CN101900773B - Underground power cable current-carrying capacity online prediction system and method - Google Patents

Abstract

The invention relates to an environment factor monitoring and finite element-based underground power cable current-carrying capacity online prediction system and an environment factor monitoring and finite element-based underground power cable current-carrying capacity online prediction method. The system consists of a temperature sensor used for measuring the air temperature, a wind speed sensor, a humidity sensor, a singlechip system and a scheduling host of a power department. The method comprises the step of predicting the current-carrying capacity of an underground power cable by combining temperature, wind speed, humidity and other data measured in real time with the calculation of a finite element temperature field. The system and the method have the advantages that: the system and the method can accurately predict the current-carrying capacity of the underground power cable on line, so that the power department can develop the capacity of conveying electric energy to the greatest degree; meanwhile, the system and the method can avoid the resource waste phenomenon of wasting the talent on a petty job on the premise of guaranteeing safe and reliable operation.

Description

The underground power cable current-carrying capacity online prediction method

Technical field

The present invention relates to a kind of underground power cable current-carrying capacity online prediction method.

Background technology

The underground high-voltage power cable is the principal mode of city electric line; Because its installation cost is higher, the characteristics of easy-maintaining not; On the basis that guarantees its long-term safety reliability service; Rationally confirm the current-carrying capacity of high voltage power cable, the ability of bringing into play its transmission of electric energy to greatest extent has great importance for power department.

At present the method confirmed of underground power cable current-carrying capacity has two kinds, and a kind of is calculated off-line, promptly according to the soil property of locality, the highest temperature in hot month, calculates according to IEC60287 or finite element; Another kind is in line computation, through the temperature of on-line monitoring cable surface, calculates the cable core temperature through Re Lu or finite element analysis method, and estimates the current-carrying capacity of cable on this basis.

The real-time current-carrying capacity of cable is and closely-related physical quantitys such as the coefficient of heat conductivity of the construction of cable, surrounding soil, face of land air, and above-mentioned two kinds of methods are the current-carrying capacity of on-line prediction cable exactly all.First method is to calculate according to the situation of the highest temperature and given soil thermal conductivity to get, when temperature or soil thermal conductivity change, and the maximum load current that cable can bear, promptly current-carrying capacity also should adjust accordingly; For second method, the temperature through cable surface can calculate the cable core temperature under preload more accurately, and will predict that it is the influence that current-carrying capacity must be considered temperature, soil thermal conductivity that maximum can be born electric current.

At present, the computing method of underground power cable current-carrying capacity are main with FEM calculation mainly, at first suppose an electric current; Maximum temperature according to boundary condition calculating power cable insulation layer if be no more than insulation course long-term work tolerable temperature, then increases electric current; If surpass tolerable temperature; Then reduce electric current, loop iteration is till the cable insulation maximum temperature equals insulation course long-term work tolerable temperature, and the electric current of this moment is the current-carrying capacity of cable.

Summary of the invention

Technical matters to be solved by this invention provides a kind of underground power cable current-carrying capacity online prediction method.

The technical solution adopted for the present invention to solve the technical problems:

A kind of underground power cable current-carrying capacity online prediction method is characterized in that:

(1) at first build underground power cable current-carrying capacity online prediction system:

Said underground power cable current-carrying capacity online prediction system is made up of the dispatching host machine (6) of the temperature sensor that is used for the Measurement of Air temperature (1), air velocity transducer (2), humidity sensor (3), SCM system (4), power department; The output terminal of said temperature sensor (1), air velocity transducer (2), humidity sensor (3) connects the respective input of said SCM system respectively, and said SCM system is connected with said dispatching host machine (6) through GSM network (5); Said temperature sensor (1) and air velocity transducer (2) be installed in said power cable directly over, said humidity sensor (3) is installed in the said power cable soil along the line;

(2) concrete steps of said underground power cable current-carrying capacity online prediction method are following:

(1) modeling:

A. soil plow-in cable:

Power cable is 1m apart from ground, and border, power cable both sides is apart from cable 10m, and in the calculating, border, said both sides normal orientation hot-fluid rate of change is 0, and promptly the soil moisture no longer changes; Below the power cable to the deep subsoil border apart from cable 10m, the earth deep layer that fetches earth boundary temperature is constant, and temperature value is 383K for the deep subsoil temperature;

B. calandria cable:

The comb top is 1m far from ground, and power cable is laid in the comb, and border, comb both sides is apart from comb 10m, and in the calculating, border, said both sides normal orientation hot-fluid rate of change is 0, and promptly the soil moisture no longer changes; Below the comb to the deep subsoil border apart from comb 10m, the earth deep layer that fetches earth boundary temperature is constant, and temperature value is 383K for the deep subsoil temperature;

C. plough groove type cable:

The ditch groove depth is 1m apart from ground, and power cable is laid in the groove, and iron plate is often laid at indoor groove top; Cement plate is often laid at outdoor groove top, and border, model both sides is apart from groove 10m, in the calculating; Border, said both sides normal orientation hot-fluid rate of change is 0, and promptly the soil moisture no longer changes; Below the groove to the deep subsoil border apart from groove 10m, the earth deep layer that fetches earth boundary temperature is constant, and temperature value is 383K for the deep subsoil temperature;

(2) subdivision: with power cable and surrounding soil subdivision thereof is little unit;

Under direct burial, calandria and three kinds of modes of plough groove type, adopting triangle or quadrilateral units is little unit with the The model subdivision;

(3) read the following data of real-time measurement: air themperature T _Air, face of land wind speed v _Air, relative moisture of the soil rh; And the coefficient of heat conductivity λ of calculating face of land cross-ventilation coefficient of heat transfer α and soil;

A. utilize following formula (1) to calculate face of land cross-ventilation coefficient of heat transfer α:

α＝7.371+6.43v _air ^0.75(1)

In the formula, α is the face of land cross-ventilation coefficient of heat transfer, the W/ (m of unit ²K); v _AirBe the face of land wind speed that air velocity transducer is measured in real time, the m/s of unit;

B. utilize formula (2) to calculate soil thermal conductivity λ:

$\mathrm{\λ}=(11.33\log \mathrm{rh}-3.4){10}^{{0.649}_{{\mathrm{\ρ}}_d}-2}---\left(2\right)$

In the formula, λ is a soil thermal conductivity, the W/ of unit (mK); ρ _dBe soil packing, units/m ³Rh is the relative moisture of the soil that humidity sensor is measured in real time;

(4) at first, choose initial current I1, the A of unit;

(5) call the finite element temperature calculation program, calculate the power cable insulation layer maximum temperature T1 under the electric current I 1, unit K;

(6) as T1 during less than power cable insulation layer long-term work insulation course tolerable temperature Tin, and Tin-T1＞0.3K, then get I2=2I1, went to for (7) step; As T1 during greater than Tin, and T1-Tin＞0.3K, then get I2=0.5I1, went to for (7) step; When | T1-Tin|≤0.5K, I1 is the power cable current-carrying capacity of asking, and withdraws from;

(7) call the finite element temperature calculation program, calculate the power cable insulation layer maximum temperature T2 under the electric current I 2;

(8) when | T2-Tin|＞0.3K, went to for (9) step; When | T2-Tin|≤0.3K, I2 is the power cable current-carrying capacity of asking, and withdraws from;

(9) get I=I2+ (I2-I1) (Tin-T2)/(T2-T1), call the finite element temperature calculation program, calculate the power cable insulation layer maximum temperature T under the electric current I;

(10) when | T-Tin|＞0.3K, get I1=I2, I2=I, T1=T2, T2=T goes to above-mentioned (9) step; When | T-Tin|≤0.3K, I is the power cable current-carrying capacity of asking, and withdraws from.

