阅读说明：本技术 一种小电阻接地系统单相接地故障保护方法 (Single-phase earth fault protection method for small-resistance earth system ) 是由 王廷凰 刘子俊 张海台 黄福全 李国栋 于 2019-10-29 设计创作，主要内容包括：本发明提供一种小电阻接地系统单相接地故障保护方法,利用时间级差和整定值配合实现出口断路器、分支开关、分界开关和配电变压器保护之间的多级配合,有选择性地切除故障线路；针对高阻接地故障时,利用变电站各出线首端检测到的故障电流行波信号进行故障选线,同时排除其他产生原因产生较大不平衡电流的情况,将接地保护定值降低,灵敏度大大提高,保证在高阻接地故障时接地保护依然能够可靠动作。发生接地故障后能快速有选择性地切除故障,当过渡电阻高达1500Ω时依然有效,对减少配电网故障停电范围和停电时间有明显的效果。(The invention provides a single-phase earth fault protection method of a small resistance earth system, which utilizes the cooperation of time step difference and setting value to realize the multi-stage cooperation among an outlet circuit breaker, a branch switch, a boundary switch and the protection of a distribution transformer and selectively remove fault lines; when the high-resistance grounding fault occurs, fault line selection is carried out by utilizing fault current traveling wave signals detected by the head ends of outgoing lines of a transformer substation, the condition that other reasons for generation generate large unbalanced current is eliminated, the grounding protection constant value is reduced, the sensitivity is greatly improved, and the grounding protection can still reliably act when the high-resistance grounding fault occurs. After the earth fault occurs, the fault can be quickly and selectively removed, and the transition resistance is still effective when the transition resistance is up to 1500 omega, so that the method has obvious effect on reducing the power failure range and the power failure time of the power distribution network fault.)

1. A single-phase earth fault protection method of a small-resistance grounding system is characterized by comprising the following steps:

for a cable closed-loop operation network adopted in provincial and above large-city high-load density areas with power supply grades of A + and A, zero-sequence current differential protection is adopted on a main line as main protection, and zero-sequence III-section protection is configured as backup protection;

for a cable open-loop operation network adopted in an A + and A-type power supply area, four-stage protection is adopted, namely main line circuit breaker protection, ring main unit outgoing line circuit breaker protection, boundary protection and distribution transformer protection; if no boundary switch exists, three-level differential protection is adopted;

for a cable open-loop operation network adopted in an B, C-class power supply area, if a ring main unit switch adopts a load switch, a ring main unit incoming line switch is not configured with protection, a circuit outlet breaker is directly tripped when a permanent single-phase ground fault occurs on a main line, and then a fault circuit is isolated through a power distribution automation system; for an overhead line looped network, the zero sequence current protection configuration adopts a four-level or three-level differential protection configuration, and a section switch on a main line adopts a load switch without protection;

for an overhead radiation network adopted in a D-type power supply area, if a load switch is adopted in a branch, the zero-sequence current protection adopts secondary protection, namely outlet circuit breaker protection and distribution transformer protection.

2. The single-phase earth fault protection method of the small-resistance earthing system according to claim 1, characterized in that for the overhead line ring network adopted in B, C type power supply area, four-stage protection is adopted, namely main line breaker protection, branch protection, boundary protection and distribution transformer protection; if no boundary switch exists, three-level differential protection is adopted.

3. The single-phase earth fault protection method of a small-resistance earthing system according to claim 1 or 2, characterized in that the requirements of the four-stage protection on the switching device are specifically: the ring main unit incoming line switch and outgoing line switch adopt circuit breakers, the overhead network branch switch adopts a circuit breaker, the demarcation switch adopts a circuit breaker, and the side of the distribution transformer is provided with a circuit breaker or a load switch-fuse combined electrical appliance.

