A multi-terminal longitudinal differential protection system and method for traction network under double-sided through AT power supply mode

Abstract

The present application belongs to the technical field of electrified railway traction power supply, and particularly relates to a traction network multi-terminal longitudinal differential protection system and method under a double-sided through AT power supply mode. The system sets multi-terminal longitudinal differential protection elements according to uplink and downlink feeders, covers multi-terminal branches composed of traction substations, AT substations and partition substations, and each branch is configured with a current transformer. The protection element includes a power frequency variation current differential element, a T-line / F-line current differential element and a low-voltage differential current starting element, which are respectively adapted to fault transient and steady state characteristics, and realize rapid fault identification and accurate removal through cooperative action. The method combines current threshold criterion, voltage criterion and opposite side allowable signal to ensure the rapidity, selectivity, sensitivity and reliability of action, realize rapid removal of fault two-end power supply, reduce the fault isolation range, and ensure the safe and stable operation of the traction network.

Description

Technical Field

[0001] This invention belongs to the field of electrified railway traction power supply technology, specifically relating to a multi-terminal longitudinal differential protection system and method for traction network under a double-sided through-type AT power supply mode. Background Technology

[0002] In current electrified railways, the single-phase power supply method is still widely used due to its simple structure and low initial investment cost. Under this power supply method, each section of the traction network is powered by only one traction substation. Its power supply capacity is directly limited by the capacity configuration of the traction substation and the output power of the feeder. With the rapid development of my country's railways towards high speed and heavy load, the demand for train traction power continues to rise, and the capacity bottleneck of the single-phase power supply mode is becoming increasingly prominent. It is not only difficult to meet the continuous high power supply demand of 10,000-ton heavy-haul trains and 350km / h high-speed trains, but also may cause trains to lower their pantographs and slow down on steep gradient sections due to insufficient power supply capacity, which seriously affects transportation efficiency and line capacity.

[0003] Meanwhile, under single-sided power supply, electrical phase separation devices must be installed between adjacent power supply sections of the traction substation to achieve electrical isolation between different power supply sections and avoid phase-to-phase short circuit accidents. However, as an inherently weak link in the traction network, the electrical phase separation poses significant operational risks and negative impacts: when a train passes through an electrical phase separation, the pantograph needs to smoothly transition from one power supply section to another. During this process, a brief power outage (loss of power) is inevitable. This not only causes a momentary loss of train traction, affecting the smoothness of operation, but may also generate an electric arc due to pantograph detachment and re-contact, leading to overvoltage and overcurrent phenomena, accelerating the wear of the pantograph contactor and the contact wire, and even causing equipment damage such as insulator breakdown and traction converter failure. Especially in complex and dangerous mountainous areas, steep slopes, long tunnels, and other special sections, the deceleration and re-acceleration process when the train passes through an electrical phase separation not only significantly increases energy consumption, but may also cause the train to "run away" due to excessive slope resistance, posing a serious threat to driving safety.

[0004] To address these challenges, the bilateral through-power supply method, with its significant technical advantages, is gaining increasing attention and favor in the construction of railways in complex and challenging mountainous areas and heavy-haul railways with steep gradients. This power supply method connects the power supply sections of two adjacent traction substations, enabling the traction network of the same section to be jointly powered by both traction substations. This not only effectively increases the total power supply capacity of the traction network, meeting the high-power traction demands of high-speed, heavy-haul trains, but also significantly reduces the number of electrical phase-splitting devices required. It can even achieve through-power supply without phase-splitting in critical sections, fundamentally reducing the negative impact of phase-splitting on train operation and improving the safety and economy of line operation.

[0005] However, it is worth noting that the electrical topology of a bilateral through-power supply system differs fundamentally from that of a single-sided power supply system. When a short-circuit fault occurs at any point within a bilateral through-power supply section (including the incoming power supply, traction network lines, and equipment within the substation), the short-circuit point will simultaneously receive short-circuit currents from both traction substations, forming a fault characteristic of dual-side power supply current feeding. Traditional traction network protection systems are designed based on single-side power supply and single-source current feeding scenarios. Their protection criteria and operating logic only analyze the fault current on a single power supply side, making it impossible to accurately identify faults within and outside the protection zone under bilateral through-power supply, and even more difficult to achieve precise fault isolation. If traditional protection schemes are used, problems such as failure to operate during faults within the protection zone (i.e., only one power supply trips, while the other side continues to feed current to the fault point, and the fault cannot be completely eliminated) or false operation during faults outside the protection zone (i.e., faults outside the protection range are judged as faults within the protection zone, leading to power outages in the normal power supply section) may occur, seriously violating the four basic requirements of relay protection: selectivity, speed, sensitivity, and reliability.

