A flexible direct current transmission system and a control method thereof

Abstract

The present invention discloses a flexible direct current (DC) power transmission system and a control method for the flexible DC power transmission system. The system includes: a flexible DC converter valve, a grid-connected transformer, and an energy-consuming device, wherein the energy-consuming device includes an energy-consuming resistor, a first supporting capacitor, and a second supporting capacitor; the valve side of the grid-connected transformer is used to connect to the flexible DC converter valve, and the grid side of the grid-connected transformer is used to connect to the AC power grid; the valve side of the grid-connected transformer adopts a star-connected winding, and the grid side of the grid-connected transformer adopts a delta-connected winding; the neutral point of the valve-side winding of the grid-connected transformer is grounded through the energy-consuming resistor; the first supporting capacitor and the second supporting capacitor are connected in series between the positive and negative DC busbars of the flexible DC converter valve, and the middle connection point of the first supporting capacitor and the second supporting capacitor is grounded. Through the above technical solution, the energy-consuming device is simplified and the cost of the flexible DC power transmission system is reduced.

Description

Technical Field

[0001] The embodiments of the present invention relate to the field of power transmission technology, and in particular to a flexible direct current transmission system and a control method for the flexible direct current transmission system. Background Art

[0002] When offshore wind power is transmitted via HVDC Flexible transmission, the offshore HVDC converter valve acts as the rectifier, converting the AC energy generated by the wind turbine into DC. The onshore HVDC converter valve acts as the inverter, converting the DC energy into AC and feeding it into the onshore AC grid. If a short-term fault occurs in the onshore AC grid, the offshore wind turbine cannot reduce its transmission power in time, and wind power continues to be transmitted to the onshore HVDC converter valve. However, the onshore HVDC converter valve is unable to transmit energy due to the AC grid fault, potentially causing overvoltage in the DC line and converter valve. To prevent a short-term AC fault from causing the entire HVDC Flexible transmission system to trip and shut down, current projects use DC energy dissipation devices to temporarily dissipate excess energy and prevent severe overvoltage in the line.

[0003] Current DC energy-consuming devices usually use multiple power semiconductor devices, half-bridge units or energy-consuming resistor submodules, etc., and require the addition of energy-consuming branches composed of a large number of power semiconductor devices. In some cases, water cooling is also required to remove the heat generated by the energy-consuming process, resulting in a high cost for the flexible DC transmission system. In addition, these additional energy-consuming devices will significantly increase the footprint of the onshore flexible DC converter valve. Summary of the Invention

[0004] The present invention provides a flexible direct current transmission system and a control method thereof to control the flexible direct current transmission system so as to simplify energy consumption devices and reduce the cost of the flexible direct current transmission system.

[0005] In a first aspect, an embodiment of the present invention provides a flexible direct current transmission system, including:

[0006] A flexible DC converter valve, a grid-connected transformer, and an energy dissipation device, wherein the energy dissipation device includes an energy dissipation resistor, a first supporting capacitor, and a second supporting capacitor;

[0007] The valve side of the grid-connected transformer is used to be connected to the flexible DC converter valve, and the grid side of the grid-connected transformer is used to be connected to the AC power grid;

[0008] The valve side of the grid-connected transformer adopts a star-connected winding, and the grid side of the grid-connected transformer adopts a delta-connected winding; the neutral point of the valve side winding of the grid-connected transformer is grounded through the energy-consuming resistor;

[0009] The first supporting capacitor and the second supporting capacitor are connected in series between the positive and negative DC busbars of the flexible DC converter valve, and an intermediate connection point between the first supporting capacitor and the second supporting capacitor is grounded.

[0010] In a second aspect, an embodiment of the present invention provides a control method for a flexible direct current transmission system, the method being applied to the flexible direct current transmission system, the method comprising:

[0011] generating a modulation signal for a bridge arm of the flexible DC converter valve, wherein the modulation signal includes a basic modulation signal and a common-mode modulation signal;

[0012] The flexible direct current transmission system is controlled based on the modulation signal, wherein the AC voltage generated by the basic modulation signal is transmitted to the grid side of the grid-connected transformer, and the common-mode current generated by the common-mode modulation signal passes through the energy-consuming resistor in the energy-consuming device.

