Power module DC capacitor state monitoring method and monitoring device

Abstract

The invention discloses a power module direct current capacitor state monitoring method and monitoring device, and belongs to the technical field of flexible direct current transmission, the power module direct current capacitor state monitoring method comprises the steps that original data are collected regularly, and the original data comprise the capacitor voltage of each sub-module, the three-phase bridge arm current and the control signal of each sub-module; original data in a set time period are collected every set time interval; original data at N moments are taken, and the capacitance capacity and the equivalent series resistance of each sub-module are calculated; and S4, averaging the capacitance capacity and the equivalent series resistance at the N moments to obtain the final capacitance capacity and equivalent series resistance. According to the invention, on-line monitoring of the capacitance value of the MMC sub-module can be realized under the condition of not additionally adding a measuring device.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible direct current power transmission, and in particular relates to a method and a device for monitoring the state of a direct current capacitor of a power module. Background Art

[0002] In flexible DC transmission projects, the type of capacitor widely used in modular multilevel converters (MMC) is dry oil-free type, and its core structure adopts the metallized polypropylene film material widely recognized internationally. The aging phenomenon of capacitors caused by the gradual degradation of internal dielectrics is usually manifested as a slow decrease in capacitor capacity and a gradual increase in equivalent series resistance. However, since the equivalent series resistance of film capacitors is significantly lower than that of electrolytic capacitors, it is difficult to accurately measure them in practical applications. Therefore, the reduction in capacitor capacity is usually used as the main indicator for evaluating the health status of film capacitors. Generally speaking, when the capacitor capacity decay reaches 3% to 5%, it is considered to have reached the end of its service life.

[0003] In order to measure the equivalent parameters of capacitors, including capacitance and equivalent series resistance, existing studies have proposed three main monitoring methods, namely, methods based on capacitor ripple current, circuit models, and data and advanced algorithms. Their dependence on hardware decreases in turn, while their dependence on software increases in turn. The method based on capacitor ripple current is to inject a controllable AC current into the circulating loop to cause a ripple voltage on the capacitor. During measurement, a bandpass filter is first used to filter out the ripple voltage and ripple current of the injected current frequency, and then the voltage and current values ​​are used to calculate the submodule capacitance through the least squares method (RLS). This method can exclude the influence of other frequency voltage and current, and the measurement accuracy reaches about 1%, but it requires additional devices and cannot meet the needs of online status monitoring of metallized film capacitors. The second method is to obtain a convergent capacitance value curve through Kalman filter (KF) filtering, and the capacitance value error fluctuation is kept within 1%. On the one hand, the influence of measurement accuracy is not considered in the verification process. On the other hand, different operating environments and measurement environments will have a greater impact on the convergence speed and measurement accuracy of this method. There is also an online monitoring method for high-voltage DC bus capacitors, which adds a short-circuit test of short duration to the normal operation of the AC converter, and uses the short-circuit current of the switch tube and the change in the DC bus voltage in the short-circuit test to accurately calculate the equivalent series resistance of the capacitor. However, this method requires adding a test circuit, which may interfere with the normal operation of the high-voltage DC system, causing system instability or failure.

[0004] In summary, the existing monitoring methods mainly stay in the offline monitoring and theoretical aspects, and it is difficult to realize the practical application of online monitoring in high-voltage systems. Some of the existing online monitoring methods for the capacitance value of MMC submodules require the addition of additional measuring devices, and some have insufficient measurement accuracy and cannot meet the engineering requirements well. In order to estimate the DC capacitance of the power module in real time in the actual flexible DC project, so as to monitor its health status, it is necessary to propose an online monitoring method for the capacitance value of MMC submodules that does not require additional measuring devices and has sufficient accuracy. Summary of the invention

[0005] The present invention provides a power module DC capacitor state monitoring method and monitoring device, which can realize online monitoring of MMC submodule capacitance value without adding additional measuring devices.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] A method for monitoring the state of a DC capacitor of a power module comprises the following steps:

