Modularized hybrid direct current unloading device and control method

Abstract

The invention provides a modular hybrid direct current unloading device and a control method, the modular hybrid direct current unloading device comprises an energy consumption valve module, a centralized dissipation resistor module and an unloading reactor, and the energy consumption valve module, the centralized dissipation resistor module and the unloading reactor are connected in series; the energy consumption valve module comprises a plurality of sub-modules which are connected in series, and each sub-module comprises a first capacitor C1, a second capacitor C2, a buffer resistor Rmus, a thyristor T1, a distributed energy consumption resistor Rsm with an anti-parallel diode D1, an IGBT module T2 with an anti-parallel diode and an anti-reverse diode D2. The buffer resistor Rmus, the thyristor T1, the distributed energy consumption resistor Rsm with the anti-parallel diode D1, the IGBT module T2 with the anti-parallel diode and the anti-reverse diode D2 are connected in series between the input end and the output end of the sub-module. According to the invention, the surplus power under various working conditions can be flexibly consumed, the rated DC voltage of the flexible DC power transmission grid-connected system is maintained to be stable, and the device cost is reduced.

Description

Technical Field

[0001] The present disclosure relates to the field of power electronics technology, and in particular to a modular hybrid DC unloading device and a control method. Background Art

[0002] At present, due to comprehensive factors such as ecological environment protection and occupation of traffic waterways, the development of offshore wind power is gradually showing a trend of advancing from nearshore to deep sea. The offshore wind power transmission grid-connected system based on flexible DC transmission has been widely used in large-scale deep sea wind farm power transmission due to its advantages of low loss, long distance and reactive power decoupling.

[0003] The offshore wind power transmission grid-connected system based on VSC-HVDC (Voltage Source Converter-High Voltage Direct Current, VSC-HVDC) is mainly composed of offshore wind farms, sending end converters (SEC), submarine cables, receiving end converters (REC), AC main grids, etc. In the offshore wind flexible direct grid-connected system, after a grounding or short circuit fault occurs in the AC side grid of the receiving end station, the grid voltage drops rapidly. At this time, due to the weak current bearing capacity of the modular multilevel converter (MMC) of the receiving end station, the current inner loop controller is saturated, the output current of the AC side of the receiving end station is limited to the rated value, the power transmission path is blocked, and the system power delivery capacity is greatly reduced. However, since the sending end converter station cannot measure the grid fault, it continues to inject power into the DC line according to the wind farm output power before the fault, resulting in a system power surplus. Power imbalance will cause the system DC capacitors (including parasitic capacitors of submarine cables, capacitors of MMC submodules and DC inter-stage capacitors, etc.) to continuously charge, causing the system DC voltage to rise rapidly, triggering overvoltage protection, and even causing the DC line to trip or causing the MMC submodule to overvoltage lock. my country's technical regulations for wind farm access to the power system require that when the voltage at the wind farm grid connection point drops to 20% of the nominal voltage, the wind turbines and reactive compensation devices in the wind farm should ensure that they will not be disconnected from the grid and operate continuously for 625ms. When the voltage at the wind farm grid connection point can be restored to 90% of the nominal voltage within 2s after the drop, the wind turbines and reactive compensation devices in the wind farm should ensure that they will not be disconnected from the grid and operate continuously.

[0004] There are currently two main solutions to solve the above-mentioned surplus power problem. One is to reduce the output of the wind farm side through coordinated control; the other is to install unloading equipment to dissipate the surplus power. The coordinated control of the converter station adopts the voltage reduction / frequency increase method, which has a series of problems such as weak regulation ability, slow response speed, and sampling difficulties. The voltage reduction / frequency increase method alone is not enough to achieve effective processing of surplus power. In order to increase the reliability of fault ride-through of the offshore wind flexible direct current grid-connected system and speed up the fault protection response speed of the HVDC system, it is a more effective solution to use high-voltage direct current unloading equipment (DC Chopper, DCC) for surplus power processing.

[0005] To prevent the device from occupying space on the offshore platform, the high-voltage DC unloading equipment is usually installed on the DC side of the onshore converter station. The existing DCC topology, that is, the topological structure of the high-voltage DC unloading equipment, can be divided into three types according to the different distribution locations of its energy dissipation resistance: centralized, distributed and hybrid. There are certain differences in the topological structure, working principle, control method, operation effect and cost of these three types of devices. Among them, the hybrid topology combines the advantages of centralized and distributed topologies, and has the advantages of higher control accuracy, better electromagnetic interference performance, and lower requirements for water cooling systems. However, the existing hybrid topology still has problems such as large number of components, high cost and intellectual property rights.

[0006] For the hybrid high-voltage direct current unloading device used in the sea breeze flexible direct current grid-connected system, it places high requirements on the high voltage resistance and current carrying capacity of the switching devices. In this application scenario, thyristors are superior to fully controlled devices such as IGBTs in many features except for the limitation that they cannot be actively shut down. At the same voltage level, the price of IGBTs is usually 3 to 5 times that of thyristors. At the same time, the voltage resistance and current carrying capacity of thyristors are also stronger than those of fully controlled devices such as IGBTs, and their conduction losses and conduction voltage drops are also smaller than those of IGBTs. Therefore, using thyristors to replace some IGBTs not only saves the device cost of the unloading device, but also improves the reliability of the unloading device in high-voltage and high-power situations. Summary of the invention

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

[0008] To achieve the above object, according to one aspect of the present disclosure, a modular hybrid DC unloading device and a control method are provided, comprising: an energy-consuming valve module, a concentrated dissipating resistance module and an unloading reactance, wherein the energy-consuming valve module, the concentrated dissipating resistance module and the unloading reactance are sequentially connected in series;

