A method for determining asymmetry of a converter based on valve current time sequence characteristics

Abstract

The application relates to a valve current time sequence feature-based converter asymmetry determination method and belongs to the technical field of high-voltage direct-current transmission technology and relay protection technology. The application considers that the reason why the system generates harmonics under AC fault is that the symmetrical system becomes an asymmetrical system in operation due to the switching-on of the converter, and the time distribution of the current flowing through each valve of the converter is uneven, so the application determines the switching-on time width of each valve and the switching-off time width of each valve according to the state and the current of each valve of the converter, and determines the switching-on width difference and the switching-off width difference of each valve of the converter, and the obtained switching-on width difference and switching-off width difference are used for judging the asymmetry of the converter. The application does not need to perform harmonic calculation, avoids the problem that the asymmetry judgment has a blind area due to the inaccuracy of the harmonic calculation, and greatly improves the accuracy of the asymmetry judgment of the converter.

Description

Technical Field

[0001] This invention relates to a method for determining converter asymmetry based on valve current timing characteristics, belonging to the field of relay protection technology for high voltage direct current transmission. Background Technology

[0002] In traditional HVDC transmission power grid projects, AC / DC disturbances such as AC system faults and converter valve faults, even if they do not induce system harmonic instability and DC blocking, still increase the risk to the safe and stable operation of the system and endanger equipment safety due to the harmonics generated by the faults. In such cases, if the converter main protection fails to operate, the 100Hz protection, as a backup protection function, needs to correctly operate in alarm and blocking mode; otherwise, system instability will occur. However, current 100Hz protection schemes in engineering projects, whether Siemens or ABB, all have the challenge of identifying converter valve faults and AC asymmetry faults in their protection logic. They cannot simultaneously meet the different requirements of the two types of faults for the protection, resulting in the inability to quickly isolate converter valve equipment faults and unnecessary DC blocking due to AC asymmetry faults, directly affecting the safe and stable operation of AC / DC hybrid transmission systems. Accurately identifying the harmonic characteristics of both internal and external faults and adopting corresponding protection schemes is crucial to meeting the requirements of the 100Hz protection action for both types of faults and protecting equipment and the power grid.

[0003] Existing research, based on the different mechanisms by which AC asymmetric faults and converter valve faults cause the generation of power frequency negative sequence components in the converter current switching function, proposes a new method to identify AC asymmetric faults and converter valve faults by utilizing the difference in the amplitude ratio of the power frequency negative sequence components of the converter current switching function under the two fault conditions. However, when using the converter's switching function for harmonic calculations, the states of each valve in the converter need to be assumed. When the converter is in bypass operation, the coupling relationship between valve current and valve state makes the assumed valve state incorrect, leading to inaccurate harmonic calculations and consequently affecting the judgment of converter asymmetry. Summary of the Invention

[0004] The purpose of this invention is to provide a method for determining converter asymmetry based on valve current timing characteristics, so as to solve the problem of low accuracy in judging converter asymmetry in current methods.

[0005] To address the aforementioned technical problems, this invention provides a method for determining converter asymmetry based on valve current timing characteristics. This method includes the following steps:

[0006] 1) Obtain the current of each valve in the converter, and determine the state of each valve based on the obtained current value. The state includes the on state and the off state.

[0007] 2) Calculate the on-time width and off-time width of each valve based on the status and current of each valve in the converter;

[0008] 3) Compare the maximum value of the on-time width and the maximum value of the off-time width of each valve in the converter with the continuous on-time width and continuous off-time width of each phase valve during normal operation to obtain the difference in the on-time width and the difference in the off-time width of the converter, and use this to determine the asymmetry of the converter.

[0009] This invention addresses the issue that harmonics generated in an AC fault system arise from the transition from a symmetrical to an asymmetrical system due to alternating conduction of the converter, resulting in uneven time distribution of current flow across the converter valves. Therefore, this invention determines the conduction and turn-off time widths of each valve based on their state and current, thereby identifying the differences in conduction and turn-off widths. These differences are then used to determine converter asymmetry. This invention eliminates the need for harmonic calculations, avoiding inaccurate harmonic calculations and blind spots in asymmetry assessment, thus significantly improving the accuracy of converter asymmetry detection.

[0010] Furthermore, the criteria for determining the asymmetry of the converter in step 3) are as follows:

[0011] The asymmetry difference of a single-bridge converter is determined based on the maximum value of the difference between conduction width and turn-off width.

[0012] The asymmetry between the two bridges is determined based on the asymmetry difference of the single-bridge converter, and the asymmetry between the two bridges is used to judge the asymmetry of the converter.

[0013] Furthermore, when the asymmetry between the two bridges exceeds a set threshold, the converter malfunctions; when the asymmetry between the two bridges does not exceed the set threshold, the converter experiences an AC asymmetry fault.

