Offshore wind power transmission system with DRU-MMC and method for AC fault ride-through of transmission end

Abstract

The present application belongs to the technical field of power grid operation, and particularly relates to a method for AC fault ride-through of a sending end of a sea wind power DRU-MMC sending-out system, which comprises starting determination on the fault of the sending end AC power grid, determining that the sending end AC power grid is faulty and the diode rectifier unit (DRU) in the sea wind power DRU-MMC sending-out system is cut off when two criteria that the AC voltage amplitude change rate is less than zero and the DC sending-out system current is equal to zero are simultaneously met; and a DC voltage deviation control loop is put into the constant DC voltage control loop of the receiving end MMC, so that the DC voltage reference value is dynamically adjusted during the fault of the sending end AC power grid, the DRU is kept on during the fault, the fault ride-through capability of the DC transmission line is significantly improved, the equipment fault risk caused by the energy consumption of the wind turbine DC load relief in the traditional method is effectively avoided, the power shortage problem of the receiving end power grid under the fault is relieved, and the continuous and stable operation of the system is ensured.

Description

Technical Field

[0001] This invention belongs to the field of power grid operation technology, and specifically relates to a method for AC fault ride-through at the sending end of an offshore wind power transmission system via DRU-MMC. Background Technology

[0002] As the global energy structure shifts towards low-carbon transformation, large-capacity offshore wind farms have become the core direction for the growth of new energy installed capacity due to their stable resources and significant economies of scale. High-voltage direct current transmission has become the mainstream solution for transmitting electricity on land. Among them, the combination of diode rectifier unit (DRU) and modular multilevel converter (MMC) is widely used in deep-sea wind power projects because it can reduce the size and cost of offshore platforms. In addition, wind turbine units need to have grid-type control capabilities to establish an offshore AC power grid.

[0003] However, the existing DRU-MMC transmission system has significant shortcomings in the event of AC failure at the sending end: some solutions rely on the wind turbine DC unloading load to dissipate redundant power. When a failure occurs, the DRU is cut off, causing the DC channel to be interrupted. The unloaded load is subjected to energy surges far exceeding the rated capacity, which can easily lead to equipment failure. At the same time, the power grid at the receiving end loses power support, threatening the grid security. Summary of the Invention

[0004] To address the aforementioned shortcomings of existing technologies, this invention provides a method for AC fault ride-through at the sending end of an offshore wind power transmission system via DRU-MMC. The offshore wind power transmission system via DRU-MMC is equipped with a DC voltage deviation control loop, which is integrated into the constant DC voltage controller of the onshore receiving-end MMC in the offshore wind power transmission system via DRU-MMC. The DC voltage deviation control loop includes a detection unit, a current deviation calculation unit, a proportional-integral controller, and a voltage reference value superposition unit, used to generate a negative DC voltage deviation command and adjust the MMC DC voltage reference value during a fault.

[0005] The methods include:

[0006] S1. Start-up judgment for AC grid fault at the sending end. When the two criteria of AC voltage amplitude change rate being less than zero and DC transmission system current being equal to zero are met simultaneously, it is determined that the AC grid at the sending end has a fault, and the diode rectifier unit DRU in the offshore wind power transmission system through DRU-MMC is cut off.

[0007] S2. Implement the active DC voltage control strategy for the receiver-end modular multilevel converter (MMC), and add a DC voltage deviation control loop to the constant DC voltage control loop of the receiver-end MMC:

[0008] S21. The actual value of DC current is detected by the detection unit in the DC voltage deviation control loop, and the current deviation between the actual value of DC current and the preset expected DC current reference value under fault is calculated by the current deviation calculation unit.

[0009] S22. Input the current deviation into the proportional-integral controller and output the DC voltage deviation command value. The DC voltage deviation command value is negative.

[0010] S23. The DC voltage deviation command value is superimposed with the rated voltage of the DC transmission link in the voltage reference value superposition unit to obtain the reference value of the DC voltage control loop of the MMC after voltage reduction, and the DC side voltage of the receiving end MMC is adjusted accordingly. When the DC side voltage of the DRU is lower than its AC side voltage amplitude, the DRU resumes conduction, the DC current is transmitted in the positive direction, and the power transmission to the receiving end grid is restored.