[0009] The energy consumption valve module includes a plurality of submodules, which are connected in series in sequence. Each of the submodules includes a first capacitor C1, a second capacitor C2, a buffer resistor R cus, thyristor T1, distributed energy dissipation resistor R with anti-parallel diode D1 sm , an IGBT module T2 with an anti-parallel diode and an anti-reverse diode D2, the snubber resistor R cus , the thyristor T1, the distributed energy dissipation resistor R with the anti-parallel diode D1 sm The IGBT module T2 with the anti-parallel diode and the anti-reverse diode D2 are sequentially connected in series between the input and output ends of the submodule, and the first capacitor C1 and the buffer resistor R cus , the thyristor T1, the distributed energy dissipation resistor R with the anti-parallel diode D1 sm The series branches are connected in parallel, and the second capacitor C2 and the buffer resistor R cus The series branch where the IGBT module T2 with the anti-parallel diode is located is connected in parallel, wherein the anti-parallel diode D1 and the distributed energy dissipation resistor R sm connected in parallel, the concentrated dissipation resistance module and the buffer resistor R cus And the distributed energy dissipation resistor R sm Used for unloading.

[0010] Optionally, the buffer resistor R cus The input end of the submodule is connected to the input end of the buffer resistor R cus The output end of 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 end is connected to the anode of the anti-parallel diode D1 and the distributed energy dissipation resistor R sm The output ends are respectively connected to the collectors of the IGBT modules T2 with anti-parallel diodes, the emitter of the IGBT modules T2 with anti-parallel diodes 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 end of the submodule.

[0011] Optionally, the positive electrode of the first capacitor C1 is connected to the input end of the submodule and the buffer resistor R cus The negative electrode 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 energy dissipation resistor R sm The output terminal connection of

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

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

[0014] Optionally, the IGBT module T2 with an 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 busbars of the flexible DC power transmission grid-connected system, the input end of the first submodule of the energy consumption valve is connected to the positive pole of the DC busbar of the flexible DC power transmission grid-connected system, and the other end of the unloading reactor is connected to the negative pole of the DC busbar of the flexible DC power transmission grid-connected system. The modular hybrid DC unloading device is used to absorb surplus power when a fault occurs in the AC side power grid of the receiving-end converter station, so as to stabilize the DC voltage of the flexible DC power transmission grid-connected system within a preset DC voltage range.

[0016] According to a second aspect of the present 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 of the flexible DC transmission grid-connected system;

[0018] When the DC voltage of the sending-end converter station of the grid-connected system of the flexible DC power transmission is less than a preset first threshold value, the grid-connected point voltage of the receiving-end converter station is a preset second threshold value, 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 power transmission grid-connected system rises to not less than the preset first threshold value, and the voltage of the grid-connected point of the receiving-end converter station is less than the preset second threshold value, the modular hybrid DC unloading device is controlled to be put into operation;

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

[0021] Optionally, the submodules of the modular hybrid DC unloading device include 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 grid-connected system of the flexible DC power transmission is less than a preset first threshold value, the grid-connected point voltage of the receiving-end converter station is a preset second threshold value, and the modular hybrid DC unloading device is controlled to be in a locked state, including:

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

[0024] Optionally, when the DC voltage of the sending-end converter station of the flexible DC power transmission grid-connected system rises to the preset first threshold value, the voltage of the grid-connected point of the receiving-end converter station is less than the preset second threshold value, and the modular hybrid DC unloading device is controlled to be put into operation, including:

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

[0026] Optionally, according to the duty cycle of the submodule of the energy-consuming valve of the modular hybrid DC unloading device, controlling the submodules of the energy-consuming valve to periodically operate in a step-by-step manner in the order of the input mode, the transition state 1, the transition state 2, and the cut-off mode includes:

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

[0028] When the submodule runs the 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 is discharged, and the second capacitor C2 is charged. The current discharged from the first capacitor C1 flows through the distributed energy dissipation resistor R sm and the snubber resistor R cus unloading, the voltage of the second capacitor C2 is greater than the voltage of the first capacitor C1, and each of the submodules outputs the voltage of the second capacitor C2;

[0029] When the submodule runs the transition state 2, the thyristor T1 is turned off, the IGBT module T2 with the anti-parallel diode is turned on, the first capacitor C1 is charged, and the second capacitor C2 is discharged. The discharged current of the second capacitor C2 flows through the distributed energy dissipation resistor R sm Unloading, the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, and each of the submodules 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 the first capacitor C1, the first capacitor C1 is charged to a first cut-off rated voltage, the second capacitor C2 is charged to a second cut-off rated voltage, and each of the submodules 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 power transmission grid-connected system drops to less than the preset first threshold, the voltage of the grid-connected point of the receiving-end converter station is equal to the preset second threshold, and the modular hybrid DC unloading device is controlled to enter a cut-off state, including:

[0032] The submodule energy consumption control signal of the submodule of the energy consumption valve of the modular hybrid DC unloading device is set to 0, and the submodules are grouped and step-by-step operated in the cut-off state. The thyristor T1 and the IGBT module T2 with the anti-parallel diode are both 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 consumption resistor R sm No current flows through the first capacitor C1, the first capacitor C1 is charged to the first cut-off rated voltage, the second capacitor C2 is charged to the second cut-off rated voltage, each of the submodules 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.

