Energy dissipation method and system for offshore wind power flexible dc grid-connected power transmission system

Abstract

The application provides an energy consumption method and system for offshore wind power flexible DC grid-connected power transmission system, and the method comprises the following steps: when a fault occurs at the inverter side of the MMC DC power transmission system, the unloading circuit and the DC energy consumption device are cooperated to consume energy. Preferably, the unloading circuit is an unloading circuit inside the wind power converter, the DC energy consumption device is a DC energy consumption device inside the MMC DC power transmission system, and the unloading circuit and the DC energy consumption device are cooperated to operate and jointly consume the accumulated unbalanced power on the submarine DC cable, so that the requirement for the energy consumption power and capacity of the DC energy consumption device is reduced, the cost is appropriately reduced, the unification of the economy and feasibility of the scheme is realized, and the like.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of offshore wind power flexible DC grid-connected power transmission system, and particularly relates to an energy consumption method and system for an offshore wind power flexible DC grid-connected power transmission system. BACKGROUND

[0002] In the technical scheme of large-capacity long-distance offshore wind power merging into a power grid, the centralized MMC (Modular Multilevel Converter) converter platform flexible DC power transmission mode is a mainstream scheme, such as the centralized MMC converter platform flexible DC power transmission mode shown in FIG. 1. Figure 1 This power transmission system is called an offshore wind power flexible DC grid-connected power transmission system in the art.

[0003] As shown in FIG. 2, in the offshore wind power flexible DC grid-connected power transmission system, there are two types of energy consumption devices: an unloading circuit inside the offshore wind power converter and a DC energy consumption device matched with the MMC DC power transmission system. Figure 1

[0004] When a fault occurs at the inverter side of the MMC DC power transmission system, the grid-side voltage decreases, the energy output by the onshore inverter station decreases, but the energy input by the offshore rectifier station does not change, and the unbalanced energy is accumulated on the submarine DC cable, resulting in an increase in the DC cable voltage. The existing method in the engineering mainly uses the DC energy consumption device in FIG. 3 to consume the unbalanced power on the submarine DC cable. The existing method has a high requirement for the power and capacity of the DC energy consumption device, and therefore has a high cost. Figure 1 SUMMARY

[0005] In order to overcome the above problems, the application provides an energy consumption method and system for an offshore wind power flexible DC grid-connected power transmission system.

[0006] The application is implemented by the following technical scheme:

[0007] The application provides an energy consumption method for an offshore wind power flexible DC grid-connected power transmission system, including the following steps:

[0008] When a fault occurs at the inverter side of the MMC DC power transmission system,

[0009] The unloading circuit and the DC energy consumption device are cooperatively used for energy consumption.

[0010] Further, the unloading circuit is an unloading circuit inside the wind power converter.

[0011] Further, the DC energy consumption device is a DC energy consumption device inside the MMC DC power transmission system.

[0012] ​​Further, the unloading circuit and the DC energy consumption device are cooperated to consume the accumulated unbalanced energy, specifically including:

[0013] The unloading circuit and the DC energy consumption device are cooperated to consume the accumulated unbalanced energy on the DC cable.

[0014] Further, the DC cable includes a submarine DC cable.

[0015] Further, the unloading circuit and the DC energy consumption device are cooperated to consume the accumulated unbalanced energy on the DC cable, specifically including:

[0016] The unloading circuit and the DC energy consumption device are cooperated to consume the accumulated unbalanced energy on the DC cable.

[0017] Further, the unloading circuit and the DC energy consumption device are cooperated to consume the accumulated unbalanced energy on the DC cable, specifically including:

[0018] First, the DC energy consumption device is used to consume the accumulated unbalanced energy on the DC cable, and then the unloading circuit is used to consume the accumulated unbalanced energy on the DC cable according to the need.

[0019] Further, the unloading circuit and the DC energy consumption device are cooperated to consume the accumulated unbalanced energy on the DC cable, specifically including:

[0020] The DC energy consumption device is used to consume the accumulated unbalanced energy on the DC cable.

[0021] According to the unbalanced energy on the DC cable after the DC energy consumption device is consumed, it is determined whether the unloading circuit needs to be used for the energy consumption operation of the DC cable.

[0022] If the unloading circuit needs to be used for the energy consumption operation of the DC cable, the unloading circuit is used to consume the remaining accumulated unbalanced energy on the DC cable.

[0023] Further, the DC energy consumption device is used to consume the accumulated unbalanced energy on the DC cable, specifically including:

[0024] It is determined whether the first voltage of the DC cable is greater than the upper limit of the first hysteresis control voltage.

[0025] If the first voltage of the DC cable is greater than the upper limit of the first hysteresis control voltage, the DC energy consumption device is used to consume the accumulated unbalanced energy on the DC cable.

