Reversing method of high-voltage direct-current transmission reversing system and high-voltage direct-current transmission reversing system

Abstract

The invention relates to a commutation method of a high-voltage direct-current transmission commutation system and the high-voltage direct-current transmission commutation system. The commutation method comprises the following steps: detecting an output current of a commutation module, and obtaining a reference current corresponding to a first direction; the output current of the commutation module is adjusted according to the output current and the reference current, so that the commutation module performs current commutation from the second direction; the first direction and the second direction are opposite directions. According to the embodiment of the invention, the output current of the commutation module can be adjusted according to the output current of any commutation module and the reference current feedback corresponding to the first direction, so that the power flow inversion can be realized by performing current commutation on the output current of the commutation module; according to the embodiment of the invention, power inversion can be realized without a direct current voltage polarity inversion mode, and a control mode for performing power flow inversion control by adopting the commutation module can be simplified.

Description

Technical Field

[0001] The present application relates to the technical field of power grids, and in particular to a commutation method of a high-voltage direct current transmission commutation system and a high-voltage direct current transmission commutation system. Background Art

[0002] With the continuous development of power grids, high-voltage direct current (LCC-HVDC) systems are becoming increasingly important in the long-distance and large-capacity power transmission process. At present, the high-voltage direct current transmission system not only needs to transmit power from the power generation center to the load center, but also needs to transmit the DC power in the reverse direction. Therefore, it is necessary to achieve power flow reversal by controlling the converters included in the high-voltage direct current transmission system.

[0003] However, when conventional technologies use inverters for power flow reversal control, there is a problem of complex control methods. Summary of the invention

[0004] Based on this, it is necessary to provide a commutation method and a high-voltage direct current transmission commutation system for the above-mentioned technical problems, which can simplify the control method of using a converter for power flow reversal control.

[0005] In a first aspect, the present application provides a commutation method for a high-voltage direct current transmission commutation system, the method being applied to a high-voltage direct current transmission commutation system, the high-voltage direct current transmission commutation system comprising at least two commutation modules, the commutation method comprising:

[0006] Detecting the output current of the commutation module, and obtaining a reference current corresponding to the first direction;

[0007] The output current of the commutation module is adjusted according to the output current and the reference current, so that the commutation module performs current commutation from a second direction; the first direction and the second direction are opposite directions.

[0008] In one embodiment, adjusting the output current of the commutation module according to the output current and the reference current includes:

[0009] Determining a current trigger angle according to the output current and the reference current;

[0010] The output current of the commutation module is adjusted according to the current trigger angle.

[0011] In one embodiment, determining the current trigger angle according to the output current and the reference current includes:

[0012] Performing a difference operation on the output current and the reference current to obtain a first control variable;

[0013] The first control variable is input into a proportional-integral controller for calculation to obtain the current trigger angle.

[0014] In one embodiment, adjusting the output current of the commutation module according to the current trigger angle includes:

[0015] A target regulating current is determined according to the current trigger angle, and the output current of the switching module is regulated to the target regulating current.

[0016] In one embodiment, the method further comprises:

[0017] Detecting the output voltage of the commutation module and obtaining a reference voltage;

[0018] Determining a voltage trigger angle according to the output voltage and the reference voltage;

[0019] The output voltage of the commutation module is adjusted according to the voltage trigger angle so that the output voltage is not less than a preset voltage threshold.

[0020] In one embodiment, obtaining a reference voltage includes:

[0021] Performing a division operation on the reference reactive power and the reference active power of the commutation module to obtain a second control amount;

[0022] Performing angle calculation on the second control amount to obtain a calculated second control amount;

[0023] Performing a product operation on the calculated second control amount and the apparent power of the commutation module to obtain a third control amount;

[0024] The third control amount and the output current are divided to obtain the reference voltage.

[0025] In one embodiment, determining the voltage trigger angle according to the output voltage and the reference voltage includes:

[0026] Performing an angle operation on the reference voltage to obtain a first angle;

[0027] Performing an angle operation on the output voltage to obtain a second angle;

[0028] Performing a difference operation on the first angle and the second angle to obtain a fourth control amount;

[0029] The fourth control variable is input into a proportional-integral controller for calculation to obtain the voltage trigger angle.

[0030] In a second aspect, the present application further provides a high-voltage direct current transmission commutation system, the high-voltage direct current transmission commutation system comprising a first converter station, a second converter station, a coupling circuit and a controller;

[0031] Wherein, the first converter station includes a first switching module; the first switching module includes at least three first switch branches connected in parallel; the second converter station includes a second switching module; the second switching module includes at least three second switch branches connected in parallel; the first switch branch includes a first group of switch circuits and a second group of switch circuits, the first group of switch circuits includes a first thyristor and a second thyristor connected in parallel, and the current conduction directions of the first thyristor and the second thyristor are opposite; the second group of switch circuits includes a third thyristor and a fourth thyristor connected in parallel, and the current conduction directions of the third thyristor and the fourth thyristor are opposite; the first thyristor and the third thyristor are connected in series, and the current conduction directions of the first thyristor and the third thyristor are the same, the second thyristor and the fourth thyristor are connected in series, and the current conduction directions of the second thyristor and the fourth thyristor are the same;

[0032] The input end of the first switching module is connected to the first external power supply circuit, the output end of the first switching module is connected to the input end of the coupling circuit, the output end of the coupling circuit is connected to the input end of the second switching module, and the output end of the second switching module is connected to the second external power supply circuit;

[0033] The controller is used to execute the steps of the method in any one of the embodiments of the first aspect above.

[0034] In one embodiment, the first converter station further includes a third switching module, the third switching module includes at least three third switch branches connected in parallel; the second converter station further includes a fourth switching module; the fourth switching module includes at least three fourth switch branches connected in parallel;

[0035] The third switch branch includes a third group of switch circuits and a fourth group of switch circuits, the third group of switch circuits includes a fifth thyristor and a sixth thyristor connected in parallel, and the current conduction directions of the fifth thyristor and the sixth thyristor are opposite; the fourth group of switch circuits includes a seventh thyristor and an eighth thyristor connected in parallel, and the current conduction directions of the seventh thyristor and the eighth thyristor are opposite; the fifth thyristor and the seventh thyristor are connected in series, and the current conduction directions of the fifth thyristor and the seventh thyristor are the same, the sixth thyristor and the eighth thyristor are connected in series, and the current conduction directions of the sixth thyristor and the eighth thyristor are the same.

