Impedance matching based method for avoiding 5th harmonic resonance of ac filter in converter station

Abstract

Impedance matching-based method for avoiding 5th harmonic resonance of AC filter in converter station, comprising: real-time monitoring of bus voltage through a mutual inductor u s and current i s , continuously extracting complex phasor and of 5th harmonic by using sliding discrete Fourier transform; calculating the current equivalent impedance of 5th harmonic on the system side by using effective disturbance samples; setting the resonance risk criterion for switching operation of AC filter; after determining the filter sequence L to be operated, judging the originally preset switching sequence of filter by using the resonance risk criterion; performing resonance risk checking on all possible filter switching sequence combinations in the filter sequence L ; increasing a standby branch to form a new to-be-operated set, and performing resonance risk checking on all possible filter switching sequence combinations of the new filter sequence to be operated. The method dynamically adjusts the switching sequence of AC filter, ensures the harmonic suppression effect, and improves the safety and harmonic resonance suppression reliability of the switching process of AC filter in converter station.

Description

Technical Field

[0001] This invention relates to the field of harmonic mitigation technology for AC filters in converter stations, specifically to a method for avoiding the fifth harmonic resonance of AC filters in converter stations based on impedance matching. Background Technology

[0002] High-voltage direct current (HVDC) converter stations generate significant harmonics through high-frequency switching of power electronic devices. AC filters are required on the busbars to shunt and absorb harmonics via LC resonant structures, compensate for reactive power, and improve the power factor, making them crucial for ensuring the safe and stable operation of the system. However, in actual operation, the performance of AC filters is easily affected by the coupling between the grid background harmonic impedance and their own impedance, leading to resonance and harmonic amplification. The volatility brought about by the high proportion of renewable energy grid integration further exacerbates the fluctuations in grid background harmonic impedance, making harmonic amplification more prominent. This results in deteriorated power quality, damage to electrical equipment, and threatens the stable operation of the power system, becoming a key issue restricting the development of HVDC systems and the clean and low-carbon transition of energy. Therefore, in-depth research on the harmonic amplification mechanism and efficient mitigation measures is urgently needed.

[0003] Currently, research on harmonic resonance amplification caused by the interaction between passive filters and system harmonic impedance mainly focuses on two dimensions: parameter design and topology optimization. In terms of parameter design, researchers consider the absence of resonance constraints as a core condition, reducing the risk of harmonic resonance from the design stage. Regarding topology optimization, introducing damping elements to suppress resonance peaks is the mainstream technique. However, for converter stations already in operation, the physical parameters and topology of the filters are fixed, making it difficult to flexibly adapt to varying resonance conditions through hardware modifications.

[0004] Furthermore, the methods described above are insufficient to accommodate the dynamic fluctuations in background harmonic impedance caused by changes in operating mode and load level. Therefore, there is an urgent need to research a control strategy that can sense impedance changes in real time and dynamically adjust the switching logic without altering the hardware configuration. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for avoiding the fifth harmonic resonance of an AC filter in a converter station based on impedance matching. This method ensures harmonic suppression effectiveness by dynamically adjusting the switching sequence of the AC filter, thereby improving the safety of the AC filter switching process and the reliability of harmonic resonance suppression in the converter station.

