Method for determining thermal life
By acquiring temperature and current measurements in electromechanical switchgear and performing trend analysis, the problem of unpredictable equipment thermal lifespan is solved, enabling equipment condition monitoring and lifespan prediction, and providing fault warnings and design improvement suggestions.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SIEMENS AG
- Filing Date
- 2024-08-01
- Publication Date
- 2026-04-21
AI Technical Summary
Existing technologies are insufficient to effectively monitor and predict the thermal life of electromechanical switching equipment, which may lead to equipment failure or fire due to changes in contact resistance and temperature rise.
By acquiring temperature measurements of switching equipment, forming a time series, performing trend analysis, calculating the remaining thermal life of the switching equipment using the temperature measurements, combining current intensity measurements for more accurate trend identification, and using a computer program to achieve life assessment.
It enables thermal condition monitoring and lifespan prediction of electromechanical switchgear, providing early warnings of impending equipment failure and helping users maintain and improve equipment design in a timely manner.
Smart Images
Figure CN121909522A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to a method for determining thermal lifetime and a computer program. Background Technology
[0002] Electromechanical switches and protective devices utilize movable contacts for switching. Due to these switching processes and environmental influences such as corrosion, changes occur on the contact surface and consequently, in contact resistance. These changes occur more abruptly during the switching process, but more slowly due to environmental factors. Increased contact resistance leads to increased power loss and consequently, increased equipment temperature. This can cause the equipment to overheat, potentially resulting in malfunction or fire.
[0003] Condition monitoring of critical temperature points (hotspots) or thermally vulnerable areas near such hotspots is essential for issuing early warnings of impending failures. In addition to determining the current condition of the equipment, predicting its remaining thermal lifespan is also beneficial. Summary of the Invention
[0004] Therefore, there is a need for methods for determining the thermal life of electromechanical switches and protection devices.
[0005] This technical problem is solved by the method according to claim 1. The method relates to a method for determining the thermal life of an electromechanical switchgear. Here, thermal life is understood as the (remaining) lifespan of the switchgear, resulting from the thermal aging of the materials used to manufacture the switchgear. Lifespan, also known as durability, refers to the period of time during which the switchgear can be used without failure. The term "electromechanical switchgear" herein includes any electromechanical switch and protection device having movable switch contacts. Here, it is irrelevant whether it refers to a multi-break interruptor switchgear or a single-break interruptor switchgear. The switch triggering method, such as an actuator, magnetic latching mechanism, or switch lock, is also irrelevant. This invention is applicable to all types of switchgear having movable switch contacts. The switchgear with contacts to which this invention is applicable follows various basic concepts. On the one hand, there are switchgear having single-break or multi-break contacts, whether linear or rotary. Furthermore, these switchgear are operated or driven differently. Switchable spring accumulators, such as bistable switch locks used in MCBs (Miniature Circuit Breakers) and MCCBs (Molded Base Circuit Breakers), or tensionable spring accumulators used in ACBs (Air Circuit Breakers), can be used. Similarly, protective contacts can be operated using magnetic actuators. Even direct connection of knife contacts, as in LBSs (Load Break Switches), is possible. Force or energy can be introduced manually, via a motor, or, in the case of a contactor, by applied voltage. This also includes all other possible forms of contact-equipped switching devices.
[0006] The switching device is connected in the load current path. Through the load current path, electrical energy is transferred from an electrical energy source, such as a power supply network providing grid voltage, to the electrical equipment, such as a motor. The load current path can be designed, for example, as a three-phase power line. The switching device can have input terminals (e.g., L1, L2, L3) and output terminals (e.g., T1, T2, T3), wherein the power line end connected to the electrical energy source (grid voltage) is connected to the input terminals, and the power line end connected to the electrical equipment is connected to the output terminals. The switching device contains a switching mechanism, such as an electromechanical switch and / or a semiconductor switch (e.g., MOSFET, IGBT), designed such that in a first switching state (“closed” switching state), the connection between the input and output terminals is electrically conductive, and in a second switching state (“open” switching state), the connection between the input and output terminals is resistively disconnected.
