MMC differential mode DC bias current calculation method and related device

Abstract

The invention discloses an MMC differential mode direct current bias current calculation method and a related device, and the method comprises the steps: firstly determining an upper and lower bridge arm average switching function and an alternating current side fundamental wave current according to a preset fundamental wave modulation signal; considering an alternating-current side 100Hz harmonic source and a direct-current side 50Hz harmonic source which generate differential-mode direct-current bias current on the alternating-current side, representing upper and lower bridge arm current; upper and lower bridge arm current and an average switching function are combined to represent upper and lower bridge arm capacitor voltage; representing upper and lower bridge arm voltages based on upper and lower bridge arm capacitor voltages and an upper and lower bridge arm average switching function; and then deriving an alternating-current side differential mode voltage based on the upper and lower bridge arm voltages, and further deriving an alternating-current side differential mode direct-current bias current according to the alternating-current side voltage. Therefore, the problem that an effective quantitative calculation method for differential mode bias current caused by alternating current and direct current harmonic coupling of the modular multilevel converter is lacked in the prior art is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of modular multilevel converters, and particularly to a method for calculating the differential-mode DC bias current of an MMC and related devices. Background Art

[0002] Modular Multilevel Converter (MMC) has been widely used in scenarios such as high-voltage DC power transmission, new energy grid connection, and medium-voltage DC distribution networks due to its advantages of good waveform quality, low switching loss, and high operation flexibility.

[0003] However, MMC has the characteristic of AC-DC harmonic coupling conduction. The harmonic source will generate differential-mode DC bias current in the AC side in coupling with modulation. This current shows pseudo-negative sequence characteristics and circulates among the three phases of the transformer without passing through the neutral point of the transformer to form a loop with the ground. When the DC bias current is too large, it will cause the magnetic saturation of the transformer core, increase the reactive power loss, and at the same time increase the leakage magnetic flux, leading to the winding temperature rise, overheating and even explosion of metal structural parts and the oil tank. The existing analysis and suppression of the DC bias current of the transformer are only applicable to the common-mode DC bias current flowing through the neutral point of the transformer to the ground, and the differential-mode bias current is no longer applicable.

[0004] Therefore, for the differential-mode DC bias current generated by the AC-DC harmonic coupling of MMC, it is necessary to propose a calculation method to facilitate the analysis of the cause of the DC bias current and calculate the specific value, so as to provide support for the stable operation of the MMC system. Summary of the Invention

[0005] The present invention provides a method for calculating the differential-mode DC bias current of an MMC and related devices, which is used to solve the problem that the existing technology lacks an effective quantitative calculation method for the differential-mode bias current caused by the AC-DC harmonic coupling of the modular multilevel converter.

[0006] In view of this, the first aspect of the present application provides a method for calculating the differential-mode DC bias current of an MMC, and the method includes:

[0007] Determine the average switching functions of the upper and lower bridge arms and the fundamental current on the AC side according to the fundamental modulation signal preset on the AC side of the modular multilevel converter;

[0008] Based on the fundamental current on the AC side, determine the harmonic source that generates the differential-mode DC bias current on the AC side, and determine the currents of the upper and lower bridge arms according to the harmonic source;

[0009] Based on the currents of the upper and lower bridge arms and the average switching functions of the upper and lower bridge arms, determine the voltages of the upper and lower bridge arms;

[0010] Based on the upper and lower arm voltages and combined with Kirchhoff's voltage equation, the differential-mode voltage on the AC side of the modular multilevel converter is derived.

[0011] Based on the AC side voltage, the differential-mode DC bias current on the AC side is derived.

[0012] Optionally, the harmonic source includes: harmonic current when there is a 100Hz harmonic disturbance on the AC side and harmonic current when there is a 50Hz harmonic current conducted via the DC line on the DC side.

[0013] Optionally, the determining of the upper and lower arm currents according to the harmonic source includes:

[0014] Considering the harmonic current when there is a 100Hz harmonic disturbance on the AC side, determining the upper and lower arm currents;

[0015] Or considering the harmonic current when there is a 50Hz harmonic current conducted via the DC line on the DC side, determining the upper and lower arm currents;

[0016] Or considering the harmonic current when there is a 100Hz harmonic disturbance on the AC side and the harmonic current when there is a 50Hz harmonic current conducted via the DC line on the DC side, determining the upper and lower arm currents.

