CN112684286A - Method for realizing small-current single-phase grounding line selection - Google Patents
Method for realizing small-current single-phase grounding line selection Download PDFInfo
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Abstract
The invention provides a method for realizing low-current single-phase grounding line selection, which is characterized in that when single-phase grounding occurs in a low-current grounding system, a zero-sequence fault signal current is obtained by combining differential filtering and filtering sequences from secondary currents of a three-phase current transformer of a line; compared with the method of the zero sequence current transformer widely adopted at present, the method for acquiring the zero sequence fault signal current is more economic, accurate and efficient, but the current transformers are arranged on three phases of a circuit, the action criterion of single-phase grounding line selection and the specific implementation scheme thereof are realized by effectively utilizing the steady-state zero sequence fault signal current, the method has the characteristics of simplicity and reliability, and can be used for a system with a neutral point not grounded and a system with the neutral point grounded through an arc suppression coil.
Description
Technical Field
The invention relates to the field of relay protection, in particular to a method for realizing small-current single-phase grounding line selection.
Background
The 6kV-35kV system in China usually adopts a small current grounding mode that a system neutral point is not grounded or is grounded through an arc suppression coil. Single phase grounding is one of the most common faults in a system. In order to ensure safe and reliable power supply of a power grid and personal equipment safety, a grounding fault line is required to be selected in time so that operation and inspection personnel can take measures to eliminate faults. Since the grounding current generated when the single-phase grounding is performed is small and the situation is complicated, it is difficult to select the line where grounding occurs by a simple and reliable method.
Although various small-current grounding line selection devices utilizing zero-sequence current and voltage have been developed and produced at home and abroad so far, most of the devices use a zero-sequence current transformer to obtain the zero-sequence current and are widely applied to systems. But the practical application effect is not satisfactory. So that at present, field operators often have to find the grounding line by adopting a manual line pulling method. The method not only can cause the user to power off for a short time so as to affect the production quality, but also can cause new faults. In addition, a large amount of manpower and material resources are consumed, and the time for repairing the fault is prolonged.
When single-phase grounding occurs in a small-current grounding system, zero-sequence fault signal current is generated and can be used for selecting a grounding line. Because the zero-sequence fault signal current is relatively small and is mixed in the strong line current, the zero-sequence fault signal current is difficult to extract by a method of directly adding three-phase currents. Zero sequence current transformers have been used for a long time to obtain zero sequence currents, but from the point of view of information processing, this method is neither economical nor efficient, and moreover has some difficulties in application on overhead lines.
The current electric power system is highly digitalized and informationized, and provides unprecedented favorable conditions for effectively extracting zero-sequence fault signal current from secondary current of a transformer. The problem is how to simply and completely and efficiently separate out zero-sequence fault signal current.
Disclosure of Invention
The invention provides a method for realizing small-current single-phase grounding line selection, which is simple and reliable in technology, effectively screens out fault current signals in three-phase line current and improves working efficiency.
The invention is realized by the following technical scheme:
a method for realizing small-current single-phase grounding line selection comprises the following steps:
step 1, obtaining three-phase line current after a fault;
step 2, carrying out differential filtering on the three-phase line current after the fault occurs, or carrying out differential filtering on the sum of the three-phase line current after the fault occurs;
step 3, screening out zero sequence fault signal current through differential filtering;
and 4, selecting a fault line by using the zero-sequence fault signal current.
Preferably, in step 1, the three-phase line currents are respectively iA、iBAnd iC。
Preferably, the three-phase line currents are arranged symmetrically.
Preferably, the three-phase fault signal currents obtained by differential filtering of the three-phase line currents are each igA、igBAnd igCObtaining zero sequence signal current i by three-phase fault signal currentgoThe calculation formula is as follows:
igo＝1/3[igA+igB+igC]。
preferably, the zero-sequence fault signal current is obtained by differential filtering of the three-phase line current by a filter-first-then-filter method and a filter-first-then-filter method.
Further, a filtering-first and filtering-second sequence method adopts a first action criterion; wherein the first action criterion is calculated as follows:
wherein: c0JIs the relative ground capacitance of line J; u shapeoIs the phase voltage of the system; kKFor a reliability factor, KKNot less than 1.2; j is the line number.
Furthermore, the first action criterion is formed by the principle of compensating the zero sequence capacitance current of the line, and after the zero sequence voltage is used for compensating the zero sequence current of each line, the magnitude of the compensation current is equal to the capacitance current of the line, and the direction is from the line to the bus.
Furthermore, the filtering-first and filtering-second method further adopts a second action criterion, wherein the second action criterion comprises an absolute value judgment method and a relative judgment method, and the absolute value judgment method comprises the following calculation method:
wherein, C0JIs the relative ground capacitance of line J; i isbpThe value is constant and is determined by the maximum unbalanced current; i is0LJIs a zero sequence fault signal current flowing through line J when single phase is grounded;
the calculation method of the relative judgment method is as follows:
Ipk＝max[IpJ] J＝I、II...
wherein J is a line number; ipk is the largest significant at Ipj when single-phase grounded, and k is the ground line.
