CN115060992A - MMC submodule capacitor aging detection method - Google Patents
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Abstract
The invention relates to an MMC sub-module capacitance aging detection method, which comprises the following steps: 1) collecting the on-off state of the sub-module; 2) when the states of the module to be tested and the reference module are the same, acquiring initial voltages of capacitors of the module to be tested and the reference module, and simultaneously accessing the module to be tested and the reference module into a system; 3) waiting for the change of the control signal, and acquiring the capacitance voltage waveforms of the module to be tested and the reference module; 4) carrying out multilayer wavelet packet decomposition on the collected capacitance voltage waveforms of the module to be detected and the reference module, and calculating the high-frequency energy ratio K after the wavelet packet decomposition R (ii) a 5) Collecting the capacitance voltage of the module to be measured and the reference module, and calculating the capacitance voltage change ratio K C (ii) a 6) Judgment of K C Less than or equal to 0.95 or K R And if the capacitance is more than or equal to 2, judging whether the capacitance of the module to be detected is aged or invalid, cutting off the module to be detected, accessing the system into a redundant module, otherwise, judging whether the capacitance of the module to be detected is not aged or invalid, sequencing the voltages of the modules, and detecting the next module. The method is beneficial to the reliability and convenience of capacitor aging detection.
Description
Technical Field
The invention belongs to the field of Modular Multilevel Converter (MMC) design, and particularly relates to a MMC submodule capacitor aging detection method.
Background
Capacitor aging is a common failure mode, and particularly in an MMC with a large number of sub-modules, the capacitor aging failure has a certain influence on the reliable and stable operation of the converter. The continuous ageing of electric capacity will lead to the open circuit trouble of electric capacity among the submodule piece, and lower capacitance value can make the voltage fluctuation rate rise, influences MMC neutron module control and the balanced problem of submodule piece voltage to the electric capacity equivalent resistance that submodule piece electric capacity ageing degree is high increases, leads to calorific capacity increase temperature rising and then leads to the further ageing of electric capacity, forms vicious circle. Therefore, faults of capacitor aging are detected and positioned, and a relative control strategy is adopted for changing, so that the method has an important effect on reliable operation of the MMC.
The failure criterion for film capacitance is a reduction in capacitance of 5% or an increase in ESR by a factor of two. The existing sub-module capacitor aging detection and positioning method for the flexible-direct power transmission system mainly comprises the following steps: firstly, injecting alternating current into a bridge arm, and estimating a current capacitance value by a least square method. According to the method, alternating current needs to be injected into the bridge arm, voltage ripples on the module capacitor can be increased, and a power supply device needs to be additionally arranged to send out the alternating current, so that the normal operation and work of the current converter are not facilitated. And secondly, calculating the capacitance value by measuring the current and voltage values of the capacitor and using Kalman filtering. The method needs to acquire current and voltage in real time, and has long calculation time. And thirdly, cutting off the capacitor submodule to be detected from the MMC, and taking the capacitor charge-discharge time as the detection data of the capacitance value of the capacitor. According to the scheme, the modules need to be cut off from the system, but the number of the submodules of the current converter is large, and the detection efficiency is low. And fourthly, under the strategy of lowest level modulation, calculating the variable quantity of the capacitor voltage under the action of the trigger signal in a single period, and calculating the current capacitance value by an iteration method, wherein an external sensor is not required, the method is limited to a system modulation method, and the method is not applicable to an MMC power transmission system with a phase-shift carrier control strategy.
Disclosure of Invention
The invention aims to provide a capacitance aging detection method for an MMC sub-module, which is favorable for reliability and convenience of capacitance aging detection.
In order to achieve the purpose, the invention adopts the technical scheme that: a capacitance aging detection method for an MMC sub-module comprises the following steps:
1) when the aging degree of the sub-module capacitor of a certain bridge arm of the MMC begins to be detected, selecting a redundant sub-module serving as a spare on the bridge arm as a standard capacitor reference module; collecting the on-off state of the sub-module;
2) when the states of the module to be tested and the reference module are the same, acquiring initial voltages of capacitors of the module to be tested and the reference module, and simultaneously accessing the module to be tested and the reference module into a system;
3) waiting for the change of the control signal, and acquiring the capacitance voltage waveforms of the module to be tested and the reference module;
4) carrying out multilayer wavelet packet decomposition on the collected capacitance voltage waveforms of the module to be detected and the reference module, and calculating the high-frequency energy ratio K after the wavelet packet decomposition R ；
5) Waiting for the change of the control signal, collecting the capacitance voltages of the module to be tested and the reference module, and calculating the capacitance voltage change ratio K C ；
6) Judgment of K C Less than or equal to 0.95 or K R And if not, judging that the module to be detected is not in the capacitor aging failure state, sequencing module voltages and detecting the next module.
