Method for determining a thermal endurance

Abstract

The invention relates to a method for determining the thermal endurance (LD(T)) of an electromechanical switch device (10) which is inserted into a load current path (14), having the following steps: - detecting a time series of temperature measurement values (Tm) which are recorded at a temperature measurement point (P) of the switch device (10); - forming a time series of calculated values which are calculated on the basis of the detected time series of temperature measurement values (Tm); - ascertaining a trend in the formed time series of calculated value; and - estimating the thermal endurance of the switch device (10) on the basis of the ascertained trend.

Description

[0001] Description

[0002] Method for determining a thermal lifetime

[0003] The present invention relates to a method for determining a thermal lifetime and a computer program.

[0004] Electromechanical switching and protective devices switch using moving contacts. These switching operations, on the one hand, and environmental influences, e.g., corrosion, on the other, cause changes in the contact surfaces and thus in the contact resistance. This change tends to occur abruptly during a switching operation, but more gradually due to environmental influences. An increase in contact resistance leads to an increase in power loss and, consequently, in the device temperature. This can cause the device to become so hot that it fails or catches fire.

[0005] Condition monitoring of a critical temperature point (hotspot) on the device or of a thermal weak point near such a hotspot is a prerequisite for issuing an early warning of an impending failure. In addition to determining the current device status, a prediction of the device's remaining thermal lifespan is also useful.

[0006] There is therefore a need for a method for determining the thermal lifetime of an electromechanical switching and protective device.

[0007] The object is achieved by a method according to claim 1. It is a method for determining the thermal lifetime of an electromechanical switching device. Thermal lifetime is understood here to be the (still remaining) lifetime of the switching device which results from the thermal aging of materials used to manufacture the switching device. The lifetime, also known as durability, refers to the period of time which the switching device can be used without failure. The term “electromechanical switching device” covers any electromechanical switching and protective device which has movable switching contacts. It is irrelevant whether it is a multiple-break switching device or a single-break switching device. The type of switching triggering, e.g. actuator, maglatch or switching lock, is also irrelevant.The invention is applicable to all types of switching devices that have movable switching contacts. The contact-based switching devices to which the present invention is applicable follow various basic concepts. On the one hand, there are switching devices that have single or multiple breaking contacts, whether linear or rotary. Furthermore, these switching devices are actuated or driven in different ways. Switchable spring actuators are used, e.g. bistable switch locks as in the MCB (= Miniature Circuit Breaker) and MCCB (= Molded Base Circuit Breaker) or tensionable spring actuators as in the ACB (= Air Circuit Breaker). A contactor contact can also be actuated using a magnetic actuator. Even direct switching of blade contacts as in the LBS (= Load Break Switch) is possible. The force or energy can be introduced manually, by motor or, in the case of a contactor, by means of an applied voltage.However, all other possible designs of contact-based switching devices are also included.

[0008] The switching device is inserted into a load current path. Electrical energy is transported via the load current path from an electrical energy source, e.g. an electrical supply network in which a mains voltage is provided, to an electrical consumer, e.g. an electric motor. The load current path can be designed, for example, as a 3-phase power line. The switching device can have input terminals (e.g. LI, L2, L3) and outgoing terminals (e.g. TI, T2, T3), with one end of the power line connected to the electrical energy source (mains voltage) being connected to the input terminals and one end of the power line connected to the consumer being connected to the outgoing terminals. A switching device, e.g. an electromechanical switch and / or a semiconductor switch (e.g.MOSFET, IGBT) is provided, which is designed in such a way that in a first switching state of the switching device (switching state "closed") a connection between the input terminals and the output terminals is electrically conductive and in a second switching state of the switching device (switching state "open") a connection between the input terminals and the output terminals is electrically blocked.

[0009] The method comprises a step in which a time series of temperature measurements is recorded, which are taken at a temperature measuring point of the switching device. A temperature measurement is a temperature value that is determined by a temperature sensor. A time series of temperature measurements is a chronologically ordered sequence of temperature measurements; the temperature measurements can be recorded at any desired point in time. A temperature measuring point is a position inside, i.e. in the interior of the device housing, and / or on an outer surface of the device housing of the switching device, at which position a temperature sensor can be positioned to measure the temperature measurement.

