A method for identifying saturation of current transformers using the alternating characteristics of differential current across intervals

Abstract

This invention discloses a current transformer saturation identification method utilizing the alternating characteristics of differential current across different zones. The method includes: acquiring current data; calculating the differential current to determine whether differential protection should be activated; calculating the accumulated differential charge to determine whether the accumulated differential charge meets the linear zone criterion; detecting whether the differential current meets the saturation zone criterion, and opening or closing the differential protection. This current transformer saturation identification method utilizing the alternating characteristics of differential current across different zones has several advantages: first, it uses integral quantization instead of traditional amplitude thresholds in differential charge analysis, which better reflects the saturation recovery characteristics of current transformers; second, sliding window integration can suppress high-frequency noise, and its integration process is equivalent to low-pass filtering; and third, it introduces an alternating characteristic verification mechanism of linear zone triggering and saturation zone tracking, which can solve the problem of false blocking. This ensures timely blocking of protection when saturation occurs outside the zone because the linear zone is followed by a saturation zone, and avoids false blocking when there is no saturation zone after a fault within the zone due to low current.

Description

Technical Field

[0001] This invention belongs to the field of power equipment relay protection technology, and relates to a current transformer saturation identification method utilizing the alternating characteristics of differential current across intervals. Background Technology

[0002] As a core component of power system relay protection, automatic control, and signal monitoring, the current transformer's transmission characteristics directly determine the operating performance of protection devices. Currently, due to limitations in equipment cost and historical maintenance conditions, a large number of Class P current transformers (Protection Current Transformers) with relatively weak transient transmission performance are still in operation in the power grid. Under transient conditions such as short-circuit faults, DC bias, and inrush current, these transformers are prone to saturation due to the aperiodic component of the primary current, leading to severe distortion of the secondary current. Of particular concern is that with the expansion of the power grid, its increasing structural complexity, and the flexibility of its operation, the inrush current generated by transformer no-load closing is characterized by a high proportion of attenuated aperiodic components, further exacerbating the transient saturation risk of Class P current transformers.

[0003] To address the impact of current transformer saturation on relay protection, several saturation identification methods have been developed, primarily including the time-difference method, the multi-derivative coordination method, and wavelet analysis. The time-difference method judges saturation based on the synchronization of fault initiation and differential current exceeding limits, but it is highly dependent on the fault occurrence time and may fail to accurately identify saturation under certain complex operating conditions. The multi-derivative coordination method determines saturation by analyzing the derivative characteristics of the current waveform, but its computational complexity is high, and it may lead to misjudgments due to noise interference in some cases. The wavelet analysis method identifies saturation by extracting high-frequency components of the signal; while it has a certain degree of accuracy, its computational complexity is high, and it requires strict signal preprocessing.

[0004] In summary, existing technologies have limitations in computational complexity, accuracy, and adaptability when identifying saturation in current transformers. Summary of the Invention

[0005] The purpose of this invention is to provide a current transformer saturation identification method that utilizes the alternating characteristics of differential current across intervals, thereby solving the problems of limitations in computational complexity, accuracy, and adaptability in the existing technology for current transformer saturation identification.

[0006] The technical solution adopted in this invention is a current transformer saturation identification method utilizing the alternating characteristics of differential current across intervals, comprising:

[0007] Step 1: Collect current data;

[0008] Step 2: Calculate the differential current to determine whether to activate the differential protection.

[0009] Step 3: Calculate the accumulated charge difference and determine whether the accumulated charge difference satisfies the linear region criterion;

[0010] Step 4: Check whether the differential current meets the saturation zone criterion, and then open or close the differential protection.

[0011] The invention is further characterized by:

[0012] Step one includes: collecting secondary current data on both sides of the protected line.

[0013] Step two includes: calculating the differential current; if the differential current reaches the start threshold, start the differential protection and proceed to step three; if the differential current does not reach the start threshold, proceed to step one.

[0014] The startup requirements are as follows:

[0015] (1),

[0016] In the formula, I 1 and I 2 represents the measured current at both ends of the differential protection. I dset This is the differential current start-up threshold.

[0017] Step 3 includes: calculating the accumulated differential charge. If the accumulated differential charge meets the linear zone criterion at any time after the differential protection is started, proceed to step 4; if the accumulated differential charge does not meet the linear zone criterion, open the differential protection.

