Method and device for suppressing low-frequency pulsation of capacitor voltage in MMC full-power frequency inverter

Abstract

This application provides a method and apparatus for suppressing low-frequency pulsation of capacitor voltage in a full-power MMC frequency converter. The method includes: when a variable-speed pumped storage unit is in variable-frequency start-up or braking mode, and the output frequency of the MMC frequency converter is within a predetermined range, generating a first voltage modulation wave component based on a predetermined high-frequency zero-sequence voltage and the three-phase output voltage of the MMC frequency converter; generating a second voltage modulation wave component based on a predetermined high-frequency bridge arm circulating current and the bridge arm circulating current of the MMC frequency converter; and generating a drive signal based on the first and second voltage modulation wave components. By injecting a high-frequency zero-sequence voltage into the three-phase output voltage and a high-frequency bridge arm circulating current into the bridge arm circulating current, capacitor voltage pulsation can be effectively suppressed.

Description

Technical Field

[0001] This application relates to the field of pumped storage technology, and in particular to a method and apparatus for suppressing low-frequency pulsation of capacitor voltage in an MMC full-power frequency converter. Background Technology

[0002] Variable-speed pumped storage units can optimize power generation efficiency, and the power absorbed under pumping conditions is adjustable, making them promising for a wide range of applications. Figure 1 The diagram illustrates a pumped-storage variable-speed generator unit based on a full-power frequency converter. The generator motor is a variable-speed motor, and its stator is connected to the power grid via the full-power frequency converter, achieving a flexible connection between the generator motor and the grid. When the generator motor speed changes, the frequency of its stator voltage also changes accordingly, while the grid frequency remains a stable power frequency. The full-power frequency converter is used to convert the alternating current with varying stator frequency into power frequency alternating current acceptable to the grid, simultaneously achieving bidirectional transmission of active and reactive power.

[0003] The back-to-back modular multilevel converter (MMC) based on half-bridge submodules is a high-performance full-power frequency converter. In variable-speed pumped storage units operating under variable-frequency start-up or braking conditions, the full-power frequency converter is required to output variable-frequency and variable-voltage AC power with a wide frequency range, typically around 0-power frequency (50Hz). When the AC output frequency of the MMC frequency converter is low, the capacitor voltage of its submodules will exhibit corresponding frequency pulsations. The pulsation amplitude is inversely proportional to the output frequency and capacitance value, and directly proportional to the load current. Therefore, effectively suppressing the capacitor voltage pulsations of the MMC frequency converter during variable-frequency start-up or braking is a problem that needs to be solved in this field. Summary of the Invention

[0004] In view of this, the purpose of this application is to provide a method and apparatus for suppressing low-frequency pulsation of capacitor voltage in a full-power MMC inverter, which can effectively suppress low-frequency pulsation of capacitor voltage.

[0005] Based on the above objectives, embodiments of this application provide a method for suppressing low-frequency ripple in the capacitor voltage of an MMC full-power inverter, including:

[0006] When the variable-speed pumped storage unit is in variable frequency start-up or braking condition, and the output frequency of the MMC inverter is within a predetermined range, a first voltage modulation wave component is generated based on the predetermined high-frequency zero-sequence voltage and the three-phase output voltage of the MMC inverter. A second voltage modulation wave component is generated based on the predetermined high-frequency bridge arm circulating current and the bridge arm circulating current of the MMC inverter. The high-frequency zero-sequence voltage is determined based on the phase output voltage amplitude, the bus DC voltage, a preset margin coefficient, and the rated operating frequency of the MMC inverter. The high-frequency bridge arm circulating current is determined based on the phase output current, the phase output voltage amplitude, the bus DC voltage, the margin coefficient, the initial phase of the fundamental voltage, and the rated operating frequency.

[0007] A driving signal is generated based on the first voltage modulation wave component and the second voltage modulation wave component.

[0008] Optionally, when the output frequency of the MMC inverter is within a predetermined range, a first voltage modulation wave component is generated based on a predetermined high-frequency zero-sequence voltage and the three-phase output voltage of the MMC inverter, including:

[0009] When the output frequency is less than a preset first low-frequency threshold, the high-frequency zero-sequence voltage is superimposed on the three-phase output voltage to obtain the first voltage modulation wave component.

[0010] Optionally, when the output frequency of the MMC inverter is within a predetermined range, a second voltage modulation wave component is generated based on a predetermined high-frequency bridge arm circulating current and the bridge arm circulating current of the MMC inverter, including:

[0011] When the output frequency is less than a preset second low-frequency threshold, the second voltage modulation wave component is obtained by adjusting the high-frequency bridge arm circulating current according to the bridge arm circulating current using a preset controller.