[0013] An embodiment of the present invention provides a flexible direct current transmission system and a control method for the flexible direct current transmission system. The system includes: a flexible direct current converter valve, a grid-connected transformer, and an energy-consuming device, wherein the energy-consuming device includes an energy-consuming resistor, a first supporting capacitor, and a second supporting capacitor; the valve side of the grid-connected transformer is used to connect to the flexible direct current converter valve, and the grid side of the grid-connected transformer is used to connect to the AC power grid; the valve side of the grid-connected transformer adopts a star-connected winding, and the grid side of the grid-connected transformer adopts a delta-connected winding; the neutral point of the valve-side winding of the grid-connected transformer is grounded through the energy-consuming resistor; the first supporting capacitor and the second supporting capacitor are connected in series between the positive and negative DC busbars of the flexible direct current converter valve, and the middle connection point of the first supporting capacitor and the second supporting capacitor is grounded. Through the above technical solution, the energy-consuming device is simplified and the cost of the flexible direct current transmission system is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The above and other features, advantages, and aspects of the various embodiments of the present disclosure will become more apparent with reference to the following detailed description in conjunction with the accompanying drawings. Throughout the drawings, the same or similar reference numerals represent the same or similar elements. It should be understood that the drawings are schematic and that the originals and elements are not necessarily drawn to scale.

[0015] Figure 1 A schematic structural diagram of a flexible direct current transmission system provided in the first embodiment of the present invention;

[0016] Figure 2 A schematic diagram of valve-side voltage of a flexible DC converter valve provided in the first embodiment of the present invention;

[0017] Figure 3 A schematic diagram of a common-mode current path provided in the first embodiment of the present invention;

[0018] Figure 4 A schematic structural diagram of another flexible direct current transmission system provided in the first embodiment of the present invention;

[0019] Figure 5 A schematic diagram of generating a basic modulation signal provided in the first embodiment of the present invention;

[0020] Figure 6 A schematic diagram of generating a common-mode modulation signal provided in the first embodiment of the present invention;

[0021] Figure 7 A schematic diagram of generating a modulation signal for each bridge arm provided in the first embodiment of the present invention;

[0022] Figure 8 A schematic diagram of system voltage of a flexible direct current transmission system provided in the first embodiment of the present invention;

[0023] Figure 9 A schematic diagram of active power emitted and received by a flexible direct current transmission system provided in the first embodiment of the present invention;

[0024] Figure 10 A schematic diagram of the voltage between the positive and negative DC bus bars provided in the first embodiment of the present invention;

[0025] Figure 11 A schematic diagram of a phase A current waveform provided in the first embodiment of the present invention;

[0026] Figure 12 A schematic diagram of a phase A current waveform detail provided in the first embodiment of the present invention;

[0027] Figure 13 A schematic diagram of a current waveform of an energy-consuming resistor connected to the neutral point of a valve-side winding of a transformer provided in the first embodiment of the present invention;

[0028] Figure 14 A schematic diagram showing details of a current waveform of an energy-consuming resistor connected to the neutral point of a valve-side winding of a transformer provided in the first embodiment of the present invention;

[0029] Figure 15 A schematic diagram of a common-mode modulation signal provided in Embodiment 1 of the present invention;

[0030] Figure 16 A schematic diagram illustrating details of a common-mode modulation signal provided in the first embodiment of the present invention;

[0031] Figure 17 A schematic diagram of an A-phase AC modulation signal waveform provided in the first embodiment of the present invention;

[0032] Figure 18 A schematic diagram of a waveform detail of an A-phase AC modulation signal provided in the first embodiment of the present invention;

[0033] Figure 19 A schematic diagram of a modulation signal waveform of an upper bridge arm of phase A provided in the first embodiment of the present invention;

[0034] Figure 20A schematic diagram of a modulation signal waveform of an upper bridge arm of phase A provided in accordance with the first embodiment of the present invention;

[0035] Figure 21 A flowchart of a control method for a flexible direct current transmission system provided in accordance with the second embodiment of the present invention. DETAILED DESCRIPTION

[0036] The present invention will be further described in detail below with reference to the accompanying drawings and examples. It will be understood that the specific embodiments described herein are intended only to illustrate the present invention and are not intended to limit the present invention. Furthermore, the embodiments and features of the embodiments of the present invention may be combined with one another unless there is a conflict. It should also be noted that, for ease of description, the drawings only illustrate portions relevant to the present invention, not all of the components.

[0037] Before discussing the exemplary embodiments in more detail, it should be mentioned that some exemplary embodiments are described as processes or methods depicted as flow charts. Although the flow charts describe the steps as sequential processes, many of the steps can be implemented in parallel, concurrently, or simultaneously. In addition, the order of the steps can be rearranged. The process can be terminated when its operation is completed, but can also have additional steps not included in the accompanying drawings. The process can correspond to a method, function, procedure, subroutine, subprogram, etc.

[0038] It should be noted that the concepts of "first" and "second" mentioned in the embodiments of the present invention are only used to distinguish different devices, modules, units or other objects, and are not used to limit the order or interdependence of the functions performed by these devices, modules, units or other objects.