[0008] S1. Regularly collect and store raw data, wherein the raw data includes the capacitor voltage of each submodule, the three-phase bridge arm current and the control signal of each submodule;

[0009] S2, collecting raw data within a set time period at set time intervals;

[0010] S3, taking the capacitor voltage of each submodule, the three-phase bridge arm current and the control signal of each submodule at N moments, and calculating the capacitor capacity and equivalent series resistance of each submodule;

[0011] S4. averaging the capacitance of each submodule at N moments to obtain the final capacitance of each submodule, and averaging the equivalent series resistance obtained at N moments to obtain the final equivalent series resistance of each submodule.

[0012] Further, step S3 includes the following steps:

[0013] S3.1, judging the working status of each submodule according to the control signal at time t;

[0014] S3.2, calculating the current flowing through the capacitor of each submodule according to the bridge arm current of the bridge arm where the submodule is located at time t and the working state of each submodule;

[0015] S3.3. Calculate the capacitance value and equivalent series resistance of the capacitor of each submodule according to the current flowing through the capacitor of each submodule and the capacitor voltage of the submodule at time t;

[0016] S3.4. According to the method described in S3.1-S3.3, calculate the capacitance value and equivalent series resistance of the capacitor of each submodule at other N-1 moments.

[0017] Furthermore, in step S3.2, the current I flowing through the capacitor of each submodule at time t is calculated by the following formula: C :

[0018]

[0019] Among them, I 桥臂 is the bridge arm current of the bridge arm where the submodule is located.

[0020] Furthermore, in step S3.2, the bridge arm current of the bridge arm where the submodule is located is obtained by fitting the collected bridge arm current.

[0021] Furthermore, in step S3.2, the ripple current amplitudes at the first M frequencies with larger ripple current amplitudes are selected to calculate the current flowing through the capacitor of each submodule.

[0022] Furthermore, in step S3.3, the capacitance C and equivalent series resistance R of the capacitor of each submodule are calculated based on the following formula:

[0023]

[0024] Among them, U C (ω) is the voltage of the capacitor, I C (ω) is the current flowing through the capacitor.

[0025] Further, in step S2, the time period is set to be an integer multiple of the set time interval.

[0026] Furthermore, the time interval is set to 2-10 seconds.

[0027] A power module DC capacitor state monitoring device, comprising:

[0028] Three current sensors are used to collect the three-phase bridge arm current and send it to the valve control cabinet;

[0029] Six optical fiber cabinets, used to collect the capacitor voltage of each submodule in the six bridge arms of the three-phase, and send the collected capacitor voltage of each submodule to the valve control cabinet;

[0030] The valve control cabinet is used to extract the switch status of each submodule, the capacitor voltage of each submodule and the three-phase bridge arm current, and send the switch status of each submodule, the capacitor voltage of each submodule and the three-phase bridge arm current to the monitoring status monitoring cabinet;

[0031] The state monitoring cabinet is used to calculate the capacitance and equivalent series resistance of each submodule according to the switch state of each submodule, the capacitor voltage of each submodule and the three-phase bridge arm current.

[0032] Furthermore, a measurement merging unit is provided between the current sensor and the valve control cabinet.

[0033] Compared with the prior art, the present invention has at least the following beneficial technical effects:

[0034] The present invention provides a method for monitoring the state of a DC capacitor of a power module. The method uses a valve control system to monitor the bridge arm voltage and current, submodule control signals and submodule voltage data for monitoring, and combines a matching algorithm to calculate the capacitance and equivalent series resistance. The required data is the data that the valve control system originally needs to collect, so there is no need to additionally install a detection device on the submodule, and real-time monitoring can be performed under production conditions to ensure the normal operation of the capacitor in the circuit, without affecting the operation of the converter valve module, thereby improving the overall performance of the circuit.

