A flexible HVDC valve and a control method and device thereof

Abstract

The application belongs to the technical field of converter valves, and particularly relates to a flexible direct current (HVDC) converter valve and a control method and device thereof. The method comprises the following steps: S1, if the DC bus voltage of the converter valve is greater than or equal to a first overvoltage threshold but less than a second overvoltage threshold, the overvoltage suppression branch of the overvoltage suppression submodule in the third-phase unit in the input state is input, and the overvoltage suppression branch of the overvoltage suppression submodule in the cut-off state is cut off; S2, if the DC bus voltage of the converter valve continues to rise and is greater than or equal to the second overvoltage threshold, the converter valve is locked and all overvoltage suppression branches of all overvoltage suppression submodules of the third-phase unit are input. The application solves the technical problem of high cost of the prior art flexible HVDC converter valve.

Description

Technical Field

[0001] This invention belongs to the field of converter valve technology, specifically relating to a flexible DC converter valve and its control method and device. Background Technology

[0002] Flexible DC transmission technology, due to its advantages such as low transmission loss over long distances, ease of fault isolation, and independent control of active and reactive power, has become a promising transmission method for offshore wind power DC transmission. However, when flexible DC transmission is used to connect isolated power generation systems to the receiving-end AC grid, a fault in the receiving-end AC grid can lead to a large surplus of power in the DC system, causing severe overvoltage and jeopardizing system safety. Existing solutions, such as AC energy dissipation devices, significantly increase the construction cost of offshore platforms and are unsuitable for offshore flexible DC transmission systems; centralized energy dissipation devices cannot maintain stable DC voltage during the commissioning phase; and traditional distributed energy dissipation devices require energy dissipation branches for all power modules of the converter valve, increasing the cost of the converter valve to some extent. Summary of the Invention

[0003] The purpose of this invention is to provide a flexible DC converter valve and its control method and device to solve the technical problem of high cost of existing flexible DC converter valves.

[0004] To address the aforementioned technical problems, the first aspect of this invention provides a flexible DC converter valve: the converter valve comprises a first phase unit, a second phase unit, and a third phase unit connected in parallel across both ends of a DC bus; the third phase unit is mainly composed of cascaded overvoltage suppression submodules with overvoltage suppression branches, and the first and second phase units are mainly composed of cascaded ordinary submodules without overvoltage suppression branches. The overvoltage suppression branch is connected in parallel across the capacitor of the submodule, and the overvoltage suppression branch includes a series-connected energy absorption element and a switching element for controlling the switching of the energy absorption element.

[0005] In one possible implementation, the energy-absorbing element is an energy-consuming resistor, a surge arrester, an energy storage battery, or a supercapacitor.

[0006] In one possible implementation, the switching element is an IGBT or an IGCT, and the overvoltage suppression branch is switched on and off by turning the IGBT or IGCT on and off.

[0007] In one possible implementation, the general submodule is a full-bridge submodule or a half-bridge submodule; the overvoltage suppression branch is a full-bridge submodule or a half-bridge submodule.

[0008] To solve the above-mentioned technical problems, a second aspect of the present invention provides a control method for a flexible DC converter valve, the method comprising: S1. If the DC bus voltage of the converter valve is greater than or equal to the first overvoltage threshold but less than the second overvoltage threshold, then the overvoltage suppression branch of the overvoltage suppression submodule in the third phase unit that is in the activated state will be activated, and the overvoltage suppression branch of the overvoltage suppression submodule that is in the deactivated state will be deactivated. S2. If the DC bus voltage of the converter valve continues to rise and is greater than or equal to the second overvoltage threshold, then the converter valve is locked and all overvoltage suppression branches of the overvoltage suppression submodules of the third phase unit are put into operation. The switching valve is a flexible-direction switching valve in any possible implementation of the first aspect of the present invention.

[0009] In one possible implementation, the method further includes: S3, if the DC bus voltage of the converter valve continues to rise and is greater than or equal to the trip voltage threshold, then the converter valve requests to trip; the trip voltage threshold is greater than a second overvoltage threshold.

[0010] In one possible implementation, the method further includes: if the DC bus voltage of the converter valve drops below the voltage control target value and remains so for a set duration, then all overvoltage suppression branches of all overvoltage suppression submodules of the third phase unit are disconnected; the voltage control target value is less than a first overvoltage threshold.