[0006] Therefore, considering the technical characteristics of the double-sided through power supply method, its traction network protection configuration must adhere to two core principles: first, simultaneously and quickly disconnect the power supply at both ends of the fault point, completely cutting off the current source at the fault point through synchronous tripping of the power supplies on both sides, thus preventing the fault from escalating; second, minimize the fault isolation range, accurately locate the faulty section, disconnect only the faulty part, and ensure normal power supply to the non-faulty sections. As a core component of electrified railways, the traction network has a relatively high failure rate for its contact network, positive feeder (T-line / F-line), and other equipment, placing more stringent demands on the performance of the protection system. Based on this, developing a multi-terminal longitudinal differential protection system for the traction network adapted to the double-sided through AT power supply method has become an urgent need to ensure the safe and efficient operation of such railways. Summary of the Invention

[0007] This invention provides a multi-terminal longitudinal differential protection system and method for traction networks under a double-sided through-type AT power supply mode. This addresses the technical problems of existing traction network protection systems based on single-sided power supply (single power source) design, which cannot adapt to the fault characteristics of short-circuit points where the short-circuit current is provided by power sources on both sides under a double-sided through-type power supply mode. These systems fail to meet the four basic requirements of relay protection: selectivity, speed, sensitivity, and reliability, and cannot achieve the protection configuration principle of simultaneously and quickly disconnecting the power sources at both ends of the fault point and minimizing the fault isolation range.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A traction network multi-terminal longitudinal differential protection system under a double-sided through-type AT power supply mode includes a double-sided through-type power supply circuit composed of an up-line feeder and a down-line feeder. Traction substations, AT substations, and section substations are all covered on the up-line feeder and down-line feeder. The up-line feeder and down-line feeder covered by the traction substation, the up-line feeder and down-line feeder covered by the section substation, and the AT substation form a multi-terminal branch, and each branch is equipped with a current transformer. Multi-terminal longitudinal differential protection elements are set on the up-line feeder or down-line feeder respectively. The multi-terminal longitudinal differential protection elements include a power frequency change current differential element, a T contact line / F positive feeder current differential element, and a low-voltage differential current starting element.

[0009] The uplink feeder is provided with an uplink T-contact line and an uplink F-positive feeder, and the downlink feeder is provided with a downlink T-contact line and a downlink F-positive feeder.

[0010] The traction substation introduces external power through incoming lines 1 and 2. The power is distributed to the up-line T contact line, up-line F positive feeder line, down-line T contact line, and down-line F positive feeder line via double-pole circuit breakers, traction substation DL1, and traction substation DL2, providing power to trains in both directions. The up-line T contact line, up-line F positive feeder line, down-line T contact line, and down-line F positive feeder line all extend along the double-sided through power supply circuit and are respectively connected to the AT substation.

[0011] The AT is equipped with circuit breakers DL3, DL4, and DL5 connected in series, as well as a three-terminal differential protection element. The input terminal of circuit breaker DL3 is electrically connected to the upstream T contact line, upstream F positive feeder, downstream T contact line, and downstream F positive feeder extending from the traction substation. The output terminal of circuit breaker DL5 is electrically connected to the upstream T contact line, upstream F positive feeder, downstream T contact line, and downstream F positive feeder corresponding to the subsequent section, forming a double-sided through electrical circuit between the traction substation, the AT station, and the section station.

[0012] The substation is equipped with circuit breakers DL6 and DL7. Circuit breaker DL6 is connected in series between the upstream T contact line and the upstream F positive feeder extending from AT, and circuit breaker DL7 is connected in series between the downstream T contact line and the downstream F positive feeder extending from AT. The output terminals of circuit breakers DL6 and DL7 are electrically connected to the upstream T contact line, upstream F positive feeder, downstream T contact line, and downstream F positive feeder of the subsequent power supply section, respectively, forming a complete double-sided through power supply circuit.