[0011] Further improvements to this technical solution include step S1, which includes:

[0012] The AC voltage amplitude of the offshore AC power grid is monitored in real time by the detection unit. Real-time trend of change, when the rate of change of AC voltage amplitude is detected When the AC voltage drops, the detection unit detects the DC current between the DRU and MMC. When the detected current value is 0, the DRU is determined to be cut off. When both of the above criteria are met, it can be determined that the AC grid at the sending end has a fault and the DC transmission line has no power transmission.

[0013] Further improvements to this technical solution include that, in step S2, the control principle of the DC voltage deviation control loop is based on the voltage-current relationship of the DC circuit. At that time, the DC side voltage of the DRU MMC DC side voltage and DC current Satisfying the formula: ;in This is a reference value for DC current. This is the equivalent resistance of a DC line.

[0014] A further improvement to this technical solution is that in step S2, when the DC current reaches the reference value, i.e. At that time, the DC side voltage of the DRU MMC DC side voltage under fault conditions The formula for DC current is: .

[0015] Further improvements to this technical solution include, in step S2, […]. and Subtracting the DC circuit formulas at the given time, we obtain the DC voltage deviation. : ,in .

[0016] A further improvement to this technical solution is that, in step S2, the DC voltage deviation control loop is implemented using a PI controller, specifically by converting the actual value of the DC current... Compared with reference value The difference is input to the PI controller, and the output is the DC voltage deviation. Furthermore, a limit of 0 is set at the difference point to prevent... Time The output causes interference.

[0017] Further improvements to this technical solution include, in step S2, the reference value of the MMC constant DC voltage control loop after voltage reduction. The calculation formula is: ;in This is the rated voltage of the DC transmission stage. To achieve DC voltage step-down control.

[0018] Further improvements to this technical solution include the following: In step S2, the implementation of the active DC voltage control strategy is divided into two stages: the first stage is from the occurrence of the fault to the DRU being turned on, during which the DC current is zero. DC voltage deviation control loop output voltage drop The first stage reduces the DC voltage of the MMC, thereby rapidly reducing the DC side voltage of the DRU until the DRU is turned on; the second stage is after the DRU is turned on, continuously based on the current deviation. Dynamically adjust the reference value of the MMC constant DC voltage control loop. To make the direct current track and stabilized at To maintain power transmission during a fault.

[0019] A further improvement to this technical solution is that the DC voltage deviation control loop is only put into operation after the AC grid fault at the sending end is determined in step S1, and is in a disabled state under normal operating conditions.

[0020] The beneficial effects of this invention are as follows:

[0021] This invention, by configuring a DC voltage deviation control loop and integrating it into the constant DC voltage controller of the onshore receiving-end MMC, enables dynamic adjustment of the DC voltage reference value during AC grid faults at the sending end. By keeping the DRU conducting during a fault, the fault ride-through capability of the DC transmission line is significantly improved, effectively avoiding the equipment failure risk caused by relying on the DC unloading of wind turbines for energy consumption in traditional methods, and ensuring the continuous and stable operation of the system.

[0022] In the event of a fault, the method of this invention can respond rapidly by generating a negative DC voltage deviation command and adjusting the MMC DC voltage reference value to restore the DRU's conduction, ensuring positive DC current transmission and thus restoring power delivery to the receiving-end grid. This process significantly reduces the active power deficit of the onshore power grid and ensures the safe and stable operation of the grid.

[0023] By reducing reliance on wind turbine unloading loads, this invention reduces the energy impact and operating time of unloading loads, thereby extending the service life of the equipment. Simultaneously, the elimination of the need for interconnecting multiple turbine clusters and inter-cluster submarine cables further reduces system costs and improves system economics.

[0024] The fault ride-through method proposed in this invention ensures the safety of wind turbines and DC transmission systems while maintaining positive DC current, thus achieving continuity of power transmission during AC faults at sea and effectively mitigating the impact of large-scale power outages on the power grid. This helps to reduce voltage fluctuations and frequency shifts in the receiving-end power grid, significantly improving the overall stability and reliability of the system. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a topology diagram of an offshore wind power transmission system via DRU-MMC.

[0027] Figure 2 This is a schematic flowchart illustrating a method according to an embodiment of the present invention.

