Offshore wind power dc collection topology circuit and dc line protection method

Abstract

The present application relates to a kind of offshore wind power dc collection topology circuit and dc line protection method, circuit includes: wind turbine side conversion unit, primary voltage boosting collection unit, secondary voltage boosting transmission unit and grid-connected conversion unit connected in turn;Based on this circuit, a kind of dc line protection method is implemented, method includes: in the connecting line between primary voltage boosting collection unit and secondary voltage boosting transmission unit, current transformer for real-time acquisition of current at both ends is respectively arranged;According to the current of acquisition, calculate differential current and braking current;According to the numerical relationship of differential current and braking current and system voltage parameter, the type of fault is discriminated and corresponding protection action is triggered.Compared with prior art, the present application realizes the unity of offshore wind power dc collection system safety and economy, and the insulation requirement of equipment is reduced by hierarchical voltage design, and the dependence on circuit breaker breaking capacity is reduced by fault current limitation, and the optimized protection criterion ensures the rapid and accurate isolation of fault.

Description

Technical Field

[0001] This invention relates to the field of wind power generation system technology, and in particular to an offshore wind power DC collection topology circuit and DC line protection method. Background Technology

[0002] With the rapid development of offshore wind power towards larger capacity and longer distances, DC aggregation technology has become the mainstream choice due to its advantages such as low transmission loss and high controllability. However, in practical engineering applications, complex aggregation systems employing multi-stage DC converters still face two major technological challenges:

[0003] First, at the system design level, existing technologies lack simulation models that can accurately reflect the collaborative working characteristics of multi-stage power electronic converters. Especially when DC-side grounding, inter-electrode short circuits, or internal faults in cascaded DC-DC converters occur, the propagation path and transient characteristics of the fault current differ fundamentally from those of traditional AC systems. In graded voltage topologies represented by "±5kV→±70kV→±500kV", the amplitude, rate of rise, and attenuation of the fault current are difficult to predict accurately using traditional methods. This leads to a lack of reliable basis for designing key system parameters, directly affecting the rationality and effectiveness of subsequent protection schemes. Chinese patent CN119093297A discloses a fault protection method suitable for offshore wind power DC systems, employing a multi-detection strategy to achieve comprehensive detection and judgment of power system faults from multiple angles and dimensions. However, it does not mention the adaptation design for multi-stage voltage conversion topologies, only targeting DC lines of a single voltage level, and cannot be compatible with the multi-stage DC-DC conversion architecture of large-scale offshore wind power.

[0004] Secondly, at the protection configuration level, the characteristic of DC system fault currents lacking a natural zero-crossing point renders traditional AC protection schemes completely ineffective. Although longitudinal differential protection is theoretically applicable to DC systems, its performance is highly dependent on the clarity of fault characteristics. Existing DC converged topologies often suffer from single voltage levels, excessively large fault currents, and slow attenuation, leading to difficulties in the breaking of protection devices. Differential protection struggles to accurately distinguish between faults within and outside the protection zone, resulting in erroneous or non-operational actions. Especially for complex faults such as inter-pole short circuits within cascaded DC-DC converters, the lack of effective fault characteristic extraction and discrimination mechanisms fails to meet the stringent reliability requirements of the system in harsh marine environments.

[0005] Therefore, there is an urgent need for a DC collection topology and supporting protection method that can actively limit the fault current amplitude and provide clear fault characteristics for longitudinal protection, so as to solve the above-mentioned technical bottlenecks and promote the large-scale safe application of offshore wind power DC collection technology. Summary of the Invention

[0006] The purpose of this invention is to overcome the defects of the prior art by providing an offshore wind power DC collection topology circuit and a DC line protection method.

[0007] The objective of this invention can be achieved through the following technical solutions:

[0008] A DC collection topology circuit for offshore wind power, the circuit comprising: a wind turbine-side converter unit, a first-stage boost collection unit, a second-stage boost transmission unit, and a grid-connected converter unit connected in sequence;

[0009] The wind turbine-side converter unit is used to convert the electrical energy output by the permanent magnet direct-drive wind turbine into low-voltage direct current; the first-stage boost-collection unit includes at least one DC-DC2 converter, used to boost the low-voltage direct current to medium voltage to complete the power collection; the second-stage boost-transmission unit includes a DC-DC1 converter, used to boost the medium-voltage direct current to high voltage to achieve long-distance power transmission; the grid-connected converter unit includes an MMC converter, used to convert high-voltage direct current into high-voltage alternating current and connect it to the AC power grid.

[0010] Furthermore, both the DC-DC1 converter and the DC-DC2 converter adopt a dual DAB cascaded structure, and both the DC-DC1 converter and the DC-DC2 converter are capable of constant DC voltage control; the dual DAB cascaded structure includes a DAB converter, an inductor and a capacitor, and the inductance value of the inductor in the DC-DC2 converter is greater than the inductance value of the inductor in the DC-DC1 converter.

