Method and device for monitoring switchgear in electrical switchgear assemblies

1. A method for determining contact wear in an electrical switchgear of an electric switchgear assembly, wherein a contact current (I f ) flowing through the switchgear during a switching action is recorded using a current transformer and is evaluated with regard to contact wear, wherein

a) in order to determine a status variable characterizing the contact wear (Cwsum), a current measuring signal (I mess ) of the current transformer is first measured as a function of the time,

b) in the event of deviations between the predicted contact current (I f ) and the current measuring signal (I mess ), a saturation of the current measuring signal (I mess ) as a measurement error (Δ) is detected,

c) in the event of detection of the measurement error (Δ) at least one characteristic current value (I char ) is determined from the current measuring signal (I mess ) and is used to determine the status variable (Cwsum), and

d) the at least one characteristic current value (I char ) is a maximum current measuring signal (I max ).

2. The method according to claim 1 , wherein

a maximum current measuring signal (I max ) of the current transformer, which occurs before reaching a quarter period of an alternating current applied to the switchgear, is used as the characteristic current value (I char ).

3. The method according to claim 1 , wherein

a) the contact current (I f ) is an overcurrent or a short-circuit current (I f ) during a switch-off action and/or

b) the status variable (Cwsum) is a measure for an arcing power during the switching action based on a contact current time integral.

4. The method according to claim 1 , wherein

a) the current measuring signal (I mess ) is recorded from a first time point (t 0 ) at the beginning of the current half-wave in which the switching action occurs, until a second time point (t max ), at which the maximum current measuring signal (I max ) occurs, and from the second time point (t max ) until a third time point (t 0 ) at the end of the current half-wave, is approximated by the maximum current measuring signal (I max ) and

b) in order to determine the status variable (Cwsum) a time integral ∫f(I mess )dt is formed over a function f(I mess ) of the recorded and approximated current measuring signal (I mess ).

5. The method according to claim 4 , wherein

a) the first time point (t 0 ) is defined as the starting time of an arc of the contact current (I f ) and is determined with a time delay based on empirical values from an opening command, a protection trigger command or a contact movement of the switchgear and

b) the time delay is corrected by comparing actual values with predicted values of the contact wear.

6. The method according to claim 4 , wherein a power function f(I mess )=I mess a where a=1.2 . . . 2.2, especially a=1.6 . . . 2.0, or a square root function f(I mess )=(I mess 2 ) 1/2 defining an effective switch-off current (I eff ) is used as the function f(I mess ) of the current measuring signal (I mess ).

7. The method according to claim 4 , wherein

a) the status variable (Cwsum) is selected to be equal to the time integral ∫f(I mess )dt times a contact wear constant c and

b) the contact wear constant c is determined from manufacturer's data based on at least one of curves giving the number of permitted switching actions as a function of an effective switch-off current per switching action (i eff ), and empirical values for a type of switchgear and switchgear usage location.

8. The method according to claim 1 , wherein

a) an effective switch-off current (I eff ) is determined for each switching action,

b) from a curve (N(I eff )) giving the number of permitted switching actions (N) as a function of the effective switch-off current (I eff ), a contact wear is determined as a percentage of the switching actions executed relative to the total number permitted for this effective switch-off current (I eff ) and

c) the percentages for all the relevant switching actions executed are summed to give a cumulative contact wear.

9. The method according to claim 1 , wherein

a) the contact wear is monitored on-line or is evaluated with reference to archived data, especially using a matched function f(I mess ) of the current measuring signal (I mess ), and/or

b) the contact wear is determined from recordings of switch-off currents (I mess ) from fault recorders or protection and control equipment having a fault recording function, wherein all recordings of the switch-off currents (I mess ) of a switchgear assembly are collected in a central data acquisition system, via at least one of a data carrier, communication, and a fault recorder collecting system.

10. A computer program for determining contact wear in an electrical switchgear of an electric switchgear assembly which can be loaded and executed on a data processing unit of a plant control system, wherein the computer program executes the steps of the method of claim 1 during implementation.

11. A device for implementing the method according to claim 1 .

12. The device according to claim 11 , wherein

a) the electric switchgear is a circuit breaker and/or

b) the current transformer is a conventional current transformer with a saturable core.

13. A high- or medium-voltage switchgear assembly with a device according to claim 11 .

14. A high- or medium-voltage switchgear assembly with a device according to claim 12 .