[0007] The method includes the step of acquiring a time series of temperature measurements recorded at a temperature measurement point on the switching equipment. The temperature measurement is a temperature value determined by a temperature sensor. The time series of temperature measurements is a sequence of temperature measurements ordered by time; wherein the temperature measurement can be acquired at any point in time. The temperature measurement point is a location inside the switching equipment, i.e., within the internal space of the equipment housing and / or on the outer surface of the equipment housing, where the temperature sensor can be positioned to measure the temperature measurement.
[0008] The method includes the step of forming a time series of calculated values based on a time series of acquired temperature measurements. The time series of calculated values is a sequence of calculated values ordered by time; wherein the time order of the calculated values corresponds to the time order of the temperature measurements. The calculated values are obtained from the temperature measurements through mathematical operations; these operations may also consist of simply multiplying by the number 1, such that the temperature measurements and the calculated values based on those temperature measurements are identical.
[0009] The method includes a step in which a trend is determined in the time series of the resulting calculated values. The trend is the long-term change in the calculated values of the time series over time. This trend can be assessed for the extreme temperature values of the switching equipment. The trend can be approximated as linear, exponential, logarithmic, etc.
[0010] The method includes the step of determining the lifespan of the switching device based on a determined trend.
[0011] This invention is based on the concept of using temperature measurement parameters for trend analysis and lifespan assessment in electromagnetic switching devices, as temperature measurement parameters best describe the overall quality of the switching device's condition. The essential difference from conventional methods for determining lifespan is that temperature measurements are used for assessment, and are assessed as a function of time, thereby allowing the determination of time trends and thermal lifespan, particularly remaining thermal lifespan. In this way, condition monitoring of the switching device can be achieved based on its quality characteristic, i.e., temperature. Furthermore, predictions about thermal lifespan can be derived. These predicted lifespans may be shorter than those predicted based on currently measured temperatures, without considering past temperature measurements.
[0012] The more measurements there are, the more reliably the trend can be depicted; therefore, averaging of the calculated values is advantageous in order to obtain sufficient smoothness of the measurements.
[0013] The aforementioned technical problem is also solved by the computer program according to claim 7. This computer program has a software code portion. The computer program is used to perform a method for determining the remaining thermal life of an electromechanical switching device. The computer program is adapted to cause the following steps to be performed when executed on an electronic computer.
[0014] The time series of temperature measurements recorded at temperature measurement points of the switching device are stored in memory. A time series of calculated values is formed based on the time series of acquired temperature measurements. The trend in the time series of calculated values is determined. The lifespan of the switching device is estimated based on the determined trend.
[0015] Advantageous designs and extensions of the invention are given in the dependent claims. Accordingly, the independent device claims are also extended based on the method according to the invention, and vice versa.
[0016] According to a preferred design of the present invention, the trend is determined by means of regression analysis.
[0017] According to a preferred embodiment of the invention, the calculated value is the same as the temperature measurement value. Therefore, the calculated value is obtained from the temperature measurement value through mathematical operations consisting solely of multiplying by the number 1. Thus, trends can be directly determined from the temperature measurement values.
[0018] According to a preferred embodiment of the invention, the calculated value is equal to the quotient QTemp, which is formed by dividing the temperature measurement value Tm by the temperature reference value Tref: QTemp = Tm / Tref. The quotient QTemp describes the ratio of the absolute measured temperature Tm to the reference temperature Tref, for example, in a brand-new switchgear at rated current. The reference temperature Tref can refer to a circuit breaker in a standard test configuration conforming to DIN EN60947-2:2020-11 Low-voltage switchgear - Part 2: Circuit breakers. The reference temperature Tref can also refer to the permissible temperature of the materials installed in the switchgear that constitute the thermally weak points of the switchgear. However, any other temperature comparison quantity can also be used as the reference temperature Tref. The reference temperature Tref can also be adjusted as necessary at the start of the calculation based on an understanding of the installation conditions.
[0019] The proposed dimensionless calculation value QTemp=Tm / Tref contains additional information: This value allows for assessment during the initial operation of the switchgear to determine whether the design and selected installation or environmental conditions are suitable for a switchgear in its new factory and testing condition. A value greater than 1 indicates that thermal conditions have deteriorated relative to standard test structures and the average new equipment; this could be due to unfavorable design (e.g., insufficient cable cross-section) or poor installation (e.g., excessive equipment packing density), suggesting a shorter lifespan for the switchgear. Therefore, recommendations for improving the thermal design of electrical switchgear can be provided to users.