[0035] At the same time, the present invention greatly reduces the measurement error and data alignment problem of the bridge arm current detection device and the bridge arm module voltage data detection, fits the current data, takes into account the influence of the dead zone of the power device on the detection, collects the cumulative data once every set time interval, stops receiving and ignores the subsequent arriving data, thereby improving the capacitance calculation accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is the structural diagram of three-phase MMC;

[0037] Figure 2 Four switching state diagrams for the half-bridge submodule;

[0038] Figure 3 It is the waveform diagram of bridge arm voltage, bridge arm current and submodule capacitor voltage;

[0039] Figure 4 It is a structural diagram of an online status detection device;

[0040] Figure 5 This is the data processing flow chart of the condition monitoring software;

[0041] Figure 6 This is a schematic diagram of the equivalent model of capacitors in series;

[0042] Figure 7 is the vector diagram of the voltage on the capacitor;

[0043] Figure 8 is the current signal borne by the capacitor;

[0044] Fig. 9 is the current signal spectrum borne by the capacitor. DETAILED DESCRIPTION

[0045] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0046] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0047] It should be noted that when an element is referred to as being "disposed on" another element, it may be directly on the other element or there may be another element centered thereon. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be another element centered thereon at the same time. The terms "upper", "lower", "front", "back", "left", "right", "top", "bottom", "inside", "outside", etc. used herein indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0048] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art of the present invention. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0049] Relationship between bridge arm current voltage and capacitor voltage under different working conditions of MMC

[0050] Three-phase MMC circuit topology Figure 1 As shown. Each phase includes two bridge arms, each bridge arm includes N series-connected submodules (SM) and two bridge arm reactors L. dc is the DC side voltage; I dc is the DC side current; i a ,i b and i c are the phase currents of each phase on the AC side; v pa ,i pa Represent the upper bridge arm voltage and current, v na ,i naRepresent the voltage and current of the lower bridge arm respectively; V C Indicates the capacitor voltage of the MMC submodule. MMC submodules generally have two structures: half-bridge topology and full-bridge topology. Only the MMC submodule with half-bridge topology shown in the figure is analyzed, which contains two IGBT switching devices, two FWD switching devices and a DC support capacitor. The upper IGBT is marked as T1, the lower IGBT is marked as T2, the upper FWD is marked as D1, and the lower FWD is marked as D2.

[0051] The switch signal combination and current direction determine the four normal operating conditions of the half-bridge submodule, which are as follows: Figure 2 As shown, the switching states of T1 and T2 are complementary.

[0052] Taking the upper bridge arm of phase A as an example, the four switching states are summarized in Table 1. When T1 applies an on signal and T2 applies an off signal, the submodule is put into the entire MMC circuit. When the bridge arm current is greater than zero, the submodule current flows in the forward direction as shown in the figure, D1 is turned on and T1 is turned off, i D1 =i pa The capacitor is in a charging state, corresponding to state (a); when the bridge arm current is less than zero, the submodule current flows in the reverse direction, T1 is turned on and D1 is turned off, i T1 =i pa And the capacitor is in a charging state, corresponding to state (b). When T2 applies an on signal and T1 applies an off signal, the submodule bypasses the MMC bridge arm. When the bridge arm current is greater than zero, the submodule current flows in the forward direction, T2 is turned on and D2 is turned off, and i T2 =i pa , corresponding to state (c); when the bridge arm current is less than zero, the submodule current flows in the reverse direction, D2 is turned on and T2 is turned off, and i D2 =i pa , corresponding to state (d).

[0053] Table 1 Four switch states of submodules

[0054]

[0055] Taking the single-phase seven-level MMC inverter as an example, the bridge arm voltage V b , bridge arm current I b And the six submodule capacitor voltages V C Waveform Figure 3 The capacitor voltage charging and discharging process is obvious, the bridge arm current cycle is 20ms and basically presents a sinusoidal change form, and there is a DC bias of about 20A.