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

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

[0035] Through the above technical solution, an energy-consuming valve module, a concentrated dissipation resistor module and a structure of unloading reactance in series are adopted, and the energy-consuming valve module includes multiple submodules, and multiple submodules are grouped and stepped, and a buffer resistor R cus , distributed energy dissipation resistor R sm The concentrated dissipation resistor consumes surplus power under different working conditions, maintains the DC voltage stability of the flexible DC transmission grid-connected system, and realizes the fault ride-through of the flexible DC transmission grid-connected system. In addition, the thyristor T1 is used in the energy-consuming valve module to replace part of the IGBT modules, which effectively reduces the device cost and improves the reliability and accuracy of load unloading in high-voltage and high-power occasions.

[0036] In the embodiments of the present disclosure, the capacitance of the first capacitor C1 is set to be smaller than the capacitance of the second capacitor C2, and the capacitance of the first capacitor C1 and the second capacitor C2 are designed asymmetrically, thereby improving the withstand voltage of the unloading device, the thyristor T1 in the sub-module, and reducing the rated operating voltage of the IGBT module in the sub-module, thereby further reducing the cost of the modular hybrid DC unloading device.

[0037] The embodiments of the present disclosure control the locking state, operating state and cutting state of the modular hybrid DC unloading device based on the DC voltage of the sending-end converter station of the flexible DC power transmission grid-connected system and the grid-connected power voltage of the receiving-end converter point, so as to timely realize the consumption of surplus power, maintain the DC voltage stability of the flexible DC power transmission grid-connected system, and realize the fault ride-through of the flexible DC power transmission grid-connected system.

[0038] In the embodiment of the present disclosure, based on the sorted sub-module capacitor voltage balancing strategy, the voltages of the first capacitor C1 and the second capacitor C2 are compared to achieve sub-module capacitor voltage balancing. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0040] Figure 1It is a schematic diagram of the topological architecture of a modular hybrid DC load shedding device applied to a grid-connected system of flexible DC power transmission according to an exemplary embodiment.

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

[0042] Figure 3 It is a schematic diagram of a topological structure of a submodule of an energy-consuming valve according to an exemplary embodiment.

[0043] Figure 4 It is a flow chart of a control method of a modular hybrid DC load unloading device according to an exemplary embodiment.

[0044] Figure 5 It is a schematic diagram of switching timing waveforms of submodules of an energy-consuming valve according to an exemplary embodiment.

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

[0046] Figure 7 It is a schematic diagram of voltage and current distribution of a submodule of an energy-consuming valve in a startup mode according to an exemplary embodiment.

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

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

[0049] Fig.10 It is a schematic diagram of simulation waveforms of a modular hybrid DC load unloading device according to an exemplary embodiment.

[0050] Fig.11 It is a schematic diagram of a current limit waveform of a switch device of any submodule in a modular hybrid DC unloading device according to an exemplary embodiment.

[0051] Fig.12 is a control block diagram of a closed-loop controller according to an exemplary embodiment. DETAILED DESCRIPTION

[0052] The present disclosure is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present disclosure, but are not intended to limit the present disclosure in any form. It should be noted that, for those of ordinary skill in the art, several variations and improvements may be made without departing from the concept of the present disclosure. These all fall within the scope of protection of the present disclosure.

[0053] Figure 1 It is a schematic diagram of the topological architecture of a modular hybrid DC load shedding device applied to a grid-connected system of flexible DC power transmission according to an exemplary embodiment. Figure 2 It is a simulation circuit diagram of a modular hybrid DC load unloading device according to an exemplary embodiment. Figure 3 It is a schematic diagram of a topological structure of a submodule of an energy-consuming valve according to an exemplary embodiment.

[0054] like Figures 1 to 3 As shown, the present disclosure provides a modular hybrid DC unloading device, comprising an energy-consuming valve module, a concentrated dissipating resistance module and an unloading reactance, wherein the energy-consuming valve module, the concentrated dissipating resistance module and the unloading reactance are sequentially connected in series;

[0055] The energy-consuming valve module includes a plurality of submodules, which are connected in series in sequence. Each submodule includes a first capacitor C1, a second capacitor C2, a buffer resistor R cus , thyristor T1, distributed energy dissipation resistor R with anti-parallel diode D1 sm , IGBT module T2 with anti-parallel diode and anti-reverse diode D2, snubber resistor R cus , thyristor T1, distributed energy dissipation resistor R with anti-parallel diode D1 sm , the IGBT module T2 with the anti-parallel diode and the anti-reverse diode D2 are connected in series between the input and output ends of the submodule, and the first capacitor C1 and the buffer resistor R cus , thyristor T1, distributed energy dissipation resistor R with anti-parallel diode D1 sm The series branches are connected in parallel, and the second capacitor C2 and the buffer resistor R cus , the series branch where the IGBT module T2 with the anti-parallel diode is located is connected in parallel, wherein the anti-parallel diode D1 and the distributed energy dissipation resistor R sm Parallel connection, concentrated dissipation resistance module, buffer resistor R cus And the distributed energy dissipation resistor R sm Used for unloading.

[0056] Through the above technical solution, an energy-consuming valve module, a concentrated dissipation resistor module and a structure of unloading reactance in series are adopted, and the energy-consuming valve module includes multiple submodules, and multiple submodules are grouped and stepped, and a buffer resistor Rcus , distributed energy dissipation resistor R sm The concentrated dissipation resistor consumes surplus power under different working conditions, maintains the DC voltage stability of the flexible DC transmission grid-connected system, and realizes the fault ride-through of the flexible DC transmission grid-connected system. In addition, the thyristor T1 is used in the energy-consuming valve module to replace part of the IGBT modules, which effectively reduces the device cost and improves the reliability and accuracy of load unloading in high-voltage and high-power occasions.

