Offshore new energy power transmission system via flexible direct current and fault ride-through cooperative control method thereof

Abstract

This invention discloses a flexible DC transmission system for offshore renewable energy and its fault ride-through coordinated control method in the field of DC transmission grid and equipment technology. The method includes: in response to a DC submarine cable fault, controlling the transformer tripping between the offshore renewable energy power station and the half-bridge converter, connecting the DC energy dissipation device in parallel with the straight submarine cable, and locking the half-bridge converter; in response to a DC overhead line fault, controlling the hybrid converter to reduce the DC voltage of the DC overhead line to 0kV and the DC current to 0A, isolating the half-bridge converter and the DC overhead line with a DC circuit breaker, and implementing an active power self-balancing control strategy for the DC energy dissipation device to achieve a balance between the active power of the offshore renewable energy power station and the active power consumed. This invention solves the problem of low efficiency and high manpower costs associated with the manual troubleshooting and resolution of DC-side faults in existing technologies.

Description

Technical Field

[0001] This invention relates to the field of DC power transmission grid and equipment technology, and in particular to a flexible DC transmission system for offshore new energy and its fault ride-through collaborative control method. Background Technology

[0002] Currently, long-distance power transmission for deep-sea new energy projects and large-scale new energy bases generally adopts flexible DC systems. Due to the large active power capacity of deep-sea new energy, it is necessary to consider the fault ride-through capability of the DC side. How to handle the faults on the DC side in a timely manner and respond accurately has become a technical challenge. Existing technologies usually use manual methods to troubleshoot and resolve DC side faults, which is inefficient and costly in terms of manpower.

[0003] Therefore, there is an urgent need for a flexible DC transmission system for offshore new energy and its fault ride-through collaborative control method to solve the above-mentioned technical problems. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a flexible DC transmission system for marine new energy and its fault ride-through collaborative control method, which can solve the technical problems of low efficiency and high manpower cost of the prior art in manually checking and solving DC side faults.

[0005] To achieve the above objectives, the present invention is implemented using the following technical solution:

[0006] In a first aspect, the present invention provides a flexible DC transmission system for offshore new energy sources, comprising:

[0007] At least one offshore new energy power station, at least one half-bridge converter connected to the offshore new energy power station via a transformer, an onshore switchgear station connected to the half-bridge converter via a straight submarine cable, a hybrid converter connected to the onshore switchgear station via a DC overhead line, and an AC power grid connected to the hybrid converter via a transformer.

[0008] The onshore switchgear station includes a DC circuit breaker connected in series with the DC submarine cable and a DC power dissipation device connected in parallel with the DC submarine cable.

[0009] Furthermore, it also includes: a dynamic compensation module for enabling offshore new energy power stations to operate synchronously without high-speed communication, avoiding system oscillations due to uneven power distribution, wherein the expression of the dynamic compensation module includes:

[0010] ;

[0011] in, This is the corrected frequency reference value. The frequency reference value before correction. and All are virtual inertia coefficients. The reactive power coefficient, This is reactive power deviation. For frequency deviation, The AC voltage reference value before correction. This is the corrected AC voltage reference value.

[0012] In a second aspect, the present invention provides a fault ride-through cooperative control method for any of the above-mentioned offshore new energy transmission systems via flexible DC transmission, further comprising:

[0013] The response is a DC submarine cable fault, which trips the transformer between the offshore new energy power station and the half-bridge converter, connects the DC energy dissipation device in parallel with the straight submarine cable, and locks out the half-bridge converter.

[0014] In response to a DC overhead line fault, the hybrid converter is controlled to reduce the DC voltage of the DC overhead line to 0kV and the DC current to 0A. The DC circuit breaker isolates the half-bridge converter and the DC overhead line. The DC energy-consuming device is put into active power self-balancing control strategy to achieve a balance between the active power and the active power consumed by the offshore new energy power station.

