A method and system for adaptive limiting control of negative sequence voltage outer loop of offshore wind power flexible HVDC station

Abstract

The application provides a kind of offshore wind power flexible direct current converter station negative sequence voltage outer loop adaptive limiting control method and system, it is related to offshore wind power through flexible direct current transmission system grid-connected control technical field.The method collects the three-phase current of the net side of flexible direct current converter station, extracts the effective value of zero sequence current and negative sequence current effective value, and the ratio of the two is as fault characteristic ratio;With the maximum value in the effective value of the six bridge arm fundamental currents of the converter valve as the total current effective value, and calculate the current margin according to the preset current limit value;According to the fault characteristic ratio, determine the negative sequence dominant or zero sequence dominant limiting mode;In the negative sequence dominant limiting mode and the current margin is greater than zero, the sum of the preset base value and the product of the proportionality coefficient and the current margin is determined as the limiting value, in the remaining cases, the limiting value is determined as the preset base value;The original negative sequence current instruction is input into the negative sequence current inner loop controller after symmetric limiting.The application can consider negative sequence fault feature visualization and converter valve overcurrent safety.

Description

Technical Field

[0001] This invention relates to the field of grid connection control technology for offshore wind power via flexible DC transmission systems, specifically to an adaptive limiting control method and system for the negative sequence voltage outer loop of an offshore wind power flexible DC converter station. Background Technology

[0002] Connecting offshore wind power to the grid via a flexible DC transmission system is a crucial technical solution for long-distance offshore wind power development. The electricity generated by offshore wind farms is collected via AC collection lines, converted to DC power through an offshore flexible DC converter station, and then transmitted to the onshore power grid via a DC submarine cable. In offshore wind power flexible DC transmission systems, the AC collection lines are typically submarine cables, which are constantly exposed to seawater and are prone to failure. Their fault characteristics directly affect the reliability of relay protection operations.

[0003] The control system of a flexible DC converter station typically employs a dual-closed-loop control strategy, with both the positive-sequence and negative-sequence control loops including an outer voltage loop and an inner current loop. The outer voltage loop generates a current command value, which is then used by the inner current loop controller to achieve current tracking. For negative-sequence component control, since the performance of traditional differential protection is poor when negative-sequence current suppression is used on the flexible DC side, existing control systems typically employ a negative-sequence voltage suppression strategy. By suppressing the negative-sequence voltage, the outer negative-sequence voltage loop generates a negative-sequence current command, achieving controllable injection of the negative-sequence current and thus revealing fault characteristics.

[0004] To prevent overcurrent in the converter valve, existing controls typically set a fixed negative sequence current limit, adjusting the negative sequence current reference value by regulating the outer loop limit of the negative sequence voltage. This method ensures that the negative sequence current command must not exceed the fixed limit regardless of the system's operating state. While this can prevent overcurrent to some extent, it is difficult to adaptively adjust based on the fault type and the current residual current capacity of the converter valve.

[0005] When a two-phase short-circuit fault or a single-phase ground fault occurs on an AC collection line, the system fault current level is relatively low, and the total current may be far below the limit value. The converter valve still has a large current margin, but the fault current characteristics themselves are weak. At this time, if the negative sequence current command is still limited to a low level by a fixed limit value, the converter station will have difficulty outputting sufficient negative sequence current to manifest the fault characteristics, which may lead to insufficient sensitivity of the differential protection installed on the AC collection line.

[0006] When a two-phase-to-ground fault occurs in an AC collection line, the fault current mainly flows through the positive-sequence and zero-sequence loops. The zero-sequence current is significant, and the fault characteristics are already quite obvious. Usually, it is not necessary to increase the negative-sequence current injection. If a large negative-sequence current output is still allowed, it may occupy the current capacity of the converter valve and even increase the risk of overcurrent.

[0007] Therefore, the existing fixed-limiting method for negative sequence current control in flexible DC converter stations cannot simultaneously adaptively determine the negative sequence voltage outer loop output limit value based on the current sequence current characteristics of the current asymmetrical fault and the real-time current margin of the converter valve. It is difficult to meet the dual requirements of negative sequence fault characteristic manifestation and converter valve overcurrent protection. Summary of the Invention

[0008] The purpose of this invention is to provide an adaptive limiting control method and system for the outer loop of negative sequence voltage in offshore wind power flexible DC converter stations, so as to at least solve the problem that when existing flexible DC converter stations use fixed negative sequence current limiting values, it is difficult to dynamically adjust the maximum allowable output value of negative sequence current based on the sequence current characteristics of asymmetrical faults and the real-time current margin of converter valves.

[0009] To achieve the above objectives, the first aspect of the present invention provides an adaptive limiting control method for the negative sequence voltage outer loop of an offshore wind power flexible DC converter station, comprising the following steps.

