Method for determining capacitance value of RC circuit capacitor on vehicle DC bus
By calculating the first and second time derivatives or definite integrals of the charging or discharging voltage of an RC circuit capacitor, the problem of low capacitance measurement accuracy in existing technologies is solved, achieving high precision and economy in capacitor aging detection.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- ALSTOM HOLDINGS SA
- Filing Date
- 2025-11-06
- Publication Date
- 2026-05-08
AI Technical Summary
In the prior art, the method for determining the capacitance value of the RC circuit capacitor on the DC bus of a vehicle has low accuracy, especially in the low charging current range, which makes it difficult to measure accurately. This leads to inaccurate detection of capacitor aging, which may result in reduced filtering capability and damage to traction converter components.
By measuring a series of values of the charging or discharging voltage of the capacitor, its first and second time derivatives or definite integrals are calculated. The capacitance value is determined by using the relationship between the negative quotient or definite integral of the first and second time derivatives and the known resistance value.
It enables high-precision measurement of capacitor capacitance, simplifies the testing process, reduces costs, ensures the accuracy of capacitor aging testing, and reduces maintenance workload.
Smart Images
Figure CN121995115A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to a method for determining the capacitance value of an RC circuit capacitor on a vehicle DC bus, a system configured to perform the method, and a vehicle comprising the system. Background Technology
[0002] Capacitors are commonly used on the DC bus of traction converters in vehicles (especially rail vehicles), specifically in RC circuits with charging and / or discharging resistors. In this regard, capacitors are particularly useful as filter capacitors. During operation, the capacitance value decreases over time due to aging. To prevent subsequent damage (especially damage to traction converter components due to decreased filtering capability), accurate monitoring of the capacitance value is necessary. If significant aging is detected, the capacitor may need to be replaced.
[0003] In the prior art, it is known that the capacitance value is determined by measuring the charging current and charging voltage of the capacitor, based on the differential equation C * dU(t) / dt (which, after simplification, yields C = I / (dU(t) / dt)), where I is the charging current, C is the capacitance value, U(t) is the charging voltage, and dU(t) / dt is the first time derivative of the charging voltage with respect to time t.
[0004] However, in practical applications, the reliability of this method may be low. The charging resistor reduces the charging current to extremely low levels, which may be below the accuracy limits of the measuring device (such as a current transformer).
[0005] Furthermore, the resolution of the A / D converter used (e.g., 1A) is insufficient to meet the requirements in low-value ranges (such as below 10A). Resolution characterizes the smallest difference between two current values that an A / D converter can distinguish. Summary of the Invention
[0006] The purpose of this invention is to provide a method for determining the capacitance value of the RC circuit capacitor on the DC bus of a vehicle. This method has higher accuracy and is simple and economical to implement and use.
[0007] According to the present invention, the above-mentioned objective is achieved by the method of claim 1, the system of claim 5, and the vehicle of claim 6.
[0008] Specifically, the present invention proposes a method for determining the capacitance value of an RC circuit capacitor on a vehicle DC bus, wherein the RC circuit includes a charging resistor or a discharging resistor, and the method includes the following steps: a) Measure a series of values for the charging or discharging voltage of the capacitor during the evaluation time interval; b) Determine the first-order time derivative of the series of charging or discharging voltage values within the evaluation time interval: - numerical value, or - Definite integral over time; c) Determine the second time derivative of the series of charging or discharging voltage values within the evaluation time interval: - numerical value, or - Definite integral over time; d) Determine the capacitance value of the capacitor using one of the following relationships: - The capacitance value can be obtained by dividing the negative quotient of the first-order time derivative and the second-order time derivative by the known resistance value of the charging or discharging resistor; or - The capacitance value can be obtained by dividing the negative quotient of the definite integral of the first-order time derivative and the definite integral of the second-order time derivative by the known resistance value of the charging or discharging resistor.
[0009] "At least one" has the same meaning as "one or more"; "partially" or "at least partially" has the same meaning as "partially or completely". In the following description, features will sometimes be described in the singular. Where applicable, such a description may alternatively or additionally include corresponding disclosures of multiple features of that class, and vice versa.
[0010] "The definite integral of the first-order time derivative" is the same as "the definite integral of the first-order time derivative with respect to time"; "The definite integral of the second-order time derivative" is the same as "the definite integral of the second-order time derivative with respect to time". "Evaluation time interval" generally refers to: all, some, or multiple segments of values within that time interval that can be used for evaluation and / or further calculations.
[0011] The vehicle may in particular be a rail vehicle, such as a locomotive or tractor-trailer (e.g., tram, metro train, light rail system train, regional train, inter-regional train, long-distance train, or high-speed train). The vehicle includes a DC bus and a traction system in which traction power is transmitted to at least one traction motor via the DC bus. Alternatively, the vehicle may be an electric bus, electric vehicle, or any electric road or rail vehicle that includes a DC bus. Alternatively, the vehicle may be a hybrid vehicle that includes a DC bus.