[0015] Furthermore, responses to DC submarine cable faults include:

[0016] Within a preset time period, the change in DC current, change in DC voltage, direction of DC current, and rate of change of DC current at the DC submarine cable measuring point are acquired. The DC submarine cable measuring point includes the first DC submarine cable measuring point of the onshore switchgear and the second DC submarine cable measuring point of the half-bridge converter.

[0017] If the change in DC current at the DC cable measuring point is greater than the preset current change value, the change in DC voltage is greater than the preset voltage change value, and the rate of change of DC current is greater than the preset rate of change of current, the current at the first DC cable measuring point flows towards the half-bridge converter, and the current at the second DC cable measuring point flows towards the onshore switch collection station, it is determined to be a DC cable fault.

[0018] Furthermore, responses to DC overhead line faults include:

[0019] Within a preset time period, the change in DC current, the change in DC voltage, the direction of DC current, and the rate of change of DC current at the DC overhead line measuring point are acquired. The DC overhead line measuring point includes the first DC overhead line measuring point of the onshore switchgear collection station and the second DC overhead line measuring point of the onshore converter station.

[0020] If the change in DC current at the overhead DC line is greater than the preset current change value, the change in DC voltage is greater than the preset voltage change value, and the rate of change of DC current is greater than the preset rate of change of current, the current at the first overhead DC line measuring point flows towards the onshore converter station, and the current at the second overhead DC line measuring point flows towards the half-bridge converter, it is determined to be a fault in the overhead DC line.

[0021] Furthermore, the active power self-balancing control strategy includes:

[0022] The active power of the offshore renewable energy power station is latched by the DC energy dissipation device, and the energy dissipation resistance of the DC energy dissipation device is adjusted to achieve a balance between the active power of the offshore renewable energy power station and the active power consumed by the DC energy dissipation device. The expression includes:

[0023] ;

[0024] in, The active power consumed by DC power-consuming devices. This is the DC voltage of the half-bridge converter. This is the energy-consuming resistor for a DC energy-consuming device.

[0025] Furthermore, it also includes:

[0026] When the difference between the DC voltage and AC voltage of the offshore new energy power station and its rated value is within the preset difference threshold, the DC energy consumption device is controlled to reduce the active power of the offshore new energy power station to 0MW according to the preset slope.

[0027] When an increase in the frequency of the AC power grid is detected, the active power output of the AC power grid is reduced to achieve power self-balancing between the offshore new energy power station, the half-bridge converter, and the DC energy-consuming device.

[0028] Furthermore, it also includes:

[0029] After a preset deionization time, the DC voltage of the onshore switchgear station rises from 0kV to the rated value. In response to the detection that the DC voltage difference between the half-bridge converter and the hybrid converter is within the preset differential voltage threshold, the DC circuit breaker is closed, and the offshore renewable energy power station resumes transmitting active power to the AC grid.

[0030] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0031] This invention first proposes a flexible DC transmission system for marine renewable energy. Based on this system, and taking into account the differences in the differential components and directions of DC current during faults in overhead DC lines and submarine DC cables, a collaborative method for fault crossing and isolation applicable to different fault points is proposed. This method enables multi-terminal flexible DC fault crossing during overhead DC line faults and fault isolation during submarine DC cable faults, thereby ensuring reliable isolation and fault crossing of DC-side faults, guaranteeing renewable energy absorption and the safe and stable operation of the system. It also achieves steady-state operation of the DC voltage and AC voltage amplitude and frequency at the offshore station during fault periods, improving the efficiency of fault response and reducing manpower costs. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of a flexible DC transmission system for offshore new energy provided in an embodiment of the present invention;

[0033] Figure 2 This is a flowchart illustrating the collaborative control method for fault ride-through of offshore new energy via flexible DC transmission system provided in an embodiment of the present invention.

[0034] Figure 3 This is a block diagram of the negative pressure control logic in the collaborative control method for fault ride-through of a flexible DC transmission system for offshore new energy provided in this embodiment of the invention.