[0010] S1, collect the three-phase current on the grid side of the converter station, extract the effective values ​​of zero-sequence and negative-sequence currents, and record the ratio of the effective value of zero-sequence current to the effective value of negative-sequence current as the fault characteristic ratio λ. S2, λ < 0.9 The first limiting mode is determined at that time. λ>1.05 The second limiting mode is determined at that time. 0.9≤λ≤1.05 The limiting mode remains the same as the previous moment. The negative sequence dominant faults corresponding to the first limiting mode include two-phase short circuits and single-phase ground faults, while the zero sequence dominant faults corresponding to the second limiting mode include two-phase ground faults. S3: At a preset sampling frequency, the instantaneous current values ​​of the six bridge arms (upper and lower arms) of the three-phase converter valve are collected through a rolling window of one power frequency cycle. A Fast Fourier Transform is performed on each sampling sequence to extract the fundamental component, and the effective value of the fundamental component is calculated based on its amplitude. The maximum value is taken as the total effective value of the current. A preset current limit is set according to the rated current and short-time overcurrent capacity of the converter valve. The current margin is obtained by subtracting the total effective value of the current from this limit. ; S4, first limiting mode and ＞0 At that time, set the preset scaling factor. k=1 ,by C+k As the outer loop output limiting value of the negative sequence voltage; first limiting mode and ≤0 At that time, the amplitude limit value is taken as C In the second limiting mode, let k=0 The amplitude limit value is taken as C And with Irrelevant; preset base value C The value ranges from 0.1 pu to 0.2 pu. S5. Using the limiting value and its opposite as the upper and lower limits, the original negative sequence current command generated by the negative sequence voltage outer loop is symmetrically limited to obtain the negative sequence current reference value. S6, input the negative sequence current reference value into the negative sequence current inner loop controller, and track and control the negative sequence current of the converter station according to the negative sequence current reference value.

[0011] Furthermore, S1 includes: real-time acquisition of the three-phase current on the grid side of the converter station. , , The three-phase currents are extracted using a method based on a second-order generalized integrator for separating positive and negative sequence currents, a method based on a notch filter for extracting sequence components, or a method based on delay signal cancellation. This yields the effective values ​​of the positive and negative sequence currents. and the effective value of zero-sequence current The effective value of the zero-sequence current With the effective value of the negative sequence current The ratio of λ to λ is used as the fault characteristic ratio.

[0012] Further, S2 includes: when the fault characteristic ratio λ is less than 0.9, determining the first limiting mode and classifying the fault as a negative-sequence dominant fault; when the fault characteristic ratio λ is greater than 1.05, determining the second limiting mode and classifying the fault as a zero-sequence dominant fault; when the fault characteristic ratio λ is greater than or equal to 0.9 and less than or equal to 1.05, maintaining the limiting mode determination result of the previous moment.

[0013] Furthermore, the negative-sequence dominant fault includes two-phase short-circuit faults and single-phase ground faults, and the zero-sequence dominant fault includes two-phase ground faults.

[0014] Furthermore, before calculating the fault characteristic ratio λ, the effective value of the negative sequence current is... and the effective value of the zero-sequence current Low-pass filtering is performed; the filtering time constant for low-pass filtering is 2ms to 5ms.

[0015] Further, S3 includes: acquiring the instantaneous values ​​of the six bridge arm currents of the converter valve, namely the three-phase upper bridge arm current and the three-phase lower bridge arm current; using a rolling window with a window length of one power frequency cycle, acquiring the instantaneous values ​​of the six bridge arm currents at a preset sampling frequency to obtain a sampling sequence corresponding to each bridge arm current; performing a fast Fourier transform on each sampling sequence to extract the fundamental component of the corresponding bridge arm current, and calculating the effective value of each bridge arm current based on the amplitude of the fundamental component; and determining the maximum value among the six effective values ​​of the bridge arm current as the total effective value of the current. A preset current limit is set based on the rated current and short-time overcurrent capacity of the converter valve, and the preset current limit is compared with the effective value of the total current. The difference between them is determined as the current margin. The preset current limit value ranges from 1.5 pu to 2.0 pu. After a fault occurs, as the scrolling window gradually slides, the data within the window gradually transitions from a mixture of pre-fault and post-fault data to entirely post-fault data. The effective value of the total current is updated accordingly, and the negative sequence voltage outer loop output limit value is adjusted synchronously in real time.

[0016] Further, S4 includes: in the first limiting mode, when the current margin When the value is greater than zero, the preset proportional coefficient k is set to 1, and the preset base value C is compared with the current margin. The sum is determined as the negative sequence voltage outer loop output limiting value; in the first limiting mode, when the current margin When the voltage is less than or equal to zero, the negative sequence voltage outer loop output limiting value is determined as the preset base value C; in the second limiting mode, the preset proportional coefficient k is determined to be 0, and the negative sequence voltage outer loop output limiting value is determined as the preset base value C, so that the negative sequence voltage outer loop output limiting value is consistent with the current margin. Irrelevant; wherein, the preset base value C ranges from 0.1pu to 0.2pu.

[0017] Further, S5 includes: S51, combining the original negative sequence current command with the negative sequence voltage outer loop output limiting value. Comparison; S52, when the original negative sequence current command is greater than the negative sequence voltage outer loop output limit value. At that time, the negative sequence current reference value is determined as the negative sequence voltage outer loop output limiting value. When the original negative sequence current command is less than the negative sequence voltage outer loop output limit value. When the negative number is the opposite of the negative sequence current reference value, the negative sequence voltage outer loop output limiting value is determined as the negative sequence voltage outer loop output limiting value. The opposite of; when the original negative sequence current command is located at the output limiting value of the outer loop of the negative sequence voltage. When the original negative sequence current command is between the negative sequence current command and its opposite, the negative sequence current reference value is used.

[0018] Further, S6 includes: S61, inputting the negative sequence current reference value into the negative sequence current inner loop controller; S62, the negative sequence current inner loop controller generates a negative sequence voltage modulation signal based on the deviation between the negative sequence current reference value and the measured negative sequence current, and generates a converter valve trigger pulse through the modulation stage to achieve closed-loop tracking control of the negative sequence current.

