Converter transformer saturation protection method and device based on small-difference differential-flow braking

Abstract

The invention discloses a converter transformer saturation protection method and device based on small difference current braking, and the method comprises the steps: collecting the network side neutral point zero-sequence current and head end current of a converter transformer, and the valve side head end current of the converter transformer, so as to calculate the periodic peak value of the neutral point zero-sequence current and the instantaneous value of the maximum absolute value of the three-phase small difference current of the converter transformer in a unit period; judging whether a saturation protection criterion is met or not, and if yes, constructing a small-difference differential current braking criterion which comprises a first criterion, setting according to a mean value of a neutral point zero-sequence current period peak value and a second criterion, setting according to a mean value of a neutral point zero-sequence current period peak value; setting according to the absolute value of the difference between any two phases in the instantaneous values when the absolute value of the three-phase small difference current of the converter transformer in the unit period is maximum; when the first criterion and the second criterion are met at the same time, the saturation protection is locked after time delay, and the saturation protection timing is stopped, otherwise, the saturation protection is opened, and the action timing is normally carried out. According to the invention, maloperation can be effectively prevented when excitation inrush current exists and distortion current caused by direct current magnetic bias does not exist.

Description

Technical Field

[0001] The present invention belongs to the technical field of direct current transmission converter transformer protection, and more specifically, relates to a converter transformer saturation protection method and device based on small differential current braking. Background Art

[0002] In order to reduce the requirements for equipment insulation level and the level of transient overvoltage in non-fault phases, the converter transformer of the high-voltage direct current transmission system is grounded through the grid-side neutral point, which is grounded together with the DC system grounding electrode. When the grounding electrode flows with DC current for a long time or the trigger angle of the converter valve is unbalanced, the DC current will flow into the grid-side neutral point of the converter transformer through the grounding electrode, generating asymmetric magnetic flux, causing distorted current to flow into the converter transformer, resulting in an increase in core temperature and loss, and even burning the equipment in severe cases. Therefore, high-voltage direct current is usually equipped with saturation protection, detecting the neutral point zero-sequence current, calculating its peak current equivalent to the DC bias current, using the inverse time integral algorithm, corresponding to the overexcitation curve provided by the converter transformer manufacturer, and the protection action when the fixed value is met.

[0003] According to statistics, in recent years, saturation protection has occurred in many converter stations when converter transformers are charged. This is because the excitation surge current flows through the neutral point of the converter transformer, and most zero-sequence current transformers are AC current transformers. When the excitation surge current is large, the neutral point current distortion caused by the excessive excitation surge current cannot be accurately identified. When the attenuation is slow, it is easy to meet the overexcitation curve of the converter transformer, causing the saturation protection to be mistakenly identified as a distorted current caused by DC bias magnetization, resulting in protection action, affecting equipment safety assessment and operation and maintenance efficiency. Similarly, when the adjacent converter transformer is charged, it will cause the operating converter transformer to generate a response surge current. The attenuation rate of the response surge current is affected by the system resistance, etc. When the system resistance is small, it causes a slow attenuation, which is easy to meet the overexcitation curve of the converter transformer, causing the saturation protection to be mistakenly identified as a distorted current caused by DC bias magnetization, resulting in protection action.

[0004] Authorized prior art document 1 (CN109103847B) discloses a converter transformer saturation protection method based on DC component estimation, which has the disadvantage that it can only avoid saturation protection misoperation caused by excitation inrush current, and has no specificity for response inrush current. Summary of the invention

[0005] In order to solve the deficiencies in the prior art, the present invention provides a saturation protection method and device for commutation transformer based on small differential current braking of commutation transformer. When there is excitation surge current or response surge current in the converter transformer and there is no distorted current due to DC bias magnetization, the small differential current braking criterion is effectively opened to prevent false operation.

[0006] The present invention adopts the following technical solution.

