CN108801501A - Cable core thermometry based on temperature gradient and thermal power conduction model - Google Patents

Abstract

The cable core thermometry based on temperature gradient and thermal power conduction model that the present invention relates to a kind of, comprises the following steps：S1：One layer of thermal insulation layer is set in the outer surface of cable, and in thermal insulation layer setting temperature sensor Tr and Tw；S2：The thermal power for the medium surfaces externally and internally that the thickness of thermal insulation layer outer surface is dw is denoted as P0 and P1；S3：The thermal power for the medium surfaces externally and internally that the thickness of thermal insulation layer inner surface is dr is denoted as P2 and P3；S4：According to step S2 and S3, the .Pn that the thermal power of the radial surfaces externally and internally of each layer dielectric layer of cable core is denoted as P4, P5 respectively ...；S5：The temperature gradient equivalent network inside cable is established in conjunction with thermal losses；S6：The temperature that thermal insulation layer surfaces externally and internally is measured by temperature sensor Tr and Tw calculates the temperature of cable core according to temperature gradient equivalent network.The method of the present invention has many advantages, such as that measurement scheme is simple in structure, while at low cost and high certainty of measurement.

Description

Cable Core Temperature Measurement Method Based on Temperature Field Gradient and Thermal Power Conduction Model

technical field

The invention belongs to the technical field of cable conductor temperature measurement, and relates to a cable core temperature measurement method based on a temperature field gradient and a thermal power conduction model.

Background technique

With the expansion of the city scale and the rapid development of economic construction, the voltage level of the cable continues to increase, and the transmission energy continues to increase. This has led to the operation management, monitoring and maintenance of power cables becoming more and more important. Operating temperature is an important parameter of the cable. When the cable is operating under rated load, the conductor temperature reaches the allowable value; once overloaded, the conductor temperature will rise sharply, accelerating insulation aging, and even thermal breakdown. Therefore, whether it is from the perspective of the safe operation of the power cable itself or from the perspective of power system dispatching needs, it is necessary to measure the conductor temperature of the power cable.

At present, there are two main methods for measuring the temperature of cable conductors. The first is the indirect measurement method, that is, the surface temperature of the cable is measured by a temperature sensor, and then the internal temperature of the core wire is calculated according to the thermodynamic method. The main method is to establish a thermodynamic model of the power cable, study the temperature distribution of each layer of the high-voltage power cable and calculate the thermal resistance of each layer, or establish a Laplace thermal circuit model and a BP neural network model to solve the thermodynamic model. However, there are two problems in this indirect measurement method of inverting the temperature by establishing a model. First, the thermal resistance and thermal capacity of the cable system are closely related to the environment, and the direct measurement of the skin temperature is greatly affected by the temperature. , it is difficult to obtain a general numerical calculation method; secondly, the temperature time constant of the cable is relatively large, generally more than several hours, so the conductor temperature cannot be obtained quickly.

Another method of cable conductor temperature measurement is the direct measurement method. At present, the distributed optical fiber method based on Raman scattering and the optical fiber Bragg grating measurement method are more researched. Because the optical fiber has good insulation, it can directly contact the cable conductor or the hot part of the joint. However, the distributed optical fiber temperature measurement system needs to pre-implant the optical fiber inside the cable, which has strict requirements on the production and transportation of the cable, and the cost is high. At the same time, the method of direct temperature measurement with optical fiber has high technical requirements for the on-site installation process, especially the way the optical fiber is connected at the joint and the way the optical fiber is drawn from the high-potential guide core to the zero-potential sensor device still has certain technical difficulties. Therefore, this technology has achieved good results in theoretical design, but there are still obstacles to its large-scale application.

In summary, how to achieve accurate measurement of cable core temperature still has great research value. Although the direct measurement method is accurate, it is extremely difficult to realize large-scale application due to the complexity of the device and high cost, while the indirect measurement can only indirectly measure the cable core temperature by measuring the skin temperature and establishing a thermodynamic model, and the measured value is affected by the temperature. No matter how the model is optimized, the influence of this factor cannot be avoided.

