Grounding transformer single-phase grounding protection method

Abstract

The invention discloses a single-phase grounding protection method for a grounding transformer, and the method comprises the steps: collecting zero-sequence current through a zero-sequence current transformer disposed at a power side of the grounding transformer and a zero-sequence current transformer disposed at a neutral point side of the grounding transformer, and inputting the zero-sequence current to respective corresponding protection function modules; setting a zero-sequence overcurrent protection setting matching principle of each protection function module; when a single-phase grounding fault occurs in a certain phase winding of the grounding transformer, zero-sequence current protection action logic is set; when a single-phase grounding fault occurs in a feed-out loop except a grounding transformer under a bus, zero-sequence current protection action logic is set. The method overcomes the defects of single-phase grounding protection of the traditional grounding transformer, effectively identifies the single-phase grounding fault of the grounding transformer, trips off the power supply side switch of the grounding transformer before the neutral point zero-sequence current protection action of the grounding transformer, eliminates the grounding fault, and prevents the whole section of bus at the low-voltage side of the main transformer from losing power.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of high-voltage power supply and distribution system relay protection, and in particular to a single-phase grounding protection method for a grounding transformer. BACKGROUND

[0002] The primary winding of a grounding transformer is actually a star coupler, which is widely used in power supply and distribution systems using a large amount of high-voltage cables, and is often used to construct an artificial neutral point to achieve small-resistance grounding of the system. Figure 1 For an application example of the grounding transformer in a small-resistance grounding system, in the figure, CB 高 is a high-voltage side switch of a main transformer, CB 低 is a low-voltage side switch of the main transformer, R is a grounding resistance led out from a neutral point of the grounding transformer, CB ZN is a power supply side switch of the grounding transformer, CB1...CB n are switches corresponding to feeder lines 1...feeder line n respectively, CT is a power supply side three-phase current transformer of the grounding transformer, ZCT R is a zero-sequence current transformer led out from the resistance side of the neutral point of the grounding transformer, 50+51 are current instantaneous trip protection and overcurrent protection function modules of the power supply side of the grounding transformer, 51G1...51G n : are zero-sequence current protection function modules corresponding to feeder lines 1...feeder line n respectively, 51G R is a neutral point zero-sequence current protection function module of the grounding transformer.

[0003] Since the zero-sequence impedance of the grounding transformer is very small, when a metallic single-phase grounding fault occurs in the system, a fault current will flow in the neutral point resistance loop, and the fault current value= is generally large, up to 100A-1000A. Among them, U N is a system phase voltage.

[0004] According to the operation setting and technical regulations of the power grid relay protection device, the current instantaneous trip protection and overcurrent protection of the grounding transformer generally act to trip the power supply side switch of the grounding transformer, and the zero-sequence current protection installed on the neutral point of the grounding transformer generally acts to trip the high-voltage side switch and the low-voltage side switch of the main transformer.

[0005] When the zero-sequence current protection installed on the neutral point of the grounding transformer acts, the high-voltage side switch and the low-voltage side switch of the main transformer are generally tripped, resulting in a large power outage range. If a single-phase grounding fault occurs in the grounding transformer itself, the switches on both sides of the main transformer will also be tripped, not only expanding the power outage range, but also not having any protection signal indicating that a single-phase grounding fault has occurred in the grounding transformer itself, making it difficult to troubleshoot the fault on site and affecting the processing and power recovery time.

[0006] In practical applications, for example,Figure 2 As shown, there is also a grounding transformer power side zero sequence current transformer ZCT configured on the grounding transformer power side ZN And a zero sequence current protection function module 51G ZN When a single-phase ground fault occurs in the grounding transformer itself, it is hoped that 51G ZN Protection action, send trip command to trip the grounding transformer power side switch CB ZN .