[0057] In high-voltage and high-power applications, at the same voltage level, the cost of thyristors is lower than that of IGBT modules. In addition, the voltage resistance and current carrying capacity of thyristors are better than those of fully controlled devices such as IGBTs, and the conduction loss and conduction voltage drop are smaller than those of IGBT modules.

[0058] like Figure 3 As shown, in a possible embodiment, the buffer resistor R cus The input terminal of the submodule is connected to the input terminal of the buffer resistor R cus The output end 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 end is connected to the anode of the anti-parallel diode D1 and the distributed energy dissipation resistor R sm The output ends are respectively connected to the collectors of the IGBT modules T2 with anti-parallel diodes, the emitter of the IGBT modules T2 with anti-parallel diodes 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 end of the sub-module.

[0059] like Figure 3 As shown, in a possible embodiment, the positive electrode of the first capacitor C1 is respectively connected to the input end of the submodule, the buffer resistor R cus The negative electrode 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 energy dissipation resistor R sm The output terminal is connected.

[0060] The positive electrode 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 sm The cathode of the second capacitor C2 is connected to the emitter of the IGBT module T2 with an anti-parallel diode and the anode of the anti-reverse diode D2 respectively.

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

[0062] The capacitance of the first capacitor C1 and the second capacitor C2 of each submodule is designed asymmetrically. In the input mode and the cut-off mode, the voltage across the thyristor T1 of the energy-consuming valve module is greater than the voltage across the IGBT module T2 with the anti-parallel diode.

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

[0064] like Figure 3 As shown, in a possible embodiment, the IGBT module T2 with an 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 a possible embodiment, a modular hybrid DC unloading device is connected in parallel between DC buses of a flexible DC power transmission grid-connected system, an input end of a first submodule of an energy-consuming valve is connected to a positive pole of the DC bus of the flexible DC power transmission grid-connected system, and the other end of the unloading reactor is connected to a negative pole of the DC bus of the flexible DC power transmission grid-connected system. The modular hybrid DC unloading device is used to absorb surplus power when a fault occurs in the AC side power grid of a receiving-end converter station, and stabilize the DC voltage of the flexible DC power 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, a modular hybrid DC unloading device ATDS-DCC (Asymmetrical Thyristor Diode Switch-DC Chopper) provided by the present invention can be applied to a grid-connected system of offshore wind flexible DC transmission based on VSC-HVDC (Voltage Source Converter-High Voltage 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 direct current 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, wherein a modular hybrid DC unloading device provided in the present invention is connected in parallel to the DC bus between the sending-end converter station and the receiving-end converter station, and the DC positive pole of the receiving-end converter station is connected to the DC negative pole of the receiving-end converter station after passing through an energy-consuming valve module, a concentrated dissipation resistance module, and an unloading reactance.

[0069] When the modular hybrid DC unloading device is put into operation, the energy-consuming valve module can be equivalent to a controlled voltage source. During the operation of the modular hybrid DC unloading device, the energy-consuming valve module switches periodically between the input mode and the cut-off mode. In addition, the concentrated dissipation resistance module and the buffer resistance R cus And the distributed energy dissipation resistor R sm All participate in unloading.

[0070] The submodules of the energy-consuming valve module include a cut-off mode, an input mode, a transition state 1 and a transition state 2. When the modular hybrid DC unloading device is put into operation, the submodules operate periodically in the order of the input mode, the transition state 1, the transition state 2 and the cut-off mode, so as to stabilize the system DC voltage in a preset DC voltage range including the rated DC voltage, thereby absorbing the surplus power of the faulty components of the onshore AC power grid on the AC side of the receiving end converter station, and realizing the fault ride-through of the grid-connected system of the offshore wind flexible DC transmission based on VSC-HVDC.

[0071] Figure 4 It is a flow chart of a control method of a modular hybrid DC load unloading device according to an exemplary embodiment.

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

[0073] S11, obtaining 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 a preset first threshold, the grid-connected 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.

[0075] Among them, 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 there is no fault in the grid-connected system of flexible DC transmission. The modular hybrid DC unloading device is in the hot standby state, and its switching devices, thyristor T1 and 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] Among them, 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, that is, 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 too high, the energy dissipation valve module is put into unloading, and the sub-modules are 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 of the sending converter station dc decreases.

[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] Among them, when the DC voltage U of the sending converter station dc drops to be less than the preset first threshold, it means that the DC voltage U of the sending converter station dc is too low. Remove the energy dissipation valve module, and the sub-modules are removed 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 of the sending converter station dc rises.

[0080] By controlling the locking 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 stability of the grid-connected system of flexible DC transmission can be maintained, and the fault ride-through of the grid-connected system of flexible DC transmission can be realized.

[0081] In a possible embodiment, the submodules of the modular hybrid DC unloading device include a cut-off mode, an input mode, a transition state 1 and a transition state 2.

[0082] Figure 5 It is a schematic diagram of switching timing waveforms of submodules of an energy-consuming valve according to an exemplary embodiment.

[0083] When the modular hybrid DC unloading device is put into operation, the energy-consuming valve module is periodically switched between the input mode and the cut-off mode, and the submodules are periodically operated in the cut-off mode, the input mode, the transition state 1, the transition state 2, and the cut-off mode.

[0084] like Figure 5 ,SM-PWM k It represents the submodule energy consumption control signal, i.e. the switching control signal, and its duty cycle is d k , k represents the group number of the submodule, ST 1,k Indicates the gate trigger signal of thyristor T1 in the submodule, ST 2,k Represents the gate drive signal of the IGBT module T2 with an anti-parallel diode.