4. The single-phase earth fault protection method of the small-resistance earthing system according to claim 3, characterized in that, for zero-sequence current III-segment protection of the main line breaker, if a zero-sequence current transformer is adopted, the line outlet breaker protection setting value is 30A, and the action time limit is 3 s; if a zero sequence current filter is adopted to obtain zero sequence current, the protection setting value of the circuit breaker at the outlet of the line is 60A, and the action time limit is 3 s;

the cable ring main unit inlet circuit breaker protection, the zero sequence current setting value is the same as the outlet circuit breaker protection; the action time limit is matched with the adjacent incoming line circuit breaker protection and matched with the ring main unit outgoing line circuit breaker protection;

for zero-sequence current protection of the middle section breaker of the radiation type overhead line network, if a zero-sequence current transformer is adopted to obtain zero-sequence current, the corresponding protection setting value is 15A; if a zero sequence current filter is adopted, the corresponding protection setting value is 30A, the action time limit is smaller than that of the outlet circuit breaker by 0.3s, and the reclosing action time limit is defined to be 1 s.

5. The single-phase earth fault protection method of the small-resistance grounding system of claim 4, wherein for the branch line zero-sequence current III-segment protection current constant value, if a zero-sequence current transformer is adopted, the corresponding protection setting value is 8A; if the zero sequence current is obtained by adopting the zero sequence current filter, the corresponding protection setting value is 15A; the action time limit is set according to a time step difference larger than the lower boundary protection or the distribution transformer grounding protection and smaller than the protection time limit of the upper main line circuit breaker; the reclosing time is set to 1 s.

6. The single-phase earth fault protection method of a small-resistance grounding system as claimed in claim 5, wherein the setting value and reclosing time of the demarcation switch and the branch line protection are the same, and the action time limit is smaller than that of the branch line by a time step difference.

7. The single-phase earth fault protection method of the small-resistance grounding system of claim 6, characterized in that when the distribution transformer adopts a load switch-fuse combined electrical apparatus, the load switch is configured with zero-sequence current III-segment protection, the protection of the small-current fault is realized by tripping the load switch, and the fuse is used for clearing the large-current fault;

the zero sequence current protection of the load switch-fuse combined electrical appliance has the action time limit smaller than that of the adjacent previous protection by a time level difference; if the action time limit is more than 1s, if a zero sequence current transformer is adopted, the corresponding protection setting value is 10% of the rated current of the distribution transformer; if the zero sequence current is obtained by adopting the zero sequence current filter, the corresponding protection setting value is 20 percent of rated current of the distribution transformer; if the action time limit is less than 1s, if a zero sequence current transformer is adopted, the corresponding protection setting value is 20% of the rated current of the distribution transformer; if the zero sequence current filter is adopted to obtain the zero sequence current, the corresponding protection setting value is 40% of rated current of the distribution transformer.

8. The single-phase earth fault protection method of the small-resistance grounding system as claimed in claim 7, wherein when the capacity of the distribution transformer is larger than a preset threshold and breaker protection is adopted, zero-sequence current I-section and III-section protection is configured as single-phase earth short-circuit protection of the tail end;

for the setting value of the I section of the zero-sequence current, if a zero-sequence current transformer is adopted, the corresponding protection setting value is 20% of the rated current of the distribution transformer; if the zero sequence current is obtained by adopting the zero sequence current filter, the corresponding protection setting value is 40 percent of rated current of the distribution transformer, and the action time is limited to 0.1 s;

for the setting value of the zero-sequence current III section, if a zero-sequence current transformer is adopted, the corresponding protection setting value is 10% of the rated current of the distribution transformer; if the zero sequence current is obtained by adopting the zero sequence current filter, the corresponding protection setting value is 20 percent of rated current of the distribution transformer; the action time is defined as 1 s;

the reclosing time is set to 1 s.

Technical Field

The invention relates to the field of power distribution network fault processing, in particular to a single-phase earth fault protection method for a small-resistance earth system.

Background

The neutral point adopts a small resistance grounding mode to limit the non-fault phase overvoltage level after single-phase grounding fault, overcomes the ferromagnetic resonance overvoltage of a non-grounding system and the series resonance overvoltage of an arc suppression coil grounding system, and has simple relay protection configuration. In China, with the development of economy, in central urban areas with dense population, the power distribution network is perfect in structure and can ensure power supply reliability, and when a line has a single-phase earth fault, the fault line is expected to be cut off at the first time in consideration of ensuring personal safety. Based on the method, in the urban power distribution networks of Shanghai, Beijing, Guangzhou, Shenzhen, Suzhou and the like in China, the grounding mode without grounding or through an arc suppression coil is changed into the low-resistance grounding mode.

