A method and system for directly grounding a single-phase sheath of a high-voltage power cable at both ends

Abstract

A method and system for directly grounding the double ends of the single-phase sheath of a high-voltage power cable, both ends of the middle phase single-core cable sheath are directly grounded; one end of the other two-phase single-core cable sheath is directly grounded, and the other end is connected to a protective device before grounding, and the metal sheath of the middle phase is used as a return line, which not only saves the equipment cost of the return line and the protector, but also avoids the risk of the return line being stolen; in particular, when the middle phase with both ends directly grounded fails, the induced voltage of the fault phase sheath is significantly reduced; and when any phase that is not directly grounded at both ends fails, the induced voltage of the fault phase sheath is slightly reduced; the grounding method proposed by the present invention is more suitable for cable loops laid in parallel. The grounding method proposed by the present invention reduces the induced voltage of the cable sheath under fault conditions, reduces the use of return lines and a protector, and reduces the construction cost and the time for arranging the return line.

Description

Technical Field

[0001] The present invention relates to the technical field of high-voltage power cables, and more specifically, to a method and system for directly grounding a single-phase sheath of a high-voltage power cable at both ends. Background Art

[0002] With the rapid development of cities, built-up areas are continuously extending outward from urban centers, and the demand for undergrounding of 110kV and above overhead lines has increased sharply. 110kV and above cables are all single-core. According to long-term operating experience, defects in the grounding system will evolve into cable failures, causing immeasurable losses. Therefore, the cable grounding system has received more and more attention from the industry, and the selection of grounding forms and the detection of grounding loop currents have also become important tasks in the design and operation and maintenance of 110kV and above cables.

[0003] In the prior art, in the process of cable engineering design, when the cable is short, or the remaining length of the cable after the cross-interconnection is not enough to complete another cross-interconnection section, the form of single-point grounding of the metal sheath is usually adopted. At present, the "GB50217-2018 Power Engineering Cable Design Specification" and the "DL / T 5221-2016 Urban Power Cable Line Design Technical Regulations" stipulate that when the metal sheath of a single-core cable of 110kV and above in the AC system is directly grounded at a single point, a parallel return line should be set along the cable. The working principle of the return line is: when a cable line with a single-point grounding of the metal sheath has a single-phase short circuit fault, the short-circuit current uses the earth as a loop, and the sheath induced voltage is very high. When the induced voltage exceeds the sheath tolerance value, it will cause a breakdown. After adding the return line, the short-circuit current is shunted through the return line, reducing the sheath induced voltage. The effect of the return line in reducing the fault induced voltage is obvious in theory, and in the process of engineering design and implementation, the setting of the return line is also a must. However, the return line only works in the event of a fault. When the system is operating normally, the return line has no effect and also increases system losses. Moreover, since both ends of the return line are grounded and not directly connected to the high-voltage part, it is often stolen during actual operation.

[0004] Prior art 1 (CN206650184U) "A single-core cable sheath grounding structure", one end of the protective sheath of each single-core cable in the three-phase single-core cable is directly grounded, and the other end is connected to the protective structure and then grounded, wherein the protective structure includes a lightning arrester and a reactor in parallel. In prior art 1, when the cable line is in normal working state, there is no circulating current in the cable metal sheath; when the line has a lightning overvoltage or an operating overvoltage, the voltage exceeds the lightning rod threshold, the lightning arrester is turned on, and the current is discharged into the earth; the reactance value of the reactor is set according to the actual situation of the project. When an asymmetric grounding fault occurs in the cable line, the saturated reactor tends to be saturated, the reactance value decreases, and the reactor is in a conducting state, so that the fault current can be quickly discharged into the earth, playing a role in protecting the insulation of the cable outer sheath, and the lightning arrester is not conducting at this time. Prior art 1 cancels the setting of the return line, avoiding system loss and theft, but each cable needs to add protection equipment, increasing the project investment, construction area and construction difficulty.

