An offshore wind power flexible direct current system asymmetric fault collaborative crossing control method

Abstract

The application discloses a kind of offshore wind power flexible DC system asymmetric fault collaborative crossing control methods, system includes offshore wind farm, AC power collection system, offshore MMC converter station and DC transmission sea cable, offshore wind farm is made of not less than one wind turbine, wind turbine is connected to grid by machine side converter and net side converter, method includes fault detection and sequence component extraction, determines system total negative sequence current instruction, controls wind turbine net side converter injection limited negative sequence current, controls flexible DC converter station to compensate negative sequence current difference four steps, with technical advantages such as considering overvoltage suppression and active transmission, guaranteeing equipment safety, improving relay protection reliability, optimizing system dynamic characteristics.

Description

Technical Field

[0001] This invention belongs to the field of power system transmission and distribution technology, specifically relating to a method for collaborative fault ride-through control of asymmetric faults in offshore wind power flexible DC systems. Background Technology

[0002] With the development of offshore wind power resources, flexible DC transmission technology based on modular multilevel converters (MMC) has become the mainstream solution for long-distance offshore wind power grid connection due to its advantages such as independent regulation of active and reactive power, no need for additional reactive power compensation devices, and no risk of commutation failure. However, the marine environment in which offshore AC collection systems are located is harsh, and cables are susceptible to anchor damage or insulation aging, resulting in a higher probability of asymmetrical faults such as single-phase grounding or two-phase short circuits.

[0003] During asymmetrical faults, significant negative sequence voltage components appear in AC systems, causing voltage increases in non-faulty phases (overvoltage), which may endanger cable and equipment insulation. Simultaneously, negative sequence voltages generate negative sequence currents in the converter; if not properly controlled, this can lead to DC-side voltage double-frequency fluctuations or converter overcurrent blocking.

[0004] Existing asymmetric fault ride-through control methods for this system can be mainly divided into the following three categories:

[0005] The first type is the scheme of completely suppressing negative sequence current. This means that both wind turbines and offshore MMCs employ negative sequence current suppression control, ensuring that the output negative sequence current is zero. The advantage of this scheme is that it maximizes the retention of converter capacity for transmitting active power. However, its disadvantages are extremely obvious: firstly, due to the lack of short-circuit current injection (especially the negative sequence component) at the fault point, the fault characteristics are weak, which may lead to insufficient sensitivity or even failure of AC-side relay protection devices; secondly, during the fault, the system lacks negative sequence current support, resulting in severe overvoltage on non-faulty phases, and the voltage recovery speed is slow after the fault is cleared.

[0006] The second type is the MMC-only negative sequence current injection scheme. This involves suppressing the negative sequence current of the wind turbine to zero, with the offshore MMC handling the negative sequence current injection to support the voltage. While this scheme can improve overvoltage issues and ensure protection operation, its disadvantage is that the offshore MMC typically bears the heavy responsibility of collecting all the power from the site. If a large amount of capacity is allocated to outputting negative sequence current, it will inevitably significantly reduce its positive sequence active power transmission capacity. In severe cases, some wind turbines may need to be shut down, resulting in wind curtailment losses.

[0007] The third type is the scheme where only the wind turbine injects negative sequence current. This means the MMC suppresses the negative sequence current, and the negative sequence current is provided collectively by the wind turbine group. This scheme attempts to reduce the burden on the MMC, but it has serious hidden dangers: wind turbines typically use two-level or three-level converters. When a large negative sequence current flows through the grid side, its DC bus will experience severe second-harmonic power fluctuations. This not only affects DC voltage stability but also couples to the turbine side through the DC bus, causing electromagnetic torque pulsation in the generator, increasing mechanical fatigue wear on the wind turbine gearbox and blades, and shortening equipment lifespan.

