Adaptive out-of-zone CT saturation determination method for busbar CT disconnection

Abstract

The invention is an adaptive CT saturation judgment method applicable to the case where the busbar CT is disconnected, which collects the busbar voltage and the current of each branch connected to the busbar; calculates the differential current and the braking current at the sampling point; and judges whether the busbar CT is disconnected. If the busbar CT is not disconnected, the saturation of the out-of-zone fault CT is identified by the sequence of the sudden changes of the small differential current and the small differential braking current sampling points, and the differential protection of the faulty busbar section is locked; if the busbar CT is disconnected, the sequence of the sudden changes of the large differential current and the large differential braking current sampling points is used to identify whether an out-of-zone fault occurs; the busbar section that needs to be locked is determined by the sampling point change rate of the small differential current of the two busbar sections connected to the busbar at the time of the fault. The present invention is applicable to busbar main connection modes with busbars, such as double busbars, double busbars with double sections, and double busbars with single sections.

Description

Technical Field

[0001] The present invention belongs to the field of electric power systems, and more specifically, relates to an adaptive out-of-zone CT saturation determination method applicable to a busbar CT disconnection situation. Background Art

[0002] Since the busbar plays the role of collecting and distributing electric energy in the power system, busbar protection plays a very important role in the field of relay protection of the power system. As the capacity of the power system increases, the losses caused by the destruction of system stability are also increasing. The action of busbar protection is bound to cause power outages in a large area. If the busbar fault in the hub substation cannot be removed in time, it may also cause an accident that destroys the stability of the power system. Therefore, setting up busbar protection with reliable action and good performance, so that it can quickly detect the location of the busbar fault and selectively remove the fault in time, is of great significance to maintaining the stability of the power system.

[0003] When a branch on the bus fails, the current of all branches on the bus flows to the fault point, which may cause serious CT saturation in the fault branch. In order to prevent the bus protection from malfunctioning due to CT saturation of the fault outside the bus zone, the CT saturation of the fault outside the zone is generally identified by distinguishing the waveform characteristics of the small differential current and the braking current, and the differential is locked. However, under abnormal operating conditions such as the bus CT disconnection, the bus small differential current and braking current are no longer reliable, and an automatic switching algorithm is required to ensure that the bus protection will not malfunction if the CT saturation of the fault outside the zone occurs under such abnormal conditions. Summary of the invention

[0004] In order to solve the deficiencies in the prior art, the object of the present invention is to provide an adaptive out-of-zone CT saturation judgment method suitable for the case of a busbar CT disconnection.

[0005] The present invention adopts the following technical solution.

[0006] The method for determining saturation of an out-of-zone CT when the busbar CT is disconnected comprises the following steps:

[0007] Step 1, collecting the bus voltage and the current of each branch connected to the bus;

[0008] Step 2, calculate the differential current and braking current of the sampling point, and determine whether the bus CT is disconnected; if the bus CT is not disconnected, execute step 3, otherwise execute step 4.

[0009] Step 3, calculate the small differential current mutation amount and the small differential braking current mutation amount, and judge whether the out-of-zone fault CT is saturated according to the sequence of the small differential current and small differential braking current sampling point mutations. If so, lock the differential protection of the faulty bus section, otherwise return to step 1;

[0010] Step 4, identifying whether CT saturation of an out-of-zone fault occurs by the sequence of mutations of the large differential current and the large differential braking current sampling points. If yes, proceed to step 5, otherwise return to step 1;

[0011] Step 5 determines the bus section that needs to be locked through the sampling point change rate of the small differential current of the two bus sections connected by the bus tie at the time of the fault.

[0012] The differential current includes a large differential current and a small differential current, wherein the large differential current is the sum of the currents of all branches on the two bus sections except the bus tie branch; the small differential current of each bus section is the sum of the currents of all branches on the bus section;

[0013] The braking current includes a large differential braking current and a small differential braking current, wherein the large differential braking current is the sum of the absolute values ​​of all branch currents on the two bus sections except the bus tie branch; the small differential current of each bus section is the sum of the absolute values ​​of all branch currents on the bus section;

[0014] The large difference differential current and small difference differential current are calculated as follows:

[0015]

[0016] The calculation method of large differential braking current and small differential braking current is as follows:

[0017]

[0018] In the formula,

[0019] I d is the differential current;

[0020] I f is the braking current;

[0021] I i is the i-th branch current;

[0022] n is the number of branches.

