CN102361322A - Technology determination method for changing crosslinked polyethylene cable alternating current line into direct current operation - Google Patents

Abstract

The invention provides a technology determination method for changing crosslinked polyethylene cable alternating current line into direct current operation. The method comprises the following steps: designing a crosslinked polyethylene cable alternating current operation scheme, calculating long-term allowed rated current-carrying capacity of a crosslinked polyethylene cable and a temperature difference between two insulated ends of the crosslinked polyethylene cable in full load operation, testing a characteristic parameter of the crosslinked polyethylene cable under direct current voltage, calculating long-term allowed direct current work voltage of the crosslinked polyethylene cable, calculating transmission capacity of a crosslinked polyethylene cable line and determining whether the crosslinked polyethylene cable satisfies a requirement of direct current operation, and checking a grounding mode of a direct current operation line of the crosslinked polyethylene cable. According to the method in the invention, present cable insulation can bear various voltage of operation under designed direct current voltage, an electrical property of the cable insulation satisfies a requirement, simultaneously present resource can be fully utilized, and long-term reliable economic transmission of electric energy is realized.

Description

The twisted polyethylene cable alternating current circuit changes the technology determination method of DC operation into

Technical field

The present invention relates to cable line running technology field, be specifically related to the technology determination method that a kind of twisted polyethylene cable alternating current circuit changes DC operation into.

Background technology

Direct current transportation has many advantages than ac transmission: 1) transfer efficiency is high, and line loss is little.2) regulate electric current and change the power direction of transfer conveniently.3) DC line is few with line, and investment cost is low, and long distance powedr transmission is saved money than exchanging.4) length of direct current cables does not receive the restriction of capacitance current, to crossing straits, transmitting electricity comparatively favourable to the island.5) DC transmission line can be used as the tie of two kinds of electrical networks, even the frequency of two kinds of electrical networks can not use simultaneously yet, and it can also reduce the short circuit current between backbone and the electrical network.6) the corona radio interference of DC line is little.

In recent years, along with related key technical problem is able to solve, direct current transportation obtains to greatly develop in global electric power system, and the quantity that puts into operation of direct current cables circuit constantly increases.The domestic extreme high voltage grade direct current overhead transmission line that built up and put into operation does not still have the operating experience of direct current cables circuit.But some long apart from electric energy transmitting, cross river, application scenario such as over strait, the actual demand of existing direct current cables circuit is put on agenda.Under some particular cases, damage mutually like one in the three-phase alternating current cable road, or when hoping that existing ac cable system carried out increase-volume, run unit also moves considering active service ac cable circuit changed under direct voltage.

According to the big electrical network in world research committee (CIGRE) work report " Upgrading and Uprating of Existing Cable Systems " in 2007; Transmission cable system to an active service; Its working voltage is become direct current with electric current from exchanging, and is a kind of possibility approach that promotes its performance.

Generally, the transmission capacity of power cable is compared exchanging meeting and is increased to some extent when adopting DC operation.Main cause is: 1) do not have induction loss in the direct current cables, therefore than interchange, allow specified transmission current higher.2) direct current cables does not have the capacitive charging current, and transmission range can endless in theory.3) the average field intensity permissible value of direct current cables system is usually than the height of ac cable system.

Become the whether feasible many aspects that depend on of DC operation from ac operation, but need consider following 3 at least: 1) whether active service cable system and annex are fit to DC operation; 2) whether the meritorious energy of transmission increases to some extent; 3) whether beneficially consider with regard to systematic function and environment aspect.

So far, the correlation experience that the ac cable system moves under direct voltage is very limited, and will be operation crosslinked polyethylene (XLPE) ac cable system after a while changes the direct voltage operation under alternating voltage pertinent literature report almost do not have.In general, cable system is if move its insulation special design of process in advance under direct voltage.Because cable is under the direct voltage effect, significantly different with insulation characterisitic under the alternating voltage effect, and ac cable system commonly used is not according to bearing direct voltage design usually, especially true to the polymer insulation cable system.In fact; Because the problems with space charge that exists in the cross-linking polyethylene materials; In very long a period of time; It is considered to be used for the direct current cables insulation, and present direct current cross-linked poly-ethylene cable material has all carried out modification through the special means such as additive of mixing, to solve the problem that influences of space charge.

