Three-phase imbalance compensation control method and system of modular multilevel converter

Abstract

The invention provides a three-phase imbalance compensation control method and system for a modular multilevel converter, and the method comprises the steps: detecting the three-phase AC voltage of a to-be-compensated three-phase imbalance side, extracting a corresponding negative-sequence voltage component, and carrying out the three-phase imbalance compensation control of the whole system through the control of the negative-sequence voltage component. The three-phase balance degree of the MMC equipment is maintained in a standard limit range, the influence of the three-phase unbalance load on the connected MMC equipment is avoided, and compared with a traditional unbalance compensation strategy, the three-phase unbalance compensation strategy is simpler, and the compensation effect is more visual.

Description

Technical Field

[0001] The present invention belongs to the field of three-phase unbalanced compensation, and in particular relates to a three-phase unbalanced compensation control method and system for a modular multi-level converter. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] The unbalanced three-phase power grid is a common phenomenon in the operation of the power grid. This phenomenon causes the three-phase voltage and current amplitudes to be inconsistent or the phase difference to be inconsistent by 120°. The unbalanced three-phase load will cause disadvantages and damage to public power facilities, and threaten the safe operation of the power grid. In the AC / DC transmission structure, the MMC device is often connected through a multi-winding transformer. At this time, the common solution to the three-phase imbalance is to compensate the negative sequence voltage and current in the unbalanced system through reactive compensation equipment to make the three phases tend to balance, that is, a static reactive generator (SVG) with a cascaded H-bridge structure is connected to one of the winding ends of the multi-winding transformer. This topology has the characteristics of modular structure, ideal output characteristics and fast reactive regulation speed.

[0004] The existing unbalanced compensation strategies of static VAR compensation devices are based on the Steinmetz principle or instantaneous reactive power theory, which compensates for the three-phase imbalance by controlling the negative sequence current and negative sequence reactive power. The above strategies have multiple conversion calculations in the calculation process, and the control process is cumbersome and complex, and the response speed is slow, and the impact of the imbalance cannot be accurately measured. Summary of the invention

[0005] In order to solve the above problems, the present invention proposes a three-phase unbalanced compensation control method and system for a modular multilevel converter. The present invention detects the three-phase AC voltage on the side that needs to compensate for the three-phase unbalanced voltage and extracts the corresponding negative-sequence voltage component. By controlling the negative-sequence voltage component, the three-phase unbalanced compensation control of the entire system is performed, so that the three-phase balance of the MMC device is maintained within the standard limit, avoiding the influence of the three-phase unbalanced load on the connected MMC device. Compared with the traditional unbalanced compensation strategy, the present invention is simpler and the compensation effect is more intuitive.

[0006] According to some embodiments, the present invention adopts the following technical solutions:

[0007] A three-phase unbalanced compensation control method for a modular multilevel converter comprises the following steps:

[0008] Obtain the grid voltage of the three-phase unbalanced load, track and extract the grid voltage phase and negative sequence voltage;

[0009] According to the extracted negative sequence voltage, the negative sequence voltage command is calculated, and the control of the unbalance degree is transferred to the control of the active component and reactive component of the negative sequence voltage;

[0010] The negative-sequence voltage is controlled based on the calculated active and reactive components of the negative-sequence voltage, and the command signal of the compensation current is obtained through PI regulation, which is then transformed to obtain the final control quantity representing the three-phase imbalance.

[0011] As an optional implementation, a phase-locked loop is used to track and extract the grid voltage phase and negative sequence voltage.

[0012] As an optional implementation, the process of calculating the negative sequence voltage command according to the extracted negative sequence voltage includes:

[0013] U -dref , Respectively represent the d-axis and q-axis command values ​​of the negative sequence voltage on the high-voltage side, U -d , U q - Represents the d and q axis feedback of the negative sequence voltage on the high voltage side, U ± Respectively represented as the amplitude of positive and negative sequence voltage, ε ref is the unbalance reference value, and the negative sequence voltage command is calculated as:

[0014]

[0015] Through the above formula, the control of imbalance is transferred to the control of the active component and reactive component of the negative sequence voltage.

[0016] As an optional implementation, the process of performing negative sequence voltage control includes:

[0017] Determine the scope of governance for imbalances;

[0018] Set an imbalance reference value. When the imbalance is less than the reference value, only maintain its own stable operation without performing power quality management. When the imbalance exceeds the limit, use the reactive power compensation device to compensate for the imbalance.

