Flexible low-frequency power transmission system and flexible low-frequency power transmission system fault ride-through method

Abstract

The invention discloses a flexible low-frequency power transmission system and a fault ride-through method of the flexible low-frequency power transmission system. The system comprises a fan, a fan side converter, a fan grid side converter, a low-frequency line and an M3C converter. When an M3C converter detects that a fault occurs at a power frequency side, on the premise that the power of a low-frequency system is stable, the voltage at the low-frequency side of the M3C converter is controlled to be reduced, so that the voltage at a grid-connected point of a fan is within a preset voltage interval for transmitting fault information; and when the fan grid-side converter detects that the voltage of the grid-connected point is within a preset voltage interval, determining that the system is in a fault ride-through state, switching the fan grid-side converter to a control mode for fan load shedding at the moment, and realizing the fan load shedding through an overspeed load shedding strategy so as to realize fault ride-through. According to the invention, low-frequency side voltage reduction signal transmission during a power frequency side fault period is realized, fan load shedding is guided, and stable fault ride-through of the flexible low-frequency power transmission system is guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of power system fault protection, and in particular to a flexible low-frequency power transmission system and a method for riding through a fault in the flexible low-frequency power transmission system. Background Art

[0002] As a new power transmission solution, low-frequency transmission technology significantly improves medium- and long-distance power transmission capabilities by reducing the system operating frequency (usually 15-20Hz). Its core advantages are reflected in reducing line losses, suppressing reactive power, and being compatible with existing AC circuit breakers. It is particularly suitable for offshore wind power transmission scenarios within the range of 70-200km.

[0003] As the core energy conversion device of low-frequency transmission systems, the modular multilevel matrix converter (M3C converter) achieves efficient, inter-frequency interconnection between the power-frequency grid and the low-frequency grid by leveraging its full-bridge submodule cascade structure and lack of a DC link. However, in flexible low-frequency transmission systems, when a fault occurs on the power-frequency side, the system must possess fault ride-through (FRT) capability. This means that during short-term voltage disturbances, the wind turbines will not disconnect from the grid, will not malfunction, and will maintain stable power transmission. This is crucial for preventing the mass disconnection of wind farms and maintaining stable grid operation.

[0004] Conventional flexible power transmission systems (FPTs) primarily focus on conventional power control and voltage stabilization, lacking effective coordinated control mechanisms between the low-frequency side and the wind turbine. When a fault occurs on the power grid, the long low-frequency line between the M3C converter and the wind turbine prevents the fault signal from reaching the wind turbine in a timely manner. This can cause the wind turbine to be unable to sense the system anomaly or respond with a delay, potentially leading to turbine overload, converter overcurrent, or false triggering of wind turbine protection, potentially impacting the safe operation of the entire wind farm.

[0005] Therefore, the existing technology urgently needs a fault ride-through solution for a flexible low-frequency transmission system. When a fault occurs on the power frequency side, it can achieve effective transmission of fault signals through coordinated voltage control on the low-frequency side while ensuring power stability, guiding the wind turbine to respond to load reduction in a timely manner, thereby achieving stable ride-through operation of the system.

[0006] This section is intended to provide a background or context to the embodiments of the invention that are recited in the claims. No statement herein is admitted to be prior art by virtue of its inclusion in this section. Summary of the Invention

[0007] In order to solve at least one technical problem in the above-mentioned background technology, the present invention proposes a flexible low-frequency power transmission system and a fault ride-through method for the flexible low-frequency power transmission system.

[0008] To achieve the above objectives, according to one aspect of the present invention, a flexible low-frequency power transmission system is provided, comprising: a wind turbine, a wind turbine machine-side converter, a wind turbine grid-side converter, a low-frequency line, and an M3C converter; the output end of the wind turbine is connected to the input end of the wind turbine machine-side converter, the wind turbine machine-side converter is connected to the wind turbine grid-side converter, the output end of the wind turbine grid-side converter is connected to the low-frequency side of the M3C converter via the low-frequency line, and the power frequency side of the M3C converter is connected to the AC power grid;

[0009] When the M3C converter detects a fault on the power frequency side, it controls the voltage on its own low-frequency side to decrease, while ensuring the power stability of the low-frequency system, so that the grid-connected point voltage of the wind turbine is within a preset voltage range for transmitting fault information. The preset voltage range is a voltage range that is lower than the normal voltage control range of the wind farm but does not enter the low voltage ride-through range; when the wind turbine grid-side converter detects that the grid-connected point voltage is within the preset voltage range, it determines that the system is in a fault ride-through state. At this time, the wind turbine machine-side converter switches to a control mode for wind turbine load shedding, and implements wind turbine load shedding through an overspeed load shedding strategy to achieve fault ride-through of the flexible low-frequency power transmission system.

[0010] Optionally, the conditions that need to be met for low-frequency system power stability are:

[0011]

[0012] Among them, U M3C is the low-frequency side voltage of the M3C converter, X L is the reactance of the low-frequency line, U W is the wind turbine grid connection point voltage, P w is the active power output by the fan.

[0013] Optionally, the preset voltage range is 0.8pu-0.97pu, where pu is the per-unit value of the grid connection point voltage.

[0014] Optionally, the M3C converter controls its own low-frequency side voltage to decrease based on a low-frequency side voltage reference value, where the low-frequency side voltage reference value is determined according to an adjustment range of the low-frequency side voltage reference value and the grid side voltage during a fault.

[0015] Optionally, the low-frequency side voltage reference value adjustment range is [u l_ref_min ,u l_ref_max ],u l_ref_min is the minimum value of the low-frequency side voltage reference value, u l_ref_max is the maximum value of the low-frequency side voltage reference value, and the minimum value of the low-frequency side voltage reference value u l_ref_minA reference value is set to ensure that the voltage at the wind turbine grid connection point does not fall below the lower limit of the preset voltage range under a fault condition;

[0016] The low-frequency side voltage reference value is determined by the following formula:

[0017]

[0018] Among them, u l_ref is the low-frequency side voltage reference value, u s_fault is the grid side voltage during the fault period.

