# Time Series RUL Estimation of Medium Voltage Connectors to Ease Predictive Maintenance Plans
## 5 Experimental Setup and Historical Data Acquisition to Validate the Models  
This paper analyzes the behavior of seven bimetallic friction-welded medium-voltage connectors of the same model (ICAU120 from the catalogue of SBI Connectors). These connectors are of Class B according to IEC 61238-1-3:2018 standard [25]. Figure 4a shows one of the connectors before being compressed to the conductor. The connectors are made of copper and aluminum and are designed to be connected to 120 mm\({}^{2}\) aluminum stranded conductors. During installation, the stranded conductors are crimped within the aluminum barrels of the connectors [31] using an hexagonal crimping tool, model BURNDI Y35-BH EP-1 HP (max compression force 120 kN) (Manchester, UK) with a remote compression head of 120 kN force. The current loop used in this work has been created by bolting the connectors to each other using M10 bolts, which were tightened by applying a torque of 35 Nm. It is noted that to minimize the contact resistance between the stranded conductor and the aluminum barrel of the connector, the internal part of the barrel is filled with a contact grease that withstands 140 \({}^{\circ}\)C.  
Figure 4: (**a**) Bimetallic aluminum–copper connector before compression. (**b**) Connectors placed in the loop to run the heat cycles illustrating the voltage drop and temperature measurements to obtain the electrical resistance. Wire equalizers allow generating an equipotential measuring point to improve the accuracy of the voltage drop measurement. Figure 3: Flux diagram illustrating the key steps of the proposed approach to determine the RUL. The final objective of this research work is to determine the RUL of substation connectors, since they are often placed on critical points of the substations, and thus, failure of a substation connector can lead to costly and severe consequences. However, due do their size and rated current, the tests needed to acquire historical data require more time, power, and energy than when using medium-voltage connectors. Therefore, the results obtained with the medium-voltage connectors studied in this work will be used to validate the proposed approach, which latter will be applied to the substation connectors. In a final application, the data (voltage drop, current and temperature) will be acquired on-line in the field, i.e., at the location where the connector is installed. In this work, the data is acquired in the AMBER high-voltage and high-current laboratory of the Universitat Politecnica de Catalunya. To acquire degradation data and to characterize the thermal and electrical behavior of the seven analyzed connectors, they were subjected to heat cycle tests following the procedure detailed in the IEC 61238-1-3:2018 standard [25]. Heat cycle tests are used to expedite the degradation process due to thermal ageing [32]. In this work, the heat cycles are a means of collecting experimental data to validate the accuracy and appropriateness of the proposed RUL approach. For such tests, the connectors are fitted on stranded aluminum conductors as shown in Figure 4b. Equalizers are used in the test loop to ensure a point of equipotential in the stranded conductor, so that the voltage drop can be measured over a specific distance. Each heat cycle consists of a heating and a cooling phase, thus producing thermal expansion and contraction effects, respectively, on the conductor and connectors, these effects have a non-negligible impact on the contact resistance, thus affecting the thermal and electrical behavior of the connectors. During the heating phase, an AC electric current is forced to flow through the loop to the point where the reference conductor temperature is 120 \({}^{\circ}\)C at thermal equilibrium. This condition was achieved by circulating an electrical current in the range of 350-380 A\({}_{\text{RMS}}\). Once the equilibrium was reached, this elevated current was maintained during 15 min as described in the IEC 61238-1-3:2018 standard.