Having recommended grade of rated voltage in CIGRE 2003 work report " Recommendations for testing DC extruded cable systems for power transmission at a rated voltage up to 250kV " is that 250kV and following electric power transfer direct current are extruded the test method that cable system carries out performance test; Can use for reference and be used for direct current cables is carried out the operating voltage design; But what it was directed against is the cable line that designs for DC operation specially, and is used for whether the examination cable line can meet the demands under the design and operation voltage of confirming; Document " Development of 500kV DC PPLP-insulated oil-filled submarine cable " can provide more insulating Design experience; But its research object is polypropylene layer platen compound inslation (PPLP) oil-filled cable, and description is the insulation thickness design process under the expection operating voltage; IEC 60287 is universally acknowledged standards that industry is confirmed the long-term rated current-carrying capacity of cable line, but it is primarily aimed at the ac cable circuit, direct current cables is had only rough a few line description contents, and the scope of application is below the direct current 5kV.

At present, though there is active service to exchange the successful case that the oil-filled cable circuit changes DC operation in the world, the theory or the technical scheme that also have no the interchange twisted polyethylene cable circuit of publishing to change DC operation into can supply to use for reference and reference.If without the technology determination of strictness, the long-time running voltage, load current, the transmission capacity that then exchange after the XLPE cable circuit changes DC operation into all can't accurately be confirmed.If working voltage, current value are too high, or the system earth mode can not satisfy the requirement of operation under the direct voltage, but will cause the cable for a long time of safe operation originally to lose efficacy in a short time, causes unexpected power outage; And if voltage, current value are low excessively, can cause the utilance of cable low on the one hand, the assets waste, not only can not promote link performance on the other hand; Even possibly not reach original transmission capacity requirement; Therefore, change the feasibility of DC operation into, need carry out sufficient theory analysis and technology determination for crosslinked polyethylene ac cable circuit; And the design of the operational factor under direct voltage also needs the test of system to check as the basis.

Summary of the invention

In order to overcome the shortcoming that above-mentioned prior art exists; The object of the present invention is to provide a kind of twisted polyethylene cable alternating current circuit to change the technology determination method of DC operation into; According to technology determination method proposed by the invention; Change twisted polyethylene cable into DC operation from the alternating current circuit, the existing cable insulation can be born the effect of long-time running voltage, various superimposed voltage and surge voltage when under the design direct voltage, moving, and the electrical property of cable insulation meets the demands; Simultaneously also can make full use of existing resource, realize long-term, reliable, the economic transmission of electric energy.

To achieve these goals, the present invention adopts following technical scheme:

A kind of twisted polyethylene cable alternating current circuit changes the technology determination method of operation under direct voltage into, comprises the steps:

Step 1: twisted polyethylene cable DC operation conceptual design,

Allow working temperature θ for a long time according to twisted polyethylene cable _cAnd the twisted polyethylene cable structure, to lay ruuning situation and transmission capacity requirement design DC operation mode be one pole or bipolar operation, selects with other conductor, the outer metal level of cable self or be greatly the path of current reflux;

Step 2: the temperature difference at crosslinking polyethylene-insulated cable insulation two ends when the calculating twisted polyethylene cable allows rated current-carrying capacity and oepration at full load for a long time,

Calculate twisted polyethylene cable and allow rated current-carrying capacity for a long time; Based on twisted polyethylene cable structure and corresponding parameters, the expection method of operation and lay environment and confirm its long-term current-carrying capacity that allows; When DC voltage moves down; If the twisted polyethylene cable circuit adopts the bipolar method of operation, then the specified permission current-carrying capacity of cable calculates with formula (1); If cable run is one pole operation, and with the earth or other conductor as current return circuit, then the specified permission current-carrying capacity of cable calculates with formula (1); If cable run is one pole operation, and with self outer metal level be protective metal shell and armor as the backflow conductor, then the specified permission current-carrying capacity of cable is with formula (2) calculating,

$I=\sqrt{\frac{{\mathrm{\θ}}_c-{\mathrm{\θ}}_0}{R^{\′}(T_1+T_2+T_3+T_4)}}---\left(1\right)$

$I=\sqrt{\frac{{\mathrm{\θ}}_c-{\mathrm{\θ}}_0}{R^{\′}[T_1+(1+{\mathrm{\λ}}_1)T_2+(1+{\mathrm{\λ}}_1+{\mathrm{\λ}}_2)(T_3+T_4)}}---\left(2\right)$

Wherein, allow the maximum current that flows through in I---cable one phase conductor, unit is A

θ _c---cable allows long-term nominal operation temperature, and unit is ℃

θ ₀---the temperature of medium on every side, unit is ℃

T ₁---unit length cable insulation thermal resistance, unit is T Ω m

T ₂---unit length cable inner covering thermal resistance, unit is T Ω m

T ₃---unit length cable serving thermal resistance, unit is T Ω m

T ₄---the thermal resistance of medium on every side, unit is T Ω m

R '---the D.C. resistance of maximum operating temperature lower conductor unit length, unit are Ω/m

