Modulation method and device of MMC topology structure of flexible DC transmission system

Abstract

The invention discloses a modulation method and device for an MMC topology structure of a flexible direct current transmission system, and relates to the technical field of power electronics. The technical scheme thereof mainly comprises the following points: dividing the flexible direct current transmission system into a shaping circuit and a two-level converter, and establishing a mathematical model of the flexible direct current transmission system working in an inverter state; calculating the conduction time of the upper and lower bridge arms of the two-level converter under asymmetric square wave modulation according to the mathematical model of the flexible direct current transmission system, and determining the range of the voltage modulation parameter of the two-level converter according to the conduction time of the upper and lower bridge arms; determining the modulation wave of the shaping circuit according to the mathematical model of the flexible direct current transmission system, determining the range of the three-phase zero-sequence voltage component according to the modulation wave and the number of submodules of the shaping circuit, obtaining the modulation signal of the submodule of the shaping circuit by using carrier pulse width modulation within the range of the three-phase zero-sequence voltage component, and outputting a wide range of sinusoidal alternating current voltage on the alternating current side of the shaping circuit.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and more specifically, to a modulation method and device for an MMC topology structure of a flexible direct current transmission system. Background Art

[0002] With the rapid development of large-scale offshore wind power technology, modular multilevel converter (MMC), as a key equipment of flexible DC transmission system, has been widely studied and applied.

[0003] The hybrid cascade multilevel converter studied in the prior art can be used as a lightweight hybrid MMC topology. The AC side voltage is shared by the two-level converter and the shaping circuit. The number of sub-modules required by the shaping circuit is only 1 / 4 of the number of sub-modules in the traditional MMC topology when the number of sub-modules is the minimum. The number of sub-modules is significantly reduced, and the device cost is also reduced accordingly, meeting the lightweight requirements of MMC technology.

[0004] As for the hybrid cascade multilevel converter studied in the prior art, the modulation strategy of the existing MMC topology structure can only enable the flexible DC transmission system to achieve stable operation of a single voltage, which increases the difficulty of building a flexible DC transmission system for offshore wind power technology and does not meet the development requirements of lightweight offshore wind power. Summary of the invention

[0005] In order to solve the problem that the modulation strategy of the existing MMC topology structure can only enable the flexible direct current transmission system to achieve stable operation of a single voltage, the present invention aims to provide a modulation method and device for the MMC topology structure of the flexible direct current transmission system. The present invention proposes a hybrid modulation strategy of a two-level converter and a shaping circuit, that is, a mathematical model of the flexible direct current transmission system working in an inverter state is established, and the range of the voltage modulation parameter of the two-level converter and the modulation wave of the shaping circuit and the number of sub-modules of the shaping circuit are determined according to the conduction time of the upper and lower bridge arms to determine the range of the three-phase zero-sequence voltage component, and the modulation signal of the sub-module of the shaping circuit is obtained by carrier pulse width modulation within the range of the three-phase zero-sequence voltage component, so that the shaping circuit of the flexible direct current transmission system is coordinated with the two-level converter, and a wide range of sinusoidal alternating current voltage is output on the alternating current side, thereby realizing the stable operation of the flexible direct current transmission system with a wide range of voltages, and meeting the development requirements of lightweight offshore wind power.

[0006] The above technical objectives of the present invention are achieved through the following technical solutions:

[0007] In a first aspect of the present application, a modulation method for an MMC topology structure of a flexible direct current transmission system is provided, the method comprising:

[0008] The flexible DC transmission system is divided into a shaping circuit and a two-level converter, and a mathematical model of the flexible DC transmission system working in the inverter state is established;

[0009] The conduction time of the upper and lower bridge arms of the two-level converter under asymmetric square wave modulation is calculated according to the mathematical model of the flexible DC transmission system, and the range of the voltage modulation parameter of the two-level converter is determined according to the conduction time of the upper and lower bridge arms;

[0010] The modulation wave of the shaping circuit is determined according to the mathematical model of the flexible DC transmission system, the range of the three-phase zero-sequence voltage component is determined according to the modulation wave and the number of sub-modules of the shaping circuit, and the modulation signal of the sub-module of the shaping circuit is obtained by using carrier pulse width modulation within the range of the three-phase zero-sequence voltage component, so that the shaping circuit of the flexible DC transmission system is coordinated with the two-level converter, and a wide range of sinusoidal AC voltage is output on the AC side of the shaping circuit.

[0011] In one implementation, the flexible DC transmission system is divided into a shaping circuit and a two-level converter, and a mathematical model of the flexible DC transmission system working in an inverter state is established, specifically including:

[0012] Connecting the flexible DC transmission system to a three-phase balanced system, determining the switching functions of the upper and lower bridge arms, determining the phase switching function according to the switching functions of the upper and lower bridge arms, and determining the first mathematical model of the two-level converter according to the phase switching function and the DC side voltage;

[0013] Calculating a second mathematical model of the shaping circuit according to the number of submodules of the shaping circuit, the switching functions of the submodules, and the capacitor voltages of the submodules;

[0014] Based on the first mathematical model of the two-level converter and the second mathematical model of the shaping circuit, the mathematical model of the flexible direct current transmission system is obtained according to Kirchhoff's voltage and current laws.

