Self-adaptive Positive-sequence Current Quick-break Protection Method for Petal-shaped Power Distribution Network Trunk Line

Abstract

The invention relates to a self-adaptive positive-sequence current quick-break protection method for a petal-shaped power distribution network trunk line. The method comprises the following steps: step 1, calculating a positive-sequence voltage phasor and a positive-sequence current amplitude at a protection installation position when a fault occurs, acquiring and storing a positive sequence impedance value of a protected line; judging a fault type, and judging a fault direction; step 2, when a fault direction element judges that a fault occurs in the forward direction, selecting a self-adaptive current quick-break protection setting formula according to the fault type, and when positive sequence current measured by protection is larger than a protection setting value, judging that the protected line has a short-circuit fault, and making a circuit breaker trip quickly. Compared with the prior art, the method provided by the invention has enough sensitivity and does not change along with the change of the line length and the system operation mode.

Claims

1. A self-adaptive positive-sequence current quick-break protection method for a petal-shaped power distribution network trunk line, comprising the following steps: step 1, calculating a positive-sequence voltage phasor {dot over (U)}.sub.1 and a positive-sequence current amplitude I.sub.1 of a protection installation position when a fault occurs based on the obtained voltage power frequency quantity {dot over (U)}.sub.apre, {dot over (U)}.sub.bpre, {dot over (U)}.sub.cpre of each phase during normal operation, voltage power frequency quantity {dot over (U)}.sub.a, {dot over (U)}.sub.b, {dot over (U)}.sub.c of each phase and current power frequency quantity İ.sub.a, İ.sub.b, İ.sub.c of each phase when a fault occurs; obtaining and storing the positive sequence impedance value Z.sub.L1 of the protected line; and utilizing the fault component of each phase current at the protection installation position to determine the fault type; at the same time, utilizing the power directional element adopting a 90° wiring mode to determine the fault direction based on the obtained fault type; step 2, electing a self-adaptive positive sequence quick-break current protection setting formula according to the fault type when the fault directional element is judged to have a fault in the forward direction, and judging that the protected line has short-circuit fault and making a circuit breaker trip quickly when the protection detects that the positive sequence current is greater than the protection setting value.

2. The self-adaptive positive-sequence current quick-break protection method for the petal-shaped power distribution network trunk line according to claim 1, the self-adaptive positive sequence quick-break current protection setting formula is as follows: $I_{ZDZ}=.Math.\frac{K_{\mathrm{rel}}{\stackrel{.}{U}}_P}{Z_{L1}}.Math.$ wherein, {dot over (U)}.sub.P={dot over (U)}.sub.1 when a three-phase short-circuit fault occurs, {dot over (U)}.sub.P={dot over (U)}.sub.1−({dot over (U)}.sub.xpre/2) when a two-phase phase-to-phase short-circuit fault occurs, {dot over (U)}.sub.xpre is voltage power frequency quantity of the non-faulty phase X during normal operation, and K.sub.rel is a reliability coefficient.

3. The self-adaptive positive-sequence current quick-break protection method for the petal-shaped power distribution network trunk line according to claim 1, wherein the voltage phase {dot over (U)}.sub.xpre during normal operation can be voltage measuring power frequency quantity 40 ms before a fault occurs.

4. The self-adaptive positive-sequence current quick-break protection method for the petal-shaped power distribution network trunk line according to claim 1, wherein when (m|İ.sub.mgC|≤|İ.sub.mgA|)∩(m|İ.sub.mgC|≤|İ.sub.mgB|), the fault type is AB two-phase short-circuit fault; when (m|İ.sub.mgA|≤|İ.sub.mgB|)∩(m|İ.sub.mgA|≤|İ.sub.mgC|), the fault type is BC two-phase short-circuit fault; when (m|İ.sub.mgB|≤|İ.sub.mgC|)∩(m|İ.sub.mgB|≤|İ.sub.mgA|), the fault type is CA two-phase short-circuit fault; when the above conditions are not met, the fault type is ABC three-phase short-circuit fault; wherein, İ.sub.mgA, İ.sub.mgB, İ.sub.mgC are separately the fault components of A-phase, B-phase and C-phase at the protection installation position respectively, and m is the setting coefficient.

5. The self-adaptive positive-sequence current quick-break protection method for the petal-shaped power distribution network trunk line according to claim 4, wherein the value range of the setting coefficient m is 4˜8.

