A method and apparatus for injecting third harmonic for control of converter circulating current

Abstract

The application discloses a third harmonic injection method and device suitable for converter circulating current control, and the method comprises the following steps: obtaining the amplitude and phase of a converter power frequency modulation wave and the amplitude and phase angle of a second harmonic modulation voltage introduced by the converter circulating current control; determining the amplitude and phase of the third harmonic injection voltage when the maximum and minimum of the positive and negative peak values of the converter alternating current modulation wave are determined according to the relationship between the amplitude and phase angle of the second harmonic modulation voltage introduced by the converter circulating current control and the amplitude and phase of the converter power frequency modulation wave. The application can improve the consistency of the positive and negative peak values of the converter modulation wave, thereby further improving the rated voltage of the valve side of the converter transformer, reducing the converter valve and related current, and reducing the system loss.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of flexible direct current transmission, and particularly relates to a third harmonic injection method and device suitable for converter circulating current control. BACKGROUND

[0002] This section is intended to provide background information to facilitate a better understanding of embodiments of the present application described in the claims. The description herein does not constitute admission of prior art.

[0003] The power range of a flexible direct current transmission system in normal operation is limited by the modulation ratio of a converter. At present, some scholars have proposed a method of over-modulation operation of a modular multilevel converter, and the third harmonic injection method is a commonly used method of over-modulation operation of a converter. The third harmonic injection method can reduce the peak value of a modulation wave, improve the modulation ratio of the converter, increase the valve-side voltage of the converter, and reduce the current of the converter, thereby reducing the operating loss of the converter valve. However, the effect of the existing third harmonic injection method needs to be further improved. SUMMARY

[0004] The circulating current control of the converter can make the alternating current modulation wave of the converter asymmetric, and the corresponding positive amplitude and negative amplitude are no longer equal. The asymmetry of the positive and negative peak values of the alternating current modulation wave is not conducive to the full use of the modulation degree of the converter. The existing technology does not consider the influence of the waveform coupling of the circulating current control on the modulation wave of the converter, and does not propose an implementation method for reducing the influence of the waveform coupling of the circulating current control of the converter on the modulation wave of the converter. The embodiments of the present application provide a third harmonic injection method suitable for circulating current control of a converter, which is used to further improve the rated voltage of the valve side of the converter transformer, reduce the converter valve and related current, and reduce system loss. The method comprises the following steps:

[0005] Obtaining the amplitude and phase of the power frequency modulation wave of the converter and the amplitude and phase angle of the second harmonic modulation voltage introduced by the circulating current control of the converter;

[0006] According to the relationship between the amplitude and phase angle of the second harmonic modulation voltage introduced by the circulating current control of the converter and the amplitude and phase of the power frequency modulation wave of the converter, the amplitude and phase of the third harmonic injection voltage when the maximum and minimum values of the positive and negative peak values of the alternating current modulation wave of the converter are determined.

[0007] The embodiments of the present application also provide a third harmonic injection device suitable for circulating current control of a converter, which is used to further improve the rated voltage of the valve side of the converter transformer, reduce the converter valve and related current, and reduce system loss. The device comprises the following steps:

[0008] The acquisition unit is used for acquiring the amplitude and phase of the converter fundamental frequency modulation wave and the amplitude and phase angle of the second harmonic modulation voltage introduced by the converter circulating current control;

[0009] The determination unit is used for determining the amplitude and phase of the third harmonic injection voltage when the maximum and minimum of the positive and negative peak values of the converter AC modulation wave are determined according to the relationship between the amplitude and phase angle of the second harmonic modulation voltage introduced by the converter circulating current control and the amplitude and phase of the converter fundamental frequency modulation wave.

[0010] The embodiment of the present application further provides a computer device, including a memory, a processor and a computer program stored in the memory and capable of running on the processor, and the processor realizes the above-mentioned third harmonic injection method suitable for the converter circulating current control when the computer program is executed.

[0011] The embodiment of the present application further provides a computer readable storage medium, which stores the computer program for executing the above-mentioned third harmonic injection method suitable for the converter circulating current control.