[0026] Further, the method for judging whether the unbalanced energy on the DC cable after the DC energy consumption device consumes needs to adopt the unloading circuit to perform the energy consumption operation of the DC cable, specifically comprising:

[0027] judging whether the second voltage of the DC cable is less than the lower limit of the first hysteresis control voltage;

[0028] if the second voltage of the DC cable is not less than the lower limit of the first hysteresis control voltage, timing the time when the second voltage of the DC cable is not less than the lower limit of the first hysteresis control voltage;

[0029] if the timing time is greater than the set time, determining that the unloading circuit needs to be adopted to perform the energy consumption operation of the DC cable.

[0030] Further, if the unloading circuit needs to participate in the energy consumption operation of the DC cable, the unloading circuit is adopted to consume the remaining unbalanced energy accumulated on the DC cable, specifically comprising:

[0031] adopting the unloading circuit to consume the remaining unbalanced energy accumulated on the DC cable;

[0032] judging whether the third voltage of the DC cable is less than the lower limit of the first hysteresis control voltage;

[0033] if the third voltage of the DC cable is less than the lower limit of the first hysteresis control voltage, cutting off the energy consumption operation of the DC cable by the unloading circuit, and continuing to judge whether the fourth voltage of the DC cable is greater than the upper limit of the first hysteresis control voltage;

[0034] if the fourth voltage of the DC cable is greater than the upper limit of the first hysteresis control voltage, re-adopting the unloading circuit to perform the energy consumption operation of the DC cable.

[0035] Further, the method further comprises the following steps:

[0036] when the grid connection point of the wind power converter fails, the unloading circuit inside the wind power converter performs the energy consumption operation.

[0037] Further, the unloading circuit inside the wind power converter performs the energy consumption operation, specifically comprising:

[0038] adopting the unloading circuit inside the wind power converter to consume the unbalanced energy accumulated on the DC bus.

[0039] Further, the unloading circuit inside the wind power converter consumes the unbalanced energy accumulated on the DC bus, specifically comprising:

[0040] judging whether the first voltage of the DC bus is greater than the upper limit of the second hysteresis control voltage;

[0041] If the first voltage of the DC bus is greater than the upper limit of the second hysteresis control voltage, the unloading circuit inside the wind power converter is used to consume the unbalanced energy accumulated on the DC bus, and it is further determined whether the second voltage of the DC bus is less than the lower limit of the second hysteresis control voltage.

[0042] If the second voltage of the DC bus is less than the lower limit of the second hysteresis control voltage, then the power consumption operation of the unloading circuit on the DC bus is cut off.

[0043] Correspondingly, the present invention provides an energy dissipation system for a flexible DC grid-connected transmission system for offshore wind power, including an unloading circuit and a DC energy dissipation device;

[0044] When a fault occurs on the inverter side of the MMC DC transmission system

[0045] Energy is consumed by the coordinated operation of the unloading circuit and the DC energy dissipation device.

[0046] Furthermore, the unloading circuit is an internal unloading circuit of the wind power converter;

[0047] The DC energy dissipation device is a DC energy dissipation device within the MMC DC transmission system.

[0048] Furthermore, the method of using an unloading circuit and a DC power dissipation device in tandem to dissipate energy specifically includes:

[0049] The unloading circuit and DC energy dissipation device work together to consume the unbalanced energy accumulated on the DC cable.

[0050] The unbalanced energy accumulated on the DC cable is consumed through a combination of unloading circuits and DC energy dissipation devices, including:

[0051] First, a DC energy dissipation device is used to consume the unbalanced energy accumulated on the DC cable. Then, as needed, an unloading circuit is used to consume the unbalanced energy accumulated on the DC cable.

[0052] Furthermore, the process of first using a DC energy-dissipating device to consume the unbalanced energy accumulated on the DC cable, and then using an unloading circuit to consume the unbalanced energy accumulated on the DC cable, specifically includes:

[0053] A DC energy-consuming device is used to dissipate the unbalanced energy accumulated on the DC cable;

[0054] Determine whether an unloading circuit is needed to perform energy dissipation on the DC cable based on the unbalanced energy on the DC cable after the DC energy dissipation device has consumed it.

[0055] If an unloading circuit is required to participate in the energy dissipation operation of the DC cable, then the unloading circuit is used to consume the remaining unbalanced energy accumulated on the DC cable.

[0056] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0057] The energy dissipation method for offshore wind power flexible DC grid-connected transmission systems provided by this invention dissipates energy in a coordinated manner using an unloading circuit and a DC energy dissipation device when a fault occurs on the inverter side of the MMC DC transmission system. Preferably, the unloading circuit is an unloading circuit inside the wind power converter, and the DC energy dissipation device is a DC energy dissipation device within the MMC DC transmission system. The unloading circuit and the DC energy dissipation device operate in coordination to jointly consume the unbalanced power accumulated on the submarine DC cable, reducing the requirements for the energy dissipation power and capacity of the DC energy dissipation device, appropriately reducing costs, and achieving a balance between the economic efficiency and feasibility of the solution. Attached Figure Description

[0058] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0059] Figure 1 This is a schematic diagram of the existing offshore wind power flexible DC grid-connected transmission system.