Furthermore, each bridge arm of the MMC converter is composed of a half-bridge submodule, and the MMC converter detects voltage output of the submodule through arranging a voltage sensor on a capacitor of the submodule; and when the bridge arm normally operates, sequencing the sub-module capacitor voltages of the bridge arm to access the system.
Further, in step 1, the switching states S of all n sub-modules on the bridge arm are collected 1 ,S 2 ,...,S n In which S is i 0 denotes no access to the system, S i 1 denotes an access system.
Further, in step 2, at S test 0 and S ref When the voltage is 0, acquiring the initial voltage U of the capacitor of the module to be tested test_0 And the initial voltage U of the capacitor of the reference module ref_0 And simultaneously connecting the module to be tested and the reference module into the system, wherein the switch state is changed into S test 1 and S ref ＝1。
Further, in step 4, decomposing the capacitor voltage waveform of the module to be tested and the capacitor voltage waveform of the reference module through i layers of wavelet packets to obtain 2 i Wavelet packet decomposition amounts of different frequency bands; energy E corresponding to jth frequency band of ith layer i,j I.e. the decomposition coefficient d of each wavelet packet of the frequency band i,j (k) Sum of squares of (c):
selecting wavelet packet decomposition coefficient and high-frequency band energy as fault characteristic value, and using energy size ratio K R As the ESR failure sign of the capacitor aging capacitor:
Further, in step 5, when the switching signal is S ref 1 and S test 1 to S ref 0 and S test When 0, the capacitance voltage U of the module to be measured is collected test_1 And the capacitor voltage U of the reference module ref_1 ；
When the module to be tested and the reference module are simultaneously connected to the cutting system, the bridge arm currents are consistent, and the currents flowing through the capacitors are consistent, so that:
wherein, C test Is the capacitance value of the module capacitor to be measured, C ref Is the capacitance value of the reference module capacitor; i.e. i arm Is bridge arm current value, t 0 For a module access system time, t 1 Removing the system time for the module;
at this time, the reciprocal of the capacitance value is in positive correlation with the voltage change rate, and K is calculated C As the capacity failure sign of the aged capacitor:
wherein, Delta U ref For reference to module voltage difference, Δ U test The voltage difference value of the module to be tested.
Further, in step 6, when the ESR of the capacitor increases twice, i.e. K R More than or equal to 2, or when the capacitance value is reduced by 5 percent, namely K C And when the capacitance is less than or equal to 0.95, the capacitance of the module to be tested is considered to be aged and invalid.
Compared with the prior art, the invention has the following beneficial effects: the method comprises the steps of collecting a charging and discharging curve of capacitance voltage when a bridge arm is connected or cut off by the submodule, calculating the voltage change rate to judge the capacitance value of the submodule, judging the ESR (equivalent series resistance) value of a capacitor by the high-frequency band energy value after the capacitor voltage wavelet packet is decomposed after the detection module is connected or cut off, and judging the capacitance aging fault of the submodule by combining the two marks, so that the reliability of capacitance aging detection of the MMC submodule is improved. Meanwhile, new equipment does not need to be added, a complex algorithm is not needed to be adopted to calculate the capacitance value in an iterative mode, and the aging condition of the capacitance of the MMC sub-module can be detected conveniently and quickly. In addition, aiming at the sub-modules with the capacitance aging failure, the redundant modules are adopted to participate in the original control strategy, so that the number of times of participating in system operation of the failed sub-modules is reduced. Therefore, the invention has strong practicability and wide application prospect.
Drawings
FIG. 1 is a flow chart of a method implementation of an embodiment of the present invention.
Fig. 2 is a schematic diagram of a half-bridge and redundancy modules of an MMC converter according to an embodiment of the present invention.
FIG. 3 is a graph of normal and aged capacitor charging curves in accordance with an embodiment of the present invention.
Fig. 4 is a schematic diagram of wavelet packet decomposition according to an embodiment of the present invention.
Detailed Description
The invention is further explained below with reference to the drawings and the embodiments.