[0014] This invention utilizes the asymmetry difference of a single-bridge converter to determine the asymmetry between two bridges, and judges the fault condition of the converter based on the magnitude of the asymmetry, which can further improve the accuracy of converter fault condition judgment.

[0015] Furthermore, the formula for calculating the asymmetry between the two bridges is:

[0016]

[0017] K represents the asymmetry between the two bridges, k YY For the asymmetric difference of the YY bridge converter, k YD This refers to the asymmetric differences in the YD bridge converter.

[0018] This invention calculates the asymmetry between two bridges based on the asymmetry difference of a single-bridge converter, which can accurately describe the asymmetry between the two bridges.

[0019] Furthermore, the calculation formula used for the asymmetry difference of a single-bridge converter is as follows:

[0020] k j =max(k H ,k L )

[0021] k j For the asymmetric difference of a single-bridge converter, j = YY or YD, indicating a YY bridge or a YD bridge; k H For the difference in conduction width, k L This refers to the difference in the shutdown width.

[0022] This invention calculates the asymmetry difference of a single-bridge converter based on the difference in conduction width and the difference in turn-off width, and can accurately describe the asymmetry of the single-bridge converter.

[0023] Furthermore, the calculation formulas used for the difference in conduction width and the difference in turn-off width are as follows:

[0024]

[0025] k H For the difference in conduction width, k L For the difference in turn-off width, m = 1, 2, 3, 4, 5, 6, t Hm t is the valve conduction width. Lm For valve shut-off width, max(t) Hm ) represents the maximum value of the conduction width of each valve, max(t) Lm ) represents the maximum value of the shut-off width of each valve, min(t) Hm ) represents the minimum conduction width of each valve, min(t) Lm ) represents the minimum shut-off width of each valve.

[0026] This invention calculates the difference in conduction width and the difference in turn-off width based on the maximum value of the conduction time width and the maximum value of the turn-off time width, and the continuous conduction time width and continuous turn-off time width of each phase valve during normal operation, thus accurately describing the difference in conduction width and the difference in turn-off width.

[0027] Furthermore, the valve opening width and valve closing width are obtained by integrating the valve in the opening state and the valve in the closing state, respectively.

[0028] Furthermore, the criteria for determining the state of each valve are as follows:

[0029]

[0030] s Hm When the valve is in the on state, s Lm When the valve is closed, i VTmLet I be the current of the m-th valve. set1 I is the first current setpoint. set2 Set the second current value. Attached Figure Description

[0031] Figure 1 This is a flowchart of the converter asymmetry determination method based on valve current timing characteristics according to the present invention;

[0032] Figure 2 This is a schematic diagram of the conduction time width, turn-off time width, and asymmetry of the two bridges of the YY bridge when the converter valve fails in an embodiment of the present invention.

[0033] Figure 3 This is a schematic diagram of the conduction time width, turn-off time width, and asymmetry of the dual bridges of the YD bridge when the converter valve fails in an embodiment of the present invention.

[0034] Figure 4 This is a schematic diagram of the conduction time width, turn-off time width, and asymmetry of the two bridges of the YY bridge when a single-phase ground fault occurs at the converter bus in an embodiment of the present invention.

[0035] Figure 5 This is a schematic diagram of the conduction time width, turn-off time width, and asymmetry of the dual bridges of the YD bridge when a single-phase ground fault occurs at the converter bus in an embodiment of the present invention. Detailed Implementation

[0036] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0037] The converter asymmetry determination method based on valve current timing characteristics of the present invention first acquires the current of each valve in the converter, and determines the state of each valve based on the acquired current value, including the on-state and off-state; then, based on the state and current of each valve, the on-time width and off-time width of each valve are calculated; finally, the maximum value of the on-time width and the maximum value of the off-time width of each valve in the converter are compared with the continuous on-time width and continuous off-time width of each phase valve during normal operation, and the asymmetry of the single-bridge converter is determined based on the difference in the on-time width and the difference in the off-time width. The implementation process of this method is as follows: Figure 1 As shown below, a detailed explanation will be provided with specific examples.

[0038] 1. Obtain the current of each valve in the converter, and determine the state of each valve based on the obtained current value. The state includes the on state and the off state.

[0039] This embodiment focuses on a pulse converter; therefore, it detects the current i of the six valves in the three-converter. VT1 i VT2 iVT3 i VT4 i VT5 i VT6 To determine the status of each valve.

[0040] The current criterion for valve conduction is expressed as follows:

[0041] i VTm ≥I set1 (1)

[0042] In the formula: I set1 The threshold values ​​for the conduction state are set as m = 1, 2, 3, 4, 5, 6.