[0010] The method comprises a step in which a time series of calculated values ​​that was calculated on the basis of the recorded time series of measured temperature values ​​is generated. The time series of calculated values ​​is a chronologically ordered sequence of calculated values; the chronological sequence of the calculated values ​​corresponds to that of the measured temperature values. The calculated values ​​are obtained from the measured temperature values ​​using a mathematical operation; this operation can also simply consist of multiplying by the number one, so that a measured temperature value and a calculated value calculated on the basis of the measured temperature value are identical. The method comprises a step in which a trend is determined in the resulting time series of calculated values. A trend is a long-term change in the calculated values ​​of the time series over time. The trend can be evaluated with regard to a limit temperature value of the switching device. The trend can be linear, but also exponential, logarithmic, etc.be approximated .

[0011] The method comprises a step in which a service life of the switching device is determined on the basis of the detected trend.

[0012] The invention is based on the idea that the measured variable temperature is used for trend analysis and service life assessment in an electromagnetic switching device, since in an electromagnetic switching device this best describes the quality of the general condition of the switching device. A key difference to conventional methods used to determine service life is that a measured temperature value is used for the assessment and evaluated as a function of time in such a way that a temporal trend and a determination of a thermal service life, in particular a remaining thermal service life, is possible. In this way, condition monitoring of a switching device based on a quality characteristic for the switching device, namely the temperature, is possible. In addition, a forecast of the thermal service life can be derived.This predicted lifetime may be shorter than a lifetime predicted based on a currently measured temperature, which does not take into account temperature measurements recorded in the past.

[0013] The more measured values ​​there are, the more reliably the trend is represented; therefore, in order to achieve sufficient smoothing of the measured values, it is advantageous to calculate an average value. The object is further achieved by a computer program according to claim 7. The computer program has software code sections. The computer program is used to carry out a method for determining a remaining thermal service life of an electromechanical switching device. The computer program is adapted to cause the following steps to be carried out when executed on an electronic computer.

[0014] Storing, in a memory, a time series of temperature measurements recorded at a temperature measuring point of the switching device. Creating a time series of calculated values ​​based on the recorded time series of temperature measurements. Determining a trend in the created time series of calculated values. Estimating the service life of the switching device based on the determined trend.

[0015] Advantageous embodiments and further developments of the invention are specified in the dependent claims. The method according to the invention can also be further developed in accordance with the dependent device claims, and vice versa.

[0016] According to a preferred embodiment of the invention, a trend is determined using a regression analysis.

[0017] According to a preferred embodiment of the invention, the calculated values ​​are identical to the measured temperature values. The calculated values ​​are thus obtained from the measured temperature values ​​using a mathematical operation that simply consists of multiplying by one. Thus, a trend can be determined directly from the measured temperature values.

[0018] According to a preferred embodiment of the invention, the calculated values ​​are equal to quotients QTemp, which are calculated from the temperature measured values ​​Tm, which are divided by a temperature reference value Tref: QTemp = Tm / Tref . The quotient QTemp describes the ratio of the absolutely measured temperature Tm to a reference temperature Tref , e.g. in a brand-new switching device at rated current . It is possible that the reference temperature Tref refers to a circuit breaker in the standard test setup of DIN EN 60947-2: 2020-11 Low-voltage switchgear and controlgear - Part 2: Circuit breakers . It is also possible that the reference temperature Tref refers to a permissible temperature of a material installed in the switching device, which material forms a thermal weak point of the switching device . However, any other temperature comparison value can also be used as the reference temperature Tref.The reference temperature Tref can also be adjusted if necessary based on knowledge of the installation situation at the beginning of the calculation.

[0019] The suggested dimensionless calculation value QTemp = Tm / Tref contains further information: with this calculation value it is possible to determine at the start of operation of the switchgear whether the design and the selected installation condition or the ambient conditions correspond to that of a switchgear in new and test condition. Values ​​greater than one indicate a deterioration in the thermal conditions compared to those of a standard test setup and an average new device; reasons could be unfavourable design (e.g. cable cross-sections that are too small) or a poor installation situation (e.g. devices that are too densely packed), which therefore indicates a shorter service life of the switchgear. This can provide users with suggestions for achieving better thermal design of the electrical switchgear.

