EP1475813A1 - Method and apparatus for controlling switching devices in electrical switchgear - Google Patents

Abstract

The method involves detecting contact current during switching with a current converter and evaluating for contact wear. A state parameter characteristic of contact wear is determined by measuring the current converter measurement signal (I mess) as a function of time (t), detecting a measurement error (delta) if it deviates from the anticipated contact current (I f) and if so deriving a characteristic current value (I char) from the measurement signal and using it to determine the state parameter. Independent claims are also included for the following: (a) a computer program for determining switching device contact wear in electrical switching systems (b) an arrangement for implementing the inventive method (c) and an electrical switching system with an inventive arrangement.

Description

TECHNICAL AREA

The invention relates to the field of secondary technology for electrical switchgear, especially the Monitoring of switches in high, medium or low voltage switchgear. It is based on a procedure a computer program and a device for determination the contact erosion of circuit breakers in one electrical switchgear and a switchgear with such a device according to the preamble of the independent Claims.

STATE OF THE ART

In most electricity supply companies today, circuit breaker maintenance is done periodically, sometimes with early maintenance, when protection trips with possibly high currents have occurred. As a rule, the switch is therefore serviced far too often, with the additional risk that damage will be caused during maintenance.
DE 102 04 849 A1 discloses a method for determining contact wear in a tripping unit. A cumulative energy converted in the circuit breaker contacts is calculated, which is proportional to the contact wear. For this purpose, the contact current I is sampled, squared during the contact separation period, multiplied by a fixed time T between samples and summed up for each contact pair in relation to each type of error or as a total. The time delay between the opening of the circuit breaker and the contact movement in the circuit breaker can be measured or estimated on the basis of typical mechanism times or those published by the manufacturer. If the adjustable threshold values for contact erosion are exceeded, a warning signal or alarm signal can be issued or the circuit breaker can be switched off or serviced. As an alternative to the I ² T measurement, the arc energy can also be determined from voltage times current or approximately from current I times time T. It is disadvantageous that current measurement errors in the case of overcurrents are not taken into account for the determination of arc energy and contact erosion. Another disadvantage is the relatively large measuring and computing effort.
EP 0 193 732 A1 discloses a monitoring and control device for switching devices and switching device combinations for determining the required maintenance times. For this purpose, a plurality of sensors measure or calculate wear states of the switching devices and generate alarm or maintenance information according to urgency. The contact erosion can directly, z. B. detected by displacement sensors, rotary angle sensors or light barriers or determined indirectly by linking the current level, switching voltage, phase angle, number of circuits, switching moments, current steepness or time constants. In particular, the contact erosion is determined indirectly by evaluating the current and temperature of the respective current path. Disadvantages are high measurement requirements and complex signal processing. Measurement errors due to saturation of the current transformer also go unnoticed.

PRESENTATION OF THE INVENTION

The object of the present invention is to provide a method, a computer program, a device and a switchgear assembly with such a device for improved and simplified monitoring of switches in electrical switchgear assemblies. According to the invention, this object is achieved by the features of the independent claims.
In a first aspect, the invention consists in a method for determining contact wear in an electrical switch, in particular in electrical switchgear for high or medium voltage, a contact current flowing through the switch during a switching operation being detected with the aid of a current transformer and evaluated with regard to contact wear , in order to determine a state variable characterizing the contact wear, a current measurement signal of the current transformer is first measured as a function of time, the presence of a measurement error is detected when deviations occur between the expected contact current and the current measurement signal, and at least one characteristic current value is detected from the current measurement signal when the measurement error is detected is determined and used to determine the state variable. The condition size must be selected so that it represents a reliable measure of the contact erosion. The expected contact current is particularly characterized by the temporal course of the contact current, in particular by reaching a maximum current at the end of a quarter or three-quarter period of the mains frequency of the nominal current applied to the switch. Depending on the switching action and type of error, other expected contact currents are also conceivable. Contact wear can also be determined with great reliability by the method if the fault current or arcing current relevant for contact erosion is not or cannot be measured correctly. The use of the characteristic current value instead of the complete current measurement signal simplifies and specifies the calculation of the contact wear. Overall, the contact wear can be calculated more accurately and the maintenance of circuit breakers and similar switching devices can be carried out instead of periodically without loss of operational safety, which means that Maintenance costs can be reduced accordingly.

