Improved M3C circulating current suppression method based on three-phase static coordinate system

Abstract

The invention discloses an improved M3C circulating current suppression method based on a three-phase static coordinate system, and belongs to the technical field of power electronic converter control. The method comprises the following steps: identifying four dominant frequency components (omega1-2omega2, 2omega1-omega2, omega1 + 2omega2 and 2omega1 + omega2) in the M3C circulating current through spectral analysis (omega1 is a power frequency side angular frequency, and omega2 is a low frequency side angular frequency); under a three-phase static coordinate system, proportional resonance controllers are independently arranged on the nine bridge arms to generate circulating current suppression voltage signals, and the four frequency components are suppressed; and meanwhile, a zero-sequence suppression voltage signal is generated through a proportional controller, so that the amplitude of zero-sequence current with the frequency of 3 omega 1 and 3 omega 2 does not exceed 5% of the rated current of the bridge arm. According to the method, coordinate transformation and a phase-locked loop are not needed, the internal loop current of the bridge arm is effectively suppressed, bridge arm current distortion is reduced, and the method is suitable for a power frequency-low frequency alternating current interconnection scene.

Description

Technical Field

[0001] This invention relates to the field of power electronic converter control technology, and in particular to an improved M3C circulating current suppression method based on a three-phase stationary coordinate system. Background Technology

[0002] As a core device in low-frequency power transmission systems, the Modular Multilevel Matrix Converter (M3C) has AC system connections on both sides in its topology, resulting in complex internal circulating current frequency components. In the M3C, the circulating current increases the peak and RMS values ​​of the arm currents, amplifies the voltage fluctuations of the submodule capacitors, and may cause voltage imbalance loss, affecting the stable operation of the M3C.

[0003] Current circulating current suppression methods are mainly divided into two categories: decoupling control based on rotating coordinate system (dq), which requires two coordinate transformations and is computationally complex, resulting in slow dynamic response and limited control accuracy; and decoupling control based on double αβ0 coordinate transformation, which requires separating the power frequency side and low frequency side current components from the bridge arm current for independent control, further increasing the complexity of system design.

[0004] The methods described above all rely on coordinate transformation and phase-locked loop (PLL) synchronization, which not only increases the real-time computational burden but also poses a risk of control instability under sudden frequency changes. Therefore, it is necessary to develop a novel circulating current suppression method based on a three-phase stationary coordinate system that eliminates the need for coordinate transformation and PLL participation. This would simplify the control structure, improve dynamic performance, effectively suppress circulating current, and ensure stable and efficient system operation under all operating conditions. Summary of the Invention

[0005] The purpose of this invention is to provide an improved M3C circulating current suppression method based on a three-phase stationary coordinate system. This method eliminates the coordinate transformation step and enables hierarchical control throughout the entire process in the three-phase stationary coordinate system, achieving effective circulating current suppression and stable system operation. To achieve the above objective, the specific steps are as follows:

[0006] Step 1: Based on the M3C circuit topology, use Kirchhoff's laws to obtain the mathematical model of M3C in the three-phase stationary coordinate system.

[0007] Step 2: Collect the input voltage, input current, output voltage, and output current of the M3C main circuit, and calculate the instantaneous power of the bridge arm.

[0008] Step 3: Analyze the power of the M3C bridge arm, derive the analytical expression for capacitor voltage fluctuation, and find that the submodule capacitor voltage contains AC components of multiple frequencies in addition to the DC component.

[0009] Step 4: After the fundamental voltage of the bridge arm interacts with the basic capacitor voltage at each frequency, a total of 8 frequency components are generated in the bridge arm voltage fluctuation. Through spectrum analysis, the dominant frequency components ω1-2ω2, ω1+2ω2, 2ω1-ω2, and 2ω1+ω2 in the M3C circulating current are identified.

[0010] Step 5: For the four dominant frequency components that cause the circulating current, set up four proportional resonant (PR) controllers independently for each bridge arm in the three-phase stationary coordinate system.

[0011] Step 6: For the zero-sequence currents with frequencies of 3ω1 and 3ω2 that increase the amplitude of the bridge arm current, a proportional (P) controller is used for each of the nine bridge arms in the three-phase stationary coordinate system.

[0012] Compared with existing technologies, the advantages of this invention are as follows: Through a hierarchical control strategy in a three-phase stationary coordinate system, the four key frequency components of circulating current and zero-sequence current can be accurately suppressed without coordinate transformation, which is beneficial to improving system stability. Compared with traditional circulating current suppression methods that rely on dual dq and dual αβ0 transformations for decoupling, this invention eliminates complex coordinate transformations and phase-locked loops, simplifies the control system structure, and directly deploys a PR controller in the three-phase stationary coordinate system to target and suppress the four types of circulating harmonic components, while simultaneously using a P controller to dynamically constrain the zero-sequence current amplitude. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the M3C main circuit topology.