Contents of the invention

In view of this, the purpose of the present invention is to provide a cable core temperature measurement method based on the temperature field gradient and thermal power conduction model, to provide a solution that can accurately measure the steady-state temperature of the cable core, the measuring equipment is simple, and the measurement results precise.

To achieve the above object, the present invention provides the following technical solutions:

The cable core temperature measurement method based on the temperature field gradient and the thermal power conduction model includes the following steps:

S1: Set a layer of heat insulation layer on the outer surface of the cable, and set temperature sensors Tr and Tw on the inner and outer surfaces of the heat insulation layer;

S2: Record the thermal power of the inner and outer surfaces of the medium with the thickness of the outer surface of the heat insulation layer as dw (very small) as P0 and P1;

S3: Record the thermal power of the inner and outer surfaces of the medium with the thickness of the inner surface of the heat insulation layer as dr (very small) as P2 and P3;

S4: According to steps S2 and S3, respectively record the thermal power of the inner and outer surfaces of each dielectric layer in the radial direction of the cable core as P4, P5....Pn, where n is the total number of inner and outer surfaces of the dielectric layer;

S5: When the thermal equilibrium steady state is reached, an equivalent network of the temperature field gradient inside the cable is established in combination with the heat loss;

S6: Measure the temperature of the inner and outer surfaces of the heat insulation layer through the temperature sensors Tr and Tw, and calculate the temperature of the cable core according to the established equivalent network of the temperature field gradient.

Further, the step S5 is specifically:

S51: After the thermal equilibrium is in a steady state, the thermal power of the inner and outer surfaces of each layer of media and the heat stored in each layer of media when each layer of media reaches a thermal steady state balance are calculated by the formula:

P ₀ ＝P ₁ ＝P ₂ ＝P ₃ ＝…＝ _Pn

Q _＝ Q0＝Q1 _{＝ Q2} ＝ _Q3 ＝…＝ _Qn

Among them, Q is the thermal energy emitted or absorbed by the surface of each layer of medium per unit time, and P _n is the thermal power emitted or absorbed by the surface of each layer of medium per unit time;

S52: formulate the temperature gradient according to the boundary conditions of the medium;

S53: Formulate calculation formulas for temperature difference and temperature gradient, medium radius, and heat transfer coefficient;

S54: Fit the relationship between the temperature gradient and the medium radius according to the temperature field simulation.

Further, in step S51, the thermal energy emitted or absorbed by the surface of each layer of medium per unit time is

Among them, T0 is the temperature of the outer surface of the heat insulation layer in contact with air, T1 is the temperature of the contact surface between the inner surface of the heat insulation layer and the medium layer, λ1 is the thermal conductivity coefficient _of the heat insulation layer, L is the length of the cable, R1 and R2 are the insulation The inner and outer radii of the thermal layer.

Further, the temperature gradient described in step S52 is:

In the formula, is the temperature gradient, i is the number of dielectric layers i∈[0,1,2,…,n/2], where i=0 represents the surface in contact with the air, i=n/2 is the surface of the cable core, λ _i is The thermal conductivity of the dielectric layer with a radius between R(i+1) and Ri, where r is the radius of the medium, and Ti is the surface temperature of the medium at the radius Ri.

Further, in step S54, the formula for calculating the temperature difference and the temperature gradient, the radius of the medium, and the thermal conductivity coefficient is:

In the formula, T is the temperature at the medium radius r.

The beneficial effects of the present invention are:

1. The measurement scheme of the present invention is simple in structure, and does not need complex equipment and high-tech and expensive sensors when detecting temperature.

2. Reduce measurement cost and realize large-scale application. Due to the optimization of the measurement method, the devices and devices used are relatively cheap, so large-scale applications can be realized.

3. Improve the measurement accuracy. The invention adopts a brand-new arrangement mode of multi-layer medium sensors, and adopts a new method for calculating temperature based on temperature field gradient and thermal power flow model, thereby improving measurement accuracy.

4. The method of measuring the skin temperature is no longer used, and a new measurement method with the addition of a heat insulating medium layer is proposed. Different from the previous method of directly measuring the temperature of the cable skin, it avoids the interference of external environmental factors and improves the accuracy of measurement.

5. It is not necessary to solve differential equations containing relational expressions of heat sources.

6. Analyze the temperature distribution characteristics of the multi-medium layer and propose a new sensor arrangement, which is different from the previous single-sensor measurement mode.