In a first exemplary embodiment, a saturation of the current measurement signal is detected as a measurement error and a maximum current measurement signal of the current transformer is used as the characteristic current value if it occurs and in particular is detected before a quarter period of an alternating current applied to the switch. The saturation of conventional current transformers often makes an exact measurement of the arc overcurrent impossible and thus falsifies the contact burn-up calculation, especially for the fault cases that cause the most contact burn-up. This can now be corrected by calculation.
The embodiment according to claim 3 has the advantage that high fault currents can be detected and the state size represents a reliable, well-predictable measure of contact erosion.
The embodiment according to claim 4 has the advantage that a very simple calculation rule for contact wear calculation is specified.
The embodiment according to claim 5 has the advantage that the reliability of the contact erosion calculation is improved by the exact determination of the arc start.
The embodiment according to claim 6 has the advantage that a selection of functions for calculating the contact erosion is specified and, if necessary, a special function can be selected for specific switches or fault current events.
The embodiment according to claim 7 has the advantage that manufacturer information can also be used for improved contact erosion calculation.
The embodiment according to claim 8 has the advantage that an additional, independent calculation of the contact wear can be carried out.
The embodiment according to claim 9 has the advantage that the contact erosion can be permanently monitored and / or subsequently determined from archived data. In particular, fault record data can be used, as z. B. in a fault recorder collection system, also called station monitoring system or SMS, are available.
In further aspects, the invention relates to a computer program for determining contact wear in an electrical switch, the method steps according to claims 1-9 being implemented by program code, furthermore an apparatus for carrying out the method and a switchgear assembly comprising the apparatus.
Further embodiments, advantages and applications of the invention result from the dependent claims and from the following description and the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1

eine schematische Darstellung zur Stromapproximation bei der erfindungsgemässen Kontaktabbrandberechnung für Leistungsschalter;

Fig. 2

ein Algorithmus zur erfindungsgemässen Kontaktabbrandberechnung in Nassi-Schneiderman Diagramdarstellung;

Fig. 3

eine Kurvendarstellung der Anzahl erlaubter Schalthandlungen als Funktion des effektiven Abschaltstroms pro Schalthandlung;

Fig. 4

ein schematisch dargestelltes erfindungsgemässes Datenerfassungssystem für die Kontaktabnutzung in einer elektrischen Schaltanlage.

In den Figuren sind gleiche Teile mit gleichen Bezugszeichen versehen.

Fig. 1

a schematic representation of current approximation in the inventive contact erosion calculation for circuit breakers;

Fig. 2

an algorithm for the contact erosion calculation according to the invention in Nassi-Schneiderman diagram representation;

Fig. 3

a graph of the number of permitted switching operations as a function of the effective switch-off current per switching operation;

Fig. 4

a schematically illustrated inventive data acquisition system for contact wear in an electrical switchgear.

In the figures, the same parts are provided with the same reference symbols.

