A parameter adjustment method for reducing current stress of enneagonal MMC

Abstract

A parameter adjustment method for reducing current stress in a 9-sided multi-layer circuit (MMC) system. The 9-sided MMC consists of three groups of bridge arms, each containing three arms, for a total of 3×3 arms. These nine arms are connected sequentially to form a 9-sided ring structure. Three AC ports are derived from the nine vertices of the ring structure. The first AC port, corresponding to vertices X, Y, and Z, is connected to the AC grid; the second AC port, corresponding to vertices R, S, and T, is connected to 50 / 3Hz low-frequency system 1; and the third AC port, corresponding to vertices U, V, and W, is connected to 50 / 3Hz low-frequency system 2. This method effectively reduces the magnitude of the stress of the integrated current periodic component in the 9-sided MMC bridge arms, thereby reducing energy loss, improving the economic efficiency of the topology, and enhancing the competitiveness of the 9-sided MMC in offshore wind power frequency-divided transmission systems.

Description

Technical Field

[0001] The present invention belongs to the field of power electronics, and in particular relates to a parameter adjustment method for reducing current stress of a nonagonal MMC. Background Art

[0002] To address the environmental pollution caused by the scarcity of traditional fossil energy and its use, actively searching for new energy sources that can replace traditional fossil energy has become a focus of attention around the world. Offshore wind power resources are abundant, do not occupy land, and have little environmental pollution, and have broad development prospects. Fractional frequency transmission system (FFTS), as a method of offshore wind power transmission, has the advantages of long transmission distance and low operation and maintenance costs, which helps wind farms to develop in deep sea. As the core device of fractional frequency transmission, the converter needs to realize the interconnection between the low-frequency system and the industrial frequency system. Therefore, studying the converter topology used in the fractional frequency transmission system is of great significance to the development of offshore wind power.

[0003] Traditional multi-port converters are typically integrated by connecting multiple single-input, single-output converters in series or parallel, sharing an AC or DC bus. This type of converter integration technology is currently mature and has improved efficiency, increased system compactness, and reduced economic costs to a certain extent. However, to further improve efficiency, reduce costs, and increase compactness, converter structures that integrate components or bridge arms are also an important approach and have been studied in the literature.

[0004] Scholars have proposed a three-port enneagonal modular multilevel converter (MMC). This topology uses nine bridge arms to directly convert energy across three AC ports without requiring a high-voltage DC bus. The enneagonal MMC offers high compactness and excellent low-frequency performance, making it suitable for high-voltage, high-power applications. However, the enneagonal MMC's reused bridge arm structure results in excessive stress on each arm, negating its significant advantage in device count compared to the M3C. This also reduces control freedom, making stress optimization impossible using traditional methods. Summary of the Invention

[0005] The purpose of the present invention is to provide a parameter adjustment method for reducing the current stress of a 9-sided MMC, so as to solve the problem of excessive bridge arm current stress caused by the reuse of 9-sided MMC bridge arms.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] A three-port 9-agon MMC topology structure is composed of nine bridge arms connected in sequence to form a 9-agon ring. The nine vertices of the 9-agon ring are R, U, Z, S, V, X, T, W, and Y in a clockwise direction. Three three-phase AC ports can be symmetrically led out, where XYZ is the first AC port, RST is the second AC port, and UVW is the third AC port. The first AC port is connected to the AC power grid, the second AC port is connected to the 50 / 3Hz low-frequency system 1, and the third AC port is connected to the 50 / 3Hz low-frequency system 2.

[0008] Furthermore, the structure of each bridge arm of the three-port 9-agonal MMC topology structure is the same. Each group of bridge arms is composed of n H-bridge units and an inductor in series. The H-bridge unit can output three levels +Vdc, 0, and -Vdc. The n H-bridge units are cascaded to output 2n+1 levels.