λ ₁---the ratio of metallic sheath loss and core loss

λ ₂---the ratio of metal armouring loss and core loss

The temperature difference at crosslinking polyethylene-insulated cable insulation two ends when calculating oepration at full load is according to twisted polyethylene cable structure and corresponding parameter, allow working temperature θ for a long time _c, the long-term permission ampacity I that is calculated calculates the temperature difference at crosslinking polyethylene-insulated cable insulation two ends when the oepration at full load with formula (3),

$\mathrm{\Δ\θ}=I^2{R}^{\′}\×\frac{{\mathrm{\ρ}}_T}{2\mathrm{\π}}\ln \frac{r_i}{r_c}---\left(3\right)$

Wherein, Δ θ---the insulation temperature difference during oepration at full load, unit is ℃

Allow the maximum current that flows through in I---cable one phase conductor, unit is A

R '---the D.C. resistance of maximum operating temperature lower conductor unit length, unit are Ω/m

ρ _T---cable XLPE insulation thermal resistance coefficient, unit are T Ω m

r _i---the insulation outer radius, unit is mm

r _c---the conductor outer radius, unit is mm

Step 3: the characterisitic parameter test of twisted polyethylene cable under direct voltage, the DC characteristic parameter of test comprise Ahmedabad that COEFFICIENT K, lightning impulse design field intensity E _PuAnd the ground connection electric branch causes time t _In,

Ahmedabad you the method for testing of COEFFICIENT K are: at first, twisted polyethylene cable is taken a sample, be not less than θ at the conductor temperature that guarantees twisted polyethylene cable then _cAnd the insulation temperature difference is not less than under the situation of Δ θ; Cable applies standard switching impulse voltage and standard lightning impulse voltage carries out breakdown test to taking a sample; The preparatory making alive of the positive and negative direct current of stack continues the sampling cable is carried out breakdown test on standard switching impulse voltage and standard lightning impulse voltage subsequently; The impact flashover voltage of impact flashover voltage when obtaining not have the direct voltage stack respectively with this when the direct voltage stack is arranged, you COEFFICIENT K calculate the Ahmedabad of twisted polyethylene cable with formula (4)

K＝(V _i-V _r)/V _dc (4)

Wherein, V _i---the impact flashover voltage when no direct voltage superposes, unit is V

V _r---the impact flashover voltage when the direct voltage stack is arranged, unit is V

V _Dc---the preparatory making alive of positive and negative direct current, unit are V

Get maximum in each calculated value of gained as your COEFFICIENT K of Ahmedabad;

Lightning impulse design field intensity E _PuMethod of testing be: at first, twisted polyethylene cable is taken a sample, is guaranteeing that conductor temperature is not less than θ then _cAnd the insulation temperature difference is not less than under the situation of Δ θ, and the standard lightning impulse voltage that the sampling cable is applied positive-negative polarity respectively carries out breakdown test, obtains the positive-negative polarity lightning impulse disruptive field intensity of crosslinking polyethylene-insulated cable insulation, get among both than low value E _Min, and introduce coefficient of safety 1.2, confirm that the lightning impulse design field intensity of crosslinking polyethylene-insulated cable insulation is E _Pu=E _Min/ 1.2;

The method of testing that the ground connection electric branch causes the time is: at first; Twisted polyethylene cable is taken a sample; Utilize pin-plate electrode to carry out electric branch to the crosslinking polyethylene-insulated cable insulation sample then and cause test; On stylus, apply the maximum short-circuit voltage of cross-linking polythene cable system, obtain 1000 ground connection and cause the pairing landing operation time of electric branch, be crosslinked polyetylene insulated ground connection electric branch under the maximum short-circuit voltage of system and cause time t _In

Step 4: calculate twisted polyethylene cable and allow direct-current working volts for a long time,

Calculate twisted polyethylene cable and allow direct-current working volts for a long time, according to the crosslinking polyethylene-insulated cable insulation thickness d, and Ahmedabad that COEFFICIENT K and lightning impulse that step 3 tests and calculates design field intensity E _Pu, calculate twisted polyethylene cable and allow direct-current working volts maximum U for a long time ₀, computing formula is formula (5);

$U_0=\frac{E_{\mathrm{pu}}\mathrm{gd}}{1.1K+1.9}---\left(5\right)$

Step 5: calculate the transmission capacity of twisted polyethylene cable circuit, and judge whether transmission capacity meets the demands,