[0019] Optional, minimum value of low-frequency side voltage reference u l_ref_min Determined by simulation analysis or estimated simulation based on the power network voltage calculation formula;

[0020] The power network voltage calculation formula is:

[0021]

[0022] Among them, U M3C is the low-frequency side voltage of the M3C converter, U W is the voltage at the wind turbine grid connection point, Δu is the line voltage drop between the wind turbine grid connection point and the M3C converter, P w is the active power output by the wind turbine, Q W is the reactive power output by the wind turbine, R L is the resistance of the low-frequency circuit, X L is the reactance of the low-frequency line.

[0023] Optionally, the wind turbine side converter switches to a control mode for wind turbine load shedding, and implements wind turbine load shedding through an overspeed load shedding strategy, so that the active power output by the wind turbine is less than or equal to the maximum transmittable power of the M3C converter under a fault condition, and the maximum transmittable power is determined based on the grid side voltage during the fault and the maximum output current of the M3C converter.

[0024] Optionally, if the M3C converter detects a symmetrical fault on the power frequency side, the M3C converter switches to a maximum power transmission mode before controlling the voltage on its own low frequency side to decrease.

[0025] Optionally, if the M3C converter detects an asymmetric fault on the power frequency side, the M3C converter switches to a negative sequence current suppression mode before controlling the voltage on its own low frequency side to decrease.

[0026] To achieve the above object, according to another aspect of the present invention, a method for fault ride-through of a flexible low-frequency power transmission system is provided, the method comprising:

[0027] When the M3C converter detects a fault on the power frequency side, it controls the voltage on its own low-frequency side to decrease, ensuring the power stability of the low-frequency system, so that the voltage at the wind turbine's grid connection point is within a preset voltage range for transmitting fault information. The preset voltage range is a voltage range that is lower than the normal voltage control range of the wind farm but does not enter the low voltage ride-through range. The power frequency side of the M3C converter is connected to the AC grid.

[0028] When the wind turbine grid-side converter detects that the grid connection point voltage is within the preset voltage range, it determines that the system is in a fault ride-through state, wherein the output end of the wind turbine grid-side converter is connected to the low-frequency side of the M3C converter through a low-frequency line, and the wind turbine grid-side converter is connected to the wind turbine machine-side converter;

[0029] When the wind turbine side converter receives information that the system is in a fault ride-through state, it switches to a control mode for wind turbine load reduction and implements wind turbine load reduction through an overspeed load reduction strategy to achieve fault ride-through of the flexible low-frequency power transmission system, wherein the input end of the wind turbine side converter is connected to the output end of the wind turbine.

[0030] Optionally, controlling the voltage on the low-frequency side thereof to decrease includes:

[0031] The M3C converter controls its own low-frequency side voltage to decrease based on a low-frequency side voltage reference value, wherein the low-frequency side voltage reference value is determined according to an adjustment range of the low-frequency side voltage reference value and the grid side voltage during a fault period.

[0032] Optionally, the switching to the control mode for wind turbine load reduction and implementing wind turbine load reduction through an overspeed load reduction strategy includes:

[0033] The wind turbine side converter switches to a control mode for wind turbine load shedding, and implements wind turbine load shedding through an overspeed load shedding strategy, so that the active power output by the wind turbine is less than or equal to the maximum transmittable power of the M3C converter under fault conditions. The maximum transmittable power is determined based on the grid side voltage during the fault and the maximum output current of the M3C converter.

[0034] Optionally, the flexible low-frequency power transmission system fault ride-through method further includes:

[0035] If the M3C converter detects a symmetrical fault on the power frequency side, the M3C converter switches to the maximum power transmission mode before controlling the voltage on its own low frequency side to decrease.

[0036] Optionally, the flexible low-frequency power transmission system fault ride-through method further includes:

[0037] If the M3C converter detects an asymmetric fault on the power frequency side, the M3C converter switches to a negative sequence current suppression mode before controlling the voltage on its own low frequency side to decrease.

[0038] The beneficial effects of the present invention are:

[0039] The present invention addresses the problems of existing flexible low-frequency power transmission systems, such as the difficulty in timely and effective transmission of fault information to the wind turbine side, delayed wind turbine response, and lack of load shedding control when a fault occurs on the power frequency side. A flexible low-frequency power transmission system based on a low-frequency side voltage reduction signal transmission mechanism is proposed. In this system, after detecting a fault on the power frequency side, the M3C converter controls its low-frequency side output voltage to decrease, causing the wind turbine grid connection point voltage to enter a preset voltage range that is lower than the normal voltage control range of the wind farm but not within the low voltage ride-through range. After sensing this voltage change, the wind turbine grid-side converter determines that the system is in a fault ride-through state, guides the wind turbine machine-side converter to switch to a control mode for load shedding, and reduces the wind turbine output power through an overspeed load shedding strategy, thereby achieving stable ride-through of the flexible low-frequency power transmission system under fault conditions. This solution can effectively improve the transmission efficiency of fault information within the system, achieve timely response and load shedding control on the wind turbine side, and enhance the fault ride-through capability and operational stability of the flexible low-frequency power transmission system. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative work. In the drawings:

[0041] Figure 1 is a schematic diagram of a flexible low-frequency power transmission system according to an embodiment of the present invention;

[0042] Figure 2 This is the topology of the offshore wind power low-frequency transmission system according to an embodiment of the present invention;

[0043] Figure 3 This is the M3C topology of an embodiment of the present invention;

[0044] Figure 4 This is the submodule structure of an embodiment of the present invention;

[0045] Figure 5 This is the fault ride-through process of an embodiment of the present invention;

[0046] Figure 6 This is a schematic diagram for verifying the effect of an embodiment of the present invention;

[0047] Figure 7It is a flow chart of a method for fault ride-through of a flexible low-frequency power transmission system according to an embodiment of the present invention. DETAILED DESCRIPTION

[0048] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0049] It will be understood by those skilled in the art that embodiments of the present invention may be provided as methods, systems, or computer program products. Thus, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware. Furthermore, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to magnetic disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0050] It should be noted that the terms "including" and "having" and any variations thereof in the specification and claims of the present invention and the above-mentioned drawings are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus comprising a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or are inherent to these processes, methods, products or apparatuses.