[0036] In one embodiment, the first external power supply circuit includes a first high-voltage power supply and a first transformer, the first high-voltage power supply is connected to one end of the first transformer through a high-voltage line, and the other end of the first transformer is connected to the input end of the first switching module; the second external power supply circuit includes a second high-voltage power supply and a second transformer, the second high-voltage power supply is connected to one end of the second transformer through a high-voltage line, and the other end of the second transformer is connected to the output end of the second switching module.

[0037] The commutation method and the commutation system of the HVDC transmission commutation system described above detect the output current of the commutation module and obtain the reference current corresponding to the first direction; adjust the output current of the commutation module according to the output current and the reference current, so that the commutation module performs current commutation from the second direction; the first direction and the second direction are opposite directions. The embodiment of the present application can adjust the output current of the commutation module according to the output current of any commutation module and the reference current feedback corresponding to the first direction, so that the power flow reversal can be achieved by current commutation of the output current of the commutation module. Therefore, the embodiment of the present application does not need to achieve power reversal by reversing the polarity of the DC voltage, and can simplify the control method of power flow reversal control using the commutation module. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the technical solutions in the embodiments of the present application or related technologies, the drawings required for use in the embodiments of the present application or related technical descriptions will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

[0039] Figure 1 A diagram showing an application environment of a commutation method for a high voltage direct current transmission commutation system in one embodiment;

[0040] Figure 2 A schematic flow chart of a commutation method of a high-voltage direct current transmission commutation system in one embodiment;

[0041] Figure 3 is a schematic flow chart of an output current regulation step in an embodiment;

[0042] Figure 4 A flowchart of the steps of determining the current trigger angle in a DC current control process in one embodiment;

[0043] Figure 5 A flowchart of the steps of determining the current trigger angle in a DC current control process in another embodiment;

[0044] Figure 6A schematic flow chart of a commutation method of a high voltage direct current transmission commutation system in another embodiment;

[0045] Figure 7 A flowchart of a step of obtaining a reference voltage in a reactive outer loop process in one embodiment;

[0046] Figure 8 A flowchart of a step of obtaining a reference voltage in a reactive outer loop process in another embodiment;

[0047] Fig. 9 A flowchart of a step of obtaining a reference voltage during a DC voltage control process in an embodiment;

[0048] Fig.10 A flowchart of a step of obtaining a reference voltage during a DC voltage control process in another embodiment;

[0049] Fig.11 A schematic diagram of the change of electrical quantity during the power flow reversal process in one embodiment;

[0050] Fig.12 A schematic diagram of a 39-node test system in one embodiment;

[0051] FIG13( a ) is a schematic diagram showing the change of DC current during a test in one embodiment;

[0052] FIG13( b ) is a schematic diagram showing changes in the trigger angle of the rectifier side during a test in one embodiment;

[0053] FIG13( c ) is a schematic diagram showing changes in the trigger angle of the inverter side during a test in one embodiment;

[0054] FIG13( d ) is a schematic diagram showing the change of DC voltage during the test process in one embodiment;

[0055] FIG13( e ) is a schematic diagram showing the change of reactive power of S1 during the test process in one embodiment;

[0056] FIG13( f ) is a schematic diagram showing the change of reactive power of S2 during the test process in one embodiment;

[0057] FIG13( g) is a schematic diagram showing changes in active power on the DC side during a test in one embodiment;

[0058] FIG13( h ) is a schematic diagram of the change of the generator speed during the test process in one embodiment;

[0059] Fig.14 A circuit diagram of a high voltage direct current transmission commutation system in one embodiment;

[0060] Fig.15A circuit diagram of a high voltage direct current transmission commutation system in another embodiment;

[0061] Fig.16 FIG. 4 is a diagram showing the internal structure of a computer device in one embodiment. DETAILED DESCRIPTION

[0062] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0063] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by technicians in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" in the specification and claims of this application and the above-mentioned figure descriptions and any variations thereof are intended to cover non-exclusive inclusions.

[0064] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0065] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0066] With the continuous development of power grids, high-voltage direct current (LCC-HVDC) systems are becoming increasingly important in the long-distance, large-capacity power transmission process. At present, with the development of cross-border interconnection, embedded HVDC and multi-terminal DC systems, power grids between different countries and regions need to be interconnected to achieve more flexible energy complementarity and optimized configuration. Therefore, higher power flow reversal requirements are put forward for LCC-HVDC converters, that is, the high-voltage direct current transmission system not only needs to transmit electricity from the power generation center to the load center, but also needs to transmit DC power in the opposite direction. Based on this, it is necessary to achieve power flow reversal by controlling the converters included in the high-voltage direct current transmission system.

[0067] However, the traditional LCC-HVDC system is limited by the unidirectional conductivity of the thyristor in the converter and can only achieve power reversal by reversing the polarity of the DC voltage. However, this method has limitations in cable solutions and multi-terminal DC systems, such as the risk of cable insulation damage and the complexity of DC voltage polarity reversal in multi-terminal systems. Therefore, when traditional technology uses converters for power flow reversal control, there is a problem of complex control methods, which makes it difficult to meet the actual operation requirements of LCC-HVDC systems with increasingly complex scale and structure for power flow reversal.

[0068] After introducing the background technology of the commutation method of the high-voltage direct current transmission commutation system provided by the embodiment of the present application, the implementation environment involved in the commutation method of the high-voltage direct current transmission commutation system provided by the embodiment of the present application will be briefly described below. The commutation method of the high-voltage direct current transmission commutation system provided by the embodiment of the present application can be applied to Figure 1 In the high-voltage direct current transmission commutation system shown, the high-voltage direct current transmission commutation system includes at least two commutation modules, and each commutation module is connected. For a detailed introduction to the high-voltage direct current transmission commutation system, reference may be made to the following specific embodiments, which will not be described in detail here. Those skilled in the art will understand that Figure 1 The structure shown in the figure is only a block diagram of a part of the structure related to the scheme of the present application, and does not constitute a limitation on the high-voltage direct current transmission commutation system to which the scheme of the present application is applied. The specific high-voltage direct current transmission commutation system may include more or fewer components than those shown in the figure, or combine certain components, or have a different arrangement of components.

[0069] In one embodiment, Figure 2 As shown, a commutation method for a high voltage direct current transmission commutation system is provided, and the method is applied to Figure 1 The high-voltage direct current transmission commutation system in FIG. 1 is taken as an example to illustrate, and the following steps are included:

[0070] S201, detecting an output current of a commutation module, and obtaining a reference current corresponding to a first direction.

[0071] Among them, the commutation module can be a commutation valve or a converter, the commutation module is used to realize power flow reversal in a high-voltage direct current transmission commutation system, the output current refers to the output current of any commutation module, the first direction refers to the current direction after the power flow reversal, and the reference current refers to the reference output current of any commutation module after the power flow reversal.

