Method and device for controlling capacitor voltage of sub-module of modular multilevel converter

Abstract

The invention discloses a modular multilevel converter sub-module capacitor voltage control method and device, and the method comprises the steps: dynamically calculating the per-unit value correction of a reference value of the capacitor voltage of an MMC sub-module according to an operation condition, and adding the per-unit value correction to the reference value of the capacitor voltage; and when the capacitor voltage control link is adopted, the capacitor voltage direct current component also has a direct current voltage offset control quantity, according to the direct current voltage offset control quantity and the rated direct current voltage of the MMC, a direct current internal potential reference value is calculated and sent to the MMC pulse control link, and according to a pulse control signal output by the MMC pulse control link, the direct current internal potential reference value is calculated. According to the method, the capacitor voltage of the MMC sub-module is controlled, the same effect as that in a non-capacitor voltage control link is achieved, the maximum peak value of the capacitor voltage is reduced, the use amount of the capacitor of the MMC sub-module is prevented from being increased, and the size and cost of the MMC are optimal.

Description

Technical Field

[0001] This invention relates to the field of modular multilevel converter technology, and more particularly to a method and apparatus for controlling the capacitor voltage of a modular multilevel converter submodule. Background Technology

[0002] Modular multilevel converters (MMCs) are the primary topology choice for flexible DC transmission converters. Traditional MMC control methods typically only include DC bus voltage control, lacking capacitor voltage control. Submodule capacitor voltage is indirectly controlled by maintaining a constant sum of the number of submodules in the upper and lower arms, ensuring a consistent ratio between the voltage of each submodule and the DC bus voltage. However, due to ripple in the submodule capacitor voltage, a ripple effect occurs. Under this control method, the DC component of the submodule capacitor voltage deviates from its rated value and varies with operating conditions.

[0003] In existing MMC submodule capacitor voltage control schemes, the rated capacitor voltage is used as the reference value for capacitor voltage control and remains constant. While the DC component control stage eliminates DC component deviation, it also increases the maximum peak capacitor voltage across the entire operating range, leading to increased capacitor requirements in the submodule. Therefore, how to retain the advantages of the capacitor voltage control stage while avoiding an increase in peak capacitor voltage, thereby optimizing the cost and size of the MMC, is a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0004] This invention provides a method and apparatus for controlling the capacitor voltage of a modular multilevel converter (MMC) submodule, which addresses the technical problem that existing MMC submodule capacitor voltage control methods lead to an increase in the maximum peak capacitor voltage across the entire operating range, resulting in increased demand for submodule capacitors and difficulty in optimizing the cost and size of the MMC.

[0005] In view of this, the first aspect of the present invention provides a method for controlling the capacitor voltage of a modular multilevel converter submodule, comprising:

[0006] Based on the fundamental frequency reference voltage modulation ratio and fundamental frequency reference voltage phase angle of the MMC pulse control loop, as well as the per-unit value of the AC current amplitude, AC current phase angle, second harmonic circulating current amplitude coefficient and second harmonic circulating current phase angle of the MMC, calculate the per-unit value correction of the MMC submodule capacitor voltage reference value.

[0007] Calculate the capacitor voltage reference value based on the per-unit correction of the MMC submodule capacitor voltage reference value and the rated value of the MMC submodule capacitor voltage.

[0008] The average value of the capacitor voltage of all MMC submodules is collected, and the difference between the average value and the capacitor voltage reference value is sent to the proportional-integral controller to obtain the DC voltage offset control quantity.

[0009] Calculate the reference value of DC internal potential based on the DC voltage offset control amount and the rated DC voltage of MMC;

[0010] The DC internal potential reference value is sent to the MMC pulse control loop, and the voltage of the MMC submodule capacitor is controlled according to the pulse control signal output by the MMC pulse control loop.