[0085] The period from t0 to t1 is the stage when the submodule is in the input mode, which is called process I; the period from t1 to t2 is the stage when the submodule is in the transition state 1, which is called process II; the period from t2 to t3 is the stage when the submodule is in the transition state 2, which is called process III; the period from t3 to t4 is the stage when the submodule is in the removal mode, which is called process IV.

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

[0087] like Figure 5 , Figure 6 As shown, before time t0, the submodule energy consumption control signal SM-PWM k =0, the submodule is in the cut-off mode, the thyristor T1 and the IGBT module T2 with the anti-parallel diode are both 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 the IGBT module, and 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 It is a schematic diagram of voltage and current distribution of a submodule of an energy-consuming valve in a startup mode according to an exemplary embodiment.

[0089] like Figure 5 and Figure 7As shown, in the period t0-t1, at time t0, the submodule energy consumption control signal SM-PWM k is 1, and a conduction signal is sent to both the thyristor T1 and the IGBT module T2 with an anti-parallel diode. The gate trigger signal ST of the thyristor T1 1,k Set to 1, the gate drive signal ST of the IGBT module T2 with anti-parallel diode 2,k Set to 1, the submodule enters the input mode, the submodule starts to unload, the thyristor T1 and the IGBT module T2 with anti-parallel diode are both turned on, then the IGBT module is also turned on, and the buffer resistor R cus , thyristor T1, distributed energy dissipation resistor R sm , IGBT module, and anti-reverse diode D2 form a conduction path, the negative electrode potentials of the first capacitor C1 and the second capacitor C2 are equal, the voltage across the second capacitor C2 is less than the voltage across the first capacitor C1, and the unloading current flows through the buffer resistor R cus , thyristor T1, distributed energy dissipation resistor R sm , IGBT module, sub-modules participate in energy dissipation, the first capacitor C1 continues to discharge to the first input rated voltage U C1,in The second capacitor C2 continues to discharge to the second input rated voltage U C2,in .

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

[0091] like Figure 5 and Figure 8 As shown, in the period t1 to t2, at time t1, the submodule energy consumption control signal SM-PWM k Set to 0, the gate drive signal ST of the IGBT module T2 with anti-parallel diode 2,k Set to 0, send a shutdown signal to the IGBT module T2 with an anti-parallel diode, the submodule enters transition state 1, and the submodule begins to be removed. At this time, the negative potentials of the first capacitor C1 and the second capacitor C2 are no longer equal, and the thyristor T1 continues to conduct because it cannot be turned off autonomously. The second capacitor C2 is charged, and the voltage of the second capacitor C2 rises. The first capacitor C1 is discharged, and the voltage of the first capacitor C1 drops. The discharge current of the first capacitor C1 flows through the distributed energy dissipation resistor R sm The load is unloaded, the voltage of the second capacitor C2 gradually becomes greater than the voltage of the first capacitor C1, and the submodule outputs the voltage of the second capacitor C2.

[0092] Fig. 9 is a schematic diagram of 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 Fig. 9 As shown, in the period t2 to t3, at time t2, the submodule energy consumption control signal SM-PWM k Set to 0, the gate trigger signal ST of thyristor T1 1,k Set to 0, the gate drive signal ST of the IGBT module T2 with anti-parallel diode 2,k Set to 1 and continue for a preset time, the IGBT module is turned on, 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 by the reverse voltage, and the submodule enters the transition state 2. The unloading branch current charges the first capacitor C1, the voltage of the first capacitor C1 rises, the unloading branch current also flows through the IGBT module, the second capacitor C2 discharges, and the discharge current flows through the distributed energy dissipation resistor R sm , the voltage of the second capacitor C2 decreases, and the voltage of the second capacitor C2 gradually becomes smaller than the voltage of the first capacitor C1, and the submodule outputs the voltage of the first capacitor C1.

[0094] like Figure 5 and Figure 6 As shown, in the period t3 to t4, at time t3, the submodule energy consumption control signal SM-PWM k =0, the gate trigger signal ST of thyristor T1 1,k Set to 0, the gate drive signal ST of the IGBT module T2 with anti-parallel diode 2,k Set to 0, turn off the IGBT module T2 with the anti-parallel diode, and the submodule enters the cut-off state. Both the thyristor T1 and the IGBT module are not conducting, 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 the IGBT module, and the unloading branch current flows through the first capacitor C1, the anti-parallel diode D1, the second capacitor C2 and the anti-reverse diode D2. The first capacitor C1 is charged to the first cut-off 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 power transmission grid-connected system, the modular hybrid DC unloading device is put into operation and the process of the above-mentioned time period t0 to t4 is repeated until the fault is eliminated, the DC bus voltage of the flexible DC power transmission grid-connected system decreases, and the modular hybrid DC unloading device exits the operation state.

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

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

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

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

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

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

[0102] As another example, when the submodule runs 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 is discharged, the second capacitor C2 is charged, and the current discharged from the first capacitor C1 flows through the distributed energy dissipation resistor R sm Unloading, 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, the thyristor T1 is turned off, the IGBT module T2 with the anti-parallel diode is turned on, the first capacitor C1 is charged, the second capacitor C2 is discharged, and the current discharged by the second capacitor C2 flows through the distributed energy dissipation resistor R sm 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, 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 the first capacitor C1, the first capacitor C1 is charged to the first cut-off rated voltage, the second capacitor C2 is charged to the second cut-off rated voltage, and each submodule outputs the sum of the voltages of the first capacitor C1 and the second capacitor C2.