Calculate the transmission capacity of twisted polyethylene cable circuit; Based on the definite DC operation scheme of step 1; The twisted polyethylene cable that step 2 is calculated allows rated current-carrying capacity for a long time, and the twisted polyethylene cable that step 4 is calculated allows direct-current working volts, calculating cable run transmission capacity for a long time; If cable run is the one pole operation, then transmission capacity is calculated with formula (6); If cable run is bipolar operation, then transmission capacity is calculated with formula (7),

P _DC＝U ₀I (6)

P _DC＝2U ₀I (7)

Wherein, P _DC---the transmission capacity of twisted polyethylene cable circuit.

Judge whether the twisted polyethylene cable transmission capacity satisfies the requirement of DC operation,, then change step 1 over to and design again, satisfy the requirement of DC operation until the twisted polyethylene cable transmission capacity if do not satisfy;

Step 6: twisted polyethylene cable DC operation line-to-ground mode is checked,

The check standard is: the discharge time constant τ of twisted polyethylene cable DC operation line-to-ground system is greater than time limit value t _Min, and said time limit value t _MinFor the ground connection electric branch of step 3 test causes 5 times of time, i.e. t _Min=5t _In, through type (8) calculates the discharge time constant τ of said twisted polyethylene cable DC operation line-to-ground system again:

τ＝L/R (8)

Wherein: L---the equivalent inductance of grounding system, unit is H

The equivalent resistance of R---grounding system, unit are Ω

If discharge time constant τ and time limit value t _MinThrough the check standard, then the twisted polyethylene cable alternating current circuit changes the technology determination method end of DC operation into; If existing earthing mode is not through checking; Then carrying out twisted polyethylene cable DC operation line-to-ground mode improves; Improve one's methods for the equivalent inductance L that increases grounding system or the equivalent resistance R that reduces grounding system until through checking, the technology determination method that the twisted polyethylene cable alternating current circuit changes DC operation into finishes.

Utilize technology determination method proposed by the invention; Can analyze and judge the feasibility that the twisted polyethylene cable alternating current circuit changes DC operation into; And confirm its under existence conditions or through relevant when transforming the back DC operation long-term permission direct-current working volts, allow ampacity, transmission capacity and these several kinds of critical technical parameters of earthing mode, make cable line under the engineering operation parameter of design, realize for a long time, safely, reliably, economical operation.No matter be performance and transmission capacity lifting to existing twisted polyethylene cable alternating current circuit, or the transformation of faulty line, the present technique decision method all is suitable for.From the several angles such as saving to increase, time and the human cost of the raising of the rational and efficient use of existing resource, circuit operational reliability and power transfer, the present invention has huge social benefit and economic interests undoubtedly.

Description of drawings

Fig. 1 is the implementing procedure figure of technology determination method proposed by the invention.

Fig. 2 changes the sketch map of DC operation into for the embodiment of the present invention alternating current circuit.

Fig. 3 is the transmission plan figure of embodiment of the present invention cable line former alternating current circuit operation.

Fig. 4 changes DC operation technical scheme figure into for the embodiment of the present invention cable line.

Embodiment

Below in conjunction with accompanying drawing and embodiment the present invention is done more detailed explanation.

As shown in Figure 1, a kind of twisted polyethylene cable alternating current circuit changes the technology determination method of operation under direct voltage into, comprises the steps:

Step 1: twisted polyethylene cable DC operation conceptual design,

Allow working temperature θ for a long time according to twisted polyethylene cable _cAnd the twisted polyethylene cable structure, to lay ruuning situation and transmission capacity requirement design DC operation mode be one pole or bipolar operation, selects with other conductor, the outer metal level of cable self or be greatly the path of current reflux;

Step 2: the temperature difference at crosslinking polyethylene-insulated cable insulation two ends when the calculating twisted polyethylene cable allows rated current-carrying capacity and oepration at full load for a long time,

Calculate twisted polyethylene cable and allow rated current-carrying capacity for a long time; Based on twisted polyethylene cable structure and corresponding parameters, the expection method of operation and lay environment and confirm its long-term current-carrying capacity that allows; When DC voltage moves down; If the twisted polyethylene cable circuit adopts the bipolar method of operation, then the specified permission current-carrying capacity of cable calculates with formula (1); If cable run is one pole operation, and with the earth or other conductor as current return circuit, then the specified permission current-carrying capacity of cable calculates with formula (1); If cable run is one pole operation, and with self outer metal level be protective metal shell and armor as the backflow conductor, then the specified permission current-carrying capacity of cable is with formula (2) calculating,