The invention has the beneficial effects as follows the current-carrying capacity of on-line prediction underground power cable more accurately; Power department can be brought into play the ability of its transmission of electric energy in view of the above to greatest extent; And can under the prerequisite that guarantees safe and reliable operation the wasting of resources phenomenon of low load with strong power can not appear.

Description of drawings

Fig. 1 is a hardware configuration of the present invention.

Fig. 2-1 is the synoptic diagram of soil direct burial underground power cable system of laying.

Fig. 2-2 is the synoptic diagram of calandria underground power cable system of laying.

Fig. 2-3 is the synoptic diagram of plough groove type underground power cable system of laying.

Fig. 3-1 is whole subdivision of the finite element of plow-in cable and surrounding soil synoptic diagram as a result.

Fig. 3-2 is the subdivision synoptic diagram as a result of the finite element cable section of plow-in cable and surrounding soil.

In Fig. 1,2,3, and 1 temperature sensor (model: DS18B20), 2 air velocity transducers (model: CQ2-FC-1), 3 humidity sensor (models: BQ8-CHR-01); 4 SCM systems (model of single-chip microcomputer: 89C52), 5GSM network, the dispatcher of 6 power departments (desk-top computer); 7 soil, 8 power cables, 9 combs; 10 cover plates, 11 back up pads, 12 grooves.

Embodiment

The concrete steps of present embodiment are:

(1) at first build underground power cable current-carrying capacity online prediction system:

Said underground power cable current-carrying capacity online prediction system is made up of the dispatching host machine (6) of the temperature sensor that is used for the Measurement of Air temperature (1), air velocity transducer (2), humidity sensor (3), SCM system (4), power department; The output terminal of said temperature sensor (1), air velocity transducer (2), humidity sensor (3) connects the respective input of said SCM system respectively, and said SCM system is connected with said dispatching host machine (6) through GSM network (5); Said temperature sensor (1) and air velocity transducer (2) be installed in said power cable directly over, said humidity sensor (3) is installed in the said power cable soil along the line;

(2) concrete steps of said power cable current-carrying capacity online prediction method are following:

(1) modeling:

A. soil plow-in cable:

Power cable is 1m apart from ground, and border, power cable both sides is apart from cable 10m, and in the calculating, border, said both sides normal orientation hot-fluid rate of change is 0, and promptly the soil moisture no longer changes; Below the power cable to the deep subsoil border apart from cable 10m, the earth deep layer that fetches earth boundary temperature is constant, and temperature value is 383K for the deep subsoil temperature;

B. calandria cable:

The comb top is 1m far from ground, and power cable is laid in the comb, and border, comb both sides is apart from comb 10m, and in the calculating, border, said both sides normal orientation hot-fluid rate of change is 0, and promptly the soil moisture no longer changes; Below the comb to the deep subsoil border apart from comb 10m, the earth deep layer that fetches earth boundary temperature is constant, and temperature value is 383K for the deep subsoil temperature;

C. plough groove type cable:

The ditch groove depth is 1m apart from ground, and power cable is laid in the groove, and iron plate is often laid at indoor groove top; Cement plate is often laid at outdoor groove top, and border, model both sides is apart from groove 10m, in the calculating; Border, said both sides normal orientation hot-fluid rate of change is 0, and promptly the soil moisture no longer changes; Below the groove to the deep subsoil border apart from groove 10m, the earth deep layer that fetches earth boundary temperature is constant, and temperature value is 383K for the deep subsoil temperature;

(2) subdivision: with power cable and surrounding soil subdivision thereof is little unit;

Under direct burial, calandria and three kinds of modes of plough groove type, adopting triangle or quadrilateral units is little unit with the The model subdivision; The cable area temperature variation is bigger, and subdivision density is higher, air-shed between air-shed and groove inner cable outside surface and trench wall between comb inner cable outside surface and comb inwall; Temperature variation is bigger, and needs calculation flow rate, and subdivision density is higher; The soil region temperature variation is less, and subdivision density is less;

