Method of free wave energy protection for half-wavelength line based on one sided current

Abstract

A method of free wave energy protection for a half-wavelength line based on a one-sided current includes: performing sampling and calculation on a current at protection measuring points to obtain variations in current sampling values; and activating a protective element to determine an occurrence time of a fault in a half-wavelength line. A free wave energy protection section includes a quick-action section, a basic section and a sensitive section, wherein the quick-action section, the basic section and the sensitive section serve as action criteria for performing protection with respect to the free wave energy of the half-wavelength line.

Claims

1. A single-side current-based free wave energy protection method for a half wavelength line, wherein a protection measuring point and a starting element are arranged on the half wavelength line, three protection sections are set for free wave energy, and the protection sections comprise a quick action section, a basic section and a sensitive section, the method comprising the following steps: Step 1: sampling and calculating, by the protection measuring point, currents to obtain variations of current sampling values; Step 2: causing the starting element to act to determine occurrence time of a fault of the half wavelength line; Step 3: calculating the variations of the current sampling values within the quick action section to obtain the free wave energy, determining whether the quick action section acts, if YES, causing the quick action section to act for protection, and if NO, proceeding to Step 4; Step 4: calculating the current sampling values within the basic section to obtain the free wave energy, determining whether the basic section acts, if YES, causing the basic section to act for protection, and if NO, proceeding to Step 5; and Step 5: determining whether the sensitive section acts according to a position of the fault and a permission command, if YES, causing the sensitive section to act for protection, and if NO, returning to Step 1.

2. The method according to claim 1, wherein Step 1 comprises: Step 1-1: sampling, by the protection measuring point, current values of a cycle before occurrence of the fault and present current values respectively to obtain a before-fault sampling value and present sampling value of each phase current, wherein a sampling rate is 48 sampling points per cycle; and Step 1-2: calculating the variations of the current sampling values according to the before-fault sampling values and present sampling values of the currents.

3. The method according to claim 1, wherein Step 2 comprises: causing the starting action to act to determine the occurrence time t of the fault of the half wavelength line: $\begin{array}{cc}\{\begin{array}{c}f\left(t\right)=i_A^2\left(t\right)+i_B^2\left(t\right)+i_C^2\left(t\right)\\ .Math.df\left(t\right).Math.>f_{\mathrm{set}}\end{array},& \left(1\right)\end{array}$ in the formula (1), .sub.set being an action fixed value and .sub.set=0.1KA.sup.2, i.sub.A(t), i.sub.B(t) and i.sub.C(t) being sudden changes of the currents of the three phases A, B and C respectively and (t) being a variation of a quadratic sum of the sudden changes of the currents of the three phases.