[0007] A first aspect of the present invention provides a commutation variable saturation protection method based on small differential current braking, characterized by comprising:

[0008] Collect the neutral point zero-sequence current on the grid side of the converter transformer, the grid side head-end current of the converter transformer, and the valve side head-end current of the converter transformer to calculate the neutral point zero-sequence current cycle peak value I fz0 and the instantaneous value I when the absolute value of the three-phase small differential current of the converter transformer is the maximum within a unit period diff_aM ,I diff_bM ,I diff_cM ;

[0009] According to the peak value of the neutral point zero-sequence current cycle I fz0 , judge whether the saturation protection criterion is met. If the saturation protection criterion is met, a small differential current braking criterion including a first criterion and a second criterion is constructed, wherein the first criterion is based on the peak value of the neutral point zero-sequence current cycle I within the set period. fz0 The second criterion is based on the instantaneous value I of the three-phase small differential current of the converter transformer when the absolute value of the three-phase small differential current is the largest within the unit period. diff_aM ,I diff_bM ,I diff_cM The absolute value of the difference between any two phases in the circuit is used for setting; if the saturation protection criterion is not met, the data is collected again;

[0010] When the first criterion and the second criterion are met at the same time, the saturation protection is locked after a delay and the saturation protection timing is stopped. Otherwise, the saturation protection is opened and the action timing is carried out normally.

[0011] Preferably, the neutral point zero-sequence current peak value I is calculated fz0 is the maximum value of the neutral point zero-sequence current within the power frequency cycle, and the three-phase small differential current of the converter transformer is calculated as the difference between the current at the first end of the converter transformer grid side and the current at the first end of the converter transformer valve side.

[0012] Preferably, the saturation protection criterion includes:

[0013] The neutral point zero-sequence current cycle peak is equivalent to the DC bias current, and the inverse time integration algorithm is used. When the time accumulation corresponding to the neutral point zero-sequence current cycle peak meets the time corresponding to the overexcitation curve, the saturation protection is activated.

[0014] Preferably, the first criterion includes: calculating the neutral point zero-sequence current cycle peak value I within six cycles according to the neutral point zero-sequence current sampling value. fz0 The mean of , and judge whether it is greater than the lower limit of the inverse time limit, expressed as the following formula,

[0015]

[0016] Where:

[0017] I fz0 is the peak value of the neutral point zero-sequence current cycle;

[0018] I set is the lower limit value of the inverse time limit;

[0019] t is the moment when the first peak current is generated;

[0020] Δt is the unit delay.

[0021] Preferably, the unit delay Δt is 20 ms.

[0022] Preferably, the second criterion includes: when one of the following three conditions is met, it is considered that the excitation inrush current or the response inrush current occurs,

[0023]

[0024] Where:

[0025] I diff_aM ,I diff_bM ,I diff_cM They are the instantaneous values ​​of the three-phase small differential currents of the commutation transformers A, B, and C when their absolute values ​​are the largest within a unit period;

[0026] I set1 is the first fixed value;

[0027] I set2 is the second fixed value;

[0028] I set3 The third fixed value.

[0029] Preferably, the first fixed value is set to I diff_aM with I diff_bM The absolute maximum value, the second constant I set2 Set to I diff_bM with I diff_cM The absolute maximum value, the third fixed value I set3 Set to I diff_cM with I diff_aM The absolute maximum value of .

[0030] Preferably, when the first criterion and the second criterion are satisfied simultaneously, the delay is set to be greater than 120 ms.

[0031] A second aspect of the present invention provides a commutation saturation protection device based on small differential current braking, and the commutation saturation protection method based on small differential current braking comprises:

[0032] A signal acquisition module, used to collect the zero-sequence current of the neutral point on the grid side of the converter transformer, the current at the first end on the grid side of the converter transformer, and the current at the first end on the valve side of the converter transformer;

[0033] Signal extraction module, used to calculate the peak value of the neutral point zero-sequence current cycle I fz0 , the instantaneous value I when the absolute value of the three-phase small differential current of the converter transformer is the largest within a unit period diff_aM ,I diff_bM ,I diff_cM ;

[0034] Saturation protection module is used to protect the neutral point zero-sequence current according to the cycle peak value I fz0 , determine whether the saturation protection criterion is met;

[0035] Differential current braking module, used to determine the peak value of the neutral point zero-sequence current cycle I fz0 and the instantaneous value I when the absolute value of the three-phase small differential current of the converter transformer is the maximum within a unit period diff_aM ,I diff_bM ,I diff_cM Check whether the small differential current braking criterion is met. If so, the saturation protection is locked and the saturation protection timing is stopped.