[0060] Figure 2 The diagram shows the voltage comparison of submarine DC cables under the following conditions: only the DC power dissipation device is activated, the unloading circuit and the DC power dissipation device are activated simultaneously, and neither the unloading circuit nor the DC power dissipation device is activated.

[0061] Figure 3 This is a schematic diagram of the voltage of a submarine DC cable when the DC power dissipation device is activated first, and then the unloading circuit is activated, as described in an embodiment of the present invention.

[0062] Figure 4 This is a schematic diagram summarizing the energy consumption method for a flexible DC grid-connected power transmission system for offshore wind power, according to an embodiment of the present invention.

[0063] Figure 5 This is a detailed flowchart illustrating the energy consumption method for a flexible DC grid-connected power transmission system for offshore wind power, according to an embodiment of the present invention. Detailed Implementation

[0064] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0065] In this document, the terms "first," "second," and other similar words are not intended to imply any order, quantity, or importance, but are merely used to distinguish different elements. The terms "one," "a," and other similar words are not intended to indicate the existence of only one of the stated things, but rather that the description refers only to one of the stated things, which may have one or more. The terms "comprising," "including," and other similar words are intended to indicate a logical relationship, not a spatial relationship. For example, "A includes B" means that logically B belongs to A, not that spatially B is located inside A. Furthermore, the meanings of the terms "comprising," "including," and other similar words should be considered open-ended, not closed. For example, "A includes B" means that B belongs to A, but B does not necessarily constitute all of A; A may also include other elements such as C, D, and E.

[0066] In this document, the terms "embodiment," "this embodiment," "an embodiment," and "one embodiment" do not imply that the description applies only to one specific embodiment, but rather that such description may also be applicable to one or more other embodiments. Those skilled in the art will understand that any description made herein with respect to one embodiment can be substituted, combined, or otherwise combined with the descriptions in one or more other embodiments. Such substitutions, combinations, or other combinations resulting in new embodiments are readily conceived by those skilled in the art and fall within the scope of protection of this invention.

[0067] like Figure 1 As shown, the existing offshore wind power flexible DC grid-connected transmission system includes a wind turbine, a permanent magnet synchronous generator (PMSG), a wind power converter, a transformer, an AC collection bus, an MMC DC transmission system, and an AC power grid.

[0068] The wind power converter includes a grid-side converter, a machine-side converter, a DC bus, and an unloading circuit. The DC bus and the unloading circuit are located between the machine-side converter and the grid-side converter.

[0069] The MMC DC transmission system includes an offshore rectifier station, an onshore inverter station, a submarine DC cable, and DC power dissipation devices, with the submarine DC cable and DC power dissipation devices located between the offshore rectifier station and the onshore inverter station.

[0070] When a fault occurs at the grid connection point of the wind power converter, the voltage at the grid connection point drops, and the power transmitted by the wind power converter to the grid side decreases. However, the power transmitted by the wind turbine to the wind power converter to the generator side remains unchanged. As a result, the power accumulates on the DC bus of the wind power converter, causing the DC bus voltage to rise. Therefore, an unloading circuit is used to dissipate energy on the DC bus.

[0071] When a fault occurs on the inverter side of the MMC DC transmission system, the grid-side voltage decreases, reducing the energy output of the onshore inverter station. However, the energy input to the offshore rectifier station remains unchanged. This imbalance energy accumulates on the submarine DC cable, causing the DC cable voltage to rise. Current engineering methods utilize DC power dissipation devices to consume the imbalance power on the submarine DC cable. However, these devices require high power and capacity, resulting in high costs.

[0072] Based on the aforementioned problems faced by existing offshore wind power flexible DC grid-connected transmission systems, this invention provides an energy consumption method for offshore wind power flexible DC grid-connected transmission systems, the overall concept of which is as follows:

[0073] When a fault occurs on the inverter side of the MMC DC transmission system, energy is consumed by a combination of unloading circuit and DC energy dissipation device.

[0074] Specifically, the energy consumption method using the unloading circuit and DC energy dissipation device in a coordinated manner includes: using the unloading circuit and DC energy dissipation device in a coordinated manner to jointly consume the unbalanced energy accumulated on the DC cable.

[0075] The unloading circuit and the DC power dissipation device work together to consume the unbalanced power accumulated on the DC cable, reducing the requirements for the power consumption and capacity of the DC power dissipation device, appropriately reducing costs, and achieving a balance between the economic efficiency and feasibility of the solution.

[0076] In a preferred embodiment, the unloading circuit includes, but is not limited to, the unloading circuit inside the wind power converter; the DC energy dissipation device includes, but is not limited to, the DC energy dissipation device in the MMC DC transmission system; and the DC cable includes, but is not limited to, the submarine DC cable.

[0077] However, in order to further reduce the energy consumption operation cost, the unloading circuit in this embodiment adopts the unloading circuit inside the existing wind power converter, and the DC energy consumption device adopts the DC energy consumption device in the existing MMC DC transmission system.