[0039] Example 1

[0040] Figure 1 This is a schematic diagram of the structure of a flexible direct current transmission system provided in the first embodiment of the present invention. The system can be used to convert direct current energy into alternating current and feed it into the alternating current grid. Figure 1 As shown, the system includes: a flexible DC converter valve 10, a grid-connected transformer 20 and an energy dissipation device 30, the energy dissipation device 30 includes an energy dissipation resistor R1, a first supporting capacitor C1 and a second supporting capacitor C2; the valve side of the grid-connected transformer 20 is used to connect to the flexible DC converter valve 10, and the grid side of the grid-connected transformer 20 is used to connect to the AC power grid; the valve side of the grid-connected transformer 20 adopts a star-connected winding, and the grid side of the grid-connected transformer 20 adopts a delta-connected winding; the neutral point of the valve-side winding of the grid-connected transformer 20 is grounded through the energy dissipation resistor R1; the first supporting capacitor C1 and the second supporting capacitor C2 are connected in series between the positive and negative DC busbars of the flexible DC converter valve 10, and the middle connection point of the first supporting capacitor C1 and the second supporting capacitor C2 is grounded.

[0041] In this embodiment, two supporting capacitors C1 and C2 are added between the positive and negative DC busbars (i.e., the DC positive busbar and the DC negative busbar) of the flexible DC converter valve 10. C1 and C2 can be the same capacitors, and the connection point of C1 and C2 is grounded. The valve-side winding of the grid-connected transformer 20 is designed to be a star connection (Y0 connection), and a high-power energy-consuming resistor R1 is connected between the neutral point (I0) of the valve-side winding and the ground. The energy-consuming resistor R1 is used to withstand the wind power energy during a short-term fault in the AC power grid. The grid-side winding of the grid-connected transformer 20 is designed to be a delta connection to ensure that the common-mode current of the valve-side winding does not flow into the AC power grid. In fact, this part of the common-mode current is grounded through the energy-consuming resistor R1, and can form a loop with the first supporting capacitor C1, the second supporting capacitor C2, and the flexible DC converter valve 10 to consume wind power energy and prevent excessive current from flowing into the AC power grid.

[0042] Figure 2 This is a schematic diagram of the valve-side voltage of a flexible DC converter valve provided in the first embodiment of the present invention. Figure 2 In the figure, only two consecutive power frequency cycles are drawn for both normal and faulty AC grid conditions, and only the waveforms of phases A and B are shown. Assume that the AC grid is normal before 0.04s and after 0.08s, and faults occur between 0.04s and 0.08s. Figure 2 As shown in the figure, before 0.04s and after 0.08s, the valve-side AC voltages of phases A and B are normal; the total valve-side voltage of phases A and B is equal to the valve-side AC voltage; and the valve-side common-mode voltage is 0. However, between 0.04s and 0.08s, the AC grid fails, and the valve-side AC voltage drops significantly.

[0043] In this embodiment, in order to consume wind power energy, each phase ( Figure 2 Only phases A and B are shown) to produce Figure 2 The valve-side common-mode voltage is shown between 0.04s and 0.08s. It is noteworthy that the amplitude and phase of the common-mode voltages of each phase are exactly the same. Thus, the total valve-side voltage of each phase is the sum of the valve-side AC voltage and the valve-side common-mode voltage. Because the valve-side winding of the grid-connected transformer is connected in Y0 configuration and the grid-side winding is connected in delta configuration, Figure 2 Of the total valve-side voltage shown in , the AC voltage portion is transmitted to the grid-connected transformer's grid side, where it, together with the grid voltage, determines the current flowing into the AC grid. The common-mode voltage portion, on the other hand, primarily falls on the energy-dissipating resistor from the valve-side winding neutral point to ground, generating a high-frequency current on the resistor and consuming wind power.

[0044] In one embodiment, the modulation signal of the bridge arm of the flexible DC converter valve includes a basic modulation signal and a common-mode modulation signal; when the AC power grid fails and the voltage between the positive and negative DC buses of the flexible DC converter valve is higher than a threshold, the AC voltage generated by the basic modulation signal is transmitted to the grid side of the grid-connected transformer, and the common-mode current generated by the common-mode modulation signal passes through the energy-consuming resistor.

[0045] Specifically, when an AC power grid fails and the DC line voltage of the flexible DC transmission system rises significantly, the AC voltage required to be generated by the flexible DC converter valve 10 will be significantly reduced due to the AC power grid failure, and the AC modulation signal of the flexible DC converter valve 10 will be significantly reduced. Based on the above system, an additional high-frequency common-mode modulation signal can be added to the flexible DC converter valve 10, thereby generating a corresponding high-frequency common-mode voltage on the valve side voltage of the grid-connected transformer 20. This common-mode voltage will generate current on the high-power energy-consuming resistor, thereby consuming wind power energy in the flexible DC transmission system.