[0036] Preferably, in the differential current braking module, the small differential current braking criterion set is specifically as follows:

[0037]

[0038] Where:

[0039] I set is the lower limit value of the inverse time limit;

[0040] t is the moment when the first peak current is generated;

[0041] Δt is the unit delay;

[0042] I set1 is the first constant, set to I diff_aM with I diff_bM The absolute maximum value of

[0043] I set2 is the second fixed value, set to I diff_bM with I diff_cM The absolute maximum value of

[0044] I set3 is the third fixed value, set to I diff_cM with I diff_aM The absolute maximum value of .

[0045] A third aspect of the present invention provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the commutation variable saturation protection method based on small differential current braking.

[0046] A fourth aspect of the present invention provides a computer device / equipment / system, comprising:

[0047] Memory, for storing computer programs / instructions;

[0048] A processor is used to execute the computer program / instructions to implement the steps of the commutation variable saturation protection method based on small differential current braking.

[0049] A fifth aspect of the present invention provides a computer program product, comprising a computer program / instruction, which, when executed by a processor, implements the steps of the commutation variable saturation protection method based on small differential current braking.

[0050] Compared with the prior art, the beneficial effects of the present invention include at least: the converter transformer saturation protection method and device based on small differential current braking of the present invention can effectively identify the excitation surge current, the response surge current and the distorted current caused by DC bias magnetization. When there is an excitation surge current in the converter transformer and there is no distorted current caused by DC bias magnetization, the small differential current braking criterion is effectively utilized to prevent false operation and ensure the safe and stable operation and operation and maintenance efficiency of the DC system. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 It is a flow chart of a commutation saturation protection method based on commutation small differential current braking provided by an embodiment of the present invention;

[0052] Figure 2 It is a schematic diagram of a small differential flow braking criterion provided by an embodiment of the present invention;

[0053] Figure 3 It is a schematic diagram of the commutation current variation difference current and the neutral point distortion current when the excitation inrush current is generated provided by an embodiment of the present invention;

[0054] Figure 4 It is a schematic diagram of the commutation current becoming small differential current and neutral point distortion current when the inrush current is generated provided by an embodiment of the present invention;

[0055] Figure 5 It is a schematic diagram of commutation difference current and neutral point distortion current when DC bias magnetization is generated provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0056] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The embodiments described in this application are only part of the embodiments of the present invention, not all of them. Based on the spirit of the present invention, other embodiments obtained by ordinary technicians in this field without creative work are all within the scope of protection of the present invention.

[0057] The present invention provides a method for preventing the saturation protection of a converter transformer from malfunctioning when there is an excitation surge current and a response surge current. A small differential current braking criterion is added to the original converter transformer saturation protection criterion. When any two-phase differential currents at the grid-side head end and the valve-side head end of the converter transformer meet the differential current magnitude relationship, the saturation protection is locked and the saturation protection timing is stopped. The present invention identifies the excitation surge current, the response surge current and the distorted current caused by DC bias magnetization through the converter transformer small differential current; calculates the periodic mean of the neutral point zero-sequence peak current to determine whether the method has the triggering conditions; calculates the difference between the instantaneous values ​​of any two-phase small differential currents at the maximum amplitude within a unit period, and determines it as an excitation surge current or a response surge current if the differential current magnitude relationship is met, otherwise it is a DC bias magnetization.

