Converter control method and device of offshore wind power flexible direct current transmission system

Abstract

The invention relates to the technical field of flexible direct-current power transmission, and discloses a converter control method and device for an offshore wind power flexible direct-current power transmission system, and the method comprises the steps: monitoring the real-time output voltage of a converter, and obtaining the power data and no-load voltage data of the converter when the real-time output voltage is not equal to a preset voltage value; based on the power data and the no-load voltage data, an output voltage reference value is obtained through reactive voltage control; constructing a virtual resistor based on virtual impedance control, and obtaining an output current reference value according to the output voltage reference value and the virtual resistor; obtaining a differential mode voltage according to the output current reference value through current inner loop control; and the real-time output current of the converter is adjusted based on the differential mode voltage, so that the real-time output voltage is adjusted to a preset voltage value according to the change along with the real-time output current. According to the invention, the accuracy of frequency information transmission in the DC power transmission system can be effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible direct current transmission, and in particular to a converter control method and device for an offshore wind power flexible direct current transmission system. Background Art

[0002] In the current DC transmission system, the offshore wind farm is isolated from the onshore power grid. The two AC systems are connected by a DC transmission line. They no longer need to maintain frequency synchronization with each other. The operating status of the onshore power grid cannot be transmitted through the DC system, which seriously hinders the ability of the wind farm to perceive the frequency fluctuations of the onshore power grid and participate in the frequency regulation of the onshore power grid. At present, most of the research on frequency transmission of DC transmission systems is based on communication means, that is, a special channel is opened between the two power grids for frequency transmission, so that the frequency information of the onshore power grid can be directly transmitted to the offshore wind farm. Alternatively, a grid following control strategy based on a phase-locked loop is adopted at the receiving-end converter to achieve passive following of the receiving-end converter by detecting the frequency fluctuations of the onshore power grid.

[0003] The current offshore wind power DC transmission system uses communication means to transmit information, which requires high costs and is difficult to ensure the accuracy and timeliness of frequency information transmission. The use of a grid-following control method will make the converter under the grid-following control strategy equivalent to a controlled current source when connected to the grid, and it will not be able to actively provide frequency support for the grid, thus eliminating the possibility of normal operation of the offshore wind power transmission system under a weak grid and failing to accurately transmit frequency information. Summary of the invention

[0004] The present invention provides a converter control method and device for an offshore wind power flexible direct current transmission system, which can effectively improve the accuracy of frequency information transmission in the direct current transmission system.

[0005] In order to solve the above technical problems, the present invention provides a converter control method for an offshore wind power flexible DC transmission system, comprising:

[0006] Monitoring the real-time output voltage of the converter, and when the real-time output voltage is not equal to a preset voltage value, acquiring power data and no-load voltage data of the converter;

[0007] Based on the power data and the no-load voltage data, obtaining an output voltage reference value through reactive voltage control;

[0008] Constructing a virtual resistor based on virtual impedance control, and obtaining an output current reference value according to the output voltage reference value and the virtual resistor;

[0009] Obtaining a differential mode voltage according to the output current reference value through a current inner loop control;

[0010] The real-time output current of the converter is adjusted based on the differential mode voltage, so that the real-time output voltage is adjusted to the preset voltage value according to the change of the real-time output current.

[0011] When the present invention detects that the real-time output voltage of the converter is not equal to the preset voltage value, the power data and no-load voltage data of the converter are acquired in real time, and based on the acquired converter data, an output voltage reference value is acquired through reactive voltage control, and the output voltage reference value is used to construct a virtual resistance through virtual impedance control to acquire an output current reference value; the output current reference value is used as the input of the current inner loop control, and the differential mode voltage is acquired through the current inner loop control; the real-time output current of the converter is adjusted through the differential mode voltage, so that the real-time output voltage is adjusted to the preset voltage value according to the change of the real-time output current, thereby ensuring the accurate output of the converter.

[0012] Further, the obtaining of the power data and no-load voltage data of the converter is specifically as follows:

[0013] Obtaining a preset reactive power reference value of the converter;

[0014] Detecting the real-time output reactive power of the converter;

[0015] Determine the reactive power reference value and the real-time output reactive power as power data of the converter;

[0016] When the real-time output reactive power of the converter is zero, the no-load voltage of the converter is detected, and the no-load voltage is determined as the no-load voltage data of the converter.

[0017] Further, the output voltage reference value is obtained through reactive voltage control based on the power data and the no-load voltage data, specifically:

[0018] Processing the power data and the no-load voltage data using a control model for reactive voltage control to obtain an output voltage reference value;

[0019] Among them, the control model of the reactive voltage control is:

[0020]

[0021] Where U sm_ref is the output voltage reference value; U sm0_ref is the no-load voltage; Q s_ref is the reactive power reference value; Q s is the real-time output reactive power; k p Preset parameters for the PI controller proportional link; k i is the preset parameter for the integral part of the PI controller; s is the Laplace transform parameter.

[0022] Furthermore, the virtual resistor is constructed based on the virtual impedance control, and the output current reference value is obtained according to the output voltage reference value and the virtual resistor, specifically:

[0023] Determining the output voltage reference value as a d-axis voltage input reference value;

[0024] Set the q-axis voltage input reference value to 0;

[0025] Based on the d-axis voltage input reference value, the q-axis voltage input reference value and the virtual resistance, respectively calculating a d-axis output current reference value and a q-axis output current reference value;

[0026] The d-axis output current reference value and the q-axis output current reference value are determined as output current reference values.

[0027] Furthermore, the current inner loop control is used to obtain the differential mode voltage according to the output current reference value, specifically:

[0028] Monitor the real-time output current of the converter;

[0029] Processing the real-time output current and the output current reference value using a control model of a current inner loop control to obtain a differential mode voltage;

[0030] Among them, the control model of the current inner loop control is:

[0031]

[0032] In the formula, u diffd (s) is the d-axis differential mode voltage; u diffq (s) is the q-axis differential mode voltage; u sd (s) is the d-axis disturbance variable; u sq (s) is the q-axis disturbance variable; i vd (s) is the real-time output current of the d-axis; i vq (s) is the real-time output current of the q axis; i vd_ref (s) is the reference value of the d-axis output current; i vq_ref (s) is the reference value of the q-axis output current; ω is the angular frequency; L is the total inductance of the converter; k pl Preset parameters for the d-axis proportional link of the current inner loop PI controller; k p2 The preset parameters for the q-axis of the proportional link of the current inner loop PI controller; k il Preset parameters for the d-axis of the integral link of the current inner loop PI controller; k i2 It is the preset parameter of the q-axis of the integral link of the current inner loop PI controller; s is the Laplace transform parameter.