[0051] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments of the present invention can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0052] This invention addresses the dynamic instability of offshore wind power transmission systems caused by power surplus after a fault on the power frequency side of the system. This approach also overcomes the drawbacks of existing conventional voltage reduction and load shedding control strategies, such as repeated switching of unit control modes and transient current surges caused by excessive voltage regulation on the low-frequency side. By improving the coordinated mechanism between voltage reduction control and wind turbine transient response, this approach provides a flexible low-frequency transmission system fault ride-through solution that eliminates the need to switch wind turbine control modes.

[0053] In one aspect of the present invention, a flexible low-frequency power transmission system is provided. Figure 1As shown, in one embodiment of the present invention, the flexible low-frequency power transmission system of the present invention includes: a wind turbine, a wind turbine machine-side converter, a wind turbine grid-side converter, a low-frequency line, and an M3C converter. The output end of the wind turbine is connected to the input end of the wind turbine machine-side converter, the wind turbine machine-side converter is connected to the wind turbine grid-side converter, the output end of the wind turbine grid-side converter is connected to the low-frequency side of the M3C converter via the low-frequency line, and the power frequency side of the M3C converter is connected to the AC power grid.

[0054] Figure 2 This is the topology of the offshore wind power low-frequency transmission system in the embodiment of the present invention. Figure 2 The structural framework of the entire flexible low-frequency transmission system is demonstrated, including: wind turbines, wind turbine-side converters, wind turbine-grid-side converters, low-frequency transmission lines, M3C converters, and industrial frequency power grids.

[0055] Figure 3 The basic topology of the M3C (Modular Multilevel Matrix Converter) is demonstrated, which includes multiple bridge arms, full-bridge submodules (SMs), inductors and other components.

[0056] Figure 4 The internal structure of a single submodule (SM) in M3C is shown, including IGBT, diode, DC capacitor, and control loop.

[0057] When the M3C converter detects a fault on the power frequency side, it controls the voltage on its own low-frequency side to decrease, while ensuring the power stability of the low-frequency system, so that the grid connection point voltage of the wind turbine is within a preset voltage range for transmitting fault information. The preset voltage range is a voltage range that is lower than the normal voltage control range of the wind farm but does not enter the low voltage ride-through range.

[0058] When the wind turbine grid-side converter detects that the grid connection point voltage is within the preset voltage range, it determines that the system is in a fault ride-through state. At this time, the wind turbine machine-side converter switches to a control mode for wind turbine load reduction, and implements wind turbine load reduction through an overspeed load reduction strategy to achieve fault ride-through of the flexible low-frequency power transmission system.

[0059] In the present invention, after the wind turbine grid-side converter detects a fault, it starts to switch the wind turbine machine-side converter control mode, and realizes wind turbine load reduction through the overspeed load reduction method, thereby achieving fault ride-through of the entire flexible low-frequency power transmission system.

[0060] In the present invention, a fault occurring on the power frequency side refers to a fault occurring between the M3C converter and the AC grid connection line.

[0061] In the present invention, under the premise of stable low-frequency system power of the M3C converter, the low-frequency side voltage is actively reduced, and the voltage is lowered outside the range of low voltage ride-through to transmit fault information to the wind turbine side converter.

[0062] In the present invention, when the M3C converter detects a fault on the power frequency side, it gives priority to the power stability of the low-frequency system, and actively reduces the output voltage of its low-frequency side under the premise of satisfying the stable operation of the system, thereby guiding the voltage at the wind turbine grid connection point to enter a preset voltage range for transmitting fault information. The preset voltage range is set below the normal voltage control range of the wind farm (such as 0.97 to 1.07 per unit), but higher than the lower limit of the low voltage ride-through (LVRT) range (generally 0.8 per unit), for example, set to 0.8pu to 0.97pu. By adjusting the grid connection point voltage within this range, it can ensure that the wind turbine side converter accurately identifies that the current system is in a fault ride-through state, while avoiding triggering the low voltage protection strategy of the wind turbine, achieving rapid response without disconnecting from the grid.

[0063] The advantage of this strategy is that it eliminates the need for communication links or additional control signals. Simply by adjusting the low-frequency output voltage of the M3C converter, fault information on the power frequency side can be effectively transmitted to the wind turbine. This results in a fast, reliable, and adaptable response mechanism. Furthermore, limiting the voltage drop to outside the low-voltage ride-through range prevents malfunction or instability of the wind turbine, thereby increasing the system's safety margin. This voltage signal-based fault ride-through instruction transmission mechanism significantly enhances the controllability and stability of the flexible low-frequency power transmission system under fault conditions.

[0064] In this invention, when the wind turbine grid-side converter detects that the voltage at the wind turbine grid connection point is within a preset voltage range (e.g., 0.8-0.97 pu), it can promptly determine that the system is in a fault ride-through state. This preset voltage range is achieved by the M3C converter actively reducing the output voltage on the low-frequency side after a fault occurs on the power frequency side, thereby transmitting a fault voltage signal to the wind turbine side.

[0065] After identifying the system as being in the fault ride-through phase, the grid-side converter triggers the turbine's machine-side converter to switch to a control mode appropriate for the fault condition, which provides turbine load shedding. Specifically, the machine-side converter implements an overspeed load shedding strategy. While the wind speed remains constant, the converter adjusts the electromagnetic torque to reduce output active power, thereby slightly increasing the turbine speed and reducing wind energy capture, achieving rapid load shedding.