[0008] In summary, existing technologies struggle to achieve a balance between suppressing overvoltage, ensuring relay protection performance, maintaining active power transmission efficiency, and protecting wind turbine equipment safety. Therefore, an optimized strategy that can synergistically utilize the capabilities of both the wind turbine and the MMC control system is urgently needed. Summary of the Invention

[0009] To address the aforementioned problems in the prior art, this invention provides an asymmetric fault collaborative ride-through control method for a flexible DC system for offshore wind power. By coordinating the control objectives of the grid-side converter and the offshore MMC (Multi-mode DC converter), it achieves a reasonable distribution of negative sequence current while meeting grid connection standards and equipment safety, effectively solving the overvoltage problem of non-faulty phases, and simultaneously considering active power transmission and equipment protection. The system includes an offshore wind farm, an AC power collection system, an offshore MMC converter station, and a DC transmission submarine cable. The offshore wind farm consists of at least one wind turbine, which is connected to the grid through a turbine-side converter and a grid-side converter.

[0010] The method includes the following steps:

[0011] S1. Fault detection and sequence component extraction: Real-time monitoring of the voltage status of the common connection point of the offshore AC power collection system. When an asymmetrical voltage drop is detected, it is determined that an asymmetrical fault has occurred in the system. The positive sequence voltage component and the negative sequence voltage component of the fault point are separated in real time using the sequence component extraction algorithm.

[0012] S2. Determine the total negative sequence current command of the system; calculate the required total negative sequence current command value of the system based on the non-faulty phase voltage suppression target;

[0013] S3. Control the wind turbine grid-side converter to inject restricted negative sequence current; set the upper limit of the negative sequence current output of the wind turbine grid-side converter, and control the wind turbine grid-side converter to inject negative sequence current into the AC power collection system not exceeding the upper limit.

[0014] S4. Control the flexible DC converter station to compensate for the negative sequence current difference; if the negative sequence current command value is greater than the upper limit of the negative sequence current of the grid-side converter of the wind turbine, calculate the difference between the total negative sequence current command value and the sum of the actual negative sequence current injected by all wind turbines, and use the difference as the target value of the negative sequence current of the offshore MMC converter station. Control the offshore MMC converter station to inject the difference between the total negative sequence current command value and the sum of the actual negative sequence current injected by all wind turbines into the AC power collection system, so as to realize the coordinated fault ride-through of wind turbines and flexible DC converter station.

[0015] Furthermore, in step S2, the calculation of the total negative sequence current command value adopts proportional-integral control, with the negative sequence voltage amplitude as the feedback quantity, to dynamically adjust the magnitude of the total negative sequence current until the voltage of the non-faulty phase is lower than the equipment insulation safety threshold.

[0016] Furthermore, in step S3, the principle for setting the upper limit of the negative sequence current output is: under the premise of meeting the requirements of the grid connection guidelines for dynamic reactive current support during faults, the amplitude of the negative sequence current is limited so that the second harmonic fluctuation amplitude of the DC bus voltage of the grid-side converter of the wind turbine does not exceed the preset safety threshold.

[0017] Furthermore, in step S3, the upper limit of the negative sequence current output is determined based on the rated current capacity of the wind turbine converter and the DC bus voltage double frequency ripple withstand capability.

[0018] Furthermore, in step S4, the control strategy of the offshore MMC converter station includes current priority management:

[0019] After calculating the required negative sequence current target value, verify the remaining current capacity of the offshore MMC converter station.

[0020] If the remaining current capacity is sufficient, then both the positive-sequence active current and the required negative-sequence current will be output simultaneously.

[0021] If the remaining current capacity is insufficient, the negative sequence current output demand will be prioritized, and the positive sequence active current will be limited and compressed until the total current does not exceed the maximum allowable current of the offshore MMC converter station.

[0022] Furthermore, the method also includes a recovery step S5 after fault clearance: when the fault clearance is detected and the AC voltage begins to recover, the offshore MMC converter station and the wind turbine grid-side converter will coordinately attenuate the negative sequence current command value according to a preset slope, smoothly transition to the normal operation state of only outputting positive sequence current, and avoid voltage overshoot.

[0023] Furthermore, the asymmetrical fault mentioned in step S1 includes a single-phase ground fault or a two-phase short-circuit fault.