[0033] Furthermore, the converter control method of the offshore wind power flexible direct current transmission system further includes:

[0034] Acquire a circulating current component in the real-time output current according to the real-time output current;

[0035] Acquire a reference value of a circulating current component in the output current reference value according to the output current reference value;

[0036] Processing the circulating current component and the circulating current component reference value using a control model of internal circulating current suppression control to obtain a common mode voltage;

[0037] Based on the common mode voltage, controlling the common mode component in the real-time output current of the converter;

[0038] Among them, the control model of the internal circulation suppression control is:

[0039]

[0040] In the formula, u comd (s) is the common mode component of d axis; u comq (s) is the common mode component of q axis; i cird (s) is the d-axis circulating current component; i cirq (s) is the q-axis circulating current component; i cird_ref (s) is the reference value of the d-axis circulating current component; i cirq_ref (s) is the reference value of the q-axis circulating current component; i cird (s) is the d-axis circulating current component; i cirq (s) is the q-axis circulating current component; ω is the angular frequency; k p3 , k p4 , k i3 and k i4 are the preset controller parameters respectively; L0 is the bridge arm inductance; s is the Laplace transform parameter.

[0041] Furthermore, the converter control method of the offshore wind power flexible direct current transmission system further includes:

[0042] respectively obtaining the output active power of the sending-end converter and the output active power of the receiving-end converter;

[0043] Obtaining the AC output frequency of the receiving-end converter;

[0044] Based on the output active power of the sending-end converter, the output active power of the receiving-end converter and the AC output frequency of the receiving-end converter, the DC capacitor self-synchronization control of the receiving-end converter is performed;

[0045] Among them, the control model of DC capacitor self-synchronous control is:

[0046]

[0047] In the formula, H c is the virtual inertia time constant of the receiving-end converter; r is the power angle of the receiving-end converter; P S is the output active power of the sending-end converter; P R is the output active power of the receiving-end converter; ω r is the AC output frequency of the receiving-end converter; ω0 is the system preset frequency steady-state value.

[0048] Furthermore, the converter control method of the offshore wind power flexible direct current transmission system further includes:

[0049] Performing frequency mirror transmission control on the sending-end converter and the receiving-end converter;

[0050] Among them, the control model of frequency mirror transfer control is:

[0051]

[0052] In the formula, Δω s is the AC frequency fluctuation value of the sending-end converter station; Δω r V is the AC frequency fluctuation value of the receiving converter station; DCs is the DC voltage of the sending-end converter; V DCr is the DC voltage of the receiving-end converter; V DC0 Preset voltage value for the system; k DC is the preset droop coefficient.

[0053] Furthermore, the converter control method of the offshore wind power flexible direct current transmission system further includes:

[0054] When the detected power grid frequency fluctuation exceeds a preset fluctuation threshold, frequency support control is performed on the sending-end converter and the receiving-end converter;

[0055] Among them, the control model of frequency support control is:

[0056]

[0057] Where P R is the output active power of the receiving-end converter; ω r is the AC frequency of the receiving converter station; V DC0 Preset voltage value for the system; P dc0 is the steady-state active power of the system; H v is the system moment of inertia; D v is the system damping coefficient; C is the system capacitance; k DCIt is the coefficient describing the droop property between frequency and voltage; D is the set droop control coefficient.

[0058] Accordingly, the present invention provides a converter control device for an offshore wind power flexible direct current transmission system, comprising: a voltage monitoring module, a reactive voltage control module, a virtual impedance control module, a current inner loop control module and a voltage adjustment module;

[0059] The voltage monitoring module is used to monitor the real-time output voltage of the converter, and when the real-time output voltage is not equal to the preset voltage value, obtain the power data and no-load voltage data of the converter;

[0060] The reactive voltage control module is used to obtain an output voltage reference value through reactive voltage control based on the power data and the no-load voltage data;

[0061] The virtual impedance control module is used to construct a virtual resistor based on the virtual impedance control, and obtain an output current reference value according to the output voltage reference value and the virtual resistor;

[0062] The current inner loop control module is used to obtain the differential mode voltage according to the output current reference value through the current inner loop control;

[0063] The voltage adjustment module is used to adjust the real-time output current of the converter based on the differential mode voltage, so that the real-time output voltage is adjusted to the preset voltage value according to the change of the real-time output current. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Figure 1 A schematic flow chart of an embodiment of a converter control method for an offshore wind power flexible DC transmission system provided by the present invention;

[0065] Figure 2 A schematic structural diagram of an embodiment of an offshore wind power flexible direct current transmission system provided by the present invention;

[0066] Figure 3 A schematic diagram of a flow chart of an embodiment of a voltage outer loop control method provided by the present invention;

[0067] Figure 4 A schematic diagram of a flow chart of an embodiment of a current inner loop control method provided by the present invention;

[0068] Figure 5 A schematic diagram of a modular multilevel converter model provided by the present invention;

[0069] Figure 6 A schematic flow chart of an embodiment of the internal circulation suppression control method provided by the present invention;

[0070] Figure 7 A topological structure diagram of the receiving-end converter to the onshore receiving-end power grid provided by the present invention;

[0071] Figure 8 A schematic structural diagram of an embodiment of a converter control device for an offshore wind power flexible DC transmission system provided by the present invention. DETAILED DESCRIPTION

[0072] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0073] The flowcharts shown in the accompanying drawings are only examples and do not necessarily include all the contents and operations / steps, nor must they be executed in the order described. For example, some operations / steps may also be decomposed, combined or partially merged, so the actual execution order may change according to actual conditions.

[0074] Some embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. In the absence of conflict, the following embodiments and features in the embodiments can be combined with each other.

[0075] Example 1

[0076] like Figure 1 As shown, it is a flow chart of an embodiment of a converter control method of an offshore wind power flexible DC transmission system provided by the present invention. The method includes steps 101 to 105, and each step is specifically as follows:

[0077] Step 101: monitor the real-time output voltage of the converter, and when the real-time output voltage is not equal to a preset voltage value, obtain power data and no-load voltage data of the converter.

[0078] Step 102: Based on the power data and the no-load voltage data, an output voltage reference value is obtained through reactive voltage control.

[0079] Step 103: construct a virtual resistor based on virtual impedance control, and obtain an output current reference value according to the output voltage reference value and the virtual resistor.

[0080] Step 104: obtaining a differential mode voltage according to the output current reference value through a current inner loop control.

[0081] Step 105: adjusting the real-time output current of the converter based on the differential mode voltage, so that the real-time output voltage is adjusted to the preset voltage value according to the change of the real-time output current.

[0082] In an embodiment of the present invention, when it is monitored that the real-time output voltage of the converter is not equal to the preset voltage value, the power data and no-load voltage data of the converter are obtained in real time. The converter data includes the power data and no-load voltage data of the converter, and the power data includes a preset reactive power reference value and the real-time output reactive power of the converter obtained by real-time detection. When the real-time output reactive power of the converter is zero, the no-load voltage of the converter is detected, and the no-load voltage is determined as the no-load voltage data of the converter.