6. The self-adaptive positive-sequence current quick-break protection method for the petal-shaped power distribution network trunk line according to claim 1, wherein the criterion of the power direction is as follows: $-180^{\circ }+\phi \le \arg \frac{{\stackrel{.}{U}}_{\mathrm{\varphi \varphi }}}{{\stackrel{.}{I}}_{\varphi }}\le \phi$ where, {dot over (U)}.sub.ϕϕ and İ.sub.ϕ are separately voltage and current phasors of the protection installation position respectively, Φ refers to A, B, C; and φ is the line impedance angle.

7. The self-adaptive positive-sequence current quick-break protection method for the petal-shaped power distribution network trunk line according to claim 2, wherein the reliability coefficient K.sub.rel is 1.2.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0027] FIG. 1 is a schematic diagram of a 10 kV petal-shaped power distribution network.

[0028] FIG. 2 is a two-phase phase-to-phase short-circuit fault composite sequence network diagram.

[0029] FIG. 3 is an equivalent circuit when three-phase short-circuit fault occurs

[0030] Explanation of labels in the figures is as follows:

[0031] In FIG. 1, E.sub.S is the equivalent phase potential of the system, Z.sub.S is the equivalent internal impedance of the system, A, B, C, D, and E represent the bus, f represents the fault point, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 are the serial numbers of the protection, LD1, LD2, LD3, LD4 represent the load, and I′.sub.f and I″.sub.f are the fault currents on both sides of the fault point.

[0032] In FIG. 2, U.sub.B1 represents the positive sequence voltage at the B bus, U.sub.f1 represents the positive sequence voltage at the fault point, I′.sub.f1 and I″.sub.f1 are the positive sequence currents on both sides of the fault point; Z.sub.m=Z.sub.AB+αZ.sub.BC, Z.sub.n=(1−α)Z.sub.BC+Z.sub.CA.

[0033] In FIG. 3, Z.sub.AB is the impedance of line AB, Z.sub.BC is the impedance of line BC, Z.sub.CA is the impedance of line CDA, α is the ratio of the distance between the fault point and bus bar B to the total length of the BC line, U.sub.A, U.sub.B, and U.sub.C are phase voltages on busbars A, B and C, and I.sub.f is the current of the fault point.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] The invention will be further described in detail below through specific examples. The following examples are only descriptive and not restrictive, and the protection scope of the invention cannot be limited by this.

[0035] A self-adaptive positive-sequence current quick-break protection method for a petal-shaped power distribution network trunk line utilizes the relationship between the positive sequence current and the positive sequence voltage at the line protection installation position to derive the positive sequence current quick-break setting value, and compares the magnitude of the positive sequence current with the setting value to realize the judgment of short-circuit fault, the specific steps are as follows:

[0036] (1) As shown in FIG. 1, it is a schematic diagram of a 10 kV petal-shaped power distribution network specifically applied in this example. A voltage acquisition device at the protection position acquires the voltage power frequency quantity {dot over (U)}.sub.apre, {dot over (U)}.sub.bpre, {dot over (U)}.sub.cpre of each phase during normal operation and the voltage power frequency quantity {dot over (U)}.sub.a, {dot over (U)}.sub.b, {dot over (U)}.sub.c of each phase when a fault occurs, and a current acquisition device acquires current power frequency quantity İ.sub.a, İ.sub.b, I.sub.c of each phase. A data processing device calculates the positive sequence voltage phasor {dot over (U)}.sub.1 and the positive sequence current amplitude I.sub.1 at the protection installation position while a fault occurs.

[0037] The positive sequence impedance value Z.sub.L1 of the protected line is stored in the protection device in advance.

[0038] (2) The fault components of each phase current at the protection installation position are used to distinguish the three-phase short-circuit fault from the two-phase short-circuit fault. The power directional element adopting the 90° wiring mode is utilized to judge the direction of the fault.

[0039] (3) The self-adaptive positive sequence quick-break current protection setting formula is selected according to the fault type when the fault directional element is judged to have a fault in the forward direction, and the protected line is judged to have a short-circuit fault and a circuit breaker is made to trip quickly when the protection detects that the positive sequence current is greater than the protection setting value. The self-adaptive positive sequence quick-break current protection setting formula is as follows:

[00003]$I_{ZDZ}=.Math.\frac{K_{rel}{\stackrel{.}{U}}_P}{Z_{L1}}.Math.$

[0040] where, {dot over (U)}.sub.P={dot over (U)}.sub.1 when a three-phase short-circuit fault occurs, {dot over (U)}.sub.P={dot over (U)}.sub.1−({dot over (U)}.sub.xpre/2) when a two-phase phase-to-phase short-circuit fault occurs, {dot over (U)}.sub.xpre is voltage power frequency quantity of the non-faulty phase X during normal operation, and K.sub.rel is a reliability coefficient.