[0019] As a further limited implementation method, when the voltage imbalance on the imbalance side to be compensated is greater than 2%, the negative sequence voltage d and q axis command values ​​±U –dqref Feedback with the negative sequence voltage d and q on the high voltage side are all negative, and the limit makes -U –dqref and The control loop output is 0, at this time U –dqref and The control loop performs unbalance compensation control; when the AC voltage unbalance on the unbalanced side to be compensated is less than -2%, ±U –dqref and The difference is positive, U –dqref and The control loop output is 0, U –dqref and The control loop performs imbalance compensation control.

[0020] As a further limited implementation method, when the AC voltage imbalance on the imbalance side to be compensated is less than 2% and greater than -2%, U –dqref and The difference is positive, -U –dqref and If the difference is negative, the output of the imbalance control loop is 0 and no imbalance compensation is performed.

[0021] As an optional implementation, the transformation is a YD transformation.

[0022] A three-phase unbalanced compensation control system for a modular multilevel converter, comprising:

[0023] A parameter extraction module is configured to obtain a grid voltage of a three-phase unbalanced load, track and extract a grid voltage phase and a negative sequence voltage;

[0024] The instruction calculation module is configured to calculate the negative sequence voltage instruction according to the extracted negative sequence voltage, and transfer the control of the imbalance degree to the control of the active component and the reactive component of the negative sequence voltage;

[0025] The control module is configured to perform negative sequence voltage control based on the calculated active and reactive components of the negative sequence voltage, obtain a command signal of the compensation current through PI regulation, and transform the final control quantity representing the three-phase imbalance.

[0026] A computer-readable storage medium is used to store computer instructions, and when the computer instructions are executed by a processor, the steps in the above method are completed.

[0027] An electronic device comprises a memory and a processor, and computer instructions stored in the memory and executed on the processor. When the computer instructions are executed by the processor, the steps in the above method are completed.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] Compared with the conventional control scheme based on three-phase unbalanced negative-sequence current, the three-phase unbalanced static VAR compensation device of the present invention has a simpler and more efficient control instruction calculation process.

[0030] The present invention can put into operation or remove the compensation device in real time according to the need of imbalance compensation, thus avoiding excessive management of the compensation device;

[0031] Compared with negative-sequence current imbalance, the negative-sequence voltage imbalance of the present invention can better measure the impact of imbalance.

[0032] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, preferred embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0034] Figure 1 It is the main circuit diagram of the angle type reactive power compensation device;

[0035] Figure 2 is the compensation control diagram of negative sequence voltage;

[0036] Figure 3 It is a negative sequence voltage control scheme for delta-connected reactive power compensation devices;

[0037] Figure 4 is the zero-sequence current injection diagram;

[0038] Figure 5 It is a schematic diagram of the system topology. DETAILED DESCRIPTION

[0039] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0040] It should be noted that the following detailed descriptions are all illustrative and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0041] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0042] In the absence of conflict, the embodiments in this application and the features in the embodiments may be combined with each other.

[0043] Embodiment 1

[0044] A three-phase unbalanced compensation control method for a modular multilevel converter comprises the following steps:

[0045] Obtain the grid voltage of the three-phase unbalanced load, track and extract the grid voltage phase and negative sequence voltage;

[0046] According to the extracted negative sequence voltage, the negative sequence voltage command is calculated, and the control of the unbalance degree is transferred to the control of the active component and reactive component of the negative sequence voltage;

[0047] The negative-sequence voltage is controlled based on the calculated active and reactive components of the negative-sequence voltage, and the command signal of the compensation current is obtained through PI regulation, which is then transformed to obtain the final control quantity representing the three-phase imbalance.

[0048] The following is a detailed introduction:

[0049] Figure 1 This is the main circuit diagram of the angular reactive power compensation device. The power distribution equipment of the present invention uses the MMC device, which can also be used for other power distribution equipment of the power grid. In the figure, u sj (j=a, b, c) is the grid-side phase voltage, i sj is the grid-side AC current, Z j Represents the equivalent impedance of the power grid, i lj is the unbalanced load AC current, i svgj It is the output current of the static VAR compensation device. The static VAR compensation device can achieve power quality management of unbalanced working conditions by outputting a negative-sequence current component with the same magnitude and opposite direction as the unbalanced load.

[0050] Figure 2 is the compensation control diagram of negative sequence voltage. In the figure, U -dref , Respectively represent the d-axis and q-axis command values ​​of the negative sequence voltage on the high-voltage side, U -d , U q - Represents the negative sequence voltage d and q axis feedback on the high voltage side, i a 、i b 、i c is the compensation current command value. The negative sequence voltage command calculation can be converted by the following formula:

[0051]

[0052] Through the above formula, the control of imbalance is transferred to the control of the active component and reactive component of the negative sequence voltage.