[0035] Figure 4 This is a block diagram of the energy consumption power self-balancing method in the fault ride-through collaborative control method of the offshore new energy transmission system via flexible DC transmission provided in this embodiment of the invention. Detailed Implementation

[0036] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the embodiments and specific features in the embodiments are detailed descriptions of the technical solution of the present application, rather than limitations thereof. In the absence of conflict, the embodiments and technical features in the embodiments can be combined with each other.

[0037] In this invention, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent three cases: A alone, A and B together, or B alone. Additionally, in this invention, the character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0038] Example 1:

[0039] This invention provides a flexible DC transmission system for offshore new energy sources, such as... Figure 1 As shown, it includes:

[0040] At least one offshore new energy power station, at least one half-bridge converter connected to the offshore new energy power station via a transformer, an onshore switchgear station connected to the half-bridge converter via a straight submarine cable, a hybrid converter connected to the onshore switchgear station via a DC overhead line, and an AC power grid connected to the hybrid converter via a transformer.

[0041] The onshore switchgear station includes a DC circuit breaker connected in series with the DC submarine cable and a DC power dissipation device connected in parallel with the DC submarine cable. The DC circuit breaker is an attached... Figure 1 The DC-DC CB in the middle, the DC power consumption device is attached Figure 1 The DBS, DC circuit breaker and DC power dissipation device mentioned are all conventional existing technologies, and will not be described in detail here.

[0042] In this embodiment, the offshore renewable energy power station may include n wind turbines. The wind turbines are designed with two control modes: grid-following control and grid-connection control. When there is no DC-side fault, the wind turbines operate in grid-following control mode. When there is a DC-side fault, the wind turbines operate in grid-connection control mode. When grid-following control is used, the wind turbines operate in maximum power point tracking mode. When grid-connection control is used, the wind turbines use grid-connection control strategies such as droop control or virtual synchronous machines to control the AC voltage amplitude and frequency at the grid connection point in real time.

[0043] Regarding the straight submarine cable and DC overhead line in this embodiment, the DC overhead line is usually limited by cost and does not have an external insulation layer, metal sheath, etc., making it prone to faults such as lightning strikes, while the DC submarine cable will not.

[0044] The half-bridge converter adopts either grid-connected control or constant DC voltage control. When there is no DC-side fault, it operates in grid-connected control VF mode. When there is a DC-side fault, it operates in constant DC voltage control mode. When the half-bridge converter is in grid-connected control mode, it controls the AC voltage amplitude and frequency of the offshore AC grid. When the constant DC voltage control mode is adopted, it controls the DC voltage of the converter.

[0045] The offshore renewable energy transmission system via flexible DC also includes:

[0046] A dynamic compensation module is used to enable offshore renewable energy power plants to operate synchronously without high-speed communication, avoiding system oscillations due to uneven power distribution. The offshore renewable energy power plants employ unified controller parameters and unified constraint parameters, and the dynamic compensation correction module compensates for regional differences in wind turbines. The expression of the dynamic compensation module includes:

[0047] ;

[0048] in, This is the corrected frequency reference value. The frequency reference value before correction. and All are virtual inertia coefficients. The reactive power coefficient, This is reactive power deviation. For frequency deviation, The AC voltage reference value before correction. This is the corrected AC voltage reference value.

[0049] In this embodiment, the aforementioned offshore new energy transmission system via flexible DC transmission may include offshore wind farms equipped with wind turbine coordination controllers, offshore converters equipped with DC control systems and DC protection hosts, onshore switchgear stations equipped with DC control hosts and DC protection hosts, and onshore converter stations equipped with DC control and DC protection hosts.

[0050] The offshore wind turbine coordination controller communicates with the wind turbine controller via gigabit Ethernet; the offshore station DC control system communicates with the wind turbine coordination control system via gigabit Ethernet; the offshore station DC control system communicates with the offshore DC protection system via gigabit Ethernet; the offshore DC control system communicates with the onshore switchgear DC protection system via gigabit Ethernet; the onshore switchgear DC control system communicates with the onshore switchgear DC protection system via gigabit Ethernet; and the DC circuit breaker communicates with the onshore switchgear DC control system via gigabit Ethernet.