[0020] According to a preferred embodiment of the invention, the calculation values ​​are quotients QTemp, which are calculated from the temperature measured values ​​Tm, which are each divided by an associated current measured value Im : QTemp = Tm / Im or by a squared associated current measured value Im : QTemp = Tm / Im 2 The associated current values ​​Im are the measured values ​​of a current flowing through the load current path, which are measured simultaneously with the temperature values ​​Tm at a current measuring point Q of the switching device. Other quotients and mathematical functions are also conceivable, such as Tm / Im 3 , In the 4 , log Im or other mathematical formulations of Im .

[0021] According to a preferred embodiment of the invention, a temperature sensor for detecting a measured temperature value in the switching device is positioned on a current path of the switching device, i.e. directly on the current path. The measuring temperature is therefore determined on a current path of the switching device via which a load current is conducted. In this case, the measuring point is on the current path. If the current sensor is electrically insulated, the measuring point can be directly on the surface of the current path, otherwise at a sufficiently large distance from the current-carrying current path to prevent electrical flashover from the current path to the temperature sensor. The advantage here is that the measuring temperature can be measured at a primary heat source of the switching device, namely a current path.

[0022] According to a preferred embodiment of the invention, a measuring temperature is determined at a measuring point on the switching device, wherein said measuring point is a thermal weak point in the switching device. The thermal weak point is a component near a hotspot or the hotspot itself, wherein the hotspot is the warmest point on the switching device. To determine the thermal weak point, the temperature-time characteristic curve of the component material as well as the temperature conditions of the component must be taken into account. The thermal weak point must be determined individually for each switching device. If the actual hotspot or the actual thermal weak point cannot be selected in a switching device, a suitable measuring point nearby can be selected as an alternative.

[0023] According to a preferred embodiment of the invention, the

[0024] The method comprises a step in which current measurement values ​​of a current flowing through the load current path are recorded at a current measurement point of the switching device by a current sensor, the current measurement values ​​being measured simultaneously with the temperature measurement values. The current measurement values ​​are preferably recorded for defined time ranges in which the current in the load current path is approximately constant. The embodiment further comprises a step in which the current range covered by the recorded current measurement values ​​is divided into two or more current classes. The embodiment further comprises a step in which the temperature measurement values ​​are each divided into the current class in which the current measurement value measured simultaneously with the temperature measurement value lies.The embodiment further comprises a step in which, in at least one of the current classes, a time series of calculated values ​​is generated based on the temperature measurements in this current class. The embodiment further comprises a step in which a trend of the generated time series of calculated values ​​is determined for each current class. The embodiment further comprises a step in which a service life of the switching device is estimated for each current class based on the determined trend. The advantage of using current classes is that a trend can be detected better and more accurately.

[0025] According to a preferred embodiment of the invention, a temperature-current value pair is formed from a temperature measurement value and the simultaneously recorded current measurement value. These value pairs are assigned to current classes which correspond to a fraction of the nominal current range In of the switching device, e.g. (0.5 ± 0.05) x In, (0.6 ± 0.05) x In, (0.7 ± 0.05) x In, etc. The temperature measurement values ​​of the temperature-current value pairs assigned to the same current class are then plotted against time and a trend is determined from this. The more measurement values ​​there are, the more accurately any existing trend will be depicted; therefore, in order to depict a trend line, a minimum running time (time with a current flow) and a minimum number of temperature measurement values ​​are required. In order to obtain the smoothest possible course of the temperature measurement values, it is advantageous to calculate an average value for each data point at which a trend analysis is carried out.The determined trend can be compared with a limit temperature value of the switching device.

[0026] The limit temperature value of the switching device can be a permissible temperature of the switching device. The permissible temperature of the switching device is defined by an institution that knows the materials used in the switching device, e.g. a manufacturer or an operator of the switching device. The permissible temperature of the switching device forms the upper limit of a first temperature range in which the switching device does not undergo faster thermal aging. In this first temperature range, the thermal lifetime decreases in proportion to the passage of time, e.g. in 24 hours the thermal lifetime also decreases by 24 hours.

