Method and system for preventing misalignment of faults in region when CT secondary neutral line is broken

Abstract

The invention discloses a method and a system for preventing misalignment of faults in a region when a CT secondary neutral line is broken. The method comprises the following steps: setting a single-phase fault phase region distinguishing element in the region formed by sequence component differential current; when the line is in non-full-phase operation, a fault in the area is judged by combining split-phase differential motion with positive and negative sequence differential current; when three phases of the line operate, only one phase of split-phase differential motion meets action conditions, single-phase jumping is carried out, when multiple phases meet action conditions and zero-sequence or negative-sequence differential current exists, single-phase jumping is carried out if the phase region judgment element judges that a single-phase fault occurs, and otherwise three-phase jumping is carried out. When the zero-sequence differential meets the action condition, if the phase region judgment element judges that a single-phase fault occurs, jumping to a single phase, and otherwise, delaying to jump to three phases; and after differential protection sends a single-phase tripping command, receiving the tripping command when judging that a fault phase has no current or positive and negative sequence differential returns. According to the invention, through the inter-region single-phase fault phase region discrimination element and the jump order receiving when the fault phase has no current or the positive and negative sequence differential motion returns, the non-overlapping condition caused by the CT secondary neutral line breakage during the single-phase fault in the line region can be prevented.

Description

Technical Field

[0001] The invention belongs to the technical field of transmission line fault identification, and relates to a method and system for preventing the non-overlapping of faults in a zone when a secondary neutral line of a CT is disconnected. Background Art

[0002] During normal operation of the line, if the secondary neutral line IN of the CT (current transformer) is broken, the current loop still maintains a state in which the positive sequence component remains unchanged and there is no negative sequence component and zero sequence component. Therefore, it is difficult for the line protection device to identify the abnormal situation of the CT secondary neutral line being broken.

[0003] After the IN line is disconnected on one side of the line, when a ground fault occurs on the line, the positive and negative sequence current components on both sides do not change much from the state when the IN line is not disconnected, but the zero sequence current component has changed significantly. There is no zero sequence current component on the IN line disconnection side, which is completely different from the state when the IN line is not disconnected, which may cause the zero sequence current differential protection to operate incorrectly. At the same time, the secondary fault phase current on the IN line disconnection side cannot flow back through the zero sequence loop, but can only flow back through the non-fault phase, resulting in abnormal changes in the non-fault phase current. However, the phase current differential protection currently in operation on the field calculates the differential current and braking current based on the same phase current on both sides, which is not suitable for the case of IN line disconnection. When the line has an in-zone or out-of-zone ground fault, it may cause the differential protection to operate incorrectly. When a single-phase ground fault occurs in the line area, the fault phase cannot be accurately identified, and a two-phase or three-phase fault may be misjudged, resulting in non-reclosing in the single-reclosing mode.

[0004] After the IN line is disconnected on one side of the line, during the non-full-phase operation of the line, there is zero-sequence load current on the primary side, but there is no zero-sequence current component on the secondary side of the IN line disconnection side, which may cause the zero-sequence current differential protection to operate incorrectly. At the same time, the secondary zero-sequence current on the IN line disconnection side has no loop to flow back and can only flow back through the running phase, resulting in abnormal changes in the running phase current, which may cause the running phase phase differential protection to operate incorrectly.

[0005] After the IN line on one side of the line is broken, even if the protection trips a single phase when a fault occurs in the line area, the non-fault phase current during the non-full-phase period will still flow back through the faulty phase. The original protection trip command may not be recovered, delaying three trips, resulting in non-reclosing in single-reclosing mode.

[0006] When the IN line on one side of the line is broken, the three-trip differential protection does not overlap in the single-phase fault area of ​​the line. There is currently no reliable prevention measure, and the failure of protection to overlap during single-phase faults is common. Summary of the invention

[0007] In order to solve the deficiencies in the prior art, the present invention provides a method and system for preventing the non-overlapping of faults in a zone when a CT secondary neutral line is broken.

[0008] The present invention adopts the following technical solution.

[0009] A first aspect of the present invention provides a method for preventing a fault in a zone from overlapping when a secondary neutral line of a CT is disconnected, comprising:

[0010] Step 1, setting a phase zone discrimination element for discriminating a single-phase fault in a zone according to the phase relationship between the sum of zero-negative sequence differential current and the positive sequence differential current, and proceeding to step 2;

[0011] Step 2, determine the operating state of the line. If the line is in a non-full-phase operating state, proceed to step 3; if the line is in a three-phase operating state, proceed to step 4;

[0012] Step 3: When the fault in the area or the disconnection of the secondary neutral line of the CT is determined based on the phase current differential and the positive and negative sequence current differential, the differential protection trips or disconnection alarm is performed;

[0013] Step 4: If the three-phase current differentials do not meet the action conditions, then go to step 5; otherwise, when only one phase current differential meets the action conditions, the protection trips once, and go to step 6; when two or three phase current differentials meet the action conditions and there is a zero-sequence or negative-sequence differential current, the phase zone discrimination element is put into operation. When the phase zone discrimination element identifies a single-phase fault in the zone, the differential protection trips once, and go to step 6; otherwise, the differential protection trips three times;

[0014] Step 5: When the zero-sequence current differential meets the action conditions, the phase zone discrimination element is put into operation. If the phase zone discrimination element determines that a single-phase fault occurs in the zone, the differential protection trips once and enters step 6. Otherwise, the differential protection trips three times after a delay.