[0066] This load shedding strategy offers the advantages of fast response, no mechanical action required, and stable control. It not only prevents wind turbines from injecting excessive power into the system during a fault, thus avoiding overload and instability in the M3C converter, but also reduces the risk of wind turbines disconnecting from the grid due to misjudgment. Compared to traditional response methods based on communication links or fixed load shedding commands, this invention combines voltage signal recognition with local autonomous control to form an adaptive load shedding response mechanism that does not rely on additional channels. This significantly improves the wind turbine's ability to perceive and quickly respond to system conditions during fault ride-through, thereby ensuring stable and safe ride-through operation of the flexible low-frequency transmission system under fault conditions.

[0067] This demonstrates that the present invention proposes a flexible low-frequency power transmission system fault ride-through solution based on improved voltage reduction load shedding control. This improved voltage reduction load shedding control approach reduces the wind turbine's transmission power by lowering the low-frequency line voltage outside the wind turbine's low-voltage ride-through range after a fault occurs. Fault ride-through in this invention refers to ride-through when a fault occurs between the M3C converter and the grid connection line.

[0068] In one embodiment of the present invention, the conditions that need to be met for low-frequency system power stability are:

[0069]

[0070] Among them, U M3C is the low-frequency side voltage of the M3C converter, X L is the reactance of the low-frequency line, U W is the wind turbine grid connection point voltage, P w is the active power output by the wind turbine. This condition is used to ensure that the flexible low-frequency transmission system can maintain stable operation during the fault ride-through process.

[0071] In one embodiment of the present invention, the process of obtaining the conditions that need to be met for the power stability of the low-frequency system is as follows:

[0072] When the phase-locked loop output phase angle θ PLL Can accurately track the low-frequency side voltage U M3C The phase angle δ satisfies U w =U wd , U wq =0, the active power output of the wind farm P w , reactive power Q w for:

[0073]

[0074] The relationship between the wind turbine outlet voltage and the M3C converter low-frequency side voltage satisfies:

[0075]

[0076] Where U wd is the voltage component of the wind turbine grid connection point in the d-axis direction, U wq is the voltage component of the wind turbine grid connection point in the q-axis direction, δ is U M3C Voltage U at the wind turbine grid connection point w The phase angle difference between d is the current component in the d-axis direction, I q is the current component in the q-axis direction.

[0077] Then, the relationship between the voltage on the low-frequency side of the converter and the voltage at the wind turbine grid connection point can be obtained:

[0078]

[0079] The power of the wind turbine grid connection point can be obtained:

[0080]

[0081] According to the principle of double closed-loop vector control, when P ref -P w >0, the power error increases, P ref is the active power reference value of the wind turbine side converter, which will output a larger current and increase the output power after passing through the PI controller; when P ref -P w <0, the PI controller will reduce the output current I d Then the output power is reduced to make it equal to the reference power, and the system reaches a stable state. Therefore, power stability requires that the active power Pw increases with the increase of the d-axis current, so the above formula needs to satisfy That is, the following relationship must be satisfied:

[0082]

[0083] Ignoring the influence of line resistance, the following can be further simplified:

[0084]

[0085] According to the above formula, we can get:

[0086]

[0087] Combining the above formula, we can conclude that the conditions that need to be met for the low-frequency system to ensure power stability are:

[0088]

[0089] By transforming the above formula, we can get the following conditions for low-frequency system power stability:

[0090]

[0091] By introducing the above-mentioned power stability judgment condition, the present invention can effectively constrain the low-frequency side voltage setting range of the M3C converter during the fault ride-through process, ensuring that the system has sufficient voltage support capability when the wind turbine output power is given, thereby maintaining the monotonicity and stability of the power regulation process. This condition can avoid insufficient power absorption capacity of the converter due to excessively low voltage, which in turn causes current oscillation, control instability or protection action, thereby improving the operational robustness and control safety of the flexible low-frequency power transmission system under fault conditions. This design of the present invention helps to achieve the safe transmission of fault information and smooth load reduction of the wind turbine, providing a stable foundation for coordinated control during fault ride-through.

[0092] In one embodiment of the present invention, the preset voltage range is 0.8pu-0.97pu, where pu is the per-unit value of the grid connection point voltage, that is, the normalized expression of the wind turbine grid connection point voltage relative to its rated voltage. The setting of the preset voltage range is determined based on the national standard requirements for the grid connection capability of wind farms. According to the current grid connection specifications, for wind farms connected to public power grids of 220kV or 330kV and below, when the grid connection point voltage is within the normal range of 97% to 107% of the rated value, the wind farm should have the ability to autonomously regulate the voltage; and when a grid fault occurs, the wind farm should have the ability to withstand low voltage ride-through (LVRT), that is, when the grid connection point voltage is within the range of 0.2 to 0.8pu, the wind turbine shall not be immediately disconnected from the grid and shall continue to operate.

[0093] In this invention, the M3C converter reduces its low-frequency output voltage to keep the wind turbine grid-connected voltage within the 0.8-0.97 pu range. This voltage range is below the normal voltage control range of the wind farm, but falls within the low-voltage ride-through protection range. Therefore, the wind turbine does not trigger protection due to misjudgment, while accurately sensing abnormal system conditions. This range design ensures that fault information on the power-frequency side is transmitted through voltage changes, guiding the wind turbine to implement timely load reduction control, without affecting the wind turbine's grid-connected stability.

[0094] The setting of this preset voltage range has clear standard basis and control boundaries, which not only avoids the risk of wind turbines being disconnected from the grid, but also provides a fast fault transmission channel that does not require communication. It is beneficial to improving the fault ride-through capability, system response speed and overall operational safety of the wind power system, and is the key foundation of the adaptive collaborative control strategy of the flexible low-frequency transmission system of the present invention.

[0095] In one embodiment of the present invention, the M3C converter controls its own low-frequency side voltage to decrease based on a low-frequency side voltage reference value, where the low-frequency side voltage reference value is determined according to an adjustment range of the low-frequency side voltage reference value and the grid side voltage during a fault.