[0012] In the embodiment of the present application, the third harmonic injection scheme suitable for the converter circulating current control is obtained by: acquiring the amplitude and phase of the converter fundamental frequency modulation wave and the amplitude and phase angle of the second harmonic modulation voltage introduced by the converter circulating current control; and determining the amplitude and phase of the third harmonic injection voltage when the maximum and minimum of the positive and negative peak values of the converter AC modulation wave are determined according to the relationship between the amplitude and phase angle of the second harmonic modulation voltage introduced by the converter circulating current control and the amplitude and phase of the converter fundamental frequency modulation wave. The consistency of the positive and negative peak values of the converter modulation wave can be improved, so that the rated voltage of the converter transformer valve side can be further improved, the converter valve and the related current can be reduced, and the system loss can be reduced. BRIEF DESCRIPTION OF DRAWINGS

[0013] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the drawings needed in the embodiments or the prior art description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort. In the drawings:

[0014] Figure 1 It is a flowchart of the third harmonic injection method suitable for the converter circulating current control in the embodiment of the present application;

[0015] Figure 2 It is a principle diagram of the third harmonic injection method suitable for the converter circulating current control in the embodiment of the present application;

[0016] Figure 3Fig. 1 is a schematic diagram of an AC modulation wave of a converter in an embodiment of the present application Figure 1 ;

[0017] Figure 4 Fig. 2 is a schematic diagram of an AC modulation wave of a converter in an embodiment of the present application Figure 2 ;

[0018] Figure 5 Fig. 3 is a schematic diagram of a third harmonic injection AC modulation voltage before the present application in an embodiment of the present application

[0019] Figure 6 Fig. 4 is a schematic diagram of a third harmonic injection AC modulation voltage after the present application in an embodiment of the present application

[0020] Figure 7 Fig. 5 is a structural schematic diagram of a third harmonic injection device suitable for a converter circulating current control in an embodiment of the present application. DETAILED DESCRIPTION

[0021] To make the purpose, technical solutions and advantages of the embodiments of the present application clearer, further detailed description will be given to the embodiments of the present application in combination with the drawings. Herein, the schematic embodiments of the present application and the descriptions thereof are used to explain the present application but not to limit the present application.

[0022] The inventor found that the technical problem existing in the prior art third harmonic injection method is that the circulating current control of the converter will make the AC modulation wave of the converter asymmetric, and the corresponding positive amplitude and negative amplitude are no longer equal. The asymmetry of the positive and negative peak values of the AC modulation wave is not conducive to the full use of the modulation degree of the converter. The prior art does not consider the influence of the waveform coupling of the circulating current control on the modulation wave of the converter, and does not propose an implementation method for reducing the influence of the waveform coupling of the circulating current control of the converter on the modulation wave of the converter.

[0023] To solve the problems existing in the prior art and make the modulation degree of the converter be used to the maximum extent, the present application proposes a third harmonic injection scheme suitable for circulating current control. The scheme determines the amplitude and phase of the third harmonic injection voltage by solving the amplitude and phase relationship of the secondary modulation voltage of the converter relative to the power frequency modulation wave, and taking the maximum value and minimum value of the positive and negative peaks of the AC modulation wave of the converter as the target. The method can further reduce the influence of the waveform coupling of the modular multilevel converter circulating current control on the modulation wave of the converter on the basis of the existing third harmonic injection method, thereby further improving the rated voltage of the converter transformer valve side and reducing the related operating current and system loss. The third harmonic injection scheme suitable for the circulating current control of the converter will be described in detail as follows.

[0024] Figure 1 Fig. 6 is a flowchart of the third harmonic injection method suitable for the circulating current control of the converter in an embodiment of the present application, as shown in Figure 1As shown, the method comprises the following steps:

[0025] Step 101: obtaining the amplitude and phase of the converter fundamental frequency modulation wave and the amplitude and phase angle of the second harmonic modulation voltage introduced by the converter circulating current control;

[0026] Step 102: determining the amplitude and phase of the third harmonic injection voltage at the maximum and minimum of the positive and negative peak values of the converter AC modulation wave according to the relationship between the amplitude and phase angle of the second harmonic modulation voltage introduced by the converter circulating current control and the amplitude and phase of the converter fundamental frequency modulation wave; the amplitude and phase of the third harmonic injection voltage at the maximum and minimum of the positive and negative peak values of the converter AC modulation wave are used for the third harmonic injection of the converter circulating current control.

