CN109638793B - Single-phase earth fault processing method suitable for small current grounding system - Google Patents
Single-phase earth fault processing method suitable for small current grounding system Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
- H02H7/261—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured involving signal transmission between at least two stations
- H02H7/262—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured involving signal transmission between at least two stations involving transmissions of switching or blocking orders
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- G01R31/08—Locating faults in cables, transmission lines, or networks
- G01R31/081—Locating faults in cables, transmission lines, or networks according to type of conductors
- G01R31/086—Locating faults in cables, transmission lines, or networks according to type of conductors in power transmission or distribution networks, i.e. with interconnected conductors
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- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
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Abstract
A single-phase earth fault processing method suitable for a low-current earth system solves the matching problem of outlet protection and line protection during single-phase earth fault in the low-current earth system, and provides a low-current earth fault protection method which utilizes the load power direction, the earth fault transient power direction and each terminal to be matched in a delayed mode respectively aiming at radial lines and hand-in-hand lines, so that the nearby isolation of the low-current earth fault is realized respectively. All upstream health sections can be cleared of faults without power outage, and in a hand-in-hand network, downstream health lines can also be directly transferred to a standby power supply. The invention can adapt to distribution network automation systems and distribution line protection devices in different modes, realizes the rapid and accurate processing of the low-current ground fault and has wide practical application value.
Description
Technical Field
The invention belongs to the field of relay protection of a power distribution network, and particularly relates to a single-phase earth fault processing method suitable for a low-current earth system.
Background
The small current ground fault location technology has become a research hotspot in recent years. The field application comprises centralized feeder automation, a distribution terminal or a fault indicator is mostly adopted to collect a transient current waveform of a ground fault, for example, a centralized feeder automation method utilizing the similarity of transient current waveforms is provided in the thesis of distribution network automation system small current ground fault transient positioning technology, a positioning main station utilizes the transient current similarity principle to determine a fault section, the positioning algorithm is complex, and different factory terminals and the main station are matched with each other with certain difficulty; in distributed feeder automation, algorithms based on transient current similarity are more difficult than in centralized mode, since less information can be utilized. In addition, local feeder automation is similar to the research on the automatic isolation technology of the single-phase earth fault of the power distribution network based on the voltage-time recloser, and the voltage-time section switch is used for isolating faults, so that the local feeder automation has a large power failure range and long time, and is mainly suitable for lines with low requirements on power supply reliability.
When a small-current ground fault occurs, small-current ground fault positioning can be achieved by using transient power characteristic differences of all sections, the principle is similar to the principle of positioning short-circuit faults of double-end power lines, the algorithm is simple, and the method can adapt to feeder automation in different modes. The method has unique advantages of realizing the automation of the small current ground fault feeder by utilizing the transient power direction, but the traditional terminal cannot obtain zero sequence voltage, so that the popularization and the application of the technology are limited. Related standards are recently formulated by national grid companies, primary and secondary equipment fusion of a power distribution switch is planned to be realized, and a newly-built power distribution switch is required to be provided with a zero-sequence voltage sensor in the forms of capacitance voltage division, resistance voltage division and the like; southern power grid companies also deploy zero-sequence voltage transformers on point-of-test distribution switches. The acquisition of the zero sequence voltage provides a development opportunity for the detection and processing technology of the small current ground fault in the transient power direction.
The method comprises the steps that a terminal in centralized feeder automation needs to report a fault transient power direction, a main station judges a fault section according to the characteristic that transient power directions of two sides of a fault point are opposite and the transient power directions of healthy sections are the same, and remotely controls switches on two sides of the fault point to trip; in distributed feeder automation, adjacent terminals need to exchange transient power direction information, and when the directions of transient power at two sides are opposite, the terminal is judged as a fault section and controls switches at two sides to trip. However, in both the centralized feeder automation mode and the distributed feeder automation mode, communication channels are required, and the system cannot adapt to a power distribution network protection system without communication conditions.