For a long time, the configuration and setting of the protection of the Chinese power distribution network emphasizes the guarantee of the safe operation of the substation equipment, the protection at the outlet circuit breaker of the medium-voltage distribution line of the substation is mainly considered, the protection on the line is not concerned enough, and the protection of a middle-section circuit breaker, branch protection, boundary protection and distribution transformer is not involved. Due to the problems of incomplete protection configuration and setting coordination, the selectivity of protection actions of the power distribution network is poor, and the fault power failure range is easily expanded. In a small resistance grounded distribution network, over 70% of short-circuit faults are single-phase ground faults, while high-resistance ground faults account for about 5% to 10% of the total number of ground faults.

At present, aiming at a single-phase earth fault of a feeder line, a stage type zero-sequence current protection scheme is mainly adopted, the influence of downstream capacitance current of a protected line and maximum unbalanced current in normal operation is considered to be avoided, the zero-sequence current protection setting value is relatively high, and the tolerable transition resistance value is generally only about 100 omega; however, in an actual power distribution network, ground faults are usually accompanied by the situations of tree obstacles, incomplete breakdown of a lightning arrester, falling of a conducting wire on the land, a pond and the like, the generated ground current is small, and the situation of refusing action often occurs in zero-sequence overcurrent protection.

Disclosure of Invention

The invention aims to provide a single-phase earth fault protection method and a single-phase earth fault protection system for a low-resistance earth system, so as to overcome the technical defect that zero-sequence overcurrent protection fails to operate and ensure the safe and stable operation of a power distribution network and power distribution equipment.

The embodiment of the invention provides a single-phase earth fault protection method for a small-resistance earth system, which comprises the following steps:

for a cable closed-loop operation network adopted in provincial and above large-city high-load density areas with power supply grades of A + and A, zero-sequence current differential protection is adopted on a main line as main protection, and zero-sequence III-section protection is configured as backup protection;

for a cable open-loop operation network adopted in an A + and A-type power supply area, four-stage protection is adopted, namely main line circuit breaker protection, ring main unit outgoing line circuit breaker protection, boundary protection and distribution transformer protection; if no boundary switch exists, three-level differential protection is adopted;

for a cable open-loop operation network adopted in an B, C-class power supply area, if a ring main unit switch adopts a load switch, a ring main unit incoming line switch is not configured with protection, a circuit outlet breaker is directly tripped when a permanent single-phase ground fault occurs on a main line, and then a fault circuit is isolated through a power distribution automation system; for an overhead line looped network, the zero sequence current protection configuration adopts a four-level or three-level differential protection configuration, and a section switch on a main line adopts a load switch without protection;

for an overhead radiation network adopted in a D-type power supply area, if a load switch is adopted in a branch, the zero-sequence current protection adopts secondary protection, namely outlet circuit breaker protection and distribution transformer protection.

The overhead line ring network adopted in the B, C-class power supply area adopts four-stage protection, namely main line circuit breaker protection, branch protection, boundary protection and distribution transformer protection; if no boundary switch exists, three-level differential protection is adopted.

The requirements of the four-stage protection on the switch device are specifically as follows: the ring main unit incoming line switch and outgoing line switch adopt circuit breakers, the overhead network branch switch adopts a circuit breaker, the demarcation switch adopts a circuit breaker, and the side of the distribution transformer is provided with a circuit breaker or a load switch-fuse combined electrical appliance.