[0012] The DC bus can be part of a vehicle traction converter. The capacitor can be any capacitor used in the DC bus of the vehicle traction converter, particularly a filter capacitor. The capacitor can be connected in series with a charging resistor (or in parallel with a discharging resistor) to form an RC circuit. This RC circuit can be an RC element. The capacitor and charging resistor can be arranged in series; the capacitor and discharging resistor can be arranged in parallel. An RC circuit can refer to a series circuit of a capacitor and a charging resistor, or a parallel circuit of a capacitor and a discharging resistor. The DC bus can refer to a DC intermediate circuit, where "DC" represents direct current. In the DC bus, when the DC voltage in the DC bus changes, the capacitor will charge or discharge.
[0013] "RC circuit on the vehicle's DC bus" can be understood as the RC circuit being electrically connected to the DC bus. The capacitor can be part of the DC bus. In embodiments using a charging resistor, the charging resistor can be part of a DC charging circuit connected to the capacitor and, consequently, to the vehicle's DC bus.
[0014] In embodiments using a discharge resistor, the discharge resistor may be part of a discharge circuit that also includes the aforementioned capacitor. For discharge to occur, the discharge circuit must be a closed circuit. For example, this closed circuit can be achieved by permanently connecting the discharge resistor to the capacitor. After the DC bus is fully charged, if the method of the present invention needs to be performed, the supply voltage can be disconnected via one or more controllable switches, allowing the DC bus to discharge through the (passive) discharge resistor. The one or more switches may be configured to switch between two states: - Power supply voltage connected to the DC bus (normal operating condition when this method is not executed); - The power supply voltage is disconnected from the DC bus (the state when this method is executed).
[0015] Alternatively, a method embodiment that simultaneously includes a charging resistor and a discharging resistor can be adopted. In this case, steps a) to d) can be performed multiple times by measuring and using a series of values of the charging voltage or the discharging voltage (depending on the selection), or steps a) to d) can be performed by combining the measurements and use of a series of values of the charging voltage and the discharging voltage. That is, this method can be performed multiple times, with at least one measurement using a series of values of the charging voltage and at least one measurement using a series of values of the discharging voltage. The results (capacitance values) obtained from the two methods can be combined (e.g., by averaging) to obtain the final capacitance value result.
[0016] This method (including all described embodiments) and the system configured to perform the method (including all described embodiments) can be executed at least partially automatically. A measuring device may be provided for measuring a series of values for charging voltage (or discharging voltage); this measuring device may be a voltmeter or include a voltmeter. The measuring device may be connected to a computing device (e.g., via wired or wireless means). The computing device may include input devices, output devices, data storage, random access memory, hard disk drive, and processor. The computing device may be configured to: - Determine the value of the first-order time derivative or definite integral; - Determine the value of the second-order time derivative or the definite integral; - Determine the capacitance value using any of the above relationships.
[0017] The charging voltage (or discharging voltage) can be a discrete value, each measured at discrete time steps, with the same preset duration (increment or step) between each time step. This preset duration (also called a "period" or "period time") can be on the order of milliseconds, such as 1ms, 2ms, 4ms, 10ms, 15ms, or 20ms. The evaluation time interval can contain multiple periods, such as 4 periods (corresponding to 5 samples), 20 periods (corresponding to 21 samples), 21 periods (corresponding to 22 samples), 39 periods (corresponding to 40 samples), or 100 periods (corresponding to 101 samples). The charging voltage (or discharging voltage) can be obtained by measuring the voltage across the capacitor.
[0018] This method may include a time measurement step, for example, to ensure that the preset duration length is consistent between time steps. Furthermore, a time value can be assigned to each charging voltage (or discharging voltage) value using a (known) preset duration between time steps (which may be a preset constant calculation cycle time) for subsequent calculations.
[0019] A series of charging voltage (or discharging voltage) values can be represented as curves or other forms of visualization. The curves of charging voltage (or discharging voltage) values can be obtained through interpolation. "Interpolation" refers only to the curve between discrete numerical points; however, the intermediate values obtained through interpolation must not be used in this method, but only for visualizing (continuous) curves and for graphically connecting (discrete) values. Alternatively / in addition, the series of charging voltage (or discharging voltage) values can be represented as numerical tables and / or stored as numerical tables.
[0020] A series of charging voltage (or discharging voltage) values can include multiple charging voltage (or discharging voltage) values, such as 5, 21, 22, 40, or 101 charging voltage (or discharging voltage) values.
[0021] Numerical differentiation is particularly useful when determining the value of the first-order time derivative of a series of charging (or discharging) voltage values. The value of the first-order time derivative can be obtained by numerically differentiating over the entire evaluation time interval (which may refer to determining the slope by calculating the differential over the entire evaluation time interval or, more precisely, by calculating the quotient of the difference) or sub-intervals of the evaluation time interval.
[0022] The first-order time derivative can be a series of charging (or discharging) voltage values and / or the slope of a curve representing the charging (or discharging) voltage values. If numerical differentiation is used, the first-order time derivative can be a series of values that change with time; alternatively, it can also be defined as a function of time.
[0023] When determining the definite integral of the first-order time derivative of a series of charging (or discharging) voltage values (based on the series of charging or discharging voltage values), it is particularly advisable to first perform numerical differentiation and then numerical integration. Specifically, one can first perform numerical differentiation on the charging (or discharging) voltage values over the entire evaluation time interval (which can refer to determining the slope by calculating the differential over the entire evaluation time interval or more precisely by calculating the quotient of the difference) or within a sub-interval of the evaluation time interval, and then perform numerical integration over the entire evaluation time interval or within a sub-interval of the evaluation time interval to obtain the definite integral of the first-order time derivative.