[0011] Optionally, the formula for calculating the per-unit correction of the MMC submodule capacitor voltage reference value is:

[0012]

[0013] in, This is the per-unit correction amount for the reference value of the capacitor voltage in the MMC submodule. Here, M is a preset constant, and M is the modulation ratio of the fundamental frequency reference voltage. This represents the per-unit value of the AC current amplitude of the MMC. The phase angle of the AC current in the MMC. The phase angle of the fundamental frequency reference voltage. The second harmonic circulating current amplitude coefficient. It is the phase angle of the second harmonic circulating current.

[0014] Optionally, the calculation of the per-unit correction for the MMC submodule capacitor voltage reference value also includes:

[0015] The preset constants are calculated based on the MMC's rated active power, the effective value of the valve-side AC line voltage of the MMC, the rated DC voltage of the MMC, the rated value of the submodule capacitor voltage of the MMC, the value of the submodule capacitor of the MMC, the number of cascaded bridge arm submodules of the MMC, and the fundamental angular frequency.

[0016] The formula for calculating the preset constant is:

[0017]

[0018] in, This is the rated active power of the MMC. This represents the effective value of the AC line voltage on the valve side of the MMC. This is the rated DC voltage of the MMC. This refers to the rated voltage of the submodule capacitors in the MMC. Here, N represents the capacitance value of the MMC submodule, and N is the number of cascaded bridge arm submodules of the MMC. This is the fundamental angular frequency.

[0019] Optionally, the formula for calculating the capacitor voltage reference value is:

[0020]

[0021] in, This is the reference value for capacitor voltage. This refers to the rated voltage of the submodule capacitors in the MMC. This is the per-unit correction amount for the reference value of the capacitor voltage in the MMC submodule.

[0022] Optionally, the formula for calculating the DC internal potential reference value is:

[0023]

[0024] in, This is the reference value for the DC internal potential. This is a DC voltage offset control quantity. This is the rated DC voltage of the MMC.

[0025] A second aspect of the present invention provides a modular multilevel converter submodule capacitor voltage control device, comprising:

[0026] The first calculation module is used to calculate the per-unit correction amount of the MMC submodule capacitor voltage reference value based on the modulation ratio and phase angle of the base frequency reference voltage of the MMC pulse control loop, as well as the per-unit value of the AC current amplitude, the phase angle of the AC current, the second harmonic circulating current amplitude coefficient and the second harmonic circulating current phase angle of the MMC.

[0027] The second calculation module is used to calculate the capacitor voltage reference value based on the per-unit correction amount of the MMC submodule capacitor voltage reference value and the rated value of the MMC submodule capacitor voltage.

[0028] The third calculation module is used to collect the average value of the capacitor voltage of all MMC sub-modules, and send the difference between the average value and the capacitor voltage reference value to the proportional-integral controller to obtain the DC voltage offset control quantity.

[0029] The fourth calculation module is used to calculate the reference value of the DC internal potential based on the DC voltage offset control amount and the rated DC voltage of the MMC.

[0030] The control module is used to send the DC internal potential reference value to the MMC pulse control loop, and control the capacitor voltage of the MMC submodule according to the pulse control signal output by the MMC pulse control loop.

[0031] Optionally, the formula for calculating the per-unit correction of the MMC submodule capacitor voltage reference value is:

[0032]

[0033] in, This is the per-unit correction amount for the reference value of the capacitor voltage in the MMC submodule. Here, M is a preset constant, and M is the modulation ratio of the fundamental frequency reference voltage. This represents the per-unit value of the AC current amplitude of the MMC. The phase angle of the AC current in the MMC. The phase angle of the fundamental frequency reference voltage. The second harmonic circulating current amplitude coefficient. It is the phase angle of the second harmonic circulating current.

[0034] Optionally, the first computing module is also used for:

[0035] The preset constants are calculated based on the MMC's rated active power, the effective value of the valve-side AC line voltage of the MMC, the rated DC voltage of the MMC, the rated value of the submodule capacitor voltage of the MMC, the value of the submodule capacitor of the MMC, the number of cascaded bridge arm submodules of the MMC, and the fundamental angular frequency.

