CN114550993A - Method and system for directly grounding two ends of single-phase sheath of high-voltage power cable - Google Patents
Method and system for directly grounding two ends of single-phase sheath of high-voltage power cable Download PDFInfo
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- H01B9/00—Power cables
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- H01R4/00—Electrically-conductive connections between two or more conductive members in direct contact, i.e. touching one another; Means for effecting or maintaining such contact; Electrically-conductive connections having two or more spaced connecting locations for conductors and using contact members penetrating insulation
- H01R4/58—Electrically-conductive connections between two or more conductive members in direct contact, i.e. touching one another; Means for effecting or maintaining such contact; Electrically-conductive connections having two or more spaced connecting locations for conductors and using contact members penetrating insulation characterised by the form or material of the contacting members
- H01R4/66—Connections with the terrestrial mass, e.g. earth plate, earth pin
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- Y04S10/00—Systems supporting electrical power generation, transmission or distribution
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Abstract
A high-voltage power cable single-phase sheath double-end direct grounding method and system, directly grounding both ends of the intermediate phase single-phase cable sheath; in addition, one end of the two single-core cable sheaths is directly grounded, the other end of the two single-core cable sheaths is grounded after being connected with the protection device, and the metal sheath of the intermediate phase is used as a return line, so that the equipment cost of the return line and the protector is saved, and the risk of stealing the return line is avoided; particularly, when the intermediate phase with two ends directly grounded fails, the induced voltage of the protective layer of the failed phase is obviously reduced; when any phase which is not directly grounded at two ends has a fault, the induced voltage of the protective layer of the fault phase is slightly reduced; the grounding mode provided by the invention is more suitable for a cable loop laid in parallel. The grounding mode provided by the invention reduces the induced voltage of the cable sheath under the fault working condition, reduces the use of a return line and a protector, and reduces the engineering construction cost and the arrangement time of the return line.
Description
Technical Field
The invention relates to the technical field of high-voltage power cables, in particular to a method and a system for directly grounding two ends of a single-phase sheath of a high-voltage power cable.
Background
With the rapid development of cities, the built-up area continuously extends outwards from the center of the urban area, and the demand for the entrance of overhead lines of 110kV or more into the ground is increased rapidly. The 110kV and above cables are all in single-core form, and according to long-term operation experience, defects of the grounding system can be changed into cable faults, which cause immeasurable loss, so the cable grounding system is more and more concerned by the industry, and the selection of the grounding form and the detection of the grounding circulation are also important work in the design, operation and maintenance of the 110kV and above cables.
In the prior art, in the cable engineering design process, when the cable is short or the remaining length of the cable after completing the cross-connection is not enough to complete a cross-connection section, a single-point grounding mode of a metal sheath is generally adopted. At present, in the regulations GB 50217 plus 2018 power engineering cable design specification and DL/T5221 plus 2016 urban power cable line design technical specification, when a single-core cable metal sleeve of 110kV or more of an alternating current system is directly grounded at a single point, a parallel return line is arranged near the cable. The working principle of the return line is as follows: when the single-point grounding cable line of the metal protective layer has a single-phase short circuit fault, the short circuit current takes the ground as a loop, the induced voltage of the protective layer is very high, and breakdown can be caused when the induced voltage exceeds the tolerance value of the protective layer. After the return line is added, the short-circuit current is shunted through the return line, and the induced voltage of the protective layer is reduced. The effect of the return line in reducing the fault-induced voltage is theoretically obvious, and the arrangement of the return line is also a necessary option in the engineering design and implementation process. However, the return line only functions in case of a fault, and when the system normally operates, the return line does not function, which additionally increases system loss, and because two ends of the return line are grounded and are not directly connected with the high-voltage part, the return line is often stolen in the actual operation process.
Prior art 1(CN206650184U) "a single core cable sheath grounding structure", each single core cable's protective sheath one end is directly grounded in the three-phase single core cable, and the other end is grounded after connecting the protection structure, wherein, the protection structure includes parallelly connected arrester and reactor. In the prior art 1, when the cable line is in a normal working state, the metal sheath of the cable has no circulating current; when the lightning overvoltage and the operation overvoltage occur on the line and the voltage exceeds the threshold value of the lightning rod, the lightning arrester is conducted, and the current is discharged to the ground; the reactance value of the reactor is set according to the actual condition of engineering, when the asymmetric earth fault occurs in the cable line, the saturable reactor tends to be saturated, the reactance value is reduced, and the reactor is in a conduction state, so that the fault current can be rapidly leaked into the ground, the effect of protecting the insulation of the outer protective layer of the cable is achieved, and the arrester is not conducted at the moment. In the prior art 1, a return line is omitted, the system loss and the theft phenomenon are avoided, but each cable needs to be additionally provided with a protection device, so that the engineering investment, the construction occupied area and the construction difficulty are increased.