[0096] In this invention, the M3C converter adjusts its low-frequency output voltage during fault ride-through to maintain the voltage at the wind turbine's grid connection point within a preset voltage range, thereby transmitting a fault signal to the wind turbine. To achieve precise control, the M3C converter performs voltage control based on a preset low-frequency voltage reference value. This low-frequency voltage reference value is dynamically set based on the power-frequency grid voltage during the current fault period and is limited to a set adjustment range.

[0097] In one embodiment of the present invention, the adjustment range of the low-frequency side voltage reference value is [u l_ref_min ,u l_ref_max ],u l_ref_min is the minimum value of the low-frequency side voltage reference value, u l_ref_max is the maximum value of the low-frequency side voltage reference value, and the minimum value of the low-frequency side voltage reference value u l_ref_min It is set as a reference value that makes the wind turbine grid connection point voltage not lower than the lower limit of the preset voltage range (such as 0.8pu) under fault conditions. The maximum value of the low-frequency side voltage reference value u l_ref_max It can be set based on the rated voltage of the low-frequency side of the M3C converter, for example, equal to the rated voltage of the low-frequency side.

[0098] In one embodiment of the present invention, the low-frequency side voltage reference value is determined by the following formula:

[0099]

[0100] Among them, u l_ref is the low-frequency side voltage reference value, u s_fault is the grid side voltage during the fault period.

[0101] During the fault period, the grid voltage u s_fault Under the premise of falling within the interval [0.2,0.9), the low-frequency side voltage reference value u l_ref Dynamically determined by the above linear function. This linear mapping relationship enables the output voltage of the M3C converter to be adaptively adjusted as the voltage on the power frequency side changes. The voltage reference value automatically decreases when the fault voltage is lower, thereby achieving a flexible and gradual voltage reduction control process and avoiding system fluctuations caused by sudden changes in regulation. This design not only meets the demand for precise control of the voltage reference value, but also ensures that the system has controllable and stable crossing capabilities at different fault depths, which is conducive to the smooth triggering of wind turbine load reduction and enhances the stability and adaptability of the flexible low-frequency power transmission system of the present invention under multiple working conditions.

[0102] In one embodiment of the present invention, the low-frequency side voltage control of the M3C converter is specifically as follows:

[0103]

[0104] Among them, u l_N is the rated voltage of the low-frequency side of the M3C converter.

[0105] In the present invention, when the grid side voltage u s_fault When it is greater than or equal to 0.9pu, the system considers that no fault has occurred on the power frequency side or the fault impact is extremely low, and does not perform the low-frequency side voltage reduction operation. At this time, the M3C converter maintains its low-frequency side voltage at the rated value.

[0106] The low-frequency side voltage control of the M3C converter described above in the present invention effectively prevents the fault ride-through process from being falsely triggered in non-fault or slight disturbance situations, avoids unnecessary voltage reduction and fan load reduction in the system, and ensures the normal operation efficiency of the system.

[0107] In one embodiment of the present invention, the minimum value u of the low-frequency side voltage reference value l_ref_min Determined by simulation analysis or estimated simulation based on the power network voltage calculation formula;

[0108] The power network voltage calculation formula is:

[0109]

[0110] Among them, U M3C is the low-frequency side voltage of the M3C converter, U W is the voltage at the wind turbine grid connection point, Δu is the line voltage drop between the wind turbine grid connection point and the M3C converter, P w is the active power output by the wind turbine, Q W is the reactive power output by the wind turbine, R L is the resistance of the low-frequency circuit, X L is the reactance of the low-frequency line.

[0111] In the present invention, in order to ensure that the low-frequency side voltage reference value used by the M3C converter during fault ride-through does not cause system power imbalance or excessive voltage drop, the minimum value u of the low-frequency side voltage reference value is l_ref_min This reference value is determined through estimation and simulation analysis using a power network model. It is used to ensure that the voltage at the wind turbine connection point does not fall below the lower limit of a preset voltage range (e.g., 0.8 pu) under the most unfavorable power output conditions (e.g., wind turbines operating at full power), thereby ensuring stable wind turbine connection and system operation.

[0112] The present invention establishes an equivalent electrical model between the wind turbine, the line, and the converter, performs simulation calculations or engineering estimates under different power output conditions and line parameter conditions, and ultimately determines the minimum U that can maintain the grid connection point voltage within a preset range. M3Cvalue, and determine the corresponding u accordingly l_ref_min This calculation method fully considers the impact of system parameters and operating conditions on voltage sag, and provides a reliable basis for setting the voltage reference value of the M3C converter during fault ride-through control.

[0113] In one embodiment of the present invention, the wind turbine-side converter switches to a control mode for wind turbine load shedding, and implements wind turbine load shedding through an overspeed load shedding strategy, so that the active power output by the wind turbine is less than or equal to the maximum transmittable power of the M3C converter under a fault condition, where the maximum transmittable power is determined based on the grid-side voltage and the maximum output current of the M3C converter during the fault period.

[0114] In the present invention, when a fault occurs on the power frequency grid side, the grid voltage drops to u s_fault ,u s_fault is the per-unit value, then the maximum transmittable power of the M3C converter after the fault is:

[0115]

[0116] Among them, P M3C_max is the maximum transmittable power of the M3C converter, i m3c_max is the maximum output current of the M3C converter, u s_fault is the grid side voltage during the fault period, k t is the coefficient.

[0117] In one embodiment of the present invention, when a fault occurs on the power frequency grid, the grid voltage drops to varying degrees, limiting the power output capacity of the M3C converter. To ensure system stability and safety during fault ride-through, the wind turbine-side converter, upon identifying a system fault, switches to a control mode for wind turbine load shedding and implements an overspeed load shedding strategy to reduce the wind turbine's active power output.

[0118] Specifically, to avoid the M3C converter from overloading and causing protection action or instability under fault conditions, the output power of the wind turbine must not exceed the maximum transmittable power P of the M3C converter under the current working condition. M3C_max The maximum transmittable power P M3C_max It is based on the grid side voltage u during the fault period s_fault and the maximum output current i of the M3C converter m3c_max Calculated.

