Current transformer saturation identification method using difference current cross-interval alternating characteristic

Abstract

The invention discloses a current transformer saturation identification method using a difference current cross-interval alternating characteristic. The method comprises the following steps: collecting current data; differential current is calculated, and whether differential protection is started or not is determined; calculating an accumulated difference charge quantity, and determining whether the accumulated difference charge quantity meets a linear region criterion; and detecting whether the differential current meets a saturation region criterion or not, and opening differential protection or locking. According to the current transformer saturation identification method based on the difference current cross-interval alternating characteristic, firstly, a traditional amplitude threshold value is replaced with integral quantization during difference charge quantity analysis, and the current transformer saturation recovery characteristic is better met; the sliding window integration can restrain high-frequency noise, and the integration process is equivalent to low-pass filtering; and thirdly, an alternating characteristic verification mechanism of linear region triggering and saturation region tracking is introduced, so that the problem of misoperation locking can be solved, timely locking protection can be carried out along with a saturation region after a linear region during external saturation, and misoperation locking can be avoided due to a non-saturation region after low current during internal faults.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of relay protection of power equipment, and relates to a current transformer saturation identification method using the cross-interval alternation characteristics of differential current. BACKGROUND

[0002] As the core link of relay protection, automatic control and signal monitoring of the power system, the transformation characteristics of the current transformer directly determine the action performance of the protection device. In the current power grid, due to the limitation of equipment cost and historical operation and maintenance conditions, a large number of P-type current transformers (Protection Current Transformer) with weak transient transformation performance are still in operation. Such transformers are prone to saturation due to the non-periodic component in the primary current under transient conditions such as short-circuit fault, DC bias, and magnetizing inrush, resulting in serious distortion of the secondary current. Especially worth noting is that, with the expansion of the power grid scale, the complication of the structure and the flexibility of the operation mode, the magnetizing inrush generated by the no-load closing of the transformer has a high proportion of decaying non-periodic components, which further aggravates the risk of transient saturation of P-type current transformers.

[0003] In order to cope with the influence of current transformer saturation on relay protection, there are currently a variety of saturation identification methods, mainly including time difference method, multi-order derivative coordination method and wavelet analysis method. The time difference method is based on the synchronization of fault starting and differential current out-of-limit for judgment, but it is strongly dependent on the fault occurrence time, and may not be able to accurately identify in some complex conditions. The multi-order derivative coordination method identifies the saturation state by analyzing the derivative characteristics of the current waveform, but the calculation complexity is high, and in some cases, false positives may occur due to noise interference. The wavelet analysis method identifies the saturation state by extracting the high-frequency component characteristics of the signal, which has a certain accuracy, but the calculation complexity is high, and the signal preprocessing requirement is strict.

[0004] In summary, the existing technology has certain limitations in calculation complexity, accuracy and adaptability in identifying current transformer saturation. SUMMARY

[0005] The purpose of the present application is to provide a current transformer saturation identification method using the cross-interval alternation characteristics of differential current, which solves the problem of certain limitations in calculation complexity, accuracy and adaptability in identifying current transformer saturation in the prior art.

[0006] The technical solution adopted by the present application is a current transformer saturation identification method using the cross-interval alternation characteristics of differential current, which comprises: Step one, collect current data; Step two, calculate the differential current and determine whether to start differential protection; Step three, calculate the accumulated differential charge quantity, and determine whether the accumulated differential charge quantity meets the linear region criterion; Step four, detect whether the differential current meets the saturation region criterion, open the differential protection or lock out.

[0007] The present application is also characterized in that: Step one includes collecting secondary current data on both sides of the protected line.

[0008] Step two includes calculating the differential current, and if the differential current reaches the starting threshold, starting the differential protection and executing step three; if the differential current does not reach the starting threshold, executing step one.

[0009] The starting threshold is shown as follows: (1), In the formula, I 1 and I 2 are the measured currents on both ends of the differential protection, I dset is the differential current starting threshold.

[0010] Step three includes calculating the accumulated differential charge quantity, and if the accumulated differential charge quantity at any time after the differential protection is started meets the linear region criterion, executing step four; if the accumulated differential charge quantity does not meet the linear region criterion, opening the differential protection.