[0006] The technical solution adopted in this invention is as follows: The method for avoiding the fifth harmonic resonance of the AC filter in a converter station based on impedance matching includes the following steps: Step 1: Monitor the bus voltage in real time using current transformers. u s and currenti s The complex phasors of the 5th harmonic are continuously extracted using the sliding discrete Fourier transform. and , The amplitude and phase information of the 5th harmonic voltage are represented by the 5th frequency domain phasor obtained after the bus time-domain voltage undergoes a sliding discrete Fourier transform. The amplitude and phase information of the 5th harmonic current are represented by the 5th frequency domain phasor obtained after the bus time-domain current is subjected to the sliding discrete Fourier transform. Step 2: Calculate the current equivalent impedance of the 5th harmonic on the system side using effective disturbance samples from the previous period of time input into the filter. ; Step 3: Set the resonance risk criterion for switching operation of AC filter; Step 4: Once the filter sequence to be operated on is determined L Then, the original preset switching sequence of the filter is judged using the resonance risk criterion from step 3: If there is no risk of resonance in each filter switching operation, then the operation will proceed according to the original preset switching sequence. If the resonance risk criterion determines that there is a resonance risk in the originally preset switching sequence, then the filter switching sequence will be readjusted. Step 5: Filter sequence L Resonance risk verification is performed using all possible filter switching sequences. If there is a switching sequence that will not cause resonance risk, then close the circuit breaker in that switching sequence. If all sequences pose a risk of resonance or are single-filter operation tasks, proceed to step 6. Step 6: Add backup branches to form a new set of filters to be operated on, and perform resonance risk verification on all possible combinations of filter switching orders for the new filter sequence to be operated on. If there are multiple switching sequences without resonance risk, the switching sequence that causes the least change to the original switching sequence table shall be selected as the final operation sequence. If there is still no switching sequence without resonance risk after adding the backup branch, then the filter switching operation should be suspended.

[0007] In step 2, in order to reduce the impact of a single measurement Random errors caused by minute fluctuations do not depend on a single instantaneous increment value, but rather utilize the accumulated values ​​over a previous period of time in the filter input. M Group of valid perturbation samples , , The calculation formulas are as follows: (1); (2); In the formula, Indicates the first i The complex phasor of the bus fifth harmonic voltage change corresponding to the effective disturbance group. Indicates the first i +1 group of effective disturbances corresponds to the 5th harmonic voltage complex phasor at the AC bus. Indicates the first i The complex phasor of the 5th harmonic voltage at the AC bus corresponding to the effective disturbance group; Indicates the first i The complex phasor of the change in the fifth harmonic current of the bus corresponding to the effective disturbance. Indicates the first i +1 group of effective disturbances corresponds to the 5th harmonic current complex phasor at the AC bus. Indicates the first i The group of effective disturbances corresponds to the complex phasor of the 5th harmonic current at the AC bus.

[0008] In step 2, since the fluctuations in the background load of the power grid are random and the amplitude is usually small, in order to further reduce the calculation error, the system calculates the amplitude of the current change in adjacent cycles in real time. Only when Exceeding the preset sensitivity threshold A sample is recorded as a valid perturbation sample only when the noise level is typically 2 to 3 times the background noise.

[0009] In step 2, the filter is used to input the data accumulated over a previous period of time. M Group of valid perturbation samples The equivalent impedance of the 5th harmonic on the current system side was calculated using the weighted least squares (WLS) method. : (3); In equation (3), This represents the weighting factor.

[0010] Considering the varying intensity of fluctuations at different times and the resulting differences in signal-to-noise ratios among samples, a weighting factor is introduced. ,definition That is, the greater the change in current, the higher the weight of the sample in the result.

[0011] In step 3, the equivalent impedance obtained in step 2 is... With the 5th harmonic impedance of the filter bank Mapping to the same complex plane, calculate the impedance matching between the filter bank and the system impedance. ; (4); Equation (4): The resistive component representing the equivalent impedance of the 5th harmonic on the system side; The resistive component representing the equivalent impedance of the fifth harmonic of the filter bank; Represents the imaginary unit; The reactance component represents the equivalent impedance of the 5th harmonic on the system side; This represents the reactance component of the equivalent impedance of the fifth harmonic of the filter bank.

[0012] by- Let the center be a circle, and set the radius to be [missing information]. The impedance matching circular region is considered the resonance risk region. If the system impedance ends within this circular region, then the following condition is satisfied: (5); This indicates a high risk of resonance. If the system impedance ends do not fall within the circular region, it indicates that there is no risk of high resonance of the 5th harmonic, the system is operating in a safe range, and the corresponding filter switching operation can be performed.

[0013] In step 3, the radius of the impedance matching circular region The selection of [the appropriate parameter] will directly determine the model's sensitivity to resonance risk and its engineering applicability. It needs to meet harmonic amplification constraints and leave a certain safety margin. Harmonic amplification constraint b 1: Based on the harmonic suppression mechanism, a proportional coefficient is set. b 1. Ensure that the fifth harmonic voltage content remains within the national standard limit when at the edge of the risk zone; Safety margin b 2: Considering impedance identification error, measurement noise, and filter component parameter deviation, a coefficient is introduced. b 2. Appropriately expand the boundary to improve the robustness of the criterion; Using the fifth harmonic impedance magnitude of the filter Based on the radius The expression is: (6).