WAYS OF CARRYING OUT THE INVENTION

Circuit breakers are designed for a certain number of mechanical switching operations or switching cycles. Are z. B. in the event of a fault, larger currents are switched off, the resulting arc burns off the contacts more than calculated in normal switching operations. In order for the circuit breaker to remain functional, the contacts must be replaced before they have completely burned down. The degree of burn-up per switching operation depends on the energy of the arc that occurs. This energy is proportional to the integral ∫I ² dt, where I denotes the current flowing during the arc duration and t the time.
According to the invention, switches 3 in electrical switchgear systems 1 are monitored for contact wear, in that a contact current I _f flowing through the switch 3 during a switching operation is detected at least approximately by a current measurement signal I _{mess of} a current transformer 30 or current sensor 30 as a function of time t, in the event of deviations A measurement error Δ is detected between the expected contact current I _f and the current measurement signal I _mess and at least one characteristic current value I _{char is} determined from the current measurement signal I _mess and is used to determine a state variable characterizing contact wear. This estimate is often a bit too conservative, but always on the safe side. The procedure can be part of a power system monitoring system.
1 shows an exemplary embodiment in which a largely sinusoidal fault current I _f is present. Saturation occurs in the current measurement signal I _mess and a current maximum I is run through at the time t _max within a quarter period of the fault current signal I _f or the mains frequency applied to the switch 3. The occurrence of the current maximum I _max is detected when the deviation or the measurement error Δ between the fault current profile I _f (t) and the current measurement _{signal profile} I _mess (t) exceeds a tolerance value Δ _min . The contact current I _f is typically an overcurrent or short-circuit current I _f during a switch-off operation, the course of which is known very precisely beforehand. In particular, a current maximum I _max that occurs in the current measurement signal I _mess before reaching a quarter period of the mains frequency is a reliable indication of a measurement error Δ. The current maximum I _max is now defined as a characteristic current value I _char and used to calculate the contact erosion state variable. The state variable should preferably be a measure of an arc power during the switching operation and in particular a contact current-time integral.
In the example according to FIG. 1, the current measurement signal I _{mess is detected} from a first point in time t ₀ at the start of the current half-wave in which the switching action occurs to a second point in time t _max at which a maximum current measurement signal I _max occurs, and from second time t _max up to a third time t ₀ at the end of the current half-wave approximated by the maximum current measurement signal I _max . The accuracy of the contact erosion calculation depends on how exactly the starting time of the arc can be determined. The first time t ₀ is to be defined as the start time of the arc of the contact current I _f . The calculation is most accurate if t _{o is known} as a binary indication in the fault record; t _o can also be determined with a time delay based on empirical values from an opening command, a protective trigger command or a contact movement of the switch 3. Any fluctuations in this time value are of minor importance compared to other influencing factors and irregularities in contact burn-off. Systematic errors caused by values of the starting time t ₀ that are too large or too small can be corrected if, for. B. on the occasion of maintenance, the expected burnup is compared with the actual one and the time delay is corrected accordingly. For safety reasons, a time delay value that is too small should be used at the beginning of a contact erosion history rather than a value that is too large, so that the contact erosion is initially overestimated in the calculation.
To determine the state variable, a time integral ∫f (I _mess ) dt is then formed via a function f (I _mess ) of the current measurement signal I _mess detected and approximated in _sections . The function f (I _mess ) of the current measurement signal I _mess is preferably a power function f (I _mess ) = I _mess ^a with a = 1, 2 ... 2.2, in particular a = 1.6 ... 2.0, used. For example, the integral ∫I _mess ² dt or ∫I _mess ^1.6 dt is determined with the current measurement signal I _mess approximated according to FIG. 1 for the approximate determination of the contact erosion. A square root function f (I _mess ) = (I _mess ² ) ^1/2 which defines an effective switch-off current I _eff can also be used as function f (I _mess ). Other functions f (I _mess ) are also possible. The time integral ∫f (I _mess ) dt via the function f (I _mess ) can be approximated by a summation of function values at interpolation points. B. are given by sampling the current measurement signal I _mess . In particular, the state variable is selected equal to the time integral ∫f (I _mess ) dt times a contact erosion constant c and the contact erosion constant c from manufacturer information, in particular from curves showing the number of permitted switching operations N (I _eff ) as a function of an effective breaking current per switching operation I _eff , and / or determined from empirical values for a switch type and switch location.
FIG. 2 shows a software algorithm for implementing the method in a computer program and computer program product in Nassi-Schneiderman representation. Initially, the variables Cwsum (= state variable for characterizing the contact erosion), I _max , cnt (= counter variable) and saturation (constant) are initialized. Then, in a while loop, which is caused by cnt in a positive (or alternatively negative, half-period of the mains AC voltage, not shown here), a sample value (cnt) of the current measurement signal is read in for each cnt value and the condition sample (cnt ) ≥I _max checked. If the condition is met, an auxiliary variable CWI and sample (cnt) are set. If the condition is not met, if cnt is less than the middle of the positive (or negative, MidthPositivePeriod, not shown here), saturation true and CWI equal to I _max ; if cnt≥MidthPositivePeriod, saturation = true CWI is set to I _max and saturation = false CWI is set to sample (cnt). Finally, the counter cnt is incremented by 1 and the auxiliary variable CWI is added to the square to the contact erosion state variable Cwsum. At the end of the half period, the summation or integration of Cwsum is complete. 1, represents the time integral over the square of the approximated current, which is given in the time interval t ₀ to t _max by the current measurement signal I _mess, corresponding to the sample values sample (cnt), and in the time interval t _max to is approximated to the next t ₀ by the current maximum I _max .
3 shows an example of a curve of a circuit breaker manufacturer, which curve correlates the maximum number of permitted switching operations N with an effective breaking current per switching operation I _eff and thus with a certain cumulative effective breaking current. If the contact wear is to be determined with the help of the integral ∫I ² dt, a switch-specific or switch-type-specific proportionality constant c must be taken into account between the integral and the contact wear, which can be specified by the switch manufacturer and / or determined by comparing measurements with calculations of the contact wear.
According to a preferred embodiment of the invention, an effective switch-off current I _eff can additionally be determined for each switching operation, contact wear as a percentage of the switching operations carried out relative to the total number of permitted switching operations from a curve over the number of permitted switching operations N (I _eff ) in function of the effective switch-off current I _eff this effective breaking current I _{eff are} determined and the percentages for all relevant switching operations carried out are added up to form a cumulative contact wear. The accumulated percentage value represents a control variable for the contact wear state variable Cwsum determined according to the invention.

