Method for suppressing direct current harmonics in flexible back-to-back system and harmonic suppressor

Abstract

A method for suppressing harmonics of direct current in a flexible direct current back-to-back system and a harmonic suppressor, comprising: obtaining electrical quantity information of a converter valve in real time, the electrical quantity information including the instantaneous value of the valve-side AC voltage, the instantaneous value of the valve-side AC current, the instantaneous value of the positive and negative DC bus voltage to ground, and the instantaneous value of the bridge arm current; determining the instantaneous value of the circulating current of each phase of the converter valve according to the electrical quantity information; multiplying the instantaneous value of the circulating current of each phase by a preset proportional coefficient respectively to generate the reference voltage superposition amount of each bridge arm; superimposing the original reference voltage of the bridge arm with the corresponding reference voltage superposition amount to generate the final reference voltage of the bridge arm, so as to generate a trigger pulse of the bridge arm switch device according to the final reference voltage. The method can suppress the harmonics of direct current in a flexible direct current back-to-back system without increasing the switching frequency of the switch device and the rated working voltage of the submodule capacitor.

Description

Technical Field

[0001] The present invention relates to the technical field of back-to-back flexible direct current transmission technology, and in particular to a method for suppressing direct current harmonics in a flexible direct current back-to-back system and a harmonic suppressor. Background Art

[0002] A flexible DC back-to-back system is a two-terminal flexible DC transmission system without DC transmission lines, primarily used for AC grid interconnection. It offers the advantages of rapid decoupling and independent control of active and reactive power, enabling power supply to passive systems. It also avoids the commutation failure common in conventional DC transmission technology, resulting in widespread attention.

[0003] The core component of the flexible direct current back-to-back system is the converter valve, and its topology is as follows: Figure 1 As shown, the system includes six bridge arms, each of which includes N HBSM submodules, each of which is equipped with an IGBT switch T1 and an IGBT switch T2. During operation, a modulation algorithm is used to convert the reference voltage of each bridge arm into a trigger pulse for the IGBT switch in each bridge arm, thereby controlling each switch and ultimately achieving control of power, voltage, and other parameters. Due to the implementation of switching frequency optimization strategies, the imbalance between the capacitor voltages of the HBSM submodules in each bridge arm of the converter valve has increased, resulting in harmonics in the DC current and affecting the stable operation of the system. Therefore, it is urgent to propose a DC current harmonic suppression strategy for flexible DC back-to-back systems. Summary of the Invention

[0004] (1) Purpose of the invention

[0005] The purpose of the present invention is to provide a method and a harmonic suppressor for suppressing direct current harmonics in a flexible direct current back-to-back system, which are used to solve the harmonic problem in the direct current when the system is running.

[0006] (2) Technical solution

[0007] To solve the above problems, the first aspect of the present application provides a method for suppressing DC current harmonics in a flexible DC back-to-back system, comprising:

[0008] Acquire electrical quantity information of the converter valve in real time, including the instantaneous value of the valve-side AC voltage, the instantaneous value of the valve-side AC current, the instantaneous value of the positive and negative DC bus voltages to ground, and the instantaneous value of the bridge arm current;

[0009] Determining the instantaneous value of the circulating current of each phase of the converter valve according to the instantaneous value of the valve-side AC voltage, the instantaneous value of the valve-side AC current, the instantaneous value of the positive and negative DC bus voltages to ground, and the instantaneous value of the bridge arm current;

[0010] Multiplying the instantaneous value of the circulating current of each phase by a preset proportional coefficient to generate a superposition amount of the reference voltage of each bridge arm;

[0011] The original reference voltage of the bridge arm is superimposed with the corresponding reference voltage superposition amount to generate a final reference voltage of the bridge arm, so as to generate a trigger pulse of the bridge arm switching device according to the final reference voltage.

[0012] Specifically, determining the instantaneous value of the circulating current of each phase of the converter valve according to the instantaneous value of the valve-side AC voltage, the instantaneous value of the valve-side AC current, the instantaneous value of the positive and negative DC bus voltages to ground, and the instantaneous value of the bridge arm current specifically includes:

[0013] Determine the instantaneous value of the valve-side active power according to the instantaneous value of the valve-side AC voltage and the instantaneous value of the valve-side AC current;

[0014] Determine the instantaneous value of the circulating current of each phase according to the following formula (I):

[0015]

[0016] Among them, i loop_j is the instantaneous value of the j-phase circulating current, j is a, b or c, P ac is the instantaneous value of active power on the valve side, I arm_pj is the instantaneous value of the upper arm current of phase j, I arm_nj is the instantaneous value of the lower arm current of phase j, U dc is the instantaneous value of the DC bus voltage.