[0023] When all the following criteria are met, the busbar CT is judged to be disconnected after a set delay:

[0024] 1) Zero-sequence current appears on the busbar;

[0025] 2) The current of a certain phase is 0, and the large differential current of this phase is balanced; the small differential current of the two bus sections connected to this phase is unbalanced, and at the same time, the sum of the small differential currents of the two bus sections connected to this phase is balanced.

[0026] The calculation method of small difference differential current mutation and large difference differential current mutation is the same, and the calculation formula is as follows:

[0027] Δi d =i d (t)-i d (tT)

[0028] In the formula,

[0029] Δi d is the differential current mutation amount;

[0030] i d is the bus differential current;

[0031] t is the current time;

[0032] T is one power frequency cycle: 0.02s.

[0033] The calculation method of small differential braking current mutation is the same as that of large differential braking current mutation. The calculation formula is as follows:

[0034] Δi f =i f (t)-i f (tT)

[0035] In the formula,

[0036] Δi f is the braking current mutation amount;

[0037] i f is the small differential braking current;

[0038] t is the current time;

[0039] T is one power frequency cycle: 0.02s.

[0040] When the busbar CT is not disconnected, judging whether the out-of-zone fault CT is saturated includes the following steps:

[0041] S301, respectively counting the number of mutations M of the small differential current and the number of mutations N of the small differential braking current of each bus within a set time, where the mutation amount Δi of the small differential current is d >Iset1, it is recorded as a small differential current mutation in the bus section, where Iset1 is the small differential current setting value; if the mutation amount of the small differential braking current Δi f >Iset2, it is recorded as a sudden change of small differential braking current in the bus section, where Iset2 is the set value of small differential braking current;

[0042] S302, if for any bus, NM≥Z1, it is determined that the CT saturation occurs due to a fault outside the bus area, and the bus differential protection is locked; wherein Z1 is a constant, and the result of Z1 multiplied by the sampling interval time is greater than 2.5ms.

[0043] When identifying whether CT saturation occurs in an external fault under the condition of disconnection of the bus-tie CT, the following steps are included:

[0044] Step S401: Count the number of mutations Q of the differential current of the main differential and the number of mutations S of the restraint current of the main differential. Among them, if the mutation amount ΔI of the differential current of the main differential d > Iset3, it is recorded as one mutation of the differential current of the main differential, where Iset3 is the set value of the differential current of the main differential; if the mutation amount ΔI of the restraint current of the main differential f > Iset4, it is recorded as one mutation of the restraint current of the main differential, where Iset4 is the set value of the restraint current of the main differential;

[0045] Step S402: If S - Q ≥ Z within a set time, it is determined that CT saturation occurs in an external fault. Z1 is a constant, and Z1 multiplied by the sampling interval is greater than 2.5 ms.

[0046] Within a set time, the number of sampling points of bus I that satisfy the mutation of the change rate of the differential current of the branch differential is W, and the number of sampling points of bus II that satisfy the mutation of the change rate of the differential current of the branch differential is U. If W ≥ Z2 and U < Z2, where Z2 is a constant, it is considered that CT saturation occurs in an external fault on bus I, and the differential protection of bus I is blocked; if U ≥ Z2 and W < Z2, it is considered that CT saturation occurs in an external fault on bus II, and the differential protection of bus II is blocked.

[0047] The mutation condition of the change rate of the differential current of the branch differential is:

[0048] Δk d1 > Δk d2

[0049] Among them, Δk d1 is the ratio of the change rate of the current sampling point of the differential current of the branch differential of a section of the bus to the change rate of the previous sampling point;

[0050] Δk d2 is the ratio of the change rate of the previous sampling point of the differential current of the branch differential of a section of the bus to the change rate of the sampling points two before the previous one;

[0051] Δk d1 =((i d (t)-i d (t - Δt)) / (i d (t - Δt)-i d (t - 2Δt))

[0052] Δk d2 =((i d (t - Δt)-i d (t - 2Δt)) / (i d (t - 2Δt)-i d (t - 3Δt))

[0053] In the formula,

[0054] Δt is the sampling interval;

[0055] i d is the busbar small differential current.

[0056] The value range of the small difference differential current setting value Iset1, the small difference braking current setting value Iset2, the large difference differential current setting value Iset3 and the large difference braking current setting value Iset4 is 0.4 to 0.6 times the differential setting value.