[0046] Figure 3 This is a schematic diagram of a common mode current path provided by the first embodiment of the present invention. Figure 2 For example, during the 0.04s-0.08s AC grid fault period, the three phases of the flexible DC converter valve are simultaneously injected with common mode voltage, and the resulting common mode current path is as follows: Figure 3 As shown: the common-mode current flows into the flexible DC converter valve from the connection point of the first supporting capacitor C1 and the second supporting capacitor C2 on the DC side, enters the positive and negative DC busbars in two ways, and then flows into the DC side of the six bridge arms respectively, flows out from the AC side of the six bridge arms, and merges into the valve side of the transformer. After passing through the valve side winding of the transformer, it merges into the valve side neutral point of the grid-connected transformer, flows through the energy-consuming resistor and returns to the ground.

[0047] In the common-mode current loop: the first supporting capacitor C1 and the second supporting capacitor C2, assuming that the capacitance of each supporting capacitor is C, then the equivalent capacitance after parallel connection is 2*C; the six bridge arm reactances L1, L2, L3, L4, L5, and L6 are connected in parallel, assuming that the inductance of each bridge arm reactance is Larm, then the equivalent inductance after parallel connection is Larm / 6; the three-phase equivalent leakage reactance on the valve side of the grid-connected transformer is connected in parallel, assuming that the inductance of each phase equivalent leakage reactance is Lt, then the equivalent inductance after parallel connection is Lt / 3; the resistance of the energy-consuming resistor is R1. To reduce the requirements for common-mode voltage, during the design process, the inductor and capacitor series resonant frequency of the entire loop should be designed to be the frequency of the common-mode modulation signal. In this way, in order to consume energy on the energy-consuming resistor R1, the common-mode voltage generated is equivalent to the voltage drop of the common-mode current on resistor R1.

[0048] The first embodiment of the present invention provides a flexible direct current transmission system that uses energy-dissipating resistors and two supporting capacitors as energy dissipation devices, effectively reducing the cost of the flexible direct current transmission system and the footprint of onshore flexible direct current converter valves. Furthermore, by properly configuring the winding connections on the valve and grid sides of the grid-connected transformer, common-mode current in the valve-side windings is prevented from flowing into the AC grid, preventing excessive current from flowing into the AC grid during short-term AC grid faults.

[0049] In one embodiment, the energy dissipation device further includes a grounding reactor and a neutral point capacitor; the intermediate connection point of the first supporting capacitor and the second supporting capacitor is grounded through the grounding reactor; and the neutral point of the valve-side winding of the grid-connected transformer is grounded through the energy dissipation resistor and the neutral point capacitor.

[0050] Figure 4 This is a structural diagram of another flexible direct current transmission system provided in the first embodiment of the present invention. Figure 4 As shown, two identical capacitors are added between the positive and negative DC busbars of the flexible DC converter valve 10: a first supporting capacitor C1 and a second supporting capacitor C2, and the connection point of C1 and C2 is grounded through a grounding reactor L7. The valve-side winding of the grid-connected transformer 20 is designed to be connected in a Y0 manner, and a high-power energy-consuming resistor R1 and a neutral-point capacitor C3 are connected between the neutral point (I0) of the valve-side winding and the ground. The energy-consuming resistor is designed to be able to withstand wind power energy during a short-term fault in the AC grid. An additional grounding high resistor (Rgnd) can also be connected between the valve-side neutral point of the grid-connected transformer 20 and the ground. The series resonant frequency of the grounding reactor L7, the first supporting capacitor C1, and the second supporting capacitor C2 connected in parallel is the common-mode modulation signal frequency. The grid-side winding of the grid-connected transformer 20 is designed to be connected in a delta manner to ensure that the common-mode current of the valve-side winding does not flow into the AC grid.

[0051] When an AC grid fault occurs and the DC line voltage of the flexible DC transmission system rises significantly, the flexible DC converter valve 10 is controlled to generate an additional common-mode modulation signal, thereby generating a corresponding high-frequency common-mode voltage on the valve-side voltage of the grid-connected transformer 20. This common-mode voltage generates a current in the energy-dissipating resistor R1, thereby consuming the wind power in the flexible DC system.

[0052] Figure 4 and Figure 1 Compared with the flexible direct current transmission system, the addition of neutral point capacitor C3 and grounding reactor L7 can make the total voltage drop of high-frequency common-mode current flowing through C1, C2 and L7 basically zero, which can reduce the fluctuation of DC circuit voltage to ground.