[0024] Compared with the prior art, the beneficial effects of this invention are:

[0025] 1. Balancing overvoltage suppression and active power transmission: Through coordinated control, the wind turbine shares some of the negative sequence current, reducing the pressure on the MMC and allowing the MMC to retain more capacity for transmitting active power, thus improving system transmission efficiency compared to solutions that rely solely on the MMC.

[0026] 2. Ensure equipment safety: The negative sequence current injected into the fan is strictly limited, which effectively suppresses the double frequency voltage ripple on the DC side of the fan, prevents torque pulsation from damaging the mechanical components of the fan, and overcomes the defects of relying solely on the fan injection scheme.

[0027] 3. Improved reliability of relay protection: Compared with the traditional solution that completely suppresses negative sequence current, the present invention ensures that sufficient negative sequence current flows through the fault point, so that the relay protection device of the AC collector system can accurately identify the fault type and operate reliably, avoiding the risk of protection failure to operate.

[0028] 4. Optimize system dynamic characteristics: The coordinated operation of the wind turbine and MMC makes the voltage support stronger during the fault period and the voltage recovery more stable after the fault is cleared, thus improving the robustness of the entire offshore wind power system. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the structure of the offshore wind power flexible DC system according to an embodiment of the present invention;

[0030] Figure 2 This is a schematic diagram illustrating the principle of the asymmetric fault cooperative ride-through control method for offshore wind power flexible DC systems according to an embodiment of the present invention.

[0031] Figure 3 This is a schematic diagram of the simulation waveform of the negative sequence current injected into the fault point of the wind turbine converter according to an embodiment of the present invention;

[0032] Figure 4 A schematic diagram of the simulation waveform of the negative sequence current injected into the fault point of the offshore MMC converter station according to an embodiment of the present invention;

[0033] Figure 5 This is a schematic diagram of the simulated voltage waveform at the common coupling point of the marine AC current collection system according to an embodiment of the present invention. Detailed Implementation

[0034] The following will provide a clear and complete description of the concept, specific steps, and technical effects of the present invention in conjunction with embodiments and accompanying drawings, so as to fully understand the purpose, solution, and effects of the present invention. It should be particularly noted that the described embodiments are merely some embodiments of the present invention, and not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without making creative contributions are all within the protection scope of the present invention.

[0035] Example 1

[0036] This embodiment provides a cooperative fault ride-through control method for asymmetric faults in an offshore wind power flexible DC system, combined with... Figure 1 The present invention provides a schematic diagram of the structure of a flexible DC system for offshore wind power. The system includes an offshore wind farm, an AC power collection system, an offshore MMC converter station, and a DC transmission submarine cable. The offshore wind farm consists of at least one wind turbine, which is connected to the grid through a turbine-side converter and a grid-side converter.

[0037] The method includes the following steps: S1, fault detection and sequence component extraction; real-time monitoring of the voltage status at the common coupling point of the offshore AC power collection system; when an asymmetrical voltage drop is detected, an asymmetrical fault is determined to have occurred in the system; and the positive and negative sequence voltage components at the fault point are separated in real time using a sequence component extraction algorithm. Asymmetrical faults include single-phase grounding faults or two-phase short-circuit faults.

[0038] S2. Determine the total negative sequence current command of the system; based on the set negative sequence voltage reference value, i.e., the non-faulty phase voltage suppression target, calculate the total negative sequence current command value required by the system. In step S2, the calculation of the total negative sequence current command value adopts proportional-integral control, using the negative sequence voltage amplitude as the feedback quantity to dynamically adjust the magnitude of the total negative sequence current until the negative sequence voltage is lower than the set threshold.