In addition, in the grounding mode provided in prior art 1, when an asymmetric ground fault occurs in a cable line, since there is no return line, fault current flows through the ground completely, as shown in fig. 1, and an induced voltage U of a metal sheath of a fault phaseSAComprises the following steps:
in the formula, R is a grounding resistor; rgThe resistance is a large ground resistance in unit length, and the influence of the minimum value on the induction voltage is small; l is the length of the cable corresponding to the distance from the fault point to the grounding point; d is the current in the groundA penetration depth; r issThe radius of the metal sheath of the cable; i isdIs a single phase earth fault current.
Non-failure two-phase metal protective layer induction voltage USBAnd USCRespectively as follows:
wherein S is the distance between the single-core cables.
By comparing the formulas (4) to (6), the fault phase induced voltage U can be foundSAOn the highest cables parallel to the fault, the induced voltage decreases with increasing distance between the cables, i.e. USC<USB。
Considering the design structure of the urban cable channel, it can be seen that although a return line is eliminated in the single-core cable grounding mode in the prior art 1, the induced voltage of the fault phase metal sheath cannot be reduced, and the induced voltage of the non-fault phase metal sheath is also higher, which brings great hidden trouble to the safe operation of the cable. In the prior art 1, under a normal working condition, the reactor presents resistance, but the power frequency is 50Hz, so that the current entering the ground through the reactor still has a certain value, the circulating current value of the sheath is still large and cannot be ignored, and the induced voltage in the sheath may be greater than the insulation tolerance strength of the metal sheath of the cable; in addition, under the overvoltage working condition, if the reactor is conducted, because the reactor is non-ideal inductance, a certain value of resistance still exists, if a large current is conducted, the reactor may explode, and if the selection of the reactor is increased, the equipment is huge in size, and the installation in the grounding box is difficult.
Therefore, a mode of directly grounding the two ends of the single-phase sheath of the high-voltage power cable is needed to be researched, and a return line is omitted, so that a more important target for ensuring safe operation of a non-fault phase is achieved while engineering construction cost, construction and other factors are considered.
Disclosure of Invention
In order to solve the defects in the prior art, the invention aims to provide a method and a system for directly grounding two ends of a single-phase sheath of a high-voltage power cable, wherein two ends of an intermediate phase are directly grounded, and the other two phases still adopt a mode that one end is directly grounded and the other end is protected and grounded, namely, a metal sheath of the intermediate phase is used as a return line, so that the equipment cost of the return line and a protector is saved, and the risk of stealing the return line is avoided; particularly, when the intermediate phase with two ends directly grounded fails, the induced voltage of the fault phase sheath is obviously reduced; when any phase which is not directly grounded at two ends has a fault, the induced voltage of the fault phase protective layer is slightly reduced; the grounding mode provided by the invention is more suitable for a cable loop laid in parallel.
The invention adopts the following technical scheme.
A high-voltage power cable single-phase sheath double-end direct grounding method, wherein the high-voltage power cable is a three-phase single-core cable, and both ends of one phase single-core cable sheath are directly grounded; one end of the other two single-core cable protective layers is directly grounded, and the other end is grounded after being connected with the protection device;
when the asymmetric ground fault occurs in the cable line, when one end of a fault phase single-core cable sheath is directly grounded and the other end of the fault phase single-core cable sheath is connected with a protection device, one part of the ground fault current takes the ground as a loop, the other part takes the single-core cable sheath with both ends directly grounded as a loop, and the induced voltage in each phase single-core cable sheath is not more than the insulation tolerance strength of a cable metal sheath;
when the asymmetric earth fault occurs in the cable line, when the two ends of the fault phase single-core cable sheath are directly grounded, the fault current takes the ground as a loop, and the induced voltage in each phase single-core cable sheath is not more than the insulation tolerance strength of the cable metal sheath.
A method for directly grounding a single-phase sheath of a high-voltage power cable at two ends comprises the following steps:
step 1, collecting engineering parameters of a high-voltage power cable;
step 5, when any phase in the high-voltage power cable has a ground fault, respectively carrying out thermal stability verification on each phase protective layer ground model by combining protection action time limit; and determining the grounding mode of the high-voltage power cable sheath according to the thermal stability verification result.
Preferably, in step 1, the engineering parameters include: voltage class, cable model, cable length, grounding resistance, metal sheath radius, metal sheath thickness, cable spacing, and protection action time limit.
Preferably, in the step 2, in the single-end direct grounding model of the sheath, one end of the single-core cable sheath is directly grounded, and the other end is grounded after being connected with the protection device;
in the sheath double-end direct grounding model, both ends of the single-core cable sheath are directly grounded;
the high-voltage power cable sheath grounding model constructed by utilizing the phase sheath grounding models comprises the following steps: one phase adopts a sheath double-end direct grounding model, and the other two phases adopt a sheath single-end direct grounding model.
