Modular hybrid dc unloading device and control method

Abstract

The present disclosure provides a modular hybrid DC unloading device and a control method, wherein the modular hybrid DC unloading device comprises: an energy consumption valve module, a concentrated dissipation resistance module and an unloading reactance, the energy consumption valve module, the concentrated dissipation resistance module and the unloading reactance are connected in series; the energy consumption valve module comprises a plurality of sub-modules, the plurality of sub-modules are connected in series, each sub-module comprises a first capacitor C1, a second capacitor C2, a buffer resistor R cus , a thyristor T1, a distributed energy consumption resistor R sm with an anti-parallel diode D1, an IGBT module T2 with an anti-parallel diode and a reverse prevention diode D2, the buffer resistor R cus , the thyristor T1, the distributed energy consumption resistor R sm with the anti-parallel diode D1, the IGBT module T2 with the anti-parallel diode and the reverse prevention diode D2 are connected in series between the input end and the output end of the sub-module. Through the present disclosure, flexible accommodation of surplus power under various working conditions is realized, the rated DC voltage of the grid-connected system of the flexible DC power transmission is maintained, and the device cost is reduced.

Description

Technical Field

[0001] This disclosure relates to the field of power electronics technology, and more specifically, to a modular hybrid DC unloading device and control method. Background Technology

[0002] Currently, due to a combination of factors such as ecological environmental protection and waterway occupation, the development of offshore wind power is gradually shifting from nearshore to deep-sea areas. Offshore wind power transmission and grid connection systems based on flexible DC transmission, with their advantages of lower losses, longer distances, and reactive power decoupling, have been widely used in power transmission for large-scale deep-sea wind farms.

[0003] The offshore wind power transmission and grid connection system based on VSC-HVDC (Voltage Source Converter-High Voltage Direct Current) mainly consists of an offshore wind farm, a sending end converter (SEC), submarine cables, a receiving end converter (REC), and the AC main grid. In the offshore wind-flexible DC grid connection system, after a ground fault or short-circuit fault occurs on the AC side of the receiving end station's grid, the grid voltage drops rapidly. At this time, due to the weak current carrying capacity of the receiving end station's modular multilevel converter (MMC) and the saturation of the current inner loop controller, the output current on the receiving end station's AC side is limited to the rated value, the power transmission path is blocked, and the system's power output capacity decreases significantly. Meanwhile, the sending end converter, unable to detect the grid fault, continues to inject power into the DC line according to the wind farm's output power before the fault, resulting in a system power surplus. Power imbalance can cause continuous charging of system DC capacitors (including submarine cable parasitic capacitance, MMC submodule capacitance, and DC inter-stage capacitance), leading to a rapid rise in system DC voltage, triggering overvoltage protection, and even causing DC line tripping or MMC submodule overvoltage lockout. my country's technical regulations for wind farm grid connection stipulate that when the grid connection voltage drops to 20% of the nominal voltage, the wind turbines and reactive power compensation devices within the wind farm should maintain continuous operation without disconnecting from the grid for 625ms. If the grid connection voltage recovers to 90% of the nominal voltage within 2 seconds after the voltage drop, the wind turbines and reactive power compensation devices within the wind farm should maintain continuous operation without disconnecting from the grid.

[0004] To address the aforementioned surplus power issue, there are currently two main solutions. One is to reduce the wind farm's output through coordinated control; the other is to install unloading equipment to dissipate the surplus power. The converter station's coordinated control uses a step-down / frequency-up method, which suffers from weak regulation capability, slow response speed, and sampling difficulties. Using the step-down / frequency-up method alone is insufficient for effective surplus power handling. To increase the reliability of the offshore wind-flexible DC grid-connected system during fault ride-through and accelerate the fault protection response speed of the HVDC system, using a high-voltage DC unloading device (DC Chopper, DCC) for surplus power handling is a more effective solution.

[0005] To prevent the equipment from occupying space on offshore platforms, high-voltage direct current (HCC) unloading equipment is typically installed on the DC side of onshore converter stations. Existing DCC topologies, i.e., the topologies of HCC equipment, can be divided into three types based on the location of their energy dissipation resistors: centralized, distributed, and hybrid. These three types of equipment differ in their topology, working principle, control method, operational performance, and cost. Among them, the hybrid topology combines the advantages of centralized and distributed topologies, offering advantages such as higher control precision, better electromagnetic interference performance, and lower requirements for water cooling systems. However, existing hybrid topologies still suffer from issues such as a large number of components, high cost, and intellectual property rights.

[0006] For hybrid HVDC unloading devices used in offshore wind-flexible DC grid-connected systems, high requirements are placed on the high voltage withstand and current carrying capacity of the switching devices. In this application scenario, thyristors, apart from their inability to actively turn off, are superior to fully controlled devices such as IGBTs in many aspects. At the same voltage level, the price of IGBTs is typically 3 to 5 times that of thyristors. Furthermore, thyristors have stronger voltage withstand and current carrying capacity than fully controlled devices like IGBTs, and their conduction losses and on-state voltage drop are also lower. Therefore, replacing some IGBTs with thyristors not only saves on the device cost of the unloading device but also improves the reliability of the unloading device in high-voltage, high-power applications. Summary of the Invention

[0007] In view of the deficiencies in the prior art, the purpose of this disclosure is to provide a modular hybrid DC unloading device and control method.

[0008] To achieve the above objectives, according to one aspect of this disclosure, a modular hybrid DC unloading device and control method are provided, comprising: an energy dissipation valve module, a centralized dissipation resistor module, and an unloading reactor, wherein the energy dissipation valve module, the centralized dissipation resistor module, and the unloading reactor are connected in series in sequence.

[0009] The energy-consuming valve module includes multiple sub-modules connected in series. Each sub-module includes a first capacitor C1, a second capacitor C2, and a buffer resistor R. cus1. Thyristor T1, 2. Distributed power dissipation resistor R with anti-parallel diode D1 sm An IGBT module T2 with an anti-parallel diode and an anti-reverse diode D2, and a buffer resistor R. cus The thyristor T1 and the distributed energy-dissipating resistor R with anti-parallel diode D1 sm The IGBT module T2 with anti-parallel diode and the anti-reverse diode D2 are connected in series between the input and output terminals of the sub-module. The first capacitor C1 and the buffer resistor R cus The thyristor T1 and the distributed energy-dissipating resistor R with anti-parallel diode D1 sm The series branch is connected in parallel, and the second capacitor C2 and the buffer resistor R cus The series branch containing the IGBT module T2 with the anti-parallel diode is connected in parallel, wherein the anti-parallel diode D1 and the distributed energy dissipation resistor R are connected in parallel. sm The centralized dissipation resistor module and the buffer resistor R are connected in parallel. cus and the distributed energy-consuming resistor R sm Used for unloading.

[0010] Optionally, the buffer resistor R cus The input terminal is connected to the input terminal of the submodule, and the buffer resistor R cus The output terminal is connected to the anode of the thyristor T1, and the cathode of the thyristor T1 is connected to the cathode of the anti-parallel diode D1 and the distributed energy dissipation resistor R. sm The input terminal is connected to the anode of the anti-parallel diode D1 and the distributed energy dissipation resistor R. sm The output terminals are respectively connected to the collector of the IGBT module T2 with anti-parallel diode, the emitter of the IGBT module T2 with anti-parallel diode is connected to the anode of the anti-reverse diode D2, and the cathode of the anti-reverse diode D2 is connected to the output terminal of the sub-module.

[0011] Optionally, the positive terminal of the first capacitor C1 is connected to the input terminal of the submodule and the buffer resistor R, respectively. cus The input terminal is connected, and the negative terminal of the first capacitor C1 is connected to the anode of the anti-parallel diode D1, the collector of the IGBT module T2 with the anti-parallel diode, and the distributed power dissipation resistor R. sm Connect the output terminal;

[0012] The positive terminal of the second capacitor C2 is connected to the cathode of the thyristor T1, the cathode of the anti-parallel diode D1, and the distributed energy dissipation resistor R, respectively. smThe input terminal is connected, and the negative terminal of the second capacitor C2 is connected to the emitter of the IGBT module T2 with anti-parallel diode and the anode of the anti-reverse diode D2.