[0051] Example 2:

[0052] Figure 2 This is a flowchart of the fault ride-through coordinated control method for offshore new energy via a flexible DC transmission system according to Embodiment 1 of the present invention. This flowchart only illustrates the logical sequence of the method described in this embodiment. Under the premise of no conflict, in other possible embodiments of the present invention, different methods may be used. Figure 2 The steps shown or described are performed in the indicated order. Specifically, the following steps are included:

[0053] Step 1: As Figure 2 As shown, the response is a DC submarine cable fault, which trips the transformer between the offshore new energy power station and the half-bridge converter, connects the DC energy dissipation device in parallel with the straight submarine cable to reduce the DC voltage of the DC submarine cable, locks out the half-bridge converter, and allows other branches without DC submarine cable faults to continue transmitting active power to the onshore AC power grid.

[0054] Step 2: In response to a DC overhead line fault, the hybrid converter is controlled to initiate negative voltage control, reducing the DC voltage of the DC overhead line to 0kV and the DC current to 0A within a preset time. The onshore converter station will no longer inject fault current into the fault point. The DC circuit breaker will isolate the half-bridge converter and the DC overhead line, and the half-bridge converter will no longer inject fault current into the fault point of the overhead line. When the DC energy consumption device receives a DC overhead line fault, it will implement an active power self-balancing control strategy: When the DC energy consumption device receives a DC-side fault, it will latch the active power of the wind turbine grid connection point. The current active power will be used as the initial target value for energy consumption. By adjusting the resistance value of the energy consumption resistor, the active power of the offshore new energy power station and the active power consumed will be balanced.

[0055] The negative pressure control logic block diagram is as follows: Figure 3 As shown, where, This refers to the DC current on the overhead line side of the hybrid converter output. The DC voltage output from the hybrid converter on the overhead DC line side. This is a reference value for the DC voltage on the overhead DC line side of the hybrid converter output.

[0056] Specifically, methods for determining a response as a DC submarine cable fault include:

[0057] Within a preset time period, the change in DC current, change in DC voltage, direction of DC current, and rate of change of DC current at the DC submarine cable measuring point are acquired. The DC submarine cable measuring point includes the first DC submarine cable measuring point of the onshore switchgear and the second DC submarine cable measuring point of the half-bridge converter.

[0058] If the change in DC current at the DC cable measuring point is greater than the preset current change value, the change in DC voltage is greater than the preset voltage change value, and the rate of change of DC current is greater than the preset rate of change of current, the current at the first DC cable measuring point flows towards the half-bridge converter, and the current at the second DC cable measuring point flows towards the onshore switch collection station, it is determined to be a DC cable fault.

[0059] Specifically, methods for determining a DC overhead line fault include:

[0060] Within a preset time period, the change in DC current, the change in DC voltage, the direction of DC current, and the rate of change of DC current at the DC overhead line measuring point are acquired. The DC overhead line measuring point includes the first DC overhead line measuring point of the onshore switchgear collection station and the second DC overhead line measuring point of the onshore converter station.

[0061] If the change in DC current at the overhead DC line is greater than the preset current change value, the change in DC voltage is greater than the preset voltage change value, and the rate of change of DC current is greater than the preset rate of change of current, the current at the first overhead DC line measuring point flows towards the onshore converter station, and the current at the second overhead DC line measuring point flows towards the half-bridge converter, it is determined to be a fault in the overhead DC line.

[0062] The active power self-balancing control strategy includes:

[0063] When a fault is detected in the DC overhead line, the DC circuit breaker is tripped at the same time as the negative voltage control is triggered, isolating the offshore converter station from the onshore converter station. It is necessary to balance the active power of the offshore converter station, DC energy consumption and offshore new energy sources.

