EP1120801B1 - Method for monitoring wear of contacts of a step switch - Google Patents

Abstract

A method of monitoring contact burnoff in tap changers operating under load in which the load current is measured and for nominal variation of the voltage of the particular tap parameters are stored which are used to calculate the burnoff rate per contact per switching operation. From these values the cumulative burnoff rate of both the switching contact and resistance contact are determined and compared with limits or threshold values.

Description

The invention relates to a method for monitoring the contact erosion in tap changers, especially the Kontaktabbrandes of arc-switching contacts, in which these contacts are designed as mechanical switching contacts.

Tap changers have been in use worldwide for many years for uninterrupted switching between different winding taps of step transformers in large numbers. Such tap changer usually consist of a selector for powerless selection of the respective winding tap of the tapped transformer to be switched to, and a diverter switch for the actual switching from the previous to the new, preselected winding tap. The diverter switch usually has switching contacts and resistance contacts. The switch contacts are used for direct connection of the respective winding tap with the load dissipation, the resistor contacts for short-term wiring, d. H. Bridging by means of one or more switching resistors. Although in recent years also diverter switch with thyristors or vacuum switching cells have become known as switching elements, nevertheless, the vast majority of all in-use diverter switch today and probably in the near future still mechanical contacts.

The switching and resistance contacts such tap changer usually consist of an arc-resistant copper-tungsten alloy. When switching these contacts are regularly arcs that melt or burn small amounts of the contact material and thus lead to a contact erosion. The contact erosion of switching and resistance contacts is thus an important criterion for assessing the state of a tap changer and for predicting necessary inspections. All switch-disconnector systems of tap-changers are somewhat dependent on the breakdown of the switching and resistance contacts in terms of their kinematics. If the contacts burn to different extents, the switching and overlapping times of the individual switching steps within the sequence can change during a load changeover in such a way that safe functioning is no longer guaranteed. For this reason, the inspection instructions for tap-changers usually specify the permissible burn-up differences or burn-up limits. If these limits are exceeded, the contacts must either be replaced by new ones or switching and resistance contacts must be interchanged. If a contact is completely burned down, it must be replaced anyway.

• Aus der DE-GM 296 19 365 und EP 0 948 006 sind rein optische Verfahren zur Restlebensdaueranzeige bzw. zur Abbrandanzeige bekannt.
• Aus den DE-OS 35 15 027 und DE-PS 40 28 721 sind Verfahren bekannt, bei denen der Bogenstrom zwischen den Kontakten ermittelt und als Kriterium verwendet wird.
• Aus der DE-PS 195 44 926 ist ein Verfahren unter Verwendung der Lichtbogenspannung bekannt.
• Die DE-OS 44 27 006 beschreibt ein Verfahren, bei dem der Kontaktdurchdruck der Schaltstücke als Ersatzkriterium für den Kontaktabbrand verwendet wird.
• Die WO97/28549 beschreibt eine entsprechende zugrundeliegende Überwachung der jeweiligen Schaltbewegung, d. h. der zeitlichen Sequenz bei der Schalthandlung.
• In der WO96/13732 ist ein Verfahren mit einer zusätzlichen Signalleitung beschrieben, deren Isolationszerstörung Kriterium für einen abgenutzten Schaltkontakt sein soll.
• Die japanische Offenlegung Hei-4-64206 beschreibt ein Verfahren, bei dem eine Berechnung in Abhängigkeit von der Zahl der von einem Stufenschalter durchgeführten Schaltungen erfolgt.

