A method for realizing line selection for small current single-phase grounding

Abstract

The present invention provides a method for realizing line selection for single-phase grounding with small current. When single-phase grounding occurs in a small current grounding system, a zero-sequence fault signal current is obtained from the secondary current of a three-phase current transformer of the line by combining differential filtering and sequence filtering. The method for obtaining the zero-sequence fault signal current is more economical, accurate and efficient than the currently widely used zero-sequence current transformer method. However, current transformers are installed on all three phases of the line. The steady-state zero-sequence fault signal current is effectively used to realize the action criterion of single-phase grounding line selection and its specific implementation scheme. The method has the characteristics of being simple and reliable, and can be used in a system with an ungrounded neutral point, and is also suitable for a system with a neutral point grounded through an arc suppression coil.

Description

Technical Field

[0001] The invention relates to the field of relay protection, and in particular to a method for realizing small current single-phase grounding line selection. Background Art

[0002] my country's 6kV-35kV system usually adopts a low-current grounding method in which the neutral point of the system is not grounded or is grounded through an arc suppression coil. Single-phase grounding is the most common fault in the system. In order to ensure the safe and reliable power supply of the power grid and the safety of personnel and equipment, it is required to select the grounding fault line in time so that the operation and inspection personnel can take measures to eliminate the fault. Since the grounding current generated by single-phase grounding is small and the situation is complicated, it is difficult to select the grounding line with a simple and reliable method.

[0003] Although many small current grounding line selection devices using zero-sequence current and voltage have been developed and produced at home and abroad, most of them use zero-sequence current transformers to obtain zero-sequence current and are widely used in systems. However, the actual application effect is not satisfactory. As a result, on-site operators often have to use the method of manually pulling the line to find the grounding line. This method not only causes users to lose power for a short time, thus affecting production quality, but may also cause new faults. In addition, it consumes a lot of manpower and material resources, delaying the time to repair the fault.

[0004] When a single-phase grounding occurs in a low-current grounding system, a zero-sequence fault signal current is generated, which can be used to select the grounding line. Since the zero-sequence fault signal current is relatively small and mixed in the strong line current, it is difficult to extract the zero-sequence fault signal current by directly adding the three-phase current. Therefore, zero-sequence current transformers have been used to obtain zero-sequence current for a long time, but from the perspective of information processing, this method is neither economical nor efficient, and there are also some difficulties in its application on overhead lines.

[0005] The current power system has been highly digitized and informationized, which provides unprecedented favorable conditions for effectively extracting zero-sequence fault signal current from the secondary current of the transformer. The question is how to separate the zero-sequence fault signal current simply, completely and efficiently. Summary of the invention

[0006] In view of the problem that the small current grounding line selection device using zero-sequence current and voltage in the prior art may cause short-term power outages for users, thus affecting production quality and may also cause new faults to occur, the present invention provides a method for realizing small current single-phase grounding line selection. The technology is simple and reliable, and can effectively screen out fault current signals in three-phase line current, thereby improving work efficiency.

[0007] The present invention is achieved through the following technical solutions:

[0008] A method for realizing low-current single-phase grounding line selection comprises the following steps:

[0009] Step 1, obtaining the three-phase line current after the fault;

[0010] Step 2, performing differential filtering on the three-phase line current after the fault occurs, or performing differential filtering on the sum of the three-phase line current after the fault occurs;

[0011] Step 3, screening out the zero-sequence fault signal current through differential filtering;

[0012] Step 4: Select the fault line using the zero-sequence fault signal current.

[0013] Preferably, in step 1, the three-phase line currents obtained are i A 、i B and i C .

[0014] Preferably, the three-phase line currents are set symmetrically.

[0015] Preferably, the three-phase fault signal currents obtained by differential filtering of the three-phase line currents are i gA 、i gB and i gC , the zero-sequence signal current i is obtained through the three-phase fault signal current go , the calculation formula is as follows:

[0016] i go =1 / 3[i gA +i gB +i gC] .

[0017] Preferably, the three-phase line current is differentially filtered to obtain the zero-sequence fault signal current by using a method of filtering first and then filtering the sequence or a method of filtering first and then filtering the sequence.

[0018] Furthermore, the filter-first-filter-then-filter sequence method adopts the first action criterion; wherein the calculation formula of the first action criterion is as follows:

[0019]

[0020] Where: C 0J is the relative capacitance of line J; U o is the phase voltage of the system; K K is the reliability coefficient, K K ≥1.2; J is the line number.

[0021] Furthermore, the first action criterion is constructed by using the principle of compensating the zero-sequence capacitive current of the line. 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 capacitive current of the line, and the direction is from the line to the bus.

[0022] Furthermore, the filter-first-filter-then-filter sequence method also adopts a second action criterion, which includes an absolute value judgment method and a relative judgment method, wherein the calculation method of the absolute value judgment method is as follows:

[0023]

[0024] Among them, C 0J is the relative capacitance of line J; I bp is a fixed value, determined by the maximum unbalanced current; I 0LJ is the zero-sequence fault signal current flowing through line J when single-phase is grounded;

[0025] The calculation method of the relative judgment method is as follows:

[0026] I pk =max[I pJ ] J=I、II...