[0024] The definite integral of the first-order time derivative can be the definite integral of the slope of the curve of the charging voltage (or discharging voltage) series and / or the charging voltage (or discharging voltage) values with respect to time.
[0025] A definite integral is typically used to calculate the area of a region in a plane, defined by the graph of a given function, the horizontal axis, and two points on the horizontal axis (the start and end points). Therefore, a definite integral yields a specific numerical value (the area enclosed by the graph of the function and the horizontal axis between the two boundary points). The start and end points of the definite integral of the first-order time derivative can be the start and end points of the evaluation time interval or sub-intervals of the evaluation time interval.
[0026] Numerical differentiation is particularly useful for determining the value of the second-order time derivative of a series of charging (or discharging) voltage values. The value of the second-order time derivative can be obtained by numerically differentiating over the entire evaluation time interval (which can be determined by differentiating the first-order time derivative values over the entire evaluation time interval or, more precisely, by calculating the quotient of the difference) or sub-intervals of the evaluation time interval. The second-order time derivative is obtained by differentiating the original given function (the series of charging or discharging voltage values) twice with respect to time. Typically, the second-order time derivative represents the curvature of the original given function.
[0027] Typically, the second time derivative differs from the first time derivative (at least when the second and first time derivatives are expressed using exponential functions, they differ by a coefficient).
[0028] The second-order time derivative can be the slope and / or gradient of the first-order time derivative series of the charging voltage (or discharging voltage) series, or it can be the curvature of the charging voltage (or discharging voltage) series. If numerical differentiation is used, the second-order time derivative can be a series of values that change with time; alternatively, the second-order time derivative can also be defined as a function of time.
[0029] When determining the definite integral of the second-order time derivative of a series of charging (or discharging) voltage values (based on the series of charging or discharging voltage values), it is particularly advisable to first perform two numerical differentiations and then one numerical integration. Specifically, one can first perform two numerical differentiations on the charging (or discharging) voltage values within the entire evaluation time interval (which can refer to determining the value by calculating the differentiation between the charging (or discharging) voltage values [first differentiation] and the first-order time derivative values [second differentiation] within the entire evaluation time interval, or more precisely by calculating the difference quotient) or within a sub-interval of the evaluation time interval, and then perform one numerical integration over the entire evaluation time interval, a sub-interval of the evaluation time interval, or another sub-interval of the evaluation time interval, thereby obtaining the definite integral of the second-order time derivative.
[0030] The definite integral of the second-order time derivative can be the integral of the slope of the first-order time derivative series of charging voltage (or discharging voltage) values over time, or it can be the integral of the curvature of the charging voltage (or discharging voltage) series of values over time.
[0031] The starting and ending points of the second-order time derivative definite integral can be the start and end points of the evaluation time interval, a sub-interval of the evaluation time interval, or another sub-interval of the evaluation time interval.
[0032] The definite integrals of the second-order time derivative and the first-order time derivative can be integrated over the same time interval. Both the definite integrals of the first-order and second-order time derivatives can be integrated over the evaluation time interval or sub-intervals of the evaluation time interval.
[0033] The sub-intervals used to determine the numerical value of the first-order time derivative and / or the definite integral of the first-order time derivative, and used to determine the numerical value of the second-order time derivative and / or the definite integral of the second-order time derivative, may have the same length.
[0034] The mathematical relationship that "the negative quotient of the first-order time derivative and the second-order time derivative, divided by the known resistance value of the charging or discharging resistor, yields the capacitance value" can be expressed as the following equation: –((dU~ / dt) / ( ~ )) / R = tau / R = C Where C is the capacitance value to be determined, and dU~ / dt is the value of the first-order time derivative of the charging voltage (or discharging voltage). ~ Here, t represents the value of the second-order time derivative of the charging voltage (or discharging voltage), R is the (known) resistance value, and t is time. The symbol "~" is used to indicate the specific values of the first-order and second-order time derivatives. The negative quotient of the first-order and second-order time derivatives is the time constant of the RC circuit, usually represented by the Greek letter "tau". The time constant tau is also equal to the product of the capacitance and resistance values, i.e., tau = C × R.
[0035] The derivation of the above equation is as follows: This equation is based on the exponential formula for charging voltage: U(t) = U_DCsupply*(1–e^(–t / tau)), where U_DCsupply is the supply voltage of the RC circuit (measurable and / or known), and U(t) is the charging voltage.
[0036] Find the first time derivative of the charging voltage: dU(t) / dt = U_DCsupply*(e^(–t / tau)) / tau Find the second time derivative of the charging voltage: (t) = –U_DCsupply*(e^(–t / tau)) At the same time, this equation is also based on the exponential formula for discharge voltage: U(t) = U(0)*e^(–t / tau), where U(0) is the initial discharge voltage of the RC circuit (measurable), and U(t) is the discharge voltage.