[0058] like Figure 1 As shown, embodiment 1 of the present invention provides a commutation saturation protection method based on commutation small differential current braking, comprising the following steps:

[0059] Step 1, collecting the zero-sequence current of the neutral point on the grid side of the converter transformer, the current at the first end on the grid side of the converter transformer, and the current at the first end on the valve side of the converter transformer;

[0060] Step 2: Calculate the neutral point zero-sequence current cycle peak value I according to the neutral point zero-sequence current, the grid-side head-end current of the converter transformer, and the valve-side head-end current of the converter transformer collected in step 1. fz0 , the instantaneous value I when the absolute value of the three-phase small differential current of the converter transformer is the largest within a unit period diff aM ,I diff bM ,I diff cM .

[0061] In a preferred but non-restrictive embodiment of the present invention, the neutral point zero-sequence current peak value is the maximum value of the neutral point zero-sequence current within the power frequency cycle (20ms), and the three-phase small differential current of the converter transformer is the difference between the current at the first end of the converter transformer grid side and the current at the first end of the converter transformer valve side.

[0062] Step 3: According to the neutral point zero-sequence current cycle peak value I in step 2 fz0 , determine whether the saturation protection criterion is met. If the saturation protection criterion is met, continue to step 4. If the saturation protection criterion is not met, return to step 1 to re-collect data.

[0063] Preferably, the saturation protection criterion specifically includes: converting the neutral point zero-sequence current cycle peak extracted in step 2 into a DC bias current, using an inverse time integration algorithm, and when the time accumulation corresponding to the neutral point zero-sequence current cycle peak meets the time corresponding to the overexcitation curve, the saturation protection is activated.

[0064] Further preferably, the overexcitation curve is provided by a converter transformer manufacturer.

[0065] Step 4: construct a small differential flow braking criterion, such as Figure 2 As shown, the small differential current braking criterion includes a first criterion and a second criterion. When the first criterion and the second criterion are met at the same time, it is determined that there is an excitation surge current or a response surge current in the neutral point zero-sequence current of the converter transformer but there is no distorted current caused by DC bias. After a delay, the saturation protection is locked and the saturation protection timing is stopped. When any condition is not met or none of them are met, it is determined that the converter transformer is saturated due to DC bias, the saturation protection is opened, and the action timing is performed normally.

[0066] The first criterion is based on the peak value of the neutral point zero-sequence current cycle I within the set period. fz0 Preferably, the peak value of the neutral point zero-sequence current cycle I within six cycles is calculated based on the neutral point zero-sequence current sampling value. fz0 The first criterion is expressed as follows:

[0067]

[0068] Where:

[0069] I fz0 is the peak value of the neutral point zero-sequence current cycle;

[0070] I set is the lower limit value of the inverse time limit;

[0071] t is the moment when the first peak current is generated;

[0072] Δt is the unit delay, usually 20ms.

[0073] The second criterion is based on the instantaneous value I of the three-phase small differential current of the converter transformer when the absolute value of the three-phase small differential current is the largest within the unit period. diff_aM ,I diff_bM ,I diff_cM The absolute value of the difference between any two phases in is set. Preferably, when the three conditions (①I diff_aM with I diff_bM The absolute value of the difference is greater than 0.5 times I set1 ; ②I diff_bM with I diff_cM The absolute value of the difference is greater than 0.5 times I set2 ; ③I diff_cM with I diff_aM The absolute value of the difference is greater than 0.5 times I set3 ) is considered to occur. The second criterion is expressed by the following formula:

[0074]

[0075] Where:

[0076] I diff_aM ,I diff_bM ,I diff_cM They are the instantaneous values ​​of the three-phase small differential currents of the commutation transformers A, B, and C when their absolute values ​​are the largest within a unit period;

[0077] I set1 is the first fixed value;

[0078] I set2 is the second fixed value;

[0079] I set3 The third fixed value.

[0080] Further preferably, the first constant value I set1 Set to I diff_aM with I diff_bM The absolute maximum value, the second constant I set2 Set to I diff_bM with I diff_cM The absolute maximum value, the third fixed value I set3 Set to I diff_cM with I diff_aM The absolute maximum value of .

[0081] Preferably, when the first criterion and the second criterion are satisfied simultaneously, the delay is set to be greater than 120 ms.