[0014] In step 4, the filter sequence to be operated L The set of filters that need to be put into operation in this operation, as determined by the reactive power control system based on the DC power ramp-up, reactive voltage regulation, or filtering performance requirements of the converter station, is represented as: (7); In the formula: For the i-th filter branch to be put into operation, i.e. the originally preset switching order, the system must ensure that the filter switching does not enter the resonance risk region each time.

[0015] The originally preset switching sequence is used as the initial fixed switching order for the converter station. The impedance matching circle criterion is used to judge the originally preset switching sequence of the filters. If there is no risk of resonance in each filter switching operation, the operation can be carried out according to the originally preset switching sequence. If the resonance risk criterion determines that there is a resonance risk in the originally preset switching sequence, then the filter switching sequence will be readjusted. Three sets of filters to be deployed For example: if the original switching sequence is F1, F2, F3, calculation using equation (5) reveals a risk of resonance during the switching process. In this case, the system recalculates other possible combinations of switching sequences for these three filter groups. S As shown in Table 1:

[0016] Calculate the 5th harmonic impedance of the filter bank for each combination during the switching process: (8); In the formula: i This represents the three intermediate states of the step-by-step switching of the three filter groups. ; Indicates the first i In the intermediate switching state, all filters on the bus side (existing filters and those put into operation this time) i The total equivalent impedance of the 5th harmonic of the filter to be applied. This indicates the total equivalent impedance of the 5th harmonic of all filters already in operation before the switching operation began. Indicates the first in the candidate sequence m The fifth harmonic impedance of the filter to be applied; Perform resonance risk verification on each intermediate state of the switching sequence. If a certain switching sequence exists... Under these conditions, the impedances of all intermediate state filter banks satisfy the following equation: (9); If the switching sequence is determined to completely avoid the risk of 5th harmonic resonance, the filter will operate according to this safe sequence. Close the circuit breakers in sequence.

[0017] In step 5, if there is a switching sequence that will not cause resonance risk, then the switches will be closed sequentially according to this switching sequence. Specifically, if there is a switching sequence that will not cause resonance risk among all switching sequences, this sequence will be selected as the execution sequence, and the switches will be closed sequentially according to the order of this sequence. In step 6, a spare branch is added to form a new set of operations. Specifically, if all switching sequences have resonance risk, it means that simply adjusting the switching order of the existing branches is still not enough to make the impedance trajectory completely bypass the resonance risk area, or if there is only a single filter switching task at present, then the spare filter branch at the end of the original switching sequence can be called.

[0018] Taking step 4 as an example, if all switching sequences have resonance risk, it means that simply adjusting the switching order of the existing branches is still not enough to make the impedance trajectory completely bypass the resonance risk area, or there is only a single filter switching task at present. In this case, the spare filter branch HP3 at the end of the original switching sequence can be called. The spare filter branch HP3 is added to the switching sequence to form a new set to be switched: the original 3 sets of filters to be switched and HP3. At this time, the objects to be switched are expanded to 4 sets, and all possible switching permutations and combinations including HP3 are regenerated. For each reconstructed switching sequence, the system will calculate the equivalent filter impedance under the switching stage according to Equation (8) state by state, and combine it with Equation (9) to carry out resonance risk prediction, eliminate all switching sequences with resonance risk, and finally determine the optimal switching scheme that meets the requirements of full resonance avoidance.

[0019] If multiple switching sequences with no resonance risk exist, the switching sequence that causes the least change to the original switching order table should be selected as the final operation sequence. Specifically, for all selected safe switching sequences, the sequence with the highest overlap with the original switching order should be selected as the final execution sequence, minimizing changes to the original switching logic while considering reactive power compensation rhythm and on-site operating habits. The overlap of sequences is quantified by a comprehensive quantification of sequence overlap using the following formula: (10); in: n This represents the total number of filter groups currently to be applied, and is the case when no backup branch is added. n Original sequence to be operated L The number of filter banks; after adding a spare branch. n To represent the number of filter banks in the expanded new set of operations. n = Original number of groups + 1.