[0013] Optionally, the capacitance of the first capacitor C1 is less than the capacitance of the second capacitor C2.

[0014] Optionally, the IGBT module T2 with anti-parallel diode includes an IGBT module and an anti-parallel diode T2. The anti-parallel diode T2 is connected in parallel with the IGBT module. The cathode of the anti-parallel diode T2 is connected to the emitter of the IGBT module, and the anode of the anti-parallel diode T2 is connected to the emitter of the IGBT module.

[0015] Optionally, the modular hybrid DC unloading device is connected in parallel between the DC buses of the flexible DC transmission grid-connected system. The input terminal of the first sub-module of the energy dissipation valve is connected to the positive terminal of the DC bus of the flexible DC transmission grid-connected system, and the other end of the unloading reactor is connected to the negative terminal of the DC bus of the flexible DC transmission grid-connected system. The modular hybrid DC unloading device is used to absorb surplus power when there is a grid fault on the AC side of the receiving-end converter station, and to stabilize the DC voltage of the flexible DC transmission grid-connected system within a preset DC voltage range.

[0016] According to a second aspect of this disclosure, a control method for a modular hybrid DC unloading device is provided, comprising:

[0017] Obtain the DC voltage of the sending-end converter station and the grid connection point voltage of the receiving-end converter station in the grid-connected system of flexible DC transmission.

[0018] When the DC voltage of the sending-end converter station of the flexible DC transmission grid-connected system is less than a preset first threshold, the grid connection point voltage of the receiving-end converter station is a preset second threshold, and the modular hybrid DC unloading device is controlled to be in a locked state.

[0019] When the DC voltage of the sending-end converter station of the flexible DC transmission grid-connected system rises to not less than the preset first threshold, the voltage of the receiving-end converter station at the grid connection point is less than the preset second threshold, and the modular hybrid DC unloading device is put into operation.

[0020] When the DC voltage of the sending-end converter station of the flexible DC transmission grid-connected system drops to less than the preset first threshold, the voltage of the receiving-end converter station at the grid connection point is equal to the preset second threshold, and the modular hybrid DC unloading device is controlled to enter the disconnection state.

[0021] Optionally, the modular hybrid DC unloading device includes a cut-off mode, an input mode, a transition state 1, and a transition state 2.

[0022] Optionally, when the DC voltage of the sending-end converter station of the flexible DC transmission grid-connected system is less than a preset first threshold, and the grid connection point voltage of the receiving-end converter station is a preset second threshold, controlling the modular hybrid DC unloading device to be in a locked state includes:

[0023] The energy consumption control signal of the sub-module of the energy consumption valve of the modular hybrid DC unloading device is set to 0, and the sub-module operates in the cut-off state. Thyristor T1 and IGBT module T2 with anti-parallel diode are in the off state. The first capacitor C1 is connected in series with the second capacitor C2 through anti-parallel diode D1, and the distributed energy consumption resistor R... sm No current flows through it, and each of the submodules outputs the sum of the voltages of the first capacitor C1 and the second capacitor C2. The output voltage of the energy dissipation valve module is equal to the rated DC voltage of the grid-connected system of the flexible DC transmission.

[0024] Optionally, when the DC voltage of the sending-end converter station of the flexible DC transmission grid-connected system rises to the preset first threshold, and the voltage at the grid connection point of the receiving-end converter station is less than the preset second threshold, controlling the modular hybrid DC unloading device to enter the operating state includes:

[0025] Based on the duty cycle of the sub-modules of the energy-consuming valve of the modular hybrid DC unloading device, the sub-modules of the energy-consuming valve are controlled to operate periodically in a step-by-step manner according to the order of the input mode, the transition mode 1, the transition mode 2, and the cut-off mode.

[0026] Optionally, controlling the sub-modules of the energy-consuming valve of the modular hybrid DC unloading device to operate periodically in a step-by-step manner according to the duty cycle of the sub-modules of the energy-consuming valve includes:

[0027] When the submodule is in the "engaged" state, the submodule power consumption control signal is set to 1, thyristor T1 and IGBT module T2 with anti-parallel diode are both turned on, and buffer resistor R... cus thyristor T1, distributed energy dissipation resistor R sm The IGBT module and the anti-reverse diode D2 form a conduction path, and the first capacitor C1 and the second capacitor C2 are connected through the distributed energy dissipation resistor R. sm Discharge to the first input rated voltage and the second input rated voltage respectively, the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, and each submodule outputs the voltage of the first capacitor C1;

[0028] When the submodule operates in transition state 1, the IGBT module T2 with anti-parallel diode is turned off, the thyristor T1 is turned on, the first capacitor C1 discharges, the second capacitor C2 charges, and the discharge current of the first capacitor C1 flows through the distributed energy dissipation resistor R. sm and the buffer resistor R cus When the load is unloaded, the voltage of the second capacitor C2 is greater than the voltage of the first capacitor C1, and each submodule outputs the voltage of the second capacitor C2.

[0029] When the submodule operates in transition state 2, the thyristor T1 is turned off, the IGBT module T2 with anti-parallel diode is turned on, the first capacitor C1 is charged, the second capacitor C2 is discharged, and the discharge current of the second capacitor C2 flows through the distributed energy dissipation resistor R. sm When unloading, the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, and each submodule outputs the voltage of the first capacitor C1;

[0030] When the submodule operates in the cut-off mode, the thyristor T1 and the IGBT module T2 with the anti-parallel diode are turned off. The first capacitor C1 is connected in series with the second capacitor C2 through the anti-parallel diode D1, and the distributed energy dissipation resistor R... sm No current flows through it. The first capacitor C1 is charged to the first cutoff rated voltage, and the second capacitor C2 is charged to the second cutoff rated voltage. Each submodule outputs the sum of the voltages of the first capacitor C1 and the second capacitor C2.

[0031] Optionally, when the DC voltage of the sending-end converter station of the flexible DC transmission grid-connected system drops below the preset first threshold, the voltage at the grid connection point of the receiving-end converter station equals the preset second threshold, and the modular hybrid DC unloading device is controlled to enter the disconnection state, the following steps are included:

[0032] The energy consumption control signal of the sub-module of the energy consumption valve of the modular hybrid DC unloading device is set to 0. The sub-modules are then operated in a stepped manner in the cut-off state. Thyristor T1 and IGBT module T2 with anti-parallel diode are both turned off. The first capacitor C1 is connected in series with the second capacitor C2 through anti-parallel diode D1, and the distributed energy consumption resistor R is connected in series. sm When no current flows through the capacitor, the first capacitor C1 is charged to the first cutoff rated voltage, and the second capacitor C2 is charged to the second cutoff rated voltage. Each submodule outputs the sum of the voltages of the first capacitor C1 and the second capacitor C2, and the modular hybrid DC unloading device is taken out of operation.

[0033] According to a third aspect of this disclosure, a closed-loop controller is provided for executing a control method for a modular hybrid DC unloading device according to any one of the second aspects of this disclosure, comprising: an overall energy dissipation controller and a sub-module capacitor voltage equalization controller, wherein the overall energy dissipation controller is used to acquire the DC voltage of the grid-connected system of flexible DC transmission, and the sub-module capacitor voltage equalization controller is used to control the operation of the thyristor T1 and the IGBT module T2 with anti-parallel diodes of the modular hybrid DC unloading device.

[0034] Compared with the prior art, the embodiments disclosed herein have at least one of the following beneficial effects:

[0035] The above technical solution employs a structure consisting of an energy-consuming valve module, a centralized dissipation resistor module, and a series-connected unloading reactor. Furthermore, the energy-consuming valve module includes multiple sub-modules, which are grouped and switched in a stepped manner. A buffer resistor R is used. cus Distributed energy-consuming resistor R sm In addition, the centralized dissipation resistor consumes surplus power under different operating conditions, maintains the DC voltage stability of the grid-connected system of flexible DC transmission, realizes fault ride-through of the grid-connected system of flexible DC transmission, and uses thyristors T1 to replace some IGBT modules in the energy dissipation valve module, effectively reducing the device cost and improving the reliability and accuracy of unloading in high-voltage and high-power applications.