[0027] In the embodiment of the present application, when the third harmonic injection method suitable for the converter circulating current control is working: the amplitude and phase of the converter fundamental frequency modulation wave are obtained through the control system of the converter, and the amplitude and phase angle of the second harmonic modulation voltage introduced by the converter circulating current control are obtained through the control system of the converter; according to the relationship between the amplitude and phase angle of the second harmonic modulation voltage introduced by the converter circulating current control and the amplitude and phase of the converter fundamental frequency modulation wave, the amplitude and phase of the third harmonic injection voltage are determined with the maximum and minimum of the positive and negative peak values of the converter AC modulation wave as the target, which can improve the consistency of the positive and negative peak values of the converter modulation wave, thereby further improving the rated voltage of the transformer valve side, reducing the converter valve and related current, and reducing the system loss.

[0028] The following will be described in detail with reference to the accompanying drawings Figure 2 to Figure 4 The various steps involved in the embodiment of the present application will be described in detail.

[0029] I. First, the above step 101 is introduced.

[0030] In one embodiment, obtaining the amplitude and phase of the converter fundamental frequency modulation wave and the amplitude and phase angle of the second harmonic modulation voltage introduced by the converter circulating current control comprises: obtaining the amplitude and phase of the converter fundamental frequency modulation wave and the amplitude and phase angle of the second harmonic modulation voltage introduced by the converter circulating current control through the control system of the converter.

[0031] In specific implementation, the phase-locked loop output λ of the system, the amplitude V ref of the converter fundamental frequency modulation voltage and the phase angle θ ref relative to the PLL phase-locked voltage can be obtained online from the control system of the converter. Wherein, the purpose of obtaining λ is to determine the initial phase angle of the AC power grid voltage, and the existing method is to use PLL (phase-locked loop) to dynamically track the initial phase angle of the AC power grid voltage, and use it as the angle reference for all calculations, such as the subsequent θ ref , which are obtained based on λ as the reference angle.

[0032] In implementation, the amplitude V ref2 and the phase angle θ ref2 of the secondary harmonic modulation voltage of the converter are obtained online by obtaining the modulation voltage of the converter circulating current controller from the control system. ref2 ref2 ref2

[0033] II. Then, the above step 102 is introduced.

[0034] In one embodiment, according to the relationship between the amplitude and the phase angle of the secondary harmonic modulation voltage introduced by the converter circulating current control and the amplitude and the phase of the power frequency modulation wave of the converter, the amplitude and the phase of the third harmonic injection voltage at which the maximum and minimum of the positive and negative peak values of the AC modulation wave of the converter are determined, can include:

[0035] According to the ratio of the amplitude of the secondary harmonic modulation voltage introduced by the converter circulating current control to the amplitude of the power frequency modulation wave of the converter, and the deviation of the phase angle of the secondary harmonic modulation voltage introduced by the converter circulating current control from the phase angle of the power frequency modulation wave of the converter, the amplitude and the phase of the third harmonic injection voltage at which the maximum and minimum of the positive and negative peak values of the AC modulation wave of the converter are determined.

[0036] In implementation, the ratio (ratio) V ref2_ratio of the amplitude of the secondary harmonic modulation voltage of the converter to the amplitude of the power frequency modulation wave is obtained based on the amplitude of the power frequency modulation voltage of the converter control system; and the deviation of the phase angle of the secondary harmonic modulation voltage of the converter from the phase angle of the modulation voltage (power frequency modulation wave voltage) of the inner loop controller is obtained based on the phase angle of the power frequency modulation voltage of the converter, θ ref2_ratio , wherein:

[0037] V ref2_ratio = V ref2 / V ref ;

[0038] θ ref2_ratio = θ ref2 -2*θ ref .

[0039] According to the ratio V ref2_ratio of the amplitude of the secondary harmonic modulation voltage of the converter to the amplitude of the power frequency modulation wave, and the deviation θ ref2_ratio of the phase angle of the secondary harmonic modulation voltage of the converter from the phase angle of the power frequency modulation wave, the amplitude θ ref3_ratio and the phase θ ref3_ratio of the third harmonic injection voltage at which the maximum and minimum of the positive and negative peak values of the AC modulation wave of the converter are determined are calculated, wherein the amplitude of the third harmonic injection voltage is V refThe unit is taken as a reference to obtain a unit value, and the advantage of unitization is universality and easy calculation. ref As a reference, the unit value is 1, and in subsequent calculations, the calculation amount caused by the change of Vref value can be reduced.