[0053] In addition, according to Figure 4In the structure shown, the series resonant frequency of the inductance and capacitance of the entire common-mode current loop can be designed to be the frequency of the common-mode modulation signal, thereby consuming wind power energy on the energy-consuming resistor R1, and the common-mode voltage generated is only equivalent to the voltage drop on the energy-consuming resistor R1.

[0054] In one embodiment, the frequency of the common-mode modulation signal is determined based on the operating parameters of the flexible DC converter valve; wherein the operating parameters include at least one of the following: the rated voltage of the power semiconductor device; the rated current of the power semiconductor device; the switching frequency of normal operation; the maximum allowable switching frequency; the maximum duration of fault ride-through; and the modulation index.

[0055] Specifically, the frequency of the common-mode modulation signal is determined based on the operating parameters (reflecting the device's capabilities) of the flexible DC converter valve 10. The higher the frequency of the common-mode modulation signal, the higher the switching frequency of the flexible DC converter valve 10 during an AC grid fault, and the smaller the two supporting capacitors C1 and C2 in the energy dissipation device 30.

[0056] Optionally, the frequency of the common-mode modulation signal is 200Hz to 400Hz. Taking a typical flexible DC project as an example, the power semiconductor device uses an insulated gate bipolar transistor (IGBT) of 4.5kV 1.5kA to 4.5kV 3kA. Its normal operating switching frequency is 150Hz, and the maximum switching frequency allowed for a short period is about 500Hz. Considering the grid fault and the requirement that the flexible DC system fault ride-through time is up to 1.5s, it is recommended to use a common-mode signal frequency f on the basis of retaining a certain margin. com 200Hz—400Hz.

[0057] Optionally, the resistance of the energy dissipation resistor satisfies the following requirement: when the fault residual voltage is a set value, the sum of the basic modulation signal and the common-mode debugging signal does not exceed a set maximum modulation index.

[0058] Specifically, considering that the modulation index of the flexible DC converter valve 10 generally has a large margin, for example, in some flexible DC transmission projects, the modulation index under rated conditions is 0.735, which has a large margin, therefore, when designing the resistance value of the energy-consuming resistor, the design standard can be that when the residual voltage of the AC grid fault is 50%, after adding the common-mode modulation signal, the sum of the basic modulation signal and the common-mode debugging signal does not exceed the total modulation index (that is, the set maximum modulation index, for example, 0.9).

[0059] For example: the rated AC voltage on the valve side is 416kV, and when the residual voltage is 50% during a grid fault:

[0060] The peak value of the AC voltage phase voltage on the valve side is approximately:

[0061] The rated voltage of the DC line is ±400kV. Based on the total modulation index of 0.9, the allowable common mode voltage peak is: 0.9×400-147=213kV;

[0062] The rated power of the system is 1100MW. When the residual voltage of the power grid is 50% after a fault, the maximum power required to be consumed by the energy dissipation resistor is: 1100×(1-50%)=550MW;

[0063] Therefore, the resistance of the energy dissipation resistor is:

[0064] Exemplarily, the capacitance C of the support capacitor can be determined according to the following method:

[0065] Select f com =250Hz; bridge arm reactance Larm=133mH; valve side single-phase transformer leakage reactance Lt=62.8mH; then

[0066]

[0067] In one embodiment, a common-mode modulation signal is generated in the following manner: a fixed deviation is added to a given DC line voltage value to obtain a target DC line voltage value controlled by the common-mode modulation signal; the target DC line voltage value is subtracted from an actual DC line voltage to obtain a DC line voltage difference; the DC line voltage difference is adjusted using a proportional integral (PI) regulator to obtain a given common-mode current amplitude; the given common-mode current amplitude is multiplied by the sine of the common-mode modulation signal frequency to obtain a given common-mode current value; the given common-mode current value is compared with a feedback value of the actual common-mode current; and a proportional resonant (PR) regulator is used to generate the common-mode modulation signal based on the comparison result.

[0068] In this embodiment, a basic modulation signal and a common-mode modulation signal are generated separately.

[0069] Figure 5 A schematic diagram of generating a basic modulation signal according to the first embodiment of the present invention is shown in FIG. dc * , the actual DC line voltage feedback is V dc , the current regulator generates a three-phase AC modulation signal u a * 、u b * 、u c * After that, there is no need to inject additional common-mode modulation signals, and the basic modulation signals output to each bridge arm are:

[0070] uau * =0.5(1-u a * );u ad * =0.5(1+u a * );

[0071] u bu * =0.5(1-u b * );u bd * =0.5(1+u b * );

[0072] u cu * =0.5(1-u c * );u cd * =0.5(1+u c * )

[0073] Figure 6 This is a schematic diagram of generating a common mode modulation signal provided by the first embodiment of the present invention. dc * Add a fixed deviation ΔV to the basis dc * (typically 10% of rated DC link voltage), we get V dc * +ΔV dc * The target DC line voltage is given as the common mode modulation signal control; the DC line voltage is given and the actual DC line voltage V dc After subtraction, the common mode current amplitude is generated by the PI regulator with a lower limit of 0; the common mode current amplitude is multiplied by the sine of the common mode signal frequency to obtain the common mode current given value i com * ; Compare the given value of common mode current with the actual common mode current feedback i com For comparison, after passing through the PR regulator, a common-mode modulation signal u is generated. com * .

