APPENDIX A: GENERATOR DIAGNOSTIC TESTING
The following factors affect the insulation systems in generators:
● High temperature
● Environment
● Mechanical effects such as thermal expansion and contraction, vibration, electromagnetic bar forces, and motor start-up forces in the end turns
● Voltage stresses during operating and transient conditions
All these factors contribute to loss of insulation integrity and reliability. These aging fac- tors interact frequently to reinforce one another’s effects. For example, high-temperature operation could deteriorate the insulation of a stator winding, loosen the winding bracing system, increase vibration, and cause erosion. At some point, high-temperature operation could lead to delamination of the core and internal discharge. This accelerates the rate of electrical aging and could lead to a winding failure.
Nondestructive diagnostic tests are used to determine the condition of the insulation and the rate of electrical aging. The description of the recommended diagnostic tests for the insulation system of motors and the conditions they are designed to detect are discussed.
Stator Insulation Tests
An electrical test is best suited to determine the condition of electrical insulation. The tests on insulation systems in electrical equipment can be divided into two categories:
1. High-potential (hipot), or voltage-withstand, tests
2. Tests that measure some specific insulation property, such as resistance or dissipation factor
Tests in the first category are performed at some elevated ac or dc voltage to confirm that the equipment is not in imminent danger of failure if operated at its rated voltage. Various stan- dards give the test voltages that are appropriate to various types and classes of equipment. They confirm that the insulation has not deteriorated below a predetermined level and that the equipment will most likely survive in service for a few more years. However, they do not give a clear indication about the condition of the insulation.
The second category of electrical tests indicates the moisture content; presence of dirt; development of flaws (voids), cracks, and delamination; and other damage to the insula- tion. A third category of tests includes the use of electrical or ultrasonic probes that can determine the specific location of damage in a stator winding. These tests require access to the air gap and energization of the winding from an external source. These tests are con- sidered an aid to visual inspection.