[0057] The beneficial effect of the present invention is that, compared with the prior art, the present invention can automatically switch the judgment criteria when the CT saturation of the out-of-zone fault occurs in the bus with bus coupling and the bus coupling CT is abnormally disconnected. Even if the small differential current and the small differential braking current are unreliable, through the waveform characteristics of the large differential current and the large differential braking current, and the sampling point change rate of the small differential current of the two bus sections connected by the bus coupling, it is still possible to judge that the CT saturation of the out-of-zone fault of the bus can be determined, and the differential protection can be locked according to the bus section to avoid false operation of the bus protection, thereby improving the reliability of the bus protection action, avoiding false operation of the protection, and maintaining the stability of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 It is a flow chart of the adaptive out-of-zone CT saturation determination method applicable to the case of busbar CT disconnection of the present invention;

[0059] Figure 2 The figure is a schematic diagram of a double bus fault branch according to a specific embodiment of the present invention.

[0060] The markings in the figure are:

[0061] PT1-I bus voltage transformer 1;

[0062] PT2-II bus voltage transformer 2;

[0063] L and 2200-circuit breaker;

[0064] CT-current transformer;

[0065] Disconnector for 2G and 1G circuit breakers. DETAILED DESCRIPTION

[0066] The present application is further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present application.

[0067] The method for determining saturation of an out-of-zone CT when the busbar CT is disconnected comprises the following steps:

[0068] Step 1, collecting the bus voltage and the current of each branch connected to the bus;

[0069] Step 2, calculate the differential current and braking current of the sampling point, and determine whether the bus CT is disconnected; if the bus CT is not disconnected, execute step 3, otherwise execute step 4.

[0070] Step 3, calculate the small differential current mutation amount and the small differential braking current mutation amount, and judge whether the out-of-zone fault CT is saturated according to the sequence of the small differential current and small differential braking current sampling point mutations. If so, lock the differential protection of the faulty bus section, otherwise return to step 1;

[0071] Step 4, identifying whether CT saturation of an out-of-zone fault occurs by the sequence of mutations of the large differential current and the large differential braking current sampling points. If yes, proceed to step 5, otherwise return to step 1;

[0072] Step 5 determines the bus section that needs to be locked through the sampling point change rate of the small differential current of the two bus sections connected by the bus tie at the time of the fault.

[0073] This application is applicable to bus main connection modes with bus tie, such as double bus, double bus double segment, double bus single segment, etc. The embodiment takes double bus as an example. Figure 1 As shown in the flowchart of the adaptive CT saturation determination method applicable to the case of busbar CT disconnection, this embodiment includes the following steps:

[0074] Step 1: collecting the bus voltage through a voltage transformer and collecting the current of each branch connected to the bus through a current transformer;

[0075] Take double busbar as an example. Figure 2 As shown in the schematic diagram of the double bus fault branch, the secondary values ​​of PT1 and PT2 on the two bus sections, that is, the voltage of the two bus sections, are collected; the secondary values ​​of the current transformer CT on the two bus sections, that is, the current of each branch, are collected.

[0076] Step 2, calculate the differential current and braking current of the sampling point, and determine whether the bus CT is disconnected. If the bus CT is not disconnected, execute step 3; if the bus CT is disconnected, execute step 4;

[0077] Specifically, the differential current includes a large differential current and a small differential current; the braking current includes a large differential braking current and a small differential braking current. The large differential current is the sum of the currents of all branches on the two bus sections except the bus coupling branch, and the large differential braking current is the sum of the absolute values ​​of the currents of all branches on the two bus sections except the bus coupling branch; the small differential current of bus I is the sum of the currents of all branches on bus I, and the small differential braking current of bus I is the sum of the absolute values ​​of the currents of all branches on bus I; the small differential current of bus II is the sum of the currents of all branches on bus II, and the small differential braking current of bus II is the sum of the absolute values ​​of the currents of all branches on bus II. Differential current I d and braking current I f The calculation formula is as follows:

[0078]

[0079]

[0080] In the formula,

[0081] I i is the busbar i-th branch current;

[0082] n is the number of branches connected to the bus.

[0083] The differential and braking currents of large and small differentials are calculated according to the above formula. Whether the bus tie CT is disconnected can be determined by the large differential current and the small differential current of bus I and bus II.