[0013] A method for multi-terminal longitudinal differential protection of traction network under bilateral through-type AT power supply includes the following steps: Determine the maximum value of the effective value of the T contact line / F positive feeder current in the half-wave integral, set the fixed threshold and floating threshold of the current, and obtain the operating equation of the differential element that reflects the change of power frequency. When the operating conditions of the differential element for the change in power frequency are met, the differential element for the change in power frequency will operate and extend for a period of time before opening the positive power supply of the output relay. When a fault occurs on the T-line or F-line within the zone, if a differential protection permission signal is received from the other side, the phase voltage related to the differential protection action is compared with the phase-to-phase voltage of the rated voltage. Based on the comparison result, when the preset voltage criterion is met, the low-voltage differential current starting element operates, opening the positive power supply of the output relay.

[0014] The operating equation of the differential element for the power frequency variation is as follows:

[0015] In the formula, To obtain the maximum value of the effective value of the T-line / F-line current in the half-wave integral, The current setting value for the differential element is the power frequency variation value; The threshold is floating.

[0016] The preset voltage criterion is specifically: determining the ratio of the phase voltage related to the differential protection action to the phase-to-phase voltage of the rated voltage. If the ratio is less than 65%, the low-voltage differential current starting element will act to open the positive power supply of the output relay, and the opening time of the positive power supply of the output relay is 7s.

[0017] The differential element for the power frequency variation is a three-terminal power frequency variation current differential element. Its protection logic is as follows: focusing on the sudden current change signal at the initial stage of the fault, the action must simultaneously meet the following two criteria:

[0018] in: : Represents T contact line / F positive feeder; : Represents the differential current, which is a change in power frequency; : Indicates the high set value of the differential operating current due to the change in power frequency; : Represents the braking current, which is a change in power frequency.

[0019] The method also includes protection logic for the T-line / F-line current differential element. This protection logic is designed for the steady-state current characteristics during the fault duration and is divided into a steady-state stage I with no time delay and a steady-state stage II with time delay backup, as detailed below: Steady-state stage I aims to quickly clear severe steady-state faults, and its action must satisfy the following steady-state stage I action equation:

[0020] Steady-state stage II action equation:

[0021] in: : Indicates the high set value of the differential operating current for the T-line / F-line. : Indicates the braking current of the T-line / F-line, : Indicates the differential current between the T line and the F line.

[0022] Compared with the prior art, the present invention has the following beneficial effects: This invention sets up multi-terminal longitudinal differential protection elements on the basis of uplink and downlink feeders, covering multi-terminal branches composed of traction substations, AT substations, and section substations. Each branch is equipped with a current transformer to form a full-link current acquisition network. This architecture design breaks through the limitations of traditional protection systems based on single-sided power supply design, accurately matching the fault characteristics of short-circuit points in double-sided through-power supply mode where the short-circuit current is provided by both power sources. It realizes multi-terminal collaborative discrimination of T contact line and F positive feeder faults, fundamentally solving the problem that traditional protection cannot adapt to the fault characteristics of double-sided through-power supply, ensuring that the protection configuration can simultaneously and quickly disconnect the power sources at both ends of the fault point, effectively reducing the fault isolation range.

[0023] Furthermore, the three-terminal power frequency change current differential element in this invention focuses on the sudden current change signal at the initial stage of a fault, and achieves early fault identification through a specific criterion combination. This avoids false tripping caused by interference in the early stage of a fault and ensures rapid fault detection, significantly improving the speed of protection action. The T-line / F-line current differential element is designed for the steady-state current characteristics during the fault's duration. The steady-state I stage with no delay can quickly clear severe steady-state faults, while the steady-state II stage with delay backup can cover minor steady-state faults or scenarios where steady-state I stage fails to operate. This achieves accurate clearing of steady-state faults and provides reliable backup protection, improving the selectivity and sensitivity of protection action. When a fault occurs within the protection zone, the low-voltage differential current starting element must simultaneously meet the requirements of receiving a differential protection permission signal from the opposite side and a specific voltage criterion to operate. This effectively avoids false tripping in scenarios such as faults outside the protection zone and voltage fluctuations, adapts to the requirements of multi-terminal coordinated action, and significantly improves the reliability of protection action.