In addition, the short-circuit fault protection of the distribution line has a mature configuration and setting scheme at present, but for a low-current ground fault, a protection scheme with complete principle, reliable action and simple realization is lacked. The acquisition of zero sequence voltage and the application of the transient power direction make it possible to realize the low-current ground fault protection by using the transient power direction.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a single-phase earth fault processing method suitable for a low-current earth system, which is realized by the following technical scheme:
the single-phase earth fault processing method suitable for the small current grounding system is applied to a single power supply radiation type line, controls the tripping of an upstream switch close to a small current grounding fault point by utilizing the cooperation of the direction of transient reactive power of a grounding fault and delay time, and comprises the following steps:
step 1) configuring different delay fixed values for outgoing line protection and line protection at each level:
the first delay fixed value is set as that the delay fixed values of the main line and the protection devices at the tail ends of all branch lines are all time T, the delay time of each protection device of the main line is sequentially increased by a time step △ T from the tail end of the line to the bus, the delay time of each protection device of each branch line is sequentially increased by a time step △ T from the load side to the branch point, and the second delay fixed value is set as that the delay fixed values of all protection devices are all infinite;
step 2), when the outgoing line protection and the line protection of each stage detect that the zero sequence voltage exceeds a preset threshold, starting fault protection, and calculating the load power direction and the grounding fault transient reactive power direction;
step 3), each protection device selects a delay fixed value according to the direction of the transient reactive power of the ground fault and the direction of the load power when necessary: when the transient reactive power of the ground fault flows to the bus from the line, the delay fixed value of the protection device is a first delay fixed value; when the transient reactive power of the ground fault flows to the line from the bus, the delay fixed value of the protection device is a second delay fixed value;
step 4), when the fault duration reaches a preset value, controlling the switch to act to remove the fault; and when the fault duration time does not reach a preset value, the protection returns.
the single-phase earth fault processing method suitable for the small-current grounding system is further designed in that △ T in the step 1) is selected according to the formula (1),
ΔT＝t1b+t2l+top+tu+tm(1)
wherein, t1bThe lead error of the current stage protection delay element; t is t2lHysteresis error of delay elements for next stage protection; t is topThe switching-off time of the sectional switch is set; t is tuIs the voltage recovery time; t is tmIs a time margin.
The single-phase earth fault processing method suitable for the small-current earthing system is further designed in that when the control switch action in the step 4) removes the fault,
for stability faults, when the stability faults reach a preset value, a fault section is directly tripped and isolated;
for intermittent arc grounding faults, define T0Accumulating the arcing time; t is1For the threshold value of the arc-quenching time, the arc-quenching time T between two arc-ignitions is determined2Is divided into two cases, when T is0Tripping after the preset delay time is accumulated;
case 1) when T2＜T1The two arcing times are counted in T0；
Case 2) when T2＞T1When the fault occurs, the circuit is determined to have two single-phase earth faults, and T is added0And clearing and restarting timing from the second arc burning.
The single-phase earth fault processing method suitable for the low-current grounding system is further designed in such a way that the preset value is set to be 1-2 hours.
The single-phase earth fault processing method suitable for the small-current earth system is applied to a hand-held line with a standby power supply, and when a single-phase earth fault occurs, transient reactive power of an upstream detection point of a fault point flows to a bus from the line; fault point downstream detection point transient state reactive power flows to the circuit by the generating line, through transient state power direction discernment fault point upper and lower stream switch, guarantees the first tripping operation of load side switch through the delay time, includes following step:
step 1) configuring different delay fixed values for outgoing line protection and line protection at each level, wherein the delay fixed values are respectively as follows:
the first delay fixed value is set as that the delay fixed value of the power outlet protection device of the fault section is time T, and the delay time of other protection devices of the circuit is sequentially increased by a time step △ T from the power supply of the fault section to the standby power supply;
the second delay fixed value is set as that the delay fixed value of the spare power supply outlet protection device is that a time step △ T is added on the basis of the first delay fixed value of the spare power supply outlet protection device, and the delay time of other protection devices of the circuit is continuously increased by a time step △ T from the spare power supply to the fault section power supply;
the third delay fixed value is set as that the delay fixed value of the outgoing line protection device of the standby power supply is time T, and the delay time of other protection devices of the circuit is sequentially increased by a time step △ T from the standby power supply to the power supply of the fault section;
the fourth delay fixed value is set as that the delay fixed value of the fault section power supply outlet protection device is that a time step △ T is added on the basis of the third delay fixed value of the fault section power supply outlet protection device, and the delay time of other protection devices of the circuit is continuously increased by a time step △ T from the fault section power supply to the standby power supply in turn;
step 3) each protection device selects a delay fixed value according to the direction of the transient reactive power of the ground fault and the direction of the load power, and when the situation that the load power flows out from the power supply of the fault section and the transient reactive power flows to the circuit from the power supply of the fault section is detected, the delay fixed value of the protection device selects a first delay fixed value; when the load power direction is detected to flow out from the power supply of the fault section and the transient reactive power flows to the power supply of the fault section from the line, the delay fixed value of the protection device selects a second delay fixed value; when the load power direction is detected to flow out from the standby power supply and the transient reactive power flows to the circuit from the standby power supply, the delay fixed value of the protection device is a third delay fixed value; when the load power direction is detected to flow out of the standby power supply and the transient reactive power flows to the standby power supply from the line, the protection device selects a fourth delay timing value;
step 4), when the fault duration reaches a preset value, controlling the switch to act to remove the fault; and when the fault duration time does not reach a preset value, the protection returns.