[0020] According to a preferred embodiment of the present invention, the calculated value is the quotient QTemp, which is obtained by dividing the temperature measurement value Tm by the corresponding current intensity measurement value Im: QTemp = Tm / Im, or by dividing by the square of the corresponding current intensity measurement value Im: QTemp = Tm / Im 2 Formation. Here, the associated current intensity measurement value Im is the measured value of the current flowing through the load current path, and this measurement value is simultaneously measured at the current intensity measurement point Q of the switching equipment along with the temperature measurement value Tm. Other different quotients and mathematical functions can also be considered, such as Tm / Im. 3 Im 4 Other mathematical expressions of log Im or Im.
[0021] According to a preferred embodiment of the invention, a temperature sensor for acquiring a temperature measurement value in a switching device is positioned on the current path of the switching device, i.e., directly on the current path. Therefore, the measured temperature is determined on the current path of the switching device, through which the load current is conducted. In this case, the measurement point is located on the current path. If the current sensor is electrically insulated, the measurement point can be located directly on the surface of the current path; otherwise, the measurement point is located at a sufficiently large distance relative to the energized current path to avoid electrical breakdown from the current path to the temperature sensor. The advantage is that the measured temperature can be measured at the main heat source of the switching device, i.e., the current path.
[0022] According to a preferred embodiment of the invention, the measured temperature is determined at a measurement point on the switchgear, wherein the measurement point is a thermally weak point of the switchgear. The thermally weak point is a component close to a hot spot or the hot spot itself, where the hot spot is the hottest location on the switchgear. To determine the thermally weak point, in addition to the temperature conditions of the component, the temperature-time characteristic curve of the component material must also be considered. The determination of the thermally weak point must be performed individually for each switchgear. If an actual hot spot or an actual thermally weak point cannot be selected in the switchgear, a suitable nearby measurement point can be selected as an alternative.
[0023] According to a preferred embodiment of the invention, the method includes the step of acquiring a current intensity measurement value of the current flowing through the load current path at a current intensity measurement point of the switching device using a current sensor, wherein the current intensity measurement value is measured simultaneously with a temperature measurement value. Preferably, the current intensity measurement value is acquired over a defined time domain, in which the current intensity in the load current path is approximately constant. The design further includes the step of dividing the current intensity range covered by the acquired current intensity measurement values into two or more current levels. The design further includes the step of assigning temperature measurement values to the current levels in which the current intensity measurement values measured simultaneously with the temperature measurement values are located. The design further includes the step of forming a time series of calculated values based on the temperature measurement values in at least one current level. The design further includes the step of determining a trend of the time series of calculated values for each current level. The design further includes the step of estimating the lifespan of the switching device based on the determined trend for each current level. The advantage of using current levels is that trends can be identified better and more accurately.
[0024] According to a preferred embodiment of the invention, temperature-current value pairs are formed from temperature measurements and simultaneously acquired current intensity measurements. These value pairs are assigned to current levels corresponding to a small portion of the rated current range In of the switching equipment, such as (0.5±0.05)xIn, (0.6±0.05)xIn, (0.7±0.05)xIn, etc. The temperature measurements of the temperature-current value pairs assigned to the same current level are then plotted against time, and a trend is determined. The more measurements, the more accurately the possible trend can be depicted; therefore, to plot a trend line, the shortest operating time (the time with current flow) and the fewest number of temperature measurements are required. To obtain the smoothest possible temperature measurement curve, averaging at each data point for trend analysis is advantageous. The determined trend can be compared with the limiting temperature values of the switching equipment.
[0025] The limit temperature value of a switchgear can be considered its permissible temperature. The permissible temperature of a switchgear is defined by an organization familiar with the materials installed in the switchgear, such as the switchgear's manufacturer or operator. The permissible temperature of a switchgear forms the upper limit of a first temperature range within which the switchgear will not thermally age faster. Within this first temperature range, the thermal life decreases accordingly over time; for example, after 24 hours, the thermal life also decreases by 24 hours.