[0015] Step 6: After the differential protection issues a single trip command, if it is determined that there is no current in the tripped phase, go to step 7; if it is determined that there is current in the tripped phase, if the positive sequence differential current or the negative sequence differential current is greater than the threshold current I K2 , the differential protection delays three trips; otherwise, the differential protection reports a CT secondary neutral line disconnection alarm and goes to step 7;

[0016] Step 7: The differential protection receives a single trip command and starts the reclosing.

[0017] Preferably, the criterion for the phase zone discriminating element to discriminate a single-phase fault in the zone is:

[0018]

[0019] Where arg is the phase sign;

[0020] are phase angle thresholds, respectively;

[0021] They are respectively the positive sequence differential current, negative sequence differential current and zero sequence differential current with phase A as the reference phase;

[0022] and They are respectively the positive sequence currents of the M side and the N side with phase A as the reference phase; and They are the negative sequence currents on the M side and the N side respectively, taking phase A as the reference phase; and They are the zero-sequence currents on the M and N sides respectively, with phase A as the reference phase.

[0023] Preferably, is -45°, is 45°, is -165°, is -75°, is 75°, is 165°.

[0024] Preferably, in step 2, the operation status of the line is determined by:

[0025] If one or two phases have a jumper open and there is no current in the corresponding phases, the line is judged to be in a non-full-phase operation state; if there is no jumper open in all three phases or there is current in all three phases, the line is judged to be in a three-phase operation state.

[0026] Preferably, the method for determining whether there is current in each phase is:

[0027] If the currents on both sides of phase A, phase B or phase C are greater than or equal to 0.05In, there is current in the corresponding phase, otherwise there is no current in the corresponding phase, and In is the secondary rated current.

[0028] Preferably, in step 3, if the operating phase split-phase current differential meets the action conditions and the positive-sequence or negative-sequence current differential protection meets the action conditions, the differential protection trips; if the operating phase split-phase current differential meets the action conditions but both the positive-sequence and negative-sequence current differential protections do not meet the action conditions, the protection reports a CT secondary neutral line break alarm;

[0029] Preferably, in step 4, the condition for determining whether there is a zero-sequence differential current or a negative-sequence differential current is:

[0030] When the following discriminants are satisfied at the same time, it is judged that there is zero-sequence differential current or negative-sequence differential current in phase A:

[0031]

[0032] When the following discriminants are satisfied at the same time, it is judged that there is zero-sequence differential current or negative-sequence differential current in phase B:

[0033]

[0034] When the following discriminants are satisfied at the same time, it is judged that there is zero-sequence differential current or negative-sequence differential current in phase C:

[0035]

[0036] In the formula, I K1 is the asymmetric current threshold;

[0037] They are respectively the positive sequence differential current, negative sequence differential current and zero sequence differential current with phase A as the reference phase.

[0038] Preferably, I K1 It is 60A.

[0039] Preferably, in step 6, I K2 Take the minimum value among 0.06In, 0.3 times the phase current differential protection setting and 0.3 times the zero-sequence current differential protection setting;

[0040] Preferably, the delay time of the three-hop differential delay is 150ms.

[0041] A second aspect of the present invention provides a system for preventing the non-reclosing of faults in a zone when a CT secondary neutral line is disconnected, comprising:

[0042] A phase zone discrimination element setting module is used to set a phase zone discrimination element for discriminating a single-phase fault in a zone according to a phase relationship between the sum of zero-negative sequence differential current and the positive sequence differential current;

[0043] The line operation status judgment module is used to judge the operation status of the line. If the line is not in full-phase operation status, it will enter the zone fault and disconnection judgment module; if the line is in three-phase operation status, it will enter the phase current differential protection single trip judgment module;

[0044] The internal fault and disconnection discrimination module is used to identify the internal fault or CT secondary neutral line disconnection according to the phase current differential and the positive and negative sequence current differential, and perform differential protection triple tripping or disconnection alarm;

[0045] The single-trip discrimination module of the phase-splitting current differential protection is used to enter the single-trip discrimination module of the zero-sequence current differential protection if the three-phase phase current differential does not meet the action conditions. Otherwise, when only one-phase current differential meets the action conditions, the protection will trip once and enter the current discrimination module; when two-phase or three-phase current differentials meet the action conditions and there is a zero-sequence or negative-sequence differential current, the phase zone discrimination element is put into operation. When the phase zone discrimination element identifies a single-phase fault in the zone, the differential protection will trip once and enter the current discrimination module, otherwise the differential protection will trip three times;

[0046] The zero-sequence current differential protection single-trip discrimination module is used to put the phase zone discrimination element into operation when the zero-sequence current differential meets the action conditions. If the phase zone discrimination element determines that a single-phase fault occurs in the zone, the differential protection will trip once and enter the current discrimination module. Otherwise, the differential protection will delay three trips.

[0047] The current discrimination module is used for the differential protection to send a single trip command. When it is judged that there is no current in the tripped phase, the differential protection single trip command recovery discrimination module is entered; when it is judged that there is current in the tripped phase, if the positive sequence differential current or the negative sequence differential current is greater than the threshold current, the differential protection delays three trips; otherwise, the differential protection reports a CT secondary neutral line disconnection alarm, and the differential protection single trip command recovery discrimination module is entered;

[0048] The differential protection single trip command recovery judgment module is used to receive the differential protection single trip command and start the reclosing.