[0078] As a preferred embodiment, the solution of this embodiment is preferably applied to an offshore wind power flexible DC grid-connected transmission system that is equipped with both an unloading circuit and a DC energy dissipation device. This makes minimal changes to the offshore wind power flexible DC grid-connected transmission system and further reduces energy consumption costs.

[0079] Specifically, the above-mentioned unloading circuit and DC energy dissipation device work together to consume the unbalanced energy accumulated on the DC cable, and the following two coordination methods are adopted:

[0080] Method 1:

[0081] The unloading circuit and the DC energy dissipation device work together to consume the unbalanced energy accumulated on the DC cable.

[0082] Method 2:

[0083] First, a DC energy dissipation device is used to consume the unbalanced energy accumulated on the DC cable. Then, as needed, an unloading circuit is used to consume the unbalanced energy accumulated on the DC cable.

[0084] It should be noted that the above-mentioned method of simultaneously consuming the unloading circuit and DC energy dissipation device to dissipate the unbalanced energy accumulated on the DC cable requires a fault to occur on the inverter side of the MMC DC transmission system, and the simultaneous operation of the unloading circuit and DC energy dissipation device must be activated immediately after the DC cable voltage rises. This method is feasible in practical applications, but it has high communication requirements. Therefore, this embodiment will mainly describe method 2 in detail below:

[0085] S1 uses a DC power dissipation device to consume the unbalanced energy accumulated on the DC cable.

[0086] S2 determines whether an unloading circuit is needed to perform energy dissipation on the DC cable based on the unbalanced energy consumed by the DC energy dissipation device.

[0087] If S3 needs to use an unloading circuit to participate in the energy dissipation operation of the DC cable, then the unloading circuit is used to consume the remaining unbalanced energy accumulated on the DC cable.

[0088] S1 employs a DC energy-dissipating device to consume the unbalanced energy accumulated on the DC cable, specifically including:

[0089] Determine whether the first voltage of the DC cable is greater than the upper limit of the first hysteresis control voltage. Here, the first voltage refers to the voltage on the DC cable before the DC energy dissipation device is used.

[0090] If the first voltage of the DC cable is greater than the upper limit of the first hysteresis control voltage, a DC energy dissipation device is used to consume the unbalanced energy accumulated on the DC cable.

[0091] If the first voltage of the DC cable is not greater than the upper limit of the first hysteresis control voltage, then continue to determine whether the first voltage of the DC cable is greater than the upper limit of the first hysteresis control voltage.

[0092] S2 determines whether an unloading circuit is needed to dissipate energy in the DC cable based on the unbalanced energy consumed by the DC energy dissipation device. Specifically, this includes:

[0093] Determine whether the second voltage of the DC cable is less than the lower limit of the first hysteresis control voltage. Here, the second voltage refers to the DC cable voltage collected in real time after the unbalanced energy accumulated on the DC cable is consumed by the DC energy dissipation device.

[0094] If the second voltage of the DC cable is less than the lower limit of the first hysteresis control voltage, the DC power dissipation device is disconnected to perform power dissipation operation on the DC cable, and the unloading circuit is not required to perform power dissipation operation on the DC cable for the time being.

[0095] If the second voltage of the DC cable is not less than the lower limit of the first hysteresis control voltage, then the time during which the second voltage of the DC cable is not less than the lower limit of the first hysteresis control voltage is timed.

[0096] If the timing time is not greater than the set time, then continue to determine whether the second voltage of the DC cable is less than the lower limit of the first hysteresis control voltage. For the time being, it is not necessary to use the unloading circuit to perform the power dissipation operation of the DC cable.

[0097] If the timing time exceeds the set time, it is determined that an unloading circuit is needed to dissipate the power of the DC cable.

[0098] If S3 requires an unloading circuit to participate in the energy dissipation operation of the DC cable, then the unloading circuit is used to consume the remaining unbalanced energy accumulated on the DC cable, specifically including:

[0099] An unloading circuit is used to dissipate the remaining unbalanced energy accumulated on the DC cable.

[0100] Determine whether the third voltage of the DC cable is less than the lower limit of the first hysteresis control voltage. Here, the third voltage refers to the DC cable voltage collected in real time after the unloading circuit consumes the remaining unbalanced energy accumulated on the DC cable.

[0101] If the third voltage of the DC cable is not less than the lower limit of the first hysteresis control voltage, then continue to determine whether the third voltage of the DC cable is less than the lower limit of the first hysteresis control voltage.

[0102] If the third voltage of the DC cable is less than the lower limit of the first hysteresis control voltage, the energy dissipation operation of the unloading circuit on the DC cable is cut off, and it is further determined whether the fourth voltage of the DC cable is greater than the upper limit of the first hysteresis control voltage. Here, the fourth voltage refers to the DC cable voltage collected in real time after the energy dissipation operation of the unloading circuit on the DC cable is cut off.

[0103] If the fourth voltage of the DC cable is greater than the upper limit of the first hysteresis control voltage, the unloading circuit is used again to dissipate energy from the DC cable.