[0036] In the embodiments of this disclosure, the capacitance value of the first capacitor C1 is set to be smaller than that of the second capacitor C2. The capacitance values ​​of the first capacitor C1 and the second capacitor C2 are designed asymmetrically to improve the voltage withstand capability of the unloading device and the thyristor T1 in the submodule, and to reduce the rated operating voltage of the IGBT module in the submodule, thereby further reducing the cost of the modular hybrid DC unloading device.

[0037] The embodiments of this disclosure control the blocking, operating, and disconnection states of the modular hybrid DC unloading device based on the DC voltage of the sending-end converter station and the grid-connected voltage of the receiving-end converter point of the flexible DC transmission grid-connected system. This enables timely absorption of surplus power, maintains the DC voltage stability of the flexible DC transmission grid-connected system, and achieves fault ride-through of the flexible DC transmission grid-connected system.

[0038] In embodiments of this disclosure, a sorted submodule capacitor voltage balancing strategy is used to compare the voltages of the first capacitor C1 and the second capacitor C2 to achieve submodule capacitor voltage balancing. Attached Figure Description

[0039] Other features, objects, and advantages of this disclosure will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0040] Figure 1This is a schematic diagram of the topology of a modular hybrid DC unloading device for a grid-connected system of flexible DC transmission, according to an exemplary embodiment.

[0041] Figure 2 This is a simulation circuit diagram of a modular hybrid DC unloading device according to an exemplary embodiment.

[0042] Figure 3 This is a schematic diagram of the topology of a submodule of an energy-consuming valve according to an exemplary embodiment.

[0043] Figure 4 This is a flowchart illustrating a control method for a modular hybrid DC unloading device according to an exemplary embodiment.

[0044] Figure 5 This is a schematic diagram of the switching timing waveform of a submodule of an energy-consuming valve according to an exemplary embodiment.

[0045] Figure 6 This is a schematic diagram illustrating the voltage and current distribution of a submodule of an energy-consuming valve in the cut-off mode, according to an exemplary embodiment.

[0046] Figure 7 This is a schematic diagram illustrating the voltage and current distribution of a submodule of an energy-consuming valve in the input mode, according to an exemplary embodiment.

[0047] Figure 8 This is a schematic diagram of the voltage and current distribution of a submodule of an energy-consuming valve in transition state 1, according to an exemplary embodiment.

[0048] Figure 9 This is a schematic diagram of the voltage and current distribution of a submodule of an energy-consuming valve in transition state 2, according to an exemplary embodiment.

[0049] Figure 10 This is a simulation waveform diagram of a modular hybrid DC unloading device according to an exemplary embodiment.

[0050] Figure 11 This is a schematic diagram of the current limit waveform of the switching device of any submodule in a modular hybrid DC unloading device according to an exemplary embodiment.

[0051] Figure 12 This is a control block diagram of a closed-loop controller according to an exemplary embodiment. Detailed Implementation

[0052] The present disclosure will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present disclosure, but do not limit the present disclosure in any way. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present disclosure. These all fall within the protection scope of the present disclosure.

[0053] Figure 1 This is a schematic diagram of the topology of a modular hybrid DC unloading device for a grid-connected system of flexible DC transmission, according to an exemplary embodiment. Figure 2 This is a simulation circuit diagram of a modular hybrid DC unloading device according to an exemplary embodiment. Figure 3 This is a schematic diagram of the topology of a submodule of an energy-consuming valve according to an exemplary embodiment.

[0054] like Figures 1 to 3 As shown, this disclosure provides a modular hybrid DC unloading device, including an energy dissipation valve module, a centralized dissipation resistor module, and an unloading reactor, wherein the energy dissipation valve module, the centralized dissipation resistor module, and the unloading reactor are connected in series in sequence.

[0055] The energy dissipation valve module includes multiple sub-modules connected in series. Each sub-module includes a first capacitor C1, a second capacitor C2, and a buffer resistor R. cus 1. Thyristor T1, 2. Distributed power dissipation resistor R with anti-parallel diode D1 sm IGBT module T2 with anti-parallel diode and anti-reverse diode D2, buffer resistor R cus 1. Thyristor T1, 2. Distributed power dissipation resistor R with anti-parallel diode D1 sm An IGBT module T2 with an anti-parallel diode and an anti-reverse diode D2 are connected in series between the input and output terminals of the submodule. The first capacitor C1 and the buffer resistor R are connected in series. cus 1. Thyristor T1, 2. Distributed power dissipation resistor R with anti-parallel diode D1 sm The series branch is connected in parallel, and the second capacitor C2 and the buffer resistor R cus The series branch containing IGBT module T2 with anti-parallel diode is connected in parallel, wherein the anti-parallel diode D1 and the distributed power dissipation resistor R are connected in parallel. sm Parallel connection, centralized dissipation resistor module, buffer resistor R cus and distributed energy dissipation resistor R sm Used for unloading.

[0056] The above technical solution employs a structure consisting of an energy-consuming valve module, a centralized dissipation resistor module, and a series-connected unloading reactor. Furthermore, the energy-consuming valve module includes multiple sub-modules, which are grouped and switched in a stepped manner. A buffer resistor R is used.cus Distributed energy-consuming resistor R sm In addition, the centralized dissipation resistor consumes surplus power under different operating conditions, maintains the DC voltage stability of the grid-connected system of flexible DC transmission, realizes fault ride-through of the grid-connected system of flexible DC transmission, and uses thyristors T1 to replace some IGBT modules in the energy dissipation valve module, effectively reducing the device cost and improving the reliability and accuracy of unloading in high-voltage and high-power applications.

[0057] In high-voltage, high-power applications, thyristors are less expensive than IGBT modules at the same voltage level. Furthermore, thyristors have better voltage withstand and current carrying capacity than fully controlled devices such as IGBTs, and their conduction losses and conduction voltage drops are lower than those of IGBT modules.

[0058] like Figure 3 As shown, in one possible embodiment, the buffer resistor R cus The input terminal is connected to the input terminal of the submodule, and the buffer resistor R cus The output terminal is connected to the anode of thyristor T1, and the cathode of thyristor T1 is connected to the cathode of anti-parallel diode D1 and the distributed energy dissipation resistor R. sm The input terminals are connected to the anode of the anti-parallel diode D1 and the distributed energy-dissipating resistor R. sm The output terminals are connected to the collector of IGBT module T2 with anti-parallel diodes, the emitter of IGBT module T2 with anti-parallel diodes is connected to the anode of anti-reverse diode D2, and the cathode of anti-reverse diode D2 is connected to the output terminal of the sub-module.

[0059] like Figure 3 As shown, in one possible embodiment, the positive terminal of the first capacitor C1 is connected to the input terminal of the submodule and the buffer resistor R, respectively. cus The input terminal is connected, and the negative terminal of the first capacitor C1 is connected to the anode of the anti-parallel diode D1, the collector of the IGBT module T2 with the anti-parallel diode, and the distributed power dissipation resistor R. sm Connect the output terminal.

[0060] The positive terminal of the second capacitor C2 is connected to the cathode of thyristor T1, the cathode of anti-parallel diode D1, and the distributed energy-dissipating resistor R, respectively. sm The input terminal is connected, and the negative terminal of the second capacitor C2 is connected to the emitter of the IGBT module T2 with anti-parallel diode and the anode of the anti-reverse diode D2, respectively.

[0061] In one possible embodiment, the capacitance of the first capacitor C1 is less than the capacitance of the second capacitor C2.

[0062] The capacitance values ​​of the first capacitor C1 and the second capacitor C2 of each submodule are designed asymmetrically. In both the power-on and power-off modes, the voltage across the thyristor T1 of the power-consuming valve module is greater than the voltage across the IGBT module T2 with the anti-parallel diode.

[0063] By asymmetrically designing the capacitance values ​​of the first capacitor C1 and the second capacitor C2, the voltage withstand capability of the unloading device and the thyristor T1 in the submodule is improved, and the rated operating voltage of the IGBT module in the submodule is reduced, thereby further reducing the cost of the modular hybrid DC unloading device.

[0064] like Figure 3 As shown, in one possible embodiment, the IGBT module T2 with anti-parallel diode includes an IGBT module and an anti-parallel diode T2. The anti-parallel diode T2 is connected in parallel with the IGBT module. The cathode of the anti-parallel diode T2 is connected to the emitter of the IGBT module, and the anode of the anti-parallel diode T2 is connected to the emitter of the IGBT module.