[0037] Find the first time derivative of the discharge voltage: dU(t) / dt = –U(0)*(e^(–t / tau)) / tau Find the second time derivative of the discharge voltage: (t) = U(0)*(e^(–t / tau)) Therefore, the negative quotient of the first and second time derivatives is the time constant tau: –(dU(t) / dt) / ( (t) ) = tau From this, we can deduce that: –((dU(t) / dt) / ( (t) )) / R = tau / R = C Since the specific values of the first and second time derivatives can be determined from a series of values of the charging voltage (or discharging voltage), the above relationship can be obtained: –((dU~ / dt) / ( ~ )) / R = tau / R = C As an alternative, especially when using numerical integration, one can use... - The definite integral (with respect to time) of the first-order time derivative, and - The definite integral (with respect to time) of the second-order time derivative. This is used to replace the values of the first and second time derivatives. From this, we can derive the following mathematical relationship: "The negative quotient of the definite integral of the first and second time derivatives, divided by the known resistance value of the charging or discharging resistor, yields the capacitance value." –(Integral(dU(t) / dt)*dt) / (Integral( (t) )*dt) / R = tau / R = C In numerical integration, "integration" specifically refers to summing the values of the sequence to be integrated (here, the values of the first or second time derivatives) and multiplying the sum by the time interval between the values (which can be a preset duration, one period, or multiple periods). The values of the preset duration, one period, or multiple periods can be eliminated from the numerator and denominator of the quotient. Therefore, the definite integrals of the first and second time derivatives can be expressed as the summation of the first (numerical) time derivatives and the summation of the second (numerical) time derivatives, respectively.
[0038] The following example illustrates the process of calculating capacitance (the generality of other expressions is not limited): Assume there is a series of 22 specific charging voltage (or discharging voltage) values, denoted as U0, U1…U21, where the numbers are indices, which can also be represented as subscripts. These 22 charging voltage (or discharging voltage) values are measured at discrete time points, with a preset period of 4ms between each time point (in this example), i.e., a time step of 4ms. The capacitance value can be calculated using the following formula: Where i is the summation index and tspan is the preset duration (in the derivation process shown in this example, the sub-interval used contains 10 cycles, i.e., 10 × 4 ms = 40 ms).
[0039] Performing the above two summations (including substituting the specific values of the charging or discharging voltage) yields: The table below shows the coefficients and symbols for each charging voltage (or discharging voltage) value: Table 1: Coefficients and Symbols for Charging Voltage (or Discharging Voltage) Values
[0040] It should be noted that in the final calculation, only 10 of the 22 charging voltage (or discharging voltage) values are actually used.
[0041] If more charging voltage (or discharging voltage) values are available, for example, 40 charging voltage (or discharging voltage) values (U0, U1…U39), then the above formula for calculating the capacitance value can be expressed as: In this example, there are 40 charging voltage (or discharging voltage) values, and the sub-interval used in the derivation is 10 cycles. The final calculation result will include all 40 charging voltage (or discharging voltage) values. If more than 40 charging voltage (or discharging voltage) values are used, the final result will include the first 20 and the last 20 charging voltage (or discharging voltage) values.
[0042] If the ratio of the number of charging voltage (or discharging voltage) values to the sub-intervals used in the derivation is greater than 4, then when the numerator and denominator are summed twice, the intermediate values of the charging voltage (or discharging voltage) will cancel each other out.
[0043] Once the capacitance value is determined, the following operations can be performed (for example): - Store the capacitance values in a database; - Compare the capacitance value with the previous capacitance value, especially for assessing the aging and / or condition of the capacitor.
[0044] It can detect capacitor aging (especially a decrease in capacitance value, particularly exceeding a threshold percentage compared to previous or rated capacitance values) and / or capacitance values below a threshold (which can be defined as aging). If aging and / or capacitance values below the threshold are detected, a signal (e.g., an alarm or warning signal) can be output, for example, to the vehicle driver, technician, or maintenance center. This signal can be transmitted wirelessly or via wired means, to an output device (such as a display or a computer with data storage), or alternatively or additionally to the vehicle's fault memory for storage.
[0045] This invention provides a precise and simple method for measuring the aging of RC circuit capacitors on a vehicle's DC bus. This method allows for precise scheduling of capacitor-related maintenance tasks (such as repair or replacement), thereby reducing costs and maintenance workload.
[0046] The implementation and use of this method are simple and economical, especially because the mathematical principles it is based on are simple to understand and easy to implement, and it can utilize existing hardware (especially the hardware that is usually equipped in rail vehicles), without the need for expensive hardware upgrades or expansions. In addition, the computational power required by this method is extremely low.
[0047] Using definite integrals of the first and second time derivatives can effectively compensate for interference in the series of charging voltage (or discharging voltage) values.
[0048] In embodiments employing a series of discharge voltage values, an additional advantage is that, even if the supply voltage changes during the execution of the method of the present invention, this embodiment is less susceptible to discrete errors.
[0049] In a preferred embodiment of the method of the present invention, the numerical differentiation method is used to determine the value of the first-order time derivative based on the following: - The entire evaluation time interval; - The first sub-interval of the evaluation time interval (which contains at least three charging voltage or discharging voltage values); When determining the value of the second-order time derivative, numerical differentiation is used based on the following terms: - The entire evaluation time interval; - The second sub-interval of the evaluation time interval (which contains at least three charging or discharging voltage values).