For zero-sequence current III-section protection of a main line circuit breaker, if a zero-sequence current transformer is adopted, the setting value of circuit outlet breaker protection is 30A, and the action time limit is 3 s; if a zero sequence current filter is adopted to obtain zero sequence current, the protection setting value of the circuit breaker at the outlet of the line is 60A, and the action time limit is 3 s;

the cable ring main unit inlet circuit breaker protection, the zero sequence current setting value is the same as the outlet circuit breaker protection; the action time limit is matched with the adjacent incoming line circuit breaker protection and matched with the ring main unit outgoing line circuit breaker protection;

for zero-sequence current protection of the middle section breaker of the radiation type overhead line network, if a zero-sequence current transformer is adopted to obtain zero-sequence current, the corresponding protection setting value is 15A; if a zero sequence current filter is adopted, the corresponding protection setting value is 30A, the action time limit is smaller than that of the outlet circuit breaker by 0.3s, and the reclosing action time limit is defined to be 1 s.

For the fixed value of the III-section protection current of the zero sequence current of the branch line, if a zero sequence current transformer is adopted, the corresponding protection setting value is 8A; if the zero sequence current is obtained by adopting the zero sequence current filter, the corresponding protection setting value is 15A; the action time limit is set according to a time step difference larger than the lower boundary protection or the distribution transformer grounding protection and smaller than the protection time limit of the upper main line circuit breaker; the reclosing time is set to 1 s.

The setting value and the reclosing time of the boundary switch and the branch line protection are the same, and the action time limit is smaller than that of the branch line by a time step difference.

When the distribution transformer adopts a load switch-fuse combined electrical appliance, the load switch is configured with zero-sequence current III-section protection, the protection of a small current fault is realized by tripping off the load switch, and the fuse is used for clearing a large current fault;

the zero sequence current protection of the load switch-fuse combined electrical appliance has the action time limit smaller than that of the adjacent previous protection by a time level difference; if the action time limit is more than 1s, if a zero sequence current transformer is adopted, the corresponding protection setting value is 10% of the rated current of the distribution transformer; if the zero sequence current is obtained by adopting the zero sequence current filter, the corresponding protection setting value is 20 percent of rated current of the distribution transformer; if the action time limit is less than 1s, if a zero sequence current transformer is adopted, the corresponding protection setting value is 20% of the rated current of the distribution transformer; if the zero sequence current filter is adopted to obtain the zero sequence current, the corresponding protection setting value is 40% of rated current of the distribution transformer.

When the capacity of the distribution transformer is larger than a preset threshold value and circuit breaker protection is adopted, zero-sequence current I-section and zero-sequence current III-section protection is configured to serve as single-phase grounding short-circuit protection of the tail end;

for the setting value of the I section of the zero-sequence current, if a zero-sequence current transformer is adopted, the corresponding protection setting value is 20% of the rated current of the distribution transformer; if the zero sequence current is obtained by adopting the zero sequence current filter, the corresponding protection setting value is 40 percent of rated current of the distribution transformer, and the action time is limited to 0.1 s;

for the setting value of the zero-sequence current III section, if a zero-sequence current transformer is adopted, the corresponding protection setting value is 10% of the rated current of the distribution transformer; if the zero sequence current is obtained by adopting the zero sequence current filter, the corresponding protection setting value is 20 percent of rated current of the distribution transformer; the action time is defined as 1 s;

the reclosing time is set to 1 s.

The embodiment of the invention provides a single-phase earth fault protection method for a small-resistance earth system, which can be completed by only detecting zero-sequence current in a line and current transient signals of each line outlet, has a simple principle, is easy to realize, and has strong practicability and economy. The embodiment of the invention provides a detailed configuration scheme aiming at single-phase earth fault protection of a small-resistance earth system under various structures and operation modes, and has low dependence on a network structure and good adaptability; when a low-resistance earth fault occurs, the fault can be selectively removed within 3s through the multi-stage protection of matching time step difference and setting value, the override trip phenomenon can not occur, and the power failure range and the power failure time are reduced; when high-resistance grounding faults occur, the faults can be reliably and selectively removed even if the transition resistance is up to more than 1500 omega through fault line selection and multi-stage protection matched with time level difference. In addition, the setting calculation method of the embodiment of the invention is simple, meets the requirement of protection on four properties, can realize the protection coordination among multiple stages of a small-resistance grounding system outlet circuit breaker, a branch line, a distribution transformer and the like, avoids the protection override motion when a grounding fault occurs, and simultaneously, the high-sensitivity grounding protection also well makes up the defect of the traditional power distribution network in detecting high-resistance grounding.

Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. Of course, not all of the advantages described above need to be achieved at the same time in the practice of any one product or method of the invention.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.

Fig. 1 is a schematic diagram of a typical structure of a closed-loop operation cable ring network in a class a + and a power supply area according to an embodiment of the present invention.

Fig. 2 shows a typical structure of an open-loop operation cable ring network in a class a + and a power supply area.

Fig. 3 shows a typical structure of an B, C power supply area overhead looped network.

Fig. 4 shows a typical structure of a radiating overhead network of a D-type power supply area.

Fig. 5 is a schematic diagram of the modulus of the current traveling wave α at the transition resistance of 200 Ω and its wavelet transform.

Fig. 6 is a schematic diagram of the modulus of the current traveling wave α at the transition resistance of 1500 Ω and its wavelet transform.

Detailed Description

The technical solutions in the embodiments of the present invention will be described clearly and completely with reference to the accompanying drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

It should be noted that, in order to avoid obscuring the present invention with unnecessary details, only the structures closely related to the solution according to the present invention are shown in the drawings, and other details not closely related to the present invention are omitted.

The embodiment of the invention provides a small current ground fault positioning method, which comprises the following steps:

(1) for small-resistance grounding systems with different power supply quality grades, the grounding protection steps are as follows:

(1.1) cable closed-loop operation network adopted for provincial and higher large city high load density areas with power supply grades of A + and A, as shown in figure 1. The main line adopts zero sequence current differential protection as main protection, thereby rapidly cutting off the main line fault and avoiding short-time power failure of users. When the grounding current is small due to grounding through the transition resistor, the differential protection has no sensitivity, so that the zero sequence III section protection is required to be configured as a backup protection to remove the fault. If the differential protection does not act within the specified time, the zero sequence backup protection is started. The protection action time limit on two sides of the ring-opening line is set to be minimum, so that the ring network is separated into two radiation type networks by the first action, and the zero sequence current protection is configured according to the protection configuration principle under the following open-loop operation mode.

(1.2) open-loop operation of the network for cables used in the class a +, a power supply area, as shown in fig. 2. In order to ensure that fault elements are selectively removed after ground faults, four-level protection schemes are adopted, namely main line circuit breaker protection, ring main unit outgoing line circuit breaker protection, boundary protection and distribution transformer protection; if no boundary switch exists, three-level differential protection is adopted. The ring main unit incoming and outgoing switches adopt circuit breakers, the demarcation switch also adopts a circuit breaker, and the side of the distribution transformer is provided with a circuit breaker or a load switch-fuse combined electrical appliance. And the outlet circuit breaker, the branch line and the like are all provided with single reclosure so as to prevent long-time power failure of loads caused by instant faults of the branch line and the user side.

(1.3) for the cable open-loop operation network adopted in the B, C-class power supply area, if the ring main unit switch adopts a load switch, the ring main unit incoming line switch is not protected, when a permanent single-phase earth fault occurs on the main line, the circuit breaker at the outlet of the circuit is directly tripped, and then the fault circuit is isolated through the power distribution automation system. And the other position protection configuration adopts a four-level or three-level differential protection configuration.

For an overhead line ring network adopted in an B, C-type power supply area, as shown in fig. 3. The zero sequence current protection configuration is the same as that in the open loop operation of the cable loop network shown in fig. 2, and a four-level or three-level difference protection configuration is adopted. When the power supply radius is short, the section switch on the main line adopts a load switch, and then protection is not configured.

(1.4) an overhead radiating network for a class D power supply area, as shown in fig. 4. If a load switch is adopted in the branch, the zero sequence current protection adopts a two-stage protection scheme, namely outlet circuit breaker protection and distribution transformer protection. When the power supply radius is longer, the trunk line is protected by a middle section breaker and is matched with upper and lower-level protection. When a section between two levels of protection has a fault, the superior protection action removes the fault, and then the non-fault line is restored to operate by means of the distribution automation system.