Compared with the prior art, the invention has the following beneficial technical effects:
the invention provides a method for realizing low-current single-phase grounding line selection, which is characterized in that when single-phase grounding occurs in a low-current grounding system, a zero-sequence fault signal current is obtained by combining differential filtering and filtering sequences from secondary currents of a three-phase current transformer of a line; compared with the method of the zero sequence current transformer widely adopted at present, the method for acquiring the zero sequence fault signal current is more economic, accurate and efficient, but the current transformers are arranged on three phases of a circuit, the action criterion of single-phase grounding line selection and the specific implementation scheme thereof are realized by effectively utilizing the steady-state zero sequence fault signal current, the method has the characteristics of simplicity and reliability, and can be used for a system with a neutral point not grounded and a system with the neutral point grounded through an arc suppression coil.
Drawings
FIG. 1 is a schematic diagram of a fault signal obtained from three-phase current in the present invention;
FIG. 2 is a schematic diagram of a filtering-first and filtering-second method according to the present invention;
FIG. 3 is a schematic diagram of a first action criterion implementation of a filter-after-filter method according to the present invention;
FIG. 4 is a schematic diagram of an implementation of the first filtering and then filtering absolute value determination method according to the present invention;
FIG. 5 is a schematic diagram of the implementation of the first action criterion of the filter-first-order-then-filter method of the present invention;
fig. 6 is a schematic diagram of the implementation of the second action criterion in the filter-first-filter-second method according to the present invention.
Detailed Description
The present invention will now be described in further detail with reference to specific examples, which are intended to be illustrative, but not limiting, of the invention.
The invention provides a method for realizing small-current single-phase grounding line selection, which comprises the following steps:
step 1, obtaining three-phase line current after a fault;
step 2, carrying out differential filtering on the three-phase line current after the fault occurs, or carrying out differential filtering on the sum of the three-phase line current after the fault occurs;
step 3, screening out zero sequence fault signal current through differential filtering;
and 4, selecting a fault line by using the zero-sequence fault signal current.
After a single-phase earth fault occurs, the secondary current obtained by the line current transformer is formed by superposing the following two currents: one is the line current before a fault occurs, i.e. under normal operating conditions, which contains various harmonic currents and capacitive currents caused by line asymmetry or symmetry and other stray currents in addition to the load current. The other is fault signal current generated by single-phase grounding. For the fault signal current generated by single-phase grounding, the fault signal current generated when small-current single-phase grounding occurs is very small, and may be only a few a, while the line current under normal operation can be as high as hundreds a, which is a strong interference signal for relatively weak fault signal current. Therefore, filtering is necessary to isolate the useful fault signal current. In digital signal processing, differential filtering can better meet the requirements, and as shown in fig. 1, a zero-sequence signal current i can be obtained from three-phase fault signal currentsgoExpressed as:
igo＝1/3[igA+igB+igC]；
the method for obtaining the zero sequence fault signal current is more economical, simple, convenient, pure and efficient than the method for obtaining the zero sequence signal current by the currently widely adopted zero sequence current transformer.
The zero-sequence fault signal current flowing in the line is i0LIs igoConsidering i0LRatio of i to igoIs more intuitive and practical, so i0LExpressed as:
ioL＝igA+igB+igC；
the method for obtaining the zero-sequence fault signal current is called a filtering-first-filtering-second-filtering method. Another method for obtaining the zero-sequence fault signal current is a filtering-first and filtering-second method, as shown in fig. 2.
When single-phase grounding occurs in a low-current grounding system, the fault current is low, the three-phase voltage still keeps symmetrical, and the normal power supply to users can be continued and the two relative ground voltages which are not grounded are increased. To ensure that the customer is powered and to prevent the fault from developing, the ground line is typically allowed to continue to operate for some time. Therefore, the primary requirement for single-phase grounding line selection is high reliability, that is, only when single-phase grounding occurs in the line, the phase selection device is activated to select the grounding line, and the line selection device should not be mistakenly activated in any other situation. Because the number of distribution lines in the low-current grounding system is large, the distribution lines are complex and various, the action principle based on fault steady-state zero-sequence fault signal current is preferably considered to realize a simple and reliable line selection method.
Wherein, the filtering-first and filtering-second method adopts a first action criterion; wherein the first action criterion is calculated as follows:
wherein: c0JIs the relative ground capacitance of line J; u shapeoIs the phase voltage of the system; kKFor a reliability factor, KKNot less than 1.2; j is a line number, and the criterion is simple and reliable and is only suitable for lines with relatively small capacitance.
The first action criterion is formed by the principle of compensating the zero sequence capacitance current of the line, and after the zero sequence voltage is used for compensating the zero sequence current of each line, the magnitude of the compensation current is equal to the capacitance current of the line, and the direction is from the line to the bus. Thus, criterion I for ungrounded lines in the event of a single-phase grounding p0. And the grounding circuit: when the neutral point is not grounded IpEqual to the sum of the full system capacitance currents; when the neutral point is grounded via the arc suppression coil, IpEqual to the residual current of the ground point. The criterion has higher sensitivity and stronger self-adaptive capacity and can be used in a system grounded by an arc suppression coil.