the single-phase earth fault processing method suitable for the small-current grounding system is further designed in that △ T in the step 1) is selected according to the formula (1),
ΔT＝t1b+t2l+top+tu+tm(1)
wherein, t1bThe lead error of the current stage protection delay element; t is t2lHysteresis error of delay elements for next stage protection; t is topThe switching-off time of the sectional switch is set; t is tuIs a voltageA recovery time; t is tmIs a time margin.
The single-phase earth fault processing method suitable for the small-current earthing system is further designed in that when the control switch action in the step 4) removes the fault,
for stability faults, when the stability faults reach a preset value, a fault section is directly tripped and isolated;
for intermittent arc grounding faults, define T0Accumulating the arcing time; t is1For the threshold value of the arc-quenching time, the arc-quenching time T between two arc-ignitions is determined2Is divided into two cases, when T is0Tripping after the preset delay time is accumulated;
case 1) when T2＜T1The two arcing times are counted in T0；
Case 2) when T2＞T1When the fault occurs, the circuit is determined to have two single-phase earth faults, and T is added0And clearing and restarting timing from the second arc burning.
The single-phase earth fault processing method suitable for the low-current grounding system is further designed in such a way that the preset value is set to be 1-2 hours.
The invention has the following advantages:
compared with a centralized feeder automation method utilizing transient waveform similarity, the method does not need a communication channel, does not have the problem of cooperation between different manufacturer terminals and a master station, and is short in isolation time; compared with a method for isolating faults by using a voltage-time type section switch, the method has the advantages that the upstream sound section of the fault point does not need to be powered off, the fault power-off range is small, and the fault processing time is short; compared with the transient power direction ground protection in a centralized feeder automation mode and a distributed feeder automation mode, the method is independent of a communication network, and the fault processing time is shorter than that in the centralized feeder automation mode.
Drawings
fig. 1 is a diagram of a single power distribution line structure (where ■ represents an outgoing line breaker; ● represents a sectionalizer). fig. 2 is a diagram of a manual distribution line structure (where ■ represents an outgoing line breaker; ● represents a sectionalizer).
Detailed Description
The present invention will be described in detail with reference to the accompanying drawings.
The short-circuit fault mostly adopts an overcurrent protection method, because the distribution line is short and the branch line is numerous, the matching between the outgoing line protection and the branch line protection and the distribution transformer protection cannot be realized only by utilizing the current constant value, the protection at each level is mostly matched by time delay, and the level is generally controlled within three levels for ensuring the rapidity. For a low-current single-phase earth fault, capacitance current exists at the upstream and downstream of a fault point; in particular, in an arc suppression coil system, due to the compensation effect of the arc suppression coil, the difference between currents at the upstream and downstream of a fault point is small, and the protection cannot be realized by an overcurrent principle. The grounding protection can be realized by utilizing the transient power direction, and the protection coordination of each stage can be realized by utilizing different time delays. Meanwhile, as the earth fault current is small, the strict requirement on the switching-off time is not required, compared with the short-circuit fault, the protection level number can be increased, and the level difference between two levels of protection can also be increased.