[0005] In addition, in the grounding method given in the prior art 1, when an asymmetric grounding fault occurs in the cable line, since there is no return line, the fault current flows entirely through the earth. Figure 1 As shown, the induced voltage U of the metal sheath of the fault phase SA for:

[0006]

[0007] Where, R is the grounding resistance; R g is the earth resistance per unit length. This value is extremely small and has little effect on the induced voltage. l is the cable length corresponding to the distance from the fault point to the grounding point. D is the penetration depth of the current in the ground. r s I is the radius of the cable metal sheath; d is the single-phase ground fault current.

[0008] Non-fault two-phase metal sheath induced voltage U SB and U SC They are:

[0009]

[0010]

[0011] Where S is the distance between single-core cables.

[0012] By comparing equations (4) to (6), we can see that the fault phase induced voltage U SA The highest, while on the cable parallel to the fault, the induced voltage decreases with the increase of the distance between the cables, that is, U SC SB .

[0013] ​Considering the design structure of the urban cable channel, it can be seen that the single-core cable grounding method such as the prior art 1, although the return line is cancelled, cannot reduce the induced voltage of the metal sheath of the fault phase, and the induced voltage of the metal sheath of the non-fault phase is also high, which brings great hidden dangers to the safe operation of the cable. In the prior art 1, under normal working conditions, the reactor is resistive, but because the power frequency is 50Hz, the current entering the ground through the reactor still has a certain value, so that the sheath circulating current value is still large and cannot be ignored. At this time, the induced voltage in the sheath may be greater than the insulation withstand strength of the cable metal sheath; in addition, under overvoltage conditions, if the reactor is turned on, since the reactor is a non-ideal inductor, there is still a certain value of resistance. If a large current is turned on, it may cause the reactor to explode. If the reactor selection is increased, the equipment will be bulky and difficult to install in the grounding box.

[0014] Therefore, it is necessary to study the method of direct grounding of both ends of the single-phase sheath of high-voltage power cables. By eliminating the return line, it is possible to take into account many factors such as engineering construction cost and construction, while also ensuring the safe operation of the non-fault phase as a more important goal. Summary of the invention

[0015] In order to solve the deficiencies in the prior art, the purpose of the present invention is to provide a method and system for directly grounding the double ends of a single-phase sheath of a high-voltage power cable, in which the two ends of the middle phase are directly grounded, and the other two phases still adopt a method of directly grounding at one end and protectively grounding at the other end, that is, the metal sheath of the middle phase is used as a return line, which not only saves the equipment cost of the return line and the protector, but also avoids the risk of the return line being stolen; in particular, when a fault occurs in the middle phase with both ends directly grounded, the induced voltage of the sheath of the faulty phase is significantly reduced; and when a fault occurs in any phase that is not directly grounded at both ends, the induced voltage of the sheath of the faulty phase is slightly reduced; the grounding method proposed by the present invention is more suitable for cable loops laid in parallel.

[0016] The present invention adopts the following technical solution.

[0017] A method for directly grounding a single-phase sheath of a high-voltage power cable at both ends, wherein the high-voltage power cable is a three-phase single-core cable, and both ends of the sheath of one phase of the single-core cable are directly grounded; one end of the sheath of the other two phases of the single-core cable is directly grounded, and the other end is connected to a protective device and then grounded;

[0018] When an asymmetric grounding fault occurs in a cable line, when one end of the sheath of the single-core cable of the fault phase is directly grounded and the other end is connected to a protective device, part of the grounding fault current takes the earth as a loop, and the other part takes the sheath of the single-core cable with both ends directly grounded as a loop. The induced voltage in the sheath of the single-core cable of each phase is not greater than the insulation withstand strength of the cable metal sheath;

[0019] When an asymmetric grounding fault occurs in a cable line, when both ends of the sheath of the single-core cable of the faulty phase are directly grounded, the fault current takes the earth as a loop, and the induced voltage in the sheath of the single-core cable of each phase is not greater than the insulation withstand strength of the cable metal sheath.