[0007] But in fact, when a metallic single-phase ground fault occurs in a phase winding of the grounding transformer, the ZCT ZN Cannot detect single-phase ground fault current, only the grounding transformer neutral point lead-out resistance side zero sequence current transformer ZCT R Can detect fault current Therefore, the grounding transformer neutral point zero sequence current protection function module 51G R Action, trip the main transformer switch on both sides.

[0008] When a single-phase metallic ground fault occurs in the system feeder 1, ZCT ZN And ZCT R Can detect single-phase ground fault current If the action time of the zero sequence current protection function module 51G1 of the feeder 1 is longer than that of 51G ZN , then the single-phase ground fault of the feeder 1 will cause the grounding transformer power side zero sequence overcurrent protection function module 51G ZN First action, trip the grounding transformer power side switch CB ZN , so that the entire system becomes an ungrounded system, and the ground fault of feeder 1 cannot be removed from the system, which may cause arc ground overvoltage, break down the weak point of system insulation, cause interphase short circuit fault, etc., so such practice is quite dangerous.

[0009] There are generally two ways to protect the single-phase ground fault of the grounding transformer: (1) Differential current method to identify single-phase fault of grounding transformer Adopt the grounding transformer zero sequence differential protection method, by setting zero sequence current transformers on the high voltage side and the neutral point side, collecting zero sequence current, calculating zero sequence differential current and zero sequence braking current, combining with the set threshold value and the proportion coefficient, to judge whether the protection condition is met, and trip the grounding transformer switch after time delay, to realize the identification and isolation of the grounding transformer itself ground fault, and avoid the full power outage caused by the trip of the main transformer high and low voltage switch. This method improves the accuracy of fault identification of the grounding transformer and the reliability of protection action, reduces the power outage range, and to some extent guarantees the safe and reliable operation of the power grid.

[0010] Problem Analysis: When using zero-sequence differential protection, the polarity of the zero-sequence current transformer must be correct; otherwise, it will lead to maloperation of the protection. During normal operation, since there is no zero-sequence component in the operating current, the polarity of the zero-sequence current transformer is generally difficult to verify; moreover, the capacity of the grounding transformer is relatively small (hundreds of kVA level), and it is generally not equipped with a relay protection device specifically designed for differential current protection.

[0011] (2) Zero-sequence power absolute value method for identifying single-phase faults in grounding transformer body This method is based on obtaining the zero-sequence voltage of the system where the grounding transformer is located, as well as the zero-sequence current on the power supply side and neutral point side of the grounding transformer, and calculating the absolute value of the zero-sequence active power on the power supply side and neutral point side of the grounding transformer; comparing the absolute value of the zero-sequence active power on the power supply side and neutral point side of the grounding transformer with a preset active power threshold value; if the comparison result meets the preset single-phase grounding fault discrimination condition of the grounding transformer body, the grounding fault protection is activated, and the power supply side switch CB of the grounding transformer is tripped. ZN .

[0012] When a single-phase ground fault occurs in the grounding transformer itself, there is a significant difference in the zero-sequence active power flowing through the power supply side and the neutral point side of the grounding transformer. The main principle is that when a metallic single-phase ground fault occurs in a certain phase winding of the grounding transformer, ZCT... ZN It is impossible to detect a single-phase ground fault current. Yes, only the zero-sequence current transformer ZCT on the neutral point lead-out resistor side of the grounding transformer. R It can detect fault current.

[0013] Problem Analysis: Although this method, based on the absolute value of zero-sequence power, avoids the current transformer polarity problem of the differential current method, it requires the acquisition of the system's zero-sequence voltage and the creation of dedicated power components, making the overall protection function relatively complex. Furthermore, there are currently almost no dedicated zero-sequence power protection relays for grounding transformers on the market, making practical application difficult. Summary of the Invention

[0014] The technical problem to be solved by the present invention is to provide a method for single-phase grounding protection of grounding transformers. This method overcomes the defects of traditional single-phase grounding protection of grounding transformers, effectively identifies the single-phase grounding fault of the grounding transformer itself, and trips the power supply side switch of the grounding transformer before the neutral point zero-sequence current protection of the grounding transformer operates, thereby clearing the grounding fault and preventing the entire low-voltage side busbar of the main transformer from losing power.

