A method and system for identifying lightning interference of a flexible direct current transmission line

Abstract

The present application belongs to the field of DC transmission line protection, and particularly relates to a flexible DC transmission line lightning interference identification method and system. The present application is characterized in that: by determining the judgment condition that the rising duration of the current fault component in a specific time window is less than the falling duration when the DC transmission line suffers from lightning interference, lightning fault and ordinary short-circuit fault, the rising and falling duration characteristics in the differential mode current fault component characteristics can be used to more accurately distinguish the fault traveling wave caused by lightning interference and other faults according to the line differential mode current fault component in the set time window, so as to avoid the maloperation of the quick-break protection and improve the protection reliability. Lightning interference identification is only performed when the line voltage mutation meets the condition, so as to avoid frequent triggering of the identification process and save the lightning interference identification calculation amount.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of DC transmission line protection, and particularly relates to a flexible DC transmission line lightning disturbance identification method and system. BACKGROUND

[0002] In traditional DC engineering applications, the data window length of the main protection criterion is generally short. Since lightning current impact often occurs in a similar time scale, this criterion can greatly improve the speed of protection discrimination, but may also be affected by lightning disturbance. Therefore, domestic and foreign manufacturers generally configure traveling wave protection and / or voltage sudden change protection in high-voltage DC transmission line protection to avoid lightning disturbance.

[0003] However, the traveling wave protection and the voltage sudden change protection generally do not configure a lightning disturbance diagnosis element alone, and only rely on protection setting to avoid lightning disturbance, which is not reliable and has certain risks. Moreover, in flexible DC engineering, when lightning disturbance occurs in the DC line, the DC line current rises and the voltage rapidly drops, which is particularly similar to other fault characteristics in the area, and also has the risk of misoperation. For the traveling wave protection, since the transient signals caused by lightning and faults are high-frequency signals, the existing traveling wave protection cannot distinguish fault traveling waves from lightning disturbance, resulting in low accuracy of lightning disturbance identification and unreliable protection. SUMMARY

[0004] The purpose of the present application is to provide a flexible DC transmission line lightning disturbance identification method and system to solve the problem of low accuracy of lightning disturbance identification caused by the difficulty of existing traveling wave protection in distinguishing fault traveling waves from lightning disturbance.

[0005] In order to achieve the above purpose, the present application provides a flexible DC transmission line lightning disturbance identification method, the steps of which are as follows:

[0006] 1) Real-time acquisition of voltage signals and positive and negative current signals of the flexible DC transmission line, and calculation of line positive and negative current fault components in combination with the current signals of the line at steady state time;

[0007] 2) Obtaining the line differential mode current fault component according to the line positive and negative current fault components;

[0008] 3) When the line voltage sudden change meets the protection starting criterion, determining the rising time and the falling time of the line differential mode current fault component in the set time window;

[0009] 4) According to the rising time and the falling time of the line differential mode current fault component, it is judged whether the lightning interference occurs to the power transmission line: if the rising time of the line differential mode current fault component is less than or equal to the falling time, or the rising time is greater than the falling time within a set upper limit, it is determined that the lightning interference occurs to the power transmission line.

[0010] The lightning interference identification method adopts the rising time and the falling time of the differential mode current fault component feature, can more accurately distinguish the lightning interference and the fault traveling wave caused by other faults, avoids the misoperation of the quick-break protection, and improves the protection reliability. Moreover, the lightning interference identification is performed only when the line voltage mutation meets the condition, the identification process is avoided from being frequently triggered, and the lightning interference identification calculation amount is saved.

[0011] Further, the protection starting criterion is specifically as follows:

[0012]

[0013] In the formula, Δu p (k) is the current time direct current line voltage mutation, u p (k) is the voltage sampling value at the current time, Δ1 is the starting criterion threshold value. p (k-1) is the voltage sampling value at the previous sampling time of the current time.