[0011] Furthermore, in the dual DAB cascaded structure, the DAB converter adopts a symmetrical dual-bridge arm structure. Each DAB converter includes a high-frequency transformer, power switching transistors, and filter elements. Each bridge arm is composed of multiple sets of power switching transistors connected in series or in parallel. The output end of the bridge arm is connected in series with the filter element and then connected to the corresponding winding of the high-frequency transformer. Electrical isolation and voltage level conversion are achieved through the transformer.

[0012] Furthermore, the MMC converter adopts a half-bridge pseudo-bipolar structure, and the upper and lower bridge arms of the MMC converter are each equipped with no less than 100 sub-levels; the MMC converter can perform constant DC voltage control and constant reactive power control, stabilize the DC line voltage at high voltage through constant DC voltage control, and maintain the reactive power balance of the system through constant reactive power control.

[0013] Furthermore, a grid interface unit is provided between the AC grid and the MMC converter. The grid interface unit includes a current transformer, a voltage transformer, and a protection switch, which are used to realize power metering and fault isolation.

[0014] A DC line protection method based on the offshore wind power DC collection topology circuit described above, the method comprising:

[0015] S1, current transformers are installed at both ends of the connection line MN between the first-stage boost collection unit and the second-stage boost transmission unit. The direction of electrical energy flowing from the wind turbine side to the AC grid is defined as the positive direction of the current. The current transformers are used to collect the current at both ends of the connection line between the first-stage boost collection unit and the second-stage boost transmission unit in real time. and ;

[0016] S2, based on the collected data and Calculate differential current and braking current ;

[0017] S3, based on differential current and braking current Based on the numerical relationship and system voltage parameters, the fault type is identified and the corresponding protection action is triggered.

[0018] Furthermore, in step S3, the fault types include positive grounding fault, inter-electrode short circuit fault, cascaded DC-DC inter-electrode short circuit fault, and external fault.

[0019] Furthermore, the specific process for determining the fault type as the positive grounding fault is as follows:

[0020] When differential current is detected The increase rate is greater than the preset increase rate threshold and > When this occurs, it is determined to be a positive grounding fault;

[0021] The specific process for determining the fault type as the inter-electrode short-circuit fault is as follows:

[0022] When differential current is detected When the surge exceeds the preset action threshold and the system current amplitude reaches more than 10 times the normal operating current, it is determined to be an inter-pole short circuit fault.

[0023] The specific process for determining the fault type as a cascaded DC-DC inter-electrode short circuit fault is as follows:

[0024] Voltage sensors are installed at both the input and output ports of each DC-DC converter to collect the inter-electrode voltage of the DC-DC converter in real time. When inter-electrode voltage is detected The voltage is reduced to below 80% of its rated voltage, while simultaneously meeting the differential current requirement. Initial negative spike and differential current appear When the voltage drops rapidly to the preset low voltage and is accompanied by oscillation, it is determined to be a short circuit fault between the electrodes of the cascaded DC-DC converter.

[0025] The specific process for determining whether a fault is an external fault or a DC line in normal operation is as follows:

[0026] When differential current is detected ≤0.1kA and braking current When the current is equal to the load current during normal system operation, it is determined to be either an external fault or a normal operating state.

[0027] Furthermore, after determining that the fault type is a positive ground fault, a trip signal is immediately sent to the circuit breakers at both ends of line MN to disconnect the faulty line.

[0028] After determining that the fault type is the inter-pole short circuit fault, a trip signal is sent to the circuit breakers and power switches of the relevant DC-DC converters at both ends of the faulty line within a preset time.

[0029] After determining that the fault type is a cascaded DC-DC inter-pole short circuit fault, immediately send a shutdown signal to all power switches of the faulty DC-DC2 converter and disconnect the collection line circuit breaker connected to the faulty DC-DC2 converter.

[0030] If the fault type is determined to be an external fault or the DC line is in normal operating condition, no protection action will be triggered.

[0031] Furthermore, the method also includes:

[0032] When the protection action is triggered, the voltage and current parameters of the faulty line are continuously monitored. When the voltage of the faulty line is detected to be above the preset threshold of the rated voltage and the current is detected to be below the preset threshold of the normal operating current, and the duration of this state reaches the preset threshold, a reset signal is sent to the relevant circuit breaker to attempt reclosing.

[0033] If differential current is detected again within a preset time after reclosing... If the value exceeds the preset threshold, it is determined to be a permanent fault, the circuit breaker is locked, and a fault alarm signal is issued.

[0034] Compared with the prior art, the beneficial effects of the present invention include:

[0035] 1. This invention addresses the DC collection topology employing graded voltage transformation (±5kV→±70kV→±500kV) in large-scale offshore wind farms. It constructs a three-stage voltage transformation architecture: low-voltage, medium-voltage, and high-voltage. This architecture matches the characteristics of the low-voltage DC output from the wind turbine side while reducing long-distance transmission losses through high-voltage transmission, thus improving the overall system transmission efficiency and facilitating subsequent fault identification and handling. Furthermore, the graded voltage design reduces the insulation requirements of each stage of equipment, and the fault current limitation reduces reliance on the breaking capacity of circuit breakers, thereby lowering equipment selection and manufacturing costs.