4. The method according to claim 1, wherein Step 3 comprises: Step 3-1: for sampling points 2 to 22 at which the starting element acts, calculating zero-sequence and negative-sequence variation phases of the variations i.sub.A(t), i.sub.B(t), i.sub.C(t) of the currents of the three phases on a point-by-point basis by using a half wave Fourier sequence filtering window, and calculating values of .sub.0+.sub.A2, .sub.0+.sub.B2 and .sub.0+.sub.C2, where .sub.0 is the variation phase value of the zero-sequence current, and .sub.A2, .sub.B2 and .sub.c2 are variation phase values of negative-sequence currents of the phases A, B and C respectively; Step 3-2: restoring the zero-sequence and negative-sequence phase sums of the phases A, B and C into sampling values i.sub.A0 (t), i.sub.B0 (t) and i.sub.C0 (t); Step 3-3: filtering zero and negative sequences in the variations of the sampling values to obtain current variation sampling values i.sub.A(t), i.sub.B(t) and i.sub.C(t) subjected to filtering of the zero sequences and the negative sequences:
i.sub.A(t)=i.sub.A(t)i.sub.A02(t)
i.sub.B(t)=i.sub.B(t)i.sub.B02(t)
i.sub.C(t)=i.sub.C(t)i.sub.C02(t)(2), in the formula (2), i.sub.A(t), i.sub.B(t) and i.sub.C(t) being the variations, caused by the fault, of the current sampling values of the currents of the phases A, B and C at a moment t; Step 3-4: constructing a quadratic sum function (t):
(t)=i.sub.A.sup.2(t)+i.sub.B.sup.2(t)+i.sub.C.sup.2(t)(3), differentiating the quadratic sum function, then calculating an absolute value to obtain |df(t)|, and integrating |df(t)| to obtain a free wave energy value; Step 3-5: determining whether the following formula (4) about the free wave energy is true: $\begin{array}{cc}\underset{iI}{.Math.}.Math.\mathrm{df}\left(t\right).Math.>250+2.5\underset{iI}{\max }.Math.\mathrm{df}\left(t\right).Math.I=\left\{-2,\mathrm{.Math.},21\right\},& \left(4\right)\end{array}$ in the formula (4), i being a sampling point and I being an integral interval, if the formula (4) is false, proceeding to Step 4, and if the formula (4) is true, proceeding to 3-6; and Step 3-6: after the sampling point 22, performing sequence filtering using a full wave Fourier data window, and converting the formula (4) into the following formula (5), determining whether the formula (5) is true: $\begin{array}{cc}\underset{iI}{.Math.}.Math.\mathrm{df}\left(t\right).Math.>150+k\underset{iI}{\max }.Math.\mathrm{df}\left(t\right).Math.I=\left\{-2,\mathrm{.Math.},45\right\},& \left(5\right)\end{array}$ in the formula (5), the value of k is reduced by a step length of 0.05 for totally 48 sampling points which are from point 2 to point 45 on a point-by-point basis and the initial value of k is set to 5, if the formula (5) is false, proceeding to Step 4, and if the formula (5) is true, causing the quick action section to act for protection.

5. The method according to claim 4, wherein Step 4 comprises: Step 4-1: calculating the zero-sequence and negative-sequence variation phases of the variations i.sub.A(t), i.sub.B(t) and i.sub.C(t) of the currents of the three phases by using the full wave Fourier sequence filtering window, and calculating the values of .sub.0+.sub.A2, .sub.0+.sub.B2 and .sub.0+.sub.C2, where .sub.A[0], .sub.B[0]and .sub.C[0]are the after-fault phase values of the currents of the phases A, B and C respectively, and .sub.A2, .sub.B2 and .sub.C2 are the variation phase values of the negative-sequence currents of the phases A, B and C respectively; Step 4-2: restoring the zero-sequence and negative-sequence phase sums of the phases A, B and C into the sampling values i.sub.A0 (t), i.sub.B0 (t) and i.sub.C0 (t); Step 4-3: filtering the zero and negative sequences in the variations of the sampling values, and obtaining, according to the formula (2), the current variation sampling values i.sub.A(t), i.sub.B(t) and i.sub.C(t) subjected to filtering of the zero sequences and the negative sequences; Step 4-4: constructing the quadratic sum function (t) according to the formula (3), differentiating the quadratic sum function, then calculating the absolute value to obtain |df(t)|, and integrating |df(t)| to obtain the free wave energy value; and Step 4-5: determining whether the following formula (6) about the free wave energy is true: $\begin{array}{cc}\underset{iI}{.Math.}.Math.\mathrm{df}\left(t\right).Math.>30+0.3\underset{iJ}{.Math.}.Math.\mathrm{df}\left(t\right).Math.+2\underset{iI}{\max }.Math.\mathrm{df}\left(t\right).Math.,& \left(6\right)\end{array}$ in the formula (6), I={4, . . . , 45} J={46, . . . , 57}, if the formula (6) is false, proceeding to Step 4, and if the formula (6) is true, causing the basic section to act for protection.