[0064] Specifically, the active power of the offshore renewable energy power station is latched by the DC energy-consuming device. The current active power of the DC energy-consuming device is used as the initial target value for energy input. The energy-consuming resistance of the DC energy-consuming device is adjusted to achieve a balance between the active power of the offshore renewable energy power station and the active power consumed by the DC energy-consuming device. The expression includes:

[0065] ;

[0066] in, The active power consumed by DC power-consuming devices. This is the DC voltage of the half-bridge converter. The power-consuming resistor of a DC power-consuming device;

[0067] The energy consumption power self-balancing block diagram is as follows: Figure 4 As shown, where, The active power of offshore renewable energy power plants. The DC current consumed by DC power-consuming devices. This is the reference value for the DC voltage of the DC power consumption device.

[0068] The collaborative control method for fault ride-through of offshore new energy via flexible DC transmission system provided in this embodiment also includes:

[0069] When the difference between the DC voltage and AC voltage of the offshore new energy power station and its rated value is within the preset stable threshold, the DC energy-consuming device is controlled to reduce the active power of the offshore new energy power station to 0MW according to the preset slope, so as to avoid the DC energy-consuming device consuming active power for a long time and being taken out of operation.

[0070] When an increase in AC grid frequency is detected, the active power output of the offshore renewable energy power station is adjusted in real time based on the droop rate formula to achieve power self-balancing among the offshore renewable energy power station, half-bridge converter, and DC energy-consuming device. The expression includes:

[0071] ;

[0072] in, This refers to the active power output in real time for offshore renewable energy power plants. This is the initial active power. This is the frequency droop factor. For the system's real-time frequency, This is the system's rated frequency.

[0073] The collaborative control method for fault ride-through of offshore new energy via flexible DC transmission system provided in this embodiment also includes:

[0074] After a preset deionization time (150ms in this embodiment), the DC voltage of the onshore switchgear station rises from 0kV to the rated value. In response to the detection that the DC voltage difference between the half-bridge converter and the hybrid converter is within the preset differential voltage threshold, the DC circuit breaker is closed. After the DC circuit breaker closes, the control mode of the offshore renewable energy power station is switched to VF grid connection control, and a command to exit grid connection control is sent to the wind turbine coordination controller. The wind turbine coordination controller sends a command to the wind turbine to switch to maximum power point tracking mode, and the offshore renewable energy power station resumes transmitting active power to the AC grid.

[0075] After the preset deionization time has elapsed, the onshore switchgear station switches to DC voltage control. During the rise of its DC voltage, the directional overcurrent micro-component protection trips again (i.e., the response to the DC overhead line fault mentioned above). It is determined to be a permanent fault in the DC overhead line, and the offshore new energy transmission system via the flexible DC transmission system needs to be shut down. At this time, the half-bridge converter is locked and the incoming line AC circuit breaker is tripped, the wind farm incoming line circuit breaker is locked and tripped, and a shutdown command is issued to the wind turbine. The entire multi-terminal DC transmission system is shut down.

[0076] Those skilled in the art will understand that embodiments of this application can be provided as methods, apparatus, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0077] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A flexible DC transmission system for offshore new energy, characterized in that, include: At least one offshore new energy power station, at least one half-bridge converter connected to the offshore new energy power station via a transformer, an onshore switchgear station connected to the half-bridge converter via a straight submarine cable, a hybrid converter connected to the onshore switchgear station via a DC overhead line, and an AC power grid connected to the hybrid converter via a transformer. The onshore switchgear station includes a DC circuit breaker connected in series with the DC submarine cable and a DC power dissipation device connected in parallel with the DC submarine cable.

2. The flexible DC transmission system for offshore new energy as described in claim 1, characterized in that, Also includes: A dynamic compensation module is used to enable offshore renewable energy power stations to operate synchronously without high-speed communication, avoiding system oscillations due to uneven power distribution. The expression of the dynamic compensation module includes: ； in, This is the corrected frequency reference value. The frequency reference value before correction. and All are virtual inertia coefficients. The reactive power coefficient, This is reactive power deviation. For frequency deviation, The AC voltage reference value before correction. This is the corrected AC voltage reference value.