[0036] The formula for calculating the preset constant is:

[0037]

[0038] in, This is the rated active power of the MMC. This represents the effective value of the AC line voltage on the valve side of the MMC. This is the rated DC voltage of the MMC. This refers to the rated voltage of the submodule capacitors in the MMC. Here, N represents the capacitance value of the MMC submodule, and N is the number of cascaded bridge arm submodules of the MMC. This is the fundamental angular frequency.

[0039] Optionally, the formula for calculating the capacitor voltage reference value is:

[0040]

[0041] in, This is the reference value for capacitor voltage. This refers to the rated voltage of the submodule capacitors in the MMC. This is the per-unit correction amount for the reference value of the capacitor voltage in the MMC submodule.

[0042] Optionally, the formula for calculating the DC internal potential reference value is:

[0043]

[0044] in, This is the reference value for the DC internal potential. This is a DC voltage offset control quantity. This is the rated DC voltage of the MMC.

[0045] As can be seen from the above technical solutions, the modular multilevel converter submodule capacitor voltage control method provided by the present invention has the following advantages:

[0046] The modular multilevel converter (MMC) submodule capacitor voltage control method provided by this invention simulates the capacitor voltage offset that exists without a capacitor voltage control loop. It dynamically calculates a per-unit correction amount for the MMC submodule capacitor voltage reference value based on operating conditions and adds this correction amount to the capacitor voltage reference value. This ensures that even with a capacitor voltage control loop, the DC component of the capacitor voltage also has a DC voltage offset control amount. Based on the DC voltage offset control amount and the rated DC voltage of the MMC, a DC internal potential reference value is calculated and sent to the MMC pulse control loop. The pulse control signal output by the MMC pulse control loop controls the MMC submodule capacitor voltage, achieving the same effect as without a capacitor voltage control loop. This reduces the maximum peak value of the capacitor voltage, avoids increasing the amount of capacitors used in the MMC submodule, and optimizes the MMC's size and cost. This solves the technical problem that existing MMC submodule capacitor voltage control methods lead to an increase in the maximum peak value of the capacitor voltage across the entire operating range, resulting in increased capacitor requirements and difficulty in optimizing the cost and size of the MMC. Attached Figure Description

[0047] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0048] Figure 1 This is a flowchart illustrating a modular multilevel converter submodule capacitor voltage control method provided in an embodiment of the present invention;

[0049] Figure 2 This is a control block diagram of the modular multilevel converter submodule capacitor voltage control method provided in this embodiment of the invention;

[0050] Figure 3 This is a graph showing the variation of the actual DC component per-unit value of the capacitor voltage with operating conditions in an embodiment of the present invention.

[0051] Figure 4 This is a comparison chart of the linear modulation margin versus the PQ boundary point by point when a conventional capacitor voltage control circuit is used, provided in an embodiment of the present invention.

[0052] Figure 5This is a schematic diagram of the structure of a modular multilevel converter submodule capacitor voltage control device provided in an embodiment of the present invention. Detailed Implementation

[0053] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0054] For easier understanding, please refer to Figure 1 and Figure 2 This invention provides an embodiment of a modular multilevel converter submodule capacitor voltage control method, comprising:

[0055] Step 101: Based on the fundamental frequency reference voltage modulation ratio and fundamental frequency reference voltage phase angle of the MMC pulse control loop, as well as the per-unit value of the AC current amplitude, AC current phase angle, second harmonic circulating current amplitude coefficient and second harmonic circulating current phase angle of the MMC, calculate the per-unit value correction of the MMC submodule capacitor voltage reference value.