[0026] The limiting temperature value of a switchgear can be considered its maximum permissible temperature. The maximum permissible temperature of a switchgear is defined by an organization familiar with the materials installed in the switchgear, such as the switchgear manufacturer or operator. The maximum permissible temperature forms the upper limit of a second temperature range in which the switchgear ages faster due to the increased temperature than in the first temperature range. Within this second temperature range, which includes all temperature values greater than the permissible temperature and less than or equal to the maximum permissible temperature, the thermal life decreases faster than the passage of time; for example, after 24 hours, the thermal life decreases by 28 hours. Materials, such as plastics, have specific temperature-time characteristic curves for their material properties. These curves take into account important material properties, such as strength or electrical insulation capabilities. The permissible temperature of a material can be determined from the temperature-time characteristic curve. As long as the permissible temperature is not exceeded, the material can withstand that temperature for a long time (e.g., 25 years) without damage. Furthermore, the maximum permissible temperature can be determined from the temperature-time characteristic curve. For the maximum permissible temperature, design limits can be derived from strength limits.
[0027] If the maximum permissible temperature is exceeded, the materials of the switchgear will be permanently weakened or damaged, and its lifespan will be reduced more significantly than in the second temperature range. For example, when the maximum permissible temperature of plastic is exceeded, the flame retardant decomposes, and the function is lost. To prevent the maximum permissible temperature from being exceeded, one or more warning thresholds can be set at lower temperatures. When these warning thresholds are reached, the operator of the switchgear receives a warning that the maximum permissible temperature is about to be reached.
[0028] According to a preferred embodiment of the invention, the temperature measurement is obtained at the hot spot, i.e., the hottest point, of the switching device. This is because the highest temperature experienced by the switching device is decisive for its overall thermal lifespan.
[0029] According to a preferred embodiment of the invention, an estimate is made of when the temperature in the switchgear will exceed the maximum permissible temperature of the switchgear, wherein the estimate is based on trend analysis of temperature measurements taken at two or more past time points. In this way, the user of the switchgear has sufficient time to plan the timely shutdown of the switchgear.
[0030] A preferred embodiment of the present invention is a computer program product comprising the computer program described above. Attached Figure Description
[0031] The invention will now be described with reference to the accompanying drawings and several embodiments. These embodiments are shown schematically and not to scale.
[0032] Figure 1 Show the switching device;
[0033] Figure 2 The T / It graph is shown, which plots the time curves of temperature measurement and current intensity measurement.
[0034] Figure 3 The Tt plot shows the time curve of the temperature measurement at the current level; and
[0035] Figure 4 The QTemp-t plot is shown, in which the time curve of the calculated value QTemp is plotted; and
[0036] Figure 5 The QTemp-t plot is shown, which plots the calculated value of QTemp for the current level over time; and
[0037] Figure 6 The flowchart is shown. Detailed Implementation
[0038] Figure 1 A switching device 10 is shown for switching load current flowing through a load current line 16 from a voltage source, such as a network transformer, to an electrical load, such as a motor. The switching device 10 has a housing 20 with load current terminals 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 terminals 15, 15', such that the switching device 10 is connected between the two ends of the load current line 16. Inside the housing 20, the load current terminals 15 are electrically connected to fixed switch contacts 11 via current paths 14.
[0039] A contact pair consists of a fixed contact 11 and a movable contact 12. In mechanical switchgear, there is at least one contact pair 11, 12 (single-break interruptor, such as ACB). Many modern switchgear uses multiple contact pairs connected in series or parallel (multi-break interruptor) depending on the requirements of the switchgear.
[0040] Figure 1 The embodiment shown has a double-break interruptible mechanical switch 11, 12, 13. When the contact bridge 13 is in a closed first position, the fixed switch contacts 11 are electrically connected via the movable contact bridge 13 carrying the movable switch contacts 12; in this position, there is a continuous electrical connection to the load current terminal 15. In the open second position of the contact bridge 13, the two fixed switch contacts 11 are electrically separated; in this position, the load current terminal 15 is current-isolated.
[0041] Actuator unit 17 enables contact bridge 13 to move back and forth between two positions; this positional change of contact bridge 13 constitutes a switching event. This switching event is triggered by manipulating actuator unit 17 to change position. Manipulation of actuator unit 17 can be performed internally within switchgear 10, for example, by thermal or magnetic triggering in a line protection switch. This control of actuator unit 17 can also be performed by external control signals, for example, in a contactor, by control commands directed via control line 19 to control current terminals 18 of actuator unit 17. The actuator unit can also be designed as a classic switch lock.