[0014] Figure 2 The block diagram for circulating current suppression control is based on a three-phase stationary coordinate system.

[0015] Figure 3 Design a flowchart for PR controller parameters;

[0016] Figure 4 This is a block diagram of zero-sequence current suppression control based on a three-phase stationary coordinate system.

[0017] Figure 5 This is a comparison diagram of the bridge arm currents before and after circulating current suppression. Detailed Implementation

[0018] To clarify the basic principles, technical solutions, and performance advantages of this invention, the invention will be further described below in conjunction with the accompanying drawings. It should be understood that the following description is merely illustrative and not intended to limit the scope of use and protection of this invention.

[0019] The purpose of this invention is to provide an improved M3C circulating current suppression method based on a three-phase stationary coordinate system. This method does not require coordinate transformation and can implement hierarchical control throughout the entire process in a three-phase stationary coordinate system, thereby achieving effective suppression of circulating current and stable system operation.

[0020] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] Step 1: First, according to Figure 1 The M3C circuit topology shown is used to solve for the mathematical model of M3C in a three-phase stationary coordinate system using Kirchhoff's laws.

[0022]

[0023] In the formula, R is the equivalent resistance of the bridge arm, L is the inductance of the bridge arm, and i gs (g represents phases u, v, w, and s represents phases a, b, c) represents the bridge arm current, U gs L is the bridge arm voltage; g For the power frequency input side filter inductor, i g L is the power frequency input current. s For low-frequency output side filter inductance, i s For low-frequency output side current, U g U is the power frequency input voltage. s For low-frequency output side voltage, U ON This is the neutral point offset voltage.

[0024] Step 2: Real-time acquisition of the main circuit's input voltage U g Input current i g Output voltage U s and output current i s Calculate the instantaneous power P of the bridge arm bi :

[0025]

[0026] In the formula, U1 and U2 are the input and output voltage amplitudes, I1 and I2 are the input and output current amplitudes, and θ and These represent the phase differences between the input and output voltages and currents, respectively, and δ is the initial phase angle between the input and output.

[0027] Step 3: Assume that the capacitance values ​​C of the submodules are equal and the average capacitance voltage u dc By deriving an analytical expression for capacitor voltage fluctuation, we can establish that the voltage remains constant.

[0028]

[0029] In the formula, N is the number of bridge arm sub-modules, C is the capacitance value of the sub-module, and u dc This represents the average voltage of the submodule capacitors.

[0030] Step 4: After the basic bridge arm voltage fluctuation interacts with the basic capacitor voltage fluctuation components at each frequency, a total of 8 frequency components are generated in the bridge arm voltage fluctuation: 3ω1, 3ω2, ω1, ω2, ω1-2ω2, ω1+2ω2, 2ω1-ω2, and 2ω1+ω2 components. Through spectral analysis (such as FFT or other equivalent methods), the four components ω1-2ω2, ω1+2ω2, 2ω1-ω2, and 2ω1+ω2 are identified as the main frequency components contained in the M3C circulating current.

[0031] Step 5: According to Figure 2 The control structure shown performs circulating current AC component suppression. For the four identified circulating current frequency components, four PR controllers are independently deployed for each bridge arm. The specific execution flow is as follows:

[0032] First, the internal circulation i is calculated directly in the three-phase stationary coordinate system. circ :

[0033] i circ =i arm -i g -i s

[0034] Where, i g i s These are the mains / low-frequency grid-side currents, i arm The current represents the bridge arm current, so no coordinate transformation is required.

[0035] Furthermore, a multi-frequency PR controller is employed, which operates independently on the nine bridge arms in a three-phase stationary coordinate system. In this embodiment, the resonant frequency of PR1 is ω1-2ω2 (18Hz); the resonant frequency of PR2 is ω1+2ω2 (82Hz); the resonant frequency of PR3 is 2ω1-ω2 (84Hz); and the resonant frequency of PR4 is 2ω1+ω2 (116Hz).

[0036] Furthermore, according to Figure 3 The design flow shown determines the PR controller parameters, and the specific expression is as follows:

[0037]

[0038] In the formula, ω0 is the resonant angular frequency (ω0=2π·f0, f0 is taken as 18Hz, 84Hz, 82Hz, 116Hz respectively), ω c The resonant bandwidth (ω) c ≈2π·0.05f0), K p1 For proportional gain, K r This represents the resonant gain. Where K... p1 Take a value of 0.5 to 2.0, K r Take values ​​from 10 to 50, bandwidth ω cThe center frequency is taken as ±5%. The above parameters can be adjusted according to the bridge arm inductance L and the sampling period.

