A DC circulating current equalization method based on zero-sequence voltage injection for unbalanced AC power grids.

Abstract

This invention relates to a method for equalizing MMC (Multi-phase Continuous Current) circulating current based on zero-sequence voltage injection in an unbalanced AC power grid. The control strategy includes: 1) real-time acquisition of the circulating current in the three-phase bridge arms of the MMC, extraction of the DC component of each phase, and comparison with reference values ​​to determine the difference in the three-phase DC circulating current; 2) real-time acquisition of the grid-side current of the three-phase MMC and per-unit processing to determine the per-unit value of the grid-side current; 3) multiplying the difference in the three-phase DC circulating current with the per-unit value of the grid-side current and summing the results to determine the DC circulating current to be adjusted; 4) designing a proportional resonant (PR) controller to generate a zero-sequence voltage regulation quantity from the DC circulating current to be adjusted; 5) injecting the zero-sequence voltage regulation quantity into the three-phase modulation wave of the MMC to equalize the three-phase DC circulating current of the MMC. The method provided by this invention has advantages such as fast dynamic response, strong versatility, and simple principle.

Description

Technical Field

[0001] This invention relates to unbalanced grid fault control in the field of power electronic multilevel converters, specifically a method for MMC DC circulating current equalization based on zero-sequence voltage injection under unbalanced grid conditions. Background Technology

[0002] Flexible DC transmission is a DC transmission technology based on voltage source converters. Due to the use of fully controlled switching devices, it has many advantages, such as eliminating the risk of commutation failure, independently controlling active and reactive power, and being able to supply power to passive networks. It has broad application prospects in areas such as grid-connected renewable energy generation and isolated power supply for offshore platforms. Modular multilevel converters (MMCs), as the most promising topology for flexible DC transmission, are characterized by high modularity and good harmonic characteristics, attracting widespread attention from experts and scholars both domestically and internationally. The operating characteristics and control strategies of MMCs under unbalanced AC grids have also become a research hotspot.

[0003] In unbalanced AC power grids, multiphase current balancing (MMC) devices typically employ three-phase current balancing control. This avoids injecting unbalanced current into the grid and promotes their own safe and stable operation. However, current balancing control cannot achieve complete balance of the three-phase arm currents in the MMC. The circulating current, composed of DC and second-harmonic positive, negative, and zero-sequence AC components, causes unbalanced current and thermal stress between the arms, posing a significant risk of shortened lifespan or even damage to the MMC power devices. Existing methods mostly focus on controlling the AC arm currents of MMCs under unbalanced grids, with insufficient research on DC circulating current balancing methods.

[0004] In flexible DC transmission systems, converter transformers often employ Y / Δ connection groups. Y-type windings on the grid side ensure reliable system grounding, while Δ-type windings on the valve side isolate zero-sequence current. Assuming there is no zero-sequence current loop on the MMC AC side, injecting zero-sequence voltage only redistributes the DC circulating current in the MMC arms, without affecting grid-connected power or current. Therefore, by designing a reasonable zero-sequence voltage injection method, the three-phase DC circulating current balance of the MMC under unbalanced AC grids can be achieved, balancing the current and thermal stresses in the three-phase arms. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides an MMC DC circulating current equalization method based on zero-sequence voltage injection for unbalanced AC power grids. This method calculates the zero-sequence voltage injection phase by combining the grid-side current and the bridge arm current, and generates the required zero-sequence voltage regulation amount through a proportional resonance (PR) controller. It eliminates the need for positive and negative sequence separation, and has advantages such as fast dynamic response, strong versatility, and simple principle. Specifically, this invention adopts the following technical solution:

[0006] A method for equalizing MMC DC circulating current based on zero-sequence voltage injection under unbalanced AC power grids includes the following steps:

[0007] Real-time acquisition of the three-phase bridge arm circulating current of MMC, extraction of the DC component of each phase and comparison with the reference value to determine the difference of the three-phase DC circulating current;

[0008] Real-time acquisition of the three-phase grid-side current of the MMC and per-unit processing to determine the per-unit value of the grid-side current;

[0009] Multiply the three-phase DC circulating current difference with the grid-side current per-unit value and sum them to determine the DC circulating current to be adjusted;

[0010] Design a PR controller to generate a zero-sequence voltage regulation value from the DC circulating current to be adjusted;

[0011] The zero-sequence voltage regulation is injected into the MMC three-phase modulation wave to balance the MMC three-phase DC circulating current.

[0012] Preferably, the unbalanced AC power grid condition is that there is a negative sequence component in the grid-side voltage and there is no zero-sequence current loop.

[0013] Preferably, the unbalanced grid-side voltage is expressed as:

[0014]

[0015] In the formula U s Represents the positive-sequence voltage amplitude, and r is the ratio of the negative-sequence voltage amplitude to the positive-sequence voltage amplitude, i.e., the voltage imbalance. ω represents the initial phase angles of the positive and negative sequence components of the grid-side voltage, respectively, while ω is the rated angular velocity. Attached Figure Description

[0016] The present invention includes the following figures:

[0017] Figure 1 A flowchart of an MMC DC circulating current equalization method based on zero-sequence voltage injection under an unbalanced AC power grid.