[0018] The formula for calculating the accumulated charge difference is as follows:

[0019] (2),

[0020] 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. This represents the differential current.

[0021] The linear region criterion is as follows:

[0022] (3),

[0023] In the formula, The current setting value represents the value that avoids the maximum unbalanced current during normal operation. This represents the minimum tuning time interval within the linear region.

[0024] The accumulated differential charge is obtained by integrating the absolute value of the differential current through a point-by-point sliding window integral.

[0025] Step four includes: detecting whether the differential current 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.

[0026] The saturation region criterion is as follows:

[0027] (4),

[0028] 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 value that avoids the maximum unbalanced current during normal operation.

[0029] The beneficial effects of this invention are: firstly, the integral quantization method replaces the traditional amplitude threshold in differential charge analysis, which is more in line with the saturation recovery characteristics of current transformers; secondly, the sliding window integral can suppress high-frequency noise, and its integration process is equivalent to low-pass filtering; thirdly, the introduction of an alternating characteristic verification mechanism of linear region triggering and saturation region tracking can solve the problem of false blocking. It can provide timely blocking protection when the linear region is saturated outside the region, and avoid false blocking when there is no saturation region after the low current during faults within the region. Attached Figure Description

[0030] Figure 1 This is a flowchart illustrating the current transformer saturation identification method utilizing the differential current cross-interval alternation characteristic of the present invention.

[0031] Figure 2 This is a schematic diagram of the simulation model structure in an embodiment of the present invention;

[0032] Figure 3 This is a schematic diagram of the secondary current of the current transformer under saturation when an external fault occurs in the simulation model of this invention embodiment;

[0033] Figure 4 This is a schematic diagram of the differential current under saturation conditions of the current transformer when an external fault occurs in the simulation model of this invention embodiment;

[0034] Figure 5 This is a schematic diagram of the accumulated differential charge in the current transformer under saturation when an external fault occurs in the simulation model of this invention.

[0035] Figure 6 This is a schematic diagram of the corresponding amplified waveform of the accumulated differential charge when the current transformer is saturated under the condition of an external fault in the simulation model of this invention embodiment;

[0036] Figure 7 This is a schematic diagram of the secondary current of the current transformer under the condition of unsaturation when a fault occurs in the simulation model in an embodiment of the present invention.

[0037] Figure 8 This is a schematic diagram of the differential current under the condition of unsaturated current transformer when a fault occurs in the simulation model of the embodiment of the present invention;

[0038] Figure 9 This is a schematic diagram of the accumulated differential charge waveform when the current transformer is unsaturated during a fault in the simulation model of this invention embodiment;

[0039] Figure 10 This 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.

[0040] 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.

[0041] 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.

[0042] 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;

[0043] 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;

[0044] 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;

[0045] 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;

[0046] 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;

[0047] 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 inrush current flows through the simulation model in this embodiment of the invention. Detailed Implementation

[0048] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0049] A current transformer saturation identification method utilizing the alternating characteristics of differential current across intervals, such as... Figure 1 As shown, it includes:

[0050] Step 1: Collect current data;

[0051] Collect secondary current data on both sides of the protected line;

[0052] Step 2: Calculate the differential current to determine whether to activate the differential protection.

[0053] 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.

[0054] The startup requirements are as follows:

[0055] (1),

[0056] 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;

[0057] Step 3: Calculate the accumulated charge difference and determine whether the accumulated charge difference satisfies the linear region criterion;

[0058] 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.

[0059] The accumulated differential charge is obtained by integrating the absolute value of the differential current through a point-by-point sliding window integral.

[0060] The formula for calculating the accumulated charge difference is as follows:

[0061] (2),

[0062] 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;

[0063] The linear region criterion is as follows:

[0064] (3),

[0065] 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;

[0066] Step 4: Check whether the differential current meets the saturation region criterion, and then open or close the differential protection.

[0067] 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.

[0068] The saturation region criterion is as follows:

[0069] (4),

[0070] 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 value that avoids the maximum unbalanced current during normal operation.