[0020] A method for directly grounding a single-phase sheath of a high-voltage power cable at two ends, comprising:

[0021] Step 1, collecting engineering parameters of high-voltage power cables;

[0022] Step 2: According to the engineering parameters, the sheath grounding models of each phase of the high-voltage power cable are established respectively. The sheath grounding models include: a sheath single-end direct grounding model and a sheath double-end direct grounding model; wherein, no separate return line is set in each sheath grounding model;

[0023] Step 3, when the high-voltage power cable is working normally, the sheath current in each phase sheath grounding model is simulated and calculated respectively; the energy loss limit in the sheath is calculated using the sheath current and sheath induced voltage limit;

[0024] Step 4, when a ground fault occurs in any phase of the high-voltage power cable, the induced voltage of the sheath grounding model of each phase is simulated and calculated respectively;

[0025] Step 5: When a ground fault occurs in any phase of the high-voltage power cable, thermal stability verification is performed on the sheath grounding model of each phase in combination with the protection action time limit; and the sheath grounding mode of the high-voltage power cable is determined based on the thermal stability verification result.

[0026] Preferably, in step 1, the engineering parameters include: voltage level, cable model, cable length, grounding resistance, metal sheath radius, metal sheath thickness, cable spacing, and protection action time limit.

[0027] Preferably, in step 2, in the sheath single-end direct grounding model, one end of the sheath of the single-core cable is directly grounded, and the other end is connected to a protective device and then grounded;

[0028] In the sheath double-end direct grounding model, both ends of the sheath of a single-core cable are directly grounded;

[0029] The high-voltage power cable sheath grounding model constructed using the sheath grounding model of each phase includes: one phase adopts a sheath double-end direct grounding model, and the other two phases adopt a sheath single-end direct grounding model.

[0030] Preferably, in the high-voltage power cable sheath grounding model, when the three-phase single-core cables are laid in parallel, the middle phase adopts a sheath double-end direct grounding model.

[0031] Preferably, step 3 comprises:

[0032] Step 3.1, when the high-voltage power cable is working normally, there is no sheath current in the sheath single-end direct grounding model, and the sheath current in the sheath double-end direct grounding model satisfies the following relationship:

[0033]

[0034] In the formula,

[0035] I sheath is the sheath current in the two-terminal direct grounding model,

[0036] The industry standard stipulates the sheath induced voltage limit, which is no more than 50V.

[0037] R g1 is the first grounding resistor, the resistance of which is determined by the soil resistivity of the grounding point where the first grounding end of the sheath is located.

[0038] R g2 is the second grounding resistor, and the resistance value of the second grounding resistor is determined by the soil resistivity of the grounding point where the second grounding end of the sheath is located.

[0039] R p The sheath resistance is determined by the conductivity of the sheath material, the sheath cross-sectional area, and the cable length.

[0040] The grounding points of each grounding terminal include: in the substation, at the terminal tower, and in the working well along the cable line;

[0041] Step 3.2, the energy loss limit in the sheath caused by the sheath current satisfies the following relationship:

[0042]

[0043] In the formula, It is the limit of energy loss in the sheath caused by sheath current.

[0044] Preferably, step 4 comprises:

[0045] Step 4.1, when the sheath of the fault phase is directly grounded at one end, the sheath of one non-fault phase is directly grounded at one end, and the sheath of the other non-fault phase is directly grounded at both ends, part of the fault current uses the earth as a loop, and the other part uses the sheath of the single-core cable with both ends directly grounded as a loop, then the induced voltage on the sheath of the fault phase is Satisfies the following relationship:

[0046]

[0047] In the formula,

[0048] Id is the single-phase ground fault current,

[0049] Z AA is the mutual impedance between the fault phase cable sheath and the fault phase cable core with the earth as the fault current loop,

[0050] Z PA It is the mutual impedance between the fault phase cable sheath and the single-core cable sheath with both ends directly grounded, with the earth as the loop.

[0051] R 1 is the grounding resistance at the power supply end,

[0052] R 2 is the transition resistance at the fault,

[0053] R g is the earth resistance, which is determined by the soil resistivity;

[0054] Step 4.2: When the sheath of the fault phase is directly grounded at both ends, and the sheaths of the two non-fault phases are directly grounded at one end, and the fault current uses the earth as the loop, the induced voltage on the sheath of the fault phase is Satisfies the following relationship:

[0055]

[0056] In the formula,

[0057] Z PP The self-impedance of the cable sheath with both ends directly grounded.