[0041] In the step (1), the voltage phase {dot over (U)}.sub.pre during normal operation can be voltage measuring power frequency quantity 40 ms before a fault occurs.

[0042] In step (2), the fault type data are as shown in table 1 after zero-sequence current is determined.

TABLE-US-00001 TABLE 1 Identification of fault type Fault types Criterions AB two-phase short circuit (m | İ.sub.mgC |≤| İ.sub.mgA |)∩(m | İ.sub.mgC |≤| İ.sub.mgB |) BC two-phase short circuit (m | İ.sub.mgA |≤| İ.sub.mgB |)∩(m | İ.sub.mgA |≤| İ.sub.mgC |) CA two-phase short circuit (m | İ.sub.mgB |≤| İ.sub.mgC |)∩(m | İ.sub.mgB |≤| İ.sub.mgA |) ABC two-phase short circuit The above conditions are not met.

[0043] In the table, İ.sub.mgA, İ.sub.mgB, İ.sub.mgC are the fault components of A-phase, B-phase and C-phase at the protection installation position respectively, and m is the setting coefficient with a value of 4˜8.

[0044] Power direction criterion is as follows:

[00004]$-180°+\phi \le \arg \frac{{\stackrel{.}{U}}_{\mathrm{\varphi \varphi }}}{{\stackrel{.}{I}}_{\varphi }}\le \phi$

[0045] where, {dot over (U)}.sub.ϕϕ and İ.sub.ϕ are separately voltage and current phasors (Φ refers to A, B, C) at the protection installation position respectively, and φ is the impedance angle of the line.

[0046] In step (3), the reliability coefficient K.sub.rel is 1.2.

[0047] The self-adaptive positive-sequence current quick-break protection based on the relationship between the positive sequence voltage and the positive sequence current at the protection installation position can identify the short-circuit fault that occurs on the protected line. The principle is as follows:

[0048] when a two-phase short-circuit fault occurs on the protected line, the fault composite sequence network diagram is as shown in FIG. 2. From FIG. 2, it can be seen that the positive sequence voltage and the positive sequence current at the protection 3 have the following relationship:

[00005]$\begin{array}{cc}{\stackrel{.}{I}}_{f1}=\frac{{\stackrel{.}{U}}_{B1}-\frac{{\stackrel{.}{E}}_s}{2}}{\alpha Z_{BC}}& \left(1\right)\end{array}$

[0049] In the actual power distribution network, the voltage drop on the impedance in the system and on the line impedance is ignored, Ė.sub.S can be replaced with voltage {dot over (U)}.sub.pre during normal operation of the non-faulty phase at the protection installation position.

[0050] In the case of a three-phase short-circuit fault, the fault equivalent circuit is as shown in FIG. 3. From FIG. 3, it can be seen that the positive sequence voltage and the positive sequence current at the protection 3 have the following relationship:

[00006]$\begin{array}{cc}{\stackrel{.}{I}}_{f1}=\frac{{\stackrel{.}{U}}_{B1}}{\alpha Z_{BC}}& \left(2\right)\end{array}$

[0051] formula (1) and formula (2) can be combined to get the self-adaptive positive sequence current quick-break protection setting value:

[00007]$\begin{array}{cc}{I}_{ZDZ}=.Math.\frac{K_{\mathrm{rel}}{\stackrel{.}{U}}_P}{Z_{L1}}.Math.& \left(3\right)\end{array}$

[0052] where, {dot over (U)}.sub.P={dot over (U)}.sub.1 when a three-phase short-circuit fault occurs, {dot over (U)}.sub.P={dot over (U)}.sub.1−({dot over (U)}.sub.xpre/2) when a two-phase phase-to-phase short-circuit fault occurs, {dot over (U)}.sub.xpre is voltage power frequency quantity of the non-faulty phase X during normal operation, and K.sub.rel is a reliability coefficient.

