System and method of determining age of a transformer
A method of determining polymerisation of transformer insulation within a transformer. The method includes the steps of measuring a first and second moisture activity of oil in a transformer, and a first and second temperature of the transformer. The method further includes calculating the ratio of the gradients of the moisture equilibrium curves associated with the moisture activity and the temperature. The calculated ratio can then be used to characterise the polymerisation of the insulation and age of the transformer.
1. A method of determining polymerisation of transformer insulation, the method comprising the steps of:
measuring a first temperature of a transformer;
measuring a first moisture activity of oil in the transformer at the first temperature;
measuring a second temperature of the transformer;
measuring a second moisture activity of oil in the transformer at the second temperature; and
calculating a degree of polymerisation of transformer insulation from the first temperature, the first moisture activity, the second temperature and the second moisture activity.
2. The method of claim 1 , wherein a moisture content of paper of the transformer is not measured or calculated and the moisture content of paper of the transformer is assumed to be substantially constant between the first temperature and the second temperature.
3. The method of claim 1 , further comprising the steps of:
calculating a first water content of oil based on the first moisture activity and the first temperature; and
calculating a second water content of oil based on the second moisture activity and the second temperature.
4. The method of claim 3 , wherein the first and second water content of oil are calculated using
WCO
=
a
w
×
10
A
B
T
,
where a w is moisture activity, WCO is the Water Content of the Oil, A and B are constants for the type of oil used, and T is temperature of the oil in Kelvin, and wherein the degree of polymerisation is calculated using
R
=
MCO
T
2
-
MCO
T
1
MCO
T
1
,
where MCO T1 is the first Water Content of the Oil at the first temperature T1 and MCO T2 is the second Water Content of the Oil at the second temperature T2.
5. The method of claim 1 , wherein the degree of polymerisation is calculated using
R
=
(
a
w
T
2
×
10
(
B
T
1
-
B
T
2
)
-
a
w
T
1
)
a
w
T
1
where a wT1 is the moisture activity at the first temperature T1 and a wT1 is the moisture activity at the second temperature T2 and B is a constant for the type of oil used, and T is temperature of the oil in Kelvin.
6. A system adapted to determine polymerisation of transformer insulation, the system comprising:
a transformer;
at least one sensor adapted to measure moisture activity of oil and temperature of the transformer; and
a processor configured to:
receive a first temperature of the transformer from the at least one sensor;
receive a first moisture activity of oil in the transformer at the first temperature from the at least one sensor;
receive a second temperature of the transformer from the at least one sensor;
a second moisture activity of oil in the transformer at the second temperature from the at least one sensor; and
calculate a degree of polymerisation of transformer insulation from the first temperature, the first moisture activity, the second temperature and the second moisture activity.
7. The system of claim 6 , wherein the system comprises two sensors comprising:
a capacitive oil sensor to measure the moisture activity of oil of the transformer; and
a temperature sensor to measure the temperature of the transformer.
8. The system of claim 6 , wherein the processor is further configured to:
calculate a first water content of oil based on the first moisture activity and the first temperature; and
calculate a second water content of oil based on the second moisture activity and the second temperature,
wherein the step of calculating the degree of polymerisation of transformer insulation comprises using the first water content of oil associated with the first moisture activity and the first temperature and the second water content of oil associated with the second moisture activity and the second temperature.
9. The system of claim 6 , wherein a moisture content of paper of the transformer is assumed to be substantially constant between the first temperature and the second temperature and the moisture content of paper of the transformer is not measured or calculated.
10. The system of claim 8 , wherein the first and second water content of oil are calculated by the processor using
WCO
=
a
w
×
10
A
-
B
T
,
where a w is moisture activity, WCO is the Water Content of the Oil, A and B are constants for the type of oil used, and T is temperature of the oil in Kelvin, and wherein the degree of polymerisation is calculated by the processor using
R
=
MCO
T
2
-
MCO
T
1
MCO
T
1
,
where MCO T1 is the first Water Content of the Oil at the first temperature T1 and MCO T2 is the second Water Content of the Oil at the second temperature T2.
11. The system of claim 6 , wherein the degree of polymerisation is calculated by the processor using
R
=
(
a
w
T
2
×
10
(
B
T
1
-
B
T
2
)
-
a
w
T
1
)
a
w
T
1
where a wT1 is the moisture activity at the first temperature T1 and a wT1 is the moisture activity at the second temperature T2 and B is a constant for the type of oil used, and T is temperature of the oil in Kelvin.