[0071] This invention addresses the problem of line differential protection maloperation caused by the saturation of the non-periodic component in the primary current of current transformers. By using the alternating characteristics of the saturation region and the linear region during external faults as a criterion, it effectively avoids the maloperation of current transformers due to saturation, achieves accurate identification of the saturation state of current transformers, and improves the safe and stable operation level of the power system.

[0072] This invention uses the alternating characteristics of the saturation and linear regions of a current transformer during an external fault as the core criterion. It achieves saturation identification through a continuous process of data acquisition, protection activation judgment, calculation of accumulated differential charge, and verification of alternating characteristics. The specific working principle is as follows:

[0073] First, continuous data acquisition and monitoring are performed to obtain real-time secondary current data on both sides of the adjacent protected lines of the transformer, providing a basic data source for subsequent analysis. Then, the differential current is calculated to determine whether the differential protection has been activated. If the threshold is not reached, data acquisition continues; if the threshold is reached, the next step of analysis is initiated.

[0074] After protection is activated, the accumulated differential charge is calculated. This accumulated differential charge quantifies the cumulative effect of the differential current, replacing the traditional amplitude threshold judgment. This method better aligns with the saturation recovery characteristics of the current transformer. Simultaneously, the sliding window integration process acts as a low-pass filter, effectively suppressing high-frequency noise interference. Next, it is determined whether the accumulated differential charge meets the linear region criterion. If it does, it proves that the current transformer has entered the linear region. To avoid false blocking based on a single criterion and enhance anti-interference capabilities, an alternating characteristic verification mechanism is introduced: when the linear region is detected, a timer is started, and the saturation region characteristics are checked within a subsequently set time period.

[0075] If both the accumulated differential charge criterion (linear region criterion, indicating the existence of a linear region) and the saturation region criterion (a saturation region following a linear region) are simultaneously met, the current transformer is determined to be saturated, and the differential protection is immediately blocked to prevent maloperation. If the criteria are not met, the protection is promptly opened to ensure reliable operation of the protection device during a fault. This process utilizes the difference between the alternating characteristics of the "linear region-saturation region" during saturation outside the zone and the absence of this alternation during faults within the zone, thus achieving accurate identification of the saturation state.

[0076] Example 1

[0077] 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:

[0078] Step 1: Collect current data;

[0079] Step 2: Calculate the differential current to determine whether to activate the differential protection.

[0080] Step 3: Calculate the accumulated charge difference and determine whether the accumulated charge difference satisfies the linear region criterion;

[0081] Step 4: Check whether the differential current meets the saturation zone criterion, and then open or close the differential protection.

[0082] Example 2

[0083] 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:

[0084] Step 1: Collect current data;

[0085] Collect secondary current data on both sides of the protected line;

[0086] Step 2: Calculate the differential current to determine whether to activate the differential protection.

[0087] Step 3: Calculate the accumulated charge difference and determine whether the accumulated charge difference satisfies the linear region criterion;

[0088] Step 4: Check whether the differential current meets the saturation zone criterion, and then open or close the differential protection.

[0089] Example 3

[0090] 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:

[0091] Step 1: Collect current data;

[0092] Step 2: Calculate the differential current to determine whether to activate the differential protection.

[0093] 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.

[0094] The startup requirements are as follows:

[0095] (1),

[0096] 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;

[0097] Step 3: Calculate the accumulated charge difference and determine whether the accumulated charge difference satisfies the linear region criterion;

[0098] Step 4: Check whether the differential current meets the saturation zone criterion, and then open or close the differential protection.

[0099] Example 4

[0100] 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:

[0101] Step 1: Collect current data;

[0102] Step 2: Calculate the differential current to determine whether to activate the differential protection.

[0103] Step 3: Calculate the accumulated charge difference and determine whether the accumulated charge difference satisfies the linear region criterion;

[0104] 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.

[0105] The formula for calculating the accumulated charge difference is as follows:

[0106] (2),

[0107] 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;

[0108] Step 4: Check whether the differential current meets the saturation zone criterion, and then open or close the differential protection.

[0109] Example 5

[0110] 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:

[0111] Step 1: Collect current data;

[0112] Step 2: Calculate the differential current to determine whether to activate the differential protection.

[0113] Step 3: Calculate the accumulated charge difference and determine whether the accumulated charge difference satisfies the linear region criterion;

[0114] 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.