[0074] Figure 7 This is a schematic diagram of generating a modulation signal for each bridge arm provided by the first embodiment of the present invention. Figure 7 As shown, setting more than V dc *The target DC link voltage is given by V dc * +ΔV dc * , and the actual DC line voltage V dc The given common-mode current amplitude is generated by the proportional-integral regulator with a lower limit of 0. The given common-mode current frequency is a preset frequency greater than the power frequency. The given common-mode current is compared with the actual common-mode current and passed through the proportional resonant regulator to generate the required common-mode modulation signal. After combining the common-mode modulation signal with the normal AC modulation signal, the modulation signals of the six bridge arms are generated:

[0075] u au * =0.5(1-u a * -u com * );u ad * =0.5(1+u a * +u com * );

[0076] u bu * =0.5(1-u b * -u com * );u bd * =0.5(1+u b * +u com * );

[0077] u cu * =0.5(1-u c * -u com * );u cd * =0.5(1+u c * +u com * )

[0078] In order to verify the effectiveness of the above flexible DC transmission system and modulation signal, the following Figure 8-Figure 20 The simulation results are shown.

[0079] Figure 8 This is a schematic diagram of the system voltage of a flexible direct current transmission system provided in the first embodiment of the present invention. Figure 8As shown in FIG, an AC grid fault occurs between 2.5s and 3.2s, and the AC three-phase voltage on the grid side drops to approximately 30%.

[0080] Figure 9 This is a schematic diagram of the active power sent and received by a flexible direct current transmission system provided in the first embodiment of the present invention. Figure 9 As shown in the figure, the DC power (active power received) is basically stable and always close to the rated power. During the AC grid fault, the AC power (active power sent by the flexible DC converter to the AC grid) is significantly reduced. This power difference needs to be absorbed by energy consumption devices.

[0081] Figure 10 This is a schematic diagram of the voltage between the positive and negative DC bus bars provided in the first embodiment of the present invention. Figure 10 As shown in the figure, when the AC voltage is normal, the voltage between the positive and negative DC busbars is stable at the rated value of 800kV. Figure 5 During the AC grid fault, the voltage between the positive and negative DC buses is stable at around 880kV. Figure 6 The control effect of the target DC line voltage is given, that is, V dc * +ΔV dc * The setting value of

[0082] Figure 11 A schematic diagram of a phase A current waveform provided in Example 1 of the present invention. Figure 12 Schematic diagram of a phase A current waveform detail provided by embodiment 1 of the present invention. Figure 11 and Figure 12 As shown in FIG, when the AC power grid is normal, the current waveform of phase A is a power frequency sine wave; during an AC power grid fault, due to the action of the common mode current, the current waveform of phase A is a power frequency sine wave superimposed with a high-frequency common mode current.

[0083] Figure 13 This is a schematic diagram of a current waveform of an energy-consuming resistor connected to the neutral point of a valve-side winding of a transformer provided in accordance with a first embodiment of the present invention. Figure 14 This is a schematic diagram of the details of the current waveform of the energy-consuming resistor connected to the neutral point of the valve-side winding of a transformer provided in the first embodiment of the present invention. Figure 13 and Figure 14 As shown in FIG, when the AC grid is normal, the current in the energy dissipation resistor is 0; during an AC grid fault, all the high-frequency common-mode current generated by the flexible DC converter valve flows through the energy dissipation resistor.

[0084] Figure 15 A schematic diagram of a common-mode modulation signal provided in Example 1 of the present invention. Figure 16This is a schematic diagram of the details of a common mode modulation signal provided in the first embodiment of the present invention. Figure 15 and Figure 16 As shown, when the AC power grid is normal, the common mode modulation signal u com * is 0; during AC grid fault, u com * is the generated high frequency common mode modulation signal.

[0085] Figure 17 A schematic diagram of an A-phase AC modulation signal waveform provided in Example 1 of the present invention. Figure 18 Schematic diagram of a phase A AC modulation signal waveform detail provided by embodiment 1 of the present invention. Figure 17 and Figure 18 As shown, the A-phase AC modulation signal u a * It is always a power frequency sine wave corresponding to the AC grid voltage.

