Alternating current fault recovery method and system for controllable power grid commutation converter

Abstract

The invention belongs to the technical field of converter alternating-current fault recovery, and provides a controllable power grid commutation converter alternating-current fault recovery method and system. Alternating-current voltage is detected, and alternating-current fault recovery is judged; a direct current control loop is shielded, so that the controllable power grid commutation converter is in a direct current voltage control mode; performing closed-loop control on the direct-current voltage, and generating a first correction value of a turn-off angle reference value of a predictive turn-off angle control link; detecting the number of actions of the arrester of the full-control sub-converter of the auxiliary branch, and generating a second correction coefficient of the reference value of the turn-off angle of the predictive turn-off angle control link; subtracting the product of the first correction and the second correction coefficient from the initial turn-off angle reference value of the predictive turn-off angle control link to obtain a final turn-off angle reference value; detecting the direct-current voltage, and judging the direct-current voltage recovery. According to the invention, the safety of lightning arrester equipment is effectively ensured.

Description

Technical Field

[0001] This invention belongs to the field of AC fault recovery technology for converters, specifically relating to a method and system for AC fault recovery of a controllable grid phase-commutation converter. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] Conventional high-voltage direct current transmission (LCC-HVDC) technology based on thyristors has advantages such as mature application, low cost, and low loss. However, since ordinary thyristors do not have self-turn-off capability, the operation of LCC-HVDC is easily affected by the AC power grid. When the AC power grid fails, LCC-HVDC is prone to commutation failure, which is a major drawback that limits the application of LCC-HVDC.

[0004] The controllable grid-commutated converter modifies the LCC-HVDC bridge arm by transforming a single bridge arm into a parallel main and auxiliary branch. Both the main and auxiliary branches consist of cascaded semi-controlled thyristor sub-converters and fully controlled sub-converters. Through the controllable turn-off characteristics of the fully controlled sub-converters, active commutation between the main and auxiliary branches and active phase commutation between different bridge arms are achieved, effectively solving the commutation failure problem. Specifically, the active turn-off of the bridge arm current is achieved by actively turning off the fully controlled sub-converter of the auxiliary branch, transferring the bridge arm current to the surge arrester connected in parallel. When the bridge arm current is transferred to the surge arrester of the fully controlled sub-converter of the auxiliary branch, the surge arrester generates a high operating voltage. This operating voltage, superimposed with the AC voltage, enhances the commutation voltage, thereby promoting forced commutation between bridge arms. Therefore, the safety of the V13 parallel surge arrester is a prerequisite for the successful implementation of forced commutation in the controllable grid-commutated converter.

[0005] Due to its active shutdown capability, the controllable grid-commutated converter not only possesses the advantage of commutation failure suppression but also operates at a smaller shutdown angle, reducing the reactive power it absorbs from the AC system and effectively supporting DC voltage, which greatly benefits the speed of AC fault recovery. However, when operating at a lower shutdown angle, the controllable grid-commutated converter requires the fully controlled sub-converters of the auxiliary branches to actively shut off the current to achieve smooth commutation between bridge arms. The smaller the shutdown angle, the greater the AC shutdown of the fully controlled sub-converters of the auxiliary branches, resulting in higher accumulated energy in their surge arresters, which is detrimental to the safety of surge arrester equipment. Summary of the Invention

[0006] To address the aforementioned problems, this invention proposes a method and system for AC fault recovery of a controllable grid phase-commutation converter. This invention fully utilizes the controllable grid phase-commutation converter's control capability to accelerate the system's AC fault recovery speed, while effectively limiting the surge arrester energy of the fully controlled sub-converter in the auxiliary branch and ensuring the safety of the surge arrester equipment.

[0007] According to some embodiments, the present invention adopts the following technical solution: A method for restoring AC faults in a controllable power grid phase-commutator includes the following steps: Detect AC voltage to determine if AC fault recovery has been achieved; The DC current control loop is shielded, so that the controllable grid-commutated converter is in DC voltage control mode; Closed-loop control of DC voltage is performed to generate the first correction value of the turn-off angle reference value of the predictive turn-off angle control loop; The number of times the surge arrester of the fully controlled sub-converter in the auxiliary branch is detected is used to generate a second correction coefficient for the reference value of the turn-off angle of the predictive turn-off angle control loop. The final cut-off angle reference value is obtained by subtracting the product of the first correction amount and the second correction coefficient from the initial cut-off angle reference value of the predictive cut-off angle control loop. Detect the DC voltage and determine if the DC voltage has recovered.