[0040] To simplify the expression, the amplitude of the power frequency modulation voltage can be set to 1 p.u., and θ ref is 0°, and in an embodiment, the maximum value expression of the absolute values of the positive and negative peak values of the AC modulation wave of the converter can be as follows:

[0041] max(|cos(ωt)-V ref2_ratio cos(2ωt+θ ref2_ratio )-V ref3_ratio cos(3ωt+θ ref3_ratio )|);

[0042] Wherein, V ref2_ratio is the ratio of the amplitude of the second harmonic modulation voltage introduced by the converter circulating current control to the amplitude of the power frequency modulation wave of the converter, θ ref2_ratio is the deviation of the phase angle of the second harmonic modulation voltage introduced by the circulating current control of the converter from the phase angle of the power frequency modulation wave of the converter, V ref3_ratio is the amplitude of the third harmonic injection voltage of the converter, θ ref3_ratio is the phase of the third harmonic injection voltage of the converter, and cos(ωt) is the power frequency modulation wave of the converter. The purpose of the max function is to find the maximum value, that is, to obtain the maximum value of the function, ω is the angular frequency, and t is the time.

[0043] In specific implementation, the third harmonic injection voltage when the maximum value of the positive and negative peak values of the AC modulation wave of the converter is the smallest can help further improve the rated voltage of the valve side of the converter transformer, reduce the converter valve and related current, and reduce system loss.

[0044] In specific implementation, the V ref3_ratio and θ ref3_ratio are calculated, and the specific solving method of the present application is not limited. In order to facilitate understanding, two solving methods are introduced below.

[0045] Further, in order to facilitate implementation, the embodiment of the present application provides a method for solving V ref3_ratio and θ ref3_ratio by table lookup, that is, determining the amplitude and phase of the third harmonic injection voltage when the maximum value of the positive and negative peak values of the AC modulation wave of the converter is the smallest by table lookup:

[0046] The second harmonic modulation voltage is calculated at different V ref2_ratio and θ ref2_ratioV is the amplitude of the absolute value of the converter voltage modulation wave, V is the amplitude of the absolute value of the second harmonic modulation voltage, V is the amplitude of the absolute value of the third harmonic injection voltage, θ is the phase of the converter voltage modulation wave, θ is the phase of the second harmonic modulation voltage, θ is the phase of the third harmonic injection voltage, and V is the amplitude of the absolute value of the third harmonic injection value of V and θ. ref3_ratio V is the amplitude of the absolute value of the converter voltage modulation wave, V is the amplitude of the absolute value of the second harmonic modulation voltage, V is the amplitude of the absolute value of the third harmonic injection voltage, θ is the phase of the converter voltage modulation wave, θ is the phase of the second harmonic modulation voltage, θ is the phase of the third harmonic injection voltage, and V is the amplitude of the absolute value of the third harmonic injection value of V and θ. ref3_ratio V is the amplitude of the absolute value of the converter voltage modulation wave, V is the amplitude of the absolute value of the second harmonic modulation voltage, V is the amplitude of the absolute value of the third harmonic injection voltage, θ is the phase of the converter voltage modulation wave, θ is the phase of the second harmonic modulation voltage, θ is the phase of the third harmonic injection voltage, and V is the amplitude of the absolute value of the third harmonic injection value of V and θ. ref2_ratio V is the amplitude of the absolute value of the converter voltage modulation wave, V is the amplitude of the absolute value of the second harmonic modulation voltage, V is the amplitude of the absolute value of the third harmonic injection voltage, θ is the phase of the converter voltage modulation wave, θ is the phase of the second harmonic modulation voltage, θ is the phase of the third harmonic injection voltage, and V is the amplitude of the absolute value of the third harmonic injection value of V and θ. ref2_ratio V is the amplitude of the absolute value of the converter voltage modulation wave, V is the amplitude of the absolute value of the second harmonic modulation voltage, V is the amplitude of the absolute value of the third harmonic injection voltage, θ is the phase of the converter voltage modulation wave, θ is the phase of the second harmonic modulation voltage, θ is the phase of the third harmonic injection voltage, and V is the amplitude of the absolute value of the third harmonic injection value of V and θ. ref3_ratio V is the amplitude of the absolute value of the converter voltage modulation wave, V is the amplitude of the absolute value of the second harmonic modulation voltage, V is the amplitude of the absolute value of the third harmonic injection voltage, θ is the phase of the converter voltage modulation wave, θ is the phase of the second harmonic modulation voltage, θ is the phase of the third harmonic injection voltage, and V is the amplitude of the absolute value of the third harmonic injection value of V and θ. ref2_ratio V is the amplitude of the absolute value of the converter voltage modulation wave, V is the amplitude of the absolute value of the second harmonic modulation voltage, V is the amplitude of the absolute value of the third harmonic injection voltage, θ is the phase of the converter voltage modulation wave, θ is the phase of the second harmonic modulation voltage, θ is the phase of the third harmonic injection voltage, and V is the amplitude of the absolute value of the third harmonic injection value of V and θ. ref2_ratio V is the amplitude of the absolute value of the converter voltage modulation wave, V is the amplitude of the absolute value of the second harmonic modulation voltage, V is the amplitude of the absolute value of the third harmonic injection voltage, θ is the phase of the converter voltage modulation wave, θ is the phase of the second harmonic modulation voltage, θ is the phase of the third harmonic injection voltage, and V is the amplitude of the absolute value of the third harmonic injection value of V and θ. ref2_ratio V is the amplitude of the absolute value of the converter voltage modulation wave, V is the amplitude of the absolute value of the second harmonic modulation voltage, V is the amplitude of the absolute value of the third harmonic injection voltage, θ is the phase of the converter voltage modulation wave, θ is the phase of the second harmonic modulation voltage, θ is the phase of the third harmonic injection voltage, and V is the amplitude of the absolute value of the third harmonic injection value of V and θ. ref2_ratio V is the amplitude of the absolute value of the converter voltage modulation wave, V is the amplitude of the absolute value of the second harmonic modulation voltage, V is the amplitude of the absolute value of the third harmonic injection voltage, θ is the phase of the converter voltage modulation wave, θ is the phase of the second harmonic modulation voltage, θ is the phase of the third harmonic injection voltage, and V is the amplitude of the absolute value of the third harmonic injection value of V and θ.

