Thermal cycling test as a quality control and/or acceptance test for HV form-wound stator coils and bars

Thermal Cycle Testing (TCT) has been commonly used for many years as a methodology to determine the ability of high-voltage form-wound stator coils and bars of large rotating machines to resist deterioration of insulation system when rapid heating and cooling during machine load cycling occurs. As a result, after many load cycles, delamination of the insulation usually occurs, intensifying discharges within the insulation and likely winding failure. The test method is described in international standards IEEE 1310 and IEC 6003418-34. The test condition somewhat resembles the operating conditions of a machine, however, not fully incorporating the real conditions such as slot simulation, vibrations etc. Further, for monitoring the insulation condition during the test a series of diagnostic tests mainly consisting of tap test, dielectric dissipation factor (Dissipation Factor) and tip-up (C&DF), partial discharge (PD) measurements etc. along with AC voltage breakdown test accompanied by dissections and microscopic examinations are often employed in the test program. Despite some limitations associated with the thermal cycling test, manufacturing flaws such as delamination, inadequate bonding, abnormal thermal insulation degradation etc. can be determined. Case studies to illustrate how the TCT, along with relevant diagnostic and other post-thermal cycling tests, as well as analysis of dissections, helped to formulate and execute informed decisions in coils/bars process qualification.