[0014] Further, in step 4), the way of judging whether the lightning interference occurs to the power transmission line according to the rising time and the falling time of the line differential mode current fault component is specifically as follows:

[0015]

[0016] If r≤Δ2, it is determined that the lightning interference occurs; if r>Δ2, it is determined that the line ground fault occurs.

[0017] In the formula, t up is the rising time of the line differential mode current fault component, t down is the falling time of the line differential mode current fault component, and Δ2 is the set lightning interference threshold value, and Δ2≥1.

[0018] Further, the calculation of the rising time and the falling time of the line differential mode current fault component is specifically as follows:

[0019]

[0020] In the formula, t up (k) is the rising time sampling point, t down (k) is the falling time sampling point; Δi1(k) is the differential mode current fault component at the current time, and Δi1(k-1) is the differential mode current fault component at the previous sampling period of the current time.

[0021] Then the rising time and the falling time are:

[0022]

[0023] In the formula, t up is the rising time of the line differential mode current fault component, t down is the falling time of the line differential mode current fault component; k = 1 is the sampling point at the starting time of the protection starting criterion, and n is the number of sampling points in the set time window.

[0024] The above calculation method normalizes the sampling point data into the rising state and the falling state, so that the statistics of the rising time and the falling time of the line differential mode current fault component are more accurate.

[0025] Further, in step 2), the line differential mode current fault component is calculated by differential mode transformation decoupling of the line positive and negative current fault components; the specific formula for calculating the line differential mode current fault component by differential mode transformation decoupling is:

[0026]

[0027] In the formula, Δi1(k) is the line differential mode current fault component; Δi P (k) and Δi N (k) are the line positive and negative current fault components, respectively, and the subscript 1 represents the differential mode component.

[0028] Further, in step 1), the calculation method of the line positive and negative current fault components is:

[0029]

[0030] In the formula, Δi P (k) and Δi N (k) are the line positive and negative current fault components, respectively; i P (k) and i N (k) are the positive and negative line current signals collected at the current time, respectively, i P (0) and i N (0) are the positive and negative line current signals at the steady state time, respectively.

[0031] Further, the starting criterion threshold value is greater than the maximum value of the voltage gradient under voltage fluctuation when the power transmission line is in normal operation.

[0032] Further, the set lightning interference threshold value Δ2 is 1.5.

[0033] In normal operation, since the current fluctuates up and down around the steady-state operating value, the rise and fall times are approximately equal, and therefore the value of Δ2 is set to 1.5 in accordance with the ratio of the rise time to the fall time of the differential mode component of the current in normal operation.

[0034] The application further provides a flexible DC transmission line lightning interference identification system, comprising a collector and a processor, the collector is used for collecting the positive and negative current signals and voltage signals of the fault end transmission line in real time, and the processor is used for executing program instructions to realize the lightning interference identification method of the flexible DC transmission line as described above. The system can realize the same beneficial effects as the lightning interference identification method of the flexible DC transmission line. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 The flowchart of the lightning interference identification method in the embodiment of the lightning interference identification method of the flexible DC transmission line of the application;

[0036] Figure 2 The differential mode current fault component waveform of the DC transmission line in the case of lightning interference in the embodiment of the lightning interference identification method of the flexible DC transmission line of the application;

[0037] Figure 3 The differential mode current fault component waveform of the DC transmission line in the case of fault caused by back strike in the embodiment of the lightning interference identification method of the flexible DC transmission line of the application;

[0038] Figure 4 The differential mode current fault component waveform of the DC transmission line in the case of fault caused by back strike in the embodiment of the lightning interference identification method of the flexible DC transmission line of the application;

[0039] Figure 5 The differential mode current fault component waveform of the DC transmission line in the case of ordinary short-circuit fault in the embodiment of the lightning interference identification method of the flexible DC transmission line of the application. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical scheme and advantages of the application more clear and obvious, the application will be further described in detail below with reference to the drawings and embodiments.

