CN1635585A - Method for inhibiting induced current of power cable sheath - Google Patents

Abstract

This invention relates to a method to use inductance to balance power cable protective layer inductance voltage and suppress the protective current, which belongs to electric engineer technique field and comprises the following steps: dividing three-phase cable into three sections with initial end three phases connected to the earth; crossing and exchanging the metal protective layer in middle part of the three-phase cable each section; computing inductance voltage on the metal layer according to the cable form and lay parameters and determining the compensation inductance to balance the voltage; connecting the compensation inductance to the third section of cable protective layer.

Description

The inhibition method of induced current of power cable sheath

Technical field

The present invention relates to a kind of employing inductance balance because of thereby the caused sheath induced voltage of the not isometric segmentation of cable suppresses the faradic method of sheath, belong to the electrical engineering technical field.

Background technology

As everyone knows, supply line generally adopts power cable as electric power transmission network.Because the demand difference of transmission capacity, the design of cable also is not quite similar.For the lower circuit of transmission capacity, generally adopt three-core cable; And require bigger circuit for transmission capacity, generally adopt single-core cable.

The public same metal sheath of three cores of three-core cable, when three-phase current was in a basic balance, the three-phase resultant current approached zero, and resultant flux also approaches zero, therefore, does not have induced voltage on the metal sheath and produces.Only when asymmetric short circuit, destroyed the symmetry of three-phase current, resultant flux no longer equals zero, and just has uneven induced voltage to produce on the metal sheath.

For single-core cable, the induced voltage of metal sheath is different fully with three-core cable.The lead of single-core cable and the relation of metal sheath can be regarded the elementary winding and the secondary winding of a transformer as.When cable conductor when the electric current, around it, produce magnetic flux, magnetic flux not only with core loop chain mutually, simultaneously also with the metal sheath of cable chain mutually, therefore on core and metal sheath, produce induced voltage.Since every special-purpose metal sheath of core, the magnetic flux that load current or short circuit current produced, and with the metal sheath interlinkage, so the induced voltage on the metal sheath exists all the time.Distance, size of current and cable length are relevant between the value of this induced voltage and core cross section, cable.Especially when cable was very long, the induced voltage on the sheath can reach higher numerical value.

At present, new cable line adopts measure such as metal sheath transposition to solve the induced voltage problem (referring to Fig. 1) of metal sheaths usually, adopt in the design of transposition connection method, by calculating draw out along wire length voltage to earth distribution map as shown in Figure 2.(as when cable being equilateral triangle when laying, voltage to earth distribute be shown in Figure 2.)

But along with the development of power construction, the transformation of cable line is increasing, improved cable line, and its length often can not satisfy three sections original transposition requirements, thereby causes the metal sheath Voltage unbalance.The metal sheath Voltage unbalance not only can cause cable line sheath circulation, and the numerical value of sheath circulation can reach the order of magnitude identical with the core electric current in some cases ^[1], cause the lot of energy loss, and might cause the cable heating, influence cable life.Therefore, must take measures to reduce or eliminate the influence of this sheath voltage.In addition, theory analysis shows that the induced voltage in the cable metal sheath is directly proportional with the product of cable length and core electric current.Under the bigger situation in cable length and core cross section, especially when system is short-circuited accident, induced voltage will reach the numerical value of harm personal safety, even cause the interruption of service of cable.Safety when contacting in order to ensure on duty and maintainer, regulation in " GB50217-94 ", induced voltage must not surpass 50V on the metal sheath.

At present, the method that generally adopts is a compensating cable both at home and abroad, promptly increases the length of cable, make the cable of its length and former three sections conversion methods isometric, and the cable ring that will put is placed in the cable trough more.This method obviously has its weak point, not only increased cost because of increasing cable length, and many payouts often is subjected to the restriction of space and laid condition.

Retrieval is found; the applying date is that 1997.12.19, application number are that 97251313.2 Chinese patent discloses a kind of cable protective layer protector; it is made up of motor, contact chip, zinc oxide valve plate, adhesive glue, silicone rubber sleeve; described zinc oxide valve plate two sides contact by the end of contact chip with two motors; and by adhesive glue it is linked as an integral body, outside adhesive glue, is arranged with a silicone rubber sleeve.It is introduced that this protector structure is simple, processing and manufacturing is easy, and volume is little, and moulding process is good, and sealing, insulation property are good, and are reliable to the protective value of cable outer layer induced voltage, are suitable for using cable protective layer protector in power cable.But; from principle; this protector mainly is to carry out overvoltage protection by release current; rather than reduce circulating current; and it is in service at present high-tension cable; when this protector puts into operation at the 220KV cable, can not well play the sheath protective effect, still cause metal sheath discharge over the ground (voltage even up to tens thousand of volts).