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

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

[0107] The submodule energy consumption control signal of the submodule of the energy consumption valve of the modular hybrid DC unloading device is set to 0, the submodule group step-by-step operation cut-off state, 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 consumption resistor R sm No current flows through the device, the first capacitor C1 is charged to the first cut-off rated voltage, the second capacitor C2 is charged to the second cut-off 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 a control method for a modular hybrid DC unloading device provided by the present invention is adopted, the dissipated power of the unloading device is regulated by grouping and periodically switching multiple groups of sub-modules of the modular hybrid DC unloading device, and the switching voltage conversion rate du / dt of the energy-consuming valve module is reduced. The switching control signal PWM of the energy-consuming valve module is set to 1, and the sub-modules are grouped and stepped, the unloading device consumes energy, and the system DC voltage decreases; the switching control signal PWM of the energy-consuming valve module is set to 0, and the sub-modules are grouped and stepped, and the offshore wind farm inputs surplus power, and the system DC voltage increases.

[0109] When the onshore AC power grid at the receiving-end converter station of the grid-connected system of flexible DC transmission fails, the surplus power coefficient is between 0-1, the modular hybrid DC unloading device enters the dynamic unloading state, and the energy-consuming valve module switches periodically between the activation and removal modes.

[0110] The average charging current and average discharging current of the system equivalent capacitor during the unloading valve control cycle meet the following requirements:

[0111]

[0112] Among them, I chargeIt represents the charging current of the equivalent capacitor on the DC side of the flexible DC transmission grid-connected system during unloading, I rated Indicates the rated current of the flexible DC transmission grid-connected system, I discharge It represents the discharge current of the equivalent capacitance on the DC side of the grid-connected system of flexible DC transmission during unloading, and k represents the group number of the submodule.

[0113] In one switching cycle of the energy-consuming valve module, the rising value and the falling value of the system DC voltage are the same, and the duty cycle D of the switching control signal PWM of the energy-consuming valve module should satisfy the following relationship:

[0114]

[0115] The solution is D=k.

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

[0117]

[0118] Among them, C eq Represents the system equivalent capacitance, U dcN represents the rated DC voltage of the system, Δt1 represents the cut-off time of the energy-consuming valve in one cycle, Δt2 represents the input time of the energy-consuming valve in one cycle, and h represents the equivalent DC side equivalent capacitance ripple level of the system as ±h.

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

[0120] The rated input power of offshore wind farm is defined as P rated The DC voltage rating of the flexible DC transmission grid-connected system (VSC-HVDC system) is U dcN The number of submodules of the energy-consuming valve module of the modular hybrid DC unloading device (ATDS-DCC device) is N, and the output voltage of the energy-consuming valve module is U sm , the concentrated dissipation resistance is R, and the voltage of the concentrated dissipation resistance 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 Indicates the rated energy dissipation resistance of the unloading branch, U dc P represents the DC voltage of the grid-connected system of flexible DC transmission. rated Indicates the rated input power of the offshore wind farm.

[0123] When the VSC-HVDC system is operating without fault, the ATDS-DCC device is in the locked state. chop =0,U sm =U dcN , the submodules are in the cut-off state, 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 submodule outputs the sum of the voltages of the first capacitor C1 and the second capacitor C2. The first cut-off rated voltage of the first capacitor C1 and the second cut-off rated voltage of the second capacitor C2 satisfy:

[0124]

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

[0126] C1=aC2(1.6)

[0127] R=pR rated (1.7)

[0128] Then the first cut-off rated voltage of the first capacitor C1 is and the second cut-off rated voltage of the second capacitor C2 satisfy:

[0129]

[0130] When the most serious 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 0p.u, and the receiving-end converter station completely loses its ability to send power. At this time, the ATDS-DCC device dissipates all the power input from the offshore wind farm, all sub-modules are put into operation, the thyristor T1 and the IGBT module T2 with anti-parallel diode are closed and turned on, and the sub-module outputs the voltage of the first capacitor C1. The power expressions of the concentrated dissipation resistance module and the energy dissipation valve module are as follows:

[0131]

[0132] Among them, P DCC It represents the power consumed by the ATDS-DCC device during unloading, P R Represents the power of the concentrated dissipation resistance module, represents the power of the distributed dissipation resistor, Indicates the power of the snubber resistor, P rated Indicates the rated input power of the offshore wind farm.

[0133] Under this working condition, the current on the concentrated dissipation resistance module and the distributed dissipation resistance and buffer resistance in each submodule is equal, which is:

[0134]

[0135] Among them, I chop Represents the unloading branch current, I rated Indicates the rated current of the flexible DC transmission grid-connected system.

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

[0137]

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

[0139]

[0140] When a fault occurs in the onshore AC power grid at 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-consuming valve module switches periodically 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 impact current caused by the submodule switching between various modes. The following is an analysis of the impact current of the submodule during the switching process:

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

[0143]

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

[0145]

[0146] At time t2, the submodule switches from transition state 1 to transition state 2. In this process, the initial values ​​of the voltages of the first capacitor C1 and the second capacitor C2 are respectively and Assume that the submodule cuts off the two processes of transition state 1 and transition state 2. chop Keep it unchanged and define time t d and t re , t d express Figure 5 The duration of the period from t1 to t2, t re express Figure 5 From time t2 to To find out and According to the pull-type transformation of the submodule circuit during the period t1 to t2, the following formula is obtained:

[0147]

[0148] Substitute the above equations (1.11) and (1.12) into the above equation (1.15), and then perform the reverse transformation to determine and The time domain expression of is:

[0149]

[0150] At t2+t re At this moment, let the voltages of C1 and C2 be and According to the pull-type transformation and corresponding calculation of the submodule circuit during the period t2 to t3, we can finally get and

[0151]

[0152] According to formula (1.16), 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 impact current on the IGBT module.