[0072] In the embodiment of the present application, optionally, the high-voltage direct current transmission commutation system can detect the output current of any commutation module in real time, and obtain the reference current corresponding to the first direction in real time. Alternatively, the high-voltage direct current transmission commutation system can also periodically detect the output current of any commutation module, and periodically obtain the reference current corresponding to the first direction. Of course, the embodiment of the present application does not limit the specific implementation method for detecting the output current and obtaining the reference current.

[0073] S202, adjusting the output current of the commutation module according to the output current and the reference current, so that the commutation module performs current commutation in a second direction; the first direction and the second direction are opposite directions.

[0074] The second direction refers to the current direction of the current output by any commutation module before the power flow is reversed, and the first direction and the second direction are opposite directions.

[0075] In the embodiment of the present application, optionally, the high-voltage direct current transmission commutation system can determine the current change amount corresponding to the above-mentioned commutation module according to the output current and the reference current, and adjust the output current of the commutation module according to the current change amount corresponding to the above-mentioned commutation module, so that the commutation module performs current commutation from the second direction, that is, the commutation module commutates from the second direction to the first direction, thereby realizing power flow reversal. Alternatively, the high-voltage direct current transmission commutation system can also determine the current triggering angle corresponding to the above-mentioned commutation module according to the output current and the reference current, and adjust the output current of the commutation module according to the current triggering angle corresponding to the above-mentioned commutation module, so that the commutation module performs current commutation from the second direction, that is, the commutation module commutates from the second direction to the first direction, thereby realizing power flow reversal. Of course, the embodiment of the present application does not limit the specific implementation method of current commutation.

[0076] In the commutation method of the above-mentioned high-voltage direct current transmission commutation system, the output current of the commutation module is detected, and the reference current corresponding to the first direction is obtained; the output current of the commutation module is adjusted according to the output current and the reference current, so that the commutation module performs current commutation from the second direction; the first direction and the second direction are opposite directions. The embodiment of the present application can adjust the output current of the commutation module according to the output current of any commutation module and the reference current feedback corresponding to the first direction, so that the power flow reversal can be achieved by current commutation of the output current of the commutation module. Therefore, the embodiment of the present application does not need to achieve power reversal by reversing the polarity of the DC voltage, and can simplify the control method of power flow reversal control using the commutation module.

[0077] In one embodiment, a method for adjusting the output current of the switching module is provided, that is, "adjusting the output current of the switching module according to the output current and the reference current" in the above S202, such as Figure 3 As shown, including:

[0078] S301, determining a current trigger angle according to an output current and a reference current.

[0079] In an embodiment of the present application, the high voltage direct current transmission commutation system can determine the current trigger angle corresponding to the above commutation module according to the output current and the reference current. In one embodiment, S301 includes:

[0080] A difference operation is performed on the output current and the reference current to obtain a first control variable.

[0081] The first control variable is input into a proportional-integral controller for operation to obtain a current trigger angle.

[0082] In the embodiment of the present application, it is assumed that the initial power flow direction is from the switching module S1 to the switching module S2. Then, if Figure 4 As shown, Figure 4 FIG. 1 is a flow chart of the steps of determining the current trigger angle in a DC current control process according to an embodiment, wherein I S1_dc_ref is the DC current reference value output by the commutation module S1 before power reversal, I S1_dc_ref1 is the DC current reference value (i.e. reference current) output by the commutation module S1 during power reversal, I S1_dc is the DC current (i.e. output current) actually output by the commutation module S1, PI refers to the proportional integral controller, α S1toS2_I Indicates the current trigger angle output during the process from S1 to S2, and Reverse Start1 is the trigger signal for power flow reversal. Figure 4 As shown, before the power flow reversal begins, the commutation module S1 is I S1_dc_ref The DC current corresponding to the commutation module S1 is controlled by the current reference value. After receiving the trigger signal of the power flow reversal, the high-voltage direct current transmission commutation system can start the power flow reversal. At this time, the DC current reference value of the commutation module S1 will become I S1_dc_ref1 , thus, the HVDC transmission commutation system can output the current I S1_dc and reference current I S1_dc_ref1 Perform difference calculation to obtain the first control quantity, and input the first control quantity into the proportional integral controller PI for calculation to obtain the current trigger angle α S1toS2_I .

[0083] In the embodiment of the present application, it is assumed that the initial power flow direction is from the switching module S2 to the switching module S1. Then, if Figure 5 As shown, Figure 5 FIG. 1 is a flow chart of the steps of determining the current trigger angle in a DC current control process according to another embodiment, wherein I S2_dc_ref is the DC current reference value output by the commutation module S2 before power reversal, I S2_dc_ref1is the DC current reference value (i.e. reference current) output by the commutation module S2 during power reversal, I S2_dc is the DC current (i.e. output current) actually output by the commutation module S2. PI refers to the proportional integral controller. α S2toS1_I Represents the current trigger angle output during the process from S2 to S1. Reverse Start2 is the trigger signal for the anti-parallel valve. V S2_dc0 is the DC voltage reference value of the no-load DC voltage of the commutation module S2, V S2_dc It is the DC voltage actually output by the commutation module S2.

[0084] Combination Figure 5 As shown, before the power flow reversal begins, the commutation module S2 is I S2_dc_ref The DC current corresponding to the switching module S2 is controlled by using the current reference value. After receiving the trigger signal of the anti-parallel valve, the high-voltage direct current transmission switching system can start the anti-parallel valve in the switching module S2. At this time, the DC current reference value of the switching module S2 will become I S2_dc_ref1 , thus, the HVDC transmission commutation system can output the current I S2_dc and reference current I S2_dc_ref1 In addition, the high-voltage direct current transmission commutation system can also use V S2_dc0 and V S2_dc Perform division operation and calculate the angle of the result after division operation to obtain the initial trigger angle α S2_initial Thus, the first control amount and the initial trigger angle α can be S2_initial Input to the proportional integral controller PI for calculation to obtain the current trigger angle α S2toS1_I For example, the initial trigger angle α of S2 is S2_initial It can be calculated according to the following formula (1):

[0085] (1)

[0086] It should be noted that the only difference between the inverter side S2 and the rectifier side S1 is that the DC current control adds the initial trigger angle control. This is because S2 needs to take on the DC current control of the considered power flow reversal process (power changes from S1 to S2 to S2 to S1).

[0087] S302, adjusting the output current of the commutation module according to the current trigger angle.

[0088] In the embodiment of the present application, the high-voltage direct current transmission commutation system can adjust the output current of the commutation module according to the current trigger angle corresponding to the commutation module, so that the commutation module performs current commutation from the second direction, that is, the commutation module commutates from the second direction to the first direction, thereby realizing power flow reversal. In one embodiment, S302 includes:

[0089] The target regulating current is determined according to the current trigger angle, and the output current of the commutation module is adjusted to the target regulating current.

