Converter fault ride-through control method based on circulating current compensation and negative sequence current injection

Abstract

The application discloses a converter fault ride-through control method based on circulating current compensation and negative sequence current injection, and belongs to the technical field of electric power control; the method comprises the following steps: collecting M3C bridge arm voltage and current in real time; performing positive and negative sequence decomposition on the collected voltage and current, and converting to the dq synchronous rotating coordinate system; calculating the imbalance power between the bridge arms according to each component of the voltage and current; calculating the required negative sequence current injection amount and circulating current injection amount according to the set power balance proportion coefficient and the imbalance power between the bridge arms, and converting the required negative sequence current injection amount and circulating current injection amount into voltage reference values; and generating a driving signal through nearest level approximation modulation. When an asymmetric fault occurs in a line, in order to balance the asymmetric voltage between each bridge arm in the M3C, the method realizes the M3C fault ride-through by injecting negative sequence current into the fault line and injecting circulating current between the converters, thereby reducing the control difficulty of the circulating current path, reserving part of the negative sequence current characteristics for line fault detection, and realizing the comprehensive optimization of the internal and external characteristics of the M3C during the fault.

Description

Technical Field

[0001] This invention belongs to the field of power control technology, specifically relating to a converter fault ride-through control method based on circulating current compensation and negative sequence current injection. Background Technology

[0002] Modular multilevel matrix converters (M3Cs) possess advantages such as superior harmonic characteristics, high modularity, and flexible adjustable power factor, demonstrating broad application prospects in low-frequency power transmission and offshore wind power grid connection. However, when asymmetrical faults occur in the power frequency grid (such as single-phase ground faults), negative sequence components appear in the grid voltage, leading to M3C arm current distortion, increased voltage fluctuations in submodule capacitors, and even overcurrent damage to devices, seriously threatening system safety. Existing technologies for fault ride-through control strategies for M3Cs mainly employ two approaches: injecting negative sequence current and injecting circulating current. The negative sequence current injection strategy detects the negative sequence component of the grid voltage and injects negative sequence current into the converter to offset the power oscillations caused by the fault. However, this method increases the difficulty of fault identification and has a singular control objective. The injected negative sequence current can mask fault characteristics, making it difficult for line protection devices to accurately detect the fault type and location, resulting in delayed fault clearing. Furthermore, this method only compensates for current balance or power fluctuations, neglecting the stability requirements of the submodule capacitor voltages within the M3C, easily leading to excessive capacitor voltage fluctuations. The circulating current injection strategy regulates the internal energy distribution to suppress capacitor voltage fluctuations by injecting a circulating current of a specific frequency into the M3C bridge arm. However, circulating current path control is complex and requires high precision. The circulating current forms a complex path among the multiple bridge arms of the M3C, and precise control of its amplitude and phase relies on high-precision models and real-time algorithms, placing extremely high demands on the controller performance.

[0003] Therefore, there is an urgent need for a control method that can coordinate negative sequence current and circulating current injection, which can reduce control complexity while suppressing fault current and retain the necessary degrees of freedom for fault detection. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a converter fault ride-through control method based on circulating current compensation and negative sequence current injection, thereby solving the problems in the prior art.

[0005] The objective of this invention can be achieved through the following technical solutions: The converter fault ride-through control method based on circulating current compensation and negative sequence current injection includes the following steps: Real-time acquisition of M3C bridge arm voltage and current; The collected voltage and current are decomposed into positive and negative order, and transformed into the dq coordinate system through Park transformation. The unbalanced power between the bridge arms is calculated based on each component of the voltage and current. The required negative sequence current injection and circulating current injection are calculated based on the set power balance ratio coefficient and the unbalanced power between the bridge arms, and the driving signal is finally generated.

[0006] Furthermore, during the acquisition of M3C bridge arm voltage and current, the sampling frequency shall not be lower than 4kHz.