[0024] Furthermore, in the three-terminal longitudinal differential protection traction network structure of this invention, the AT substation forms a unified electrical terminal through series circuit breakers, the section substations interconnect feeders through circuit breakers, and the current transformer accurately collects multi-terminal current data and forms an effective linkage with the logic of the three types of protection elements. This collaborative design of hardware circuits and protection logic ensures that when a fault occurs, the faulty section can be accurately located, and only the fault-related part can be isolated, avoiding the expansion of the power outage area. This perfectly implements the protection configuration principle of reducing the fault isolation range, ensuring the normal operation of the rest of the power supply system and improving the overall power supply reliability.

[0025] Furthermore, this invention fundamentally solves the problem that traditional protection methods cannot meet the four basic requirements of selectivity, speed, sensitivity, and reliability of relay protection under bilateral through-power supply mode by comprehensively employing technical means such as architecture adaptation, transient and steady-state collaborative discrimination, dual reliability verification, and hardware and logic linkage. The various technical means work together to form a complete protection system, enabling rapid identification, precise isolation, and reliable protection of various faults, providing strong support for the safe and stable operation of the traction network under bilateral through-power supply mode. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the three-end longitudinal differential protection traction network structure in an embodiment of the present invention. Detailed Implementation

[0027] To further understand the content of this invention, the invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the embodiments are merely illustrative and not limiting of the invention.

[0028] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0029] This embodiment proposes a traction network multi-terminal longitudinal differential protection system under the bilateral through-type AT power supply mode, including an up feeder and a down feeder. The up feeder and the down feeder form a bilateral through-type power supply circuit, and the up feeder and the down feeder cover three parts: traction substation, AT station and section station. The traction substation introduces external power through incoming lines 1 and 2. Through switching equipment such as double-pole circuit breakers, traction substation DL1, and traction substation DL2, the power is distributed to the up-line T contact line, up-line F positive feeder line, down-line T contact line, and down-line F positive feeder line to supply power to bidirectional trains.

[0030] The AT is equipped with circuit breakers DL3, DL4, and DL5 connected in series, as well as a three-terminal differential protection element, to realize AT power supply and improve the power supply efficiency and voltage quality of the traction network.

[0031] The section is equipped with circuit breakers DL6 and DL7 for sectional control of the traction network, which can isolate faulty areas and prevent the fault from spreading to the entire power supply system.

[0032] like Figure 1As shown, a three-terminal longitudinal differential protection system is set up for each of the upstream or downstream feeders. The upstream system consists of traction substation DL2, section substation circuit breaker DL6, and AT substation circuit breakers DL3-DL5. The currents of the three circuit breakers in the AT substation are connected in series to the device, merging into a single electrical terminal to form a three-terminal longitudinal differential protection element. The downstream / upstream feeders of the traction substation, the downstream / upstream feeders of the section substation, and the AT substation form multi-terminal branches, each equipped with a current transformer. When a fault occurs in the upstream contact network, all the aforementioned circuit breakers trip simultaneously. The downstream feeder consists of traction substation DL1, section substation DL7, and AT substations DL3-DL5. The currents of the three circuit breakers in the AT substation are connected in series to the device, merging into a single electrical terminal to form a three-terminal longitudinal differential protection element. When a fault occurs in the downstream feeder contact network, all the aforementioned circuit breakers trip simultaneously.

[0033] The longitudinal differential protection element includes the following three types of components: power frequency variation current differential element, T-line / F-line current differential element, and low-voltage differential current starting element. All of these components can implement T-line / F-line differential logic.

[0034] The power frequency change current differential element is used to reflect the power frequency change, and its starting element's operating equation is:

[0035] in: Indicates: Take the maximum value of the half-wave integral of the effective current values ​​of the two lines (T line / F line); It is an adjustable fixed threshold, that is, the current setting of the differential element for power frequency variation; The threshold is a floating value that automatically adjusts according to changes in the quantity. A value of 1.25 ensures the threshold is always slightly higher than the unbalanced output. The power frequency variation current differential element activates and extends for 7 seconds to open the positive power supply of the output relay. For devices suitable for frequent starts, the power frequency variation current starting element needs to be opened with low voltage.

[0036] When a fault occurs on the T-line or F-line within the zone, the short-circuit current supplied by the fault point may be small due to its distance from the relatively weaker power supply side. This may cause the differential current element of the power frequency variation on that side to fail to meet the operating conditions and thus not operate. In this case, if a differential protection permission signal is received from the protection device on the opposite side (i.e., the relatively stronger power supply side), and the phase voltage and phase-to-phase voltage values ​​related to this differential protection operation are both less than 65% of the rated voltage, the low-voltage differential current starting element will operate to open the positive power supply of the output relay for 7 seconds.