[0015] In one implementation, the conduction time of the upper and lower bridge arms of the two-level converter under asymmetric square wave modulation is calculated according to the mathematical model of the flexible direct current transmission system, and the voltage modulation parameters of the two-level converter are determined according to the conduction time of the upper and lower bridge arms, specifically:

[0016] The charge and discharge time of the submodule of the shaping circuit is controlled by using the zero-sequence component of the three-phase voltage to determine the on time of the upper and lower bridge arms and the off time of the upper and lower bridge arms;

[0017] The relationship between the duty cycle of the upper and lower bridge arms and the zero-sequence component of the three-phase voltage is determined according to the ratio of the continuous conduction time of the upper bridge arm and the fundamental frequency cycle time; when the zero-sequence component of the three-phase voltage is greater than or equal to zero, the continuous conduction time is the off-time minus the on-time; when the zero-sequence component of the three-phase voltage is less than zero, the continuous conduction time is the off-time minus the on-time plus the fundamental frequency cycle time;

[0018] The voltage modulation parameters are calculated according to the AC side phase voltage amplitude and the DC side voltage. According to the capacitor voltage balance of the submodule of the shaping circuit, the energy exchange between the shaping circuit and the external circuit is required to be zero within one cycle, thereby obtaining the constraints of the voltage modulation parameters and the duty cycle.

[0019] Substitute the duty cycle of the upper and lower bridge arms into the constraints to determine the range of the voltage modulation parameters.

[0020] In one implementation, the relationship between the duty cycle of the upper and lower bridge arms and the zero-sequence component of the three-phase voltage is: Among them, V0 represents the zero-sequence component of the three-phase voltage, U m Indicates the AC side phase voltage amplitude;

[0021] The relationship between the voltage modulation parameters and the duty cycle constraints is: Where τ represents the duty cycle, k u Represents the voltage modulation parameter.

[0022] In one implementation, the modulation wave of the shaping circuit is determined according to the mathematical model of the flexible DC transmission system, specifically:

[0023] The mathematical model of the flexible DC transmission system is expressed as: Among them, v a (t) is the output voltage on the AC side, S a (t) represents the phase switching function, U dc Indicates the DC side output voltage, u wa (t) represents the sum of the submodule voltages input by the shaping circuit;

[0024] According to the mathematical model of the flexible DC transmission system, the modulation wave expression of the shaping circuit is determined as follows:

[0025] Among them, u wa(ref) (t) represents the modulated wave of the sum of the submodule voltages input by the shaping circuit.

[0026] In a second aspect of the present application, a modulation device for an MMC topology structure of a flexible direct current transmission system is provided, the device comprising:

[0027] A mathematical model building module is used to divide the flexible DC transmission system into a shaping circuit and a two-level converter, and to build a mathematical model of the flexible DC transmission system working in an inverter state;

[0028] The first parameter adjustment module is used to calculate the conduction time of the upper and lower bridge arms of the two-level converter under asymmetric square wave modulation according to the mathematical model of the flexible direct current transmission system, and determine the range of the voltage modulation parameter of the two-level converter according to the conduction time of the upper and lower bridge arms;

[0029] The second parameter adjustment module is used to determine the modulation wave of the shaping circuit according to the mathematical model of the flexible direct current transmission system, determine the range of the three-phase zero-sequence voltage component according to the modulation wave and the number of sub-modules of the shaping circuit, and obtain the modulation signal of the sub-module of the shaping circuit by using carrier pulse width modulation within the range of the three-phase zero-sequence voltage component, so that the shaping circuit of the flexible direct current transmission system is coordinated with the two-level converter, and a wide range of sinusoidal alternating current voltage is output on the alternating current side of the shaping circuit.

[0030] In one embodiment, the mathematical model building module is further used to:

[0031] Connecting the flexible DC transmission system to a three-phase balanced system, determining the switching functions of the upper and lower bridge arms, determining the phase switching function according to the switching functions of the upper and lower bridge arms, and determining the first mathematical model of the two-level converter according to the phase switching function and the DC side voltage;

[0032] Calculating a second mathematical model of the shaping circuit according to the number of submodules of the shaping circuit, the switching functions of the submodules, and the capacitor voltages of the submodules;

[0033] Based on the first mathematical model of the two-level converter and the second mathematical model of the shaping circuit, the mathematical model of the flexible direct current transmission system is obtained according to Kirchhoff's voltage and current laws.

[0034] In one implementation, the first parameter adjustment module is further configured to:

[0035] The charge and discharge time of the submodule of the shaping circuit is controlled by using the zero-sequence component of the three-phase voltage to determine the on time of the upper and lower bridge arms and the off time of the upper and lower bridge arms;

[0036] The relationship between the duty cycle of the upper and lower bridge arms and the zero-sequence component of the three-phase voltage is determined according to the ratio of the continuous conduction time of the upper bridge arm and the fundamental frequency cycle time; when the zero-sequence component of the three-phase voltage is greater than or equal to zero, the continuous conduction time is the off-time minus the on-time; when the zero-sequence component of the three-phase voltage is less than zero, the continuous conduction time is the off-time minus the on-time plus the fundamental frequency cycle time;

[0037] The voltage modulation parameters are calculated according to the AC side phase voltage amplitude and the DC side voltage. According to the capacitor voltage balance of the submodule of the shaping circuit, the energy exchange between the shaping circuit and the external circuit is required to be zero within one cycle, thereby obtaining the constraints of the voltage modulation parameters and the duty cycle.