[0017] Optionally, the expression of the differential-mode DC bias current on the AC side is:

[0018] ;

[0019] In the formula, is the amplitude of the differential-mode DC bias current on the AC side, is the amplitude of the differential-mode DC bias current coupled from the 100Hz on the AC side to the AC side, is the amplitude of the harmonic current coupled from the 100Hz on the AC side to the DC side, is the fundamental modulation ratio, is the initial phase of the 50Hz harmonic current on the DC side generated by the modulation coupling of the 100Hz harmonic current on the AC side, is the amplitude of the 50Hz harmonic current conducted via the DC line, is the initial phase of the 50Hz harmonic current conducted via the DC line.

[0020] The second aspect of this application provides a system for calculating the differential-mode DC bias current of an MMC, and the system includes:

[0021] The first calculation unit is used to determine the average switching function of the upper and lower arms and the fundamental current on the AC side according to the fundamental modulation signal preset on the AC side of the modular multilevel converter.

[0022] A second calculation unit, configured to determine a harmonic source that generates a differential-mode DC bias current on the AC side based on the fundamental current on the AC side, and determine the upper and lower arm currents according to the harmonic source;

[0023] A third calculation unit, configured to determine the upper and lower arm voltages based on the upper and lower arm currents and the average switching functions of the upper and lower arms;

[0024] A fourth calculation unit, configured to derive the differential-mode voltage on the AC side of the modular multilevel converter based on the upper and lower arm voltages and in combination with Kirchhoff's voltage equation;

[0025] A fifth calculation unit, configured to derive the differential-mode DC bias current on the AC side based on the voltage on the AC side.

[0026] Optionally, the harmonic source includes: harmonic currents when there is a 100 Hz harmonic disturbance on the AC side and harmonic currents when there is a 50 Hz harmonic current conducted via the DC line on the DC side.

[0027] Optionally, the determining the upper and lower arm currents according to the harmonic source includes:

[0028] considering the harmonic currents when there is a 100 Hz harmonic disturbance on the AC side, and determining the upper and lower arm currents;

[0029] or considering the harmonic currents when there is a 50 Hz harmonic current conducted via the DC line on the DC side, and determining the upper and lower arm currents;

[0030] or considering the harmonic currents when there is a 100 Hz harmonic disturbance on the AC side and the harmonic currents when there is a 50 Hz harmonic current conducted via the DC line on the DC side, and determining the upper and lower arm currents.

[0031] Optionally, the expression of the differential-mode DC bias current on the AC side is:

[0032] ;

[0033] In the formula, is the amplitude of the differential-mode DC bias current on the AC side, is the amplitude of the differential-mode DC bias current coupled from the 100 Hz on the AC side to the AC side, is the amplitude of the harmonic current coupled from the 100 Hz on the AC side to the DC side, is the fundamental modulation ratio, is the initial phase of the 50 Hz harmonic current on the DC side generated by the modulation coupling of the 100 Hz harmonic current on the AC side, is the amplitude of the 50 Hz harmonic current conducted via the DC line, is the initial phase of the 50 Hz harmonic current conducted via the DC line.

[0034] In the third aspect of the present invention, a device for calculating the differential-mode DC bias current of an MMC is provided. The device includes a processor and a memory:

[0035] The memory is used to store program code and transmit the program code to the processor;

[0036] The processor is used to execute the steps of the method for calculating the differential-mode DC bias current of the MMC as described in the first aspect above according to the instructions in the program code.

[0037] In the fourth aspect of the present invention, a computer-readable storage medium is provided. The computer-readable storage medium is used to store program code, and the program code is used to execute the method for calculating the differential-mode DC bias current of the MMC as described in the first aspect above.