[0041] Embodiment of the lightning interference identification method of the flexible DC transmission line

[0042] The embodiment provides a technical scheme of a lightning interference identification method of a flexible DC transmission line, referring to Figure 1 , and the specific steps are as follows:

[0043] 1) Real-time acquisition of voltage signal and positive and negative electrode current signal of flexible HVDC transmission line, and calculation of positive and negative electrode current fault components of the line at steady state time respectively; in this embodiment, in order to reduce the difficulty of obtaining voltage and current signals required for lightning interference identification, the steady-state current signal of the line at the time, the current voltage signal and the positive and negative electrode current signal of the transmission line at the local end (i.e. the line end performing the lightning interference identification method process) are directly acquired for lightning interference identification, since the relevant circuit data can be directly acquired from the local end and directly used for judgment, the cost of data acquisition can be reduced.

[0044] Wherein, the calculation method of the positive and negative electrode current fault components of the line is:

[0045]

[0046] In the formula: Δi P (k), Δi N (k) are the positive and negative electrode current fault components of the line; i P (k), i N (k) are the positive and negative electrode current signals collected at the current time, i P (0), i N (0) are the positive and negative electrode current signals of the line at the steady state time.

[0047] 2) Obtain the line differential mode current fault component according to the positive and negative electrode current fault components of the line.

[0048] For the flexible HVDC bipolar system, the differential mode transformation method can be used to construct the transformation matrix Q to decouple the bipolar line into common mode and differential mode system. In this embodiment, the differential mode current fault component is obtained from the positive and negative electrode current fault components by the differential mode transformation matrix Q as follows:

[0049]

[0050] Therefore, in this embodiment, the positive and negative electrode current fault components of the line are decoupled by differential mode transformation, and the specific formula for calculating the line differential mode current fault component is:

[0051]

[0052] In the formula: Δi1(k) is the line differential mode current fault component; Δi P (k), Δi N (k) are the positive and negative electrode current fault components of the line, and subscript 1 represents the differential mode component.

[0053] 3) When the line voltage mutation meets the protection starting criterion, the rising duration and falling duration of the line differential mode current fault component within the set time window are determined.

[0054] The protection starting criterion that needs to be met by the line voltage mutation variable is specifically as follows:

[0055]

[0056] In the formula, Δu p (k) is the line voltage mutation variable at the current moment, u p (k) is the voltage sampling value at the current moment, u p (k-1) is the voltage sampling value at the previous moment of the current moment, Δ1 is a starting criterion threshold value, and Δ2≥1; the moments mentioned in the embodiment all refer to sampling moments. Generally, the starting criterion threshold value is greater than the maximum value of the voltage gradient under normal operation voltage fluctuation, and also needs to ensure that the internal fault of the DC power grid is accurately and quickly started; in the embodiment, the starting criterion threshold value Δ1 is 0.02. After the line voltage mutation variable meets the protection starting criterion, the step of lightning disturbance identification is continued, so as to avoid frequent triggering of the identification process and save the lightning disturbance identification cost.

[0057] The calculation of the rising duration and the falling duration of the line differential mode current fault component is specifically as follows:

[0058]

[0059] In the formula, t up (k) is the rising time sampling point, t down (k) is the falling time sampling point; Δi1(k) is the differential mode current fault component at the current moment, and Δi1(k-1) is the differential mode current fault component at the previous sampling period of the current moment;

[0060] The rising duration and the falling duration are as follows:

[0061]

[0062] In the formula, t up is the rising duration of the line differential mode current fault component, t down is the falling duration of the line differential mode current fault component; k=1 is the sampling point at the starting moment of the protection starting criterion, and n is the sampling point number in the set time window. In the flexible DC line protection, the line protection is generally required to be fast, for example, a certain project requires the protection to be exported within 3ms, so a 1ms time window is generally taken, and if the sampling rate is 20KHz, n is 20. The above calculation method normalizes the sampling point data into the rising state and the falling state, so that the statistics of the rising duration and the falling duration of the line differential mode current fault component are more accurate.