[0027] Wherein, J is the line number; Ipk is the significant maximum among Ipj when single-phase is grounded, and k is the grounding line.

[0028] Compared with the prior art, the present invention has the following beneficial technical effects:

[0029] The present invention provides a method for realizing line selection for single-phase grounding with small current. When single-phase grounding occurs in a small current grounding system, a zero-sequence fault signal current is obtained from the secondary current of a three-phase current transformer of the line by combining differential filtering and sequence filtering. The method for obtaining the zero-sequence fault signal current is more economical, accurate and efficient than the currently widely used zero-sequence current transformer method. However, current transformers are installed on all three phases of the line. The steady-state zero-sequence fault signal current is effectively used to realize the action criterion of single-phase grounding line selection and its specific implementation scheme. The method has the characteristics of being simple and reliable, and can be used in a system with an ungrounded neutral point, and is also suitable for a system with a neutral point grounded through an arc suppression coil. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 A schematic diagram of a fault signal obtained from three-phase current in the present invention;

[0031] Figure 2 It is a schematic diagram of the method of filtering first and filtering later in the present invention;

[0032] Figure 3 This is a schematic diagram of the implementation principle of the first action criterion of the filter-first-filter-then-filter sequence method in the present invention;

[0033] Figure 4 It is a principle diagram of the absolute value judgment method of the first filtering and then filtering sequence method in the present invention;

[0034] Figure 5 It is a principle diagram for realizing the first action criterion of the filter-first-filter-then-filter method in the present invention;

[0035] Figure 6 This is a schematic diagram of the implementation of the second action criterion of the first-filter-then-filter method in the present invention. DETAILED DESCRIPTION

[0036] The present invention is further described in detail below in conjunction with specific embodiments, which are intended to explain the present invention rather than to limit it.

[0037] The present invention provides a method for realizing low-current single-phase grounding line selection, comprising the following steps:

[0038] Step 1, obtaining the three-phase line current after the fault;

[0039] Step 2, performing differential filtering on the three-phase line current after the fault occurs, or performing differential filtering on the sum of the three-phase line current after the fault occurs;

[0040] Step 3, screening out the zero-sequence fault signal current through differential filtering;

[0041] Step 4: Select the fault line using the zero-sequence fault signal current.

[0042] After a single-phase grounding fault occurs, the secondary current obtained by the line current transformer is composed of the following two currents: one is the line current before the fault occurs, that is, under normal operating conditions, which, in addition to the load current, also contains various harmonic currents and capacitive currents caused by line asymmetry or symmetry, as well as other stray currents. The other is the fault signal current generated by single-phase grounding. For the fault signal current generated by single-phase grounding, since the fault signal current generated when a small current single-phase grounding occurs is very small, it may be only a few A, while the line current under normal operation can be as high as hundreds of A, it is a strong interference signal for the relatively weak fault signal current. Therefore, it is necessary to use filtering methods to separate the useful fault signal current. In digital signal processing, differential filtering can better meet the above requirements, such as Figure 1 As shown, the zero-sequence signal current i can be obtained from the three-phase fault signal current go , expressed as:

[0043] i go =1 / 3[i gA +i gB +i gC ];

[0044] The zero-sequence fault signal current obtained in this way is more economical, simple, pure and efficient than the zero-sequence signal current obtained by the zero-sequence current transformer which is widely used now.

[0045] In fact, the zero-sequence fault signal current flowing through the line is i 0L That's i go Considering i 0L Than i go More intuitive and practical, so i 0L It is expressed as:

[0046] i oL =i gA +i gB +i gC ;

[0047] The above method of obtaining zero-sequence fault signal current is called the method of filtering first and filtering second. Based on the same principle, another method of obtaining zero-sequence fault signal current is the method of filtering first and filtering second. Figure 2 shown.

[0048] In the present invention, when single-phase grounding occurs in a small current grounding system, the fault current is small, the three-phase voltage remains symmetrical, the user can continue to be powered normally, and the voltage between the two phases that are not grounded increases. In order to ensure the power supply to the user and prevent the fault from expanding, the grounding line is usually allowed to continue to operate for a period of time. Therefore, the primary requirement for single-phase grounding line selection is high reliability, that is, only when single-phase grounding occurs inside the line, the phase selection device will operate to select the grounding line, and the line selection device should not malfunction under any other circumstances. Since the number of distribution lines in the small current grounding system is huge and complex and diverse, priority is given to a simple and reliable line selection method based on the action principle of the fault steady-state zero-sequence fault signal current.

[0049] Among them, the first filtering and then filtering sequence method adopts the first action criterion; the calculation formula of the first action criterion is as follows:

[0050]

[0051] Where: C 0J is the relative capacitance of line J; U o is the phase voltage of the system; K K is the reliability coefficient, K K ≥1.2; J is the line number. This criterion is simple and reliable and is only applicable to lines with relatively small capacitance.