[0012] Optionally, when the output frequency of the MMC inverter is within a predetermined range, a second voltage modulation wave component is generated based on a predetermined high-frequency bridge arm circulating current and the bridge arm circulating current of the MMC inverter, further comprising:

[0013] When the output frequency is greater than the second low-frequency threshold and less than the first low-frequency threshold, the high-frequency bridge arm circulating current is attenuated according to the output frequency. After the controller adjusts the attenuated high-frequency bridge arm circulating current according to the bridge arm circulating current, the second voltage modulation wave component is obtained.

[0014] Optionally, attenuating the high-frequency bridge arm circulating current according to the output frequency includes:

[0015] The attenuation coefficient is determined based on the output frequency;

[0016] The high-frequency bridge arm circulating current is attenuated according to the attenuation coefficient.

[0017] Optionally, the controller is a dual proportional resonant controller; after adjusting the high-frequency bridge arm circulating current according to the bridge arm circulating current using a preset controller, the second voltage modulation wave component is obtained, including:

[0018] Calculate the difference between the high-frequency bridge arm circulating current and the bridge arm circulating current;

[0019] The difference is input into the dual proportional resonant controller, which then outputs the second voltage modulation wave component.

[0020] Optionally, the high-frequency zero-sequence voltage is determined according to formula (20):

[0021]

[0022] Where M is the ratio of the phase output voltage amplitude to the half-bus DC voltage, U dc K is the DC bus voltage. cm ω is the margin coefficient. cm The angular frequency is determined based on the rated operating frequency.

[0023] Optionally, the high-frequency bridge arm circulating current is determined according to formula (21):

[0024]

[0025] Where, θ x Let i be the initial phase of the output voltage of phase x. x Let x be the output current of phase x, where x = a, b, c, ω out It is the AC output angular frequency of the MMC frequency converter.

[0026] Optionally, the transfer function of the dual proportional resonant controller is:

[0027]

[0028] Among them, K p K is the proportional gain coefficient. r ω is the resonant gain coefficient. i To consider the resonant term bandwidth required by -3dB, s is the Laplace operator, ω out This refers to the AC output angular frequency of the MMC inverter.

[0029] This application embodiment also provides a low-frequency pulsation suppression device for capacitor voltage of MMC full-power frequency converter, including:

[0030] The modulation module is used to generate a first voltage modulation wave component based on a predetermined high-frequency zero-sequence voltage and the three-phase output voltage of the MMC inverter when the output frequency of the variable-speed pumped storage unit is in variable frequency start-up or braking condition, and to generate a second voltage modulation wave component based on a predetermined high-frequency bridge arm circulating current and the bridge arm circulating current of the MMC inverter. The high-frequency zero-sequence voltage is determined based on the phase output voltage amplitude, the bus DC voltage, a preset margin coefficient, and the rated operating frequency of the MMC inverter. The high-frequency bridge arm circulating current is determined based on the phase output current, the phase output voltage amplitude, the bus DC voltage, the margin coefficient, the initial phase of the fundamental voltage, and the rated operating frequency.

[0031] The driving module is used to generate a driving signal based on the first voltage modulation wave component and the second voltage modulation wave component.

[0032] As can be seen from the above, the method and apparatus for suppressing low-frequency pulsation of capacitor voltage in a full-power MMC inverter provided in this application, when the variable-speed pumped storage unit is in variable-frequency start-up or braking condition and the output frequency of the MMC inverter is low, generates a first voltage modulation wave component based on a predetermined high-frequency zero-sequence voltage and the three-phase output voltage of the MMC inverter, and generates a second voltage modulation wave component based on a predetermined high-frequency bridge arm circulating current and the bridge arm circulating current of the MMC inverter; a drive signal is generated based on the first and second voltage modulation wave components. By injecting a high-frequency zero-sequence voltage into the three-phase output voltage and a high-frequency bridge arm circulating current into the bridge arm circulating current, capacitor voltage pulsation can be effectively suppressed. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of this application 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 embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 This is a schematic diagram of the grid connection structure of the variable speed generator unit according to an embodiment of this application;

[0035] Figure 2 This is a schematic diagram of the method flow of an embodiment of this application;

[0036] Figure 3 This is a schematic diagram of the topology of the MMC frequency converter and its sub-modules according to an embodiment of this application;

[0037] Figure 4 This is a schematic diagram of the control model according to an embodiment of this application;

[0038] Figure 5 This is a schematic diagram of the enable signal in an embodiment of this application;

[0039] Figure 6 This is a schematic diagram illustrating the relationship between the attenuation coefficient and the output frequency in an embodiment of this application.

[0040] Figure 7 This is a schematic diagram of the frequency response curve of the dual proportional resonant controller according to an embodiment of this application;

[0041] Figure 8 This is a schematic diagram of the simulated capacitor voltage waveform in an embodiment of this application;

[0042] Figure 9 This is a block diagram of the device structure according to an embodiment of this application;

[0043] Figure 10 This is a block diagram of the electronic device structure according to an embodiment of this application. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of this disclosure clearer, the following detailed description is provided in conjunction with specific embodiments and the accompanying drawings.

