Monitoring arrangement for monitoring a surge arrester and method for establishing monitoring data related to a surge arrester

1. A monitoring arrangement for monitoring a surge arrester which is connected to a live wire of a power network at a connection point on the live wire in order to protect an electrical apparatus of the power network against incoming surges, the monitoring arrangement comprising:

a conductor element, which is configured to be electrically connected to ground and to the surge arrester so that electric current impulses passing through the surge arrester will pass through this conductor element;

an analogue measuring circuit for sensing current impulses passing through the conductor element and producing analogue measuring signals representing such current impulses;

an analog-to-digital converter connected to the analogue measuring circuit and configured to sample the analogue measuring signals from the analogue measuring circuit in order to convert these analogue measuring signals into digital signals; and

an electronic processing device for receiving said digital signals from the analog-to-digital converter and producing monitoring data based on these digital signals;

wherein:

that the electronic processing device is configured to establish, based on digital signals produced by the analog-to-digital converter for a current impulse through the surge arrester caused by an incoming surge in the live wire, a voltage peak value representing the highest voltage at said connection point during the incoming surge; and

that the electronic processing device is configured to establish, based on said voltage peak value and by means of a calculation model or look-up table, a maximum-voltage value representing an estimated maximum voltage across the electrical apparatus during the incoming surge in question.

2. The monitoring arrangement according to claim 1 , wherein the analog-to-digital converter is configured to sample the analogue measuring signals from the analogue measuring circuit at a sampling frequency of 20 MHz or higher, preferably of 35-45 MHz.

3. The monitoring arrangement according to claim 1 , wherein:

that the monitoring arrangement includes a base module and an auxiliary module detachably mountable to the base module;

that the conductor element and the analogue measuring circuit are arranged in the base module;

that the analog-to-digital converter and the electronic processing device are arranged in the auxiliary module;

that the base module includes a first connector member, which is connected to the analogue measuring circuit; and

that the auxiliary module includes a second connector member, which is connected to the analog-to-digital converter, wherein the second connector member is configured to be engaged with the first connector member when the auxiliary module is mounted to the base module to thereby allow the analog-to-digital converter to receive said analogue measuring signals from the analogue measuring circuit via the first and second connector members.

4. The monitoring arrangement according to claim 1 , wherein the monitoring arrangement includes a data storage medium for storing the maximum-voltage values established by the electronic processing device.

5. The monitoring arrangement according to claim 4 , wherein the monitoring arrangement includes communication unit for wireless transmission of maximum-voltage values stored on the data storage medium.

6. The monitoring arrangement according to claim 1 , wherein:

that the electronic processing device is configured to established, based on said digital signals for the current impulse, a front time value and a current peak value representing the front time and the highest current, respectively, of the current impulse; and

that the electronic processing device is configured to take said front time value and current peak value into account when establishing the voltage peak value.

7. The monitoring arrangement according to claim 6 , wherein:

that the electronic processing device is configured to establish, based on said front time value and current peak value, a first voltage value representing the highest discharge voltage across the surge arrester during the incoming surge in question, as derived from the voltage-current characteristic of the surge arrester;

that the electronic processing device is configured to establish, based on said digital signals for the current impulse and an inductance value representing the estimated total inductance of the electric path from the connection point to the surge arrester, across the surge arrester and from the surge arrester to ground, a second voltage value representing the highest inductance-related voltage drop occurring in said electric path during the incoming surge in question; and

that the electronic processing device is configured to establish the voltage peak value as the sum of the first voltage value and the second voltage value.

8. The monitoring arrangement according to claim 1 , wherein:

that the electronic processing device is configured to establish a steepness value representing the steepness of the incoming surge, wherein the electronic processing device is configured to establish the steepness value as the quotient between the voltage peak value and the front time value;

that the electronic processing device is configured to establish a multiplication factor from a stored lock-up table with the voltage peak value and the steepness value as input values in the look-up table; and

that the electronic processing device is configured to establish the maximum-voltage value as the product of the voltage peak value and the multiplication factor.

9. The monitoring arrangement according to claim 1 , wherein the analogue measuring circuit includes an inductive sensing member for sensing current impulses passing through the conductor element.

