Station domain protection method and device based on channel alarm information

Abstract

The present invention discloses a station domain protection method and device based on channel alarm information, the method comprising: obtaining alarm information of multiple local line channels and / or multiple wide area line channels in a substation, and counting the number of alarms; if the number of alarms is equal to a first threshold, if the number of channel alarms of multiple lines is equal to a second threshold, it is determined that the communication power supply is undervoltage and a first protection strategy is adopted for protection; if the number of channel alarms of any one of the multiple lines is equal to a third threshold, it is determined that the DC of the adjacent station is undervoltage and a second protection strategy is adopted for protection; if the number of alarms is greater than or equal to a fourth threshold, if the number of channel alarms of multiple lines is equal to the first threshold or the second threshold, the first protection strategy is adopted for protection. The present invention can correctly identify line fault problems by detecting the number of alarms, invest corresponding protection in different fault conditions, and improve the stability of substations and power systems.

Description

Technical Field

[0001] The present invention relates to the field of communication technology, and in particular to a station domain protection method and device based on channel alarm information. Background Art

[0002] Smart substations have the basic requirements of digitalization of all station information, networking of communication platforms, standardization of information sharing, integration of system functions, compact structural design, intelligence of high-voltage equipment and visualization of operating status. They can support real-time online analysis and control decisions of the power grid, thereby improving the reliability and economy of the entire power grid operation. Their disadvantage is that they are too dependent on the normal operation of the station communication power supply. How to correctly judge the working condition of the communication power supply losing pressure and thus invest in corresponding protection is a problem that needs to be solved urgently.

[0003] Currently, the communication power supply undervoltage alarm signal issued by the device is usually used for judgment. However, when multiple communication power supplies are configured, it will be impossible to judge the specific situations of multiple alarm signals, resulting in complete loss of communication and affecting emergency processing. Summary of the invention

[0004] The object of the present invention is to provide a station domain protection method and device based on channel alarm information to solve the problem of low efficiency in judging different fault conditions in the prior art.

[0005] To achieve the above object, the present invention provides a station domain protection method based on channel alarm information, comprising:

[0006] Acquire alarm information of multiple local line channels and / or multiple wide area line channels in the substation, and count the number of alarms of the lines containing the alarm information;

[0007] If the number of alarms is equal to the first threshold, the number of channel alarms of the multiple lines is detected. If the number of channel alarms of the multiple lines is equal to the second threshold, it is determined that the communication power supply is undervoltage and the first protection strategy is adopted for protection. If the number of channel alarms of any one of the multiple lines is equal to the third threshold, it is determined that the DC voltage of the neighboring station is undervoltage and the second protection strategy is adopted for protection.

[0008] If the number of alarms is greater than or equal to the fourth threshold, the communication power supply is determined to be depressurized and the number of channel alarms of each line is detected. If the number of channel alarms of multiple lines is equal to the first threshold or the second threshold, the first protection strategy is used for protection.

[0009] Preferably, the station domain protection method based on channel alarm information further includes:

[0010] If the alarm number is equal to the fifth threshold, it is determined to be a general channel failure and it is determined not to take protective measures.

[0011] Preferably, if the number of channel alarms of the plurality of lines is equal to the second threshold, it is determined that the communication power supply is de-voltaged and a first protection strategy is adopted for protection, including:

[0012] If the number of channel alarms of the plurality of lines is equal to the second threshold, after the target delay, it is determined that the communication power supply is depressurized and the phase-to-phase and ground distance relays are used to determine the fault and serve as close backup protection.

[0013] Preferably, if the number of channel alarms of any one of the multiple lines is equal to the third threshold, it is determined that the DC voltage of the neighboring station is lost and the second protection strategy is adopted for protection, including:

[0014] If the channel alarm number of any one of the multiple lines is equal to the third threshold, the negative sequence directional element, the zero sequence directional element and the power frequency variation directional element are used to judge the fault and provide remote backup protection for the neighboring station outgoing line.