$I=\sqrt{\frac{{\mathrm{\θ}}_c-{\mathrm{\θ}}_0}{R^{\′}(T_1+T_2+T_3+T_4)}}---\left(1\right)$

$I=\sqrt{\frac{{\mathrm{\θ}}_c-{\mathrm{\θ}}_0}{R^{\′}[T_1+(1+{\mathrm{\λ}}_1)T_2+(1+{\mathrm{\λ}}_1+{\mathrm{\λ}}_2)(T_3+T_4)}}---\left(2\right)$

Wherein, allow the maximum current that flows through in I---cable one phase conductor, unit is A

θ _c---cable allows long-term nominal operation temperature, and unit is ℃

θ ₀---the temperature of medium on every side, unit is ℃

T ₁---unit length cable insulation thermal resistance, unit is T Ω m

T ₂---unit length cable inner covering thermal resistance, unit is T Ω m

T ₃---unit length cable serving thermal resistance, unit is T Ω m

T ₄---the thermal resistance of medium on every side, unit is T Ω m

R '---the D.C. resistance of maximum operating temperature lower conductor unit length, unit are Ω/m

λ ₁---the ratio of metallic sheath loss and core loss

λ ₂---the ratio of metal armouring loss and core loss

The temperature difference at crosslinking polyethylene-insulated cable insulation two ends when calculating oepration at full load is according to twisted polyethylene cable structure and corresponding parameter, allow working temperature θ for a long time _c, the long-term permission ampacity I that is calculated calculates the temperature difference at crosslinking polyethylene-insulated cable insulation two ends when the oepration at full load with formula (3),

$\mathrm{\Δ\θ}=I^2{R}^{\′}\×\frac{{\mathrm{\ρ}}_T}{2\mathrm{\π}}\ln \frac{r_i}{r_c}---\left(3\right)$

Wherein, Δ θ---the insulation temperature difference during oepration at full load, unit is ℃

Allow the maximum current that flows through in I---cable one phase conductor, unit is A

R '---the D.C. resistance of maximum operating temperature lower conductor unit length, unit are Ω/m

ρ _T---cable XLPE insulation thermal resistance coefficient, unit are T Ω m

r _i---the insulation outer radius, unit is mm

r _c---the conductor outer radius, unit is mm

Step 3: the characterisitic parameter test of twisted polyethylene cable under direct voltage, the DC characteristic parameter of test comprise Ahmedabad that COEFFICIENT K, lightning impulse design field intensity E _PuAnd the ground connection electric branch causes time t _In,

Ahmedabad you the method for testing of COEFFICIENT K are: at first, twisted polyethylene cable is taken a sample, be not less than θ at the conductor temperature that guarantees twisted polyethylene cable then _cAnd the insulation temperature difference is not less than under the situation of Δ θ; Cable applies standard switching impulse voltage and standard lightning impulse voltage carries out breakdown test to taking a sample; The preparatory making alive of the positive and negative direct current of stack continues the sampling cable is carried out breakdown test on standard switching impulse voltage and standard lightning impulse voltage subsequently; The impact flashover voltage of impact flashover voltage when obtaining not have the direct voltage stack respectively with this when the direct voltage stack is arranged, you COEFFICIENT K calculate the Ahmedabad of twisted polyethylene cable with formula (4)

K＝(V _i-V _r)/V _dc (4)

Wherein, V _i---the impact flashover voltage when no direct voltage superposes, unit is V

V _r---the impact flashover voltage when having voltage DC stacked, unit are V

V _Dc---the preparatory making alive of positive and negative direct current, unit are V

Get maximum in each calculated value of gained as your COEFFICIENT K of Ahmedabad;

Lightning impulse design field intensity E _PuMethod of testing be: at first, twisted polyethylene cable is taken a sample, is guaranteeing that conductor temperature is not less than θ then _cAnd the insulation temperature difference is not less than under the situation of Δ θ, and the standard lightning impulse voltage that the sampling cable is applied positive-negative polarity respectively carries out breakdown test, obtains the positive-negative polarity lightning impulse disruptive field intensity of crosslinking polyethylene-insulated cable insulation, get among both than low value E _Min, and introduce coefficient of safety 1.2, confirm that the lightning impulse design field intensity of crosslinking polyethylene-insulated cable insulation is E _Pu=E _Min/ 1.2;