[0056] It should be noted that the modulation strategy of MMC is implemented through a pulse control loop. In this embodiment of the invention, the modulation ratio M and the phase angle of the base frequency reference voltage of the pulse control loop are first obtained. And obtain the per-unit value of the AC current amplitude of MMC. AC current phase angle Second harmonic circulating current amplitude coefficient Phase angle of second harmonic circulation Based on the fundamental frequency reference voltage modulation ratio M and the fundamental frequency reference voltage phase angle. AC current amplitude per unit value AC current phase angle Second harmonic circulating current amplitude coefficient Phase angle of second harmonic circulation Calculate the per-unit correction amount of the MMC submodule capacitor voltage reference value. MMC submodule capacitor voltage reference value per unit correction amount The formula for calculation is:

[0057]

[0058] in, This is the per-unit correction amount for the reference value of the capacitor voltage in the MMC submodule. Here, M is a preset constant, and M is the modulation ratio of the fundamental frequency reference voltage. This represents the per-unit value of the AC current amplitude of the MMC. The phase angle of the AC current in the MMC. The phase angle of the fundamental frequency reference voltage. The second harmonic circulating current amplitude coefficient. It is the phase angle of the second harmonic circulating current.

[0059] In one embodiment, a preset constant is set. The method of obtaining it is:

[0060] Obtain the rated active power of MMC The effective value of the AC line voltage on the valve side of the MMC. The rated DC voltage of MMC MMC submodule capacitor voltage rating MMC submodule capacitance values The number N of cascaded bridge arm submodules and the fundamental angular frequency of MMC. According to the rated active power of MMC The effective value of the AC line voltage on the valve side of the MMC. The rated DC voltage of MMC MMC submodule capacitor voltage rating MMC submodule capacitance values Calculate the preset constants based on the number N of cascaded bridge arm submodules of the MMC and the fundamental angular frequency. The specific calculation formula is as follows:

[0061]

[0062] in, This is the rated active power of the MMC. This represents the effective value of the AC line voltage on the valve side of the MMC. This is the rated DC voltage of the MMC. This refers to the rated voltage of the submodule capacitors in the MMC. Here, N represents the capacitance value of the MMC submodule, and N is the number of cascaded bridge arm submodules of the MMC. The fundamental angular frequency .

[0063] Step 102: Calculate the capacitor voltage reference value based on the per-unit correction amount of the MMC submodule capacitor voltage reference value and the rated value of the MMC submodule capacitor voltage.

[0064] It should be noted that the per-unit correction amount is based on the reference value of the MMC submodule capacitor voltage. and MMC submodule capacitor voltage rating Calculate the reference value of capacitor voltage The specific calculation formula is as follows:

[0065]

[0066] in, This is the reference value for capacitor voltage. This refers to the rated voltage of the submodule capacitors in the MMC. This is the per-unit correction amount for the reference value of the capacitor voltage in the MMC submodule.

[0067] Step 103: Collect the average value of the capacitor voltage of all MMC submodules, and send the difference between the average value and the capacitor voltage reference value to the proportional-integral controller to obtain the DC voltage offset control quantity.

[0068] It should be noted that the average value of the capacitor voltages of all MMC submodules is collected. , average and capacitor voltage reference value The difference is fed into the proportional-integral controller to obtain the DC voltage offset control quantity. .

[0069] Step 104: Calculate the reference value of DC internal potential based on the DC voltage offset control amount and the rated DC voltage of MMC.

[0070] It should be noted that, based on the DC voltage offset control quantity and the rated DC voltage of MMC Calculate the reference value of DC internal potential The specific calculation formula is as follows:

[0071]

[0072] in, This is the reference value for the DC internal potential. This is a DC voltage offset control quantity. This is the rated DC voltage of the MMC.

[0073] Step 105: Send the DC internal potential reference value into the MMC pulse control loop, and control the capacitor voltage of the MMC submodule according to the pulse control signal output by the MMC pulse control loop.

[0074] It should be noted that the DC internal potential reference value The signal is fed into the MMC pulse control circuit, and the pulse control signal output by the MMC pulse control circuit is used to control the capacitor voltage of the MMC submodule.