[0083] In an embodiment of the present invention, based on the acquired converter data, an output current reference value is obtained through voltage outer loop control. The voltage outer loop control includes two parts: reactive voltage control and virtual impedance control. Based on the acquired converter data, an output voltage reference value is obtained through reactive voltage control, and the output voltage reference value is used to construct a virtual resistance through virtual impedance control to obtain an output current reference value.

[0084] In the embodiment of the present invention, after obtaining the output current reference value through the voltage outer loop control, the output current reference value is used as the input of the current inner loop control, and the virtual resistance constructed in the current inner loop control is combined to output the differential mode voltage.

[0085] In the embodiment of the present invention, the real-time output current of the converter is adjusted according to the differential mode voltage, so that the real-time output voltage is adjusted to a preset voltage value according to the change of the real-time output current, thereby ensuring the accurate output of the converter.

[0086] In an embodiment of the present invention, a dual closed-loop control of voltage outer loop control and current inner loop control is performed on the converter, wherein the voltage outer loop is responsible for setting the reference value of the voltage and detecting the actual output voltage. When the actual voltage deviates from the reference value, the voltage outer loop adjusts its output and uses the output of the voltage outer loop as the reference value of the current inner loop. The current inner loop is responsible for tracking this reference value and adjusting the current to return the actual voltage to the reference value. Therefore, through the rapid response and precise control of the current, the stable regulation of the output voltage amplitude can be achieved.

[0087] In summary, the embodiment of the present invention provides a converter control method for an offshore wind power flexible direct current transmission system, monitors the real-time output voltage of the converter, and when the real-time output voltage is not equal to the preset voltage value, obtains the power data and no-load voltage data of the converter; based on the power data and no-load voltage data, obtains the output voltage reference value through reactive voltage control; constructs a virtual resistor based on virtual impedance control, and obtains the output current reference value according to the output voltage reference value and the virtual resistor; obtains the differential mode voltage according to the output current reference value through the current inner loop control; adjusts the real-time output current of the converter based on the differential mode voltage, so that the real-time output voltage is adjusted to the preset voltage value according to the change of the real-time output current. The present invention can effectively improve the accuracy of frequency information transmission in a direct current transmission system.

[0088] Example 2

[0089] See also Figure 2 , a schematic diagram of the structure of an embodiment of the offshore wind power flexible direct current transmission system provided by the present invention. The offshore wind power flexible direct current transmission system is used to connect the onshore power grid to the offshore wind farm. The offshore wind farm is connected to the sending-end converter station through a step-up transformer. The alternating current sent by the wind farm is at a lower voltage level, which is maintained at around 110kV or 220kV after being stepped up by the transformer. The sending-end converter and the receiving-end converter are connected with a high-voltage DC bus, which is responsible for transmitting the active power and reactive power of the system. At the receiving-end converter station, the converter has an inverter function, which converts direct current into alternating current, and is connected to the power grid after re-voltage regulation by the transformer.

[0090] like Figure 1 As shown, it is a flow chart of an embodiment of a converter control method of an offshore wind power flexible DC transmission system provided by the present invention, including steps 101 to 105, each of which is specifically as follows:

[0091] Step 101: monitor the real-time output voltage of the converter, and when the real-time output voltage is not equal to a preset voltage value, obtain power data and no-load voltage data of the converter.

[0092] Furthermore, in the embodiment of the present invention, the power data and no-load voltage data of the converter are obtained, specifically:

[0093] Obtaining a preset reactive power reference value of the converter;

[0094] Detecting the real-time output reactive power of the converter;

[0095] Determine the reactive power reference value and the real-time output reactive power as power data of the converter;

[0096] When the real-time output reactive power of the converter is zero, the no-load voltage of the converter is detected, and the no-load voltage is determined as the no-load voltage data of the converter.

[0097] In an embodiment of the present invention, the converter control method of the offshore wind power flexible direct current transmission system is used to control the real-time output voltage of the converter to be equal to the preset voltage value. Therefore, when the real-time output voltage is not equal to the preset voltage value, the converter data is collected in real time, and corresponding control processing is performed based on the converter data. Among them, the converter data includes the power data and no-load voltage data of the converter, and the power data includes a preset reactive power reference value and the real-time output reactive power of the converter obtained by real-time detection. When the real-time output reactive power of the converter is zero, the no-load voltage of the converter is detected, and the no-load voltage is determined as the no-load voltage data of the converter.

[0098] See also Figure 3 , is a flow chart of an embodiment of the voltage outer loop control method provided by the present invention. The voltage outer loop control of the converter can be divided into two parts: reactive voltage control and virtual impedance control. The series connection of reactive voltage control and virtual impedance control enables the voltage outer loop control to obtain a reference value of the converter output current according to the reactive power change of the system. The voltage outer loop control is described in detail below:

[0099] Step 102: Based on the power data and the no-load voltage data, an output voltage reference value is obtained through reactive voltage control.

[0100] In the embodiment of the present invention, reactive voltage control belongs to fixed value control, and the control variable has a long change cycle and is relatively stable, so voltage is selected as the control variable. There are many forms of voltage outer loop control loops. A reactive-voltage PI controller can be selected as the outer loop control strategy, and a proportional-integral link can be set to correct the difference between the converter output voltage and the reference value. Reactive voltage control can be expressed as:

[0101]

[0102] Where U sm_ref is the output voltage reference value; U sm0_ref is the no-load voltage; Q s_ref is the reactive power reference value; Q s is the real-time output reactive power; k p Preset parameters for the PI controller proportional link; k i is the preset parameter for the integral part of the PI controller; s is the Laplace transform parameter.

[0103] Step 103: construct a virtual resistor based on virtual impedance control, and obtain an output current reference value according to the output voltage reference value and the virtual resistor.

[0104] In the embodiment of the present invention, when the converter is running, if the system is not a pure resistive system or a pure inductive system, but has a resistive-inductive characteristic, there will be a problem of interference between reactive power and active power. Therefore, by using the cascade structure of the control loop, virtual impedance control can be applied to the converter to achieve decoupling of active and reactive power. Since virtual impedance does not have the thermal effect of resistance and does not generate additional loss to the system, it has obvious advantages over actual resistance.

[0105] Further, in an embodiment of the present invention, a virtual resistor is constructed based on virtual impedance control, and an output current reference value is obtained according to the output voltage reference value and the virtual resistor, specifically:

[0106] Determining the output voltage reference value as a d-axis voltage input reference value;

[0107] Set the q-axis voltage input reference value to 0;

[0108] Based on the d-axis voltage input reference value, the q-axis voltage input reference value and the virtual resistance, respectively calculating a d-axis output current reference value and a q-axis output current reference value;

[0109] The d-axis output current reference value and the q-axis output current reference value are determined as output current reference values.