[0038] It can be seen from the above technical solutions that the present invention has the following advantages:

[0039] A method for calculating the differential-mode DC bias current of an MMC provided by an embodiment of the present invention first determines the average switching functions of the upper and lower bridge arms and the fundamental current on the AC side according to a preset fundamental modulation signal; then, considering the harmonic sources that generate the differential-mode DC bias current on the AC side and combining the average switching functions of the upper and lower bridge arms, the currents and voltages of the upper and lower bridge arms are determined; then, in combination with Kirchhoff's voltage equation, the differential-mode voltage on the AC side of the modular multilevel converter is derived, and thus the differential-mode DC bias current on the AC side is derived from the AC side voltage. Thereby, the problem that the prior art lacks an effective quantitative calculation method for the differential-mode bias current caused by the AC-DC harmonic coupling of the modular multilevel converter is solved; further, the method of the present invention superimposes the closed-loop control result of the DC component of the valve-side current on the basis of the inner-loop control of the modular multilevel converter, optimizes and adjusts the DC bias of the valve-side voltage of the modular multilevel converter, and can realize the suppression of the DC bias current of the commutation transformer. Description of the Drawings

[0040] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0041] Figure 1 Schematic diagram of the flow of a method for calculating the differential-mode DC bias current of an MMC provided by an embodiment of the present invention Figure 1 ;

[0042] Figure 2Flow schematic of a method for calculating the differential-mode DC bias current of an MMC provided by an embodiment of the present invention Figure 2 ;

[0043] Figure 3 Structural schematic diagram of a system for calculating the differential-mode DC bias current of an MMC provided by an embodiment of the present invention. Detailed implementation manners

[0044] To make the objectives, features, and advantages of the present invention more obvious and understandable, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the embodiments described below are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0045] Term explanation:

[0046] Differential-mode DC bias current: When there are second-order positive-sequence harmonics in the AC system or fundamental-frequency harmonics in the DC system, a DC bias current will be generated on the AC side of the converter station. This current only circulates between the three-phase windings of the transformer and does not flow through the neutral-point grounding circuit.

[0047] Common-mode DC bias current: It mainly appears when the bipolar DC system operates in the monopole ground return mode or is affected by solar magnetic storms. At this time, the neutral-point potential of the converter transformer shifts to form a DC loop, and the current flows into the winding through the neutral point and forms a path through the ground.

[0048] Please refer to Figure 1 and 2 , a method for calculating the differential-mode DC bias current of an MMC provided by an embodiment of the present invention includes:

[0049] Step 101: Determine the average switching functions of the upper and lower bridge arms and the fundamental-frequency current on the AC side according to the fundamental-frequency modulation signal preset on the AC side of the modular multilevel converter.

[0050] In this embodiment, first, a fundamental-frequency modulation signal is set, and the specific expression is:

[0051] ;

[0052] Based on the above fundamental-frequency modulation signal, the average switching functions of the upper and lower bridge arms are determined, and the specific expression is:

[0053] ;

[0054] And the fundamental-frequency current on the AC side, the specific expression is:

[0055] ;

[0056] It should be noted that the fundamental modulation signal refers to a waveform that changes according to a specific frequency and amplitude, and is used to modulate the carrier signal to generate the required output waveform. In the present invention, the fundamental modulation signal is used to control the on / off of the switching device, thereby realizing the effective regulation of the DC bias current. It can be understood that by constructing a mathematical model containing the fundamental modulation signal, the influence mechanism on the DC bias current can be deeply analyzed. This helps to optimize the modulation strategy and further improve the accuracy and efficiency of the differential-mode DC bias current calculation.

[0057] It should be noted that the average switching functions of the upper and lower bridge arms play a key role in the calculation of the differential-mode DC bias current. This function describes the average effect of the switching states of the upper and lower bridge arms of the inverter and is a bridge connecting the fundamental modulation signal and the fundamental current on the AC side. Specifically, the average switching function of the upper bridge arm is defined as the ratio of the conduction time of the upper bridge arm switch, and the average switching function of the lower bridge arm is defined as the ratio of the conduction time of the lower bridge arm switch. These two functions change with the fundamental modulation signal and jointly determine the waveform and amplitude of the fundamental current on the AC side. During the calculation of the differential-mode DC bias current, it is necessary to accurately measure or estimate the average switching states of the upper and lower bridge arms to accurately solve the magnitude and direction of the differential-mode DC bias current.

[0058] It should be noted that the fundamental current on the AC side, as a key current component of the converter output, its waveform and amplitude are directly affected by the average switching functions of the upper and lower bridge arms. During the operation of the converter, the fundamental current on the AC side forms a loop through the load to transfer energy. And the average switching states of the upper and lower bridge arms, that is, the ratio of the conduction time of the switch, determine the specific performance of the fundamental current on the AC side.