[0015] Preferably, if the number of alarms is greater than or equal to the fourth threshold, it is determined that the communication power supply is depressurized and the number of channel alarms of each line is detected; if the number of channel alarms of multiple lines is equal to the first threshold or the second threshold, the first protection strategy is adopted for protection, including:

[0016] If the number of channel alarms of the plurality of said lines is equal to the first threshold, after the target delay, the phase-to-phase and ground distance relays are used to judge the fault and serve as close backup protection;

[0017] If the number of channel alarms of the plurality of lines is equal to the second threshold, the phase-to-phase and ground distance relays are used to determine the fault and serve as close backup protection.

[0018] The present invention also provides a station area protection device based on channel alarm information, comprising:

[0019] A statistical module, used to obtain alarm information of multiple local line channels and / or multiple wide area line channels in the substation, and count the number of alarms of the lines containing the alarm information;

[0020] A first protection module is used to detect the number of channel alarms of multiple lines if the number of alarms is equal to the first threshold, and if the number of channel alarms of the multiple lines is equal to the second threshold, determine that the communication power supply is undervoltage and adopt the first protection strategy for protection, and if the number of channel alarms of any one of the multiple lines is equal to the third threshold, determine that the DC of the neighboring station is undervoltage and adopt the second protection strategy for protection;

[0021] The second protection module is used to determine that the communication power supply has lost voltage and detect the number of channel alarms for each line if the number of alarms is greater than or equal to the fourth threshold; if the number of channel alarms for multiple lines is equal to the first threshold or the second threshold, the first protection strategy is used for protection.

[0022] Preferably, a third protection module is further included, and the third protection module is used to:

[0023] If the alarm number is equal to the fifth threshold, it is determined to be a general channel failure and it is determined not to take protective measures.

[0024] Preferably, the first protection module is further used for:

[0025] If the number of channel alarms of the plurality of lines is equal to the second threshold, after the target delay, it is determined that the communication power supply is depressurized and the phase-to-phase and ground distance relays are used to determine the fault and serve as close backup protection.

[0026] Preferably, the first protection module is further used for:

[0027] If the channel alarm number of any one of the multiple lines is equal to the third threshold, the negative sequence directional element, the zero sequence directional element and the power frequency variation directional element are used to judge the fault and provide remote backup protection for the neighboring station outgoing line.

[0028] Preferably, the second protection module is further used for:

[0029] If the number of channel alarms of the plurality of said lines is equal to the first threshold, after the target delay, the phase-to-phase and ground distance relays are used to judge the fault and serve as close backup protection;

[0030] If the number of channel alarms of the plurality of lines is equal to the second threshold, the phase-to-phase and ground distance relays are used to determine the fault and serve as close backup protection.

[0031] Compared with the prior art, the beneficial effects of the present invention are:

[0032] The alarm information of multiple local line channels and / or multiple wide area line channels in the substation is obtained, and the number of alarms of the lines containing alarm information is counted. If the number of alarms is equal to the first threshold, the number of channel alarms of multiple lines is detected. If the number of channel alarms of multiple lines is equal to the second threshold, it is determined that the communication power supply is undervoltage and the first protection strategy is adopted for protection. If the number of channel alarms of any one of the multiple lines is equal to the third threshold, it is determined that the DC of the adjacent station is undervoltage and the second protection strategy is adopted for protection. If the number of alarms is equal to the fourth threshold, it is determined that the communication power supply is undervoltage and the number of channel alarms of each line is detected. If the number of channel alarms of multiple lines is equal to the first threshold or the second threshold, the first protection strategy is adopted for protection. According to the number of alarms of the obtained channels, correlation analysis is performed, and the fault condition is judged according to the different number of alarms of the statistical lines, so as to put in corresponding protection measures to ensure the stable operation of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the technical solution of the present invention, the drawings required for use in the implementation mode will be briefly introduced below. Obviously, the drawings described below are only some implementation modes of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0034] Figure 1 It is a flow chart of a station domain protection method based on channel alarm information provided by a certain embodiment of the present invention;

[0035] Figure 2 It is a structural diagram of a station area protection device based on channel alarm information provided by a certain embodiment of the present invention. DETAILED DESCRIPTION

[0036] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0037] It should be understood that the step numbers used in this document are only for convenience of description and are not intended to limit the order in which the steps are executed.

