Offshore wind power flexible direct current system and method based on active power dynamic balance cooperation

Abstract

The application relates to an offshore wind power flexible direct system and method based on active power dynamic balance cooperation, which comprises an offshore wind farm, an offshore converter station, an onshore converter station and an onshore alternating current power grid; the offshore wind farm is fed into the offshore converter station; the direct current side of the offshore converter station is connected with the direct current side of the onshore converter station; the alternating current side of the onshore converter station is connected with the onshore alternating current power grid through a coupling or a converter transformer and an incoming line alternating current circuit breaker; wherein the onshore converter station adopts an improved modular multilevel converter with an integrated energy dissipation function; at least one set of alternating current side active power dynamic balance devices are arranged between the incoming line alternating current circuit breaker and the coupling or the converter transformer of the onshore converter station, and are connected with the grid side point of common connection of the coupling or the converter transformer through a step-down transformer; and the active power dynamic balance cooperation is completed through the coordinated cooperation of the improved modular multilevel converter with the integrated energy dissipation function and the alternating current side active power dynamic balance devices.

Description

Technical Field

[0001] This invention relates to a flexible DC transmission system and method for offshore wind power based on dynamic balance coordination of active power, and belongs to the field of flexible DC transmission technology. Background Technology

[0002] Large-capacity flexible DC transmission systems offer advantages such as autonomous voltage control, independent power adjustment, and highly flexible control, making them a primary grid connection technology for future large-scale offshore wind power development. The main power flow direction of this system involves the offshore wind farm sequentially feeding energy into the onshore AC grid via an offshore converter station, submarine cable, and onshore converter station. However, when an AC grid failure occurs, the power generated by the offshore wind farm continues to flow into the flexible DC system, resulting in a large surplus of power. This can lead to overvoltage in the DC system, severely impacting the stable operation of the entire system and the safety of equipment.

[0003] The main technical means to solve the problem of flexible DC system for offshore wind power in the existing technology is to configure active power dynamic balancing device or energy dissipation system (energy dissipation device) on the DC side of the onshore station. There are currently three main technical routes: centralized, distributed and hybrid. The main difference between the three is whether the resistor responsible for energy dissipation or power balancing is centrally configured or distributed.

[0004] However, regardless of which method is used, the active power balancing device consists of a large number of cascaded sub-modules (including controllable devices, module capacitors, etc.) forming an energy-consuming arm and energy-consuming resistor, which is relatively expensive. In addition, it requires the additional configuration of many DC-side devices, such as DC-side disconnect switches, DC through-wall bushings, current-limiting reactors, measuring equipment, surge arresters, etc., which have high voltage levels, high technical difficulty, and relatively complex control. Summary of the Invention

[0005] To address the aforementioned problems, the purpose of this invention is to provide an offshore wind power flexible DC system and method based on dynamic balance coordination of active power, which can ensure stable system operation and equipment safety.

[0006] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0007] In a first aspect, the present invention provides an offshore wind power flexible DC system based on active power dynamic balance coordination, characterized in that it includes an offshore wind farm, an offshore converter station, an onshore converter station, and an onshore AC grid; the offshore wind farm feeds into the offshore converter station; the DC side of the offshore converter station is connected to the DC side of the onshore converter station via positive and negative DC submarine cables; the AC side of the onshore converter station is connected to the onshore AC grid through a connecting or converter transformer and an incoming AC circuit breaker; wherein, the onshore converter station adopts an improved modular multilevel converter with integrated energy dissipation function, and at least one set of AC-side active power dynamic balance devices is configured between the incoming AC circuit breaker and the connecting or converter transformer of the onshore converter station, and connected to the grid-side common connection point of the connecting or converter transformer through a step-down transformer, and the active power dynamic balance coordination is achieved through the coordinated cooperation of the improved modular multilevel converter with integrated energy dissipation function and the AC-side active power dynamic balance devices.

[0008] Furthermore, the improved modular multilevel converter with integrated energy dissipation function adopts a three-phase six-bridge arm structure. Each phase includes an upper bridge arm and a lower bridge arm. Each bridge arm includes N cascaded new improved sub-modules, namely energy dissipation sub-modules. Each bridge arm is also equipped with a bridge arm reactor to limit the current rise rate during commutation and faults within the bridge arm.