[0049] Compared with the prior art, the beneficial effects of the present invention include at least:

[0050] Based on the main judgment criterion of the original longitudinal current differential protection function, the present invention proposes a single-phase fault phase zone discrimination element in the area formed according to the phase relationship between the sum of the zero-negative sequence differential current phasor and the positive-sequence differential current phasor. The element has high sensitivity and is not affected by transition resistance, and realizes auxiliary discrimination of the action phase zone of the single-phase fault in the line area. The differential protection determines the A-phase, B-phase and C-phase action phase zones of the single-phase grounding fault in the line area according to the phase relationship between the sum of the zero-negative sequence differential current phasor and the positive-sequence differential current phasor, and can prevent the three-phase tripping and the erroneous tripping of the non-fault phase when the CT secondary neutral line is broken in the single-phase grounding fault in the line area.

[0051] When the differential protection of the present invention trips a single phase, on the basis of the original protection judging that the fault phase has no current and retracting the tripping command, a method for judging that the positive sequence differential current and the negative sequence differential current both return and also retracting the tripping command is proposed, so that the protection single tripping starts the reclosing and the situation of identifying the disconnection of the secondary neutral line of the CT is completed, and the problem that the protection issues a single tripping command when a single-phase grounding fault occurs in the line area after the secondary neutral line of the CT is disconnected, but the fault phase continues to have current and does not meet the tripping command retraction condition, resulting in a protection delay of three trips, can be solved, and the situation that the reclosing fails in the single reclosing mode can be prevented;

[0052] The present invention proposes that when there is still current in the fault phase after the protection sends a single trip command, and the positive sequence differential current and the negative sequence differential current are less than or equal to the threshold current, it is judged that the CT secondary neutral line is abnormal; during the non-full-phase operation of the line, when the phase-by-phase current differential protection of the operating phase meets the action conditions, and both the positive sequence current differential and the negative sequence current differential do not meet the action conditions, it is judged that the CT secondary neutral line is abnormal. The above method can solve the problem that the original protection cannot effectively identify the CT secondary neutral line break when there is a fault in the line area.

[0053] The present invention proposes to use a high ratio positive sequence differential current when judging whether each phase has a zero sequence differential current or a negative sequence differential current. As a floating threshold, it can effectively prevent the generation of zero-negative sequence unbalanced current in the secondary during a three-phase short circuit fault. Since the phase of the unbalanced current is random, the phase zone discrimination element may misjudge it as a single-phase fault phase zone problem. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 This is a schematic diagram of the fault in the line area after the secondary neutral line of the CT is broken;

[0055] Figure 2 This is a schematic diagram of a method for preventing the non-reclosing of faults in a zone when the secondary neutral line of a CT is disconnected according to the present invention;

[0056] Figure 3 This is the operating phase area schematic diagram for a single-phase grounding fault within the line area. DETAILED DESCRIPTION

[0057] 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 embodiments of a part of the present invention, rather than all embodiments. Based on the spirit of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work belong to the protection scope of the present invention.

[0058] Embodiment 1 of the present invention provides a method for preventing the non-reclosing of faults in the zone when the secondary neutral line of a CT is disconnected, which can be applied to Figure 1 The situation where a fault occurs in the line area after the secondary neutral line of the CT is broken, where point F is a fault in the line area, including a single-phase grounding fault, a two-phase grounding short-circuit fault, a two-phase ungrounded short-circuit fault or a three-phase short-circuit fault. The three-phase primary currents are Ia, Ib, and Ic respectively. The longitudinal differential protection device collects the A-phase current, B-phase current, C-phase current and zero-sequence current through the current secondary circuit. Terminals IA, IB, IC and IN' are polarity terminals, terminals IA', IB', IC' and IN are non-polarity terminals, and terminals IA', IB', IC' and IN' are short-circuited. In the event of a grounding short-circuit fault, the zero-sequence current flows back through the neutral line IN, the A-phase current is taken from the current transformer between terminals IA and IA', the B-phase current is taken from the current transformer between terminals IB and IB', the C-phase current is taken from the current transformer between terminals IC and IC', and the external zero-sequence current 3I0 is taken from the current transformer between terminals IN and IN'. In the figure, IN is disconnected at K. When there is a ground short circuit fault, there is no current in the external 3I0, and the currents of phases A, B, and C flow back through each phase. For example, if a phase A ground fault occurs in the line area, the differential protection trips A, and the A phase circuit breaker B MaThe current of phases B and C will flow back through phase A, resulting in current in phase A. The previous differential protection tripping command A cannot be withdrawn. After the three tripping commands are delayed, the current can disappear only after all three circuit breakers are tripped, and the tripping command can be withdrawn, increasing the risk of false operation of the failure protection and causing the single-reclosing mode reclosing to fail to reclose. Figure 2 As shown, the method includes:

[0059] Step 1, setting a single-phase fault phase zone discrimination element in the zone formed according to the phase relationship between the sum of the zero-negative sequence differential current phasor and the positive-sequence differential current phasor, and entering step 2;

[0060] More preferably, if Figure 3 As shown, the single-phase fault phase zone discrimination element in the zone is composed of sequence component differential flow, which satisfies the following relationship.

