Condition assessment of instrument transformers using dielectric response analysis

Dielectric response analysis is an advanced technique, providing information about the insulation condition of the measured test object. It is typically used for power transformers, where the water content in the solid cellulosic insulation can be assessed. Water in oil-paper/pressboard insulations is decreasing the dielectric strength, accelerating the ageing and causing the bubble effect at high temperatures. Also instrument transformers are oil-paper insulated and therefore exposed to the harming effects of water. The condition assessment of the insulation system of instrument transformers has not played an important role up to now since typical measurement methods which might be used for power transformers are not applicable here. For example oil sampling is often avoided due to the small oil volume. However, the increasing age is leading to a higher risk of explosions, which can damage surrounding parts and cause high follow-up costs. This paper introduces a new approach to use the dielectric response analysis for condition assessment of instrument transformers.

The measurement of the dielectric response in a wide frequency range (e.g. 100 μHz up to 5 kHz) provides information about the insulation condition and especially for oil-paper/pressboard insulations about the water content in the solid insulation. The measured curve is formed by the superposition of the dielectric response of the cellulosic material and the oil. It is reflecting the conductivity and polarization effects of each material. Due to the high content of cellulosic material, the oil has a minor impact on the dielectric response of an instrument transformer and the curve is highly determined by the behavior of the cellulosic material.

The resulting curve can be used for the assessment of the insulation condition in various ways. Qualitative comparisons, e.g. between instrument transformers of the same type and age, might give general information which of these is in better condition. The dissipation factor and the capacitance trend can also deliver valuable data. Furthermore, the water content in the solid insulation can be assessed. An automatic analysis algorithm compares the measured dielectric properties of the actual instrument transformer with modeled dielectric properties to calculate the water content in the solid insulation. This paper describes the modeling in concerns of water content, oil conductivity and geometry. Furthermore, the influences of temperature and ageing products are explained.

To confirm the validity of this assessment, measurements on several instrument transformers in service were performed. Dielectric response curves and the trend of the capacitances for new and aged instrument transformers with different water contents are discussed. Selected examples presented in this paper show, that the dielectric response is well-modeled and the water content can be derived. Also ageing effects can be observed. To avoid explosions of their instrument transformers, this method is now regularly used by TNB.