Numerous methods have already become known with which such contact wear monitoring or, derived therefrom, a remaining service life determination at tap changers or at other high-voltage switch contacts should take place. The known methods can be divided into different groups:

• From DE-GM 296 19 365 and EP 0 948 006 purely optical method for remaining life display or burn-up display are known.
• From DE-OS 35 15 027 and DE-PS 40 28 721 methods are known in which the arc current between the contacts is determined and used as a criterion.
• From DE-PS 195 44 926 a method using the arc voltage is known.
• DE-OS 44 27 006 describes a method in which the contact pressure of the switching pieces is used as a substitute criterion for the contact erosion.
• WO97 / 28549 describes a corresponding underlying monitoring of the respective switching movement, ie the temporal sequence in the switching operation.
• In WO96 / 13732 a method with an additional signal line is described, the insulation destruction is to be criterion for a worn switch contact.
• Japanese Laid-Open Publication Hei-4-64206 describes a method in which calculation is made according to the number of circuits made by a tap changer.

From DE 195 30 776 C1 a method for monitoring a diverter switch for a tap changer is also known, occurring during the load switching arcs are recorded in time and by comparing the times between the individual arcs and possibly an additional comparison of the length of the individual arcs each setpoint values, which are stored non-volatile as characteristic variables of the respective tap changer, a function monitoring takes place. With this method, it is possible to detect indirectly by comparing the actual times between the individual arcs with the corresponding setpoints if the life of contacts has been exceeded, ie. H. whose contact erosion has progressed beyond the permissible level. A direct detection and monitoring of contact erosion is not possible.

Finally, from DE-OS 27 27 378 a device for controlling the operability of switching devices is generally known, wherein the burn-off takes place here from load current measurements by means of a current transformer. For the special conditions on tap changers this device is not suitable.

The methods described have not been able to prevail in tap changers for a variety of reasons. Direct optical and mechanical processes are impractical because of the location of the contacts to be monitored inside the - usually oil-filled - tap changer, processes which require additional measuring leads or the like, which must be led from the interior of the tap changer to the outside, are unsuitable for reasons of dielectric strength , Methods based on the arc current, the arc voltage or the number of completed circuits do not lead to reliable results.

The object of the invention is to provide a method for monitoring the contact erosion in tap changers, which allows reliable and simple as possible and yet exactly the burning of the contacts without requiring visual inspection or required measurements on each contact and corresponding to deviations from a predetermined amount addition Error messages generated.

This object is achieved by a method according to claim 1. The subclaims relate to particularly advantageous developments of the invention.

In a particularly advantageous manner, in the method according to the invention, the contact erosion of each contact is determined on the basis of a burnup rate A. The individual process steps can be carried out in a computer in which the characteristic parameters of the respective tap changer type whose contacts the contact erosion is to be monitored in advance, as well as Abbrandgrenzwerte, when exceeding a warning or other message to be generated, are not stored volatile.

Dabei ist J der von dem zu überwachenden Kontakt des Stufenschalters abzuschaltende Strom. Er ergibt sich rechnerisch auf bekannte Weise aus dem aktuellen Laststrom des Transformators, der gemessen wird, der aktuellen Stufenspannung zwischen zwei benachbarten Wicklungsanzapfungen, zwischen denen umgeschaltet wird, und der jeweiligen Ausführung des Stufenschalters.
Die Größen a und b sind stufenschalterspezifische Kenngrößen, die vorab - wie weiter oben bereits erläutert wurde - nicht flüchtig gespeichert werden. Der Faktor a liegt dabei im Bereich von 10^-5 ...10^-2. Für den bekannten Stufenschalter Typ M der Anmelderin etwa beträgt a vorzugsweise 8,5 · 10^-5.
Der Wert für b liegt im Bereich von 0,8...2,2. Für den bereits erwähnten Stufenschalter Typ M beträgt b vorzugsweise 1,16.As already stated, in the method according to the invention the contact erosion of the respective switching or resistance contact in volume units of the burnable contact material, approximately in mm ³ , is determined on the basis of a specific burn rate A. This burn rate A with the physical unit mm ³ / circuit, ie volume unit / circuit, is a material and current-dependent characteristic.
The rate of burn A results particularly advantageous as follows: $$\mathrm{A}\left[\frac{{\mathrm{mm}}^{\mathrm{3}}}{\mathrm{circuit}}\right]\mathrm{=}\mathrm{a}\mathrm{\cdot }{\mathrm{J}}^{\mathrm{b}}$$