[0061]

[0062] In the formula, are phase angle thresholds, respectively;

[0063] They are respectively the positive sequence differential current, negative sequence differential current and zero sequence differential current with phase A as the reference phase;

[0064] and They are respectively the positive sequence currents of the M side and the N side with phase A as the reference phase; and They are the negative sequence currents on the M side and the N side respectively, taking phase A as the reference phase; and They are the zero-sequence currents on the M and N sides respectively, with phase A as the reference phase. -45° is preferred, Preferably 45°, Preferably -165°, Preferably -75°, Preferably 75°, The preferred angle is 165°, which can achieve correct phase selection during a single-phase grounding fault.

[0065] The sum of the zero-sequence differential current and the negative-sequence differential current phasors, and the phase relationship with the positive-sequence differential current phasor constitute the above-mentioned single-phase fault phase zone discrimination element in the zone. For a grounded system, when a single-phase grounding fault occurs, when the phase of the sum of the zero-sequence differential current phasors with each phase as the reference phase is between -45° and 45° relative to the positive-sequence differential current phasor with phase A as the reference phase, it is judged as the A-phase fault phase zone; when it is between -165° and -75°, it is judged as the B-phase fault phase zone; when it is between 75° and 165°, it is judged as the C-phase fault phase zone.

[0066] For the single-phase fault phase zone discrimination element within the zone, if a single-phase grounding fault occurs, the faulty phase will be selected; if a two-phase grounding short circuit, a two-phase ungrounded short circuit or a three-phase short circuit fault occurs, it will not be judged as within the A-phase, B-phase or C-phase fault phase zone.

[0067] Step 2, determine the operating state of the line. If the line is in a non-full-phase operating state, proceed to step 3; if the line is in a three-phase operating state, proceed to step 4;

[0068] Further preferably, it is first determined whether the line is in full-phase operation or non-full-phase operation. If one or two phases have a jumper open and the corresponding phases have no current, it is determined that the line is in a non-full-phase operation state; if none of the three phases have a jumper open or all three phases have current, it is determined that the line is in a three-phase operation state. The threshold for determining whether phase A, phase B, and phase C have current is 0.05In, where In is the secondary rated current. When the current on both sides of the phase is greater than or equal to 0.05In, it is determined to have flow, indicating that the phase is in operation; when one side of the phase has a current less than 0.05In, it is determined to have no flow, but when the load is small, there is also no flow, and it needs to be further determined by the jumper. If the phase has a jumper, it is determined to be in a tripped state, and if the phase has no jumper, it is determined to be in an operating state. If there is flow on both sides of the phase and there is a jumper, it is mainly determined to have flow, and it is also determined to be in an operating state.

[0069] Step 3: When the fault in the area or the disconnection of the secondary neutral line of the CT is determined based on the phase current differential and the positive and negative sequence current differential, the differential protection trips or disconnection alarm is performed;

[0070] Further preferably, when it is determined that the line is not in full-phase operation state, it means that a phase has tripped, which may be a protection trip or a switch trip. When a fault occurs in the operating phase again, the protection needs to trip three times.

[0071] The positive sequence current differential protection and negative sequence current differential protection are less affected by the disconnection of the CT secondary neutral line. If the phase current differential of the operating phase meets the action conditions, and the positive sequence current differential protection or the negative sequence current differential protection meets the action conditions, it means that the line operating phase has an internal fault and the differential protection trips.

[0072] If the operating phase current differential meets the action conditions, but the positive sequence current differential protection and the negative sequence current differential protection do not meet the action conditions, the CT secondary neutral line is broken. The protection reports the CT secondary neutral line broken alarm and returns.

[0073] Longitudinal current differential protection includes phase current differential protection and zero-sequence current differential protection.

[0074] This step still uses the original phase-sequence current differential protection as the main criterion, and the positive-sequence current differential protection and negative-sequence current differential protection also use the usual calculation method. The details are as follows:

[0075] (1) The action criteria of the phase current differential protection shall satisfy the following relationship at the same time.

[0076]

[0077] In the formula, are the Φ-phase currents at the M side and the N side of the line MN at the current moment respectively;

[0078] Φ is phase A, phase B or phase C;

[0079] I DZ It is the setting value of phase current differential protection;

[0080] k is the braking coefficient of the phase current differential protection.

[0081] (2) The action criteria of the positive sequence current differential protection shall satisfy the following relationship at the same time.

[0082]

[0083] In the formula, They are the positive sequence currents on the M side and N side of line MN respectively;

[0084] I DZ1 is the positive sequence current differential protection action threshold;

[0085] k1 is the positive sequence current differential protection braking coefficient.

[0086] (3) The action criterion of the negative sequence current differential protection satisfies the following relationship at the same time.

[0087]

[0088] In the formula, They are the negative sequence currents on the M side and N side of line MN respectively;

[0089] I DZ2 is the negative sequence current differential protection action threshold;

[0090] k2 is the negative sequence current differential protection braking coefficient.

[0091] Step 4, based on the above-mentioned action criterion of the phase current differential protection, if the phase current differential meets the action conditions, and only one phase meets the differential action conditions, the protection trips once, and enters step 6; if the phase current differential meets the action conditions, and two or three phases meet the differential action conditions, when there is a zero-sequence or negative-sequence differential current, the phase area discrimination element is put into operation, and the differential protection trips once when the phase area discrimination element judges that it is a single-phase fault, and enters step 6, otherwise the differential protection trips three times; if the three-phase current differentials do not meet the action conditions, enter step 5;

[0092] Further preferably, only one phase meets the phase current differential action condition, and this phase is the fault phase, and enters the single-trip logic of the differential protection, protecting the single-trip.