[0086] Figure 19 A schematic diagram of a modulation signal waveform of the upper bridge arm of phase A provided in the first embodiment of the present invention. Figure 20 Schematic diagram of the waveform details of the modulation signal of the upper bridge arm of phase A provided by the first embodiment of the present invention. Figure 19 and Figure 20 As shown, the A phase upper bridge arm modulation signal u au * The controller is based on the common mode modulation signal u com * and A-phase AC modulation signal u a * Generate: u au * =0.5(1-u a * -u com * ), which includes industrial frequency sinusoidal modulation signal and high frequency common mode modulation signal.

[0087] according to Figures 8 to 20 The simulation results show that the energy dissipation device of this embodiment can utilize the short-term overload capacity of the flexible DC converter valve and the energy dissipation resistor to dissipate excess wind power during AC grid faults. The entire system operates smoothly, and AC grid faults no longer affect the normal operation of offshore wind power generation.

[0088] Example 2

[0089] Figure 21A flow chart of a control method for a flexible direct current transmission system provided in the second embodiment of the present invention is applicable to the case of controlling a flexible direct current transmission system. Specifically, the control method for the flexible direct current transmission system can be implemented by software and / or hardware and integrated into a controller. The flexible direct current transmission system includes a flexible direct current converter valve, a grid-connected transformer, and an energy dissipation device, wherein the energy dissipation device includes an energy dissipation resistor, a first supporting capacitor, and a second supporting capacitor. The energy dissipation resistor can form a grounded loop with the first supporting capacitor and the second supporting capacitor, so that the energy dissipation resistor can be used to withstand wind power energy during a short-term fault in the AC power grid.

[0090] It should be noted that for technical details not fully described in this embodiment, reference can be made to any of the above embodiments.

[0091] like Figure 21 As shown, the method specifically includes the following steps:

[0092] S110: Generate a modulation signal for the bridge arm of the flexible DC converter valve, where the modulation signal includes a basic modulation signal and a common-mode modulation signal.

[0093] S120. Control the flexible direct current transmission system based on the modulation signal, wherein the AC voltage generated by the basic modulation signal is transmitted to the grid side of the grid-connected transformer, and the common-mode current generated by the common-mode modulation signal passes through the energy-consuming resistor in the energy-consuming device.

[0094] A second embodiment of the present invention provides a control method for a flexible DC transmission system. By adding an additional high-frequency common-mode modulation signal to the flexible DC converter valve, a corresponding high-frequency common-mode voltage is generated on the valve-side voltage of the grid-connected transformer. This common-mode voltage induces a current in the energy-dissipating resistor, which flows through the ground return path formed by the first and second support capacitors, thereby consuming wind power in the flexible DC transmission system. This ensures that the common-mode current in the valve-side winding does not flow into the AC grid, preventing excessive current from flowing into the AC grid during a short-term AC grid fault.

[0095] Optionally, the method also includes: determining the frequency of the common-mode modulation signal based on the operating parameters of the flexible DC converter valve; wherein the operating parameters include at least one of the following: the rated voltage of the power semiconductor device; the rated current of the power semiconductor device; the switching frequency of normal operation; the maximum allowable switching frequency; the maximum duration of fault ride-through; and the modulation index.

[0096] Optionally, the process of generating the common-mode modulation signal includes:

[0097] A fixed deviation is added to a given DC line voltage value to obtain a target DC line voltage value controlled by a common-mode modulation signal; the target DC line voltage value is subtracted from the actual DC line voltage to obtain a DC line voltage difference; the DC line voltage difference is adjusted by a proportional-integral regulator to obtain a given amplitude of the common-mode current; the given amplitude of the common-mode current is multiplied by the sine of the common-mode modulation signal frequency to obtain a given value of the common-mode current; the given value of the common-mode current is compared with a feedback value of the actual common-mode current; and a common-mode modulation signal is generated based on the comparison result by a proportional resonant regulator.

[0098] The control method of a flexible direct current transmission system provided in the second embodiment can be applied to the flexible direct current transmission system provided in any of the above embodiments, and has corresponding functions and beneficial effects.

[0099] It should be understood that the various forms of the processes shown above can be used to reorder, add, or delete steps. For example, the steps described in the present invention can be performed in parallel, sequentially, or in a different order, as long as the desired results of the technical solution of the present invention can be achieved. This is not limited herein.

[0100] The above specific embodiments do not limit the scope of protection of the present invention. Those skilled in the art will appreciate that various modifications, combinations, sub-combinations, and substitutions may be made based on design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention are intended to be included within the scope of protection of the present invention.