[0009] The proposed parameter adjustment method can reduce the stress of the comprehensive current period component of the three-port 9-sided MMC bridge arm. The parameter adjustment method includes the following specific steps:

[0010] Step 1: Obtain phase current signals of the AC grid, low-frequency system 1, and low-frequency system 2. The AC grid ports are ix, iy, and iz; the low-frequency system 1 ports are ir, is, and it; and the low-frequency system 2 ports are iu, iv, and iw.

[0011] Step 2: Based on the obtained system phase current signal, on the basis of the enneagonal MMC equivalent circuit model and combined with Kirchhoff's current law, the current periodic component expressions i11, i2 of the nine bridge arms of the enneagonal MMC are obtained. 12 , i13, i21, i22, i23, i31, i32, i33, isr, iwv, ixz, iuw, ivu, and izy are the equivalent line currents of the corresponding ports of the enneagonal MMC;

[0012] i 11 =i sr +i uw +i zy ,i 21 =i ts +i vu +i xz ,i 31 =i rt +i wv +i yx

[0013] i 12 =i sr +i vu +i zy ,i 22 =i ts +i wv+i xz ,i 32 =i rt +i uw +i yx

[0014] i 13 =i sr +i vu +i xz ,i 23 =i ts +i wv +i yx ,i 33 =i rt +i uw +i zy

[0015] Step 3: Combine the same-frequency components in the current period component expressions of the nine bridge arms of the enneagonal MMC obtained in step 2 using the following formula, where the first term on the right side of the equation is the parameter expression that affects the bridge arm current stress.

[0016]

[0017] Where A1 and A2 are the equivalent line current amplitudes corresponding to the two low-frequency AC ports, respectively; ω is the angular velocity corresponding to the two low-frequency AC ports; θ1 and θ2 are the equivalent line current phases corresponding to the two low-frequency AC ports, respectively; θ3 is the phase of the combined sinusoidal quantity;

[0018] The enneagonal MMC has nine bridge arms, and nine parameter expressions are obtained;

[0019]

[0020]

[0021]

[0022] Where A11, A12, A13, A21, A22, A23, A31, A32, and A33 are the parameter expressions of the nine bridge arms, and I2 and I3 are the port currents of port 2 and port 3, respectively. are the power factor angles of port 2 and port 3, Ψ1 and Ψ2 are the phase difference between port 2 and port 1, and the phase angle difference between port 3 and port 1, respectively.

[0023] Step 4: Sum the nine parameter expressions affecting the bridge arm current stress obtained in step 3 to establish the objective function J as follows, where the independent variable of the objective function is the phase angle difference between the two ports with the same frequency;

[0024]

[0025] Step 5: By analyzing the objective function, the relationship between the stress of the comprehensive current periodic component of the enneagonal MMC bridge arm and the phase angle difference between the two ports with the same frequency is obtained. Based on this relationship, the phase angle difference corresponding to the lowest stress of the comprehensive current periodic component of the bridge arm is selected.

[0026] The beneficial effects of the present invention are:

[0027] The present invention can reduce the stress of the comprehensive current periodic component of the enneagonal MMC bridge arm, thereby reducing the construction and maintenance costs of the converter, reducing energy loss, and enhancing the competitiveness of the enneagonal MMC compared to the cascaded matrix converter (M3C) in the application of offshore wind power frequency division transmission systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The accompanying drawings illustrate various embodiments generally by way of example and not limitation, and together with the description and claims, serve to explain embodiments of the invention. Where appropriate, the same reference numerals are used throughout the drawings to refer to the same or similar parts. Such embodiments are illustrative and are not intended to be exhaustive or exclusive of the embodiments of the present apparatus or method.