[0093] If two or three phases meet the conditions for the phase-splitting current differential protection operation, first determine whether there is zero-sequence differential current and negative-sequence differential current. If there is no zero-sequence differential current and negative-sequence differential current, it is determined to be a three-phase short circuit and the differential protection will trip three times. When there is a zero-sequence differential current or a negative-sequence differential current, the single-phase fault phase zone discrimination element in the area is put into use. Through the phase situation of the sum of the zero-sequence differential current phasors relative to the positive-sequence differential current phasor, it is determined whether the conditions in the A-phase fault phase zone, the B-phase fault phase zone or the C-phase fault phase zone are met. If any of the fault phase zones are not met, the differential protection will trip three times. If any of the fault phase zones are met, the single-trip logic of the differential protection is entered and the protection will trip single-trip.

[0094] When the following relationship is satisfied at the same time, it is judged that there is zero-sequence differential current or negative-sequence differential current in phase A.

[0095]

[0096] When the following relationship is satisfied at the same time, it is judged that there is zero-sequence differential current or negative-sequence differential current in phase B.

[0097]

[0098] When the following relationship is satisfied at the same time, it is judged that there is zero-sequence differential current or negative-sequence differential current in phase C.

[0099]

[0100] In the formula, I K1 For the asymmetric current threshold, the preferred primary current is 60A, which has a higher sensitivity.

[0101] Using a high-ratio positive-sequence differential current as the floating threshold can effectively prevent the generation of zero-negative-sequence unbalanced current in the secondary during a three-phase short-circuit fault. Since the phase of the unbalanced current is random, the phase zone discrimination element may misjudge it as a single-phase fault phase zone problem.

[0102] Step 5: If the zero-sequence current differential meets the action conditions, the phase zone discrimination element is put into operation. When the phase zone discrimination element determines that it is a single-phase fault, the differential protection trips once, and the process goes to step 6; otherwise, the differential protection trips three times after a delay;

[0103] Further preferably, the phase current differential does not meet the action condition, but the zero-sequence current differential meets the action condition, the phase zone discrimination element is put into operation, and the differential protection delays three trips when the phase zone discrimination element does not judge it as a single-phase fault, and the protection single trips when the phase zone discrimination element judges it as a single-phase fault, and enters step 6;

[0104] This step still uses the original zero-sequence current differential protection as the main criterion. The action criterion of the zero-sequence current differential protection is:

[0105]

[0106] In the formula, They are three times the zero-sequence current on the M side and N side of line MN respectively;

[0107] I DZ0 It is the setting value of zero-sequence current differential protection;

[0108] k0 is the braking coefficient of zero-sequence current differential protection.

[0109] The zero-sequence current differential protection action time is fixed with a 100ms delay.

[0110] Through the phase situation of the sum of the zero negative sequence differential current phasor relative to the positive sequence differential current phasor, it is judged whether the conditions in the A phase fault phase zone, the B phase fault phase zone or the C phase fault phase zone are met. If any fault phase zone is not met, the differential protection enters the delayed three-trip logic; if any fault phase zone is met, the single-trip logic of the differential protection is entered, and the protection is single-trip.

[0111] Step 6: After the differential protection issues a single trip command, if it is determined that there is no current in the tripped phase, go to step 7; if it is determined that there is current in the tripped phase, if the positive sequence differential current or the negative sequence differential current is greater than the threshold current I K2 , the differential protection delays three trips; otherwise, the differential protection reports a CT secondary neutral line disconnection alarm and goes to step 7;

[0112] Further preferably, after the differential protection issues a single trip command, when it is determined that there is no current in the tripped phase, the secondary neutral line of the CT is normal; when it is determined that there is current in the tripped phase, if the positive sequence differential current or the negative sequence differential current is greater than the threshold current I K2 , indicating that the primary fault current did not trip, which is a circuit breaker failure, and the protection delay is three trips.

[0113] After determining that there is no current in the tripped phase, it is considered that the fault current has been removed and the CT secondary neutral line is not broken.

[0114] After judging that there is current in the tripping phase, if both the positive sequence differential current and the negative sequence differential current are less than or equal to the threshold current I K2 , indicating that the primary fault has been cleared, and the CT secondary neutral line is broken. The secondary fault phase current cannot be cleared, and the differential protection reports a CT secondary neutral line break alarm.

[0115] I K2 Take the minimum value among 0.06In, 0.3 times the phase current differential protection setting and 0.3 times the zero-sequence current differential protection setting, and the positive sequence differential current and negative sequence differential current are both less than I K2, it is determined that the primary fault has been eliminated.

[0116] The time of the differential delay three jumps is preferably 150ms, which is less than the failure protection action time.

[0117] The protection sends a three-trip command. If the circuit breaker is not malfunctioning, the circuit breaker will cut off the three-phase current, and there will be no current in the three phases. The protection can then take back the three-trip command normally.

[0118] In summary, the protection alarm of the CT secondary neutral line disconnection alarm has the following modes:

[0119] 1) Determine that the line is in a non-full-phase operation state. If the phase-by-phase current differential of the operating phase meets the action conditions but both the positive-sequence and negative-sequence current differential protections do not meet the action conditions, the protection reports a CT secondary neutral line break alarm;

[0120] 2) After determining that there is current in the tripping phase, if both the positive sequence differential current and the negative sequence differential current are less than or equal to the threshold current I K2 , the protection reports the CT secondary neutral line break alarm.

