Method for estimating capacitor degradation and device performing the same

Abstract

The present disclosure provides a method for estimating capacitor degradation and a device performing the same. The present invention relates to a method for estimating a degree of degradation of a capacitor of a power filter connected with an adjustable speed drive (ASD) or a power converter. The present invention also relates to an adjustable speed drive and a power converter for performing the method.

Description

Technical Field

[0001] This invention relates to a method for estimating the degree of degradation of capacitors in a power filter connected to an adjustable speed driver (ASD) or power converter.

[0002] The present invention also relates to providing an adjustable speed driver and a power converter for performing the method.

[0003] This invention generally relates to the field of adjustable speed drives and power converters, and their use in conjunction with power filters. Power filters are essential components in drive systems, counteracting the effects of inverter power device switching on motor lifespan. Background Technology

[0004] In known applications, the LC output filter is a passive component of the drive system. It is not always possible to disconnect the LC output filter from the drive system to check its health. In some critical applications, stopping the drive is not permitted, making it impossible to perform a health check on a powered LC filter.

[0005] Known solutions address the disconnection of filters from drive systems. Depending on the scale of the application (e.g., in a petrochemical plant), there may be hundreds of drivers using LC filters. Disconnecting these drivers can result in substantial downtime and associated high downtime costs. Therefore, it is not always possible to disconnect multiple drivers to perform health checks on the filters and their capacitors. Summary of the Invention

[0006] The purpose of this invention is to provide an improved method for estimating the degree of degradation of a capacitor, an improved adjustable speed driver (ASD), and a power converter that overcome the above-mentioned problems.

[0007] This objective is achieved by the method according to claim 1, the adjustable speed driver (ASD) according to claim 13, and the power converter according to claim 14.

[0008] Preferred embodiments of the present invention are described in the dependent claims.

[0009] Claim 1 provides a method for estimating the degree of degradation of capacitors in a power filter connected to an adjustable speed driver (ASD) or power converter. In a preferred embodiment of the invention, the method includes the steps of:

[0010] Inject a controlled perturbation and measure the response of at least one of the capacitors.

[0011] Determine the undegraded resonant frequency of the capacitor.

[0012] A second controllable perturbation is injected, and the capacitor's response is measured.

[0013] Determine the deterioration resonant frequency of the capacitor.

[0014] The degree of capacitor degradation is determined based on these two resonant frequencies.

[0015] A capacitor may be part of a power filter and / or may be connected to a system. In the case of an input power filter, the system may include a power supply, a power filter, and a power converter. In the case of an output filter, the system may include a power converter / driver, a power filter, and a motor. According to the invention, AC signals (preferably AC signals of different frequencies) can be injected into at least one capacitor within a finite time. This allows sensing and calculating the resonant frequency of the power filter, which is related to the state of the filter capacitor. Once a capacitor deteriorates over time, its resonant frequency shifts to a higher frequency. Deep degradation of the capacitor can lead to unpredictable events such as explosions or other equipment failures. The invention detects the degree of change in the resonant frequency and estimates the amount of capacitor degradation based on the change in the resonant frequency. The invention estimates the degree of degradation of capacitors in a power filter, which can be used by predictive maintenance procedures. The invention uses the power converter itself as a sensor to detect capacitor degradation.

[0016] This invention provides a method for estimating the health status of a filter without disconnecting the driver. By monitoring capacitor degradation, timely action can be taken to prevent driver failure or damage. Therefore, the safety, reliability, and security of the driver can be significantly improved. This invention can avoid equipment failure and unpredictable operational stoppages. Therefore, further cost reductions can be achieved.

[0017] In a preferred embodiment of the present invention, the undegraded resonant frequency is theoretically determined according to Formula 1:

[0018]

[0019] In another preferred embodiment of the invention, the degraded resonant frequency is theoretically determined according to the formula...

[0020] Equation 2 determines:

[0021]

[0022] Equivalent inductance L ech It is the combined inductance of the entire system, namely the motor inductance L. motor and filter inductor L fi The combination of inductors.

[0023] In another preferred embodiment of the invention, the degree of degradation of the capacitor is determined according to Formula 4:

[0024]

[0025] In another preferred embodiment of the invention, the method steps are repeated periodically, specifically, based on a local timer value.