[0009] Furthermore, each of the energy-consuming submodules includes a first to a third insulated-gate bipolar transistor, a bypass switch, a bypass thyristor, a module capacitor, and a first energy-consuming resistor. The bypass switch and the bypass thyristor are connected in parallel to the second insulated-gate bipolar transistor. The first insulated-gate bipolar transistor, the second insulated-gate bipolar transistor, and the module capacitor form a half-bridge structure. The third insulated-gate bipolar transistor is connected in series with the first energy-consuming resistor and then in parallel with the module capacitor.

[0010] Furthermore, the active power dynamic balance is achieved through the coordinated operation of the improved modular multilevel converter with integrated energy dissipation function and the AC-side active power dynamic balancing device, including:

[0011] After a fault occurs, the improved modular multilevel converter of the onshore converter station enters the energy consumption mode to absorb surplus power. According to the set ride-through time, it relies on the energy consumption control strategy of the converter submodule to achieve fault ride-through. If the fault ride-through cannot be achieved by relying solely on the energy consumption control strategy of the converter submodule within the set ride-through time, it exits the converter energy consumption mode and uses the surplus power to complete the fault ride-through by activating the AC side active power dynamic balancing device, thus completing the active power dynamic balancing coordination.

[0012] Furthermore, the first energy-consuming resistor is a metal-type press-fit power resistor or a ceramic-type water-cooled resistor, wherein the resistance value R1 of the first energy-consuming resistor is determined by:

[0013]

[0014] In the formula, N is the number of sub-modules in each bridge arm, and U c The rated submodule voltage is η1, where η1 is the efficiency coefficient and P is the efficiency coefficient. dcmax This represents the maximum power on the DC side.

[0015] Furthermore, the energy consumption capacity allocation relationship between the improved modular multilevel converter and the AC-side active power dynamic balancing device is determined by the following formula:

[0016]

[0017] In the formula, P dcmax E1 represents the maximum power on the DC side; E2 and E1 represent the energy consumption capacities of the improved modular multilevel converter and the AC side active power dynamic balancing device, respectively. 1max , Δt 2max All of these are set fault travel times.

[0018] Furthermore, the AC-side active power dynamic balancing device can be a single group or multiple groups. Each group of the AC-side active power dynamic balancing device adopts a three-phase circuit structure. Each phase mainly consists of three single-phase thyristor valves and a second energy-consuming resistor connected in series. The three phases are connected in a delta configuration. The method for determining the resistance value of the second energy-consuming resistor R2 is as follows:

[0019]

[0020] In the formula, M represents the number of AC-side active power dynamic balancing devices, and U l The line voltage of the AC system connected to the AC-side active power dynamic balancing device is given by η2, where η2 is the efficiency coefficient and P is the line voltage of the AC system connected to the active power dynamic balancing device. dcmax This represents the maximum power on the DC side.

[0021] Secondly, the present invention also provides a power surplus collaborative control method for offshore wind power flexible DC systems based on active power dynamic balance collaboration, comprising:

[0022] S1. Real-time monitoring of DC voltage and transmission power of offshore wind power flexible DC systems;

[0023] S2. After a fault occurs, the improved modular multilevel converter of the onshore converter station enters the energy consumption mode according to the energy consumption strategy of the converter submodule to absorb the surplus power.

[0024] S3. If the surplus power criterion is met, proceed to step S4; otherwise, proceed to step S5.

[0025] S4. If fault ride-through is achieved solely by the converter submodule energy consumption control strategy within the set ride-through time, proceed to step S6.

[0026] S5. If the fault ride-through cannot be achieved by relying solely on the converter submodule energy consumption control strategy within the set ride-through time, the incoming line circuit breaker will be tripped. After the incoming line circuit breaker is fully tripped, the set time will be delayed to exit the converter energy consumption mode. The initial number of AC side active power dynamic balance devices will be calculated based on the surplus power.

[0027] S6, the flexible vertical system completes fault crossing.

[0028] Furthermore, the improved modular multilevel converter of the onshore converter station has two operating modes: normal mode and energy consumption mode;

[0029] In normal mode, all submodules' third insulated gate bipolar transistors are in a latched state.