[0019] The second aspect of the present invention provides an adaptive limiting control system for the outer loop of negative sequence voltage in an offshore wind power flexible DC converter station, comprising a current acquisition module, a sequence component extraction module, a fault mode determination module, a current margin calculation module, an adaptive limiting module, and a negative sequence current inner loop controller.

[0020] The current acquisition module is used to acquire the three-phase current on the grid side of the offshore wind power flexible DC converter station, and obtain the instantaneous values ​​of the three-phase upper arm current and the three-phase lower arm current of the converter valve, totaling six arm currents. The sequence component extraction module is used to extract the sequence components of the three-phase currents to obtain the effective values ​​of the zero-sequence current and the negative-sequence current. The fault mode determination module is used to determine the ratio of the effective value of the zero-sequence current to the effective value of the negative-sequence current as the fault characteristic ratio λ, and determine the first limiting mode or the second limiting mode based on the fault characteristic ratio λ. The current margin calculation module is used to determine the fundamental effective values ​​of the six arm currents, and determine the maximum value among the six fundamental effective values ​​as the total effective value of the current. And the preset current limit value is compared with the effective value of the total current. The difference between them is determined as the current margin. The adaptive limiting module is used in the first limiting mode and the current margin When the value is greater than zero, the preset base value C, the preset proportional coefficient k, and the current margin are used. The sum of the products is determined as the negative sequence voltage outer loop output limiting value, in the first limiting mode and the current margin. When the negative sequence voltage is less than or equal to zero, and in the second limiting mode, the output limiting value of the negative sequence voltage outer loop is determined as the preset base value C, and the original negative sequence current command is symmetrically limited using the output limiting value of the negative sequence voltage outer loop to obtain a negative sequence current reference value; the negative sequence current inner loop controller is used to receive the negative sequence current reference value and perform tracking control on the negative sequence current.

[0021] A third aspect of the present invention provides an electronic device, including a processor and a memory, wherein the memory stores a computer program, and when the computer program is executed by the processor, it implements the above-described adaptive limiting control method for the negative sequence voltage outer loop of an offshore wind power flexible DC converter station.

[0022] A fourth aspect of the present invention provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements the above-described adaptive limiting control method for the negative sequence voltage outer loop of an offshore wind power flexible DC converter station.

[0023] Compared with the prior art, the present invention has the following beneficial effects: First, the present invention determines the fault characteristic ratio based on the ratio of the effective value of zero-sequence current to the effective value of negative-sequence current, and distinguishes between negative-sequence dominant faults and zero-sequence dominant faults accordingly, so that the negative-sequence voltage outer loop limiting control can combine the sequence current characteristics of different asymmetrical faults for sub-mode processing.

[0024] Second, this invention establishes a correlation between the negative sequence voltage outer loop output limiting value and the current current margin of the converter station. Under negative sequence-dominant fault conditions, when the current margin is large, the allowable negative sequence current command amplitude is increased to enhance the negative sequence fault characteristics; when the current margin decreases, the limiting value is reduced to reduce the overcurrent risk caused by additional negative sequence injection; when the current margin is zero or negative, the limiting value is maintained at a preset base value to retain the minimum negative sequence output and avoid the limiting value from returning to zero.

[0025] Third, under zero-sequence dominant faults, the present invention maintains the negative sequence voltage outer loop output limiting value at a preset base value level, so that when the fault already has obvious zero-sequence characteristics, it will not further enhance the negative sequence current output, thereby reducing unnecessary current capacity occupation, while retaining the minimum negative sequence current maintenance control function.

[0026] Fourth, the present invention sets a limiting mode holding mechanism near the fault characteristic ratio threshold, which can reduce the possibility of repeated switching of control mode caused by measurement noise and transient fluctuations, and improve the stability of the control process.

[0027] Fifth, the present invention only requires adding adaptive limiting processing between the negative sequence voltage outer loop and the negative sequence current inner loop, and uses parameters such as fault characteristic ratio, current margin, preset base value and proportional coefficient to achieve control, which is easy to implement in the existing flexible DC converter station control system. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of a flexible direct transmission system for offshore wind power provided in an embodiment of the present invention; Figure 2 This is an overall flowchart of an adaptive limiting control method for the negative sequence voltage outer loop of an offshore wind power flexible DC converter station, provided by an embodiment of the present invention. Figure 3 This is a fault characteristic ratio waveform diagram provided by an embodiment of the present invention under a 30-ohm two-phase short circuit fault at the midpoint of an offshore wind power AC collection line. Figure 4 The waveform diagram of the ratio braking coefficient under a 30-ohm two-phase short circuit fault at the midpoint of the offshore wind power AC collection line provided in this embodiment of the invention; Figure 5 The fault current waveform diagram under a two-phase short-circuit metallic fault at the midpoint of the offshore wind power AC collection line is provided in an embodiment of the present invention. Figure 6A waveform diagram of the ratio braking coefficient under a two-phase short-circuit metallic fault at the midpoint of an offshore wind power AC collection line, provided in an embodiment of the present invention. Figure 7 This is a fault characteristic ratio waveform diagram provided by an embodiment of the present invention under a two-phase short-circuit grounding metallic fault at the midpoint of an offshore wind power AC collection line; Figure 8 The waveform diagram of the ratio braking coefficient under a two-phase short-circuit grounding metallic fault at the midpoint of the offshore wind power AC collection line is provided in an embodiment of the present invention. Figure 9 A comparison diagram of the ratio braking characteristics before and after the strategy improvement under a single-phase grounding metallic fault at the midpoint of the offshore wind power AC collection line, provided in an embodiment of the present invention. Figure 10 This is a composite sequence network diagram for a single-phase ground fault in a line, provided in an embodiment of the present invention. Figure 11 The zero-sequence current phase diagram on both sides of the line is provided for an embodiment of the present invention. Detailed Implementation