[0029] Figure 1 A three-port 9-gon MMC topology structure according to an embodiment of the present invention;

[0030] Figure 2 A mathematical model of a three-port 9-gon MMC topology structure according to an embodiment of the present invention is provided;

[0031] Figure 3 This is a waveform diagram of the voltage and current at each port of a three-port 9-agonal MMC under different phase angle differences between ports with the same frequency when the MMC is running according to an embodiment of the present invention;

[0032] Figure 4 This is a waveform diagram of power distribution of each port under different phase angle differences between ports with the same frequency when a three-port 9-agonal MMC is running in accordance with an embodiment of the present invention;

[0033] Figure 5 This is a waveform diagram of the capacitor voltage of the bridge arm submodule under different phase angle differences between ports with the same frequency when a three-port 9-agon MMC is running according to an embodiment of the present invention;

[0034] Figure 6 This is a waveform diagram of the current periodic components of the nine bridge arms under different phase angle differences between the same-frequency ports when a three-port 9-agonal MMC is running according to an embodiment of the present invention. DETAILED DESCRIPTION

[0035] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments in this application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0036] The technical solution adopted by the present invention is: a three-port 9-agonal MMC topology structure is composed of nine bridge arms connected in sequence to form a 9-agonal ring, the nine vertices of the 9-agonal ring are R, U, Z, S, V, X, T, W, and Y in a clockwise direction, and three three-phase AC ports can be symmetrically led out, where XYZ is the first AC port, RST is the second AC port, and UVW is the third AC port; the first AC port is connected to the AC power grid, the second AC port is connected to the 50 / 3Hz low-frequency system 1; and the third AC port is connected to the 50 / 3Hz low-frequency system 2.

[0037] The proposed parameter adjustment method can reduce the stress of the comprehensive current period component of the three-port 9-sided MMC bridge arm. The parameter adjustment method includes the following specific steps:

[0038] Step 1: Obtain phase current signals of the AC grid, low-frequency system 1, and low-frequency system 2. The AC grid ports are ix, iy, and iz; the low-frequency system 1 ports are ir, is, and it; and the low-frequency system 2 ports are iu, iv, and iw.

[0039] Step 2: Based on the acquired system phase current signals and the equivalent circuit model of the enneagonal MMC, and in combination with Kirchhoff's current law, the expressions for the periodic components of the current in the nine bridge arms of the enneagonal MMC are obtained: i11, i12, i13, i21, i22, i23, i31, i32, and i33. isr, iwv, ixz, iuw, ivu, and izy are the equivalent line currents at the corresponding ports of the enneagonal MMC.

[0040] i 11 =i sr +i uw +i zy ,i 21 =i ts +i vu +i xz ,i 31 =i rt +i wv +i yx

[0041] i 12 =i sr +i vu +i zy ,i 22 =i ts +i wv +i xz ,i 32 =i rt +i uw +i yx

[0042] i 13 =i sr +i vu +i xz ,i 23 =i ts +i wv +i yx ,i 33 =i rt +i uw +i zy

[0043] Step 3: Combine the same-frequency components in the current periodic component expressions of the nine bridge arms of the enneagonal MMC obtained in step 2 using the following formula, where the first term on the right side of the equation is the parameter expression that affects the current stress of the bridge arm.

[0044]

[0045] Where A1 and A2 are the equivalent line current amplitudes corresponding to the two low-frequency AC ports, respectively; ω is the angular velocity corresponding to the two low-frequency AC ports; θ1 and θ2 are the equivalent line current phases corresponding to the two low-frequency AC ports, respectively; and θ3 is the phase of the combined sinusoidal quantity.

[0046] The enneagonal MMC has nine bridge arms, so nine parameter expressions are obtained.

[0047]

[0048]

[0049]

[0050] Where A11, A12, A13, A21, A22, A23, A31, A32, and A33 are the parameter expressions of the nine bridge arms, and I2 and I3 are the port currents of port 2 and port 3, respectively. are the power factor angles of port 2 and port 3, Ψ1 and Ψ2 are the phase difference between port 2 and port 1, and the phase angle difference between port 3 and port 1, respectively.