[0026] In another preferred embodiment of the invention, the timer value is a function of the operating time of the power converter or the adjustable speed driver.

[0027] In another preferred embodiment of the invention, the determination of the undegraded resonant frequency of the filter including the capacitor includes nameplate estimation, static state measurement and / or operational state measurement.

[0028] In another preferred embodiment of the invention, the deterioration resonant frequency of the capacitor includes measurements of the static state and / or the operating state.

[0029] In another preferred embodiment of the invention, the power filter includes an input power filter and / or an output power filter. Therefore, the power filter can be connected between the power grid and the power converter, or between the speed drive and the motor.

[0030] In another preferred embodiment of the invention, software for operating an adjustable speed driver or power converter is provided for measuring the resonance of a system including a filter capacitor by injecting a controlled disturbance and for measuring the transfer function of the system including the filter capacitor.

[0031] In another preferred embodiment of the invention

[0032] The injected frequency is monotonically varying within a given range near the degraded resonant frequency and / or the undegraded resonant frequency.

[0033] And / or, the injection frequency is randomized to minimize perturbation.

[0034] And / or, the injection ignores the critical frequency region to avoid control problems.

[0035] And / or, the injection amplitude is manually and / or automatically adjusted to minimize disturbances, and / or, the injection duration is manually and / or automatically adjusted to minimize disturbances, and / or, when capacitor degradation exceeds a given level, the method steps are repeated at an automatically increased frequency or repetition rate to monitor its accelerated degradation.

[0036] The injected frequency is randomized to minimize disturbances to the motor, the entire application, or general audio disturbances. Injection can ignore critical frequency regions to avoid control problems such as aliasing, EMI interference, mechanical resonance, and / or application disturbances. When capacitor degradation exceeds a given level, the repetition rate of this method can be increased to monitor accelerated capacitor degradation.

[0037] In another preferred embodiment of the invention, the length and / or amplitude of the injection is adapted to the speed and / or load in the operating conditions of the adjustable speed driver or power converter, and / or the method steps are repeated at a manually and / or automatically adjusted rate.

[0038] Alternatively or additionally, the length and / or amplitude of the injection can be adapted to the speed and / or load of the operating conditions of the device controlled by the ASD or connected to the power converter. The repetition rate of the method steps can be automatically adjusted to produce a sufficient number of measurements to achieve accurate estimation.

[0039] In another preferred embodiment of the invention

[0040] The degree of degradation is compared with a threshold to issue a warning, alarm, and / or trip signal.

[0041] And / or, the degree of degradation is determined separately for each phase of the multiphase signal, and / or, the time series of capacitor degradation is used to estimate the end-of-life time for predictive / preventive maintenance.

[0042] And / or, capacitor degradation is used to update the control parameters of adjustable speed drivers or power converters to improve control performance.

[0043] And / or, the determined resonant frequency is used to ignore operating conditions of the adjustable speed driver or power converter that could excite the resonant frequency.

[0044] Therefore, harmonic stress can be minimized, thereby extending the lifespan of the power supply filter.

[0045] The present invention also relates to an adjustable speed driver for performing at least the method according to claim 13. Software for the adjustable speed driver is provided for measuring the resonance of a system including a capacitor by injecting controlled disturbances and for measuring the transfer function of the system including a capacitor.

[0046] The present invention also relates to a power converter for performing at least the method according to claim 14. Software for the power converter is provided for measuring the resonance of a capacitor by injecting controlled perturbations and for measuring the transfer function of a system including the capacitor. Attached Figure Description

[0047] Further advantages and details of the invention are described with reference to the accompanying drawings. The drawings show:

[0048] Figure 1 Block diagram of an adjustable speed driver connected to the output power filter and the motor;

[0049] Figure 2 Block diagram of a power converter connected to an AC power supply via an LCL input power filter;

[0050] Figure 3 A diagram illustrating the degradation process of a capacitor over time, showing the evolution of capacitance C and ESR;

[0051] Figure 4 The electrical diagram showing the system transfer function is illustrated.