[0065] In one possible embodiment, a modular hybrid DC unloading device is connected in parallel between the DC buses of the flexible DC transmission grid-connected system. The input end of the first sub-module of the energy dissipation valve is connected to the positive terminal of the DC bus of the flexible DC transmission grid-connected system, and the other end of the unloading reactor is connected to the negative terminal of the DC bus of the flexible DC transmission grid-connected system. The modular hybrid DC unloading device is used to absorb surplus power when there is a grid fault on the AC side of the receiving-end converter station, and to stabilize the DC voltage of the flexible DC transmission grid-connected system within a preset DC voltage range.

[0066] The preset DC voltage range includes the rated DC voltage.

[0067] like Figure 1 As shown, the modular hybrid DC unloading device ATDS-DCC (Asymmetrical Thyristor Diode Switch-DC Chopper) provided in this disclosure can be applied to the grid-connected system of offshore flexible DC transmission based on VSC-HVDC (Voltage Source Converter-HighVoltage Direct Current, VSC-HVDC) to solve the system surplus power problem caused by AC side faults at the receiving end converter station.

[0068] like Figure 1As shown, the grid-connected system of offshore wind flexible DC transmission based on VSC-HVDC (Voltage Source Converter-High Voltage Direct Current, VSC-HVDC) includes an offshore wind farm, a sending-end converter station, a receiving-end converter station, and an onshore AC power grid. Among them, a modular hybrid DC unloading device provided in this disclosure is connected in parallel on the DC bus between the sending-end converter station and the receiving-end converter station. The DC positive terminal of the receiving-end converter station is connected to the DC negative terminal of the receiving-end converter station after passing through an energy dissipation valve module, a centralized dissipation resistor module, and an unloading reactor.

[0069] When the modular hybrid DC unloading device is put into operation, the energy dissipation valve module can be equivalent to a controlled voltage source. During the operation of the modular hybrid DC unloading device, the energy dissipation valve module periodically switches between the activation mode and the deactivation mode. Furthermore, the centralized dissipation resistor module and the buffer resistor R... cus and distributed energy dissipation resistor R sm All participated in unloading.

[0070] The energy dissipation valve module includes a cut-off mode, an input mode, a transition mode 1, and a transition mode 2. When the modular hybrid DC unloading device is put into operation, the sub-modules operate periodically in the order of input mode, transition mode 1, transition mode 2, and cut-off mode to stabilize the system DC voltage within a preset DC voltage range including the rated DC voltage. This allows the excess power of faulty devices in the onshore AC grid on the AC side of the receiving-end converter station to be absorbed, thus realizing fault ride-through of the grid-connected system of offshore wind flexible DC transmission based on VSC-HVDC.

[0071] Figure 4 This is a flowchart illustrating a control method for a modular hybrid DC unloading device according to an exemplary embodiment.

[0072] like Figure 4 As shown, this disclosure provides a control method for a modular hybrid DC unloading device, including steps S11 to S14.

[0073] S11, obtain the DC voltage of the sending-end converter station and the grid connection point voltage of the receiving-end converter station of the flexible DC transmission grid-connected system.

[0074] S12, when the DC voltage of the sending-end converter station of the flexible DC transmission grid-connected system is less than the preset first threshold, the grid connection point voltage of the receiving-end converter station is the preset second threshold, and the modular hybrid DC unloading device is controlled to be in a locked state.

[0075] Wherein, when the DC voltage of the sending converter station of the grid-connected system of flexible DC transmission is less than a preset first threshold, the grid-connected voltage of the receiving converter station is a preset second threshold, i.e., the rated DC voltage, indicating that the grid-connected system of flexible DC transmission has no fault, and the modular hybrid DC unloading device is in the hot standby state, and its switching devices, the thyristor T1 and the IGBT module T2 with an antiparallel diode, are both turned off. The first capacitor C1 and the second capacitor C2 in the sub-module of the energy dissipation valve module are connected in series, and the energy dissipation valve module outputs the rated DC voltage.

[0076] S13. When the DC voltage of the sending converter station of the grid-connected system of flexible DC transmission rises to be not less than the preset first threshold and the voltage at the grid connection point of the receiving converter station is less than the preset second threshold, control the modular hybrid DC unloading device to enter the operating state.

[0077] [[ID=??]]Wherein, when the DC voltage of the sending converter station of the grid-connected system of flexible DC transmission rises to be not less than the preset first threshold and the voltage at the grid connection point of the receiving converter station is less than the preset second threshold, i.e., the voltage at the grid connection point of the receiving converter station drops to (1-k)p.u., 0<k<1, the DC voltage of the sending converter station is on the high side, the energy dissipation valve module is put into unloading, the sub-modules are grouped and put in step by step, the concentrated dissipation resistor R and the distributed energy dissipation resistor R sm participate in energy dissipation, and the system equivalent capacitor C eq , and the DC voltage U dc of the sending converter station drops.

[0078] S14. When the DC voltage of the sending converter station of the grid-connected system of flexible DC transmission drops to be less than the preset first threshold and the voltage at the grid connection point of the receiving converter station is equal to the preset second threshold, control the modular hybrid DC unloading device to enter the cut-off state.

[0079] Wherein, when the DC voltage U dc of the sending converter station drops to be less than the preset first threshold, it indicates that the DC voltage U dc of the sending converter station is on the low side. Cut off the energy dissipation valve module, the sub-modules are grouped and cut off step by step, the energy dissipation valve module outputs the rated DC voltage, there is no current in the unloading branch, and the system equivalent capacitor C eq is charged, and the DC voltage U dc of the sending converter station rises.

[0080] By controlling the blocking state, operating state and cut-off state of the modular hybrid DC unloading device based on the DC voltage of the sending converter station and the grid connection point voltage of the receiving converter point of the grid-connected system of flexible DC transmission, the surplus power can be consumed in time, the DC voltage of the grid-connected system of flexible DC transmission can be maintained stable, and the fault ride-through of the grid-connected system of flexible DC transmission can be realized.

[0081] It seems there is an unclear part in your original text at ID=6, which is marked as "??" in the translation. Please check and correct it if necessary.In one possible embodiment, the modular hybrid DC unloading device includes a cut-off mode, an input mode, a transition state 1, and a transition state 2.

[0082] Figure 5 This is a schematic diagram of the switching timing waveform of a submodule of an energy-consuming valve according to an exemplary embodiment.

[0083] During operation, the modular hybrid DC unloading device periodically switches between the power-on mode and the power-off mode of the energy-consuming valve module, and the sub-modules periodically operate in the power-off mode, power-on mode, transition mode 1, transition mode 2, and power-off mode.

[0084] like Figure 5 SM-PWM k This represents the submodule power consumption control signal, i.e., the switching control signal, with a duty cycle of d. k , k represents the group number of the submodule, ST 1,k ST represents the gate trigger signal of thyristor T1 within the submodule. 2,k This indicates the gate drive signal for IGBT module T2 with anti-parallel diodes.

[0085] The time period from t0 to t1 is the stage where the submodule is in the input mode, which is called process I; the time period from t1 to t2 is the stage where the submodule is in the transition mode 1, which is called process II; the time period from t2 to t3 is the stage where the submodule is in the transition mode 2, which is called process III; and the time period from t3 to t4 is the stage where the submodule is in the cut-off mode, which is called process IV.

[0086] Figure 6 This is a schematic diagram illustrating the voltage and current distribution of a submodule of an energy-consuming valve in the cut-off mode, according to an exemplary embodiment.

[0087] like Figure 5 , Figure 6 As shown, before time t0, the submodule power consumption control signal SM-PWM k When the value is 0, the submodule is in cut-off mode. Both thyristor T1 and IGBT module T2 with anti-parallel diodes are turned off. The first capacitor C1 is connected in series with the second capacitor C2 through the anti-parallel diode D1, and the distributed energy-dissipating resistor R... sm Since no current flows through the IGBT module, the voltage output by the submodule is the sum of the voltage of the first capacitor C1 and the voltage of the second capacitor C2.

[0088] Figure 7 This is a schematic diagram illustrating the voltage and current distribution of a submodule of an energy-consuming valve in the input mode, according to an exemplary embodiment.