[0058] Preferably, in step 4,

[0059] Z AA Satisfies the following relationship:

[0060]

[0061] Z PA Satisfies the following relationship:

[0062]

[0063] Z PP Satisfies the following relationship:

[0064]

[0065] In the above formula,

[0066] l is the cable length corresponding to the distance from the fault point to the grounding point,

[0067] D is the penetration depth of the ground current,

[0068] DA The distance between the sheath of a single-core cable directly grounded at both ends and the sheath of the fault phase,

[0069] r s is the radius of the cable metal sheath,

[0070] R p is the sheath resistance.

[0071] Preferably, step 5 comprises:

[0072] Step 5.1, according to the short-circuit duration t determined by the main protection time limit of the ground fault, the design considers the system power supply short-circuit overcurrent I, and calculates the heat generation Q of the double-terminal directly grounded phase shielding layer according to the following relationship:

[0073] Q=I 2 ·t……(1)

[0074] Step 5.2, calculate the thermal stability verification coefficient C according to the cable model and cable specifications, and satisfy the following relationship:

[0075]

[0076]

[0077] In the formula,

[0078] J is the heat equivalent coefficient,

[0079] q is the heat capacity per unit volume of the cable conductor,

[0080] θ m The maximum temperature allowed for the cable conductor during the short-circuit action time.

[0081] θ p is the maximum operating temperature of the cable conductor before a short circuit occurs,

[0082] α is the resistance temperature coefficient of the cable conductor at 20°C,

[0083] ρ is the resistivity of the cable conductor at 20°C,

[0084] k is the ratio of the AC resistance to the DC resistance of the cable core conductor,

[0085] S is the effective cross-section of the metal sheath,

[0086] η is the correction factor that takes into account the influence of the heat capacity of the cable conductor filler;

[0087] Step 5.3, combine formulas (1) to (3) to calculate the temperature rise of the metal sheath of the double-terminal directly grounded phase under short-circuit fault;

[0088] In step 5.4, the calculated result of the sheath temperature rise is compared with the tolerable temperature of the cable outer sheath. If the temperature rise of the metal sheath of the double-end direct grounding phase is less than the tolerable temperature of the outer sheath, this grounding method can be adopted; otherwise, after adjusting the grounding method, repeat steps 5.1 to 5.3.

[0089] Preferably, in step 5.2, the process of setting the short circuit duration includes:

[0090] Step 5.2.1, collecting the protection action time limits of each section of the zero-sequence impedance protection and the protection action time limits of each section of the zero-sequence overcurrent protection in the ground fault main protection;

[0091] Step 5.2.2: The short-circuit duration is the time between the action of the four-stage zero-sequence overcurrent protection and the failure of the three-stage zero-sequence impedance protection.

[0092] A high-voltage power cable single-phase sheath double-end direct grounding system, the system comprising: a first grounding device and a second grounding device, one end of the three-phase single-core cable is grounded through the first grounding device, and the other end of the three-phase single-core cable is grounded through the second grounding device;

[0093] One end of the three-phase single-core cable is directly grounded to the ground via a first grounding device;

[0094] The other end of the three-phase single-core cable is grounded through a second grounding device, wherein the sheath of one phase single-core cable is directly grounded; and the sheaths of the other two phase single-core cables are connected to the protective device and then grounded.