[0011] The calculation formula of the accumulated differential charge quantity is as follows: (2), In the formula, represents the accumulated differential charge quantity, represents the current time, represents the time window length of the sliding window integration, i.e. the minimum time section of the linear region to be determined, represents the differential current.

[0012] The linear region criterion is shown as follows: (3), In the formula, represents the current setting value, i.e. the maximum unbalanced current to be avoided during normal operation; represents the minimum setting time section of the linear region.

[0013] The accumulated differential charge quantity is the integral of the absolute value of the differential current by point-by-point sliding window integration.

[0014] Step four includes detecting whether the differential current meets the saturation region criterion within 5 ms after entering the linear region, and if the differential current meets the saturation region criterion, locking out the differential protection; if the differential current does not meet the saturation region criterion, opening the differential protection.

[0015] The saturation criterion is as follows: (4), In the formula, represents the differential current, represents the maximum value of the absolute value of the differential current within 5 ms after the linear region, represents the current setting value, that is, the maximum unbalanced current during normal operation.

[0016] The beneficial effects of the present application are: first, the integral quantization is used to replace the traditional amplitude threshold in differential charge analysis, which is more in line with the saturation recovery characteristics of the current transformer; second, the sliding window integration can suppress high-frequency noise, and the integral process is equivalent to low-pass filtering; third, the alternating characteristic verification mechanism of linear region triggering and saturation region tracking is introduced, which can solve the problem of false blocking, that is, when the saturation region is outside the region, the saturation region can be blocked in time due to the linear region, and when the fault is inside the region, the saturation region is avoided due to the low current. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a flowchart of the current transformer saturation identification method using the differential current cross-region alternating characteristic of the present application; Figure 2 is a simulation model structure diagram in an embodiment of the present application; Figure 3 is a secondary side current diagram under the current transformer saturation condition when the simulation model has an out-of-region fault in an embodiment of the present application; Figure 4 is a differential current diagram under the current transformer saturation condition when the simulation model has an out-of-region fault in an embodiment of the present application; Figure 5 is an accumulated differential charge diagram under the current transformer saturation condition when the simulation model has an out-of-region fault in an embodiment of the present application; Figure 6 is a corresponding amplification waveform diagram of the accumulated differential charge under the current transformer saturation condition when the simulation model has an out-of-region fault in an embodiment of the present application; Figure 7 is a secondary side current diagram under the current transformer unsaturation condition when the simulation model has an in-region fault in an embodiment of the present application; Figure 8 is a differential current diagram under the current transformer unsaturation condition when the simulation model has an in-region fault in an embodiment of the present application; Figure 9 is an accumulated differential charge waveform diagram under the current transformer unsaturation condition when the simulation model has an in-region fault in an embodiment of the present application; Figure 10This is a schematic diagram of the corresponding magnified waveform of the accumulated differential charge waveform when the current transformer is unsaturated under the fault condition in the simulation model of the embodiment of the present invention. Figure 11 This is a schematic diagram of the secondary current of the current transformer under saturation when a fault occurs in the simulation model in an embodiment of the present invention. Figure 12 This is a schematic diagram of the differential current under saturation conditions of the current transformer when a fault occurs in the simulation model in an embodiment of the present invention. Figure 13 This is a schematic diagram of the differential charge quantity of the current transformer under saturation conditions when a fault occurs in the simulation model in this embodiment of the invention; Figure 14 This is a schematic diagram of the corresponding amplified waveform of the differential charge quantity when the current transformer is saturated under the fault condition in the simulation model of the embodiment of the present invention; Figure 15 This is a schematic diagram of the secondary current of the current transformer under saturation when the inrush current flows through the simulation model in this embodiment of the invention; Figure 16 This is a schematic diagram of the differential current under saturation conditions of the current transformer when the excitation inrush current flows through the simulation model in this embodiment of the invention; Figure 17 This is a schematic diagram of the differential charge quantity of the current transformer under saturation conditions when the excitation inrush current flows through the simulation model in this embodiment of the invention; Figure 18 This is a schematic diagram of the amplified waveform of the differential charge quantity of the current transformer under saturation conditions when the excitation inrush current flows through the simulation model in this embodiment of the invention. Detailed Implementation

