High-frequency harmonic suppression method for switching of modular multilevel converter

Abstract

The invention provides a modular multi-level converter switching high-frequency harmonic suppression method, which comprises the following steps of: adjusting a sub-module communication period between a valve control system and a modular multi-level converter to enable the duration of a valve control period in a pole control system to be the duration of a plurality of sub-module communication periods; and grouping and sorting the to-be-switched sub-modules in the same valve control period for at least one time, and sequentially grouping and switching the sub-modules based on a grouping and sorting sequence. According to the invention, switching control is carried out on the sub-modules of the modular multilevel converter, so that the amplitude of hundreds of kilohertz high-frequency oscillation in the flexible direct-current power transmission system is obviously reduced, oscillation energy is inhibited from the source, and capacitor equipment in a station is protected from the risk of insulation breakdown; based on a control period adjusting technology, oscillation is weakened from the source, the suppression effect is good, and long-term operation can be achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electronic power devices, and in particular to a method for suppressing high-frequency harmonics in the switching of a modular multilevel converter. BACKGROUND

[0002] Flexible HVDC (VSC-HVDC) has become an important part of modern power systems due to its flexible power regulation capability and efficient energy transmission characteristics, and is widely used in new energy grid connection, grid interconnection, island power supply and offshore wind power transmission. In the application of this technology, a number of landmark projects have been put into operation. However, as the scale of the flexible DC project expands and the application scenarios become more complex, the technical challenges faced by the flexible DC converter in high-frequency harmonic characteristics and impedance modeling are increasingly prominent.

[0003] In recent years, there have been cases of insulation breakdown of capacitive devices such as wall bushings, resistance-capacitance voltage dividers, and transformer bushings in some flexible HVDC projects, causing the flexible HVDC system to shut down, and similar risks exist in other flexible DC projects. After a special study, the preliminary conclusion is that the presence of persistent 100 kHz harmonic current to ground in the device causes local overheating, which leads to a decrease in the insulation performance of the device, posing a threat to the safe and stable operation of the power system.

[0004] The core of the flexible DC converter is the modular multilevel converter (MMC), and the high-frequency switching behavior of its sub-modules will generate high-frequency harmonics in the range of 100 kHz to 1 MHz in the system. These harmonics form a conduction path through the converter, the converter transformer, the bushing, and the DC voltage divider, posing a threat to the safe operation of power equipment. The traditional oscillation analysis and suppression measures currently focus on 10 kHz and below wideband oscillation, which is a system stability problem. However, the ultra-high frequency oscillation that occurs near the 100 kHz frequency band is a new challenge for flexible DC systems, and its long-term existence will seriously threaten the reliability of equipment operation.

[0005] As a new problem in the development of flexible DC, ultra-high frequency oscillation has been studied in the early stage, but in order to restore the operation of the faulty project as soon as possible, only temporary measures have been taken: replacing the damaged equipment with a model with a smaller stray capacitance to ground to reduce the leakage current and reduce heat accumulation; at the same time, according to the simulation results, the equipment operation boundary is proposed to the manufacturer, requiring the design of equipment that can withstand ultra-high frequency oscillation. However, replacing the equipment still faces the impact of long-term ultra-high frequency oscillation, and the risk of damage still exists.

[0006] Currently, the suppression methods adopted by the flexible direct current engineering mainly rely on passive resistance methods, but such passive measures have significant limitations: first, the design lacks pertinence, can only cover a specific frequency range, and is difficult to adapt to the wide frequency characteristics of the hundred-kilohertz-level oscillation; second, the increase of passive elements will introduce additional loss and equipment volume, reducing the system efficiency, so it is difficult to fundamentally solve the problem of ultra-high frequency oscillation, and the equipment still has the risk of damage. In contrast, the suppression measures based on active control can start from the source of oscillation and flexibly adjust the behavior of the sub-module to weaken the high-frequency oscillation, but the related technical research still needs to be further improved.