[0036] 2. By utilizing the inherent current-limiting characteristics and electrical isolation advantages of the dual active bridge (DAB) converter, this invention can solve the protection selectivity problem caused by the large current amplitude and slow decay during DC system faults, and achieve rapid and accurate fault diagnosis and isolation.

[0037] 3. In this invention, the topology circuit utilizes the inherent characteristics of the power conservation principle of the dual active bridge (DAB) converter to achieve electrical isolation and adaptive limitation of fault current. The DAB converter achieves electrical isolation through a high-frequency transformer and precisely regulates power transmission through phase offset control. When a short-circuit fault occurs on the low-voltage side, although the wind turbine's control system attempts to maintain voltage stability, the transmitted power is limited by the rated capacity of the power electronic devices. According to the power conservation law, the DAB module cannot obtain excess power from the high-voltage side, thus naturally limiting the output current within a safe range. This self-limiting current characteristic effectively prevents the impact of the high-voltage side short-circuit capacity and strictly limits the fault impact to the occurrence area, creating ideal conditions for protection configuration.

[0038] 4. In this invention, both the DC-DC1 converter and the DC-DC2 converter adopt dual DAB cascaded control with constant DC voltage, which can realize high power expansion and improve voltage regulation accuracy, adapting to the dynamic power fluctuation scenario of wind farms.

[0039] 5. The DC line protection method of this invention designs differentiated criteria for different fault types. For positive grounding faults, it is based on the relationship between the amplitude of differential current and braking current; for inter-pole short circuit faults, it combines the current amplitude multiple characteristics; and it supplements the voltage threshold detection of cascaded DC-DC faults to achieve accurate fault identification in all scenarios. The longitudinal differential protection criteria combine multi-dimensional characteristics of current and voltage to achieve millisecond-level fault isolation and can accurately distinguish between faults inside / outside the zone and different fault types, eliminating false tripping and failure to trip. Through active control, the destructive short-circuit current is transformed into a fault mode with controllable amplitude and clear characteristics, thereby ensuring the safety and selective protection of equipment under extreme fault conditions.

[0040] 6. Based on the topological self-limiting current characteristic, the protection device can be set with more sensitive action settings, which solves the contradiction between sensitivity and selectivity in traditional protection due to excessive fault current, and improves the high-resistance fault detection capability. Attached Figure Description

[0041] Figure 1 This is a circuit diagram of the DC collection topology for offshore wind power in this invention;

[0042] Figure 2 This is a schematic diagram of the DAB converter of the present invention;

[0043] Figure 3 This is a schematic diagram of the dual DAB cascade structure of the present invention;

[0044] Figure 4 This is a schematic diagram of the MMC converter of the present invention;

[0045] Figure 5 This is a flowchart of the method of the present invention;

[0046] Figure 6 This is a flowchart illustrating the process of determining the fault type in this invention.

[0047] Figure 7 This is a differential and braking current diagram when the fault in the region of this invention is a positive ground fault;

[0048] Figure 8 This is a differential and braking current diagram when the fault in the region of this invention is an inter-electrode short-circuit fault;

[0049] Figure 9 This is a diagram of the positive and negative electrode currents when the fault in the region of this invention is a short circuit between the electrodes of a cascaded DC-DC converter.

[0050] Figure 10 This is a diagram of the inter-electrode voltage when the fault in the region of this invention is a short circuit between the electrodes of a cascaded DC-DC converter. Detailed Implementation

[0051] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0052] Example 1

[0053] This embodiment discloses a DC collection topology circuit for offshore wind power, as shown in the circuit diagram. Figure 1 As shown, it includes: a wind turbine-side converter unit, a first-stage boosting and collecting unit, a second-stage boosting and transmission unit, and a grid-connected converter unit connected in sequence.

[0054] The wind turbine-side converter unit is used to convert the electrical energy output by the permanent magnet direct drive wind turbine into low-voltage direct current.

[0055] The first-stage boosting and collection unit includes at least one DC-DC converter for boosting low-voltage DC power to medium voltage to complete power collection.

[0056] The secondary boost transmission unit includes a DC-DC converter for boosting medium-voltage DC power to high voltage to achieve long-distance power transmission;

[0057] The grid-connected converter unit includes an MMC converter, which is used to convert high-voltage direct current into high-voltage alternating current and connect it to the AC power grid.

[0058] Specifically, in this embodiment, the wind turbine-side converter unit is used to convert the electrical energy output by multiple permanent magnet direct-drive wind turbine units into ±5kV DC power.