[0104] If the fourth voltage of the DC cable is not greater than the upper limit of the first hysteresis control voltage, then continue to determine whether the fourth voltage of the DC cable is greater than the upper limit of the first hysteresis control voltage.

[0105] As a preferred embodiment, the energy dissipation method for offshore wind power flexible DC grid-connected transmission systems further includes the following steps:

[0106] When a fault occurs at the grid connection point of the wind turbine converter, the unloading circuit inside the converter performs energy dissipation operations. Specifically, when a fault occurs at the grid connection point of the wind turbine generator, the grid connection point voltage drops, the power delivered to the grid side of the wind turbine converter decreases, while the power delivered from the wind turbine generator to the turbine side of the wind turbine converter remains unchanged. As a result, power accumulates on the DC bus of the wind turbine converter, causing the DC bus voltage to rise. In response, the unloading circuit inside the wind turbine converter dissipates the unbalanced energy accumulated on the DC bus.

[0107] More specifically, the unloading circuit inside the wind power converter is used to dissipate the unbalanced energy accumulated on the DC bus, including the following steps:

[0108] Determine whether the first voltage of the DC bus is greater than the upper limit of the second hysteresis control voltage. Here, the first voltage is the voltage of the DC bus before the energy dissipation operation is performed using the unloading circuit inside the wind power converter.

[0109] If the first voltage of the DC bus is not greater than the upper limit of the second hysteresis control voltage, then continue to determine whether the first voltage of the DC bus is greater than the upper limit of the second hysteresis control voltage.

[0110] If the first voltage of the DC bus is greater than the upper limit of the second hysteresis control voltage, the unloading circuit inside the wind power converter is used to consume the unbalanced energy accumulated on the DC bus, and it is further determined whether the second voltage of the DC bus is less than the lower limit of the second hysteresis control voltage. The second voltage here is the DC bus voltage collected in real time after the unloading circuit inside the wind power converter consumes the unbalanced energy accumulated on the DC bus.

[0111] If the second voltage of the DC bus is less than the lower limit of the second hysteresis control voltage, then the power consumption operation of the unloading circuit on the DC bus is cut off.

[0112] If the second voltage of the DC bus is not less than the lower limit of the second hysteresis control voltage, then continue to determine whether the second voltage of the DC bus is less than the lower limit of the second hysteresis control voltage.

[0113] like Figure 2The figure shows a comparison of the submarine DC cable voltage under the following conditions: only the DC power consumption device is activated, the unloading circuit and the DC power consumption device are activated simultaneously, and neither the unloading circuit nor the DC power consumption device is activated.

[0114] When only the DC power dissipation device operates, its power consumption is 1000MW; when both the unloading circuit and the DC power dissipation device operate simultaneously, the power consumption is 500MW each. When neither the unloading circuit nor the DC power dissipation device operates, the submarine DC cable voltage rises rapidly. It is evident that the operation of either the DC power dissipation device alone or both the unloading circuit and the DC power dissipation device operate simultaneously can limit the submarine DC cable voltage between the upper and lower hysteresis limits, achieving good fault ride-through performance. However, using the simultaneous operation of the unloading circuit and the DC power dissipation device reduces the requirements for the power consumption and capacity of the DC power dissipation device, thereby reducing costs.

[0115] like Figure 3 As shown, this is a schematic diagram of the submarine DC cable voltage when the DC energy dissipation device is activated first and then the unloading circuit is activated in an embodiment of the present invention. At t=6s, a fault (three-phase grounding fault) occurs on the inverter side of the MMC DC transmission system, the DC energy dissipation device is activated, and the DC cable voltage rises slightly; at t=7s, the unloading circuit is activated, and the MMC submarine DC cable voltage oscillates between the upper and lower limits of the hysteresis control.

[0116] The scheme of adopting the DC energy-consuming device to act first and the unloading circuit to act later has moderate requirements for communication speed, reduces the requirements for the power consumption and capacity of the DC energy-consuming device, effectively reduces costs, and can achieve good control effect. It can achieve a balance between economy and feasibility and is suitable for offshore wind power flexible DC grid-connected transmission systems.

[0117] The following example illustrates a flexible DC grid-connected transmission system for offshore wind power, equipped with both unloading circuits and DC energy dissipation devices. It details a method where the unbalanced energy accumulated on the DC cables is first dissipated using the DC energy dissipation devices within the MMC DC transmission system, and then, as needed, the unloading circuit within the wind power converter is used to dissipate the accumulated unbalanced energy on the DC cables. Figure 4 As shown:

[0118] The P1 offshore wind power flexible DC grid-connected transmission system was successfully launched.

[0119] P2 allows the unloading circuit inside the wind power converter and the DC energy dissipation device in the MMC DC transmission system to operate.

[0120] P3-1 uses the unloading circuit inside the wind power converter for energy dissipation operation as needed, including:

[0121] When a fault occurs at the grid connection point of the wind power converter, the DC voltage bus rises, and it is determined whether the unbalanced energy on the DC bus needs to be consumed.