[0043] The current criterion for valve shut-off is expressed as follows:

[0044] i VTm <I set2 (2)

[0045] In the formula: I set2 Set a threshold value for the off state.

[0046] 2. Calculate the on-time width and off-time width of each valve based on the status and current of each valve in the converter.

[0047] In this embodiment, based on the on-time and off-time of each valve in the converter as described in step 1, the on-state and off-state of each valve are constructed, and the on-time width and off-time width of each valve are calculated. The specific process includes:

[0048] Step 21: Based on the on-time and off-time of each valve in the converter from Step 1, construct the on-state and off-state of each valve. The criteria are as follows:

[0049]

[0050] In the formula: s Hm When the valve is in the on state, s Lm For the valve to be in the closed state, m = 1, 2, 3, 4, 5, 6.

[0051] Step 22: Based on the on and off states of each valve in Step 1, construct a continuous integral of the valve on-state during the on-time period, and calculate the valve on-time width: [Calculate the valve on-time width S]. Hm The integrator performs integration, and its value is output as the maximum value by the MAXHOLD function and held until S. Hm The integrator is set to zero when a low level is encountered; similarly, a continuous integral of the valve's off-state during the off-state is constructed to determine the valve off-state width. The criteria are as follows:

[0052]

[0053] In the formula: t Hm t is the valve conduction time width. Lm Let m be the valve shut-off time width, where m = 1, 2, 3, 4, 5, 6.

[0054] 3. Calculate the difference in the conduction width and the difference in the turn-off width of the converter, and use this to determine the asymmetry of the converter.

[0055] Step 31: Calculate the difference between the maximum and minimum conduction time widths of the six valves in Step 2, and then compare this difference with the continuous conduction width of the normally operating valves to obtain the conduction width difference. Similarly, calculate the difference between the maximum and minimum shut-off time widths of the six valves in Step 2, and then compare this difference with the continuous shut-off width of the normally operating valves to obtain the shut-off width difference. See the formula below.

[0056]

[0057] In the formula: k H For the difference in conduction width, k L For the difference in turn-off width, m = 1, 2, 3, 4, 5, 6, t Hm t is the valve conduction width. Lm For valve shut-off width, max(t) Hm ) represents the maximum value of the 6-valve conduction width, max(t) Lm ) represents the maximum value of the shut-off width of valve 6, min(t) Hm ) represents the minimum value of the 6-valve conduction width, min(t) Lm ) represents the minimum shut-off width of valve 6, t HV Valve conduction width during normal operation t LV Valve shut-off width during normal operation

[0058] Step 32: Based on Step 31, use the maximum value of the difference between the conduction width and the turn-off width to determine the asymmetry of the single-bridge converter. The expression is as follows:

[0059] k j =max(k H ,k L (6)

[0060] In the formula: k j For the asymmetric difference of a single-bridge converter, j = YY or YD, indicating a YY bridge or a YD bridge.

[0061] Step 33: Based on Step 32, construct the asymmetry degree between the two bridges by utilizing the difference between the asymmetries of a single bridge and the relative relationship between the maximum asymmetry value of a single bridge. The expression is as follows:

[0062]

[0063] In the formula: K is the asymmetry between the two bridges, k YY For the asymmetric difference of the YY bridge converter, k YD This refers to the asymmetric differences in the YD bridge converter.

[0064] Step 34: Based on Step 33, construct a criterion for identifying converter valve faults and AC asymmetry faults using the asymmetry degree in the DC 100Hz protection system. The expression is as follows:

[0065] K > K set (8)

[0066] Where: K set This is the setpoint value.

[0067] If the asymmetry K satisfies step 34, it is determined to be a converter valve failure; otherwise, it is determined to be an AC asymmetry failure. In this embodiment, K... set It is set to 0.8, but other values ​​can also be set as alternative implementations.