Preferably, in the sheath grounding model of the high-voltage power cable, when the three-phase single-core cable is laid in parallel, the middle phase adopts a sheath double-end direct grounding model.
Preferably, step 3 comprises:
step 3.1, under the normal working condition of the high-voltage power cable, the sheath current does not exist in the sheath single-end direct grounding model, and the sheath current in the sheath double-end direct grounding model meets the following relational expression:
in the formula (I), the compound is shown in the specification,
Isheathfor sheath currents in the two-terminal direct ground model,
Rg1the resistance of the first grounding resistor is determined by the soil resistivity of the grounding point of the first grounding terminal of the passivation layer,
Rg2the resistance of the second grounding resistor is determined by the soil resistivity of the grounding point of the second grounding terminal of the sheath,
Rpthe resistance of the sheath resistor is determined by the conductivity of the sheath material, the sectional area of the sheath and the length of the cable;
wherein, the grounding point that each ground terminal is located includes: in a transformer substation, at a terminal tower, in a working well along a cable;
step 3.2, the energy loss limit value in the sheath caused by the sheath current meets the following relational expression:
in the formula (I), the compound is shown in the specification,
is the limit of energy loss in the sheath due to sheath current.
Preferably, step 4 comprises:
step 4.1, when the protective layer of the fault phase adopts single-end direct grounding, the protective layer of one non-fault phase adopts single-end direct groundingWhen the other non-fault phase sheath is directly grounded at two ends, one part of fault current takes the earth as a return circuit, and the other part takes the single-core cable sheath with two directly grounded ends as a return circuit, so that the induced voltage on the fault phase sheath
Satisfies the following relation:
in the formula (I), the compound is shown in the specification,
Idin order to achieve a single-phase earth fault current,
ZAAthe fault phase cable sheath and the fault phase cable core use the ground as the mutual impedance when the fault current loop is formed,
ZPAthe mutual impedance is formed when the fault phase cable sheath and the single core cable sheath with both ends directly grounded use the ground as a loop,
R1is a power end ground resistor,
R2as a result of the transition resistance at the fault,
Rgis the earth resistance, which is determined by the soil resistivity;
step 4.2, when the two ends of the protective layer of the fault phase are directly grounded, the two protective layers of the non-fault phases are directly grounded by the single end, and the fault current takes the earth as a loop, the induced voltage on the protective layer of the fault phase is
Satisfies the following relation:
in the formula (I), the compound is shown in the specification,
ZPPthe self-impedance of the sheath, both ends of which are directly grounded.
Preferably, in step 4,
ZAAsatisfies the following relation:
ZPAsatisfies the following relation:
ZPPsatisfies the following relation:
in the above-mentioned formula, the compound of formula,
l is the cable length corresponding to the distance from the fault point to the grounding point,
d is the current penetration depth in the ground,
DAthe distance between the single-core cable sheath with both ends directly grounded and the fault phase sheath,
rsthe radius of the metal sheath of the cable,
Rpis the sheath resistance.
Preferably, step 5 comprises:
step 5.1, according to the short circuit duration t determined by the main protection time limit of the ground fault, designing and considering a system power supply short circuit overcurrent I, and calculating the calorific value Q of the double-end direct grounding phase protective layer according to the following relational expression:
Q＝I2·t……(1)
step 5.2, calculating a thermal stability check coefficient C according to the cable model and the cable specification, and satisfying the following relational expression:
in the formula (I), the compound is shown in the specification,
j is a thermal-power equivalent coefficient,
q is the heat capacity per unit volume of the cable conductor,
θmthe maximum temperature allowed for the cable conductor during the short-circuit action time,
θpfor the maximum operating temperature of the cable conductor before a short circuit occurs,
alpha is the temperature coefficient of resistance of the cable conductor at 20 ℃,
p is the resistivity of the cable conductor at 20 ℃,
k is the ratio of the AC resistance to the DC resistance of the cable core conductor,
s is the effective cross section of the metal protective layer,
eta is a correction coefficient for accounting for the influence of the heat capacity of the filler containing the cable conductor;
step 5.3, simultaneous formulas (1) to (3) are used for checking the temperature rise of the metal protective layer of the two-end direct grounding phase under the short-circuit fault;
step 5.4, comparing the calculation result of the temperature rise of the protective layer with the bearable temperature of the outer protective layer of the cable, and if the temperature rise of the metal protective layer of the double-end direct grounding phase is less than the bearable temperature of the outer protective layer, adopting the grounding mode; otherwise, after the docking mode is adjusted, repeating the steps 5.1 to 5.3.