[0122] If it is necessary to consume the unbalanced energy on the DC bus, the wind power converter unloading circuit is activated for energy consumption.

[0123] If it is not necessary to consume the unbalanced energy on the DC bus, then disconnect the wind power converter unloading circuit for energy consumption operation.

[0124] More specifically, determine whether it is necessary to dissipate the unbalanced energy on the DC bus; if it is necessary, engage the wind turbine converter unloading circuit for energy dissipation; if it is not necessary, disconnect the wind turbine converter unloading circuit for energy dissipation. The specific steps include the following: Figure 5 As shown:

[0125] Determine whether the first voltage of the DC bus is greater than the upper limit of the second hysteresis control voltage.

[0126] If the first voltage of the DC bus is not greater than the upper limit of the second hysteresis control voltage, then continue to determine whether the first voltage of the DC bus is greater than the upper limit of the second hysteresis control voltage.

[0127] If the first voltage of the DC bus is greater than the upper limit of the second hysteresis control voltage, the unloading circuit inside the wind power converter will be activated to consume the unbalanced energy accumulated on the DC bus, and the second voltage of the DC bus will be determined to be less than the lower limit of the second hysteresis control voltage.

[0128] If the second voltage of the DC bus is less than the lower limit of the second hysteresis control voltage, the unloading circuit inside the wind power converter will cut off the energy consumption operation of the DC bus.

[0129] If the second voltage of the DC bus is not less than the lower limit of the second hysteresis control voltage, then continue to determine whether the second voltage of the DC bus is less than the lower limit of the second hysteresis control voltage.

[0130] P3-2 utilizes the DC energy dissipation devices within the MMC DC transmission system and / or the unloading circuits within the wind power converter for energy dissipation operations as needed, including:

[0131] When a fault occurs on the inverter side of the MMC DC transmission system, the voltage of the submarine DC cable increases, and it is determined whether the unbalanced energy on the submarine DC cable needs to be consumed.

[0132] If it is necessary to consume the unbalanced energy on the submarine DC cable, the DC energy dissipation device inside the MMC DC transmission system or the unloading circuit inside the wind power converter can be used to consume the unbalanced energy accumulated on the submarine DC cable.

[0133] If it is not necessary to consume the unbalanced energy on the submarine DC cable, then the DC energy consumption device inside the MMC DC transmission system and the unloading circuit inside the wind power converter can be disconnected for energy consumption operation.

[0134] More specifically, determine whether it is necessary to dissipate the unbalanced energy on the submarine DC cable. If it is necessary, activate the DC energy dissipation device within the MMC DC transmission system or the unloading circuit within the wind turbine converter to dissipate the unbalanced energy accumulated on the submarine DC cable. If it is not necessary to dissipate the unbalanced energy on the submarine DC cable, disconnect the DC energy dissipation device within the MMC DC transmission system and the unloading circuit within the wind turbine converter for energy dissipation operations. Specific operational steps include the following: Figure 5 As shown:

[0135] Determine whether the first voltage of the submarine DC cable is greater than the upper limit of the first hysteresis control voltage.

[0136] If the first voltage of the submarine DC cable is not greater than the upper limit of the first hysteresis control voltage, then continue to determine whether the first voltage of the submarine DC cable is greater than the upper limit of the first hysteresis control voltage.

[0137] If the first voltage of the submarine DC cable is greater than the upper limit of the first hysteresis control voltage, the DC energy dissipation device inside the MMC DC transmission system will be activated to consume the unbalanced energy accumulated on the DC cable, and the second voltage of the submarine DC cable will be determined to be less than the lower limit of the first hysteresis control voltage.

[0138] If the second voltage of the submarine DC cable is less than the lower limit of the first hysteresis control voltage, the DC energy dissipation device inside the MMC DC transmission system will be disconnected to perform the energy dissipation operation of the DC cable. For the time being, it is not necessary to use the unloading circuit inside the wind power converter to perform the energy dissipation operation of the DC cable.

[0139] If the second voltage of the submarine DC cable is not less than the lower limit of the first hysteresis control voltage, then the time during which the second voltage of the submarine DC cable is not less than the lower limit of the first hysteresis control voltage is timed.

[0140] If the timing time is not greater than the set time (the set time is shown as 1s in the figure), then continue to determine whether the second voltage of the submarine DC cable is less than the lower limit of the first hysteresis control voltage. For the time being, it is not necessary to use the unloading circuit inside the wind power converter to perform energy dissipation operation on the submarine DC cable.

[0141] If the timing time exceeds the set time, it is determined that the unloading circuit inside the wind power converter needs to be used to dissipate energy in the submarine DC cable. The unloading circuit inside the wind power converter is activated to consume the remaining unbalanced energy accumulated on the DC cable. Then, it is determined whether the third voltage of the submarine DC cable is less than the lower limit of the first hysteresis control voltage.