[0075] The DC port voltage of the MMC is primarily determined by the sum of the voltages of the upper and lower bridge arms. The sum of the output voltages of the upper and lower bridge arms is defined as the common-mode voltage of the bridge arms, which can be expressed as:

[0076]

[0077] in, Let be the common-mode voltage of the bridge arm at time t. This represents the sum of the submodule capacitor voltages applied to the upper bridge arm at time t. This represents the sum of the submodule capacitor voltages applied to the lower bridge arm at time t. For the pulse control signal of the upper bridge arm, This is the pulse control signal for the lower bridge arm. This represents the DC component of the capacitor voltage in the submodule. This refers to the voltage fluctuation component of the capacitor in the upper bridge arm submodule. The voltage fluctuation component of the lower bridge arm submodule capacitor is denoted as N, and the number of bridge arm submodules cascaded is denoted as N.

[0078] In the above formula, This is the DC component effect of the capacitor voltage. This is the effect of capacitor voltage ripple. Bridge arm common-mode voltage. The DC component in the value is actually the DC port voltage of the MMC. It is determined not only by the DC component of the N-fold submodule capacitor voltage, but also by the capacitor voltage ripple. (The remaining text appears to be incomplete and requires further context.) For example, in the modulated wave There is also a fundamental frequency AC component, which is present in the capacitor voltage ripple. It also contains a fundamental frequency AC component, therefore it also contains a fundamental frequency component. and Multiplication will inevitably produce a DC component. Similarly, and Multiplication will inevitably produce a DC component. This indicates that the capacitor voltage ripple will affect the bridge arm common-mode voltage during the pulse modulation process. The DC component in the circuit has an impact, which in turn affects the DC port characteristics. When the MMC is connected to a DC line, due to the DC voltage closed-loop control, the DC line voltage usually operates at the MMC's rated DC voltage. That is, satisfying To satisfy this condition, the DC component of the submodule capacitor voltage must necessarily meet the following condition:

[0079]

[0080] To keep the calculation expressions concise and consistent, per-unit values ​​will be used in the following calculation process. It can be rewritten as:

[0081]

[0082] in, for per-unit value, This represents the per-unit value of the capacitor voltage in the MMC submodule relative to its rated value. The specific expression is:

[0083]

[0084] Without a dedicated capacitor voltage control circuit, when the DC line voltage is controlled to its rated value, the ripple effect deviation is not observed in the DC port voltage, but rather manifests in the DC component of the submodule capacitor voltage. If the DC port voltage operates at its rated value during steady-state operation, to offset the DC component effect of the ripple, there will inevitably be a deviation between the DC component of the submodule capacitor voltage and its rated value; it will not operate exactly at the rated value as ideally assumed. This deviation will vary with operating conditions, especially with the magnitude of the AC output current and the power factor angle.

[0085] An analysis is conducted using a specific example of a 1250 MW / 400 kV MMC. Figure 3 The variation of the DC component of the capacitor voltage with operating conditions is given. When the MMC is in the inductive operating range, i.e., -π < When <0, the actual value of the DC component of the capacitor voltage is greater than the rated value. When MMC is in the capacitive operating range, i.e., 0 < When π < π, the actual value of the DC component of the capacitor voltage is less than the rated value. Conventional MMC control methods typically only include DC bus voltage control, lacking capacitor voltage control. In this case, the DC component of the submodule capacitor voltage will exist as follows: Figure 4 The offset shown varies with different operating conditions.