[0020] j This represents the position number in the sequence to be submitted. ; In the original fixed cutting order table, the first j The position number of the priority filter in the fixed priority of the whole station constitutes the reference priority sequence; the spare branch itself has a definite position in the original priority table, and after being added, the reference sequence can be naturally expanded according to the original priority. For the first k In the candidate cutting sequence, the first j The position number of the position filter in the fixed order table of the whole station; Let be the sum of the deviations between the k-th candidate sequence and the original cutting order. The smaller the value, the higher the overlap between the sequence and the original cutting logic, and the smaller the change to the original cutting rhythm. Therefore, it should be selected first. The smallest sequence is used as the final execution order.

[0021] This invention provides a method for avoiding the fifth harmonic resonance of an AC filter in a converter station based on impedance matching. The technical effects are as follows: 1) This invention utilizes the background load fluctuations that naturally occur in the power grid as an excitation source. Through the combination algorithm of SDFT and WLS, the system's fifth harmonic background impedance can be identified without injecting additional disturbance signals into the power grid.

[0022] 2) By mapping the system identification impedance and the filter impedance to the same complex impedance plane, this invention transforms the abstract frequency domain resonance analysis into an intuitive geometric position discrimination. Through reasonable design of radius parameters, it ensures that the 5th harmonic content at the risk boundary still meets the national standard safety limit, providing a scientific quantitative boundary for filter switching.

[0023] 3) This invention is mainly based on software optimization of the control algorithm, without the need to add expensive hardware filtering devices or compensation equipment, and the modification and maintenance costs are low.

[0024] 4) This invention uses the weighted least squares method to obtain the fifth harmonic impedance of the power grid in real time, constructs a resonance risk assessment model on the complex impedance plane, and dynamically adjusts the switching path of the AC filter to avoid resonance conditions and ensure the safety of filter switching. Attached Figure Description

[0025] The present invention will be further described below with reference to the accompanying drawings and examples; Figure 1 This is a topology diagram of a DC power transmission project.

[0026] Figure 2 This is a diagram showing the configuration of AC filters at an inverter station of a DC transmission project.

[0027] Figure 3 This is a structural diagram of the AC filter in the inverter station of a DC transmission project.

[0028] Figure 4(a) is a graph showing the THD variation curve under the original strategy in the single-group filter switching condition of the present invention. Figure 4(b) is a graph showing the variation of the 5th harmonic content under the single-group filter switching condition of the present invention.

[0029] Figure 5(a) is a graph showing the THD variation under the resonance avoidance strategy in the single-group filter switching condition of the present invention. Figure 5(b) is a graph showing the variation of the 5th harmonic content under the single-group filter switching condition of the present invention.

[0030] Figure 6(a) is a graph showing the THD variation curve under the original strategy in the case of multiple filter switching of the present invention; Figure 6(b) is a graph showing the variation of the 5th harmonic content under the switching conditions of multiple filters in this invention.

[0031] Figure 7(a) is a graph showing the THD variation under the resonance avoidance strategy in the case of multiple filter switching of the present invention. Figure 7(b) is a graph showing the variation of the 5th harmonic content under the switching conditions of multiple filters in this invention. Detailed Implementation