[0008] As an alternative implementation method, the process of detecting AC voltage and determining AC fault recovery includes: real-time acquisition of the three-phase-to-ground AC voltage on the grid side of the controllable grid commutator converter. U a , U b , U c ; Compare U a , U b , U c With AC voltage setting value U acset The size, when U a , U b , U c All continuous t 1. Time greater than AC voltage setting value U acset If the AC voltage on the grid side is determined to have returned to the normal range, the AC fault recovery method will be initiated.

[0009] As a further step, AC voltage setting value U acset The value range is 0.85pu~0.9pu. t The value of 1 ranges from 0 to 5ms.

[0010] As an alternative implementation, the process of shielding the DC current control loop includes: putting the controllable grid commutator in DC voltage control mode and setting the DC current reference value to a negative value less than a set value. U NEG .

[0011] As an alternative implementation, the process of performing closed-loop control of the DC voltage to generate a first correction value for the turn-off angle reference value of the predictive turn-off angle control loop includes: The deviation d is obtained by subtracting the actual value from the reference value of the DC voltage. U dc ; Deviation d U dc Multiply by a first correction factor greater than zero k 1. Obtain the first correction amount d of the reference value of the turn-off angle of the predictive turn-off angle control loop. Gam 1.

[0012] As an alternative implementation method, the calculation process for the second correction coefficient is as follows: k 2 = max[(1-0.1)] N ar ), 0] In the formula, k 2 is the second correction factor. N ar This represents the maximum number of times the surge arresters of the fully controlled sub-converters in each auxiliary branch operate during AC fault recovery.

[0013] As an alternative implementation method, the process of detecting DC voltage and determining DC voltage recovery includes: Real-time acquisition of DC port voltage of controllable grid commutator U dc ; Compare U dc With DC voltage setting value U dcset The size, when U dc continuous t 2 times greater than U dcset If so, it can be determined that the DC voltage has returned to the normal range.

[0014] As a further step, DC voltage setting value U dcset The value range is 0.9pu~0.95pu. t The value of 2 ranges from 0 to 10 ms.

[0015] As an alternative implementation, after determining that the DC voltage has recovered, the system switches to normal control mode to continue operation, and the DC current reference value returns to its normal value; the first correction coefficient... k 1 and second correction coefficient k 2 is set to 0.

[0016] A controllable grid phase-commutator AC fault recovery system includes: The AC detection module is configured to detect AC voltage and determine whether an AC fault has been resolved. The shielded control loop module is configured as a shielded DC current control loop, enabling the controllable grid-commutated converter to be in DC voltage control mode. The first correction calculation module is configured to perform closed-loop control of the DC voltage and generate the first correction of the reference value of the turn-off angle of the predictive turn-off angle control loop. The second correction coefficient calculation module is configured to detect the number of times the arrester of the fully controlled sub-converter in the auxiliary branch operates and generate the second correction coefficient of the reference value of the turn-off angle of the predictive turn-off angle control link. The shut-off angle reference value calculation module is configured to subtract the product of the first correction amount and the second correction coefficient from the initial shut-off angle reference value of the predictive shut-off angle control loop to obtain the final shut-off angle reference value. The DC detection module is configured to detect DC voltage and determine when the DC voltage has recovered.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention can effectively accelerate the power recovery speed of the controllable grid phase converter after the AC fault ends, while avoiding excessive energy accumulation in the surge arrester of the sub-converter, thus ensuring the safety of the surge arrester equipment.

[0018] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0019] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0020] Figure 1 This is a schematic diagram of a controllable grid phase-commutation converter topology according to one embodiment; Figure 2 This is a flowchart of an AC fault recovery method for a controllable grid phase-commutation converter, according to one embodiment. Detailed Implementation

[0021] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0022] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0023] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0024] Where there is no conflict, the embodiments and features described in this application may be combined with each other.

[0025] Example 1 A method for recovering AC faults in a controllable power grid phase-commutator includes: S1: Detects AC voltage to determine if AC fault has been resolved; S2: Shield the DC current control loop to put the controllable grid commutator in DC voltage control mode; S3: Perform closed-loop control on the DC voltage to generate the first correction value of the turn-off angle reference value of the predictive turn-off angle control loop; S4: Detect the number of times the surge arrester of the fully controlled sub-converter in the auxiliary branch operates, and generate the second correction coefficient of the reference value of the turn-off angle of the predictive turn-off angle control loop; S5: Subtract the product of the first correction amount and the second correction coefficient from the initial cut-off angle reference value of the predictive cut-off angle control loop to obtain the final cut-off angle reference value. S6: Detect DC voltage and determine if DC voltage has recovered; S7: Switch to normal control mode and continue operation.