[0142] If the third voltage of the submarine DC cable is not less than the lower limit of the first hysteresis control voltage, then continue to determine whether the third voltage of the submarine DC cable is less than the lower limit of the first hysteresis control voltage.

[0143] If the third voltage of the submarine DC cable is less than the lower limit of the first hysteresis control voltage, the energy dissipation operation of the unloading circuit inside the wind power converter on the submarine DC cable is cut off, and it is further determined whether the fourth voltage of the submarine DC cable is greater than the upper limit of the first hysteresis control voltage.

[0144] If the fourth voltage of the submarine DC cable is greater than the upper limit of the first hysteresis control voltage, the unloading circuit inside the wind power converter will be used again to dissipate energy from the submarine DC cable.

[0145] If the fourth voltage of the submarine DC cable is not greater than the upper limit of the first hysteresis control voltage, then continue to determine whether the fourth voltage of the submarine DC cable is greater than the upper limit of the first hysteresis control voltage.

[0146] The first voltage of the aforementioned DC bus corresponds to U in the figure. DC11 The second voltage of the DC bus corresponds to U in the figure. DC12 The upper limit of the second hysteresis control voltage corresponds to U in the figure. DC1_max The lower limit of the second hysteresis control voltage corresponds to U in the figure. DC1_min .

[0147] The first voltage of the aforementioned submarine DC cable corresponds to U in the figure. DC21 The second voltage of the submarine DC cable corresponds to U in the figure. DC22 The third voltage of the submarine DC cable corresponds to U in the figure. DC23 The fourth voltage of the submarine DC cable corresponds to U in the figure. DC24 The upper limit of the first hysteresis control voltage corresponds to U in the figure. DC2_max The lower limit of the first hysteresis control voltage corresponds to U in the figure. DC2_min .

[0148] Corresponding to the above-mentioned energy dissipation method for offshore wind power flexible DC grid-connected transmission systems, this embodiment of the invention also provides an energy dissipation system for offshore wind power flexible DC grid-connected transmission systems, including an unloading circuit and a DC energy dissipation device.

[0149] When a fault occurs on the inverter side of the MMC DC transmission system, energy is consumed by a combination of unloading circuit and DC energy dissipation device.

[0150] In a preferred embodiment, the unloading circuit is an unloading circuit inside the wind power converter, and the DC energy dissipation device is a DC energy dissipation device in the MMC DC transmission system.

[0151] Specifically, energy is dissipated through a combination of unloading circuits and DC power dissipation devices, including the following two methods:

[0152] Method 1: Use a combination of unloading circuit and DC energy dissipation device to consume the unbalanced energy accumulated on the DC cable;

[0153] Method 2: First, use a DC energy dissipation device to consume the unbalanced energy accumulated on the DC cable, and then use an unloading circuit to consume the unbalanced energy accumulated on the DC cable as needed.

[0154] Method 2: First, a DC energy dissipation device is used to consume the unbalanced energy accumulated on the DC cable, and then an unloading circuit is used to consume the unbalanced energy accumulated on the DC cable. Specifically, this includes:

[0155] A DC energy-consuming device is used to dissipate the unbalanced energy accumulated on the DC cable;

[0156] Determine whether an unloading circuit is needed to perform energy dissipation on the DC cable based on the unbalanced energy on the DC cable after the DC energy dissipation device has consumed it.

[0157] If an unloading circuit is required to participate in the energy dissipation operation of the DC cable, then the unloading circuit is used to consume the remaining unbalanced energy accumulated on the DC cable.

[0158] Other specific operations are the same as the energy consumption methods described above, and will not be repeated here.

[0159] The above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still make modifications or equivalent substitutions to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention are within the protection scope of the claims of the present invention pending approval.

Claims

1. An energy dissipation method for offshore wind power flexible DC grid-connected transmission systems, characterized in that, Includes the following steps: When a fault occurs on the inverter side of the MMC DC transmission system The unbalanced energy accumulated on the DC cable is consumed through a combination of unloading circuits and DC energy dissipation devices. Specifically, this includes: The method of using a DC energy dissipation device to consume the unbalanced energy accumulated on a DC cable includes: determining whether the first voltage of the DC cable is greater than the upper limit of the first hysteresis control voltage; if the first voltage of the DC cable is greater than the upper limit of the first hysteresis control voltage, then using a DC energy dissipation device to consume the unbalanced energy accumulated on the DC cable; the first voltage is the voltage on the DC cable before using the DC energy dissipation device. The determination of whether an unloading circuit is needed to dissipate energy in the DC cable after the unbalanced energy on the DC cable is consumed by the DC energy dissipation device includes: determining whether the second voltage of the DC cable is less than the lower limit of the first hysteresis control voltage; if the second voltage of the DC cable is not less than the lower limit of the first hysteresis control voltage, timing the time during which the second voltage of the DC cable is not less than the lower limit of the first hysteresis control voltage; if the timing time is longer than the set time, it is determined that an unloading circuit is needed to dissipate energy in the DC cable; the second voltage is the DC cable voltage collected in real time after the unbalanced energy accumulated on the DC cable is consumed by the DC energy dissipation device. If an unloading circuit is required to participate in the energy dissipation operation of the DC cable, then the unloading circuit is used to consume the remaining unbalanced energy accumulated on the DC cable. The unloading circuit is the unloading circuit inside the wind power converter, and the DC energy dissipation device is the DC energy dissipation device in the MMC DC transmission system.