[0059] The first-stage boost-collection unit includes multiple DC-DC-2 converters to boost ±5kV DC power to ±70kV to complete power collection.

[0060] The secondary boost transmission unit includes a DC-DC converter, which is used to boost ±70kV DC power to ±500kV to achieve long-distance power transmission;

[0061] The grid-connected converter unit includes an MMC converter, which is used to convert ±500kV DC power into 230kV AC power and connect it to the AC grid.

[0062] In terms of topology, this invention adopts a multi-stage DC-DC converter architecture, where energy flows from the permanent magnet direct-drive wind turbine to the AC grid: each wind turbine generates ±5kV DC power through a turbine-side converter, which is then boosted to ±70kV via DC-DC2 for primary collection, and further boosted to ±500kV via DC-DC1 for long-distance transmission, finally being connected to the 230kV AC grid via an MMC converter. This not only optimizes transmission efficiency and reduces transmission losses, but more importantly, it creates clear partitioning in the electrical structure, providing natural boundary conditions for protection configurations.

[0063] The key innovation of this topology lies in utilizing the inherent characteristics of the dual active bridge (DAB) converter to achieve electrical isolation and adaptive fault current limiting. The DAB achieves electrical isolation through a high-frequency transformer and precisely regulates power transmission through phase offset control. When a short-circuit fault occurs on the low-voltage side, although the control system attempts to maintain voltage stability, the transmitted power is limited by the rated capacity of the power electronic devices, according to the law of conservation of power (…). Since the DAB cannot draw excess power from the high-voltage side, it naturally limits the output current to a safe range. This "self-limiting" characteristic effectively prevents the impact of short-circuit capacity on the high-voltage side, strictly limiting the impact of faults to the area where they occur, and creating ideal conditions for protection configuration.

[0064] Both the DC-DC1 and DC-DC2 converters employ a dual-DAB cascaded structure, as shown in the diagram. Figure 3As shown, both the DC-DC1 and DC-DC2 converters are capable of constant DC voltage control. Both converters include a DAB converter, an inductor, and a capacitor. The inductor is connected in series on one side of the DAB converter, and the capacitor is connected in parallel on both sides. The inductance value of the inductor in the DC-DC2 converter is greater than that in the DC-DC1 converter. The inductance value of the filter inductor in the DC-DC2 converter is 0.05H ± 0.005H, and the capacitance value of the energy storage capacitor in the DC-DC2 converter is 2mF ± 0.1mF. In contrast, the inductance value of the filter inductor in the DC-DC1 converter is 0.002H, and the capacitance value of the energy storage capacitor is 2mF ± 1mF.

[0065] The core of the DC-DC2 converter is the aggregation of multiple units. It needs to be connected in parallel to the ±5kV DC output of multiple wind turbines to achieve centralized voltage boosting of distributed power. Therefore, it usually adopts an architecture of multiple DC-DC2 units connected in parallel to adapt to the distributed power input characteristics of wind farms.

[0066] The core of the DCDC1 converter is high-voltage, high-power transmission. It needs to receive the ±70kV combined power from multiple DCDC2 converters and further boost the voltage to ±500kV to reduce long-distance transmission losses. Therefore, it has a higher power level and usually adopts a high-capacity architecture of single-unit dual-DAB cascade.

[0067] In a dual-DAB cascaded structure, each DAB converter is as follows: Figure 2 The structure employs a symmetrical double-bridge arm design, including a high-frequency transformer, power switching transistors, and filter elements. Each bridge arm consists of multiple sets of power switching transistors connected in series or in parallel. The output end of each bridge arm is connected in series with a filter element and then connected to the corresponding winding of the high-frequency transformer. Electrical isolation and voltage level conversion are achieved through the transformer.

[0068] Specifically, the high-frequency transformer is used to achieve electrical isolation, the power switching transistor is used to regulate power transmission, and the filter components include inductors and capacitors. The inductor has an inductance value of 0.002H, and the capacitors include 2mF and 1mF capacitors.

[0069] MMC converters, such as Figure 4 As shown, a half-bridge pseudo-bipolar structure is adopted, and the upper and lower bridge arms of the MMC converter are each equipped with no less than 100 sub-levels. The MMC converter can perform constant DC voltage control and constant reactive power control. The constant DC voltage control stabilizes the DC line voltage at ±500kV, and the constant reactive power control maintains the reactive power balance of the system.

[0070] Specifically, such as Figure 4 As shown, the core module of the MMC converter includes the MMC NLM main circuit module and the MMCEdoQ control module, with the following connection relationship:

[0071] 1) DC side connection:

[0072] The positive and negative terminals of the DC bus (DCP, DCN) are directly connected to the DC ports (DC+, DC-) of the MMC NLM.

[0073] DC side monitoring parameters (DC current Idc3P, DC voltage Edc) are connected to the MMC NLM, and Edc is fed back to the Vdc control module in MMCEdoQ to provide a reference for DC voltage control.