[0056] DC capacitor online monitoring solution

[0057] 1) Capacitor online monitoring device structure and data error processing method

[0058] The above different working conditions can provide the bridge arm voltage and current fluctuation data, as well as the DC capacitor voltage fluctuation data. In the actual evaluation of the power module capacitance, it is necessary to pay attention to several key error factors in the valve control measurement data. The valve control mechanism relies on communication means to obtain information such as bridge arm current readings, power module capacitor voltage values, and power module switching status. The evaluation process first involves a preliminary estimate of the capacitor current, and then the capacitor capacity is calculated based on this estimate. In this process, there are three main sources of error:

[0059] (1) The measurement deviation of the bridge arm current sensor and the self-test error of the capacitor voltage inside the power module.

[0060] (2) The driving dead zone effect in the operation of power semiconductor devices (IGBT). Specifically, the dead zone time of high-voltage power semiconductor devices is usually more than 10 microseconds, and the valve control system receives the power device trigger signal from the power module controller through communication, not the actual gate pulse signal of the power device. There is a time difference between the two caused by the gate drive dead zone, and the time difference will change according to the direction of the bridge arm current. Therefore, the power device trigger signal needs to be adjusted according to the real-time bridge arm current direction to accurately calculate the actual working state of the power device, and then accurately estimate the capacitor current.

[0061] (3) The synchronization error between the various parts of the valve control system causes a deviation between the acquired bridge arm current feedback and the power module trigger state at the sampling time, and this deviation changes slowly and continuously.

[0062] Since the sampling of the bridge arm current and the communication cycle of the power module are independent of each other and no synchronization strategy is adopted, it is impossible to maintain a strict multiple relationship between them, which naturally introduces a time difference between the two. In other words, in the latest feedback information received by the valve control system about the submodule switch state, bridge arm current and capacitor voltage, the feedback of the bridge arm current may precede the capacitor voltage at some times and lag behind at other times. This synchronization inconsistency between the bridge arm current and the capacitor voltage further leads to inaccuracy in the estimation of the capacitor equivalent series resistance.

[0063] In order to meet this challenge, the main control box in the valve control cabinet will regularly send the latest bridge arm current sampling data, power module switch status and capacitor voltage to the valve control monitoring system through high-speed optical fiber. The period is 0.5s-2s, and the power module data from the pulse communication panel cabinet and the bridge arm CT monitoring current often have time misalignment deviations. In the operation of the health status monitoring cabinet, 10s of cumulative data are collected every 5 seconds. After a batch of data transmitted by high-speed optical fiber is collected, it is temporarily stored. The health status monitoring cabinet will temporarily (5 seconds) stop receiving and ignore the subsequent data, and instead perform data processing and calculation work. The data processing flow is as follows Figure 5 As shown, the problem of large error deviation in measurement data was solved based on this data processing and analysis.

[0064] 2) Capacitance parameter calculation method

[0065] Consider the series equivalent model of metallized film capacitors such as Figure 6 As shown in the figure, the resistance R in the figure is the equivalent series resistance of the capacitor during actual operation, which is generally around 100mΩ. A vector diagram analysis of the voltage it bears is performed, as shown in the figure. Figure 7 shown.

[0066] Depend on Figure 7 It can be seen that at the same frequency, the voltage U flowing through the capacitor is C and current I C Satisfies the following relationship.

[0067]

[0068] Where: C is the capacitance of the metallized film capacitor, and R is the equivalent series resistance of the capacitor.

[0069] During actual operation, the voltage and current signals that the metallized film capacitor is subjected to contain harmonic components at multiple frequencies. Spectral analysis of these two signals is performed to obtain the voltage amplitude and current amplitude at different frequencies. The voltage amplitude and current amplitude at different frequencies are then substituted into formula (2-1) to calculate the capacitance value of the metallized film capacitor and the equivalent series resistance of the capacitor.

[0070] Referring to the system solution structure, according to the online monitoring data error elimination method, the capacitor state parameter calculation algorithm can be different. For example, the least squares method, time domain data calculation and other software algorithms can be used to calculate the capacitor parameters.