[0027] The limit temperature value of the switching device can be a maximum permissible temperature of the switching device. The maximum permissible temperature of the switching device is defined by an institution that knows the materials used in the switching device, e.g. a manufacturer or an operator of the switching device. The maximum permissible temperature forms the upper limit of a second temperature range, in which the switching device thermally ages more quickly due to an increased temperature than in the first temperature range. In this second temperature range, which includes all temperature values ​​that are greater than the permissible temperature and less than or equal to the maximum permissible temperature, the thermal lifetime decreases faster than time passes, e.g. in 24 hours the thermal lifetime decreases by 28 hours. Materials, e.g. plastics, have a specific temperature-time characteristic curve for a material property.This temperature-time characteristic takes into account the safeguarding of important material properties, such as strength or electrical insulation capacity. The permissible temperature of the material can be determined from the temperature-time characteristic. As long as the permissible temperature is not exceeded, the material can easily withstand this permissible temperature over a long period of time, e.g. 25 years, without suffering damage. Furthermore, the maximum permissible temperature can also be determined from the temperature-time characteristic. A design limit for the maximum permissible temperature can be derived from a strength limit value.

[0028] If the maximum permissible temperature is exceeded, a material in the switchgear will be permanently weakened or damaged, and its service life will be reduced more than in the second temperature range. For example, if the maximum permissible temperature of a plastic is exceeded, the flame retardant will decompose and the device will no longer function. To prevent the maximum permissible temperature from being exceeded, one or more warning thresholds can be implemented at lower temperatures. When these thresholds are reached, the switchgear operator is warned that the maximum permissible temperature is approaching.

[0029] According to a preferred embodiment of the invention, the temperature measurements are recorded at a hotspot, i.e., the hottest point, in a switching device. The reason for this is that the highest temperature to which a switching device is exposed is crucial for its overall thermal lifetime.

[0030] According to a preferred embodiment of the invention, an estimate is made as to when the temperature in the switching device will be greater than a maximum permissible temperature of the switching device. This estimate is based on a trend analysis of temperature measurements recorded at two or more previous points in time. In this way, a user of the switching device has sufficient time to plan a timely shutdown of the switching device. A preferred embodiment of the invention is a computer program product comprising a computer program as described above.

[0031] In the following, the invention is explained using several embodiments with the aid of the accompanying drawings. Each drawing is schematic and not to scale.

[0032] Fig. 1 a switching device;

[0033] Fig. 2 shows a T / It diagram in which the time courses of temperature and current measurements are plotted;

[0034] Fig. 3 is a Tt diagram showing the time course of temperature measurements of a current class; and

[0035] Fig. 4 a QTemp-t diagram in which the time course of calculated values ​​QTemp is entered; and

[0036] Fig. 5 is a QTemp-t diagram in which the time course of calculated values ​​QTemp of a current class is entered; and

[0037] Fig. 6 is a flow chart.

[0038] Fig. 1 shows a switching device 10 for switching a load current flowing through a load current line 16 from a voltage source, e.g., a mains transformer, to an electrical load, e.g., an electric motor. The switching device 10 has a housing 20 with load current connections 15, 15', which are divided into input terminals 15 and output terminals 15'. The load current line 16 is electrically connected to the load current connections 15, 15' such that the switching device 10 is connected between two ends of the load current line 16. Inside the housing 20, the load current connections 15 are each electrically connected to a fixed switching contact 11 by means of a current path 14.

[0039] A contact pair consists of a fixed contact 11 and a moving contact 12. In a mechanical switching device, there is at least one contact pair 11, 12 (single-circuit breaker such as an ACB). Many modern switching devices use multiple contact pairs in series or parallel (multiple-circuit breaker), depending on the requirements of the switching device.

[0040] The embodiment shown in Fig. 1 has a double-break mechanical switch 11, 12, 13. The fixed switching contact 11 is electrically connected by a movable contact bridge 13, which carries movable switching contacts 12, if the contact bridge 13 is in a first, closed position; in this position there is a continuous electrical connection of the load current connections 15. In a second, open position of the contact bridge 13, the two fixed switching contacts 11 are electrically separated; in this position the load current connections 15 are galvanically isolated.