[0038] It should be understood that the terms used in the present specification are only for the purpose of describing specific embodiments and are not intended to limit the present invention. As used in the present specification and the appended claims, unless the context clearly indicates otherwise, the singular forms "a", "an" and "the" are intended to include plural forms.

[0039] The terms “include” and “comprising” indicate the presence of described features, integers, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or combinations thereof.

[0040] The term "and / or" means and includes any and all possible combinations of one or more of the associated listed items.

[0041] See also Figure 1 , a certain embodiment of the present invention provides a station domain protection method based on channel alarm information. Figure 1 As shown, the station domain protection method based on channel alarm information includes steps S101 to S103. The specific steps are as follows:

[0042] S101: Acquire alarm information of multiple local line channels and / or multiple wide area line channels in a substation, and count the number of alarms of the lines containing the alarm information.

[0043] Specifically, the wide area protection device in the smart substation is used to detect channel alarm information. The wide area protection device in the smart substation should have the function of counting the number of channel alarms and can collect information within the station, including the channel alarm information of each outgoing line. When the local channel is abnormal, the wide area protection device can receive the channel alarm signal sent by the local protection device through the GOOSE network. When the wide area channel is abnormal, the wide area protection device can receive the channel alarm signal sent to it by the wide area communication equipment.

[0044] After obtaining the alarm information of multiple local line channels and / or multiple wide area line channels in the substation, the wide area protection device can identify which line the channel alarm information comes from and how many alarm signals this line has issued, and perform different processing on the line protection according to different alarm numbers. The alarm information value of the line at this time is counted. If no channel alarm occurs, the detection process ends. If a channel alarm is detected, the next step is entered for corresponding processing.

[0045] S102: If the number of alarms is equal to the first threshold, detect the number of channel alarms of multiple lines. If the number of channel alarms of multiple lines is equal to the second threshold, determine that the communication power supply has lost voltage and adopt the first protection strategy for protection. If the number of channel alarms of any one of the multiple lines is equal to the third threshold, determine that the DC voltage of the neighboring station has lost voltage and adopt the second protection strategy for protection.

[0046] Specifically, when the wide area protection device detects the occurrence of a channel alarm, it enters a logic: the number of alarms for each line is detected, and the number of channel alarms is counted as the alarm information value.

[0047] The number of alarms of each line is detected one by one, and no processing is performed if the number of alarms is equal to 0. The number of alarms detected is equal to 1 as the fifth threshold, and if the number of alarms is equal to the fifth threshold, it is determined to be a general channel fault and it is determined that no protection measures are required.

[0048] The detected alarm number equal to 2 is taken as the first threshold. If the alarm number is equal to the first threshold, there are generally two situations. One situation is that if the channel alarm numbers of multiple lines are equal to the second threshold, that is, the value 1, it means that the alarm information comes from different lines, and it is believed that the communication power supply of this station has lost voltage. Since there is still differential protection as the main protection, in order to ensure selectivity, after a delay, the line that sends the alarm signal is protected by the first protection strategy, that is, the phase-to-phase and ground distance relays are used to determine the fault.

[0049] The protection criteria are as follows:

[0050] 1) Power frequency variable distance relay.

[0051] The power frequency variation distance relay measures the amplitude of the power frequency variation of the working voltage, and its action equation is:

[0052] |ΔU OP |>U Z ;

[0053] For phase-to-phase faults:

[0054]

[0055] For ground fault:

[0056]

[0057] In the formula, ΔU OP is the power frequency variation of the working voltage, U Z is the action voltage, and the memory value of the working voltage before the fault is taken. is the phase-to-phase voltage, is the phase-to-phase current, is the phase voltage, is the phase current, K is the zero-sequence compensation coefficient, I0 is the zero-sequence current, Z ZD To set the distance impedance of the power frequency variation, external adjustment is required.

[0058] 2) Low voltage distance relay.

[0059] When the positive sequence polarization voltage is high, the distance relay polarized by the positive sequence voltage has good directionality. When the positive sequence voltage drops below 10%, it enters the three-phase low voltage program and is polarized by the positive sequence voltage memory. The low voltage distance relay compares the phase of the working voltage and the polarization voltage:

[0060]

[0061] In the formula, is the operating voltage, is the polarization voltage, Z ZD To set the impedance, To memorize the positive sequence voltage before the fault.