[0045] It should be noted that, unless otherwise defined, the technical or scientific terms used in the embodiments of this application should have the ordinary meaning understood by one of ordinary skill in the art to which this disclosure pertains. The terms "first," "second," and similar terms used in the embodiments of this application do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed after the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are only used to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0046] like Figure 2 As shown in the figure, this application provides a method for suppressing low-frequency ripple in capacitor voltage of an MMC full-power frequency converter, applied to a back-to-back modular multilevel full-power frequency converter based on a half-bridge submodule (hereinafter referred to as MMC frequency converter). The method includes:

[0047] S201: When the variable speed pumped storage unit is in the variable frequency start-up or braking condition, when the output frequency of the MMC inverter is within the predetermined range, a first voltage modulation wave component is generated according to the predetermined high-frequency zero-sequence voltage and the three-phase output voltage of the MMC inverter, and a second voltage modulation wave component is generated according to the predetermined high-frequency bridge arm circulating current and the bridge arm circulating current of the MMC inverter.

[0048] S202: Generate a driving signal based on the first voltage modulation wave component and the second voltage modulation wave component.

[0049] like Figure 1-3 As shown, in this embodiment, the stator of the variable-speed pumped storage unit is connected to the power grid via an MMC frequency converter, which converts the stator AC power into grid-acceptable AC power. The MMC frequency converter is a symmetrical structure composed of multiple sub-modules, which adopt a half-bridge structure. When the variable-speed pumped storage unit is in variable-frequency start-up or braking mode, and the output frequency of the MMC frequency converter is low, the capacitor voltage of its sub-modules will exhibit corresponding frequency pulsations. To suppress the low-frequency pulsations of the capacitor voltage, when the output frequency (frequency of output voltage and output current) of the MMC frequency converter is detected to be within a predetermined range, a first voltage modulation wave component is generated based on the pre-generated high-frequency zero-sequence voltage and the three-phase output voltage of the MMC frequency converter. Simultaneously, a second voltage modulation wave component is generated based on the pre-generated high-frequency bridge arm circulating current and the bridge arm circulating current of the MMC frequency converter. Based on the first and second voltage modulation wave components, a drive signal for driving the machine-side converter of the MMC frequency converter is generated. In this way, when the output frequency of the MMC inverter is low, the injection of high-frequency zero-sequence voltage and high-frequency bridge arm circulating current can effectively suppress low-frequency pulsation of capacitor voltage and ensure system stability.

[0050] In some embodiments, when the output frequency of the MMC inverter is within a predetermined range, a first voltage modulation wave component is generated based on a predetermined high-frequency zero-sequence voltage and the three-phase output voltage of the MMC inverter, including:

[0051] When the output frequency of the MMC inverter is less than the preset first low-frequency threshold, the high-frequency zero-sequence voltage is superimposed on the three-phase output voltage to obtain the first voltage modulation wave component. That is, when the output frequency of the MMC inverter is lower than the first low-frequency threshold, capacitor voltage pulsation is likely to occur. By injecting the generated high-frequency zero-sequence voltage into the three-phase output voltage, the first voltage modulation wave component used to suppress capacitor voltage pulsation is obtained.

[0052] In some embodiments, when the output frequency of the MMC inverter is within a predetermined range, a second voltage modulation wave component is generated based on a predetermined high-frequency bridge arm circulating current and the bridge arm circulating current of the MMC inverter, including:

[0053] When the output frequency is less than the preset second low-frequency threshold, the high-frequency bridge arm circulating current is adjusted by the preset controller according to the bridge arm circulating current to obtain the second voltage modulation wave component; or,

[0054] When the output frequency is greater than the second low-frequency threshold and less than the first low-frequency threshold, the high-frequency bridge arm circulating current is attenuated according to the output frequency. After the controller adjusts the attenuated high-frequency bridge arm circulating current according to the bridge arm circulating current, the second voltage modulation wave component is obtained.

[0055] In this embodiment, when the output frequency of the MMC inverter is lower than the second low-frequency threshold, the difference between the generated high-frequency bridge arm circulating current and the real-time detected bridge arm circulating current of the MMC inverter is input to the controller, which outputs a second voltage modulation wave component to suppress capacitor voltage pulsation. The second low-frequency threshold is less than the first low-frequency threshold. When the output frequency of the MMC inverter is between the second and first low-frequency thresholds, the high-frequency bridge arm circulating current is first attenuated, and then the difference between the attenuated high-frequency bridge arm circulating current and the real-time detected bridge arm circulating current is input to the controller, which outputs a second voltage modulation wave component to suppress capacitor voltage pulsation.