[0039] S3. Control the injection of restricted negative sequence current into the grid-side converter of the wind turbine; set an upper limit for the negative sequence current output of the grid-side converter of the wind turbine, and control the injection of negative sequence current into the AC collector system by the grid-side converter of the wind turbine to not exceed the upper limit. In step S3, the principle for setting the upper limit of the negative sequence current output is: under the premise of meeting the requirements of the grid connection guidelines for dynamic reactive current support during faults, limit the amplitude of the negative sequence current so that the second harmonic fluctuation amplitude of the DC bus voltage of the grid-side converter of the wind turbine does not exceed the preset safety threshold. In step S3, the upper limit of the negative sequence current output is determined based on the rated current capacity of the wind turbine converter and the second harmonic ripple withstand capability of the DC bus voltage.

[0040] S4. Control the flexible DC converter station to compensate for the negative sequence current difference; if the negative sequence current command value is greater than the upper limit of the negative sequence current of the wind turbine grid-side converter, calculate the difference between the total negative sequence current command value and the sum of the actual negative sequence current injected by all wind turbines. Use this difference as the target value of the negative sequence current for the offshore MMC converter station, and control the offshore MMC converter station to inject the difference between the total negative sequence current command value and the sum of the actual negative sequence current injected by all wind turbines into the AC collector system, thereby achieving coordinated fault ride-through between the wind turbines and the flexible DC converter station. In step S4, the control strategy of the offshore MMC converter station includes current priority management:

[0041] After calculating the required negative sequence current target value, verify the remaining current capacity of the offshore MMC converter station.

[0042] If the remaining current capacity is sufficient, then both the positive-sequence active current and the required negative-sequence current will be output simultaneously.

[0043] If the remaining current capacity is insufficient, the negative sequence current output demand will be prioritized, and the positive sequence active current will be limited and compressed until the total current does not exceed the maximum allowable current of the offshore MMC converter station.

[0044] The asymmetric fault collaborative ride-through control method for a flexible DC system for offshore wind power in this embodiment also includes a recovery step after fault clearing: when the fault clearing is detected and the AC voltage begins to recover, the offshore MMC converter station and the grid-side converter of the wind turbine unit collaboratively attenuate the negative sequence current command value according to the preset slope, smoothly transition to the normal operation state of only outputting positive sequence current, and avoid voltage overshoot.

[0045] Compared with the prior art, the beneficial effects of this invention are:

[0046] 1. Balancing overvoltage suppression and active power transmission: Through coordinated control, the wind turbine shares some of the negative sequence current, reducing the pressure on the MMC and allowing the MMC to retain more capacity for transmitting active power, thus improving system transmission efficiency compared to solutions that rely solely on the MMC.

[0047] 2. Ensure equipment safety: The negative sequence current injected into the fan is strictly limited, which effectively suppresses the double frequency voltage ripple on the DC side of the fan, prevents torque pulsation from damaging the mechanical components of the fan, and overcomes the defects of relying solely on the fan injection scheme.

[0048] 3. Improved reliability of relay protection: Compared with the traditional solution that completely suppresses negative sequence current, the present invention ensures that sufficient negative sequence current flows through the fault point, so that the relay protection device of the AC collector system can accurately identify the fault type and operate reliably, avoiding the risk of protection failure to operate.

[0049] 4. Optimize system dynamic characteristics: The coordinated operation of the wind turbine and MMC makes the voltage support stronger during the fault period and the voltage recovery more stable after the fault is cleared, thus improving the robustness of the entire offshore wind power system.

[0050] Example 2

[0051] This embodiment uses a typical offshore wind power transmission system via an MMC-HVDC (offshore MMC converter station-high voltage direct current transmission) system as an example. Combined with... Figure 1The schematic diagram of the offshore wind power flexible DC system in this embodiment of the invention includes an offshore wind farm, an AC power collection system, an offshore MMC converter station, and a DC transmission submarine cable. Multiple full-power converter wind turbines are collected by 35kV power collection cables to a 66kV bus, then connected to the offshore MMC converter station (flexible DC converter station), and finally transmitted to the onshore power grid via a high-voltage DC submarine cable.