[0052] Figures 5a to 5c The transfer function of the system admittance Y (aliased as current);

[0053] Figure 6 : Frequency domain characteristics of capacitance changes observed in the output current of the driver;

[0054] Figure 7a , Figure 7b The effect of capacitance drop observed in the Bode plot and detected by the signal injection method;

[0055] Figure 8 : A diagram of the laboratory setup used to test the present invention;

[0056] Figures 9a to 9c : The resonant frequency recorded in response to the balance change of the filter capacitor;

[0057] Figure 10 : The accuracy of the degradation estimation for balanced capacitors;

[0058] Figure 11 : A block diagram of the components used to calculate CF degradation;

[0059] Figure 12 : A flowchart illustrating three methods for collecting baseline resonant frequencies; and

[0060] Figure 13 The flowchart for monitoring capacitor degradation is shown. Detailed Implementation

[0061] Figure 1 A block diagram of an adjustable speed driver (ASD) according to the prior art is shown. The ASD is connected to the output power filter and the motor.

[0062] Adjustable speed drivers are used with motors to achieve controllable speed and power supply required for applications such as industrial applications. ASD provides a pulse-width modulated voltage at its output for this controllable movement of the motor.

[0063] The drawback of this modulation is the voltage stress on the motor windings. Therefore, an output power supply filter Cf is used to smooth the stress caused by high-voltage modulation, thereby providing lower electrical stress to the motor insulation material. Furthermore, the motor has lower losses, resulting in lower temperatures, which helps reduce fire hazards.

[0064] Figure 1 A typical connection of the output power filter between the ASD and the motor is shown. As operating time increases, the physical properties of the capacitor Cf change, resulting in a degradation process. Known causes of capacitor degradation include internal effects triggered by the current and voltage loads applied to the capacitor. These internal effects include thermal stress, which accelerates the aging of the capacitor (specifically, its internal structure and material properties). Other internal effects include corona effects and voltage breakdown effects, which alter the material structure and its overall properties.

[0065] Other causes of capacitor degradation involve external effects (i.e., environmental conditions such as ambient temperature, ventilation, humidity, and / or pressure). Other causes of capacitor degradation are time-related and involve the aging of the capacitor's internal materials. Mechanical factors (e.g., mounting conditions and external mechanical stresses, which alter the capacitor's structure and physical properties) also contribute to capacitor degradation.

[0066] Once the capacitor deteriorates, the risk of controlled motor failure and improper operation increases.

[0067] This invention provides a method and apparatus for estimating the degradation of capacitors in a power filter connected to an ASD (Automatic Filter Detector). This invention directly utilizes the ASD to detect the degradation of the filter capacitor Cf.

[0068] This method injects AC currents of different frequencies over a finite period of time. This allows the sensing and calculation of the resonant frequency of the power supply filter, which depends on the filter capacitor Cf. As the capacitor deteriorates over time, the resonant frequency of the power supply filter shifts to a higher frequency, and the invention described herein makes it possible to detect the degree of change and estimate the amount of degradation of the capacitor Cf.

[0069] This invention enables the detection of both symmetrical and asymmetrical degradation of capacitor Cf in three-phase applications.

[0070] Figure 2A block diagram of a power converter connected to an AC power source via an LCL input power filter is shown. It illustrates the typical use of an input power filter in active front-end power converter applications. Since the power converter has a controllable power switch on the grid side, the input filter is typically used to filter out pulse-width modulation (PWM) noise. Examples of such applications include: generator units in photovoltaic and wind power applications, regenerative power converters used in power drives, and so on. The capacitor Cf will be subjected to the same type of electrical stress due to PWM and will experience the same process as described above. Figure 1 The same aging process described above.

[0071] Figure 3 This is a diagram illustrating the degradation process of a capacitor over time, showing the evolution of capacitance C and ESR. Two values ​​are indicated: the healthy capacitor C... n and end of life C EOL The following methods are used to monitor capacitor degradation.

[0072] If capacitor losses can be estimated, temperature is a good indicator of capacitor degradation. Therefore, one approach is to compare temperatures for the same amount of loss. For a given amount of loss, a higher temperature indicates a greater degree of capacitor degradation. The challenge lies in estimating the losses, because even with some simplifying assumptions, this will require knowledge of the equivalent series resistance (ESR). Temperature sensors can be directly used in tripping logic within drivers.