[0062] In case of positive direction fault, the phase ratio equation of the relay is:

[0063]

[0064] 3) Ground distance relay.

[0065]

[0066] In the formula, Z ZD is the ground distance impedance constant, is the corresponding phase positive sequence voltage, θ is the offset angle, its function is to shift the directional impedance characteristics to the I quadrant when applied in short lines, so as to expand the capacity of allowable fault transition resistance, and the value range is 0°, 15°, and 30°.

[0067] In case of positive direction fault, the phase ratio equation of the relay is:

[0068]

[0069] 4) Phase distance relay.

[0070]

[0071] In the formula, Z ZD is the ground distance impedance constant, To correspond to the positive sequence voltage between phases, θ is the offset angle, which is used to shift the directional impedance characteristics to the I quadrant when applied in short lines to expand the capacity of the allowable fault transition resistance. The value range is 0°, 15°, and 30°.

[0072] In case of positive direction fault, the phase ratio equation of the relay is:

[0073]

[0074] In another case, if the number of channel alarms of any one of the multiple lines is equal to the third threshold value, that is, the value 2, it is determined that the alarm information at this time comes from the same line. At this time, it is considered that the DC power supply of the neighboring station is under voltage, and the second protection strategy is used to protect the line, that is, the negative sequence directional element, the zero sequence directional element, and the power frequency variation directional element are used to judge the fault. The protection judgment criteria are as follows:

[0075] 1) Negative sequence directional element.

[0076] The positive action area is:

[0077]

[0078] In the formula, and They are the negative sequence current at the protection installation and the negative sequence voltage on the line side.

[0079] 2) Zero-sequence directional element.

[0080] The positive action area is:

[0081]

[0082] In the formula, To protect the zero-sequence voltage at the installation location, In order to protect the zero-sequence current at the installation site, the positive direction of the zero-sequence current is from the busbar pointing to the line.

[0083] 3) Power frequency variation direction element.

[0084] The power frequency variation direction element uses the phase angle of the power frequency variation impedance calculated from the power frequency variation of the fault voltage and fault current to determine the fault direction. In the case of a positive direction fault, the action criterion is:

[0085]

[0086] In the formula, the subscript 12 indicates that the electrical quantity includes positive and negative sequence components, and no zero sequence component. d To simulate impedance, its impedance angle is consistent with the line impedance angle, Z com To compensate for impedance, the ratio of system impedance to line impedance under maximum operating mode is When Z com =0, otherwise Z com Take half of the "power frequency variation impedance". ΔU 12 , ΔI 12 When used, it must be greater than a fixed threshold, which is 0.1Un and 0.1In respectively.

[0087] S103: If the number of alarms is greater than or equal to the fourth threshold, determine that the communication power supply is depressurized and detect the number of channel alarms for each line. If the number of channel alarms for multiple lines is equal to the first threshold or the second threshold, use the first protection strategy for protection.

[0088] Specifically, if the number of alarms is greater than or equal to the fourth threshold, that is, if the total number of channel alarms is equal to 3 or above, it means that the communication power supply of this station has lost voltage, and the number of channel alarms of each line is detected. For the line whose number of alarms is equal to the first threshold, that is, the value 1, the first protection strategy is used for protection after a certain delay (the reason for the certain delay is that for the line with one channel alarm, there must be another set of fast protection that can be used, and the first protection strategy is used as a close backup). For the line whose number of alarms is equal to the second threshold, that is, the value 2, the first protection strategy is directly used for protection (at this time, the line has no local differential or wide-area longitudinal protection, and no delay is required).

[0089] Aiming at the situation where the substation is equipped with two communication power supplies, the present invention performs correlation analysis based on the collected channel alarm information, and judges the fault condition based on the different situations of the overall alarm number and the line alarm number, so as to decide how to reasonably invest in protection, and finally correctly cut off the fault to ensure the stable operation of the power system.

[0090] See also Figure 2 Another embodiment of the present invention provides a station domain protection device based on channel alarm information, comprising:

[0091] The statistical module 11 is used to obtain the alarm information of multiple local line channels and / or multiple wide area line channels in the substation, and count the number of alarms of the lines containing the alarm information.