[0030] The energy consumption mode is when the third insulated gate bipolar transistor of the converter submodule is in the on state.

[0031] Furthermore, the power consumption strategy for the converter submodule includes:

[0032] Based on the DC power, determine the number N of energy-consuming submodules to be put into each phase upper and lower bridge arm. SM :

[0033]

[0034] In the formula, P SM P represents the power consumption of the submodule. dc For DC power, U c R1 is the module capacitor voltage, and R1 is the first energy-consuming resistor;

[0035] The capacitor voltages of each phase submodule are sorted according to a fixed frequency, and the N with the highest capacitor voltage is selected. SM Each submodule;

[0036] For the above N SM The submodule is controlled by turning on the third insulated gate bipolar transistor corresponding to the submodule, causing the submodule to enter the power consumption mode.

[0037] Thirdly, the present invention also provides an onshore converter station, wherein the onshore converter station adopts an improved modular multilevel converter with integrated energy dissipation function, and at least one set of AC-side active power dynamic balancing device is provided between the incoming AC circuit breaker of the onshore converter station and the connecting or converter transformer, and the AC-side active power dynamic balancing device is connected to the grid-side common connection point of the connecting or converter transformer through a step-down transformer.

[0038] Furthermore, the improved modular multilevel converter with integrated energy dissipation function adopts a three-phase six-bridge-arm structure. Each phase includes an upper bridge arm and a lower bridge arm. Each bridge arm includes N cascaded and series-connected novel improved sub-modules, namely energy-dissipating sub-modules. Each bridge arm is also equipped with a bridge arm reactor to limit the rate of current rise during commutation and faults within the bridge arm. Each energy-dissipating sub-module includes a first to a third insulated-gate bipolar transistor, a bypass switch, a bypass thyristor, a module capacitor, and a first energy-dissipating resistor. The bypass switch and bypass thyristor are connected in parallel. The second insulated-gate bipolar transistor is connected in series. The first insulated-gate bipolar transistor, the second insulated-gate bipolar transistor, and the module capacitor form a half-bridge structure. The third insulated-gate bipolar transistor is connected in series with the first energy-dissipating resistor and then in parallel with the module capacitor. The AC-side active power dynamic balancing device can be a single group or multiple groups. Each group of the AC-side active power dynamic balancing device adopts a three-phase circuit structure. Each phase is mainly composed of three single-phase thyristor valves and a second energy-dissipating resistor connected in series. The three phases are connected in a delta configuration.

[0039] Because the present invention adopts the above technical solution, it has the following characteristics:

[0040] 1. This invention includes an offshore wind farm, an offshore converter station, an onshore converter station, and an onshore AC power grid. The onshore converter station adopts an improved modular multilevel converter with integrated energy dissipation function. At least one set of AC-side active power dynamic balancing devices is configured between the incoming AC circuit breaker of the onshore converter station and the connecting or converter transformer, and is connected to the grid-side common connection point of the connecting or converter transformer through a step-down transformer. During a set crossover time, the improved modular multilevel converter with integrated energy dissipation function and the AC-side active power dynamic balancing device coordinate to complete the dynamic balancing of active power, so as to realize the reasonable and effective distribution of surplus power in the converter valve and the AC-side active power dynamic balancing device, ensuring economy and compactness.

[0041] 2. This invention, by configuring energy-consuming resistors in sub-modules as distributed energy dissipation units for converter valves, can effectively cope with fault scenarios with small surplus power, and at the same time effectively suppress overcurrent caused by energy imbalance in a single module.

[0042] 3. This invention replaces the DC side active power dynamic balancing device with an AC side active power dynamic balancing device, thereby reducing engineering costs and improving economic efficiency.

[0043] 4. The improved converter of this invention optimizes and adjusts the converter topology based on the traditional modular multilevel converter. It also has the advantages of overvoltage suppression and surplus power smoothing during normal operation and fault conditions. It has the advantages of fewer new devices, clear functions, compact configuration, optimized footprint, and high economy.

[0044] In summary, this invention is applicable to offshore wind farms with flexible DC grid connection and can be extended to onshore flexible DC systems for islanded access to new energy applications. Attached Figure Description

[0045] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts. In the drawings:

[0046] Figure 1 This is a schematic diagram of an offshore wind power flexible DC system based on active power dynamic balance coordination, according to an embodiment of the present invention.