[0029] Example 1 This embodiment provides an adaptive limiting control method for the negative sequence voltage outer loop of an offshore wind power flexible DC converter station. For example... Figure 1 As shown, the wind farm first receives voltage boosting from an offshore transformer, and then connects to an offshore MMC (Modular Multilevel Converter) station via an AC collection line to complete the AC-DC conversion. The control system of the offshore MMC converter station includes a negative sequence voltage outer loop controller and a negative sequence current inner loop controller. The negative sequence voltage outer loop controller generates the original negative sequence current command based on the negative sequence voltage deviation, and the negative sequence current inner loop controller generates a modulation signal based on the negative sequence current reference value to control the converter valve to output the corresponding negative sequence current.

[0030] In this embodiment, an adaptive limiting processing loop is set between the negative sequence voltage outer loop and the negative sequence current inner loop. The adaptive limiting processing loop takes the fault characteristic ratio and current margin as inputs, determines the output limiting value of the negative sequence voltage outer loop in real time, and outputs the limited negative sequence current reference value to the negative sequence current inner loop controller.

[0031] like Figure 2 As shown, the control method in this embodiment includes the following steps.

[0032] S1: Collect three-phase current, extract sequence components, and calculate fault characteristic ratio.

[0033] Three-phase current is collected in real time by current transformers installed on the grid side of the offshore flexible DC converter station. , , The acquired analog signals are converted into digital signals by an analog-to-digital converter and then sent to a controller or digital signal processor.

[0034] Sequence component extraction is performed on the three-phase currents to obtain the effective value of the positive sequence current. Effective value of negative sequence current and the effective value of zero-sequence current Sequence component extraction can be performed using methods such as positive and negative sequence separation based on a second-order generalized integrator, sequence component extraction based on a notch filter, or sequence component extraction based on a delay signal cancellation method.

[0035] In one specific implementation, the sequence component extraction adopts a positive and negative sequence separation method based on a second-order generalized integrator. The basic process is as follows: the electrical quantities in the three-phase stationary coordinate system are converted to the two-phase stationary coordinate system through Clark transformation, then orthogonal signals are constructed through a second-order generalized integrator, and finally the positive and negative sequence components are separated through the positive and negative sequence calculation module.

[0036] The specific calculation process for the fault characteristic ratio is as follows.

[0037] Based on the effective value of negative sequence current and the effective value of zero-sequence current Calculate the fault characteristic ratio λ, which is determined according to formula (1): (1); in, This represents the effective value of the zero-sequence current. This represents the effective value of the negative sequence current.

[0038] To avoid misjudgments caused by measurement noise and transient fluctuations, the effective value of the negative sequence current can be... and the effective value of zero-sequence current The fault characteristic ratio λ is calculated after low-pass filtering. The filtering time constant for low-pass filtering can be 2ms to 5ms; in specific implementations, a filtering time constant of 3ms can be used.

[0039] S2. Determine the limiting mode based on the fault characteristic ratio. The limiting mode is determined based on the fault characteristic ratio λ. When λ < 1, it is determined to be a negative-sequence dominant fault, mainly including two-phase short-circuit faults and single-phase ground faults, and enters the first limiting mode; when λ ≥ 1, it is determined to be a zero-sequence dominant fault, mainly including two-phase ground faults, and enters the second limiting mode. For normal operation, weak disturbances, or three-phase short-circuit faults, no separate fault characteristic ratio calculation start threshold is set; even if the value of the fault characteristic ratio is affected by small-sequence components, since there is no significant zero-sequence current or negative-sequence current, the adjustment of the negative-sequence limiting loop will not have a substantial impact on system operation.

[0040] In one specific implementation, a hysteresis interval is set to avoid the fault characteristic ratio λ switching frequently around the threshold 1: when λ < 0.9, it is determined to be the first limiting mode; when 0.9 ≤ λ ≤ 1.05, the limiting mode determination result of the previous moment is maintained; when λ > 1.05, it is determined to be the second limiting mode.

[0041] For single-phase grounding faults, the sampling location on the network side of the offshore flexible DC converter station can be determined according to... Figure 10 The boundary conditions between the composite sequence network and the single-phase ground fault are listed below for the single-phase ground fault: (2); in, This represents the positive-sequence current vector on the wind farm side. This represents the positive sequence current phasor on the offshore flexible DC converter station side. This represents the negative sequence current phasor on the offshore flexible DC converter station side. This represents the zero-sequence current vector on the wind farm side. This represents the zero-sequence current phasor on the side of the offshore flexible DC converter station; This represents the A-phase current phasor on the wind farm side. This represents the A-phase current phasor on the offshore flexible DC converter station side; Figure 10 middle, , These are the positive sequence voltages at the wind turbine side and the converter station side, respectively. The three sequence currents at the outlets on both sides are denoted as follows: , , and , , The three-sequence impedances of the lines on both sides are respectively , , and , The three-sequence voltage at the fault point, This is the transition resistance.