[0056] In some methods, the timing of injecting high-frequency zero-sequence voltage can be controlled by an enable signal, and the timing of injecting high-frequency bridge arm circulating current can be controlled by an enable signal and an attenuation coefficient.

[0057] Combination Figure 5 As shown in Equation (1), the enable signal Ena can be expressed as:

[0058]

[0059] According to formula (1), when the output frequency f of the MMC inverter out When the frequency is greater than or equal to 0 and less than or equal to the first low-frequency threshold, a high-frequency zero-sequence voltage is injected into the three-phase output voltage. Optionally, the first low-frequency threshold can be set to 15Hz.

[0060] Combination Figure 6 As shown in formula (2), the attenuation signal K ih It can be represented as:

[0061]

[0062] According to formula (2), when the output frequency of the MMC inverter is greater than or equal to 0 and less than the second low-frequency threshold, the high-frequency bridge arm circulating current is directly injected into the bridge arm circulating current of the MMC inverter; when the output frequency is greater than or equal to the second low-frequency threshold and less than the first low-frequency threshold, the high-frequency bridge arm circulating current is injected into the bridge arm circulating current after being attenuated according to the attenuation coefficient; when the output frequency is greater than the first low-frequency threshold, there is no need to inject the high-frequency bridge arm circulating current. The attenuation coefficient is determined based on the output frequency, and the attenuation coefficient has a linear attenuation relationship with the output frequency; the high-frequency bridge arm circulating current decreases linearly with the increase of the output frequency. Optionally, the second low-frequency threshold can be set to 10Hz.

[0063] In some embodiments, the controller employs a dual proportional resonant controller; after adjusting the high-frequency bridge arm circulating current according to the bridge arm circulating current using a preset controller, a second voltage modulation wave component is obtained, including:

[0064] Calculate the difference between the high-frequency bridge arm circulating current and the bridge arm circulating current;

[0065] The difference is input into the dual proportional resonant controller, which then outputs the second modulated wave component.

[0066] In this embodiment, a dual proportional resonant controller is used to perform precise closed-loop control of the high-frequency arm circulating current. When the output frequency of the MMC inverter is lower than the second low-frequency threshold, the difference between the high-frequency arm circulating current and the actual arm circulating current of the inverter is calculated, and then the difference is input to the dual proportional resonant controller, which outputs the corresponding second voltage modulation wave component. When the output frequency is between the second low-frequency threshold and the first low-frequency threshold, the difference between the attenuated high-frequency arm circulating current and the actual arm circulating current is calculated, and this difference is input to the dual proportional resonant controller, which outputs the corresponding second voltage modulation wave component.

[0067] The high-frequency bridge arm circulating current has an output fundamental frequency component i x and high-frequency component sin(ω) cm The product term of t), therefore, ω exists in the high-frequency bridge arm circulation. cm ±ω out Two main frequency components. Combined Figure 7 As shown, the dual proportional resonant controller can provide high gain at two frequency points, enabling accurate tracking of two frequency components.

[0068] In some methods, the transfer function of the dual proportional resonant controller in the s-domain is:

[0069]

[0070] Among them, K p K is the proportional gain coefficient. r ω is the resonant gain coefficient. iTo consider the resonant term bandwidth required by -3dB, s is the Laplace operator, ω cm ω is the angular frequency of the high-frequency bridge arm circulating current. out This refers to the output angular frequency of the MMC inverter.

[0071] The following detailed description of the method for determining high-frequency zero-sequence voltage and high-frequency bridge arm circulating current, with reference to specific embodiments, provides a detailed explanation.

[0072] Combination Figure 3 The circuit topology diagram of the MMC full-power frequency converter shown is given, with DC bus voltage U. dc SM is a cascaded half-bridge sub-module, L0 is the bridge arm inductance, and R0 is the bridge arm equivalent series resistance. The voltage at the midpoint of the DC bus between the a, b, and c phase voltage output terminals of the MMC inverter is u. x (x = a, b, c) can be represented as:

[0073]

[0074] Among them, U x Let x be the output voltage amplitude of phase x, and θ be the initial phase.

[0075] The output current of the MMC frequency converter is i x , represented as:

[0076]

[0077] Among them, I x Let x be the amplitude of the output current of phase x.