[0089] like Figure 5 and Figure 7As shown, during the time period t0 to t1, at time t0, the submodule energy consumption control signal SM-PWM... k When the value is 1, a conduction signal is sent to both thyristor T1 and IGBT module T2 with anti-parallel diodes, and the gate trigger signal ST of thyristor T1 is also sent. 1,k Set to 1, the gate drive signal ST of IGBT module T2 with anti-parallel diode. 2,k When the value is set to 1, the submodule enters the input mode and begins to unload. Both thyristor T1 and IGBT module T2 with anti-parallel diodes conduct, and the IGBT module also conducts. The buffer resistor R... cus thyristor T1, distributed energy dissipation resistor R sm The IGBT module and the anti-reverse diode D2 form a conductive path. The negative terminals of the first capacitor C1 and the second capacitor C2 have the same potential. The voltage across the second capacitor C2 is less than the voltage across the first capacitor C1. The unloading current flows through the buffer resistor R. cus thyristor T1, distributed energy dissipation resistor R sm The IGBT module and its sub-modules participate in energy dissipation. The first capacitor C1 continues to discharge until the first applied rated voltage U is reached. C1,in The second capacitor C2 continues to discharge until the second rated input voltage U is reached. C2,in .

[0090] Figure 8 This is a schematic diagram of the voltage and current distribution of a submodule of an energy-consuming valve in transition state 1, according to an exemplary embodiment.

[0091] like Figure 5 and Figure 8 As shown, during the time period t1 to t2, at time t1, the submodule power consumption control signal SM-PWM k Set to 0, and set the gate drive signal ST of IGBT module T2 with anti-parallel diode to 0. 2,k Set to 0, send a turn-off signal to IGBT module T2 with anti-parallel diode, and the sub-module enters transition state 1, initiating the disengagement of the sub-module. At this time, the negative potentials of the first capacitor C1 and the second capacitor C2 are no longer equal. Thyristor T1 remains on because it cannot turn off autonomously, the second capacitor C2 charges, and its voltage rises. The first capacitor C1 discharges, and its voltage drops. The discharge current of the first capacitor C1 flows through the distributed energy dissipation resistor R. sm As the load is released, the voltage of the second capacitor C2 gradually exceeds the voltage of the first capacitor C1, and the submodule outputs the voltage of the second capacitor C2.

[0092] Figure 9 This is a schematic diagram of the voltage and current distribution of a submodule of an energy-consuming valve in transition state 2, according to an exemplary embodiment.

[0093] like Figure 5 and Figure 9 As shown, during the time period t2 to t3, at time t2, the submodule power consumption control signal SM-PWM is... k Set to 0, and set the gate trigger signal ST of thyristor T1 to 0. 1,k Set to 0, and set the gate drive signal ST of IGBT module T2 with anti-parallel diode to 0. 2,k Set to 1 and maintain for a preset duration. The IGBT module is turned on, and the negative potentials of the first capacitor C1 and the second capacitor C2 are equal. The voltage of the second capacitor C2 is greater than the voltage of the first capacitor C1. The thyristor T1 is turned off due to reverse voltage, and the submodule enters transition state 2. The unloading branch current charges the first capacitor C1, causing its voltage to rise. The unloading branch current also flows through the IGBT module, and the second capacitor C2 discharges. The discharge current flows through the distributed energy dissipation resistor R. sm The voltage of the second capacitor C2 drops, and the voltage of the second capacitor C2 gradually becomes less than the voltage of the first capacitor C1. The submodule outputs the voltage of the first capacitor C1.

[0094] like Figure 5 and Figure 6 As shown, during the time period t3 to t4, the submodule power consumption control signal SM-PWM at time t3 is... k The gate trigger signal ST of thyristor T1 is set to 0. 1,k Set to 0, and set the gate drive signal ST of IGBT module T2 with anti-parallel diode to 0. 2,k Setting the value to 0 turns off IGBT module T2 with its anti-parallel diode, and the submodule enters the cut-off state. Neither thyristor T1 nor the IGBT module is conducting. The first capacitor C1 is connected in series with the second capacitor C2 through the anti-parallel diode D1, and the distributed energy-dissipating resistor R... sm With no current flowing through the IGBT module, the unloading branch current flows through the first capacitor C1, the anti-parallel diode D1, the second capacitor C2, and the reverse protection diode D2. The first capacitor C1 is charged to the first cutoff rated voltage U. C1,out The second capacitor C2 is charged to the second cut-off rated voltage U. C2,out .

[0095] When a fault occurs in the onshore AC power grid on the AC side of the receiving-end converter station of the flexible DC transmission grid-connected system, the modular hybrid DC unloading device is put into operation, repeating the process from t0 to t4 until the fault is cleared, the DC bus voltage of the flexible DC transmission grid-connected system drops, and the modular hybrid DC unloading device is taken out of operation.

[0096] In one possible embodiment, S12 may include:

[0097] The energy dissipation control signal of the sub-module of the energy dissipation valve of the modular hybrid DC unloading device is set to 0, the sub-module is in the off state, thyristor T1 and IGBT module T2 with anti-parallel diode are in the off state, the first capacitor C1 is connected in series with the second capacitor C2 through anti-parallel diode D1, and the distributed energy dissipation resistor R sm No current flows through it. Each submodule outputs the sum of the voltages of the first capacitor C1 and the second capacitor C2. The output voltage of the energy dissipation valve module is equal to the rated DC voltage of the grid-connected flexible DC transmission system.

[0098] Specifically, when the submodule is in the cut-off state, the submodule power consumption control signal SM-PWM is... k Set to 0, and set the gate trigger signal ST of thyristor T1 to 0. 1,k Set to 0.

[0099] In one possible embodiment, S13 may include:

[0100] Based on the duty cycle of the sub-modules of the energy-consuming valve in the modular hybrid DC unloading device, the sub-modules of the energy-consuming valve are controlled to operate periodically in a step-by-step manner in the order of input mode, transition mode 1, transition mode 2, and cut-off mode.

[0101] As an example, when the submodule is in the active state, the submodule power consumption control signal is set to 1, and both thyristor T1 and IGBT module T2 with anti-parallel diode are turned on, and buffer resistor R... cus thyristor T1, distributed energy dissipation resistor R sm The IGBT module and the anti-reverse diode D2 form a conduction path, and the first capacitor C1 and the second capacitor C2 are connected through the distributed energy dissipation resistor R. sm Discharge to the first input rated voltage and the second input rated voltage respectively. The voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2. Each submodule outputs the voltage of the first capacitor C1.

[0102] As another example, when the submodule is operating in transition state 1, the IGBT module T2 with the anti-parallel diode is turned off, the thyristor T1 is turned on, the first capacitor C1 discharges, the second capacitor C2 charges, and the discharge current of the first capacitor C1 flows through the distributed energy dissipation resistor R. sm When the load is released, the voltage of the second capacitor C2 is greater than the voltage of the first capacitor C1, and each submodule outputs the voltage of the second capacitor C2.

[0103] As another example, the submodule operates in transition state 2, with thyristor T1 turned off and IGBT module T2 with anti-parallel diodes turned on. The first capacitor C1 charges, and the second capacitor C2 discharges. The discharge current of the second capacitor C2 flows through the distributed energy-dissipating resistor R. sm When unloading, the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2. Each submodule outputs the voltage of the first capacitor C1.

[0104] As another example, in the submodule's cut-off mode, thyristor T1 and IGBT module T2 with anti-parallel diodes are turned off. The first capacitor C1 is connected in series with the second capacitor C2 through anti-parallel diode D1, and the distributed power-dissipating resistor R... sm No current flows through it. The first capacitor C1 is charged to the first cutoff rated voltage, and the second capacitor C2 is charged to the second cutoff rated voltage. Each submodule outputs the sum of the voltages of the first capacitor C1 and the second capacitor C2.

[0105] By using a sorting-based submodule capacitor voltage balancing strategy, the voltages of the first capacitor C1 and the second capacitor C2 are compared to achieve submodule capacitor voltage balancing.