[0038] Substitute the duty cycle of the upper and lower bridge arms into the constraints to determine the range of the voltage modulation parameters.

[0039] In one implementation, the relationship between the duty cycle of the upper and lower bridge arms and the zero-sequence component of the three-phase voltage is: Among them, V0 represents the zero-sequence component of the three-phase voltage, U m Indicates the AC side phase voltage amplitude;

[0040] The relationship between the voltage modulation parameters and the duty cycle constraints is: Where τ represents the duty cycle, k u Represents the voltage modulation parameter.

[0041] In one implementation, the second parameter adjustment module is further configured to:

[0042] The mathematical model of the flexible DC transmission system is expressed as: Among them, v a (t) is the output voltage on the AC side, S a (t) represents the phase switching function, U dc Indicates the DC side output voltage, u wa (t) represents the sum of the submodule voltages input by the shaping circuit;

[0043] According to the mathematical model of the flexible DC transmission system, the modulation wave expression of the shaping circuit is determined as follows:

[0044] Among them, u wa(ref) (t) represents the modulated wave of the sum of the submodule voltages input by the shaping circuit.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] The present invention proposes a hybrid modulation strategy of a two-level converter and a shaping circuit, that is, a mathematical model of a flexible direct current transmission system working in an inverter state is established, the range of the voltage modulation parameters of the two-level converter and the modulation wave of the shaping circuit and the number of sub-modules of the shaping circuit are determined according to the conduction time of the upper and lower bridge arms to determine the range of the three-phase zero-sequence voltage component, and the modulation signal of the sub-module of the shaping circuit is obtained by using carrier pulse width modulation within the range of the three-phase zero-sequence voltage component, so that the shaping circuit of the flexible direct current transmission system is coordinated with the two-level converter, and a wide range of sinusoidal alternating current voltage is output on the AC side of the shaping circuit, thereby realizing the stable operation of the flexible direct current transmission system with a wide range of voltages and meeting the development requirements of lightweight offshore wind power. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0048] Figure 1 A schematic flow chart of a modulation method for an MMC topology structure of a flexible direct current transmission system provided in an embodiment of the present application;

[0049] Figure 2 A main circuit topology diagram of the flexible direct current transmission system provided in an embodiment of the present application;

[0050] Figure 3 A schematic diagram of asymmetric square wave modulation provided in an embodiment of the present application;

[0051] Figure 4 A schematic diagram of carrier phase-shift pulse width modulation provided in an embodiment of the present application;

[0052] Figure 5 A schematic diagram of control signals of a single submodule of a shaping circuit provided in an embodiment of the present application;

[0053] Figure 6 A functional block diagram of a modulation device of an MMC topology structure of a flexible direct current transmission system provided in an embodiment of the present application. DETAILED DESCRIPTION

[0054] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0055] It should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly indicate the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0056] Embodiment 1

[0057] It should be understood that the modulation method of the MMC topology structure of the flexible DC transmission system provided in the embodiment of the present application, wherein the MMC topology structure of the flexible DC transmission system is the prior art, and its main circuit structure is as follows Figure 2 As shown, this is the prior art, and the embodiment of the present application does not make any redundant description on the MMC topology structure of the flexible DC transmission system. Figure 2 The FSM part in the figure represents the full-bridge submodule of the shaping circuit, which can also be called a submodule. Figure 2 DS1-DS6 are all conduction switches, and are all composed of IGBTs. The sub-modules and conduction switches are all existing technologies and no unnecessary description is given.

[0058] Please refer to Figure 1 , Figure 1 A modulation method for an MMC topology structure of a flexible direct current transmission system provided in an embodiment of the present application is as follows: Figure 1 As shown, the method comprises the following steps:

[0059] S110, dividing the flexible direct current transmission system into a shaping circuit and a two-level converter, and establishing a mathematical model of the flexible direct current transmission system working in an inverter state.

[0060] In this embodiment, the flexible DC transmission system is divided into a shaping circuit and a two-level converter. Figure 2 The main circuit topology of the flexible DC transmission system shown in FIG. 1 is divided into two parts: a two-level converter and a shaping circuit. The two-level converter guides the current to flow through the upper and lower bridge arms, outputs a two-level waveform, and bears part of the voltage, reducing the number of full-bridge submodules. The shaping circuit works as a series active filter, shaping the two-level waveform output by the two-level converter by adding and removing the series submodules, and outputs a desired sinusoidal AC waveform on the AC side.

[0061] As a specific embodiment, the flexible direct current transmission system is connected to a three-phase balanced system, the switching functions of the upper and lower bridge arms are determined, the phase switching function is determined according to the switching functions of the upper and lower bridge arms, and the first mathematical model of the two-level converter is determined according to the phase switching function and the DC side voltage;

[0062] Calculating a second mathematical model of the shaping circuit according to the number of submodules of the shaping circuit, the switching functions of the submodules, and the capacitor voltages of the submodules;

[0063] Based on the first mathematical model of the two-level converter and the second mathematical model of the shaping circuit, the mathematical model of the flexible direct current transmission system is obtained according to Kirchhoff's voltage and current laws.