[0017] Specifically, the positive direction of the valve-side AC current is when it flows into the valve, the positive direction of the upper arm current is when it flows from DC to AC, the positive direction of the lower arm current is when it flows from AC to DC, and the positive direction of the active power is when it flows in the rectification direction.

[0018] Specifically, determining the instantaneous value of the DC bus voltage according to the positive and negative DC bus voltages to ground, and determining the instantaneous value of the valve-side active power according to the instantaneous value of the valve-side AC voltage and the instantaneous value of the valve-side AC current, specifically includes:

[0019] Determine the instantaneous value of the DC bus voltage according to the following formula (II):

[0020] U dc =U dc+ -U dc- (Ⅱ)

[0021] Among them, U dc+ is the instantaneous value of the positive DC bus voltage to ground, U dc- is the instantaneous value of the negative DC bus voltage to ground;

[0022] Determine the instantaneous value of the valve-side active power according to the following formula (III):

[0023] P ac =u a i a +u b ib +u c i c (III)

[0024] Among them, u a is the instantaneous value of the AC voltage on the valve side of phase a, u b is the instantaneous value of the AC voltage on the valve side of phase b, u c is the instantaneous value of the AC voltage on the valve side of phase C;

[0025] i a is the instantaneous value of the AC current on the valve side of phase a, i b is the instantaneous value of the AC current on the b-phase valve side, i c is the instantaneous value of the AC current on the valve side of phase C.

[0026] Specifically, multiplying the instantaneous value of the circulating current of each phase by a preset proportional coefficient to generate a superposition amount of the reference voltage of each bridge arm includes:

[0027] The reference voltage superposition of each bridge arm is generated according to the following formula (IV):

[0028]

[0029] Among them, k p is the preset proportional coefficient, Δu pj is the superposition of the reference voltage of the upper bridge arm of phase j, Δu nj is the superposition of the reference voltage of the lower bridge arm of phase j.

[0030] Specifically, the step of superimposing the original reference voltage of the bridge arm with the corresponding reference voltage superposition amount to generate the final reference voltage of the bridge arm includes:

[0031] The final reference voltage is generated according to the following formula (V):

[0032]

[0033] Among them, u pj is the original reference voltage of the upper bridge arm of phase j, Δu pj is the superposition of the upper arm reference voltage of phase j, u' pj is the final reference voltage of the upper bridge arm of phase j, u nj is the original reference voltage of the lower bridge arm of phase j, Δu nj is the superposition of the reference voltage of the lower bridge arm of phase j, u' nj is the final reference voltage of the j-phase lower bridge arm.

[0034] A second aspect of the present application provides a flexible DC back-to-back system DC current harmonic suppressor, comprising:

[0035] An acquisition module is used to acquire electrical quantity information of the converter valve in real time, wherein the electrical quantity information includes the instantaneous value of the valve-side AC voltage, the instantaneous value of the valve-side AC current, the instantaneous value of the positive and negative DC bus voltages to ground, and the instantaneous value of the bridge arm current;

[0036] a phase circulating current determination module, configured to determine the instantaneous value of the phase circulating current of the converter valve according to the instantaneous value of the valve-side AC voltage, the instantaneous value of the valve-side AC current, the instantaneous value of the positive and negative DC bus voltages to ground, and the instantaneous value of the bridge arm current;

[0037] A first generating module is configured to multiply the instantaneous value of the circulating current of each phase by a preset proportional coefficient to generate a superposition amount of the reference voltage of each bridge arm;

[0038] The second generating module is used to superimpose the original reference voltage of the bridge arm and the corresponding reference voltage superposition amount to generate a final reference voltage of the bridge arm, so as to generate a trigger pulse of the bridge arm switching device according to the final reference voltage.