[0052] Combination Figure 2 The schematic diagram illustrates the principle of the asymmetric fault cooperative ride-through control method for offshore wind power flexible DC systems according to an embodiment of the present invention. When a single-phase ground fault (such as phase A grounding) occurs in the offshore AC collector system, the control system executes the following process:

[0053] Step 1: Fault Identification and Sequential Component Separation

[0054] Both the wind turbine and the MMC controller are equipped with a fast positive and negative sequence separation module. During normal operation, the negative sequence voltage is close to zero. When a fault occurs, the voltage of phase A drops, while the voltages of phases B and C rise. The controller detects that the negative sequence voltage amplitude exceeds 0.1 pu within 1 / 4 of the power frequency cycle, triggering the asymmetric fault ride-through control mode.

[0055] Step 2: Determine the negative sequence current command for the wind turbine.

[0056] The grid-side converter (GSC) of a wind turbine is first calculated to determine its maximum allowable current. To prevent excessive second-harmonic fluctuations in the DC bus voltage, a negative sequence current limit is set for the fan. This limit is set at 0.2 pu, and is only used to meet the basic requirement of generating inductive reactive current during a fault.

[0057] negative sequence current command output by the fan Take the smaller of the following two values: Where K is the reactive power support coefficient. At this time, the wind turbine is mainly responsible for providing a basic short-circuit current path to ensure that the protection device can detect the fault, while protecting its own mechanical structure from torque pulsation impact.

[0058] Step 3: Calculate the compensation command for the offshore MMC converter station

[0059] The offshore MMC, acting as the system's balancing node, is responsible for controlling the AC voltage. Based on the current negative sequence voltage value, the controller calculates the total negative sequence current injected into the PCC point required to suppress the negative sequence voltage to 0.1 pu. Subsequently, the MMC controller calculates the negative sequence current it needs to handle:

[0060]

[0061] in This represents the negative sequence current value injected by the grid-side converter of the i-th wind turbine into the fault point.

[0062] Step 4: Limiting and Priority Control of Offshore MMC Converter Stations

[0063] The MMC controller will calculate the negative sequence current command. With the positive sequence current command required to maintain power transmission Perform vector synthesis. If the synthesized current amplitude exceeds the IGBT allowable current of the MMC... Then the amplitude limiting logic will be activated: maintain The positive-sequence active current remains unchanged, and is proportionally reduced until the total current meets the requirements. This strategy ensures the insulation safety of the equipment during short-term faults.

[0064] Step 5: Fault Recovery

[0065] When the fault is cleared, the negative sequence voltage disappears. After the system detects that the voltage has returned to normal, the wind turbine and MMC linearly exit the negative sequence current injection within 50-100ms, smoothly returning to the maximum power point tracking (MPPT) operation state before the fault.

[0066] Through the above implementation methods, the present invention comprehensively utilizes the characteristics of a large number of wind turbines and their wide distribution, as well as the advantages of MMC's large capacity and flexible control, to form a complementary fault ride-through mechanism, effectively solving the various drawbacks of a single control strategy.

[0067] The system parameters in this embodiment are shown in Table 1:

[0068] Table 1 System Parameters

[0069]

[0070] A simulation platform was built in the electromagnetic transient simulation software PSCAD / EMTDC to simulate a single-phase ground fault on the grid side of the point of common coupling (PCC) of an offshore AC power distribution system. The system had already entered steady state before t=2.1 s; it was assumed that a single-phase ground fault occurred on the grid side of the PCC of the offshore AC power distribution system at t=2.1 s. Figure 3 and Figure 4 Simulation results of negative sequence current injected into the fault point of the wind turbine converter and the offshore MMC converter station are presented. The vertical axis represents the negative sequence current value injected into the fault point, and the horizontal axis represents time. Figure 5 Simulation results of the overvoltage characteristics at the point of common coupling (PCC) of an offshore AC current collector system are presented. The vertical axis represents the voltage value at the PCC, and the horizontal axis represents time. As shown in the figure, when a system fault occurs, the wind turbine grid-side converter and the offshore MMC converter station jointly bear the negative sequence current injected into the fault point, achieving reliable fault ride-through of the system. The simulation results demonstrate the effectiveness of this invention.

