CN109188144B - Modular multilevel converter submodule capacitor aging on-line monitoring method - Google Patents
Modular multilevel converter submodule capacitor aging on-line monitoring method Download PDFInfo
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Abstract
The invention provides an online monitoring method for capacitor aging of a submodule of a modular multilevel converter, which comprises the following steps: s1, data selection: collecting the switching function S of the submodule over a period of time m Delta tiBridge arm current iarmWhere Δ t is the control of the inverterPeriod, m is the number of data continuously collected, when SiWhen the value is 1, the submodule is put into; siWhen the value is 0, the submodule is cut off; i is 1,2,3 … n, representing n submodules; s2, feature quantity extraction: judging the direction of the bridge arm current and the switching state of the submodules according to the data collected in the step S1, and calculating two characteristic quantities of the switching frequency f, the ratio lambda of the conduction times of the charging state to the conduction times of the discharging state of each submodule; s3, judging the aging state: and comparing the feature quantities of the submodules according to the feature quantities obtained by calculation in the step S2, judging the relative aging degree of the submodules, and identifying the submodules with abnormal aging, wherein the submodules with larger feature quantities have more serious aging.
Description
Technical Field
The invention relates to the technical field of power transmission and distribution, in particular to an online aging monitoring method for a submodule capacitor of a modular multilevel converter.
Background
The modularized multi-level converter is a flexible direct current transmission converter widely applied at present, the output of multi-level can be flexibly realized due to the characteristics of modularized structural design, the harmonic content of alternating voltage is greatly reduced, the investment of a filter is reduced, and meanwhile, active and reactive decoupling control can be realized due to the adoption of fully-controlled devices such as IGBT and the like. The sub-module capacitor is an important element in the modular multilevel converter, and plays a role in maintaining the voltage at two ends of the sub-module, and once the capacitor fails, the state of each sub-module is different, and the performance of the converter is affected. The modular multilevel converter submodule capacitor failure mainly comprises capacitor open circuit fault, short circuit fault and aging. At present, research on submodule capacitor failure mainly focuses on diagnosis of two faults of open circuit and short circuit of a capacitor, and fault detection and positioning are achieved by means of changes of capacitance voltage and current waveforms at the moment of fault occurrence. The detection and positioning of the aging of the sub-module capacitor are not discussed at present, for the capacitor, under the conditions of overhigh temperature and humidity and over-current and over-voltage, the aging gradually occurs, so that the capacitance value of the capacitor is reduced, the equivalent series resistance value is increased, the aging is a process with slowly changing parameters, and the aging does not bring sudden change of the parameters to the system, and has no obvious influence on voltage and current waveforms, so that the fault diagnosis method for the short circuit and the open circuit of the capacitor is not suitable for the capacitor aging.
The capacitor aging can increase the equivalent series resistance value, so that the heating value is increased, the temperature is increased, the capacitor aging is accelerated, and a vicious circle is formed. When part of the sub-module capacitors in the MMC are aged, the aging of the capacitors and the switching devices in the sub-modules can be accelerated by the vicious cycle, and hidden dangers are brought to the operation of the converter station. Therefore, for the MMC, the running time of a capacitor with serious aging is reduced, the aging state of the capacitor of the sub-modules on one bridge arm is balanced, and the service life of equipment can be prolonged to a certain extent. To realize this, only the relative aging degree of each sub-module capacitor needs to be known, and as for a specific capacitance value, a more accurate result can be obtained by using an off-line measurement method when equipment is periodically overhauled.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides an online monitoring method for the aging of a submodule capacitor of a modular multilevel converter based on submodule historical switching information, which takes a historical switching function of a submodule and the direction of bridge arm current as data, extracts two characteristic quantities of the submodule, namely the cumulative conduction times, the ratio of the conduction times in a charging state to the conduction times in a discharging state, judges the relative aging degree of the capacitor through the comparison of the characteristic quantities of different submodules, and distinguishes the capacitor with serious aging.
Specifically, the invention provides an online aging monitoring method for a submodule capacitor of a modular multilevel converter, which comprises the following steps:
S1and data selection: the switching function S of the submodule is acquired within a predetermined time length m delta tiBridge arm current iarmThe control period of the current converter is delta t, and data of continuous m points are collected together, wherein SiWhen the value is 1, the submodule is put into; siWhen the value is 0, the submodule is cut off; i is 1,2,3 … n, representing n submodules;
s2, feature quantity extraction: judging the direction of the bridge arm current and the switching state of the submodules according to the data collected in the step S1, and calculating two characteristic quantities of the switching frequency f, the ratio lambda of the conduction times of the charging state to the conduction times of the discharging state of each submodule;
s3, judging the aging state: and comparing the feature quantities of the submodules according to the feature quantities obtained by calculation in the step S2, judging the relative aging degree of the submodules, and identifying the submodules with abnormal aging, wherein the submodules with larger feature quantities have more serious aging.