[0064] It should be understood that the structures of the upper bridge arms of the B-phase unit and the C-phase unit are the same as those of the upper bridge arms of the A-phase unit, and the current paths are also similar. We will not go into details here. For example, taking the A-phase as an example, we will mathematically model the flexible DC transmission system working in the inverter working state. The flexible DC transmission system is connected to the three-phase balanced system. The parameters of each sub-module of the shaping circuit are exactly the same, and only the fundamental wave output component of the system is considered. For the two-level converter, it is known that the switch states of the upper and lower bridge arms of each phase are complementary, and S is defined. aj is the switching function of the upper and lower bridge arms of phase a, and it is known that:

[0065] Among them, 1 means that all switch tubes are turned on, and 0 means that all switch tubes are turned off; P and N represent the upper and lower bridge arms respectively.

[0066] Since the upper and lower bridge arms cannot be turned on together, the following equation (2) exists:

[0067] S aP +S aN =1 (2)

[0068] After finding the upper and lower bridge arm switching functions, define the A phase switching function S a As shown in formula (3):

[0069] S a =S aP -S aN (3), S a 1 means the upper bridge arm switch of phase A is turned on, S a -1 means that the lower bridge arm switch of phase A is turned on, and the output voltage U aO (t) are as follows: That is the first mathematical model, where U dc is the DC side voltage.

[0070] Voltage u of phase A shaping circuit wa (t) can be expressed as the sum of the voltages of the input submodules, that is, (5), i.e., the second mathematical model, where n is the number of submodules in phase A, S ai(H) represents the switching function of the i-th submodule of phase A, S ai(H) =1 (positive input state), S ai(H) =0 (cut-off state), Sai(H) =-1 (negative input state); U ca(i) is the capacitor voltage of the i-th submodule of the A-phase shaping circuit.

[0071] The mathematical models of the shaping circuit and the two-level converter have been derived above. The mathematical model of the flexible DC transmission system is determined according to Kirchhoff's voltage and current laws as shown in equations (6)-(8):

[0072] v a (t) = U m sin(ωt) (6);

[0073]

[0074]

[0075] Among them, v a (t) is the output voltage on the AC side, U m is the AC side phase voltage amplitude, i a (t) is the output current on the AC side, I m is the AC phase current amplitude, ω is the synchronous angular frequency, is the angle between voltage and current.

[0076] S120, calculating the conduction time of the upper and lower bridge arms of the two-level converter under asymmetric square wave modulation according to the mathematical model of the flexible direct current transmission system, and determining the range of the voltage modulation parameter of the two-level converter according to the conduction time of the upper and lower bridge arms.

[0077] In this embodiment, according to the mathematical model of the flexible DC transmission system described in the above embodiment, the conduction time of the upper and lower bridge arms of the two-level converter of the flexible DC transmission system under the asymmetric square wave modulation strategy and the voltage modulation parameter range of the asymmetric modulation strategy are determined. This embodiment adopts an asymmetrical square wave modulation (ASWM) scheme, and its working principle is as follows: Figure 3 As shown, by introducing the three-phase voltage zero-sequence component V0, the energy exchange balance of the shaping circuit is guaranteed, and the variability of the output voltage range on the AC side is met under the same DC voltage level.

[0078] S130, determining the modulation wave of the shaping circuit according to the mathematical model of the flexible DC transmission system, determining the range of the three-phase zero-sequence voltage component according to the modulation wave and the number of sub-modules of the shaping circuit, and obtaining the modulation signal of the sub-module of the shaping circuit by using carrier pulse width modulation within the range of the three-phase zero-sequence voltage component, so that the shaping circuit of the flexible DC transmission system cooperates with the two-level converter, and outputs a wide range of sinusoidal AC voltage on the AC side of the shaping circuit.

[0079] In this embodiment, according to formula (8) described in the above embodiment, the modulation wave expression of the shaping circuit is obtained as follows: And the maximum value of the modulation wave is

[0080] To avoid the two-level converter from entering an overmodulation state (i.e., the amplitude modulation ratio k c >1), derive the range of the zero-sequence voltage control component according to the number of submodules n.

[0081] First, define the amplitude modulation ratio k c equal

[0082] When the number of submodules is large enough, that is, n satisfies equation (28), the value range of the three-phase zero-sequence voltage component V0 is determined by equation (29), as shown below:

[0083]

[0084]

[0085] When n is small, that is, n satisfies formula (30), the range of V0 is determined by formula (31).

[0086]

[0087]

[0088] According to the reference value u of the modulation wave of the shaping circuit wa(ref) (t), the modulation signal of each submodule is obtained by using the carrier phase-shift pulse width modulation strategy, so that the shaping circuit can complete the coordination with the two-level converter, and the AC end outputs a sinusoidal AC voltage.