[0106] In one possible embodiment, S14 may include:

[0107] The energy consumption control signal of the sub-module of the energy consumption valve of the modular hybrid DC unloading device is set to 0, and the sub-modules are operated in a stepped cut-off state in groups. Thyristor T1 and IGBT module T2 with anti-parallel diode are both turned off. The first capacitor C1 is connected in series with the second capacitor C2 through anti-parallel diode D1, and the distributed energy consumption resistor R... sm With no current flowing through it, the first capacitor C1 is charged to the first cutoff rated voltage, and the second capacitor C2 is charged to the second cutoff rated voltage. Each submodule outputs the sum of the voltages of the first capacitor C1 and the second capacitor C2, and the modular hybrid DC unloading device exits operation.

[0108] When using the control method of the modular hybrid DC unloading device provided in this disclosure, the power dissipation of the unloading device is adjusted by the centralized and periodic switching of multiple sub-modules of the modular hybrid DC unloading device, reducing the switching voltage conversion rate du / dt of the energy-consuming valve module. When the switching control signal PWM of the energy-consuming valve module is set to 1, the sub-modules are switched on in a step-by-step manner, the unloading device consumes energy, and the system DC voltage drops. When the switching control signal PWM of the energy-consuming valve module is set to 0, the sub-modules are switched off in a step-by-step manner, the offshore wind farm inputs surplus power, and the system DC voltage rises.

[0109] When the onshore AC grid of the receiving-end converter station of the flexible DC transmission grid experiences a fault, the surplus power coefficient is between 0 and 1. The modular hybrid DC unloading device enters a dynamic unloading state, and the energy consumption valve module periodically switches between the input and output modes.

[0110] The average charging current and average discharging current of the system's equivalent capacitance during the unloading valve control cycle satisfy the following:

[0111]

[0112] Among them, I chargeI represents the charging current during the unloading of the DC-side equivalent capacitor of a grid-connected flexible DC transmission system. rated I represents the rated current of the grid-connected system for flexible DC transmission. discharge The discharge current during the unloading of the DC-side equivalent capacitor of the grid-connected flexible DC transmission system is represented by k, where k represents the group number of the submodule.

[0113] Within one switching cycle of the energy dissipation valve module, the rise and fall values ​​of the system DC voltage are the same. The duty cycle D of the PWM control signal for switching the energy dissipation valve module should satisfy the following relationship:

[0114]

[0115] Solving for D, we get D = k.

[0116] The control cycle (hereinafter referred to as the valve control cycle) T of the energy-consuming valve module s for:

[0117]

[0118] Among them, C eq U represents the system's equivalent capacitance. dcN Δt1 represents the system's rated DC voltage, Δt2 represents the time the energy-consuming valve is turned off within one cycle, and h represents the system's equivalent DC side equivalent capacitance ripple level as ±h.

[0119] In one possible embodiment, the parameters of the modular hybrid DC unloading device are designed as follows:

[0120] Define the rated input power of an offshore wind farm as P rated The rated DC voltage of the grid-connected flexible DC transmission system (VSC-HVDC system) is U. dcN The modular hybrid DC unloading device (ATDS-DCC device) has N sub-modules for its energy-consuming valve module, and the output voltage of the energy-consuming valve module is U. sm The lumped dissipation resistor is R, and the voltage across the lumped dissipation resistor is U. R The unloading branch current is I chop Define the rated energy dissipation resistance R of the unloading branch. rated :

[0121]

[0122] Among them, R rated U represents the rated energy dissipation resistance of the unloading branch. dc P represents the DC voltage of the grid-connected system for flexible DC transmission. rated This indicates the rated input power of the offshore wind farm.

[0123] When the VSC-HVDC system is operating without faults, the ATDS-DCC device is in a locked state. chop =0, U sm =U dcN All submodules are in the off state. Thyristor T1 and IGBT module T2 with anti-parallel diode are both turned off. The first capacitor C1 is connected in series with the second capacitor C2 through anti-parallel diode D1. The submodule outputs the sum of the voltages of the first capacitor C1 and the second capacitor C2. The first off rated voltage of the first capacitor C1 is... The second cutoff rated voltage of the second capacitor C2 satisfy:

[0124]

[0125] Define constants a and p, 1 < p < 2, a > 1, satisfying:

[0126] C1 = aC2(1.6)

[0127] R = pR rated (1.7)

[0128] Then the first cutoff rated voltage of the first capacitor C1 The second cutoff rated voltage of the second capacitor C2 satisfy:

[0129]

[0130] When the most severe fault occurs in the onshore AC grid of the receiving-end converter station of the VSC-HVDC system, the voltage of the onshore AC grid drops to 0 p.u., and the receiving-end converter station completely loses its power transmission capability. At this time, the ATDS-DCC device dissipates all the power input from the offshore wind farm, all sub-modules are activated, thyristor T1 and IGBT module T2 with anti-parallel diodes are both closed and conducting, and the voltage of the first capacitor C1 of the sub-module outputs... The power expressions for the centralized dissipation resistor module and the energy dissipation valve module are as follows:

[0131]

[0132] Among them, P DCC P represents the power consumed during the unloading of the ATDS-DCC unit. R This indicates the power of the centralized dissipation resistor module. Power representing distributed dissipation resistance, P represents the power of the buffer resistor. rated This indicates the rated input power of the offshore wind farm.

[0133] Under this operating condition, the current in the centralized dissipation resistor module and the distributed dissipation resistors and buffer resistors in each sub-module is equal, and is:

[0134]

[0135] Among them, I chop I represents the unloaded branch current. rated This indicates the rated current of the grid-connected system for flexible DC transmission.

[0136] Then R+R sm +R cus =2R rated From (1.6) and (1.7), we can obtain:

[0137]

[0138] At this time, the submodule is in the engaged state, and the first engaged voltage U of the first capacitor C1 is... C1,in The second input voltage of the second capacitor C2 satisfies:

[0139]

[0140] When a fault occurs in the onshore AC power grid of the receiving-end converter station of the VSC-HVDC system, the surplus power factor is between 0 and 1, the ATDS-DCC device enters a dynamic unloading state, and the energy consumption valve module periodically switches between the input mode and the cut-off mode.

[0141] The submodule parameter design of the ATDS-DCC device also needs to consider limiting the inrush current caused by the submodule switching between different modes. The following is a detailed analysis of the inrush current during the switching process of each submodule:

[0142] At time t0, the submodule switches from the cut-off state to the input state, and the circuit diagram changes from... Figure 7 Switch to Figure 6 initial state At the moment of switching, the potential difference between the first capacitor C1 and the second capacitor C2 generates an inrush current on the thyristor T1. The magnitude of the inrush current is... for:

[0143]

[0144] At time t1, the submodule switches from the input state to transition state 1, and the circuit diagram changes from... Figure 7 Switch to Figure 8 Initial state At the moment of switching, the voltage across C1 generates an inrush current across thyristor T1, the magnitude of which is... for:

[0145]

[0146] At time t2, the submodule switches from transition state 1 to transition state 2. During this process, the initial values ​​of the voltages of the first capacitor C1 and the second capacitor C2 are respectively... and Assuming the submodule removes transition states 1 and 2 in two processes, I chop Keeping constant, define time t d and t re , t d express Figure 5 The duration of the time interval t1 to t2, t re express Figure 5 From time t2 to The duration of time. To determine... and Based on the Laplace transform of the submodule circuit during the time interval t1 to t2, the following equation is obtained:

[0147]

[0148] Substituting equations (1.11) and (1.12) into equation (1.15), and then performing a reverse pull transformation, we can determine... and Time-domain expression:

[0149]

[0150] At t2+t re At time t, let the voltages of C1 and C2 be t. and Based on the Laplace transform and corresponding calculations performed on the sub-module circuit during the time period t2 to t3, the final result can be obtained. and

[0151]

[0152] From equation (1.16), the following condition is satisfied at this time:

[0153]

[0154] At time t2, the submodule switches from transition state 1 to transition state 2, and the mode switching generates an inrush current on the IGBT module.

[0155]

[0156] When thyristor T1 is turned off, the duration of the reverse voltage applied across thyristor T1 must be greater than the turn-off time t of thyristor T1. qFurthermore, when thyristor T1 is turned on, thyristor T1 cannot withstand reverse voltage. The timing sequence of the submodule switching operation must meet the following requirements:

[0157] t re ≥t q (1.20)

[0158] Among them, t re This indicates the duration of the reverse voltage applied across thyristor T1.