[0007] Furthermore, the positive and negative order decomposition process includes: The fault voltage and current are converted to synchronous rotating coordinates using Clark transformation. Quantity; Based on a second-order generalized integrator-orthogonal signal generator and a positive and negative sequence component calculator, at rest Real-time decoupled calculation of components is achieved in a coordinate system; The voltage phase angle is obtained using a phase-locked loop, and the positive and negative sequence components of the voltage and current on the dq axis are obtained through Park transformation.

[0008] Furthermore, the negative sequence current injection amount is determined by the negative sequence voltage deviation ratio on the power frequency side, and the circulating current injection amount is adaptively adjusted according to the power imbalance between bridge arms. The two are coordinated by a weighting coefficient to suppress the voltage fluctuation of the submodule capacitor.

[0009] Furthermore, the negative sequence current injection amount satisfies: in, , These represent the active and reactive components of the injected negative sequence current, respectively; K is the power distribution coefficient. , These are the active and reactive components of the positive sequence current on the power frequency side, respectively. , These are the active components of the positive-sequence voltage and negative-sequence voltage on the power frequency side, respectively.

[0010] Furthermore, the circulating injection amount satisfies: in, The required circulating flow rate; K is the power distribution factor; , The active and reactive components of the negative sequence current are injected respectively. This refers to the active component of the positive sequence voltage on the power frequency side. This represents the active component of the positive sequence voltage on the low-frequency side. This is the negative sequence voltage offset angle.

[0011] Furthermore, the process of generating the driving signal is as follows: the negative sequence current injection amount and the circulating current injection amount are superimposed on the modulation wave, a switching signal is generated by the nearest level approximation modulation method, and distributed to the IGBT driving unit of each sub-module.

[0012] The converter fault ride-through control system based on circulating current compensation and negative sequence current injection executes the above control method, including: Bridge arm voltage and current sampling unit: used for real-time acquisition of M3C bridge arm voltage and current; Unbalanced power calculation unit: used to decompose the collected voltage and current into positive and negative order, and transform them into the dq coordinate system through Park transformation, and calculate the unbalanced power between bridge arms based on each component of voltage and current; Negative sequence current and circulating current injection unit: used to calculate the required negative sequence current injection and circulating current injection based on the set power balance ratio coefficient and the unbalanced power between bridge arms, and finally generate the drive signal.

[0013] A computer storage medium storing a readable program that, when executed, instructs a computing device to perform the converter fault ride-through control method based on circulating current compensation and negative sequence current injection as described above.

[0014] An electronic device includes: a processor, a memory, a communication interface, and a communication bus, wherein the processor, the memory, and the communication interface communicate with each other through the communication bus; The memory is used to store at least one executable instruction that causes the processor to perform operations corresponding to the converter fault ride-through control method based on circulating current compensation and negative sequence current injection described above.

[0015] The beneficial effects of this invention are: 1. The converter fault ride-through control method based on the coordinated injection of negative-sequence current and circulating current proposed in this invention can effectively improve the operational stability of modular multilevel matrix converters under asymmetrical grid fault conditions, and is particularly suitable for scenarios with strict fault ride-through requirements, such as offshore wind power frequency division transmission. When a voltage drop or asymmetrical fault occurs in the power frequency grid, the proposed coordinated injection mechanism can dynamically allocate the injection ratio of negative-sequence current and circulating current according to the real-time calculated power imbalance, thereby suppressing the voltage fluctuation of the submodule capacitors within a safe threshold.