the method comprises the following steps of I, selecting a line with the most detection points as a trunk line based on a single-power distribution line structure diagram shown in the attached figure 1, wherein the transient power direction is characterized in that ①, when the trunk line has a single-phase earth fault, the reactive power direction of a detection point of the trunk line at the upstream of a fault point flows to a bus from the line, the rest detection points all flow to the line from the bus, ②, when the branch line has a fault, the transient reactive power of the detection point at the upstream of the fault point in the branch line and the detection point of the trunk line at the upstream of the branch line (collectively called as the upstream detection point of the fault point) flows to the bus from the line, and.
For a radiation type line, only an upstream switch close to a fault point needs to be tripped, and the transient power direction and the delay time are matched to control the power supply side switch of the fault section to trip. The specific idea is as follows: regardless of the main line switch or the branch line switch, the transient reactive power flows to the bus through the line and trips after the time delay is reached.
The specific principle is described with reference to fig. 1. To ensure close upstream of the fault pointTripping switch, main line switches (QF)1～QL41) The delay time is increased from the load side to the power side sequentially and is T, T + delta T, T +2 delta T, T +3 delta T, T +4 delta T, T +5 delta T, T +6 delta T, T +7 delta T respectively; the branch line switch needs to be matched with the trunk line switch, and the branch line switch and the trunk line switch are sequentially increased from the load side to the power supply side from the T side according to the rule. The method can make the delay time of the branch line switch less than that of the upstream main line switch because the main line switch has the most number. Whether the main line fails or the branch line fails, the upstream switch immediately adjacent to the failure point can be ensured to trip first.
With switch QL12、QL21Take single-phase earth fault between them as an example, the section switch QF1、QL11And QL12And when the local transient reactive power is detected, the local transient reactive power flows to the bus through the line, and the timing is respectively started. T +5 delta T post-switch QL12Trip out of fault, QF1、QL11And the switch returns, and the upstream sound section restores normal operation.
Unlike a short-circuit fault, the ground fault current is very small. Therefore, in the low-current ground fault protection, the load switch provided with the protection device can remove the fault in addition to the breaker.
The details of this technique will be described based on the structure diagram of the power distribution line with backup shown in fig. 2.
In the radiation type line protection scheme, only the power supply side switch of the fault section needs to be controlled to trip. For a hand-pulling circuit with a standby power supply, a load side switch of a fault section needs to be controlled to trip, and the downstream sound circuit is safely transferred to the standby power supply.
If the power side switch trips first, the fault characteristics at the load side switch also disappear immediately, so that it is necessary to ensure that the load side switch trips first. When a single-phase earth fault occurs, transient reactive power of an upstream detection point of a fault point flows to a bus from a line; transient reactive power of a downstream detection point of the fault point flows to the line from the bus. According to the scheme, the upstream switch and the downstream switch of the fault point are identified by utilizing the transient power direction, and the load side switch is ensured to trip firstly by utilizing the delay time.
And setting the delay time. As shown in FIG. 2The power supply 1 is a fault section power supply, the power supply 2 is a standby power supply, the power supply 1 supplies power, the load power direction flows out from the power supply 1, transient reactive power flows to a circuit from a bus, and each switch (QF)1～QF2) The delay time is respectively set to T, T + delta T, T +2 delta T, T +3 delta T, T +4 delta T, T +5 delta T, T +6 delta T, T +7 delta T; each switch (QF) when transient reactive power flows from the line to the bus1～QF2) The delay time is respectively set to be T +15 delta T, T +14 delta T, T +13 delta T, T +12 delta T, T +11 delta T, T +10 delta T, T +9 delta T, T +8 delta T. The power is supplied by a power supply 2, the load power flows out from the power supply 2, the transient reactive power flows to a circuit from a bus, and each switch (QF)1～QF2) The delay time is respectively set as T +7 delta T, T +6 delta T, T +5 delta T, T +4 delta T, T +3 delta T, T +2 delta T, T + delta T, T; each switch (QF) when transient reactive power flows from the line to the bus1～QF2) The delay time is respectively set to be T +8 delta T, T +9 delta T, T +10 delta T, T +11 delta T, T +12 delta T, T +13 delta T, T +14 delta T, T +15 delta T
The delay time of the scheme is jointly determined by the transient power direction and the power supply. Therefore, a method for identifying a power supply source by using the load power direction is provided: if the load power direction of the switch flows out from the power supply 1, the power supply of the switch is judged to be the power supply 1; otherwise, power supply 2.