[0071] During the operation of the MMC, the capacitor has two working states: the on state and the off state. In the on state, the current flowing through the capacitor is equal to the bridge arm current; in the off state, the capacitor is short-circuited and the current flowing through the capacitor is zero. Therefore, in order to obtain the capacitor current signal, the current signal flowing through the capacitor is first obtained through the bridge arm current signal and the control signal, as shown in formula (2-2).

[0072]

[0073] The voltage signal borne by the capacitor is the submodule voltage signal, which is a known signal of the MMC system. The control signal and the bridge arm current signal are also monitoring signals of the control system. Therefore, this method can realize the online monitoring of the metallized film capacitor without adding additional sensors. The capacitance value and equivalent series resistance of the capacitor can be monitored at the same time, and the state of the metallized film capacitor can be judged from two aspects, but this method needs to obtain the submodule voltage signal, the bridge arm current signal and the switch state information of the submodule.

[0074] Example 1

[0075] Reference Figure 4 A method and device for detecting the online state of a DC capacitor of a power module includes three current sensors, six optical fiber cabinets, a valve control cabinet and a monitoring state monitoring cabinet. The output ends of the three current sensors are all connected to a measurement merging unit, the input ends of the six optical fiber cabinets are respectively connected to the sub-modules of the upper and lower bridge arms of the three phases, and the output ends are all connected to the input end of the valve control cabinet, the output end of the measurement merging unit is connected to the input end of the valve control cabinet, and the output end of the valve control cabinet is connected to the input end of the monitoring state monitoring cabinet.

[0076] The measurement merging unit converts the analog signals of six flow sensors into digital signals, thereby collecting and measuring the six bridge arm currents I 桥臂 , and sent to the valve control cabinet through optical fiber; six optical fiber cabinets are used to collect the six bridge arm voltages of the three-phase upper and lower bridge arms and the capacitor voltages of each submodule, and send the collected bridge arm voltages and capacitor voltages of each submodule to the valve control cabinet, which extracts the switch status of each submodule, the capacitor voltage of each submodule and the three-phase bridge arm current I 桥臂 , and the switch state of each submodule, the capacitor voltage of each submodule and the three-phase bridge arm current I 桥臂 The status monitoring cabinet sends the status information to the monitoring status monitoring cabinet, which calculates the status of each submodule according to the switch status, capacitor voltage and three-phase bridge arm current I 桥臂 Calculate the capacitance and equivalent series resistance of each submodule.

[0077] Example 2

[0078] Reference Figure 5 , a method for detecting the online state of a DC capacitor of a power module, comprising the following steps:

[0079] S1. The valve control cabinet regularly collects raw data, which includes: the bridge arm voltage of the three-phase upper and lower bridge arms, the capacitor voltage of each submodule, and the three-phase bridge arm current I 桥臂 and control signals of each submodule; the time step of regular acquisition is 0.5s-2s;

[0080] S2, valve control cabinet caches original data;

[0081] S3, the health status monitoring cabinet collects raw data within a set time period from the valve control cabinet at set time intervals;

[0082] The set time interval can be 5s, 2s, 10s, etc.; the set time period can be 10s, 15s, 20s, etc.; preferably, the set time period is an integer multiple of the set time interval;

[0083] S4, performing dead zone compensation on the switch state of the power device according to the direction of the bridge arm current; specifically, fitting the bridge arm current data to obtain fitting current data;

[0084] S5, select the capacitor voltage of each submodule and the three-phase bridge arm current I at N moments 桥臂 and the control signals of each submodule, and calculate the capacitance and equivalent series resistance of each submodule. Specifically, the following steps are included:

[0085] S5.1. Determine the working status of the submodule according to the control signal at time t;

[0086] S5.2, the three-phase bridge arm current I at time t 桥臂 Substitute the working status of the submodule into the following formula to calculate the current I flowing through the capacitor of each submodule C :

[0087]

[0088] S5.3, the current I flowing through the capacitors of each submodule at time t C and the capacitor voltage U of the submodule C Substitute the following formula to calculate the capacitance value of the metallized film capacitor and the equivalent series resistance of the capacitor;

[0089]

[0090] S5.4. Calculate the capacitance value of the submodule and the equivalent series resistance of the capacitor at other N-1 moments according to the method described in S5.1-S5.3;

[0091] S6. averaging the capacitances obtained at N moments to obtain a final capacitance, and averaging the equivalent series resistances obtained at N moments to obtain a final equivalent series resistance.

