INSTRUMENT TRANSFORMER

Abstract

An instrument transformer of a type designed for using an insulation medium containing SF.sub.6, said instrument transformer including a housing enclosing an insulation space and further including an electrical active part arranged in the insulation space, said insulation space containing a dielectric insulation medium. The instrument transformer is characterized in that the dielectric insulation medium contains a gaseous mixture including from 3 to 5 mol-% of heptafluoroisobutyronitrile, from 4 to 11 mol-% of oxygen (O.sub.2) and from 84 to 93 mol-% of nitrogen (N.sub.2).

Claims

1. Instrument transformer of a type designed for using an insulation medium containing SF.sub.6, said instrument transformer comprising a housing enclosing an insulation space and further comprising an electrical active part arranged in the insulation space, said insulation space containing a dielectric insulation medium, the dielectric insulation medium containing a gaseous mixture comprising from 3 to 5 mol-% of heptafluoroisobutyronitrile, from 4 to 11 mol-% of oxygen (O.sub.2) and from 84 to 93 mol-% of nitrogen (N.sub.2).

2. Instrument transformer according to claim 1, wherein the amount of oxygen (O.sub.2) in the gaseous mixture is from 4 to 6 mol-%.

3. Instrument transformer according to claim 1, wherein the amount of heptafluoroisobutyronitrile in the gaseous mixture is from 3.5 to 4.5 mol-%.

4. Instrument transformer according to claim 1, wherein the amount of nitrogen (N.sub.2) in the gaseous mixture is from 89.5 to 92.5 mol-%.

5. Instrument transformer according to claim 1, wherein its rated minimum operating temperature is ?5? C. or lower.

6. Instrument transformer according to claim 1, wherein its rated minimum operating temperature is ?30? C.

7. Instrument transformer according to claim 1, wherein the dielectric insulation medium is present in the insulation space at a pressure ranging from 3 bar absolute to 12 bar absolute.

8. Instrument transformer according to claim 1, wherein the instrument transformer is of the type designed for using SF.sub.6 at a pressure of 5 bar absolute or above, preferably 6 bar absolute or above.

9. Instrument transformer according to claim 1, wherein the dielectric insulation medium contains less than 5 mol-% of carbon dioxide.

10. Instrument transformer according to claim 1, wherein the insulation space is sealed by a sealing component comprising a sealing material selected from the group consisting of EPDM rubber, nitrile rubber and butyl rubber.

11. Instrument transformer according to claim 1, wherein the instrument transformer is one of a high voltage instrument transformer, a current transformer and substation voltage transformer.

12. Instrument transformer according to claim 1, wherein the amount of oxygen (O.sub.2) in the gaseous mixture is about 5 mol-%.

13. Instrument transformer according to claim 1, wherein the amount of heptafluoroisobutyronitrile in the gaseous mixture is about 4 mol-%.

14. Instrument transformer according to claim 1, wherein the amount of nitrogen (N.sub.2) in the gaseous mixture is about 91 mol-%.

15. Instrument transformer according to claim 1, wherein the instrument transformer is of the type designed for using SF.sub.6 at a pressure of 6 bar absolute or above.

16. Instrument transformer according to claim 1, wherein the dielectric insulation medium contains less than 2 mol-% of carbon dioxide.

17. Instrument transformer according to claim 1, wherein the dielectric insulation medium is at least essentially devoid of carbon dioxide.

Description

[0042] The present disclosure is further illustrated by means of the following working example in connection with

[0043] FIG. 1 showing a side view of an instrument transformer according to the present disclosure in the form of a current transformer;

[0044] FIG. 2 showing the instrument transformer of FIG. 1 in longitudinal section through a first section plane; and

[0045] FIG. 3 showing the instrument transformer of FIG. 1 in longitudinal section through a second section plane.

EXAMPLE

[0046] An instrument transformer designed for using SF.sub.6 as dielectric insulation medium has been provided.

[0047] An alternative insulation medium containing 91 mol-% of nitrogen, 4 mol-% of heptafluoroisobutyronitrile and 5 mol-% of oxygen has then been filled into the insulation space by means of feed pipe connected to a respective filling valve in the housing enclosing the insulation space.

[0048] The instrument transformer thus filled successfully passed dielectric tests according to IEC regarding lightening impulse withstand voltage, switching impulse withstand voltage, power frequency withstand voltage and partial discharge measurement.

[0049] The dew point of the mixture was determined by constantly cooling down slowly the fluid and monitoring the respective pressure of the fluid, the drop in the pressure indicating the point where condensation starts. Thereby, a dew point of the mixture at ?36? C. was determined, i.e. lower than the dew point of isolated heptafluoroisobutyronitrile at the same partial pressure, which is at ?29? C.

[0050] The alternative gas mixture was further found to be compatible with most materials used in the instrument transformer designed for using SF.sub.6 as dielectric insulation medium. Thus, no design change and major material change are necessary.

[0051] Regarding gas tightness, EPDM O-rings used as standard SF.sub.6-sealing components in the apparatus have shown an acceptable degree of permeation of the alternative gas mixture used. Specifically, the permeation of nitrogen through the EPDM O-rings were found to be reduced by a factor of 7 compared to the permeation of carbon dioxide.

[0052] In the specific embodiment shown in FIG. 1, the insulation transformer (1) comprises a filling valve (2) at its bottom. Via this filling valve (2), the insulation medium is introduced into the insulation space (4) enclosed by the housing (6) to surround electrical active parts arranged in the insulation space, in particular a coil (8) and the portion of main conductor (10) arranged in the insulation space, as shown in FIGS. 2 and 3. The insulation space is sealed by a number of sealing components, of which a main sealing component (12) is shown in FIG. 3.