[0011] In one possible implementation, in S1, if the bridge arm current is in the charging direction, then the n overvoltage suppression submodules with lower voltage in the third phase unit are activated, and the n ordinary submodules with lower voltage in the first and second phase units are also activated.

[0012] In one possible implementation, in S1, if the bridge arm current is in the discharge direction, then the n overvoltage suppression submodules with higher voltage in the third phase unit are put into operation, and the n ordinary submodules with higher voltage in the first and second phase units are also put into operation.

[0013] To address the aforementioned technical problems, a third aspect of the present invention provides a control device for a flexible DC converter valve, comprising a processor for executing a computer program to implement the steps of the method in any possible implementation of the second aspect of the present invention.

[0014] The beneficial effects of this invention are: This invention fully utilizes the surplus power absorption capacity of the third phase unit, which can effectively reduce the overvoltage level of the converter valve; it can eliminate the need for DC power consumption devices in the converter valve system, reducing the initial investment in the converter valve equipment, lowering system costs, and improving the economic efficiency of the converter valve. This invention solves the technical problem of high cost in existing flexible DC converter valves. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the converter valve topology in an embodiment of the control method for the flexible DC converter valve of the present invention; Figure 2 This is a schematic diagram of the overvoltage suppression branch topology in an embodiment of the control method for the flexible DC converter valve of the present invention; Figure 3 This is a flowchart illustrating the control process of the converter valve in the embodiment of the control method for the flexible DC converter valve of the present invention. Figure 4 This is a control flowchart of the overvoltage suppression strategy in the implementation of the control method for the flexible DC converter valve of the present invention. Figure 5 This is a control flowchart of the overvoltage suppression strategy two in the implementation of the control method for the flexible DC converter valve of the present invention; Figure 6 This diagram illustrates the DC bus voltage suppression effect in an embodiment of the control method for the flexible DC converter valve of the present invention. Figure 7 This is a schematic diagram of the device architecture in an embodiment of the control device for the flexible DC converter valve of the present invention. Detailed Implementation

[0016] This invention fully utilizes the surplus power absorption capacity of the third phase unit to effectively reduce the overvoltage level of the converter valve, eliminating the need for DC power dissipation devices in the converter valve system, reducing initial investment in the converter valve equipment, lowering system costs, and improving the economic efficiency of the converter valve. This invention solves the technical problem of high cost in existing flexible DC converter valves.

[0017] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be further described in detail below with reference to the accompanying drawings.

[0018] Implementation method of control method for flexible DC converter valve: This invention provides a converter valve with overvoltage suppression capability and a control method thereof. Based on this topology, the first and second phase units of the flexible DC converter valve are constructed by cascading conventional flexible DC submodules, and the third phase unit is constructed by cascading flexible DC submodules with overvoltage suppression capability. The third phase unit has two overvoltage suppression operating modes based on two overvoltage thresholds of the bus voltage, limiting DC bus overvoltage. Applying the converter valve topology and control method with overvoltage suppression capability provided in this invention to an offshore wind power transmission system, by fully utilizing the surplus power absorption capacity of the third phase unit, the overvoltage level of the converter valve can be effectively reduced, eliminating the need for DC energy-consuming devices, reducing the initial investment in the converter valve equipment, lowering the system cost, and improving the economic efficiency of the converter valve while maintaining overvoltage suppression capability.

[0019] This embodiment provides a converter valve topology (i.e., a flexible DC converter valve) with overvoltage suppression capability, such as... Figure 1As shown, it consists of a first phase unit, a second phase unit, and a third phase unit connected in parallel. One end of each phase unit is connected to the positive bus, and the other end is connected to the negative bus. The first and second phase units are composed of several conventional flexible DC converter valve full-bridge sub-modules and half-bridge sub-modules (i.e., Figure 1 The third phase unit consists of several flexible DC converter valve full-bridge sub-modules and half-bridge sub-modules (i.e., SM) cascaded together; the third phase unit consists of several flexible DC converter valve full-bridge sub-modules and half-bridge sub-modules with overvoltage suppression capability. Figure 1 The ESM (Electronic Module for Converter) in the diagram is constructed by cascading. The flexible DC converter valve submodule (ESM) with overvoltage suppression capability is composed of a conventional flexible DC submodule (SM) and an overvoltage suppression branch, with the overvoltage suppression branch connected in parallel across the submodule capacitor.