[0092] Implementation Verification

[0093] In the actual engineering operation process, the measurement system will inevitably be interfered by some environmental factors, and noise signals will be introduced during the measurement process. Considering the distribution characteristics of the voltage and current signals borne by the metallized film capacitor in various frequency bands, a frequency band with a high signal-to-noise ratio should be selected to monitor the status of the capacitor.

[0094] The three-phase nine-level MMC simulation experiment model is used as an example to illustrate the model parameters. The model parameters are shown in Table 2, and the nearest level approach (NLM) modulation method is adopted.

[0095] Table 2 Three-phase nine-level MMC simulation model parameters

[0096]

[0097] The voltage borne by the metallized film capacitor is the MMC submodule voltage. According to the spectrum analysis of the submodule voltage in the above model, the amplitude of the voltage borne by the capacitor in different frequency bands is sorted out as shown in Table 3.

[0098] Table 3 The amplitude of the voltage borne by the capacitor in different frequency bands

[0099]

[0100] It can be seen from the above table that the ripple voltage amplitude that the capacitor bears is the highest at 50Hz, which can reach 17V, followed by the amplitude at 100Hz, which reaches 10V, and the remaining voltage amplitudes are all lower than 0.2V.

[0101] The current signal borne by the metallized film capacitor needs to be calculated based on the control signal and the bridge arm current signal. The current signal borne by the capacitor is calculated using the above simulation model as follows: Figure 8 shown.

[0102] Perform spectrum analysis on the signal in the above figure, and the results are as follows Fig. 9 shown.

[0103] According to the spectrum analysis in the above figure, the frequency point with the largest ripple current amplitude is 100Hz, followed by 50Hz. In addition, the amplitude of the ripple current is also large at the four frequency points of 150Hz, 400Hz, 500Hz and 800Hz.

[0104] On the one hand, considering that when calculating the state parameters of the capacitor, the voltage component and current component at a fixed frequency that the capacitor is subjected to are simultaneously involved in the calculation, it is necessary to select a frequency point with a large voltage amplitude and a large current amplitude for calculation. On the other hand, considering that the noise interference in the on-site operating environment is mainly power frequency noise, using only the calculation result at a frequency point of 50Hz as the final result may cause a large error. Therefore, this method uses the average value of the calculation results at the two frequency points of 50Hz and 100Hz as the final calculation result.

[0105] Table 4 Calculation results

[0106]

[0107] When the uncertainty of voltage and current measurement is 1%, the error of the calculated capacitance value of the metallized film capacitor can be kept within 0.2% and the error of the calculated equivalent series resistance can be kept within 0.5% by using the online detection algorithm of spectrum analysis. Both can meet the requirements of online monitoring.

[0108] The term "consisting of" describing a combination shall include the identified elements, ingredients, parts or steps and other elements, ingredients, parts or steps that do not substantially affect the basic novel characteristics of the combination. The use of the terms "comprising" or "including" to describe a combination of elements, ingredients, parts or steps herein also contemplates embodiments that consist essentially of these elements, ingredients, parts or steps. By using the term "may", it is intended to illustrate that any of the attributes described that "may" be included are optional.

[0109] Multiple elements, ingredients, parts or steps can be provided by a single integrated element, ingredient, part or step. Alternatively, a single integrated element, ingredient, part or step can be divided into separate multiple elements, ingredients, parts or steps. The disclosure "one" or "an" used to describe an element, ingredient, part or step is not intended to exclude other elements, ingredients, parts or steps.