Description of drawings

In order to make the purpose, technical scheme and beneficial effect of the present invention clearer, the present invention provides the following drawings for illustration:

Fig. 1 is a schematic diagram of cable structure and temperature sensor arrangement of the present invention;

Fig. 2 is a schematic diagram of calculation of cable core temperature according to an embodiment of the present invention;

Figure 3 is the same medium in the curve;

Fig. 4 is the relation curve of temperature gradient and r;

Fig. 5 is a schematic diagram of the temperature field gradient equivalent network.

Detailed ways

The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

As shown in Figure 1, the cable structure comprises the inner cable core 4, the intermediate dielectric layer 3, and the measurement method proposed by the present invention sets the heat insulation layer 5 at the cable outer layer, and the heat insulation layer external temperature sensor Tw and the heat insulation layer internal temperature Sensor Tr.

Since the cable can only obtain the temperature of the skin in general, this method adopts the method of wrapping a layer of dielectric heat insulation layer 5 on the cable skin to obtain the temperature of two points on the inner and outer surfaces of the heat insulation layer and to avoid the influence of environmental factors. The short-term measurement process will not cause the temperature of the cable core to rise when the medium and the heat insulation layer are added. Since the measurement of the present invention is aimed at the steady-state temperature, heat balance is reached on the surface of each dielectric layer at this time, and the total power on each dielectric layer is equal to the thermal power conducted by the heat source (cable core).

The specific implementation plan is as follows:

As shown in Figure 3, the thermal power P ₀ and P ₁ of the inner and outer surfaces of the heat insulation layer are firstly obtained through the thermal simulation model, and the P ₂ and P ₃ of the inner surface of the heat insulation layer are also obtained, as well as the dielectric power of each layer in the direction of the cable core. Thermal power P ₃ , P ₄ , P ₅ . . . Pn of inner and outer surfaces.

In the steady state of heat balance, the surface temperature of each layer of medium reaches a heat balance, and the temperature also reaches a stable temperature value, that is:

P ₀ ＝P ₁ ＝P ₂ ＝P ₃ ＝…＝ _Pn

Q _＝ Q0＝Q1 _{＝ Q2} ＝ _Q3 ＝…＝ _Qn

Q is the thermal energy emitted (or absorbed) per unit time by the surface of each layer of media, and P _n is the thermal power emitted (or absorbed) per unit time by the surface of each layer of media.

As shown in the figure, R2 and R1 are the inner and outer radii of the heat insulation layer, the cable core radius R5, λ1 is the heat conduction coefficient of the heat insulation layer, λ2 is the heat conduction coefficient of medium III, λ3 is the heat conduction coefficient of medium II, and λ4 is the heat conduction of medium I coefficient.

ρ ₁ is the heat flow per unit area of the outer surface of the insulation layer, ρ ₂ is the heat flow per unit area of the outer surface of medium III, ρ ₃ is the heat flow per unit area of the outer surface of medium II, ρ ₄ is the heat flow per unit area of the outer surface of medium Ⅰ, h is the heat insulation layer and air conversion coefficient, S is the surface area of the cable, and L is the length of the cable.

For the heat insulation layer (R1>r>R2), there is convective heat exchange with the air, the outer surface of the heat insulation layer and the air belong to the third type of boundary condition, and the inner and outer temperatures of the outer surface are Ta+ and Ta respectively.

S=2πrL

Tb＝T| _r＝R2 (boundary condition)

For medium 3 (R2>r>R3)

For medium 2 (R3>r>R4)

For medium 1 (R4>r>R5)

The relationship between temperature difference and temperature gradient, radius, and thermal conductivity:

Insulation layer (R1>r>R2):

For medium 3 (R2>r>R3):

For medium 2 (R3>r>R4):

For medium 1 (R4>r>R5):

From the above formulas, it can be seen that in the same medium both satisfy is a fixed value, Satisfy the curve relationship shown in Figure 3.

According to the fitted temperature gradient curve, the outer surface temperature Tc, Td, Te (cable core temperature) of each medium can be solved sequentially.