(3) read the following data of real-time measurement: air themperature T _Air, face of land wind speed v _Air, relative moisture of the soil rh; And the coefficient of heat conductivity λ of calculating face of land cross-ventilation coefficient of heat transfer α and soil;

A. utilize following formula (1) to calculate face of land cross-ventilation coefficient of heat transfer α:

α＝7.371+643v _air ^0.75 (1)

In the formula, α is the face of land cross-ventilation coefficient of heat transfer, the W/ (m of unit ²K); v _AirBe the face of land wind speed that air velocity transducer is measured in real time, the m/s of unit;

B. utilize formula (2) to calculate soil thermal conductivity λ:

$\mathrm{\λ}=(11.33\log \mathrm{rh}-3.4){10}^{{0.649}_{{\mathrm{\ρ}}_d}-2}---\left(2\right)$

In the formula, λ is a soil thermal conductivity, the W/ of unit (mK); ρ _dBe soil packing, units/m ³Rh is the relative moisture of the soil that humidity sensor is measured in real time.

(4) at first, choose initial current I1, the A of unit;

(5) call the finite element temperature calculation program, calculate the power cable insulation layer maximum temperature T1 under the electric current I 1, unit K;

(6) as T1 during less than power cable insulation layer long-term work insulation course tolerable temperature Tin, and Tin-T1＞0.3K, then get I2=2I1, went to for (7) step; As T1 during greater than Tin, and T1-Tin＞0.3K, then get I2=0.5I1, went to for (7) step; When | T1-Tin|≤0.5K, I1 is the power cable current-carrying capacity of asking, and withdraws from;

(7) call the finite element temperature calculation program, calculate the power cable insulation layer maximum temperature T2 under the electric current I 2;

(8) when | T2-Tin|＞0.3K, went to for (9) step; When | T2-Tin|≤0.3K, I2 is the power cable current-carrying capacity of asking, and withdraws from;

(9) get I=I2+ (I2-I1) (Tin-T2)/(T2-T1), call the finite element temperature calculation program, calculate the power cable insulation layer maximum temperature T under the electric current I;

(10) when | T-Tin|＞0.3K, get I1=I2, I2=I, T1=T2, T2=T goes to above-mentioned (9) step; When | T-Tin|≤0.3K, I is the power cable current-carrying capacity of asking, and withdraws from.

In the present embodiment, condition of convergence 0.3K actual and thermometric error decision by engineering.Temperature measurement accuracy is 0.1-0.2K, and in conjunction with the actual requirement of engineering, the condition of convergence is got 0.3K in the present embodiment.

With Shijiazhuang summer be example, actual measurement face of land air themperature is 35 degree, wind speed is 1.5m/s, soil moisture is 18.5%, calculating soil thermal conductivity thus is 0.8W/ (m ²K), the cross-ventilation coefficient of heat transfer in the face of land is 16W/ (m ²K), the single loop of soil direct-burried shown in Fig. 2-1 yi word pattern is arranged 800mm ²The YJLW02XLPE power cable single-end earthed, buries ground degree of depth 1000mm, cable spacing 200mm, and calculating current-carrying capacity of cable is 892A.

Comb lays 800mm shown in Fig. 2-2 ²The calculating current-carrying capacity of YJLW02XLPE power cable is 858A.

Groove lays 800mm shown in Fig. 2-3 ²The calculating current-carrying capacity of YJLW02XLPE power cable is 1085A.

The temperature field FEM calculation method that the present invention relates to is prior art (seeing above-mentioned background technology part), and is following in the face of temperature field FEM calculation method introduction down:

Because underground power cable length is approximately infinitely great with respect to xsect, thereby underground power cable crowd's steady temperature field belongs to two-dimentional steady heat conduction problem.

The temperature control equation that heat source region (like cable conductor, insulating medium, metal screen layer and armor) is arranged is as shown in the formula shown in (3).