[0050] In this embodiment, the evaluation time interval may include at least 4 cycles (corresponding to 5 sample values), 8 cycles (corresponding to 9 sample values), 12 cycles (corresponding to 13 sample values), 16 cycles (corresponding to 17 sample values), 20 cycles (corresponding to 21 sample values), or more cycles.
[0051] The first sub-interval may contain, for example, at least 2 cycles, 4 cycles, 10 cycles, 20 cycles or more cycles, or it may contain the entire evaluation time interval. The value (slope) of the first-order time derivative can be obtained by dividing the difference between the charging voltage (or discharging voltage) at the beginning and end of the evaluation time interval or the first sub-interval by the evaluation time interval (which may be its time span in milliseconds) or the first sub-interval (which may be its time span in milliseconds).
[0052] The second sub-interval may contain, for example, at least 2 cycles, 4 cycles, 10 cycles, 20 cycles, or more cycles. The value of the second-order time derivative (curvature) can be obtained by dividing the difference in the slope of the charging voltage (or discharging voltage) at the beginning and end of the evaluation time interval (specifically, its time span, in milliseconds) or the second sub-interval by the evaluation time interval or the second sub-interval (specifically, its time span, in milliseconds).
[0053] The length of the second sub-space may be the same as the length of the first sub-space, or it may be greater or less than the length of the first sub-space; the second sub-space may at least partially contain the first sub-space, or vice versa.
[0054] The above embodiments of the method of the present invention can calculate the capacitance value more accurately because the embodiments take into account a relatively long time interval during the calculation process.
[0055] In another preferred embodiment of the method of the present invention, multiple capacitance values can be determined, wherein: - If the predefined statistical dispersion measure of the multiple capacitance values is not exceeded, they are considered usable; - If the predefined statistical dispersion measure for the multiple capacitance values is exceeded, they are deemed unavailable.
[0056] "Deemed usable" can be understood as not discarding the relevant capacitance values, but outputting these capacitance values (especially for further evaluation of aging); "deemed unusable" can be understood as discarding the relevant capacitance values or not using these capacitance values (especially not for evaluating the aging of capacitors). The plurality of capacitance values can be determined by performing the method of the present invention multiple times (at least partially). The above embodiments of the method can be performed at least partially automatically, and the method can be performed multiple times (at least partially) by a computing device, and / or the plurality of capacitance values can be determined to be usable or unusable.
[0057] In a simplified embodiment, the maximum value C_max and the minimum value C_min among multiple capacitance values can be determined. The dispersion of the multiple capacitance values is determined by calculating (C_max-C_min) / C_max. If the result is lower than a preset threshold, the multiple capacitance values can be considered usable; if the result is higher than the preset threshold, the multiple capacitance values can be considered unusable.
[0058] In embodiments employing a series of charging voltage values, if the supply voltage of the RC circuit changes during the determination of multiple capacitance values, the capacitance values may exhibit significant dispersion. In such cases, the method of the present invention may be (at least partially) inapplicable, and the determined capacitance values may be invalid (in the above mathematical derivation, the supply voltage of the RC circuit is considered a constant). The above embodiments are able to identify such capacitance values as unusable and valid capacitance values as usable, thereby ensuring that the capacitance values used to assess aging conditions are valid and have high reference value.
[0059] In another preferred embodiment of the method of the present invention, if a change in the supply voltage of the RC circuit is detected during the evaluation time interval, the capacitor value (and multiple capacitor values where applicable) is deemed unusable.
[0060] Changes in the supply voltage can be measured using a measuring device (especially a voltmeter). If a change in the supply voltage is detected, the computing device can automatically deem the capacitance value (and multiple capacitance values, if applicable) as unusable. "Deemed unusable" can be understood as discarding the relevant capacitance value or not using these capacitance values (especially not for assessing the aging of capacitors).
[0061] The above embodiments ensure that the capacitance values used to assess aging are valid and of high reference value. This embodiment particularly requires that the RC circuit include a charging resistor, and that subsequent calculations are performed using a series of charging voltage values to obtain the capacitance value.
[0062] Furthermore, the present invention also proposes a system for determining the capacitance value of an RC circuit capacitor on a vehicle DC bus, wherein the RC circuit includes a charging resistor or a discharging resistor, the system being configured to perform the method of the present invention, and the system comprising: a) A measuring device configured to measure a series of values of the charging voltage or discharging voltage of a capacitor during an evaluation time interval; b) A computing device, configured as follows: Determine the first time derivative of the series of values of the charging voltage or discharging voltage within the evaluation time interval. - numerical value, or - Definite integral over time; And configured to also be able to: Determine the second time derivative of the series of values of the charging voltage or discharging voltage within the evaluation time interval. - numerical value, or - Definite integral over time; It is also configured to be able to determine the capacitance value of the capacitor using one of the following relationships: - The capacitance value can be obtained by dividing the negative quotient of the first-order time derivative and the second-order time derivative by the known resistance value of the charging resistor or discharging resistor. The capacitance value can be obtained by dividing the negative quotient of the definite integral of the first-order time derivative and the definite integral of the second-order time derivative by the known resistance value of the charging resistor or the discharging resistor.