[0121] Step 7: The differential protection receives a single trip command and starts the reclosing.

[0122] The differential protection receiving and jumping command method in the present invention includes:

[0123] 1) After the protection sends out a single trip command, if it is determined that there is no current in the phase, the protection can withdraw the single trip command;

[0124] 2) After the protection issues a single trip command, it is determined that there is current in the phase, but the positive sequence differential current and the negative sequence differential current are both less than or equal to the threshold current I K2 , indicating that the primary side fault has been removed and the protection can withdraw the single trip command.

[0125] High-voltage line protection is usually put into single-phase reclosing mode. When the current of the primary fault phase is cut off and the protection single trip command is withdrawn, the reclosing switch is started to complete the closing function of the reclosing switch, which can prevent the situation where the fault in the zone is not reclosed when the CT secondary neutral line is broken.

[0126] Embodiment 2 of the present invention provides a system for malfunction of current differential protection caused by a CT secondary neutral line break, comprising:

[0127] A phase zone discrimination element setting module is used to set a phase zone discrimination element for discriminating a single-phase fault in a zone according to a phase relationship between the sum of zero-negative sequence differential current and the positive sequence differential current;

[0128] The line operation status judgment module is used to judge the operation status of the line. If the line is not in full-phase operation status, it will enter the zone fault and disconnection judgment module; if the line is in three-phase operation status, it will enter the phase current differential protection single trip judgment module;

[0129] The internal fault and disconnection discrimination module is used to identify the internal fault or CT secondary neutral line disconnection according to the phase current differential and the positive and negative sequence current differential, and perform differential protection triple tripping or disconnection alarm;

[0130] The single-trip discrimination module of the phase-splitting current differential protection is used to enter the single-trip discrimination module of the zero-sequence current differential protection if the three-phase phase current differential does not meet the action conditions. Otherwise, when only one-phase current differential meets the action conditions, the protection will trip once and enter the current discrimination module; when two-phase or three-phase current differentials meet the action conditions and there is a zero-sequence or negative-sequence differential current, the phase zone discrimination element is put into operation. When the phase zone discrimination element identifies a single-phase fault in the zone, the differential protection will trip once and enter the current discrimination module, otherwise the differential protection will trip three times;

[0131] The zero-sequence current differential protection single-trip discrimination module is used to put the phase zone discrimination element into operation when the zero-sequence current differential meets the action conditions. If the phase zone discrimination element determines that a single-phase fault occurs in the zone, the differential protection will trip once and enter the current discrimination module. Otherwise, the differential protection will delay three trips.

[0132] The current discrimination module is used for the differential protection to send a single trip command. When it is judged that there is no current in the tripped phase, the differential protection single trip command recovery discrimination module is entered; when it is judged that there is current in the tripped phase, if the positive sequence differential current or the negative sequence differential current is greater than the threshold current, the differential protection delays three trips; otherwise, the differential protection reports a CT secondary neutral line disconnection alarm, and the differential protection single trip command recovery discrimination module is entered;

[0133] The differential protection single trip command recovery judgment module is used to receive the differential protection single trip command and start the reclosing.

[0134] Furthermore, it involves the following specific functions:

[0135] The single-phase fault phase zone discrimination element in the zone is used to judge that when the phase of the sum of the zero-negative sequence differential current phasors of each phase as the reference phase is between -45° and 45° relative to the positive-sequence differential current phasor with phase A as the reference phase, it is judged as the A-phase fault phase zone; when it is between -165° and -75°, it is judged as the B-phase fault phase zone; when it is between 75° and 165°, it is judged as the C-phase fault phase zone.

[0136] Three-phase operation judgment of the line: if one or two phases have a jumper open and there is no current in the corresponding phases, the line is judged to be in a non-full-phase operation state; if there is no jumper open in all three phases or there is current in all three phases, the line is judged to be in a three-phase operation state.

[0137] Three-trip judgment of differential protection: When the line is not in full-phase state, the operating phase split-phase current differential meets the action conditions, and at the same time, the positive-sequence or negative-sequence current differential protection meets the action conditions, the differential protection will trip three times; when the line is in three-phase operation state, when two-phase or three-phase split-phase current differential meets the action conditions, if there is no zero-sequence and negative-sequence differential current, or it is not in the phase selection area of ​​the phase area judgment element, the differential protection will trip three times.

[0138] Single-trip discrimination of differential protection: When the line is in three-phase operation state, the differential protection will trip when one-phase split-phase current differential meets the action conditions; when two-phase or three-phase split-phase current differential meets the action conditions, if there is zero-sequence or negative-sequence differential current, and the phase zone discrimination element judges it as a single-phase fault, the differential protection will trip; when the zero-sequence current differential protection meets the action conditions, the phase zone discrimination element judges it as a single-phase fault, and the differential protection will trip.

[0139] Differential protection delay three trip judgment: When the line is in three-phase operation state and the zero-sequence current differential protection meets the action conditions, the phase area discrimination element does not judge it as a single-phase fault, and the differential delay three trips; after the differential protection single trips, there is still current in the tripped phase, and the positive sequence differential current or negative sequence differential current is greater than the threshold current I K2 , the differential protection delay is three trips.