[0015] To solve the above-mentioned technical problems, the single-phase grounding protection method for grounding transformers of the present invention includes the following steps: Step 1: Zero-sequence current acquisition, using the zero-sequence current transformer ZCT configured on the power supply side of the grounding transformer. ZNZero-sequence current transformer ZCT configured on the neutral point side R The zero-sequence current is collected, and the secondary current of each zero-sequence current transformer is input to its corresponding protection function module 51G. ZN and 51G R ; Step 2: Coordination principle of zero-sequence overcurrent protection setting. The zero-sequence current protection setting value is calculated based on the small resistance grounding current limiting value of the actual power supply and distribution system. Protection Function Module 51G ZN Zero-sequence overcurrent protection operating current and operating time settings, and zero-sequence protection function module 51G for the feeder n below the bus. n The setpoint values ​​are consistent; Protection Function Module 51G R The current setting values ​​for stage I and stage II of the zero-sequence overcurrent protection are consistent, and are also present in the protection function module 51G. ZN A reliability factor of 1.1 is taken based on the current setting value, and the protection function module 51G is used. R I-stage action time setting value and protection function module 51G ZN The action time setting value is consistent and less than that of the protection function module 51G. R Operating time of stage II of zero-sequence overcurrent protection; Protection Function Module 51G R The setting time of the zero-sequence overcurrent protection stage II is greater than the setting time of the zero-sequence current protection of any feeder circuit under the bus, but less than the time specified on the grounding resistance nameplate for withstanding the current limit value. Zero-sequence current protection and protection function module 51G for each feeder circuit of the busbar ZN The current setting value of zero-sequence current protection avoids the system capacitive current; Step 3: If a single-phase ground fault occurs in a winding of the grounding transformer, the zero-sequence current protection operation logic is as follows: When the protection function module 51G is satisfied ZN The zero-sequence current protection did not activate, and the protection function module 51G... R If the zero-sequence overcurrent protection stage I trips, it will trigger the tripping signal to the grounding transformer power supply side switch CB. ZN The instruction disconnects the faulty grounding transformer from the system; if due to the protection function module 51G R Zero-sequence overcurrent stage I protection fails to operate, or an abnormality in the trip circuit causes switch CB to malfunction. ZN If the circuit breaker is not tripped, and the single-phase ground fault point of the grounding transformer cannot be isolated from the system, then the protection function module 51G... R Zero-sequence overcurrent protection stage II is still active. After a time delay, the protection function module 51G will activate. R Zero-sequence overcurrent protection (Stage II) trips the switches CB on both sides of the main transformer. 高 and CB低 Commands are used to implement remote backup protection functions; Step 4: If a single-phase ground fault occurs in the feeder circuit other than the grounding transformer on the busbar, the zero-sequence current protection operation logic is as follows: When the protection function module 51G is satisfied ZN Zero-sequence current protection tripped, and the protection function module 51G was activated. R If the zero-sequence overcurrent protection stage I also trips, then wait for the protection function module 51G to activate. R The zero-sequence overcurrent protection operates in stage II; if, during the waiting process, a fault feeder circuit on the busbar (excluding the grounding transformer) experiences a self-protection function module 51G. n Action, and cause the corresponding switch CB to move. n If the circuit breaker trips and the fault point is cleared, then the protection function module 51G... R Zero-sequence overcurrent protection stages I and II return to normal upon the disappearance of the fault current and no longer issue trip commands; if during the waiting period, the protection function module 51G of the fault circuit... n Protection fails to operate, or corresponding switch CB n Tripping circuit fault, switch CB n 51G protection module for preventing operation R The first stage of the zero-sequence overcurrent protection remains active, awaiting the protection function module 51G. R After the stage II delay of the zero-sequence overcurrent protection arrives, a tripping signal is issued to the switches CB on both sides of the main transformer. 高 and CB 低 The command enables remote backup protection.