[0047] In addition, the relationship between the second harmonic modulation voltage and the third harmonic injection voltage at the minimum positive and negative peak values of the converter AC modulation wave can also be curve fitted. Through solving the fitted curve, the relationship between the amplitude and phase of the third harmonic injection voltage and the amplitude and phase of the second harmonic modulation voltage can be obtained, and the solving result of the fitted curve is used for online solving of the third harmonic injection voltage. Compared with the above method of determining the amplitude and phase of the third harmonic injection voltage at the minimum positive and negative peak values of the converter AC modulation wave through the table lookup method, the calculation amount can be reduced.

[0048] III. Next, the above step 103 is introduced.

[0049] According to the solved V and θ, the final third harmonic injection voltage is calculated, that is, the amplitude and phase V and θ of the third harmonic voltage injected into the converter are determined. ref3_ratio ref3_ratio ref3 ref3 The determined V and θ are taken as the final system third harmonic injection value, and the determination methods of V and θ are as follows: ref3 ref3 ref3 ref3_ratio ref ref3 ref3_ratio ref

[0050]

[0051]

[0052] ​​​​​​​​​​​​​IV. The other protection points related to the embodiments of the application are introduced below.

[0053] In one embodiment, the converter circulating current suppression can be to suppress the circulating current of the converter to zero, and the converter circulating current injection control can be to control the circulating current of the converter to a specified amplitude and initial phase angle.

[0054] In specific implementation, the converter circulating current suppression to suppress the circulating current of the converter to zero and the converter circulating current injection control to control the circulating current of the converter to a specified amplitude and initial phase angle can further improve the rated voltage of the converter transformer valve side, reduce the converter valve and related current, and reduce system loss.

[0055] Further, the calculation obtains the third harmonic injection voltage when the maximum of the positive and negative peak values of the converter AC modulation wave is the minimum, which can reduce the inconsistency of the positive and negative peak values of the converter AC modulation wave, effectively avoid the positive and negative overmodulation of the converter, and avoid the negative overmodulation problem caused by only taking the minimum of the positive peak value of the AC modulation wave as the target. Figure 3 and Figure 4 As shown in the formulas (1) and (2), the positive overmodulation of the converter refers to the positive peak value of the modulation wave of the converter being greater than Udc, where Udc is the DC voltage between the DC ports of the converter; and the negative overmodulation of the converter refers to the negative waveform of the modulation wave of the converter existing a phenomenon less than 0.