3. A fault ride-through cooperative control method for a flexible DC transmission system for offshore new energy sources according to any one of claims 1-2, characterized in that, Also includes: The response is a DC submarine cable fault, which trips the transformer between the offshore new energy power station and the half-bridge converter, connects the DC energy dissipation device in parallel with the straight submarine cable, and locks out the half-bridge converter. In response to a DC overhead line fault, the hybrid converter is controlled to reduce the DC voltage of the DC overhead line to 0kV and the DC current to 0A. The DC circuit breaker isolates the half-bridge converter and the DC overhead line. The DC energy-consuming device is put into active power self-balancing control strategy to achieve a balance between the active power and the active power consumed by the offshore new energy power station.

4. The collaborative control method for fault ride-through of a flexible DC transmission system for offshore new energy as described in claim 3, characterized in that, Responses to DC submarine cable faults include: Within a preset time period, the change in DC current, change in DC voltage, direction of DC current, and rate of change of DC current at the DC submarine cable measuring point are acquired. The DC submarine cable measuring point includes the first DC submarine cable measuring point of the onshore switchgear and the second DC submarine cable measuring point of the half-bridge converter. If the change in DC current at the DC cable measuring point is greater than the preset current change value, the change in DC voltage is greater than the preset voltage change value, and the rate of change of DC current is greater than the preset rate of change of current, the current at the first DC cable measuring point flows towards the half-bridge converter, and the current at the second DC cable measuring point flows towards the onshore switch collection station, it is determined to be a DC cable fault.

5. The collaborative control method for fault ride-through of a flexible DC transmission system for offshore new energy as described in claim 3, characterized in that, Responses to DC overhead line faults include: Within a preset time period, the change in DC current, the change in DC voltage, the direction of DC current, and the rate of change of DC current at the DC overhead line measuring point are acquired. The DC overhead line measuring point includes the first DC overhead line measuring point of the onshore switchgear collection station and the second DC overhead line measuring point of the onshore converter station. If the change in DC current at the overhead DC line is greater than the preset current change value, the change in DC voltage is greater than the preset voltage change value, and the rate of change of DC current is greater than the preset rate of change of current, the current at the first overhead DC line measuring point flows towards the onshore converter station, and the current at the second overhead DC line measuring point flows towards the half-bridge converter, it is determined to be a fault in the overhead DC line.

6. The collaborative control method for fault ride-through of a flexible DC transmission system for offshore new energy as described in claim 3, characterized in that, The active power self-balancing control strategy includes: The active power of the offshore renewable energy power station is latched by the DC energy dissipation device, and the energy dissipation resistance of the DC energy dissipation device is adjusted to achieve a balance between the active power of the offshore renewable energy power station and the active power consumed by the DC energy dissipation device. The expression includes: ； in, The active power consumed by DC power-consuming devices. This is the DC voltage of the half-bridge converter. This is the energy-consuming resistor for a DC energy-consuming device.

7. The collaborative control method for fault ride-through of a flexible DC transmission system for offshore new energy as described in claim 3, characterized in that, Also includes: When the difference between the DC voltage and AC voltage of the offshore new energy power station and its rated value is within the preset difference threshold, the DC energy consumption device is controlled to reduce the active power of the offshore new energy power station to 0MW according to the preset slope. When an increase in the frequency of the AC power grid is detected, the active power output of the AC power grid is reduced to achieve power self-balancing between the offshore new energy power station, the half-bridge converter, and the DC energy-consuming device.

8. The collaborative control method for fault ride-through of offshore new energy via flexible DC transmission system according to claim 3, characterized in that, Also includes: After a preset deionization time, the DC voltage of the onshore switchgear station rises from 0kV to the rated value. In response to the detection that the DC voltage difference between the half-bridge converter and the hybrid converter is within the preset differential voltage threshold, the DC circuit breaker is closed, and the offshore renewable energy power station resumes transmitting active power to the AC grid.

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