[0084] When all the following criteria are met, the busbar CT is judged to be disconnected after a set delay:

[0085] 1) Zero-sequence current appears on the busbar;

[0086] 2) The current of a certain phase is 0, and the large differential current of this phase is balanced; the small differential current of the two bus sections connected to this phase is unbalanced, and at the same time, the sum of the small differential currents of the two bus sections connected to this phase is balanced.

[0087] Specifically, taking the disconnection of busbar A phase as an example,

[0088] Bus coupler CT disconnection criterion 1: Zero sequence current appears on the bus coupler;

[0089] Bus CT disconnection criterion 2: The bus A current is 0, and the large differential current of phase A is balanced; the small differential current of bus I phase A and the small differential current of bus II phase A are unbalanced, and at the same time, the sum of the small differential currents of bus I and bus II of phase A is balanced.

[0090] If the above two criteria are met at the same time, the busbar A phase CT is judged to be broken after a fixed delay. The B and C phases are the same as the A phase.

[0091] If there is no CT disconnection in the bus coupler, proceed to step 3. If there is a CT disconnection in the bus coupler, proceed to step 4.

[0092] Step 3: Determine whether CT saturation of an out-of-zone fault occurs by the sequence of mutations of the small differential current and the small differential braking current sampling points. If yes, lock the differential protection of the corresponding busbar section, otherwise return to step 1;

[0093] When the CT is saturated, the magnetic flux of the core of the current transformer cannot change suddenly. When the fault starts and the primary current passes through the zero point in the opposite direction, the current transformer has a linear transfer area, and the secondary current and the primary current satisfy a linear relationship. The size of the linear transfer area is related to the saturation degree of the current transformer. When the CT is saturated for a fault outside the busbar area, the differential current is 0 in the linear area, and the braking current changes suddenly. Therefore, this feature can be used to identify faults outside the area.

[0094] The calculation method of small difference differential current mutation and large difference differential current mutation is the same, and the calculation formula is as follows:

[0095] Δi d =i d (t)-i d (tT)

[0096] In the formula,

[0097] Δi d is the differential current mutation amount;

[0098] i d is the bus differential current;

[0099] t is the current time;

[0100] T is one power frequency cycle: 0.02s.

[0101] The calculation method of small differential braking current mutation is the same as that of large differential braking current mutation. The calculation formula is as follows:

[0102] Δi f =i f (t)-i f (tT)

[0103] In the formula,

[0104] Δi f is the braking current mutation amount;

[0105] i f is the small differential braking current;

[0106] t is the current time;

[0107] T is one power frequency cycle: 0.02s.

[0108] Taking the double busbar as an example, the small differential current mutation value Δi of busbar I is d1 The algorithm is:

[0109] Δi d1 =i d1 (t)-i d1 (tT)

[0110] Where:

[0111] t is the current time;

[0112] T is a power frequency cycle: 0.02s;

[0113] i d1 is the I bus small differential current;

[0114] The small differential braking current mutation of bus I Δi f1 The algorithm is:

[0115] Δi f1 =i f1 (t)-i f1 (tT)

[0116] In the formula,

[0117] t is the current time;

[0118] T is a power frequency cycle: 0.02s;

[0119] i f1 It is the I bus small differential braking current.

[0120] The sampling point data window is used to count the number of sudden changes in the small differential current of bus I and the number of sudden changes in the small differential braking current of bus I. The sudden change amount Δi of the small differential current of bus I d1 >Iset1, recorded as 1, otherwise recorded as 0, where Iset1 is the set value of the I bus differential current. The sudden change of the I bus differential braking current Δi f1 >Iset2, recorded as 1, otherwise recorded as 0, where Iset1 is the I bus small differential braking current setting value. Preferably, the I bus small differential current setting value ranges from 0.4 to 0.6 times the differential setting value; the I bus small differential braking current setting value ranges from 0.4 to 0.6 times the differential setting value.

[0121] Within the set time, the number of sudden changes of the I bus small differential current that meet the conditions is M, and the number of sudden changes of the I bus small differential braking current that meet the conditions is N. If NM≥Z, that is, the I bus small differential braking current mutates before the I bus small differential current, it is considered that the I bus external fault CT saturation occurs, and the I bus differential protection is locked. The II bus criterion is the same as that of the I bus. Z is a constant. In a preferred but non-limiting embodiment of the present invention, the result of Z multiplied by the sampling interval is greater than 2.5ms.