[0037] In this embodiment, the three-terminal longitudinal differential protection element has two types of core components that enable accurate full-cycle discrimination of faults in the T-contact line and F-positive feeder line. These components are adapted to the transient characteristics in the initial stage of the fault and the steady-state characteristics during the sustained period, ensuring the speed, selectivity, and reliability of the protection action. Specifically, the power frequency change current differential element of the three-terminal longitudinal differential protection element focuses on the sudden current change signal in the initial stage of the fault. Its action must simultaneously meet two criteria, as described in the following formula:

[0038] in: : Represents T contact line / F positive feeder; : Represents the differential current representing the change in power frequency; the calculation formula for the differential current representing the change in power frequency is as follows:

[0039] In the formula, This represents the power frequency change current at terminal M of the three terminals of the three-terminal longitudinal differential protection element. Terminal M is the traction substation terminal, which is one of the core power supply terminals of the three-terminal longitudinal differential protection. The current data is collected by the current transformer on the feeder side of this terminal. This represents the power frequency change current at the N terminal of the three-terminal longitudinal differential protection element. The N terminal is the partition terminal, which is the intermediate node terminal of the three-terminal longitudinal differential protection. The current data is collected by the current transformer on the feeder side of this terminal. This indicates the power frequency change current at terminal S of the three terminals of the three-terminal longitudinal differential protection element. Terminal S is the AT terminal, which is a unified electrical terminal formed by merging the currents of the three circuit breakers DL3 to DL5 after they are connected in series. The current data is collected by the current transformer connected in series at this terminal.

[0040] : This represents the braking current due to the change in power frequency. The formula for calculating the braking current due to the change in power frequency is as follows:

[0041] : indicates the high set value of the differential operating current for the power frequency change current, wherein the high set value of the differential operating current is the greater of the maximum value of 1.5 times the locomotive current and the maximum value of 2.5 times the locomotive current.

[0042] The T-line / F-line current differential element of the three-terminal longitudinal differential protection component is designed for the steady-state current characteristics during fault persistence. It is divided into a steady-state stage I with no time delay and a steady-state stage II with time delay backup. The steady-state stage I aims to quickly clear severe steady-state faults, and its operation must satisfy the following steady-state stage I operating equation:

[0043] in: : This represents the T-line / F-line differential current, and the calculation formula for the T-line / F-line differential current is as follows:

[0044] : This represents the braking current of the T-line / F-line, and the calculation formula for the T-line / F-line braking current is as follows:

[0045] The steady-state stage II serves as backup protection for the steady-state stage I, covering scenarios involving minor steady-state faults or failure of steady-state stage I to operate. Its operation must satisfy the steady-state stage II operating equation:

[0046] in: : This indicates the high set value of the differential operating current of the T-line / F-line current. The high set value of the differential operating current of the T-line / F-line current is the larger of the maximum value of 1.5 times the locomotive current and the maximum value of 2.5 times the locomotive current, which is suitable for relatively minor fault scenarios.

[0047] Meanwhile, to avoid conflicts in steady-state I actions and ensure protection selectivity, steady-state II actions are subject to a 25ms delay after the action criteria are met before triggering the protection action.