In this case, J is the current to be disconnected from the contact of the tap changer to be monitored. It results arithmetically in a known manner from the current load current of the transformer which is measured, the current step voltage between two adjacent winding taps, between which is switched, and the respective embodiment of the tap changer.
The variables a and b are step-switch-specific parameters, which are not stored in advance - as already explained above - not volatile. The factor a is in the range of 10 ^-5 ... 10 ^-2 . For example, for the known tap-changer type M of the Applicant, a is preferably 8.5 · 10 ^-5 .
The value for b is in the range of 0.8 ... 2.2. For the already mentioned tap-changer type M, b is preferably 1.16.

According to a particularly advantageous embodiment of the invention, the described determination of the burn rate A is surrounded by a tolerance band, which must be taken into account for reliable statements.

Bei dieser Ermittlung der Abbrandrate wird, wie erläutert, gewissen Schwankungen durch den Sicherheitszuschlag s pauschal Rechnung getragen.It has been shown that the contact erosion is subject to diverse, unpredictable and difficult computationally described influences that can lead to certain fluctuations. Thus, a safety margin s in the order of about 10 ... 12% is introduced. This covers all the usual fluctuations that occur in practice. After this development of the invention, the burn rate A then results as follows: $$\mathrm{A}\left[\frac{{\mathrm{mm}}^{\mathrm{3}}}{\mathrm{circuit}}\right]\mathrm{=}\mathrm{a}\mathrm{\cdot }{\mathrm{J}}^{\mathrm{b}}\mathrm{\cdot }\mathrm{s}$$

In this determination of the burning rate, as explained, certain fluctuations due to the security surcharge are generally taken into account.

lässt sich dann zur Ermittlung der Abbrandrate wie folgt einsetzen: $$\mathrm{Abbrand}\left[\frac{{\mathrm{mm}}^{\mathrm{3}}}{\mathrm{Schaltung}}\right]\mathrm{=}\mathrm{f}\mathrm{\cdot }\mathrm{a}\mathrm{\cdot }{\mathrm{J}}^{\mathrm{b}}\mathrm{\cdot }\mathrm{s}$$

According to a further advantageous embodiment of the invention, it is also possible to increase the accuracy of the determination of the burn rate even further, for example for life-prediction of the contacts by an adjustment of the burn rate is made, a safety margin is therefore no longer flat rate, but determined iteratively becomes. For this purpose, the actual contact erosion is measured according to a representative number of contacts. This can happen, for example, in the context of an already required routine inspection. The actual volume burnup for each contact is determined from the measured values and compared with the volume burnup determined according to the invention. A correction factor $$\mathrm{f}\mathrm{=}\frac{{\mathrm{volume\; burnoff}}_{\mathrm{measured}}}{\mathrm{accumulated}{\mathrm{volume\; burnoff}}_{\mathrm{calculated}}}$$

can then be used to determine the burning rate as follows: $$\mathrm{combustion}\left[\frac{{\mathrm{mm}}^{\mathrm{3}}}{\mathrm{circuit}}\right]\mathrm{=}\mathrm{f}\mathrm{\cdot }\mathrm{a}\mathrm{\cdot }{\mathrm{J}}^{\mathrm{b}}\mathrm{\cdot }\mathrm{s}$$

In the method according to the invention, the computationally determined burning rate A is integrated into a method for monitoring the contact erosion. Thus, the method according to the invention as a whole includes both the determination of the burn rate A and the subsequent determination of the cumulative contact burnup at the respective switching contact, as well as the subsequent generation of situation-related warning or other messages.

The particular advantage of the method according to the invention lies in the fact that a monitoring possibility of contact erosion of the contacts in the tap changer results in a simple manner, without these contacts themselves having to be accessible for visual inspections, measurements on them or in any other way.

A further advantage of the invention is that the method according to the invention can be readily implemented in a complex tap-changer and / or transformer monitoring system.