[0086] To achieve better dynamic performance, or to use the submodule capacitor voltage as an indicator of AC / DC power balance and to realize DC bus voltage, an increasing number of applications employ average capacitor voltage control loops. The goal is to ensure that the DC component of the capacitor voltage is precisely equal to the reference voltage. When using conventional capacitor voltage control strategies, the DC component control loop naturally eliminates deviations in the capacitor voltage's DC component. However, this elimination of DC component deviation also alters the peak and trough values ​​of the reference waveform, affecting the linear modulation range and the maximum peak value of the capacitor voltage. Conventional capacitor voltage control loops control the DC component of the capacitor voltage, which also influences the peak value of the capacitor voltage. Figure 4Curves showing the DC component and peak value of the capacitor voltage varying with the PQ operating range boundaries, with and without a conventional capacitor voltage control loop, are presented. Without the capacitor voltage control loop, the actual DC component of the capacitor voltage is affected by the ripple effect, and its value varies with operating conditions. The conventional capacitor voltage control loop ensures that the DC component of the capacitor voltage is strictly equal to the reference value. Compared to the case without capacitor voltage control, the conventional capacitor voltage control loop reduces the DC component in the inductive operating region and increases the DC component in the capacitive operating range, and this difference gradually increases with the increase of output reactive power. Therefore, as... Figure 4 As shown, the use of conventional capacitor voltage control leads to a significant increase in the peak capacitor voltage in the capacitive operating region, which in turn leads to an increase in the maximum peak capacitor voltage across the entire operating range, resulting in an increase in the demand for sub-mode capacitors.

[0087] The modular multilevel converter (MMC) submodule capacitor voltage control method provided by this invention simulates the capacitor voltage offset that exists without a capacitor voltage control loop. It dynamically calculates a per-unit correction amount for the MMC submodule capacitor voltage reference value based on operating conditions and adds this correction amount to the capacitor voltage reference value. This ensures that even with a capacitor voltage control loop, the DC component of the capacitor voltage also has a DC voltage offset control amount. Based on the DC voltage offset control amount and the rated DC voltage of the MMC, a DC internal potential reference value is calculated and sent to the MMC pulse control loop. The pulse control signal output by the MMC pulse control loop controls the MMC submodule capacitor voltage, achieving the same effect as without a capacitor voltage control loop. This reduces the maximum peak value of the capacitor voltage, avoids increasing the amount of capacitors used in the MMC submodule, and optimizes the MMC's size and cost. This solves the technical problem that existing MMC submodule capacitor voltage control methods lead to an increase in the maximum peak value of the capacitor voltage across the entire operating range, resulting in increased capacitor requirements and difficulty in optimizing the cost and size of the MMC.

[0088] For easier understanding, please refer to Figure 5 This invention provides an embodiment of a modular multilevel converter submodule capacitor voltage control device, comprising:

[0089] The first calculation module is used to calculate the per-unit correction amount of the MMC submodule capacitor voltage reference value based on the modulation ratio and phase angle of the base frequency reference voltage of the MMC pulse control loop, as well as the per-unit value of the AC current amplitude, the phase angle of the AC current, the second harmonic circulating current amplitude coefficient and the second harmonic circulating current phase angle of the MMC.

[0090] The second calculation module is used to calculate the capacitor voltage reference value based on the per-unit correction amount of the MMC submodule capacitor voltage reference value and the rated value of the MMC submodule capacitor voltage.

[0091] The third calculation module is used to collect the average value of the capacitor voltage of all MMC sub-modules, and send the difference between the average value and the capacitor voltage reference value to the proportional-integral controller to obtain the DC voltage offset control quantity.

[0092] The fourth calculation module is used to calculate the reference value of the DC internal potential based on the DC voltage offset control amount and the rated DC voltage of the MMC.

[0093] The control module is used to send the DC internal potential reference value to the MMC pulse control loop, and control the capacitor voltage of the MMC submodule according to the pulse control signal output by the MMC pulse control loop.

[0094] In one embodiment, the formula for calculating the per-unit correction of the MMC submodule capacitor voltage reference value is:

[0095]

[0096] in, This is the per-unit correction amount for the reference value of the capacitor voltage in the MMC submodule. Here, M is a preset constant, and M is the modulation ratio of the fundamental frequency reference voltage. This represents the per-unit value of the AC current amplitude of the MMC. The phase angle of the AC current in the MMC. The phase angle of the fundamental frequency reference voltage. The second harmonic circulating current amplitude coefficient. It is the phase angle of the second harmonic circulating current.