[0110] In an embodiment of the present invention, in virtual impedance control, the output voltage reference value output in the reactive voltage control is used as the d-axis voltage input reference value, and the q-axis voltage input reference value is set to 0, and a virtual resistance is constructed, so that the d-axis output current reference value and the q-axis output current reference value can be obtained.

[0111] Step 104: obtaining a differential mode voltage according to the output current reference value through a current inner loop control.

[0112] See also Figure 4 , is a flow chart of an embodiment of the current inner loop control method provided by the present invention, after the current inner loop control is connected in series with the above-mentioned voltage outer loop control, an output current reference value is received and a differential mode voltage is output.

[0113] See also Figure 5 , is a schematic diagram of a modular multi-level converter model provided by the present invention. According to Kirchhoff's law, Figure 5 The KVL equations are written for the upper and lower bridge arms of a single phase of the modular multilevel converter:

[0114]

[0115] In the formula, u sj Represents the AC side power supply voltage; Lac Represents the load inductance on the AC side; i vj represents the AC side current; u pj Represents the upper bridge arm voltage; u nj Represents the voltage of the lower bridge arm; i pj Represents the upper arm current; i nj represents the lower bridge arm current; j represents any one of the three phases a, b, c; R0 represents the bridge arm resistance; L0 represents the bridge arm inductance; u oo' Represents the voltage difference between the neutral points on both sides; U dc Represents the DC side voltage.

[0116] Define the differential mode voltage of the upper and lower bridge arms as u diffj , the common mode voltage of the upper and lower bridge arms is u comj , the circulation is i cirj , j represents any one of the three phases a, b, c. Thus, the following relationship is obtained:

[0117]

[0118] Simplifying the above relationship, we can get:

[0119]

[0120] Set L and R according to the following formula:

[0121]

[0122] This leads to the following formula:

[0123]

[0124] The Park transformation can be used to transform the stationary three-phase coordinates in the above formula into rotating two-phase coordinates:

[0125]

[0126] The relationship between the differential mode voltage and the output current is obtained by using Laplace transform on the two-phase coordinates of the above equation:

[0127] (R+Ls)i vd (s)=-u sd (s)+u diffd (s)+ωLi vq (s)

[0128] (R+Ls)i vq (s)=-u sq (s)+u diffq (s)-ωLi vd (s)

[0129] The relationship between the differential mode voltage and the output current in the above equation is the theoretical basis of the current inner loop control. The current inner loop control regulates the operating state of the converter by continuously adjusting the values ​​of these two current variables. diffd and u diffq is the control variable and is the window for operating the inner loop regulation. sd and u sq is the disturbance variable, and due to the electromagnetic coupling between the d-axis and the q-axis, the expression of the d-axis component contains the current component of the q-axis, and the expression of the q-axis component contains the d-axis component. The working process of the inner loop current controller is to receive the output current reference value i given by the outer loop controller vd_ref and i vq_ref , thereby calculating the reference value u of the required control command diffd_ref and u diffq_ref , and then through the control variable u diffd and u diffq To make the final output current i vd and i vq Equal to the reference value given by the outer loop controller.

[0130] However, at this time, a stable output current cannot be obtained based on the relationship between the obtained differential mode voltage and the output current. The introduction of negative feedback control can improve the stability and response speed of the system, and dynamically adjust the input differential mode voltage value according to the deviation between the output current and the rated value for correction.

[0131] The following is an explanation of the introduced negative feedback control:

[0132] Set the intermediate current control variable V d (s) and V q (s):

[0133] V d (s)=-u sd (s)+u diffd (s)+ωLi vq (s)

[0134] V q (s)=-u sq (s)+u diffq (s)-ωLi vd (s)

[0135] Simplifying the above formula, we can get:

[0136] (R+Ls)i vd (s) = V d (s)

[0137] (R+Ls)i vq (s) = V q (s)

[0138] According to the above formula, the output variable i is established vd (s), i vq (s) and the intermediate variable V d (s), V q The transfer function between (s):

[0139]

[0140] According to the relevant information of negative feedback in the automatic control principle, in order to make the output variable i vd (s), i vq (s) can always be stable at its reference value i vd_ref and i vq_ref Nearby, it is necessary to adjust the output variable i vd (s), i vq The actual value of (s) affects the control variable V d (s) and V q (s) is corrected, so it is necessary to introduce a negative feedback system to V d (s), V q (s) is used for regulation. Since the PI regulator can achieve good results in real-time tracking of DC quantities, the PI regulator can be used to correct the error. Thus, the intermediate control variable V d (s), V q The expression of (s) is:

[0141]

[0142] Substitute the above formula into the intermediate current control variable V d (s) and V q The setting formula of (s) can obtain the control model of the current inner loop control adopted in this embodiment:

[0143]

[0144] In the formula, u diffd (s) is the d-axis differential mode voltage under Laplace transformation, u diffq (s) is the q-axis differential mode voltage under Laplace transformation, and then the differential mode voltage command U of the converter is obtained by inverse transformation back to the three-phase coordinate system. diff_abc .

[0145] Step 105: adjusting the real-time output current of the converter based on the differential mode voltage, so that the real-time output voltage is adjusted to the preset voltage value according to the change of the real-time output current.

[0146] In an embodiment of the present invention, a dual closed-loop control of voltage outer loop control and current inner loop control is performed on the converter, wherein the voltage outer loop is responsible for setting the reference value of the voltage and detecting the actual output voltage. When the actual voltage deviates from the reference value, the voltage outer loop adjusts its output and uses the output of the voltage outer loop as the reference value of the current inner loop. The current inner loop is responsible for tracking this reference value and adjusting the current to return the actual voltage to the reference value. Therefore, through the rapid response and precise control of the current, the stable regulation of the output voltage amplitude can be achieved.

[0147] Since circulating current is inevitably generated during the operation of the system, the existence of this part of the current will affect the normal operation of the system and cause the converter output waveform to be distorted. Therefore, it is necessary to design a circulating current suppression link to reduce the circulating current during system operation. The following is an explanation of the internal circulating current suppression control of an embodiment of the present invention:

[0148] See also Figure 6 , is a flow chart of an embodiment of the internal circulating current suppression control method provided by the present invention. The design method of the internal circulating current suppression control is similar to the current inner loop control method. According to Kirchhoff's law, the analytical expression of the internal circulating current of the modular multilevel converter model can be obtained as follows:

[0149]

[0150] In the formula, i cirj is the circulating current of the modular multilevel converter model, I dc is the DC current, I r2m is the 2-fold frequency current, ω is the angular frequency, θ t is the initial phase angle.