[0032] A fifth-harmonic resonance avoidance method for AC filters in converter stations based on impedance matching is proposed. This method involves real-time acquisition of bus voltage and current, extraction of the fifth-harmonic complex phasor using SDFT, screening of valid disturbance samples exceeding a sensitivity threshold, and introducing a weighting factor. The equivalent impedance of the fifth harmonic on the system side is identified using WLS. The identified impedance and filter impedance are mapped to the same complex plane, constructing a radius centered on the negative value of the filter impedance, considering harmonic amplification constraints and safety margins. The resonant risk circle is defined as the boundary; the preset switching sequence of the filter is verified using this risk circle criterion; if a risk exists, a safe switching sequence is reconstructed by traversing permutations and combinations; if the safety is still not met, the switching operation is temporarily suspended. This invention can achieve early prediction and active avoidance of the risk of 5th harmonic resonance, improve the safety and stability of the system during filter switching, and prevent abnormal amplification of harmonic voltage. It includes the following steps: Step 1: Monitor the bus voltage in real time using current transformers. u s and current i s The sliding discrete Fourier transform (SDFT) was used to continuously extract the complex phasors of the 5th harmonic. and For the sampling sequence Its 5th harmonic complex phasor at time k It can be represented as: ; In the formula: N This represents the number of sampling points per cycle. Using this recursive formula, the 5th harmonic voltage phasor of the bus can be obtained. and current phasor The real and imaginary parts of .

[0033] Step 2: To reduce the impact of a single measurement The random error caused by small fluctuations does not depend on a single instantaneous increment value, but rather utilizes the filter to input M groups of effective perturbation samples accumulated over a previous period. , , The calculation formulas are as follows: ; ; Because the fluctuations in the background load of the power grid are random and the amplitude is usually small, in order to further reduce calculation errors, the system calculates the amplitude of the current change in adjacent cycles in real time. Only when Exceeding the preset sensitivity threshold A sample is considered valid only when its noise level is typically 2-3 times the background noise level. Considering the varying intensity of fluctuations at different times and the resulting differences in signal-to-noise ratios, a weighting factor is introduced. ,definition That is, the greater the current change, the higher the weight of the sample in the result. The equivalent impedance of the 5th harmonic on the current system side is calculated by WLS. : ; Step 3: Obtain the With the 5th harmonic impedance of the filter bank Mapped to the same complex plane. Calculate the impedance matching between the filter bank and the system impedance. : ; by- Let the center be a circle, and set the radius to be . The circular region is designated as the resonance risk region. If the system impedance ends within this circular region, then the following condition is satisfied: ; This indicates a high risk of resonance.

[0034] Impedance matching circle radius The selection of the harmonic amplification factor will directly determine the model's sensitivity to resonance risk and its engineering applicability. It needs to meet the harmonic amplification constraint and leave a certain safety margin.

[0035] Harmonic Amplification Constraint b 1: Based on the harmonic suppression mechanism, a proportional coefficient is set. b 1. Ensure that the 5th harmonic voltage content remains within the national standard limit when at the edge of the risk zone.

[0036] safety margin b 2: Considering impedance identification error, measurement noise, and filter component parameter deviation, a coefficient is introduced. b2. Appropriately expand the boundary to improve the robustness of the criterion.

[0037] Using the fifth harmonic impedance magnitude of the filter Based on the radius The expression is: ; Step 4: Once the filter sequence to be operated on is determined L Then, the original preset switching order of the filter is judged using the criteria in step 3. If there is no risk of resonance in each filter switching operation, the operation can be carried out in the original order; if the impedance matching circle criterion determines that there is a risk of resonance in the original order, the filter switching order is readjusted.

[0038] Step 5: [Regarding...] L The resonance risk is checked by combining all possible filter switching sequences. If there is a safe switching sequence that will not cause resonance, the circuit is closed in sequence according to this new sequence. If all sequences have resonance risk or are single-group filter operation tasks, proceed to step 6.

[0039] Step 6: Add a spare branch to form a new set of operations to be performed. Perform resonance risk verification on all possible filter switching sequence combinations of the new sequence. If there are multiple sets of operation sequences without resonance risk, select the sequence that changes the original switching order table the least as the final operation sequence; if there is still no switching sequence without resonance risk after adding the spare branch, then postpone the filter switching operation.