[0095] The beneficial effect of the present invention is that, compared with the prior art, the grounding method proposed by the present invention reduces the induced voltage of the cable sheath under fault conditions, reduces the use of return lines and a protector, and reduces the engineering construction cost and the return line layout time. BRIEF DESCRIPTION OF THE DRAWINGS

[0096] Figure 1 It is an equivalent circuit diagram under the grounding method in prior art 1;

[0097] Figure 2 It is a flowchart of a method for directly grounding a single-phase sheath of a high-voltage power cable at both ends according to the present invention;

[0098] Figure 3 Schematic diagram of a high-voltage power cable single-phase sheath double-end direct grounding system in one embodiment of the present invention;

[0099] Figure 3 The reference numerals in the drawings are described as follows:

[0100] 1A-high voltage power cable phase A; 1B-high voltage power cable phase B; 1C-high voltage power cable phase C;

[0101] 2- Protection device at the other end of high-voltage power cable phase A; 3- Protection device at the other end of high-voltage power cable phase C. DETAILED DESCRIPTION

[0102] The present application is further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present application.

[0103] A method for directly grounding a single-phase sheath of a high-voltage power cable at both ends, wherein the high-voltage power cable is a three-phase single-core cable, and both ends of the sheath of one phase of the single-core cable are directly grounded; one end of the sheath of the other two phases of the single-core cable is directly grounded, and the other end is connected to a protective device and then grounded;

[0104] When an asymmetric grounding fault occurs in a cable line, when one end of the sheath of the single-core cable of the fault phase is directly grounded and the other end is connected to a protective device, part of the grounding fault current takes the earth as a loop, and the other part takes the sheath of the single-core cable with both ends directly grounded as a loop. The induced voltage in the sheath of the single-core cable of each phase is not greater than the insulation withstand strength of the cable metal sheath;

[0105] When an asymmetric grounding fault occurs in a cable line, when both ends of the sheath of the single-core cable of the faulty phase are directly grounded, the fault current takes the earth as a loop, and the induced voltage in the sheath of the single-core cable of each phase is not greater than the insulation withstand strength of the cable metal sheath.

[0106] According to Q / GDW 11316-2014 "Power Cable Line Test Procedure", the cable outer sheath should be able to withstand a DC voltage of 10kV without breakdown, that is, the insulation withstand strength of the cable metal sheath is 10kV.

[0107] like Figure 2 , a method for double-end direct grounding of a single-phase sheath of a high-voltage power cable, comprising steps 1 to 6.

[0108] Step 1: Collect engineering parameters of high-voltage power cables.

[0109] Specifically, in step 1, the engineering parameters include: voltage level, cable model, cable length, grounding resistance, metal sheath radius, metal sheath thickness, cable spacing, and protection action time limit.

[0110] Step 2: According to the engineering parameters, the sheath grounding models of each phase of the high-voltage power cable are established respectively. The sheath grounding models include: a sheath single-end direct grounding model and a sheath double-end direct grounding model; wherein, no separate return line is set in each sheath grounding model.

[0111] Specifically, in step 2, in the sheath single-end direct grounding model, one end of the sheath of the single-core cable is directly grounded, and the other end is connected to the protection device and then grounded;

[0112] In the sheath double-end direct grounding model, both ends of the sheath of a single-core cable are directly grounded;

[0113] The high-voltage power cable sheath grounding model constructed using the sheath grounding model of each phase includes: one phase adopts a sheath double-end direct grounding model, and the other two phases adopt a sheath single-end direct grounding model.

[0114] Specifically, in the high-voltage power cable sheath grounding model, when the three-phase single-core cable is laid in parallel, the middle phase adopts the sheath double-end direct grounding model.

[0115] Step 3: When the high-voltage power cable is working normally, the sheath current in each phase sheath grounding model is simulated and calculated respectively; the energy loss limit in the sheath is calculated using the sheath current and sheath induced voltage limit.

[0116] Specifically, step 3 includes:

[0117] Step 3.1, when the high-voltage power cable is working normally, there is no sheath current in the sheath single-end direct grounding model, and the sheath current in the sheath double-end direct grounding model satisfies the following relationship:

[0118]

[0119] In the formula,

[0120] I sheath is the sheath current in the two-terminal direct grounding model,

[0121] The industry standard stipulates the sheath induced voltage limit, which is no more than 50V.

[0122] R g1 is the first grounding resistor, the resistance of which is determined by the soil resistivity of the grounding point where the first grounding end of the sheath is located.

