Method and apparatus for accelerated testing of power cable insulation for susceptibility to treeing

Claim 2

An accelerated method of testing the insulation of an insulated conductor for susceptibility to treeing, said insulation having an inner annular surface adjacent to the conductor and an outer annular surface, said method comprising the steps of: wetting at least a portion of at least one of said surfaces of the insulation of a sample of the insulated conductor with a predetermined wetting agent; and applying an alternating current electrical potential for a prolonged period of time across the insulation between the wetted surface portion and the adjacent portion of the other surface of the insulation, said alternating current potential having a voltage at least equal to the rated voltage of the insulated conductor and a frequency substantially higher than the normal operating frequency of the insulated conductor to provide an electrical stress on the insulation and promote a reaction involving the wetting agent and the insulation material to cause an accelerated development of electrochemical trees in insulation susceptible to treeing.

Claim 3

The method defined in claim 1 wherein the conductor of the sample is maintained at approximately its normal operating temperature during said step of applying.

Claim 4

The method defined in claim 2 wherein the conductor is maintained at approximately its normal operating temperature by inductively heating the metal of the conductor.

Claim 5

The method defined in claim 1 wherein the conductor is maintained at a temperature in the range from 70* to 90* C during said step of applying.

Claim 6

The method defined in claim 1 wherein said wetting agent is a liquid having a dipole moment at least equal to one.

Claim 7

The method defined in claim 1 wherein the inner surface of the insulation is wetted by introducing the wetting agent into the insulated conductor inside the insulation.

Claim 8

The method defined in claim 1 wherein the conductor is a stranded conductor and the inner surface of the insulation is wetted by introducing the wetting agent between the strands of the conductor.

Claim 9

The method defined in claim 6 wherein the outer surface of the insulation is also wetted and wherein the alternating current electrical potential is applied across the insulation between adjacent wetted portions of the inner and outer surfaces.

Claim 10

The method defined in claim 1 wherein the insulated conductor further includes an annular conductive shield surrounding the insulation and wherein the alternating current electrical potential is applied between the conductor and the annular shield.

Claim 11

The method defined in claim 1 wherein the outer surface of the insulation is wetted and wherein the alternating current electrical potential is applied between the conductor and the wetted outer surface of the insulation.

Claim 12

The method defined in claim 1 wherein the voltage of said alternating current electrical potential is at least equal to the rated voltage of the insulated conductor.

Claim 13

The methOd defined in claim 1 wherein the voltage of said alternating current electrical potential is in the range from one to five time the rated voltage of the insulated conductor.

Claim 14

The method defined in claim 1 wherein the frequency of said alternating current electrical potential is at least 400 Hz.

Claim 15

The method defined in claim 1 wherein the frequency of said alternating current electrical potential is in the range from 5 kHz to 10 kHz.

Claim 16

The method of claim 1 wherein the sample is maintained in a U-shape while said alternating current electrical potential is being applied.

Claim 17

An accelerated method of testing the solid insulation of a stranded conductor power cable for susceptibility to gradual electro-chemical deterioration, said insulation having an inner annular surface adjacent to the conductor and an outer annular surface, said method comprising the steps of: introducing a liquid having a dipole moment at least as high as that of water into the strands of the conductor in a sample of the cable; heating the conductor of the sample to at least the normal operating temperature of the cable; and applying an alternating current electrical potential across the insulation of the sample for a prolonged period of time between the inner and outer surfaces of said insulation, said alternating current potential having a voltage at least equal to the rated voltage of the cable and a frequency of at least 400 Hz to provide an electrical stress on the insulation and promote a reaction involving the wetting agent and the insulation material to cause an accelerated development of electro-chemical trees in cables susceptible to electro-chemical deterioration.

Claim 18

An accelerated method of testing the solid insulation of a stranded conductor power cable for susceptibility to gradual electro-chemical deterioration, said insulation having an inner annular surface adjacent to the conductor and an outer annular surface, said method comprising the steps of: introducing a liquid having a dipole moment at least as high as that of water into the strands of the conductor in a sample of the cable; heating the conductor of the sample to at least the normal operating temperature of the cable; and applying an alternating current electrical potential across the insulation of the sample for a prolonged period of time between the inner and outer surfaces of said insulation, said alternating current potential having a voltage at least equal to the rated voltage of the cable and a frequency of at least 400 Hz to provide an electrical stress on the insulation and promote a reaction involving the wetting agent and the insulation material to cause an accelerated development of electro-chemical trees in cables susceptible to electro-chemical deterioration.

Claim 19

The method defined in claim 16 wherein said alternating current electrical potential has a voltage in the range from one to five times the rated voltage of the cable.

Claim 20

The method defined in claim 17 wherein said alternating current electrical potential has a frequency in the range from 5 to 10 kHz.

Claim 21

The method defined in claim 18 wherein the conductor is heated to a temperature in the range from 70* to 90* C.

Claim 22

The method defined in claim 19 wherein the conductor is heated by inductive heating of the conductor metal.

Claim 23

The method defined in claim 20 wherein the sample is maintained in a U-shape during said step of applying.

Claim 24

The method defined in claim 16 wherein at least a portion of the exterior of the sample is immersed in said liquid during said step of applying.

Claim 25

The method defined in claim 22 wherein said alternating current electrical potential is applied between the conductor of the sample and the liquid in which the exterior of the sample is immersed.

Claim 26

The method defined in claim 23 wherein the sample is maintained in a U-shape during said step of applying.

Claim 27

The method defined in claim 16 wherein said cable further includes a conductive shield surrounding the outer surface of the insulation and wherein said alternating current electrical potential is applied between the conductor of the sample and the shield.

Claim 28

The method defined in claim 25 wherein at least a portion of the exterior of the sample is immersed in said liquid during said step of applying.

Claim 29

The method defined in claim 26 wherein the sample is maintained in a U-shape during said step of applying.