[0040] Verification Example: To fully verify the effectiveness of the fifth harmonic resonance avoidance strategy for the current filter proposed in this invention, simulation verification was performed using a combination of MATLAB and PSCAD / EMTDC electromagnetic transient simulation. Based on the operating parameters of a real HVDC project, a simulation model was constructed. Figure 1 The HVDC system shown has an AC system voltage of 220 kV, a DC voltage of ±200 kV, a DC transmission power of 1200 MW, a converter transformer capacity of 603.73 MVA per unit, and rated voltages of 230 / 173 kV for the Y / Y and Y / Δ transformers, respectively. ACF-1M is the first large group of AC filters, containing: 2 groups of BP11 / 13, 1 group of HP24 / 36, and 1 group of HP3; ACF-2M is the second large group of AC filters, containing: 1 group of BP11 / 13, 2 groups of HP24 / 36, and 1 group of HP3, as shown... Figure 2 As shown. The structures and parameters of each type of filter are as follows. Figure 3As shown in Table 1, the switching sequence of the filters at this station is as follows: BP11 / 13, HP24 / 36, HP3, BP11 / 13, HP24 / 36, BP11 / 13, HP24 / 36, HP3. Under rated operating conditions, only 7 groups need to be engaged, with the last group, HP3, typically used as a backup branch.

[0041] The effectiveness of the proposed method in switching a single filter group was verified by simulation under the following conditions: Initially, the system had 3 groups of filters (BP11 / 13 + 2 groups of filters (HP24 / 36) + 1 group of filters (HP3). At t=2 s, based on the power increase requirement, the 7th filter group (HP24 / 36) was switched on in the original order. The THD and 5th harmonic content changes at this time are shown in Figures 4(a) and 4(b). After switching on this filter, the 5th harmonic content increased significantly and remained above the national standard limit, with THD reaching approximately 3.7%, indicating that the system triggered severe 5th harmonic parallel resonance. After activating the resonance avoidance strategy proposed in this invention, the system identified the risk of the branch to be switched on through real-time impedance identification, optimizing the switching order to: […]. t =4 s to activate the backup branch HP3, then at t =6s to put the target branch HP24 / 36 into operation. The simulation curves in Figures 5(a) and 5(b) show that the early input of HP3 effectively optimized the system impedance distribution. When HP24 / 36 was put into operation, the 5th harmonic remained stable at around 1.3% and the THD was controlled at around 1.8%, neither of which exceeded the safety limit.

[0042] To further verify the effectiveness and reliability of the proposed resonant avoidance strategy in a scenario of continuous switching of multiple filters, a continuous adjustment condition was set up where the DC power gradually increases from a low power level to a high power level. At the start of the simulation, the filters already in operation were 1 BP11 / 13 + 1 HP24 / 36 + 1 HP3. When the DC power gradually increased from 360 MW to 1080 MW, the control system needed to continuously activate multiple AC filters to meet the reactive power and filtering requirements. According to the original sequence, 1 BP11 / 13, 1 HP24 / 36, and 1 BP11 / 13 needed to be activated sequentially. The THD and 5th harmonic content variation curves of the system at this time are shown in Figure 6(a) and Figure 6(b). t When the third filter group BP11 / 13 is put into operation at 6 s, the system impedance is highly matched with the equivalent impedance of the filter, which causes the 5th harmonic content and THD to exceed the limit.

[0043] After activating the strategy of this invention, it was detected that the third step of the original sequence would cause the impedance trajectory to fall within the risk circle. Therefore, the switching path was planned as: BP11 / 13, HP24 / 36, HP3, BP11 / 13. As shown in Figures 7(a) and 7(b), in t=6 s After the standby branch HP3 was put into operation, the harmonic content dropped significantly because its impedance characteristics canceled out the capacitive effect of the system; subsequently in t When the last filter was applied at 8 s, the system ran smoothly without any resonant amplification.

[0044] The simulation results comparing Figures 4(a), 4(b), 5(a), 5(b), 6(a), 6(b), 7(a), and 7(b) show that the strategy of the present invention, by actively reconstructing the switching path, ensures that the equivalent impedance of the system can avoid the resonance risk region under various complex operating conditions, which is significantly better than the traditional fixed sequence strategy.

