that hundreds oftap-connectors were running unacceptably hot. Concern over this issue prompted PG&E toevaluate all tap-connector technologies relevant to its overhead primary and secondarydistribution systems. All evaluations of tap-connectors were carried out according to acceptedtesting procedures, such as those of the American National Standard Institute (ANSI).
In reviewing the ANSI evaluation test, a concern was raised that the testing uses "new/unused"electrical conductor to evaluate connector performance. This is inconsistent with field conditionswhere connectors are installed on conductors that have generally been exposed to theenvironment for many years.
With this concern in mind, PG&E undertook the tap-connector evaluation program using threesimultaneously-operated test circuits adapted respectively with new/unused aluminumconductor (as recommended by ANSI C119.4), with cleaned service-aged aluminumconductor obtained from the field, and with as-received/uncleaned service-aged aluminumconductor.
Connector evaluations using cleaned service-aged conductor would be consistent withPG&E's standard installation practice. Evaluations using the as-received/uncleanedservice-aged conductor were carried out for reference purposes only, to determine the effects onelectrical connectibility of the presence of unusually thick contaminant deposits on the conductor.
Thermal Cycling Tests
Ten types of tap-connectors were evaluated in the thermal cycling tests. The generic connectordesigns are illustrated in Fig. 1 and included:
For the thermal cycling tests, four connectors of each type were installed for a total of 40connectors per loop, using 397.5 kcmil AAC conductor. As mentioned earlier, separate loopswere prepared using new/unused conductor, cleaned service-aged conductor andas-received/uncleaned service-aged conductor.
The test loops were prepared in accordance with ANSI C119.4 test specifications, using weldedaluminum electrical equalizer plates to allow electrical resistance measurements across aconnector after selected thermal cycling intervals. A thermocouple was attached to the center ofeach connector, identified by the arrow in Fig. 1, to monitor the temperature excursions duringthermal cycling.
Finally, a 0.61 m length of 397.5 kcmil AAC conductor, terminated at each end with an electricalequalizer plate, was connected in series with each test loop to provide a reference temperatureduring test-cycling. The reference temperature was measured at the middle of the conductorlength. With the exception of the as-received/uncleaned service-aged condition, all conductorswere wire-brush cleaned before installation of the connectors.All loops were mechanically supported on wooded racks in an open laboratory environment andwere electrically energized using AC electrical current. Electrical current was passed through theloops and cycled a maximum of 500 times, with identical current-on and current-off time intervalsof 90 minutes. Temperature was measured once every three hours at the end of each current-oncycle. For loops that used new/unused conductor, the electrical current was adjusted to raisethe temperature of the reference 397.5 kcmil AAC conductor 100 degrees Celcius above the ambienttemperature in accordance to the ANSI C119.4 test standard. A slightly modified procedure wasused to test connectors using service-aged conductors.
According to ANSI C119.4 specifications, a connector is deemed to fail when any of thefollowing conditions are met:
Two bolted wedge connectors failed by thermal runaway and one bolted wedge device failed byRSF. All four formed-wire connectors failed by thermal runaway. Generally, failure by thermalrunaway occurred well in advance of the 500th current cycle.
The 2-bolt parallel groove (PG) connectors did not fare well when two of these devices failed byRSF and the remaining failed by TDF. Finally, the fired wedge-connectors operated at relativelylow temperatures. This suggests superior contact resistance properties in these devices.
In the tests carried out with the cleaned service-aged and as-received/uncleanedservice-aged conductors, several anomalies were recorded. First, it required approximately 750A (rather than the 633 A for the new/unused conductor) to raise the service-aged conductor byapproximately 100 degrees Celcius above ambient. This difference in heating current stems from the differentthermal emissivities of the two conductors. Clearly, the rough and dark surface ofas-received/uncleaned service-aged conductor enhanced the thermal emissivity properties ofthe conductor, thus requiring a larger current to produce a selected temperature rise.