[0110] It should be understood that the above description is for illustration and not for limitation. Many embodiments and many applications beyond the examples provided will be apparent to those skilled in the art upon reading the above description. Therefore, the scope of the present teachings should not be determined with reference to the above description, but rather with reference to the foregoing claims and the full scope of equivalents to which such claims are entitled. For the purpose of comprehensiveness, all articles and references, including disclosures of patent applications and publications, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein in the foregoing claims is not intended to be a waiver of such subject matter, nor should it be considered that the applicant has not considered such subject matter to be part of the disclosed inventive subject matter.

Claims

1. A method for monitoring the state of a DC capacitor of a power module, characterized in that: The following steps are involved: S1. Regularly collect and store raw data, wherein the raw data includes the capacitor voltage of each submodule, the three-phase bridge arm current and the control signal of each submodule; S2, collecting raw data within a set time period at set time intervals; S3, taking the capacitor voltage of each submodule, the three-phase bridge arm current and the control signal of each submodule at N moments, and calculating the capacitor capacity and equivalent series resistance of each submodule; S4. averaging the capacitance of each submodule at N moments to obtain the final capacitance of each submodule, and averaging the equivalent series resistance obtained at N moments to obtain the final equivalent series resistance of each submodule.

2. A method for monitoring the state of a DC capacitor of a power module according to claim 1, characterized in that: The step S3 comprises the following steps: S3.1, judging the working status of each submodule according to the control signal at time t; S3.2, calculating the current flowing through the capacitor of each submodule according to the bridge arm current of the bridge arm where the submodule is located at time t and the working state of each submodule; S3.

3. Calculate the capacitance value and equivalent series resistance of the capacitor of each submodule according to the current flowing through the capacitor of each submodule and the capacitor voltage of the submodule at time t; S3.

4. According to the method described in S3.1-S3.3, calculate the capacitance value and equivalent series resistance of the capacitor of each submodule at other N-1 moments.

3. A method for monitoring the state of a DC capacitor of a power module according to claim 2, characterized in that: In step S3.2, the current I flowing through the capacitor of each submodule at time t is calculated by the following formula: C : Among them, I 桥臂 is the bridge arm current of the bridge arm where the submodule is located.

4. A method for monitoring the state of a DC capacitor of a power module according to claim 2, characterized in that: In the step S3.2, the bridge arm current of the bridge arm where the submodule is located is obtained by fitting the collected bridge arm current.

5. A method for monitoring the state of a DC capacitor of a power module according to claim 2, characterized in that: In step S3.2, the ripple current amplitudes at the first M frequencies with larger ripple current amplitudes are selected to calculate the current flowing through the capacitor of each submodule.

6. A method for monitoring the state of a DC capacitor of a power module according to claim 2, characterized in that: In step S3.3, the capacitance C and the equivalent series resistance R of the capacitor of each submodule are calculated based on the following formula: Among them, U C (ω) is the voltage of the capacitor, I C (ω) is the current flowing through the capacitor.

7. A method for monitoring the state of a DC capacitor of a power module according to claim 1, characterized in that: In step S2, the time period is set to be an integer multiple of the time interval.

8. A method for monitoring the state of a DC capacitor of a power module according to claim 1 or 7, characterized in that: The set time interval is 2-10 seconds.

9. A power module DC capacitor status monitoring device, characterized in that: include: Three current sensors are used to collect the three-phase bridge arm current and send it to the valve control cabinet; Six optical fiber cabinets, used to collect the capacitor voltage of each submodule in the six bridge arms of the three-phase, and send the collected capacitor voltage of each submodule to the valve control cabinet; The valve control cabinet is used to extract the switch status of each submodule, the capacitor voltage of each submodule and the three-phase bridge arm current, and send the switch status of each submodule, the capacitor voltage of each submodule and the three-phase bridge arm current to the monitoring status monitoring cabinet; The state monitoring cabinet is used to calculate the capacitance and equivalent series resistance of each submodule according to the switch state of each submodule, the capacitor voltage of each submodule and the three-phase bridge arm current.

10. A power module DC capacitor status monitoring device according to claim 9, characterized in that: A measurement merging unit is arranged between the current sensor and the valve control cabinet.

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