[0123] R g2 is the second grounding resistor, and the resistance value of the second grounding resistor is determined by the soil resistivity of the grounding point where the second grounding end of the sheath is located.

[0124] R p The sheath resistance is determined by the conductivity of the sheath material, the sheath cross-sectional area, and the cable length.

[0125] The grounding points of each grounding terminal include: in the substation, at the terminal tower, and in the working well along the cable line;

[0126] exist When the voltage is limited to 50V, the first ground terminal is in the substation or at the terminal tower. g1Less than 1Ω, the second grounding terminal is in a working well along the cable, the environment is poor, R g2 Within 4Ω, it is estimated that the sheath current I sheath It should be around 10A.

[0127] Step 3.2, the energy loss limit in the sheath caused by the sheath current satisfies the following relationship:

[0128]

[0129] In the formula, It is the limit of energy loss in the sheath caused by sheath current.

[0130] Step 4: When a ground fault occurs in any phase of the high-voltage power cable, the induced voltage of the sheath grounding model of each phase is simulated and calculated respectively.

[0131] Specifically, step 4 includes:

[0132] Step 4.1, when the sheath of the fault phase is directly grounded at one end, the sheath of one non-fault phase is directly grounded at one end, and the sheath of the other non-fault phase is directly grounded at both ends, part of the fault current uses the earth as a loop, and the other part uses the sheath of the single-core cable with both ends directly grounded as a loop, then the induced voltage on the sheath of the fault phase is Satisfies the following relationship:

[0133]

[0134] In the formula,

[0135] I d is the single-phase ground fault current,

[0136] Z AA is the mutual impedance between the fault phase cable sheath and the fault phase cable core with the earth as the fault current loop,

[0137] Z PA It is the mutual impedance between the fault phase cable sheath and the single-core cable sheath with both ends directly grounded, with the earth as the loop.

[0138] R 1 is the grounding resistance at the power supply end,

[0139] R 2 is the transition resistance at the fault,

[0140] R g is the earth resistance, which is determined by the soil resistivity;

[0141] Step 4.2: When the sheath of the fault phase is directly grounded at both ends, and the sheaths of the two non-fault phases are directly grounded at one end, and the fault current uses the earth as the loop, the induced voltage on the sheath of the fault phase is Satisfies the following relationship:

[0142]

[0143] In the formula,

[0144] Z PP The self-impedance of the cable sheath with both ends directly grounded.

[0145] When a double-end grounded phase fails, the grounding method proposed in this paper is more effective in reducing the induced voltage on the fault phase than when the return line is arranged in a "30-70" manner; when a non-double-end grounded phase fails, the grounding method proposed in this paper is less effective in reducing the induced voltage on the fault phase than when the return line is arranged in a "30-70" manner; for parallel cable loops, double-end grounding should be performed in the middle phase and the induced voltage should be calculated.

[0146] Specifically, in step 4,

[0147] Z AA Satisfies the following relationship:

[0148]

[0149] Z PA Satisfies the following relationship:

[0150]

[0151] Z PP Satisfies the following relationship:

[0152]

[0153] In the above formula,

[0154] l is the cable length corresponding to the distance from the fault point to the grounding point,

[0155] D is the penetration depth of the ground current,

[0156] D A The distance between the sheath of a single-core cable directly grounded at both ends and the sheath of the fault phase,

[0157] r s is the radius of the cable metal sheath,

[0158] R p is the sheath resistance.

[0159] In a preferred embodiment of the present invention, a cable and grounding system model is built in PSCAD, wherein the cable adopts a frequency-variable model, the voltage level is 110kV, and the length is 500 meters. For fault conditions, a single-phase grounding fault with a transition resistance of 5Ω is set at phase A at 400 meters of the cable line. The fault triggering time is 1s and the duration is 0.5s. In the three-phase one-end direct grounding and one-end protective grounding mode, the effective value of the sheath induced voltage after the grounding fault occurs in phase A is 4.05kV; in the single-phase double-end grounding system, the effective value of the sheath induced voltage after the grounding fault occurs in phase A is 2.65kV, and the induced voltage is reduced by 35%, which can significantly reduce the fault induced voltage.