[0122] Step 4: Identify whether the out-of-zone fault CT saturation occurs by the sequence of the large differential current and large differential braking current sampling point mutations. If yes, execute step 5; otherwise, return to step 1.

[0123] That is, the large differential braking current of bus I changes suddenly before the large differential current of bus I. When the bus tie CT is disconnected and the CT of the out-of-zone fault branch is saturated, the small differential current and the small differential braking current change suddenly at the same time in the linear region. The out-of-zone fault CT saturation criterion in step 3 cannot be applied to the scenario of this step. Therefore, the mutation sequence of the large differential current and the large differential braking current is judged to identify whether an out-of-zone fault occurs, and then the locked bus section is determined by judging the sampling point change rate of the small differential current and the small differential braking current at the moment of the fault.

[0124] The mutation criterion of the sampling point of large differential current and large differential braking current is similar to step 3, specifically:

[0125] Large differential current sudden change Δi d The algorithm is:

[0126] Δi d =i d (t)-i d (tT)

[0127] t is the current time;

[0128] T is a power frequency cycle: 0.02s;

[0129] i d is the large differential current;

[0130] Large differential braking current mutation Δi f The algorithm is:

[0131] Δi f =i f (t)-i f (tT)

[0132] In the formula,

[0133] t is the current time;

[0134] T is a power frequency cycle: 0.02s;

[0135] i f It is a large differential braking current.

[0136] The sampling point data window is used to count the number of mutations of the large differential current and the number of mutations of the large differential braking current. At any sampling point, if the mutation amount of the large differential current Δi d >Iset3, it is recorded as a large differential current mutation, where Iset3 is the large differential current setting value; if the mutation amount of the large differential braking current Δi f >Iset4, it is recorded as a sudden change of large differential braking current, where Iset4 is the set value of the large differential braking current.

[0137] Preferably, the value range of the large differential current setting value Iset3 is 0.4 to 0.6 times the differential setting value; the value range of the large differential braking current setting value Iset4 is 0.4 to 0.6 times the differential setting value.

[0138] Specifically, the differential setting is a standard value calculated based on the relay protection setting procedures and actual grid parameters.

[0139] Within the set time, the number of mutations of the large differential current that meets the conditions is Q, and the number of mutations of the large differential braking current that meets the conditions is S. If SQ ≥ Z1, that is, the large differential braking current mutates before the large differential current, it is considered that the out-of-zone fault CT is saturated and step 5 is executed, otherwise, step 1 is executed. Z1 is a constant, and preferably, the result of Z1 multiplied by the sampling interval is greater than 2.5ms.

[0140] Step 5, determining the bus section that needs to be blocked by the sampling point change rate of the small differential current of the two bus sections at the time of the fault.

[0141] When an out-of-zone fault occurs when the bus-coupled CT is disconnected, if one branch CT is saturated, in the saturated linear region at the beginning of the fault, the small differential current of busbars I and II is a power frequency sine wave; when entering the saturated nonlinear region, the small differential current of the busbar connected to the CT saturated branch is distorted, and the small differential current of the other busbar is still a power frequency sine wave. According to the waveform characteristics of the two busbars when the out-of-zone fault CT is saturated, the busbar section that needs to be locked can be selected by calculating the change rate of the small differential current of busbars I and II.

[0142] Within the set time, the number of sampling points of Bus I that satisfy the sudden change in the differential current change rate of the small differential is W, and the number of sampling points of Bus II that satisfy the sudden change in the differential current change rate of the small differential is U. If W≥Z2 and U<Z2, where Z2 is a constant, it is considered that the CT of Bus I is saturated during an external fault, and the differential protection of Bus I is blocked; if U≥Z2 and W<Z2, it is considered that the CT of Bus II is saturated during an external fault, and the differential protection of Bus II is blocked.

[0143] The condition for the sudden change in the differential current change rate of the small differential is:

[0144] Δk d1 >Δk d2

[0145] Among them, Δk d1 is the ratio of the change rate of the current sampling point of the small differential current of a section of bus to the change rate of the previous sampling point;

[0146] Δk d2 is the ratio of the change rate of the previous sampling point of the small differential current of a section of bus to the change rate of the two previous sampling points;

[0147] Δk d1 =((i d (t)-i d (t-Δt)) / (i d (t-Δt)-i d (t-2Δt))

[0148] Δk d2 =((i d (t-Δt)-i d (t-2Δt)) / (i d (t-2Δt)-i d (t-3Δt))

[0149] In the formula,

[0150] Δt is the sampling interval time;

[0151] i d is the small differential current of the bus.