[0016] Furthermore, the zero-sequence current of each feeder circuit of the busbar and the power supply side of the grounding transformer is collected using a dedicated zero-sequence current transformer, or by synthesizing the zero-sequence current through three phase current transformers, or by calculating the zero-sequence current after collecting the three-phase current through a relay protection device. Because the single-phase grounding protection method for grounding transformers of this invention adopts the above-mentioned technical solution, namely, this method collects zero-sequence current through zero-sequence current transformers configured on the power supply side and neutral point side of the grounding transformer, and inputs them into their respective corresponding protection function modules; sets the zero-sequence overcurrent protection setting coordination principle for each protection function module; sets the zero-sequence current protection operation logic when a single-phase grounding fault occurs in a certain phase winding of the grounding transformer; and sets the zero-sequence current protection operation logic when a single-phase grounding fault occurs in the feeder circuit of the busbar other than the grounding transformer. This method overcomes the defects of traditional single-phase grounding protection for grounding transformers, effectively identifies single-phase grounding faults in the grounding transformer itself, and trips the power supply side switch of the grounding transformer before the neutral point zero-sequence current protection of the grounding transformer operates, thus clearing the grounding fault and preventing the entire low-voltage side busbar of the main transformer from losing power. Attached Figure Description

[0017] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments: Figure 1 This is a schematic diagram illustrating the application of a grounding transformer in a low-resistance grounding system. Figure 2 Schematic diagram of zero-sequence current protection configured on the power supply side of the grounding transformer; Figure 3 This is a flowchart of the single-phase grounding protection method for grounding transformers according to the present invention; Figure 4 This is a schematic diagram of single-phase grounding protection for a grounding transformer in a 35kV power supply and distribution system. Detailed Implementation

[0018] Implementation, for example Figure 3 As shown, the single-phase grounding protection method for grounding transformers of the present invention includes the following steps: Step 1: Zero-sequence current acquisition, using the zero-sequence current transformer ZCT configured on the power supply side of the grounding transformer. ZN Zero-sequence current transformer ZCT configured on the neutral point side R The zero-sequence current is collected, and the secondary current of each zero-sequence current transformer is input to its corresponding protection function module 51G. ZN and 51G R ; Step 2: Coordination principle of zero-sequence overcurrent protection setting. The zero-sequence current protection setting value is calculated based on the small resistance grounding current limiting value of the actual power supply and distribution system. Protection Function Module 51G ZN Zero-sequence overcurrent protection operating current and operating time settings, and zero-sequence protection function module 51G for the feeder n below the bus. n The setpoint values ​​are consistent; Protection Function Module 51G R The current setting values ​​for stage I and stage II of the zero-sequence overcurrent protection are consistent, and are also present in the protection function module 51G. ZN A reliability factor of 1.1 is taken based on the current setting value, and the protection function module 51G is used. R I-stage action time setting value and protection function module 51G ZN The action time setting value is consistent and less than that of the protection function module 51G. R The operating time of stage II of zero-sequence overcurrent protection; Protection Function Module 51G R The setting time of the zero-sequence overcurrent protection stage II is greater than the setting time of the zero-sequence current protection of any feeder circuit under the bus, but less than the time specified on the grounding resistance nameplate for withstanding the current limit value. Zero-sequence current protection and protection function module 51G for each feeder circuit of the busbar ZNThe current setting value of zero-sequence current protection avoids the system capacitive current; Step 3: If a single-phase ground fault occurs in a winding of the grounding transformer, the zero-sequence current protection operation logic is as follows: When the protection function module 51G is satisfied ZN The zero-sequence current protection did not activate, and the protection function module 51G... R If the zero-sequence overcurrent protection stage I trips, it will trigger the tripping signal to the grounding transformer power supply side switch CB. ZN The instruction disconnects the faulty grounding transformer from the system; if due to the protection function module 51G R Zero-sequence overcurrent stage I protection fails to operate, or an abnormality in the trip circuit causes switch CB to malfunction. ZN If the circuit breaker is not tripped, and the single-phase ground fault point of the grounding transformer cannot be isolated from the system, then the protection function module 51G... R Zero-sequence overcurrent protection stage II is still active. After a time delay, the protection function module 51G will activate. R Zero-sequence overcurrent protection (Stage II) trips the switches CB on both sides of the main transformer. 高 and CB 低 Commands are used to implement remote backup protection functions; Step 4: If a single-phase ground fault occurs in the feeder circuit other than the grounding transformer on the busbar, the zero-sequence current protection operation logic is as follows: When the protection function module 51G is satisfied ZN Zero-sequence current protection tripped, and the protection function module 51G was activated. R If the zero-sequence overcurrent protection stage I also trips, then wait for the protection function module 51G to activate. R The zero-sequence overcurrent protection operates in stage II; if, during the waiting process, a fault feeder circuit on the busbar (excluding the grounding transformer) experiences a self-protection function module 51G. n Action, and cause the corresponding switch CB to move. n If the circuit breaker trips and the fault point is cleared, then the protection function module 51G... R Zero-sequence overcurrent protection stages I and II return to normal upon the disappearance of the fault current and no longer issue trip commands; if during the waiting period, the protection function module 51G of the fault circuit... n Protection fails to operate, or corresponding switch CB n Tripping circuit fault, switch CB n 51G protection module for preventing operation R The first stage of the zero-sequence overcurrent protection remains active, awaiting the protection function module 51G. R After the stage II delay of the zero-sequence overcurrent protection arrives, a tripping signal is issued to the switches CB on both sides of the main transformer. 高 and CB 低 The command enables remote backup protection.