[0016] 2. This invention avoids the masking of fault characteristics by coordinating the injection ratio of negative sequence current and circulating current. Traditional single negative sequence current injection will significantly suppress fault current, which may lead to a decrease in the detection sensitivity of line protection devices and make it difficult to accurately identify the fault type and location. This invention, by reasonably allocating control quantities, ensures the stability of the M3C's own operation while retaining the necessary fault characteristics for fault detection, and improves the reliability of system fault detection. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A schematic diagram of a flexible low-frequency power transmission system at sea; Figure 2 This is a structural diagram of the M3C fault ride-through control system based on circulating current compensation and negative sequence current injection of the present invention. Figure 3 This is the control block diagram for the unbalanced power calculation unit; Figure 4 This is a block diagram of the negative sequence current and circulating current injection unit control. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Example 1 In this embodiment, a converter fault ride-through control method based on circulating current compensation and negative sequence current injection is proposed, including the following steps: S1, real-time acquisition of M3C bridge arm voltage and current; During the acquisition of M3C bridge arm voltage and current, the voltage values ​​of the nine bridge arms inside the M3C are acquired at a frequency of no less than 4kHz. , With current value , ( , ).

[0021] S2, decompose the collected voltage and current into positive and negative order, and transform them into the dq coordinate system by Park transformation, and calculate the unbalanced power between bridge arms based on each component of voltage and current; The specific steps for positive and negative sequence decomposition of voltage and current include: The fault voltage and current are converted to synchronous rotating coordinates using Clark transformation. The components, and based on the second-order generalized integrator-orthogonal signal generator (SOGI-QSG) and positive and negative sequence component calculator (PNSC), are at rest. Real-time decoupled calculation of components is achieved in a coordinate system. The voltage phase angle is obtained using a phase-locked loop (PLL), and the positive and negative sequence components of the voltage and current on the dq axis are obtained through Park transformation, yielding the positive sequence voltage and current dq axis components at power frequency. , , , The negative sequence voltage dq-axis component on the power frequency side , , , Low-frequency side positive sequence voltage and current dq axis components , , , Low-frequency side negative sequence voltage dq axis component , , , .

[0022] S3 calculates the required negative sequence current injection and circulating current injection based on the set power balance ratio coefficient and the unbalanced power between bridge arms; and finally generates a drive signal to achieve stable operation of M3C during faults.

[0023] The power imbalance P between the M3C bridge arms is calculated based on the above components, and the power distribution coefficient K is determined by the negative sequence component deviation of the fault voltage in the asymmetrical fault circuit, where K satisfies: in, This represents the effective value of the negative sequence voltage on the power frequency side. This is the effective value of the positive sequence voltage on the power frequency side.

[0024] In this process, part of the power imbalance KP is balanced by negative sequence current injection, and the remaining power imbalance is balanced by circulating current injection, thereby suppressing the voltage waveform of the submodule capacitor.

[0025] The negative sequence current in the negative sequence current injection method should satisfy the following: in, , These represent the active and reactive components of the injected negative sequence current, respectively; K is the power distribution coefficient. , These are the active and reactive components of the positive sequence current on the power frequency side, respectively. , These are the active components of the positive-sequence voltage and negative-sequence voltage on the power frequency side, respectively.

[0026] Taking the au bridge arm as an example, the injected circulation should meet the following requirements: in, The required circulating flow rate; K is the power distribution factor; , These are the active and reactive components of the negative sequence current on the power frequency side, respectively. This refers to the active component of the positive sequence voltage on the power frequency side. This represents the active component of the positive sequence voltage on the low-frequency side. This is the negative sequence voltage offset angle.

[0027] The method for generating the drive signal is as follows: the negative sequence current component and the circulating current injection amount are superimposed on the modulation wave, and the switching signal is generated by the nearest level approximation modulation and distributed to the IGBT drive unit of each sub-module.

[0028] Based on a similar inventive concept, embodiments of the present invention also provide a computer storage medium storing a readable program that, when run by a processor, can execute the above-described converter fault ride-through control method based on circulating current compensation and negative sequence current injection.

[0029] Based on a similar inventive concept, this invention provides an electronic device, including: a processor, a memory, a communication interface, and a communication bus, wherein the processor, the memory, and the communication interface communicate with each other through the communication bus; The memory is used to store at least one executable instruction, which causes the processor to perform the operation corresponding to the converter fault ride-through control method based on circulating current compensation and negative sequence current injection described above.