[0152] In this embodiment, Bus I is taken as an example.

[0153] Δk d11 =((i d1 (t)-i d1 (t-Δt)) / (i d1 (t-Δt)-i d1 (t-2Δt))

[0154] Δk d12 =((i d1 (t-Δt)-id1 (t-2Δt)) / (i d1 (t-2Δt)-i d1 (t-3Δt))

[0155] Δk d21 =((i d2 (t)-i d2 (t-Δt)) / (i d2 (t-Δt)-i d2 (t-2Δt))

[0156] Δk d22 =((i d2 (t-Δt)-i d2 (t-2Δt)) / (i d2 (t-2Δt)-i d2 (t-3Δt))

[0157] In the formula,

[0158] Δk d11 is the ratio of the current point change rate of the small differential current of bus I to the previous point change rate;

[0159] Δk d12 is the ratio of the change rate of the previous point of the small differential current of bus I to the change rate of the previous two points;

[0160] Δk d21 is the ratio of the change rate of the small differential current of bus II at the current point to the change rate at the previous point;

[0161] Δk d22 It is the ratio of the change rate of the first point of the small differential current of bus II to the change rate of the first two points;

[0162] t is the current time;

[0163] Δt is the sampling interval;

[0164] i d1 is the small differential current of bus I;

[0165] i d2 is the small differential current of bus II.

[0166] The sampling point data window is used to count the changes in the small differential current of each bus section. Within the set time, the statistics satisfy Δk d11 >Δk d12 The number of sampling points of the small differential current of bus I is W, satisfying Δk d21 >Δk d22The number of sampling points of the differential current of the small differential of bus II is U. If W≥Z2 and U<Z2, it is considered that the CT of the external fault of bus I is saturated, and the differential protection of bus I is blocked. Otherwise, return to step 1. Z2 is a constant. Preferably, the value of Z2 is 3.

[0167] The applicant of the present invention has made a detailed description and illustration of the embodiments of the present invention in combination with the accompanying drawings. However, those skilled in the art should understand that the above embodiments are only the preferred implementation schemes of the present invention, and the detailed description is only to help readers better understand the spirit of the present invention, rather than a limitation on the protection scope of the present invention. On the contrary, any improvement or modification based on the spirit of the present invention should fall within the protection scope of the present invention.

Claims

1. An adaptive out-of-zone CT saturation determination method applicable to the case of busbar CT disconnection, characterized in that: It includes the following steps: Step 1, collect the bus voltage and the currents of each branch connected to the bus; Step 2, calculate the differential current and restraint current at the sampling point, and at the same time determine whether the bus-tie CT is broken; if the bus-tie CT is not broken, execute Step 3, otherwise execute Step 4. Step 3, calculate the sudden change amount of the differential current of the small differential and the sudden change amount of the restraint current of the small differential, and determine whether the CT of the external fault is saturated according to the sequence of the sudden changes of the sampling points of the differential current of the small differential and the restraint current of the small differential. If so, block the differential protection of the faulty bus section, otherwise return to Step 1; S301, respectively counting the number of mutations M of the small differential current and the number of mutations N of the small differential braking current of each bus within a set time, where the mutation amount Δi of the small differential current is d >Iset1, it is recorded as a small differential current mutation in the bus section, where Iset1 is the small differential current setting value; if the mutation amount of the small differential braking current Δi f >Iset2, it is recorded as a sudden change of small differential braking current in the bus section, where Iset2 is the set value of small differential braking current; S302, if for any bus, N - M ≥ Z1, it is determined that the CT of the external fault of this bus is saturated, and the differential protection of this bus is blocked; where Z1 is a constant, and the result of multiplying Z1 by the sampling interval time is greater than 2.5 ms; Step 4, identify whether the CT of the external fault is saturated through the sequence of the sudden changes of the sampling points of the differential current of the large differential and the restraint current of the large differential. If so, execute Step 5, otherwise return to Step 1; Step S401, counting the number of sudden changes Q of the large differential current and the number of sudden changes S of the large differential braking current; wherein the sudden change amount ΔI of the large differential current is d >Iset3, it is recorded as a sudden change of the large differential current, where Iset3 is the set value of the large differential current; the sudden change of the large differential braking current ΔI f >Iset4, it is recorded as a sudden change in the large differential braking current, where Iset4 is the set value of the large differential braking current; Step S402, within the set time, if S - Q ≥ Z, it is determined that the CT of the external fault is saturated, Z1 is a constant, and Z1 multiplied by the sampling interval is greater than 2.5 ms; Step 5, determine the bus section to be blocked through the change rate of the sampling points of the differential current of the small differential of the two bus sections connected by the bus-tie at the fault moment; Within the set time, the number of sampling points of Bus I that satisfy the sudden change of the change rate of the differential current of the small differential is W, and the number of sampling points of Bus II that satisfy the sudden change of the change rate of the differential current of the small differential is U. If W ≥ Z2 and U < Z2, where Z2 is a constant, it is considered that the CT of the external fault of Bus I is saturated, and the differential protection of Bus I is blocked; if U ≥ Z2 and W < Z2, it is considered that the CT of the external fault of Bus II is saturated, and the differential protection of Bus II is blocked.