[0074] 2) Connection between main circuit and control module:

[0075] The AC port of the MMC NLM is connected to the AC port of the MMCEdoQ control module to output the converted AC power to the control module.

[0076] The MMC NLM receives the control reference signal (Ref) output by MMCEdoQ and adjusts the switching state of the submodule through nearest level modulation (NLM) to achieve DC-AC power conversion.

[0077] 3) Internal connections of the control module:

[0078] Vdc control (DC voltage control): Receives DC voltage feedback Edc and reference signal, outputs adjustment command to MMC NLM, and stabilizes DC side voltage;

[0079] Q control (reactive power control): Interacts with the AC side to adjust the reactive power on the AC side and ensure the grid-connected power factor;

[0080] Auxiliary signals (Treference, AC Fit, ACFlic_flag): Treference is the control reference base, AC Fit enables AC side electrical parameter adaptation, and ACFlic_flag is used to identify the AC side operating or fault status.

[0081] Final result:

[0082] Stabilize DC-side voltage: Maintain stable ±500kV DC bus voltage through Vdc control module to adapt to power fluctuations in DC collection system;

[0083] Reactive power control on the AC side: The reactive power on the AC side (230kV power grid) is adjusted through the Q control module to ensure voltage stability and qualified power factor during grid connection;

[0084] In conjunction with grid connection logic: Through AC Fit (AC side adaptation) and ACFlic_flag (AC side status flag), safe and stable grid connection with the AC power grid is achieved.

[0085] The AC power grid is configured with the following parameters: voltage 230kV, frequency 50Hz, rated capacity 1000MVA, X / R ratio 7, and short-circuit ratio SCR=5. A power grid interface unit is also provided between the AC power grid and the MMC converter. The power grid interface unit includes a current transformer, a voltage transformer, and a protection switch, which are used to realize power metering and fault isolation.

[0086] Example 2

[0087] This embodiment, based on the offshore wind power DC collection topology circuit disclosed in Embodiment 1 above, discloses a DC line protection method, the specific method of which is as follows: Figure 5 As shown, it includes:

[0088] S1. Current transformers are installed at both ends of the connection line MN between the first-stage boost collection unit and the second-stage boost transmission unit. The direction of electrical energy flowing from the wind turbine side to the AC grid is defined as the positive direction of the current. The current at both ends of MN is collected in real time through the current transformers. and ;

[0089] S2, based on the collected data and Calculate differential current and braking current ;

[0090] S3, based on differential current and braking current Based on the numerical relationship and system voltage parameters, the fault type is identified and the corresponding protection action is triggered;

[0091] S4. After the protection action is triggered, the voltage and current parameters of the faulty line are continuously monitored. When the voltage of the faulty line is detected to be above the preset threshold of the rated voltage and the current is detected to be below the preset threshold of the normal operating current, and the duration of this state reaches the preset threshold, a reset signal is sent to the relevant circuit breaker to attempt reclosing.

[0092] If differential current is detected again within a preset time after reclosing... If the value exceeds the preset threshold, it is determined to be a permanent fault, the circuit breaker is locked, and a fault alarm signal is issued; otherwise, return to S2 to continue differential current operation. and braking current The calculation.

[0093] The sampling frequency of the current transformer shall not be less than 10kHz, and the sampling frequency of the voltage sensor shall not be less than 5kHz.

[0094] The braking current and differential current can be calculated using the following formula:

[0095]

[0096] Under normal operating conditions or when the fault point is outside the line:

[0097] Current at points M and N and Equal in size, opposite in direction, that is .

[0098] The differential current and braking current can be calculated using the following formula:

[0099]

[0100] The differential current is theoretically zero, the braking current is the load current, the protection is reliable, and it does not operate.

[0101] In step S3, the fault types include faults within the zone: positive ground fault, inter-electrode short circuit fault, and cascaded DC-DC inter-electrode short circuit fault, as well as faults outside the zone.

[0102] The specific process for determining the fault type as a positive ground fault is as follows: Figure 6 As shown, it is:

[0103] Fault diagnosis within the area:

[0104] (1) The differential and braking current diagrams during a positive ground fault are as follows: Figure 7 As shown in the diagram, the circled lines represent differential current, and the squared lines represent braking current.

[0105] Criterion: When differential current is detected The increase rate is greater than the preset increase rate threshold and > When this occurs, it is determined to be a positive grounding fault.

[0106] During a fault, the current at points M and N and The directions are the same. At this time, the differential current... The value increases significantly, approximately equal to the fault current; while The value is relatively small, at this time there is > The differential current is greater than the braking current. The protection immediately trips, opening the circuit breakers at both ends of the line.

