The insulation in the majority of power transformers consists of both oil and cellulose (paper/pressboard). The solid insulation is divided into major and minor insulation structures. The major insulation system includes barriers, spacers and clamps while the minor insulation consists of winding insulation.
Cellulosic transformer insulation serves three functions. Foremost, it acts as a dielectric by storing electrical charge when the transformer is energised and, thereby, isolates transformer components that exist at different voltages. It also fulfils a mechanical function by supporting the windings, and contributes to better thermal health of the transformer by creating cooling ducts for the oil.
The oil must provide sufficient dielectric strength, provide sufficient cooling, preserve the core and coil assembly by filling voids in the insulating materials, and minimise contact of oxygen with cellulose and other materials that are at risk for oxidation.
Insulation performs its roles best when it is clean, dry, relatively void-free, and utilised within a certain temperature bandwidth. The following electrical field tests provide information about the integrity of the transformer insulation.
- Power factor/Tan delta/dissipation factor/capacitance (@ line frequency); a power factor/dissipation factor test provides a general sense of how clean, dry, and relatively void-free the insulation system is, confirmation that the system’s electrical characteristics have not deteriorated with age and operating stresses, and therefore an overall impression of how efficiently the insulation is performing in providing electrical isolation. Capacitance reflects the amount of electrical charge being effectively stored by the insulation and is influenced by changes in the physical attributes of the insulation system being tested.
- Variable frequency power factor/dissipation factor (VFPF); a variable frequency power factor test provides context to a line-frequency power factor/dissipation factor measurement so that a more informed assessment of the insulation system can be made. Specifically, by performing power factor/dissipation factor measurements at additional frequencies, the tester confirms that a seemingly good line-frequency power factor/dissipation factor is indeed good (which sometimes may not be the case), detects problems earlier than they would in a 50/60 Hz power factor/dissipation factor test, and more.
- Tan delta/Power factor/dissipation factor tip-up; used to determine whether a voltage sensitive problem exists in the dielectric of a transformer. A positive indication may suggest the presence of a localised problem, such as tracking (as long as the insulation system under test is not prohibitively large which undermines the sensitivity of the test), or partial discharge type activity.
- Dielectric frequency response (DFR/DRA); dielectric response measurements provide a moisture and oil conductivity assessment of transformers. This is an internationally accepted test that is growing in use. Of the two principal dielectric response methods, an AC method called Dielectric Frequency Response (DFR, also Frequency Domain Spectroscopy, FDS) is preferred due to its robustness against noise.
- Exciting current; an exciting current test provides information regarding the integrity of the turn-to-turn winding insulation and electrical tracking problems in the transformer insulation (for example, tracking somewhere along a winding turn(s) to ground or phase-to-phase electrical tracking).
- Partial discharge (PD);
- DC insulation resistance;
- Dielectric breakdown test on oil; as an insulating liquid, oil’s most important property is a high dielectric strength. The dielectric breakdown voltage is an important measurement of the electrical stress which an insulating oil can withstand without failure. This test checks that the dielectric breakdown voltage of the oil in the main tank is above a minimum threshold. This may be done in the laboratory or in the field.