[0092] The first protection module 12 is used to detect the number of channel alarms of multiple lines if the number of alarms is equal to the first threshold; if the number of channel alarms of the multiple lines is equal to the second threshold, it is determined that the communication power supply has lost voltage and the first protection strategy is adopted for protection; if the number of channel alarms of any one of the multiple lines is equal to the third threshold, it is determined that the DC voltage of the adjacent station has lost voltage and the second protection strategy is adopted for protection.

[0093] The second protection module 13 is used to determine that the communication power supply has lost voltage and detect the number of channel alarms for each line if the number of alarms is greater than or equal to the fourth threshold. If the number of channel alarms for multiple lines is equal to the first threshold or the second threshold, the first protection strategy is used for protection.

[0094] Preferably, a third protection module is further included, and the third protection module is used to determine that it is a general channel fault and determine not to take protection measures if the number of alarms is equal to a fifth threshold.

[0095] Preferably, the first protection module is also used for: if the number of channel alarms of the plurality of lines is equal to the second threshold, after a target delay, determining that the communication power supply is depressurized and using phase-to-phase and ground distance relays to determine the fault and serve as close backup protection.

[0096] Preferably, the first protection module is also used to use negative-sequence directional elements, zero-sequence directional elements and power frequency variation directional elements to judge the fault if the number of channel alarms of any one of the multiple lines is equal to the third threshold, so as to provide remote backup protection for the neighboring station outgoing line.

[0097] The second protection module is further used for: if the number of channel alarms of the plurality of lines is equal to the first threshold, after a target delay, using phase-to-phase and ground distance relays to determine the fault and use them as close backup protection;

[0098] If the number of channel alarms of the plurality of lines is equal to the second threshold, the phase-to-phase and ground distance relays are used to determine the fault and serve as close backup protection.

[0099] For the specific definition of the station domain protection device based on channel alarm information, please refer to the definition of the station domain protection method based on channel alarm information above, which will not be repeated here. Each module in the above-mentioned station domain protection device based on channel alarm information can be implemented in whole or in part by software, hardware and a combination thereof. The above-mentioned modules can be embedded in or independent of the processor in the computer device in the form of hardware, or can be stored in the memory of the computer device in the form of software, so that the processor can call and execute the operations corresponding to the above modules.

[0100] The above is a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several improvements and modifications without departing from the principle of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A station domain protection method based on channel alarm information, characterized in that: include: Acquire alarm information of multiple local line channels and / or multiple wide area line channels in the substation, and count the number of alarms of the lines containing the alarm information; If the number of alarms is equal to the first threshold, the number of channel alarms of multiple lines is detected; if the number of channel alarms of multiple ground line channels and / or multiple wide area line channels is equal to the second threshold, it is determined that the communication power supply has lost voltage and the first protection strategy is adopted for protection; if the number of channel alarms of any one of the multiple ground line channels and / or multiple wide area line channels is equal to the third threshold, it is determined that the DC voltage of the neighboring station has lost voltage and the second protection strategy is adopted for protection; the third threshold is greater than the second threshold; The first protection strategy refers to using phase-to-phase and ground distance relays to judge faults; the second protection strategy refers to using negative sequence directional elements, zero sequence directional elements and power frequency variation directional elements to judge faults; If the number of alarms is greater than or equal to the fourth threshold, the communication power supply is determined to be depressurized and the number of channel alarms for each line is detected; if the number of channel alarms of multiple ground line channels and / or multiple wide area line channels is equal to the first threshold or the second threshold, the first protection strategy is adopted for protection; the fourth threshold is greater than the first threshold.

2. The station domain protection method based on channel alarm information according to claim 1 is characterized in that: Also includes: If the number of alarms is equal to the fifth threshold, it is determined to be a general channel fault and it is determined not to take protective measures; The fifth threshold is smaller than the first threshold.

3. The station domain protection method based on channel alarm information according to claim 1 is characterized in that: If the number of channel alarms of the plurality of ground line channels and / or the plurality of wide area line channels is equal to the second threshold, it is determined that the communication power supply is under voltage and a first protection strategy is adopted for protection, including: If the number of channel alarms of multiple ground line channels and / or multiple wide area line channels is equal to the second threshold, after the target delay, the communication power supply is determined to be depressurized and the phase-to-phase and ground distance relays are used to determine the fault and serve as close backup protection.