[0030] Based on a similar inventive concept, embodiments of the present invention also provide a computer program product, including computer instructions, which instruct a computing device to perform the operations corresponding to the above-described converter fault ride-through control method based on circulating current compensation and negative sequence current injection.

[0031] Example 2 In this embodiment, a flexible low-frequency power transmission system at sea is used as an example to illustrate the present invention; such as Figure 1As shown, the offshore flexible low-frequency power transmission system includes: an offshore wind farm, a low-frequency transformer, a low-frequency transmission line, and an M3C frequency converter station. The offshore wind farm outputs low-frequency (16-20Hz) electricity, which is stepped up by the low-frequency transformer and transmitted to the shore. The M3C frequency converter station then converts the low-frequency electricity generated by the offshore wind farm into power frequency electricity and integrates it into the power grid. When an asymmetrical fault occurs on the power frequency side, the negative sequence current generated by the fault will cause voltage imbalance in the bridge arms of the M3C. If this is not suppressed, it will eventually lead to the M3C going out of control.

[0032] like Figure 2 As shown, the converter fault ride-through control system based on circulating current compensation and negative sequence current injection includes: Bridge arm voltage and current sampling unit: used for real-time acquisition of M3C bridge arm voltage and current; Unbalanced power calculation unit: used to perform Park transformation on the collected voltage and current to convert them to the dq coordinate system, decompose them into positive and negative order, and calculate the unbalanced power between bridge arms based on each component of voltage and current; Negative sequence current and circulating current injection unit: used to calculate the required negative sequence current injection and circulating current injection based on the set power balance ratio coefficient and the unbalanced power between bridge arms, and finally generate the drive signal to realize the stable operation of M3C during faults.

[0033] Specifically: The unbalanced power calculation unit calculates the unbalanced power between bridge arms in real time. Taking an asymmetrical fault on the power frequency side as an example, the voltage and current on the power frequency side generate negative sequence components. This unit transforms the voltage and current on the power frequency side to the dq coordinate system and performs positive and negative sequence decomposition to obtain the positive and negative sequence components of the voltage and current on the power frequency side in the dq coordinate system. The structural diagram is as follows Figure 3 As shown, its expression is: in, , , These are the three-phase voltages on the power frequency side; , , These are the three-phase currents on the power frequency side; , These are the active components of the positive-sequence voltage and negative-sequence voltage on the power frequency side, respectively. , These are the active and reactive components of the positive sequence current on the power frequency side, respectively. , These are the active and reactive components of the negative sequence current on the power frequency side, respectively. It is the fundamental frequency on the power frequency side; The negative sequence voltage deviation angle.

[0034] The expression for the bridge arm power is: ; Among them, the bridge arm current Bridge arm voltage ; Taking the au bridge arm as an example, substituting the voltage and current expressions for phase a and phase u, we can obtain... The specific form of the power includes a DC component and AC components of various frequencies. The AC component in the bridge arm power causes periodic fluctuations in the submodule capacitor voltage, while the DC component causes the bridge arm capacitor to continuously discharge or charge, resulting in voltage imbalance. Therefore, measures need to be taken to balance the DC component of the power to improve the fault ride-through capability of the M3C.

[0035] The expression for the DC component of power is: in, , These are the active and reactive components of the positive sequence current on the power frequency side, respectively. , These are the active and reactive components of the negative sequence current on the power frequency side, respectively. , These are the active components of the positive-sequence voltage and negative-sequence voltage on the power frequency side, respectively. This represents the active component of the positive sequence current on the low-frequency side. This represents the active component of the positive sequence voltage on the low-frequency side. This is the negative sequence voltage offset angle on the power frequency side.