[0089] The principle of carrier phase shift pulse width modulation is as follows Figure 5 As shown in the figure, for a flexible DC transmission system converter with n submodules per phase, n bipolar triangular carriers V are required, which are spaced π / n electrical degrees apart from each other. cr(i) (t) is compared with the modulation wave, and the trigger signal obtained by comparison corresponds to the switch state of each submodule, where each triangular carrier has the same frequency f cr and amplitude nU C Because a full-bridge module can output three levels, namely (-U C ,0,U C), the output voltages of n submodules are superimposed to obtain 2n+1 level step waves. After obtaining the modulation signal of each submodule, the modulation signal of each switch tube (T1, T2, T3, T4) of the submodule is derived. For a single submodule, the carrier signals of the left and right bridge arms differ by 180 electrical degrees, and in order to avoid the switch tube of the same bridge arm of the submodule being directly turned on, the trigger signals of T1 and T3 are inverted, and the trigger signals of T2 and T4 are inverted. According to Figure 5 The control signal schematic diagram of a single submodule of the shaping circuit can realize the inversion of the trigger signals of T1 and T3 and the inversion of the trigger signals of T2 and T4, so that each submodule of the final shaping circuit has four pulse signals to control the conduction of the four switch tubes respectively.

[0090] In summary, the modulation method of the MMC topology structure of the flexible direct current transmission system provided in this embodiment proposes a hybrid modulation strategy of a two-level converter and a shaping circuit, that is, a mathematical model of a flexible direct current transmission system working in an inverter state is established, and the range of the voltage modulation parameter of the two-level converter and the modulation wave of the shaping circuit and the number of sub-modules of the shaping circuit are determined according to the conduction time of the upper and lower bridge arms to determine the range of the three-phase zero-sequence voltage component, and the modulation signal of the sub-module of the shaping circuit is obtained by using carrier pulse width modulation within the range of the three-phase zero-sequence voltage component, so that the shaping circuit of the flexible direct current transmission system is coordinated with the two-level converter, and a wide range of sinusoidal alternating current voltage is output on the alternating current side, thereby realizing the stable operation of a wide range of voltage of the flexible direct current transmission system and meeting the development requirements of lightweight offshore wind power.

[0091] In one implementation, the conduction time of the upper and lower bridge arms of the two-level converter under asymmetric square wave modulation is calculated according to the mathematical model of the flexible direct current transmission system, and the voltage modulation parameters of the two-level converter are determined according to the conduction time of the upper and lower bridge arms, specifically:

[0092] The charge and discharge time of the submodule of the shaping circuit is controlled by using the zero-sequence component of the three-phase voltage to determine the on time of the upper and lower bridge arms and the off time of the upper and lower bridge arms;

[0093] The relationship between the duty cycle of the upper and lower bridge arms and the zero-sequence component of the three-phase voltage is determined according to the ratio of the continuous conduction time of the upper bridge arm and the fundamental frequency cycle time; when the zero-sequence component of the three-phase voltage is greater than or equal to zero, the continuous conduction time is the off-time minus the on-time; when the zero-sequence component of the three-phase voltage is less than zero, the continuous conduction time is the off-time minus the on-time plus the fundamental frequency cycle time;

[0094] The voltage modulation parameters are calculated according to the AC side phase voltage amplitude and the DC side voltage. According to the capacitor voltage balance of the submodule of the shaping circuit, the energy exchange between the shaping circuit and the external circuit is required to be zero within one cycle, thereby obtaining the constraints of the voltage modulation parameters and the duty cycle.

[0095] Substitute the duty cycle of the upper and lower bridge arms into the constraints to determine the range of the voltage modulation parameters.

[0096] Specifically, it should be understood that the structures of the upper bridge arms of the B-phase unit and the C-phase unit are the same as those of the upper bridge arms of the A-phase unit, and the current paths are also similar, which will not be described here. For example, taking the A-phase of the three-phase circuit as an example, its phase switching function is determined by the following formula (9). Wherein, sgn(x) is a sign function, and V0 controls the charging and discharging time of the shaping circuit submodule to ensure that its energy exchange with the external circuit is zero within one cycle.

[0097] S a (t) = sgn(v a (t)-V0)(9);

[0098] According to formula (9), the on time t1 of the upper bridge arm of phase A and the off time t2 of the lower bridge arm can be obtained.

[0099]

[0100]

[0101] The upper arm (e.g. Figure 2 DS1) continuous conduction time T C The ratio to the fundamental frequency cycle time T is defined as the duty cycle τ.

[0102] in

[0103]

[0104]

[0105] According to the upper arm (e.g. Figure 2 DS1) continuous conduction time T C The relationship between the on-time t1 of the upper bridge arm of phase A and the off-time t2 of the lower bridge arm is obtained by substituting equations (10) and (11) into equation (12). The relationship between the duty cycle of the upper and lower bridge arms and the zero-sequence component of the three-phase voltage is as follows:

[0106] In order to maintain the stability of the DC voltage, at least one upper bridge arm switch group and one lower bridge arm switch group are in the on state at any time for the two-level converter part, that is, the following equations (15) and (16) are satisfied. Where τ a , τ b and τ c is the corresponding duty cycle.