Claims

1. A method for avoiding the fifth harmonic resonance of an AC filter in a converter station based on impedance matching, characterized in that... Includes the following steps: Step 1: Monitor the bus voltage in real time using current transformers. u s and current i s The complex phasors of the 5th harmonic are continuously extracted using the sliding discrete Fourier transform. and , This indicates the amplitude and phase information of the 5th harmonic voltage. This indicates the amplitude and phase information of the 5th harmonic current; Step 2: Calculate the current equivalent impedance of the 5th harmonic on the system side using effective disturbance samples from the previous period of time input into the filter. ; Step 3: Set the resonance risk criterion for switching operation of AC filter; Step 4: Once the filter sequence to be operated on is determined L Then, the original preset switching sequence of the filter is judged using the resonance risk criterion from step 3: If there is no risk of resonance in each filter switching operation, then the operation will proceed according to the original preset switching sequence. If the resonance risk criterion determines that there is a resonance risk in the originally preset switching sequence, then the filter switching sequence will be readjusted. Step 5: Process the filter sequence L Resonance risk verification is performed using all possible filter switching sequences. If there is a switching sequence that will not cause resonance risk, then close the circuit breaker in that switching sequence. If all sequences pose a risk of resonance or are single-filter operation tasks, proceed to step 6. Step 6: Add backup branches to form a new set of filters to be operated on, and perform resonance risk verification on all possible combinations of filter switching orders for the new filter sequence to be operated on. If there are multiple switching sequences without resonance risk, the switching sequence that causes the least change to the original switching sequence table shall be selected as the final operation sequence. If there is still no switching sequence without resonance risk after adding the backup branch, then the filter switching operation should be suspended.

2. The method for avoiding the 5th harmonic resonance of the AC filter in a converter station based on impedance matching according to claim 1, characterized in that: In step 2, in order to reduce the impact of a single measurement Random errors caused by minute fluctuations do not depend on a single instantaneous increment value, but rather utilize the accumulated values ​​over a previous period of time in the filter input. M Group of valid perturbation samples , , The calculation formulas are as follows: （1）； （2）； In the formula, Indicates the first i The complex phasor of the bus fifth harmonic voltage change corresponding to the effective disturbance group. Indicates the first i +1 group of effective disturbances corresponds to the 5th harmonic voltage complex phasor at the AC bus. Indicates the first i The complex phasor of the 5th harmonic voltage at the AC bus corresponding to the effective disturbance group; Indicates the first i The complex phasor of the change in the fifth harmonic current of the bus corresponding to the effective disturbance. Indicates the first i +1 group of effective disturbances corresponds to the 5th harmonic current complex phasor at the AC bus. Indicates the first i The group of effective disturbances corresponds to the complex phasor of the 5th harmonic current at the AC bus.

3. The method for avoiding the 5th harmonic resonance of the AC filter in a converter station based on impedance matching according to claim 2, characterized in that: In step 2, since the fluctuations in the background load of the power grid are random and the amplitude is small, in order to further reduce the calculation error, the system calculates the amplitude of the current change in adjacent cycles in real time. Only when Exceeding the preset sensitivity threshold Only when this condition is met is it recorded as a valid perturbation sample.

4. The method for avoiding the 5th harmonic resonance of the AC filter in a converter station based on impedance matching according to claim 3, characterized in that: In step 2, the filter is used to input the data accumulated over a previous period of time. M Group of valid perturbation samples The equivalent impedance of the 5th harmonic on the current system side was calculated using the weighted least squares (WLS) method. : （3）； In equation (3), Indicates the weighting factor; Considering the varying intensity of fluctuations at different times and the resulting differences in signal-to-noise ratios among samples, a weighting factor is introduced. ,definition That is, the greater the change in current, the higher the weight of the sample in the result.

5. The method for avoiding the 5th harmonic resonance of the AC filter in a converter station based on impedance matching according to claim 4, characterized in that: In step 3, the equivalent impedance obtained in step 2 is... With the 5th harmonic impedance of the filter bank Mapping to the same complex plane, calculate the impedance matching between the filter bank and the system impedance. ; （4）； Equation (4): The resistive component representing the equivalent impedance of the 5th harmonic on the system side; The resistive component representing the equivalent impedance of the fifth harmonic of the filter bank; Represents the imaginary unit; The reactance component represents the equivalent impedance of the 5th harmonic on the system side; The reactance component represents the equivalent impedance of the fifth harmonic of the filter bank. by- Let the center be a circle, and set the radius to be . The impedance matching circular region is used as the resonance risk region; If the system impedance ends within a circular region, then the following conditions are met: （5）； This indicates a high risk of resonance. If the system impedance ends do not fall within the circular region, it indicates that there is no risk of high resonance of the 5th harmonic, the system is operating in a safe range, and the corresponding filter switching operation can be performed.