[0151] By transforming the above equation, we can get:

[0152]

[0153] In the formula, R0 represents the bridge arm resistance; L0 represents the bridge arm inductance; i cird (t) represents the d-axis circulating current component, i cirq (t) represents the q-axis circulating current component, u comd (t) represents the common mode component of the d-axis voltage, u comq (t) represents the common-mode component of the q-axis voltage.

[0154] By performing Laplace transform on the above equation, we can obtain the frequency domain form of the internal circulation dynamic equation of the modular multilevel converter model:

[0155] (R0+L0s)i cird (s)=-u comd (s)-2ωL0i cirq (s)

[0156] (R0+L0s)i cirq (s)=-u comq (s)-2ωL0i cird (s)

[0157] The following V″ d (s) and V″ q (s) To set:

[0158] V″ d (s)=-u comd (s)-2ωL0i cirq (s)

[0159] V″ q (s)=-u comq (s)+2ωL0i cird (s)

[0160] According to the negative feedback theory, in order to make the output variable i cird (s), i cirq (s) tracks its reference value i cird_ref =0, i cirq_ref =0, a negative feedback control system needs to be constructed, and the PI controller has a good performance in tracking DC quantity, so the PI controller can be used for control, thus:

[0161]

[0162] In the formula, k p3 , k p4 , k i3 and k i4 are the preset parameters of the PI controller respectively.

[0163] Thus, the control model of the internal circulation suppression control adopted in this embodiment can be obtained, that is, the actual control variable u comd (s),u comq The expression of (s) is:

[0164]

[0165] The d-axis common mode component u comd (s) and q-axis common mode component u comq (s) is used as the output voltage command reference of the modular multilevel converter model, and is transformed back to the three-phase coordinate system to obtain the common-mode voltage command u of the converter. com_abc .

[0166] See also Figure 7 , which is a topological structure diagram of the receiving-end converter to the onshore receiving-end power grid provided by the present invention. Figure 7If the transmission loss of the converter and DC line is ignored, the dynamic equation of the capacitor on the DC side of the receiving converter can be expressed as:

[0167]

[0168] Where P S is the output active power of the converter at the sending end; P R is the active power at the receiving end converter; C is the equivalent capacitance value on the DC side; V DCr is the DC voltage value at the receiving converter.

[0169] The receiving-end converter is directly connected to the onshore power grid, and the active power P transmitted from the receiving-end converter to the onshore power grid is R It can be expressed as:

[0170]

[0171] Where V r is the AC voltage value at the receiving converter station; V g is the AC voltage value of the onshore power grid; X l is the line impedance; δ r V r With V g The voltage phase angle difference between them.

[0172] In order to make the receiving converter station simulate the performance of the synchronous generator, the mathematical equation of the synchronous generator is also listed for comparison:

[0173]

[0174] Where, P m is the mechanical power of the synchronous motor; P e is the electromagnetic power; J is the rotor moment of inertia; E f is the induced potential; ω is the angular velocity of the generator.

[0175] Combining the above equations, it can be obtained that the receiving-end converter station and the synchronous generator have similar equation forms, and there is also a corresponding relationship between the variables. The DC side capacitor voltage of the receiving-end converter corresponds to the angular velocity of the generator rotor, and the capacitor size corresponds to the size of the rotational inertia.

[0176] In order to further emulate the characteristics of the synchronous generator, the swing equation of the synchronous generator is considered, and the voltage fluctuation at the DC line is often artificially coupled with the frequency fluctuation at the AC side of the receiving converter station, thereby obtaining the equation:

[0177] V DCr -V DC0 =k DC ·(ω r-ω0)

[0178] Where V DCr is the DC voltage value at the receiving converter, k DC is the set droop coefficient, ω r is the output frequency of the AC side at the receiving converter station; ω0 is the preset steady-state value of the system frequency; V DC0 Preset voltage value for the system.

[0179] That is, the output AC frequency of the receiving-end converter station has a self-synchronous property similar to the output AC frequency of the synchronous generator, realizing the simulation of the synchronous generator characteristics by the receiving-end converter station.

[0180] Virtual inertia time constant H at the receiving converter station c and the power angle δ r for:

[0181]

[0182] Finally, the control model of the DC capacitor self-synchronous control of this embodiment is obtained, that is, the mathematical expression at the receiving end converter station is:

[0183]

[0184] When the grid frequency fluctuates, the active power transmitted by the receiving-end converter to the grid fluctuates accordingly, causing the capacitor voltage to change accordingly. The change trends of the two are consistent. If the grid frequency increases, resulting in a phase angle difference δ r Decrease, output active power P R The decrease in the DC side capacitance forces the increase of the AC side output frequency of the receiving end converter to suppress the active power P R When the system finally reaches a steady state, the output frequency of the receiving-end converter is once again equal to the new frequency after the grid frequency fluctuation occurs, and the active power P R No longer decreases, that is, the receiving-end converter tracks the grid frequency changes. When the grid frequency fluctuates, the receiving-end converter responds naturally and always keeps in sync with the grid frequency. At the same time, the control model of the DC capacitor self-synchronous control also realizes the mapping of voltage information to frequency fluctuations. The change of grid frequency is directly reflected in the change of voltage, which makes it possible for the sending-end converter station to extract frequency information from voltage information. The present invention can map the change of system frequency to voltage through the self-synchronous control of capacitors, realizing the communication-free transmission of frequency information.

[0185] At the sending-end converter, the converter is mainly responsible for receiving the frequency information transmitted from the receiving-end power grid and transmitting it to the wind farm. Due to the use of a non-communication frequency information transmission method, the receiving-end converter station couples the frequency information with the DC bus voltage, and synchronously characterizes the frequency change of the onshore power grid through the voltage change, which is an "encryption" process. Therefore, the sending-end converter station needs to "decrypt" the voltage information to restore the frequency change information. The following is an explanation of the frequency mirror transmission control:

[0186] In the embodiment of the present invention, in the control strategy of the double-end network type, the control strategy of the sending-end converter is such that the capacitor self-synchronization control mode of the sending-end converter station can be expressed as:

[0187] V DCs -V DC0 =k DC ·(ω s -ω0)

[0188] Where V DCs is the DC side capacitor voltage of the sending-end converter; ω s is the output AC frequency of the AC side of the sending-end converter.