[0063] The features described above in the embodiments of the method of the present invention are also applicable to the system of the present invention. For the system of the present invention, please refer entirely to the embodiments of the method of the present invention, and vice versa.
[0064] Furthermore, the present invention also proposes a vehicle incorporating the system of the present invention. The aforementioned features of the embodiments of the method and system of the present invention are equally applicable to the vehicle of the present invention. Reference can be made entirely to the embodiments of the method and system of the present invention, and vice versa, regarding the vehicle of the present invention. In particular, the vehicle can be a rail vehicle. Attached Figure Description
[0065] The exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which the figures illustrate: Figure 1 : A schematic diagram of an embodiment of the method of the present invention; Figure 2 Example charging voltage versus time curve (during execution) Figure 1 The measurements were obtained during the process of the method embodiment shown, where the time axis (horizontal axis) shows each period as 4ms; Figure 3 Used for execution Figure 1 The diagram shown is a schematic representation of an embodiment of the system of the present invention using the method of the present invention. Figure 4 : A schematic diagram of another embodiment of the system of the present invention.
[0066] The same reference numerals in different figures indicate the same, substantially the same, or similar features. Not every feature with reference numerals in every figure will be described repeatedly in the figure descriptions. Detailed Implementation
[0067] Figure 1 A schematic diagram of an embodiment of the method of the present invention is shown. Figure 3 It shows the method for execution Figure 1 The diagram shows an embodiment of the system 1 of the present invention, which is the method of the present invention. In the first step S1, a voltmeter VUt ( Figure 3 The system measures 21 values U0-U20 of the charging voltage Ut (which can be the DC bus voltage) and outputs them to the computing device CD. The capacitor CA ( Figure 3 ) through charging voltage Ut ( Figure 3 ) at DC bus DCL ( Figure 3 Charge it on the device. Figure 3 The charging circuit CC is shown. Capacitor CA is part of the DC bus (in the section representing the DC bus, only capacitor CA is shown; other parts of the DC bus are not shown). Resistor RE is provided as a charging resistor. Resistor RE is an additional component, specifically used to limit the current during the charging of capacitor CA. Resistor RE is connected to the DC bus DCL, but is not considered part of the DC bus. The DC bus DCL is part of the traction converter of the rail vehicle (i.e., the traction freight car of a tram). Capacitor CA is connected in series with resistor RE ( Figure 3 Together, they form an RC circuit on the DC bus DCL. This can be achieved using a second voltmeter VUS in the charging circuit CC. Figure 3 )Measure the supply voltage value US ( Figure 3 And output it to the computing device CD (this step is optional).
[0068] Figure 2 The diagram shows 21 values of the charging voltage Ut, U0-U20 (these values are interpolated to form a curve of the charging voltage Ut). These values were measured at 21 time points, with a preset duration of 4 ms between each time point, i.e., a time step of 4 ms. Value U0 was measured 8 ms after the start of the charging process, when the charging voltage was 0V. The 20 time durations (or periods) between the 21 time points correspond to the evaluation time interval T1. Figure 2 The evaluation time interval is from index 0 to index 20 and has a length of 80ms.
[0069] The following are examples of known or measured parameters: - The known resistance value R of resistor RE is 68 ohms; - The known rated capacitance value CN of capacitor CA is 2.7 millifarads; - The measured supply voltage value is US: 750 volts; - The charging voltage Ut value U0: 32.0 volts; - The charging voltage Ut value U5: 106.1 volts; - The charging voltage Ut value U10: 172.5 volts; - Charging voltage Ut value U15: 232.1 volts; - The value of the charging voltage Ut, U20: 285.6 volts.
[0070] In the second step S2, the quotient of the first and second time derivatives of the charging voltage Ut is determined by numerical differentiation within 10 cycles (10 cycles or 40ms is represented as the time duration tspan, i.e., tspan=40ms).
[0071] The value of the first-order time derivative is (U15 – U5) / tspan, calculated using the sub-intervals from index 5 to index 15 in the evaluation time interval T1.
[0072] The value of the second-order time derivative is [(U20 – U10) / tspan – (U10 – U0) / tspan] / tspan, which is calculated using the sub-intervals from index 10 to index 20 and from index 0 to index 10 in the evaluation time interval T1.
[0073] Figure 2 The aforementioned differences (U15 – U5, U10 – U0, U20 – U10) are plotted on the curve of charging voltage Ut. The time axis (horizontal axis) shows 20 cycles (4 ms each) and 21 time points.
[0074] It should be noted that if different time durations are used when calculating the first and second time derivatives (e.g., 50ms for the first time derivative and 20ms for the second time derivative), then different identifiers must be used for these time durations. For simplicity, the time duration tspan is used in both cases above because the sub-interval lengths used are the same, both corresponding to the time duration tspan.