[0042] Inside the housing 20, a temperature sensor 23 is arranged at the first measurement point P, the temperature measurement point. The first measurement point P is located on the current path 14 of the switching device 10. Ideally, this is the hottest location of the switching device 10, the so-called "hot spot". The temperature measurement value obtained by the temperature sensor 23 is transmitted to the evaluation unit 30 via the data line 24.
[0043] Inside the housing 20, a current sensor 21 is also arranged at the second measurement point Q, the current intensity measurement point. The second measurement point Q is located on the current path 14 of the switching device 10. The current measurement value obtained by the current sensor 21 is transmitted to the evaluation unit 30 via the data line 22.
[0044] The evaluation unit 30 has a calculation unit that can determine the thermal lifetime LD(T) of the switchgear 10 based on the received temperature measurement value. The thermal lifetime value determined by the evaluation unit 30 can be transmitted to the human-machine interface (HMI) 32 via the transmission medium 31 and communicated to the user of the switchgear 10 via the HMI 32.
[0045] Figure 2 The T / It plot is shown, where temperature T and current intensity I are plotted on the y-axis, and time t is plotted on the x-axis. The plot shows the time curves for the temperature measurement Tm and the current intensity measurement Im. The current intensity measurements Im1,..., Im7 are obtained for a defined time domain Δt1 to Δt7, in which the current intensity Im in the load current path is approximately constant. Then, temperature-current value pairs [Tm_i, Im_i], i=1,...,7 are formed from the temperature measurements Tm1,...,Tm7 and the simultaneously obtained current measurements Im1,..., Im7, respectively, and these temperature-current value pairs are plotted in the plot.
[0046] A switching event occurs at time 45.
[0047] Temperature-current pairs [Tm_i, Im_i] are assigned to current ratings 46, which correspond to a small portion of the rated current range In of the switching equipment, such as (0.5±0.05)xIn, (0.6±0.05)xIn, (0.7±0.05)xIn, etc. Then, the temperature measurements of those temperature-current pairs [Tm_i, Im_i] assigned to the same current rating are plotted against time, as shown below. Figure 3 As shown.
[0048] Figure 3 A Tt plot is shown, where temperature T is plotted on the y-axis and time t on the x-axis. The plot shows temperature measurements 43 assigned to the same current level, here (0.5 ± 0.05) xIn, of the temperature-current value pairs [Tm_i, Im_i]. A straight line 47 is fitted (regression analysis) to these temperature measurements 43. The slope of the straight line 47 reflects the trend of the temperature measurements 43: the temperature measurements increase slowly over time, i.e., with the number of years.
[0049] The more temperature measurements 43 there are, the more reliably the trend can be depicted; therefore, to plot a trend line, the shortest operating time of the switching equipment and the minimum number of temperature measurements are required. To obtain the smoothest possible temperature measurement curve, averaging at each data point for trend analysis is advantageous. The identified trend can be compared to the switching equipment's limit temperature value 48.
[0050] The limiting temperature value 48 extending parallel to the t-axis of the switchgear can be, for example, the permissible temperature Tzul or the maximum permissible temperature Tmax. For a temperature measurement Tm less than or equal to the permissible temperature Tzul, the thermal life LD of the switchgear simply decreases over time. For a temperature measurement Tm greater than the permissible temperature Tzul and less than or equal to the maximum permissible temperature Tmax, the decrease in the thermal life LD of the switchgear is significantly greater than the decrease in time. For a temperature measurement Tm greater than the maximum permissible temperature Tmax, the material of the switchgear undergoes permanent weakening or damage.
[0051] Figure 4A QTemp-t plot is shown, where the calculated value QTemp is plotted on the y-axis and time t is plotted on the x-axis. The plot includes the calculated values QTemp 44, which are calculated as the quotient of the temperature measurement Tm and the temperature reference Tref: QTemp = Tm / Tref. The quotient QTemp describes the ratio of the absolute measured temperature Tm to the reference temperature Tref, obtained in a brand-new switchgear at rated current. A regression line 49 is fitted (regression analysis) to these calculated values 44. The slope of line 49 reflects the trend of the calculated values 44: at the beginning of the switchgear's life, the quotient QTemp is 1, corresponding to line 50 extending parallel to the t-axis, but it slowly increases over time (days, months, years).