[0039] Specifically, the phase circulation determination module is used to:

[0040] Determine the instantaneous value of the valve-side active power according to the instantaneous value of the valve-side AC voltage and the instantaneous value of the valve-side AC current;

[0041] Determine the instantaneous value of the circulating current of each phase according to the following formula (I):

[0042]

[0043] Among them, i loop_j is the instantaneous value of the j-phase circulating current, j is a, b or c, P ac is the instantaneous value of active power on the valve side, I arm_pj is the instantaneous value of the upper arm current of phase j, I arm_nj is the instantaneous value of the lower arm current of phase j.

[0044] Specifically, the positive direction of the valve-side AC current is when it flows into the valve, the positive direction of the upper arm current is when it flows from DC to AC, the positive direction of the lower arm current is when it flows from AC to DC, and the positive direction of the active power is when it flows in the rectification direction.

[0045] Specifically, the phase circulation determination module is used to:

[0046] Determine the instantaneous value of the DC bus voltage according to the following formula (II):

[0047] U dc =U dc+ -U dc- (Ⅱ)

[0048] Among them, U dc+ is the instantaneous value of the positive DC bus voltage to ground, U dc- is the instantaneous value of the negative DC bus voltage to ground;

[0049] Determine the instantaneous value of the valve-side active power according to the following formula (III):

[0050] P ac =u a i a +u b i b +u c i c (III)

[0051] Among them, u a is the instantaneous value of the AC voltage on the valve side of phase a, u b is the instantaneous value of the AC voltage on the valve side of phase b, u c is the instantaneous value of the AC voltage on the valve side of phase C;

[0052] i a is the instantaneous value of the AC current on the valve side of phase a, i b is the instantaneous value of the AC current on the b-phase valve side, i c is the instantaneous value of the AC current on the valve side of phase C.

[0053] Specifically, the first generation module is used to:

[0054] The reference voltage superposition of each bridge arm is generated according to the following formula (IV):

[0055]

[0056] Among them, k p is the preset proportional coefficient, Δu pj is the superposition of the reference voltage of the upper bridge arm of phase j, Δu nj is the superposition of the reference voltage of the lower bridge arm of phase j.

[0057] Specifically, the second generation module is used to:

[0058] The final reference voltage is generated according to the following formula (V):

[0059]

[0060] Among them, u pj is the original reference voltage of the upper bridge arm of phase j, Δu pj is the superposition of the upper arm reference voltage of phase j, u' pj is the final reference voltage of the upper bridge arm of phase j, u nj is the original reference voltage of the lower bridge arm of phase j, Δu nj is the superposition of the reference voltage of the lower bridge arm of phase j, u' nj is the final reference voltage of the j-phase lower bridge arm.

[0061] (3) Beneficial effects

[0062] The above technical solution of the present invention has the following beneficial technical effects:

[0063] This method can suppress the DC current harmonics of the flexible DC back-to-back system without increasing the switching frequency of the switching devices and the rated operating voltage of the sub-module capacitors. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Figure 1 This is the topological structure diagram of the converter valve of this flexible direct current back-to-back system;

[0065] Figure 2 The present invention provides a flow chart of a method for suppressing DC current harmonics in a flexible DC back-to-back system;

[0066] Figure 3 A schematic diagram of a DC current harmonic suppression strategy for a flexible DC back-to-back system according to a specific embodiment of the present invention;

[0067] Figure 3 middle:

[0068] P ac is the calculated instantaneous value of active power on the AC valve side; U dc is the calculated instantaneous value of the DC bus voltage; I arm_pj is the instantaneous value of the upper arm current of phase j; I arm_nj is the instantaneous value of the lower arm current of phase j;

[0069] i d_ref is the reference value of the inner loop of active current; i q_ref is the reference value of the inner loop of reactive current; u pj is the original reference voltage of the upper bridge arm of phase j; u nj is the original reference voltage of the lower bridge arm of phase j; u' pj is the final reference voltage of the upper bridge arm of phase j; u' nj is the final reference voltage of the lower bridge arm of phase j; △u pj is the superposition of the reference voltage of the upper bridge arm of phase j; △u nj is the superposition of the reference voltage of the lower bridge arm of phase j. DETAILED DESCRIPTION

[0070] To make the objectives, technical solutions, and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings. It should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessary confusion of the concepts of the present invention.