[0107] Differential current The sharp pulse originates from the sudden change in current direction and capacitor discharge at the moment of the fault. Under normal operating conditions, the current across the protected line MN... With The differential currents are equal in magnitude but opposite in direction. Theoretically, the answer is zero. When a ground fault occurs at the positive terminal, the potential at the fault point drops sharply, causing the currents at both terminals M and N to point towards the fault point during the fault transient process; that is, their directions become the same. At this time, They no longer cancel each other out, and their value increases rapidly, approximately equal to the total fault current flowing to the fault point. (In the waveform...) The sharp peak current of approximately 3.5 kA was primarily caused by the instantaneous discharge of the DC bus capacitor near the fault point; this current had a high amplitude but decayed rapidly. Subsequently, the current-limiting control of the DC-DC converter in the topology quickly intervened, limiting the current fed into the fault point to a low level. It quickly falls back from the peak and stabilizes at a steady-state value that is higher than normal but lower than the peak.

[0108] Braking current The gradual change reflects the characteristics of the through current. During an in-zone fault, and Since they are in the same direction, their vector difference is much smaller than their vector sum, therefore The amplitude is also much smaller Waveform display The slight fluctuations only occur during faults, which is precisely what this mathematical relationship reflects. Persistently greater than The state clearly satisfies the differential protection's operating criteria. > This triggers the protection device to act immediately.

[0109] (2) Differential and braking current diagrams during inter-pole short circuit faults are shown below. Figure 8 As shown in the diagram, the circled lines represent differential current, and the squared lines represent braking current.

[0110] Criterion: When differential current is detected When the surge exceeds the preset action threshold and the system current amplitude reaches more than 10 times the normal operating current, it is determined to be an inter-pole short circuit fault.

[0111] The fault point is directly connected to the positive and negative terminals, and does not flow through the normal load path.

[0112] Current path: The fault current flows directly from the positive power supply to the negative power supply, with extremely low path impedance, resulting in a very large fault current (10-12 times its original value). For line differential protection, the fault current surges into the fault point from both ends of the line. At this point, the differential current will rapidly exceed the operating threshold, causing the protection device to trip.

[0113] At the instant an inter-pole short-circuit fault occurs, a low-impedance path is formed between the positive and negative poles at the fault point, causing a sharp increase in current across the line and a surge into the fault point. This process first triggers a momentary discharge of the nearby DC bus capacitor, generating an extremely high-amplitude inrush current, which in turn causes the braking current to... The fault current quickly reaches its peak value, providing a clear fault activation signal for the protection device. Subsequently, the fast current control loop of the DC-DC converter in the system quickly intervenes, actively limiting the output current of the power devices by adjusting the switching strategy, forcibly suppressing the fault current from its peak value to a safe level, which is manifested as a rapid current decay and oscillating transition process in the waveform.

[0114] After entering the fault transient steady state, the differential current The current gradually decreases to near zero, reflecting the restoration of dynamic balance between the currents at both ends of the line under active current limiting control; while the braking current... The current then stabilizes at a new platform higher than normal, and its amplitude is the steady-state short-circuit current limited by the system. This waveform characteristic not only verifies the reliability of the longitudinal differential protection criterion—that it can achieve rapid fault identification using the initial current peak value—but also highlights the core advantage of the power electronic converter in this topology: by actively controlling the destructive short-circuit current to transform it into a fault mode with controllable amplitude and clear characteristics, thereby ensuring the safety and selective protection of the equipment under extreme fault conditions.

[0115] (3) The inter-electrode current diagram when the cascaded DC-DC converter is short-circuited is as follows: Figure 9 As shown in the diagram, the circled line represents the positive current, and the boxed line represents the negative current. The voltage between the terminals of the DC-DC converter is as follows: Figure 10 As shown:

[0116] Criterion: Voltage sensors are installed at both the input and output ports of each DC-DC converter to collect the inter-electrode voltage of the DC-DC converter in real time. When inter-electrode voltage is detected The voltage is reduced to below 80% of its rated voltage, while simultaneously meeting the differential current requirement. Initial negative spike and differential current appear When the voltage drops rapidly to a preset low level and is accompanied by oscillation, it is determined to be a short circuit fault between the electrodes of the cascaded DC-DC converter.

[0117] The typical fault location is a DC-to-electrode short circuit in the second DC-DC converter. A current transformer is installed across the two terminals of the DC-DC converter, and the current flowing out of the positive terminal is... The current flowing out of the negative terminal is Under normal operating conditions = - When a fault occurs, it causes a local voltage collapse. The DC bus capacitor closest to the fault point will discharge instantaneously through its extremely low short-circuit impedance. This discharge current is very large and rapid, resulting in the first sharp negative spike in the current waveform. During this phase, the current is primarily determined by the natural response of the physical circuitry, before the control system can react in time.

[0118] Within milliseconds after the fault, the current rapidly decreased from 1.6kA, accompanied by oscillations, because the fast current inner loop control of the DC-DC converter began to intervene. The control system detected a sharp increase in current, far exceeding its reference or safety limits, and thus quickly adjusted the drive signal of the switching transistor to attempt to bring the current back within a controllable range. This control adjustment process triggers a transient response in the system, manifested as a decrease in current and oscillations.