For example, maintenance of the switch 3 can be initiated at the first point in time at which the state variable Cwsum exceeds a limit value or the accumulated percentage reaches 100% minus a residual safety margin for the next one or two shutdown operations with the maximum I _eff permissible for this switch 3.
FIG. 4 shows a schematic representation of a data acquisition system for the inventive determination of the contact wear state variable Cwsum and / or the accumulated percentage value from N (I _eff ). Switchgear 1 has switches 3, typically circuit breakers 3, which are equipped with current transformers 30 or current sensors 30, typically conventional current transformers 30 with a saturable core. For example, instrument transformers with 1% accuracy and accounting converters with 0.1% - 0.5% accuracy are saturated at the high currents that cause the most contact wear. As a result, conventional contact erosion estimates with the integral ∫I _mess ² dt become very inaccurate and in any case too small, making them unsuitable or risky for determining need-based maintenance times. In contrast, classic protective transformers for overcurrent functions have a large measuring range without saturation, but are relatively imprecise for small currents, so that they typically belong to an accuracy class of 2% - 5%. For these converters, too, an improved contact erosion calculation can be achieved by the invention by selecting a characteristic current value I _char with which the measurement error Δ in the current measurement signal I _{mess can} be corrected in such a way that the state variable Cwsum and in particular a contact erosion-relevant arcing power are determined as precisely as possible is achieved. The current transformers 30 are connected to means 4 for data acquisition at electrical switches 3, in particular with fault recorders 4, protective devices 4 or control devices 4. These data acquisition means 4 are connected via serial communication 5 or via data carriers 5 to a central acquisition unit 6 for calculating contact wear and preferably to a database 7 for data on contact wear.
With the help of this device 2 for contact erosion calculation, the method described above can be implemented. In particular, the contact wear can be monitored online, ie continuously during operation, or evaluated retrospectively from archived data, in particular with a function f (I _mess ) of the current measurement signal I _mess adapted to a switch type or switch _location . The contact wear can be determined from recordings of switch-off currents I _mess from fault recorders 4 or protection and control devices 4 with fault record function, all records of the switch-off currents I _{mess of} a switchgear assembly 1 being collected centrally, in particular in an existing or specially designed fault recorder collection system 4 -6, also called SMS or station monitoring system. The invention also extends to such a device 2 for contact erosion calculation, which is integrated, for example, in the system control system (not shown) of the switchgear 1, and to an electrical switchgear 1 which comprises such a device 2. Overall, there is an improved condition-controlled instead of periodic maintenance of switches 3 and their switch contacts.

LIST OF REFERENCE NUMBERS

1

Electrical switchgear

2

Data acquisition system for contact wear

3

Electrical switch, circuit breaker

30

Current transformer, current sensor

4

Data acquisition means on electrical switches; Fault recorder, protection device, control device

5

Serial communication, data carrier

6

Central data acquisition; Means of calculation of contact wear

7

Database for contact wear data

I

Contact current, arc current

I _char

characteristic current value

I _eff

effective current

I _f

fault current

I _max

maximum current

I _mess

Current measurement signal

t, t ₀ , t _max time

cnt, CWI, Cwsum, sample variables PositivePeriod, MidthPositivePeriod, saturation constants

N

Number of switching operations permitted

Claims (13)