[0051] Step 4: Sum the nine parameter expressions affecting the bridge arm current stress obtained in step 3 to establish the objective function J as follows, where the independent variable of the objective function is the phase angle difference between the two ports with the same frequency.

[0052]

[0053] Step 5: By analyzing the objective function, the relationship between the stress of the comprehensive current periodic component of the enneagonal MMC bridge arm and the phase angle difference between the two ports with the same frequency is obtained. Based on this relationship, the phase angle difference corresponding to the lowest stress of the comprehensive current periodic component of the bridge arm is selected.

[0054] To illustrate the present invention, the proposed method was verified on the RTLAB simulation platform. The system parameters used in the simulation were: port 1 line voltage of 6 kV and frequency of 50 Hz; port 2 line voltage of 6 kV, phase current of 100 A, frequency of 50 / 3 Hz; port 3 line voltage of 6 kV, phase current of 100 A, frequency of 50 / 3 Hz. The converter parameters used in the simulation were: H-bridge unit capacitance of 10 mF, H-bridge unit capacitance voltage of 3535 V, converter arm inductance of 0.25 mH, converter arm submodule number of 4, and switching frequency of 1000 Hz.

[0055] like Figure 3 As shown in the figure, the voltage and current waveforms of each port under different phase angle differences between the same frequency ports of the 9-sided MMC are shown. Figure 3 (a)-(b) and Figure 3 (c)-(d) The phase angle difference between the same frequency ports is 0° and 180° respectively. At the same time, under two different phase angle differences, the voltage and current waveforms of the three ports of the enneagonal MMC are good, and the amplitudes are maintained near the rated values. The power waveforms of each port of the enneagonal MMC are shown in Figure 2. Figure 4 shown. Figure 4 (a) and Figure 4 (b) shows the power waveforms for the same-frequency ports with a phase angle difference of 0° and 180°, respectively. The only difference between the two operating conditions is the phase angle difference, resulting in identical power waveforms. Port 1 absorbs 2.08 MW of active power, while ports 2 and 3 each deliver 1.04 MW. Therefore, as shown in the figure, the power waveforms of the two low-frequency ports overlap on the oscilloscope. The power supplied to port 1 is provided by ports 2 and 3. Calculations show that the three ports are in a power balance state. Figure 5 The waveforms of the average capacitor voltages of the four submodules in the enneagonal MMC bridge arms b11, b12, and b13 are shown when the phase angle differences between the same-frequency ports are 0° and 180°, respectively. As can be seen from the figures, the average submodule capacitor voltages in all three bridge arms fluctuate around the given unit direct voltage of 3535V, maintaining stability. This indicates that the power constant component in the bridge arms has been successfully eliminated, ensuring the stable operation of the enneagonal MMC. Figure 6Figures (a)-(c) and (d)-(f) show the waveforms of the nine cyclical components of the current in the nine arms of the enneagonal MMC, respectively, when the phase angle difference between the ports at the same frequency is 0° and 180°. The magnitude of the current stress in each arm is marked in the figures. The combined cyclical component stress of the bridge arm is the sum of the stresses of the cyclical components of the current in the nine arms. Calculations show that the sum of the current stresses in the nine arms is 2493.4A when the phase angle difference between the ports at the same frequency is 0°, and 1844A when the phase angle difference between the ports at the same frequency is 180°. The minimum value of the combined cyclical component stress in the enneagonal MMC arms is 26.04% lower than the maximum value.

[0056] The above is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with this technical field, within the technical scope disclosed by the present invention, can make equivalent replacements or changes based on the technical solutions and inventive concepts of the present invention, which should be covered by the scope of protection of the present invention.