[0073] The equivalent series resistance (ESR) of a capacitor is another known indicator of capacitor degradation. Corresponding methods require appropriate definitions and characterizations of the ESR (loss factor) to consider at what frequency and under what measurement conditions. Failure to predefine these and a poor understanding of the dependencies can lead to estimation errors, as other influences such as temperature and humidity can occur during the estimation process. One limitation is the need for sensors that can handle both voltage and current in the capacitor.

[0074] Capacitor losses are another way to predict capacitor degradation. Capacitor losses themselves are not a risk, but they are related to internal changes within the capacitor and therefore to degradation. A decrease in capacitance carries an additional risk: as the LC resonant frequency increases towards the switching frequency, an increase in internal losses can be identified.

[0075] Two other, less common methods for monitoring capacitor degradation are monitoring isolation resistance and thermal impedance. These methods are not frequently used in practice due to the need for advanced measuring equipment.

[0076] This invention can be implemented using a driver, an output power filter, and a motor. The admittance of the corresponding system is derived through analysis and calculation and plotted as a frequency response transfer function, such as... Figure 3As shown. If a voltage u exists at the ASD terminal. ASD The current is then expressed at different frequencies. This method helps to obtain a basic understanding of the sensitive region as a function of frequency, and thus a basic understanding of where strong characteristics of the capacitor can be expected.

[0077] Calculate the following transfer function:

[0078] Y ASD (s)=i ASD / u ASD =Driver current based on drive voltage

[0079] Y cap (s)=i f / u ASD =Capacitor current based on driving voltage

[0080] Y moto (s)=i s / u ASD =Motor current based on drive voltage

[0081] Figure 4 An equivalent circuit with the system's transfer function is shown.

[0082] Figures 5a to 5c The Bode plots of the transfer functions of the filter and the motor are shown. Two cases are plotted: one with an output power supply filter (solid line) and the other without (dashed line) to examine its impact on the transfer function. Figure 5a A Bode plot for the combination of filter and motor is shown. Figure 5b The Bode plot is shown only for the filter, while Figure 5c A Bode plot is shown for the motor only.

[0083] The characteristics of capacitance as seen in current are based on the following formula:

[0084] i ASD =Y ASD (s)*u ASD

[0085] For the strong characteristics of the capacitor in the driver current, a high factor is desired, i.e., a high voltage u. ASD and / or high admittance Y ASD (s).

[0086] The resonant frequency provides a strong indication of the degradation of the filter capacitor. To obtain this information, the LC resonant frequency must first be excited using an injection voltage signal generated by ASD. Changes in the resonant frequency are measured via the driver's output current sensor. This deviation in the resonant frequency is correlated with changes in the capacitance value.

[0087] Characteristics of capacitance change, such as Figure 6 The capacitance is shown in the frequency domain. Here, the capacitance is shown decreasing from an initial 100% healthy value to 50% and 30% of its initial value, respectively.

[0088] When the capacitance Cf decreases by 20%, the characteristics of the capacitance change are as follows: Figure 7a As shown in the frequency domain. Figure 7b In this study, the same feature was detected in the time domain by injecting a frequency scanning signal and detecting the maximum amplitude of the current according to the Discrete Fourier Transform (DFT). Figure 7b It shows Figure 7a The analog demodulated signals and their resonant frequencies are shown in the two cases.

[0089] This invention proposes measuring the resonant frequency of a healthy output power supply filter and repeating this measurement over time to observe any changes in its resonant frequency. The degree of capacitor degradation can be calculated based on the following formula:

[0090] f resBaseline =1 / (2π√(L) ech Cf Baselin e)), Formula 1

[0091] Taken from baseline time, assuming a healthy capacitor.

[0092] f resDegraded =1 / (2π√(L) ech Cf Degraded )), Formula 2

[0093] Taken from a later time, assuming it is a deteriorated capacitor.

[0094] Formulas 1 and 2 are theoretical representations. The frequencies mentioned can be determined through measurement.

[0095] The ratio between the two scenarios is:

[0096] Cf Degraded =Cf Baseline (f r e sBaseline / f r e sDegraded )^2 Formula 3

[0097] The percentage degradation of the Cf capacitor can be calculated as follows:

[0098] Deterioration=100(f resBase;ine / f resDegraded )^2[%] Formula 4

[0099] Therefore, the estimation of degradation can follow these steps:

[0100] - Inject a controllable disturbance and measure the resonant frequency of the system.

