3 minute read
Thermal cycle testing of wound stator coils
Rapid temperature changes within motor windings can stress insulation and, in time, lead to dangerous conditions or system breakdowns. Hayward Tyler explains how thermal cycle testing can be used to simulate operating environments and test the bonding between the copper widings and their insulation.
Large rotating machines are subjected to rapid transitions from low power to full power and vice versa. For example, hydrogenerators (peaking duty and pumped storage), synchronous condensers, and gas turbine generators are often raised from idle to full power in a matter of minutes, are then operated at full power for hours, and then rapidly reduced to zero output. This load cycling leads to rapid temperature changes within the stator winding. As a result, an alternating shear stress develops within the ground insulation system (IEEE 1310).
If the bond between the copper and the insulation is not adequate, the copper may separate from the insulation. This results in the formation of voids between the insulation and the copper that may permit relative movement of the copper strands/turns, leading to abrasion of the insulation. Also, voids can develop between the layers of the groundwall insulation as a result of delamination. In high-voltage coils, these voids can lead to partial discharges and, under certain circumstances, to a puncture of the insulation (IEEE 1310).
Hence, a thermal cycling test under a controlled condition can simulate the effect of the above to verify the quality of the bonding between the copper and the insulations. This practice can be accelerated by repeatedly heating and cooling the power cables without any hold time at the maximum or minimum temperatures. Verification tests such as Megger, PI and PD can then be carried out at intervals to assess the dielectric condition of the power cables.
Hayward Tyler has a pressure & temperature test rig capable of conducting temperature cycling tests and thermal ageing tests. For temperature cycling, the test chamber can house up to six 2m cable and penetrator samples to test at operational temperatures ranging from ambient to 80°C. For thermal ageing tests, the chamber can be used for durations of up to six months and at temperatures up to 120°C thanks to pressure capabilities that stop the fluid from boiling. While in both test examples, Glythermin can be tested based on Polypropylene glycol (PPG).
PARTIAL DISCHARGE MEASUREMENT OF INSULATIONS
Partial discharge (PD) is a localised electrical discharge that only partially bridges the insulation between conductors and which can or cannot occur adjacent to a conductor. Partial discharges are, in general, a consequence of local electrical stress concentrations in the insulation or on the surface of the insulation. Generally, such discharges appear as pulses having a duration of much less than 1sec (IEC 60270). Partial discharge measurements are a means to identify these potential insulation failures, which can result in an electrical breakdown and can endanger operators.
PD activity may occur in insulating materials for many reasons. In solid insulations, a defect in the manufacturing process could lead to a gas-filled void. Gas bubbles in liquid insulations alsocould be a reason for PD. ‘Corona’ is another form of partial discharge that occurs in gaseous media around conductors which are remote from solid or liquid insulation. Partial discharges are often accompanied by the emission of sound, light, heat, and chemical reactions. Hayward Tyler has test capabilities to carry out partial discharge measurements on a wide range of new or in-service electrical products up to 50 kV. A fully shielded enclosure and a PD free high-voltage transformer are used to conduct the measurements, which can guarantee minimum impact from external sources of PDs. chevron-