[0018] Figure 2 A control block diagram illustrating the specific implementation of an MMC DC circulating current equalization method based on zero-sequence voltage injection under an unbalanced AC power grid.

[0019] Figure 3 A schematic diagram of the model structure based on the Matlab / Simulink simulation environment to verify the effectiveness of specific embodiments.

[0020] Figure 4 Simulation waveforms of the MMC DC circulating current equalization method based on zero-sequence voltage injection in a specific embodiment are shown. Detailed Implementation

[0021] The following is in conjunction with the appendix Figure 1 The present invention will be described in further detail below.

[0022] like Figure 1 A method for equalizing MMC DC circulating current based on zero-sequence voltage injection under unbalanced AC power grids includes the following steps:

[0023] Step 1: Real-time acquisition of MMC three-phase bridge arm circulating current under unbalanced AC power grid, extraction of DC components of each phase and comparison with reference values ​​to determine the difference of three-phase DC circulating current;

[0024] Step 2: Real-time acquisition of the three-phase grid-side current of the MMC and per-unit processing to determine the per-unit value of the grid-side current;

[0025] Step 3: Multiply the three-phase DC circulating current difference with the grid-side current per-unit value and sum them to determine the DC circulating current to be adjusted;

[0026] Step 4: Design a PR controller to generate a zero-sequence voltage regulation value from the DC circulating current to be adjusted;

[0027] Step 5: Inject the zero-sequence voltage regulation into the MMC three-phase modulation wave to balance the MMC three-phase DC circulating current.

[0028] The unbalanced AC power grid condition is that there is a negative sequence component in the grid-side voltage and there is no zero-sequence current loop.

[0029] The unbalanced grid-side voltage can be expressed as:

[0030]

[0031] In the formula U s Represents the positive-sequence voltage amplitude, and r is the ratio of the negative-sequence voltage amplitude to the positive-sequence voltage amplitude, i.e., the voltage imbalance. ω represents the initial phase angles of the positive and negative sequence components of the grid-side voltage, respectively, while ω is the rated angular velocity.

[0032] Provided that there is no zero-sequence current loop on the AC side, zero-sequence voltage injection will only redistribute the active power between the MMC arms, and will not affect the grid-connected power or current.

[0033] According to the superposition principle, the negative sequence component in the AC output voltage of the MMC under current balance control should be equal to the negative sequence voltage on the grid side. The amplitudes are equal but the phases are opposite, thus avoiding negative-sequence output current. The positive-sequence component of the MMC AC-side output voltage can be calculated from the system power reference value, the grid-side positive-sequence voltage, and the grid-connected equivalent impedance. Let the injected zero-sequence voltage amplitude and phase be... At this time, the voltages of the upper and lower arms of the MMC should meet the following requirements.

[0034]

[0035] In the formula These represent the positive and negative sequence voltage amplitudes of the MMC output, respectively. These are the initial phase angles θ of the positive and negative sequence voltages output by the MMC. j = [0, -2π / 3, 2π / 3] T .

[0036] The j-phase bridge arm current of the MMC can be considered as consisting of DC circulating current, AC current, and positive, negative, and zero-sequence second harmonic circulating current components.

[0037]

[0038] Assuming ideal voltage equalization of the MMC submodules, and ignoring bridge arm impedance voltage drop and MMC system losses, the average power of phase j should be zero.

[0039]

[0040] The zero-sequence voltage injection control objective is set as three-phase DC circulating current balancing, i.e., I dca =I dcb =I dcc The zero-sequence voltage amplitude and phase can be calculated to satisfy...

[0041]

[0042] To extract the zero-sequence voltage phase angle in equation (5), the DC circulating current is introduced into the control loop as a negative feedback quantity. The three-phase DC circulating current error ΔI dcj satisfy

[0043]

[0044] Introducing a grid-side current product element, since only the phase information of the grid-side current is needed, can prevent current amplitude variations from affecting DC circulating current equalization performance by normalizing the product element.

[0045]

[0046] The required zero-sequence voltage regulation u is obtained by summing the three terms. ZSV

[0047]

[0048] Design a baseband PR controller with the following transfer function expression:

[0049]

[0050] In the formula kp k is the proportionality coefficient. r For the resonant gain, ω f It is the resonant angular frequency.

[0051] zero-sequence voltage regulation amount u ZSV The final zero-sequence voltage injection quantity is generated by the PR controller. Superimposed on the MMC reference modulation wave, the MMC three-phase DC circulating current balance is achieved under unbalanced AC power grid. Figure 2 This is a control block diagram for a specific implementation of the MMC DC circulating current equalization method based on zero-sequence voltage injection under an unbalanced AC power grid.