6. The method according to claim 5, wherein Step 5 comprises: determining whether a criterion formula (7) for the sensitive section is true according to the position of the fault and the permission command: $\begin{array}{cc}\underset{iI}{.Math.}.Math.\mathrm{df}\left(t\right).Math.>8+2\underset{iI}{\max }.Math.\mathrm{df}\left(t\right).Math.I=\left\{L,\mathrm{.Math.},H\right\},& \left(7\right)\end{array}$ in the formula (7), L being a lower limit of the integral interval and H being an upper limit of the integral interval; if the permission command is received and an estimation result of the position of the fault is more than 2,000 km, causing the sensitive section to act for protection, and determining that the integral interval I comprises the sampling points 2 to 45; if the permission command is received and the estimation result of the position of the fault is within a range of {(15001), . . . ,(1500+1)}1500 km, causing the sensitive section to act for protection, and determining that the integral interval I comprises the sampling points 2 to 45; if the permission command is received and the estimation result of the position of the fault is within a range {, . . . , (15001)}, 1500 km, 100 km, causing the sensitive section to act for protection, and determining that the integral interval I is L to H, wherein $L=\frac{24}{1000-a}1-\frac{12000}{1000-a}-2\mathrm{and}H=48-L;$ and when the permission command is not received or the estimation result of the position of the fault is within a range of {0, . . . , }, 100 km, performing latching protection, and returning to Step 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a flowchart of a free wave energy-based half wavelength line protection method according to the disclosure.

(2) FIG. 2 is a schematic diagram of an analogue simulation system for a half wavelength line according to a specific application example of the disclosure.

(3) FIG. 3 is a schematic diagram of calculation points of a free wave energy protection section I according to a specific application example of the disclosure.

(4) FIG. 4 is a schematic diagram of calculation points of a free wave energy protection section II according to a specific application example of the disclosure.

(5) FIG. 5 is a schematic diagram of an integral interval of a free wave energy protection section III according to a specific application example of the disclosure.

(6) FIG. 6 is a logic diagram of free wave energy protection formed by a free wave energy protection section I, section II and section III according to a specific application example of the disclosure.

(7) FIG. 7 is a schematic diagram of a free wave in case of a three-phase short-circuit fault at a point F1 at an outlet of a half wavelength line according to a specific application example of the disclosure.

(8) FIG. 8 is a schematic diagram of a free wave in case of a three-phase short-circuit fault at a point F8 at a reverse outlet of a half wavelength line according to a specific application example of the disclosure.

(9) FIG. 9 is a schematic diagram of a free wave in case of a three-phase short-circuit fault at a point F8 at a reverse outlet of a half wavelength line according to a specific application example of the disclosure.

DETAILED DESCRIPTION

(10) The technical solutions in the embodiments of the disclosure will be clearly and completely described below in combination with the drawings in the embodiments of the disclosure. Obviously, the described embodiments are not all embodiments but only part of embodiments of the disclosure. All other embodiments obtained by those skilled in the art on the basis of the embodiments of the disclosure without creative work fall within the scope of protection of the disclosure.

(11) As shown in FIG. 1, an embodiment of the disclosure provides a single-side current-based free wave energy protection method for a half wavelength line, a protection measuring point and a starting element being arranged on the half wavelength line, protection sections with integral intervals being set for free wave energy and the protection sections including a quick action section, a basic section and a sensitive section, herein the starting element is a composition content of a starting algorithm of a program. The method includes the following steps.

(12) In Step 1, the protection measuring point samples and calculates currents to obtain variations of current sampling values.

(13) In Step 2, the starting element acts to determine occurrence time of a fault of the half wavelength line.

(14) In Step 3, the variations of the current sampling values are calculated within the quick action section to obtain the free wave energy, it is determined whether the quick action section acts, if YES, the quick action section acts for protection, and if NO, proceed to Step 4.

(15) In Step 4, the current sampling values are calculated within the basic section to obtain the free wave energy, it is determined whether the basic section acts, if YES, the basic section acts for protection, and if NO, proceed to Step 5.

(16) In Step 5, it is determined whether the sensitive section acts according to a position of the fault and a permission command, if YES, the sensitive section acts for protection, and if NO, return to Step 1.

(17) In another embodiment of the disclosure, Step 1 includes the following steps.

(18) In Step 1-1, the protection measuring point samples current values of a cycle before occurrence of the fault and present current values respectively to obtain a before-fault sampling value and present sampling value of each phase current, herein a sampling rate is 48 sampling points per cycle.