[0082] It can be understood that the added commutation small differential current criterion extracts the waveform characteristics of the commutation small differential current and utilizes the characteristic that the three-phase small differential currents of DC bias are all biased to the same side of the time axis to effectively distinguish between the excitation surge current, the response surge current and the distorted current caused by DC bias, so as to avoid the false operation of the saturation protection caused by the excitation and response surge current. When all the criterion conditions are met, the saturation protection action timing is stopped. When any condition is not met, the saturation protection is opened and the action timing is carried out normally. The small differential current braking criterion is only used as an auxiliary opening condition of the saturation protection, does not change the original saturation protection logic, and is easy to implement.

[0083] Embodiment 2 of the present invention provides a commutation saturation protection device based on small differential current braking, and the commutation saturation protection method based on small differential current braking as described in Embodiment 1 is executed, including:

[0084] A signal acquisition module, used to collect the zero-sequence current of the neutral point on the grid side of the converter transformer, the current at the first end on the grid side of the converter transformer, and the current at the first end on the valve side of the converter transformer;

[0085] Signal extraction module, used to calculate the peak value of the neutral point zero-sequence current cycle I fz0, the instantaneous value I when the absolute value of the three-phase small differential current of the converter transformer is the largest within a unit period diff_aM ,I diff_bM ,I diff_cM ;

[0086] Saturation protection module is used to protect the neutral point zero-sequence current according to the cycle peak value I fz0 , determine whether the saturation protection criterion is met;

[0087] Differential current braking module, used to determine the peak value of the neutral point zero-sequence current cycle I fz0 and the instantaneous value I when the absolute value of the three-phase small differential current of the converter transformer is the maximum within a unit period diff_aM ,I diff_bM ,I diff_cM Check whether the small differential current braking criterion is met. If so, the saturation protection is locked and the saturation protection timing is stopped.

[0088] Preferably, in the differential current braking module, the small differential current braking criterion set is specifically as follows:

[0089]

[0090] Where:

[0091] I set is the lower limit value of the inverse time limit;

[0092] t is the moment when the first peak current is generated;

[0093] Δt is the unit delay, generally 20ms;

[0094] I set1 is the first constant, set to I diff_aM with I diff_bM The absolute maximum value of

[0095] I set2 is the second fixed value, set to I diff_bM with I diff_cM The absolute maximum value of

[0096] I set3 is the third fixed value, set to I diff_cM with I diff_aM The absolute maximum value of .

[0097] Embodiment 3 of the present invention provides a computer-readable storage medium on which a computer program is stored. When the program is executed by a processor, the steps of the method described in Embodiment 1 are implemented.

[0098] Embodiment 4 of the present invention provides a computer device / equipment / system, including:

[0099] Memory, for storing computer programs / instructions;

[0100] A processor is used to execute the computer program / instructions to implement the steps of the method described in Example 1.

[0101] Embodiment 5 of the present invention provides a computer program product, including a computer program / instruction, which implements the steps of the method described in Embodiment 1 when executed by a processor.

[0102] Application examples:

[0103] In order to further clearly introduce the technical solution of the present invention and the beneficial technical effects brought by it, the following application examples are introduced. According to RTDS simulation and engineering site data and waveforms, the current characteristics of the converter transformer when the excitation surge current, response surge current and distorted current when DC bias occurs have many similarities, such as non-periodic components and discontinuous characteristics, but there are obvious differences in the extraction of the three-phase small differential current of the converter transformer. Therefore, the converter transformer saturation protection method based on the converter transformer small differential current braking provided in Example 1 is applied to analyze the three-phase small differential current of the converter transformer to find out the differences, which can realize the identification of the excitation surge current, response surge current and distorted current caused by DC bias.