[0071] See also Figure 2 The present invention provides a method for suppressing direct current harmonics in a flexible direct current back-to-back system, comprising:

[0072] Step 101: Acquire electrical quantity information of the converter valve in real time, wherein the electrical quantity information includes the instantaneous value of the valve-side AC voltage, the instantaneous value of the valve-side AC current, the instantaneous value of the positive and negative DC bus voltages to ground, and the instantaneous value of the bridge arm current;

[0073] Step 102: determining the instantaneous value of the circulating current of each phase of the converter valve according to the instantaneous value of the valve-side AC voltage, the instantaneous value of the valve-side AC current, the instantaneous value of the positive and negative DC bus voltages to ground, and the instantaneous value of the bridge arm current;

[0074] Step 103: multiplying the instantaneous value of the circulating current of each phase by a preset proportional coefficient to generate a superposition amount of the reference voltage of each bridge arm;

[0075] Step 104: superimposing the original reference voltage of the bridge arm with the corresponding reference voltage superposition amount to generate a final reference voltage of the bridge arm, so as to generate a trigger pulse of the bridge arm switch device according to the final reference voltage.

[0076] This method suppresses the DC current harmonics of the flexible DC back-to-back system without increasing the switching frequency of the switching devices and the rated working voltage of the sub-module capacitors.

[0077] Specifically, determining the instantaneous value of the circulating current of each phase of the converter valve according to the instantaneous value of the valve-side AC voltage, the instantaneous value of the valve-side AC current, the instantaneous value of the positive and negative DC bus voltages to ground, and the instantaneous value of the bridge arm current specifically includes:

[0078] The instantaneous value of the DC bus voltage is determined based on the positive and negative DC bus voltages to ground, and the instantaneous value of the valve-side active power is determined based on the instantaneous value of the valve-side AC voltage and the instantaneous value of the valve-side AC current;

[0079] Determine the instantaneous value of the circulating current of each phase according to the following formula (I):

[0080]

[0081] Among them, i loop_j is the instantaneous value of the j-phase circulating current, j is a, b or c, P ac is the instantaneous value of active power on the valve side, I arm_pj is the instantaneous value of the upper arm current of phase j, I arm_nj is the instantaneous value of the lower arm current of phase j, U dc is the instantaneous value of the DC bus voltage.

[0082] By using formula (I) to calculate the inter-phase circulating current, the DC component of the inter-phase circulating current can be eliminated, thereby avoiding the phenomenon of an increase in the sub-module operating voltage due to the existence of the DC component.

[0083] Specifically, the positive direction of the valve-side AC current is when it flows into the valve, the positive direction of the upper arm current is when it flows from DC to AC, the positive direction of the lower arm current is when it flows from AC to DC, and the positive direction of the active power is when it flows in the rectification direction.

[0084] Specifically, determining the instantaneous value of the DC bus voltage according to the positive and negative DC bus voltages to ground, and determining the instantaneous value of the valve-side active power according to the instantaneous value of the valve-side AC voltage and the instantaneous value of the valve-side AC current, specifically includes:

[0085] Determine the instantaneous value of the DC bus voltage according to the following formula (II):

[0086] U dc =U dc+ -U dc- (Ⅱ)

[0087] Among them, U dc+ is the instantaneous value of the positive DC bus voltage to ground, U dc- is the instantaneous value of the negative DC bus voltage to ground;

[0088] Determine the instantaneous value of the valve-side active power according to the following formula (III):

[0089] P ac =u a i a +u b i b +u c i c (III)

[0090] Among them, u a is the instantaneous value of the AC voltage on the valve side of phase a, u b is the instantaneous value of the AC voltage on the valve side of phase b, u c is the instantaneous value of the AC voltage on the valve side of phase C;

[0091] i a is the instantaneous value of the AC current on the valve side of phase a, i b is the instantaneous value of the AC current on the b-phase valve side, i c is the instantaneous value of the AC current on the valve side of phase C.

[0092] Specifically, multiplying the instantaneous value of the circulating current of each phase by a preset proportional coefficient to generate a superposition amount of the reference voltage of each bridge arm includes:

[0093] The reference voltage superposition of each bridge arm is generated according to the following formula (IV):

[0094]

[0095] Among them, k p is the preset proportional coefficient, Δu pjis the superposition of the reference voltage of the upper bridge arm of phase j, Δu nj is the superposition of the reference voltage of the lower bridge arm of phase j.