(19) In Step 1-2, the variations of the current sampling values are calculated according to the before-fault sampling values and present sampling values of the currents.

(20) In another embodiment of the disclosure, Step 2 includes that:

(21) the starting action acts to determine the occurrence time t of the fault of the half wavelength line according to a formula (1):

(22) $\begin{array}{cc}\{\begin{array}{c}f\left(t\right)=i_A^2\left(t\right)+i_B^2\left(t\right)+i_C^2\left(t\right)\\ .Math.df\left(t\right).Math.>f_{\mathrm{set}}\end{array},& \left(1\right)\end{array}$

(23) in the formula (1), .sub.set being an action fixed value and .sub.set=0.1 KA.sup.2, i.sub.A(t), i.sub.B(t) and i.sub.C(t) being sudden changes of the currents of the three phases A, B and C respectively and (t) being a variation of a quadratic sum of the sudden changes of the currents of the three phases.

(24) In another embodiment of the disclosure, Step 3 includes the following steps.

(25) In Step 3-1, for sampling points 2 to 22 at which the starting element acts, zero-sequence and negative-sequence variation phases of the variations i.sub.A(t), i.sub.B(t) and i.sub.C(t) of the currents of the three phases are calculated on a point-by-point basis by using a half wave Fourier sequence filtering window, and

(26) values of .sub.0+.sub.A2, .sub.0+.sub.B2 and .sub.0+.sub.C2 are calculated, where .sub.0 is the variation phase value of the zero-sequence current, and .sub.A2, .sub.B2 and .sub.C2 are variation phase values of negative-sequence currents of the phases A, B and C respectively.

(27) In Step 3-2, the zero-sequence and negative-sequence phase sums of the phases A, B and C are restored into sampling values i.sub.A02(t), i.sub.B02(t) and i.sub.C02(t).

(28) In Step 3-3, zero and negative sequences in the variations of the sampling values are filtered to obtain current variation sampling values i.sub.A(t), i.sub.B(t) and i.sub.C(t) subjected to filtering of the zero sequences and the negative sequences:
i.sub.A(t)=i.sub.A(t)i.sub.A02(t)
i.sub.B(t)=i.sub.B(t)i.sub.B02(t)
i.sub.C(t)=i.sub.C(t)i.sub.C02(t)(2),

(29) in the formula (2), i.sub.A(t), i.sub.B(t) and i.sub.C(t) being the variations, caused by the fault, of the current sampling values of the currents of the phases A, B and C at a moment t.

(30) In Step 3-4, a quadratic sum function (t) is constructed:
(t)=i.sub.A.sup.2(t)+i.sub.B.sup.2(t)+i.sub.C.sup.2(t)(3),

(31) the quadratic sum function is differentiated, then an absolute value is calculated to obtain |df(t)|, and |df(t)| is integrated to obtain a free wave energy value.

(32) In Step 3-5, it is determined whether the following formula (4) about the free wave energy is true:

(33) $\begin{array}{cc}\underset{iI}{.Math.}.Math.\mathrm{df}\left(t\right).Math.>250+2.5\underset{iI}{\max }.Math.\mathrm{df}\left(t\right).Math.I=\left\{-2,\mathrm{.Math.},21\right\},& \left(4\right)\end{array}$

(34) in the formula (4), i being a sampling point and I being an integral interval,

(35) if the formula (4) is false, proceed to Step 4, and if the formula (4) is true, proceed to Step 3-6.

(36) In Step 3-6, after the sampling point 22, sequence filtering is performed using a full wave Fourier data window, and the formula (4) is converted into the following formula (5), it is determined whether the formula (5) is true:

(37) $\begin{array}{cc}\underset{iI}{.Math.}.Math.\mathrm{df}\left(t\right).Math.>150+k\underset{iI}{\max }.Math.\mathrm{df}\left(t\right).Math.I=\left\{-2,\mathrm{.Math.},45\right\},& \left(5\right)\end{array}$

(38) in the formula (5), the value of k is reduced by a step length of 0.05 for the totally 48 sampling points which are from point 2 to point 45 on a point-by-point basis and the initial value of k is set to 5.