[0189] If the frequency fluctuates at the onshore power grid, the output frequency ω of the AC side of the receiving converter r The same magnitude of change will also occur, let the change value be Δω r According to the control model of DC capacitor self-synchronous control, the frequency fluctuation information will be synchronously reflected in the voltage change on the DC side of the receiving converter:

[0190]

[0191] The bus voltage fluctuation on the DC side of the receiving converter station will cause the bus voltage fluctuation on the DC side of the sending converter station, and then cause the frequency fluctuation on the AC side of the sending converter station. Assuming that there is no loss in the voltage amplitude during DC line transmission, the fluctuation value of the frequency on the AC side of the sending converter station is Δω s , the control model of the frequency mirror transfer control in this embodiment can be obtained:

[0192]

[0193] That is, the frequency change on the AC side at the sending-end converter station is completely equal to the frequency change on the receiving-end converter. It can be further concluded that the frequency change of the sending-end converter is equal to the frequency fluctuation of the onshore power grid, thus achieving the purpose of transmitting the frequency fluctuation information at the onshore power grid to the sending-end converter. The wind farm only needs to detect the frequency change information of the sending-end converter to know the frequency change of the onshore power grid. Since a meshing control strategy is adopted at both ends of the DC system, it is also called a dual-end meshing control strategy. The present invention realizes the mirror transmission of frequency information from the power grid to the wind farm by proposing a dual-end meshing control architecture, thereby improving the accuracy of information transmission.

[0194] The access of a large number of wind power and new energy sources may lead to a weakening of the power grid. A weak power grid refers to a situation where the nonlinear loads in the line account for a large proportion and the power grid is inductive. When the location of the equipment connected to the power grid changes, the inductive reactance of the power grid will also change. A weak power grid is used to define this non-ideal power grid. Severe frequency fluctuations may occur during the operation of a weak power grid.

[0195] When the grid frequency fluctuates greatly, in order to maintain the stable operation of the system, it is necessary to first provide frequency support to the grid. In order to further accelerate the support speed during frequency fluctuations, a network control strategy that enhances the frequency support effect is obtained by setting the frequency support control on the original basis. The following is an explanation of the frequency support control:

[0196] The self-synchronous control strategy of the converters at both ends is modified, and the modified strategy is shown in the following formula:

[0197] V DCr -V DCr_ref =k DC ·(ω r -ω0)

[0198] V DCs -V DCs_ref =k DC ·(ω s -ω0)

[0199] Where V DCr_ref and V DCs_ref are the voltage reference values ​​of the receiving-end converter station and the sending-end converter station, respectively, which are obtained by the voltage-active power droop control strategy, as shown in the following formula:

[0200] V DCr_ref =D(P DCr -P refr )+V DC0

[0201] V DCs_ref =D(P DCs -P refs )+VDC0

[0202] Where D is the set droop control coefficient; P DCr is the active power of the receiving converter station; P refr is the active power reference of the receiving converter station, both of which are taken as positive when entering the receiving converter station from the DC side; P DCs and P refs It is the active power flowing into the sending-end converter station from the DC side and the active power reference value.

[0203] When the system is running stably, the active power transmitted under the double-end network system correction strategy is:

[0204]

[0205] Ignore line loss, that is, P dc =P DCr =-P DCs , substituting into the above formula, we can get:

[0206]

[0207] Set the D value to make 2D》 Then update the above formula to:

[0208]

[0209] The above formula represents the double-terminal grid control using additional active power-voltage droop control. By properly setting P refr With P refs By adjusting the value of , the artificial control of the active power transmitted by the system can be realized. At the same time, after adopting the active power-voltage droop control strategy, the DC transmission power is shown as follows:

[0210]

[0211] Ignore transmission loss, that is, P dc =P DCr =-P DCs , and also set 2D》 Simplify the above formula to:

[0212]

[0213] In P refr With P refs For the case of a fixed value set in advance, linearization near the operating point can be obtained as follows:

[0214] P dc =P dc0 +ΔP dc

[0215]

[0216] If a frequency change occurs at the onshore grid, The transmission power changes in the opposite direction to the grid frequency, providing support for the grid. Substitute the above equation into the linearized capacitor dynamic equation on the DC side of the receiving converter station, which is: We can get:

[0217]

[0218] D v Δω r The existence of plays a damping role. When the system frequency fluctuates, it can speed up the synchronization process and quickly provide frequency support, making the system run more smoothly. The present invention adopts frequency support control, so that the system can also operate stably under weak power grid conditions.

[0219] In summary, the embodiments of the present invention provide a converter control method for an offshore wind power flexible direct current transmission system, which monitors the real-time output voltage of the converter. When the real-time output voltage is not equal to a preset voltage value, an output current reference value is obtained through a current outer loop control based on power data and no-load voltage data, and a differential mode voltage is obtained according to the output current reference value through a current inner loop control, and then the real-time output current of the converter is adjusted, thereby effectively improving the accuracy of frequency information transmission in the direct current transmission system.

[0220] Example 3

[0221] See also Figure 8 , is a structural schematic diagram of an embodiment of a converter control device of an offshore wind power flexible DC transmission system provided by the present invention, the device includes a voltage monitoring module 201, a reactive voltage control module 202, a virtual impedance control module 203, a current inner loop control module 204 and a voltage adjustment module 205;

[0222] The voltage monitoring module 201 is used to monitor the real-time output voltage of the converter, and when the real-time output voltage is not equal to the preset voltage value, obtain the power data and no-load voltage data of the converter;

[0223] The reactive voltage control module 202 is used to obtain an output voltage reference value through reactive voltage control based on the power data and the no-load voltage data;

[0224] The virtual impedance control module 203 is used to construct a virtual resistor based on the virtual impedance control, and obtain an output current reference value according to the output voltage reference value and the virtual resistor;

[0225] The current inner loop control module 204 is used to obtain the differential mode voltage according to the output current reference value through the current inner loop control;

[0226] The voltage adjustment module 205 is used to adjust the real-time output current of the converter based on the differential mode voltage, so that the real-time output voltage is adjusted to the preset voltage value according to the change of the real-time output current.

[0227] Furthermore, in the embodiment of the present invention, the power data and no-load voltage data of the converter are obtained, specifically:

[0228] Obtaining a preset reactive power reference value of the converter;

[0229] Detecting the real-time output reactive power of the converter;

[0230] Determine the reactive power reference value and the real-time output reactive power as power data of the converter;

[0231] When the real-time output reactive power of the converter is zero, the no-load voltage of the converter is detected, and the no-load voltage is determined as the no-load voltage data of the converter.

[0232] Further, in an embodiment of the present invention, based on the power data and the no-load voltage data, an output voltage reference value is obtained through reactive voltage control, specifically:

[0233] Processing the power data and the no-load voltage data using a control model for reactive voltage control to obtain an output voltage reference value;

[0234] Among them, the control model of the reactive voltage control is:

[0235]

[0236] Where U sm_ref is the output voltage reference value; U sm0_ref is the no-load voltage; Q s_ref is the reactive power reference value; Q s is the real-time output reactive power; k p Preset parameters for the PI controller proportional link; k i is the preset parameter for the integral part of the PI controller; s is the Laplace transform parameter.