[0047] Figure 2 A schematic diagram of the existing conventional modular multilevel converter structure in an offshore converter station;

[0048] Figure 3 This is a schematic diagram of an improved modular multilevel converter with integrated energy dissipation function in an onshore converter station according to an embodiment of the present invention.

[0049] Figure 4 This is a multi-group AC-side active power dynamic balance and coordinated configuration scheme according to an embodiment of the present invention;

[0050] Figure 5 This is a flowchart of the active power dynamic balance and collaborative power surplus control method for offshore wind power flexible DC system according to an embodiment of the present invention. Detailed Implementation

[0051] It should be understood that the terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting. Unless the context clearly indicates otherwise, the singular forms “a,” “an,” and “described” as used herein may also include the plural forms. The terms “comprising,” “including,” “containing,” and “having” are inclusive and therefore indicate the presence of the stated features, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, elements, components, and / or combinations thereof. The method steps, processes, and operations described herein are not construed as requiring them to be performed in a particular order described or illustrated unless the order of performance is explicitly indicated. It should also be understood that additional or alternative steps may be used.

[0052] Although terms such as first, second, third, etc., may be used in this document to describe multiple elements, components, regions, layers, and / or segments, these elements, components, regions, layers, and / or segments should not be limited by these terms. These terms may be used only to distinguish one element, component, region, layer, or segment from another. Unless the context clearly indicates otherwise, terms such as "first," "second," and other numerical terms used herein do not imply order or sequence. Therefore, the first element, component, region, layer, or segment discussed below may be referred to as the second element, component, region, layer, or segment without departing from the teachings of the exemplary embodiments.

[0053] For ease of description, spatial relative terms may be used in the text to describe the relationship of one element or feature relative to another element or feature as shown in the figure. These relative terms include, for example, "inside," "outside," "middle," "outer," "below," "above," etc. Such spatial relative terms are intended to include different orientations of the device in use or operation, other than those depicted in the figure.

[0054] This invention provides an offshore wind power flexible DC system, method, and onshore converter station based on active power dynamic balance coordination. The system includes an offshore wind farm, an offshore converter station, an onshore converter station, and an onshore AC grid. The offshore wind farm feeds into the offshore converter station. The DC side of the offshore converter station is connected to the DC side of the onshore converter station. The AC side of the onshore converter station is connected to the onshore AC grid via a connecting or converter transformer and an incoming AC circuit breaker. The onshore converter station employs an improved modular multilevel converter with integrated energy dissipation function. At least one set of AC-side active power dynamic balancing devices is configured between the incoming AC circuit breaker and the connecting or converter transformer of the onshore converter station, and connected to the grid-side common connection point of the connecting or converter transformer via a step-down transformer. Active power dynamic balance coordination is achieved through the coordinated operation of the improved modular multilevel converter with integrated energy dissipation function and the AC-side active power dynamic balancing devices. The energy coordination control method proposed in this invention is simple, effective, and highly implementable. It can achieve a reasonable distribution of surplus energy between the improved converter and the AC side active power dynamic balancing device. It can be applied to offshore wind farms with flexible DC grid connection and can be extended to onshore flexible DC system island access to new energy applications.

[0055] Exemplary embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0056] Example 1: The offshore wind power flexible DC system based on active power dynamic balance coordination provided in this example includes an offshore wind farm 1, an offshore converter station 2, positive and negative submarine DC cables 3, an onshore converter station 4, and an onshore AC power grid 5.

[0057] Offshore wind farm 1 is fed into offshore converter station 2 via an offshore AC booster station and AC submarine cable.

[0058] The DC side of the offshore converter station 2 is connected to the DC side of the onshore converter station 4 via positive and negative DC submarine cables 3. For example... Figure 2 As shown, the offshore converter station 2 can adopt a modular multilevel converter (MMC) based on a traditional half-bridge submodule (the submodule is the minimum energy working unit of the modular multilevel converter). The above structure is an existing traditional structure, and will not be described in detail.

