Flexible direct current converter valve control method and system

Abstract

The invention relates to a flexible direct current converter valve control method and system, and belongs to the technical field of flexible direct current converter valve control. When the current direction of a certain bridge arm of the converter valve is judged to be the charging direction, at least one submodule with the lowest voltage is input in the next control period, and at least one submodule with the highest voltage is cut off; when it is judged that the current direction of a certain bridge arm of the converter valve is the discharging direction, at least one submodule with the highest voltage is input in the next control period, and at least one submodule with the lowest voltage is cut off; if the instantaneous value of the current of a certain bridge arm of the converter valve at the current moment is greater than or equal to the lower limit threshold value of the set threshold value interval and less than or equal to the upper limit threshold value of the set threshold value interval, judging that the current direction of the bridge arm is the same as the current direction of the bridge arm at the moment before the moment of entering the set threshold value interval most recently from the current moment; the lower limit threshold is negative, and the upper limit threshold is positive.

Description

Technical Field

[0001] The present invention relates to a flexible DC converter valve control method and system, belonging to the technical field of flexible DC converter valve control. Background Art

[0002] The MMC converter valve is the core system of the modular multi-level flexible direct current transmission system (MMC-HVDC). The reliable operation of the MMC converter valve is the basis for the superior performance of the flexible direct current transmission system to be reflected and exerted. After the development practice of multiple projects, the MMC converter valve has reliable detection and protection methods for sub-module overvoltage, undervoltage, communication failure with the converter station control and protection system, valve bridge arm overcurrent, valve bridge arm overall overvoltage and other failures. The MMC converter valve control device is responsible for sending the control instructions of opening and closing to the sub-module, and the sub-module is responsible for sending the sub-module voltage and status information to the converter valve control device.

[0003] The MMC converter valve control device determines whether each submodule is put into operation or removed in the next control cycle based on whether the instantaneous value of the bridge arm current received in each control cycle is positive or negative, and combined with the voltage sorting of the submodules of each bridge arm. When the bridge arm current direction is positive (charging direction), the MMC converter valve control device will put the submodule with lower voltage into operation and remove the submodule with higher voltage in the next control cycle. When the bridge arm current direction is negative (discharging direction), the submodule with lower voltage will be removed and the submodule with higher voltage will be put into operation in the next control cycle.

[0004] Under special working conditions, the bridge arm current may be almost close to zero. In addition, the zero drift of the bridge arm current sampling causes the instantaneous value of the bridge arm current to frequently fluctuate above and below zero. As a result, the direction of the bridge arm current determined by the converter valve control device switches between the charging direction and the discharging direction at a high frequency, causing the converter valve control device to frequently switch on and off the submodules with the highest or lowest submodule voltages, triggering the submodule device-level protection, thereby bypassing the submodule and reducing the reliability of the system operation. Summary of the invention

[0005] The object of the present invention is to provide a flexible DC converter valve control method and system to solve the problem that the instantaneous value of the bridge arm current frequently fluctuates above and below zero, causing the converter valve control device to frequently switch on and off individual converter valve submodules.

[0006] In order to solve the above technical problems, the present invention provides a flexible DC converter valve control method. If it is determined that the current direction of a bridge arm of the converter valve is a charging direction, at least one submodule with the lowest voltage will be put into operation and at least one submodule with the highest voltage will be removed in the next control cycle; if it is determined that the current direction of a bridge arm of the converter valve is a discharging direction, at least one submodule with the highest voltage will be put into operation and at least one submodule with the lowest voltage will be removed in the next control cycle; it is characterized in that if the instantaneous value of the current of a bridge arm of the converter valve at the current moment is greater than or equal to the lower threshold of the set threshold interval and less than or equal to the upper threshold of the set threshold interval, it is determined that the current direction of the bridge arm is the same as the current direction of the bridge arm at the moment before the most recent time it entered the set threshold interval from the current moment; the lower threshold is a negative number, and the upper threshold is a positive number.

[0007] Furthermore, if the instantaneous value of the current of a certain bridge arm of the converter valve at the current moment is greater than the upper limit threshold of the set threshold interval, it is determined that the current direction of the bridge arm is the charging direction; if the instantaneous value of the current of a certain bridge arm of the converter valve at the current moment is less than the lower limit threshold of the set threshold interval, it is determined that the current direction of the bridge arm is the discharging direction.

[0008] Furthermore, the lower limit threshold of the threshold interval is set to range from -40A to -10A.

[0009] Furthermore, the upper threshold of the threshold interval is set to range from 10A to 40A.

[0010] Beneficial effect: The present invention provides a new flexible DC converter valve control method. The present invention sets the direction of the bridge arm current when the instantaneous value of the bridge arm current is in a set threshold interval near zero to be the same as the direction of the bridge arm current at a moment before the instantaneous value of the bridge arm current most recently entered the threshold interval. Therefore, even if the instantaneous value of the bridge arm current fluctuates around 0, the converter valve will only maintain the current direction of the bridge arm at the current direction at a moment before the most recent entry into the threshold interval, thereby avoiding frequent switching of the direction of the bridge arm current, that is, maintaining the way in which the sub-modules of the converter valve are put into operation and removed, thereby avoiding high-frequency repeated switching of the sub-modules with the highest or lowest voltage; thereby avoiding sub-module failures caused by high-frequency switching of sub-modules due to fluctuations of the instantaneous value of the bridge arm current near 0, thereby ensuring the stability of system operation.

[0011] The present invention also provides a flexible DC converter valve control system, comprising a processor, wherein the processor is used to execute a computer program to implement the steps of the above-mentioned flexible DC converter valve control method.