[0028] Figure 3 This is a block diagram of the DC voltage deviation control loop.

[0029] Figure 4 This is a schematic diagram illustrating the implementation process of the AC fault ride-through method at the sending end of the offshore wind power transmission system via DRU-MMC. Detailed Implementation

[0030] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the specific embodiments. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0032] The existing offshore wind power transmission system topology via DRU-MMC is as follows: Figure 1 As shown, an offshore wind farm consists of several wind turbines, each employing a permanent magnet synchronous generator. The generated electricity is fed into the offshore AC power grid (i.e., the sending-end grid) via back-to-back converters and an offshore booster station. The sending-end rectifier station uses a 12-pulse DRU, which not only has a higher DC voltage output capability but also significantly reduces AC-side characteristic harmonic currents and DC-side characteristic harmonic voltages. An AC filter is also installed at the rectifier station to filter out harmonic currents on the DRU's AC side while simultaneously providing the reactive power consumed during DRU operation, working with the wind turbines to maintain the reactive power balance of the entire system. After rectification by the DRU, the wind power becomes high-voltage DC, which is transmitted to the onshore receiving-end inverter station via a DC submarine cable. The receiving-end MMC uses a hybrid MMC based on full-bridge and half-bridge submodules. Because the full-bridge submodule has negative-level output capability, the hybrid MMC offers more flexible DC voltage regulation and can reduce DC voltage more significantly under normal operating conditions. After the DC power is inverted into AC power by the MMC, it is stepped down and connected to the land-based AC system (i.e., the receiving-end AC system).

[0033] exist Figure 1 In the system shown, the wind turbine-side converter uses constant DC voltage control to maintain stable DC link voltage between back-to-back converters, thereby providing a stable DC power supply to the wind turbine grid-side converter. The wind turbine grid-side converter adopts a grid-type control strategy, regulating the active power of the wind turbine by controlling the amplitude of the AC output voltage and regulating the reactive power by controlling the operating frequency of the offshore AC grid. However, the current inner loop reference value is limited. After current limiting during a fault, the wind turbine outputs a constant current, which is equivalent to a current source externally.

[0034] For the receiving-end MMC of the DC transmission system, constant DC voltage control and constant reactive power control are adopted. In the control loop, the outer loop is constant DC power supply and constant reactive power control, with a given reference value. and After passing through the PI controller, the reference value of the grid-side current (i.e., differential mode current) is obtained. and The two current reference values ​​enter the inner current loop and, after passing through a PI controller and inverse Park transform, are used to obtain the differential mode voltage reference value. (j=a,b,c). Common-mode voltage reference value obtained through the circulating current suppression control loop. and Through voltage synthesis, reference values ​​for the upper and lower bridge arm voltages are obtained. and Finally, a control signal is generated using the nearest-level approximation voltage modulation method and applied to the MMC.

[0035] In offshore wind power transmission systems via DRU-MMC, the receiving-end modular multilevel converter (MMC) employs constant DC voltage control and constant reactive power control strategies. Its core objective is to maintain stable DC voltage and reactive power balance, ensuring efficient power transmission to the onshore grid. This control strategy is implemented through a multi-loop control structure, including outer-loop voltage / reactive power control, inner-loop current control, circulating current suppression, and modulation.

[0036] Among them, the outer loop constant DC voltage and constant reactive power control are:

[0037] The outer loop control uses the DC voltage reference value. and reactive power reference value As input, the d-axis and q-axis reference values ​​of the grid-side current are generated by the PI controller. and The specific process is as follows:

[0038] DC voltage deviation Output via PI controller It is used to regulate DC voltage.

[0039] reactive power deviation Output via PI controller It is used to control reactive power.

[0040] The parameters of the outer loop PI controller are tuned according to the dynamic response requirements of the system to ensure rapid tracking of the reference value and reduce overshoot.

[0041] Inner current loop control and inverse Park transform:

[0042] The inner current loop receives the output of the outer loop. and After comparing with the actual current value, a differential-mode voltage reference value is generated through a PI controller and inverse Park transform. (in The specific process is as follows:

[0043] The dq-axis current deviation is output as a dq-axis voltage component by the PI controller, and then converted into a differential-mode voltage reference value in a three-phase stationary coordinate system through inverse Park transformation.