[0160] Step 5: When a ground fault occurs in any phase of the high-voltage power cable, thermal stability verification is performed on the sheath grounding model of each phase in combination with the protection action time limit; and the sheath grounding mode of the high-voltage power cable is determined based on the thermal stability verification result.

[0161] According to IEEE Std 635-2003, the metal sheath must be able to withstand the short-circuit current amplitude and duration, and ensure that the inner buffer layer and outer sheath of the metal sheath are not damaged. In addition, since the fault current flows through the double-end grounded phase aluminum sheath, it is necessary to perform thermal stability verification on the phase aluminum sheath. In the preferred embodiment of the present invention, the thermal stability verification is carried out according to the "Power Engineering Cable Design Specification" GB / T50217:

[0162] Specifically, step 5 includes:

[0163] Step 5.1, according to the short-circuit duration t determined by the main protection time limit of the ground fault, the design considers the system power supply short-circuit overcurrent I, and calculates the heat generation Q of the double-terminal directly grounded phase shielding layer according to the following relationship:

[0164] Q=I 2 ·t……(1)

[0165] Step 5.2, calculate the thermal stability verification coefficient C according to the cable model and cable specifications, and satisfy the following relationship:

[0166]

[0167]

[0168] In the formula,

[0169] J is the thermal equivalent coefficient, which can be taken as J = 1;

[0170] q is the heat capacity per unit volume of the cable conductor, and the value for aluminum core cable is 2.48;

[0171] θ m The maximum temperature allowed for the cable conductor during the short-circuit action time;

[0172] θ p The maximum operating temperature of the cable conductor before a short circuit occurs;

[0173] α is the resistance temperature coefficient of the cable conductor at 20°C, and the aluminum core cable is 0.00403;

[0174] ρ is the resistivity of the cable conductor at 20°C, and the resistivity of the aluminum core cable is 0.031×10 -4 ;

[0175] k is the ratio of the AC resistance to the DC resistance of the cable core conductor, which is 1.010;

[0176] S is the effective cross-section of the metal sheath,

[0177] η is the correction factor that takes into account the influence of the heat capacity of the cable conductor filling material. For 110kV lines, the value can be η = 1;

[0178] Step 5.3, combining equations (1) to (3), calculates the temperature rise of the metal sheath of the double-terminal directly grounded phase under a short-circuit fault;

[0179] In step 5.4, the calculated result of the sheath temperature rise is compared with the tolerable temperature of the cable outer sheath. If the temperature rise of the metal sheath of the double-end direct grounding phase is less than the tolerable temperature of the outer sheath, this grounding method can be adopted; otherwise, after adjusting the grounding method, repeat steps 5.1 to 5.3.

[0180] Preferably, in step 5.2, the process of setting the short circuit duration includes:

[0181] Step 5.2.1, collecting the protection action time limits of each section of the zero-sequence impedance protection and the protection action time limits of each section of the zero-sequence overcurrent protection in the ground fault main protection;

[0182] Step 5.2.2: The short-circuit duration is the time between the action of the four-stage zero-sequence overcurrent protection and the failure of the three-stage zero-sequence impedance protection.

[0183] In the preferred embodiment of the present invention, considering that the main protection for ground fault of 110kV line is zero-sequence impedance protection and zero-sequence overcurrent protection: the backup three-stage protection action time limit of zero-sequence impedance protection is 2s; the backup four-stage action time limit of zero-sequence overcurrent protection is 0.9s. Considering that if the four-stage zero-sequence overcurrent protection does not act at 0.9s, but the three-stage zero-sequence impedance protection acts at 2.0s, it means that the fault transition resistance is high and the short-circuit overcurrent is small. At this time, the aluminum sheath must be able to withstand the heat caused by the fault overcurrent. Therefore, when performing thermal stability verification in this article, the fault duration is set to the four-stage zero-sequence overcurrent protection action time limit, that is: the effective value of the short-circuit overcurrent is set to 40kA, and the duration is 1.0s: then formula (1) satisfies the following relationship:

[0184] Q=I 2 t=1.6×10 9 J……(7)

[0185] Take a 110 kV 700 mm 2 For example, the inner diameter of the aluminum sheath is 42.7 mm and the outer diameter is 44.9 mm, so the cross-sectional area of ​​the aluminum sheath is 605 mm 2 From equations (1) and (3), it can be seen that the temperature of the aluminum sheath can rise from 40°C to 106°C during the 1s fault duration. Since the softening temperature of the polyethylene sheath outside the aluminum sheath is between 125°C and 135°C, it will not be damaged during the 1s fault duration.

[0186] like Figure 3 , a high-voltage power cable single-phase sheath double-end direct grounding system, wherein the high-voltage power cable includes a high-voltage power cable A phase 1A, a high-voltage power cable B phase 1B, and a high-voltage power cable C phase 1C; the system includes: a first grounding device and a second grounding device, one end of the three-phase single-core cable is grounded through the first grounding device, and the other end of the three-phase single-core cable is grounded through the second grounding device;

[0187] One end of the three-phase single-core cable is directly grounded to the ground via a first grounding device.

[0188] The other end of the three-phase single-core cable is connected to the second grounding device, where the sheath of one phase of the single-core cable is directly grounded; the sheaths of the other two phases of the single-core cable are connected to the protection device and then grounded. The second grounding device includes: a protection device 2 for the other end of the high-voltage power cable phase A and a protection device 3 for the other end of the high-voltage power cable phase C.

[0189] In the preferred embodiment of the present invention, the two ends of the middle phase are directly grounded, and the other two phases are still directly grounded at one end and protected at the other end. That is, the metal sheath of the middle phase is used as the return line, which not only saves the equipment cost of the return line and the protector, but also avoids the risk of the return line being stolen. A section of cable of about 600 meters, if equipped with equal length 240mm 2 The engineering budget of the copper core return line is about 100,000 yuan to 200,000 yuan, so the grounding method proposed in the present invention has more obvious economic advantages.

[0190] The applicant of the present invention has made a detailed explanation and description of the implementation examples of the present invention in conjunction with the drawings in the specification. However, those skilled in the art should understand that the above implementation examples are only preferred implementation schemes of the present invention, and the detailed description is only to help readers better understand the spirit of the present invention, but not to limit the scope of protection of the present invention. On the contrary, any improvements or modifications based on the inventive spirit of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for direct grounding of a single-phase sheath of a high-voltage power cable at both ends, wherein the high-voltage power cable is a three-phase single-core cable, characterized in that: Directly ground both ends of the sheath of one phase single-core cable; directly ground one end of the sheath of the other two phase single-core cables, and connect the other end to the protective device before grounding; When an asymmetric grounding fault occurs in a cable line, when one end of the sheath of the single-core cable of the fault phase is directly grounded and the other end is connected to a protective device, part of the grounding fault current takes the earth as a loop, and the other part takes the sheath of the single-core cable with both ends directly grounded as a loop. The induced voltage in the sheath of the single-core cable of each phase is not greater than the insulation withstand strength of the cable metal sheath; When an asymmetric grounding fault occurs in a cable line, when both ends of the sheath of the single-core cable of the faulty phase are directly grounded, the fault current takes the earth as a loop, and the induced voltage in the sheath of the single-core cable of each phase is not greater than the insulation withstand strength of the cable metal sheath.

2. A high-voltage power cable single-phase sheath double-end direct grounding system implemented by using the high-voltage power cable single-phase sheath double-end direct grounding method according to claim 1, the system comprising: The first grounding device and the second grounding device, one end of the three-phase single-core cable is grounded through the first grounding device, and the other end of the three-phase single-core cable is grounded through the second grounding device, characterized in that: One end of the three-phase single-core cable is directly grounded to the ground via a first grounding device; The other end of the three-phase single-core cable is grounded through a second grounding device, wherein the sheath of one-phase single-core cable is directly grounded; the sheaths of the other two-phase single-core cables are connected to the protection device and then grounded.

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