Preferably, in step 5.2, the tuning process for the short circuit duration includes:
step 5.2.1, collecting each section of protection action time limit of zero sequence impedance protection and each section of protection action time limit of zero sequence overcurrent protection in the main protection of the ground fault;
and 5.2.2, taking the time between the four-section zero-sequence overcurrent protection actions and the three-section zero-sequence impedance protection actions as the short circuit duration.
A high voltage power cable single phase sheath double ended direct grounding system, said system comprising: one end of each three-phase single-core cable is grounded through the first grounding device, and the other end of each three-phase single-core cable is grounded through the second grounding device;
one end of the three-phase single-core cable realizes direct grounding of three phases through a first grounding device;
the other end of the three-phase single-core cable is realized through a second grounding device, and a sheath of one-phase single-core cable is directly grounded; the other two single-core cable sheaths are connected with the protective device and then grounded.
Compared with the prior art, the invention has the beneficial effects that: the grounding mode provided by the invention reduces the induced voltage of the cable sheath under the fault working condition, reduces the use of a return line and a protector, and reduces the engineering construction cost and the arrangement time of the return line.
Drawings
Fig. 1 is an equivalent circuit diagram in the grounding mode in the prior art 1;
FIG. 2 is a block diagram illustrating the steps of a single-phase sheath double-end direct grounding method for high-voltage power cable according to the present invention;
FIG. 3 is a schematic diagram of a single-phase sheath double-ended direct grounding system for a high-voltage power cable according to an embodiment of the present invention;
the reference numerals in fig. 3 are explained as follows:
1A-phase A of the high-voltage power cable; 1B-phase of the high-voltage power cable; 1C-high voltage power cable C phase;
2-a protection device at the other end of the phase A of the high-voltage power cable; 3-protection device for the other end of C phase of high-voltage power cable.
Detailed Description
The present application is further described below with reference to the accompanying drawings. The following examples are only for illustrating the technical solutions of the present invention more clearly, and the protection scope of the present application is not limited thereby.
A high-voltage power cable single-phase sheath double-end direct grounding method, wherein the high-voltage power cable is a three-phase single-core cable, and both ends of one phase single-core cable sheath are directly grounded; one end of the other two single-core cable protective layers is directly grounded, and the other end is grounded after being connected with the protection device;
when the asymmetric ground fault occurs in the cable line, when one end of a fault phase single-core cable sheath is directly grounded and the other end of the fault phase single-core cable sheath is connected with a protection device, one part of the ground fault current takes the ground as a loop, the other part takes the single-core cable sheath with both ends directly grounded as a loop, and the induced voltage in each phase single-core cable sheath is not more than the insulation tolerance strength of a cable metal sheath;
when the asymmetric earth fault occurs in the cable line, when the two ends of the fault phase single-core cable sheath are directly grounded, the fault current takes the ground as a loop, and the induced voltage in each phase single-core cable sheath is not more than the insulation tolerance strength of the cable metal sheath.
According to the specification of Q/GDW 11316-2014 Power Cable line test regulations, the cable outer sheath can bear 10kV of direct-current voltage without breakdown, namely the insulation resistance of the cable metal sheath is 10 kV.
Referring to fig. 2, a method for directly grounding two ends of a single-phase sheath of a high-voltage power cable includes steps 1 to 6.
Step 1, collecting engineering parameters of a high-voltage power cable.
Specifically, in step 1, the engineering parameters include: voltage class, cable model, cable length, grounding resistance, metal sheath radius, metal sheath thickness, cable spacing, and protection action time limit.
Specifically, in the step 2, in the single-end direct grounding model of the sheath, one end of the single-core cable sheath is directly grounded, and the other end is grounded after being connected with the protection device;
in the sheath double-end direct grounding model, both ends of the single-core cable sheath are directly grounded;
the high-voltage power cable sheath grounding model constructed by utilizing the phase sheath grounding models comprises the following steps: one phase adopts a sheath double-end direct grounding model, and the other two phases adopt a sheath single-end direct grounding model.
Specifically, in the high-voltage power cable sheath grounding model, when a three-phase single-core cable adopts a parallel laying mode, a sheath double-end direct grounding model is adopted for an intermediate phase.
Specifically, step 3 includes:
step 3.1, under the normal working condition of the high-voltage power cable, the sheath current does not exist in the sheath single-end direct grounding model, and the sheath current in the sheath double-end direct grounding model meets the following relational expression:
in the formula (I), the compound is shown in the specification,
Isheathfor sheath current in the two-terminal direct ground model,
Rg1the resistance of the first grounding resistor is determined by the soil resistivity of the grounding point of the first grounding terminal of the passivation layer,
Rg2the resistance of the second grounding resistor is determined by the soil resistivity of the grounding point of the second grounding terminal of the sheath,
Rpthe resistance of the sheath resistor is determined by the conductivity of the sheath material, the sectional area of the sheath and the length of the cable;
wherein, the grounding point that each ground terminal is located includes: in a transformer substation, at a terminal tower, in a working well along a cable;
in that
Is limited to 50V, and is groundedEnd in transformer substation or terminal tower, then Rg1Less than 1 omega, the second grounding end is arranged in the work well along the cable, the environment is poor, and R isg2Within 4 omega, and estimating the sheath current IsheathShould be around 10A.