[0104] like Figure 3 As shown in (a), for a three-phase converter transformer, due to its phase difference of 120°, the excitation inrush current of each phase is not exactly the same, and the maximum inrush current value occurs when the voltage passes through zero when the converter transformer is put into operation. By calculating the three-phase small differential current of the converter transformer, the three-phase small differential current in the excitation inrush current is biased toward one side of the time axis. There may be only one or two phases with a large differential current (the differential current value that meets the lower limit value of the saturation protection inverse time limit). When there is a large differential current in both phases, the two-phase differential currents are biased toward one side of the time axis, meeting the small differential current braking criterion. Figure 3 As shown in (b), when there are large differential currents in the three phases, with phase A biased to one side of the time axis and phases B and C biased to the other side of the time axis, the small differential current braking criterion is still met.

[0105] like Figure 4 As shown in (a), when the surge current occurs, only phase B has a large differential current among the three-phase small differential currents, and the differential currents of the other two phases are very small, which meets the small differential current braking criterion; Figure 4 As shown in (b), when the surge current occurs, there is a large differential current between phase A and phase C in the three-phase small differential current, and the differential currents of the two phases are biased to one side of the time axis, satisfying the small differential current braking criterion.

[0106] like Figure 5 As shown, the distorted current that occurs during DC bias magnetization also has three-phase small differential currents that are biased to one side, but the three-phase small differential currents are all biased to the same side of the time axis, and the amplitudes of the three-phase differential currents are basically the same, which does not meet the small differential current braking criterion.

[0107] It can be understood that by calculating the average value of the zero-sequence peak current to meet the lower limit value of the saturation protection as the trigger condition, the small differential current braking criterion can be used to effectively identify the excitation surge current, response surge current and distorted current caused by DC bias magnetization in the zero-sequence current. When the criterion is met, the saturation protection is locked to prevent it from malfunctioning due to the excitation surge current or response surge current.

[0108] Compared with the prior art, the beneficial effects of the present invention include at least: the converter transformer saturation protection method and device based on small differential current braking of the present invention can effectively identify the excitation surge current, the response surge current and the distorted current caused by DC bias magnetization. When there is an excitation surge current in the converter transformer and there is no distorted current caused by DC bias magnetization, the small differential current braking criterion is effectively utilized to prevent false operation and ensure the safe and stable operation and operation and maintenance efficiency of the DC system.

[0109] The present disclosure may be a system, a method and / or a computer program product. The computer program product may include a computer-readable storage medium carrying computer-readable program instructions for causing a processor to implement various aspects of the present disclosure.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents, and any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A commutation variable saturation protection method based on small differential current braking, characterized in that: include: Collect the neutral point zero-sequence current on the grid side of the converter transformer, the grid side head-end current of the converter transformer, and the valve side head-end current of the converter transformer to calculate the neutral point zero-sequence current cycle peak value I fz0 and the instantaneous value I when the absolute value of the three-phase small differential current of the converter transformer is the maximum within a unit period diff_aM ,I diff_bM ,I diff_cM ; According to the peak value of the neutral point zero-sequence current cycle I fz0 , judge whether the saturation protection criterion is met. If the saturation protection criterion is met, a small differential current braking criterion including a first criterion and a second criterion is constructed, wherein the first criterion is based on the peak value of the neutral point zero-sequence current cycle I within the set period. fz0 The second criterion is based on the instantaneous value I of the three-phase small differential current of the converter transformer when the absolute value of the three-phase small differential current is the largest within the unit period. diff_aM ,I diff_bM ,I diff_cM The absolute value of the difference between any two phases in the circuit is used for setting; if the saturation protection criterion is not met, the data is collected again; When the first criterion and the second criterion are met at the same time, the saturation protection is locked after a delay and the saturation protection timing is stopped. Otherwise, the saturation protection is opened and the action timing is carried out normally.

2. The commutation variable saturation protection method based on small differential current braking according to claim 1 is characterized in that: Calculate the neutral point zero sequence current peak value I fz0 is the maximum value of the neutral point zero-sequence current within the power frequency cycle, and the three-phase small differential current of the converter transformer is calculated as the difference between the current at the first end of the converter transformer grid side and the current at the first end of the converter transformer valve side.