4. The station domain protection method based on channel alarm information according to claim 3 is characterized in that: If the channel alarm number of any one of the plurality of ground line channels and / or the plurality of wide area line channels is equal to the third threshold, it is determined that the DC voltage of the neighboring station is lost and the second protection strategy is adopted for protection, including: If the channel alarm number of any one of the multiple ground line channels and / or multiple wide area line channels is equal to the third threshold, the negative sequence directional element, the zero sequence directional element and the power frequency variation directional element are used to judge the fault and provide remote backup protection for the neighboring station outgoing line.

5. The station domain protection method based on channel alarm information according to claim 4 is characterized in that: If the number of alarms is greater than or equal to the fourth threshold, determining that the communication power supply is depressurized and detecting the number of channel alarms of each line, if the number of channel alarms of multiple ground line channels and / or multiple wide area line channels is equal to the first threshold or the second threshold, adopting the first protection strategy for protection, including: If the number of channel alarms of multiple ground line channels and / or multiple wide area line channels is equal to the first threshold, after the target delay, the phase-to-phase and ground distance relays are used to determine the fault and serve as close backup protection; If the number of channel alarms of multiple ground line channels and / or multiple wide area line channels is equal to the second threshold, phase-to-phase and ground distance relays are used to determine the fault and serve as close backup protection.

6. A station area protection device based on channel alarm information, characterized in that: include: A statistical module, used to obtain alarm information of multiple local line channels and / or multiple wide area line channels in the substation, and count the number of alarms of the lines containing the alarm information; A first protection module is configured to detect the number of channel alarms of multiple lines if the number of alarms is equal to a first threshold, determine that the communication power supply is depressurized and adopt a first protection strategy for protection if the number of channel alarms of multiple ground line channels and / or multiple wide area line channels are all equal to a second threshold, and determine that the DC voltage of the neighboring station is depressurized and adopt a second protection strategy for protection if the number of channel alarms of any one of the multiple ground line channels and / or multiple wide area line channels is equal to a third threshold; the third threshold is greater than the second threshold; The first protection strategy refers to using phase-to-phase and ground distance relays to judge faults; the second protection strategy refers to using negative sequence directional elements, zero sequence directional elements and power frequency variation directional elements to judge faults; The second protection module is used to determine that the communication power supply has lost pressure and detect the number of channel alarms for each line if the number of alarms is greater than or equal to a fourth threshold, and to adopt the first protection strategy for protection if the number of channel alarms of multiple ground line channels and / or multiple wide area line channels is equal to the first threshold or the second threshold; the fourth threshold is greater than the first threshold.

7. The station area protection device based on channel alarm information according to claim 6 is characterized in that: The third protection module is further included, and the third protection module is used to: If the number of alarms is equal to a fifth threshold, it is determined to be a general channel fault and it is determined not to take protective measures; the fifth threshold is less than the first threshold.

8. The station area protection device based on channel alarm information according to claim 6 is characterized in that: The first protection module is further used for: If the number of channel alarms of multiple ground line channels and / or multiple wide area line channels is equal to the second threshold, after the target delay, the communication power supply is determined to be depressurized and the phase-to-phase and ground distance relays are used to determine the fault and serve as close backup protection.

9. The station area protection device based on channel alarm information according to claim 8, characterized in that: The first protection module is further used for: If the channel alarm number of any one of the multiple ground line channels and / or multiple wide area line channels is equal to the third threshold, the negative sequence directional element, the zero sequence directional element and the power frequency variation directional element are used to judge the fault and provide remote backup protection for the neighboring station outgoing line.

10. The station area protection device based on channel alarm information according to claim 9, characterized in that: The second protection module is further used for: If the number of channel alarms of multiple ground line channels and / or multiple wide area line channels is equal to the first threshold, after the target delay, the phase-to-phase and ground distance relays are used to determine the fault and serve as close backup protection; If the number of channel alarms of multiple ground line channels and / or multiple wide area line channels is equal to the second threshold, phase-to-phase and ground distance relays are used to determine the fault and serve as close backup protection.

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