[0018] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0019] A current transformer saturation identification method utilizing the alternating characteristics of differential current across intervals, such as... Figure 1 As shown, it includes: Step 1: Collect current data; Collect secondary current data on both sides of the protected line; Step 2: Calculate the differential current to determine whether to activate the differential protection. Calculate the differential current. If the differential current reaches the start threshold, activate the differential protection and proceed to step three; if the differential current does not reach the start threshold, proceed to step one. The startup requirements are as follows: (1), In the formula, I 1 and I 2 represents the measured current at both ends of the differential protection.I dset The differential current starts the threshold; Step three, calculate the accumulated differential charge amount, and determine whether the accumulated differential charge amount meets the linear region criterion; Calculate the accumulated differential charge amount, if the accumulated differential charge amount meets the linear region criterion at any time after the differential protection is started, execute step four, if the accumulated differential charge amount does not meet the linear region criterion, open the differential protection; The accumulated differential charge amount is the integral of the absolute value of the differential current by point-by-point sliding window integration; The calculation formula of the accumulated differential charge amount is as follows: (2), In the formula, The accumulated differential charge amount is represented by, The current time is represented by, The time window length of the sliding window integration, that is, the minimum time section of the linear region to be determined, The differential current is represented by; The linear region criterion is as follows: (3), In the formula, The current setting value, that is, the maximum unbalanced current during normal operation, is represented by, The minimum setting time section of the linear region is represented by; Step four, detect whether the differential current meets the saturation region criterion, open the differential protection or block; Detect whether the differential current meets the saturation region criterion within 5ms after entering the linear region, if the differential current meets the saturation region criterion, block the differential protection, if the differential current does not meet the saturation region criterion, open the differential protection; The saturation region criterion is as follows: (4), In the formula, The differential current is represented by, The maximum value of the absolute value of the differential current within 5ms after the linear region is represented by, The current setting value, that is, the maximum unbalanced current during normal operation, is represented by.

[0020] The application aims at the problem that the line differential protection malfunctions due to the saturation of the current transformer caused by the non-periodic component in the primary current, uses the alternating characteristics of the saturation region and the linear region during external fault as the criterion, effectively avoids the saturation malfunction of the current transformer, accurately identifies the saturation state of the current transformer, and improves the safe and stable operation level of the power system.

[0021] The application takes the alternating characteristics of the saturation zone and the linear zone of the current transformer during external fault as the core criterion, and realizes saturation recognition through the coherent process of data acquisition, protection start judgment, accumulated differential charge calculation and alternating characteristic verification. The specific working principle is as follows: First, data is continuously acquired and monitored to obtain the secondary side current data of the protected line on both sides of the transformer in real time, providing a basic data source for subsequent analysis. Then, the differential current is calculated to determine whether the differential protection is started. If the threshold is not reached, the data is continuously acquired, and if the threshold is reached, the next step of analysis is entered.

[0022] After starting the protection, the accumulated differential charge is calculated. The accumulated differential charge quantifies the cumulative effect of the differential current, replacing the traditional amplitude threshold judgment, which is more in line with the saturation recovery characteristics of the current transformer. At the same time, the sliding window integration process is equivalent to low-pass filtering, which can effectively suppress high-frequency noise interference. Next, it is determined whether the accumulated differential charge meets the linear zone criterion. If it meets the criterion, it is proved that the current transformer enters the linear zone. To avoid false blocking by a single criterion and enhance the anti-interference ability, an alternating characteristic verification mechanism is introduced: when the linear zone is detected, a timer is started, and whether the saturation zone characteristics appear within a certain period of time is detected; If both the accumulated differential charge criterion (linear zone exists) and the saturation zone criterion (linear zone is followed by a saturation zone) are met, it is determined that the current transformer is saturated, and the differential protection is immediately blocked to avoid misoperation; if the criterion is not met, the protection is opened in time to ensure that the protection device can reliably act during a fault. This process takes advantage of the difference between the alternating characteristics of the "linear zone-saturation zone" during external saturation and the absence of such alternation during internal faults, achieving accurate identification of the saturation state.

[0023] Embodiment 1 This embodiment proposes a current transformer saturation recognition method using the differential current across interval alternating characteristics, as shown in Figure 1 , which includes: Step one, acquire current data; Step two, calculate the differential current to determine whether to start the differential protection; Step three, calculate the accumulated differential charge to determine whether the accumulated differential charge meets the linear zone criterion; Step four, detect whether the differential current meets the saturation zone criterion, and open or block the differential protection.