$\mathrm{\λ}(\frac{{\∂}^{2}T}{\∂x^2}+\frac{{\∂}^{2}T}{\∂y^2})+q_v=0---\left(3\right)$

In the formula, q _vBe volume heat generation rate, W/m ³

The temperature control equation in no thermal source solid dielectric zone (like other layers of power cable, soil and comb etc.) is suc as formula shown in (4).

$\frac{{\&PartialD;}^{2}T}{\&PartialD;x^2}+\frac{{\&PartialD;}^{2}T}{\&PartialD;{\mathrm{<figure-callout\; id="2"\; label="y"\; filenames="HSA00000175331100011.png"\; state="\{\{state\}\}">y</figure-callout>}}^2}=0---\left(4\right)$

Lay with groove for comb and to lay under the dual mode, power cable and have air on every side between solid dielectric, there is the characteristic of the natural convection of being heated in air, and calculate in this part regional temperature field needs Solving Coupled continuity equation, the equation of momentum and energy equation.Respectively as shown in the formula shown in (5), (6), (7).

$\frac{\&PartialD;u}{\&PartialD;x}+\frac{\&PartialD;v}{\&PartialD;y}=0---\left(5\right)$

In the formula, u, v are the component of flow field velocity vector at x and y axle, and unit is m/s.

$\left\{\begin{array}{c}\mathrm{\&rho;}(u\frac{\&PartialD;u}{\&PartialD;x}+v\frac{\&PartialD;u}{\&PartialD;y})=-\frac{\&PartialD;p}{\&PartialD;x}+\mathrm{\&eta;}(\frac{{\&PartialD;}^{2}u}{\&PartialD;x^2}+\frac{{\&PartialD;}^{2}u}{\&PartialD;y^2})+\mathrm{\&rho;g\&alpha;}(T-T_r)\cos \mathrm{\&theta;}\\ \mathrm{\&rho;}(u\frac{\&PartialD;v}{\&PartialD;x}+v\frac{\&PartialD;v}{\&PartialD;y})=-\frac{\&PartialD;p}{\&PartialD;y}+\mathrm{\&eta;}(\frac{{\&PartialD;}^{2}v}{\&PartialD;x^2}+\frac{{\&PartialD;}^{2}v}{\&PartialD;y^2})+\mathrm{\&rho;g\&alpha;}(T-T_r)\sin \mathrm{\&theta;}\end{array}\right.---\left(6\right)$

In the formula, T _rBe the fluid reference temperature; α is a volume expansivity, and unit is K ^-1ρ is a fluid density, and unit is kg/m ³P is the pressure in flow field, and unit is Pa; η is a hydrodynamic force viscosity, and unit is Pas.

$u\frac{\&PartialD;T}{\&PartialD;x}+v\frac{\&PartialD;T}{\&PartialD;y}-\mathrm{\&lambda;}(\frac{{\&PartialD;}^{2}T}{\&PartialD;x^2}+\frac{{\&PartialD;}^{2}T}{\&PartialD;y^2})=0---\left(7\right)$

Find the solution the temperature field and need confirm the border of whole temperature field, under the soil directly buried installation situation, suppose deep soil temperature constant (being the lower boundary among Fig. 2-1), shown in (8):

$\left.\begin{array}{c}\frac{{\&PartialD;}^{2}T}{\&PartialD;x^2}+\frac{{\&PartialD;}^{2}T}{\&PartialD;{\mathrm{<figure-callout\; id="2"\; label="y"\; filenames="HSA00000175331100011.png"\; state="\{\{state\}\}">y</figure-callout>}}^2}=0\\ T(x,y){|}_{\mathrm{\&Gamma;}}=T_{\mathrm{soil}}\end{array}\right\}---\left(8\right)$

In the formula, T _SoilFor the deep soil temperature, get 383K.