[0119] This formula fully considers the mutual influence between the safe current carrying capacity of the M3C converter and the commutation control margin in the case of grid voltage drop, and provides a quantitative basis for setting the load reduction target on the wind turbine side. wLess than or equal to the P calculated above M3C_max , which can effectively avoid converter overload operation and improve the power coordination and dynamic stability of the system during fault ride-through.

[0120] In one embodiment of the present invention, if the M3C converter detects a symmetrical fault on the power frequency side, the M3C converter switches to a maximum power transmission mode before controlling the voltage on its own low frequency side to decrease.

[0121] In one embodiment of the present invention, if the M3C converter detects an asymmetric fault on the power frequency side, the M3C converter switches to a negative sequence current suppression mode before controlling the voltage on its own low frequency side to decrease.

[0122] In the present invention, after a fault occurs, the M3C converter detects the fault type and adjusts the M3C converter to different power transmission modes based on the detected fault information; if a symmetrical fault occurs, it switches to the maximum power transmission mode; if an asymmetrical fault occurs, it switches to the negative sequence current suppression mode.

[0123] In one embodiment of the present invention, when the M3C converter detects a fault on the power frequency side, it will first determine the fault type based on the acquired fault voltage and current information, and further select a matching power transmission control mode to enhance the system's fault response capability. Specifically, if a symmetrical fault (such as a three-phase short circuit) is detected on the power frequency side, the M3C converter switches to the maximum power transmission mode, maintaining wind power transmission as much as possible while ensuring voltage stability, preventing power accumulation on the system side, and reducing system power fluctuations. When an asymmetric fault (such as a single-phase or two-phase short circuit) is detected, the M3C converter switches to the negative sequence current suppression mode, which is used to suppress the negative sequence current component caused by the asymmetric voltage, prevent current imbalance in the system, and ensure safe operation of the equipment.

[0124] The dynamic switching of the above control modes occurs before the M3C converter performs the low-frequency side voltage reduction operation, which helps to stabilize the current output characteristics and power regulation direction in advance when the system state suddenly changes, thereby creating a stable foundation for subsequent voltage reduction operations.

[0125] The advantage of this solution lies in enabling classified responses and targeted control for different types of faults, avoiding the incompatibility issues caused by unified control strategies in specific fault scenarios and improving the adaptability and flexible control capabilities of the M3C converter. Furthermore, the mode switching process is implemented using the existing converter control platform, independent of external communication systems. This offers technical advantages such as fast response, simple implementation, and strong system robustness, helping to achieve stable ride-through operation of flexible low-frequency power transmission systems under various fault conditions.

[0126] Figure 6This is a diagram verifying the effect of the control strategy of the present invention, which is used to show the dynamic response process of key parameters of the system before and after the low-frequency side voltage reduction and overspeed load reduction control strategy proposed by the present invention is adopted after a fault occurs on the power frequency side. Figure 6 The middle curve includes typical quantities such as wind turbine grid connection point voltage, active power, M3C converter current, capacitor voltage, etc., which respectively correspond to the comparison of the operation results under no control and the scheme of the present invention. Figure 6 It can be seen that when the strategy of the present invention is not adopted, the wind turbine power release is delayed after the fault occurs, and the M3C converter has obvious overcurrent and voltage oscillations. However, after applying the control scheme of the present invention, the wind turbine quickly responds to the fault voltage signal, realizes the load reduction process, and the active power is quickly reduced to the tolerable range of the M3C converter. The voltage and current fluctuations are significantly reduced, and the overall system tends to be stable. Figure 6 It is verified that the control method proposed in the present invention has good stability and coordination during the fault ride-through process, and can effectively improve the anti-disturbance capability and safe operation level of the flexible low-frequency power transmission system.

[0127] Based on the same inventive concept, embodiments of the present invention also provide a method for fault ride-through of a flexible low-frequency power transmission system, as described in the following embodiments. Because the principles underlying the method for fault ride-through of a flexible low-frequency power transmission system are similar to those of the flexible low-frequency power transmission system, the embodiments of the method for fault ride-through of a flexible low-frequency power transmission system can be found in the embodiments of the flexible low-frequency power transmission system, and any repetitions will not be repeated.

[0128] Figure 7 FIG. 1 is a flow chart of a method for riding through a fault in a flexible low-frequency power transmission system according to an embodiment of the present invention. Figure 7 As shown, in one embodiment of the present invention, the flexible low-frequency power transmission system fault ride-through method of the present invention includes steps S101 to S103.

[0129] Step S101: When the M3C converter detects a fault on the power frequency side, it controls the voltage on its own low-frequency side to decrease, while ensuring the power stability of the low-frequency system, so that the grid connection point voltage of the wind turbine is within a preset voltage range for transmitting fault information. The preset voltage range is a voltage range that is lower than the normal voltage control range of the wind farm but does not enter the low voltage ride-through range, wherein the power frequency side of the M3C converter is connected to the AC power grid.

[0130] In step S102, when the wind turbine grid-side converter detects that the grid-connected point voltage is within the preset voltage range, it is determined that the system is in a fault ride-through state, wherein the output end of the wind turbine grid-side converter is connected to the low-frequency side of the M3C converter through a low-frequency line, and the wind turbine grid-side converter is connected to the wind turbine machine-side converter.

[0131] In step S103, upon receiving information indicating that the system is in a fault ride-through state, the wind turbine side converter switches to a control mode for wind turbine load shedding, and implements wind turbine load shedding through an overspeed load shedding strategy to achieve fault ride-through of the flexible low-frequency power transmission system, wherein the input end of the wind turbine side converter is connected to the output end of the wind turbine.

[0132] Figure 5 The overall process of fault ride-through control according to an embodiment of the present invention is shown as follows: Figure 5 As shown, the overall process of the fault ride-through control of the present invention includes: 1. The M3C converter performs fault detection; 2. The M3C converter switches to the maximum power transmission mode after detecting the fault; 3. The M3C converter performs voltage reduction control to transmit the fault information; 4. The wind turbine grid-side converter receives the fault information through voltage detection; 5. The wind turbine machine-side converter performs overspeed load reduction control to perform fault ride-through.