Summary of the invention

The objective of the invention is to: at the shortcoming of above prior art existence, the faradic inhibition method of a kind of cable sheath is proposed, this method is by taking measure simple and cost economy, come balance owing to three sections not isometric cable sheath overvoltage that produce of cable, thereby reduce sheath circulation greatly, reduce the wastage, improve the ampacity of cable, prolong the useful life of cable.

In order to reach above purpose, the applicant has set up the Mathematical Modeling of cable by board design, and according to the electric parameter of cable, the equivalent circuit that can set up cable as shown in Figure 3.Wherein R is the core resistance of cable, L _iBe the interior sense of cable, L _eBe the diseases caused by external factors of cable, R _jBe the resistance of cable metal sheath, C is the electric capacity of cable, and insulation resistance Ri ignores at this.Work inductance L=the L of single-phase cable _i+ L _eFor three-phase circuit, the work inductance of each cable is also relevant with the form of laying, and is determined on a case-by-case basis.

By above parameter and model, can calculate the induced potential in the cable metal sheath theoretically.Draw the inhibition method of induced current of power cable sheath of the present invention thus, the steps include:

1) with the interconnected ground connection of sheath three-phase of three sections cable initiating terminals;

2) metal sheath with each section of threephase cable connects in the coordinated transposition of the two exchange point places at middle part;

3), calculate the induced voltage on the metal sheath, and determine the external compensating inductance value that makes three-phase induction voltage in a basic balance according to the cable laying form with lay parameter and change;

4) compensating inductance is connected on outward on the cable sheath,, suppresses circulating current with balance sheath induced voltage.

The process of more than calculating the external inductance value be used to suppress circulating current can reduce:

A, calculating sheath induced voltage

Lay with the cable triangle and to be example, induced voltage Usa, Usb, the Usc of three-phase sheath unit length are calculated as follows respectively:

${\stackrel{\·}{U}}_{\mathrm{Sa}}=-j{\stackrel{\·}{I}}_a{X}_S,V/m$

${\stackrel{\·}{U}}_{\mathrm{Sb}}=-j{\stackrel{\·}{I}}_b{X}_S,V/m$

${\stackrel{\·}{U}}_{\mathrm{Sc}}=-j{\stackrel{\·}{I}}_c{X}_S,V/m$

Wherein $X_S=2\mathrm{\ω}\left(\ln \frac{2S}{D_S}\right)\×{10}^{-7}\mathrm{\Ω}/m$

With j in the following formula---be the complex operation factor

ω---be angular frequency, ω=2 π f

Ia, Ib, Ic---be respectively each phase load electric current of threephase cable

S---cable center's axle base

Ds---cable sheath average diameter

B, the external compensating inductance value of calculating;

$X_a=\mathrm{imag}\left(\frac{({\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_1*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sa}}+{\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_2*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sc}}+{\mathrm{<figure-callout\; id="3"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_3*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sb}})}{{\stackrel{\&CenterDot;}{I}}_a}\right)$

$X_b=\mathrm{imag}\left(\frac{({\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_1*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sb}}+{\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_2*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sa}}+{\mathrm{<figure-callout\; id="3"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_3*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sc}})}{{\stackrel{\&CenterDot;}{I}}_b}\right)$

$X_c=\mathrm{imag}\left(\frac{({\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_1*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sc}}+{\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_2*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sb}}+{\mathrm{<figure-callout\; id="3"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_3*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sa}})}{{\stackrel{\&CenterDot;}{I}}_c}\right)$

With X _a, X _b, X _cCan obtain the inductance value that each phase compensates divided by 2 π f respectively.

In following formula

X _a, X _b, X _c---be respectively the reactance value of three corresponding compensation

L ₁, L ₂, L ₃---be respectively the length of the 1st, 2,3 section cable

L _a, L _b, L _c---be respectively the inductance value of three corresponding compensation, computing formula is respectively: $L_a=\frac{X_a}{2\mathrm{\&pi;f}},$ $L_b=\frac{{\mathrm{<figure-callout\; id="2"\; label="X\; b"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">X</figure-callout>}}_b}{2\mathrm{\&pi;f}},$ $L_c=\frac{{\mathrm{<figure-callout\; id="2"\; label="X\; c"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">X</figure-callout>}}_c}{2\mathrm{\&pi;f}}$

F---frequency of supply

Like this, when owing to reasons such as renovation of power lines exploitations, the 3rd segment length is during less than raw footage, by calculating the inductance parameters of determining required compensation, connect on the cable end piece sheath suitably that the inductance of size compensates, can make the also balance as far as possible of three-phase induction voltage on the cable sheath.As a result, the circulation in the cable sheath will reduce greatly.

The present invention's thinking that breaks traditions, theory analysis by science, the ingenious Mathematical Modeling of having set up, realized the faradic inhibition of cable sheath with simple and practical method, because the relative cable considerably cheaper of cost of inductance, nor be subjected to the restriction of laid condition, and therefore adopt cost economy of the present invention, practical.Obviously, compared with prior art, the present invention has outstanding progress.