[0053] Negative sequence voltage control specifically includes the following steps:

[0054] 1. Determine the scope of imbalance control. According to the national standard "GB / T 15543-2008", the definition of imbalance is that the voltage imbalance at the compensation point must be less than 2% overall and must not exceed 4% in a short period of time.

[0055] 2. Design the control logic of the imbalance degree: In order to avoid excessive treatment of the reactive power compensation device, the reference value of the imbalance degree is set to 2%. (1) When the imbalance degree is less than 2%, the reactive power compensation device only maintains its own stable operation and does not perform power quality treatment; (2) When the imbalance degree exceeds the limit, the reactive power compensation device can perform imbalance compensation in real time. Therefore, this control logic enables the reactive power compensation device to have a real-time switching function.

[0056] 3. Collect the grid-side voltage on the side that needs unbalance compensation, and extract the negative sequence component for unbalance control.

[0057] 4. Through the conversion of the above formula, the negative sequence voltage is converted into the control components of the power component and the reactive component.

[0058] 5. According to Figure 2 As shown in the figure, the negative sequence voltage command and the feedback difference are adjusted by the PI controller to output the compensation current command on the system side that needs to compensate for the imbalance. Among them, the PI controller limit function realizes the control loop. When the three-phase imbalance meets the requirements, the PI controller outputs 0 after limiting, and the reactive compensation device does not compensate. When the imbalance does not meet the requirements, the controller operates normally to manage the power quality.

[0059] The specific steps are as follows:

[0060] (1) When the voltage unbalance on the unbalanced side to be compensated is greater than 2%, ±U –dqref and are all negative, and the limit makes -U –dqref and The control loop output is 0, at this time U –dqref and The control loop performs unbalance compensation control; when the AC voltage unbalance on the unbalanced side to be compensated is less than -2%, ±U –dqref and The difference is positive, U –dqref and The control loop output is 0, U –dqref and The control loop performs unbalance compensation control;

[0061] (2) When the AC voltage imbalance on the side to be compensated is less than 2% and greater than -2%, U –dqref and The difference is positive, -U –dqref and If the difference is negative, the output of the imbalance control loop is 0 and no imbalance compensation is performed.

[0062] Figure 3This is the negative sequence voltage control scheme of the delta-connected reactive power compensation device. In the figure, the subscript i represents the three links (ab, bc, ca) of the reactive power compensation device, us compensation represents the grid voltage on the side to be compensated, and i ab 、i bc 、i ac Represents the in-angle current of the reactive compensation device, u smi Represents the average voltage of each link submodule, Represents the submodule capacitor voltage reference value, theoretically 1 (per unit), sinθ i Represents the phase of each link of the reactive compensation device, i 0 is the injected zero-sequence current.

[0063] As can be seen from the figure, the compensation strategy of the reactive compensation device based on negative sequence voltage control is mainly composed of a phase-locked loop, an outer loop and a current inner loop. The phase-locked loop is used to track the phase of the power grid and realize the extraction of positive and negative sequence components. The outer loop is divided into a negative sequence voltage control loop and a DC voltage control loop. The negative sequence voltage control loop is used for power quality management under unbalanced conditions, and the DC voltage loop can balance the capacitor voltage of the submodule of the reactive compensation device. The current inner loop has 4 input components, namely the negative sequence compensation component, the submodule capacitor voltage balance component, the reactive compensation device current feedback and the zero sequence current injection. Among them, the zero sequence current injection is used to balance the power of each link of the angular reactive compensation device.

[0064] Figure 4 Figure 2 is a diagram of zero-sequence current injection, where sinωt and cosωt are the sine and cosine values ​​of the phase angle within the reactive power compensation device. 0f It is the negative sequence current command feedforward. By injecting zero sequence current, the line voltage and phase current phasor of the delta-connected reactive power compensation device are in a vertical relationship, achieving the balance of the chain link power.

[0065] Figure 5 The system topology diagram is shown in Figure 1. The three-winding transformer is connected to the MMC device, the three-phase unbalanced load, and the static VAR compensation device. If unbalance compensation is to be performed on the MMC side, and the source of the unbalance is on the three-phase unbalanced load side, a YD transformation is required because there is a transformer and voltage level transformation between the two sides.