[0075] In the third step S3, the negative quotient of the first-order time derivative and the second-order time derivative is determined, and this quotient is equal to the time constant tau of the RC circuit. tau = – {(U15 – U5) / tspan} / {[(U20 – U10) / tspan – (U10 – U0) / tspan] / tspan}<=> tau = – (U15 – U5) / ((U20 – U10) – (U10 – U0))*tspan<=> tau = – (U15 – U5) / (U20 + U0 – 2*U10)*tspan<=> tau = – (232.1 V – 106.1 V) / (285.6 V + 32.0 V – 2*172.5 V)*40 ms =183.9 ms Therefore, the capacitance C of capacitor CA can be calculated: C = tau / R = 183.9 ms / 68 Ohm = 2.705 mF The test results showed that the relative deviation between the capacitance value C and the rated capacitance value CN (2.7 millifarads) was +0.2%. (2.705 mF – 2.7 mF) / 2.7 mF = 0.2% After a period of time (e.g., 5 or 10 years), repeat the above calculations (step S1, step S2, step S3). New values or results are marked with the suffix "-new". Ensure the following new values are obtained (values not listed remain unchanged, and new values are not included in the calculations). Figure 2 (as shown in the image) - The new value of the charging voltage Ut, U0-new: 33.6 volts; - The new value for the charging voltage Ut, U5-new: 111.2 volts; - New value for charging voltage Ut, U10-new: 180.4 volts; - New value for charging voltage Ut U15-new: 242.1 volts; - New value for charging voltage Ut U20-new: 297.2 volts.
[0076] Repeat the above calculation process using the new values to obtain: tau-new = – (U15-new – U5-new) / (U20-new + U0-new – 2*U10-new)*tspan<=> tau-new = – (242.1 V – 111.2 V) / (297.2 V + 33.6 V – 2*180.4 V)*40 ms= 174.5 ms. Therefore, the new capacitance value C-new of capacitor CA can be calculated: C-new: 2.567 millifarads.
[0077] The above calculation process is performed by a computing device CD (CD). Figure 3The process is executed, and the results are output via an output device OD. The output device OD can be, for example, a display and / or monitor, for displaying the capacitance value C, the new capacitance value C-new, and their relative deviations from the rated capacitance value CN. Alternatively or additionally, the output device can be an interface connected to a data storage device (e.g., a fault memory for a rail vehicle).
[0078] During the process of determining the charging voltage value, the computing device CD continuously checks the supply voltage value US. Upon detection, the supply voltage value US remains unchanged, and this process continues... Figure 1 This is referred to as step S4. Since the above measurement and calculation process is not affected by the change in the supply voltage value US, the calculation device CD recognizes the capacitance value C and the new capacitance value C-new as usable.
[0079] As an alternative to or supplement to step S4, the computing device CD can be configured to perform a predefined statistical measure of dispersion. In this case, multiple capacitance values must first be determined by at least partially executing the method of the present invention to detect possible dispersion and discard erroneous capacitance values with excessive dispersion. The supply voltage value US is not required in this case.
[0080] In step S5, the relative deviation between the new capacitance value C-new and the rated capacitance value CN (2.7 millifarads) was detected to be -4.9%. (2.567 mF – 2.7 mF) / 2.7 mF = –4.9% The decrease in capacitance value relative to the rated value indicates that capacitor CA has aged during the aforementioned time period.
[0081] Using the difference quotient (based on a time duration tspan of 40ms) instead of the differential quotient (based on an infinitesimal time interval) introduces systematic errors. To limit the relative error to within 0.2%, it is recommended that the ratio of the time duration tspan used for differentiation to the time constant tau be greater than 5. In the calculation example above, additional errors are introduced due to rounding of intermediate results. Both the systematic and additional errors are small and have no impact on the reference value of the aging determination results (or their impact is negligible).
[0082] The above method embodiments (more precisely, the first step S1, the second step S2, and the third step S3) can also be used, with necessary modifications, to address the discharge voltage Ud of capacitor CA (see...). Figure 4 The series of numerical calculations are used to determine the capacitance value C. For example... Figure 4 As shown, the required system 10 of the present invention and Figure 3The difference in the system 1 of the present invention shown is that: the resistor RD is a discharge resistor, connected in parallel with the capacitor CA to form a closed discharge circuit DSC. The resistor RD is a passive discharge resistor. (The last sentence appears to be incomplete and possibly contains errors.) Figure 3 The present invention system 1 shown is the same, and the capacitor CA is part of the DC bus (in the part representing the DC bus, only the capacitor CA is shown, and other parts of the DC bus are not shown).
[0083] The discharge voltage Ud is measured using a voltmeter VUd and is used to replace the charging voltage Ut values U0-U20 in the further calculations described above. After a period of time (e.g., 5 or 10 years), when this method (steps S1, S2, and S3) is repeated, the new value of the discharge voltage Ud will be detected, and the above calculations will be performed. Finally, if necessary modifications are made, step S5 can be used to evaluate the aging condition of the capacitor CA.
[0084] In this embodiment of the method, step S4 is not performed because the supply voltage may not be available at this time, and it is not necessary to use the supply voltage. Instead, the computing device CD is configured to perform a predefined statistical dispersion metric. In this case, multiple capacitance values need to be determined by at least partially performing the method of the present invention in order to detect possible dispersion and discard erroneous capacitance values with excessive dispersion. The supply voltage value US is not needed in this case.
[0085] The present invention system 10, as well as the method of determining the capacitance value (here, capacitance C) by means of a discharge resistor (here, resistor RD), has an advantage: it is not affected by external factors such as fluctuations in the supply voltage. Therefore, the method of determining the capacitance value by means of a discharge resistor is particularly reliable.