[0007] At the same time, the current research on wideband oscillation mainly focuses on the frequency band below 10 kHz, and the research on oscillation suppression measures for the hundred-kilohertz-level (above 100 kHz) is very limited. During the sub-module switching process, the dynamic coupling effect of stray capacitance and inductance is significantly enhanced in the high-frequency band, resulting in complex and variable impedance characteristics. Traditional low-frequency optimization cannot effectively suppress such oscillation, and traditional oscillation suppression measures and analysis have no obvious effect on the hundred-kilohertz frequency band, and there is a technical gap in the related field.

[0008] In summary, it is currently urgent to solve the problem of hundred-kilohertz-level ultra-high frequency oscillation caused by the high-frequency switching of the modular multilevel converter sub-module in the flexible direct current transmission system, in order to cope with the threat of insulation breakdown caused by the oscillation to capacitive devices through the conduction path. SUMMARY

[0009] The present application provides a method for actively improving the sub-module switching of a modular multilevel converter, thereby solving the above problems.

[0010] To achieve the above-mentioned purpose, the technical solution adopted by the present application is as follows:

[0011] The present application provides a method for suppressing high-frequency harmonics during the switching of a modular multilevel converter, which includes the following steps:

[0012] Adjust the sub-module communication period between the valve control system and the modular multilevel converter, so that the valve control control period in the pole control system is N times the length of the sub-module communication period, and N is a positive integer;

[0013] Group and sort the to-be-switched sub-modules in the same valve control control period at least once, and sequentially group and switch the sub-modules based on the sequence of the grouping and sorting;

[0014] In the grouping and sorting, when the grouping number is 1, the total number of groups ranges from 2 to N; when the grouping number is greater than 1, the total number of groups is equal to N.

[0015] Optionally, in the grouping sequence, the interval time between groups is equal.

[0016] Optionally, in the sequence of groups, the number of sub-modules in at least some groups is the same.

[0017] Optionally, in the sequence of groups, the switching time of at least some groups is the same.

[0018] Optionally, the number of sub-modules to be switched in the valve control control period is M, and M is greater than or equal to N.

[0019] When the number of groups is 1, the total number of groups is Z, and the number of sub-modules to be switched in each group is M / Z.

[0020] If M is not an integer multiple of Z, the remainder K corresponds to the number of sub-modules that are merged into the last group of the sequence of groups, and the number of sub-modules to be switched in the last group of the sequence of groups is the sum of the integer part of M / Z and K.

[0021] Optionally, the number of sub-modules to be switched in the valve control control period is M, and M is greater than or equal to N.

[0022] When the number of groups is greater than 1, the number of sub-modules to be switched in each group is M / N.

[0023] If M is not an integer multiple of N, the remainder K corresponds to the number of sub-modules that are merged into the last group of the sequence of groups, and the number of sub-modules to be switched in the last group of the sequence of groups is the sum of the integer part of M / N and K.

[0024] Optionally, when grouping, the number of sub-modules in the previous group of the sequence is less than or equal to the number of sub-modules in the subsequent group of the sequence.

[0025] Optionally, the voltage step amplitude generated by switching sub-modules each time the group is less than or equal to the rated voltage of the capacitor of a single sub-module.

[0026] Optionally, the pole control system calculates the current high-frequency harmonic energy distribution frequency band in real time by monitoring the harmonic spectrum of the AC side of the modular multilevel converter.

[0027] Based on the current high-frequency harmonic energy distribution frequency band, the value of N is adjusted.

[0028] Optionally, when the current high-frequency harmonic energy distribution frequency band is within a preset frequency band range, the value of N is linearly increased until the current high-frequency harmonic energy distribution frequency band is out of the preset frequency band range.

[0029] Compared with the prior art, the present application has the following beneficial effects:

[0030] The application significantly reduces the amplitude of the high-frequency oscillation of the flexible DC power transmission system in the hundreds of kilohertz range by switching control of the sub-modules of the modular multilevel converter, and inhibits the oscillation energy from the source to protect the capacitor-type equipment in the station from the risk of insulation breakdown; the control cycle adjustment technology is used to weaken the oscillation from the source, and the inhibition effect is good, and the system can be operated for a long time. BRIEF DESCRIPTION OF DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments will be briefly introduced as follows. Obviously, the drawings in the following description only represent some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor.