Overall, it can be reliably indicated by the method according to the invention when an exchange of the contacts in the respective tap changer is actually required. This avoids, on the one hand, that for alleged safety reasons premature contacts are exchanged, which is unnecessary and expensive, but on the other hand it is also avoided that an actual necessary contact exchange is not recognized or delayed inadmissible, which can lead to malfunctions and serious damage.

Figur 1

zeigt den Ablaufplan eines ersten erfindungsgemäßen Verfahrens

Figur 2

zeigt den Ablaufplan eines weiter entwickelten zweiten erfindungsgemäßen Verfahrens.

The invention will be explained in more detail by way of example below.

FIG. 1

shows the flowchart of a first method according to the invention

FIG. 2

shows the flowchart of a further developed second inventive method.

First, the method shown in Figure 1 will be explained in more detail.

At the beginning, an input and non-volatile storage of specific tap changer parameters or characteristic data, the respective permissible burnup limit values of the individual contacts and the rated step voltages of each possible circuit, ie operating position of the tap changer, takes place. By this first step, an initialization, ie the adaptation to the respective tap changer whose contacts are to be monitored. An index n is set to zero; the system is ready for use. Furthermore, the current step switch position is determined by a position reporting device. If the step switch is now actuated by a switching pulse, a corresponding motor drive moves the tap changer in the direction "higher" or "lower" depending on the direction of rotation. In this and each further operation, the index n is increased by 1. At the same time, the load current J _{L is} measured. Furthermore, the corresponding nominal step voltage for the current circuit is read from the non-volatile memory. At the same time it is detected in which direction the circuit was made, and it will both the new Step switch position as well as the respective switching off diverter switch side whose contacts are each switching arc determined.

Subsequently, the corresponding switching currents are calculated separately for the switching contact and the resistance contact. The switching current of the switching contact J _SK is given by: $${\mathrm{J}}_{\mathrm{SK}}\mathrm{=}\frac{{\mathrm{J}}_{\mathrm{L}}}{\mathrm{parsec}}$$

The switching current of the resistance contact J _WK results after: $${\mathrm{J}}_{\mathrm{wk}}\mathrm{=}\frac{{\mathrm{U}}_{\mathrm{S}}\mathrm{+}{\mathrm{J}}_{\mathrm{L}}\mathrm{\cdot }\frac{{\mathrm{R}}_{\tilde{\mathrm{u}}}}{{\mathrm{S}}_{\mathrm{res}}}}{\mathrm{2}\mathrm{\cdot }{\mathrm{R}}_{\tilde{\mathrm{u}}}}$$

In these formulas, ParSek means the number of parallel sectors of the diverter switch, ie, the parallel circuits of individual switch contacts, U _S is the respective rated step voltage, and S _res is the resulting current division. R _ü is the size of the switching resistance.

und die Abbrandrate des Widerstandskontaktes nach $${\mathrm{A}}_{\mathrm{WK}}\mathrm{=}\mathrm{a}\mathrm{\cdot }{{\mathrm{J}}_{\mathrm{WK}}}^{\mathrm{b}}\mathrm{\cdot }\mathrm{s}$$

Subsequently, the burn rates are calculated. Various possibilities for calculating these burnup rates have already been discussed above. In this case, shown in FIG. 1, the burning rate of the switching contact A _{SK is} calculated $${\mathrm{A}}_{\mathrm{SK}}\mathrm{=}\mathrm{a}\mathrm{\cdot }{{\mathrm{J}}_{\mathrm{SK}}}^{\mathrm{b}}\mathrm{\cdot }\mathrm{s}$$

and the burning rate of the resistance contact after $${\mathrm{A}}_{\mathrm{WK}}\mathrm{=}\mathrm{a}\mathrm{\cdot }{{\mathrm{J}}_{\mathrm{WK}}}^{\mathrm{b}}\mathrm{\cdot }\mathrm{s}$$

Here, a and b are the specific factors already explained, s is the safety margin also explained, which is set out here on a flat-rate basis.