[0056] In order to verify the effectiveness of the embodiments of the application, the inventors give a calculation example under a symmetric bipolar structure, and the main parameters of the example are shown in Table 1:

[0057] <![CDATA[ System voltage ]]> ±525 kV ]]> ​ Rated transmission power ]]> ​ 2000 MW ]]> ​ <![CDATA[ Reactive power ]]> -900 Mvar ]]> ​ Number of single bridge arm power modules ]]> ​ 259 ]]> ​ Sub-module rated operating voltage ]]> ​ 2.134 ]]> ​ Sub-module DC capacitance value ]]> ​ 10 mF ]]> ​

[0058] Table 1

[0059] As can be seen from the calculation results of the example, as shown in the formulas (1) and (2), the positive overmodulation of the converter refers to the positive peak value of the modulation wave of the converter being greater than Udc, where Udc is the DC voltage between the DC ports of the converter; and the negative overmodulation of the converter refers to the negative waveform of the modulation wave of the converter existing a phenomenon less than 0. Figure 5 and Figure 6 As shown in the formulas (1) and (2), the positive overmodulation of the converter refers to the positive peak value of the modulation wave of the converter being greater than Udc, where Udc is the DC voltage between the DC ports of the converter; and the negative overmodulation of the converter refers to the negative waveform of the modulation wave of the converter existing a phenomenon less than 0. Figure 5 and Figure 6In the formula, X, O are two measurement scales, X represents the voltage value measured at the position of the X scale, O is inhibited with X, Delta is the measurement value of the X scale minus the measurement value of the O scale, Min represents the minimum value in the measurement between the X scale and the O scale, Max represents the maximum value corresponding to Min, Diff represents max minus min, sec is the English abbreviation of second), after the third harmonic injection method provided by the embodiment of the present application is adopted, the positive peak value of the alternating current modulation wave of the converter is increased, but the absolute value of the negative peak value is reduced, the symmetry of the peak value of the alternating current modulation wave is higher, and the maximum value of the absolute value of the modulation wave peak value is reduced by about 5.4kV, thereby further improving the margin of the modulation ratio, so that the rated voltage of the transformer valve side can be further improved, the current of the converter valve and the related current can be reduced, and the system loss can be reduced.

[0060] In summary, the third harmonic injection method suitable for converter circulating current control provided by the embodiment of the present application has the following advantages:

[0061] The third harmonic online injection method suitable for circulating current control provided by the present application can further improve the highest modulation degree available for the converter on the basis of the existing third harmonic voltage injection method, which is beneficial to improve the transformer valve side voltage, reduce the converter valve and the related current, and reduce the system loss. At the same time, the third harmonic injection method disclosed in the present application aims to obtain the amplitude and phase of the third harmonic injected when the maximum value of the absolute values of the positive and negative peak values of the alternating current modulation wave of the converter is minimum in the existing power frequency modulation and second circulating current control. This method considers the risk of positive and negative overmodulation of the converter at the same time, takes the minimum maximum value of the absolute values of the positive and negative peak values of the alternating current modulation wave of the converter as the control target, can reduce the inconsistency of the positive and negative amplitudes of the alternating current modulation wave of the converter, and can avoid the risk that the minimum value of the modulation wave of the converter is less than 0 when only considering reducing the positive peak value of the alternating current modulation wave of the converter.

[0062] In the embodiment of the present application, a third harmonic injection device suitable for converter circulating current control is also provided, as described in the following embodiment. Since the principle of solving the problem of the device is similar to that of the third harmonic injection method suitable for converter circulating current control, the implementation of the device can be referred to the implementation of the third harmonic injection method suitable for converter circulating current control, and the repeated parts will not be described herein.

[0063] Figure 7 The structure diagram of the third harmonic injection device suitable for converter circulating current control in the embodiment of the present application is shown in FIG. 1, Figure 7 As shown in the figure, the device comprises:

[0064] The acquisition unit 01 is configured to acquire the amplitude and phase of the power frequency modulation wave of the converter and the amplitude and phase angle of the second harmonic modulation voltage introduced by the converter circulating current control.

[0065] The determining unit 02 is configured to determine the amplitude and phase of the third harmonic injection voltage at which the maximum and minimum of the positive and negative peak values of the AC modulation wave of the converter occur according to the relationship between the amplitude and phase of the secondary harmonic modulation voltage introduced by the converter circulating current control and the amplitude and phase of the power frequency modulation wave of the converter.