10. A method for establishing monitoring data related to a surge arrester which is connected to a live wire of a power network at a connection point on the live wire in order to protect an electrical apparatus of the power network against incoming surges, wherein the method includes the steps of:

a) producing an analogue current signal by means of an analogue measuring circuit which senses current impulses passing through a conductor element electrically connected to ground and to the surge arrester, wherein said analogue current signal represents a current impulse which passes through the surge arrester and the conductor element and which is associated with an incoming surge in the live wire;

b) sampling the analogue current signal from the analogue measuring circuit by means of an analog-to-digital converter in order to convert this analogue current signal into digital signals representing said current impulse;

c) establishing, based on said digital signals, a voltage peak value representing the highest voltage at said connection point during the incoming surge; and

d) establishing, based on said voltage peak value and by means of a calculation model or look-up table, a maximum-voltage value representing an estimated maximum voltage across the electrical apparatus during the incoming surge in question.

11. The method according to claim 10 , wherein the analog-to-digital converter samples the analogue current signal from the analogue measuring circuit at a sampling frequency of 20 MHz or higher.

12. The method according to claim 10 , wherein said maximum-voltage value is stored on a data storage medium.

13. The method according to claim 10 , wherein a front time value and a current peak value representing the front time and the highest current, respectively, of the current impulse is established based on said digital signals for the current impulse, wherein said front time value and current peak value are taken into account when establishing the voltage peak value.

14. The method according to claim 13 , wherein:

that a first voltage value, which represents the highest discharge voltage across the surge arrester during the incoming surge in question as derived from the voltage-current characteristic of the surge arrester, is established based on said front time value and current peak value and the voltage-current characteristic of the surge arrester;

that a second voltage value, which represents the highest inductance-related voltage drop occurring in the electric path from the connection point to the surge arrester, across the surge arrester and from the surge arrester to ground during the incoming surge in question, is established based on said front time value and current peak value and an inductance value representing the estimated total inductance of said electric path; and

that the voltage peak value is established as the sum of the first voltage value and the second voltage value.

15. The method according to claim 13 , wherein:

that a steepness value, which represents the steepness of the incoming surge, is established as the quotient between the voltage peak value and the front time value;

that a multiplication factor is established from a stored lock-up table using the voltage peak value and the steepness value as input values in the look-up table; and

that the maximum-voltage value is established as the product of the voltage peak value and the multiplication factor.

16. The monitoring arrangement according to claim 2 , wherein:

that the monitoring arrangement includes a base module and an auxiliary module detachably mountable to the base module;

that the conductor element and the analogue measuring circuit are arranged in the base module;

that the analog-to-digital converter and the electronic processing device are arranged in the auxiliary module;

that the base module includes a first connector member, which is connected to the analogue measuring circuit; and

that the auxiliary module includes a second connector member, which is connected to the analog-to-digital converter, wherein the second connector member is configured to be engaged with the first connector member when the auxiliary module is mounted to the base module to thereby allow the analog-to-digital converter to receive said analogue measuring signals from the analogue measuring circuit via the first and second connector members.

17. The monitoring arrangement according to claim 2 , wherein the monitoring arrangement includes a data storage medium for storing the maximum-voltage values established by the electronic processing device.

18. The monitoring arrangement according to claim 2 , wherein:

that the electronic processing device is configured to establish a steepness value representing the steepness of the incoming surge, wherein the electronic processing device is configured to establish the steepness value as the quotient between the voltage peak value and the front time value;

that the electronic processing device is configured to establish a multiplication factor from a stored lock-up table with the voltage peak value and the steepness value as input values in the look-up table; and

that the electronic processing device is configured to establish the maximum-voltage value as the product of the voltage peak value and the multiplication factor.

19. The method according to claim 11 , wherein said maximum-voltage value is stored on a data storage medium.

20. The method according to claim 14 , wherein:

that a steepness value, which represents the steepness of the incoming surge, is established as the quotient between the voltage peak value and the front time value;

that a multiplication factor is established from a stored lock-up table using the voltage peak value and the steepness value as input values in the look-up table; and

that the maximum-voltage value is established as the product of the voltage peak value and the multiplication factor.

21. The method according to claim 10 , wherein the analog-to-digital converter samples the analogue current signal from the analogue measuring circuit at a sampling frequency of 35-45 MHz.