[0053] From formula (3), the protection range η of quick-break protection can be obtained:

[00008]$\begin{array}{cc}\eta =\frac{1}{K_{\mathrm{rel}}}& \left(4\right)\end{array}$

[0054] The protection range η is only related to the reliability coefficient K.sub.rel, and has nothing to do with the line length and the operation mode of the system. When the reliability coefficient K.sub.rel is 1.2, the protection range of quick-break current protection is η=83.3%.

[0055] The power direction criterion can be combined to obtain the operation criterion of the self-adaptive positive sequence current quick-break protection:

[00009]$\begin{array}{cc}\{\begin{array}{c}{I}_1\ge I_{ZDZ}\\ -180\mathrm{¦°}+\phi \le \arg \frac{U_{\mathrm{\varphi \varphi }}}{I_{\varphi }}\le \phi \end{array}& \left(5\right)\end{array}$

[0056] It can be seen from formula (4) that when a fault occurs within the protection range of the protected line in the forward direction, the positive sequence fault current is greater than the setting value. Therefore, the self-adaptive positive sequence current quick-break protection can quickly and accurately identify short-circuit faults within the protection range.

[0057] In this example, PSCAD/EMTDC software is used in the 10 kV petal-shaped power distribution network system as shown in FIG. 1. The reference capacity of the system is 100 MVA, the reference voltage is 10.5 kV, and the internal impedance is 0.23Ω. The unit inductance and resistance of the cable line are X1=0.063 Ω/km and R1=0.047 Ω/km; the lengths of lines AB, BC, CD, DE, and EA are 2 km, 2 km, 1 km, 1 km, 2 km respectively. A load with a rated capacity of 2 MVA and a rated power factor of 0.9 is accessed to each node.

[0058] 1) Fault Identification of Self-Adaptive Positive Sequence Current Quick-Break Protection

[0059] In order to verify the influence of different fault types and fault positions on the self-adaptive positive sequence current quick-break protection, the phase-to-phase short-circuit fault and the three-phase short-circuit fault occurred at the BC line α=0.1, 0.4, 0.6, 0.9 are simulated, and the Operation condition of the protection 3 is as shown in Table 1.

TABLE-US-00002 TABLE 1 Operation condition of BC line self-adaptive positive sequence current quick-break protection Three-phase Two-phase phase-to-phase short-circuit fault short-circuit fault I.sub.ZDZ/ I.sub.l/ Operation I.sub.ZDZ/ I.sub.l/ Operation α Protection kA kA condition kA kA condition 0.1 3 1.511 12.480 Operate 0.759 6.308 Operate 0.4 3 5.124 10.649 Operate 2.565 5.393 Operate 0.6 3 6.941 9.623 Operate 3.473 4.881 Operate 0.9 3 8.965 8.290 Nooperate 4.458 4.214 No operate

[0060] It can be seen from Table 1:

[0061] Within α≤0.833 and I.sub.d1>I.sub.ZDZ, protection 3 will all operate; within α>0.833, protection 3 will not operate, and the quick-break protection at both ends of the line has a definite protection range, which can ensure sufficient sensitivity.

[0062] 2) Influence of Line Length on Protection Range

[0063] In order to verify the influence of different line lengths on the range of self-adaptive positive-sequence current quick-break protection, the simulation results of CD line protection are shown in Table 2. In Table 2, β is the ratio of the distance between the fault point and bus C to the total length of the CD line.

TABLE-US-00003 TABLE 2 Operation condition of line protection 5 Three-phase Two-phase phase-to-phase short-circuit fault short-circuit fault I.sub.ZDZ/ I.sub.l/ Operation I.sub.ZDZ/ I.sub.l/ Operation β kA kA condition kA kA condition 0.1 0.948 7.689 Operate 0.48 3.834 Operate 0.4 3.443 7.12 Operate 1.727 3.55 Operate 0.6 4.89 6.757 Operate 2.451 3.368 Operate 0.9 6.753 6.229 No Operate 3.382 3.104 No Operate

[0064] It can be seen by combining table 1 with table 2:

[0065] The line length does not affect the protection range of the self-adaptive positive sequence current quick-break protection. Even in a short line, the protection still has a certain protection range, which can ensure the sensitivity of the self-adaptive positive sequence current quick-break protection.

[0066] Although the examples and figures of the invention are disclosed for illustrative purposes, those skilled in the art can understand that various substitutions, changes and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the scope of the invention is not limited to the content disclosed in the examples and figures.