[0237] Further, in an embodiment of the present invention, a virtual resistor is constructed based on virtual impedance control, and an output current reference value is obtained according to the output voltage reference value and the virtual resistor, specifically:

[0238] Determining the output voltage reference value as a d-axis voltage input reference value;

[0239] Set the q-axis voltage input reference value to 0;

[0240] Based on the d-axis voltage input reference value, the q-axis voltage input reference value and the virtual resistance, respectively calculating a d-axis output current reference value and a q-axis output current reference value;

[0241] The d-axis output current reference value and the q-axis output current reference value are determined as output current reference values.

[0242] Further, in an embodiment of the present invention, the differential mode voltage is obtained according to the output current reference value through the current inner loop control, specifically:

[0243] Monitor the real-time output current of the converter;

[0244] Processing the real-time output current and the output current reference value using a control model of a current inner loop control to obtain a differential mode voltage;

[0245] Among them, the control model of the current inner loop control is:

[0246]

[0247] In the formula, u diffd (s) is the d-axis differential mode voltage; u diffq (s) is the q-axis differential mode voltage; u sd (s) is the d-axis disturbance variable; u sq (s) is the q-axis disturbance variable; i vd (s) is the real-time output current of the d-axis; i vq (s) is the real-time output current of the q axis; i vd_ref (s) is the reference value of the d-axis output current; i vq_ref (s) is the reference value of the q-axis output current; ω is the angular frequency; L is the total inductance of the converter; k pl Preset parameters for the d-axis proportional link of the current inner loop PI controller; k p2 The preset parameters for the q-axis of the proportional link of the current inner loop PI controller; k il Preset parameters for the d-axis of the integral link of the current inner loop PI controller; k i2 It is the preset parameter of the q-axis of the integral link of the current inner loop PI controller; s is the Laplace transform parameter.

[0248] Furthermore, in an embodiment of the present invention, the converter control method of the offshore wind power flexible direct current transmission system further includes:

[0249] Acquire a circulating current component in the real-time output current according to the real-time output current;

[0250] Acquire a reference value of a circulating current component in the output current reference value according to the output current reference value;

[0251] Processing the circulating current component and the circulating current component reference value using a control model of internal circulating current suppression control to obtain a common mode voltage;

[0252] Based on the common mode voltage, controlling the common mode component in the real-time output current of the converter;

[0253] Among them, the control model of the internal circulation suppression control is:

[0254]

[0255] In the formula, u comd (s) is the common mode component of the d-axis; u comq (s) is the common mode component of q axis; i cird (s) is the d-axis circulating current component; i cirq (s) is the q-axis circulating current component; i cird_ref (s) is the reference value of the d-axis circulating current component; i cirq_ref (s) is the reference value of the q-axis circulating current component; i cird (s) is the d-axis circulating current component; i cirq (s) is the q-axis circulating current component; ω is the angular frequency; k p3 , k p4 , k i3 and k i4 are the preset controller parameters respectively; L0 is the bridge arm inductance; s is the Laplace transform parameter.

[0256] Furthermore, in an embodiment of the present invention, the converter control method of the offshore wind power flexible direct current transmission system further includes:

[0257] respectively obtaining the output active power of the sending-end converter and the output active power of the receiving-end converter;

[0258] Obtaining the AC output frequency of the receiving-end converter;

[0259] Based on the output active power of the sending-end converter, the output active power of the receiving-end converter and the AC output frequency of the receiving-end converter, the DC capacitor self-synchronization control of the receiving-end converter is performed;

[0260] Among them, the control model of DC capacitor self-synchronous control is:

[0261]

[0262] In the formula, H c is the virtual inertia time constant of the receiving-end converter; r is the power angle of the receiving-end converter; P S is the output active power of the sending-end converter; P Ris the output active power of the receiving-end converter; ω r is the AC output frequency of the receiving-end converter; ω0 is the system preset frequency steady-state value.

[0263] Furthermore, in an embodiment of the present invention, the converter control method of the offshore wind power flexible direct current transmission system further includes:

[0264] Performing frequency mirror transmission control on the sending-end converter and the receiving-end converter;

[0265] Among them, the control model of frequency mirror transfer control is:

[0266]

[0267] In the formula, Δω s is the AC frequency fluctuation value of the sending-end converter station; Δω r V is the AC frequency fluctuation value of the receiving converter station; DCs is the DC voltage of the sending-end converter; V DCr is the DC voltage of the receiving-end converter; V DC0 Preset voltage value for the system; k DC is the preset droop coefficient.

[0268] Furthermore, in an embodiment of the present invention, the converter control method of the offshore wind power flexible direct current transmission system further includes:

[0269] When the detected power grid frequency fluctuation exceeds a preset fluctuation threshold, frequency support control is performed on the sending-end converter and the receiving-end converter;

[0270] Among them, the control model of frequency support control is:

[0271]

[0272] Where P R is the output active power of the receiving-end converter; ω r is the AC frequency of the receiving converter station; V DC0 Preset voltage value for the system; P dc0 is the steady-state active power of the system; H v is the system moment of inertia; D v is the system damping coefficient; C is the system capacitance; k DC It is the coefficient describing the droop property between frequency and voltage; D is the set droop control coefficient.

[0273] In summary, the embodiment of the present invention provides a converter control device for an offshore wind power flexible direct current transmission system, which monitors the real-time output voltage of the converter based on the organic combination of modules. When the real-time output voltage is not equal to the preset voltage value, the output current reference value is obtained through the current outer loop control based on the power data and the no-load voltage data, and the differential mode voltage is obtained according to the output current reference value through the current inner loop control, and then the real-time output current of the converter is adjusted, thereby effectively improving the accuracy of frequency information transmission in the direct current transmission system.

[0274] The specific embodiments described above further illustrate the purpose, technical solutions and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. It is particularly pointed out that for those skilled in the art, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the scope of protection of the present invention.

Claims

1. A converter control method for an offshore wind power flexible direct current transmission system, characterized in that: include: Monitoring the real-time output voltage of the converter, and when the real-time output voltage is not equal to a preset voltage value, acquiring power data and no-load voltage data of the converter; Based on the power data and the no-load voltage data, obtaining an output voltage reference value through reactive voltage control; Constructing a virtual resistor based on virtual impedance control, and obtaining an output current reference value according to the output voltage reference value and the virtual resistor; Obtaining a differential mode voltage according to the output current reference value through a current inner loop control; The real-time output current of the converter is adjusted based on the differential mode voltage, so that the real-time output voltage is adjusted to the preset voltage value according to the change of the real-time output current.