(2) The setting steps for the multi-stage grounding protection are as follows:

(2.1) for the zero-sequence current III-section protection of the main line circuit breaker, if a zero-sequence current transformer is adopted, the overhead line outlet circuit breaker protection setting value avoids the current of the line capacitor, the setting value is 30A, and the action time limit is 3 s; if the zero sequence current filter is adopted to obtain the zero sequence current, the unbalanced current of the circuit during cold starting needs to be avoided, the setting value is 60A, and the action time limit is the same.

The protection of the incoming circuit breaker of the cable ring main unit is the same as that of the outgoing circuit breaker, the zero sequence current setting value is the same as that of the outgoing circuit breaker, and the action time limit is matched with the protection of the adjacent incoming circuit breaker and the protection of the outgoing circuit breaker of the ring main unit. The time step difference is selected to be 0.3s in consideration of the mechanical action time, arc quenching time, etc. of the breaker switch.

The zero sequence current protection of the middle section circuit breaker of the radiation type overhead line network is realized, if a zero sequence current transformer is adopted to obtain zero sequence current, the maximum capacitance current of a downstream line is avoided, and the setting value can be uniformly selected to be 15A; if a zero sequence current filter is adopted, the influence of unbalanced current in cold starting of a downstream line can be avoided, the value can be uniformly selected to be 30A, and the action time limit is smaller than the protection of an outlet circuit breaker by 0.3 s. The reclosing time is set to 1 s.

(2.2) the fixed value of the zero sequence current III section protection current of the branch line should firstly avoid the maximum capacitance current at the downstream of the branch line, and if a zero sequence current transformer is adopted, the set value can be uniformly selected to be 8A; if the zero sequence current filter is adopted to obtain the zero sequence current, the value can be uniformly selected to be 15A. The action time limit is set according to a time step difference larger than that of the lower boundary protection or the distribution transformer grounding protection and smaller than that of the upper main line breaker protection time limit. The reclosing time is set to 1 s.

And (2.3) setting values and reclosing time of the boundary switch and the branch line protection are the same. The action time limit is smaller than the action time limit of the branch line by a time step difference.

(2.4) when the distribution transformer adopts a load switch-fuse combined electrical apparatus, the rated current of the fuse is selected according to the rated current of the distribution transformer which is more than 2 times, the load switch is provided with zero sequence current III-section protection, the protection of small current faults is realized by tripping the load switch, and the fuse is used for clearing large current faults. The zero sequence current protection of the load switch-fuse combined electric appliance has action time limit smaller than that of the adjacent previous protection by a time level difference. If the action time limit is more than 1s, if a zero sequence current transformer is adopted, the unbalanced current is very small, and the fixed value is set to be 10% of the rated current of the distribution transformer; if the zero sequence current filter is adopted to obtain the zero sequence current, the unbalanced current generated by load cold start is avoided, and 20% of rated current of the distribution transformer is selected. If the action time limit is less than 1s, the setting value is doubled; if a zero sequence current transformer is adopted, the unbalanced current is very small, and the fixed value is set to be 20% of the rated current of the distribution transformer; if the zero sequence current filter is adopted to obtain the zero sequence current, the unbalanced current generated by load cold start is avoided, and 40% of rated current of the distribution transformer is selected.

When the capacity of the distribution transformer is large and the breaker protection is adopted, the I-section and III-section protection of zero sequence current is configured to be used as single-phase grounding short-circuit protection of the tail end. Setting value of the zero sequence current I section is the same as that of the upper operation time limit, and the operation time limit is set to be 0.1 s; the setting value of the zero sequence current III section is the same as the setting value when the action time limit is large, and the action time limit is set to be 1 s. The reclosing time is set to 1 s.