[0019] Preferably, the zero-sequence current of each feeder circuit of the busbar and the power supply side of the grounding transformer is collected using a dedicated zero-sequence current transformer, or by synthesizing the zero-sequence current through three phase current transformers, or by collecting the three-phase current through a relay protection device and then calculating the zero-sequence current.

[0020] like Figure 4 As shown, a 35kV power supply and distribution system and its zero-sequence protection function configuration and zero-sequence current protection setting value are shown.

[0021] Fault Scenario 1: If a single-phase ground fault occurs in one phase winding of a 35kVA grounding transformer, the zero-sequence current transformer ZCT... ZN No single-phase ground fault current was detected; the corresponding protection module 51G... ZN The zero-sequence current protection will not activate; the zero-sequence current transformer ZCT on the neutral point lead-out resistor side of the grounding transformer will not activate. R The 51G protection module can detect fault current. R The zero-sequence overcurrent protection stage I is activated. After a 0.1-second delay, the normally open contact of the relay protection closes, and the grounding transformer power supply side switch CB... ZN The trip circuit is energized, and switch CB... ZN The circuit breaker trips, disconnecting the grounding transformer from the system, isolating the fault point, temporarily ensuring production, and allowing equipment management personnel to address the grounding transformer fault point after a planned shutdown; if the fault is due to the 51G protection module... R Zero-sequence overcurrent stage I protection fails to operate, or an abnormality in the trip circuit causes switch CB to malfunction. ZN Without tripping, the single-phase ground fault point of the grounding transformer cannot be isolated from the system, so the protection function module 51G... R The zero-sequence overcurrent protection stage II is still active. After a certain delay, the protection function module 51G will activate. R The zero-sequence overcurrent protection stage II sends a command to trip the switches on both sides of the main transformer, realizing the remote backup protection function.