[0024] Embodiment 2 This embodiment proposes a current transformer saturation recognition method using the differential current across interval alternating characteristics, as shown in Figure 1 , which includes: Step one, acquire current data; Acquire the secondary side current data of the protected line on both sides; Step two, calculate the differential current to determine whether to start the differential protection; Step 3: Calculate the accumulated charge difference and determine whether the accumulated charge difference satisfies the linear region criterion; Step 4: Check whether the differential current meets the saturation zone criterion, and then open or close the differential protection.

[0025] Example 3 This embodiment proposes a current transformer saturation identification method utilizing the alternating characteristics of differential current across intervals, such as... Figure 1 As shown, it includes: Step 1: Collect current data; Step 2: Calculate the differential current to determine whether to activate the differential protection. Calculate the differential current. If the differential current reaches the start threshold, activate the differential protection and proceed to step three. If the differential current does not reach the start threshold, proceed to step one. The startup requirements are as follows: (1), In the formula, I 1 and I 2 represents the measured current at both ends of the differential protection. I dset The differential current start-up threshold; Step 3: Calculate the accumulated charge difference and determine whether the accumulated charge difference satisfies the linear region criterion; Step 4: Check whether the differential current meets the saturation zone criterion, and then open or close the differential protection.

[0026] Example 4 This embodiment proposes a current transformer saturation identification method utilizing the alternating characteristics of differential current across intervals, such as... Figure 1 As shown, it includes: Step 1: Collect current data; Step 2: Calculate the differential current to determine whether to activate the differential protection. Step 3: Calculate the accumulated charge difference and determine whether the accumulated charge difference satisfies the linear region criterion; Calculate the accumulated differential charge. If the accumulated differential charge at any time after the differential protection is started meets the linear zone criterion, proceed to step four; if the accumulated differential charge does not meet the linear zone criterion, open the differential protection. The formula for calculating the accumulated charge difference is as follows: (2), In the formula, This represents the accumulated difference in charge. Indicates the current moment. The time window length representing the sliding window integration is the minimum time interval of the linear region that needs to be determined. Indicates differential current; Step 4: Check whether the differential current meets the saturation zone criterion, and then open or close the differential protection.

[0027] Example 5 This embodiment proposes a current transformer saturation identification method utilizing the alternating characteristics of differential current across intervals, such as... Figure 1 As shown, it includes: Step 1: Collect current data; Step 2: Calculate the differential current to determine whether to activate the differential protection. Step 3: Calculate the accumulated charge difference and determine whether the accumulated charge difference satisfies the linear region criterion; Calculate the accumulated differential charge. If the accumulated differential charge at any time after the differential protection is started meets the linear zone criterion, proceed to step four; if the accumulated differential charge does not meet the linear zone criterion, open the differential protection. The linear region criterion is as follows: (3), In the formula, The current setting value represents the value that avoids the maximum unbalanced current during normal operation. This represents the minimum settling time interval within the linear region; The accumulated differential charge is obtained by integrating the absolute value of the differential current through a point-by-point sliding window integral. Step 4: Check whether the differential current meets the saturation zone criterion, and then open or close the differential protection.

[0028] Example 6 This embodiment proposes a current transformer saturation identification method utilizing the alternating characteristics of differential current across intervals, such as... Figure 1 As shown, it includes: Step 1: Collect current data; Step 2: Calculate the differential current to determine whether to activate the differential protection. Step 3: Calculate the accumulated charge difference and determine whether the accumulated charge difference satisfies the linear region criterion; Step 4: Check whether the differential current meets the saturation zone criterion, and then open or close the differential protection. The differential current is checked to see if it meets the saturation zone criterion within 5ms after entering the linear zone. If the differential current meets the saturation zone criterion, the differential protection is blocked; if the differential current does not meet the saturation zone criterion, the differential protection is opened. The saturation region criterion is as follows: (4), In the formula, Represents differential current. This represents the maximum absolute value of the differential current within 5ms after the linear region. The current setting value represents the maximum unbalanced current during normal operation.