[0078] Define u as the sum of the port voltages of all submodules in the upper bridge arm. px The sum of the port voltages of all submodules in the lower bridge arm is u. nx Ignoring the voltage drop across the bridge arm inductance, we get:

[0079]

[0080] Define the upper arm current as i px The lower bridge arm current is i nx The current flowing through the upper and lower arms of the same phase simultaneously is called the arm circulating current i. zx Then we have:

[0081]

[0082] Based on the instantaneous power balance of the input and output of each phase of the MMC frequency converter, the bridge arm circulating current can also be expressed as:

[0083] i zx =u x i x / u dc (8)

[0084] According to Kirchhoff's current law, the output current of each phase can also be obtained from the difference between the currents of the upper and lower bridge arms, expressed as:

[0085] i x =i px -i nx (9)

[0086] The high-frequency zero-sequence voltage u cm The high-frequency bridge arm circulating current i is superimposed on the three-phase output voltage of the MMC frequency converter. cmx The frequency of both the high-frequency zero-sequence voltage and the high-frequency bridge arm circulating current injected into phase x is f. cm The angular frequencies are all ω cm The bridge arm voltage is obtained as follows:

[0087]

[0088] The currents of the upper and lower bridge arms are:

[0089]

[0090] The instantaneous power of the upper arm is the product of the arm voltage and the arm circulating current, expressed as:

[0091] p px =u px i px =(U dc / 2-u x -u cm (i) zx +i x / 2+i cmx ) = p x_cm +p x_dm (12)

[0092] The instantaneous power of the lower arm is the product of the arm voltage and the arm circulating current, expressed as:

[0093] p nx =u nx i nx =(U dc / 2+u x +u cm (i) zx -i x / 2+i cmx ) = p x_cm -p x_dm (13)

[0094] in,

[0095]

[0096]

[0097] According to formula (14), the p included in the instantaneous power of the upper and lower bridge arms x_cm The first item High-frequency bridge arm circulation i containing high-frequency components cmx The second item High-frequency zero-sequence voltage u containing high-frequency components cm According to formula (15), the p included in the instantaneous power of the upper and lower bridge arms x_dm In the terms, the first term is the low-frequency component, and the second term is u. cm i cmx High-frequency bridge arm circulation i containing high-frequency components cmx and high-frequency zero-sequence voltage u cm The third item u x i cmx High-frequency bridge arm circulation i containing high-frequency components cmx , fourth item High-frequency zero-sequence voltage u containing high-frequency components cm For the low-frequency component of the instantaneous power of the bridge arm (which generates the low-frequency pulsating component of the capacitor voltage), the high-frequency bridge arm circulating current and high-frequency zero-sequence voltage can be used to suppress and cancel it, thereby achieving the effect of suppressing the low-frequency pulsation of the capacitor voltage.

[0098] To determine the high-frequency bridge arm circulating current and high-frequency zero-sequence voltage that can suppress low-frequency pulsations, we first express the high-frequency zero-sequence voltage and high-frequency bridge arm circulating current as follows, based on the property that multiplying two sinusoidal functions of the same frequency produces zero-order and second-harmonic components:

[0099]

[0100] The second term in formula (15) can be expressed as:

[0101]

[0102] To cancel out the low-frequency component shown in the first term of formula (15), the following must be satisfied:

[0103]

[0104] Based on formulas (17) and (18), formula (15) can be simplified to:

[0105]

[0106] According to formula (19), the pulsating power on the bridge arm can be modulated from the low frequency band to the high frequency band.

[0107] For high-frequency zero-sequence voltage and high-frequency bridge arm circulating current, the following constraints need to be met:

[0108] 1) The same high-frequency zero-sequence voltage is superimposed on the three-phase output voltage command, and the high-frequency bridge arm circulating current of each phase is superimposed on the corresponding phase bridge arm circulating current command.

[0109] 2) To reduce the current stress on the bridge arm, the circulating current of the high-frequency bridge arm should be as small as possible, and the high-frequency zero-sequence voltage should be as large as possible;

[0110] 3) The high-frequency zero-sequence voltage should not cause overmodulation in the upper and lower bridge arms. At the fundamental output voltage u... x Injecting high-frequency zero-sequence voltage u cm The amplitude of the subsequent phase voltage should not exceed U. dc / 2, the high-frequency zero-sequence voltage should decrease as the modulation ratio increases;

[0111] 4) Considering factors such as dead zone, sampling error, and quantization error, it is difficult to completely cancel out low-frequency pulsations. In practical applications, the high-frequency zero-sequence voltage should have a certain margin.

[0112] Based on the above analysis, the high-frequency zero-sequence voltage and the high-frequency bridge arm circulating current are determined, and are expressed as follows:

[0113]

[0114]

[0115] Among them, K cm The set margin coefficient has a value range of 1.0 to 1.2; θ x Let f be the initial phase of the fundamental output voltage of phase x; considering that the capacity of the full-power inverter of the variable-speed pumped-storage unit is usually large, the system switching frequency will not be too high, and the frequency f of the high-frequency zero-sequence voltage and the high-frequency bridge arm circulating current is... cm f can be taken cm =6f n f n The rated operating frequency of the MMC frequency converter is ω. cm According to frequency f cm Sure.