[0059] Step 102: Based on the fundamental current on the AC side, determine the harmonic source that generates the differential-mode DC bias current on the AC side, and determine the currents of the upper and lower bridge arms according to the harmonic source.

[0060] In one embodiment, the harmonic sources in step 102 include: harmonic currents when there is a 100Hz harmonic disturbance on the AC side and harmonic currents when there is a 50Hz harmonic current conducted through the DC line on the DC side;

[0061] Further, in one embodiment, determining the currents of the upper and lower bridge arms according to the harmonic source includes: considering the harmonic currents when there is a 100Hz harmonic disturbance on the AC side and determining the currents of the upper and lower bridge arms;

[0062] or considering the harmonic currents when there is a 50Hz harmonic current conducted through the DC line on the DC side and determining the currents of the upper and lower bridge arms;

[0063] Or consider the harmonic current when there is a 100Hz harmonic disturbance on the AC side and the harmonic current when there is a 50Hz harmonic current conducted through the DC line on the DC side, and determine the upper and lower bridge arm currents.

[0064] It should be noted that the modular multilevel converter has the characteristic of AC-DC harmonic coupling and conduction. The 100Hz positive-sequence harmonic on the AC side will generate a 50Hz harmonic voltage on the DC side after modulation coupling, and a differential-mode DC bias voltage on the AC side, further generating a differential-mode DC bias current. In addition, the 50Hz harmonic on the DC side will generate a differential-mode DC bias voltage on the AC side after modulation coupling, further generating a differential-mode DC bias current. It can be seen that whether it is the 100Hz positive-sequence harmonic source on the AC side or the 50Hz harmonic source on the DC side, both will generate a differential-mode DC bias current through modulation coupling on the AC side. Therefore, this embodiment considers the differential-mode DC bias current generated by the influence of one or a combination of the above two situations. The specific descriptions of the harmonic sources generated by the above two situations are as follows:

[0065] a) When there is a 100Hz harmonic disturbance on the AC side, the harmonic current is expressed as

[0066] ;

[0067] The differential-mode DC bias current generated by the 100Hz harmonic current on the AC side through modulation coupling is expressed as I gh0 , and the 50Hz harmonic current (i gh1 ) on the DC side generated by the 100Hz harmonic current on the AC side through modulation coupling is expressed as

[0068] ;

[0069] b) When there is a 50Hz harmonic current conducted through the DC line on the DC side, the harmonic current is expressed as

[0070] ;

[0071] The following is an explanation with two harmonic sources:

[0072] It should be noted that when considering the simultaneous presence of a 100Hz harmonic source on the AC side and a 50Hz harmonic source conducted through the DC line, the upper bridge arm current can be expressed as:

[0073] ;

[0074] The lower bridge arm current is expressed as:

[0075] ;

[0076] Among them, I dc is the DC current on the DC side, igh1 is the harmonic current coupled from the AC side at 100 Hz to the DC side, i dh1 is the harmonic current conducted through the DC line; i g1 is the fundamental current on the AC side, i gh2 is the 100 Hz harmonic current on the AC side, I gh0 is the differential-mode DC bias current coupled from the AC side at 100 Hz to the AC side; I gdc is the differential-mode DC bias current on the AC side of the MMC. It should be noted that I gdc contains two parts of sources, corresponding to the 100 Hz harmonic source on the AC side and the 50 Hz harmonic source on the DC side respectively.

[0077] It can be understood that in practical applications, by accurately analyzing the current components of the upper and lower bridge arms, the operating state of the converter under different working conditions can be deeply understood. In particular, when there are multiple harmonic sources, such as the 100 Hz harmonic source on the AC side and the 50 Hz harmonic source conducted through the DC line, the currents of the upper and lower bridge arms will exhibit complex waveform characteristics. These characteristics not only reflect the influence of the harmonic sources but also embody the interaction of the internal parameters of the converter. Therefore, when designing and optimizing the converter, the characteristics of these current components must be fully considered to ensure the stable operation of the converter under various working conditions.