[0101] - Detect the resonant frequency of a healthy capacitor based on measurements.

[0102] -The resonant frequency of the deteriorated capacitor will be detected later based on measurements.

[0103] - Calculate the degree of degradation according to Formula 4.

[0104] Through such Figure 8 The setup shown validates the method described above using an output power filter connected to the induction motor. The power filter is modified to allow the capacitor to gradually decrease in value, simulating the aging process of the capacitor. This change is achieved by turning off the capacitor for each phase using switches S1uvw…S3uvw separately. Therefore, depending on the choice of switches, symmetrical or unbalanced degradation of the capacitor can be simulated. This can be done while the motor is running or stationary.

[0105] The ASD software was modified to allow the resonance of the power supply filter to be measured by adding excitation and measuring the transfer function of the system.

[0106] When the change in the filter's resonant frequency is measured while the motor is running or stationary, the following information is obtained: Figures 9a to 9c The results are shown. Figures 9a to 9c This shows how the resonant frequency shifts to a higher value when the capacitor deteriorates.

[0107] Figures 9a to 9c The system resonance is shown for different degrees of capacitor degradation. In this example, the capacitor degradation is balanced, meaning the same degradation occurs in all three phases. The amount of capacitor variation is indicated in each graph. In the example shown, the motor speed is 700 RPM, and the ASD operates at a typical switching frequency of 5 kHz.

[0108] exist Figure 9a In this context, the capacitor in each phase is at 100% of its initial capacitance. Figure 9b In the middle, the capacitor of each phase is at 80% of its initial capacitance, while... Figure 9c In this process, the capacitor in each phase is at 60% of its initial capacitance.

[0109] Once the resonant frequency is measured, the degradation can be calculated based on the capacitance using Formula 4.

[0110] The accuracy of the estimated degradation can be compared with that of capacitance measurements performed using calibrated instruments. Figure 10 Examples of the differences are given in the text.

[0111] Figure 11A block diagram showing the main components of the invention as described herein is presented. These components may be software functions implemented in existing ASDs. The invention is based on... Figure 11 The process is centered around a step called "Cf degradation".

[0112] This block diagram illustrates an ASD (Automatic Filter System) connected to the AC mains and via an output power filter to the motor. The ASD comprises at least two components:

[0113] a. A power supply card containing all hardware, analog and digital circuits, power switches, etc.

[0114] b. Controller card, which has all the software, control, and protection features of ASD.

[0115] The ASD controller card primarily provides ASD control, which is the main function of ASD. The ASD controller card provides all the features required for hardware control and motor control.

[0116] The ASD control also provides a secondary function of monitoring capacitor degradation (Cf). The corresponding algorithm is implemented in the control card SW, which receives internal and external signals and calculates capacitor degradation in the power supply filter. This information is later used for warnings and / or fed back to the ASD control to improve stability and control performance.

[0117] The sub-components of the "Cf Degradation" section are as follows:

[0118] 1. Control Unit – This unit is responsible for synchronizing (via the signal “sync”) the entire process, which includes injecting AC current and measuring its effects on the ASD output current and voltage. The injection process takes only a short time, but it is repeated periodically based on a local timer. This method minimizes ripple in the output motor torque and speed. This function also estimates when the correct time to inject AC current is to avoid disturbing the system and causing faults and trips. It also checks whether the ASD is under consistently stable operating conditions suitable for performing measurements and avoiding erroneous results. For the same reason, the amplitude of the AC injection is also set by this unit to avoid excessive disturbance.

[0119] 2. AC voltage injection – Aggregating information blocks (amplitude, frequency range, enable flags) received from the control unit into the required reference voltage. ACinjection This is then sent to the ASD controller to be added to the AC output function. The injection begins upon receiving a synchronization signal from the control unit. This function synchronizes the injection to minimize disturbances to the normal operation of the ASD output current, thereby avoiding transients and erroneous results.

[0120] 3. Current sensor Is – This function receives the output current Is1, 2, 3 from the ASD current sensor after filtering for offset, noise, or common-mode signals.