[0059] The AC side of the onshore converter station 4 is connected to the onshore AC power grid 5 via a connection or converter transformer 7 and an incoming AC circuit breaker S1. The onshore converter station 4 adopts an improved modular multilevel converter with integrated energy dissipation function.

[0060] At least one set of AC-side active power dynamic balancing devices 6 is configured between the incoming AC circuit breaker S1 of the onshore converter station 4 and the connecting or converter transformer 7, and is connected to the grid-side common connection point 9 of the connecting or converter transformer 7 through a step-down transformer 8.

[0061] Among them, the dynamic balance of active power is achieved through the coordinated operation of an improved modular multilevel converter with integrated energy dissipation function and an AC-side active power dynamic balancing device.

[0062] In a preferred embodiment, the improved modular multilevel converter with integrated energy dissipation function is an improvement and optimization of the existing mature modular multilevel converter topology based on half-bridge submodules. For example... Figure 3 As shown in (a), the improved modular multilevel converter with integrated energy dissipation function adopts a three-phase six-bridge-arm structure. Each phase consists of an upper bridge arm and a lower bridge arm. Each bridge arm includes N cascaded new improved sub-modules, namely energy dissipation sub-modules SM1...SM2. N Each bridge arm is equipped with a bridge arm reactor to limit the rate of current rise during commutation and faults within the bridge arm.

[0063] Each energy-dissipating submodule consists of two insulated-gate bipolar transistors (IGBTs) T1 and T2 with anti-parallel diodes forming a basic half-bridge configuration, a bypass switch, a bypass thyristor, a DC capacitor, an energy-dissipating resistor, and an IGBT T3. Compared to conventional half-bridge submodules, the energy-dissipating resistor replaces the conventional equalizing or discharging resistor, and the added IGBT T3 acts as a switching switch for the energy-dissipating resistor, enabling controllable and adjustable energy dissipation.

[0064] Specifically, such as Figure 3 As shown in (b), each energy-consuming submodule includes first to third insulated-gate bipolar transistors T1 to T3, a bypass switch K1, a bypass thyristor Ty1, a module capacitor C1, and an energy-consuming resistor R1. The bypass switch K1 and the bypass thyristor Ty1 are connected in parallel to the second insulated-gate bipolar transistor T2. The first insulated-gate bipolar transistor T1, the second insulated-gate bipolar transistor T2, and the module capacitor C1 form a half-bridge structure. The third insulated-gate bipolar transistor T3 is connected in series with the energy-consuming resistor R1 and then in parallel with the module capacitor C1.

[0065] Furthermore, the power-consuming resistor R1 can be a metal-type press-fit power resistor with metal contact electrodes on both sides. When in use, metal cold plates need to be pressed on both sides to ensure a certain clamping force; a ceramic-type water-cooled resistor can also be used.

[0066] Furthermore, the energy consumption capacity allocation relationship between the improved modular multilevel converter and the AC-side active power dynamic balancing device is determined by the following formula:

[0067]

[0068] In the formula, P dcmax E1 represents the maximum power on the DC side; E2 and E1 represent the energy consumption capacities of the improved modular multilevel converter and the AC side active power dynamic balancing device, respectively. Preferably, Δt 1max =100ms, Δt 2max =1.5s~2s.

[0069] Furthermore, the method for determining the value of the energy-consuming resistor R1 in the improved modular multilevel converter submodule is as follows:

[0070]

[0071] In the formula, N is the number of sub-modules in each bridge arm, and U c The rated submodule voltage is η1, which is the efficiency coefficient (a constant less than 1, preferably 0.95).

[0072] Furthermore, the maximum energy consumed during the primary power surplus processing of the improved modular multilevel converter submodule energy-consuming resistor R1 is... The following is confirmed:

[0073]

[0074] In the formula, N is the number of sub-modules of each bridge arm, and ρ1 is the margin coefficient, which can be preferably 1.1.

[0075] In a preferred embodiment, such as Figure 4 As shown, the AC side active power dynamic balancing device can be a single group or multiple groups. The single group AC side active power dynamic balancing device adopts a three-phase circuit structure. Each phase is mainly composed of single-phase thyristor valves (thyristors connected in anti-parallel) Z1 to Z3 and energy-consuming resistor R2 connected in series. The three phases are connected in a delta connection manner.