[0096] Specifically, the step of superimposing the original reference voltage of the bridge arm with the corresponding reference voltage superposition amount to generate the final reference voltage of the bridge arm includes:

[0097] The final reference voltage is generated according to the following formula (V):

[0098]

[0099] Among them, u pj is the original reference voltage of the upper bridge arm of phase j, Δu pj is the superposition of the upper arm reference voltage of phase j, u' pj is the final reference voltage of the upper bridge arm of phase j, u nj is the original reference voltage of the lower bridge arm of phase j, Δu nj is the superposition of the reference voltage of the lower bridge arm of phase j, u' nj is the final reference voltage of the j-phase lower bridge arm.

[0100] The present invention also provides a flexible DC back-to-back system DC current harmonic suppressor, comprising:

[0101] An acquisition module is used to acquire electrical quantity information of the converter valve in real time, wherein the electrical quantity information includes the instantaneous value of the valve-side AC voltage, the instantaneous value of the valve-side AC current, the instantaneous value of the positive and negative DC bus voltages to ground, and the instantaneous value of the bridge arm current;

[0102] a phase circulating current determination module, configured to determine the instantaneous value of the phase circulating current of the converter valve according to the instantaneous value of the valve-side AC voltage, the instantaneous value of the valve-side AC current, the instantaneous value of the positive and negative DC bus voltages to ground, and the instantaneous value of the bridge arm current;

[0103] A first generating module is configured to multiply the instantaneous value of the circulating current of each phase by a preset proportional coefficient to generate a superposition amount of the reference voltage of each bridge arm;

[0104] The second generating module is used to superimpose the original reference voltage of the bridge arm and the corresponding reference voltage superposition amount to generate a final reference voltage of the bridge arm, so as to generate a trigger pulse of the bridge arm switching device according to the final reference voltage.

[0105] Specifically, the phase circulation determination module is used to:

[0106] The instantaneous value of the DC bus voltage is determined based on the positive and negative DC bus voltages to ground, and the instantaneous value of the valve-side active power is determined based on the instantaneous value of the valve-side AC voltage and the instantaneous value of the valve-side AC current;

[0107] Determine the instantaneous value of the circulating current of each phase according to the following formula (I):

[0108]

[0109] Among them, i loop_j is the instantaneous value of the j-phase circulating current, j is a, b or c, P ac is the instantaneous value of active power on the valve side, I arm_pj is the instantaneous value of the upper arm current of phase j, I arm_nj is the instantaneous value of the lower arm current of phase j.

[0110] Specifically, the positive direction of the valve-side AC current is when it flows into the valve, the positive direction of the upper arm current is when it flows from DC to AC, the positive direction of the lower arm current is when it flows from AC to DC, and the positive direction of the active power is when it flows in the rectification direction.

[0111] Specifically, the phase circulation determination module is used to:

[0112] Determine the instantaneous value of the DC bus voltage according to the following formula (II):

[0113] U dc =U dc+ -U dc- (Ⅱ)

[0114] Among them, U dc+ is the instantaneous value of the positive DC bus voltage to ground, U dc- is the instantaneous value of the negative DC bus voltage to ground;

[0115] Determine the instantaneous value of the valve-side active power according to the following formula (III):

[0116] P ac =u a i a +u b i b +u c i c (III)

[0117] Among them, u a is the instantaneous value of the AC voltage on the valve side of phase a, u b is the instantaneous value of the AC voltage on the valve side of phase b, u c is the instantaneous value of the AC voltage on the valve side of phase C;

[0118] i a is the instantaneous value of the AC current on the valve side of phase a, i b is the instantaneous value of the AC current on the b-phase valve side, i c is the instantaneous value of the AC current on the valve side of phase C.

[0119] Specifically, the first generation module is used to:

[0120] The reference voltage superposition of each bridge arm is generated according to the following formula (IV):

[0121]

[0122] Among them, k p is the preset proportional coefficient, Δu pj is the superposition of the reference voltage of the upper bridge arm of phase j, Δu nj is the superposition of the reference voltage of the lower bridge arm of phase j.