[0042] The phases of the zero-sequence current components on both sides of the line are basically the same, and their magnitudes are shunted according to the zero-sequence impedance on both sides of the fault point, such as... Figure 11 As shown. Let If m is approximately a positive real number, then: (3); At the sampling location on the network side of the offshore flexible DC converter station, the fault characteristics are compared to... It can be represented as: (4); Therefore, when a single-phase ground fault occurs on the line, the fault characteristic ratio λ falls into the negative sequence dominant range; this embodiment further uses the hysteresis range composed of 0.9 and 1.05 to determine the amplitude limiting mode.

[0043] S3, calculate the fundamental RMS value and current margin of the six bridge arm currents.

[0044] According to the preset current limit With the effective value of total current Calculate current margin The current margin Determine according to formula (5): (5); in, Indicates the preset current limit. This represents the actual measured RMS value of the total current. The RMS value of the total current... The value is the maximum effective value among the six bridge arm currents (upper and lower phases) of the converter valve. Specifically, a rolling window with a window length of one power frequency cycle (20ms) is used to collect the instantaneous values ​​of the six bridge arm currents in real time at a preset sampling frequency, forming a sampling sequence corresponding to each bridge arm current. A Fast Fourier Transform is performed on each sampling sequence to extract the fundamental component of the corresponding bridge arm current. The effective value of each bridge arm current is calculated based on the amplitude of the fundamental component, and the maximum effective value among the six bridge arm currents is taken as the total effective value of the current. Preset current limit This preset current limit can be pre-set based on the rated current and short-time overcurrent capacity of the converter valve. Its physical meaning is the maximum effective value of the current that the converter valve is allowed to pass under the current operating conditions. The value can range from 1.5pu to 2.0pu, and in specific implementations, 2.0pu can be used.

[0045] The total current RMS value is calculated using a rolling window FFT method, with a window length of one power frequency cycle. After a fault occurs, as the window gradually slides, the data within the window gradually transitions from a mixture of pre-fault and post-fault data to entirely post-fault data. The total current RMS value is updated accordingly, and the negative sequence voltage outer loop output limiting value is adjusted synchronously in real time.

[0046] when When the current is greater than 0, it indicates that the current total current is lower than the preset current limit, and the system still has current capacity available for use; when When =0, it indicates that the current total current has reached the preset current limit; when When the current is less than 0, it indicates that the current total current exceeds the preset current limit and the system is in an overcurrent state.

[0047] S4 determines the output limiting value of the negative sequence voltage outer loop.

[0048] Based on the limiting mode and current margin corresponding to the fault characteristic ratio λ Determine the output limiting value of the negative sequence voltage outer loop. The negative sequence voltage outer loop output limiting value. This indicates the maximum amplitude that the negative sequence current command output by the negative sequence voltage outer loop can reach within the current control cycle.

[0049] When in the first limiting mode and with current margin When the value is greater than zero, that is, when the system has a negative sequence dominant fault and the system has a usable current margin, the negative sequence voltage outer loop output limit value is... Determine according to formula (6): (6) Where C is the preset base value, representing the minimum negative sequence current base value that is allowed to be output when the current margin is zero; k is the proportional coefficient. The preset base value C can range from 0.1pu to 0.2pu. In specific implementations, C can be 0.2pu, and k can be 1. When in the first limiting mode and the current margin is zero... When the value is less than or equal to zero, the negative sequence voltage outer loop output will be limited. The preset base value C is set to retain a minimum amount of negative-order output.

[0050] In the first limiting mode, the negative sequence voltage outer loop output limiting value With current margin There is a positive correlation: when the current margin is large, the limit value automatically increases, allowing the negative sequence voltage outer loop to output a larger negative sequence current command to enhance the negative sequence fault characteristics; when the current margin decreases, the limit value automatically decreases to limit the negative sequence current output.

[0051] When in the second limiting mode, corresponding to a zero-sequence dominant fault, the negative-sequence voltage outer loop output limiting value is... Determine according to formula (7): (7) At this time, the negative sequence voltage outer loop output limiting value With current margin It is irrelevant and does not change with the current margin, meaning that the negative sequence outer loop limit value is no longer adjusted, and the negative sequence current output capability is limited to the preset base value level.

[0052] In one specific implementation, when λ < 1, the proportional coefficient k is 1; when λ ≥ 1, the proportional coefficient k is 0; when the current margin... When the negative sequence voltage is less than or equal to zero, the outer loop output limiting value is... Take the preset base value C. The two limiting modes can be uniformly expressed according to formula (8): (8) S5 limits the original negative sequence current command.

[0053] The negative sequence voltage outer loop generates the original negative sequence current command based on the negative sequence voltage deviation. The original negative sequence current command The negative sequence voltage outer loop output limiting value determined in step S4 After limiting, the final negative sequence current reference value is obtained. The amplitude limiting process is determined according to formula (9): (9) When the absolute value of the original negative sequence current command generated by the negative sequence voltage outer loop exceeds the current maximum allowable amplitude, it is truncated to the output limit value of the negative sequence voltage outer loop or the opposite of the output limit value of the negative sequence voltage outer loop according to the sign of the original negative sequence current command; when the original negative sequence current command is between the output limit value of the negative sequence voltage outer loop and its opposite, the original negative sequence current command is transmitted completely.