[0026] The process of detecting AC voltage and determining AC fault recovery includes: Real-time acquisition of three-phase AC voltage to ground on the grid side of the controllable power grid commutator converter. U a , U b , U c ; Compare U a , U b , U c With AC voltage setting value Uacset The size, when U a , U b , U c All continuous t 1 time greater than U acset If the grid-side AC voltage is determined to have returned to the normal range, the AC fault recovery method is initiated; wherein... U acset A value of 0.85 PU to 0.9 PU is acceptable. t 1 can be set to 0~5ms.

[0027] Specifically, the shielded DC current control loop puts the controllable grid-connected phase converter in DC voltage control mode. The method involves setting the DC current reference value to an extremely small negative value. U NEG For example, -10pu.

[0028] The process of performing closed-loop control on the DC voltage to generate a first correction value for the turn-off angle reference value in the predictive turn-off angle control loop includes: The deviation d is obtained by subtracting the actual value from the reference value of the DC voltage. U dc ; For d U dc Multiply by a first correction factor greater than zero k 1. Obtain the first correction amount d of the reference value of the turn-off angle of the predictive turn-off angle control loop. Gam 1.

[0029] The formula for calculating the second correction factor is as follows: k 2 = max[(1-0.1)] N ar ), 0] in, k 2 is the second correction factor. N ar This represents the maximum number of times the surge arresters of the fully controlled sub-converters in each auxiliary branch operate during AC fault recovery.

[0030] The process of detecting DC voltage and determining DC voltage recovery includes: Real-time acquisition of DC port voltage of controllable grid commutator U dc ; Compare U dc With DC voltage setting value U dcset The size, when Udc continuous t 2 times greater than U dcset If the DC voltage returns to the normal range, then it is determined that the DC voltage has recovered to the normal range; among which, U dcset A value of 0.9 pu~0.95 pu can be selected. t 2 can be set to 0~10ms.

[0031] Switching to normal control mode for continued operation includes: The DC current reference value has returned to normal. First correction factor k 1 and second correction coefficient k 2 is set to 0.

[0032] The topology of the controllable grid phase-commutation converter of this invention is as follows: Figure 1 As shown, the converter consists of six identical bridge arms: V1, V2, V3, V4, V5, and V6. Taking bridge arm V1 as an example, each bridge arm is composed of four sub-converters: V11, V12, V13, and V14. V11 and V12 are connected in series to form the main branch, and V13 and V14 are connected in series to form the auxiliary branch. The main and auxiliary branches are connected in parallel. V11 and V14 are semi-controlled thyristor converters. V12 is composed of a fully controlled sub-converter branch and a semi-controlled thyristor bypass branch connected in parallel. V13 is a fully controlled converter. Each sub-converter is equipped with corresponding damping and voltage equalization branches. Sub-converters V12, V13, and V14 are also connected in parallel with surge arresters to limit overvoltage at their terminals. When the thyristor bypass branch of V12 is not triggered, the thyristor bypass branch is in a blocked state, and the V12 sub-converter is controlled by the fully controlled sub-converter of V12.

[0033] like Figure 1 As shown, for the six arms of the hybrid converter, the triggering commutation process is similar to that of a conventional LCC-HVDC. The arm triggering sequence is V1-V2-V3-V4-V5-V6-V1-…, with the six arm trigger pulses evenly spaced, each 60° electrical angle apart. The arm commutation process is lateral commutation. The commutation sequence for the upper half of the bridge is V4-V6-V2-V4-…, and the commutation sequence for the lower half of the bridge is V1-V3-V5-V1-…, with each trigger pulse width being 120° electrical angle.

[0034] Controllable grid commutation converters have two operating modes: natural commutation and forced commutation. In natural commutation mode, the system is in normal operation, and the commutation current gradually decreases. When the main branch current decreases to a set value, V12 actively turns off, while V13 and V14 turn on, transferring the current from the main branch to the auxiliary branch. After the main branch current crosses zero, the main thyristor converter V11 experiences reverse voltage and enters the blocking recovery period. After a specified delay, V13 turns off with zero current, and the entire bridge arm regains its blocking capability, completing natural commutation. In forced commutation mode (due to AC system fault or control system activation), the commutation current increases and cannot decrease to the set value. When the main branch current-carrying time exceeds the allowable time or the control system actively enters forced commutation mode, V12 actively turns off, while V13 and V14 turn on, forcing the main branch current to transfer to the auxiliary branch. After V11 regains its blocking capability, V13 actively turns off, transferring the current to its parallel surge arrester, forming a higher operating voltage, completing forced commutation between bridge arms. Therefore, the safety of the V13 parallel surge arrester is a prerequisite for the successful implementation of the forced commutation mode of the controllable power grid commutator.