The method of testing that the ground connection electric branch causes the time is: at first; Twisted polyethylene cable is taken a sample; Utilize pin-plate electrode to carry out electric branch to the crosslinking polyethylene-insulated cable insulation sample then and cause test; On stylus, apply the maximum short-circuit voltage of cross-linking polythene cable system, obtain 1000 ground connection and cause the pairing landing operation time of electric branch, be crosslinked polyetylene insulated ground connection electric branch under the maximum short-circuit voltage of system and cause time t _In

Step 4: calculate twisted polyethylene cable and allow direct-current working volts for a long time,

Calculate twisted polyethylene cable and allow direct-current working volts for a long time, according to the crosslinking polyethylene-insulated cable insulation thickness d, and Ahmedabad that COEFFICIENT K and lightning impulse that step 3 tests and calculates design field intensity E _Pu, calculate twisted polyethylene cable and allow direct-current working volts maximum U for a long time ₀, computing formula is formula (5);

$U_0=\frac{E_{\mathrm{pu}}\mathrm{gd}}{1.1K+1.9}---\left(5\right)$

Step 5: calculate the transmission capacity of twisted polyethylene cable circuit, and judge whether transmission capacity meets the demands,

Calculate the transmission capacity of twisted polyethylene cable circuit; Based on the definite DC operation scheme of step 1; The twisted polyethylene cable that step 2 is calculated allows rated current-carrying capacity for a long time, and the twisted polyethylene cable that step 4 is calculated allows direct-current working volts, calculating cable run transmission capacity for a long time; If cable run is the one pole operation, then transmission capacity is calculated with formula (6); If cable run is bipolar operation, then transmission capacity is calculated with formula (7),

P _DC＝U ₀I (6)

P _DC＝2U ₀I (7)

Wherein, P _DC---the transmission capacity of twisted polyethylene cable circuit.

Judge whether the twisted polyethylene cable transmission capacity satisfies the requirement of DC operation,, then change step 1 over to and design again, satisfy the requirement of DC operation until the twisted polyethylene cable transmission capacity if do not satisfy;

Step 6: twisted polyethylene cable DC operation line-to-ground mode is checked,

The check standard is: the discharge time constant τ of twisted polyethylene cable DC operation line-to-ground system is greater than time limit value t _Min, and said time limit value t _MinFor the ground connection electric branch of step 3 test causes 5 times of time, i.e. t _Min=5t _In, through type (8) calculates the discharge time constant τ of said twisted polyethylene cable DC operation line-to-ground system again:

τ＝L/R (8)

Wherein: L---the equivalent inductance of grounding system, unit is H

The equivalent resistance of R---grounding system, unit are Ω

If discharge time constant τ and time limit value t _MinThrough the check standard, then the twisted polyethylene cable alternating current circuit changes the technology determination method end of DC operation into; If existing earthing mode is not through checking; Then carrying out twisted polyethylene cable DC operation line-to-ground mode improves; Improve one's methods for the equivalent inductance L that increases grounding system or the equivalent resistance R that reduces grounding system until through checking, the technology determination method that the twisted polyethylene cable alternating current circuit changes DC operation into finishes.

As shown in Figure 2, when single loop three-phase alternating current cable changes DC operation into, utilize wherein two cables respectively as the both positive and negative polarity of direct current, and with the 3rd as cable in stock; Perhaps with the positive pole of a phase cable as direct current, and with the negative pole of remaining two-phase cable parallel connection as direct current.Under latter event, total ohmic loss can further reduce.If ac cable is the double loop, have 6 cables, so, utilize all 6 cables to carry out the direct current transmission operation.That is to say, exchange three loops that two loops become direct current.

In order to represent the practicality of technology determination method proposed by the invention more intuitively; Utilizing technology determination method proposed by the invention that an active service is exchanged the single telegram in reply cable of XLPE road transforms; Change DC operation into by interchange, technical scheme and transformation result are following.

There is a single loop crosslinked polyethylene ac cable circuit in applying unit, is the seabed composite cable, rated voltage 35kV, and the about 40km of length, insulation thickness is 10.5mm.The operation under alternating voltage originally of this cable line, it exchanges, and transmission plan is as shown in Figure 3, and in service one breakdown fault takes place mutually, so need to discuss the post-fault scheme of circuit.Existing trial changes this loop line road into DC operation, utilize wherein two intactly to constitute positive and negative 10kV DC power transmission line mutually, and the residue third phase is that fault is used as earth connection.The technical scheme that this active service interchange crosslinked polyethylene submarine cable circuit changes direct current transportation into is as shown in Figure 4.