2. The converter control method of the offshore wind power flexible DC transmission system according to claim 1, characterized in that: The obtaining of the power data and no-load voltage data of the converter is specifically as follows: Obtaining a preset reactive power reference value of the converter; Detecting the real-time output reactive power of the converter; Determine the reactive power reference value and the real-time output reactive power as power data of the converter; When the real-time output reactive power of the converter is zero, the no-load voltage of the converter is detected, and the no-load voltage is determined as the no-load voltage data of the converter.

3. The converter control method of the offshore wind power flexible DC transmission system according to claim 2 is characterized in that: The obtaining of the output voltage reference value through reactive voltage control based on the power data and the no-load voltage data is specifically as follows: Processing the power data and the no-load voltage data using a control model for reactive voltage control to obtain an output voltage reference value; Among them, the control model of the reactive voltage control is: Where U sm_ref is the output voltage reference value; U sm0_ref is the no-load voltage; Q s_ref is the reactive power reference value; Q s is the real-time output reactive power; k p Preset parameters for the PI controller proportional link; k i is the preset parameter for the integral part of the PI controller; s is the Laplace transform parameter.

4. The converter control method of the offshore wind power flexible DC transmission system according to claim 3 is characterized in that: The virtual resistance is constructed based on the virtual impedance control, and the output current reference value is obtained according to the output voltage reference value and the virtual resistance, specifically: Determining the output voltage reference value as a d-axis voltage input reference value; Set the q-axis voltage input reference value to 0; Based on the d-axis voltage input reference value, the q-axis voltage input reference value and the virtual resistance, respectively calculating a d-axis output current reference value and a q-axis output current reference value; The d-axis output current reference value and the q-axis output current reference value are determined as output current reference values.

5. The converter control method of the offshore wind power flexible DC transmission system according to claim 4, characterized in that: The current inner loop control is used to obtain the differential mode voltage according to the output current reference value, specifically: Monitor the real-time output current of the converter; Processing the real-time output current and the output current reference value using a control model of a current inner loop control to obtain a differential mode voltage; Among them, the control model of the current inner loop control is: In the formula, u diffd (s) is the d-axis differential mode voltage; u diffq (s) is the q-axis differential mode voltage; u sd (s) is the d-axis disturbance variable; u sq (s) is the q-axis disturbance variable; i vd (s) is the real-time output current of the d-axis; i vq (s) is the real-time output current of the q axis; i vd_ref (s) is the reference value of the d-axis output current; i vq_ref (s) is the reference value of the q-axis output current; ω is the angular frequency; L is the total inductance of the converter; k pl Preset parameters for the d-axis proportional link of the current inner loop PI controller; k p2 The preset parameters for the q-axis of the proportional link of the current inner loop PI controller; k il Preset parameters for the d-axis of the integral link of the current inner loop PI controller; k i2 It is the preset parameter of the q-axis of the integral link of the current inner loop PI controller; s is the Laplace transform parameter.

6. The converter control method of the offshore wind power flexible DC transmission system according to claim 5, characterized in that: Also includes: Acquire a circulating current component in the real-time output current according to the real-time output current; Acquire a reference value of a circulating current component in the output current reference value according to the output current reference value; Processing the circulating current component and the circulating current component reference value using a control model of internal circulating current suppression control to obtain a common mode voltage; Based on the common mode voltage, controlling the common mode component in the real-time output current of the converter; Among them, the control model of the internal circulation suppression control is: In the formula, u comd (s) is the common mode component of the d-axis; u comq (s) is the common mode component of q axis; i cird (s) is the d-axis circulating current component; i cirq (s) is the q-axis circulating current component; i cird_ref (s) is the reference value of the d-axis circulating current component; i cirq_ref (s) is the reference value of the q-axis circulating current component; i cird (s) is the d-axis circulating current component; i cirq (s) is the q-axis circulating current component; ω is the angular frequency; k p3 , k p4 , k i3 and k i4 are the preset controller parameters respectively; L0 is the bridge arm inductance; s is the Laplace transform parameter.

7. The converter control method of the offshore wind power flexible DC transmission system according to claim 1, characterized in that: Also includes: respectively obtaining the output active power of the sending-end converter and the output active power of the receiving-end converter; Obtaining the AC output frequency of the receiving-end converter; Based on the output active power of the sending-end converter, the output active power of the receiving-end converter and the AC output frequency of the receiving-end converter, the DC capacitor self-synchronization control of the receiving-end converter is performed; Among them, the control model of DC capacitor self-synchronous control is: In the formula, H c is the virtual inertia time constant of the receiving-end converter; r is the power angle of the receiving-end converter; P S is the output active power of the sending-end converter; P R is the output active power of the receiving-end converter; ω r is the AC output frequency of the receiving-end converter; ω0 is the system preset frequency steady-state value.

8. The converter control method of the offshore wind power flexible DC transmission system according to claim 7, characterized in that: Also includes: Performing frequency mirror transmission control on the sending-end converter and the receiving-end converter; Among them, the control model of frequency mirror transfer control is: In the formula, Δω s is the AC frequency fluctuation value of the sending-end converter station; Δω r V is the AC frequency fluctuation value of the receiving converter station; DCs is the DC voltage of the sending-end converter; V DCr is the DC voltage of the receiving-end converter; V DC0 Preset voltage value for the system; k DC is the preset droop coefficient.

9. The converter control method of the offshore wind power flexible DC transmission system according to claim 8, characterized in that: Also includes: When the detected power grid frequency fluctuation exceeds a preset fluctuation threshold, frequency support control is performed on the sending-end converter and the receiving-end converter; Among them, the control model of frequency support control is: Where P R is the output active power of the receiving-end converter; ω r V is the AC frequency of the receiving converter station; DC0 Preset voltage value for the system; P dc0 is the system steady-state active power; H v is the system moment of inertia; D v is the system damping coefficient; C is the system capacitance; k DC It is the coefficient describing the droop property between frequency and voltage; D is the set droop control coefficient.

10. A converter control device for an offshore wind power flexible DC transmission system, characterized in that: include: Voltage monitoring module, reactive voltage control module, virtual impedance control module, current inner loop control module and voltage adjustment module; The voltage monitoring module is used to monitor the real-time output voltage of the converter, and when the real-time output voltage is not equal to the preset voltage value, obtain the power data and no-load voltage data of the converter; The reactive voltage control module is used to obtain an output voltage reference value through reactive voltage control based on the power data and the no-load voltage data; The virtual impedance control module is used to construct a virtual resistor based on the virtual impedance control, and obtain an output current reference value according to the output voltage reference value and the virtual resistor; The current inner loop control module is used to obtain the differential mode voltage according to the output current reference value through the current inner loop control; The voltage adjustment module is used to adjust the real-time output current of the converter based on the differential mode voltage, so that the real-time output voltage is adjusted to the preset voltage value according to the change of the real-time output current.

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