[0140] CT secondary neutral line break judgment: During the three-phase operation of the line, when there is still current in the tripped phase after the differential protection single trips, if the positive sequence differential current and the negative sequence differential current are both less than or equal to the threshold current I K2 , the protection reports a CT secondary neutral line break alarm; during non-full-phase operation of the line, the operating phase phase current differential protection meets the action conditions, but the positive-sequence differential protection or the negative-sequence differential protection does not meet the action conditions, and the protection reports a CT secondary neutral line break alarm.

[0141] Differential protection single trip command withdrawal judgment: After the differential protection single trips, if it is judged that there is no current in the tripped phase, the single trip command will be withdrawn; if it is judged that there is current in the tripped phase, if the positive sequence differential current and the negative sequence differential current are both less than or equal to the threshold current I K2 , retract the single trip command. For the single-phase reclosing mode, the reclosing starts at the same time as the protection single trip command is retracted.

[0142] Compared with the prior art, the beneficial effects of the present invention include at least:

[0143] Based on the main judgment criterion of the original longitudinal current differential protection function, the present invention proposes a single-phase fault phase zone discrimination element in the area formed according to the phase relationship between the sum of the zero-negative sequence differential current phasor and the positive-sequence differential current phasor. The element has high sensitivity and is not affected by transition resistance, and realizes auxiliary discrimination of the action phase zone of the single-phase fault in the line area. The differential protection determines the A-phase, B-phase and C-phase action phase zones of the single-phase grounding fault in the line area according to the phase relationship between the sum of the zero-negative sequence differential current phasor and the positive-sequence differential current phasor, and can prevent the three-phase tripping and the erroneous tripping of the non-fault phase when the CT secondary neutral line is broken in the single-phase grounding fault in the line area.

[0144] When the differential protection of the present invention trips a single phase, on the basis of the original protection judging that the fault phase has no current and retracting the tripping command, a method for judging that the positive sequence differential current and the negative sequence differential current both return and also retracting the tripping command is proposed, so that the protection single tripping starts the reclosing and the situation of identifying the disconnection of the secondary neutral line of the CT is completed, and the problem that the protection issues a single tripping command when a single-phase grounding fault occurs in the line area after the secondary neutral line of the CT is disconnected, but the fault phase continues to have current and does not meet the tripping command retraction condition, resulting in a protection delay of three trips, can be solved, and the situation that the reclosing fails in the single reclosing mode can be prevented;

[0145] The present invention proposes that when there is still current in the fault phase after the protection sends a single trip command, and the positive sequence differential current and the negative sequence differential current are less than or equal to the threshold current, it is judged that the CT secondary neutral line is abnormal; during the non-full-phase operation of the line, when the phase-separated differential protection meets the action conditions, and both the positive sequence current differential and the negative sequence current differential do not meet the action conditions, it is judged that the CT secondary neutral line is abnormal; the protection reports the CT secondary neutral line disconnection alarm. The above method can solve the problem that the original protection cannot effectively identify the CT secondary neutral line disconnection when there is a fault in the line area.

[0146] The present invention proposes to use a high-ratio positive-sequence differential current as a floating threshold when judging whether each phase has a zero-sequence differential current or a negative-sequence differential current. This can effectively prevent the generation of a zero-negative-sequence unbalanced current in the secondary during a three-phase short-circuit fault. Since the phase of the unbalanced current is random, the phase zone discrimination element may misjudge it as a single-phase fault phase zone.

[0147] 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 method for preventing the non-reclosing of faults in the zone when the secondary neutral line of the CT is disconnected, characterized in that: include: Step 1, setting a phase zone discrimination element for discriminating a single-phase fault in a zone according to the phase relationship between the sum of zero-negative sequence differential current and the positive sequence differential current, and proceeding to step 2; Step 2, determine the operating state of the line. If the line is in a non-full-phase operating state, proceed to step 3; if the line is in a three-phase operating state, proceed to step 4; Step 3: When the fault in the area or the disconnection of the secondary neutral line of the CT is determined based on the phase current differential and the positive and negative sequence current differential, the differential protection trips or disconnection alarm is performed; Step 4: If the three-phase current differentials do not meet the action conditions, then go to step 5; otherwise, when only one phase current differential meets the action conditions, the protection trips once, and go to step 6; when two or three phase current differentials meet the action conditions and there is a zero-sequence or negative-sequence differential current, the phase zone discrimination element is put into operation. When the phase zone discrimination element identifies a single-phase fault in the zone, the differential protection trips once, and go to step 6; otherwise, the differential protection trips three times; Step 5: When the zero-sequence current differential meets the action conditions, the phase zone discrimination element is put into operation. If the phase zone discrimination element determines that a single-phase fault occurs in the zone, the differential protection trips once and enters step 6. Otherwise, the differential protection trips three times after a delay. Step 6: After the differential protection issues a single trip command, if it is determined that there is no current in the tripped phase, go to step 7; When judging whether there is current in the tripping phase, if the positive sequence differential current or the negative sequence differential current is greater than the threshold current I K2 , differential protection delay three jumps; Otherwise, the differential protection reports a CT secondary neutral line disconnection alarm and goes to step 7; Step 7: The differential protection receives a single trip command and starts the reclosing.