[0054] S6 outputs the negative sequence current reference value to the negative sequence current inner loop controller.

[0055] The negative sequence current reference value obtained in step S5 The output is sent to the negative sequence current inner loop controller. The negative sequence current inner loop controller then uses the received negative sequence current reference value... The deviation between the measured negative sequence current and the actual negative sequence current is used to generate a negative sequence voltage modulation signal through proportional-integral regulation. This signal is then used to generate a trigger pulse for the converter valve through modulation stages such as pulse width modulation or nearest-level approximation modulation, thereby achieving closed-loop tracking control of the negative sequence current.

[0056] Example 2 This embodiment takes a scenario where a two-phase short circuit occurs at the midpoint of an AC busbar line via a 30-ohm transition resistor, and the current margin is large. After the fault occurs, the control system executes the steps in Embodiment 1 to obtain the effective value of the total current and the current margin. Since the zero-sequence current of a two-phase short circuit fault is zero, such as Figure 3 As shown, the fault characteristic ratio λ (which is 0) is less than 1, and the control system identifies it as a negative sequence dominant fault and enters the first limiting mode.

[0057] In the first limiting mode, due to the large current margin, the determined negative sequence voltage outer loop output limiting value is at a high level, allowing the negative sequence voltage outer loop to output a large negative sequence current command. After the negative sequence voltage outer loop generates the original negative sequence current command based on the negative sequence voltage deviation, it is limited by the limiter. Finally, the negative sequence current reference value is not truncated by the limiter, and the converter station outputs a sufficiently strong negative sequence current. The negative sequence characteristics of the two-phase short-circuit fault are manifested, such as... Figure 4 As shown, the differential protection can operate reliably; at the same time, the fault current level through the transition resistor is low, and there is no risk of overcurrent in the converter valve.

[0058] Example 3 This embodiment takes a scenario where a two-phase short-circuit metallic fault occurs on an AC collection line with a small current margin as an example. The control system identifies it as a negative-sequence dominant fault and enters the first limiting mode. However, because the fault point is close to the converter station, the total current is high and the current margin is small, so the determined negative-sequence voltage outer loop output limiting value is at a low level.

[0059] When the initial negative-sequence current command generated by the outer loop of the negative-sequence voltage exceeds the current limit value, the limiter activates, truncating the initial negative-sequence current command to the limit value level, ultimately limiting the negative-sequence current reference value. The negative-sequence current output by the converter station is controlled within a safe range, such as... Figure 5 As shown, the minimum instantaneous current value is -1.887 pu, and its absolute value is 1.887 pu, which does not exceed the preset current limit of 2.0 pu due to additional negative sequence injection; Figure 6 As shown, the differential protection can still operate reliably.

[0060] Example 4 This embodiment uses a scenario of a two-phase-to-ground metallic fault occurring on an AC convergence line as an example. After the fault occurs, the control system executes the steps in Embodiment 1. Because the zero-sequence current of a two-phase-to-ground fault is significant, such as... Figure 7 As shown, if the fault characteristic ratio λ is greater than or equal to 1, the control system identifies it as a zero-sequence dominant fault and enters the second limiting mode.

[0061] In the second limiting mode, the output limiting value of the negative sequence voltage outer loop is fixed at a preset base value and does not change with the current margin. The original negative sequence current command generated by the negative sequence voltage outer loop is limited by the fixed limiting value before being output, and the negative sequence current is limited to the base value level. The zero-sequence current generated by the two-phase ground fault itself provides sufficient fault characteristics for the differential protection, such as... Figure 8 As shown, differential protection can operate reliably without additional negative sequence injection.

[0062] Example 5 This embodiment illustrates a comparison of the protection adaptability of the present invention with that of existing fixed-limit control methods. In the prior art, the negative sequence voltage outer loop output typically uses a fixed limit value, such as 0.1pu or 0.2pu. Regardless of whether the total system current is high or low, or whether the fault type is a two-phase short circuit or a two-phase ground fault, the negative sequence current command must not exceed this fixed limit value.

[0063] In the adaptive limiting method of this invention, the negative sequence voltage outer loop output limiting value dynamically changes according to the fault characteristic ratio and current margin. This allows for adaptively increasing the negative sequence current injection amount based on the fault current level when it is necessary to manifest negative sequence fault characteristics, thereby improving the adaptability of the AC differential protection. Taking a single-phase grounding metallic fault as an example, a comparison of the ratio braking characteristics before and after the strategy improvement is shown below. Figure 9 As shown. Figure 9 middle, The ratio braking coefficient is set to a value of 0.8 in this embodiment.

[0064] Example 6 This embodiment provides an adaptive limiting control system for the negative sequence voltage outer loop of an offshore wind power flexible DC converter station. The control system includes a current acquisition module, a sequence component extraction module, a fault mode determination module, a current margin calculation module, an adaptive limiting module, and a negative sequence current inner loop controller.

[0065] The current acquisition module is used to acquire the three-phase current on the grid side of the offshore wind power flexible DC converter station, and sends the acquired analog signal to the controller after analog-to-digital conversion. The sequence component extraction module is used to extract the RMS values ​​of the positive sequence current, negative sequence current, and zero sequence current from the three-phase current.