[0052] Figure 3 A schematic diagram of the model structure based on the Matlab / Simulink simulation environment is shown to verify the effectiveness of the specific implementation. The MMC has a rated capacity of 20 MVA, a DC-side voltage of 20 kV, and a grid-side voltage of 10 kV. During the simulation, the MMC operates in rectification mode with a power factor of 1. At 0.2 s, a single-phase (phase a) ground fault and a two-phase (phase a, phase b) ground fault occur in the grid, respectively. At 0.7 s, the aforementioned MMC DC circulating current equalization method based on zero-sequence voltage injection under unbalanced AC grid conditions is applied.

[0053] Figure 4 The waveforms shown are simulations of the MMC DC circulating current equalization method based on zero-sequence voltage injection in a specific embodiment. From top to bottom, the waveforms represent grid-side voltage, grid-side current, MMC three-phase circulating current, submodule voltage, and zero-sequence voltage regulation.

[0054] Figure 4 (a) Simulation results show that after a single-phase (a-phase) ground fault occurs at 0.2s, the steady-state values ​​of the MMC three-phase DC circulating current are -233A, -233A, and -533A, respectively. After the MMC DC circulating current equalization method based on zero-sequence voltage injection is applied at 0.7s, the three-phase DC circulating current is equalized in about 0.02s, and there is almost no overshoot during the adjustment process. Figure 4 (b) Simulation results show that after a two-phase (a and b) ground fault occurs in 0.2s, the MMC three-phase DC circulating currents are approximately 0A, -500A, and -500A, respectively. After applying the MMC DC circulating current balancing method based on zero-sequence voltage injection, the adjustment time is approximately 0.10s, and the maximum overshoot is only 10A. Upon reaching steady state, due to the balancing of the three-phase DC circulating currents, the MMC three-phase bridge arm currents tend to balance, and the maximum current peak value decreases. Therefore, the imbalance between the MMC three-phase bridge arm current stress and thermal stress can be avoided.

[0055] The advantages of this invention over the prior art are:

[0056] This invention provides a DC circulating current balancing method for MMCs based on zero-sequence voltage injection under unbalanced AC power grids. It eliminates the need for positive and negative sequence separation and has advantages such as fast dynamic response, strong versatility, and simple principle. It can achieve three-phase DC circulating current balancing of MMCs under various unbalanced AC power grids, which helps to reduce the peak value of the three-phase bridge arm current of the MMC, balance the current stress and thermal stress of the three-phase bridge arm, and improve the safe and stable operation capability of the MMC under unbalanced AC power grids.

[0057] The contents not described in detail in this specification are existing technologies known to those skilled in the art.

Claims

1. A method for balancing DC circulating current of MMC under unbalanced AC grid based on zero sequence voltage injection, characterized in that, Includes the following steps: Real-time acquisition of the three-phase bridge arm circulating current of the MMC, extraction of the DC component of each phase and comparison with the reference value to determine the difference of the three-phase DC circulating current; Real-time acquisition of three-phase grid-side current in MMC; The zero-sequence voltage regulation is determined by multiplying the three-phase DC circulating current difference of each phase by the corresponding grid-measured current value; the calculation includes: Introduce a grid-side current product element; ; The three summations give the required zero sequence voltage regulation u ZSV : ; in, and These represent the magnitude of the negative sequence voltage and the initial phase angle of the MMC output, respectively. U dc This is the DC bus voltage of the MMC. The initial phase angle of the MMC output positive sequence current, Δ I dca Δ I dcb Δ I dcc These are the three-phase DC circulating current error values. i sa , i sb , i sc These are the three-phase AC output currents, I s This refers to the amplitude of the AC output current. ω That is the rated angular velocity; Design a proportional resonant PR controller to generate a zero-sequence voltage injection quantity by means of the zero-sequence voltage regulation quantity; The zero-sequence voltage injection amount is injected into the MMC three-phase modulation wave to balance the MMC three-phase DC circulating current.

2. The method of claim 1, wherein the zero sequence voltage injection based MMC DC circulating current balancing method under unbalanced AC power grid is characterized by, The unbalanced AC power grid condition is that there is a negative sequence component in the grid-side voltage and there is no zero-sequence current loop.

3. The method of claim 1, wherein the zero sequence voltage injection based MMC DC circulating current balancing method under unbalanced AC power grid is characterized by, The unbalanced grid-side voltage is expressed as: ; In the formula, U s Represents the positive sequence voltage amplitude. r The voltage imbalance is the ratio of the magnitude of the negative-sequence voltage to the magnitude of the positive-sequence voltage. u sa , u sb , u sc These are the grid-side voltages of phases a, b, and c of the power grid, respectively. These are the positive sequence voltages of phases a, b, and c, respectively. These are the negative sequence voltages of phases a, b, and c, respectively. These are the initial phase angles of the positive and negative sequence components of the grid-side voltage, respectively. ω That is the rated angular velocity.

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