[0159] Let t re =t Q , t Q For greater than or equal to t q The constant, t Q Substituting equations (1.8), (1.11), and (1.12) into equation (1.18), we can solve for t. d Expressions concerning a and p:

[0160]

[0161] In one possible embodiment, when selecting the thyristor T1 and the IGBT module, their rated currents should be greater than all the current peaks calculated above.

[0162] The design of parameters a and p for a cost-based asymmetric ATDS-DCC device submodules is presented. In practical engineering, the thyristor T1 and IGBT modules within the module are connected in series; therefore, the cost of the thyristor T1 and IGBT modules is proportional to their rated operating voltage. The ratio of the cost of a single submodule in an asymmetric ATDS-DCC device to the cost of a symmetric ATDS-DCC device at the same voltage level can be calculated using the following formula:

[0163]

[0164] Under the condition of satisfying the current limit, the range of values ​​for a and p is traversed to determine... The minimum value.

[0165] In one possible embodiment, the modular hybrid DC unloading device of this application is verified using a simulation model of a ±800kV / 1100MW symmetrical unipolar offshore wind flexible DC power transmission and grid connection system.

[0166] The simulation parameters are shown in Table 1:

[0167]

[0168]

[0169] Table 1

[0170] In the offshore wind-powered DC transmission grid-connected system, the back-to-back modular multilevel converter (BTB-MMC) adopts an average value model. The receiving-end modular multilevel converter (REC-MMC) uses voltage outer loop control, while the sending-end modular multilevel converter (SEC-MMC) uses PQ control to simulate the operation mode of the wind farm-side converter station.

[0171] According to the fault ride-through requirements of the offshore wind flexible DC power transmission grid-connected system, the peak value of the DC bus voltage during the unloading period of the ATDS-DCC device should be less than 1.1 pu, the ripple should be less than 10%, and the peak value of the submodule capacitor voltage should be less than 1.8 kV.

[0172] Simulations verified the effectiveness of the ATDS-DCC device proposed in this application and the correctness of the parameter design.

[0173] To accelerate the simulation, the number of submodule switching groups was reduced to 4, and a single submodule structure was used to represent all series-connected submodules within a group. For submodules within the same group, their submodule power consumption control signals (i.e., the submodule switching control signal SM-PWM) are identical, with the same timing sequence; therefore, their distributed power consumption resistance and submodule capacitance can be considered as series-connected. The simulation uses one submodule equivalent to 70 series-connected submodules, with a distributed resistance of R / 4 and a capacitance of C. sm / 70. Regarding the grid connection point voltage U PCC Simulation was performed when the voltage dropped to 0.5 PU.

[0174] Under the operating conditions shown in Table 1, the system operates normally before the simulation time t = 0.7s. After t = 0.7s, the effective value of the equivalent power supply output voltage on the AC side of the onshore converter station MMC is modified to 0.5pu (208.21kV) to simulate a three-phase short-circuit fault in the onshore AC power grid, with a surplus power factor k = 0.5.

[0175] The system DC voltage U was detected after 0.025s. dc When the voltage exceeds 1.05 pu, the ATDS-DCC device is enabled and put into operation to absorb surplus power and maintain the stability of the system's DC voltage. At t = 1.3s, the effective value of the output voltage of the equivalent power supply on the AC side of the MMC of the onshore converter station recovers to 1 p.u., the fault in the offshore wind-flexible DC power transmission and grid connection system is restored, and the ATDS-DCC device enters the lockout state and exits operation.

[0176] Figure 10 This is a simulation waveform diagram of a modular hybrid DC unloading device according to an exemplary embodiment.

[0177] like Figure 10 As shown, P REC P represents the power transmitted from the AC side of the receiving station's MMC. SECU represents the AC input power of the sending station's MMC. pcc I represents the grid connection point voltage on the AC side of the REC-MMC. ac This indicates the grid-connected current on the AC side of the REC-MMC. U dc I represents the system DC voltage. chopper U represents the unloaded branch current. sm This indicates the output voltage of the energy dissipation valve module in the ATDS-DCC device. This indicates the capacitor voltage of the ATDS-DCC device submodule.

[0178] To make the waveforms more intuitive, all physical quantities are expressed in per-unit values. Figure 10 As can be seen, the ATDS-DCC device topology reduces the operating voltage of the IGBTs in the submodules during the device unloading process, thereby reducing the cost of the device.

[0179] Based on the parameter design method proposed in this disclosure, t is taken as... re =t q =65μs, t is calculated d =778.95μs.

[0180] Figure 11 This is a schematic diagram of the current limit waveform of the switching device of any submodule in a modular hybrid DC unloading device according to an exemplary embodiment.

[0181] like Figure 11 As shown in the figure, the simulation results indicate that the peak current of the switching device is close to the calculated value, which verifies the correctness of the peak current calculation method for the switching device. The specific calculated values ​​and the obtained simulation values ​​are shown in Table 2:

[0182]

[0183] Table 2

[0184] Figure 12 This is a control block diagram of a closed-loop controller according to an exemplary embodiment.

[0185] Figure 12 As shown, PWM represents the submodule switching control signal, ST2a and ST2b represent IGBT drive signals, and ST1 represents the thyristor drive signal.

[0186] like Figure 12As shown, in one possible embodiment, this disclosure also provides a closed-loop controller for executing a control method for a modular hybrid DC unloading device provided in this disclosure. The closed-loop controller includes: an overall energy dissipation controller and a sub-module capacitor voltage equalization controller. The overall energy dissipation controller is used to obtain the DC voltage of the grid-connected system of flexible DC transmission, and the sub-module capacitor voltage equalization controller is used to control the operation of the thyristor T1 and the IGBT module T2 with anti-parallel diodes in the modular hybrid DC unloading device.

[0187] The closed-loop controller adjusts the DC current in the unloading branch by changing the modulation index of the PWM modulation wave, thereby realizing DC injection control of the unloading branch and controlling the power consumption of the centralized dissipation resistor and the distributed power consumption resistor in the submodule under the corresponding operating conditions.

[0188] The specific embodiments of this disclosure have been described above. It should be understood that this disclosure is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the substantive content of this disclosure. The above-described preferred features can be used in any combination without conflict.

Claims

1. A modular hybrid DC unloading device, characterized in that, include: The energy dissipation valve module, the centralized dissipation resistor module, and the unloading reactor are connected in series. The energy-consuming valve module includes multiple sub-modules connected in series. Each sub-module includes a first capacitor C1, a second capacitor C2, and a buffer resistor R. cus 1. Thyristor T1, 2. Distributed power dissipation resistor R with anti-parallel diode D1 sm An IGBT module T2 with an anti-parallel diode and an anti-reverse diode D2, and a buffer resistor R. cus The thyristor T1 and the distributed energy-dissipating resistor R with anti-parallel diode D1 sm The IGBT module T2 with anti-parallel diode and the anti-reverse diode D2 are connected in series between the input and output terminals of the sub-module. The first capacitor C1 and the buffer resistor R cus The thyristor T1 and the distributed energy-dissipating resistor R with anti-parallel diode D1 sm The series branch is connected in parallel, and the second capacitor C2 and the buffer resistor R cus The series branch containing the IGBT module T2 with the anti-parallel diode is connected in parallel, wherein the anti-parallel diode D1 and the distributed energy dissipation resistor R are connected in parallel. sm The centralized dissipation resistor module and the buffer resistor R are connected in parallel. cus and the distributed energy-consuming resistor R sm Used for unloading.

2. The modular hybrid DC unloading device according to claim 1, characterized in that, The buffer resistor R cus The input terminal is connected to the input terminal of the submodule, and the buffer resistor R cus The output terminal is connected to the anode of the thyristor T1, and the cathode of the thyristor T1 is connected to the cathode of the anti-parallel diode D1 and the distributed energy dissipation resistor R. sm The input terminal is connected to the anode of the anti-parallel diode D1 and the distributed energy dissipation resistor R. sm The output terminals are respectively connected to the collector of the IGBT module T2 with anti-parallel diode, the emitter of the IGBT module T2 with anti-parallel diode is connected to the anode of the anti-reverse diode D2, and the cathode of the anti-reverse diode D2 is connected to the output terminal of the sub-module.