[0039] Furthermore, the outputs of each PR controller are superimposed to generate a circulating current suppression voltage u. circ_ref :

[0040]

[0041] Step Six: Press Figure 4 The control structure shown performs zero-sequence current suppression. In a three-phase stationary coordinate system, a P controller is used to suppress the zero-sequence current amplitude at frequencies 3ω1 and 3ω2 for the nine bridge arms. Specifically, this includes:

[0042] First, the zero-sequence current component i is separated by a band-pass filter (BPF). circ_zero :

[0043]

[0044] Among them, i a i b i c This refers to the three-phase current on the low-frequency output side. In this embodiment, It is a bandpass filter with a center frequency of 3f1 = 150Hz and a bandwidth of 10Hz; It is a bandpass filter with a center frequency of 3f2 = 48Hz and a bandwidth of 10Hz.

[0045] Furthermore, i circ_zero Input to controller P generates zero-sequence suppression voltage signal u zero_ref The specific expression is:

[0046] u zero_ref =K p2 ·i circ_zero ,

[0047] Proportional Gain K p2 Based on the real-time dynamic adjustment of the zero-sequence current amplitude, the target satisfies |i circ_zero |≤0.05I rated The nine bridge arms of the M3C independently generate zero-sequence suppression voltage signals, so that the amplitude of the zero-sequence current does not exceed 5% of the rated bridge arm current.

[0048] Perform FFT analysis on the bridge arm currents, such as Figure 5As shown, the measured spectrum deviates from the theoretical circulating current characteristic frequency by ≤1Hz, verifying the accuracy of the mechanism. Subsequently, the three-phase stationary coordinate system hierarchical strategy of this invention was applied, and the waveforms and spectra before and after control were observed. The circulating current harmonics were significantly attenuated, the zero-sequence component converged synchronously, and the system was dynamically stable without additional oscillations. This fully verifies that this invention can stably and effectively suppress the M3C circulating current without coordinate transformation and phase-locked loop.

[0049] This invention proposes a novel hierarchical control method based on a three-phase stationary coordinate system for circulating current suppression in M3C (Multi-phase Current Control). Its advantages include independent control without any coordinate transformation or phase-locked loop (PLL), a simplified structure, low computational requirements, and fast dynamic response. Through the synergistic action of the PR (Positive Regulator) and P (Positive Controller) controllers, targeted suppression of circulating current and its zero-sequence current is achieved, making it suitable for power frequency-low frequency AC interconnection scenarios.

[0050] The above description represents a specific embodiment and advantages of the present invention, but the scope of protection of the present invention is not limited thereto. For those skilled in the art, variations and modifications can be made to the above embodiments without fundamentally departing from the technical spirit and principles described in the present invention, and these variations and modifications should also be considered within the scope of protection of the present invention.

Claims

1. An improved M3C circulating current suppression method based on a three-phase stationary coordinate system, characterized in that, include: Step 1: Identify the amplitudes of the four dominant frequency components ω1-2ω2, ω1+2ω2, 2ω1-ω2, and 2ω1+ω2 in the M3C circulating current through spectrum analysis, where ω1 represents the angular frequency on the M3C power frequency side and ω2 represents the angular frequency on the M3C low frequency side. Step 2: In the three-phase stationary coordinate system, set proportional resonant controllers for each of the 9 bridge arms in M3C to independently generate circulating current suppression voltage signals to suppress the 4 dominant frequency components in Step 1. Step 3: In the three-phase stationary coordinate system, based on the zero-sequence current components of the nine bridge arms, set up a proportional controller to generate a zero-sequence suppression voltage signal so that the amplitude of the zero-sequence current at frequencies 3ω1 and 3ω2 does not exceed 5% of the rated current of the bridge arm.

2. The M3C circulating current suppression method according to claim 1, characterized in that, The transfer function of the proportional resonant controller mentioned in step two is: In the formula, K p1 For proportional gain, K r ω is the resonant gain, ω0 is the resonant angular frequency, ω c The resonant bandwidth; Among them, the resonant angular frequency ω0 precisely matches the above four dominant frequency components, and the resonant bandwidth ω c Set to ±5% of the corresponding center frequency.

3. The M3C circulating current suppression method according to claim 1, characterized in that, The proportional controller output described in step three satisfies: Wherein, the proportional gain K p2 Based on the zero-sequence current i circ_zero Real-time dynamic amplitude adjustment ensures that the zero-sequence current amplitude strictly satisfies |i circ_zero |≤0.05I rated The current shall not exceed 5% of the rated current of the bridge arm.

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