Observations on the Polarization Index Test as Applied to Strip-On-Edge Field Windings of Salient Pole Hydro Generators

A test commonly used to determine the condition of salient pole hydro generator stator and field windings is the Insulation Resistance and Polarization Index test. The insulation systems applied to field windings are simpler and contain fewer components than those used in stator windings, which can result in reduced levels of absorption (polarization) current being observed during the test. This has led some to conclude that Polarization Index test results are of little value when the test is applied to hydro-generator field windings, particularly those with “strip-on-edge” field windings. This paper reviews the basic theory of the test, field winding insulation systems, when and how to perform the test, and the current industry standard guidance on the applicability of the test to salient pole field windings. A summary of the results of Polarization Index tests on both service-aged and new “strip-on-edge” field windings is presented, and a further review of the usefulness of the test for field windings is proposed.