[0063] 4) According to the rising time and the falling time of the line differential mode current fault component, it is judged whether the lightning interference occurs to the power transmission line: if the rising time of the line differential mode current fault component is less than or equal to the falling time, or the rising time is greater than the falling time within a set upper limit (i.e. the rising time is greater than the falling time to a small extent), it is determined that the lightning interference occurs to the power transmission line.

[0064] Figure 2 、 Figure 3 、 Figure 4 and Figure 5 The waveforms of the differential mode current fault components collected in four cases of lightning interference, back strike leading to fault, shielding leading to fault and ordinary short circuit fault of the DC power transmission line are shown in FIGS. 1, 2, 3 and 4 respectively; the back strike leading to fault and the shielding leading to fault both belong to lightning fault, and the lightning fault also belongs to one kind of line grounding fault, and the ordinary short circuit fault also belongs to line grounding fault. According to the above waveform images, it can be determined that when the lightning fault and the ordinary short circuit fault occur to the line, the rising time of the differential mode current fault component is greater than the falling time within a certain time window after the protection is started; when the lightning interference occurs to the line, the rising time of the differential mode current fault component is less than the falling time within the certain time window after the protection is started. Based on this feature, considering the system error and normal fluctuation, the condition for judging whether the lightning interference occurs to the power transmission line is summarized as follows: within the certain time window after the protection is started, the rising time of the differential mode current fault component is less than or equal to the falling time, or the rising time is greater than the falling time within a set upper limit range.

[0065] In this embodiment, the judgment condition is specifically represented by the following way:

[0066]

[0067] If r≤Δ2, it is determined that the lightning interference occurs; if r>Δ2, it is determined that the line grounding fault occurs;

[0068] wherein, t up is the rising time of the line differential mode current fault component, t down is the falling time of the line differential mode current fault component, and Δ2 is the set lightning interference threshold value. In order to meet the reliability and selectivity of the protection, the lightning interference threshold value Δ2 is set to a value slightly greater than 1, and the specific value is adjusted according to the ratio of the rising time to the falling time of the current mode component in the normal operation, i.e. the rising time of the differential mode current fault component is greater than the falling time within a set upper limit range. In the normal operation, since the current fluctuates around the steady-state operating value, the rising and falling times are approximately equal, and in this embodiment, considering a certain reliability coefficient, Δ2 is set to 1.5.

[0069] In other embodiments, the above judgment condition can also be expressed in other ways, such as taking the difference between the falling time length and the rising time length of the line differential mode current fault component as a judgment index, and if the difference Δt=t down -t up greater than or equal to a set difference threshold value, or the difference is less than the set difference threshold value to a certain upper limit range, it is determined that lightning disturbance occurs, otherwise it is determined that line ground fault occurs.

[0070] Flexible DC transmission line lightning disturbance identification system embodiment

[0071] The embodiment provides a flexible DC transmission line lightning disturbance identification system, which comprises a collector and a processor; the collector is used for collecting positive and negative current signals and voltage signals of a fault end transmission line in real time, and the processor is used for executing program instructions to realize the lightning disturbance identification method in the above-mentioned flexible DC transmission line lightning disturbance identification method embodiment. Since the processor processing principle, content and corresponding beneficial effects of the system are described in detail in the above-mentioned flexible DC transmission line lightning disturbance identification method embodiment, they will not be described here again.

[0072] The application has the characteristics that: by determining the judgment condition that the rising time length of the current fault component in a specific time window is less than the falling time length when the DC transmission line suffers from lightning disturbance, lightning fault and ordinary short-circuit fault, the rising and falling time length characteristics in the differential mode current fault component characteristics can be used to distinguish the lightning disturbance and the fault traveling wave caused by other faults more accurately according to the line differential mode current fault component in the set time window, the speed-break protection misoperation is avoided, and the protection reliability is improved. Lightning disturbance identification is only performed when the line voltage mutation meets the condition, the identification process is prevented from being frequently triggered, and the lightning disturbance identification calculation amount is saved.