2. The energy dissipation method for a flexible DC grid-connected power transmission system for offshore wind power according to claim 1, wherein the DC cable includes a submarine DC cable.

3. The energy dissipation method for a flexible DC grid-connected transmission system for offshore wind power according to claim 1, characterized in that, The method of using unloading circuit and DC energy dissipation device to jointly consume the unbalanced energy accumulated on DC cable specifically includes: using unloading circuit and DC energy dissipation device to operate simultaneously to jointly consume the unbalanced energy accumulated on DC cable.

4. The energy dissipation method for a flexible DC grid-connected transmission system for offshore wind power according to claim 1, characterized in that, If an unloading circuit is required to participate in the energy dissipation operation of the DC cable, the unloading circuit is used to consume the remaining unbalanced energy accumulated on the DC cable. Specifically, this includes: using the unloading circuit to consume the remaining unbalanced energy accumulated on the DC cable; determining whether the third voltage of the DC cable is less than the lower limit of the first hysteresis control voltage; if the third voltage of the DC cable is less than the lower limit of the first hysteresis control voltage, the energy dissipation operation of the unloading circuit on the DC cable is cut off, and the fourth voltage of the DC cable is determined to be greater than the upper limit of the first hysteresis control voltage; if the fourth voltage of the DC cable is greater than the upper limit of the first hysteresis control voltage, the unloading circuit is used again to perform the energy dissipation operation on the DC cable.

5. The energy dissipation method for a flexible DC grid-connected transmission system for offshore wind power according to claim 1, characterized in that, It also includes the following steps: when a fault occurs at the grid connection point of the wind power converter, the unloading circuit inside the wind power converter performs energy dissipation operation.

6. The energy dissipation method for a flexible DC grid-connected transmission system for offshore wind power according to claim 5, characterized in that, The unloading circuit inside the wind power converter performs energy dissipation operations, specifically including: using the unloading circuit inside the wind power converter to dissipate the unbalanced energy accumulated on the DC bus.

7. The energy dissipation method for a flexible DC grid-connected transmission system for offshore wind power according to claim 6, characterized in that, The method of using the unloading circuit inside the wind power converter to consume the unbalanced energy accumulated on the DC bus specifically includes: determining whether the first voltage of the DC bus is greater than the upper limit of the second hysteresis control voltage; if the first voltage of the DC bus is greater than the upper limit of the second hysteresis control voltage, then using the unloading circuit inside the wind power converter to consume the unbalanced energy accumulated on the DC bus, and continuing to determine whether the second voltage of the DC bus is less than the lower limit of the second hysteresis control voltage; if the second voltage of the DC bus is less than the lower limit of the second hysteresis control voltage, then cutting off the energy consumption operation of the unloading circuit on the DC bus.

8. An energy-consuming system for a flexible DC grid-connected transmission system for offshore wind power, characterized in that, The energy-consuming system includes an unloading circuit and a DC energy-consuming device; When a fault occurs on the inverter side of the MMC DC transmission system The unbalanced energy accumulated on the DC cable is consumed through a combination of unloading circuits and DC energy dissipation devices. Specifically, this includes: The method of using a DC energy dissipation device to consume the unbalanced energy accumulated on a DC cable includes: determining whether the first voltage of the DC cable is greater than the upper limit of the first hysteresis control voltage; if the first voltage of the DC cable is greater than the upper limit of the first hysteresis control voltage, then using a DC energy dissipation device to consume the unbalanced energy accumulated on the DC cable; the first voltage is the voltage on the DC cable before using the DC energy dissipation device. The determination of whether an unloading circuit is needed to dissipate energy in the DC cable after the unbalanced energy on the DC cable is consumed by the DC energy dissipation device includes: determining whether the second voltage of the DC cable is less than the lower limit of the first hysteresis control voltage; if the second voltage of the DC cable is not less than the lower limit of the first hysteresis control voltage, timing the time during which the second voltage of the DC cable is not less than the lower limit of the first hysteresis control voltage; if the timing time is longer than the set time, it is determined that an unloading circuit is needed to dissipate energy in the DC cable; the second voltage is the DC cable voltage collected in real time after the unbalanced energy accumulated on the DC cable is consumed by the DC energy dissipation device. If an unloading circuit is required to participate in the energy dissipation operation of the DC cable, then the unloading circuit is used to consume the remaining unbalanced energy accumulated on the DC cable. The unloading circuit is the unloading circuit inside the wind power converter, and the DC energy dissipation device is the DC energy dissipation device in the MMC DC transmission system.

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