(39) If the formula (5) is false, proceed to Step 4, and if the formula (5) is true, the quick action section (the section I) acts for protection.

(40) In another embodiment of the disclosure, Step 4 includes the following steps.

(41) In Step 4-1, the zero-sequence and negative-sequence variation phases of the variations i.sub.A(t), i.sub.B(t) and i.sub.C(t) of the currents of the three phases are calculated by using the full wave Fourier sequence filtering window, and the values of .sub.0+.sub.A2, .sub.0+.sub.B2 and .sub.0+.sub.C2 are calculated, where .sub.A[0], .sub.B[0] and .sub.C[0] are the after-fault phase values of the currents of the phases A, B and C respectively, and .sub.A2, .sub.B2 and .sub.C2 are the variation phase values of the negative-sequence currents of the phases A, B and C respectively.

(42) In Step 4-2, the zero-sequence and negative-sequence phase sums of the phases A, B and C are restored into the sampling values i.sub.A02(t), i.sub.B02(t) and i.sub.C02(t).

(43) In Step 4-3, the zero and negative sequences in the variations of the sampling values are filtered, and the current variation sampling values i.sub.A(t), i.sub.B(t) and i.sub.C(t) subjected to filtering of the zero sequences and the negative sequences are obtained according to the formula (2).

(44) In Step 4-4, the quadratic sum function (t) is constructed according to the formula (3), the quadratic sum function is differentiated, then the absolute value is calculated to obtain |df(t)|, and |df(t)| is integrated to obtain the free wave energy value.

(45) In Step 4-5, it is determined whether the following formula (6) about the free wave energy is true:

(46) 0$\begin{array}{cc}\underset{iI}{.Math.}.Math.\mathrm{df}\left(t\right).Math.>30+0.3\underset{iJ}{.Math.}.Math.\mathrm{df}\left(t\right).Math.+2\underset{iI}{\max }.Math.\mathrm{df}\left(t\right).Math.,& \left(6\right)\end{array}$

(47) in the formula (6), I={4, . . . , 45} J={46, . . . , 57},

(48) if the formula (6) is false, proceed to Step 4, and if the formula (6) is true, the basic section (the section II) acts for protection.

(49) herein, Step 5 includes that:

(50) it is determined whether a criterion formula (7) for the sensitive section (the section III) is true according to the position of the fault and the permission command:

(51) $\begin{array}{cc}\underset{iI}{.Math.}.Math.\mathrm{df}\left(t\right).Math.>8+2\underset{iI}{\max }.Math.\mathrm{df}\left(t\right).Math.I=\left\{L,\mathrm{.Math.},H\right\},& \left(7\right)\end{array}$

(52) in the formula (7), L being a lower limit of the integral interval and H being an upper limit of the integral interval;

(53) if the permission command is received and an estimation result of the position of the fault is more than 2,000 km, the sensitive section acts for protection, and it is determined that the integral interval I includes the sampling points 2 to 45;

(54) if the permission command is received and the estimation result of the position of the fault is within a range of {(15001), . . . , (1500+1)} 1500 km, the sensitive section acts for protection, and it is determined that the integral interval I includes the sampling points 2 to 45;

(55) if the permission command is received and the estimation result of the position of the fault is within a range {, . . . , (15001)}, 1500 km, 100 km, the sensitive section acts for protection, and it is determined that the integral interval I is L to H, where

(56) $L=\frac{24}{1000-a}1-\frac{12000}{1000-a}-2\mathrm{and}H=48-L;$
and

(57) when the permission command is not received or the estimation result of the position of the fault is within a range of {0, . . . , }, 100 km, latching protection is performed, and return to Step 1.

(58) An embodiment of the disclosure provides a specific application example of a free wave energy-based half wavelength line protection method, herein the free wave energy-based half wavelength line protection method is implemented in an analogue simulation system for a half wavelength line, and the analogue simulation system for the half wavelength line is shown in FIG. 2 as follows.