[0078] Step 103: Determine the voltages of the upper and lower bridge arms based on the currents of the upper and lower bridge arms and the average switching functions of the upper and lower bridge arms.

[0079] It should be noted that in this embodiment, first, based on the currents of the upper and lower bridge arms and the average switching functions, the capacitor voltages of the upper and lower bridge arms can be expressed. The expressions for the voltages of the upper and lower bridge arms obtained are:

[0080] ;

[0081] Next, based on the currents of the upper and lower bridge arms and the average switching functions of the bridge arms, the voltages of the upper and lower bridge arms can be expressed. The expressions for the voltages of the upper and lower bridge arms obtained are:

[0082] ;

[0083] It can be understood that in this embodiment, during the process of determining the voltages of the upper and lower bridge arms based on the currents of the upper and lower bridge arms and the average switching functions of the upper and lower bridge arms, the influence of the complex current components and harmonic sources inside the converter is fully considered. Through an accurate mathematical model, the currents of the upper and lower bridge arms are combined with the average switching functions, so as to accurately reflect the voltage characteristics of the converter under different working conditions. This step is the basis for calculating the differential-mode DC bias current subsequently and is of great significance for ensuring the accuracy and reliability of the entire calculation method.

[0084] Step 104: Based on the upper and lower arm voltages and combined with Kirchhoff's voltage equation, the differential-mode voltage on the AC side of the modular multilevel converter is derived.

[0085] It should be noted that in this embodiment, the AC side voltage of the MMC can be derived based on the upper and lower arm voltages and Kirchhoff's voltage equation. The specific expression is:

[0086] ;

[0087] The coefficient of t must be zero, otherwise the AC side voltage will gradually become infinitely large.

[0088] Step 105: Based on the AC side voltage, the differential-mode DC bias current on the AC side is derived.

[0089] It should be noted that in this embodiment, the differential-mode DC bias current I gdc on the AC side is derived based on the AC side voltage, and the specific expression is as follows:

[0090] ;

[0091] This expression shows that the differential-mode DC bias current is jointly determined by the 100Hz harmonic current on the AC side and the 50Hz harmonic current on the DC side. The value of the differential-mode DC bias current can be calculated according to the above formula.

[0092] A method for calculating the differential-mode DC bias current of an MMC provided by an embodiment of the present invention first determines the average switching functions of the upper and lower arms and the fundamental current on the AC side according to a preset fundamental modulation signal; then considers the harmonic sources that generate the differential-mode DC bias current on the AC side, and determines the currents and voltages of the upper and lower arms in combination with the average switching functions of the upper and lower arms; then, in combination with Kirchhoff's voltage equation, the differential-mode voltage on the AC side of the modular multilevel converter is derived, so as to derive the differential-mode DC bias current on the AC side according to the AC side voltage. Thus, the problem that the existing technology lacks an effective quantitative calculation method for the differential-mode bias current caused by the AC-DC harmonic coupling of the modular multilevel converter is solved; further, the method of the present invention superimposes the closed-loop control result of the DC component of the valve-side current on the basis of the inner-loop control of the modular multilevel converter, optimizes and adjusts the DC bias of the valve-side voltage of the modular multilevel converter, and can realize the suppression of the DC bias current of the commutation transformer.

[0093] The above is a method for calculating the differential-mode DC bias current of an MMC provided in an embodiment of the present invention. The following is a system for calculating the differential-mode DC bias current of an MMC provided in an embodiment of the present invention.

[0094] Please refer to Figure 3 , a system for calculating the differential-mode DC bias current of an MMC provided in an embodiment of the present invention includes:

[0095] The first calculation unit 201 is configured to determine the average switching functions of the upper and lower arms and the fundamental current on the AC side of the modular multilevel converter according to the preset fundamental modulation signal on the AC side of the modular multilevel converter.

[0096] The second calculation unit 202 is configured to determine the harmonic sources that generate differential-mode DC bias current on the AC side based on the fundamental current on the AC side, and determine the currents of the upper and lower arms according to the harmonic sources.

[0097] The third calculation unit 203 is configured to determine the voltages of the upper and lower arms based on the currents of the upper and lower arms and the average switching functions of the upper and lower arms.