[0097] In one embodiment, the first computing module is further configured to:

[0098] The preset constants are calculated based on the MMC's rated active power, the effective value of the valve-side AC line voltage of the MMC, the rated DC voltage of the MMC, the rated value of the submodule capacitor voltage of the MMC, the value of the submodule capacitor of the MMC, the number of cascaded bridge arm submodules of the MMC, and the fundamental angular frequency.

[0099] The formula for calculating the preset constant is:

[0100]

[0101] in, This is the rated active power of the MMC. This represents the effective value of the AC line voltage on the valve side of the MMC. This is the rated DC voltage of the MMC. This refers to the rated voltage of the submodule capacitors in the MMC. Here, N represents the capacitance value of the MMC submodule, and N is the number of cascaded bridge arm submodules of the MMC. This is the fundamental angular frequency.

[0102] In one embodiment, the formula for calculating the capacitor voltage reference value is:

[0103]

[0104] in, This is the reference value for capacitor voltage. This refers to the rated voltage of the submodule capacitors in the MMC. This is the per-unit correction amount for the reference value of the capacitor voltage in the MMC submodule.

[0105] In one embodiment, the formula for calculating the DC internal potential reference value is:

[0106]

[0107] in, This is the reference value for the DC internal potential. This is a DC voltage offset control quantity. This is the rated DC voltage of the MMC.

[0108] The modular multilevel converter submodule capacitor voltage control device provided in this invention is used to execute the modular multilevel converter submodule capacitor voltage control method provided in this invention. Its principle and the technical effects achieved are the same as those of the modular multilevel converter submodule capacitor voltage control method provided in this invention, and will not be repeated here.

[0109] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working processes of the systems, devices, and units described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0110] The above-described embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for controlling the capacitor voltage of a modular multilevel converter submodule, characterized in that, include: Based on the fundamental frequency reference voltage modulation ratio and fundamental frequency reference voltage phase angle of the MMC pulse control loop, as well as the per-unit value of the AC current amplitude, AC current phase angle, second harmonic circulating current amplitude coefficient and second harmonic circulating current phase angle of the MMC, calculate the per-unit value correction of the MMC submodule capacitor voltage reference value. Calculate the capacitor voltage reference value based on the per-unit correction of the MMC submodule capacitor voltage reference value and the rated value of the MMC submodule capacitor voltage. The average value of the capacitor voltage of all MMC submodules is collected, and the difference between the average value and the capacitor voltage reference value is sent to the proportional-integral controller to obtain the DC voltage offset control quantity. Calculate the reference value of DC internal potential based on the DC voltage offset control amount and the rated DC voltage of MMC; The DC internal potential reference value is sent to the MMC pulse control loop, and the voltage of the MMC submodule capacitor is controlled according to the pulse control signal output by the MMC pulse control loop.

2. The modular multilevel converter submodule capacitor voltage control method according to claim 1, characterized in that, The formula for calculating the per-unit correction of the MMC submodule capacitor voltage reference value is: in, This is the per-unit correction amount for the reference value of the capacitor voltage in the MMC submodule. Here, M is a preset constant, and M is the modulation ratio of the fundamental frequency reference voltage. This represents the per-unit value of the AC current amplitude of the MMC. The phase angle of the AC current in the MMC. The phase angle of the fundamental frequency reference voltage. The second harmonic circulating current amplitude coefficient. It is the phase angle of the second harmonic circulating current.

3. The modular multilevel converter submodule capacitor voltage control method according to claim 2, characterized in that, The calculation of the per-unit correction for the MMC submodule capacitor voltage reference value also includes: The preset constants are calculated based on the MMC's rated active power, the effective value of the valve-side AC line voltage of the MMC, the rated DC voltage of the MMC, the rated value of the submodule capacitor voltage of the MMC, the value of the submodule capacitor of the MMC, the number of cascaded bridge arm submodules of the MMC, and the fundamental angular frequency. The formula for calculating the preset constant is: in, This is the rated active power of the MMC. This represents the effective value of the AC line voltage on the valve side of the MMC. This is the rated DC voltage of the MMC. This refers to the rated voltage of the submodule capacitors in the MMC. Here, N represents the capacitance value of the MMC submodule, and N is the number of cascaded bridge arm submodules of the MMC. This is the fundamental angular frequency.