(59) (1) Occurrence Time of a Fault is Determined Using a Starting Element:

(60) $\begin{array}{cc}\{\begin{array}{c}f\left(t\right)=i_A^2\left(t\right)+i_B^2\left(t\right)+i_C^2\left(t\right)\\ .Math.df\left(t\right).Math.>f_{\mathrm{set}}\end{array},& \left(1\right)\end{array}$

(61) in the formula: .sub.set=0.1 kA.sup.2, and i.sub.A(t), i.sub.B(t) and i.sub.C(t) being sudden changes of currents of three phases A, B and C.

(62) Action time of the starting element is time when a protection element senses occurrence of the fault. In the embodiment of the disclosure, descriptions will be made with a sampling rate of 48 points per cycle as an example.

(63) (2) A Free Wave Energy Protection Section I

(64) The free wave energy protection section I is a quick section. Point-by-point sequence filtering and point-by-point integration are performed by using a half wave Fourier algorithm. Sequence filtering refers to filtering of zero-sequence and negative-sequence components, and integration refers to calculation of energy.

(65) {circle around (1)} Sequence Filtering

(66) For points 2 to 22 at which the starting element acts, zero and negative-sequence variation phase sums .sub.0+.sub.A2, .sub.0+.sub.B2 and .sub.0+.sub.C2 of variations i.sub.A(t), i.sub.B(t) and i.sub.C(t) of the currents of the three phases are calculated on a point-by-point basis by using a half wave Fourier sequence filtering window. The zero and negative-sequence phase sums are restored into sampling values i.sub.A02(t), i.sub.B02(t) and i.sub.C02(t) of the zero-sequence and negative-sequence sums. Then, the zero and negative-sequence components in variations of the sampling values are filtered using the following formula (2):
i.sub.A(t)=i.sub.A(t)i.sub.A02(t)
i.sub.B(t)=i.sub.B(t)i.sub.B02(t)
i.sub.C(t)=i.sub.C(t)i.sub.C02(t)(2).

(67) {circle around (2)} Integration

(68) A quadratic sum function is constructed on the basis of the formula (2) at first:
(t)=i.sub.A.sup.2(t)+i.sub.B.sup.2(t)+i.sub.C.sup.2(t)(3),

(69) the quadratic sum function is differentiated, then an absolute value is calculated to obtain |df(t)|, and then |df(t)| is integrated, thereby calculating free wave energy. Then, it is determined whether the free wave energy protection section I acts according to whether the following formula (4) and formula (5) are met:

(70) $\begin{array}{cc}\underset{iI}{.Math.}.Math.\mathrm{df}\left(t\right).Math.>250+2.5\underset{iI}{\max }.Math.\mathrm{df}\left(t\right).Math.I=\left\{-2,\mathrm{.Math.},21\right\}.& \left(4\right)\end{array}$

(71) After the sampling point 22, sequence filtering is performed using a full wave Fourier data window instead. Meanwhile, a fixed value is reduced. That is, the formula (4) is converted into the following formula:

(72) $\begin{array}{cc}\underset{iI}{.Math.}.Math.\mathrm{df}\left(t\right).Math.>150+k\underset{iI}{\max }.Math.\mathrm{df}\left(t\right).Math.I=\left\{-2,\mathrm{.Math.},45\right\},& \left(5\right)\end{array}$

(73) in the formula (5), the value of k is reduced by a step length of 0.05 for the totally 48 sampling points which are from 2 to point 45 on a point-by-point basis and the initial value of k is set to 5.

(74) Selection calculation for the sampling points in the above calculation is shown in FIG. 3.

(75) (3) A Free Wave Energy Protection Section II (Basic Section)

(76) {circle around (1)} Sequence Filtering

(77) In this protection section, after the starting element acts, the zero-sequence and negative-sequence variation phase sums .sub.0+.sub.A2, .sub.0 .sub.B2 and .sub.0+.sub.C2 of i.sub.A(t), i.sub.B(t) and i.sub.C(t) are calculated by using the full wave Fourier sequence filtering window. The zero and negative-sequence phase sums are restored into zero and negative-sequence sampling value sums i.sub.A02(t), i.sub.B02(t) and i.sub.C02(t). Then, sequence filtering over the variations of the sampling values is completed using the formula (2).