Claims

1. A flexible direct current transmission system, characterized in that: include: A flexible DC converter valve, a grid-connected transformer, and an energy dissipation device, wherein the energy dissipation device includes an energy dissipation resistor, a first supporting capacitor, and a second supporting capacitor; The valve side of the grid-connected transformer is used to be connected to the flexible DC converter valve, and the grid side of the grid-connected transformer is used to be connected to the AC power grid; The valve side of the grid-connected transformer adopts a star-connected winding, and the grid side of the grid-connected transformer adopts a delta-connected winding; The neutral point of the valve-side winding of the grid-connected transformer is grounded through the energy-consuming resistor; The first supporting capacitor and the second supporting capacitor are connected in series between the positive and negative DC busbars of the flexible DC converter valve, and the middle connection point between the first supporting capacitor and the second supporting capacitor is grounded; The modulation signal of the bridge arm of the flexible DC converter valve includes a basic modulation signal and a common mode modulation signal; The frequency of the common-mode modulation signal is determined according to the operating parameters of the flexible DC converter valve; The operating parameters include at least one of the following: rated voltage of the power semiconductor device; rated current of the power semiconductor device; normal operating switching frequency; maximum allowable switching frequency; maximum duration of fault ride-through; and modulation index.

2. The system according to claim 1, wherein: The energy dissipation device also includes a grounding reactance and a neutral point capacitor; An intermediate connection point between the first supporting capacitor and the second supporting capacitor is grounded via the grounding reactance; The neutral point of the valve-side winding of the grid-connected transformer is grounded through the energy-consuming resistor and the neutral-point capacitor.

3. The system according to claim 1 or 2, characterized in that When the AC power grid fails and the voltage between the positive and negative DC busbars of the flexible DC converter valve is higher than a threshold value, the AC voltage generated by the basic modulation signal is transmitted to the grid side of the grid-connected transformer, and the common-mode current generated by the common-mode modulation signal passes through the energy-consuming resistor.

4. The system according to claim 1, wherein: The frequency of the common-mode modulation signal is 200 Hz to 400 Hz.

5. The system according to claim 3, wherein: The resistance of the energy dissipation resistor satisfies: When the fault residual voltage is a set value, the sum of the basic modulation signal and the common-mode modulation signal does not exceed a set maximum modulation index.

6. The system according to claim 3, wherein: The common-mode modulation signal is generated according to the following method: Adding a fixed deviation to a given DC line voltage value to obtain a target DC line voltage value controlled by the common-mode modulation signal; Subtracting the target DC line voltage from the actual DC line voltage to obtain a DC line voltage difference; The DC line voltage difference is adjusted by a proportional-integral regulator to obtain a given amplitude of the common-mode current; Multiplying the given amplitude of the common-mode current by the sine of the common-mode modulation signal frequency to obtain a given value of the common-mode current; comparing the given value of the common-mode current with the feedback value of the actual common-mode current; The common-mode modulation signal is generated by a proportional resonant regulator based on the comparison result.

7. A control method for a flexible direct current transmission system, characterized in that: The method is applied to a flexible direct current transmission system, the flexible direct current transmission system including a flexible direct current converter valve, a grid-connected transformer, and an energy consumption device, the energy consumption device including an energy consumption resistor, a first supporting capacitor, and a second supporting capacitor; the method includes: generating a modulation signal for a bridge arm of the flexible DC converter valve, wherein the modulation signal includes a basic modulation signal and a common-mode modulation signal; Controlling the flexible direct current transmission system based on the modulation signal, wherein the AC voltage generated by the basic modulation signal is transmitted to the grid side of the grid-connected transformer, and the common-mode current generated by the common-mode modulation signal passes through the energy-consuming resistor in the energy-consuming device; Determining the frequency of the common-mode modulation signal according to the operating parameters of the flexible DC converter valve; The operating parameters include at least one of the following: rated voltage of the power semiconductor device; rated current of the power semiconductor device; normal operating switching frequency; maximum allowable switching frequency; maximum duration of fault ride-through; and modulation index.

8. The method according to claim 7, characterized in that The process of generating the common-mode modulation signal includes: Adding a fixed deviation to a given DC line voltage value to obtain a target DC line voltage value controlled by the common-mode modulation signal; Subtracting the target DC line voltage from the actual DC line voltage to obtain a DC line voltage difference; The DC line voltage difference is adjusted by a proportional-integral regulator to obtain a given amplitude of the common-mode current; Multiplying the given amplitude of the common-mode current by the sine of the common-mode modulation signal frequency to obtain a given value of the common-mode current; comparing the given value of the common-mode current with the feedback value of the actual common-mode current; The common-mode modulation signal is generated by a proportional resonant regulator based on the comparison result.

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