[0020] Overvoltage suppression branch topology such as Figure 2 As shown, it consists of a fully controlled power electronic switch (i.e., switching element) and an energy absorption element connected in series. The fully controlled power electronic switch can be an IGBT or an IGCT, and the energy absorption element can be a power-consuming resistor, a surge arrester, an energy storage battery, or a supercapacitor. By controlling the on and off states of the fully controlled power electronic switch, the switching on and off states of the power-consuming branch can be achieved.

[0021] The original three-phase unit's energy self-balancing flexible DC converter valve patent, which included energy-dissipating branches (i.e., overvoltage suppression branches), controlled the activation and deactivation of these branches by monitoring the capacitor voltage of each submodule. When a submodule's capacitor voltage exceeded a threshold, its energy-dissipating resistor was activated. Since the faulty phase could be one or more of the three phases, different fault points resulted in different bridge arms experiencing voltage overshoot. The original solution monitored the submodule voltages of all six bridge arms in real time and activated the corresponding leakage resistors based on the voltage magnitude. This implementation, however, only has a single-phase unit's submodule equipped with an energy-dissipating branch. It controls the activation and deactivation of the leakage resistors by monitoring the DC bus voltage, without controlling the voltage magnitude of each submodule's capacitor. Regardless of which phase or phases of the AC system on the valve side are affected by the fault, after the fault occurs, the capacitors of some bridge arms of the converter valve will be charged, resulting in an increase in the DC bus voltage on the system's DC side. At this point, ensuring that the converter valve bridge arm submodules are not charged to the overvoltage protection setting is sufficient to achieve fault ride-through. The method involves strategically controlling the switching of energy-dissipating resistors in phase units with energy-dissipating branches, thereby stabilizing the DC bus voltage within a certain range. When a receiving-end AC fault occurs in the flexible DC system, causing the DC bus voltage to rise to the overvoltage suppression branch's activation threshold, the activation and deactivation status of the third phase unit submodule's overvoltage suppression branch is controlled based on the DC bus overvoltage level and the submodule's operating status. This absorbs surplus power from the DC system, suppresses DC bus overvoltage, and assists the flexible DC system in completing AC fault ride-through.

[0022] Specifically, the flowchart of the converter valve overvoltage suppression method in this embodiment is as follows: Figure 3As shown, when a receiving-end AC fault occurs in the flexible DC system, causing the DC bus voltage to rise to the overvoltage suppression branch action threshold, the activation and deactivation status of the overvoltage suppression branch of the third phase unit submodule is controlled according to the DC bus overvoltage level and the submodule operating status. This absorbs the surplus power of the DC system, suppresses the DC bus overvoltage, and assists the flexible DC system in completing AC fault ride-through.

[0023] The specific implementation process is as follows: When the converter valve is in steady state operation, the overvoltage suppression branch of the third phase unit submodule is disconnected, and the control system sets the bus level 1 overvoltage threshold U. b1 (i.e., the first overvoltage threshold), bus level 2 overvoltage threshold U b2 (i.e., the second overvoltage threshold), bus trip overvoltage threshold U u (i.e., trip voltage threshold) and bus voltage control target value U q And monitor the bus voltage value U in real time. t Different strategies are implemented based on the DC bus voltage and the overvoltage threshold; when the DC bus voltage rises to the level 1 overvoltage threshold U... b1 And less than the busbar level 2 overvoltage threshold U b2 (i.e. U b1 ≤U t <U b2 When the DC bus voltage drops to the control target value U, the converter valve remains unlocked, and overvoltage suppression strategy one is executed; q If the overvoltage suppression of the converter valve is successful, then the DC bus voltage continues to rise to the bus level 2 overvoltage threshold U. b2 (i.e. U t ≥U b2 The converter valve is locked, and overvoltage suppression strategy two is executed.