[0133] In one embodiment of the present invention, the conditions that need to be met for low-frequency system power stability are:

[0134]

[0135] Among them, U M3C is the low-frequency side voltage of the M3C converter, X L is the reactance of the low-frequency line, U W is the wind turbine grid connection point voltage, P w is the active power output by the fan.

[0136] In one embodiment of the present invention, the preset voltage range is 0.8 pu-0.97 pu, where pu is the per-unit value of the grid connection point voltage.

[0137] In one embodiment of the present invention, controlling the low-frequency side voltage to decrease in step S101 specifically includes:

[0138] The M3C converter controls its own low-frequency side voltage to decrease based on a low-frequency side voltage reference value, wherein the low-frequency side voltage reference value is determined according to an adjustment range of the low-frequency side voltage reference value and the grid side voltage during a fault period.

[0139] In one embodiment of the present invention, the adjustment range of the low-frequency side voltage reference value is [u l_ref_min ,u l_ref_max ],u l_ref_min is the minimum value of the low-frequency side voltage reference value, u l_ref_max is the maximum value of the low-frequency side voltage reference value, and the minimum value of the low-frequency side voltage reference value u l_ref_min A reference value is set to ensure that the voltage at the wind turbine grid connection point does not fall below the lower limit of the preset voltage range under a fault condition;

[0140] The low-frequency side voltage reference value is determined by the following formula:

[0141]

[0142] Among them, u l_ref is the low-frequency side voltage reference value, u s_fault is the grid side voltage during the fault period.

[0143] In one embodiment of the present invention, the minimum value u of the low-frequency side voltage reference value l_ref_min Determined by simulation analysis or estimated simulation based on the power network voltage calculation formula;

[0144] The power network voltage calculation formula is:

[0145]

[0146] Among them, U M3C is the low-frequency side voltage of the M3C converter, U W is the voltage at the wind turbine grid connection point, Δu is the line voltage drop between the wind turbine grid connection point and the M3C converter, P w is the active power output by the wind turbine, Q W is the reactive power output by the wind turbine, R L is the resistance of the low-frequency circuit, X L is the reactance of the low-frequency line.

[0147] In one embodiment of the present invention, switching to a control mode for wind turbine load reduction in step S103 and implementing wind turbine load reduction through an overspeed load reduction strategy specifically includes:

[0148] The wind turbine side converter switches to a control mode for wind turbine load shedding, and implements wind turbine load shedding through an overspeed load shedding strategy, so that the active power output by the wind turbine is less than or equal to the maximum transmittable power of the M3C converter under fault conditions. The maximum transmittable power is determined based on the grid side voltage during the fault and the maximum output current of the M3C converter.

[0149] In one embodiment of the present invention, the flexible low-frequency power transmission system fault ride-through method of the present invention further includes:

[0150] If the M3C converter detects a symmetrical fault on the power frequency side, the M3C converter switches to the maximum power transmission mode before controlling the voltage on its own low frequency side to decrease.

[0151] In one embodiment of the present invention, the flexible low-frequency power transmission system fault ride-through method of the present invention further includes:

[0152] If the M3C converter detects an asymmetric fault on the power frequency side, the M3C converter switches to a negative sequence current suppression mode before controlling the voltage on its own low frequency side to decrease.

[0153] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A flexible low-frequency power transmission system, characterized in that: include: Wind turbines, wind turbine-side converters, wind turbine-grid-side converters, low-frequency lines, and M3C converters; The output end of the wind turbine is connected to the input end of the wind turbine machine-side converter, the wind turbine machine-side converter is connected to the wind turbine grid-side converter, the output end of the wind turbine grid-side converter is connected to the low-frequency side of the M3C converter through the low-frequency line, and the power frequency side of the M3C converter is connected to the AC power grid; When the M3C converter detects a fault on the power frequency side, it controls the voltage on its own low-frequency side to decrease, while ensuring the power stability of the low-frequency system, so that the grid-connected point voltage of the wind turbine is within a preset voltage range for transmitting fault information. The preset voltage range is a voltage range that is lower than the normal voltage control range of the wind farm but does not enter the low voltage ride-through range; when the wind turbine grid-side converter detects that the grid-connected point voltage is within the preset voltage range, it determines that the system is in a fault ride-through state. At this time, the wind turbine machine-side converter switches to a control mode for wind turbine load shedding, and implements wind turbine load shedding through an overspeed load shedding strategy to achieve fault ride-through of the flexible low-frequency power transmission system.

2. The flexible low-frequency power transmission system according to claim 1, characterized in that: The conditions that need to be met for low-frequency system power stability are: Among them, U M3C is the low-frequency side voltage of the M3C converter, X L is the reactance of the low-frequency line, U W is the wind turbine grid connection point voltage, P w is the active power output by the fan.

3. The flexible low-frequency power transmission system according to claim 1, characterized in that: The preset voltage range is 0.8pu-0.97pu, where pu is the per-unit value of the grid connection point voltage.

4. The flexible low-frequency power transmission system according to claim 1, characterized in that: The M3C converter controls its own low-frequency side voltage to decrease based on a low-frequency side voltage reference value, wherein the low-frequency side voltage reference value is determined according to an adjustment range of the low-frequency side voltage reference value and the grid side voltage during a fault period.

5. The flexible low-frequency power transmission system according to claim 4, characterized in that: The low-frequency side voltage reference value adjustment range is [u l_ref_min ,u l_ref_max ],u l_ref_min is the minimum value of the low-frequency side voltage reference value, u l_ref_max is the maximum value of the low-frequency side voltage reference value, and the minimum value of the low-frequency side voltage reference value u l_ref_min A reference value is set to ensure that the voltage at the wind turbine grid connection point does not fall below the lower limit of the preset voltage range under a fault condition; The low-frequency side voltage reference value is determined by the following formula: Among them, u l_ref is the low-frequency side voltage reference value, u s_fault is the grid side voltage during the fault period.