3. The modular hybrid DC unloading device according to claim 2, characterized in that, The positive terminal of the first capacitor C1 is connected to the input terminal of the sub-module and the buffer resistor R, respectively. cus The input terminal is connected, and the negative terminal of the first capacitor C1 is connected to the anode of the anti-parallel diode D1, the collector of the IGBT module T2 with the anti-parallel diode, and the distributed power dissipation resistor R. sm Connect the output terminal; The positive terminal of the second capacitor C2 is connected to the cathode of the thyristor T1, the cathode of the anti-parallel diode D1, and the distributed energy dissipation resistor R, respectively. sm The input terminal is connected, and the negative terminal of the second capacitor C2 is connected to the emitter of the IGBT module T2 with anti-parallel diode and the anode of the anti-reverse diode D2.

4. The modular hybrid DC unloading device according to claim 3, characterized in that, The capacitance of the first capacitor C1 is less than the capacitance of the second capacitor C2.

5. The modular hybrid DC unloading device according to claim 3, characterized in that, The IGBT module T2 with anti-parallel diode includes an IGBT module and an anti-parallel diode T2. The anti-parallel diode T2 is connected in parallel with the IGBT module. The cathode of the anti-parallel diode T2 is connected to the emitter of the IGBT module, and the anode of the anti-parallel diode T2 is connected to the emitter of the IGBT module.

6. The modular hybrid DC unloading device according to claim 1, characterized in that, The modular hybrid DC unloading device is connected in parallel between the DC buses of the flexible DC transmission grid-connected system. The input end of the first sub-module of the energy dissipation valve is connected to the positive terminal of the DC bus of the flexible DC transmission grid-connected system, and the other end of the unloading reactor is connected to the negative terminal of the DC bus of the flexible DC transmission grid-connected system. The modular hybrid DC unloading device is used to absorb surplus power when there is a grid fault on the AC side of the receiving-end converter station, and to stabilize the DC voltage of the flexible DC transmission grid-connected system within a preset DC voltage range.

7. A control method for a modular hybrid DC unloading device, characterized in that, include: Obtain the DC voltage of the sending-end converter station and the grid connection point voltage of the receiving-end converter station in the grid-connected system of flexible DC transmission. When the DC voltage of the sending-end converter station of the flexible DC transmission grid-connected system is less than a preset first threshold, the grid connection point voltage of the receiving-end converter station is a preset second threshold, and the modular hybrid DC unloading device is controlled to be in a locked state. When the DC voltage of the sending-end converter station of the flexible DC transmission grid-connected system rises to not less than the preset first threshold, the voltage of the receiving-end converter station at the grid connection point is less than the preset second threshold, and the modular hybrid DC unloading device is put into operation. When the DC voltage of the sending-end converter station of the flexible DC transmission grid-connected system drops to less than the preset first threshold, the voltage of the receiving-end converter station at the grid connection point is equal to the preset second threshold, and the modular hybrid DC unloading device is controlled to enter the disconnection state.

8. The method according to claim 7, characterized in that, The modular hybrid DC unloading device includes a cut-off mode, an input mode, a transition mode 1, and a transition mode 2.

9. The method according to claim 8, characterized in that, When the DC voltage of the sending-end converter station of the flexible DC transmission grid-connected system is less than a preset first threshold, and the grid connection point voltage of the receiving-end converter station is a preset second threshold, the modular hybrid DC unloading device is controlled to be in a locked state, including: The energy consumption control signal of the sub-module of the energy consumption valve of the modular hybrid DC unloading device is set to 0. The sub-module operates the cut-off mode. Thyristor T1 and IGBT module T2 with anti-parallel diode are in the off state. The first capacitor C1 is connected in series with the second capacitor C2 through anti-parallel diode D1, and the distributed energy consumption resistor R is connected. sm No current flows through it, and each of the submodules outputs the sum of the voltages of the first capacitor C1 and the second capacitor C2. The output voltage of the energy dissipation valve module is equal to the rated DC voltage of the grid-connected system of the flexible DC transmission.

10. The method according to claim 8, characterized in that, When the DC voltage of the sending-end converter station of the flexible DC transmission grid-connected system rises to the preset first threshold, and the voltage at the grid connection point of the receiving-end converter station is less than the preset second threshold, the modular hybrid DC unloading device is controlled to enter the operating state, including: Based on the duty cycle of the sub-modules of the energy-consuming valve of the modular hybrid DC unloading device, the sub-modules of the energy-consuming valve are controlled to operate periodically in a step-by-step manner according to the order of the input mode, the transition mode 1, the transition mode 2, and the cut-off mode.

11. The method according to claim 10, characterized in that, The step of controlling the sub-modules of the energy-consuming valve of the modular hybrid DC unloading device to operate periodically in a stepped manner according to the order of the input mode, the transition mode 1, the transition mode 2, and the cut-off mode, based on the duty cycle of the sub-modules of the energy-consuming valve, includes: When the submodule operates in the input mode, the submodule power consumption control signal is set to 1, and both thyristor T1 and IGBT module T2 with anti-parallel diode are turned on, and buffer resistor R... cus thyristor T1, distributed energy dissipation resistor R sm The IGBT module and the anti-reverse diode D2 form a conduction path, and the first capacitor C1 and the second capacitor C2 are connected through the distributed energy dissipation resistor R. sm Discharge to the first input rated voltage and the second input rated voltage respectively, the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, and each submodule outputs the voltage of the first capacitor C1; When the submodule operates in transition state 1, the IGBT module T2 with anti-parallel diode is turned off, the thyristor T1 is turned on, the first capacitor C1 discharges, the second capacitor C2 charges, and the discharge current of the first capacitor C1 flows through the distributed energy dissipation resistor R. sm and the buffer resistor R cus When the load is unloaded, the voltage of the second capacitor C2 is greater than the voltage of the first capacitor C1, and each submodule outputs the voltage of the second capacitor C2. When the submodule operates in transition state 2, the thyristor T1 is turned off, the IGBT module T2 with anti-parallel diode is turned on, the first capacitor C1 is charged, the second capacitor C2 is discharged, and the discharge current of the second capacitor C2 flows through the distributed energy dissipation resistor R. sm When unloading, the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, and each submodule outputs the voltage of the first capacitor C1; When the submodule operates in the cut-off mode, the thyristor T1 and the IGBT module T2 with the anti-parallel diode are turned off. The first capacitor C1 is connected in series with the second capacitor C2 through the anti-parallel diode D1, and the distributed energy dissipation resistor R... sm No current flows through it. The first capacitor C1 is charged to the first cutoff rated voltage, and the second capacitor C2 is charged to the second cutoff rated voltage. Each submodule outputs the sum of the voltages of the first capacitor C1 and the second capacitor C2.

12. The method according to claim 8, characterized in that, When the DC voltage of the sending-end converter station of the flexible DC transmission grid-connected system drops below the preset first threshold, the voltage at the grid connection point of the receiving-end converter station equals the preset second threshold, and the modular hybrid DC unloading device is controlled to enter the disconnection state, including: The energy consumption control signal of the sub-module of the energy consumption valve of the modular hybrid DC unloading device is set to 0. The sub-modules operate in a stepped manner in the cut-off mode. Thyristor T1 and IGBT module T2 with anti-parallel diode are both turned off. The first capacitor C1 is connected in series with the second capacitor C2 through anti-parallel diode D1, and the distributed energy consumption resistor R is connected in series. sm When no current flows through the capacitor, the first capacitor C1 is charged to the first cutoff rated voltage, and the second capacitor C2 is charged to the second cutoff rated voltage. Each submodule outputs the sum of the voltages of the first capacitor C1 and the second capacitor C2, and the modular hybrid DC unloading device is taken out of operation.

13. A closed-loop controller, characterized in that, A control method for executing the modular hybrid DC unloading device according to any one of claims 7 to 12 includes: an overall energy dissipation controller and a sub-module capacitor voltage equalization controller, wherein the overall energy dissipation controller is used to acquire the DC voltage of the grid-connected system of flexible DC transmission, and the sub-module capacitor voltage equalization controller is used to control the operation of the thyristor T1 and the IGBT module T2 with anti-parallel diodes of the modular hybrid DC unloading device.

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