[0022] Fault Scenario 2: If a single-phase ground fault occurs in a feeder circuit, such as a single-phase ground fault in the feeder line to the downstream electrical room, the zero-sequence current transformer ZCT... ZN and ZCT R All can detect single-phase ground fault current, protection function module 51G ZN Zero-sequence current protection tripped, opening the normally closed contacts of the relay protection; protection function module 51G. R The zero-sequence overcurrent protection stage I also tripped, the normally open contact of the relay protection closed, and the grounding transformer power supply side switch CB... ZN The trip circuit is not energized, switch CB ZNIf the circuit breaker does not trip, the zero-sequence current protection module 51G1 of the feeder to the downstream electrical room will trip after a 0.5-second delay, and then switch CB1 will trip. If the protection module 51G1 fails to trip, or if there is a fault in the tripping circuit of switch CB1, and switch CB1 fails to trip, the neutral point protection module 51G1 of the grounding transformer will trip. R Both Stage I and Stage II protection of the zero-sequence overcurrent protection will activate, ultimately controlled by the protection function module 51G. R After a 5-second delay, the zero-sequence overcurrent protection stage II sends a trip command to the switches CB on both sides of the main transformer. 低 and CB 高 This enables remote backup protection.

[0023] During implementation, the following matters must be noted regarding relay protection settings: 1. Zero-sequence current protection for any feeder circuit under the busbar (including the 51G grounding transformer protection module) ZN The current setting value of the zero-sequence current protection must avoid the system capacitive current and can be set consistently (converted to the primary side current). For example, in the example, the zero-sequence current setting value of all feeder circuits is 1A (because the zero-sequence current transformer is 100 / 5, the current converted to the primary side is 20A). 2. Protection Function Module 51G R The current setting value for stage I of the zero-sequence overcurrent protection can be consistent with the current setting value for stage II, but this must be done in the protection function module 51G. ZN A certain reliability coefficient (generally 1.1) is applied based on the current setting value. The operating time settings for both can be the same, but must be less than that of the 51G protection function module. R The operating time of the zero-sequence overcurrent protection stage II. For example, the protection function module 51G in the example. R The zero-sequence overcurrent protection stage I and stage II current settings are 1.1A, and the operating time is the same as that of the protection function module 51G. ZN The time is 0.1 seconds, which is less than the 51G protection function module. R The setting time for stage II of zero-sequence overcurrent protection is 5 seconds; 3. Protection function module 51G R The setting time of the zero-sequence overcurrent protection stage II must be greater than the setting time of the zero-sequence current protection for any feeder circuit under the bus, but must be less than the time during which the grounding resistance can withstand the current limiting value. For example, in the example, a setting of 5 seconds is greater than the maximum operating time of the zero-sequence current protection for the feeder circuit under the bus (0.5 seconds) and less than the time during which the grounding resistance can withstand the current limiting value (10 seconds).

[0024] When a single-phase ground fault occurs in the grounding transformer itself, this method can trip the power supply side switch CBzn of the grounding transformer before the neutral point zero-sequence current protection trips, thus cutting off the fault point of the grounding transformer and preventing the entire low-voltage side busbar of the main transformer from losing power.

[0025] Compared with the differential current method and the absolute value of active power method, this method does not require the installation of a relay protection device with dedicated differential current protection function, nor does it require the use of a protection device with a dedicated power calculation module. Figure 2 Under the conventional relay protection zero-sequence overcurrent function configuration shown, by simply setting appropriate zero-sequence current protection settings, applying the zero-sequence current protection action node of the relay protection device, and setting relevant output action logic interlocks, the grounding fault of the grounding transformer itself can be determined, reducing the scope of power outages. This method is simple, practical, reliable, and incurs almost no cost.