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
The embodiment provides a capacitance aging detection method for an MMC sub-module, and the implementation scheme is as follows:
as shown in fig. 2, each bridge arm of the MMC current converter is composed of a half-bridge sub-module, the MMC current converter adopts a voltage-sharing control strategy, and voltage output of the sub-module is detected by installing a voltage sensor on a capacitor of the sub-module. And when the bridge arm normally operates, sequencing the sub-module capacitor voltages of the bridge arm to access the system.
When the aging degree of the sub-module capacitor of a certain bridge arm of the MMC begins to be detected, the bridge arm is selected as a standby redundant sub-module (whether the capacitance value of the bridge arm is a normal standard capacitance value or not can be measured by exiting the system first) to serve as a standard capacitor reference module, and then as shown in fig. 1, the method comprises the following steps:
1) collecting the switching states S of all n submodules on a bridge arm 1 ,S 2 ,...,S n In which S is i 0 denotes no access to the system, S i 1 denotes an access system.
2) At S test Is 0 and S ref When the voltage is 0, acquiring the initial voltage U of the capacitor of the module to be tested test_0 And the initial voltage U of the reference module ref_0 And simultaneously accessing the module to be tested and the reference module into the system. At this time, the switch state changes to S test 1 and S ref ＝1。
3) Waiting for control signal changes, i.e. when switching signal is changed from S ref 0 and S test 0 to S ref 1 and S test And when the voltage is 1, acquiring the capacitance voltage waveforms of the module to be tested and the reference module.
4) Carrying out multilayer wavelet packet decomposition on the collected capacitance voltage waveforms of the module to be detected and the reference module, and calculating the high-frequency energy ratio K after the wavelet packet decomposition R 。
As the capacitor ages, the ESR increases, causing the sub-modules to cut and switch in and out with a corresponding increase in the voltage difference between the capacitors, and also with an increase in energy. The 98% change of the energy achieved by the low-frequency band percentage is not obvious, and the energy of the high-frequency band is selected as the fault characteristic. Decomposing the capacitor voltage waveform of the module to be tested and the capacitor voltage waveform of the reference module through i layers of wavelet packets to obtain 2 i The amount of wavelet packet decomposition for each different frequency band.
And collecting the capacitance voltage of the sub-module to be tested and the capacitance voltage of the reference sub-module at a sampling frequency of 10 kHz. Energy E corresponding to jth frequency band of ith layer i,j I.e. the decomposition coefficient d of each wavelet packet of the frequency band i,j (k) Sum of squares of (c):
because the fault characteristics of low-frequency energy are not obvious, wavelet packet decomposition coefficients and high-frequency band energy are selected as fault characteristic values, and the energy size ratio K is used R As the ESR failure sign of the capacitor aging capacitor:
Decomposing with 3 layers of wavelet packet to obtain 2 3 Band energy is for example: the high-frequency energy of the capacitance voltage of the submodule to be tested is
Reference submodule capacitor voltage high frequency energy is
Capacitance failure value:
5) waiting for control signal change, i.e. when switching signal is changed from S ref 1 and S test 1 to S ref 0 and S test When the capacitance voltage is 0, the capacitance voltages of the module to be measured and the reference module are collected, and the capacitance voltage change ratio K is calculated C 。
When the converter is connected into the system for a period of time, along with the alternating voltage conversion, the number of the submodules of the converter connected into the system changes, at the moment, the module to be tested and the reference module are simultaneously cut off the system, and at the moment, the switch state is S test Is 0 and S ref Collecting the capacitance voltage U of the module to be measured as 0 test_1 And the capacitor voltage U of the reference module ref_1 。
When the module to be tested and the reference module are simultaneously connected to the cutting system, the bridge arm currents are consistent, and the currents flowing through the capacitors are consistent, so that:
wherein, C test Is the capacitance value of the module capacitor to be measured, C ref Is the capacitance value of the reference module capacitor; i all right angle arm Is bridge arm current value, t 0 For a module access system time, t 1 The system time is cut for the module.
At this time, the reciprocal of the capacitance value is in positive correlation with the voltage change rate, and K is calculated C As the capacity failure sign of the aged capacitor:
wherein, Delta U ref For reference to module voltage difference, Δ U test The voltage difference value of the module to be tested.
6) When the ESR of the capacitor rises twice as much as the original value, namelyK R More than or equal to 2, or when the capacitance value is reduced by 5 percent, namely K C And when the capacitance is less than or equal to 0.95, the capacitance of the module to be tested is considered to be aged and invalid.
Judgment of K C Less than or equal to 0.95 or K R And if not, judging that the module to be detected is not in the capacitor aging failure state, sequencing module voltages and detecting the next module.