[0032] Figure 1 is a method flowchart in the specific embodiment of the present application;

[0033] Figure 2 is a bridge arm voltage waveform diagram in multiple scenarios in the specific embodiment of the present application;

[0034] Figure 3a is a traditional switching phase-frequency characteristic diagram of the sub-module;

[0035] Figure 3b is a switching phase-frequency characteristic diagram of the sub-module in the specific embodiment of the present application.

[0036] In the figure: 1, ideal voltage curve, 2, first voltage curve, 3, second voltage curve, 4, valve control control cycle. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the embodiments of the present application more clear, the technical solutions in the embodiments of the present application will be described clearly and completely in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0038] It should be noted that similar reference numerals and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further defined and explained in the subsequent drawings.

[0039] It should be noted that the methods used in the present application are conventional methods unless otherwise specified; the raw materials and devices used are conventional commercially available products unless otherwise specified, and their sources are not specifically limited.

[0040] It should be further explained that, in order to facilitate understanding, the method steps in the embodiments of the present application are described in a certain order, but those skilled in the art can change the order of the steps according to actual needs, so it cannot be regarded as a limitation; further, in the description of the following specific embodiments, the upper and lower indexes of each parameter should be understood as the distinguishing marks of the similar indexes without special explanation, representing the parameters of the related or corresponding devices of the index, and cannot be understood as specific models or special marks.

[0041] The embodiment of the present application provides a method for suppressing high-frequency harmonics by switching a modular multilevel converter, which is suitable for the sub-module switching control of a modular multilevel converter (MMC) in a flexible direct current transmission system. Through fine timing optimization and grouping strategy, the method realizes effective suppression of hundred-kilohertz-level ultra-high-frequency harmonics, thereby reducing the risk of insulation breakdown of capacitive devices caused by high-frequency leakage current.

[0042] Specifically, as shown in the figure, Figure 1 The method comprises the following steps:

[0043] Firstly, the pole control system adjusts the communication rate matching between the valve control system and the sub-modules by shortening the sub-module communication period, so that the length of the valve control control period is the length of N sub-module communication periods, wherein N is a positive integer. This design divides independent time windows for multiple switching operations in the same valve control control period, ensuring that each grouping and switching operation can be completed within the corresponding sub-module communication period, avoiding control delay caused by timing conflicts. For example, if the valve control control period is 100 μs, and N = 5, the sub-module communication period is adjusted to 20 μs, and the pole control system can issue the switching command 5 times within 100 μs, providing a time basis for grouping and switching.

[0044] After completing the communication period adaptation, the system performs grouping and sorting for the to-be-switched sub-modules within the same valve control control period, and sequentially performs grouping and switching based on the sorting result. The grouping strategy is:

[0045] When the grouping number is 1, the total number of groups is in the range of 2 to N; specifically, taking N = 5 as an example, when the grouping operation is single grouping, it is divided into 2 groups, 3 groups, 4 groups or 5 groups at a time.

[0046] When the grouping number is greater than 1, the total number of groups is fixed to N; specifically, taking twice grouping and N = 5 as an example, when the grouping operation is twice grouping, the first grouping is divided into 2 primary groups; then in the second grouping, at least one of the primary groups is further grouped, so that a total of 5 groups are obtained.

[0047] Further, in the above grouping switching process, when the total number of groups is less than N, then the switching operation is performed by merging the communication periods of multiple sub-modules; when the total number of groups is equal to N, then each sub-module communication period performs the switching operation of a group of sub-modules.

[0048] The above grouping strategy not only ensures the flexibility of the switching process, which can be dynamically combined and adjusted according to the actual situation, but also avoids the rise of control complexity caused by excessive splitting by limiting the upper limit of the number of groups.