[0090] In the embodiment of the present application, the high-voltage direct current transmission commutation system can determine the target regulation current of the commutation module according to the above-mentioned current trigger angle, and adjust the output current of the commutation module to the target regulation current, so that the commutation module performs current commutation from the second direction, that is, the current direction of the commutation module is commutated from the second direction to the first direction, thereby realizing power flow reversal.

[0091] In this embodiment, the current trigger angle can be accurately determined based on the output current and the reference current, so that the output current of the switching module can be feedback-adjusted based on the accurate current trigger angle, so that the current direction of the switching module is switched from the second direction to the first direction, thereby realizing power flow reversal.

[0092] In one embodiment, a method for adjusting the output voltage of a commutation module is provided, namely, a commutation method of the above-mentioned high-voltage direct current transmission commutation system, such as Figure 6 As shown, it also includes:

[0093] S203, detecting the output voltage of the commutation module and obtaining a reference voltage.

[0094] The output voltage refers to the output voltage of any commutation module, and the reference voltage refers to the reference output voltage of any commutation module after power flow reversal.

[0095] In the embodiment of the present application, optionally, the high-voltage direct current transmission commutation system can detect the output voltage of any commutation module in real time, and obtain the reference voltage in real time. Alternatively, the high-voltage direct current transmission commutation system can also periodically detect the output voltage of any commutation module, and periodically obtain the reference voltage corresponding to the first direction. Of course, the embodiment of the present application does not limit the specific implementation method for detecting the output voltage and obtaining the reference voltage.

[0096] S204, determining a voltage trigger angle according to the output voltage and the reference voltage.

[0097] In the embodiment of the present application, optionally, the high-voltage direct current transmission commutation system can directly determine the voltage trigger angle according to the output voltage and the reference voltage. Alternatively, the high-voltage direct current transmission commutation system can also first determine the fourth control amount according to the output voltage and the reference voltage, and then determine the voltage trigger angle according to the fourth control amount. Of course, the embodiment of the present application does not limit the specific implementation method for determining the voltage trigger angle.

[0098] S205, adjusting the output voltage of the commutation module according to the voltage trigger angle so that the output voltage is not less than a preset voltage threshold.

[0099] In an embodiment of the present application, exemplarily, the high-voltage direct current transmission commutation system can determine the target regulation voltage of the commutation module according to the above-mentioned voltage trigger angle, and adjust the output voltage of the commutation module to the target regulation voltage, so that the output voltage of the commutation module is always not less than a preset voltage threshold.

[0100] In this embodiment, the output voltage of the commutation module can be detected, and the reference voltage can be obtained. Therefore, the voltage trigger angle can be accurately determined according to the output voltage and the reference voltage. Afterwards, the output voltage of the commutation module can be accurately adjusted according to the accurate voltage trigger angle so that the output voltage is not less than the preset voltage threshold.

[0101] In one embodiment, a method for obtaining a reference voltage is provided, namely, the “obtaining a reference voltage” in the above S203, including:

[0102] A division operation is performed on the reference reactive power and the reference active power of the commutation module to obtain a second control variable.

[0103] An angle operation is performed on the second control amount to obtain a second control amount after the operation.

[0104] The calculated second control amount and the apparent power of the commutation module are multiplied to obtain a third control amount.

[0105] A division operation is performed on the third control variable and the output current to obtain a reference voltage.

[0106] In the embodiment of the present application, it is assumed that the initial power flow direction is from the switching module S1 to the switching module S2. Then, if Figure 7 As shown, Figure 7 FIG. 1 is a flowchart of the steps of obtaining a reference voltage in a reactive outer loop process in an embodiment, wherein Q S1_ref is the reactive power reference value (i.e., reference reactive power) at the commutation module S1, V S1_dc_ref1 is the DC voltage reference value (i.e., reference voltage) during power reversal, P S1 is the active power flowing into the commutation module S1 (i.e., the reference active power), S S1is the apparent power flowing into commutation module S1. Figure 7 As shown, the HVDC transmission commutation system can be used to commutate the reference reactive power Q of the commutation module S1. S1_ref and reference active power P S1 Perform a division operation to obtain a second control quantity, and perform an angle operation on the second control quantity to obtain the calculated second control quantity. Then, the calculated second control quantity and the apparent power S of the commutation module S1 can be compared. S1 Perform product operation to obtain the third control quantity, and then multiply the third control quantity and the output current I S1_dc Perform division operation to obtain the reference voltage V S1_dc_ref1 For example, according to the steady-state operation formula of the rectifier, the formula for calculating the reference voltage can be shown as follows (2):

[0107] (2)

[0108] In the embodiment of the present application, it is assumed that the initial power flow direction is from the switching module S2 to the switching module S1. Then, if Figure 8 As shown, Figure 8 FIG. 1 is a flowchart of the steps of obtaining a reference voltage in a reactive outer loop process in another embodiment, wherein Q S2_ref is the reactive power reference value (i.e., reference reactive power) at the commutation module S2, V S2_dc_ref1 is the DC voltage reference value (i.e., reference voltage) during power reversal, P S2 is the active power flowing into the commutation module S2 (i.e., the reference active power), S S2 is the apparent power flowing into commutation module S2. Figure 7 As shown, the HVDC transmission commutation system can be used to commutate the reference reactive power Q of the commutation module S2. S2_ref and reference active power P S2 Perform a division operation to obtain a second control quantity, and perform an angle operation on the second control quantity to obtain the calculated second control quantity. Then, the calculated second control quantity and the apparent power S of the commutation module S2 can be compared. S2 Perform product operation to obtain the third control quantity, and then multiply the third control quantity and the output current I S2_dc Perform division operation to obtain the reference voltage V S2_dc_ref1 .

[0109] In this embodiment, the reference reactive power and the reference active power of the commutation module can be divided to obtain the second control quantity, and the second control quantity can be angle-operated to obtain the calculated second control quantity. Thus, the calculated second control quantity and the apparent power of the commutation module can be multiplied to obtain the third control quantity, and the third control quantity and the output current can be divided to accurately determine the reference voltage corresponding to the commutation model.

[0110] In one embodiment, a method for determining a voltage trigger angle is provided, namely, the step of “determining a voltage trigger angle according to an output voltage and a reference voltage” in S204 above, including:

[0111] An angle operation is performed on the reference voltage to obtain a first angle.

[0112] An angle operation is performed on the output voltage to obtain a second angle.

[0113] A difference operation is performed on the first angle and the second angle to obtain a fourth control amount.