[0048] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can be appropriately combined to form other embodiments that can be understood by those skilled in the art. The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A multi-terminal longitudinal differential protection system for traction network under a bilateral through-type AT power supply mode, characterized in that, It includes a double-sided through power supply circuit consisting of an uplink feeder and a downlink feeder, with traction substations, AT substations, and section substations covered on both the uplink and downlink feeders. The uplink and downlink feeders covered by the traction substations, the uplink and downlink feeders covered by the section substations, and the AT substations form multi-terminal branches, each branch equipped with a current transformer. Multi-terminal longitudinal differential protection elements are set up on either the uplink or downlink feeder, including power frequency variation current differential elements, T contact line / F positive feeder current differential elements, and low-voltage differential current starting elements. The up feeder is equipped with an up T contact line and an up F positive feeder, and the down feeder is equipped with a down T contact line and a down F positive feeder. The traction substation introduces external power through the No. 1 and No. 2 incoming lines. The power is distributed to the up T contact line, up F positive feeder, down T contact line, and down F positive feeder via double-pole circuit breakers, traction substation DL1, and traction substation DL2 to supply power to the trains in both directions. The up T contact line, up F positive feeder, down T contact line, and down F positive feeder all extend along the double-sided through power supply circuit and are respectively connected to the AT substation. The AT is equipped with circuit breakers DL3, DL4, and DL5 connected in series, as well as a three-terminal differential protection element. The input terminal of circuit breaker DL3 is electrically connected to the upstream T contact line, upstream F positive feeder, downstream T contact line, and downstream F positive feeder extending from the traction substation. The output terminal of circuit breaker DL5 is electrically connected to the upstream T contact line, upstream F positive feeder, downstream T contact line, and downstream F positive feeder corresponding to the subsequent section, forming a double-sided through electrical circuit between the traction substation, the AT station, and the section station. The substation is equipped with circuit breakers DL6 and DL7. Circuit breaker DL6 is connected in series between the upstream T contact line and the upstream F positive feeder extending from AT, and circuit breaker DL7 is connected in series between the downstream T contact line and the downstream F positive feeder extending from AT. The output terminals of circuit breakers DL6 and DL7 are electrically connected to the upstream T contact line, upstream F positive feeder, downstream T contact line, and downstream F positive feeder of the subsequent power supply section, respectively, forming a complete double-sided through power supply circuit.

2. A method for multi-terminal longitudinal differential protection of traction network under bilateral through-type AT power supply, based on the multi-terminal longitudinal differential protection system of traction network under bilateral through-type AT power supply as described in claim 1, characterized in that... Includes the following steps: Determine the maximum value of the effective value of the T contact line / F positive feeder current in the half-wave integral, set the fixed threshold and floating threshold of the current, and obtain the operating equation of the differential element that reflects the change of power frequency. When the operating conditions of the differential element for the change in power frequency are met, the differential element for the change in power frequency will operate, and after a period of time, the positive power supply of the output relay will be opened. When a fault occurs on the T-line or F-line within the zone, if a differential protection permission signal is received from the other side, the phase voltage related to the differential protection action is compared with the phase-to-phase voltage of the rated voltage. Based on the comparison result, when the preset voltage criterion is met, the low-voltage differential current starting element operates, opening the positive power supply of the output relay.

3. The method for multi-terminal longitudinal differential protection of traction network under bilateral through-type AT power supply as described in claim 2, characterized in that, The operating equation of the differential element for the power frequency variation is as follows: In the formula, To obtain the maximum value of the effective value of the T-line / F-line current in the half-wave integral, The current setting value for the differential element is the power frequency variation value; The threshold is floating.

4. The method for multi-terminal longitudinal differential protection of traction network under double-sided through-type AT power supply as described in claim 2, characterized in that, The preset voltage criterion is specifically: determining the ratio of the phase voltage related to the differential protection action to the phase-to-phase voltage of the rated voltage. If the ratio is less than 65%, the low-voltage differential current starting element will act to open the positive power supply of the output relay, and the opening time of the positive power supply of the output relay is 7s.

5. A method for multi-terminal longitudinal differential protection of traction network under double-sided through-type AT power supply as described in claim 2, characterized in that, The differential element for the power frequency variation is a three-terminal power frequency variation current differential element. Its protection logic is as follows: focusing on the sudden current change signal at the initial stage of the fault, the action must simultaneously meet the following two criteria: in: : Represents T contact line / F positive feeder; : Represents the differential current, which is a change in power frequency; : Indicates the high set value of the differential operating current due to the change in power frequency; : Represents the braking current, which is a change in power frequency.

6. The method for multi-terminal longitudinal differential protection of traction network under double-sided through-type AT power supply as described in claim 2, characterized in that, The method also includes protection logic for the T-line / F-line current differential element. This protection logic is designed for the steady-state current characteristics during the fault duration and is divided into a steady-state stage I with no time delay and a steady-state stage II with time delay backup, as detailed below: Steady-state stage I aims to quickly clear severe steady-state faults, and its action must satisfy the following steady-state stage I action equation: Steady-state stage II action equation: in: : Indicates the high set value of the differential operating current for the T-line / F-line. : Indicates the braking current of the T-line / F-line, : Indicates the differential current between the T line and the F line.

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