[0044] The inverse Park transform is based on the grid voltage phase to ensure coordinate alignment; the inner current loop bandwidth is higher than the outer loop, improving dynamic response speed.

[0045] Circulating current suppression and voltage synthesis:

[0046] A common-mode voltage reference value is generated through a circulating current suppression control circuit. , and differential mode voltage reference value By combining the data, reference values ​​for the upper and lower bridge arm voltages are obtained. and The specific process is as follows:

[0047] The circulating current suppression controller detects the circulating current in the bridge arm and outputs a common-mode voltage to balance the energy of the submodule.

[0048] Voltage synthesis formula: .

[0049] Common-mode voltage suppresses circulating current oscillations and ensures voltage equalization between MMC bridge arms; the synthesized voltage reference value is used for modulation.

[0050] Nearest level approximation modulation and control signal generation:

[0051] The upper and lower bridge arm voltage reference values ​​are used to generate actual control signals through the nearest level approximation (NLM) modulation method, which drive the switching devices of the MMC submodule.

[0052] NLM modulation reduces switching frequency and losses, improving efficiency; the modulation stage is synchronized with the control system to ensure real-time performance. However, in the event of a fault, the DRU is cut off, causing a DC channel interruption. The unloaded load is subjected to an energy surge far exceeding the rated capacity, which can easily lead to equipment failure. At the same time, the receiving-end power grid loses power support, threatening grid security.

[0053] To address the aforementioned technical problems, the present invention provides the following... Figure 2 This paper illustrates a method for AC fault ride-through at the sending end of an offshore wind power transmission system via DRU-MMC. The offshore wind power transmission system via DRU-MMC is equipped with a DC voltage deviation control loop, such as... Figure 3 As shown, the DC voltage deviation control loop is integrated into the constant DC voltage controller of the onshore receiving-end MMC in the offshore wind power transmission system via DRU-MMC. The DC voltage deviation control loop includes a detection unit, a current deviation calculation unit, a proportional-integral controller, and a voltage reference value superposition unit. It is used to generate a negative DC voltage deviation command and adjust the MMC DC voltage reference value during faults. Depending on different requirements, the order of the steps in this flowchart can be changed, and some steps can be omitted.

[0054] like Figure 2 As shown, the method includes:

[0055] S1. Start-up judgment for AC grid fault at the sending end. When the two criteria of AC voltage amplitude change rate being less than zero and DC transmission system current being equal to zero are met simultaneously, it is determined that the AC grid at the sending end has a fault, and the diode rectifier unit DRU in the offshore wind power transmission system through DRU-MMC is cut off.

[0056] S2. Implement the active DC voltage control strategy for the receiver-end modular multilevel converter (MMC), and add a DC voltage deviation control loop to the constant DC voltage control loop of the receiver-end MMC:

[0057] S21. The actual value of DC current is detected by the detection unit in the DC voltage deviation control loop, and the current deviation between the actual value of DC current and the preset expected DC current reference value under fault is calculated by the current deviation calculation unit.

[0058] S22. Input the current deviation into the proportional-integral controller and output the DC voltage deviation command value. The DC voltage deviation command value is negative.

[0059] S23. The DC voltage deviation command value is superimposed with the rated voltage of the DC transmission link in the voltage reference value superposition unit to obtain the reference value of the DC voltage control loop of the MMC after voltage reduction, and the DC side voltage of the receiving end MMC is adjusted accordingly. When the DC side voltage of the DRU is lower than its AC side voltage amplitude, the DRU resumes conduction, the DC current is transmitted in the positive direction, and the power transmission to the receiving end grid is restored.

[0060] To facilitate understanding of the present invention, the following description further illustrates the AC fault ride-through method for offshore wind power transmitted through the DRU-MMC system, based on the principle of the present invention and the process of AC fault ride-through at the sending end of the offshore wind power transmitted through the DRU-MMC system in the embodiments.

[0061] First, step S1 includes: detecting the AC voltage amplitude of the offshore AC power grid in real time using a detection unit. Real-time trend of change, when the rate of change of AC voltage amplitude is detected When the AC voltage drops, the detection unit detects the DC current between the DRU and MMC. When the detected current value is 0, the DRU is determined to be cut off. When both of the above criteria are met, it can be determined that the AC grid at the sending end has a fault and the DC transmission line has no power transmission.