[0123] Specifically, the second generation module is used to:

[0124] The final reference voltage is generated according to the following formula (V):

[0125]

[0126] Among them, u pj is the original reference voltage of the upper bridge arm of phase j, Δu pj is the superposition of the upper arm reference voltage of phase j, u' pj is the final reference voltage of the upper bridge arm of phase j, u nj is the original reference voltage of the lower bridge arm of phase j, Δu nj is the superposition of the reference voltage of the lower bridge arm of phase j, u' nj is the final reference voltage of the j-phase lower bridge arm.

[0127] The specific description and effects are shown in the method examples and will not be repeated here.

[0128] The following is a specific embodiment of the present invention:

[0129] See also Figure 3 This embodiment provides a strategy for suppressing DC current harmonics in a flexible DC back-to-back system, the implementation of which includes the following steps:

[0130] ①Measure the instantaneous value of the valve-side AC voltage, the instantaneous value of the valve-side AC current, the positive and negative DC bus voltages to ground, the instantaneous value of the bridge arm current and other electrical quantities, and calculate the instantaneous value of the DC bus voltage and the instantaneous value of the valve-side active power;

[0131] ②According to the measured electrical quantities, calculate the instantaneous value of the circulating current of each phase;

[0132] ③ Multiply the instantaneous value of each phase circulating current by a certain proportional coefficient to generate the reference voltage superposition of each bridge arm;

[0133] ④ The generated superposition quantities are superimposed on the original reference voltages of the bridge arms to generate the final reference voltage.

[0134] Among them: ① The instantaneous value of the valve side AC voltage, the instantaneous value of the valve side AC current, the positive and negative DC bus voltages to the ground, the instantaneous value of the bridge arm current and other electrical quantities are measured, and the instantaneous value of the DC bus voltage and the instantaneous value of the valve side active power are calculated: The control object of the present invention is the circulating current of each phase. In order to remove the DC component in the circulating current without introducing a delay, the present invention selects a method of calculating the DC component of each phase by using the AC valve side power and the DC bus voltage, and subtracting it from half of the sum of the upper and lower bridge arm currents of each phase. To implement this method, it is necessary to first measure the valve side AC voltage, the valve side AC current, the positive and negative DC bus voltages to the ground, the bridge arm current and other electrical quantities, and obtain the instantaneous value of the DC bus voltage and the instantaneous value of the valve side active power by calculation, and specifically determine the instantaneous value of the valve side active power according to formula (III);

[0135] It should also be noted that the positive direction of the measured valve-side AC current is the current flowing into the valve, the positive direction of the measured upper bridge arm current is the current flowing from DC to AC, the positive direction of the measured lower bridge arm current is the current flowing from AC to DC, and the calculated power is in the rectification direction.

[0136] ②According to the measured electrical quantities, calculate the instantaneous value of the circulating current of each phase:

[0137] The present invention selects a method to calculate the DC component of each phase by using the AC valve-side power and the DC bus voltage, and to perform a subtraction with half of the sum of the upper and lower bridge arm currents of each phase to realize the calculation of the instantaneous value of the circulating current of each phase. The instantaneous value of the valve-side active power has been calculated by the previous step. Then, the DC component of each phase is calculated by using the AC valve-side power and the DC bus voltage, and to perform a subtraction with half of the sum of the upper and lower bridge arm currents of each phase to calculate the instantaneous value of the circulating current of each phase after the DC component is removed. Specifically, the instantaneous value of the circulating current of each phase is calculated according to formula (I);

[0138] ③ Multiply the instantaneous value of each phase circulating current by a certain proportional coefficient to generate the reference voltage superposition of each bridge arm:

[0139] After calculating the instantaneous value of the circulating current of each phase, it is necessary to calculate the superimposed amount on the reference voltage of each bridge arm. According to the principle of harmonic suppression, a proportional controller is designed to multiply the circulating current of each phase by a specific coefficient to obtain the superimposed amount of the reference voltage of each bridge arm. The calculation method is shown in formula (IV).

[0140] ④ The generated superposition quantities are superimposed on the original reference voltage of each bridge arm to generate the final reference voltage, and the trigger pulse of the switching device is generated to suppress the DC current harmonics of the system:

[0141] Suppression of DC current harmonics is achieved by adding the superposition amount to the reference voltage and then adjusting the number of submodules in real time. Therefore, the superposition amount of the calculated reference voltage of each bridge arm is added to the initial reference voltage of each bridge arm to generate a final reference voltage, thereby achieving suppression of DC current harmonics. The final reference voltage can be calculated according to formula (V);

[0142] By calculating the final reference voltage, the trigger pulses of each switching device are calculated to control the switching devices and ultimately suppress the DC current harmonics.

