Method and device for determining the relative humidity of an insulating liquid filled electric apparatus

1. A method for determining the relative humidity of an insulating liquid filled electric apparatus, the method comprising the steps of:

providing a container containing insulating liquid and gas, a communication unit providing communication between the gas of the container and gas of an external environment, and dehydration of gas passing through the communication unit:

providing an electric unit and enclosing the electric unit within the container such that the electric unit is completely submerged within the liquid and below an interface between the liquid and the gas of the container;

continuously measuring by a sensor and storing by a computing unit a measurement of the relative humidity of the gas in the container,

calculating by the computing unit an average value of the relative humidity of the gas based on said stored humidity measurements during a certain time period, and

determining by the computing unit the relative humidity of the insulating liquid based on said average value of the relative humidity of the gas and a relationship between the relative humidity of the gas and the relative humidity of the insulating liquid at equilibrium.

2. The method according to claim 1 , wherein the method further comprises indicating a malfunction of the insulating liquid if said determined relative humidity of the insulating liquid exceeds a certain value.

3. The method according to claim 1 , wherein the method further comprises:

continuously measuring and storing the temperature of the insulating liquid of the container,

calculating an average value of the temperature in the container based on said stored temperature measurements during said time period,

producing a relationship between the relative humidity of the gas and the relative humidity of the insulating liquid at equilibrium in dependence on the average value of the temperature, and

determining the relative humidity of the insulating liquid based on the produced relationship.

4. The method according to claim 1 , wherein the average value of the relative humidity is based on stored humidity measurements during a time period of more than one day.

5. The method according to claim 1 , wherein the method further comprises:

detecting when the temperature in the insulating liquid drops between a first and a second temperature, and the difference between the first and the second temperature is larger than a certain value,

calculating the difference in the relative humidity of the gas in the container at the first temperature and at the second temperature, and

indicating a malfunction of the communication unit based on said calculated relative humidity difference.

6. The method according to claim 5 , wherein the malfunction of the communication unit is indicated when said calculated relative humidity difference is less than a certain value.

7. The method according to claim 1 , wherein the electric unit is a tap changer and wherein such method detects a malfunction of the tap changer.

8. A device for determining the relative humidity of an insulating liquid filled electric apparatus, the device comprising:

a container containing insulating liquid and gas;

a communication unit providing communication between the gas of the container and gas of an external environment, and providing dehydration of gas passing through the communication unit;

an electric unit enclosed within the container, the electric unit being completely submerged within the liquid and below an interface between the liquid and the gas of the container;

a first sensor is configured to continuously measure the temperature in the insulating liquid of the container;

a second sensor configured to continuously measure the relative humidity of the gas;

a computing unit configured to receive and store measurement values from the second sensor, and to calculate an average value of the relative humidity in the gas based on said stored humidity measurements during a certain time period, to determine the relative humidity of the insulating liquid based on said average value of the relative humidity of the gas and a relationship between the relative humidity of the gas and the relative humidity of the insulating liquid at equilibrium.

9. The device according to claim 8 , wherein the computing unit is configured to indicate a malfunction of the insulating liquid if said determined relative humidity of the insulating liquid exceeds a certain value.

10. The device according to claim 8 , wherein the computing unit is configured to receive and store measurement values from the first sensor and the second sensor, and to calculate an average value of the temperature in the container based on said stored temperature measurements during a certain time period, and to produce a relationship between the relative humidity of the gas and the relative humidity of the insulating liquid at equilibrium in dependence on the average value of the temperature, and to determine the relative humidity of the insulating liquid based on said average value of the relative humidity in the gas and the produced relationship between the relative humidity of the gas and the relative humidity of the insulating liquid at equilibrium.

11. The device according to claim 8 , wherein the computing unit is configured to receive and store measurement values from the first sensor and the second sensor, to detect when the temperature in the insulating liquid drops between a first and a second temperature, and the difference between the first and the second temperature is larger than a certain value, to calculate the difference in the relative humidity of the gas in the container at the first temperature and at the second temperature, and to indicate a malfunction of the communication unit based on said calculated humidity difference.

12. The device according to claim 8 , wherein the electric unit is a tap changer.