[0107] τ a +τ b +τ c≥1 (15)

[0108] (1-τ a )+(1-τ b )+(1-τ c )≥1 (16)

[0109] Due to the rotational symmetry of the flexible DC transmission system, the duty cycles of the three phases are equal in steady-state operation. According to formulas (15) and (16), the range of τ can be obtained as follows:

[0110] When 1 / 3≤τ<1 / 2, V0 is positive; when 1 / 2≤τ<2 / 3, V0 is negative; when τ=1 / 2, V0 is 0. The voltage modulation parameter k u Defined as:

[0111] according to Figure 2 For the main circuit topology shown, the instantaneous power flowing into the shaping circuit is:

[0112]

[0113] Since the active power flowing into the converter is:

[0114] Substituting formula (20) into formula (19) to determine, the instantaneous power flowing into the shaping circuit is rewritten as the following formula (21):

[0115]

[0116] When V0≥0 or when V0<0, the energy exchange between the shaping circuit and the external circuit is equal. Combining the two cases, the energy exchange expression between the shaping circuit and the external environment in one cycle is:

[0117] To ensure the capacitor voltage balance of the submodules of the shaping circuit, it is required that the energy exchange between the shaping circuit and the external circuit is 0 within one cycle, and W wa =0, the relationship between the voltage modulation parameters and the duty cycle constraints is obtained as follows: Where τ represents the duty cycle, k u Represents the voltage modulation parameter.

[0118] And according to the range of τ, k is obtained u The range is:

[0119] As those skilled in the art can understand, when the voltage modulation parameters are known, the value of the corresponding duty cycle τ can be calculated, and the value of the three-phase zero-sequence voltage component V0 can be inferred, and finally the conduction time of the upper and lower bridge arms can be obtained, thereby obtaining the modulation signal of the two-level converter.

[0120] As a specific embodiment, the modulation wave of the shaping circuit is determined according to the mathematical model of the flexible DC transmission system. Specifically, the mathematical model of the flexible DC transmission system is expressed as: Among them, v a (t) is the output voltage on the AC side, S a (t) represents the phase switching function, U dc Indicates the DC side output voltage, u wa (t) represents the sum of the submodule voltages input by the shaping circuit;

[0121] According to the mathematical model of the flexible DC transmission system, the modulation wave expression of the shaping circuit is determined as follows:

[0122] Among them, u wa(ref) (t) represents the modulated wave of the sum of the submodule voltages input by the shaping circuit.

[0123] Embodiment 2

[0124] Embodiment 2 of the present application provides a modulation device of an MMC topology structure of a flexible DC power transmission system on the basis of Embodiment 1. It should be noted that the modulation device provided in Embodiment 2 is based on the same inventive concept of the modulation method of Embodiment 1. Since the principles of solving the problems of these devices are the same as those of the present invention, Figure 1 A method shown is similar, so the implementation of these devices can refer to Figure 1 The embodiments of the method shown are not described in detail herein.

[0125] Please refer to Figure 6 , Figure 6 A functional block diagram of a modulation device of an MMC topology structure of a flexible DC transmission system provided in an embodiment of the present application is shown in FIG. Figure 6 As shown, the device comprises:

[0126] A mathematical model building module 610 is used to divide the flexible DC power transmission system into a shaping circuit and a two-level converter, and to build a mathematical model of the flexible DC power transmission system working in an inverter state;

[0127] A first parameter adjustment module 620, configured to calculate the conduction time of the upper and lower bridge arms of the two-level converter under asymmetric square wave modulation according to the mathematical model of the flexible DC transmission system, and determine the range of the voltage modulation parameter of the two-level converter according to the conduction time of the upper and lower bridge arms;

[0128] The second parameter adjustment module 630 is used to determine the modulation wave of the shaping circuit according to the mathematical model of the flexible direct current transmission system, determine the range of the three-phase zero-sequence voltage component according to the modulation wave and the number of sub-modules of the shaping circuit, and obtain the modulation signal of the sub-module of the shaping circuit by using carrier pulse width modulation within the range of the three-phase zero-sequence voltage component, so that the shaping circuit of the flexible direct current transmission system is coordinated with the two-level converter, and a wide range of sinusoidal AC voltage is output on the AC side of the shaping circuit.

[0129] It can be seen that the modulation device provided in the above embodiment proposes a hybrid modulation strategy of a two-level converter and a shaping circuit, that is, a mathematical model of a flexible direct current transmission system working in an inverter state is established, and the range of the voltage modulation parameter of the two-level converter and the modulation wave of the shaping circuit and the number of sub-modules of the shaping circuit are determined according to the conduction time of the upper and lower bridge arms to determine the range of the three-phase zero-sequence voltage component, and the modulation signal of the sub-module of the shaping circuit is obtained by using carrier pulse width modulation within the range of the three-phase zero-sequence voltage component, so that the shaping circuit of the flexible direct current transmission system is coordinated with the two-level converter, and a wide range of sinusoidal alternating current voltage is output on the AC side, thereby realizing the stable operation of a wide range of voltage of the flexible direct current transmission system and meeting the development requirements of lightweight offshore wind power.

[0130] In one embodiment, the mathematical model building module is further used to:

[0131] Connecting the flexible DC transmission system to a three-phase balanced system, determining the switching functions of the upper and lower bridge arms, determining the phase switching function according to the switching functions of the upper and lower bridge arms, and determining the first mathematical model of the two-level converter according to the phase switching function and the DC side voltage;

[0132] Calculating a second mathematical model of the shaping circuit according to the number of submodules of the shaping circuit, the switching functions of the submodules, and the capacitor voltages of the submodules;

[0133] Based on the first mathematical model of the two-level converter and the second mathematical model of the shaping circuit, the mathematical model of the flexible direct current transmission system is obtained according to Kirchhoff's voltage and current laws.