Description of drawings

The present invention is further illustrated below in conjunction with accompanying drawing.

Fig. 1 is existing cable sheath transposition syndeton schematic diagram.

Fig. 2 is three sections on cable coordinated transposition induced voltage distribution map when isometric.

Fig. 3 is an equivalent circuit schematic diagram of setting up the cable Mathematical Modeling.

Fig. 4 is the structural representation of the embodiment of the invention.

Fig. 5 is the induced voltage distribution map of Fig. 4 embodiment.

Fig. 6 is the quantitative induced voltage distribution map of Fig. 4 embodiment

Embodiment

Embodiment

In the present embodiment transformer station to adopt the separate unit main transformer capacity be 50,000 kilovolt-amperes 110kV transformer, the about I=500A of the every phase load electric current of cable, cable center's between centers apart from S=250mm, cable sheath average diameter D _S=69.3mm forms the three-phase loop by three single-core cables, and circuit has adopted equilateral triangle to lay.Under the balanced load condition, U _Sa, U _Sb, U _ScThree-phase induction voltage is:

U _Sa＝U _Sb＝U _Sc＝U _S＝IX _S＝500×1.1247×10 ^-4＝0.0621(V/m)

Wherein Xs is according to formula $X_S=2\mathrm{\&omega;}\left(\ln \frac{2S}{D_S}\right)\&times;{10}^{-7}\mathrm{\&Omega;}/m$ Calculate.This shows and find out,

When cable is very long (in the actual motion, the total length of 110kV cable line generally can reach 5～6 kilometers), the sheath induced voltage will be very high.And when system was short-circuited accident, the numerical value of induced voltage will reach 10 ³The V/m order of magnitude [2].

When laying, threephase cable A, B, C are divided into three sections of I, II, III, the sheath of threephase cable initiating terminal is the interconnected ground connection of plain edition jointing box 1 place's three-phase, and the metal sheath of each section cable connects by insulation joint box 2,3 difference coordinated transpositions at two exchange point places afterwards.

Because track remodelling, the length of three sections cables of threephase cable is respectively 500m, 500m, 300m.Cable is that equilateral triangle lays, and at operation at rating, transposition back three-phase induction voltage calculates according to following formula respectively:

${\stackrel{\&CenterDot;}{U}}_{\mathrm{Sa}}=-j{\stackrel{\&CenterDot;}{I}}_a{X}_S,V/m$

${\stackrel{\&CenterDot;}{U}}_{\mathrm{Sb}}=-j{\stackrel{\&CenterDot;}{I}}_b{X}_S,V/m$

${\stackrel{\&CenterDot;}{U}}_{\mathrm{Sc}}=-j{\stackrel{\&CenterDot;}{I}}_c{X}_S,V/m$

Wherein $X_S=2\mathrm{\&omega;}\left(\ln \frac{2S}{D_S}\right)\&times;{10}^{-7}\mathrm{\&Omega;}/m$

Again according to U _a=L ₁* U _Sa+ L ₂* U _Sc+ L ₃* U _Sb

U _b＝L ₁*U _Sb+L ₂*U _Sa+L ₃*U _Sc

U _c＝L ₁*U _Sc+L ₂*U _Sb+L ₃*U _Sa

Get each phase sheath induced voltage Ua=Ub=Uc=12.4166V, circulating current for the 76.9231A computing formula is: $I_h=\left|\frac{{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sa}}}{j{X}_S({\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_1+{\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_2+l_3)}\right|,$ Because the three-phase induction balance of voltage, do not have circulation in the sheath this moment.If will make no current in the sheath, then according to following calculating:

$X_a=\mathrm{imag}\left(\frac{({\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_1*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sa}}+{\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_2*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sc}}+{\mathrm{<figure-callout\; id="3"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_3*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sb}})}{{\stackrel{\&CenterDot;}{I}}_a}\right)=0.0248$

$X_b=\mathrm{imag}\left(\frac{({\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_1*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sb}}+{\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_2*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sa}}+{\mathrm{<figure-callout\; id="3"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_3*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sc}})}{{\stackrel{\&CenterDot;}{I}}_b}\right)=0.0248$

$X_c=\mathrm{imag}\left(\frac{({\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_1*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sc}}+{\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_2*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sb}}+{\mathrm{<figure-callout\; id="3"\; label="L"\; filenames="A20041006604900121.png"\; state="\{\{state\}\}">L</figure-callout>}}_3*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sa}})}{{\stackrel{\&CenterDot;}{I}}_c}\right)=0.0248$

Must need the compensation size is the reactance of 0.0248 Ω, and promptly getting external compensating inductance divided by 2 π f respectively is 0.079047mh.