[0066] In one embodiment, the determining unit can be specifically configured to determine the amplitude and phase of the third harmonic injection voltage at which the maximum and minimum of the positive and negative peak values of the AC modulation wave of the converter occur according to the ratio of the amplitude of the secondary harmonic modulation voltage introduced by the converter circulating current control to the amplitude of the power frequency modulation wave of the converter and the deviation of the phase of the secondary harmonic modulation voltage introduced by the converter circulating current control from the phase of the power frequency modulation wave of the converter.

[0067] In one embodiment, the amplitude of the power frequency modulation voltage can be set as 1 p.u., and the phase θ ref is 0°, and the expression of the maximum of the positive and negative peak values of the AC modulation wave of the converter can be:

[0068] max(|cos(ωt)-V ref2_ratio cos(2ωt+θ ref2_ratio )-V ref3_ratio cos(3ωt+θ ref3_ratio )|);

[0069] wherein V ref2_ratio is the ratio of the amplitude of the secondary harmonic modulation voltage introduced by the converter circulating current control to the amplitude of the power frequency modulation wave of the converter, θ ref2_ratio is the deviation of the phase of the secondary harmonic modulation voltage introduced by the converter circulating current control from the phase of the power frequency modulation wave of the converter, V ref3_ratio is the amplitude of the third harmonic injection voltage of the converter, θ ref3_ratio is the phase of the third harmonic injection voltage of the converter, cos(ωt) is the power frequency modulation wave of the converter, ω is the angular frequency, and t is the time.

[0070] In one embodiment, the converter circulating current control includes converter circulating current suppression or converter circulating current injection control.

[0071] In one embodiment, the converter circulating current suppression is to suppress the circulating current of the converter to be zero, and the converter circulating current injection control is to control the circulating current of the converter to be a specified amplitude and initial phase.

[0072] In one embodiment, obtaining the amplitude and phase of the power frequency modulation wave of the converter and the amplitude and phase of the secondary harmonic modulation voltage introduced by the converter circulating current control can include:

[0073] The control system of the converter obtains the amplitude and phase of the frequency modulation wave of the converter, and the amplitude and phase angle of the second harmonic modulation voltage introduced by the converter circulating current control.

[0074] The embodiment of the present application also provides a computer device, including a memory, a processor and a computer program stored in the memory and executable on the processor, and the processor realizes the above-mentioned third harmonic injection method for the circulating current control of the converter when executing the computer program.

[0075] The embodiment of the present application also provides a computer readable storage medium, which stores the computer program for executing the above-mentioned third harmonic injection method for the circulating current control of the converter.

[0076] The beneficial technical effect of the third harmonic injection scheme for the circulating current control of the converter provided by the embodiment of the present application is that:

[0077] The present application discloses a third harmonic online injection method for circulating current control, which can further reduce the influence of waveform coupling on the converter modulation wave when the modular multilevel converter circulating current control is controlled on the basis of the existing third harmonic injection method, thereby further improving the highest modulation degree available for the converter, and being beneficial to improving the transformer valve side voltage and reducing the converter loss.

[0078] Meanwhile, the present application discloses a third harmonic voltage injection method based on the minimum value of the maximum value of the absolute value of the positive and negative peak values of the alternating current modulation wave of the converter, which considers the risks of positive and negative overmodulation of the converter, and takes the minimum value of the maximum value of the absolute value of the positive and negative peak values of the alternating current modulation wave of the converter as the control target, so that the inconsistency of the positive and negative amplitude of the alternating current modulation wave of the converter can be reduced, and the risk that the minimum value of the converter modulation wave is less than 0 caused by only considering reducing the positive peak value of the alternating current modulation wave of the converter can be avoided.

[0079] Those skilled in the art should understand that the embodiments of the present application can be provided as a method, a system or a computer program product. Therefore, the present application can adopt a completely hardware embodiment, a completely software embodiment or an embodiment combining software and hardware aspects. 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 disk storage, CD-ROM, optical storage, etc.) containing computer usable program codes.