[0116] M is the modulation ratio, defined as the ratio of the amplitude of the phase output voltage to the DC voltage of the half-bus. It changes with the amplitude of the fundamental frequency of the output voltage, and is expressed as:

[0117]

[0118] Combination Figure 4 As shown, when the output frequency of the MMC inverter is lower than the first low-frequency threshold, the high-frequency zero-sequence voltage u cm Enable and inject three-phase output voltage u xref_oThat is, by superimposing the same high-frequency zero-sequence voltage onto the three-phase output voltages a, b, and c respectively, the first voltage modulation component u is obtained. xref_1 , represented as:

[0119] u xref_1 =u xref_o +u cm (twenty three)

[0120] When the output frequency of the MMC inverter is lower than the second low-frequency threshold, the high-frequency bridge arm current i of phase x will be... cmx Enable, calculate the high-frequency bridge arm current i cmx Bridge arm circulation i in phase x zx The difference between them is input into a dual proportional resonant controller, which outputs the second voltage modulation wave component u. xref_2 .

[0121] When the output frequency of the MMC inverter is between the second low-frequency threshold and the first low-frequency threshold, the high-frequency bridge arm current i of phase x will be... cmx Enable the circuit and, after attenuation by the attenuation coefficient, calculate the attenuated high-frequency bridge arm current i. cmx Bridge arm circulation i in phase x zx The difference between them is input into a dual proportional resonant controller, which outputs the second voltage modulation wave component u. xref_2 .

[0122] After obtaining the first voltage modulation component and the second voltage modulation component, a drive signal for driving the frequency converter is generated based on the first voltage modulation component and the second voltage modulation component, so that the MMC frequency converter operating according to the drive signal can effectively suppress low-frequency pulsation of capacitor voltage.

[0123] like Figure 8 As shown, a simulation experiment of the capacitor voltage was conducted according to the method provided in this application. When the output frequency of the MMC inverter is 5Hz, a high-frequency zero-sequence voltage is injected into the three-phase output voltage, and a high-frequency bridge arm circulating current is injected into the bridge arm circulating current to suppress low-frequency pulsation of the capacitor voltage. At t=15s, the injection of the high-frequency zero-sequence voltage and the high-frequency bridge arm circulating current is stopped. After 1s, the high-frequency zero-sequence voltage and the high-frequency bridge arm circulating current are injected again. The experimental results show that when the output frequency is below a certain threshold, the injection of high-frequency zero-sequence voltage and high-frequency bridge arm circulating current can effectively suppress low-frequency pulsation of the capacitor voltage. After stopping the injection, the capacitor voltage pulsation increases, with an amplitude of 13%. After re-injection, the low-frequency pulsation converges rapidly. This indicates that the high-frequency injection method provided in this application can effectively suppress low-frequency pulsation of the capacitor voltage.

[0124] It should be noted that the method in this embodiment can be executed by a single device, such as a computer or server. The method can also be applied in a distributed scenario, where multiple devices cooperate to complete the task. In such a distributed scenario, one of these devices may execute only one or more steps of the method in this embodiment, and the multiple devices will interact with each other to complete the method described.

[0125] It should be noted that the above description describes specific embodiments of this specification. Other embodiments are within the scope of the appended claims. In some cases, the actions or steps recorded in the claims may be performed in a different order than that shown in the embodiments and still achieve the desired results. Furthermore, the processes depicted in the drawings do not necessarily require the specific or sequential order shown to achieve the desired results. In some embodiments, multitasking and parallel processing are also possible or may be advantageous.

[0126] like Figure 9 As shown in the illustration, this application also provides a low-frequency ripple suppression device for capacitor voltage in a full-power MMC inverter, comprising:

[0127] The modulation module is used to generate a first voltage modulation wave component based on a predetermined high-frequency zero-sequence voltage and the three-phase output voltage of the MMC inverter when the output frequency of the variable-speed pumped storage unit is in variable frequency start-up or braking condition, and to generate a second voltage modulation wave component based on a predetermined high-frequency bridge arm circulating current and the bridge arm circulating current of the MMC inverter. The high-frequency zero-sequence voltage is determined based on the phase output voltage amplitude, the bus DC voltage, a preset margin coefficient, and the rated operating frequency of the MMC inverter. The high-frequency bridge arm circulating current is determined based on the phase output current, the phase output voltage amplitude, the bus DC voltage, the margin coefficient, the initial phase of the fundamental voltage, and the rated operating frequency.

[0128] The driving module is used to generate a driving signal based on the first voltage modulation wave component and the second voltage modulation wave component.

[0129] For ease of description, the above devices are described in terms of function, divided into various modules. Of course, in implementing the embodiments of this application, the functions of each module can be implemented in one or more software and / or hardware.