Step 3.2, the energy loss limit value in the sheath caused by the sheath current meets the following relational expression:
in the formula (I), the compound is shown in the specification,
is the limit of energy loss in the sheath due to sheath current.
And 4, when any phase in the high-voltage power cable has a ground fault, respectively carrying out simulation calculation on the induction voltage of the sheath grounding model of each phase.
Specifically, step 4 includes:
step 4.1, when the protective layer of the fault phase adopts single-end direct grounding, the protective layer of one non-fault phase adopts single-end direct grounding, and the protective layer of the other non-fault phase adopts double-end direct grounding, one part of the fault current takes the earth as a loop, the other part takes the single-core cable protective layer with both ends directly grounded as a loop, and the induced voltage on the protective layer of the fault phase
Satisfies the following relation:
in the formula (I), the compound is shown in the specification,
Idin order to achieve a single-phase earth fault current,
ZAAthe fault phase cable sheath and the fault phase cable core use the ground as the mutual impedance when the fault current loop is formed,
ZPAfor fault phase cable sheath and twoThe single core cable sheath with both ends directly grounded uses the earth as the mutual impedance when the loop is formed,
R1is a power end ground resistor,
R2as a result of the transition resistance at the fault,
Rgis the earth resistance, which is determined by the soil resistivity;
step 4.2, when the two ends of the protective layer of the fault phase are directly grounded, the two protective layers of the non-fault phases are directly grounded by the single end, and the fault current takes the earth as a loop, the induced voltage on the protective layer of the fault phase is
Satisfies the following relation:
in the formula (I), the compound is shown in the specification,
ZPPis the self-impedance of the sheath with both ends directly grounded.
When the double-end grounding phase has a fault, the reduction effect of the grounding mode on the induced voltage of the sheath layer of the fault phase is superior to the reduction effect when the return line is arranged in a 'three-seven-opening' mode; when the non-double-end grounding phase has a fault, the reduction effect of the grounding mode on the induced voltage of the sheath of the fault phase is inferior to the reduction effect when the return line is arranged in a 'three-seven-opening' mode; for a parallel cable loop, the induced voltage is calculated by double-end grounding in the middle phase.
Specifically, in the step 4,
ZAAsatisfies the following relation:
ZPAthe following relation is satisfied:
ZPPsatisfies the following relation:
in the above-mentioned formula, the compound of formula,
l is the cable length corresponding to the distance from the fault point to the grounding point,
d is the current penetration depth in the ground,
DAthe distance between the single-core cable sheath with both ends directly grounded and the fault phase sheath,
rsthe radius of the metal sheath of the cable,
Rpis the sheath resistance.
In the preferred embodiment of the 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 m. For the fault condition, a single-phase earth fault with the transition resistance of 5 omega is arranged on the A phase at 400 meters of the cable line, the fault triggering time is 1s, and the duration is 0.5 s. Under the mode that one end of three phases is directly grounded and the other end of the three phases is protected and grounded, the effective value of the induced voltage of the protective layer after the A phase has a ground fault is 4.05 kV; in a single-phase double-end grounding system, the effective value of the sheath induction voltage after the A phase has a grounding fault is 2.65kV, the induction voltage is reduced by 35 percent, and the fault induction voltage can be obviously reduced.
Step 5, when any phase in the high-voltage power cable has a ground fault, respectively carrying out thermal stability verification on each phase protective layer ground model by combining protection action time limit; and determining the grounding mode of the high-voltage power cable sheath according to the thermal stability verification result.