Under new direct current transportation scheme, move, on the one hand, can guarantee original transmission capacity; In addition, though this scheme has increased rectification, inversion intrinsic assets input partly, than laying a former specification 35kV single-core cable again to recover required raw material and the laid down cost of Alternating Current Power Supply, estimating economizes on the use of funds reaches 4,000 ten thousand yuan of RMB.

Claims (1)

1. a twisted polyethylene cable alternating current circuit changes the technology determination method of under direct voltage, moving into, comprises the steps:

Step 1: twisted polyethylene cable DC operation conceptual design,

Allow working temperature θ for a long time according to twisted polyethylene cable _cAnd the twisted polyethylene cable structure, to lay ruuning situation and transmission capacity requirement design DC operation mode be one pole or bipolar operation, selects with other conductor, the outer metal level of cable self or be greatly the path of current reflux;

Step 2: the temperature difference at crosslinking polyethylene-insulated cable insulation two ends when the calculating twisted polyethylene cable allows rated current-carrying capacity and oepration at full load for a long time,

Calculate twisted polyethylene cable and allow rated current-carrying capacity for a long time; Based on twisted polyethylene cable structure and corresponding parameters, the expection method of operation and lay environment and confirm its long-term current-carrying capacity that allows; When DC voltage moves down; If the twisted polyethylene cable circuit adopts the bipolar method of operation, then the specified permission current-carrying capacity of cable calculates with formula (1); If cable run is one pole operation, and with the earth or other conductor as current return circuit, then the specified permission current-carrying capacity of cable calculates with formula (1); If cable run is one pole operation, and with self outer metal level be protective metal shell and armor as the backflow conductor, then the specified permission current-carrying capacity of cable is with formula (2) calculating,

$I=\sqrt{\frac{{\mathrm{\θ}}_c-{\mathrm{\θ}}_0}{R^{\′}(T_1+T_2+T_3+T_4)}}---\left(1\right)$

$I=\sqrt{\frac{{\mathrm{\θ}}_c-{\mathrm{\θ}}_0}{R^{\′}[T_1+(1+{\mathrm{\λ}}_1)T_2+(1+{\mathrm{\λ}}_1+{\mathrm{\λ}}_2)(T_3+T_4)}}---\left(2\right)$

Wherein, allow the maximum current that flows through in I---cable one phase conductor, unit is A

θ _c---cable allows long-term nominal operation temperature, and unit is ℃

θ ₀---the temperature of medium on every side, unit is ℃

T ₁---unit length cable insulation thermal resistance, unit is T Ω m

T ₂---unit length cable inner covering thermal resistance, unit is T Ω m

T ₃---unit length cable serving thermal resistance, unit is T Ω m

T ₄---the thermal resistance of medium on every side, unit is T Ω m

R '---the D.C. resistance of maximum operating temperature lower conductor unit length, unit are Ω/m

λ ₁---the ratio of metallic sheath loss and core loss

λ ₂---the ratio of metal armouring loss and core loss

The temperature difference at crosslinking polyethylene-insulated cable insulation two ends when calculating oepration at full load is according to twisted polyethylene cable structure and corresponding parameter, allow working temperature θ for a long time _c, the long-term permission ampacity I that is calculated calculates the temperature difference at crosslinking polyethylene-insulated cable insulation two ends when the oepration at full load with formula (3),

$\mathrm{\Δ\θ}=I^2{R}^{\′}\×\frac{{\mathrm{\ρ}}_T}{2\mathrm{\π}}\ln \frac{r_i}{r_c}---\left(3\right)$

Wherein, Δ θ---the insulation temperature difference during oepration at full load, unit is ℃

Allow the maximum current that flows through in I---cable one phase conductor, unit is A

R '---the D.C. resistance of maximum operating temperature lower conductor unit length, unit are Ω/m

ρ _T---cable XLPE insulation thermal resistance coefficient, unit are T Ω m

r _i---the insulation outer radius, unit is mm

r _c---the conductor outer radius, unit is mm

Step 3: the characterisitic parameter test of twisted polyethylene cable under direct voltage, the DC characteristic parameter of test comprise Ahmedabad that COEFFICIENT K, lightning impulse design field intensity E _PuAnd the ground connection electric branch causes time t _In,

Ahmedabad you the method for testing of COEFFICIENT K are: at first, twisted polyethylene cable is taken a sample, be not less than θ at the conductor temperature that guarantees twisted polyethylene cable then _cAnd the insulation temperature difference is not less than under the situation of Δ θ; Cable applies standard switching impulse voltage and standard lightning impulse voltage carries out breakdown test to taking a sample; The preparatory making alive of the positive and negative direct current of stack continues the sampling cable is carried out breakdown test on standard switching impulse voltage and standard lightning impulse voltage subsequently; The impact flashover voltage of impact flashover voltage when obtaining not have the direct voltage stack respectively with this when the direct voltage stack is arranged, you COEFFICIENT K calculate the Ahmedabad of twisted polyethylene cable with formula (4)