[0130] The apparatus described above is used to implement the corresponding methods in the foregoing embodiments and has the beneficial effects of the corresponding method embodiments, which will not be repeated here.

[0131] Figure 10This embodiment illustrates a more specific hardware structure of an electronic device, which may include a processor 1010, a memory 1020, an input / output interface 1030, a communication interface 1040, and a bus 1050. The processor 1010, memory 1020, input / output interface 1030, and communication interface 1040 are interconnected internally via the bus 1050.

[0132] The processor 1010 can be implemented using a general-purpose CPU (Central Processing Unit), microprocessor, application-specific integrated circuit (ASIC), or one or more integrated circuits, and is used to execute relevant programs to implement the technical solutions provided in the embodiments of this specification.

[0133] The memory 1020 can be implemented in the form of ROM (Read Only Memory), RAM (Random Access Memory), static storage device, dynamic storage device, etc. The memory 1020 can store the operating system and other applications. When the technical solutions provided in the embodiments of this specification are implemented by software or firmware, the relevant program code is stored in the memory 1020 and is called and executed by the processor 1010.

[0134] The input / output interface 1030 is used to connect input / output modules to realize information input and output. Input / output modules can be configured as components within the device (not shown in the figure) or externally connected to the device to provide corresponding functions. Input devices may include keyboards, mice, touchscreens, microphones, various sensors, etc., while output devices may include displays, speakers, vibrators, indicator lights, etc.

[0135] The communication interface 1040 is used to connect a communication module (not shown in the figure) to enable communication between this device and other devices. The communication module can communicate via wired means (such as USB, Ethernet cable, etc.) or wireless means (such as mobile network, WIFI, Bluetooth, etc.).

[0136] Bus 1050 includes a pathway for transmitting information between various components of the device, such as processor 1010, memory 1020, input / output interface 1030, and communication interface 1040.

[0137] It should be noted that although the above-described device only shows the processor 1010, memory 1020, input / output interface 1030, communication interface 1040, and bus 1050, in specific implementations, the device may also include other components necessary for normal operation. Furthermore, those skilled in the art will understand that the above-described device may only include the components necessary for implementing the embodiments of this specification, and not necessarily all the components shown in the figures.

[0138] The electronic devices described above are used to implement the corresponding methods in the foregoing embodiments and have the beneficial effects of the corresponding method embodiments, which will not be repeated here.

[0139] The computer-readable medium of this embodiment includes permanent and non-permanent, removable and non-removable media, and information storage can be implemented by any method or technology. Information can be computer-readable instructions, data structures, program modules, or other data. Examples of computer storage media include, but are not limited to, phase-change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), other types of random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technologies, CD-ROM, digital versatile optical disc (DVD) or other optical storage, magnetic tape, magnetic magnetic disk storage or other magnetic storage devices, or any other non-transfer medium that can be used to store information accessible by a computing device.

[0140] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of this disclosure (including the claims) is limited to these examples; within the framework of this disclosure, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of different aspects of the embodiments of this application as described above, which are not provided in the details for the sake of brevity.

[0141] Additionally, to simplify the description and discussion, and to avoid obscuring the embodiments of this application, the well-known power / ground connections to integrated circuit (IC) chips and other components may or may not be shown in the provided drawings. Furthermore, the apparatus may be shown in block diagram form to avoid obscuring the embodiments of this application, and this also takes into account the fact that the details of the implementation of these block diagram apparatuses are highly dependent on the platform on which the embodiments of this application will be implemented (i.e., these details should be fully understood by those skilled in the art). While specific details (e.g., circuits) have been set forth to describe exemplary embodiments of this disclosure, it will be apparent to those skilled in the art that the embodiments of this application can be implemented without these specific details or with variations thereof. Therefore, these descriptions should be considered illustrative rather than restrictive.

[0142] Although this disclosure has been described in conjunction with specific embodiments thereof, many substitutions, modifications, and variations of these embodiments will be apparent to those skilled in the art from the foregoing description. For example, other memory architectures (e.g., dynamic RAM (DRAM)) may be used with the embodiments discussed.

[0143] The embodiments of this application are intended to cover all such substitutions, modifications, and variations that fall within the broad scope of the appended claims. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the embodiments of this application should be included within the protection scope of this disclosure.