[0035] For example, the rated grid-side AC voltage of the controllable grid-side phase-commutator is 230kV, the rated DC voltage is 500kV, and the rated DC current is 1.2kA. The initial turn-off angle reference value of the predictive turn-off angle control element is... Gam 0 = 0.2967 rad (17°).

[0036] Example 2 Methods for restoring AC faults in controllable grid phase-commutator converters, such as Figure 2 As shown, it includes the following steps: 1. Detect AC voltage to determine if an AC fault has been resolved. This includes: First, the three-phase AC voltage to ground on the grid side of the controllable grid commutator converter is acquired in real time. U a , U b , U c ; Then, compare U a , U b , U c With AC voltage setting value U acset The size, when U a , U b , U c All continuous t 1 time greater than U acsetIf the AC voltage on the grid side is determined to have returned to the normal range, the AC fault recovery method will be initiated.

[0037] in, U acset Take 0.9 pu, t 1. Take 5ms.

[0038] 2. Shield the DC current control loop to put the controllable grid-connected phase converter in DC voltage control mode. Specifically, set the DC current reference value to a minimum negative value. U NEG .

[0039] in, U NEG Take -10 pu.

[0040] 3. After closed-loop control of the DC voltage, it serves as the first correction value for the turn-off angle reference in the predictive turn-off angle control loop. This includes: First, the deviation d is obtained by subtracting the actual value from the reference value of the DC voltage. U dc ; Then, for d U dc Multiply by a first correction factor greater than zero k 1. Obtain the first correction amount d of the reference value of the turn-off angle of the predictive turn-off angle control loop. Gam 1.

[0041] For example, the DC voltage reference value is 1 pu, and the actual value is 0.6 pu. k 1. Taking 0.35, the quantity d is calculated. U dc =0.4, d Gam 1 = 0.14 rad (8.02°).

[0042] 4. The formula for calculating the second correction factor is as follows: k 2 = max[(1-0.1)] N ar ), 0] In the formula, k 2 is the second correction factor. N ar This represents the maximum number of times the surge arresters of the fully controlled sub-converters in each auxiliary branch operate during AC fault recovery.

[0043] For example, N ar =2, calculated to k 2 = 0.8.

[0044] 5. Subtract the product of the first correction amount and the second correction coefficient from the initial cut-off angle reference value of the predictive cut-off angle control loop to obtain the final cut-off angle reference value.

[0045] Among them, the initial shut-off angle reference value Gam 0 = 0.2967 rad (17°), first correction d Gam 1 = 0.14 rad (8.02°), second correction factor k 2=0.8, and the final cutoff angle reference value is calculated to be 0.1847rad ​​(10.58°).

[0046] 6. Detect DC voltage and determine if DC voltage has recovered. This includes: First, the DC port voltage of the controllable grid commutator is acquired in real time. U dc ; Then, compare. U dc With DC voltage setting value U dcset The size, when U dc continuous t 2 times greater than U dcset If so, it can be determined that the DC voltage has returned to the normal range.

[0047] in, U dcset The value can be between 0.9 pu and 0.95 pu; here, we'll use 0.9 pu. t 2. Take 10ms.

[0048] 7. Switch to normal control mode to continue operation. This includes: The DC current reference value has returned to normal. First correction factor k 1 and second correction coefficient k 2 is set to 0.

[0049] This invention can effectively accelerate the power recovery speed of the controllable grid phase converter after the AC fault ends, while avoiding excessive energy accumulation in the surge arrester of the sub-converter, thus ensuring the safety of the surge arrester equipment.

[0050] Example 3 A controllable grid phase-commutator AC fault recovery system includes: The AC detection module is configured to detect AC voltage and determine whether an AC fault has been resolved. The shielded control loop module is configured as a shielded DC current control loop, enabling the controllable grid-commutated converter to be in DC voltage control mode. The first correction calculation module is configured to perform closed-loop control of the DC voltage and generate the first correction of the reference value of the turn-off angle of the predictive turn-off angle control loop. The second correction coefficient calculation module is configured to detect the number of times the arrester of the fully controlled sub-converter in the auxiliary branch operates and generate the second correction coefficient of the reference value of the turn-off angle of the predictive turn-off angle control link. The shut-off angle reference value calculation module is configured to subtract the product of the first correction amount and the second correction coefficient from the initial shut-off angle reference value of the predictive shut-off angle control loop to obtain the final shut-off angle reference value. The DC detection module is configured to detect DC voltage and determine when the DC voltage has recovered.