[0062] The DC voltage deviation control loop is only put into operation after a fault is detected in the AC power grid at the sending end in step S1, and is disabled under normal operating conditions.

[0063] In step S1, when a fault occurs in the sending-end AC grid, the grid-side converter activates the current inner loop limiting function to limit the d-axis and q-axis current reference values ​​to a preset safe value, so that the wind turbine outputs a constant current and outputs stable power in the fault steady state.

[0064] In the offshore wind power transmission system via DRU-MMC, the wind turbine-side converter adopts constant DC voltage control to maintain the DC link voltage stability of the back-to-back converters; the wind turbine grid-side converter adopts grid-type control, which regulates active power by controlling the AC side output voltage amplitude and regulates reactive power by controlling the grid frequency at the sending end, and the current inner loop is limited during faults to make the wind turbine equivalent to a current source.

[0065] Secondly, in step S2, the control principle of the DC voltage deviation control loop is based on the voltage-current relationship of the DC loop. At that time, the DC side voltage of the DRU MMC DC side voltage and DC current Satisfying the formula: ;in This is a reference value for DC current. This is the equivalent resistance of a DC line.

[0066] Next, in step S2, when the DC current reaches the reference value, that is... At that time, the DC side voltage of the DRU MMC DC side voltage under fault conditions The formula for DC current is: .

[0067] In addition, in step S2, and Subtracting the DC circuit formulas at the given time, we obtain the DC voltage deviation. : ,in .

[0068] Then, in step S2, the DC voltage deviation control loop is implemented using a PI controller, specifically by converting the actual value of the DC current... Compared with reference value The difference is input to the PI controller, and the output is the DC voltage deviation. Furthermore, a limit of 0 is set at the difference point to prevent... Time The output causes interference.

[0069] Furthermore, in step S2, the reference value of the MMC constant DC voltage control loop after voltage reduction... The calculation formula is: ;in This is the rated voltage of the DC transmission stage. To achieve DC voltage step-down control.

[0070] In step S2, the implementation of the DC voltage active control strategy is divided into two stages: the first stage is from the occurrence of the fault to the DRU being turned on, during which the DC current is zero. DC voltage deviation control loop output voltage drop The first stage reduces the DC voltage of the MMC, thereby rapidly reducing the DC side voltage of the DRU until the DRU is turned on; the second stage is after the DRU is turned on, continuously based on the current deviation. Dynamically adjust the reference value of the MMC constant DC voltage control loop. To make the direct current track and stabilized at To maintain power transmission during a fault.

[0071] Specifically, after obtaining the reference value of the MMC constant DC voltage control loop... Then, based on the reference value of the MMC DC voltage control loop... Reduce MMC DC side voltage To simultaneously reduce the DC-side voltage of the DRU This makes the DC side voltage of the DRU... The voltage amplitude is lower than that of the AC side of the DRU, thus restoring the conduction of the DRU.

[0072] Constant DC voltage control ensures Quickly track reference values ​​to reduce voltage fluctuations during faults, prevent DRU cutoff, and maintain continuous power transmission.

[0073] When the DRU is turned on, the DC side voltage of the DRU (denoted as...) ) and MMC DC side voltage (denoted as These are not the same; there is a voltage difference between them, which reflects the power loss in the DC transmission line. Therefore, strictly speaking, the DC voltage controlled by MMC is... The following relationship exists between the AC and DC side voltages of the DRU:

[0074] (1)

[0075] In the formula This is the effective value of the AC bus voltage. To calculate the leakage reactance of the converter transformer referred to the valve side, For the converter transformer turns ratio, This is the DC-side current, from which the DC-side voltage of the DRU can be seen. It is mainly determined by its AC side voltage.

[0076] When DRU is deactivated =0, the DC side of the DRU is equivalent to an open circuit. and Numerically equal, when MMC controls When changing, It will also change synchronously. If it is under MMC control at this time... reduce, Synchronous reduction, when When the voltage level is lower than the AC side voltage of the DRU, the DRU can resume conduction.