[0143] Thus, the suppression of DC current harmonics in the flexible DC back-to-back system has been achieved. This method can suppress DC current harmonics in the flexible DC back-to-back system without increasing the switching frequency of the switching devices or the rated operating voltage of the submodule capacitors.

[0144] Those skilled in the art will appreciate that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to magnetic disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code.

[0145] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes 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 steps in the process. 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.

[0146] These computer program instructions may also be stored in a computer readable memory that can direct a computer or other programmable data processing device to work 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 The function specified in one or more boxes.

[0147] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing the instructions executed on the computer or other programmable device for implementing the process. Figure 1 a process or multiple processes and / or boxes Figure 1 The steps for the function specified in one or more boxes.

[0148] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. A method for suppressing direct current harmonics in a flexible back-to-back system, characterized in that: include: Acquire electrical quantity information of the converter valve in real time, wherein the electrical quantity information includes the instantaneous value of the valve side AC voltage, the instantaneous value of the valve side AC current, the instantaneous value of the positive and negative DC bus voltages to ground, and the instantaneous value of the bridge arm current; Determine the instantaneous value of the circulating current of each phase of the converter valve according to the instantaneous value of the valve-side AC voltage, the instantaneous value of the valve-side AC current, the instantaneous value of the positive and negative DC bus voltages to ground, and the instantaneous value of the bridge arm current; Multiplying the instantaneous value of the circulating current of each phase by a preset proportional coefficient to generate a reference voltage superposition amount of each bridge arm; Superimposing the original reference voltage of the bridge arm with the corresponding reference voltage superposition amount to generate a final reference voltage of the bridge arm, so as to generate a trigger pulse of the bridge arm switch device according to the final reference voltage; The step of determining the instantaneous value of the circulating current of each phase of the converter valve according to the instantaneous value of the valve-side AC voltage, the instantaneous value of the valve-side AC current, the instantaneous value of the positive and negative DC bus voltages to ground, and the instantaneous value of the bridge arm current specifically includes: Determine the instantaneous value of the DC bus voltage according to the instantaneous value of the positive and negative DC bus voltages to ground, and determine the instantaneous value of the valve-side active power according to the instantaneous value of the valve-side AC voltage and the instantaneous value of the valve-side AC current; Determine the instantaneous value of each phase circulation current according to the following formula (I): Among them, i loop_j is the instantaneous value of the j-phase circulating current, j is a, b or c, P ac is the instantaneous value of active power on the valve side, I arm_pj is the instantaneous value of the upper bridge arm current of phase j, I arm_nj is the instantaneous value of the lower bridge arm current of phase j, U dc is the instantaneous value of the DC bus voltage.

2. The suppression method according to claim 1, characterized in that: The positive direction of the AC current on the valve side is when it flows into the valve, the positive direction of the upper bridge arm current is when it flows from DC to AC, the positive direction of the lower bridge arm current is when it flows from AC to DC, and the positive direction of the active power is when it flows in the rectification direction.

3. The suppression method according to claim 2, characterized in that: The method of determining the instantaneous value of the DC bus voltage according to the instantaneous value of the positive and negative DC bus voltages to ground, and determining the instantaneous value of the valve-side active power according to the instantaneous value of the valve-side AC voltage and the instantaneous value of the valve-side AC current, specifically includes: Determine the instantaneous value of the DC bus voltage according to the following formula (II): IN dc =U dc+ -IN dc- (Ⅱ) Among them, U dc+ is the instantaneous value of the positive DC bus voltage to ground, U dc- is the instantaneous value of the negative DC bus voltage to ground; Determine the instantaneous value of the valve side active power according to the following formula (III): P ac =in a and a +in b and b +in c and c (Ⅲ) Among them, u a is the instantaneous value of the AC voltage on the valve side of phase a, u b is the instantaneous value of the AC voltage on the valve side of phase b, u c is the instantaneous value of the AC voltage on the valve side of phase c; i a is the instantaneous value of the AC current on the valve side of phase a, i b is the instantaneous value of the AC current on the valve side of phase b, i c is the instantaneous value of the AC current on the valve side of phase C.