The normal direction gradient on border, both sides is 0 among Fig. 2-1, does not promptly receive the influence of power cable heating away from the temperature field of power cable regional soil, shown in (9):

$\left.\begin{array}{c}\frac{{\&PartialD;}^{2}T}{\&PartialD;x^2}+\frac{{\&PartialD;}^{2}T}{\&PartialD;{\mathrm{<figure-callout\; id="2"\; label="y"\; filenames="HSA00000175331100011.png"\; state="\{\{state\}\}">y</figure-callout>}}^2}=0\\ \mathrm{\&lambda;}\frac{\&PartialD;T}{\&PartialD;n}{|}_{\mathrm{\&Gamma;}}=0\end{array}\right\}---\left(9\right)$

Coboundary among Fig. 2-1 is the ground table boundary, and the heat in the soil is diffused in the air through the form of convection heat transfer, shown in (10):

$\left.\begin{array}{c}\frac{{\&PartialD;}^{2}T}{\&PartialD;x^2}+\frac{{\&PartialD;}^{2}T}{\&PartialD;{\mathrm{<figure-callout\; id="2"\; label="y"\; filenames="HSA00000175331100011.png"\; state="\{\{state\}\}">y</figure-callout>}}^2}=0\\ -\mathrm{\&lambda;}\frac{\&PartialD;T}{\&PartialD;n}{|}_{\mathrm{\&Gamma;}}=\mathrm{\&alpha;}(T-T_{\mathrm{air}}){|}_{\mathrm{\&Gamma;}}\end{array}\right\}---\left(10\right)$

In following formula (8), (9) and (10), Γ is an integral boundary.

For soil direct-burried power cable, after whole field domain carries out subdivision, utilize the Jia Lvejin formula, can get the FEM calculation equation, shown in (11):

KT＝P (11)

In the formula, K is the whole audience territory cell matrix relevant with coefficient of heat conductivity and cell configuration, and T is a whole audience territory cell temperature matrix, and P is the whole audience territory cell matrix relevant with heating.

Common whole field domain adopts little triangle to carry out subdivision, and the subdivision result is as shown in Figure 3.Temperature T on each triangular element is the linear function of coordinate x and y, is expressed as: T=a ₁+ a ₂X+a ₃Y, a ₁, a ₂And a ₃Determine by the temperature value on the Atria summit.Utilize this linear function that the variation formula of formula (3), (8), (9), (10) is prolonged triangular element and carry out integration, can get the k value and the p value of each unit, the last integrated following formula (11) that gets, its expansion form is:

$\left[\begin{array}{cccc}{k}_{11}& {\mathrm{<figure-callout\; id="12"\; label="k"\; filenames="HSA00000175331100023.png"\; state="\{\{state\}\}">k</figure-callout>}}_{12}& \&CenterDot;\&CenterDot;\&CenterDot;& k_{1n}\\ k_{21}& k_{22}& \&CenterDot;\&CenterDot;\&CenterDot;& k_{2n}\\ \&CenterDot;\&CenterDot;\&CenterDot;& \&CenterDot;\&CenterDot;\&CenterDot;& \&CenterDot;\&CenterDot;\&CenterDot;& \&CenterDot;\&CenterDot;\&CenterDot;\\ k_{n1}& {\mathrm{<figure-callout\; id="2"\; label="k\; n"\; filenames="HSA00000175331100011.png"\; state="\{\{state\}\}">k</figure-callout>}}_n& \&CenterDot;\&CenterDot;\&CenterDot;& k_{\mathrm{nn}}\end{array}\right]\left[\begin{array}{c}{T}_1\\ {\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="HSA00000175331100011.png"\; state="\{\{state\}\}">T</figure-callout>}}_2\\ \&CenterDot;\&CenterDot;\&CenterDot;\\ T_n\end{array}\right]=\left[\begin{array}{c}{p}_1\\ {\mathrm{<figure-callout\; id="2"\; label="p"\; filenames="HSA00000175331100011.png"\; state="\{\{state\}\}">p</figure-callout>}}_2\\ \&CenterDot;\&CenterDot;\&CenterDot;\\ p_3\end{array}\right]---\left(12\right)$

With Gaussian elimination following formula (12) is found the solution, can solve the temperature value of each node, thereby obtain the temperature field distribution of whole field domain and the maximum temperature of power cable.