[0098] The fourth calculation unit 204 is configured to derive the differential-mode voltage on the AC side of the modular multilevel converter based on the voltages of the upper and lower arms and in combination with Kirchhoff's voltage equation.

[0099] The fifth calculation unit 205 is configured to derive the differential-mode DC bias current on the AC side based on the voltage on the AC side.

[0100] A differential-mode DC bias current calculation system for a MMC provided by an embodiment of the present invention first determines the average switching functions of the upper and lower arms and the fundamental current on the AC side according to a preset fundamental modulation signal; then considers the harmonic sources that generate differential-mode DC bias current on the AC side, and determines the currents and voltages of the upper and lower arms in combination with the average switching functions of the upper and lower arms; and then derives the differential-mode voltage on the AC side of the modular multilevel converter in combination with Kirchhoff's voltage equation, so as to derive the differential-mode DC bias current on the AC side according to the voltage on the AC side. Thereby, the problem that the prior art lacks an effective quantitative calculation method for the differential-mode bias current caused by the AC-DC harmonic coupling of the modular multilevel converter is solved; further, the method of the present invention superimposes the closed-loop control result of the DC component of the valve-side current on the basis of the inner-loop control of the modular multilevel converter, and optimizes and adjusts the DC bias of the valve-side voltage of the modular multilevel converter, and can realize the suppression of the DC bias current of the commutation transformer.

[0101] Further, an embodiment of the present invention also provides a differential-mode DC bias current calculation device for a MMC. The device includes a processor and a memory:

[0102] The memory is configured to store program code and transmit the program code to the processor;

[0103] The processor is configured to execute the steps of the differential-mode DC bias current calculation method for a MMC as described in the above method embodiment according to the instructions in the program code.

[0104] Furthermore, the embodiments of the present invention also provide a computer-readable storage medium, which is used to store program codes for executing the MMC differential-mode DC bias current calculation method described in the above method embodiments.

[0105] Those skilled in the art can clearly understand that for the convenience and brevity of description, the specific working processes of the above-described systems, devices, and units can refer to the corresponding processes in the foregoing method embodiments and will not be elaborated herein.

[0106] In several embodiments provided by the present invention, it should be understood that the disclosed systems, devices, and methods can be implemented in other ways. For example, the device embodiments described above are merely illustrative. For example, the division of units is only a logical function division, and there may be other division methods in actual implementation. For example, multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the displayed or discussed couplings, direct couplings, or communication connections to each other can be through some interfaces, indirect couplings, or communication connections of devices or units, and can be in electrical, mechanical, or other forms.

[0107] The units described as separate components may or may not be physically separated, and the components displayed as units may or may not be physical units, that is, they can be located in one place or distributed to multiple network units. Some or all of the units can be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0108] In addition, the functional units in various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically alone, or two or more units can be integrated into one unit. The above integrated units can be implemented in the form of hardware or in the form of software functional units.

[0109] When the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on such an understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium and includes several instructions for causing a computer device (which may be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the methods of the various embodiments of the present invention. The foregoing storage medium includes: various media that can store program codes, such as USB flash drives, mobile hard disks, read-only memories (ROMs), random access memories (RAMs), magnetic disks, or optical discs.

[0110] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for calculating the differential-mode DC bias current of an MMC, characterized in that including: Determine the average switching functions of the upper and lower bridge arms and the fundamental current on the AC side according to the fundamental modulation signal preset on the AC side of the modular multilevel converter; Based on the fundamental current on the AC side, determine the harmonic sources that generate differential-mode DC bias current on the AC side, and determine the currents of the upper and lower bridge arms according to the harmonic sources; Based on the currents of the upper and lower bridge arms and the average switching functions of the upper and lower bridge arms, determine the voltages of the upper and lower bridge arms; Based on the voltages of the upper and lower bridge arms and in combination with Kirchhoff's voltage equation, derive the differential-mode voltage on the AC side of the modular multilevel converter; Based on the voltage on the AC side, derive the differential-mode DC bias current on the AC side.

2. The MMC differential-mode DC bias current calculation method according to claim 1, wherein The harmonic sources include: harmonic currents when there is a 100Hz harmonic disturbance on the AC side and harmonic currents when there is a 50Hz harmonic current conducted through the DC line on the DC side.