[0134] In one implementation, the first parameter adjustment module is further configured to:

[0135] The charge and discharge time of the submodule of the shaping circuit is controlled by using the zero-sequence component of the three-phase voltage to determine the on time of the upper and lower bridge arms and the off time of the upper and lower bridge arms;

[0136] The relationship between the duty cycle of the upper and lower bridge arms and the zero-sequence component of the three-phase voltage is determined according to the ratio of the continuous conduction time of the upper bridge arm and the fundamental frequency cycle time; when the zero-sequence component of the three-phase voltage is greater than or equal to zero, the continuous conduction time is the off-time minus the on-time; when the zero-sequence component of the three-phase voltage is less than zero, the continuous conduction time is the off-time minus the on-time plus the fundamental frequency cycle time;

[0137] The voltage modulation parameters are calculated according to the AC side phase voltage amplitude and the DC side voltage. According to the capacitor voltage balance of the submodule of the shaping circuit, the energy exchange between the shaping circuit and the external circuit is required to be zero within one cycle, thereby obtaining the constraints of the voltage modulation parameters and the duty cycle.

[0138] Substitute the duty cycle of the upper and lower bridge arms into the constraints to determine the range of the voltage modulation parameters.

[0139] In one implementation, the relationship between the duty cycle of the upper and lower bridge arms and the zero-sequence component of the three-phase voltage is: Among them, V0 represents the zero-sequence component of the three-phase voltage, U m Indicates the AC side phase voltage amplitude;

[0140] The relationship between the voltage modulation parameters and the duty cycle constraints is: Where τ represents the duty cycle, k u Represents the voltage modulation parameter.

[0141] In one implementation, the second parameter adjustment module is further configured to:

[0142] The mathematical model of the flexible DC transmission system is expressed as: Among them, v a (t) is the output voltage on the AC side, S a (t) represents the phase switching function, U dc Indicates the DC side output voltage, u wa (t) represents the sum of the submodule voltages input by the shaping circuit;

[0143] According to the mathematical model of the flexible DC transmission system, the modulation wave expression of the shaping circuit is determined as follows:

[0144] Among them, u wa ( ref )(t) represents the modulation wave of the sum of the sub-module voltages input by the shaping circuit.

[0145] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A modulation method for an MMC topology structure of a flexible direct current transmission system, characterized in that: Methods include: The flexible direct current transmission system is divided into a shaping circuit and a two-level converter, and a mathematical model of the flexible direct current transmission system working in the inverter state is established; wherein, the mathematical model of the flexible direct current transmission system working in the inverter state is established, specifically including: connecting the flexible direct current transmission system to a three-phase balanced system, determining the switching functions of the upper and lower bridge arms, determining the phase switching function according to the switching functions of the upper and lower bridge arms, and determining the first mathematical model of the two-level converter according to the phase switching function and the DC side voltage; calculating the second mathematical model of the shaping circuit according to the number of sub-modules of the shaping circuit, the switching function of the sub-module, and the capacitor voltage of the sub-module; based on the first mathematical model of the two-level converter and the second mathematical model of the shaping circuit, the mathematical model of the flexible direct current transmission system is obtained by Kirchhoff's voltage and current laws; wherein, the expression of the mathematical model of the flexible direct current transmission system is: Among them, v a (t) is the output voltage on the AC side, S a (t) represents the phase switching function, U dc Indicates the DC side output voltage, u wa (t) represents the sum of the submodule voltages input by the shaping circuit; The conduction time of the upper and lower bridge arms of the two-level converter under asymmetric square wave modulation is calculated according to the mathematical model of the flexible DC transmission system, and the range of the voltage modulation parameter of the two-level converter is determined according to the conduction time of the upper and lower bridge arms; The modulation wave of the shaping circuit is determined according to the mathematical model of the flexible DC transmission system, the range of the three-phase zero-sequence voltage component is determined according to the modulation wave and the number of sub-modules of the shaping circuit, and the modulation signal of the sub-module of the shaping circuit is obtained by using carrier pulse width modulation within the range of the three-phase zero-sequence voltage component, so that the shaping circuit of the flexible DC transmission system is coordinated with the two-level converter, and a wide range of sinusoidal AC voltage is output on the AC side of the shaping circuit; wherein, the expression of the modulation wave of the shaping circuit is: Among them, u wa(ref) (t) represents the modulated wave of the sum of the submodule voltages input by the shaping circuit.