(3) For high-resistance ground faults, reducing the zero-sequence current protection setting value is an effective measure for improving the protection sensitivity in the high-resistance fault. The difference of the amplitude and the polarity of each outgoing line current traveling wave signal during the fault is utilized to determine that the system has the fault and select a fault line, so that the protection setting value of the circuit breaker at the line outlet does not need to consider the capacitance current to ground of the line when other outgoing line faults are avoided, and meanwhile, the zero sequence current can also be judged to be the unbalanced current caused by the line fault but not other reasons. The zero sequence current setting value only needs to avoid zero sequence current caused by three-phase load unbalance during normal operation of the system, simultaneously the linear range and the measurement error of the zero sequence current transformer are considered, the starting current at least reaches 0.5 percent of the full range of the zero sequence transformer, and the setting value can be determined to be 3.2A. After the transformer substation determines a fault line, if the transition resistance is small, the grounding protection selectively removes the fault, and the maximum protection action time is 3s because the action time of the circuit outlet breaker is limited to 3 s. Considering the tripping time of the circuit breaker, if the circuit breaker is connected with the ground within 3.1s for protection, the system recovers to operate; if the grounding protection does not act, the high-sensitivity grounding protection of the fault line is started, and the action time limit is set according to the step difference, so that the fault is selectively removed.

The setting method of the embodiment of the invention has simple principle, and provides a detailed method for the small-resistance grounding system power distribution network in various operation modes; the protection has better reliability, the coordination among various levels such as a small-resistance grounding system outlet circuit breaker, a branch line, a distribution transformer and the like can be realized, and the protection override action is avoided when a grounding fault occurs; for high-resistance ground faults, the protection still can selectively and reliably act when the transition resistance is as high as 1500 omega.

Taking a typical power grid model as an example:

a Matlab-Simulink software tool was used to build 10kV cable open loop operating network and overhead line radial network models, as shown in FIGS. 2 and 4.

During simulation, overhead lines f1 (2.6 km away from an outlet on a main line), f2 (0.3 km away from a branch point on a branch B4) and f3 (primary side of a distribution transformer T6) are respectively arranged to generate single-phase ground faults, and transition resistances are respectively 10 omega, 200 omega and 1500 omega; in the simulation, a cable looped network f1 (the distance between a L1 line and an outlet is 0.8km), f2 (the distance between the L1 line and the outlet of a 2# looped network cabinet is 0.2km) and f3 (the primary side of a distribution transformer T2) are respectively arranged to generate single-phase ground faults, and the transition resistances are respectively 10 omega, 200 omega and 1500 omega. After the fault, the zero sequence current of each protection installation position is detected, the zero sequence current traveling wave signal of each outgoing line is extracted and subjected to wavelet transformation, and the traveling wave and the wavelet transformation waveform thereof are obtained as shown in fig. 5. The system protection configuration and the setting values are shown in table 1 and table 2, respectively. The zero sequence current and protection behavior are shown in tables 3 and 4.

TABLE 1 overhead radiation network earthing protection configuration and setting

TABLE 2 Cable Looped network earthing protection configuration and setting

Table 3 zero sequence current effective value and action condition of protective installation (overhead radiation network)

Note:_↑-ground protection enabled;_↓-a ground protection return;_×-ground protection is not enabled;_Hhigh sensitivity grounding

Table 4 zero sequence current effective value and action condition of protective installation (cable ring network)

Because the influence of the structure and the length of the line on the zero-sequence current is small when the high-resistance fault occurs, the zero-sequence current and the action condition are basically the same as those shown by an overhead network, and the list is not written.

The protection is selectively and reliably operated at low-resistance grounding as shown in tables 3 and 4, and the data in table 3 shows that the outlet protection does not operate when the transition resistance is 200 Ω, although the transition resistance tolerance of each stage of circuit is different, the transition resistance tolerance of the middle breaker, the branch and the distribution transformer is gradually enhanced, but the reliable operation at the transition resistance higher than 1000 Ω still can not be performed, when the grounding protection sensitivity is not enough, the transformer substation detects the current traveling wave signal and performs wavelet transformation, selects the fault circuit by the difference of the traveling wave amplitude and the polarity, starts the high-sensitivity grounding protection after 3.1s, the relation of the current traveling wave α modulus and the wavelet transformation polarity of each outlet when the transition resistance is 200 Ω and 1500 Ω is as shown in fig. 5, and the traveling wave line selection method can still correctly select the fault circuit, and then the correct operation of each stage of high-sensitivity grounding protection.

The foregoing is directed to embodiments of the present invention, and it is understood that various modifications and improvements can be made by those skilled in the art without departing from the spirit of the invention.