und für den Widerstandskontakt zu $${\mathrm{GA}}_{\mathrm{WK}}^{\mathrm{n}}\mathrm{=}{\mathrm{GA}}_{\mathrm{WK}}^{\mathrm{n}-1}\mathrm{+}{\mathrm{A}}_{\mathrm{WK}}$$

Subsequently, the cumulative Volumenabbrand is determined, ie it is added for switching and resistance contact at each circuit in this circuit computationally determined burn to Gesamtabbrand, which has resulted from all previous circuits, and stored as a new Volumenabbrand. The cumulative volume erosion for the switching contact results in: $${\mathrm{GA}}_{\mathrm{SK}}^{\mathrm{n}}\mathrm{=}{\mathrm{GA}}_{\mathrm{SK}}^{\mathrm{n}-1}\mathrm{+}{\mathrm{A}}_{\mathrm{SK}}$$

and for the resistance contact too $${\mathrm{GA}}_{\mathrm{WK}}^{\mathrm{n}}\mathrm{=}{\mathrm{GA}}_{\mathrm{WK}}^{\mathrm{n}-1}\mathrm{+}{\mathrm{A}}_{\mathrm{WK}}$$

und für den Widerstandskontakt ergibt sich $${\mathrm{GAd}}_{\mathrm{WK}}^{\mathrm{n}}\mathrm{=}\frac{{\mathrm{GA}}_{\mathrm{WK}}^{\mathrm{n}}}{\mathrm{F}}\mathrm{\cdot }\mathrm{k}$$

The variable n designates the already explained index, which is increased by 1 each time the tap changer is actuated. Subsequently, the burnup in mm contact thickness is calculated from this cumulative volume burnup in mm ³ . For the switching contact results $${\mathrm{GAd}}_{\mathrm{SK}}^{\mathrm{n}}\mathrm{=}\frac{{\mathrm{GA}}_{\mathrm{SK}}^{\mathrm{n}}}{\mathrm{F}}\mathrm{\cdot }\mathrm{k}$$

and for the resistance contact results $${\mathrm{GAd}}_{\mathrm{WK}}^{\mathrm{n}}\mathrm{=}\frac{{\mathrm{GA}}_{\mathrm{WK}}^{\mathrm{n}}}{\mathrm{F}}\mathrm{\cdot }\mathrm{k}$$

F is the respective contact surface of the corresponding contact, k is a switch-specific correction factor. The burnup values calculated in this way thus represent the total, cumulative burnup of the respective contact in mm, d. H. the deviation from the contact thickness when new.

With these values, a comparison is finally made with the previously non-volatile stored limit values. It is checked whether a corresponding percentage of each allowable contact erosion is reached or whether a certain percentage of permissible Abbrandunterschiede between switching contact on the one hand and resistance contact on the other hand is reached. In both cases, corresponding warning messages can be output or the tap changer can also be stopped in a manner known per se. In the first case, when the allowable contact erosion of a contact is reached, an exchange is necessary. If a corresponding warning message is generated already at 90% of this limit, a corresponding inspection can be prepared. In the second case, when the allowable burnup difference between contact and resistance contact is exceeded, but the contact wear itself has not yet reached the limit, the contacts do not necessarily have to be replaced with new ones. In such cases it may be sufficient to exchange switching and resistance contacts.

lässt sich als Korrekturfaktor in die Berechnung der Abbrandrate wie folgt einbringen: $$\mathrm{Abbrand}\left[\frac{{\mathrm{mm}}^{\mathrm{3}}}{\mathrm{Schaltung}}\right]\mathrm{=}\mathrm{f}\mathrm{\cdot }\mathrm{a}\mathrm{\cdot }{\mathrm{J}}^{\mathrm{b}}\mathrm{\cdot }\mathrm{s}$$