[0041] An actuator unit 17 is able to move the contact bridge 13 back and forth between the two positions; such a change in position of the contact bridge 13 represents a switching event. Such a switching event is triggered by the actuator unit 17 being controlled to change position. The actuator unit 17 can be controlled internally in the switching device 10, e.g. in the case of a circuit breaker by a thermally or magnetically induced tripping. Such control of the actuator unit 17 can also be carried out by an external control signal, e.g. in the case of a contactor by a control command which is passed via a control line 19 to a control current connection 18 of the actuator unit 17. The actuator unit can also be designed as a conventional switching lock. A temperature sensor 23 is arranged inside the housing 20 at a first measuring point P, a temperature measuring point.The first measuring point P is located on the current path 14 of the switching device 10. Ideally, this is the warmest point of the switching device 10, the so-called "hot spot." The temperature measured values ​​recorded by the temperature sensor 23 are transmitted to an evaluation unit 30 via a data line 24.

[0042] Inside the housing 20, a current sensor 21 is also arranged at a second measuring point Q, a current measuring point. The second measuring point Q is located on the current path 14 of the switching device 10. The current measured values ​​detected by the current sensor 21 are transmitted to the evaluation unit 30 via a data line 22.

[0043] The evaluation unit 30 has a computing unit that can determine a thermal lifetime LD(T) of the switching device 10 based on the received temperature measurements. A thermal lifetime value determined by the evaluation unit 30 can be transmitted to an HMI 32 via a transmission medium 31 and communicated to a user of the switching device 10 via the HMI 32.

[0044] Fig. 2 shows a T / It diagram in which the temperature T and current I are plotted on the y-axis and the time t on the x-axis. The time courses of temperature measured values ​​Tm and current measured values ​​Im are entered in the diagram. The current measured values ​​Iml, ..., Im7 are recorded for defined time periods Atl to At7, in which the current Im in the load current path is approximately constant. Then a temperature-current value pair [Tm_i, Im_i] , i=l, ..., 7, is created from a temperature measured value Tml, . . . , Tm7 and the simultaneously recorded current measured value Iml, ..., Im7, which are plotted in the diagram.

[0045] At time 45, a switching event occurs. The temperature-current value pairs [Tm_i, Im_i] are assigned to current classes 46, each corresponding to a fraction of the rated current range In of the switching device, e.g., (0.5 ± 0.05) x In, (0.6 ± 0.05) x In, (0.7 ± 0.05) x In, etc. Then, the temperature measured values ​​of those temperature-current value pairs [Tm_i, Im_i] assigned to the same current class are plotted against time, as shown in Fig. 3.

[0046] Fig. 3 shows a Tt diagram in which temperature T is plotted on the y-axis and time t on the x-axis. The temperature measured values ​​43 of those temperature-current value pairs [Tm_i, Im_i] that are assigned to the same current class, here the current class (0.5 ± 0.05) x In, are plotted in the diagram. A straight line 47 is fitted to these temperature measured values ​​43 (regression analysis). The slope of the straight line 47 reflects the trend of the temperature measured values ​​43: they increase slowly over time, i.e., over years.

[0047] The more temperature measurement values ​​43 are available, the more reliably any existing trend can be mapped. Therefore, to map a trend line, a minimum operating time of the switching device and a minimum number of temperature measurement values ​​are required. To obtain the smoothest possible progression of the temperature measurement values, averaging is advantageous for each data point at which a trend analysis is performed.

[0048] The determined trend can be compared with a limit temperature value 48 of the switching device.

[0049] The limit temperature value 48 of the switching device, which runs parallel to the t-axis, can be, for example, a permissible temperature Tzul or a maximum permissible temperature Tmax of the switching device. For temperature measurement values ​​Tm that are less than or equal to the permissible temperature Tzul, the thermal lifetime LD of the switching device simply decreases with the elapsed time. For temperature measurement values ​​Tm that are greater than the permissible temperature Tzul and less than or equal to the maximum permissible temperature Tmax, the thermal lifetime LD of the switching device decreases significantly more than the elapsed time. For temperature measurement values ​​Tm that are greater than the maximum permissible temperature Tmax, the material of the switching device is permanently weakened or damaged.

[0050] Fig. 4 shows a QTemp-t diagram in which a calculated value QTemp is plotted on the y-axis and time t on the x-axis. The diagram shows the calculated values ​​QTemp 44, which are calculated as the quotient of the measured temperature values ​​Tm and a temperature reference value Tref: QTemp = Tm / Tref. The quotient QTemp describes the ratio of the absolutely measured temperature Tm to a reference temperature Tref, which results from a brand-new switching device at rated current. A regression line 49 is fitted to these calculated values ​​44 (regression analysis). The slope of the straight line 49 represents the trend of the calculated values ​​44 : At the beginning of the service life of the switching device, the value of the quotient QTemp is 1 , corresponding to the line 50 running parallel to the t-axis, but increases slowly over time (days, months, years).