[0077] like Figure 4 As shown, when a fault occurs in the offshore AC power grid, the AC voltage drops, the DRU (Digital Runner Unit) is cut off, the DC link current suddenly drops to zero, and there is no power transmission on the DC line. Part of the power generated by the wind turbine is fed into the fault point, and part is consumed by the unloading load of the wind turbine, resulting in a power deficit in the receiving-end power grid and a significant energy surge to the unloaded load. The proposed fault ride-through method introduces a DC voltage deviation control loop. By detecting the DC current, it outputs the DC voltage drop, thereby reducing the receiving-end DC voltage. When the voltage drops below the AC side voltage amplitude of the DRU, the DRU is restarted, ensuring positive DC current transmission and restoring power transmission to the receiving-end power grid, while simultaneously alleviating the pressure on the unloaded load.

[0078] The system's fault ride-through method consists of two steps: initiation judgment and execution of the active control strategy for the DC voltage of the receiving end MMC.

[0079] Step 1: Startup Detection

[0080] When a fault occurs in the AC power grid at the offshore sending end, its AC voltage will drop significantly within a very short time. Since faults are usually accompanied by short-circuit current surges and voltage fluctuations, the AC voltage amplitude will decrease rapidly. When the AC voltage amplitude of the DRU is lower than the DC voltage, the diodes on its bridge arm will be reverse-biased and automatically cut off, causing the DC transmission current to gradually decrease until it drops to zero. At this point, the system will no longer transmit power.

[0081] To quickly and accurately determine whether a fault has occurred in the sending-end AC grid, and thus whether to execute the receiving-end MMC DC voltage active control strategy in step two, this invention sets the following two criteria based on the electrical characteristics of the sending-end AC fault:

[0082] 1. When the rate of change of AC voltage amplitude is detected When a continuous downward trend in AC voltage is detected, it can be preliminarily determined that a voltage drop has occurred in the AC system. This indicator can sensitively reflect the occurrence of transient disturbances on the AC side;

[0083] 2. When the DC output system current is detected When the value equals 0, it can be determined that the DRU rectifier unit has entered the cutoff state, and the DC side no longer outputs current, indicating that the AC fault has caused the power transmission to be interrupted.

[0084] Since the amplitude of AC voltage can be directly measured, the rate of change of amplitude is used. This allows for a faster and more convenient assessment of voltage drops. When both of the above criteria are met simultaneously, a fault in the sending-end AC grid can be determined. At this time, the AC voltage amplitude not only drops rapidly but also becomes less than the DC-side voltage of the DRU, causing the DRU to be in a cutoff state. The DC transmission link of the system stops power transmission, and the receiving-end MMC active DC voltage control strategy can then be implemented.

[0085] Step 2: Implement the active control strategy for the DC voltage of the receiving-end MMC:

[0086] Once it is confirmed that the DRU has been shut down due to an AC fault, an active buck control strategy can be implemented on the receiving-end MMC. The specific process is as follows: In the constant DC voltage control loop of the MMC, a DC voltage deviation control loop is activated. By detecting the DC current, the DC voltage deviation control loop outputs a buck command value ΔUdc. <0), rapidly reducing the reference value of the MMC constant DC voltage control loop. Under the action of MMC, the voltage of the DC transmission link is reduced quickly and smoothly. When the DC side voltage of DRU is lower than the AC side voltage amplitude, DRU resumes conduction and can continue to reduce the voltage until the DC current rises to a specified value. This allows the wind farm to continue transmitting power to the onshore grid through the DRU-MMC DC channel during a fault, thereby reducing the active power deficit of the onshore grid and alleviating the working pressure of wind turbine unloading.

[0087] 1. The specific principle of DC voltage deviation control is as follows:

[0088] Record the reference value for the expected DC current under fault conditions. ( >0), the actual DC current is .when < At that time, according to the principle of DC circuit, the DC side voltage of DRU MMC DC side voltage and DC current The following conditions must be met:

[0089] (2)

[0090] In the formula This is the equivalent resistance of a DC line.

[0091] when = At that time, in the DC circuit, there are:

[0092] (3)

[0093] In the formula The DC current under fault conditions is The corresponding DC-side voltage value of the MMC at that time. From equation (1), it can be seen that... It is mainly controlled by the AC side voltage of the DRU, so it can be considered as a constant in equations (2) and (3).