[0036] When the active power input of the M3C equals the active power output, that is... The first three terms cancel each other out, so only the DC component caused by the negative sequence offset needs to be balanced.

[0037] When the injected negative sequence current is At this time, the negative sequence current will balance the unbalanced current between all bridge arms. Simultaneously, the injected negative sequence current will also cancel the negative sequence current of the faulty line, increasing the difficulty of line fault detection. Therefore, according to the power balance factor K, a portion of the negative sequence current is injected: in, , These represent the active and reactive components of the injected negative sequence current, respectively; K is the power distribution coefficient. , These are the active and reactive components of the positive sequence current on the power frequency side, respectively. , These are the active components of the positive-sequence voltage and negative-sequence voltage on the power frequency side, respectively.

[0038] The remaining power DC that needs to be balanced is: Where K is the power allocation coefficient; , These are the active and reactive components of the negative sequence current on the power frequency side, respectively. This refers to the active component of the positive sequence voltage on the power frequency side. This is the negative sequence voltage offset angle.

[0039] This component is balanced by circulating injection.

[0040] When the injected circulation is At that time, the bridge arm current of bridge arm au is: Substituting the formula into the power expression and extracting the DC component, the unbalanced power of the bridge arm caused by circulating current injection is: in, For injection circulation; This represents the positive-sequence active component on the low-frequency side.

[0041] This power is used to balance the remaining unbalanced components, that is: in, Inject the desired circulating current component; K is the power distribution factor; , These are the active and reactive components of the negative sequence current on the power frequency side, respectively. This refers to the active component of the positive sequence voltage on the power frequency side. This represents the active component of the positive sequence voltage on the low-frequency side. This is the negative sequence voltage offset angle.

[0042] The required circulating current is: in, Inject the desired circulating current component; K is the power distribution factor; , These are the active and reactive components of the negative sequence current on the power frequency side, respectively. This refers to the active component of the positive sequence voltage on the power frequency side. This represents the active component of the positive sequence voltage on the low-frequency side. This is the negative sequence voltage offset angle.

[0043] The control block diagram of the negative sequence current and circulating current injection unit is as follows: Figure 4 As shown, the M3C signal is finally modulated using the nearest-level approximation modulation method based on the obtained negative sequence current and circulating current.

[0044] The nearest-level approximation modulation includes the following steps: Step 1: Convert the calculated negative sequence current and circulating current into reference voltage signals for each phase arm of the M3C bridge. Step 2: Compare these reference voltages with the available discrete levels of the bridge arm, and select the closest integer level as the output by rounding or rounding up. Step 3: Determine the number of inputs for each submodule based on the selected level value, and generate the corresponding switch sequence; Step 4: Use pulse width modulation to drive the power device to achieve precise modulation of the M3C signal.

[0045] The methods of the present invention can be implemented in hardware, firmware, or as software or computer code that can be stored in a recording medium (such as a CD-ROM, RAM, floppy disk, hard disk, or magneto-optical disk), or as computer code originally stored on a remote recording medium or a non-transitory machine-readable medium and subsequently stored on a local recording medium, downloaded via a network. Thus, the methods described herein can be processed by software stored on a recording medium using a general-purpose computer, a dedicated processor, or programmable or dedicated hardware (such as an ASIC or FPGA). It is understood that the computer, processor, microprocessor controller, or programmable hardware includes storage components (e.g., RAM, ROM, flash memory, etc.) capable of storing or receiving software or computer code that, when accessed and executed by the computer, processor, or hardware, implements the methods described herein. Furthermore, when a general-purpose computer accesses the code used to implement the methods shown herein, the execution of the code transforms the general-purpose computer into a dedicated computer for performing the methods shown herein.