[0076] To match the surplus power of large-capacity offshore wind power flexible DC system, the AC side active power dynamic balancing device adopts a grouped arrangement. Multiple AC side active power dynamic balancing devices are configured according to system requirements, and they are gathered at the common bus 10 of the AC side active power dynamic balancing device. Then, they are connected to the grid-side common connection point 9 of the converter (connection) transformer 7 via the step-down transformer 8.

[0077] Furthermore, the method for determining the resistance value of the energy-consuming resistor R2 of each AC-side active power dynamic balancing device is as follows:

[0078]

[0079] In the formula, M represents the number of AC-side active power dynamic balancing devices, and U l η2 is the line voltage of the AC system connected to the AC side active power dynamic balancing device, and η2 is the efficiency coefficient, which is a constant less than 1, preferably 0.95.

[0080] Furthermore, the maximum energy consumed during the primary power surplus processing of the improved modular multilevel converter submodule energy-consuming resistor R2 is... The following is confirmed:

[0081]

[0082] In the formula, ρ2 is the margin coefficient, which can be preferably 1.1.

[0083] Example 2: Figure 5 As shown in this embodiment, the power surplus collaborative control method for offshore wind power flexible DC systems based on active power dynamic balance collaboration includes:

[0084] S1. Real-time monitoring of DC voltage and transmission power of offshore wind power flexible DC systems;

[0085] S2. After a fault is detected, the improved modular multilevel converter of the land-based converter station enters the energy consumption mode according to the sub-module energy consumption resistor control strategy to absorb the surplus power.

[0086] Specifically, the improved modular multilevel converter of the onshore converter station has two operating modes: normal mode and energy consumption mode.

[0087] In normal mode, the third insulated gate bipolar transistor T3 of all submodules is in a latched state;

[0088] The energy consumption mode is that the third insulated gate bipolar transistor T3 of the converter submodule is in the on state.

[0089] Furthermore, the converter energy consumption control strategy is as follows:

[0090] A1. Determine the number N of energy-consuming sub-modules to be put into operation for each phase's upper and lower bridge arms based on the DC power. SM :

[0091]

[0092] In the formula, P SM P represents the power consumption of the submodule. dc Uc represents DC power, and Uc represents the module capacitor voltage.

[0093] A2. Sort the capacitor voltages of each phase submodule according to a fixed frequency, and select N with the highest capacitor voltage. SM Each submodule can be sorted using methods such as quicksort or bubble sort, and this is just one example; it is not limited to these methods.

[0094] A3. Regarding the above N SM The submodule is controlled by turning on the third insulated gate bipolar transistor T3 connected in series with the submodule, causing the submodule to enter the power consumption mode.

[0095] S3. If the surplus power criterion is met, proceed to step S4; otherwise, proceed to step S5.

[0096] Specifically, the surplus power criterion is as follows: the improved modular multilevel converter cannot absorb all the surplus power P within the set fault ride-through time, that is, the maximum energy that the converter can absorb exceeds the surplus power of the maximum fault ride-through time:

[0097]

[0098] In the formula, P dc For DC power, Δt 2max This represents the maximum fault travel time.

[0099] S4. If fault ride-through is achieved solely by the converter submodule energy consumption control strategy within the set ride-through time, proceed to step S6.

[0100] S5. If the fault ride-through cannot be achieved by relying solely on the converter submodule energy consumption control strategy within the set ride-through time, trip the incoming circuit breaker S1. After the incoming circuit breaker has fully tripped, delay △t3, exit the converter energy consumption mode, and calculate the initial number of AC side active power dynamic balancing devices based on the surplus power.

[0101] Based on the principle of energy balance, the initial number of AC active power dynamic balancing devices is determined as follows:

[0102]

[0103] In the formula, P chopper This refers to the rated power of a single AC active power dynamic balancing device.

[0104] The number of AC-side active power dynamic balancing devices can be dynamically adjusted based on changes in DC voltage. A PI controller can be introduced, which calculates the difference between the target DC voltage value and the measured DC voltage value, and then outputs the result through a proportional-integral control loop to adjust the number of AC-side active power dynamic balancing devices in operation.