[0121] 4. Calculation of Cf Degradation – This function executes Formula 4 to determine how much the capacitor has degraded since the last baseline value was recorded. This function can also perform regression curves using the collected time series data to determine the end-of-life of the capacitor. One output of this function is an estimated Cf value based on the calculated degradation, which can be fed back to the ASD control to improve its control stability and performance. Based on user selection, another output is used for warning and trip logic. Timer values ​​can vary based on ASD operating time and measurement availability, as well as application-specific constraints.

[0122] Figure 12 The baseline mode is shown to differentiate between different options used to measure capacitor degradation.

[0123] Nameplate estimates are used for power filters that have been in use for some time, in which capacitors may have deteriorated, making the initial capacitor values ​​unmeasurable.

[0124] The Standstill state is used when execution during the Running state is impossible or not expected by the user. It can be used, for example, during a shutdown.

[0125] - The running state requires less user intervention, but relies on the learning phase to collect data during application runtime.

[0126] Figure 13 The monitoring phase for capacitor degradation is described. Monitoring can be performed in a static state or in an operating state, depending on the user's choice. In the case of static state monitoring, the system is powered up to the same level used during the baseline phase. This decouples the load dependence and ensures that the resonant frequency is estimated under the same conditions, thereby estimating the capacitance.

[0127] In the case of operational status monitoring, motor load and speed are also measured, and the monitoring results are used when comparing them to baseline values. When the exact load value is missing from the baseline dataset, interpolation is used. The output can be used in various ways to provide warnings, alarms, trips, or to improve the control performance of the power converter.

Claims

1. A method for estimating the degree of degradation of one or more capacitors in a power filter connected to an adjustable speed driver or power converter, wherein, The method is performed during the operation of the adjustable speed driver or power converter, and the method includes: A controllable disturbance, including an AC signal, is injected into the power filter, the adjustable speed driver, or the power converter. Measure the response to determine the degraded resonant frequency f of the capacitor. resDegraded ; Determine the undegraded resonant frequency f of the capacitor. resBaseline ;as well as Based on the undegraded resonant frequency f resBaseline and the degraded resonant frequency f resDegraded Determine the degree of degradation of the capacitor. The degree of degradation is determined according to the following formula: 。 2. The method according to claim 1, wherein, The injection of the controlled disturbance and the measurement of the response are repeated periodically.

3. The method according to claim 1, wherein, The steps of the method are repeated periodically based on a local timer value.

4. The method according to claim 3, wherein, The timer value is a function of the operating time of the adjustable speed driver or the power converter.

5. The method according to claim 1, wherein, Determine the undegraded resonant frequency f resBaseline This includes: nameplate estimation, static state measurement, and / or operational state measurement.

6. The method according to claim 1, wherein, The degraded resonant frequency f resDegraded This includes static state measurements and / or operational state measurements.

7. The method according to claim 1, wherein, The power filter includes an input power filter and / or an output power filter.

8. The method according to claim 1, wherein, Software for operating the adjustable speed driver or the power converter is provided for performing the method.

9. The method according to claim 1, wherein, The injected frequency is at the degraded resonant frequency f resDegraded And / or the undegraded resonant frequency f resBaseline It is monotonically changing within a given range nearby, and / or, The injection frequency is randomized to minimize perturbation, and / or, The critical frequency region was avoided to prevent control problems, and / or, The injection amplitude and / or duration are manually and / or automatically adjusted to minimize disturbances, and / or, When the capacitor deteriorates beyond a given level, the repetition rate of the method is automatically increased.

10. The method according to claim 1, wherein, The length and / or amplitude of the injection are adapted to the speed and / or load operating conditions of the adjustable speed driver or the power converter.

11. The method according to claim 1, wherein, The degree of degradation is compared with a threshold to generate a warning, alarm, and / or trip signal, and / or, Degradation is determined separately for each phase of a multiphase system, and / or, Use the time series of capacitor degradation to estimate the end-of-life time, and / or, The degree of degradation is used to update the control parameters of the adjustable speed driver or the power converter, and / or, The determined resonant frequency is used to avoid operating conditions that would excite resonance.

12. An adjustable speed drive configured to perform at least the method according to claim 8.

13. A power converter configured to perform at least the method according to claim 8.

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