6. The flexible low-frequency power transmission system according to claim 5, characterized in that: Minimum value of low-frequency side voltage reference value u l_ref_min Determined by simulation analysis or estimated simulation based on the power network voltage calculation formula; The power network voltage calculation formula is: Among them, U M3C is the low-frequency side voltage of the M3C converter, U W is the voltage at the wind turbine grid connection point, Δu is the line voltage drop between the wind turbine grid connection point and the M3C converter, P w is the active power output by the wind turbine, Q W is the reactive power output by the wind turbine, R L is the resistance of the low-frequency circuit, X L is the reactance of the low-frequency line.

7. The flexible low-frequency power transmission system according to claim 1, characterized in that: The wind turbine side converter switches to a control mode for wind turbine load shedding, and implements wind turbine load shedding through an overspeed load shedding strategy, so that the active power output by the wind turbine is less than or equal to the maximum transmittable power of the M3C converter under fault conditions. The maximum transmittable power is determined based on the grid side voltage during the fault and the maximum output current of the M3C converter.

8. The flexible low-frequency power transmission system according to claim 1, characterized in that: If the M3C converter detects a symmetrical fault on the power frequency side, the M3C converter switches to the maximum power transmission mode before controlling the voltage on its own low frequency side to decrease.

9. The flexible low-frequency power transmission system according to claim 1, characterized in that: If the M3C converter detects an asymmetric fault on the power frequency side, the M3C converter switches to a negative sequence current suppression mode before controlling the voltage on its own low frequency side to decrease.

10. A method for fault ride-through of a flexible low-frequency power transmission system, characterized in that: include: When the M3C converter detects a fault on the power frequency side, it controls the voltage on its own low-frequency side to decrease, ensuring the power stability of the low-frequency system, so that the voltage at the wind turbine's grid connection point is within a preset voltage range for transmitting fault information. The preset voltage range is a voltage range that is lower than the normal voltage control range of the wind farm but does not enter the low voltage ride-through range. The power frequency side of the M3C converter is connected to the AC grid. When the wind turbine grid-side converter detects that the grid connection point voltage is within the preset voltage range, it determines that the system is in a fault ride-through state, wherein the output end of the wind turbine grid-side converter is connected to the low-frequency side of the M3C converter through a low-frequency line, and the wind turbine grid-side converter is connected to the wind turbine machine-side converter; When the wind turbine side converter receives information that the system is in a fault ride-through state, it switches to a control mode for wind turbine load reduction and implements wind turbine load reduction through an overspeed load reduction strategy to achieve fault ride-through of the flexible low-frequency power transmission system, wherein the input end of the wind turbine side converter is connected to the output end of the wind turbine.

11. The method for flexible low-frequency power transmission system fault ride-through according to claim 10, characterized in that: The conditions that need to be met for low-frequency system power stability are: Among them, U M3C is the low-frequency side voltage of the M3C converter, X L is the reactance of the low-frequency line, U W is the wind turbine grid connection point voltage, P w is the active power output by the fan.

12. The flexible low-frequency power transmission system fault ride-through method according to claim 10, characterized in that: The preset voltage range is 0.8pu-0.97pu, where pu is the per-unit value of the grid connection point voltage.

13. The flexible low-frequency power transmission system fault ride-through method according to claim 10, characterized in that: The controlling of the voltage reduction on the low-frequency side thereof includes: The M3C converter controls its own low-frequency side voltage to decrease based on a low-frequency side voltage reference value, wherein the low-frequency side voltage reference value is determined according to an adjustment range of the low-frequency side voltage reference value and the grid side voltage during a fault period.

14. The flexible low-frequency power transmission system fault ride-through method according to claim 13, characterized in that: The low-frequency side voltage reference value adjustment range is [u l_ref_min ,u l_ref_max ],u l_ref_min is the minimum value of the low-frequency side voltage reference value, u l_ref_max is the maximum value of the low-frequency side voltage reference value, and the minimum value of the low-frequency side voltage reference value u l_ref_min A reference value is set to ensure that the voltage at the wind turbine grid connection point does not fall below the lower limit of the preset voltage range under a fault condition; The low-frequency side voltage reference value is determined by the following formula: Among them, u l_ref is the low-frequency side voltage reference value, u s_fault is the grid side voltage during the fault period.

15. The flexible low-frequency power transmission system fault ride-through method according to claim 14, characterized in that: Minimum value of low-frequency side voltage reference value u l_ref_min Determined by simulation analysis or estimated simulation based on the power network voltage calculation formula; The power network voltage calculation formula is: Among them, U M3C is the low-frequency side voltage of the M3C converter, U W is the voltage at the wind turbine grid connection point, Δu is the line voltage drop between the wind turbine grid connection point and the M3C converter, P w is the active power output by the wind turbine, Q W is the reactive power output by the wind turbine, R L is the resistance of the low-frequency circuit, X L is the reactance of the low-frequency line.

16. The flexible low-frequency power transmission system fault ride-through method according to claim 10, characterized in that: The switching to the control mode for fan load reduction and implementing the fan load reduction through the overspeed load reduction strategy includes: The wind turbine side converter switches to a control mode for wind turbine load shedding, and implements wind turbine load shedding through an overspeed load shedding strategy, so that the active power output by the wind turbine is less than or equal to the maximum transmittable power of the M3C converter under fault conditions. The maximum transmittable power is determined based on the grid side voltage during the fault and the maximum output current of the M3C converter.

17. The method for fault ride-through of a flexible low-frequency power transmission system according to claim 10, characterized in that: Also includes: If the M3C converter detects a symmetrical fault on the power frequency side, the M3C converter switches to the maximum power transmission mode before controlling the voltage on its own low frequency side to decrease.

18. The flexible low-frequency power transmission system fault ride-through method according to claim 10, characterized in that: Also includes: If the M3C converter detects an asymmetric fault on the power frequency side, the M3C converter switches to a negative sequence current suppression mode before controlling the voltage on its own low frequency side to decrease.

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