[0114] The fourth control variable is input into the proportional-integral controller for calculation to obtain the voltage trigger angle.

[0115] In the embodiment of the present application, it is assumed that the initial power flow direction is from the switching module S1 to the switching module S2. Then, if Fig. 9 As shown, Fig. 9 FIG. 1 is a flow chart of the steps of obtaining a reference voltage during a DC voltage control process in an embodiment, wherein V S1_dc_ref is the DC voltage reference value before power reversal, V S1_dc_ref1 is the DC voltage reference value during power reversal, V S1_dc0 is the DC voltage reference value of the no-load DC voltage of the commutation module S1; α S1_Qcontrol is the trigger angle corresponding to the reactive power control at the commutation module S1, α S1_Icontrol is the trigger angle corresponding to the current control, α S1_γcontrol is the trigger angle corresponding to the fixed cut-off angle control loop, α S1_initial represents the initial trigger angle at the reversing module S1 when the anti-parallel valve is started, α S2toS1_Q Indicates the voltage trigger angle of reactive power control output in the flow direction from S1 to S2, α S1toS2_Q Indicates the voltage trigger angle of reactive power control output in the flow direction from S2 to S1, X T Represents the reactance of the transformer.

[0116] Combination Fig. 9 As shown, the HVDC transmission commutation system can be used to control the reference voltage V S1_dc_ref1 Perform angle calculation to obtain the first angle and the output voltage V S1_dc Angle calculation is performed to obtain the second angle. Thus, the difference calculation can be performed on the first angle and the second angle to obtain the fourth control amount. Exemplarily, according to the converter steady-state operation formula, the fourth control amount α S1_Qref It can be expressed as shown in the following formula (3):

[0117] (3)

[0118] Among them, α S1_Qref is the firing angle calculated based on the DC voltage reference, α S1_Qref exist Fig. 9 Not shown, its specific position is the same as α S2_Qref The position is similar, so you can refer to Fig.10 Medium α S2_Qref Position. S1_dc0 is the no-load DC voltage of S1, B is the number of 6-pulsation converter valves, T1 is the transformer ratio of S1, V LL1 is the measured line voltage of S1.

[0119] In order to minimize the DC transient process that occurs when the reverse parallel valve is triggered, the Fig. 9 As shown, the initial trigger angle α can also be S1_initial As the input of PI, at this time, the initial trigger angle α S1_initial It can be expressed as the measured line voltage V S1_dc and no-load DC voltage V S1_dc0 The function is shown in the following formula (4):

[0120] (4)

[0121] That is, it can be understood that the fourth control amount and the initial trigger angle α S1_initia Input to the proportional integral controller for calculation to obtain the voltage trigger angle α S2toS1_Q .

[0122] In the embodiment of the present application, it is assumed that the initial power flow direction is from the switching module S2 to the switching module S1. Then, if Fig.10 As shown, Fig.10 FIG. 1 is a flowchart of the steps of obtaining a reference voltage in a DC voltage control process according to another embodiment, wherein V S2_dc_ref is the DC voltage reference value before power reversal, V S2_dc_ref1 is the DC voltage reference value during power reversal, V S2_dc0 is the DC voltage reference value of the no-load DC voltage of the commutation module S2; α S2_Qcontrol is the trigger angle corresponding to the reactive power control at the commutation module S2, α S2_Icontrol is the trigger angle corresponding to the current control, α S2_γcontrol is the trigger angle corresponding to the fixed cut-off angle control loop, α S2_initial represents the initial trigger angle at the reversing module S2 when the anti-parallel valve is started, α S1toS2_Q Indicates the voltage trigger angle of reactive power control output in the flow direction from S2 to S1, X T Represents the reactance of the transformer.

[0123] Combination Fig.10As shown, the HVDC transmission commutation system can be used to control the reference voltage V S2_dc_ref1 Perform angle calculation to obtain the first angle and the output voltage V S2_dc Perform angle calculation to obtain the second angle. Thus, the difference calculation can be performed on the first angle and the second angle to obtain the fourth control amount α S2_Qref After that, the fourth control variable can be directly input into the proportional integral controller for calculation to obtain the voltage trigger angle α S1toS2_Q Exemplarily, the HVDC transmission commutation system may use a PI controller to minimize the control error of the fourth control variable to generate α S2_Qcontrol , and then from α S2_Qcontrol , α S2_Icontrol , α S2_γcontrol Select the minimum value in S2 to get the final voltage trigger angle α S1toS2_Q .

[0124] In this embodiment, the reference voltage can be operated at an angle to obtain a first angle, the output voltage can be operated at an angle to obtain a second angle, and the first angle and the second angle can be operated at a difference to obtain a fourth control amount. Thus, the fourth control amount can be input into a proportional-integral controller for operation to accurately determine the voltage trigger angle.

[0125] In an optional embodiment, a commutation method of a high-voltage direct current transmission commutation system is provided, which is applied to the high-voltage direct current transmission commutation system, comprising:

[0126] like Fig.11 As shown, Fig.11 FIG. 1 is a schematic diagram of the change of electrical quantity during the flow reversal process in one embodiment. In order to achieve flow reversal, I S1_dc_ref1 The DC current will gradually decrease according to the command of the controller in the HVDC transmission commutation system, resulting in a DC current such as Fig.11 The specific control method of reducing the current is as described in the above embodiments and Figure 4-Figure 5 At the same time, in the time period t1-t2, at the switching module S2, once the power flow reversal is started, the controller will switch to Figure 8 and Fig.10 The reactive power control process shown in the figure, at this time, the reactive power of S2 will be controlled at QS2_ref (i.e. Fig.11 Q2 in the circuit breaker is used to prevent the reactive power consumption of S2 from decreasing due to the reduction of DC current during the power flow reversal, which will increase the reactive power of the converter station and the AC network and cause overvoltage problems on the AC side. Due to the increase in the cut-off angle caused by the current margin and the reduction of DC current, α S2_Qcontrol The final trigger angle of S2 is passed to α S1toS2_Q .

[0127] During the t2-t3 period, at a lower DC voltage level, S2 will operate at a higher turn-off angle, causing additional stress on the thyristor valve. To shorten this phase, once the DC voltage V S1_dc To achieve Fig.11 The predetermined voltage threshold V S1_dcmin In the embodiment of the present application, the slope rate of the DC current is increased between t2 and t3, so that the DC voltage reference value is maintained at V S1_dcmin , thereby maintaining a constant DC voltage level between t2 and t3. S1_dc When it decreases to a smaller value, the valves in S1 and S2 that are in the on state will be blocked at time t3. When the valve is blocked, the DC voltage will remain unchanged because the DC current is very small.