[0052] The first action criterion is based on the principle of compensating the zero-sequence capacitance current of the line. 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 capacitance current of the line, and the direction is from the line to the busbar. Therefore, when a single-phase grounding occurs, the criterion I of the non-grounded linep = 0. For grounded circuit: When the neutral point is not grounded, I p , which is equal to the sum of the capacitive currents of the entire system; when the neutral point is grounded through the arc suppression coil, I p This criterion has high sensitivity and strong adaptability, and can be used in arc suppression coil grounding systems.

[0053] The filter-first-filter-then-filter sequence method also adopts a second action criterion, which includes an absolute value judgment method and a relative judgment method. The calculation method of the absolute value judgment method is as follows:

[0054]

[0055] Among them, C 0J is the relative capacitance of line J; I bp is a fixed value, determined by the maximum unbalanced current; I 0LJ is the zero-sequence fault signal current flowing through line J when single-phase is grounded;

[0056] The calculation method of the relative judgment method is as follows:

[0057] I pk =max[I pJ ], J = I, II...

[0058] Where, J is the line number; I pk When single-phase grounding, I pj The significant largest one among them, k is the ground line.

[0059] The principle implemented by the first action criterion in the filter-first-filter-then-filter method is as follows: Figure 3 As shown; the principle of using the absolute value judgment method in the second action criterion is as follows Figure 4 As shown; the first filter sequence and then the filtering include the first action criterion and the second action criterion; the principle of the first action criterion being implemented by the absolute value judgment method is as follows Figure 5 As shown; the principle of the second action criterion using the absolute value judgment method is as follows Figure 6 shown.

[0060] When the relative judgment method is used to implement the second action criterion, as long as the zero-sequence current I p Collect them together and compare them. p The value is significantly greater than other I p value, we can determine the I p The line with the value is the ground line. If the I pIf the values ​​are similar and none of them is significantly larger, it can be determined that the grounding point is outside all the compared lines, such as on the busbar. Another possibility is that when the compensation error of some lines is large, more than one I p If the value is significantly larger than the other values, it can be determined that the grounding point is within one of the lines.

[0061] in Figures 3 to 6 It can be realized by using the existing microcomputer protection device. Figures 3 to 6 Where t1 / 0 is a time-delay action link. Its main function is to prevent the zero-sequence fault signal current generated by the line switch operation and the single-phase grounding transient process from causing the judgment to malfunction. It can be taken that t1≥0.04s-0.06s; according to Figures 3 to 6 As shown in the figure, the judgment result must be made using the data after t1s of grounding, so the full-cycle differential filtering formula cannot be directly applied. g (n) = i(n)-i(nK) for operation. In order to achieve the same effect of full-cycle differential filtering, the alternative processing method is as follows: after a single-phase grounding occurs, store the current data i- of the cycle before the fault. Then extract the current data i+ corresponding to one or more cycles after t1s after the fault and i-. Then perform differential operation on i- and i+ to obtain the required fault signal current i g .

Claims

1. A method for realizing low current single-phase grounding line selection, characterized in that: The steps include: Step 1, obtaining the three-phase line current after the fault; Step 2, performing differential filtering on the three-phase line current after the fault occurs, or performing differential filtering on the sum of the three-phase line current after the fault occurs; Step 3, screening out the zero-sequence fault signal current through differential filtering; Step 4, selecting the fault line using the zero-sequence fault signal current; The zero-sequence fault signal current is obtained by differential filtering using the method of filtering first and then filtering sequence; The first filter and then filter sequence method adopts the first action criterion or the second action criterion; wherein the first action criterion is suitable for lines with relatively small capacitance, and the calculation formula of the first action criterion is as follows: in: is the capacitance of line J relative to ground; is the phase voltage of the system; is the reliability coefficient, ≥1.2; J is the line number; The first action criterion is formed by compensating the zero-sequence capacitive current of the line. 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 capacitive current of the line, and the direction is from the line to the busbar. When single-phase grounding occurs, the criterion zero-sequence current I p =0; when the neutral point of the grounded circuit is not grounded, the zero-sequence current I p Equal to the sum of the capacitive currents of the entire system; when the neutral point is grounded through the arc suppression coil, the zero-sequence current I p Equal to the residual current at the grounding point; The second action criterion includes an absolute value judgment method or a relative judgment method, wherein the calculation method of the absolute value judgment method is as follows: in, is the capacitance of line J relative to ground; It is a fixed value, determined by the maximum unbalanced current; is the 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: I pk =max[I pJ ] J=I、II... Where, J is the line number; I pk When single-phase grounding, I pJ The significant largest one among them, k is the ground line.

2. A method for realizing low current single-phase grounding line selection according to claim 1, characterized in that: In step 1, the three-phase line currents obtained are i A 、i B and i C .

3. A method for realizing low current single-phase grounding line selection according to claim 1, characterized in that: The three-phase line currents are arranged symmetrically.

4. A method for realizing low current single-phase grounding line selection according to claim 1, characterized in that: The three-phase fault signal currents obtained by differential filtering of the three-phase line current are i gA 、i gB and i gC , the zero-sequence signal current i is obtained through the three-phase fault signal current go , the calculation formula is as follows: i go =1 / 3[i gA +i gB +i gC ]。

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