[0012] The flexible DC converter valve control system can achieve the same beneficial effects as the above-mentioned flexible DC converter valve control method. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a flow chart of the flexible DC converter valve control method in the embodiment of the flexible DC converter valve control method of the present invention. DETAILED DESCRIPTION

[0014] The specific implementation of the present invention will be further described below in conjunction with the accompanying drawings.

[0015] Flexible DC converter valve control method embodiment The present embodiment provides a technical solution for a flexible DC converter valve control method, which maintains the direction of the bridge arm current when the instantaneous value of the bridge arm current is in a set threshold interval near zero at the direction of the bridge arm current at the moment before the instantaneous value of the bridge arm current last entered the threshold interval; avoids frequent switching of the direction of the bridge arm current, that is, maintains the mode of switching on and off the sub-modules of the converter valve, thereby avoiding high-frequency repeated switching of several sub-modules with the highest or lowest voltage; thereby avoiding sub-module failure caused by high-frequency switching of sub-modules due to fluctuations of the instantaneous value of the bridge arm current near 0, thereby ensuring the stability of system operation.

[0016] The following is a detailed description: The flow chart of the flexible DC converter valve control method is as follows: Figure 1 As shown, if the current direction of a bridge arm of the converter valve is determined to be the charging direction, at least one submodule with the lowest voltage is put into operation in the next control cycle, and at least one submodule with the highest voltage is removed; if the current direction of a bridge arm of the converter valve is determined to be the discharging direction, at least one submodule with the highest voltage is put into operation in the next control cycle, and at least one submodule with the lowest voltage is removed; if the instantaneous value of the current of a bridge arm of the converter valve at the current moment is greater than or equal to the lower threshold of the set threshold interval and less than or equal to the upper threshold of the set threshold interval, it is determined that the current direction of the bridge arm is the same as the current direction of the bridge arm at the moment before the most recent entry into the set threshold interval from the current moment; the lower threshold of the set threshold interval is a negative number, and the upper threshold of the set threshold interval is a positive number. Specifically, the range of the lower threshold of the set threshold interval is -40A~-10A. The range of the upper threshold of the set threshold interval is 10A~40A; in this embodiment, the lower threshold of the set threshold interval is -30A, and the upper threshold is 30A.

[0017] For example, when the instantaneous value of the bridge arm current at the current moment was a negative value (the direction of the bridge arm current was in the discharge direction) the moment before it last entered the set threshold interval, the direction of the bridge arm current continues to maintain the discharge direction; when the instantaneous value of the bridge arm current at the current moment was a positive value (the direction of the bridge arm current was in the charging direction) the moment before it entered the set threshold interval, the direction of the bridge arm current continues to maintain the charging direction.

[0018] Furthermore, in the flexible DC converter valve control method of this embodiment, if the instantaneous value of a bridge arm current of the converter valve at the current moment is greater than the upper threshold of the set threshold interval, the current direction of the bridge arm is determined to be the charging direction; if the instantaneous value of a bridge arm current of the converter valve at the current moment is less than the lower threshold of the set threshold interval, the current direction of the bridge arm is determined to be the discharging direction. Since once the instantaneous value of the bridge arm current exceeds the threshold interval, that is, the absolute value of the current exceeds the set smaller range, the switching of the converter valve submodule is controlled according to the bridge arm current direction corresponding to the actual bridge arm current instantaneous value, even if the bridge arm current direction at the moment before the bridge arm current instantaneous value last entered the set threshold interval is opposite to the bridge arm current direction corresponding to the actual bridge arm current instantaneous value, it will only occur when the current is small, and will not cause too much impact on the normal operation of the converter valve.

[0019] Flexible DC converter valve control system implementation example This embodiment provides a technical solution for a flexible DC converter valve control system. The system includes a processor, which is used to execute a computer program to implement the steps of a flexible DC converter valve control method.

[0020] Since the specific implementation process and principle of the flexible DC converter valve control system of this embodiment have been described in detail in the flexible DC converter valve control method embodiment, they will not be repeated here.

Claims

1. A flexible DC converter valve control method, if it is determined that the current direction of a bridge arm of the converter valve is a charging direction, then at least one submodule with the lowest voltage is put into operation in the next control cycle, and at least one submodule with the highest voltage is removed; if it is determined that the current direction of a bridge arm of the converter valve is a discharging direction, then at least one submodule with the highest voltage is put into operation in the next control cycle, and at least one submodule with the lowest voltage is removed; characterized in that, If the instantaneous value of the current in a bridge arm of the converter valve at the current moment is greater than or equal to the lower threshold of the set threshold interval and less than or equal to the upper threshold of the set threshold interval, it is determined that the current direction of the bridge arm is the same as the current direction of the bridge arm at the moment before the most recent moment of entering the set threshold interval from the current moment; the lower threshold is a negative number, and the upper threshold is a positive number.

2. The flexible DC converter valve control method according to claim 1, characterized in that: If the instantaneous value of the current of a certain bridge arm of the converter valve at the current moment is greater than the upper limit threshold of the set threshold interval, then the current direction of the bridge arm is determined to be the charging direction; if the instantaneous value of the current of a certain bridge arm of the converter valve at the current moment is less than the lower limit threshold of the set threshold interval, then the current direction of the bridge arm is determined to be the discharging direction.

3. The flexible DC converter valve control method according to claim 1 or 2, characterized in that: The lower limit of the threshold range is set to -40A~-10A.

4. The flexible DC converter valve control method according to claim 1 or 2, characterized in that: The upper threshold of the threshold interval is set to range from 10A to 40A.

5. A flexible DC converter valve control system, comprising a processor, characterized in that: The processor is used to execute a computer program to implement the steps of the flexible DC converter valve control method according to any one of claims 1 to 4.

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