[0024] like Figure 4 The diagram shows the implementation flowchart of overvoltage suppression strategy one. The specific implementation process is as follows: The converter valve remains unlocked. If the bridge arm current is in the charging direction, the first and second phase units' bridge arms respectively activate the n sub-modules with lower voltages. The third phase unit's bridge arm, in addition to activating the n sub-modules with lower voltages, activates the overvoltage suppression branch of the activated sub-modules. If the bridge arm current is in the discharging direction, the first and second phase units' bridge arms respectively activate the n sub-modules with higher voltages. The third phase unit's bridge arm, in addition to activating the n sub-modules with higher voltages, activates the overvoltage suppression branch of the activated sub-modules. The overvoltage suppression branch of the activated sub-modules in the third phase unit absorbs surplus power, limiting the DC bus voltage. If the DC bus voltage drops to the control target value U... q If this continues for a certain period of time, the converter valve overvoltage suppression is successful, and the overvoltage suppression branch of the third phase unit submodule is disconnected; if the DC bus voltage continues to rise to the bus level 2 overvoltage threshold U... b2The converter valve is locked, and overvoltage suppression strategy two is executed. That is, the overvoltage suppression branch of the submodule in the third phase unit in the activated state is activated, and the overvoltage suppression branch of the submodule in the deactivated state is deactivated.

[0025] like Figure 5 The diagram shown is a flowchart of the implementation of overvoltage suppression strategy two. The specific implementation process is as follows: the sixth bridge arm submodule of the converter valve is locked; the overvoltage suppression branches of all submodules in the third phase unit are activated, limiting the DC bus voltage; the overvoltage suppression branches of the submodules in the third phase unit that are activated absorb surplus power, limiting the DC bus voltage; if the DC bus voltage drops to the control target value U... q If the overvoltage suppression of the converter valve is successful and continues for a certain period of time, the overvoltage suppression branch of the third phase unit submodule will be disconnected, the converter valve will remain locked, and it will wait for the next instruction from the system; if the DC bus voltage continues to rise to the system trip voltage U, the overvoltage suppression will be successful. u The converter valve requests a trip, and the system shuts down. This means the overvoltage suppression branches of all submodules in the third phase unit are activated.

[0026] Using two overvoltage suppression methods, strategy one and strategy two, to reduce the DC bus voltage to the control target value U q The fault ride-through is successful. If the DC bus voltage rises to the trip overvoltage threshold U... u If so, the fault crossing will fail.

[0027] The DC bus voltage suppression effect diagram according to the method provided by the present invention is shown in the figure below. Figure 6 As shown in the figure. It can be seen that, according to the method provided by the present invention, after an AC fault occurs at the receiving end at 1.5s, the DC bus voltage is limited to within 1120kV. The overvoltage suppression strategy 2 effectively limits the DC bus overvoltage until the fault is cleared at 2s.

[0028] Implementation method of control device for flexible DC converter valve: A schematic diagram of the control device for the flexible DC converter valve in this embodiment is shown below. Figure 7 As shown, the system includes a memory, a processor, a system bus, and a computer program stored in the memory. The processor and the memory communicate and exchange data via the system bus. The processor executes the computer program to implement the steps of the flexible DC-DC converter valve control method described in the embodiments of the present invention. The specific control method for the flexible DC-DC converter valve has been described in sufficient detail in the above embodiments and will not be repeated here. The processor can be a microprocessor (MCU) or other processing device; the memory can be any type of memory that stores information using electrical energy, such as non-volatile storage media (including computer programs, databases), or other types of memory.

[0029] Implementation method of flexible DC converter valve: A flexible DC converter valve comprises a first-phase unit, a second-phase unit, and a third-phase unit connected in parallel across a DC bus. The first and second-phase units are mainly composed of interconnected ordinary sub-modules, while the third-phase unit is mainly composed of interconnected overvoltage suppression sub-modules. Each overvoltage suppression sub-module includes an ordinary sub-module and an overvoltage suppression branch connected in parallel across the capacitor of the ordinary sub-module. The overvoltage suppression branch includes an energy absorption element connected in series and a switching element for controlling the switching of the energy absorption element. The energy absorption element can be a power-dissipating resistor, surge arrester, energy storage battery, or supercapacitor. The switching element is an IGBT or IGCT, and the overvoltage suppression branch is switched on and off by the conduction and cutoff of the IGBT or IGCT. The ordinary sub-module is either a full-bridge sub-module or a half-bridge sub-module.