[0071] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Therefore, any modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for cooperative fault ride-through control of an offshore wind power flexible DC system, characterized in that, The system includes an offshore wind farm, an AC power collection system, an offshore MMC converter station, and a DC transmission submarine cable. The offshore wind farm consists of at least one wind turbine, which is connected to the grid through a turbine-side converter and a grid-side converter. The method includes the following steps: S1. Fault detection and sequence component extraction: Real-time monitoring of the voltage status of the common connection point of the offshore AC power collection system. When an asymmetrical voltage drop is detected, it is determined that an asymmetrical fault has occurred in the system. The positive sequence voltage component and the negative sequence voltage component of the fault point are separated in real time using the sequence component extraction algorithm. S2. Determine the total negative sequence current command of the system; calculate the required total negative sequence current command value of the system based on the non-faulty phase voltage suppression target; S3. Control the wind turbine grid-side converter to inject restricted negative sequence current; set the upper limit of the negative sequence current output of the wind turbine grid-side converter, and control the wind turbine grid-side converter to inject negative sequence current into the AC power collection system not exceeding the upper limit. S4. Control the flexible DC converter station to compensate for the negative sequence current difference; if the negative sequence current command value is greater than the upper limit of the negative sequence current of the grid-side converter of the wind turbine, calculate the difference between the total negative sequence current command value and the sum of the actual negative sequence current injected by all wind turbines, and use the difference as the target value of the negative sequence current of the offshore MMC converter station. Control the offshore MMC converter station to inject the difference between the total negative sequence current command value and the sum of the actual negative sequence current injected by all wind turbines into the AC power collection system, so as to realize the coordinated fault ride-through of wind turbines and flexible DC converter station.

2. The method for cooperative fault ride-through control of asymmetric faults in a flexible DC system for offshore wind power according to claim 1, characterized in that, In step S2, the calculation of the total negative sequence current command value adopts proportional-integral control, with the negative sequence voltage amplitude as the feedback quantity, and the magnitude of the total negative sequence current is dynamically adjusted until the voltage of the non-faulty phase is lower than the equipment insulation safety threshold.

3. The asymmetric fault cooperative ride-through control method for offshore wind power flexible DC systems according to claim 1, characterized in that, In step S3, the principle for setting the upper limit of the negative sequence current output is: under the premise of meeting the requirements of the grid connection guidelines for dynamic reactive current support during faults, the amplitude of the negative sequence current is limited so that the second harmonic fluctuation amplitude of the DC bus voltage of the grid-side converter of the wind turbine does not exceed the preset safety threshold.

4. The asymmetric fault cooperative ride-through control method for offshore wind power flexible DC systems according to claim 1, characterized in that, In step S3, the upper limit of the negative sequence current output is determined based on the rated current capacity of the wind turbine converter and the DC bus voltage double frequency ripple withstand capability.

5. The method for cooperative fault ride-through control of a flexible DC system for offshore wind power according to claim 1, characterized in that, In step S4, the control strategy of the offshore MMC converter station includes current priority management: After calculating the required negative sequence current target value, verify the remaining current capacity of the offshore MMC converter station. If the remaining current capacity is sufficient, then both the positive-sequence active current and the required negative-sequence current will be output simultaneously. If the remaining current capacity is insufficient, the negative sequence current output demand will be prioritized, and the positive sequence active current will be limited and compressed until the total current does not exceed the maximum allowable current of the offshore MMC converter station.

6. The asymmetric fault cooperative ride-through control method for offshore wind power flexible DC systems according to claim 1, characterized in that, The method also includes a recovery step S5 after fault clearance: when the fault clearance is detected and the AC voltage begins to recover, the offshore MMC converter station and the wind turbine grid-side converter will coordinately attenuate the negative sequence current command value according to the preset slope, smoothly transition to the normal operation state of only outputting positive sequence current, and avoid voltage overshoot.

7. The asymmetric fault cooperative ride-through control method for offshore wind power flexible DC systems according to claim 1, characterized in that, The asymmetrical faults mentioned in step S1 include single-phase ground faults or two-phase short-circuit faults.

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