[0155]

[0156] When the thyristor T1 is turned off, the duration of the reverse voltage applied to both ends of the thyristor T1 must be greater than the turn-off time t q, and when thyristor T1 is turned on, thyristor T1 cannot withstand reverse voltage. The submodule switching action sequence must meet:

[0157] t re ≥t q (1.20)

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

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

[0160]

[0161] In a possible embodiment, when selecting the thyristor T1 and the IGBT module, the rated currents of the two should be greater than all the current peaks obtained by the above calculations.

[0162] Design of the submodule parameters a and p of the asymmetric ATDS-DCC device based on cost. In actual engineering, the thyristor T1 and IGBT module in the module are connected in series, so the cost of the thyristor T1 and IGBT module is proportional to its rated operating voltage. The ratio of the cost of a single submodule in the asymmetric ATDS-DCC device to the cost of a symmetrical ATDS-DCC device at the same voltage level can be calculated by the following formula:

[0163]

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

[0165] In a possible embodiment, a simulation model of a ±800kV / 1100MW symmetrical monopole sea wind flexible DC power transmission grid-connected system is used to verify the modular hybrid DC unloading device of the present application.

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

[0167]

[0168]

[0169] Table 1

[0170] Among them, the back-to-back modular multilevel converter BTB-MMC in the offshore wind flexible direct current power transmission grid-connected system adopts the average value model. The receiving-end modular multilevel converter REC-MMC adopts voltage outer loop control, and the sending-end modular multilevel converter SEC-MMC adopts PQ control to simulate the operation mode of the wind farm side converter station.

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

[0172] The simulation verifies the effectiveness of the ATDS-DCC device proposed in this application and the correctness of the parameter design.

[0173] In order to speed up the simulation, the number of submodule switching groups is reduced to 4 groups, and a single submodule structure is used to be equivalent to all series submodules in one group. For submodules in the same group, their submodule energy consumption control signals, that is, the submodule switching control signals SM-PWM, are the same, and the action timing is the same, so their distributed energy consumption resistance and submodule capacitance can be regarded as series. In the simulation, one submodule is used to be equivalent to 70 submodules in series, with a distributed resistance of R / 4 and a capacitance of C. sm / 70. For the grid voltage U PCC The simulation was performed by dropping to 0.5 pu.

[0174] Under the working 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 output voltage of the equivalent power supply on the AC side of the onshore converter station MMC is modified to 0.5pu (208.21kV), simulating a three-phase short-circuit fault in the onshore AC power grid, and the surplus power coefficient k = 0.5.

[0175] After 0.025s, the system DC voltage U dc >1.05pu, the ATDS-DCC device is enabled and put into operation to absorb the surplus power and maintain the stability of the system DC voltage. At t=1.3s, the effective value of the output voltage of the equivalent power supply on the AC side of the onshore converter station MMC is restored to 1p.u., the sea breeze soft DC power is sent out and the grid-connected system fault is restored, and the ATDS-DCC device enters the locked state and exits operation.

[0176] Fig.10 It is a schematic diagram of simulation waveforms of a modular hybrid DC load unloading device according to an exemplary embodiment.

[0177] like Fig.10 As shown, P REC Indicates the power sent out by the MMC AC side of the receiving station, P SECIndicates the input power of the MMC AC side at the sending station, U pcc Indicates the grid connection point voltage on the AC side of REC-MMC, I ac Indicates the grid-connected point current on the AC side of REC-MMC. dc Indicates the system DC voltage, I chopper Indicates the unloading branch current, U sm Indicates the output voltage of the energy consumption valve module of the ATDS-DCC device. Indicates the ATDS-DCC device submodule capacitor voltage.

[0178] To make the waveform more intuitive, all physical quantities are expressed in per unit. Fig.10 It can be seen that the ATDS-DCC device topology reduces the operating voltage of the IGBT in the submodule during the device unloading process, thereby reducing the cost of the device.

[0179] According to the parameter design method proposed in this disclosure, take t re =t q = 65μs, and t d =778.95μs.

[0180] Fig.11 It is a schematic diagram of a current limit waveform of a switch device of any submodule in a modular hybrid DC unloading device according to an exemplary embodiment.

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

[0182]

[0183] Table 2

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

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

[0186] like Fig.12As shown, in a possible embodiment, the present disclosure further provides a closed-loop controller for executing a control method for a modular hybrid DC unloading device provided by the present disclosure, the closed-loop controller comprising: an overall energy dissipation controller and a sub-module capacitor voltage balancing controller, the overall energy dissipation controller is used to obtain the DC voltage of the grid-connected system of the flexible DC transmission, and the sub-module capacitor voltage balancing controller is used to control the action of the thyristor T1 and the IGBT module T2 with an anti-parallel diode of the modular hybrid DC unloading device.

[0187] The closed-loop controller adjusts the DC current in the unloading branch by changing the modulation degree of the PWM modulation wave, realizes the DC injection control of the unloading branch, and thus controls the energy consumption power of the concentrated dissipation resistor and the distributed energy consumption resistor in the submodule under the corresponding working conditions.

[0188] The above describes the specific embodiments of the present disclosure. It should be understood that the present disclosure is not limited to the above specific embodiments, and those skilled in the art can make various deformations or modifications within the scope of the claims, which does not affect the essence of the present disclosure. The above preferred features can be used in any combination without conflicting with each other.