[0094] Subtracting equation (2) from equation (3), we can obtain the result that the DC current is insufficient when there is an AC fault. The resulting DC voltage deviation for:

[0095] (4)

[0096] From equation (4), we can obtain that the difference between the actual value and the reference value of the DC current under fault conditions is related to the DC voltage deviation. Since the relationship is linear, a PI controller can be used to design a DC voltage deviation control loop. This loop outputs a DC voltage deviation signal, which acts on the MMC constant DC voltage control loop, and also affects the DC current. Quickly track and reach the reference value .

[0097] The DC voltage deviation control block diagram proposed in this invention is shown in Figure 4. First, and The difference is output as a DC voltage deviation after passing through a PI controller. Since the DC voltage deviation loop is only put into use after a fault occurs, It starts increasing from 0 and eventually stabilizes under the action of the PI controller. Therefore, there will always be ≤ ,and <0; however, to prevent fluctuations during the DC current increase process > This situation, in turn, affects the output of △Udc, and the control loop in and A limit is set at the difference, with the upper limit set to 0. Secondly, the voltage deviation output by the DC voltage deviation loop... Rated voltage of DC transmission link After summing, the reference value of the step-down MMC constant DC voltage control loop is obtained. The difference between the actual DC voltage value and the actual DC voltage value is then used to obtain the d-axis current reference value via a PI controller. During this process, the reduced reference value was analyzed. Tracking enables the reduction of DC voltage.

[0098] 2. Implementation process of the active control strategy for the DC voltage of the receiving-end MMC:

[0099] ① From the occurrence of the fault to the DRU being turned on:

[0100] When a voltage dip fault occurs in the offshore AC power grid, the amplitude of the AC voltage at the sending end decreases rapidly, causing the DRU to switch from the on state to the off state. At this time, the DC current... =0, the DC channel can be considered an open circuit, therefore the voltage on the DC side of the DRU is equal to the voltage on the DC side of the receiving end MMC, i.e.

[0101] (5)

[0102] At this point, the deviation between the actual current value and the reference value of the DC voltage deviation control loop is at its maximum. To eliminate this deviation, the DC voltage deviation control loop outputs a larger DC voltage adjustment amount. This rapidly reduces the voltage of the DC transmission stage. .when When the voltage drops below the amplitude of the AC phase voltage of the DRU, the diode gains a forward bias and begins to conduct again, establishing a new power transmission path for the system.

[0103] ② DRU turns on until the DC current rises to the reference value:

[0104] After the DRU is turned on, according to the circuit principle, the DC side voltage of the DRU... MMC DC side voltage and DC current The relationship is shown in equation (2). From equation (1), we can know the DC side voltage of the DRU. It is determined by its AC side voltage, while MMC actually controls its DC side voltage. .

[0105] When the DRU is just turned on The DC voltage deviation control loop continues to output the voltage deviation in real time based on the current deviation, adjusting the DC voltage reference value to ensure that the DC voltage at the receiving end is relatively small. Further reduction, prompting Gradually increasing. From the perspective of circuit principles, as shown in equation (2), as... The reduction, With a relatively fixed voltage, the voltage difference between the sending and receiving ends of the DC link increases, thereby... Continuously increasing. When When the current rises to the reference value, the current deviation tends to zero, and the controller output... The system stops changing and enters a new steady-state operating range. At this point, the DRU is stably turned on, and power is re-established through the DC transmission channel, achieving rapid current recovery and smooth power transmission during the fault.