In addition, every connector heated by 750 A exceeded the temperature of the service-agedcontrol conductor within the first current-on cycle, and several of the connectors showed earlysigns of rapid thermal runaway. This rapid deterioration probably stemmed from the presence ofthick contaminant deposits on the conductor surfaces, such as thick oxide films, which hinderedthe generation of electrical interfaces of low contact resistance. In light of these observations, theperformance evaluation for these tests was carried out using a modified procedure.
In the modified test procedure, current cycling was initiated using an AC current value of only 50A. The current was then raised in small steps and again cycled. Current was finally set at 633 Aand cycled for 129 periods. This procedure did not yield the immediate (1st cycle) failuresmentioned earlier for two possible reasons:
Note in Fig. 3 that the temperature of the reference cleaned service-aged conductor was only91.7 degrees Celcius, corresponding to a rise of only 64.1 degrees Celcius above ambient. In this test, a length ofnew/unused reference conductor was also connected to the test loop, for reference purposesonly. Its temperature reached 134.3 degrees Celcius. The temperature of many of the connectors was slightlylarger than 100 degrees Celcius, or only slightly higher than the temperature reached when the connectors wereinstalled with new/unused conductor (Fig. 2). Note in Fig. 3 that the fired wedge-connectorsand the "H" compression connectors performed best, despite observations of TDF failures in allthese connectors.
Under conditions where the as-received/uncleaned service-aged conductor was used, the dataof Fig. 4 show that the reference conductor temperature only rose to 100 degrees Celcius (a 69 degrees Celcius temperaturerise) and only the fired wedge-connectors and the compression sleeve device survived the test. Allother connectors failed by thermal runaway in a time interval considerably shorter than the timerequired to complete the 129 current cycles.
Because the conductor was uncleaned in this case, it is surmised that this latter test is the mostsevere of the three tests described in this section and the results therefore provides an excellentevaluation of the robustness of the surviving devices.
In summary, fired wedge-connectors perform best overall in all the tests described in this section.Fired wedge-connector technology therefore appears superior to all others evaluated in thepresent work. Although compression "H" connectors performed well withservice-aged/cleaned conductor, they did not pass the more severe test using theas-received/uncleaned service-aged conductor. Similarly, the compression sleeve connectorsperformed well with as-received/uncleaned service-aged conductor but, surprisingly,performed less well with cleaned service-aged conductor.
Overall then, the present evaluation indicates that neither compression H nor compressionsleeve connector technology ranks as highly as wedge-connector technology.
Harsh-Environment Tests
The current cycling tests described in the previous section, and specifically the tests using serviceaged conductors, were found effective in ranking connector performance. Although firedwedge-connectors ranked highest, it may be argued that the ultimate reliability test for theconnector is an evaluation of its performance in a harsh environment where the device is exposedto current cycling in a corrosive atmosphere.
Under these conditions, connector failure would presumably be accelerated through the ingress ofcorrosive contaminant into the electrical interfaces of the connector during interfacial motioninduced by current/thermal cycling. For this reason, fired wedge and other types of connectorswere subjected to current cycling in a controlled saline environment.
The performance evaluations were carried out in a corrosion chamber of adequate dimensions tohouse a test loop consisting of at least six connectors in series, and in which a 5 per cent salt fogwas introduced. The chamber was calibrated in accordance with ASTM B117-90 specifications.The conductors were adapted with welded equalizer plates to allow electrical resistancemeasurements across a connector following selected thermal cycling intervals. A thermocouplewas attached to the center of each connector to monitor the temperature excursions duringthermal cycling.
The test loop was mechanically supported on wooden racks in the corrosion chamber and waselectrically energized using AC electrical current. Electrical current was passed through the loopand cycled at least 500 times, with current-on and current-off time intervals of 5 minutes and 55minutes respectively. Salt fog was introduced into the chamber with a time period of 2 hours.