Preferably, the specific step of S2 is (where the subscript i denotes the ith sub-module, the subscript j denotes the jth data collected, a, b, c and x are intermediate variables required for calculation, f is the sub-module switching frequency, and λ is the ratio of the number of on-times in the charging state to the number of on-times in the discharging state):
s21, when i is equal to 1,
s22, making the sub-module to accumulate the conducting times ai(0) 0, sub-module charging conducting times bi(0) 0, sub-module discharge turn-on number ci(0)＝0，j＝1，
S23, calculating x ═ Si(j+1)-Si(j) If x>0, the sub-module is conducted once, so that the cumulative conduction times of the sub-module is added with 1, namely ai(j)＝ai(j-1)+1，
S24, if iarmjIf the current is more than 0, namely the bridge arm current is more than 0, the capacitor is in a charging state, the charging conduction times of the sub-modules are increased by 1, namely bi(j)＝bi(j-1) +1, if iarmjIf the current of the bridge arm is less than 0, the capacitor is in a discharging state, the discharging conduction times of the sub-modules are increased by 1, namely ci(j)＝ci(j-1)+1，
S25, if j is m, then go to step S26, otherwise let j add 1 to itself, and repeat steps S23 to S25,
s26, calculating the intermediate variable ai(m)、bi(m)、ciThe value of (m) can be given as:
switching frequency of submodule i
Ratio of conduction times of charge state to conduction times of discharge state of submodule i
S27, if i is equal to n, the feature quantity of all the sub-modules is calculated, the step S2 is ended, otherwise, i is added with 1 on the basis of the sub-modules, S22 to S27 are repeated, and the feature quantity of the next sub-module is calculated until the feature quantity of all the sub-modules is calculated;
the subscript i represents the ith submodule, the subscript j represents the jth acquired data, a, b, c and x are intermediate variables required for calculation, f is the switching frequency of the submodule, and lambda is the ratio of the conduction times of the charging state to the conduction times of the discharging state.
Preferably, when Δ t is 0.0001, i.e., the inverter control frequency is 10kHz, the predetermined time period m Δ t in step S1 should be greater than or equal to 0.5S, and corresponding m should be greater than or equal to 5000, and when Δ t is 0.0002, i.e., the inverter control frequency is 5kHz, the predetermined time period m Δ t in step S1 should be not greater than or equal to 1S, and corresponding m should be greater than or equal to 5000.
Preferably, the method for determining the relative aging state of the capacitor in step S3 is to compare the switching frequency and the charging/discharging conduction number ratio of each sub-module, when the switching frequency and the charging/discharging conduction number ratio is greater than the threshold value of the switching frequency and the charging/discharging conduction number ratio, the capacitor of the corresponding sub-module is seriously aged and is determined to be abnormally aged, and when the switching frequency and the charging/discharging conduction number ratio is greater than the threshold value of the switching frequency and the charging/discharging conduction number ratio, the capacitor of the corresponding sub-module is slightly aged.
Compared with the prior art, the invention has the following beneficial effects:
the invention is based on the operation principle of the modular multilevel converter, counts the switching function and the bridge arm current direction of the sub-modules, extracts two characteristic quantities of the switching frequency and the ratio of the conduction times of the charging state to the conduction times of the discharging state from the switching function and the bridge arm current direction, judges the relative aging state of the sub-module capacitor by comparing the characteristic quantities of the sub-modules, and identifies the capacitor with abnormal aging. The method realizes the online monitoring of the aging state of the sub-module capacitor, and the acquired information is the numerical value calculated in the capacitor voltage balance measurement of the modular multilevel converter, so that the required original data can be directly obtained only from the converter controller, a monitoring device does not need to be additionally installed for each sub-module, and the larger economic cost can not be brought.
Drawings
FIG. 1 is a flow chart of a sub-module feature extraction method of the present invention;
FIG. 2 is a graph showing the relationship between the ratio of the number of conduction times of charge and discharge with statistical time when the control period is 0.0001;
FIG. 3 is a graph showing the relationship between the ratio of the number of conduction times of charge and discharge with statistical time when the control period is 0.0002;
FIG. 4 is a graph of the switching frequency of a submodule versus the percentage of submodule capacitance drop for an exemplary implementation;
FIG. 5 is a graph of the charge-discharge turn-on ratio of a neutron module versus the percentage of sub-module capacitance drop for an example implementation.
Detailed Description
Exemplary embodiments, features and aspects of the present invention will be described in detail below with reference to the accompanying drawings. In the drawings, like reference numbers can indicate functionally identical or similar elements. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated.