[0068] To verify the feasibility of this invention, during actual power grid operation, a valve short-circuit fault was set in the converter and a single-phase ground fault was set at the converter bus. The conduction time width and turn-off time width of each valve, as well as the asymmetry of the double bridge, were recorded. The feasibility of using asymmetry to distinguish between converter valve faults and AC asymmetry faults was verified. The verification results are as follows: Figure 2 , Figure 3 , Figure 4 and Figure 5 As shown. Among them. Figure 2 t in YH1 t YH2 t YH3 t YH4 t YH5 t YH6 t represents the on-time width of each valve in the YY bridge converter. YL1 t YL2 t YL3 t YL4 t YL5 t YL6 K represents the turn-off time width of each valve in the YY bridge converter. YY For the asymmetric difference of the YY bridge, K YD Let YD represent the asymmetry difference of the bridge, and K represent the asymmetry of the two bridges. Figure 3 t in DH1 t DH2 t DH3 t DH4 t DH5 t DH6 t represents the on-time width of each valve in the YD bridge converter. DL1 t DL2 tDL3 t DL4 t DL5 t DL6 K represents the turn-off time width of each valve in the YD bridge converter. YY For the asymmetric difference of the YY bridge, K YD Let YD represent the asymmetry difference of the bridge, and K represent the asymmetry of the two bridges. Figure 4 t in YH1 t YH2 t YH3 t YH4 t YH5 t YH6 t represents the on-time width of each valve in the YY bridge converter. YL1 t YL2 t YL3 t YL4 t YL5 t YL6 K represents the turn-off time width of each valve in the YY bridge converter. YY For the asymmetric difference of the YY bridge, K YD Let YD represent the asymmetry difference of the bridge, and K represent the asymmetry of the two bridges. Figure 5 Chinese DH1 t DH2 t DH3 t DH4 t DH5 t DH6 t represents the on-time width of each valve in the YD bridge converter. DL1 t DL2 t DL3 t DL4 t DL5 t DL6 K represents the turn-off time width of each valve in the YD bridge converter. YY For the asymmetric difference of the YY bridge, K YD Let YD represent the asymmetry difference of the bridge, and K represent the asymmetry of the two bridges.

[0069] This shows that when the converter valve experiences a short-circuit fault, the asymmetry K > 0.8, indicating a converter valve fault; when the converter bus experiences a single-phase ground fault, the asymmetry K < 0.8, indicating an AC asymmetry fault. The judgment results are consistent with the set fault area, verifying the feasibility of this method.

Claims

1. A method for determining converter asymmetry based on valve current timing characteristics, characterized in that, The determination method includes the following steps: 1) Obtain the current of each valve in the converter, and determine the state of each valve based on the obtained current value. The state includes the on state and the off state. 2) Calculate the on-time width and off-time width of each valve based on the status and current of each valve in the converter; 3) Compare the maximum values ​​of the on-time width and off-time width of each valve in the converter with the continuous on-time width and continuous off-time width of each phase valve during normal operation to obtain the converter's on-time width difference and off-time width difference. This difference is used to determine the converter's asymmetry. The criteria for determining the converter's asymmetry are as follows: The asymmetry difference of a single-bridge converter is determined based on the maximum value of the difference between conduction width and turn-off width. The asymmetry between the two bridges is determined based on the asymmetry difference of the single-bridge converter, and the asymmetry between the two bridges is used to judge the asymmetry of the converter. The formula for calculating the asymmetry between the two bridges is: K represents the asymmetry between the two bridges. Due to the asymmetry difference of the YY bridge converter, This refers to the asymmetric differences in the YD bridge converter.

2. The method for determining converter asymmetry based on valve current timing characteristics according to claim 1, characterized in that... The current criterion expression for determining whether the valve is in the on state is as follows: In the formula: To set the threshold value for the conduction state, i VTm Valve current; The current criterion expression for determining whether the valve is in the closed state is as follows: In the formula: Set a threshold value for the off state, i VTm This represents the valve current.

3. The method for determining converter asymmetry based on valve current timing characteristics according to claim 1, characterized in that, When the asymmetry between the two bridges exceeds the set threshold, the converter malfunctions; when the asymmetry between the two bridges does not exceed the set threshold, the converter experiences an AC asymmetry fault.

4. The method for determining converter asymmetry based on valve current timing characteristics according to claim 1, characterized in that, The formula used to calculate the asymmetry difference of a single-bridge converter is: The asymmetric difference of single-bridge converters =YY or YD indicates YY bridge or YD bridge; Due to the difference in conduction width, This refers to the difference in the shutdown width.

5. The method for determining converter asymmetry based on valve current timing characteristics according to claim 1, characterized in that, The calculation formulas used for the difference in conduction width and the difference in turn-off width are as follows: Due to the difference in conduction width, Due to the difference in turn-off width, =1,2,3,4,5,6 For valve conduction width, This refers to the valve shut-off width. This represents the maximum value of the conduction width of each valve. This represents the maximum value of the shut-off width for each valve. This represents the minimum conduction width of each valve. t is the minimum shut-off width of each valve. HV For the continuous conduction width of the valve during normal operation, t LV This refers to the continuous shut-off width of the valve during normal operation.

6. The method for determining converter asymmetry based on valve current timing characteristics according to claim 5, characterized in that, The valve opening width and valve closing width are obtained by integrating the valve in the opening state and the valve closing state, respectively.

7. The method for determining converter asymmetry based on valve current timing characteristics according to claim 1, characterized in that, The criteria for determining the state of each valve are as follows: The valve is in the on state. When the valve is closed, i VTm Let I be the current of the m-th valve. set1 I is the first current setpoint. set2 Set the second current value.

Images