According to the IEEE Std 635-2003 standard, the metal passivation layer must be able to withstand the magnitude and duration of the short-circuit current, and ensure that the inner buffer layer and the outer passivation layer of the metal passivation layer are not damaged. In addition, since a fault current flows in the double-ended grounded phase aluminum sheath, it is necessary to perform a thermal stability verification on the phase aluminum sheath. In the preferred embodiment of the invention, thermal stability verification is carried out according to the design Specification of Power engineering Cable GB/T50217:
specifically, step 5 comprises:
step 5.1, according to the short circuit duration t determined by the main protection time limit of the ground fault, designing and considering a system power supply short circuit overcurrent I, and calculating the calorific value Q of the double-end direct grounding phase protective layer according to the following relational expression:
Q＝I2·t……(1)
step 5.2, calculating a thermal stability check coefficient C according to the cable model and the cable specification, and satisfying the following relational expression:
in the formula (I), the compound is shown in the specification,
j is a thermal equivalent coefficient, and the desirable value is that J is 1;
q is the unit volume heat capacity of the cable conductor, and the aluminum core cable is 2.48;
θmthe maximum temperature allowed by the cable conductor in the short circuit action time;
θpthe maximum working temperature of the cable conductor before short circuit occurs;
alpha is the temperature coefficient of resistance of the cable conductor at 20 ℃, and the aluminum core cable is 0.00403;
rho is the resistivity of the cable conductor at 20 ℃, and the aluminum core cable is 0.031 multiplied by 10-4；
k is the ratio of the alternating current resistance to the direct current resistance of the cable core conductor, and the value is 1.010;
s is the effective cross section of the metal protective layer,
eta is a correction coefficient for accounting the influence of the heat capacity of the filler of the cable conductor, and the value of eta is 1 for a 110kV circuit;
step 5.3, performing joint vertical type (1) to (3) to check the temperature rise of the metal protective layer of the two-end direct grounding phase under the short-circuit fault;
step 5.4, comparing the calculation result of the temperature rise of the protective layer with the bearable temperature of the outer protective layer of the cable, and if the temperature rise of the metal protective layer of the double-end direct grounding phase is less than the bearable temperature of the outer protective layer, adopting the grounding mode; otherwise, after the docking mode is adjusted, repeating the steps 5.1 to 5.3.
Preferably, in step 5.2, the tuning process for the short circuit duration includes:
step 5.2.1, collecting each section of protection action time limit of zero sequence impedance protection and each section of protection action time limit of zero sequence overcurrent protection in the main protection of the ground fault;
and 5.2.2, taking the time between the four-section zero-sequence overcurrent protection actions and the three-section zero-sequence impedance protection actions as the short circuit duration.
In the preferred embodiment of the invention, the main protection of the 110kV line grounding fault is considered to be zero-sequence impedance protection and zero-sequence overcurrent protection: the backup three-section protection action time limit of zero sequence impedance protection is 2 s; the action time limit of the backup four sections of the zero sequence overcurrent protection is 0.9 s. Considering that if the four-segment zero-sequence overcurrent protection does not act in 0.9s, but the three-segment zero-sequence impedance protection acts in 2.0s, the fault transition resistance is higher, the short-circuit overcurrent is smaller, and the aluminum protective layer can bear the heating caused by the fault overcurrent certainly, so that when the thermal stability verification is carried out, the fault duration is set to be within the action time limit of the four-segment zero-sequence overcurrent protection, namely: the short-circuit overcurrent effective value is set to be 40kA, and the duration is 1.0 s: then equation (1) satisfies the following relationship:
Q＝I2·t＝1.6×109J……(7)
at a certain 110kV 700mm2For example, the inner diameter of the aluminum sheath is 42.7mm, the outer diameter is 44.9mm, and the cross-sectional area of the aluminum sheath is 605mm2. The combination of formula (1) and formula (3) shows that the temperature of the aluminum sheath can be increased from 40 ℃ to 106 ℃ in the failure duration of 1s, and the polyethylene sheath on the outer layer of the aluminum sheath has the softening temperature of 125-135 ℃, so that the aluminum sheath cannot be damaged in the failure duration of 1 s.
Referring to fig. 3, a single-phase sheath two-end direct grounding system for 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 comprises: one end of each three-phase single-core cable is grounded through the first grounding device, and the other end of each three-phase single-core cable is grounded through the second grounding device;
one end of the three-phase single-core cable realizes direct grounding of three phases through the first grounding device.
The other end of the three-phase single-core cable is realized through a second grounding device, wherein a sheath of one-phase single-core cable is directly grounded; the other two single-core cable sheaths are connected with the protective device and then grounded. The second grounding device includes: the protection device 2 at the other end of the phase A of the high-voltage power cable and the protection device 3 at the other end of the phase C of the high-voltage power cable.
In the preferred embodiment of the invention, two ends of the intermediate phase are directly grounded, and the other two phases are still grounded in a mode of directly grounding one end and protecting the other end, namely, the metal protective layer of the intermediate phase is used as the return line, so that the equipment cost of the return line and the protector is saved, the risk of stealing the return line is avoided, and a cable with the length of about 600 meters is provided, if the cable with the length of 240mm is provided2The copper core return wire has the engineering budget of 10-20 ten thousand yuan approximately, so the grounding mode provided by the invention has more obvious economic advantages.