2. The method for preventing the non-overlapping of faults in the zone when the secondary neutral line of a CT is disconnected according to claim 1 is characterized in that: The criterion for the phase zone discriminating element to discriminate a single-phase fault in the zone is: Where arg is the phase sign; are phase angle thresholds, respectively; They are respectively the positive sequence differential current, negative sequence differential current and zero sequence differential current with phase A as the reference phase; and They are respectively the positive sequence currents of the M side and the N side with phase A as the reference phase; and They are the negative sequence currents on the M side and the N side respectively, taking phase A as the reference phase; and They are the zero-sequence currents on the M and N sides respectively, with phase A as the reference phase.

3. The method for preventing the non-overlapping of faults in the zone when the secondary neutral line of a CT is disconnected according to claim 2 is characterized in that: is -45°, is 45°, is -165°, is -75°, is 75°, is 165°.

4. The method for preventing the non-overlapping of faults in the zone when the secondary neutral line of a CT is disconnected according to claim 1 is characterized in that: In step 2, the operation status of the line is determined as follows: If one or two phases have a jumper open and there is no current in the corresponding phases, the line is judged to be in a non-full-phase operation state; if there is no jumper open in all three phases or there is current in all three phases, the line is judged to be in a three-phase operation state.

5. The method for preventing the non-overlapping of faults in the zone when the secondary neutral line of a CT is disconnected according to claim 4, characterized in that: The method for determining whether there is current in each phase is: If the currents on both sides of phase A, phase B or phase C are greater than or equal to 0.05In, there is current in the corresponding phase, otherwise there is no current in the corresponding phase, and In is the secondary rated current.

6. The method for preventing the non-overlapping of faults in the zone when the secondary neutral line of a CT is disconnected according to claim 1 is characterized in that: In step 3, if the operating phase current differential meets the action conditions and the positive-sequence or negative-sequence current differential protection meets the action conditions, the differential protection trips; if the operating phase current differential meets the action conditions but both the positive-sequence and negative-sequence current differential protections do not meet the action conditions, the protection reports a CT secondary neutral line break alarm.

7. The method for preventing the non-overlapping of faults in the zone when the secondary neutral line of a CT is disconnected according to claim 1 is characterized in that: In step 4, the conditions for determining whether there is a zero-sequence differential current or a negative-sequence differential current are: When the following discriminants are satisfied at the same time, it is judged that there is zero-sequence differential current or negative-sequence differential current in phase A: When the following discriminants are satisfied at the same time, it is judged that there is zero-sequence differential current or negative-sequence differential current in phase B: When the following discriminants are satisfied at the same time, it is judged that there is zero-sequence differential current or negative-sequence differential current in phase C: In the formula, I K1 is the asymmetric current threshold; They are respectively the positive sequence differential current, negative sequence differential current and zero sequence differential current with phase A as the reference phase.

8. The method for preventing the non-overlapping of faults in the zone when the secondary neutral line of a CT is disconnected according to claim 7, characterized in that: I K1 It is 60A.

9. The method for preventing the non-overlapping of faults in the zone when the secondary neutral line of a CT is disconnected according to claim 1, characterized in that: In step 6, I K2 Take the minimum value among 0.06In, 0.3 times the phase current differential protection setting and 0.3 times the zero-sequence current differential protection setting.

10. The method for preventing the non-overlapping of faults in the zone when the secondary neutral line of a CT is disconnected according to claim 1, characterized in that: The delay time of the differential delay three jumps is 150ms.

11. A system for preventing the non-reclosing of faults in the zone when the secondary neutral line of a CT is disconnected, using the method according to any one of claims 1 to 10, characterized in that: The system comprises: A phase zone discrimination element setting module is used to set a phase zone discrimination element for discriminating a single-phase fault in a zone according to a phase relationship between the sum of zero-negative sequence differential current and the positive sequence differential current; The line operation status judgment module is used to judge the operation status of the line. If the line is not in full-phase operation status, it will enter the zone fault and disconnection judgment module; if the line is in three-phase operation status, it will enter the phase current differential protection single trip judgment module; The internal fault and disconnection discrimination module is used to identify the internal fault or CT secondary neutral line disconnection according to the phase current differential and the positive and negative sequence current differential, and perform differential protection triple tripping or disconnection alarm; The single-trip discrimination module of the phase-splitting current differential protection is used to enter the single-trip discrimination module of the zero-sequence current differential protection if the three-phase phase current differential does not meet the action conditions. Otherwise, when only one-phase current differential meets the action conditions, the protection will trip once and enter the current discrimination module; when two-phase or three-phase current differentials meet the action conditions and there is a zero-sequence or negative-sequence differential current, the phase zone discrimination element is put into operation. When the phase zone discrimination element identifies a single-phase fault in the zone, the differential protection will trip once and enter the current discrimination module, otherwise the differential protection will trip three times; The zero-sequence current differential protection single-trip discrimination module is used to put the phase zone discrimination element into operation when the zero-sequence current differential meets the action conditions. If the phase zone discrimination element determines that a single-phase fault occurs in the zone, the differential protection will trip once and enter the current discrimination module. Otherwise, the differential protection will delay three trips. The current discrimination module is used for the differential protection to send a single trip command. When it is judged that there is no current in the tripped phase, the differential protection single trip command recovery discrimination module is entered; when it is judged that there is current in the tripped phase, if the positive sequence differential current or the negative sequence differential current is greater than the threshold current, the differential protection delays three trips; otherwise, the differential protection reports a CT secondary neutral line disconnection alarm, and the differential protection single trip command recovery discrimination module is entered; The differential protection single trip command recovery judgment module is used to receive the differential protection single trip command and start the reclosing.

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