oder anders: A_neu = f · A_alt. Damit ergeben sich korrigierte Abbrandraten für jeden Kontakt, die nun nicht mehr ausschließlich vom Schaltstrom abhängig sind, sondern auch durch den Korrekturfaktor f bestimmt werden.FIG. 2 shows a correspondingly further developed further method according to the invention. In this case, further method steps are added to the method already explained with reference to FIG. 1, which make the entire method adaptive. So far it has been described that the contact erosion is subject to certain fluctuations, which is taken into account by the security surcharge s flat rate. If now the accuracy of the burn-up calculation is to be increased, this can be done by adjusting the burn-up rate according to these further method steps. For this purpose, after a representative number of switches of the tap changer, z. B. after 10,000 circuits per contact, the actual contact erosion, usually in mm contact thickness measured. This can be done as part of a routine inspection. The volume burnup for each contact is determined from the measured values and compared with the respective calculated volume burnup of this contact according to the method according to the invention. The quotient $$\mathrm{f}\mathrm{=}\frac{{\mathrm{volume\; burnoff}}_{\mathrm{measured}}}{\mathrm{accumulated}{\mathrm{volume\; burnoff}}_{\mathrm{calculated}}}$$

can be incorporated as a correction factor in the calculation of the burn rate as follows: $$\mathrm{combustion}\left[\frac{{\mathrm{mm}}^{\mathrm{3}}}{\mathrm{circuit}}\right]\mathrm{=}\mathrm{f}\mathrm{\cdot }\mathrm{a}\mathrm{\cdot }{\mathrm{J}}^{\mathrm{b}}\mathrm{\cdot }\mathrm{s}$$

or otherwise: A _new = f · A _old . This results in corrected burn rates for each contact, which are no longer exclusively dependent on the switching current, but are also determined by the correction factor f.

At each inspection, new correction factors f are determined and taken into account again; the result is the following recursion: $${\mathrm{A}}_{\mathrm{i}}\mathrm{=}{\mathrm{f}}_{\mathrm{i}}\mathrm{\cdot }{\mathrm{A}}_{\mathrm{i}\mathrm{-}\mathrm{1}}\mathrm{,}$$

The index i indicates the number of inspections carried out, ie. H. Measurements of the actual volume burnup. Thus, the accuracy of the method according to the invention is continuously improved; the system is self-learning.

Claims (3)

1. Method of monitoring contact burning in tap changers having switch contacts and resistance contacts, comprising the following features:

   - permanent storage of the values of the respective nominal tap voltage (U_s), the limit values for the permissible contact burning of the switch contacts and the resistance contacts and the characteristic magnitudes a, b and k specific to the tap changer,

   - determination of the instantaneous setting of the tap changer, increasing an index n in the case of each switching over, i.e. actuation of the tap changer, measuring of the respective value of the load current (J_L) and reading-out of the permanently stored corresponding value for the nominal tap voltage (U_s),

   - calculation of the switching currents of the respective switch contact (J_SK) being switched off and of the resistance contact (J_WK) by means of the equations $${\mathrm{J}}_{\mathrm{SK}}\mathrm{=}\frac{{\mathrm{J}}_{\mathrm{L}}}{\mathrm{ParSek}}$$

   

   $${\mathrm{J}}_{\mathrm{wk}}\mathrm{=}\frac{{\mathrm{U}}_{\mathrm{S}}\mathrm{+}{\mathrm{J}}_{\mathrm{L}}\mathrm{\cdot }\frac{{\mathrm{R}}_{\ddot{\mathrm{u}}}}{{\mathrm{S}}_{\mathrm{res}}}}{\mathrm{2}\mathrm{\cdot }{\mathrm{R}}_{\ddot{\mathrm{u}}}}$$

   

   
   wherein ParSek represents the number of parallel sectors, R_ü represents the level of the switching-over resistance and S_res represents the resultant current division,

   - calculation of the respective rate of burning of the switch contact (A_SK) and of the resistance contact (A_WK) by means of the equations $${\mathrm{A}}_{\mathrm{SK}}\mathrm{=}\mathrm{a}\mathrm{\cdot }{{\mathrm{J}}_{\mathrm{SK}}}^{\mathrm{b}}$$