As shown in fig. 1, the invention provides an online aging monitoring method for a capacitor of a modular multilevel converter based on historical switching information of sub-modules, which utilizes the historical switching information of the sub-modules to extract two characteristic quantities of the switching frequency of the sub-modules, the ratio of the conduction times of a charging state to the conduction times of a discharging state, and distinguishes the abnormally aged capacitor by comparing the characteristic quantities of different sub-modules. Which comprises the following steps:
s1, data selection: the switching function S of the submodule is acquired within a predetermined time length m delta tiBridge arm current iarmThe control period of the current converter is delta t, and data of continuous m points are collected together, wherein SiWhen the value is 1, the submodule is put into; siWhen the value is 0, the submodule is cut off; i is 1,2,3 … n, representing n submodules;
s2, feature quantity extraction: and judging the direction of the bridge arm current and the switching state of the submodules according to the data collected in the step S1, and calculating two characteristic quantities of the switching frequency f, the ratio lambda of the conduction times of the charging state to the conduction times of the discharging state of each submodule within a certain time. The method comprises the following steps:
s21, when i is equal to 1,
s22, making the sub-module to accumulate the conducting times ai(0) 0, sub-module charging conducting times bi(0) 0, sub-module discharge turn-on number ci(0)＝0，j＝1，
S23, calculating x ═ Si(j+1)-Si(j) If x>0, the sub-module is conducted once, so that the cumulative conduction times of the sub-module is added with 1, namely ai(j)＝ai(j-1)+1，
S24, if iarmjIf the current is more than 0, namely the bridge arm current is more than 0, the capacitor is in a charging state, the charging conduction times of the sub-modules are increased by 1, namely bi(j)＝bi(j-1) +1, otherwise, if the bridge arm current is less than 0, the capacitor is in a discharging state, and the sub-module discharging conduction times are increased by 1, namely ci(j)＝ci(j-1)+1，
S25, if j is m, then go to step S26, otherwise let j add 1 to itself, and repeat steps S23 to S25,
s26, calculating the intermediate variable ai(m)、bi(m)、ciThe value of (m) can be given as:
switching frequency of submodule i
Ratio of conduction times of charge state to conduction times of discharge state of submodule i
S27, if i is equal to n, the characteristic quantity of all sub-modules is calculated, the step S2 is ended, otherwise, i is added with 1 on the basis of the sub-module, S22 to S27 are repeated, the characteristic quantity of the next sub-module is calculated,
the subscript i represents the ith submodule, the subscript j represents the jth acquired data, a, b, c and x are intermediate variables required for calculation, f is the switching frequency of the submodule, and lambda is the ratio of the conduction times of the charging state to the conduction times of the discharging state.
S3, judging the aging state: and comparing the feature quantities of the submodules according to the feature quantities obtained by calculation in the step S2, judging the relative aging degree of the submodules, and identifying the submodules with abnormal aging. The larger the feature size, the more severely the sub-module ages.
In the calculation process, considering the requirement of the actual engineering on the aging degree of the capacitor, the calculation results of the two characteristic quantities are basically large or small, so the calculation methods of the two situations are only listed.
The system parameters of the built simulation model of the modular multilevel converter in the embodiment are shown in table 1, and the transmission active power is 2MW and the transmission reactive power is 0.
TABLE 1MMC System parameters
Because the switching frequency and the charging and discharging conducting number ratio are statistics, and the accuracy of the numerical values is related to the number of statistical data, the simulation is carried out by adopting the model built by the embodiment, the control periods are respectively set to be 0.0001s and 0.0002s, the data statistics is started at the moment of 1s, the charging and discharging conducting number ratio is calculated, and the relation between the charging and discharging conducting number ratio and the statistical time is obtained. As shown in fig. 2 and 3, it can be seen that the charge/discharge on-number ratio starts to stabilize after the statistical time reaches 0.5s when the control period is 0.0001s, and the charge/discharge on-number ratio starts to stabilize after the statistical time reaches 1s when the control period is 0.0002 s. Based on this, in the present embodiment, the control cycle is set to 0.0002s, and the statistical time m Δ t of the data is set to 1.5 s.
The capacitance value of the upper bridge arm submodule 1 of the phase a in the modular multilevel converter is gradually reduced, and two characteristic quantities of the switching frequency of the submodule 1 and the ratio of the conduction times of the charging state to the conduction times of the discharging state within 1.5s are calculated by adopting the capacitance online monitoring method provided by the invention, and the results are shown in fig. 4 and 5. It can be seen that both the switching frequency and the ratio of the number of conduction times in the charging state to the number of conduction times in the discharging state show a trend of increasing first and then decreasing as the capacitance value decreases. In practical engineering, when the capacitance value is decreased by more than 20%, the capacitor is generally considered to have failed, so it can be considered from fig. 4 and 5 that the two feature quantities extracted by the present invention have a tendency of increasing with decreasing capacitance value, and a larger feature quantity means a smaller capacitance value and a more serious aging.