[0049] Specifically, as shown in Figure 2 , the sub-module communication period is shortened, so that each valve control control period 4 contains 3 identical sub-module communication periods (N=3), and the total number of sub-modules to be switched in the valve control control period 4 is divided into 3 groups, and the grouping and switching are performed in the same valve control control period 4. Thus, the ideal state of the bridge arm voltage is shown as the ideal voltage curve 1; the traditional synchronous switching is shown as the second voltage curve 3, which is a staircase waveform, and because a large number of sub-modules are switched at the same time, the step of part of the staircase is 3Uc (Uc is the sub-module capacitor voltage), thereby exciting high-frequency harmonic voltage. These harmonics are conducted to the capacitor devices through the station propagation path (such as the converter transformer and the bushing), generating an ultra-high frequency oscillation current with an amplitude of several amperes, which causes overheating and insulation breakdown of the devices; combined with Figure 3a , the harmonic amplitude in the bridge arm voltage spectrum is high, the bushing leakage current time domain waveform oscillates violently, and the amplitude reaches several amperes, and the heat accumulation is significant; in this embodiment, because of the sequential grouping and switching, the voltage curve is closer to the ideal voltage curve, thereby obtaining the first voltage curve 2, and the step between groups is only Uc; combined with Figure 3b , after using the method of this embodiment, the harmonic amplitude is reduced by 50%, the bushing current oscillation amplitude is reduced to less than 50%, the heat accumulation effect is greatly weakened, and the capacitor devices in the station are protected from the risk of insulation breakdown.

[0050] Further, the grouping of the sub-modules to be switched has a time sequence feature, that is, in the same valve control control period, the timing of the switching of each group is inconsistent, and the sequential grouping and switching are adopted. Thus, each group is assigned a sequence in the grouping, and the switching is performed in turn in the corresponding period.

[0051] To ensure the smoothness of the switching process, the interval time between each group in the grouping sequence needs to be kept equal. The uniform time interval makes the step change of the bridge arm voltage regular, avoiding additional high-frequency components caused by time sequence disorder. For example, in a control period of N=4, the switching interval of the 4 groups of sub-modules is 1 / 4 of the valve control control period, so that the step change of the voltage waveform is uniformly distributed, and is closer to the ideal sinusoidal waveform. This design can effectively reduce the harmonic interference caused by asymmetric timing, and lay the foundation for high-frequency harmonic suppression.

[0052] In the number of sub-modules, the number of sub-modules in at least part of the group in the grouping sequence needs to be consistent. The balanced grouping design can make the influence amplitude of each switching on the voltage relatively uniform, avoiding the intensification of voltage fluctuation caused by too large difference in the number of sub-modules in single switching. For example, when the total number of sub-modules to be switched is 10 and the total number of groups is 4, 3 groups can be set to contain 2 sub-modules each, and 1 group contains 4 sub-modules, so that the stability of the overall switching is ensured by the consistency of the number of groups. At the same time, the execution switching time of at least part of the groups needs to be kept equal, which further reduces the harmonic interference through the symmetrization operation. For example, in the 4-group switching, the switching duration of the first 3 groups is t, and the 4th group can be extended to 2t due to the larger number, but the time consistency of the first 3 groups can still ensure the overall smoothness of the voltage waveform.

[0053] Further, in the case where the cycle multiple N is fixed, the number of sub-modules to be switched in the valve control cycle is M (M≥N), and the system can also use different calculation methods for the number of sub-modules to be switched in the group according to the number of groups, as follows:

[0054] When the number of groups is 1, the total number of groups is Z (Z≤N), if M cannot be divided by Z, the remainder K corresponds to the sub-modules merged into the last group of the grouping sequence, and the number of sub-modules to be switched in the group is modified to the sum of the integer part of M / Z and K. For example, when M=11, N=5 and Z=3, the integer part of 11 / 3 is 3, and the remainder is 2, so the last group contains 3+2=5 sub-modules. Thus, taking N=4 as an example, the sub-modules are switched in 3 groups in the cycle, and the sub-module communication cycle has 4, and since the last group contains a larger number of sub-modules, the first and second sub-module communication cycles respectively perform the switching tasks of the first two groups of sub-modules in sequence, and the third and fourth sub-module communication cycles jointly perform the switching task of the last group of sub-modules. This design ensures that the distribution of the number of sub-modules in each group is as balanced as possible, avoiding the concentration of single switching load.