Above result of calculation proves, when cable is a symmetry when laying, the external compensating inductance value of three-phase equates.Afterwards, the external compensating inductance ground connection (referring to Fig. 4) by separately respectively at plain edition jointing box 4 places of the 3rd section cable sheath terminal.The induced voltage of compensation back cable sheath distributes as shown in Figure 5 over the ground.

Facts have proved, present embodiment adopts at the terminal inductance that connects of sheath, to compensate the not influence that brings of the induced voltage of isometric generation of three sections cables, thereby sheath induced voltage and circulation have been reduced greatly, improve the ampacity of cable, prolonged the useful life of cable, and be not subjected to the restriction of space and laid condition, both eliminate the influence of sheath induced voltage, significantly reduced the improvement cost of cable again.

Experimental results show that, after the compensation, present embodiment has been eliminated the not influence of isometric generation of three sections cables substantially, thereby sheath induced voltage and circulation have been reduced greatly, improve the ampacity of cable, prolonged the useful life of cable, and be not subjected to the restriction of space and laid condition, both eliminate the influence of sheath induced voltage, significantly reduced the improvement cost of cable again.

Attached: the symbol physical significance in the specification is as follows:

The electric capacity of C cable

Ds cable sheath average diameter

The f frequency of supply

The I load current

Ia, Ib, Ic are respectively each phase core electric current of threephase cable

I _bCirculating current

J is the complex operation factor

The work inductance of the single-phase cable of L, L=L _i+ L _e

L ₁, L ₂, L ₃Be respectively the length of the 1st, 2,3 section cable

L _a, L _b, L _cBe respectively the inductance value of three corresponding compensation

L _eThe diseases caused by external factors of cable

L _iThe interior sense of cable

The core resistance of R cable

The Ri insulation resistance is ignored at this.

R _jThe resistance of cable metal sheath

S threephase cable center axle base

Ua, Ub, Uc are respectively the sheath induced voltage after three sections transpositions of cable

Usa, Usb, Usc are respectively the induced voltage of three-phase sheath unit length

ω is an angular frequency, ω=2 π f

X _a, X _b, X _cBe respectively the reactance value of three corresponding compensation

X _SReactance for the unit length metal sheath

[1] chief editor of The Department of Electric Power of State Economic and Trade Commission, power cable, Beijing: China Electric Power Publishing House, in September, 2002

[2] Liu Ziyu compiles, the electric insulation structure design principle, and the first volume, Beijing: mechanical industry is published and is established in September, 1981

Claims (4)

1, a kind of inhibition method of induced current of power cable sheath the steps include:

1) with the interconnected ground connection of sheath three-phase of three sections cable initiating terminals;

2) metal sheath with each section of threephase cable connects in the coordinated transposition of the two exchange point places at middle part;

It is characterized in that:

3), calculate the induced voltage on the metal sheath, and determine the external compensating inductance value that makes three-phase induction voltage in a basic balance according to the cable laying form with lay parameter and change;

4) compensating inductance is connected on outward on the cable sheath,, suppresses circulating current with balance sheath induced voltage.

2. according to the inhibition method of the described induced current of power cable sheath of claim 1, it is characterized in that: described step 3) is according to the cable laying form and does not hold length to lay the parameter variation, calculate the induced voltage on the metal sheath, and determine the external compensating inductance value that makes three-phase induction voltage in a basic balance; Described step 4) is with on the external cable sheath endways of compensating inductance, with balance sheath induced voltage, suppresses circulating current.

3. according to the inhibition method of claim 1 or 2 described induced current of power cable sheath, it is characterized in that:

A, calculating sheath induced voltage

$\mathrm{Usa}=\mathrm{Usb}={\stackrel{\&CenterDot;}{U}}_{\mathrm{Sa}}=-j{\stackrel{\&CenterDot;}{I}}_a{X}_{S}V/m$

$X_S=2\mathrm{\&omega;}\left(\ln \frac{2S}{D_S}\right)\&times;{10}^{-7}\mathrm{\&Omega;}/m$

B, the external compensating inductance value of calculating;

$\mathrm{Xa}=\mathrm{Xb}=X_c=\mathrm{imag}\left(\frac{(L_1*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sc}}+L_2*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sb}}+L_3*{\stackrel{\&CenterDot;}{U}}_{\mathrm{Sa}})}{{\stackrel{\&CenterDot;}{I}}_c}\right)$

Xa, Xb, Xc are obtained the inductance value that each phase compensates divided by 2 π f respectively.

4, according to the inhibition method of the described induced current of power cable sheath of claim 3, it is characterized in that: the sheath of described threephase cable initiating terminal is the interconnected ground connection of jointing box place three-phase, and the metal sheath of each section cable connects by the insulation joint box coordinated transposition at the exchange point place.

Images