The present applicant has described and illustrated embodiments of the present invention in detail with reference to the accompanying drawings, but it should be understood by those skilled in the art that the above embodiments are merely preferred embodiments of the present invention, and the detailed description is only for the purpose of helping the reader to better understand the spirit of the present invention, and not for limiting the scope of the present invention, and on the contrary, any improvement or modification made based on the spirit of the present invention should fall within the scope of the present invention.
Claims (11)
1. A method for directly grounding the two ends of the single-phase sheath of a high-voltage power cable, wherein the high-voltage power cable is a three-phase single-core cable, is characterized in that,
directly grounding both ends of the sheath of one phase of single-core cable; one end of the other two single-core cable protective layers is directly grounded, and the other end is grounded after being connected with the protection device;
when the asymmetric ground fault occurs in the cable line, when one end of a fault phase single-core cable sheath is directly grounded and the other end of the fault phase single-core cable sheath is connected with a protection device, one part of the ground fault current takes the ground as a loop, the other part takes the single-core cable sheath with both ends directly grounded as a loop, and the induced voltage in each phase single-core cable sheath is not more than the insulation tolerance strength of a cable metal sheath;
when the asymmetric earth fault occurs in the cable line, when the two ends of the fault phase single-core cable sheath are directly grounded, the fault current takes the ground as a loop, and the induced voltage in each phase single-core cable sheath is not more than the insulation tolerance strength of the cable metal sheath.
2. The method of claim 1, wherein the method comprises grounding the single-phase sheath of the high-voltage power cable,
the method comprises the following steps:
step 1, collecting engineering parameters of a high-voltage power cable;
step 2, establishing each phase of sheath grounding model of the high-voltage power cable respectively according to the engineering parameters, wherein the sheath grounding model comprises the following steps: the single-end direct grounding model of the sheath and the double-end direct grounding model of the sheath; wherein, each protective layer grounding model is not provided with an independent return wire;
step 3, respectively carrying out simulation calculation on the sheath current in each phase sheath grounding model under the normal working condition of the high-voltage power cable; calculating the energy loss limit value in the sheath by using the sheath current and the sheath induction voltage limit value;
step 4, when any phase of the high-voltage power cable has a ground fault, respectively carrying out simulation calculation on the induction voltage of each phase of the sheath grounding model;
step 5, when any phase in the high-voltage power cable has a ground fault, respectively carrying out thermal stability verification on each phase protective layer ground model by combining protection action time limit; and determining the grounding mode of the high-voltage power cable sheath according to the thermal stability verification result.
3. The method of claim 2, wherein the single-phase sheath of the high-voltage power cable is directly grounded at both ends,
in step 1, the engineering parameters include: voltage class, cable model, cable length, grounding resistance, metal sheath radius, metal sheath thickness, cable spacing, and protection action time limit.
4. The method of claim 2, wherein the single-phase sheath of the high-voltage power cable is directly grounded at both ends,
in the step 2, in the single-end direct grounding model of the sheath, one end of the single-core cable sheath is directly grounded, and the other end of the single-core cable sheath is grounded after being connected with the protection device;
in the sheath double-end direct grounding model, both ends of the single-core cable sheath are directly grounded;
the high-voltage power cable sheath grounding model constructed by utilizing the phase sheath grounding models comprises the following steps: one phase adopts a sheath double-end direct grounding model, and the other two phases adopt a sheath single-end direct grounding model.
5. The method of claim 4, wherein the single-phase sheath of the high-voltage power cable is directly grounded at both ends,
in the high-voltage power cable sheath grounding model, when a three-phase single-core cable adopts a parallel laying mode, a sheath double-end direct grounding model is adopted in an intermediate phase.
6. The method of claim 2, wherein the single-phase sheath of the high-voltage power cable is directly grounded at both ends,
the step 3 comprises the following steps:
step 3.1, under the normal working condition of the high-voltage power cable, the sheath current does not exist in the sheath single-end direct grounding model, and the sheath current in the sheath double-end direct grounding model meets the following relational expression:
in the formula (I), the compound is shown in the specification,
Isheathfor sheath currents in the two-terminal direct ground model,
Rg1the resistance of the first grounding resistor is determined by the soil resistivity of the grounding point of the first grounding terminal of the passivation layer,
Rg2the resistance of the second grounding resistor is determined by the soil resistivity of the grounding point of the second grounding terminal of the sheath,
Rpthe resistance of the sheath resistor is determined by the conductivity of the sheath material, the sectional area of the sheath and the length of the cable;
wherein, the grounding point that each ground terminal is located includes: in a transformer substation, at a terminal tower, in a working well along a cable;
step 3.2, the energy loss limit value in the sheath caused by the sheath current meets the following relational expression:
in the formula (I), the compound is shown in the specification,
is the limit of energy loss in the sheath due to sheath current.