Lay (seeing Fig. 2-2) and groove power cable laying (seeing Fig. 2-3) for comb, after whole field domain carries out subdivision, need calculate with alternative manner.

List the Contact Coupled boundary condition in solid dielectric zone and air fluid zone, the borderline condition that wherein is coupled need satisfy following two conditions:

It is continuous that coupling border w goes up temperature:

T _w| ₁＝T _w| ₂ (13)

Heat flow density on the coupling border w is continuous:

q _n| ₁＝q _n| ₂ (14)

Suppose the borderline Temperature Distribution of coupling; One of them zone 1 (like solid dielectric) is found the solution; Draw borderline local heat flux density of coupling and thermograde; Use formula (12) and formula (13) then and find the solution another zone 2 (like air dielectrics), to draw new Temperature Distribution on the coupling border.Distribute as the input in zone 1 with this, repeat aforementioned calculation up to convergence, the temperature field that provides whole field domain at last distributes, and obtains the maximum temperature of power cable insulation.

Lay with groove for comb and to lay two kinds of situation; All between power cable and outside soil, there is air layer; Its temperature field computation process comprises the The Coupling of solid and fluid, can adopt subregion to calculate, and the border coupling scheme are carried out the temperature field of whole field domain and calculated.

Subregion calculates, the implementation step of border coupling process is:

1. respectively the physical problem in power cable body, air, the soil is set up governing equation;

2. list each regional boundary condition, wherein solid area and air section be coupled borderline condition for temperature on the coupling border continuously and heat flow density continuous, promptly satisfy following formula (13) and formula (14).

3. electric power, air and soil are all carried out finite element solving (promptly whole by finding the solution with the mode of soil direct-burried) by conduction heat transfer, obtain the initial temperature field distribution.

4. according to the result of calculation in above-mentioned the 3rd step, find the solution convective-diffusive equation and radiation equation in the air-shed, to draw new Temperature Distribution on the coupling border.

5. calculate of the input of the boundary temperature of solid and fluid with above-mentioned the 4th step, find the solution the conduction governing equation, calculate the temperature field of power cable and soil region as power cable and soil.

6. repeat the above-mentioned 3-5 step, till satisfying the condition of convergence.The variable quantity of promptly working as temperature and fluid velocity is all less than 10 ^-10The time, can end iteration.Provide the final temperature field distribution and the maximum temperature of power cable.

From the above; As long as given air themperature, air wind speed, sun light intensity, soil moisture just can calculate face of land convection transfer rate, the coefficient of heat conductivity of soil; Thereby the temperature field that calculates whole field domain distributes, and finally calculates the maximum temperature of cable insulation.Then through alternative manner, with regard to the measurable current-carrying capacity that goes out the power cable under the current environment condition.

Claims (1)

1. underground power cable current-carrying capacity online prediction method is characterized in that:

(1) at first build underground power cable current-carrying capacity online prediction system:

Said underground power cable current-carrying capacity online prediction system is made up of the dispatching host machine (6) of the temperature sensor that is used for the Measurement of Air temperature (1), air velocity transducer (2), humidity sensor (3), SCM system (4), power department; The output terminal of said temperature sensor (1), air velocity transducer (2), humidity sensor (3) connects the respective input of said SCM system respectively, and said SCM system is connected with said dispatching host machine (6) through GSM network (5); Said temperature sensor (1) and air velocity transducer (2) be installed in said power cable directly over, said humidity sensor (3) is installed in the said power cable soil along the line;

(2) concrete steps of said underground power cable current-carrying capacity online prediction method are following:

(1) modeling:

A. soil plow-in cable:

Power cable is 1m apart from ground, and border, power cable both sides is apart from cable 10m, and in the calculating, border, said both sides normal orientation hot-fluid rate of change is 0, and promptly the soil moisture no longer changes; Below the power cable to the deep subsoil border apart from cable 10m, the earth deep layer that fetches earth boundary temperature is constant, and temperature value is 383K for the deep subsoil temperature;

B. calandria cable:

The comb top is 1m far from ground, and power cable is laid in the comb, and border, comb both sides is apart from comb 10m, and in the calculating, border, said both sides normal orientation hot-fluid rate of change is 0, and promptly the soil moisture no longer changes; Below the comb to the deep subsoil border apart from comb 10m, the earth deep layer that fetches earth boundary temperature is constant, and temperature value is 383K for the deep subsoil temperature;

C. plough groove type cable:

The ditch groove depth is 1m apart from ground, and power cable is laid in the groove, and iron plate is often laid at indoor groove top; Cement plate is often laid at outdoor groove top, and border, model both sides is apart from groove 10m, in the calculating; Border, said both sides normal orientation hot-fluid rate of change is 0, and promptly the soil moisture no longer changes; Below the groove to the deep subsoil border apart from groove 10m, the earth deep layer that fetches earth boundary temperature is constant, and temperature value is 383K for the deep subsoil temperature;

(2) subdivision: with power cable and surrounding soil subdivision thereof is little unit;

Under direct burial, calandria and three kinds of modes of plough groove type, adopting triangle or quadrilateral units is little unit with the The model subdivision;

(3) read the following data of real-time measurement: air themperature T _Air, face of land wind speed v _Air, relative moisture of the soil rh; And the coefficient of heat conductivity λ of calculating face of land cross-ventilation coefficient of heat transfer α and soil;

A. utilize following formula (1) to calculate face of land cross-ventilation coefficient of heat transfer α:

α＝7.371+6.43v _air ^0.75(1)

In the formula, α is the face of land cross-ventilation coefficient of heat transfer, the W/ (m of unit ²K); v _AirBe the face of land wind speed that air velocity transducer is measured in real time, the m/s of unit;

B. utilize formula (2) to calculate soil thermal conductivity λ:

$\mathrm{\&lambda;}=(11.33\log \mathrm{rh}-3.4){10}^{{0.649}_{{\mathrm{\&rho;}}_d}-2}---\left(2\right)$

In the formula, λ is a soil thermal conductivity, the W/ of unit (mK); ρ _dBe soil packing, units/m ³Rh is the relative moisture of the soil that humidity sensor is measured in real time;

(4) at first, choose initial current I1, the A of unit;

(5) call the finite element temperature calculation program, calculate the power cable insulation layer maximum temperature T1 under the electric current I 1, unit K;

(6) as T1 during less than power cable insulation layer long-term work insulation course tolerable temperature Tin, and Tin-T1＞0.3K, then get I2=2I1, went to for (7) step; As T1 during greater than Tin, and T1-Tin＞0.3K, then get I2=0.5I1, went to for (7) step; When | T1-Tin|≤0.5K, I1 is the power cable current-carrying capacity of asking, and withdraws from;

(7) call the finite element temperature calculation program, calculate the power cable insulation layer maximum temperature T2 under the electric current I 2;

(8) when | T2-Tin|＞0.3K, went to for (9) step; When | T2-Tin|≤0.3K, I2 is the power cable current-carrying capacity of asking, and withdraws from;

(9) get I=I2+ (I2-I1) (Tin-T2)/(T2-T1), call the finite element temperature calculation program, calculate the power cable insulation layer maximum temperature T under the electric current I;

(10) when | T-Tin|＞0.3K, get I1=I2, I2=I, T1=T2, T2=T goes to above-mentioned (9) step; When | T-Tin|≤0.3K, I is the power cable current-carrying capacity of asking, and withdraws from.

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