K＝(V _i-V _r)/V _dc (4)

Wherein, V _i---the impact flashover voltage when no direct voltage superposes, unit is V

V _r---the impact flashover voltage when the direct voltage stack is arranged, unit is V

V _Dc---the preparatory making alive of positive and negative direct current, unit are V

Get maximum in each calculated value of gained as your COEFFICIENT K of Ahmedabad;

Lightning impulse design field intensity E _PuMethod of testing be: at first, twisted polyethylene cable is taken a sample, is guaranteeing that conductor temperature is not less than θ then _cAnd the insulation temperature difference is not less than under the situation of Δ θ, and the standard lightning impulse voltage that the sampling cable is applied positive-negative polarity respectively carries out breakdown test, obtains the positive-negative polarity lightning impulse disruptive field intensity of crosslinking polyethylene-insulated cable insulation, get among both than low value E _Min, and introduce coefficient of safety 1.2, confirm that the lightning impulse design field intensity of crosslinking polyethylene-insulated cable insulation is E _Pu=E _Min/ 1.2;

The method of testing that the ground connection electric branch causes the time is: at first; Twisted polyethylene cable is taken a sample; Utilize pin-plate electrode to carry out electric branch to the crosslinking polyethylene-insulated cable insulation sample then and cause test; On stylus, apply the maximum short-circuit voltage of cross-linking polythene cable system, obtain 1000 ground connection and cause the pairing landing operation time of electric branch, be crosslinked polyetylene insulated ground connection electric branch under the maximum short-circuit voltage of system and cause time t _In

Step 4: calculate twisted polyethylene cable and allow direct-current working volts for a long time,

Calculate twisted polyethylene cable and allow direct-current working volts for a long time, according to the crosslinking polyethylene-insulated cable insulation thickness d, and Ahmedabad that COEFFICIENT K and lightning impulse that step 3 tests and calculates design field intensity E _Pu, calculate twisted polyethylene cable and allow direct-current working volts maximum U for a long time ₀, computing formula is formula (5);

$U_0=\frac{E_{\mathrm{pu}}\mathrm{gd}}{1.1K+1.9}---\left(5\right)$

Step 5: calculate the transmission capacity of twisted polyethylene cable circuit, and judge whether transmission capacity satisfies the DC operation requirement,

Calculate the transmission capacity of twisted polyethylene cable circuit; Based on the definite DC operation scheme of step 1; The twisted polyethylene cable that step 2 is calculated allows rated current-carrying capacity for a long time, and the twisted polyethylene cable that step 4 is calculated allows direct-current working volts, calculating cable run transmission capacity for a long time; If cable run is the one pole operation, then transmission capacity is calculated with formula (6); If cable run is bipolar operation, then transmission capacity is calculated with formula (7),

P _DC＝U ₀I (6)

P _DC＝2U ₀I (7)

Wherein, P _DC---the transmission capacity of twisted polyethylene cable circuit.

Judge whether the twisted polyethylene cable transmission capacity satisfies the requirement of DC operation,, then change step 1 over to and design again, satisfy the requirement of DC operation until the twisted polyethylene cable transmission capacity if do not satisfy;

Step 6: twisted polyethylene cable DC operation line-to-ground mode is checked,

The check standard is: the discharge time constant τ of twisted polyethylene cable DC operation line-to-ground system is greater than time limit value t _Min, and said time limit value t _MinFor the ground connection electric branch of step 3 test causes 5 times of time, i.e. t _Min=5t _In, through type (8) calculates the discharge time constant τ of said twisted polyethylene cable DC operation line-to-ground system again:

τ＝L/R (8)

Wherein: L---the equivalent inductance of grounding system, unit is H

The equivalent resistance of R---grounding system, unit are Ω

If discharge time constant τ and time limit value t _MinThrough the check standard, then the twisted polyethylene cable alternating current circuit changes the technology determination method end of DC operation into; If existing earthing mode is not through checking; Then carrying out twisted polyethylene cable DC operation line-to-ground mode improves; Improve one's methods for the equivalent inductance L that increases grounding system or the equivalent resistance R that reduces grounding system until through checking, the technology determination method that the twisted polyethylene cable alternating current circuit changes DC operation into finishes.

Images