7. The method of claim 6, wherein the single-phase sheath of the high-voltage power cable is directly grounded at both ends,
step 4 comprises the following steps:
step 4.1, when the protective layer of the fault phase adopts single-end direct grounding, the protective layer of one non-fault phase adopts single-end direct grounding, and the protective layer of the other non-fault phase adopts single-end direct groundingWhen the two ends of the sheath are directly grounded, one part of the fault current takes the earth as a return circuit, and the other part takes the single-core cable sheath with the two ends directly grounded as a return circuit, so that the induced voltage on the fault phase sheath
Satisfies the following relation:
in the formula (I), the compound is shown in the specification,
Idin order to achieve a single-phase earth fault current,
ZAAthe fault phase cable sheath and the fault phase cable core use the ground as the mutual impedance when the fault current loop is formed,
ZPAthe mutual impedance is formed when the fault phase cable sheath and the single core cable sheath with both ends directly grounded use the ground as a loop,
R1is a power end ground resistor,
R2as a result of the transition resistance at the fault,
Rgis the earth resistance, which is determined by the soil resistivity;
step 4.2, when the two ends of the protective layer of the fault phase are directly grounded, the two protective layers of the non-fault phases are directly grounded by the single end, and the fault current takes the earth as a loop, the induced voltage on the protective layer of the fault phase is
The following relation is satisfied:
in the formula (I), the compound is shown in the specification,
ZPPis the self-impedance of the sheath with both ends directly grounded.
8. The method of claim 7, wherein the single-phase sheath of the high-voltage power cable is directly grounded at both ends,
in the step 4, the process of the method,
ZAAsatisfies the following relation:
ZPAthe following relation is satisfied:
ZPPsatisfies the following relation:
in the above-mentioned formula, the compound of formula,
l is the cable length corresponding to the distance from the fault point to the grounding point,
d is the current penetration depth in the ground,
DAthe distance between the single-core cable sheath with both ends directly grounded and the fault phase sheath,
rsthe radius of the metal sheath of the cable,
Rpis the sheath resistance.
9. The method of claim 7, wherein the single-phase sheath of the high-voltage power cable is directly grounded at both ends,
the step 5 comprises the following steps:
step 5.1, according to the short circuit duration t determined by the main protection time limit of the ground fault, designing and considering a system power supply short circuit overcurrent I, and calculating the calorific value Q of the double-end direct grounding phase protective layer according to the following relational expression:
Q＝I2·t......(1)
step 5.2, calculating a thermal stability check coefficient C according to the cable model and the cable specification, and satisfying the following relational expression:
in the formula (I), the compound is shown in the specification,
j is a thermal-power equivalent coefficient,
q is the heat capacity per unit volume of the cable conductor,
θmthe maximum temperature allowed for the cable conductor during the short-circuit action time,
θpfor the maximum operating temperature of the cable conductor before a short circuit occurs,
alpha is the temperature coefficient of resistance of the cable conductor at 20 ℃,
p is the resistivity of the cable conductor at 20 ℃,
k is the ratio of the AC resistance to the DC resistance of the cable core conductor,
s is the effective cross section of the metal protective layer,
eta is a correction coefficient for accounting for the influence of the heat capacity of the filler containing the cable conductor;
step 5.3, performing joint vertical type (1) to (3) to check the temperature rise of the metal protective layer of the two-end direct grounding phase under the short-circuit fault;
step 5.4, comparing the calculation result of the temperature rise of the protective layer with the bearable temperature of the outer protective layer of the cable, and if the temperature rise of the metal protective layer of the double-end direct grounding phase is less than the bearable temperature of the outer protective layer, adopting the grounding mode; otherwise, after the docking mode is adjusted, repeating the steps 5.1 to 5.3.
10. The method of claim 9, wherein the method comprises grounding the single-phase sheath of the high-voltage power cable,
in step 5.2, the setting process for the short circuit duration comprises the following steps:
step 5.2.1, collecting each section of protection action time limit of zero sequence impedance protection and each section of protection action time limit of zero sequence overcurrent protection in the main protection of the ground fault;
and 5.2.2, taking the time between the four-section zero-sequence overcurrent protection actions and the three-section zero-sequence impedance protection actions as the short circuit duration.
11. A system for direct double-ended grounding of a single-phase sheath of a high voltage power cable, implemented using the method of direct double-ended grounding of a single-phase sheath of a high voltage power cable according to any one of claims 1 to 10, said system comprising: a first grounding device and a second grounding device, wherein one end of each three-phase single-core cable is grounded through the first grounding device, and the other end of each three-phase single-core cable is grounded through the second grounding device,
one end of the three-phase single-core cable realizes direct grounding of three phases through a first grounding device;
the other end of the three-phase single-core cable is realized through a second grounding device, and a sheath of one-phase single-core cable is directly grounded; the other two single-core cable sheaths are connected with the protective device and then grounded.
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