The filtering-first and filtering-second sequence method also adopts a second action criterion, the second action criterion comprises an absolute value judgment method and a relative judgment method, wherein the calculation method of the absolute value judgment method comprises the following steps:
wherein, C0JIs the relative ground capacitance of line J; i isbpThe value is constant and is determined by the maximum unbalanced current; i is0LJIs a zero sequence fault signal current flowing through line J when single phase is grounded;
the calculation method of the relative judgment method is as follows:
Ipk＝max[IpJ],J＝I、II...
wherein J is a line number; i ispkTo be grounded in a single phase, IpjK is a ground line.
The principle implemented by the first action criterion in the filter-first-then-filter-sequence method is shown in fig. 3; the principle of the implementation by the absolute value judgment method in the second action criterion is shown in fig. 4; the filtering-first and filtering-second comprises a first action criterion and a second action criterion; the principle of the first action criterion implemented by an absolute value judgment method is shown in fig. 5; the principle of the second action criterion implemented by absolute value judgment is shown in fig. 6.
When the relative judgment method is adopted to realize the second action criterion, only the zero sequence current I after the outgoing line compensation on the bus of the substation can be usedpCollected together for comparison. If find I of one of the linespValue significantly greater than other IpValue, i.e. the I can be determinedpThe line of values is the ground line. If I of all linespIf the values are all comparable, none are significantly larger, then it can be determined that the ground point is outside all compared lines, e.g., on the bus. It is also possible that more than one I may occur when some of the line compensation errors are largepA value significantly greater than the other values, it can be determined that the ground point is in one of the lines.
Wherein fig. 3 to 6 can be implemented by using the existing microcomputer protection device, t in fig. 3 to 61And/0 is a delay action link. The method has the main function of preventing the zero-sequence fault signal current generated in the line switch operation and single-phase grounding transient process from causing the criterion misoperation. Can take t1Not less than 0.04s-0.06 s; t occurs according to the illustration of figures 3 to 6 as it must be used for grounding1The judgment result made by the data after s, therefore, the whole-cycle differential filtering formula i cannot be directly appliedg(n) i (n) -i (n-K). In order to achieve the equivalent effect of the full-cycle differential filtering, the flexible processing method is as follows: and storing the current data i-of the cycle before the fault after the single-phase grounding occurs. And extracting current data i + corresponding to one or more cycles after t1s and the fault. Then the i-and i + are subjected to differential operation to obtain the required fault messageSign current ig。
Claims (8)
1. A method for realizing low-current single-phase grounding line selection is characterized by comprising the following steps:
step 1, obtaining three-phase line current after a fault;
step 2, carrying out differential filtering on the three-phase line current after the fault occurs, or carrying out differential filtering on the sum of the three-phase line current after the fault occurs;
step 3, screening out zero sequence fault signal current through differential filtering;
and 4, selecting a fault line by using the zero-sequence fault signal current.
2. The method for realizing the small-current single-phase grounding line selection according to claim 1, wherein the three-phase line currents obtained in step 1 are i respectivelyA、iBAnd iC。
3. The method for realizing low-current single-phase grounding line selection according to claim 1, wherein the three-phase line current is symmetrically arranged.
4. The method of claim 1, wherein the three-phase fault signal currents obtained by differentially filtering the three-phase line currents are igA、igBAnd igCObtaining zero sequence signal current i by three-phase fault signal currentgoThe calculation formula is as follows:
igo＝1/3[igA+igB+igC]。
5. the method of claim 1, wherein the three-phase line current is filtered to obtain zero-sequence fault signal current by differential filtering.
6. The method for realizing low-current single-phase grounding line selection according to claim 5, wherein the filtering-first and filtering-second sequence method adopts a first action criterion; wherein the first action criterion is calculated as follows:
wherein: c0JIs the relative ground capacitance of line J; u shapeoIs the phase voltage of the system; kKFor a reliability factor, KKNot less than 1.2; j is the line number.
7. The method as claimed in claim 6, wherein the first action criterion is based on the principle of compensating the zero sequence capacitor current of the line, and when the zero sequence current of each line is compensated by the zero sequence voltage, the magnitude of the compensation current is equal to the capacitor current of the line, and the direction is from the line to the bus.
8. The method of claim 6, wherein the filtering-first filtering-second filtering sequence method further adopts a second action criterion, the second action criterion includes an absolute value judgment method and a relative judgment method, and the absolute value judgment method is calculated as follows:
wherein, C0JIs the relative ground capacitance of line J; i isbpThe value is constant and is determined by the maximum unbalanced current; i is0LJIs a zero sequence fault signal current flowing through line J when single phase is grounded;
the calculation method of the relative judgment method is as follows:
Ipk＝max[IpJ]J＝I、II...
whereinJ is a line number; i ispkTo be grounded in a single phase, IpjK is a ground line.
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