[0066] Embodiment 2

[0067] A three-phase unbalanced compensation control system for a modular multilevel converter, comprising:

[0068] A parameter extraction module is configured to obtain a grid voltage of a three-phase unbalanced load, track and extract a grid voltage phase and a negative sequence voltage;

[0069] The instruction calculation module is configured to calculate the negative sequence voltage instruction according to the extracted negative sequence voltage, and transfer the control of the imbalance degree to the control of the active component and the reactive component of the negative sequence voltage;

[0070] The control module is configured to perform negative sequence voltage control based on the calculated active and reactive components of the negative sequence voltage, obtain a command signal of the compensation current through PI regulation, and transform the final control quantity representing the three-phase imbalance.

[0071] Those skilled in the art will appreciate that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0072] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0073] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0074] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0075] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made by those skilled in the art within the spirit and principle of the present invention without creative labor shall be included in the protection scope of the present invention.

Claims

1. A three-phase unbalanced compensation control method for a modular multilevel converter, characterized in that: The following steps are involved: Obtain the grid voltage of the three-phase unbalanced load, track and extract the grid voltage phase and negative sequence voltage; According to the extracted negative sequence voltage, the negative sequence voltage command is calculated, and the control of the unbalance degree is transferred to the control of the active component and reactive component of the negative sequence voltage; The negative-sequence voltage is controlled based on the calculated active and reactive components of the negative-sequence voltage, and the command signal of the compensation current is obtained through PI regulation, which is then transformed to obtain the final control quantity representing the three-phase imbalance.

2. The three-phase unbalanced compensation control method of a modular multilevel converter according to claim 1, characterized in that: A phase-locked loop is used to track and extract the grid voltage phase and negative sequence voltage.

3. The three-phase unbalanced compensation control method of a modular multilevel converter according to claim 1, characterized in that: According to the extracted negative sequence voltage, the process of calculating the negative sequence voltage command includes: U -dref , Respectively represent the d-axis and q-axis command values ​​of the negative sequence voltage on the high-voltage side, U -d , U q - Represents the d and q axis feedback of the negative sequence voltage on the high voltage side, U ± Respectively represented as the amplitude of positive and negative sequence voltage, ε ref is the unbalance reference value, and the negative sequence voltage command is calculated as: Through the above formula, the control of imbalance is transferred to the control of the active component and reactive component of the negative sequence voltage.

4. The three-phase unbalanced compensation control method of a modular multilevel converter according to claim 1, characterized in that: The process of negative sequence voltage control includes: Determine the scope of governance for imbalances; Set an imbalance reference value. When the imbalance is less than the reference value, only maintain its own stable operation without performing power quality management. When the imbalance exceeds the limit, use the reactive power compensation device to compensate for the imbalance.

5. The three-phase unbalanced compensation control method of a modular multilevel converter according to claim 4, characterized in that: When the voltage imbalance on the unbalanced side to be compensated is greater than 2%, the negative sequence voltage d and q axis command values ​​±U –dqref Feedback with the negative sequence voltage d and q on the high voltage side are all negative, and the limit makes -U –dqref and The control loop output is 0, at this time U –dqref and The control loop performs unbalance compensation control; when the AC voltage unbalance on the unbalanced side to be compensated is less than -2%, ±U –dqref and The difference is positive, U –dqref and The control loop output is 0, U –dqref and The control loop performs imbalance compensation control.

6. The three-phase unbalanced compensation control method of a modular multilevel converter according to claim 4, characterized in that: When the AC voltage unbalance on the side to be compensated is less than 2% and greater than -2%, U –dqref and The difference is positive, -U –dqref and If the difference is negative, the output of the imbalance control loop is 0 and no imbalance compensation is performed.

7. The three-phase unbalanced compensation control method of a modular multilevel converter according to claim 1, characterized in that: The transformation is a YD transformation.

8. A three-phase unbalanced compensation control system for a modular multilevel converter, characterized in that: include: A parameter extraction module is configured to obtain a grid voltage of a three-phase unbalanced load, track and extract a grid voltage phase and a negative sequence voltage; The instruction calculation module is configured to calculate the negative sequence voltage instruction according to the extracted negative sequence voltage, and transfer the control of the imbalance degree to the control of the active component and the reactive component of the negative sequence voltage; The control module is configured to perform negative sequence voltage control based on the calculated active and reactive components of the negative sequence voltage, obtain a command signal of the compensation current through PI regulation, and transform the final control quantity representing the three-phase imbalance.

9. A computer-readable storage medium, characterized in that: Used to store computer instructions, which, when executed by a processor, complete the steps of the method according to any one of claims 1 to 7.

10. An electronic device, characterized in that: The method comprises a memory and a processor, and computer instructions stored in the memory and executed on the processor, wherein when the computer instructions are executed by the processor, the steps in the method according to any one of claims 1 to 7 are completed.

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