Respectively reducing the capacitance value of the sub-module 1 in the upper bridge arm of the phase a by 10%, reducing the capacitance value of the sub-module 7 by 30%, reducing the capacitance value of the sub-module 15 by 20%, and keeping the capacitance values of the other sub-modules normal. By adopting the capacitance online monitoring method provided by the invention, two characteristic quantities of the switching frequency and the ratio of the conduction times of the charging state to the conduction times of the discharging state of each submodule within 1.5s are calculated, and the result is shown in table 2, so that the characteristic quantity corresponding to the submodule with capacitance aging is larger than that of the normal submodule, and the more serious the aging degree is, the larger the value of the characteristic quantity is.
TABLE 2 values of sub-module characteristic quantities during aging of a plurality of sub-modules
Finally, it should be noted that: the above-mentioned embodiments are only used for illustrating the technical solution of the present invention, and not for limiting the same; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.
Claims (4)
1. A modular multilevel converter submodule capacitor aging on-line monitoring method is characterized in that: which comprises the following steps:
s1, data selection: the switching function S of the submodule is acquired within a predetermined time length m delta tiBridge arm current iarmThe control period of the current converter is delta t, and data of continuous m points are collected together, wherein SiWhen the value is 1, the submodule is put into; siWhen the value is 0, the submodule is cut off; i is 1,2,3 … n, representing n submodules;
s2, feature quantity extraction: judging the direction of the bridge arm current and the switching state of the submodules according to the data collected in the step S1, and calculating the switching frequency f of each submodule and two characteristic quantities of the ratio lambda of the conduction times of the charging state to the conduction times of the discharging state;
s3, judging the aging state: and obtaining an aging value according to the two characteristic quantities obtained by the calculation in the step S2, judging the relative aging degree of each submodule, and when the aging value of a certain submodule is greater than an aging threshold, judging the submodule as the submodule with abnormal aging.
2. The modular multilevel converter submodule capacitor aging on-line monitoring method of claim 1, wherein: the specific steps of S2 are:
s21, when i is equal to 1,
s22, making the sub-module to accumulate the conducting times ai(0) 0, sub-module charging conducting times bi(0) 0, sub-module discharge turn-on number ci(0)＝0，j＝1，
S23, calculating x ═ Si(j+1)-Si(j) If x>0, the sub-module is conducted once, so that the cumulative conduction times of the sub-module is added with 1, namely ai(j)＝ai(j-1)+1，
S24, if iarmjIf the current is more than 0, namely the bridge arm current is more than 0, the capacitor is in a charging state, the charging conduction times of the sub-modules are increased by 1, namely bi(j)＝bi(j-1) +1, if iarmjIf the current of the bridge arm is less than 0, the capacitor is in a discharging state, the discharging conduction times of the sub-modules are increased by 1, namely ci(j)＝ci(j-1)+1，
S25, if j is m, then go to step S26, otherwise let j add 1 to itself, and repeat steps S23 to S25,
s26, calculating the intermediate variable ai(m)、bi(m)、ciThe value of (m) can be given as:
switching frequency of submodule i
Ratio of conduction times of charge state to conduction times of discharge state of submodule i
S27, if i is equal to n, the feature quantity of all the sub-modules is calculated, the step S2 is ended, otherwise, i is added with 1 on the basis of the sub-modules, S22 to S27 are repeated, and the feature quantity of the next sub-module is calculated until the feature quantity of all the sub-modules is calculated;
the subscript i represents the ith submodule, the subscript j represents the jth acquired data, a, b, c and x are intermediate variables required for calculation, f is the switching frequency of the submodule, and lambda is the ratio of the conduction times of the charging state to the conduction times of the discharging state.
3. The modular multilevel converter submodule capacitor aging on-line monitoring method of claim 1, wherein:
when Δ t is 0.0001S, i.e., the inverter control frequency is 10kHz, the predetermined time period m Δ t in step S1 should be greater than or equal to 0.5S, and corresponding m should be greater than or equal to 5000, and when Δ t is 0.0002S, i.e., the inverter control frequency is 5kHz, the predetermined time period m Δ t in step S1 should be not greater than or equal to 1S, and corresponding m should be greater than or equal to 5000.
4. The modular multilevel converter submodule capacitor aging on-line monitoring method of claim 1, wherein: the method for judging the relative aging state of the capacitor in the step S3 is to compare the switching frequency and the charging and discharging conduction number ratio of each submodule, when the switching frequency and the charging and discharging conduction number ratio is greater than the threshold value of the switching frequency and the charging and discharging conduction number ratio, the capacitor of the corresponding submodule is seriously aged and is judged to be abnormally aged, and when the switching frequency and the charging and discharging conduction number ratio is less than the threshold value of the switching frequency and the charging and discharging conduction number ratio, the capacitor of the corresponding submodule is slightly aged.
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