Claims

1. A method for single-phase grounding protection of a grounding transformer, characterized in that... Includes the following steps: Step 1: Zero-sequence current acquisition, using the zero-sequence current transformer ZCT configured on the power supply side of the grounding transformer. ZN Zero-sequence current transformer ZCT configured on the neutral point side R The zero-sequence current is collected, and the secondary current of each zero-sequence current transformer is input to its corresponding protection function module 51G. ZN and 51G R ; Step 2: Coordination principle of zero-sequence overcurrent protection setting. The zero-sequence current protection setting value is calculated based on the small resistance grounding current limiting value of the actual power supply and distribution system. Protection Function Module 51G ZN Zero-sequence overcurrent protection operating current and operating time settings, and zero-sequence protection function module 51G for the feeder n below the bus. n The setpoint values ​​are consistent; Protection Function Module 51G R The current setting values ​​for stage I and stage II of the zero-sequence overcurrent protection are consistent, and are also present in the protection function module 51G. ZN A reliability factor of 1.1 is taken based on the current setting value, and the protection function module 51G is used. R I-stage action time setting value and protection function module 51G ZN The action time setting value is consistent and less than that of the protection function module 51G. R The operating time of stage II of zero-sequence overcurrent protection; Protection Function Module 51G R The setting time of the zero-sequence overcurrent protection stage II is greater than the setting time of the zero-sequence current protection of any feeder circuit under the bus, but less than the time specified on the grounding resistance nameplate for withstanding the current limit value. Zero-sequence current protection and protection function module 51G for each feeder circuit of the busbar ZN The current setting value of zero-sequence current protection avoids the system capacitive current; Step 3: If a single-phase ground fault occurs in a winding of the grounding transformer, the zero-sequence current protection operation logic is as follows: When the protection function module 51G is satisfied ZN The zero-sequence current protection did not activate, and the protection function module 51G... R If the zero-sequence overcurrent protection stage I trips, it will trigger the tripping signal to the grounding transformer power supply side switch CB. ZN The instruction is to disconnect the faulty grounding transformer from the system; If due to the protection function module 51G R Zero-sequence overcurrent stage I protection fails to operate, or an abnormality in the trip circuit causes switch CB to malfunction. ZN If the circuit breaker is not tripped, and the single-phase ground fault point of the grounding transformer cannot be isolated from the system, then the protection function module 51G... R Zero-sequence overcurrent protection stage II is still active. After a time delay, the protection function module 51G will activate. R Zero-sequence overcurrent protection (Stage II) trips the switches CB on both sides of the main transformer. 高 and CB 低 Commands are used to implement remote backup protection functions; Step 4: If a single-phase ground fault occurs in the feeder circuit other than the grounding transformer on the busbar, the zero-sequence current protection operation logic is as follows: When the protection function module 51G is satisfied ZN Zero-sequence current protection tripped, and the protection function module 51G was activated. R If the zero-sequence overcurrent protection stage I also trips, then wait for the protection function module 51G to activate. R The zero-sequence overcurrent protection operates in stage II; if, during the waiting process, a fault feeder circuit on the busbar (excluding the grounding transformer) experiences a self-protection function module 51G. n Action, and cause the corresponding switch CB to move. n If the circuit breaker trips and the fault point is cleared, then the protection function module 51G... R Zero-sequence overcurrent protection stages I and II return to normal upon the disappearance of the fault current and no longer issue trip commands; if during the waiting period, the protection function module 51G of the fault circuit... n Protection fails to operate, or corresponding switch CB n Tripping circuit fault, switch CB n 51G protection module for preventing operation R The first stage of the zero-sequence overcurrent protection remains active, awaiting the protection function module 51G. R After the stage II delay of the zero-sequence overcurrent protection arrives, a tripping signal is issued to the switches CB on both sides of the main transformer. 高 and CB 低 The command enables remote backup protection.

2. The method for single-phase grounding protection of a grounding transformer according to claim 1, characterized in that: The zero-sequence current of each feeder circuit of the busbar and the power supply side of the grounding transformer is collected using a dedicated zero-sequence current transformer, or by synthesizing the zero-sequence current through three phase current transformers, or by collecting the three-phase current through a relay protection device and then calculating the zero-sequence current.

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