[0046] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A converter fault ride-through control method based on circulating current compensation and negative sequence current injection, characterized in that, Includes the following steps: Real-time acquisition of M3C bridge arm voltage and current; The collected voltage and current are decomposed into positive and negative order, and transformed into the dq coordinate system through Park transformation. The unbalanced power between the bridge arms is calculated based on each component of the voltage and current. The required negative sequence current injection and circulating current injection are calculated based on the set power balance ratio coefficient and the unbalanced power between the bridge arms, and the driving signal is finally generated.

2. The converter fault ride-through control method based on circulating current compensation and negative sequence current injection according to claim 1, characterized in that, During the acquisition of M3C bridge arm voltage and current, the sampling frequency shall not be lower than 4kHz.

3. The converter fault ride-through control method based on circulating current compensation and negative sequence current injection according to claim 1, characterized in that, The positive and negative order decomposition process includes: The fault voltage and current are converted to synchronous rotating coordinates using Clark transformation. Quantity; Based on a second-order generalized integrator-orthogonal signal generator and a positive and negative sequence component calculator, at rest Real-time decoupled calculation of components is achieved in a coordinate system; The voltage phase angle is obtained using a phase-locked loop, and the positive and negative sequence components of the voltage and current on the dq axis are obtained through Park transformation.

4. The converter fault ride-through control method based on circulating current compensation and negative sequence current injection according to claim 1, characterized in that, The negative sequence current injection amount is determined by the negative sequence voltage deviation ratio on the power frequency side, and the circulating current injection amount is adaptively adjusted according to the power imbalance between bridge arms. The two are coordinated by a weighting coefficient to suppress the voltage fluctuation of the submodule capacitor.

5. The converter fault ride-through control method based on circulating current compensation and negative sequence current injection according to claim 4, characterized in that, The negative sequence current injection amount satisfies: in, , These represent the active and reactive components of the injected negative sequence current, respectively; K is the power distribution coefficient. , These are the active and reactive components of the positive sequence current on the power frequency side, respectively. , These are the active components of the positive-sequence voltage and negative-sequence voltage on the power frequency side, respectively.

6. The converter fault ride-through control method based on circulating current compensation and negative sequence current injection according to claim 4, characterized in that, The circulating injection amount satisfies: in, The required circulating flow rate; K is the power distribution factor; , The active and reactive components of the negative sequence current are injected respectively. This refers to the active component of the positive sequence voltage on the power frequency side. This represents the active component of the positive sequence voltage on the low-frequency side. This is the negative sequence voltage offset angle.

7. The converter fault ride-through control method based on circulating current compensation and negative sequence current injection according to claim 1, characterized in that, The process of generating the drive signal is as follows: the negative sequence current injection amount and the circulating current injection amount are superimposed on the modulation wave, and a switching signal is generated by the nearest level approximation modulation method and distributed to the IGBT drive unit of each sub-module.

8. A converter fault ride-through control system based on circulating current compensation and negative sequence current injection, executing the control method according to any one of claims 1-7, characterized in that, include: Bridge arm voltage and current sampling unit: used for real-time acquisition of M3C bridge arm voltage and current; Unbalanced power calculation unit: used to decompose the collected voltage and current into positive and negative order, and transform them into the dq coordinate system through Park transformation, and calculate the unbalanced power between bridge arms based on each component of voltage and current; Negative sequence current and circulating current injection unit: used to calculate the required negative sequence current injection and circulating current injection based on the set power balance ratio coefficient and the unbalanced power between bridge arms, and finally generate the drive signal.

9. A computer storage medium storing a readable program, characterized in that, When the program is running, it can instruct the computing device to execute the converter fault ride-through control method based on circulating current compensation and negative sequence current injection as described in any one of claims 1-7.

10. An electronic device, characterized in that, include: The processor, memory, communication interface, and communication bus are provided, wherein the processor, memory, and communication interface communicate with each other via the communication bus. The memory is used to store at least one executable instruction, which causes the processor to perform the operation corresponding to the converter fault ride-through control method based on circulating current compensation and negative sequence current injection as described in any one of claims 1-7.

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