[0066] The fault mode determination module is used to determine the ratio of the zero-sequence current RMS value to the negative-sequence current RMS value as the fault characteristic ratio λ, and to determine the first or second limiting mode based on the fault characteristic ratio λ. The current margin calculation module is used to determine the fundamental RMS values ​​of the six bridge arm currents, and to determine the maximum value among the six fundamental RMS values ​​as the total current RMS value. And the preset current limit value is compared with the effective value of the total current. The difference between them is determined as the current margin. The current margin calculation module uses a rolling window FFT method with a window length of one power frequency cycle to calculate the fundamental effective value of the current in the six bridge arms; after a fault occurs, the total effective value of the current is updated as the window slides, and the updated current margin is provided to the adaptive limiting module.

[0067] The adaptive limiting module is used in the first limiting mode and the current margin When the value is greater than zero, the preset base value C, the preset proportional coefficient k, and the current margin are used. The sum of the products is determined as the negative sequence voltage outer loop output limiting value, in the first limiting mode and the current margin. When the negative sequence voltage is less than or equal to zero, and in the second limiting mode, the output limiting value of the negative sequence voltage outer loop is determined as the preset base value C, and the original negative sequence current command generated by the negative sequence voltage outer loop is symmetrically limited using the output limiting value of the negative sequence voltage outer loop to obtain a negative sequence current reference value. The negative sequence current inner loop controller is used to receive the negative sequence current reference value and to track and control the negative sequence current of the offshore wind power flexible DC converter station.

[0068] Example 7 This embodiment provides an electronic device, including a processor and a memory. The memory stores a computer program, and when the processor executes the computer program, it performs the negative sequence voltage outer loop adaptive limiting control method for offshore wind power flexible DC converter stations as described in any one of Embodiments 1 to 5.

[0069] This embodiment also provides a computer-readable storage medium storing a computer program. When the computer program is executed by a processor, it performs the negative sequence voltage outer loop adaptive limiting control method for offshore wind power flexible DC converter stations as described in any one of embodiments one to five.

[0070] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, and is not intended to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions made by those skilled in the art to the technical solution of the present invention do not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A method for adaptive limiting control of the negative sequence voltage outer loop of an offshore wind power flexible DC converter station, characterized in that, include: S1, collect the three-phase current on the grid side of the converter station, extract the effective values ​​of zero-sequence and negative-sequence currents, and record the ratio of the effective value of zero-sequence current to the effective value of negative-sequence current as the fault characteristic ratio λ. S2, λ < 0.9 The first limiting mode is determined at that time. λ>1.05 The second limiting mode is determined at that time. 0.9≤λ≤1.05 The limiting mode remains the same as the previous moment. The negative sequence dominant faults corresponding to the first limiting mode include two-phase short circuits and single-phase ground faults, while the zero sequence dominant faults corresponding to the second limiting mode include two-phase ground faults. S3: At a preset sampling frequency, the instantaneous current values ​​of the six bridge arms (upper and lower arms) of the three-phase converter valve are collected through a rolling window of one power frequency cycle. A Fast Fourier Transform is performed on each sampling sequence to extract the fundamental component, and the effective value of the fundamental component is calculated based on its amplitude. The maximum value is taken as the total effective value of the current. A preset current limit is set according to the rated current and short-time overcurrent capacity of the converter valve. The current margin is obtained by subtracting the total effective value of the current from this limit. ; S4, first limiting mode and ＞0 At that time, set the preset scaling factor. k=1 ,by C+k As the outer loop output limiting value of the negative sequence voltage; first limiting mode and ≤0 At that time, the amplitude limit value is taken as C ; In the second limiting mode, k=0 The amplitude limit value is taken as C And with Irrelevant; preset base value C The value ranges from 0.1 pu to 0.2 pu. S5. Using the limiting value and its opposite as the upper and lower limits, the original negative sequence current command generated by the negative sequence voltage outer loop is symmetrically limited to obtain the negative sequence current reference value. S6, input the negative sequence current reference value into the negative sequence current inner loop controller, and track and control the negative sequence current of the converter station according to the negative sequence current reference value.

2. The adaptive limiting control method for the negative sequence voltage outer loop of an offshore wind power flexible DC converter station according to claim 1, characterized in that, S1 includes: S11, Real-time acquisition of the three-phase current on the grid side of the converter station. , , ; S12, using a positive-sequence separation method based on a second-order generalized integrator, a sequence component extraction method based on a notch filter, or a sequence component extraction method based on delay signal cancellation, the three-phase current is extracted to obtain the effective value of the positive-sequence current and the effective value of the negative-sequence current. and the effective value of the zero-sequence current ; S13, the effective value of the zero-sequence current With the effective value of the negative sequence current The ratio of λ to λ is used as the fault characteristic ratio λ.

3. The adaptive limiting control method for the negative sequence voltage outer loop of an offshore wind power flexible DC converter station according to claim 2, characterized in that, S2 includes: S21, when the fault characteristic ratio λ is less than 0.9, the first limiting mode is determined, and the fault is identified as the negative sequence dominant type fault; S22, when the fault characteristic ratio λ is greater than 1.05, the second limiting mode is determined, and the fault is identified as the zero-sequence dominant fault; S23, when the fault characteristic ratio λ is greater than or equal to 0.9 and less than or equal to 1.05, the amplitude limiting mode determination result of the previous moment is maintained; wherein, the negative sequence dominant fault includes two-phase short circuit fault and single-phase ground fault, and the zero sequence dominant fault includes two-phase ground fault.