[0030] The specific topology of the flexible DC converter valve is the same as that of the converter valve described in the implementation method of the flexible DC converter valve control method. The topology of the flexible DC converter valve has been described in sufficient detail in the above implementation method of the flexible DC converter valve control method, and will not be repeated here.

[0031] This invention has the following characteristics: This invention provides a converter valve and control method with overvoltage suppression capability. The first and second phase units of the flexible DC converter valve based on this topology are constructed by cascading conventional flexible DC submodules, while the third phase unit is constructed by cascading flexible DC submodules with overvoltage suppression capability. The third phase unit has two overvoltage suppression operating modes based on two overvoltage thresholds of the bus voltage, limiting DC bus overvoltage. Advantages of this invention: By fully utilizing the surplus power absorption capability of the third phase unit, the overvoltage level of the converter valve can be effectively reduced, eliminating the need for DC energy-consuming devices, reducing the initial investment in the converter valve equipment, lowering system costs, and improving the economic efficiency of the converter valve while maintaining overvoltage suppression capability.

[0032] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still make modifications to the technical solutions described in the foregoing embodiments without creative effort, or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A flexible direct current converter valve, characterized in that, The converter valve consists of a first phase unit, a second phase unit, and a third phase unit connected in parallel across the DC bus. The third phase unit is mainly composed of cascaded overvoltage suppression submodules with overvoltage suppression branches. The first and second phase units are mainly composed of cascaded ordinary submodules without overvoltage suppression branches. The overvoltage suppression branch is connected in parallel across the capacitor of the submodule. The overvoltage suppression branch includes energy absorption elements connected in series and switching elements for controlling the switching of the energy absorption elements.

2. The flexible DC converter valve according to claim 1, characterized in that, The energy absorption element is an energy-consuming resistor, a surge arrester, an energy storage battery, or a supercapacitor.

3. The flexible DC converter valve according to claim 1, characterized in that, The switching element is an IGBT or an IGCT, and the overvoltage suppression branch is switched on and off by turning on and off the IGBT or IGCT.

4. The flexible DC converter valve according to any one of claims 1 to 3, characterized in that, The ordinary submodule is a full-bridge submodule or a half-bridge submodule; the overvoltage suppression branch is a full-bridge submodule or a half-bridge submodule.

5. A control method for a flexible DC converter valve, characterized in that, The method includes: S1. If the DC bus voltage of the converter valve is greater than or equal to the first overvoltage threshold but less than the second overvoltage threshold, then the overvoltage suppression branch of the overvoltage suppression submodule in the third phase unit that is in the activated state will be activated, and the overvoltage suppression branch of the overvoltage suppression submodule that is in the deactivated state will be deactivated. S2. If the DC bus voltage of the converter valve continues to rise and is greater than or equal to the second overvoltage threshold, then the converter valve is locked and all overvoltage suppression branches of the overvoltage suppression submodules of the third phase unit are put into operation. The converter valve is the flexible DC converter valve as described in any one of claims 1 to 4.

6. The control method for the flexible DC converter valve according to claim 5, characterized in that, The method further includes: S3, if the DC bus voltage of the converter valve continues to rise and is greater than or equal to the trip voltage threshold, the converter valve requests to trip; the trip voltage threshold is greater than the second overvoltage threshold.

7. The control method for the flexible DC converter valve according to claim 5 or 6, characterized in that, The method further includes: if the DC bus voltage of the converter valve drops below the voltage control target value and continues for a set duration, then all overvoltage suppression branches of all overvoltage suppression submodules of the third phase unit are cut off; the voltage control target value is less than the first overvoltage threshold.

8. The control method for the flexible DC converter valve according to claim 5, characterized in that, In S1, if the bridge arm current is in the charging direction, then the n overvoltage suppression submodules with lower voltage in the third phase unit will be activated, and the n ordinary submodules with lower voltage in the first and second phase units will also be activated.

9. The control method for the flexible DC converter valve according to claim 5 or 8, characterized in that, In S1, if the bridge arm current is in the discharge direction, then the n overvoltage suppression submodules with higher voltage in the third phase unit will be put into operation, and the n ordinary submodules with higher voltage in the first and second phase units will also be put into operation.

10. A control device for a flexible DC converter valve, comprising a processor, characterized in that, The processor is used to execute a computer program to implement the steps of the control method for the flexible DC converter valve as described in any one of claims 5 to 9.

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