[0105] S6, the flexible vertical system completes fault crossing.

[0106] Example 3: This example also provides an onshore converter station 4. The onshore converter station 4 adopts an improved modular multilevel converter with integrated energy dissipation function. At least one set of AC-side active power dynamic balancing device 6 is installed between the incoming AC circuit breaker of the onshore converter station 4 and the connecting or converter transformer. The AC-side active power dynamic balancing device 6 is connected to the grid-side common connection point 9 of the connecting or converter transformer 7 through a step-down transformer 8. The structure of the improved modular multilevel converter with integrated energy dissipation function and the AC-side active power dynamic balancing device is not described in detail; please refer to the content of Example 1.

[0107] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to interchangeably. Each embodiment focuses on describing the differences from other embodiments. In the description of this specification, the terms "one embodiment," "some implementations," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the embodiments in this specification. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described can be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0108] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A flexible DC-DC transmission system for offshore wind power based on dynamic balance and coordination of active power, characterized in that, It includes an offshore wind farm, an offshore converter station, an onshore converter station, and an onshore AC power grid; the offshore wind farm feeds into the offshore converter station; the DC side of the offshore converter station is connected to the DC side of the onshore converter station via positive and negative DC submarine cables; the AC side of the onshore converter station is connected to the onshore AC power grid through a coupling or converter transformer and an incoming AC circuit breaker; The onshore converter station employs an improved modular multilevel converter with integrated energy dissipation function. At least one set of AC-side active power dynamic balancing devices is configured between the incoming AC circuit breaker and the connecting or converter transformer of the onshore converter station, and connected to the grid-side common connection point of the connecting or converter transformer via a step-down transformer. The improved modular multilevel converter with integrated energy dissipation function and the AC-side active power dynamic balancing devices work in coordination to achieve dynamic power balancing, including: After a fault occurs, the improved modular multilevel converter of the onshore converter station enters the energy consumption mode to absorb surplus power. According to the set ride-through time, it relies on the energy consumption control strategy of the converter submodule to achieve fault ride-through. If the fault ride-through cannot be achieved by relying solely on the energy consumption control strategy of the converter submodule within the set ride-through time, it exits the converter energy consumption mode and uses the surplus power to complete the fault ride-through by activating the AC side active power dynamic balancing device, thus completing the active power dynamic balancing coordination. The energy consumption capacity allocation relationship between the improved modular multilevel converter and the AC side active power dynamic balancing device is determined by the following formula: In the formula, P dcmax This represents the maximum power on the DC side. E 1 and E 2 represents the energy consumption capacity of the improved modular multilevel converter and the AC-side active power dynamic balancing device, respectively. All of these are the set fault travel times.

2. The offshore wind power flexible DC system based on active power dynamic balance coordination according to claim 1, characterized in that, The improved modular multilevel converter with integrated energy dissipation function adopts a three-phase six-bridge arm structure. Each phase includes an upper bridge arm and a lower bridge arm. Each bridge arm includes N cascaded and connected new improved sub-modules, namely energy dissipation sub-modules. Each bridge arm is also equipped with a bridge arm reactor to limit the current rise rate during commutation and fault within the bridge arm. Each of the energy-consuming submodules includes a first to a third insulated-gate bipolar transistor (IGBT), a bypass switch, a bypass thyristor, a module capacitor, and a first energy-consuming resistor. The bypass switch and the bypass thyristor are connected in parallel to the second IGBT. The first IGBT, the second IGBT, and the module capacitor form a half-bridge structure. The third IGBT is connected in series with the first energy-consuming resistor and then in parallel with the module capacitor.

3. The offshore wind power flexible DC system based on dynamic balance coordination of active power as described in claim 1 or 2, characterized in that, The first energy-consuming resistor is a metal-type press-fit power resistor or a ceramic-type water-cooled resistor, wherein the resistance value of the first energy-consuming resistor is... Determination method: In the formula, N The number of modules for each bridge arm submodule Uc This is the rated submodule voltage. For efficiency coefficient, Pdcmax This represents the maximum power on the DC side.