[0051] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art without creative effort within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for recovering AC faults in a controllable power grid phase-commutator, characterized in that, Includes the following steps: Detect AC voltage to determine if AC fault recovery has been achieved; The DC current control loop is shielded, so that the controllable grid-commutated converter is in DC voltage control mode; Closed-loop control of DC voltage is performed to generate the first correction value of the turn-off angle reference value of the predictive turn-off angle control loop; The number of times the surge arrester of the fully controlled sub-converter in the auxiliary branch is detected is used to generate a second correction coefficient for the reference value of the turn-off angle of the predictive turn-off angle control loop. The final cut-off angle reference value is obtained by subtracting the product of the first correction amount and the second correction coefficient from the initial cut-off angle reference value of the predictive cut-off angle control loop. Detect the DC voltage and determine if the DC voltage has recovered.

2. The method for restoring AC faults in a controllable power grid phase-commutator as described in claim 1, characterized in that, The process of detecting AC voltage and determining AC fault recovery includes: real-time acquisition of the three-phase AC voltage to ground on the grid side of the controllable power grid commutator converter. U a , U b , U c ; Compare U a , U b , U c With AC voltage setting value U acset The size, when U a , U b , U c All continuous t 1. Time greater than AC voltage setting value U acset If the AC voltage on the grid side is determined to have returned to the normal range, the AC fault recovery method will be initiated.

3. The method for restoring AC faults in a controllable power grid phase-commutator as described in claim 2, characterized in that, AC voltage setting value U acset The value range is 0.85pu~0.9pu. t The value of 1 ranges from 0 to 5ms.

4. The method for restoring AC faults in a controllable power grid phase-commutator as described in claim 1, characterized in that, The process of shielding the DC current control loop includes: putting the controllable grid commutator in DC voltage control mode and setting the DC current reference value to a negative value less than the set value. U NEG .

5. The method for restoring AC faults in a controllable power grid phase-commutator as described in claim 1, characterized in that, The process of generating the first correction value of the turn-off angle reference value for the predictive turn-off angle control loop by performing closed-loop control of the DC voltage includes: The deviation d is obtained by subtracting the actual value from the reference value of the DC voltage. U dc ; Deviation d U dc Multiply by a first correction factor greater than zero k 1. Obtain the first correction amount d of the reference value of the turn-off angle of the predictive turn-off angle control loop. Gam 1.

6. The method for restoring AC faults in a controllable power grid phase-commutator as described in claim 1, characterized in that, The calculation process for the second correction factor is as follows: k 2=max[(1-0.1 N ar ), 0] In the formula, k 2 is the second correction factor. N ar This represents the maximum number of times the surge arresters of the fully controlled sub-converters in each auxiliary branch operate during AC fault recovery.

7. The method for recovering AC faults in a controllable power grid phase-commutator as described in claim 1, characterized in that, The process of detecting DC voltage and determining DC voltage recovery includes: Real-time acquisition of DC port voltage of controllable grid commutator U dc ; Compare U dc With DC voltage setting value U dcset The size, when U dc continuous t 2 times greater than U dcset If so, it can be determined that the DC voltage has returned to the normal range.

8. The method for recovering AC faults in a controllable power grid phase-commutator as described in claim 7, characterized in that, DC voltage setting value U dcset The value range is 0.9pu~0.95pu. t The value of 2 ranges from 0 to 10 ms.

9. The method for restoring AC faults in a controllable power grid phase-commutator as described in claim 1, characterized in that, After the DC voltage recovers, switch to normal control mode to continue operation, and the DC current reference value returns to normal; first correction coefficient. k 1 and second correction coefficient k 2 is set to 0.

10. A controllable power grid phase-commutator AC fault recovery system, characterized in that, include: The AC detection module is configured to detect AC voltage and determine whether an AC fault has been resolved. The shielded control loop module is configured as a shielded DC current control loop, enabling the controllable grid-commutated converter to be in DC voltage control mode. The first correction calculation module is configured to perform closed-loop control of the DC voltage and generate the first correction of the reference value of the turn-off angle of the predictive turn-off angle control loop. The second correction coefficient calculation module is configured to detect the number of times the arrester of the fully controlled sub-converter in the auxiliary branch operates and generate the second correction coefficient of the reference value of the turn-off angle of the predictive turn-off angle control link. The shut-off angle reference value calculation module is configured to subtract the product of the first correction amount and the second correction coefficient from the initial shut-off angle reference value of the predictive shut-off angle control loop to obtain the final shut-off angle reference value. The DC detection module is configured to detect DC voltage and determine when the DC voltage has recovered.

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