[0119] After entering fault steady state, the current Instead of returning to the pre-fault level of 0.6kA, the voltage stabilizes at a new steady-state value, such as 0.2kA. This is because the DC-DC converter has switched to current-limiting control mode, where the controller limits the output current to a preset safety margin to protect the power semiconductor devices from overcurrent damage. Simultaneously, the transition resistance in the fault circuit causes the inter-electrode voltage to drop from the normal setting (20kV) to a lower level.

[0120] To accurately identify such faults, this solution installs a voltage detection device at the DCDC2 port to monitor the inter-electrode voltage in real time. Once the voltage is detected to drop below 80% of the original set value, the internal fault criterion is formed by combining the aforementioned current characteristics, including initial surge, dynamic response, and steady-state current limiting performance. This comprehensive identification method effectively activates the corresponding protection action and reliably identifies DC inter-electrode short-circuit faults in the DCDC2.

[0121] (4) The fault is outside the zone or the DC line is in normal operating condition:

[0122] When differential current is detected ≤0.1kA and braking current When the current is equal to the load current during normal system operation, it is determined to be an external fault or normal operating condition, and no protection action is triggered.

[0123] Thanks to the topology's limitation on fault current, the protection device can employ more sensitive setting parameters. The isolation and self-current limiting characteristics of the DC-DC converter ensure that any fault on the low-voltage side cannot obtain significant short-circuit capacity support from the high-voltage side. Therefore, the absolute amplitude of the fault current is significantly limited to a lower level, improving the detection capability for high-resistance faults. Simultaneously, clear electrical boundaries ensure the selectivity of protection actions, avoiding cascading tripping. Simulation verification shows that this scheme can achieve millisecond-level fault isolation, significantly improving system reliability.

[0124] This invention achieves a balance between safety and economy in offshore wind power DC collection systems through innovative topology design and protection strategies. Tiered voltage design reduces equipment insulation requirements, fault current limiting reduces reliance on circuit breaker breaking capacity, and optimized protection criteria ensure rapid and accurate fault isolation. This complete solution provides crucial technical support for large-scale offshore wind power development.

[0125] Example 3

[0126] Based on Embodiment 2, this embodiment provides an electronic device, including: one or more processors and a memory, wherein the memory stores one or more programs, the one or more programs including instructions for executing the aforementioned DC line protection method.

[0127] At the hardware level, the electronic device includes a processor, an internal bus, a network interface, memory, and non-volatile memory, and may also include other hardware required for the business operations. The processor reads the corresponding computer program from the non-volatile memory into memory and then runs it to implement the aforementioned DC line protection method. Of course, in addition to software implementation, this invention does not exclude other implementation methods, such as logic devices or a combination of hardware and software, etc. That is to say, the execution subject of the following processing flow is not limited to individual logic units, but can also be hardware or logic devices.

[0128] Memory may include non-persistent storage in computer-readable media, such as random access memory (RAM) and / or non-volatile memory, such as read-only memory (ROM) or flash RAM. Memory is an example of computer-readable media.

[0129] Computer-readable media include both permanent and non-permanent, removable and non-removable media that can store information using any method or technology. Information can be computer-readable instructions, data structures, modules of programs, or other data. Examples of computer storage media include, but are not limited to, phase-change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), other types of random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technologies, CD-ROM, digital versatile optical disc (DVD) or other optical storage, magnetic tape, disk storage or other magnetic storage devices, or any other non-transferable medium that can be used to store information accessible by a computing device. As defined herein, computer-readable media does not include transient computer-readable media, such as modulated data signals and carrier waves.