Claims

1. A method of identifying lightning disturbance of a flexible DC power transmission line, characterized by, The steps are as follows: 1) Real-time acquisition of voltage signals and positive and negative electrode current signals of the flexible HVDC transmission line, and calculation of positive and negative electrode current fault components of the line respectively in combination with the current signals of the line at the steady state time; 2) Obtaining the line differential mode current fault component according to the positive and negative electrode current fault components of the line; 3) When the line voltage mutation meets the protection starting criterion, determining the rising duration and falling duration of the line differential mode current fault component within a set time window; 4) According to the rising duration and falling duration of the line differential mode current fault component, judging whether the transmission line is subjected to lightning interference: if the rising duration of the line differential mode current fault component is less than or equal to the falling duration, or the degree that the rising duration is greater than the falling duration is within a set upper limit, it is determined that the transmission line is subjected to lightning interference.

2. The method of claim 1, wherein, The protection starting criterion is specifically as follows: where: Δu p (k) is the DC line voltage jump variable at the current time p (k) is the voltage sampling value at the current time p (k-1) is the voltage sampling value at the previous time of the current time, and Δ1 is the starting criterion threshold value.

3. The method of claim 1, wherein, In step 4), the condition for determining that the transmission line is subjected to lightning interference is specifically as follows: If r≤Δ2, it is determined that lightning interference occurs; if r>Δ2, it is determined that a line grounding fault occurs; Wherein, t up is the rise time of the line differential mode current fault component, t down is the fall time of the line differential mode current fault component, and Δ2 is a set lightning interference threshold value, and Δ2≥1.

4. The method of lightning disturbance identification for a flexible HVDC power line according to any one of claims 1-3, characterized in that, The calculation of the rising duration and falling duration of the line differential mode current fault component is specifically as follows: where: t up (k) is the rising time sampling point, t down (k) is the falling time sampling point; Δi1(k) is the differential mode current fault component at the current time, and Δi1(k-1) is the differential mode current fault component at the previous sampling period before the current time; Then the rising duration and the falling duration are: wherein: t up is the rise time of the line differential mode current fault component, t down is the fall time of the line differential mode current fault component; k = 1 is the sampling point at the protection start criterion start time, and n is the sampling point number within the set time window.

5. The method of lightning disturbance identification for a flexible HVDC power line according to any one of claims 1-3, characterized in that, In step 2), the line differential mode current fault component is calculated by differential mode transformation decoupling of the positive and negative electrode current fault components of the line; the specific formula for calculating the line differential mode current fault component by differential mode transformation decoupling is: In the formula, Δi1(k) is the line differential mode current fault component; Δi P (k), Δi N (k) are the line positive and negative current fault components, respectively, with subscript 1 representing the differential mode component.

6. The method of lightning disturbance identification for a flexible HVDC power line according to any one of claims 1-3, characterized in that, In step 1), the calculation method of the positive and negative electrode current fault components of the line is as follows: where Δi P (k), Δi N (k) are the positive and negative line current fault components, respectively; i P (k), i N (k) are the positive and negative line current signals collected at the current time, respectively, i P (0), i N (0) are the positive and negative line current signals at the steady state, respectively.

7. The method of claim 2, wherein, The starting criterion threshold value is greater than the maximum value of the voltage gradient under voltage fluctuation when the transmission line is in normal operation.

8. The method of claim 3, wherein, The set lightning interference threshold value Δ2 is 1.

5.

9. A flexible HVDC power line lightning disturbance identification system comprising a collector and a processor, characterized in that, The collector is used for real-time acquisition of positive and negative electrode current signals and voltage signals of the fault end transmission line, and the processor is used for executing program instructions to realize the lightning interference identification method of the flexible HVDC transmission line according to any one of claims 1-8.

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