4. The adaptive limiting control method for the negative sequence voltage outer loop of an offshore wind power flexible DC converter station according to claim 3, characterized in that, Before calculating the fault characteristic ratio λ, the effective value of the negative sequence current is... and the effective value of the zero-sequence current Low-pass filtering is performed; the filtering time constant for low-pass filtering is 2ms to 5ms.

5. The adaptive limiting control method for the negative sequence voltage outer loop of an offshore wind power flexible DC converter station according to claim 1, characterized in that, S3 includes: S31, Obtain the instantaneous values ​​of the six bridge arm currents of the converter valve, including the three-phase upper bridge arm current and the three-phase lower bridge arm current; S32, using a rolling window with a window length of one power frequency cycle, the instantaneous values ​​of the currents of the six bridge arms are collected at a preset sampling frequency to obtain the sampling sequence corresponding to each bridge arm current; S33, Perform a fast Fourier transform on each of the sampling sequences to extract the fundamental component of the corresponding bridge arm current, and calculate the effective value of each bridge arm current based on the amplitude of the fundamental component. S34, determine the maximum value among the six bridge arm current RMS values ​​as the total current RMS value. ; S35, set the preset current limit value according to the rated current and short-time overcurrent capability of the converter valve, and set the preset current limit value in relation to the total current. Stream RMS value The difference between them is used to obtain the current margin; The preset current limit value ranges from 1.5 pu to 2.0 pu.

6. The adaptive limiting control method for the negative sequence voltage outer loop of an offshore wind power flexible DC converter station according to claim 5, characterized in that, S4 includes: S41, in the first limiting mode, when the current margin When the value is greater than zero, the preset proportional coefficient k is set to 1, and the preset base value C is compared with the current margin. The sum of these values ​​is determined as the outer loop output limiting value of the negative sequence voltage; S42, in the first limiting mode, when the current margin When the value is less than or equal to zero, the output limiting value of the negative sequence voltage outer loop is determined as the preset base value C; S43, in the second limiting mode, the preset proportional coefficient k is set to 0, and the negative sequence voltage outer loop output limiting value is set to the preset base value C, so that the negative sequence voltage outer loop output limiting value is consistent with the current margin. Irrelevant; The preset base value C ranges from 0.1 pu to 0.2 pu.

7. The adaptive limiting control method for the negative sequence voltage outer loop of an offshore wind power flexible DC converter station according to claim 6, characterized in that, S5 includes: S51, the original negative sequence current command and the negative sequence voltage outer loop output limiting value are combined. Compare; S52, when the original negative sequence current command is greater than the negative sequence voltage outer loop output limit value. At that time, the negative sequence current reference value is determined as the negative sequence voltage outer loop output limiting value. When the original negative sequence current command is less than the negative sequence voltage outer loop output limit value. When the negative number is the opposite of the negative sequence current reference value, the negative sequence voltage outer loop output limiting value is determined as the negative sequence voltage outer loop output limiting value. The opposite of; when the original negative sequence current command is located at the output limiting value of the outer loop of the negative sequence voltage. When the original negative sequence current command is between the negative sequence current command and its opposite, the negative sequence current reference value is used.

8. A negative sequence voltage outer loop adaptive limiting control system for an offshore wind power flexible DC converter station, characterized in that, include: The current acquisition module is used to acquire the three-phase current on the grid side of the offshore wind power flexible DC converter station, and to obtain the instantaneous values ​​of the current of the six bridge arms, namely the three-phase upper bridge arm current and the three-phase lower bridge arm current of the converter valve. The sequence component extraction module is used to extract the sequence components of the three-phase current to obtain the effective value of the zero-sequence current and the effective value of the negative-sequence current. The fault mode determination module is used to determine the ratio of the effective value of the zero-sequence current to the effective value of the negative-sequence current as the fault characteristic ratio λ, and to determine the first limiting mode or the second limiting mode based on the fault characteristic ratio λ. The current margin calculation module is used to determine the fundamental RMS values ​​of the six bridge arm currents using a rolling window Fast Fourier Transform with a window length of one power frequency cycle, and to determine the maximum value among the six fundamental RMS values ​​as the total current RMS value. And the preset current limit value is compared with the effective value of the total current. The difference between them is determined as the current margin. ; An adaptive limiting module is configured to operate in the first limiting mode and with the current margin... When the value is greater than zero, the preset base value C, the preset proportional coefficient k, and the current margin are used. The sum of the products is determined as the negative sequence voltage outer loop output limiting value, in the first limiting mode and the current margin. When the value is less than or equal to zero and in the second limiting mode, the output limiting value of the negative sequence voltage outer loop is determined as the preset base value C, and the original negative sequence current command generated by the negative sequence voltage outer loop is symmetrically limited using the output limiting value of the negative sequence voltage outer loop to obtain the negative sequence current reference value. The negative sequence current inner loop controller is used to receive the negative sequence current reference value and track and control the negative sequence current of the offshore wind power flexible DC converter station according to the negative sequence current reference value.

9. An electronic device, characterized in that, It includes a processor and a memory, the memory storing a computer program, which, when executed by the processor, implements the negative sequence voltage outer loop adaptive limiting control method for offshore wind power flexible DC converter stations as described in any one of claims 1 to 7.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program, which, when executed by a processor, implements the negative sequence voltage outer loop adaptive limiting control method for offshore wind power flexible DC converter stations as described in any one of claims 1 to 7.

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