Claims

1. A method for suppressing low-frequency pulsation of capacitor voltage in MMC full-power frequency converter, characterized in that, The method comprises the following steps: When the output frequency of the MMC frequency converter is in a predetermined range, a first voltage modulation wave component is generated according to a predetermined high-frequency zero-sequence voltage and three-phase output voltages of the MMC frequency converter, including: when the output frequency is less than a preset first low-frequency threshold, the high-frequency zero-sequence voltage is superimposed on the three-phase output voltages respectively to obtain the first voltage modulation wave component; A second voltage modulation wave component is generated according to a predetermined high-frequency bridge arm circulating current and a bridge arm circulating current of the MMC frequency converter, including: when the output frequency is less than a preset second low-frequency threshold, the second voltage modulation wave component is obtained by adjusting the high-frequency bridge arm circulating current according to the bridge arm circulating current by using a preset controller; Wherein, the high-frequency zero-sequence voltage is determined according to a phase output voltage amplitude, a bus DC voltage, a preset margin coefficient and a rated working frequency of the MMC frequency converter; the high-frequency bridge arm circulating current is determined according to a phase output current, the phase output voltage amplitude, the bus DC voltage, the margin coefficient, an initial phase of a fundamental voltage and the rated working frequency; A driving signal is generated according to the first voltage modulation wave component and the second voltage modulation wave component; The high-frequency zero-sequence voltage is determined according to formula (20): （20） wherein M is the ratio of the amplitude of the phase output voltage to the DC bus voltage, U dc is the DC bus voltage, K cm is a margin coefficient, ω cm is an angular frequency determined from the rated operating frequency; The high-frequency bridge arm circulating current is determined according to formula (21): （21） wherein θ x is x the initial phase of the output voltage of the phase, i x is x the output current of the phase, x = a , b , c , ω out is the angular frequency of the AC output of the MMC converter.

2. The method of claim 1, wherein, When the output frequency of the MMC frequency converter is in a predetermined range, a second voltage modulation wave component is generated according to a predetermined high-frequency bridge arm circulating current and a bridge arm circulating current of the MMC frequency converter, further comprising: When the output frequency is greater than the second low-frequency threshold and less than the first low-frequency threshold, the high-frequency bridge arm circulating current is attenuated according to the output frequency, and the second voltage modulation wave component is obtained by adjusting the attenuated high-frequency bridge arm circulating current according to the bridge arm circulating current by using the controller.

3. The method of claim 2, wherein, Attenuating the high-frequency bridge arm circulating current according to the output frequency comprises: Determining an attenuation coefficient according to the output frequency; Attenuating the high-frequency bridge arm circulating current according to the attenuation coefficient.

4. The method of claim 1, wherein, The controller is a double proportional-resonant controller; The second voltage modulation wave component is obtained by adjusting the high-frequency bridge arm circulating current according to the bridge arm circulating current by using a preset controller, including: Calculating a difference between the high-frequency bridge arm circulating current and the bridge arm circulating current; Inputting the difference into the double proportional-resonant controller, and outputting the second voltage modulation wave component from the double proportional-resonant controller.

5. The method of claim 4, wherein, The transfer function of the double proportional-resonant controller is: （3） wherein, K p is a proportional gain coefficient, K r is a resonant gain coefficient, ω i is a resonant term bandwidth considering the -3dB requirement, s is a Laplace operator, is an AC output angular frequency of the MMC converter.

6. The device for suppressing low-frequency pulsation of capacitor voltage of MMC full-power frequency converter, characterized in that, The method comprises the following steps: A modulation module is configured to generate a first voltage modulation wave component according to a predetermined high-frequency zero-sequence voltage and three-phase output voltages of the MMC frequency converter when the output frequency of the MMC frequency converter is in a predetermined range under the variable-speed pumped storage unit in the variable-frequency starting or braking working condition, including: when the output frequency is less than a preset first low-frequency threshold, the high-frequency zero-sequence voltage is superimposed on the three-phase output voltages respectively to obtain the first voltage modulation wave component; The second voltage modulation wave component is generated according to a predetermined high-frequency bridge arm circulating current and a bridge arm circulating current of the MMC frequency converter, and comprises: when the output frequency is less than a preset second low-frequency threshold, the second voltage modulation wave component is obtained by adjusting the high-frequency bridge arm circulating current according to the bridge arm circulating current by using a preset controller. The high-frequency zero-sequence voltage is determined according to a phase output voltage amplitude, a bus DC voltage, a preset margin coefficient and a rated working frequency of the MMC frequency converter; and the high-frequency bridge arm circulating current is determined according to a phase output current, the phase output voltage amplitude, the bus DC voltage, the margin coefficient, an initial phase of a fundamental voltage and the rated working frequency. The drive module is configured to generate a drive signal according to the first voltage modulation wave component and the second voltage modulation wave component. The high-frequency zero-sequence voltage is determined according to formula (20): （20） wherein M is a ratio of the amplitude of the phase output voltage to the DC bus voltage, U dc is a DC bus voltage, K cm is a margin coefficient, ω cm is an angular frequency determined from the rated operating frequency; The high-frequency bridge arm circulating current is determined according to formula (21): （21） wherein θ x is x the initial phase of the output voltage of the phase, i x is x the output current of the phase, x = a , b , c , ω out is the angular frequency of the AC output of the MMC converter.

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