6. The method for avoiding the 5th harmonic resonance of the AC filter in a converter station based on impedance matching according to claim 5, characterized in that: In step 3, the radius of the impedance matching circular region The selection of [the appropriate parameter] will directly determine the model's sensitivity to resonance risk and its engineering applicability, satisfying harmonic amplification constraints while leaving a certain safety margin. Harmonic amplification constraint b 1: Set a scaling factor b 1. Ensure that the fifth harmonic voltage content remains within the national standard limit when at the edge of the risk zone; safety margin b 2: Considering impedance identification error, measurement noise, and filter component parameter deviation, a coefficient is introduced. b 2. Appropriately expand the boundary to improve the robustness of the criterion; Using the fifth harmonic impedance magnitude of the filter Based on, radius The expression is: （6）。 7. The method for avoiding the 5th harmonic resonance of the AC filter in a converter station based on impedance matching according to claim 6, characterized in that: In step 4, the filter sequence to be operated L The set of filters that need to be put into operation in this operation, as determined by the reactive power control system based on the DC power ramp-up, reactive voltage regulation, or filtering performance requirements of the converter station, is represented as: (7); In the formula: This refers to the i-th filter branch to be put into operation, which is the originally preset switching order; The original preset switching sequence is used as the initial fixed switching order adopted by the converter station; the original preset switching sequence of the filter is judged by the impedance matching circle criterion. If there is no risk of resonance in each filter switching operation, it can be operated according to the original preset switching sequence. If the resonance risk criterion indicates that there is a resonance risk in the originally preset switching sequence, the filter switching sequence will be readjusted.

8. The method for avoiding the 5th harmonic resonance of the AC filter in a converter station based on impedance matching according to claim 7, characterized in that: In step 5, if there is a switching sequence that will not cause resonance risk, then the switches will be closed sequentially according to this switching sequence. Specifically, if there is a switching sequence that will not cause resonance risk among all switching sequences, this sequence will be selected as the execution sequence, and the switches will be closed sequentially according to the order of this sequence to put the filters into operation.

9. The method for avoiding the 5th harmonic resonance of the AC filter in a converter station based on impedance matching according to claim 8, characterized in that: In step 6, an additional backup branch is added to form a new set of operations. Specifically, if all switching sequences have resonance risk, it means that simply adjusting the switching order of the existing branches is still not enough to make the impedance trajectory completely bypass the resonance risk area, or if there is only a single filter switching task at present, then the backup filter branch at the end of the original switching sequence can be called. If there are multiple switching sequences without resonance risk, the switching sequence that causes the least change to the original switching sequence table shall be selected as the final operation sequence. Specifically, for all the selected safe cutting sequences, the sequence with the highest overlap with the original cutting order is selected as the final execution order.

10. The method for avoiding the 5th harmonic resonance of the AC filter in a converter station based on impedance matching according to claim 9, characterized in that: The sequence overlap is comprehensively quantified using the order deviation method, and the calculation formula is as follows: （10）； in: n This represents the total number of filter groups currently to be applied, and is the case when no backup branch is added. n Original sequence to be operated L The number of filter banks; after adding a spare branch. n’ To represent the number of filter banks in the expanded new set of operations. n’ =Original number of groups + 1; j This represents the position number in the sequence to be submitted. ; In the original fixed cutting order table, the first j The position number of the priority filter in the fixed priority of the whole station constitutes the reference priority sequence; the spare branch itself has a definite position in the original priority table, and after being added, the reference sequence can be naturally expanded according to the original priority. For the first k In the candidate cutting sequence, the first j The position number of the position filter in the fixed order table of the whole station; Let be the sum of the deviations between the k-th candidate sequence and the original cutting order. The smaller the value, the higher the overlap between the sequence and the original cutting logic, and the smaller the change to the original cutting rhythm. Therefore, it should be selected first. The smallest sequence is used as the final execution order.

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