84. The Ramped Direct High-Voltage Method of Generator Stator Winding Insulation Testing Rux L.M., Bertenshaw D.R. (Proceedings, Hydro 2003, Vol. I, Croatia, pp. 539). Abstract Deregulation of the electric utility industry has increased the need to monitor the state of power plant equipment, such as critical generators and motors, in order to improve availability and reduce life cycle costs via condition-based maintenance. To achieve these goals, nondestructive diagnostic tests are necessary to evaluate the quality and condition of high-voltage insulation systems. This paper describes the ramped direct high-voltage method of assessing insulation condition as well a test equipment available to perform the test. Ramped voltage test curves representing the most common types of stator winding insulations are provided and interpreted in terms of the variations in the measured current versus applied voltage response. Test results obtained from in-service machines are included to illustrate the types of stator winding problems that have been detected and identified using ramped voltage tests. The dielectric materials and physical structure of an insulation system are fundamentally responsible for its mechanical and electrical properties. Insulation defects, damage, and aging can manifest as anomalies or changes in these properties. The measured current versus applied voltage response obtained during ramped direct high-voltage tests can be used to detect such changes, and to monitor the condition and rate of degradation in service. This information may also help determine when and what corrective actions are needed. As demonstrated herein, the ramped voltage test is an effective and practical means of identifying stator windings with cracks and fissures, tape separations and delamination, moisture absorption, incomplete resin cure, and conductive surface contamination. When attempting to relate an I-V response to the quality and condition of the insulation it must be recognized that a variety of conduction, polarization, and ionization mechanisms exist which can affect the behaviour of the current. An abnormal current characteristic may be caused by contaminants such as moisture either in the insulation or on exposed surfaces, internal void ionization, surface corona, conduction across internal or external creepage paths, and many other possible deficiencies. In some cases, distinctly different insulation problems appear to affect the measured current in essentially identical ways. Although ramped voltage tests are relatively simple to perform, knowledge of high-voltage insulation system and testing is required to properly interpret anomalous test results. Because the measured current must be visually unraveled to assess insulation condition and diagnose problems, experience with interpretation and/or additional information is sometimes required to properly evaluate the test results. Although the diagnostic capabilities of ramped voltage testing are well-known in North America, this method has not enjoyed worldwide use, primarily because the testing equipment was not commercially available until recently. In the past, only a few electric utilities with the requisite in-house resources have been able to build ramped voltage test instruments and incorporate this technique into their motor and generator maintenance programs. Given the increased availability of test equipment, however, more generator maintenance personnel will be afforded the opportunity to conduct ramped voltage tests and interpret the results, and thereby benefit from this diagnostic tool. Top Keywords Deregulation, Utility industry, Diagnostic, Insulations, Stator winding, Voltage tests. Top |