[0106] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for AC fault ride-through at the sending end of an offshore wind power transmission system via DRU-MMC, characterized in that, The offshore wind power transmission system via DRU-MMC is equipped with a DC voltage deviation control loop. The DC voltage deviation control loop is integrated into the constant DC voltage controller of the onshore receiving-end MMC in the offshore wind power transmission system via DRU-MMC. The DC voltage deviation control loop includes a detection unit, a current deviation calculation unit, a proportional-integral controller, and a voltage reference value superposition unit. It is used to generate a negative DC voltage deviation command and adjust the MMC DC voltage reference value during a fault. The methods include: S1. Start-up judgment for AC grid fault at the sending end. When the two criteria of AC voltage amplitude change rate being less than zero and DC transmission system current being equal to zero are met simultaneously, it is determined that the AC grid at the sending end has a fault, and the diode rectifier unit DRU in the offshore wind power transmission system through DRU-MMC is cut off. S2. Implement the active DC voltage control strategy for the receiver-end modular multilevel converter (MMC), and add a DC voltage deviation control loop to the constant DC voltage control loop of the receiver-end MMC: S21. The actual value of DC current is detected by the detection unit in the DC voltage deviation control loop, and the current deviation between the actual value of DC current and the preset expected DC current reference value under fault is calculated by the current deviation calculation unit. S22. Input the current deviation into the proportional-integral controller and output the DC voltage deviation command value. The DC voltage deviation command value is negative. S23. The DC voltage deviation command value is superimposed with the rated voltage of the DC transmission link in the voltage reference value superposition unit to obtain the reference value of the DC voltage control loop of the MMC after voltage reduction, and the DC side voltage of the receiving end MMC is adjusted accordingly. When the DC side voltage of the DRU is lower than its AC side voltage amplitude, the DRU resumes conduction, the DC current is transmitted in the positive direction, and the power transmission to the receiving end grid is restored.

2. The method for AC fault ride-through at the sending end of the offshore wind power transmission system via DRU-MMC as described in claim 1, characterized in that, Step S1 includes: The AC voltage amplitude of the offshore AC power grid is monitored in real time by the detection unit. Real-time trend of change, when the rate of change of AC voltage amplitude is detected When the AC voltage drops, the detection unit detects the DC current between the DRU and MMC. When the detected current value is 0, the DRU is determined to be cut off. When both of the above criteria are met, it can be determined that the AC grid at the sending end has a fault and the DC transmission line has no power transmission.

3. The method for AC fault ride-through at the sending end of the offshore wind power transmission system via DRU-MMC as described in claim 1, characterized in that, In step S2, the control principle of the DC voltage deviation control loop is based on the voltage-current relationship of the DC loop. At that time, the DC side voltage of the DRU MMC DC side voltage and DC current Satisfying the formula: ;in This is a reference value for DC current. This is the equivalent resistance of a DC line.

4. The method for AC fault ride-through at the sending end of the offshore wind power transmission system via DRU-MMC according to claim 3, characterized in that, In step S2, when the DC current reaches the reference value, that is... At that time, the DC side voltage of the DRU MMC DC side voltage under fault conditions The formula for DC current is: .

5. The method for AC fault ride-through at the sending end of the offshore wind power transmission system via DRU-MMC according to claim 4, characterized in that, In step S2, and Subtracting the DC circuit formulas at the given time, we obtain the DC voltage deviation. : ,in .

6. The method for AC fault ride-through at the sending end of the offshore wind power transmission system via DRU-MMC according to claim 5, characterized in that, In step S2, the DC voltage deviation control loop is implemented using a PI controller, specifically by converting the actual value of the DC current... Compared with reference value The difference is input to the PI controller, and the output is the DC voltage deviation. Furthermore, a limit of 0 is set at the difference point to prevent... Time The output causes interference.

7. The method for AC fault ride-through at the sending end of an offshore wind power transmission system via DRU-MMC according to claim 6, characterized in that, In step S2, the reference value of the MMC constant DC voltage control loop after voltage reduction is... The calculation formula is: ;in This is the rated voltage of the DC transmission stage. To achieve DC voltage step-down control.

8. The method for AC fault ride-through at the sending end of the offshore wind power transmission system via DRU-MMC according to claim 7, characterized in that, In step S2, the implementation of the DC voltage active control strategy is divided into two stages: the first stage is from the occurrence of the fault to the DRU being turned on, during which the DC current is zero. DC voltage deviation control loop output voltage drop This reduces the DC voltage of the MMC, thereby rapidly reducing the DC side voltage of the DRU until the DRU is turned on; The second stage is after the DRU is turned on, continuously based on the current deviation. Dynamically adjust the reference value of the MMC constant DC voltage control loop. To make the direct current track and stabilized at To maintain power transmission during a fault.

9. The method for AC fault ride-through at the sending end of an offshore wind power transmission system via DRU-MMC according to claim 1, characterized in that, The DC voltage deviation control loop is only put into operation after a fault is detected in the AC power grid at the sending end in step S1, and is disabled under normal operating conditions.

Images