[0051] Fig. 5 shows a QTemp-t diagram in which a calculated value QTemp is plotted on the y-axis and the time t on the x-axis. The calculated values ​​QTemp 44 are entered in the diagram and are calculated as a quotient: the dividend (numerator) of the quotient are the temperature measurement values ​​Tm of those temperature-current value pairs [Tm_i, Im_i] that are assigned to the same current class, here the current class (0.8 ± 0.05) x In, the divisor (denominator) of the quotient is a temperature reference value Tref: QTemp = Tm / Tref. The quotient QTemp describes the ratio of the absolutely measured temperature Tm to a reference temperature Tref that results from a brand new switching device at rated current. A straight line 51 is fitted to these calculated values ​​53 (regression analysis).The slope of the straight line 51 represents the trend of the calculated values ​​53: At the beginning of the service life of the switching device, the value of the quotient QTemp is below a temperature limit value, corresponding to the line 52 running parallel to the t-axis, but increases slowly over time (days, months, years) and approaches the temperature limit value 52. Fig. 6 is a flow diagram of a method according to the invention. In a first step 601, a time series of temperature measurements is recorded, which are taken at a temperature measuring point of the switching device. In a second step 602, a time series of calculated values ​​is formed, which was calculated on the basis of the recorded time series of temperature measurements. In a third step 603, a trend is determined in the formed time series of calculated values. In a fourth step 604, a service life of the switching device is estimated on the basis of the determined trend.

Claims

Patent claims 1. Method for determining a thermal lifetime (LD(T)) of an electromechanical switching device (10) which is inserted into a load current path (14), comprising the following steps: - recording a time series of temperature measurement values ​​(Tm) recorded at a temperature measurement point (P) of the switching device (10); - Creating a time series of calculated values ​​based on the recorded time series of temperature measurements (Tm); - Determining a trend in the generated time series of calculated values; - Estimating the service life of the switching device (10) based on the determined trend.

2. Method according to claim 1, wherein the calculated values ​​are identical to the temperature measured values ​​(Tm).

3. Method according to claim 1, wherein the calculated values ​​are quotients (QTemp) formed from the temperature measurement values ​​(Tm) divided by a temperature reference value (Tref): QTemp = Tm / Tref.

4. Method according to claim 1, wherein the calculation values ​​are quotients (QTemp) formed from the temperature measurement values ​​(Tm) which are each divided by an associated current measurement value (Im): Tm / Im or by a squared associated current measurement value (Im): Tm / Im 2 are divided, wherein the associated current measurement values ​​(Im) are the measurement values ​​of a current (I) flowing through the load current path (14), which are each measured simultaneously with the temperature measurement values ​​(Tm) at a current measurement point (Q) of the switching device (10).

5. Method according to one of claims 2 to 4, comprising the following steps: - detecting current measurement values ​​(Im) of a current (I) flowing through the load current path (14) at a current measurement point (Q) of the switching device (10), wherein the current measurement values ​​(Im) are each measured simultaneously with the temperature measurement values ​​(Tm); - dividing a current range covered by the recorded current measurements into two or more current classes; - Classification of the temperature measurement values ​​into the current class in which the current measurement value measured at the same time as the temperature measurement value lies; Forming, in at least one of the current classes, a time series of calculated values ​​calculated on the basis of the temperature measured values ​​(Tm) in this current class; - Determination of a trend of the generated time series of calculated values ​​per current class; - Estimating the service life of the switching device (10) based on the determined trend for each current class.

6. Method according to one of the preceding claims, wherein the determination of a trend is carried out using a regression analysis.

7. Computer program with software code sections for carrying out a method for determining a remaining thermal lifetime (LD(T)) of an electromechanical switching device (10), comprising the following steps: - storing (310) in a memory a time series of temperature measurement values ​​(Tm) recorded at a temperature measurement point (P) of the switching device (10); - Creating a time series of calculated values ​​based on the recorded time series of temperature measurements (Tm); - Determining a trend in the generated time series of calculated values; - Estimating the service life of the switching device (10) based on the determined trend.

8. A computer program product comprising a computer program according to claim 7.

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