The foregoing is directed to preferred embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. However, any simple modification, equivalent change and modification of the above embodiments according to the technical essence of the present invention are within the protection scope of the technical solution of the present invention.
Claims (7)
1. The MMC submodule capacitor aging detection method is characterized by comprising the following steps:
1) when the aging degree of the sub-module capacitor of a certain bridge arm of the MMC begins to be detected, selecting a redundant sub-module serving as a spare on the bridge arm as a standard capacitor reference module; collecting the on-off state of the sub-module;
2) when the states of the module to be tested and the reference module are the same, acquiring initial voltages of capacitors of the module to be tested and the reference module, and simultaneously accessing the module to be tested and the reference module into a system;
3) waiting for the change of the control signal, and acquiring the capacitance voltage waveforms of the module to be tested and the reference module;
4) carrying out multilayer wavelet packet decomposition on the collected capacitance voltage waveforms of the module to be detected and the reference module, and calculating the high-frequency energy ratio K after the wavelet packet decomposition R ；
5) Waiting for the change of the control signal, collecting the capacitance voltages of the module to be tested and the reference module, and calculating the capacitance voltage change ratio K C ；
6) Judgment of K C Less than or equal to 0.95 or K R Whether or not more than or equal to 2If yes, judging that the module to be detected is in a capacitor aging failure state, cutting off the module to be detected, accessing the system into the redundant module, otherwise judging that the module to be detected is not in the capacitor aging failure state, sequencing module voltages, and detecting the next module.
2. The MMC submodule capacitor aging detection method of claim 1, wherein each bridge arm of the MMC converter is composed of half-bridge submodules, and the MMC converter detects the voltage output of the submodules by installing a voltage sensor on the capacitors of the submodules; and when the bridge arm normally operates, sequencing the sub-module capacitor voltages of the bridge arm to access the system.
3. The MMC submodule capacitor aging detection method of claim 1, wherein in step 1, the switch states S of all n submodules on a bridge arm are collected 1 ,S 2 ,...,S n In which S is i 0 denotes no access to the system, S i 1 denotes an access system.
4. The MMC sub-module capacitance aging detection method of claim 1, wherein in step 2, at S test 0 and S ref When the voltage is 0, acquiring the initial voltage U of the capacitor of the module to be tested test_0 And the initial voltage U of the capacitor of the reference module ref_0 And simultaneously connecting the module to be tested and the reference module into the system, wherein the switch state is changed into S test 1 and S ref ＝1。
5. The MMC submodule capacitor aging detection method of claim 1, wherein in step 4, 2 is obtained by decomposing a module capacitor voltage waveform to be tested and a reference module capacitor voltage waveform through i-layer wavelet packets i Wavelet packet decomposition amounts of different frequency bands; energy E corresponding to jth frequency band of ith layer i,j I.e. the decomposition coefficient d of each wavelet packet of the frequency band i,j (k) Sum of squares of (c):
selecting wavelet packet decomposition coefficient and high-frequency band energy as fault characteristic value, and using energy size ratio K R As the ESR failure sign of the capacitor aging capacitor:
6. The MMC sub-module capacitor aging detection method of claim 1, wherein in step 5, when the switching signal is S ref 1 and S test 1 to S ref 0 and S test When 0, the capacitance voltage U of the module to be measured is collected test_1 And the capacitor voltage U of the reference module ref_1 ；
When the module to be tested and the reference module are simultaneously connected to the cutting system, the bridge arm currents are consistent, and the currents flowing through the capacitors are consistent, so that:
wherein, C test Is the capacitance value of the module capacitor to be measured, C ref Is the capacitance value of the reference module capacitor; i.e. i arm Is a bridgeArm current value, t 0 For a module access system time, t 1 Removing the system time for the module;
at this time, the reciprocal of the capacitance value is in positive correlation with the voltage change rate, and K is calculated C As the capacity failure sign of the aged capacitor:
wherein, Delta U ref For reference module voltage difference, Δ U test The voltage difference value of the module to be tested.
7. The MMC sub-module capacitance aging detection method of claim 1, wherein in step 6, when the ESR of the capacitance internal resistance rises twice the original value, K R More than or equal to 2, or when the capacitance value is reduced by 5 percent, namely K C And when the capacitance is less than or equal to 0.95, the capacitance of the module to be tested is considered to be aged and invalid.
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN115792735A (en) * | 2022-11-28 | 2023-03-14 | 清华大学 | Method, device, equipment and medium for online diagnosis of direct current bus capacitor aging |
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