4. The suppression method according to claim 3, characterized in that: The step of multiplying the instantaneous value of the circulating current of each phase by a preset proportional coefficient to generate a reference voltage superposition amount of each bridge arm includes: The reference voltage superposition of each bridge arm is generated according to the following formula (IV): Among them, k p is the preset proportional coefficient, Δu pj is the superposition of the reference voltage of the upper bridge arm of phase j, Δu nj It is the superposition amount of the reference voltage of the lower bridge arm of phase j.

5. The suppression method according to claim 1, characterized in that: The method of superimposing the original reference voltage of the bridge arm with the corresponding reference voltage superposition amount to generate the final reference voltage of the bridge arm includes: The final reference voltage is generated according to the following formula (V): Among them, u pj is the original reference voltage of the upper bridge arm of phase j, Δu pj is the superposition of the reference voltage of the upper bridge arm of phase j, u' pj is the final reference voltage of the upper bridge arm of phase j, u nj is the original reference voltage of the lower bridge arm of phase j, Δu nj is the superposition of the reference voltage of the lower bridge arm of phase j, u' nj It is the final reference voltage of the lower bridge arm of phase j.

6. A DC current harmonic suppressor for a flexible back-to-back system, characterized in that: include: An acquisition module is used to acquire electrical quantity information of the converter valve in real time, wherein the electrical quantity information includes the instantaneous value of the valve side AC voltage, the instantaneous value of the valve side AC current, the instantaneous value of the positive and negative DC bus voltages to ground, and the instantaneous value of the bridge arm current; The phase circulating current determination module is used to determine the instantaneous value of the phase circulating current of the converter valve according to the instantaneous value of the valve side AC voltage, the instantaneous value of the valve side AC current, the instantaneous value of the positive and negative DC bus voltages to ground and the instantaneous value of the bridge arm current; A first generating module is used to multiply the instantaneous value of the circulating current of each phase by a preset proportional coefficient to generate a reference voltage superposition amount of each bridge arm; A second generating module, used for superimposing the original reference voltage of the bridge arm with the corresponding reference voltage superposition amount to generate a final reference voltage of the bridge arm, so as to generate a trigger pulse of the bridge arm switch device according to the final reference voltage; The phase circulation determination module is used to: Determine the instantaneous value of the DC bus voltage according to the instantaneous value of the positive and negative DC bus voltages to ground, and determine the instantaneous value of the valve-side active power according to the instantaneous value of the valve-side AC voltage and the instantaneous value of the valve-side AC current; Determine the instantaneous value of each phase circulation current according to the following formula (I): Among them, i loop_j is the instantaneous value of the j-phase circulating current, j is a, b or c, P ac is the instantaneous value of active power on the valve side, I arm_pj is the instantaneous value of the upper bridge arm current of phase j, I arm_nj is the instantaneous value of the lower bridge arm current of phase j.

7. The suppressor according to claim 6, characterized in that The positive direction of the AC current on the valve side is when it flows into the valve, the positive direction of the upper bridge arm current is when it flows from DC to AC, the positive direction of the lower bridge arm current is when it flows from AC to DC, and the positive direction of the active power is when it flows in the rectification direction.

8. The suppressor according to claim 6 or 7, characterized in that: The phase circulation determination module is used to: Determine the instantaneous value of the DC bus voltage according to the following formula (II): IN dc =U dc+ -IN dc- (Ⅱ) Among them, U dc+ is the instantaneous value of the positive DC bus voltage to ground, U dc- is the instantaneous value of the negative DC bus voltage to ground; Determine the instantaneous value of the valve side active power according to the following formula (III): P ac =in a and a +in b and b +in c and c (Ⅲ) Among them, u a is the instantaneous value of the AC voltage on the valve side of phase a, u b is the instantaneous value of the AC voltage on the valve side of phase b, u c is the instantaneous value of the AC voltage on the valve side of phase c; i a is the instantaneous value of the AC current on the valve side of phase a, i b is the instantaneous value of the AC current on the valve side of phase b, i c is the instantaneous value of the AC current on the valve side of phase C.

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