4. The offshore wind power flexible DC system based on active power dynamic balance coordination according to claim 1, characterized in that, The AC-side active power dynamic balancing device can be a single group or multiple groups. Each group of the AC-side active power dynamic balancing device adopts a three-phase circuit structure. Each phase mainly consists of three single-phase thyristor valves and a second energy-consuming resistor connected in series. The three phases are connected in a delta configuration. The method for determining the resistance value of the second energy-consuming resistor R2 is as follows: In the formula, M represents the number of AC-side active power dynamic balancing devices. Ul The line voltage of the AC system connected to the AC-side active power dynamic balancing device. For efficiency coefficient, Pdcmax This represents the maximum power on the DC side.

5. A power surplus collaborative control method for offshore wind power flexible DC systems based on active power dynamic balance collaboration as described in any one of claims 1 to 4, characterized in that... include: S1. Real-time monitoring of DC voltage and transmission power of offshore wind power flexible DC systems; S2. After a fault occurs, the improved modular multilevel converter of the onshore converter station enters the energy consumption mode according to the energy consumption strategy of the converter submodule to absorb the surplus power. S3. If the surplus power criterion is met, proceed to step S4; otherwise, proceed to step S5. S4. If fault ride-through is achieved solely by the converter submodule energy consumption control strategy within the set ride-through time, proceed to step S6. S5. If the fault ride-through cannot be achieved by relying solely on the converter submodule energy consumption control strategy within the set ride-through time, the incoming line circuit breaker will be tripped. After the incoming line circuit breaker is fully tripped, the set time will be delayed to exit the converter energy consumption mode. The initial number of AC side active power dynamic balance devices will be calculated based on the surplus power. S6, the flexible vertical system completes fault crossing.

6. The power surplus cooperative control method according to claim 5, characterized in that, The improved modular multilevel converter of the onshore converter station has two operating modes: normal mode and energy consumption mode. In normal mode, all sub-modules' third insulated gate bipolar transistors are in a locked state; in energy consumption mode, some sub-modules of the converter have their third insulated gate bipolar transistors in a conducting state. The converter submodule energy consumption strategy includes: determining the number N of energy consumption submodules to be deployed for each phase's upper and lower arms based on the DC power. SM : In the formula, P SM The power consumption of the submodule P dc DC power U c This refers to the module capacitor voltage. R 1 is the first energy-consuming resistor; the capacitor voltages of each phase submodule are sorted according to a fixed frequency, and N with the higher capacitor voltage is selected. SM Each submodule; for the above N SM The submodule is controlled by turning on the third insulated gate bipolar transistor corresponding to the submodule, causing the submodule to enter the power consumption mode.

7. An onshore converter station for implementing the offshore wind power flexible DC system based on active power dynamic balance coordination as described in any one of claims 1-5, characterized in that, The onshore converter station adopts an improved modular multilevel converter with integrated energy dissipation function. At least one set of AC-side active power dynamic balancing devices is set between the incoming AC circuit breaker and the connecting or converter transformer of the onshore converter station. The AC-side active power dynamic balancing devices are connected to the grid-side common connection point of the connecting or converter transformer through a step-down transformer.

8. The onshore converter station according to claim 7, characterized in that, The improved modular multilevel converter with integrated energy dissipation function adopts a three-phase six-bridge arm structure. Each phase includes an upper bridge arm and a lower bridge arm. Each bridge arm includes N cascaded and connected new improved sub-modules, namely energy dissipation sub-modules. Each bridge arm is also equipped with a bridge arm reactor to limit the current rise rate during commutation and faults within the bridge arm. Each energy dissipation sub-module includes a first to a third insulated-gate bipolar transistor, a bypass switch, a bypass thyristor, a module capacitor, and a first energy dissipation resistor. The bypass switch and the bypass thyristor are connected in parallel to the second insulated-gate bipolar transistor. The first insulated-gate bipolar transistor, the second insulated-gate bipolar transistor, and the module capacitor form a half-bridge structure. The third insulated-gate bipolar transistor is connected in series with the first energy dissipation resistor and then in parallel with the module capacitor. The AC-side active power dynamic balancing device can be a single group or multiple groups. Each group of the AC-side active power dynamic balancing device adopts a three-phase circuit structure. Each phase is mainly composed of three single-phase thyristor valves and a second energy-consuming resistor connected in series. The three phases are connected in a delta configuration.

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