[0080] The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Figure 1 one or more flow or blocks Figure 1 one or more flow or blocks

[0081] These computer program instructions can also be stored in a computer readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer readable memory produce an article of manufacture including instructions which implement the function specified in the flowchart block or blocks. Figure 1 one or more flow or blocks Figure 1 one or more flow or blocks

[0082] The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Figure 1 one or more flow or blocks Figure 1 one or more flow or blocks

[0083] The above-described specific embodiments, the purpose, technical solutions and advantages of the present application are further described in detail, it should be understood that the above-described is only a specific embodiment of the present application, and is not used to limit the protection scope of the present application, any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A third harmonic injection method suitable for use in control of circulating currents in a converter, characterized in that, The method comprises the following steps: obtaining the amplitude and phase of the frequency modulation wave of the converter and the amplitude and phase of the second harmonic modulation voltage introduced by the converter circulating current control; determining the amplitude and phase of the third harmonic injection voltage with the maximum positive and negative peak values of the AC modulation wave of the converter being equal and the minimum value according to the relationship between the amplitude and phase of the second harmonic modulation voltage introduced by the converter circulating current control and the amplitude and phase of the frequency modulation wave of the converter; the converter circulating current control comprises converter circulating current suppression or converter circulating current injection control; the converter circulating current suppression is to suppress the circulating current of the converter to be zero, and the converter circulating current injection control is to control the circulating current of the converter to be a specified amplitude and initial phase angle.

2. The triplen harmonic injection method suitable for use in a current source converter circulating current control according to claim 1, characterized in that, The method for determining the amplitude and phase of the third harmonic injection voltage with the maximum positive and negative peak values of the AC modulation wave of the converter being equal and the minimum value according to the relationship between the amplitude and phase of the second harmonic modulation voltage introduced by the converter circulating current control and the amplitude and phase of the frequency modulation wave of the converter comprises the following steps: determining the amplitude and phase of the third harmonic injection voltage with the maximum positive and negative peak values of the AC modulation wave of the converter being equal and the minimum value according to the ratio of the amplitude of the second harmonic modulation voltage introduced by the converter circulating current control to the amplitude of the frequency modulation wave of the converter and the deviation of the phase angle of the second harmonic modulation voltage introduced by the converter circulating current control from the phase angle of the frequency modulation wave of the converter.

3. The triplen harmonic injection method suitable for use in a current source converter circulating current control according to claim 1, wherein, The method for obtaining the amplitude and phase of the frequency modulation wave of the converter and the amplitude and phase of the second harmonic modulation voltage introduced by the converter circulating current control comprises the following steps: obtaining the amplitude and phase of the frequency modulation wave of the converter and the amplitude and phase of the second harmonic modulation voltage introduced by the converter circulating current control through the control system of the converter.

4. A third harmonic injection device suitable for use in a current source converter circulating current control, characterized in that, The method comprises the following steps: an obtaining unit, configured to obtain the amplitude and phase of the frequency modulation wave of the converter and the amplitude and phase of the second harmonic modulation voltage introduced by the converter circulating current control; a determining unit, configured to determine the amplitude and phase of the third harmonic injection voltage with the maximum positive and negative peak values of the AC modulation wave of the converter being equal and the minimum value according to the relationship between the amplitude and phase of the second harmonic modulation voltage introduced by the converter circulating current control and the amplitude and phase of the frequency modulation wave of the converter; the converter circulating current control comprises converter circulating current suppression or converter circulating current injection control; the converter circulating current suppression is to suppress the circulating current of the converter to be zero, and the converter circulating current injection control is to control the circulating current of the converter to be a specified amplitude and initial phase angle.

5. The triplen harmonic injection device suitable for use in a current source converter circulating current control according to claim 4, characterized in that, The determining unit is specifically configured to: determine the amplitude and phase of the third harmonic injection voltage with the maximum positive and negative peak values of the AC modulation wave of the converter being equal and the minimum value according to the ratio of the amplitude of the second harmonic modulation voltage introduced by the converter circulating current control to the amplitude of the frequency modulation wave of the converter and the deviation of the phase angle of the second harmonic modulation voltage introduced by the converter circulating current control from the phase angle of the frequency modulation wave of the converter.

6. The third harmonic injection device suitable for use in a current source inverter circulating current control of claim 4, wherein, The obtaining unit is specifically configured to: obtain the amplitude and phase of the frequency modulation wave of the converter and the amplitude and phase of the second harmonic modulation voltage introduced by the converter circulating current control through the control system of the converter.

7. A computer device comprising a memory, a processor, and a computer program stored on the memory and executable on the processor, characterized in that, The processor realizes the method according to any one of claims 1 to 3 when executing the computer program.

8. A computer-readable storage medium, characterized in that, The computer readable storage medium stores a computer program for executing the method of any one of claims 1 to 3.

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