[0055] When the number of groups is greater than 1, the total number of groups is fixed as N, and the number of sub-modules to be switched in the group is M / N; if M cannot be divided by N, the remainder corresponds to the sub-modules merged into the last group of the grouping sequence, and the number of sub-modules to be switched in the group is modified to the sum of the integer part of M / N and K. For example, when M=14 and N=5, the integer part of 14 / 5 is 2, and the remainder is 4, so the total number of groups is 5, the first 4 groups each contain 2 sub-modules to be switched, and the last group contains 2+4=6 sub-modules to be switched. This design is beneficial to reduce the voltage mutation at the initial stage of switching by postponing the execution time sequence of most of the switched sub-modules on the premise of simplifying the grouping calculation amount and preset conditions.

[0056] Further, considering the scenario where the system operation speed and computing resources are sufficient, in another embodiment, the number of sub-modules to be switched in the previous group in the grouping sequence is less than or equal to the number of sub-modules to be switched in the subsequent group. The gradual number allocation makes the voltage step present an increasing trend, reducing the high-frequency harmonics caused by sudden changes. For example, when the grouping number is 1, N = 4, and the total number of groups is 4, the number of sub-modules to be switched in the valve control period is M = 10, so the number of sub-modules in each group can be set in time sequence as 2, 2, 3, and 3; in each sub-module communication period, a group of switching tasks is executed, and the impact strength of the voltage step is reduced in a step-by-step increasing manner. At the same time, by detecting and confirming that the voltage step amplitude of the bridge arm generated by each grouping switching is controlled within the rated voltage of a single sub-module, this constraint limits the energy source of high-frequency harmonics from the physical layer, avoiding the excitation of strong harmonic signals due to large voltage changes.

[0057] Further, in order to achieve the above-mentioned dynamic suppression of ultra-high frequency oscillation, the pole control system needs to monitor the harmonic spectrum of the AC side of the converter in real time, calculate the distribution frequency band of the current high-frequency harmonic energy, and adjust the N value based on the characteristics of the frequency band. When the high-frequency harmonic energy is concentrated in the preset ultra-high frequency band (100 kHz to 1 MHz), the system linearly increases the N value until the harmonic energy is out of the frequency band. For example, when it is detected that the harmonic energy of the 200 kHz frequency band is excessive, the N value can be gradually increased from 4 to 8 for the subsequent same scenario, thereby reducing the single voltage step amplitude by increasing the grouping switching number and specifically weakening the harmonic energy of the 200 kHz frequency band. This dynamic adjustment mechanism enables the system to adapt to changes in harmonic characteristics in real time, ensuring the persistence of the suppression effect. In practical applications, this method can flexibly adjust parameters according to system operating conditions: when the high-frequency harmonic energy is low, the N value can be reduced to reduce control complexity; when the harmonic energy increases, the N value is increased to strengthen the suppression effect. Through this adaptive adjustment, the system can achieve an optimal balance between suppression effect and operating efficiency, meeting the high-frequency harmonic control needs in different scenarios. At the same time, the flexibility of the grouping strategy makes this method adaptable to MMC converters of different sizes, having wide engineering application value.

[0058] The method of the embodiment reduces the generation of hundred-kilohertz ultra-high frequency harmonics from the source by optimizing the communication period, controlling the grouping time sequence, evenly distributing the number of sub-modules, and dynamically adjusting the N value. It replaces passive measures with active control strategies, achieving high-frequency harmonic suppression without additional hardware investment; it reduces the bridge arm voltage step amplitude through fine switching, directly weakening the oscillation energy; and it ensures the specific suppression of ultra-high frequency harmonics in different frequency bands through a dynamic adaptation mechanism. These technical features work together to effectively reduce the high-frequency leakage current and heat accumulation risk of capacitive devices, providing a reliable guarantee for the safe and stable operation of flexible DC power transmission systems.