[0115] The linear region criterion is as follows:

[0116] (3),

[0117] 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;

[0118] The accumulated differential charge is obtained by integrating the absolute value of the differential current through a point-by-point sliding window integral.

[0119] Step 4: Check whether the differential current meets the saturation zone criterion, and then open or close the differential protection.

[0120] Example 6

[0121] 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:

[0122] Step 1: Collect current data;

[0123] Step 2: Calculate the differential current to determine whether to activate the differential protection.

[0124] Step 3: Calculate the accumulated charge difference and determine whether the accumulated charge difference satisfies the linear region criterion;

[0125] Step 4: Check whether the differential current meets the saturation region criterion, and then open or close the differential protection.

[0126] 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.

[0127] The saturation region criterion is as follows:

[0128] (4),

[0129] 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 value that avoids the maximum unbalanced current during normal operation.

[0130] This invention takes a 110kV distribution line as an example, and establishes a simulation model using the electromagnetic transient simulation software PSCAD (Power Systems Computer Aided Design). Figure 2 The simulation model shown primarily studies a 110kV / 10kV transformer with saturation characteristics and its adjacent primary side lines. Transformer and line parameters are shown in Tables 1 and 2. The active load per phase is 1MW (approximately no-load to ensure normal model operation), the system frequency is 50Hz, and the overhead line length is 8km.

[0131] Table 1 Parameters of 110kV Overhead Line

[0132]

[0133] Table 2 Parameters of SF10-50000 / 110 Transformer

[0134]

[0135] The specific simulation results are explained below:

[0136] Time window Take 3ms. Take 0.6I n The accumulated differential charge setting value is 0.047 A·s, where I nThis is the rated current of the current transformer.

[0137] When an external fault occurs and the current transformer is not saturated, the differential protection will not be activated.

[0138] When an external fault occurs, the current transformer saturation status is as follows: Figures 3 to 6 As shown, it can be seen that the differential current exists in a linear region. The current in the linear region is almost zero, and the corresponding accumulated differential charge is significantly below 0.047 A·s, thus the protection is blocked.

[0139] When a fault occurs within the affected area, the current transformer may be unsaturated, as follows: Figures 7 to 10 As shown, it can be seen that there is no linear region for the differential current, so the minimum accumulated differential charge is 0.5 A·s, and the protection is activated.

[0140] When a fault occurs within the affected area, and the current transformer becomes saturated, such as... Figures 11 to 14 As shown, it can be seen that there is no linear region for the differential current at this time, so the minimum accumulated differential charge is 0.1 A·s, and the protection is open.

[0141] When the inrush current flows and the current transformer is saturated, such as Figures 15 to 18 As shown, it can be seen that the differential current exists in a linear region. The current in the linear region is almost zero, and the corresponding accumulated differential charge is below 0.047 A·s, which triggers the protection lockout.

[0142] Based on the above analysis and Table 3, it can be seen that the present invention can promptly activate the differential protection for faults within the zone, while avoiding maloperation of the differential protection caused by inrush current and fault current outside the zone, which fully demonstrates the effectiveness of the present invention.

[0143] Table 3 Fault simulation results

[0144]

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 method for improving the accuracy of differential protection. Step one, collecting current data; Step two, calculating differential current and determining whether to start differential protection; Calculating differential current, if the differential current reaches the starting threshold, starting differential protection, and executing step three; If the differential current does not reach the starting threshold, executing step one; Step three, calculating accumulated differential charge and determining whether the accumulated differential charge meets the linear region criterion; Calculating accumulated differential charge, if the accumulated differential charge meets the linear region criterion at any time after the differential protection is started, executing step four; if the accumulated differential charge does not meet the linear region criterion, opening the differential protection; The calculation formula of the accumulated differential charge is 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; Step four, detecting whether the differential current meets the saturation region criterion, opening the differential protection or blocking; Detecting whether the differential current meets the saturation region criterion within 5 ms after entering the linear region, if the differential current meets the saturation region criterion, blocking the differential protection; if the differential current does not meet the saturation region criterion, opening the differential protection.

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

3. The method according to claim 1, wherein the method is characterized by, The starting threshold is 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.

4. The method according to claim 1, wherein, The linear region criterion is 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.

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

6. The method according to claim 1, wherein The saturation region 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, i.e., the maximum unbalanced current during normal operation.

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