[0128] In the t3-t4 period, the reverse parallel valve can be triggered by triggering the Reverse Start2 signal. Before triggering, in order to minimize the instantaneous change of the DC voltage, the initial trigger angle α can be set to S1_initial and α S2_initial The trigger angle passed to the final output, such as Figure 5 and Fig. 9 Once the reverse parallel valve is triggered, the DC current will start to flow in the reverse direction, as shown in Fig.11 As shown, S1 will then control its reactive power consumption as Figure 7 and Fig. 9 As shown, S2 will control the DC current with a positive slope, such as Figure 5 As shown, in this way, the DC current I S1_dc The power reversal process is completed at time t4 when the DC current increases in the reverse direction with a trajectory similar to that of the t1-t3 stage and the DC voltage returns to the rated value when the DC current flows in the opposite direction to the reference value.

[0129] In addition, in order to verify the effectiveness of the control method in the embodiment of the present application, a 39-node test system was built in the PSCAD / EMTDC software for verification. Fig.12 As shown, Fig.12 Schematic diagram of a 39-node test system in one embodiment, wherein: Fig.12The IEEE 39-node transmission system, 39 nodes may include but are not limited to bus nodes, transformer nodes and power supply nodes, wherein the proposed bidirectional LCC HVDC system (i.e., high-voltage direct current transmission commutation system) is used to replace the AC line between bus No. 16 and bus No. 19. Before the power flow is reversed, the HVDC system transmits 250MW of active power from S1 to S2. The purpose of power flow reversal is to reverse the direction of the power flow, that is, to transmit 250MW of active power from S2 to S1. Before the power reversal, the control mode is: S1 is DC current control, and S2 is DC voltage control.

[0130] As shown in Figure 13, Figure 13 shows the simulation results of an embodiment of the present application in the above-mentioned test system, wherein Figure 13 (a) is a schematic diagram of the change of DC current during the test process in an embodiment, Figure 13 (b) is a schematic diagram of the change of the trigger angle on the rectifier side during the test process in an embodiment, Figure 13 (c) is a schematic diagram of the change of the trigger angle on the inverter side during the test process in an embodiment, Figure 13 (d) is a schematic diagram of the change of DC voltage during the test process in an embodiment, Figure 13 (e) is a schematic diagram of the change of S1 reactive power during the test process in an embodiment, Figure 13 (f) is a schematic diagram of the change of S2 reactive power during the test process in an embodiment, Figure 13 (g) is a schematic diagram of the change of active power on the DC side during the test process in an embodiment, and Figure 13 (h) is a schematic diagram of the change of generator speed during the test process in an embodiment.

[0131] Assuming that the power reversal is initiated at 0.5 seconds, it can be seen from Figure 13 (a) that the DC current accurately follows the current reference throughout the power reversal period, drops to zero, and then rises in the opposite direction, and the ramp down of the DC current is initiated by the increase in the firing angle at S1, as shown by Alpha_S1_Icontrol in Figure 13 (b). At the same time, S2 controls the reactive power consumption by reducing the firing angle, as shown by Alpha_S2_Qcontrol in Figure 13 (c). When the DC voltage reaches the threshold of 0.7 pu, as shown in Figure 13 (a), the current ramp rate increases, and as shown in Figure 13 (c), the S2 firing angle tends to stabilize, as shown in Figure 13 (d), and the DC voltage remains stable as shown in Figure 13 (f). It can be seen from Figure 13 (f) that the reactive power consumption of S2 is controlled at the reference value by the given reactive power control. As shown in Figure 13 (e), the reactive power consumption of S1 is slightly reduced due to the reduction of the DC current. As can be seen in Figure 13 (a), at around 5.1s, the DC current reaches zero and the forward-conducting valve is closed. As shown in Figures 13(b) and 13(c), the triggering angles of the anti-parallel valves of Alpha_S1_Qcontrol and Alpha_S2_Icontrol are calculated based on the AC and DC voltage measurements to minimize transients on the DC side. At about 6.1s, the anti-parallel valves are triggered to increase the DC current in the opposite direction. As can be seen in Figure 13 (e), the reactive power consumption of S1 is well controlled during the ramp-up process. As shown in Figure 13 (g), the reverse trajectory of the active power is similar to that of the DC current. Figure 13 (h) shows the generator speeds of G4 (whose speed is ω4) and G5 (whose speed is ω5) on the AC network S1 side and G8 (whose speed is ω8) and G9 (whose speed is ω9) on the AC network S2 side. It can be seen from Figure 13 (h) that due to the power reversal, the generator speeds of G4 and G5 increase slightly because the loads of these two generators decrease net, and the generator speeds of G4 and G5 are the same as their speeds in the same synchronous region. At the same time, due to the net increase in the loads of the generators, the speeds of the generators at G8 and G9 decrease slightly.

[0132] In the commutation method of the above-mentioned high-voltage direct current transmission commutation system, the output current of any commutation module can be adjusted according to the output current of any commutation module and the reference current feedback corresponding to the first direction, so that the power flow reversal can be achieved by current commutation of the output current of the commutation module. Therefore, the embodiment of the present application does not need to achieve power reversal by reversing the polarity of the DC voltage, and can simplify the control method of using the commutation module for power flow reversal control. In this way, the DC current slope control in the process of power flow reversal can be added to the constant DC current control to achieve the current ramp-down control in the early stage of power flow reversal. In addition to current control, each converter station can also add reactive power outer loop control to the DC voltage control outer loop, which can realize low-voltage operation and control the reactive power during the reversal period by adding the voltage reference value obtained by reactive power calculation, and reduce the reactive loss of the converter station during power flow reversal by adjusting the slope of current change and reactive power control, so as to achieve the current ramp control during power flow reversal to prevent the power flow reversal device from overvoltage, and finally achieve the power flow reversal demand stably and quickly when the power flow of the current reversal converter changes. Therefore, the embodiments of the present application can solve the problem in the related art that the system can only reverse the voltage when the power flow reverses, while retaining the advantages of the LCC-HVDC direct current transmission system such as large capacity and low loss, so as to enable the LCC-HVDC system to be more flexibly connected to the large power grid or multi-terminal DC system, help achieve the complementarity and optimal configuration of multi-regional energy, and improve the flexibility and reliability of power grid interconnection.

[0133] It should be understood that, although the steps in the flowcharts involved in the above embodiments are displayed in sequence according to the indication of the arrows, these steps are not necessarily executed in sequence according to the order indicated by the arrows. Unless there is a clear explanation in this article, the execution of these steps is not strictly limited in order, and these steps can be executed in other orders. Moreover, at least a part of the steps in the flowcharts involved in the above embodiments may include multiple steps or multiple stages, and these steps or stages are not necessarily executed at the same time, but can be executed at different times, and the execution order of these steps or stages is not necessarily carried out in sequence, but can be executed in turn or alternately with other steps or at least a part of the steps or stages in other steps.