[0059] In addition, the implementation of the method does not require modification of the existing converter hardware structure, and can be realized only by optimization of the control logic at the software level, with low engineering modification cost. The core logic can be integrated in the existing pole control system, and the real-time suppression of the ultra-high frequency harmonic can be realized by upgrading the control algorithm, thereby providing technical support for safe operation of the device of the flexible DC power transmission project. By strictly following the grouping method and the control strategy, the method can effectively fill the gap of the active suppression technology of the hundred-kilohertz-level ultra-high frequency oscillation, overcome the limitations of the traditional measures, and provide strong guarantee for stable operation of the power system.

[0060] Finally, it should be noted that the above content is only used to illustrate the technical solutions of the present application, and is not a limitation on the protection scope of the present application. Simple modifications or equivalent replacements of the technical solutions of the present application made by those skilled in the art do not deviate from the essence and scope of the technical solutions of the present application.

Claims

1. A method for suppressing high-frequency harmonics during switching of a modular multilevel converter, characterized in that: Includes the following steps: By adjusting the sub-module communication cycle between the valve control system and the modular multilevel converter, the duration of the valve control cycle in the polar control system is made to be N times the duration of the sub-module communication cycle, where N is a positive integer. The sub-modules to be switched within the same valve control cycle are grouped and sorted at least once, and the sub-modules are switched sequentially based on the grouping sequence of the grouping and sorting. In the grouping sorting, when the number of groupings is 1, the total number of groups ranges from 2 to N; when the number of groupings is greater than 1, the total number of groups is equal to N.

2. The method for suppressing high-frequency harmonics by switching a modular multilevel converter according to claim 1, characterized in that: In the grouping sequence, the time intervals between groups are equal.

3. The method for suppressing high-frequency harmonics by switching a modular multilevel converter according to claim 1 or 2, characterized in that: In the grouping sequence, at least some groups have the same number of sub-modules.

4. The method for suppressing high-frequency harmonics by switching a modular multilevel converter according to claim 3, characterized in that: In the grouping sequence, at least some groups have equal switching times.

5. The method for suppressing high-frequency harmonics by switching a modular multilevel converter according to claim 1, characterized in that: The number of sub-modules to be switched during the valve control cycle is M, and M is greater than or equal to N; When the number of groupings is 1, the total number of groups is Z, and the number of sub-modules to be cut within a group is M / Z; If M is not an integer multiple of Z, the number of sub-modules corresponding to the remainder K is merged into the last group of the grouping sequence, and the number of sub-modules to be cut in the last group of the grouping sequence is the sum of the integer part of M / Z and K.

6. The method for suppressing high-frequency harmonics by switching a modular multilevel converter according to claim 1, characterized in that: The number of sub-modules to be switched during the valve control cycle is M, and M is greater than or equal to N; When the number of groupings is greater than 1, the number of sub-modules to be deployed within a group is M / N; If M is not an integer multiple of N, the number of sub-modules corresponding to the remainder K is merged into the last group of the grouping sequence, and the number of sub-modules to be cut in the last group of the grouping sequence is the sum of the integer part of M / N and K.

7. The method for suppressing high-frequency harmonics by switching a modular multilevel converter according to claim 1, characterized in that: When grouping, the number of submodules in the previous group of the sequence is less than or equal to the number of submodules in the next group of the sequence.

8. The method for suppressing high-frequency harmonics by switching a modular multilevel converter according to claim 1, characterized in that: The voltage step amplitude of the bridge arm generated by each group switching of submodules shall not exceed the rated capacitor voltage of a single submodule.

9. The method for suppressing high-frequency harmonics by switching a modular multilevel converter according to claim 1, characterized in that: The polar control system monitors the AC side harmonic spectrum of the modular multilevel converter and calculates the current high-frequency harmonic energy distribution band in real time. Based on the current high-frequency harmonic energy distribution band, adjust the N value.

10. The method for suppressing high-frequency harmonics by switching a modular multilevel converter according to claim 9, characterized in that: When the current high-frequency harmonic energy distribution frequency band is within the preset frequency band range, the N value is linearly increased until the current high-frequency harmonic energy distribution frequency band leaves the preset frequency band range.

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