[0130] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A topology circuit for offshore wind power DC collection, characterized in that, The circuit includes: a wind turbine-side converter unit, a first-stage boost and collection unit, a second-stage boost and transmission unit, and a grid-connected converter unit connected in sequence; The wind turbine-side converter unit is used to convert the electrical energy output by the permanent magnet direct drive wind turbine into low-voltage direct current. The first-stage boosting and collection unit includes at least one DC-DC converter for boosting low-voltage DC power to medium voltage to complete power collection. The secondary boost transmission unit includes a DC-DC converter for boosting medium-voltage DC power to high voltage to achieve long-distance power transmission; The grid-connected converter unit includes an MMC converter, which is used to convert high-voltage direct current into high-voltage alternating current and connect it to the AC power grid. The offshore wind power DC collection topology circuit operates based on a DC line protection method, which includes: S1, current transformers are installed at both ends of the connection line MN between the first-stage boost collection unit and the second-stage boost transmission unit. The direction of electrical energy flowing from the wind turbine side to the AC grid is defined as the positive direction of the current. The current transformers are used to collect the current at both ends of the connection line between the first-stage boost collection unit and the second-stage boost transmission unit in real time. and ; S2, according to the collected with computing the differential current and the braking current ; S3, according to the differential current and the braking current numerical relationship and system voltage parameters, to determine the fault type and trigger the corresponding protection action; In S3, the fault types include positive grounding fault, inter-electrode short circuit fault, cascaded DC-DC inter-electrode short circuit fault, and fault outside the zone; The specific process for determining the fault type as the positive ground fault is as follows: When the differential current is detected the increase rate is greater than a preset increase rate threshold and > a positive electrode ground fault is determined. The specific process for determining the fault type as the inter-electrode short-circuit fault is as follows: When the differential current When the differential current jumps over the preset action threshold and the system current amplitude reaches more than 10 times of the normal operation current, it is determined as an inter-pole short circuit fault. The specific process for determining the fault type as a cascaded DC-DC inter-electrode short circuit fault is as follows: Voltage sensors are installed at both the input and output ports of each DC-DC converter to collect the inter-electrode voltage of the DC-DC converter in real time. When inter-electrode voltage is detected The voltage is reduced to below 80% of its rated voltage, while simultaneously meeting the differential current requirement. Initial negative spike and differential current appear When the voltage drops rapidly to the preset low voltage and is accompanied by oscillation, it is determined to be a short circuit fault between the electrodes of the cascaded DC-DC converter. The specific process for determining whether a fault is an external fault or a DC line in normal operation is as follows: When differential current is detected ≤0.1kA and braking current When the current is equal to the load current during normal system operation, it is determined to be an external fault or a normal operating state. After determining that the fault type is a positive ground fault, immediately send a trip signal to the circuit breakers at both ends of line MN to disconnect the faulty line; After determining that the fault type is the inter-pole short circuit fault, a trip signal is sent to the circuit breakers and power switches of the relevant DC-DC converters at both ends of the faulty line within a preset time. After determining that the fault type is a cascaded DC-DC inter-pole short circuit fault, immediately send a shutdown signal to all power switches of the faulty DC-DC2 converter and disconnect the collection line circuit breaker connected to the faulty DC-DC2 converter. If the fault type is determined to be an external fault or the DC line is in normal operating condition, no protection action will be triggered.

2. The topology circuit according to claim 1, wherein, Both the DC-DC1 converter and the DC-DC2 converter adopt a dual-DAB cascaded structure, and both the DC-DC1 converter and the DC-DC2 converter are capable of constant DC voltage control. The dual-DAB cascaded structure includes two DAB converters, an inductor and a capacitor, and the inductance value of the inductor in the DC-DC2 converter is greater than the inductance value of the inductor in the DC-DC1 converter.

3. The topology of DC power collection of offshore wind power according to claim 2, characterized in that, In the dual DAB cascaded structure, the DAB converter adopts a symmetrical dual-bridge arm structure. Each DAB converter includes a high-frequency transformer, power switching transistors and filter elements. Each bridge arm is composed of multiple sets of power switching transistors connected in series or in parallel. The output end of the bridge arm is connected in series with the filter element and then connected to the corresponding winding of the high-frequency transformer. Electrical isolation and voltage level conversion are achieved through the transformer.

4. The topology circuit according to claim 1, wherein, The MMC converter adopts a half-bridge pseudo-bipolar structure. The upper and lower bridge arms of the MMC converter are each equipped with no less than 100 sub-levels. The MMC converter can perform constant DC voltage control and constant reactive power control. The constant DC voltage control stabilizes the DC line voltage at high voltage, and the constant reactive power control maintains the reactive power balance of the system.

5. The topology of direct current collection of offshore wind power according to claim 1, characterized in that, A grid interface unit is also provided between the AC grid and the MMC converter. The grid interface unit includes a current transformer, a voltage transformer, and a protection switch, which are used to realize power metering and fault isolation.

6. A DC line protection method based on the offshore wind power DC collection topology circuit according to any one of claims 1-5, characterized in that, The method includes: S1, current transformers are installed at both ends of the connection line MN between the first-stage boost collection unit and the second-stage boost transmission unit. The direction of electrical energy flowing from the wind turbine side to the AC grid is defined as the positive direction of the current. The current transformers are used to collect the current at both ends of the connection line between the first-stage boost collection unit and the second-stage boost transmission unit in real time. and ; S2, based on the collected data and Calculate differential current and braking current ; S3, according to the differential current and the numerical relationship between the braking current and the system voltage parameters, to determine the fault type and trigger the corresponding protection action; S4. After the protection action is triggered, the voltage and current parameters of the faulty line are continuously monitored. When the voltage of the faulty line is detected to be above the preset threshold of the rated voltage and the current is detected to be below the preset threshold of the normal operating current, and the duration of this state reaches the preset threshold, a reset signal is sent to the relevant circuit breaker to attempt reclosing. If differential current is detected again within a preset time after reclosing... If the value exceeds the preset threshold, it is determined to be a permanent fault, the circuit breaker is locked, and a fault alarm signal is issued; otherwise, return to S2 to continue differential current operation. and braking current The calculation.

Images