[0134] In one embodiment, Fig.14 As shown, a high-voltage direct current transmission commutation system is provided, the high-voltage direct current transmission commutation system comprising a first converter station S1, a second converter station S2, a coupling circuit 13 and a controller (not shown in the figure);

[0135] Among them, the first converter station S1 includes a first switching module 11; the first switching module 11 includes at least three first switch branches connected in parallel; the second converter station S2 includes a second switching module 12; the second switching module 12 includes at least three second switch branches connected in parallel; the first switch branch includes a first group of switch circuits and a second group of switch circuits, the first group of switch circuits includes a first thyristor and a second thyristor connected in parallel, the current conduction directions of the first thyristor and the second thyristor are opposite, that is, the first thyristor and the second thyristor form a bidirectional conduction valve; the second group of switch circuits includes a third thyristor and a fourth thyristor connected in parallel, the current conduction directions of the third thyristor and the fourth thyristor are opposite, that is, the third thyristor and the fourth thyristor form another bidirectional conduction valve; the first thyristor and the third thyristor are connected in series, and the current conduction directions of the first thyristor and the third thyristor are the same, the second thyristor and the fourth thyristor are connected in series, and the current conduction directions of the second thyristor and the fourth thyristor are the same;

[0136] The input end of the first switching module 11 is connected to the first external power supply circuit 14, the output end of the first switching module 11 is connected to the input end of the coupling circuit 13, the output end of the coupling circuit 13 is connected to the input end of the second switching module 12, and the output end of the second switching module 12 is connected to the second external power supply circuit 15;

[0137] A controller is used to execute the commutation method in any of the above embodiments.

[0138] Wherein, every two thyristors connected in reverse parallel can form a bidirectional conducting valve, which can also be called an anti-parallel valve or an anti-parallel valve. Fig.14 As shown, the coupling circuit 13 may include two inductors Ls, two resistors R, and a capacitor C. The two inductors Ls and the two resistors R are connected in series, one end of the capacitor C is connected between the two resistors R, and the other end of the capacitor C is grounded. Assume that the positive polarity of active power, reactive power, and DC current is as follows: Fig.14 As shown, Ls is the DC smoothing reactor, R is the equivalent DC resistance, Q Si is the reactive power flowing into Si (i=1, 2), P Si is the active power flowing into Si (i=1, 2), S Si is the apparent power flowing into Si (i=1, 2), V Si_dc is the DC voltage at Si, Z s1 is the equivalent network impedance of the first external power circuit 14 in the AC system, Z s2 is the equivalent network impedance of the second external power supply circuit 15 in the AC system.

[0139] In the above-mentioned high-voltage direct current transmission commutation system, a thyristor can be reversely connected in parallel to each bridge arm of the unidirectionally conducting LCC-HVDC converter in the related technology to ensure that the converter uses a bidirectional commutation valve, thereby establishing a reverse current conduction loop. In this way, based on the above-mentioned bidirectional commutation valve, power reversal can be achieved by reversing the direction of the direct current without reversing the polarity of the direct current voltage, which can simplify the control method of using the commutation module for power flow reversal control and the high-voltage direct current transmission commutation system.

[0140] In one embodiment, Fig.15 As shown, Fig.15 Schematic diagram of a circuit of a high-voltage direct current transmission commutation system in another embodiment, wherein the first converter station S1 further comprises a third commutation module 16, the third commutation module 16 comprises at least three third switch branches connected in parallel; the second converter station S2 further comprises a fourth commutation module 17; the fourth commutation module 17 comprises at least three fourth switch branches connected in parallel;

[0141] The third switch branch includes a third group of switch circuits and a fourth group of switch circuits. The third group of switch circuits includes a fifth thyristor and a sixth thyristor connected in parallel. The current conduction directions of the fifth thyristor and the sixth thyristor are opposite, that is, the fifth thyristor and the sixth thyristor form a bidirectional conduction valve; the fourth group of switch circuits includes a seventh thyristor and an eighth thyristor connected in parallel. The current conduction directions of the seventh thyristor and the eighth thyristor are opposite, that is, the seventh thyristor and the eighth thyristor form a bidirectional conduction valve; the fifth thyristor and the seventh thyristor are connected in series, and the current conduction directions of the fifth thyristor and the seventh thyristor are the same, and the sixth thyristor and the eighth thyristor are connected in series, and the current conduction directions of the sixth thyristor and the eighth thyristor are the same.

[0142] In one embodiment, in combination Fig.14 or Fig.15 As shown, the first external power supply circuit 14 includes a first high-voltage power supply U1 and a first transformer T1, the first high-voltage power supply U1 is connected to one end of the first transformer T1 through a high-voltage line (a, b, c), and the other end of the first transformer T1 is connected to the input end of the first switching module 11; the second external power supply circuit 15 includes a second high-voltage power supply U2 and a second transformer T2, the second high-voltage power supply U2 is connected to one end of the second transformer T2 through a high-voltage line (a, b, c), and the other end of the second transformer T2 is connected to the output end of the second switching module 12.

[0143] In an exemplary embodiment, a controller is provided. The controller may be a computer device, and the computer device may be a terminal. The internal structure diagram thereof may be as shown in FIG. Fig.16As shown. The computer device includes a processor, a memory, an input / output interface, a communication interface, a display unit and an input device. The processor, the memory and the input / output interface are connected through a system bus, and the communication interface, the display unit and the input device are connected to the system bus through the input / output interface. The processor of the computer device is used to provide computing and control capabilities. The memory of the computer device includes a non-volatile storage medium and an internal memory. The non-volatile storage medium stores an operating system and a computer program. The internal memory provides an environment for the operation of the operating system and the computer program in the non-volatile storage medium. The input / output interface of the computer device is used to exchange information between the processor and an external device. The communication interface of the computer device is used to communicate with an external terminal in a wired or wireless manner, and the wireless manner can be implemented through WIFI, a mobile cellular network, near field communication (NFC) or other technologies. When the computer program is executed by the processor, a commutation method of a high-voltage direct current transmission commutation system is implemented. The display unit of the computer device is used to form a visually visible picture, which can be a display screen, a projection device or a virtual reality imaging device. The display screen can be a liquid crystal display screen or an electronic ink display screen, and the input device of the computer device can be a touch layer covering the display screen, or a button, trackball or touchpad set on the computer device shell, or an external keyboard, touchpad or mouse.