[0086] List of reference numerals 1: The system of this invention 10: The system of this invention C: Capacitance of the capacitor CA: Capacitor CC: Charging circuit CD: Computing device CN: Rated capacitance value of the capacitor C-new: The new capacitance value of the capacitor. DCL: DC bus DSC: Discharge Circuit OD: Output device R: Resistance value RD: (Discharge) Resistor RE: (Charging) Resistor S1-S5: Steps 1-5 T1: Evaluation time interval tspan: Duration U0-new-U20-new: New value for charging voltage U0-U20: Charging voltage values Ud: Discharge voltage US: Supply voltage value Ut: Charging voltage VUd: Voltmeter VUS: Second voltmeter VUt: Voltmeter
Claims
1. A method for determining the capacitance value (C) of a capacitor (CA) in an RC circuit on a vehicle's DC bus (DCL), wherein, The RC circuit includes a charging resistor (RE) or a discharging resistor (RD), and the method includes the following steps: a) During the evaluation time interval (T1), measure a series of values (U0-U20; U0-new-U20-new) of the charging voltage (Ut) or discharging voltage (Ud) of the capacitor (CA). b) Determine the first-order time derivative of the series of values (U0-U20; U0-new-U20-new) of the charging voltage (Ut) or discharging voltage (Ud) within the evaluation time interval (T1): - numerical value, or - Definite integral over time; c) Determine the second-order time derivative of the series of values (U0-U20; U0-new-U20-new) of the charging voltage (Ut) or discharging voltage (Ud) within the evaluation time interval (T1): - numerical value, or - Definite integral over time; d) Determine the capacitance value (C) of the capacitor (CA) using one of the following relationships: - The negative quotient of the first-order time derivative and the second-order time derivative, wherein the capacitance value (C) is obtained by dividing the negative quotient by the known resistance value (R) of the charging resistor (RE) or discharging resistor (RD); or - The negative quotient of the definite integral of the first-order time derivative and the definite integral of the second-order time derivative, wherein the capacitance value (C) is obtained by dividing the negative quotient by the known resistance value (R) of the charging resistor (RE) or discharging resistor (RD).
2. The method according to claim 1, characterized in that, The numerical differentiation method is used to determine the value of the first-order time derivative based on the following terms: - The entire evaluation time interval (T1); or - The first sub-interval of the evaluation time interval (T1), wherein the first sub-interval includes at least three values of the charging voltage (Ut) or discharging voltage (Ud) (U0-U20; U0-new-U20-new). The numerical differentiation method is used to determine the value of the second-order time derivative based on the following terms: - The entire evaluation time interval (T1); or - The second sub-interval of the evaluation time interval (T1), wherein the second sub-interval contains at least three values of the charging voltage (Ut) or discharging voltage (Ud) (U0-U20; U0-new-U20-new).
3. The method according to claim 1 or 2, characterized in that, Determine multiple capacitance values (C), where: - If the predefined statistical dispersion measure of the plurality of capacitance values (C) is not exceeded, then the plurality of capacitance values (C) are deemed available; and / or - If the predefined statistical dispersion measure of the plurality of capacitance values (C) is exceeded, the plurality of capacitance values (C) are deemed unavailable.
4. The method according to any one of claims 1-3, characterized in that, If a change in the supply voltage (US) of the RC circuit is detected during the evaluation time interval (T1), the capacitor value (C) and / or, if applicable, the plurality of capacitor values (C) are deemed unusable.
5. A system (1, 10) for determining the capacitance value (C) of a capacitor (CA) in an RC circuit on a vehicle's DC bus (DCL), wherein, The RC circuit includes a charging resistor (RE) or a discharging resistor (RD), and the system (1) is configured to perform the method according to any one of claims 1-4, the system (1) comprising: a) A measuring device configured to measure a series of values (U0-U20; U0-new-U20-new) of the charging voltage (Ut) or discharging voltage (Ud) of the capacitor (CA) within an evaluation time interval (T1). b) A computing device configured to: Determine the first-order time derivative of the series of values (U0-U20; U0-new-U20-new) of the charging voltage (Ut) or discharging voltage (Ud) within the evaluation time interval (T1): - numerical value, or - Definite integral over time; And the computing device is configured to be able to: Determine the second-order time derivative of the series of values (U0-U20; U0-new-U20-new) of the charging voltage (Ut) or discharging voltage (Ud) within the evaluation time interval (T1): - numerical value, or - Definite integral over time; Furthermore, the computing device is configured to determine the capacitance value (C) of the capacitor (CA) using one of the following relationships: - The negative quotient of the value of the first time derivative and the value of the second time derivative, wherein the capacitance value (C) is obtained by dividing the negative quotient by the known resistance value (R) of the charging resistor (RE) or the discharging resistor (RD). - The negative quotient of the definite integral of the first-order time derivative and the definite integral of the second-order time derivative, wherein the capacitance value (C) is obtained by dividing the negative quotient by the known resistance value (R) of the charging resistor (RE) or discharging resistor (RD).
6. A vehicle comprising the system (1, 10) according to claim 5.
7. The vehicle according to claim 6, characterized in that, The vehicle in question is a rail vehicle.