In normal operation, the interconnection switch QL4The switch is in an open state, and the other switches are in a closed state. Section QF1～QL3The power is supplied by a power supply 1, and the 1 st set value is used; section QF2～QL5Supplied by the power supply 2, using the set 2 value.
If switch QL1、QL2A single-phase earth fault occurs between the two, the switch QL2、QL3When local transient reactive power flowing from the bus to the line is detected, single-phase earth fault timing is respectively started, and a switch QL is switched after T +2 delta T2Trip, switch QL3Returning under the condition of pressure loss; switch QF1、QL1When local transient reactive power flowing from the line to the bus is detected, single-phase earth fault timing is respectively started, and a switch QL is switched after T +14 delta T1Tripping; the voltage returns to normal, QF1And returning. Downstream of fault point of closing recovery of interconnection switch QL4Power supply is carried out in a healthy section; therefore, the fault isolation is successful, all healthy sections are not subjected to power failure, and the interconnection switch can be closed after the fault isolation is successful, so that the normal operation of the downstream healthy sections is recovered.
The above description is only a preferred embodiment of the present invention, and is not intended to limit the present invention in any way, and any person skilled in the art may modify or modify the technical details disclosed above into equivalent embodiments with equivalent variations. However, any simple modification, equivalent change and modification of the above embodiments according to the technical essence of the present invention are within the protection scope of the technical solution of the present invention.
Claims (6)
1. A single-phase earth fault processing method suitable for a low-current grounding system is applied to a single-power-supply radiation type line, and controls an upstream switch close to a low-current grounding fault point to trip by utilizing the cooperation of the transient reactive power direction of the grounding fault and the delay time, and is characterized by comprising the following steps:
step 1) configuring different delay fixed values for outgoing line protection and line protection at each level:
the first delay fixed value is set as that the delay fixed values of the main line and the protection devices at the tail ends of all branch lines are all time T, the delay time of each protection device of the main line is sequentially increased by a time step △ T from the tail end of the line to the bus, the delay time of each protection device of each branch line is sequentially increased by a time step △ T from the load side to the branch point, and the second delay fixed value is set as that the delay fixed values of all protection devices are all infinite;
step 2), when the outgoing line protection and the line protection of each stage detect that the zero sequence voltage exceeds a preset threshold, starting fault protection, and calculating the load power direction and the grounding fault transient reactive power direction;
step 3), each protection device selects a delay fixed value according to the direction of the transient reactive power of the ground fault and the direction of the load power when necessary: when the transient reactive power of the ground fault flows to the bus from the line, the delay fixed value of the protection device is a first delay fixed value; when the transient reactive power of the ground fault flows to the line from the bus, the delay fixed value of the protection device is a second delay fixed value;
step 4), when the fault duration reaches a preset value, controlling the switch to act to remove the fault; when the fault duration time does not reach a preset value, protection is returned;
when the control switch action in the step 4) removes the fault,
for stability faults, when the stability faults reach a preset value, a fault section is directly tripped and isolated;
for intermittent arc grounding faults, define T0Accumulating the arcing time; t is1For the threshold value of the arc-quenching time, the arc-quenching time T between two arc-ignitions is determined2Is divided into two cases, when T is0Tripping after the preset delay time is accumulated;
case 1) when T2＜T1The two arcing times are counted in T0；
Case 2) when T2＞T1When the fault occurs, the circuit is determined to have two single-phase earth faults, and T is added0Clearing, and restarting timing from the second arc burning;
the preset value is set to 1-2 hours.