[0144] Those skilled in the art will understand that Fig.16 The structure shown in the figure is only a block diagram of a part of the structure related to the solution of the present application, and does not constitute a limitation on the computer device to which the solution of the present application is applied. The specific computer device may include more or fewer components than those shown in the figure, or combine certain components, or have a different arrangement of components.

[0145] In one embodiment, a computer device is provided, including a memory and a processor, wherein a computer program is stored in the memory, and the processor implements the steps in the above-mentioned method embodiments when executing the computer program.

[0146] Those skilled in the art can understand that all or part of the processes in the above-mentioned embodiments can be completed by instructing the relevant hardware through a computer program, and the computer program can be stored in a non-volatile computer-readable storage medium. When the computer program is executed, it can include the processes of the embodiments of the above-mentioned methods. Among them, any reference to the memory, database or other medium used in the embodiments provided in this application can include at least one of non-volatile memory and volatile memory. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory, optical memory, high-density embedded non-volatile memory, resistive random access memory (ReRAM), magnetic random access memory (MRAM), ferroelectric random access memory (FRAM), phase change memory (PCM), graphene memory, etc. Volatile memory can include random access memory (RAM) or external cache memory, etc. As an illustration and not limitation, RAM can be in various forms, such as static random access memory (SRAM) or dynamic random access memory (DRAM). The database involved in each embodiment provided in this application may include at least one of a relational database and a non-relational database. Non-relational databases may include distributed databases based on blockchains, etc., but are not limited to this. The processor involved in each embodiment provided in this application may be a general-purpose processor, a central processing unit, a graphics processor, a digital signal processor, a programmable logic device, a data processing logic device based on quantum computing, an artificial intelligence (AI) processor, etc., but are not limited to this.

[0147] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this application.

[0148] The above embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the present application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

Claims

1. A commutation method for a high voltage direct current transmission commutation system, characterized in that: The method is applied to a high-voltage direct current transmission commutation system, wherein the high-voltage direct current transmission commutation system comprises at least two commutation modules, and the commutation method comprises: Detecting the output current of the commutation module, and obtaining a reference current corresponding to the first direction; The output current of the commutation module is adjusted according to the output current and the reference current, so that the commutation module performs current commutation from a second direction; the first direction and the second direction are opposite directions.

2. The method according to claim 1, characterized in that: The step of adjusting the output current of the switching module according to the output current and the reference current comprises: Determining a current trigger angle according to the output current and the reference current; The output current of the commutation module is adjusted according to the current trigger angle.

3. The method according to claim 2, characterized in that The step of determining the current trigger angle according to the output current and the reference current includes: Performing a difference operation on the output current and the reference current to obtain a first control variable; The first control variable is input into a proportional-integral controller for calculation to obtain the current trigger angle.

4. The method according to claim 2, characterized in that The step of adjusting the output current of the commutation module according to the current trigger angle includes: A target regulating current is determined according to the current trigger angle, and the output current of the switching module is regulated to the target regulating current.

5. The method according to any one of claims 1 to 4, characterized in that The method further comprises: Detecting the output voltage of the commutation module and obtaining a reference voltage; Determining a voltage trigger angle according to the output voltage and the reference voltage; The output voltage of the commutation module is adjusted according to the voltage trigger angle so that the output voltage is not less than a preset voltage threshold.

6. The method according to claim 5, characterized in that The obtaining of the reference voltage comprises: Performing a division operation on the reference reactive power and the reference active power of the commutation module to obtain a second control amount; Performing angle calculation on the second control amount to obtain a calculated second control amount; Performing a product operation on the calculated second control amount and the apparent power of the commutation module to obtain a third control amount; The third control amount and the output current are divided to obtain the reference voltage.

7. The method according to claim 5, characterized in that The step of determining a voltage trigger angle according to the output voltage and the reference voltage comprises: Performing an angle operation on the reference voltage to obtain a first angle; Performing an angle operation on the output voltage to obtain a second angle; Performing a difference operation on the first angle and the second angle to obtain a fourth control amount; The fourth control variable is input into a proportional-integral controller for calculation to obtain the voltage trigger angle.

8. A high voltage direct current transmission commutation system, characterized in that: The high-voltage direct current transmission commutation system comprises a first converter station, a second converter station, a coupling circuit and a controller; Wherein, the first converter station includes a first switching module; the first switching module includes at least three first switch branches connected in parallel; the second converter station includes a second switching module; the second switching module includes at least three second switch branches connected in parallel; the first switch branch includes a first group of switch circuits and a second group of switch circuits, the first group of switch circuits includes a first thyristor and a second thyristor connected in parallel, and the current conduction directions of the first thyristor and the second thyristor are opposite; the second group of switch circuits includes a third thyristor and a fourth thyristor connected in parallel, and the current conduction directions of the third thyristor and the fourth thyristor are opposite; the first thyristor and the third thyristor are connected in series, and the current conduction directions of the first thyristor and the third thyristor are the same, the second thyristor and the fourth thyristor are connected in series, and the current conduction directions of the second thyristor and the fourth thyristor are the same; The input end of the first switching module is connected to the first external power supply circuit, the output end of the first switching module is connected to the input end of the coupling circuit, the output end of the coupling circuit is connected to the input end of the second switching module, and the output end of the second switching module is connected to the second external power supply circuit; The controller is used to execute the commutation method according to any one of claims 1 to 7.

9. The system according to claim 8, characterized in that The first converter station further includes a third switching module, the third switching module includes at least three third switch branches connected in parallel; the second converter station further includes a fourth switching module; the fourth switching module includes at least three fourth switch branches connected in parallel; The third switch branch includes a third group of switch circuits and a fourth group of switch circuits, the third group of switch circuits includes a fifth thyristor and a sixth thyristor connected in parallel, and the current conduction directions of the fifth thyristor and the sixth thyristor are opposite; the fourth group of switch circuits includes a seventh thyristor and an eighth thyristor connected in parallel, and the current conduction directions of the seventh thyristor and the eighth thyristor are opposite; the fifth thyristor and the seventh thyristor are connected in series, and the current conduction directions of the fifth thyristor and the seventh thyristor are the same, the sixth thyristor and the eighth thyristor are connected in series, and the current conduction directions of the sixth thyristor and the eighth thyristor are the same.

10. The system according to claim 8, characterized in that The first external power supply circuit includes a first high-voltage power supply and a first transformer, the first high-voltage power supply is connected to one end of the first transformer through a high-voltage line, and the other end of the first transformer is connected to the input end of the first switching module; the second external power supply circuit includes a second high-voltage power supply and a second transformer, the second high-voltage power supply is connected to one end of the second transformer through a high-voltage line, and the other end of the second transformer is connected to the output end of the second switching module.

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