Diagnostic measurements on instrument transformers – Part II
Part 1 of this article, published in Volume 3 Issue 4, pages 100ff, describes the measurements of excitation, winding resistance, turns ratio and accuracy as the most common diagnostic measurements on instrument transformers (current and voltage transformers) for condition and reliability assessment. Case studies show the failures which can be derived from the results and underline the importance of conducting regular diagnostic tests. Part 2 gives more details about the measurements of capacitance and dissipation/power factor, short circuit impedance, dielectric response analysis and partial discharge.
Keywords: instrument transformer, electro-magnetic circuit, insulation, diagnostic tests
7. Capacitance and dissipation/power factor measurement
The dissipation factor is measured by comparing the current of a test object to a known reference (“ideal” capacitive current). The phase difference between the reference current and the test object current is determined. Calculating the tangent of δ gives the dissipation/loss factor.
The capacitance and dissipation/power factor measurement is a well-established method to evaluate the insulation condition. An ideal (loss-free) insulation consists of a vacuum capacity also referred to as the geometrical capacity C0. If insulation material other than vacuum is being used, one or more polarization processes can be observed. They represent the electrical behavior of the used insulation material(s). Polarization processes cause losses, for example due to a rotation of dipoles. This will furthermore increase the capacitance measured due to a dielectric constant greater than 1. In addition, the insulation material has a certain conductivity which creates conductive losses . A dissipation factor measurement measures a combination of these losses, see Figure 9.
Figure 9. Insulation and its losses
A voltage tip-up test (ramping up of the test voltage) can be used to check whether or not there is any PD activity present. An increase in the dissipation factor at a certain inception voltage indicates possible PD activity. This is a common diagnostic tool on generators and motors. However, a dissipation factor measurement does not give an exact localization of PD. It can only give an overall representation of the insulation condition.
A capacitance and dissipation / power factor measurement on the capacitive stack of a CVT can reveal any possible insulation degradation or even shorted capacitive layers. The physical construction of the capacitive stack is similar to that of condenser bushings. If a capacitive layer should break down, the overall capacitance of the stack will increase.
Likewise, if the dissipation factor increases, it is an indication of an aging process taking place (moisture ingress, partial discharge, etc.).