3. The commutation variable saturation protection method based on small differential current braking according to claim 1 is characterized in that: Saturation protection criteria include: The neutral point zero-sequence current cycle peak is equivalent to the DC bias current, and the inverse time integration algorithm is used. When the time accumulation corresponding to the neutral point zero-sequence current cycle peak meets the time corresponding to the overexcitation curve, the saturation protection is activated.

4. The commutation variable saturation protection method based on small differential current braking according to claim 1 is characterized in that: The first criterion includes: calculating the neutral point zero-sequence current cycle peak value I within six cycles according to the neutral point zero-sequence current sampling value fz0 The mean of , and judge whether it is greater than the lower limit of the inverse time limit, expressed as the following formula, Where: I fz0 is the peak value of the neutral point zero-sequence current cycle; I set is the lower limit value of the inverse time limit; t is the moment when the first peak current is generated; Δt is the unit delay.

5. The commutation variable saturation protection method based on small differential current braking according to claim 4 is characterized in that: The unit delay Δt is taken as 20ms.

6. The commutation variable saturation protection method based on small differential current braking according to claim 1 is characterized in that: The second criterion includes: when one of the following three conditions is met, it is considered that the excitation inrush current or the response inrush current occurs, Where: I diff_aM ,I diff_bM ,I diff_cM They are the instantaneous values ​​of the three-phase small differential currents of the commutation transformers A, B, and C when their absolute values ​​are the largest within a unit period; I set1 is the first fixed value; I set2 is the second fixed value; I set3 The third fixed value.

7. The commutation variable saturation protection method based on small differential current braking according to claim 6 is characterized in that: The first constant is set to I diff_aM with I diff_bM The absolute maximum value, the second fixed value I set2 Set to I diff_bM with I diff_cM The absolute maximum value, the third fixed value I set3 Set to I diff_cM with I diff_aM The absolute maximum value of .

8. The commutation variable saturation protection method based on small differential current braking according to claim 1 is characterized in that: When the first criterion and the second criterion are satisfied simultaneously, the delay is set to be greater than 120 ms.

9. A commutation saturation protection device based on small differential current braking, running the commutation saturation protection method based on small differential current braking as described in any one of claims 1 to 8, characterized in that: include: A signal acquisition module, used to collect the zero-sequence current of the neutral point on the grid side of the converter transformer, the current at the first end on the grid side of the converter transformer, and the current at the first end on the valve side of the converter transformer; Signal extraction module, used to calculate the peak value of the neutral point zero-sequence current cycle I fz0 , the instantaneous value I when the absolute value of the three-phase small differential current of the converter transformer is the largest within a unit period diff_aM ,I diff_bM ,I diff_cM ; Saturation protection module is used to protect the neutral point zero-sequence current according to the cycle peak value I fz0 , determine whether the saturation protection criterion is met; Differential current braking module, used to determine the peak value of the neutral point zero-sequence current cycle I fz0 and the instantaneous value I when the absolute value of the three-phase small differential current of the converter transformer is the maximum within a unit period diff_aM ,I diff_bM ,I diff_cM Check whether the small differential current braking criterion is met. If so, the saturation protection is locked and the saturation protection timing is stopped.

10. The commutation variable saturation protection device based on small differential current braking according to claim 9 is characterized in that: In the differential current braking module, the small differential current braking criteria set are as follows: Where: I set is the lower limit value of the inverse time limit; t is the moment when the first peak current is generated; Δt is the unit delay; I set1 is the first constant, set to I diff_aM with I diff_bM The absolute maximum value of I set2 is the second fixed value, set to I diff_bM with I diff_cM The absolute maximum value of I set3 is the third fixed value, set to I diff_cM with I diff_aM The absolute maximum value of .

11. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps of the method described in any one of claims 1 to 8 are implemented.

12. A computer device / equipment / system, characterized in that: include: Memory, for storing computer programs / instructions; A processor, configured to execute the computer program / instructions to implement the steps of the method according to any one of claims 1 to 8.

13. A computer program product comprising a computer program / instructions, characterized in that: When the computer program / instructions are executed by a processor, the steps of the method according to any one of claims 1 to 8 are implemented.

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