2. The method according to claim 1, characterized in that According to the mathematical model of the flexible DC transmission system, the conduction time of the upper and lower bridge arms of the two-level converter under asymmetric square wave modulation is calculated, and the voltage modulation parameters of the two-level converter are determined according to the conduction time of the upper and lower bridge arms, which are specifically: The charge and discharge time of the submodule of the shaping circuit is controlled by using the zero-sequence component of the three-phase voltage to determine the on time of the upper and lower bridge arms and the off time of the upper and lower bridge arms; The relationship between the duty cycle of the upper and lower bridge arms and the zero-sequence component of the three-phase voltage is determined according to the ratio of the continuous conduction time of the upper bridge arm and the fundamental frequency cycle time; when the zero-sequence component of the three-phase voltage is greater than or equal to zero, the continuous conduction time is the off-time minus the on-time; when the zero-sequence component of the three-phase voltage is less than zero, the continuous conduction time is the off-time minus the on-time plus the fundamental frequency cycle time; The voltage modulation parameters are calculated according to the AC side phase voltage amplitude and the DC side voltage. According to the capacitor voltage balance of the submodule of the shaping circuit, the energy exchange between the shaping circuit and the external circuit is required to be zero within one cycle, thereby obtaining the constraints of the voltage modulation parameters and the duty cycle. Substitute the duty cycle of the upper and lower bridge arms into the constraints to determine the range of the voltage modulation parameters.

3. The method according to claim 2, characterized in that The relationship between the duty cycle of the upper and lower bridge arms and the zero-sequence component of the three-phase voltage is: Among them, V0 represents the zero-sequence component of the three-phase voltage, U m Indicates the AC side phase voltage amplitude; The relationship between the voltage modulation parameters and the duty cycle constraints is: Where τ represents the duty cycle, k u Represents the voltage modulation parameter.

4. A modulation device of an MMC topology structure of a flexible direct current transmission system, characterized in that: The device includes: A mathematical model building module is used to divide the flexible direct current transmission system into a shaping circuit and a two-level converter, and to establish a mathematical model of the flexible direct current transmission system working in an inverter state; wherein, the mathematical model of the flexible direct current transmission system working in an inverter state is established, specifically including: connecting the flexible direct current transmission system to a three-phase balanced system, determining the switching functions of the upper and lower bridge arms, determining the phase switching function according to the switching functions of the upper and lower bridge arms, and determining the first mathematical model of the two-level converter according to the phase switching function and the DC side voltage; calculating the second mathematical model of the shaping circuit according to the number of sub-modules of the shaping circuit, the switching function of the sub-module, and the capacitor voltage of the sub-module; based on the first mathematical model of the two-level converter and the second mathematical model of the shaping circuit, the mathematical model of the flexible direct current transmission system is obtained by Kirchhoff's voltage and current laws; wherein, the expression of the mathematical model of the flexible direct current transmission system is: Among them, v a (t) is the output voltage on the AC side, S a (t) represents the phase switching function, U dc Indicates the DC side output voltage, u wa (t) represents the sum of the submodule voltages input by the shaping circuit; The first parameter adjustment module is used to calculate the conduction time of the upper and lower bridge arms of the two-level converter under asymmetric square wave modulation according to the mathematical model of the flexible direct current transmission system, and determine the range of the voltage modulation parameter of the two-level converter according to the conduction time of the upper and lower bridge arms; The second parameter adjustment module is used to determine the modulation wave of the shaping circuit according to the mathematical model of the flexible direct current transmission system, determine the range of the three-phase zero-sequence voltage component according to the modulation wave and the number of sub-modules of the shaping circuit, and obtain the modulation signal of the sub-module of the shaping circuit by using carrier pulse width modulation within the range of the three-phase zero-sequence voltage component, so that the shaping circuit of the flexible direct current transmission system is coordinated with the two-level converter, and a wide range of sinusoidal alternating current voltage is output on the alternating current side of the shaping circuit; wherein, the expression of the modulation wave of the shaping circuit is: Among them, u wa(ref) (t) represents the modulated wave of the sum of the submodule voltages input by the shaping circuit.

5. The device according to claim 4, characterized in that The first parameter adjustment module is further used for: The charge and discharge time of the submodule of the shaping circuit is controlled by using the zero-sequence component of the three-phase voltage to determine the on time of the upper and lower bridge arms and the off time of the upper and lower bridge arms; The relationship between the duty cycle of the upper and lower bridge arms and the zero-sequence component of the three-phase voltage is determined according to the ratio of the continuous conduction time of the upper bridge arm and the fundamental frequency cycle time; when the zero-sequence component of the three-phase voltage is greater than or equal to zero, the continuous conduction time is the off-time minus the on-time; when the zero-sequence component of the three-phase voltage is less than zero, the continuous conduction time is the off-time minus the on-time plus the fundamental frequency cycle time; The voltage modulation parameters are calculated according to the AC side phase voltage amplitude and the DC side voltage. According to the capacitor voltage balance of the submodule of the shaping circuit, the energy exchange between the shaping circuit and the external circuit is required to be zero within one cycle, thereby obtaining the constraints of the voltage modulation parameters and the duty cycle. Substitute the duty cycle of the upper and lower bridge arms into the constraints to determine the range of the voltage modulation parameters.

6. The device according to claim 5, characterized in that The relationship between the duty cycle of the upper and lower bridge arms and the zero-sequence component of the three-phase voltage is: Among them, V0 represents the zero-sequence component of the three-phase voltage, U m Indicates the AC side phase voltage amplitude; The relationship between the voltage modulation parameters and the duty cycle constraints is: Where τ represents the duty cycle, k u Represents the voltage modulation parameter.

Images