Claims

1. A parameter adjustment method for reducing the current stress of a 9-sided MMC, characterized in that: Derive the expression for the periodic component of the bridge arm current; Extract the influencing parameters of the bridge arm current periodic component stress; Establish the objective function; The specific steps include: Step 1: Obtain phase current signals of the AC grid, low-frequency system 1, and low-frequency system 2. The AC grid ports are ix, iy, and iz; the low-frequency system 1 ports are ir, is, and it; and the low-frequency system 2 ports are iu, iv, and iw. Step 2: Based on the obtained system phase current signal, on the basis of the enneagonal MMC equivalent circuit model and combined with Kirchhoff's current law, the current period component expression of the nine bridge arms of the enneagonal MMC is obtained: 11 , i12, i13, i21, i22, i23, i31, i32, i33; among them, isr, iwv, ixz, iuw, ivu, izy are the equivalent line currents of the corresponding ports of the enneagonal MMC respectively; i 11 =i sr +i uw +i zy ,i 21 =i ts +i vu +i xz ,i 31 =i rt +i wv +i yx i 12 =i sr +i vu +i zy ,i 22 =i ts +i wv +i xz ,i 32 =i rt +i uw +i yx i 13 =i sr +i vu +i xz ,i 23 =i ts +i wv +i yx ,i 33 =i rt +i uw +i zy Step 3: Combine the same-frequency components in the current period component expressions of the nine bridge arms of the enneagonal MMC obtained in step 2 using the following formula, where the first term on the right side of the formula is a parameter expression that affects the bridge arm current stress; Where A1 and A2 are the equivalent line current amplitudes corresponding to the two low-frequency AC ports, respectively; ω is the angular velocity corresponding to the two low-frequency AC ports; θ1 and θ2 are the equivalent line current phases corresponding to the two low-frequency AC ports, respectively; θ3 is the phase of the combined sinusoidal quantity; The enneagonal MMC has nine bridge arms, and nine parameter expressions are obtained; Where A11, A12, A13, A21, A22, A23, A31, A32, and A33 are the parameter expressions of the nine bridge arms, I2 and I3 are the port currents of port 2 and port 3, φ2 and φ3 are the power factor angles of port 2 and port 3, Ψ1 and Ψ2 are the phase difference between port 2 and port 1, and the phase angle difference between port 3 and port 1, respectively. Step 4: Sum the nine parameter expressions affecting the bridge arm current stress obtained in step 3 to establish the objective function J as follows, where the independent variable of the objective function is the phase angle difference between the two ports with the same frequency; Step 5: By analyzing the objective function, the relationship between the stress of the comprehensive current periodic component of the enneagonal MMC bridge arm and the phase angle difference between the two ports with the same frequency is obtained. Based on this relationship, the phase angle difference corresponding to the lowest stress of the comprehensive current periodic component of the bridge arm is selected.

2. The method according to claim 1, characterized in that The expression of the periodic component of the bridge arm current is derived from the three-port 9-gon MMC topology structure; The extraction of the influencing parameters of the bridge arm current periodic component stress is performed by combining the same frequency components in the bridge arm current periodic component expression to analyze and calculate the influencing parameters of the bridge arm current stress; The objective function is established by summing the influencing parameters of the current stress of the nine bridge arms.

3. The method according to claim 2, characterized in that The three-port enneagonal MMC topology structure is composed of nine bridge arms connected in sequence to form a enneagonal ring. The nine vertices of the enneagonal ring are R, U, Z, S, V, X, T, W, and Y in a clockwise direction, and three three-phase AC ports can be symmetrically led out.

4. The method according to claim 3, characterized in that The three-phase AC port, XYZ is the first AC port, RST is the second AC port, UVW is the third AC port; wherein, the first AC port is connected to the AC power grid, the second AC port is connected to the 50 / 3Hz low-frequency system 1; the third AC port is connected to the 50 / 3Hz low-frequency system 2.

5. The method according to claim 3, characterized in that The structure of each bridge arm of the three-port 9-agonal MMC topology is the same. Each group of bridge arms is composed of n H-bridge units connected in series with an inductor. The H-bridge unit can output three levels: +Vdc, 0, and -Vdc. The n H-bridge units are cascaded to output 2n+1 levels.

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