2. the single-phase earth fault handling method suitable for a low-current grounding system according to claim 1, wherein △ T in step 1) is selected according to formula (1),
ΔT＝t1b+t2l+top+tu+tm(1)
wherein, t1bThe lead error of the current stage protection delay element; t is t2lHysteresis error of delay elements for next stage protection; t is topThe switching-off time of the sectional switch is set; t is tuIs the voltage recovery time; t is tmIs a time margin.
3. A single-phase earth fault processing method suitable for a small current grounding system is applied to a hand-held line with a standby power supply, and when a single-phase earth fault occurs, transient reactive power of an upstream detection point of a fault point flows to a bus from the line; transient reactive power of a downstream detection point of a fault point flows to a circuit from a bus, a downstream switch and an upstream switch of the fault point are identified through the direction of the transient power, and a load side switch is ensured to trip first through delay time, and the method is characterized by comprising the following steps:
step 1) configuring different delay fixed values for outgoing line protection and line protection at each level, wherein the delay fixed values are respectively as follows:
the first delay fixed value is set as that the delay fixed value of the power outlet protection device of the fault section is time T, and the delay time of other protection devices of the circuit is sequentially increased by a time step △ T from the power supply of the fault section to the standby power supply;
the second delay fixed value is set as that the delay fixed value of the spare power supply outlet protection device is that a time step △ T is added on the basis of the first delay fixed value, and the delay time of other protection devices of the circuit is continuously increased by a time step △ T from the spare power supply to the fault section power supply;
the third delay fixed value is set as that the delay fixed value of the outgoing line protection device of the standby power supply is time T, and the delay time of other protection devices of the circuit is sequentially increased by a time step △ T from the standby power supply to the power supply of the fault section;
the fourth delay fixed value is set as that the delay fixed value of the fault section power supply outlet protection device is that a time step △ T is added on the basis of the third delay fixed value, and the delay time of other protection devices of the circuit is continuously increased by a time step △ T from the fault section power supply to the standby power supply;
step 2), when the outgoing line protection and the line protection of each stage detect that the zero sequence voltage exceeds a preset threshold, starting fault protection, and calculating the load power direction and the grounding fault transient reactive power direction;
step 3) each protection device selects a delay fixed value according to the direction of the transient reactive power of the ground fault and the direction of the load power, and when the situation that the load power flows out from the power supply of the fault section and the transient reactive power flows to the circuit from the power supply of the fault section is detected, the delay fixed value of the protection device selects a first delay fixed value; when the load power direction is detected to flow out from the power supply of the fault section and the transient reactive power flows to the power supply of the fault section from the line, the delay fixed value of the protection device selects a second delay fixed value; when the load power direction is detected to flow out from the standby power supply and the transient reactive power flows to the circuit from the standby power supply, the delay fixed value of the protection device is a third delay fixed value; when the load power direction is detected to flow out of the standby power supply and the transient reactive power flows to the standby power supply from the line, the protection device selects a fourth delay timing value;
step 4), when the fault duration reaches a preset value, controlling the switch to act to remove the fault; and when the fault duration time does not reach a preset value, the protection returns.
4. the single-phase earth fault handling method for small current grounding system according to claim 3, wherein △ T in step 1) is selected according to formula (1),
ΔT＝t1b+t2l+top+tu+tm(1)
wherein, t1bThe lead error of the current stage protection delay element; t is t2lHysteresis error of delay elements for next stage protection; t is topThe switching-off time of the sectional switch is set; t is tuIs the voltage recovery time; t is tmIs a time margin.
5. The single-phase earth fault handling method suitable for small current grounding system of claim 3, wherein when the control switch action in step 4) removes the fault,
for stability faults, when the stability faults reach a preset value, a fault section is directly tripped and isolated;
for intermittent arc grounding faults, define T0Accumulating the arcing time; t is1For the threshold value of the arc-quenching time, the arc-quenching time T between two arc-ignitions is determined2Is divided into two cases, when T is0Tripping after the preset delay time is accumulated;
case 1) when T2＜T1The two arcing times are counted in T0；
Case 2) when T2＞T1When the fault occurs, the circuit is determined to have two single-phase earth faults, and T is added0And clearing and restarting timing from the second arc burning.
6. The single-phase earth fault handling method for a low-current grounding system according to claim 3, wherein said preset value is set to 1-2 hours.
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