Claims

1. A modular hybrid DC unloading device, characterized in that: include: An energy-consuming valve module, a concentrated dissipating resistance module and a load-removing reactance, wherein the energy-consuming valve module, the concentrated dissipating resistance module and the load-removing reactance are sequentially connected in series; The energy consumption valve module includes a plurality of submodules, which are connected in series in sequence. Each of the submodules includes a first capacitor C1, a second capacitor C2, a buffer resistor R cus , thyristor T1, distributed energy dissipation resistor R with anti-parallel diode D1 sm , an IGBT module T2 with an anti-parallel diode and an anti-reverse diode D2, the snubber resistor R cus , the thyristor T1, the distributed energy dissipation resistor R with the anti-parallel diode D1 sm The IGBT module T2 with the anti-parallel diode and the anti-reverse diode D2 are sequentially connected in series between the input and output ends of the submodule, and the first capacitor C1 and the buffer resistor R cus , the thyristor T1, the distributed energy dissipation resistor R with the anti-parallel diode D1 sm The series branches are connected in parallel, and the second capacitor C2 and the buffer resistor R cus The series branch where the IGBT module T2 with the anti-parallel diode is located is connected in parallel, wherein the anti-parallel diode D1 and the distributed energy dissipation resistor R sm connected in parallel, the concentrated dissipation resistance module and the buffer resistor R cus And the distributed energy dissipation resistor R sm Used for unloading.

2. The modular hybrid DC unloading device according to claim 1, characterized in that: The snubber resistor R cus The input end of the submodule is connected to the input end of the buffer resistor R cus The output end of 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 end is connected to the anode of the anti-parallel diode D1 and the distributed energy dissipation resistor R sm The output ends are respectively connected to the collectors of the IGBT modules T2 with anti-parallel diodes, the emitter of the IGBT modules T2 with anti-parallel diodes 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 end of the submodule.

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

4. The modular hybrid DC unloading device according to claim 3, characterized in that: The capacitance of the first capacitor C1 is smaller 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 an anti-parallel diode comprises an IGBT module and an anti-parallel diode T2, wherein 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 busbars of the flexible DC power transmission grid-connected system, the input end of the first submodule of the energy consumption valve is connected to the positive pole of the DC busbar of the flexible DC power transmission grid-connected system, and the other end of the unloading reactor is connected to the negative pole of the DC busbar of the flexible DC power transmission grid-connected system. The modular hybrid DC unloading device is used to absorb surplus power when a fault occurs in the AC side power grid of the receiving-end converter station, and stabilize the DC voltage of the flexible DC power 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 of the flexible DC transmission grid-connected system; When the DC voltage of the sending-end converter station of the grid-connected system of the flexible DC power transmission is less than a preset first threshold value, the grid-connected point voltage of the receiving-end converter station is a preset second threshold value, 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 power transmission grid-connected system rises to not less than the preset first threshold value, and the voltage of the grid-connected point of the receiving-end converter station is less than the preset second threshold value, the modular hybrid DC unloading device is controlled to be put into operation; When the DC voltage of the sending-end converter station of the flexible DC power transmission grid-connected system drops to less than the preset first threshold, the voltage of the grid-connected point of the receiving-end converter station is equal to the preset second threshold, and the modular hybrid DC unloading device is controlled to enter a cut-off state.

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

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

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

11. The method according to claim 10, characterized in that According to the duty cycle of the submodule of the energy-consuming valve of the modular hybrid DC unloading device, the submodules of the energy-consuming valve are controlled to operate periodically in a step-by-step manner in the order of the input mode, the transition state 1, the transition state 2, and the cut-off mode, including: When the submodule is in the input state, the submodule energy consumption control signal of the submodule is set to 1, the thyristor T1 and the IGBT module T2 with the anti-parallel diode are both turned on, and the buffer resistor R cus , thyristor T1, distributed energy dissipation resistor R sm , IGBT module, anti-reverse diode D2 form a conduction path, the first capacitor C1 and the second capacitor C2 are connected through the distributed energy dissipation resistor R sm Discharging to a first input rated voltage and a second input rated voltage respectively, the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, and each of the submodules outputs the voltage of the first capacitor C1; When the submodule runs the 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 is discharged, and the second capacitor C2 is charged. The current discharged from the first capacitor C1 flows through the distributed energy dissipation resistor R sm and the buffer resistor R is unloaded, the voltage of the second capacitor C2 is greater than the cus The voltage of the first capacitor C1, each of the submodules outputs the voltage of the second capacitor C2; When the submodule runs the transition state 2, the thyristor T1 is turned off, the IGBT module T2 with the anti-parallel diode is turned on, the first capacitor C1 is charged, and the second capacitor C2 is discharged. The discharged current of the second capacitor C2 flows through the distributed energy dissipation resistor R sm Unloading, the voltage of the first capacitor C1 is greater than the voltage of the second capacitor C2, and each of the submodules 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 the first capacitor C1, the first capacitor C1 is charged to a first cut-off rated voltage, the second capacitor C2 is charged to a second cut-off rated voltage, and each of the submodules 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 grid-connected system of the flexible DC power transmission drops to less than the preset first threshold value, the voltage of the grid-connected point of the receiving-end converter station is equal to the preset second threshold value, and the modular hybrid DC unloading device is controlled to enter a cut-off state, including: The submodule energy consumption control signal of the submodule of the energy consumption valve of the modular hybrid DC unloading device is set to 0, and the submodules are grouped and step-by-step operated in the cut-off state. The thyristor T1 and the IGBT module T2 with the anti-parallel diode are both 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 consumption resistor R sm No current flows through the first capacitor C1, the first capacitor C1 is charged to the first cut-off rated voltage, the second capacitor C2 is charged to the second cut-off rated voltage, each of the submodules 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.

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

Images