The current cycles were synchronized with the saline fog cycles as indicated in Fig. 5.Temperature was measured once at the end of each current-on cycle and electrical resistance wasmeasured at the end of each current-off cycle. The electrical current was adjusted during the first25 cycles to generate a maximum temperature of 150 degrees Celcius on the hottest connector. Typicalexcursions of the temperature and electrical resistance in fired wedge-connectors, and inconnectors of other designs, are illustrated in Fig. 6.
The curves of Fig. 6(a) show the temperature variations in two bolted connectors. Note that oneof these connectors failed after approximately 490 cycles. Although the second bolted connectorsurvived for the entire test duration (650 cycles), it operated at a more elevated temperature thanthe fired wedge-connectors. The variations in electrical resistance are shown in Fig. 6(b).
The resistance of the bolted connectors was significantly larger than that of the firedwedge-connectors. In contrast with the performance of bolted connectors, and of other types ofconnectors not identified in Fig. 6, no fired wedge-connectors ever showed evidence of imminentfailure in the test. The observation in Fig. 6(a) that the fired wedge-connectors operated at lowertemperatures than the bolted connectors indicates that the power loss was significantly larger inthe bolted connectors.
In summary, fired wedge-connectors have performed well in all the harsh-environment testscarried out to date. Fired wedge-connector technology thus appears superior also underharsh-environment conditions.
Wedge-Connector Technology
The performance attributes of fired wedge-connectors are related to key design characteristicsthat allow the generation of stable electrical contact interfaces.
Because the C-member deforms plastically and generates the force that secures the installedwedge and conductors in place, the mechanical properties of the C-member are the mostimportant to the reliable mechanical function of wedge-connectors.
The mechanical properties were evaluated from force-deformation curves on unas-sembledC-members. These curves were generated bodies using a tensile pull technique in which theC-body is held in a mechanical tensile machine by two cylindrical pins inserted into the curvedends of the C-member. The pins are held in a yoke at inclination angles that match the wedgeangles of the connector and a tensile force is then applied.
The force associated with the increasing C-member deformation is monitored. Typicalforce/displacement plots are shown by the curves of Fig. 8. This data was generated fromC-bodies approximately 89 mm in length, with narrow-end and wide-end dimensions of 22 mmand 49 mm respectively. Note that the C-body deforms elasto-plastically after a displacement ofthe ends of the C-member of approximately 0.5 mm.
The average elastic restoring force following a stretch of 7.6 mm is 21,400 N, with an averageelastic compliance of 1.9 mm. This compliance is large and indicates that the mechanical forceholding the conductors in place in an assembled connector should remain largely unaffected bysmall changes in conductor dimensions. These dimensional changes may arise in practice fromconductor creep or other conductor compaction mechanisms.
The mechanical stresses associated with these displacements, and developed both within theC-member and on the conductor surfaces, were evaluated from a computer model. It was foundthat the stresses developed in the internal flat surface of the C-body exceed the yield value of thematerial. The stresses in the electrical interfaces with the conductors were found to be smallerthan the yield value for AA6061.
In summary, the C-body is characterized by an elastic compliance capable of compensating forcable compaction, and is thus responsible for maintaining a nearly constant mechanical load onconductors during the life of the connector. This property, along with the relatively smallmechanical stresses generated in the electrical contact interfaces that prevent significant conductorcreep flow, accounts in part for the superior performance of fired wedge-connectors in currentcycling and harsh environment tests.
Conclusion
The article has presented experimental evidence that the performance of fired wedge-connectorsunder current/thermal-cycling conditions is superior to that of most other types of tap-connectors.Fired wedge-connectors also perform acceptably under harsh-environment conditions.
It has also pointed out some of the factors responsible for the reliable performance of firedwedge-connectors. These include:
J.D. Sprecher is with Pacific Gas and Electric. J. Schindler, B. Johnson, G. Menechella and R.S.Timsit are with AMP of Canada. This article was excerpted from a paper presented at ESMO-95.