2. The adaptive external CT saturation discrimination method applicable to the case of bus-tie CT disconnection according to claim 1, characterized in that the differential current includes the differential current of the large differential and the differential current of the small differential. Among them, the differential current of the large differential is the sum of the currents of all branches except the bus-tie branch on the two buses; the differential current of the small differential of each bus is the sum of the currents of all branches on this bus; the restraint current includes the restraint current of the large differential and the restraint current of the small differential. Among them, the restraint current of the large differential is the sum of the absolute values of the currents of all branches except the bus-tie branch on the two buses; the differential current of the small differential of each bus is the sum of the absolute values of the currents of all branches on this bus; The calculation methods of the differential current of the large differential and the differential current of the small differential are as follows: The calculation methods of the restraint current of the large differential and the restraint current of the small differential are as follows: In the formula, I d is the differential current; I f is the braking current; I i is the i-th branch current; n is the number of branches.

3. The adaptive external CT saturation discrimination method applicable to the case of bus-tie CT disconnection according to claim 1, characterized in that when all the following criteria are met, the disconnection of the bus-tie CT is determined after a set delay: 1) There is zero-sequence current on the bus-tie; 2) The current of a certain phase is 0, and the differential current of the large differential of this phase is balanced; the differential currents of the small differentials of the two bus sections connected by this phase are unbalanced, and at the same time the sum of the differential currents of the small differentials of the two bus sections connected to this phase is balanced.

4. The adaptive out-of-zone CT saturation determination method applicable to the case of busbar CT disconnection according to claim 1 is characterized in that: The calculation method of small difference differential current mutation and large difference differential current mutation is the same, and the calculation formula is as follows: Δi d =i d (t)-i d (t-T) In the formula, Δi d is the differential current mutation amount; i d is the bus differential current; t is the current time; T is one power frequency cycle: 0.02s.

5. The adaptive out-of-zone CT saturation determination method applicable to the case of busbar CT disconnection according to claim 1 is characterized in that: The calculation method of small differential braking current mutation is the same as that of large differential braking current mutation. The calculation formula is as follows: Δi f =i f (t)-i f (t-T) In the formula, Δi f is the braking current mutation amount; i f is the small differential braking current; t is the current time; T is one power frequency cycle: 0.02s.

6. The adaptive out-of-zone CT saturation determination method applicable to the case of busbar CT disconnection according to claim 5 is characterized in that: The sudden change condition of the small differential current change rate is: Δk d1 >Δk d2 Among them, Δk d1 It is the ratio of the change rate of the current sampling point of a busbar small differential current to the change rate of the previous sampling point; Δk d2 It is the ratio of the change rate of the previous sampling point of the small differential current of a section of busbar to the change rate of the previous two sampling points; Δk d1 =((i d (t)-i d (t-Δt)) / (i d (t-Δt)-i d (t-2Δt)) Δk d2 =((i d (t-Δt)-i d (t-2Δt)) / (i d (t-2Δt)-i d (t-3Δt)) In the formula, Δt is the sampling interval; i d is the busbar small differential current.

7. The adaptive out-of-zone CT saturation determination method applicable to the case of busbar CT disconnection according to claim 1 is characterized in that: The value range of the small difference differential current setting value Iset1, the small difference braking current setting value Iset2, the large difference differential current setting value Iset3 and the large difference braking current setting value Iset4 is 0.4 to 0.6 times the differential setting value.

Images