(78) {circle around (2)} Integration

(79) Similar to the free wave energy protection section I, the quadratic sum function (t)=i.sub.A.sup.2(t)+i.sub.B.sup.2(t)+i.sub.C.sup.2(t) is constructed for the variations, subjected to sequence filtering, of the sampling values, the quadratic sum function is differentiated, and then the absolute value is calculated to obtain |df(t)|. Then, |df(t)| is integrated to obtain the free wave energy. A formula (6) is an expression of a low-fixed value section criterion of the free wave energy protection section II:

(80) $\begin{array}{cc}\underset{iI}{.Math.}.Math.\mathrm{df}\left(t\right).Math.>30+0.3\underset{iJ}{.Math.}.Math.\mathrm{df}\left(t\right).Math.+2\underset{iI}{\max }.Math.\mathrm{df}\left(t\right).Math..& \left(6\right)\end{array}$

(81) In the above two formulae: I={4, . . . , 45} J={46, . . . , 57}.

(82) Selection calculation for the sampling points in the above calculation is shown in FIG. 4.

(83) (4) A Free Wave Energy Protection Section III

(84) The free wave energy protection section III changes the integral interval according to a distance measurement result and a permission command of an opposite side, thereby improving sensitivity of the free wave energy protection section III to ensure that protection may reliably act under the condition of determining that a fault in a region occurs. A criterion formula is expressed as follows:

(85) $\begin{array}{cc}\underset{iI}{.Math.}.Math.\mathrm{df}\left(t\right).Math.>8+2\underset{iI}{\max }.Math.\mathrm{df}\left(t\right).Math.I=\left\{L,\mathrm{.Math.},H\right\},& \left(7\right)\end{array}$

(86) in the formula, L being a lower limit of the integral interval, H being an upper limit of the integral interval, and its value being shown in FIG. 5.

(87) As shown in FIG. 5, when the permission command is received and an estimation result of a position of the fault is more than 2,000 km, the integral interval includes the sampling points 2 to 45; when the estimation result of the position of the fault is within a range of {(15001), . . . , (1500+1)} 1500 km, the integral interval includes the sampling points 2 to 45; when the estimation result of the position of the fault is within a range {, . . . , (15001)}, 1500 km, 100 km, the integral interval is L to H, where

(88) $L=\frac{24}{1000-a}1-\frac{12000}{1000-a}-2\mathrm{and}H=48-L;$
and when the estimation result of the position of the fault is within a range of {0, . . . , }, 100 km, latching protection is performed.

(89) (5) Free Wave Energy Protection

(90) The free wave energy protection section I, section II and section III form free wave energy protection together. A specific logic is shown in FIG. 6, herein a free wave in case of a three-phase short-circuit fault at a point F1 at an outlet of the half wavelength line, a free wave in case of a three-phase short-circuit fault at a point F8 at the reverse outlet of the half wavelength line and a free wave in case of a three-phase short-circuit fault at the point F9 outside the forward region of the half wavelength line are shown in FIGS. 7, 8 and 9 respectively.

(91) The above embodiments are adopted not to limit but only to describe the technical solutions of the disclosure. Although the disclosure has been described in detail with reference to the embodiments, those skilled in the art may still make modifications or equivalent replacements to specific implementation modes of the disclosure. Any modifications or equivalent replacements made without departing from the spirit and scope of the disclosure fall within the scope of protection of the claims of the disclosure applying for approval.

INDUSTRIAL APPLICABILITY

(92) In the embodiments of the disclosure, the currents are sampled and calculated to obtain a free wave energy protection algorithm capable of effectively distinguishing faults in and outside the region through the protection measuring point. The algorithm includes the free wave energy protection quick action section, basic section and sensitive section. In such a manner, the technical solutions provided by the embodiments of the disclosure reduce the sampling rate of a conventional wave process-based algorithm, furthermore solve the problem of influence of a capacitive voltage transformer on the conventional wave process-based algorithm, have the advantages of accuracy, efficiency and reliability, consider both the reliability and sensitivity of the protection action over the half wavelength line, and ensure the safe and reliable operation of the half wavelength line.