Obtain the temperature Ta and Tb of the inner and outer surfaces of the heat insulation layer through the temperature sensor, so as to obtain the relationship between the temperature difference and the heat flow Q, and simulate and fit it according to the temperature field Curve—The relationship between temperature gradient and r is shown in Figure 4, and the value of the cable core temperature Te can be obtained by combining the above formula principle.

Finally, the temperature field gradient equivalent network of the internal temperature of the cable is established as shown in Figure 5, where DC represents the heat source of the cable core, Ra, Rb, Rc, and Rd represent the thermal resistance of each dielectric layer, and an experimental measurement platform is built to complete the temperature measurement. Calibration and correction of measurement curves.

Finally, it is noted that the above preferred embodiments are only used to illustrate the technical solutions of the invention and not limit them. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that it may be possible in form and details. Various changes can be made to it without departing from the scope defined by the claims of the present invention.

Claims (5)

1. the cable core thermometry based on temperature gradient and thermal power conduction model, it is characterised in that：Including as follows Step：

S1：One layer of thermal insulation layer is set in the outer surface of cable, and temperature sensor Tr is respectively set in the surfaces externally and internally of thermal insulation layer And Tw；

S2：The thermal power for the medium surfaces externally and internally that the thickness of thermal insulation layer outer surface is dw is denoted as P0 and P1；

S3：The thermal power for the medium surfaces externally and internally that the thickness of thermal insulation layer inner surface is dr is denoted as P2 and P3；

S4：According to step S2 and S3, respectively by the thermal power of the radial surfaces externally and internally of each layer dielectric layer of cable core be denoted as P4, P5 ... .Pn, wherein n are the surfaces externally and internally sum of dielectric layer；

S5：After reaching thermal balance stable state, the temperature gradient equivalent network inside cable is established in conjunction with thermal losses；

S6：The temperature that the thermal insulation layer surfaces externally and internally is measured by temperature sensor Tr and Tw, according to the temperature field ladder established Spend the temperature that equivalent network calculates cable core.

2. the cable core thermometry according to claim 1 based on temperature gradient and thermal power conduction model, It is characterized in that：The step S5 is specially：

S51：After thermal balance stable state, the thermal power of each layer medium surfaces externally and internally of the every layer of medium when reaching hot homeostasis and The heat being stored in each layer medium meets following relationship：

P₀=P₁=P₂=P₃=...=P_n

Q=Q₀=Q₁=Q₂=Q₃=...=Q_n

Wherein, Q is the thermal energy for each layer dielectric surface unit interval sending out or absorbing, P_nIt is sent out for each layer dielectric surface unit interval The thermal power for going out or absorbing；

S52：According to dielectric boundaries condition intended temperature gradient；

S53：Intended temperature difference and temperature gradient, medium radius, the calculating formula of the coefficient of heat conduction；

S54：The relationship of temperature gradient and medium radius is fitted according to Temperature Field Simulation.

3. the cable core thermometry according to claim 2 based on temperature gradient and thermal power conduction model, It is characterized in that：The thermal energy that each layer dielectric surface unit interval sends out or absorbs in step S51 is

Wherein, T0 is the temperature that thermal insulation layer contacts outer surface with air, and T1 is the temperature of thermal insulation layer inner surface and dielectric layer interface Degree, λ₁For the coefficient of heat conduction of thermal insulation layer, L is cable length, and R1, R2 are the interior outer radius of thermal insulation layer.

4. the cable core thermometry according to claim 3 based on temperature gradient and thermal power conduction model, It is characterized in that：Temperature gradient is described in step S52：

In formula,For temperature gradient, i is medium number of plies i ∈ [0,1,2 ..., n/2], and when wherein i=0 represents to be contacted with air Surface, i=n/2 be cable wicking surface, λ_iIt is the coefficient of heat conduction of the radius in R (i+1) to the dielectric layer between Ri, r is Jie Matter radius, Ti are that radius is dielectric surface temperature at Ri.

5. the cable core thermometry according to claim 4 based on temperature gradient and thermal power conduction model, It is characterized in that：Step S54 moderates difference and temperature gradient, the calculating formula of medium radius, the coefficient of heat conduction are：

In formula, T is that medium radius is temperature at r.