4. The modular multilevel converter submodule capacitor voltage control method according to claim 1, characterized in that, The formula for calculating the capacitor voltage reference value is: in, This is the reference value for capacitor voltage. This refers to the rated voltage of the submodule capacitors in the MMC. This is the per-unit correction amount for the reference value of the capacitor voltage in the MMC submodule.

5. The modular multilevel converter submodule capacitor voltage control method according to claim 1, characterized in that, The formula for calculating the DC internal potential reference value is: in, This is the reference value for the DC internal potential. This is a DC voltage offset control quantity. This is the rated DC voltage of the MMC.

6. A modular multilevel converter submodule capacitor voltage control device, characterized in that, include: The first calculation module is used to calculate the per-unit correction amount of the MMC submodule capacitor voltage reference value based on the modulation ratio and phase angle of the base frequency reference voltage of the MMC pulse control loop, as well as the per-unit value of the AC current amplitude, the phase angle of the AC current, the second harmonic circulating current amplitude coefficient and the second harmonic circulating current phase angle of the MMC. The second calculation module is used to calculate the capacitor voltage reference value based on the per-unit correction amount of the MMC submodule capacitor voltage reference value and the rated value of the MMC submodule capacitor voltage. The third calculation module is used to collect the average value of the capacitor voltage of all MMC sub-modules, and send the difference between the average value and the capacitor voltage reference value to the proportional-integral controller to obtain the DC voltage offset control quantity. The fourth calculation module is used to calculate the reference value of the DC internal potential based on the DC voltage offset control amount and the rated DC voltage of the MMC. The control module is used to send the DC internal potential reference value to the MMC pulse control loop, and control the capacitor voltage of the MMC submodule according to the pulse control signal output by the MMC pulse control loop.

7. The modular multilevel converter submodule capacitor voltage control device according to claim 6, characterized in that, The formula for calculating the per-unit correction of the MMC submodule capacitor voltage reference value is: in, This is the per-unit correction amount for the reference value of the capacitor voltage in the MMC submodule. Here, M is a preset constant, and M is the modulation ratio of the fundamental frequency reference voltage. This represents the per-unit value of the AC current amplitude of the MMC. The phase angle of the AC current in the MMC. The phase angle of the fundamental frequency reference voltage. The second harmonic circulating current amplitude coefficient. It is the phase angle of the second harmonic circulating current.

8. The modular multilevel converter submodule capacitor voltage control device according to claim 7, characterized in that, The first calculation module is also used for: The preset constants are calculated based on the MMC's rated active power, the effective value of the valve-side AC line voltage of the MMC, the rated DC voltage of the MMC, the rated value of the submodule capacitor voltage of the MMC, the value of the submodule capacitor of the MMC, the number of cascaded bridge arm submodules of the MMC, and the fundamental angular frequency. The formula for calculating the preset constant is: in, This is the rated active power of the MMC. This represents the effective value of the AC line voltage on the valve side of the MMC. This is the rated DC voltage of the MMC. This refers to the rated voltage of the submodule capacitors in the MMC. Here, N represents the capacitance value of the MMC submodule, and N is the number of cascaded bridge arm submodules of the MMC. This is the fundamental angular frequency.

9. The modular multilevel converter submodule capacitor voltage control device according to claim 6, characterized in that, The formula for calculating the capacitor voltage reference value is: in, This is the reference value for capacitor voltage. This refers to the rated voltage of the submodule capacitors in the MMC. This is the per-unit correction amount for the reference value of the capacitor voltage in the MMC submodule.

10. The modular multilevel converter submodule capacitor voltage control device according to claim 6, characterized in that, The formula for calculating the DC internal potential reference value is: in, This is the reference value for the DC internal potential. This is a DC voltage offset control quantity. This is the rated DC voltage of the MMC.

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