3. The MMC differential-mode DC bias current calculation method according to claim 2, wherein The determining the currents of the upper and lower bridge arms according to the harmonic sources includes: Considering the harmonic currents when there is a 100Hz harmonic disturbance on the AC side, determine the currents of the upper and lower bridge arms; Or considering the harmonic currents when there is a 50Hz harmonic current conducted through the DC line on the DC side, determine the currents of the upper and lower bridge arms; Or considering the harmonic currents when there is a 100Hz harmonic disturbance on the AC side and the harmonic currents when there is a 50Hz harmonic current conducted through the DC line on the DC side, determine the currents of the upper and lower bridge arms.

4. The MMC differential-mode DC bias current calculation method according to claim 3, wherein The expression of the differential-mode DC bias current on the AC side is: ； In the formula, is the amplitude of the differential-mode DC bias current on the AC side, is the amplitude of the differential-mode DC bias current coupled from the 100 Hz on the AC side to the AC side, is the amplitude of the harmonic current coupled from the 100 Hz on the AC side to the DC side, is the fundamental modulation ratio, is the initial phase of the 50 Hz harmonic current on the DC side generated by the modulation coupling of the 100 Hz harmonic current on the AC side, is the amplitude of the 50 Hz harmonic current conducted through the DC line, is the initial phase of the 50 Hz harmonic current conducted through the DC line.

5. A differential-mode DC bias current calculation system for MMC, characterized in that, including: A first calculation unit for determining the average switching functions of the upper and lower bridge arms and the fundamental current on the AC side according to the fundamental modulation signal preset on the AC side of the modular multilevel converter; A second calculation unit for determining the harmonic sources that generate differential-mode DC bias current on the AC side based on the fundamental current on the AC side, and determining the currents of the upper and lower bridge arms according to the harmonic sources; A third calculation unit for determining the voltages of the upper and lower bridge arms based on the currents of the upper and lower bridge arms and the average switching functions of the upper and lower bridge arms; A fourth calculation unit for deriving the differential-mode voltage on the AC side of the modular multilevel converter based on the voltages of the upper and lower bridge arms and in combination with Kirchhoff's voltage equation; A fifth calculation unit for deriving the differential-mode DC bias current on the AC side based on the voltage on the AC side.

6. The MMC differential-mode DC bias current calculation system according to claim 5, wherein The harmonic sources include: harmonic currents when there is a 100Hz harmonic disturbance on the AC side and harmonic currents when there is a 50Hz harmonic current conducted through the DC line on the DC side.

7. The MMC differential-mode DC bias current calculation system according to claim 6, wherein The determining the currents of the upper and lower bridge arms according to the harmonic sources includes: Considering the harmonic currents when there is a 100Hz harmonic disturbance on the AC side, determine the currents of the upper and lower bridge arms; Or considering the harmonic currents when there is a 50Hz harmonic current conducted through the DC line on the DC side, determine the currents of the upper and lower bridge arms; Or considering the harmonic currents when there is a 100Hz harmonic disturbance on the AC side and the harmonic currents when there is a 50Hz harmonic current conducted through the DC line on the DC side, determine the currents of the upper and lower bridge arms.

8. The MMC differential-mode DC bias current calculation system according to claim 7, wherein The expression of the differential-mode DC bias current on the AC side is: ； Wherein, is the amplitude of the differential-mode DC bias current on the AC side, is the amplitude of the differential-mode DC bias current coupled from the 100 Hz on the AC side to the AC side, is the amplitude of the harmonic current coupled from the 100 Hz on the AC side to the DC side, is the fundamental modulation ratio, is the initial phase of the 50 Hz harmonic current on the DC side generated by modulation coupling of the 100 Hz harmonic current on the AC side, is the amplitude of the 50 Hz harmonic current conducted through the DC line, is the initial phase of the 50 Hz harmonic current conducted through the DC line.

9. A device for calculating the differential-mode DC bias current of an MMC, characterized in that The device includes a processor and a memory: The memory is used to store program codes and transmit the program codes to the processor; The processor is configured to execute the MMC differential-mode DC bias current calculation method according to any one of claims 1-4 based on the instructions in the program code.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium is configured to store program code for executing the MMC differential-mode DC bias current calculation method according to any one of claims 1-4.

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