[0052] Figure 5 A QTemp-t plot is shown, where the calculated values QTemp are plotted on the y-axis and time t is plotted on the x-axis. The calculated values QTemp 44 are plotted in this plot, and these calculated values are obtained as quotients: the dividend (numerator) of the quotient is the temperature measurement Tm assigned to the temperature-current value pair [Tm_i, Im_i] of the same current rating, here (0.8±0.05)xIn, and the divisor (denominator) of the quotient is the temperature reference value Tref: QTemp = Tm / Tref. The quotient QTemp describes the ratio of the absolute measured temperature Tm to the reference temperature Tref, which is obtained in a brand-new switchgear at rated current. A straight line 51 is fitted (regression analysis) to these calculated values 53. The slope of the straight line 51 reflects the trend of the calculated values 53: at the beginning of the switchgear's life, the value of the quotient QTemp is below the temperature limit, corresponding to line 52 extending parallel to the t-axis, but slowly rises and approaches the temperature limit 52 over time (days, months, years).
[0053] Figure 6 This is a flowchart of the method according to the present invention. In the first step 601, a time series of temperature measurement values recorded at temperature measurement points of the switching device is acquired. In the second step 602, a time series of calculated values is formed based on the acquired time series of temperature measurement values. In the third step 603, a trend in the time series of the formed calculated values is determined. In the fourth step 604, the lifespan of the switching device is estimated based on the determined trend.
Claims
1. A method for determining the thermal life (LD(T)) of an electromechanical switchgear (10) connected in a load current path (14), the method comprising the following steps: - Obtain the time series of temperature measurement values (Tm) recorded at the temperature measurement point (P) of the switching device (10); - Form a time series of calculated values based on the time series of the acquired temperature measurements (Tm); - Determine the trend in the time series of the resulting calculated values; - Estimate the lifespan of the switchgear (10) based on the identified trends.
2. The method according to claim 1, wherein, The calculated value is the same as the measured temperature value (Tm).
3. The method according to claim 1, wherein, The calculated value is the quotient (QTemp), which is formed by dividing the temperature measurement value (Tm) by the temperature reference value (Tref): QTemp = Tm / Tref.
4. The method according to claim 1, wherein, The calculated value is the quotient (QTemp), which is obtained by dividing the temperature measurement value (Tm) by the corresponding current intensity measurement value (Im): Tm / Im, or by dividing by the square of the corresponding current intensity measurement value (Im): Tm / Im. 2 The formation is such that the associated current intensity measurement value (Im) is the measurement value of the current (I) flowing through the load current path (14), and these measurements are taken simultaneously with the temperature measurement value (Tm) at the current intensity measurement point (Q) of the switching device (10).
5. The method according to any one of claims 2 to 4, comprising the following steps: - Obtain the current intensity measurement value (Im) of the current (I) flowing through the load current path (14) at the current intensity measurement point (Q) of the switching device (10), wherein the current intensity measurement value (Im) is measured simultaneously with the temperature measurement value (Tm); - Divide the current intensity range covered by the acquired current intensity measurements into two or more current levels; -Assign the temperature measurement value to the current level of the current intensity measurement value that was measured at the same time as the temperature measurement value; - In at least one current level, a time series of calculated values based on temperature measurements (Tm) at that current level is formed; - Determine the trend of the time series of the calculated values for each current level; - The lifespan of the switching device (10) is estimated based on the trends determined for each current level.
6. The method according to any one of the preceding claims, in, Regression analysis is used to determine trends.
7. A computer program having a software code portion, the computer program being used to perform a method for determining the remaining thermal life (LD(T)) of an electromechanical switchgear (10), comprising the steps of: - Store in memory (310) the time series of temperature measurement values (Tm) recorded at the temperature measurement point (P) of the switching device (10); - Form a time series of calculated values based on the time series of the acquired temperature measurements (Tm); - Determine the trend in the time series of the resulting calculated values; - Estimate the lifespan of the switchgear (10) based on the identified trends.
8. A computer program product comprising the computer program according to claim 7.