Method for determining contact wear in an electrical switch (3), in particular in electrical switchgear (1) for high or medium voltage, a contact current (I _f ) flowing through the switch (3) during a switching operation with the aid of a current transformer (30) is recorded and evaluated with regard to contact wear, characterized in that a) to determine a state variable characterizing the contact wear (Cwsum), first a current measurement signal (I _mess ) of the current transformer (30) is measured as a function of time (t), b) if deviations occur between the expected contact current (I _f ) and the current measurement signal (I _mess ), the presence of a measurement error (Δ) is detected and c) upon detection of the measurement error (Δ) from the current measurement signal (I _mess ), at least one characteristic current value (I _char ) is determined and used to determine the state variable (Cwsum). The method according to claim 1, characterized in that a) a saturation of the current measurement signal (I _mess ) is detected as a measurement error (Δ) and b) a maximum current measurement signal (I _max ) of the current transformer (30) is used as the characteristic current value (I _char ), which occurs before reaching a quarter period of an alternating current present at the switch (30). The method according to any one of the preceding claims, characterized in that a) the contact current (I _f ) is an overcurrent or short-circuit current (I _f ) during a switch-off operation and / or b) the state variable (Cwsum) is a measure of an arc power during the switching operation, in particular a contact current-time integral. The method according to any one of the preceding claims, characterized in that a) the current measurement signal (I _mess ) is detected from a first point in time (t ₀ ) at the beginning of the current half-wave in which the switching action occurs to a second point in time (t _max ) at which a maximum current measurement signal (I _max ) occurs , and from the second point in time (t _max ) to a third point in time (t ₀ ) at the end of the current half-wave is approximated by the maximum current measurement signal (I _max ) and b) to determine the state 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). The method according to claim 4, characterized in that a) the first point in time (t ₀ ) is defined as the start time of an arc of the contact current (I _f ) and is known as a binary indication in the fault record or is determined with a time delay based on empirical values from an opening command, a protective trigger command or a contact movement of the switch (3) will and b) in particular that the time delay is corrected by comparing actual values with expected values of contact wear. The method according to any one of claims 4-5, characterized in that a function f (I _mess ) = I _mess ^a with a = 1, 2 ... 2.2 as a function f (I _mess ) of the current measurement signal (I _mess ) , in particular a = 1.6 ... 2.0, or a square root function f (I _mess ) = (I _mess ² ) ^1/2 defining an effective cut-off current (I _eff ) is used. The method according to any one of claims 4-6, characterized in that a) the state variable (Cwsum) is chosen equal to the time integral ∫f (I _mess ) dt times a contact erosion constant c and b) the contact erosion constant c is determined from manufacturer's information, in particular from curves of the number of permitted switching operations as a function of an effective breaking current per switching operation (I _eff ), and / or from empirical values for a switch type and switch location. The method according to any one of the preceding claims, characterized in that a) an effective breaking current (I _eff ) is determined for each switching _action , b) from a curve (N (I _eff )) over the number of permitted switching operations (N) as a function of the effective breaking current (I _eff ) a contact wear is determined as a percentage of the executed switching operations relative to the total number of permitted switching operations at this effective breaking current (I _eff ) and c) the percentages for all relevant executed switching operations are added up to a cumulative contact wear. The method according to any one of the preceding claims, characterized in that a) the contact wear is monitored online or is evaluated retrospectively from archived data, in particular with an adapted function f (I _mess ) of the current measurement signal (I _mess ), and / or b) the contact wear is determined from records of switch-off currents (I _mess ) from fault recorders (4) or protection and control devices (4) with a fault record function, all records of the switch-off currents (I _mess ) of a switchgear (1) being recorded in a central data acquisition (6 ) are collected, in particular by data carrier (5) or by communication (5) or in a fault recorder collection system (4-6). A computer program for determining contact wear in an electrical switch (3), especially in electric switchgear (1) for high or medium voltage, which is on a data processing unit (6), especially in a plant control system of the switchgear (1), loaded and executed, characterized characterized in that the computer program executes the steps of the method according to one of claims 1-9 when executed. Device (2) for performing the method according to a of claims 1-9. Device (2) according to claim 11, characterized in that a) the electrical switch (3) is a circuit breaker (3) and / or b) the current transformer (30) is a conventional current transformer (30) with a saturable core. Electrical switchgear (1), in particular high or medium voltage switchgear (1), characterized by a device (2) according to one of claims 11-12.

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