[0029] An embodiment of the present application takes 110kV distribution line as an example, and establishes a simulation model as shown in the drawing by using electromagnetic transient simulation software PSCAD (Power Systems Computer Aided Design), the main research object is a 110kV / 10kV transformer with saturation characteristics and its adjacent line at the primary side, the transformer parameters and line parameters are shown in Table 1 and Table 2. Each phase active load is 1MW (approximately no load, ensuring normal operation of the model), the system frequency is 50Hz, and the length of overhead line is 8km. Figure 2

[0030] Table 1 110kV overhead line parameters

[0031] Table 2 SF10-50000 / 110 type transformer parameters

[0032] The specific simulation conditions are as follows: Time window Take 3ms, Take 0.6I n , the accumulated differential charge quantity setting value is 0.047A·s, wherein I n is the rated current of the current transformer.

[0033] When the external fault occurs and the current transformer is not saturated, the differential protection will not be started.

[0034] When the external fault occurs and the current transformer is saturated, as shown in the drawing, Figures 3 to 6 it can be seen that there is a linear region in the differential current, the current in the linear region is almost zero, and the corresponding accumulated differential charge quantity is significantly below 0.047A·s, and the protection is blocked.

[0035] When the internal fault occurs and the current transformer is not saturated, as shown in the drawing, Figures 7 to 10 it can be seen that there is no linear region in the differential current, and therefore the minimum accumulated differential charge quantity is also 0.5A·s, and the protection is opened.

[0036] When the internal fault occurs and the current transformer is saturated, as shown in the drawing, Figures 11 to 14 it can be seen that there is no linear region in the differential current, and therefore the minimum accumulated differential charge quantity is also 0.1A·s, and the protection is opened.

[0037] When the magnetizing inrush current flows through the current transformer and the current transformer is saturated, as shown in the drawing, Figures 15 to 18 ​As shown, at this time, it can be seen that the differential current has a linear region, the current of the linear region is almost zero, and the accumulated differential charge amount is below 0.047 A·s, and the protection is locked.

[0038] Based on the above analysis and Table 3, it can be seen that for the internal fault, the differential protection can be opened in time, and the differential protection misoperation caused by the magnetizing inrush current and the external fault current is avoided, which fully proves the effectiveness of the application.

[0039] Table 3 Fault simulation results

Claims

1. A method for identifying saturation of a current transformer using the differential current cross-region alternation characteristic, characterized in that, The application relates to a differential protection method and device for power transmission lines. Step one, collecting current data; Step two, calculating differential current and determining whether to start differential protection; Step three, calculating accumulated differential charge and determining whether the accumulated differential charge meets linear region criteria; Step four, detecting whether the differential current meets saturation region criteria and opening or locking differential protection.

2. The method according to claim 1, wherein, The step one comprises collecting secondary current data on both sides of a protected line.

3. The method according to claim 1, wherein, The step two comprises calculating differential current, starting differential protection if the differential current reaches a starting threshold, and executing step three if the differential current does not reach the starting threshold.

4. The method according to claim 3, wherein, The starting threshold is shown as follows: （1）， wherein I 1 and I 2 are the measured currents at the two ends of the differential protection, I dset is the differential current activation threshold.

5. The method according to claim 1, wherein, The step three comprises calculating accumulated differential charge, executing step four if the accumulated differential charge meets linear region criteria at any time after differential protection is started, and opening differential protection if the accumulated differential charge does not meet the linear region criteria.

6. The method according to claim 5, wherein the method is characterized by, The calculation formula of the accumulated differential charge is shown as follows: （2）， In the formula, represents the accumulated differential charge amount, represents the current time, represents the time window length of the sliding window integration, that is, the minimum time section of the linear region that needs to be determined, represents the differential current.

7. The method according to claim 5, wherein the method is characterized by, The linear region criteria is shown as follows: （3）， In the formula, represents the current setting value, i.e. the maximum unbalanced current during normal operation; represents the minimum setting time section of the linear region.

8. The method according to claim 1, wherein, The accumulated differential charge is the integral of the absolute value of differential current by point-by-point sliding window integration.

9. The method according to claim 1, wherein the method is characterized by, The step four comprises detecting whether the differential current meets saturation region criteria within 5 ms after entering the linear region, locking differential protection if the differential current meets the saturation region criteria, and opening differential protection if the differential current does not meet the saturation region criteria.

10. The method according to claim 1, wherein, The saturation region criteria is shown as follows: （4）， In the formula, represents the differential current, represents the maximum value of the absolute value of the differential current within 5 ms after the linear region, represents the current setting value, i.e. the maximum unbalanced current during normal operation.

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