   

   $${\mathrm{A}}_{\mathrm{WK}}\mathrm{=}\mathrm{a}\mathrm{\cdot }{{\mathrm{J}}_{\mathrm{WK}}}^{\mathrm{b}}$$

   

   - summating the burning rates (A_SK, A_WK) for the total volume burning of the switch contact $\left({\mathrm{GA}}_{\mathrm{SK}}^{\mathrm{n}}\right)$

   

   and for the resistance contact $\left({\mathrm{GA}}_{\mathrm{WK}}^{\mathrm{n}}\right)$

   

   by means of the equations $${\mathrm{GA}}_{\mathrm{SK}}^{\mathrm{n}}\mathrm{=}{\mathrm{GA}}_{\mathrm{SK}}^{\mathrm{n}-1}\mathrm{+}{\mathrm{A}}_{\mathrm{SK}}$$

   

   $${\mathrm{GA}}_{\mathrm{WK}}^{\mathrm{n}}\mathrm{=}{\mathrm{GA}}_{\mathrm{WK}}^{\mathrm{n}-1}\mathrm{+}{\mathrm{A}}_{\mathrm{WK}}$$

   

   - calculation of the respective burning in millimetres of contact thickness for the switch contact $\left({\mathrm{GAd}}_{\mathrm{SK}}^{\mathrm{n}}\right)$

   

   and for the resistance contact $\left({\mathrm{GAd}}_{\mathrm{WK}}^{\mathrm{n}}\right)$

   

   with consideration of the respect contact area (F) according to the equations $${\mathrm{GAd}}_{\mathrm{WK}}^{\mathrm{n}}\mathrm{=}\frac{{\mathrm{GA}}_{\mathrm{WK}}^{\mathrm{n}}}{\mathrm{F}}\mathrm{\cdot }\mathrm{k}$$

   

   $${\mathrm{GAd}}_{\mathrm{WK}}^{\mathrm{n}}\mathrm{=}\frac{{\mathrm{GA}}_{\mathrm{WK}}^{\mathrm{n}}}{\mathrm{F}}\mathrm{\cdot }\mathrm{k}$$

   

   - comparison of these values $\left({\mathrm{GAd}}_{\mathrm{SK}}^{\mathrm{n}}\right)$

   

   and $\left({\mathrm{GAd}}_{\mathrm{WK}}^{\mathrm{n}}\right)$

   

   with the permanently stored limit values and generation of reports in the case of exceeding of the limit values or percentage limits thereof.
2. Method according to claim 1, wherein the respective burning rates (A_SK, A_WK) are determined from the calculated switching currents (J_SK, J_WK) according to the equations $${\mathrm{A}}_{\mathrm{SK}}\mathrm{=}\mathrm{a}\mathrm{\cdot }{{\mathrm{J}}_{\mathrm{SK}}}^{\mathrm{b}}\mathrm{\cdot }\mathrm{s}$$

   

   $${\mathrm{A}}_{\mathrm{WK}}\mathrm{=}\mathrm{a}\mathrm{\cdot }{{\mathrm{J}}_{\mathrm{WK}}}^{\mathrm{b}}\mathrm{\cdot }\mathrm{s}$$

   

   wherein s is an additional safety margin.
3. Method according to claim 1 or 2, wherein after a greater number of switchings the actual contact burning is measured, an actual volume burning is calculated therefrom, a factor f is ascertained therefrom according to the equation $$\frac{{\mathrm{volume\hspace{1em}burning}}_{\mathrm{measured}}}{\mathrm{cumulative\hspace{1em}}{\mathrm{volume\hspace{1em}burning}}_{\mathrm{calculated}}}\mathrm{=}\mathrm{f}$$

   

   and subsequently the respective burning rate is corrected according to the equation A_neu = f · A_alt and in future the corrected values (A_neu) are used in the method.

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