CN106599383B - A method for obtaining transient temperature rise between two cables - Google Patents

Abstract

The invention relates to a method for obtaining transient temperature rise between two cables, comprising the following steps: 1) separately obtaining the intrinsic coefficient of each cable in the two cables, and constructing a finite element physical model for heat dissipation of the two cables; 2) Determine the parameters in the finite element physical model, generate the transient temperature rise model between the two cables, and adjust the parameters after verification; 3) Take the ambient temperature as the temperature reference point to obtain the cable thermal load of the actual operation of the first cable, Calculate the transient temperature rise between the two cables according to the transient temperature rise model between the two cables, that is, the transient temperature rise of the thermal load of the first cable to the core wire of the second cable. The method includes the following steps: compared with the prior art, the present invention has the advantages of fast calculation, convenient calculation, easy expansion and the like.

Description

A method for obtaining transient temperature rise between two cables

technical field

The invention relates to the technical field of power cable operation, in particular to a method for obtaining transient temperature rise between two circuits of cables.

Background technique

Due to the particularity of power cable operation, it is generally impossible to obtain the core temperature of the power cable through direct measurement, especially the real-time transient temperature of the core. Therefore, technicians have proposed a variety of methods to calculate the core temperature of the power cable. Engineering formulas or approximate formulas based on numerical solutions and test results. For example, based on the IEC60287 standard, it is a classic calculation method for calculating the core temperature of power cables, and a solution method for ampacity based on numerical calculation. For the cable in actual operation, the numerical method is used to solve the problem. Considering the complexity of the working conditions, the required calculation amount is huge, the efficiency in the actual implementation is very low, and the required calculation time is very long. It is possible to know the transient temperature of the power cable core in real time. Compared with the time of numerical calculation, this real-time performance cannot be guaranteed.

Due to the limitation of calculation, it is often used to install light temperature measurement and other devices to obtain the cable skin temperature during operation, and then calculate the cable core temperature. The principle is shown in Figure 1. It also played a certain role in actual operation.

Among them, I ₁ is the heat loss of the cable, U ₁ is the core temperature, U ₀ is the skin temperature, R ₁ is the "core-skin" thermal resistance, and C ₁ is the "core-skin" heat capacity.

However, there are some defects in this type of method: 1) Essentially, this type of method only realizes the measurement and estimation of the temperature rise process of a single cable, and cannot separate the mutual influence between cables, so it cannot realize the temporary calculation under the condition of multiple cable load changes. 2) It depends heavily on the health status of the temperature measurement device, and defects such as temperature measurement deviation and transmission communication will directly restrict the selection of cable equipment operation. Considering that there are several sections of a cable that need to be monitored, it is necessary to install multiple sets of temperature measuring devices, so the reliability of the overall system is extremely low, which is also the fundamental reason why this type of method cannot be directly applied on a large scale; / The overall reliability of the system needs to enhance the redundant design, or even the method of combining multiple sets, which not only increases the initial investment of the device / system, but also brings a huge workload to the subsequent operation and maintenance.

Contents of the invention

The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a method for obtaining the transient temperature rise between two cables based on the time domain response, which is fast in calculation, convenient in calculation and easy to expand.

The purpose of the present invention can be achieved through the following technical solutions:

A method for obtaining the transient temperature rise between two cables, which is used to obtain the transient temperature rise of the heat load of the first cable to the core wire of the second cable, comprising the following steps:

1) Obtain the intrinsic coefficient of each of the two cables, including cable density, cable specific heat capacity, harmonic thermal conductivity, and the soil density, soil specific heat capacity, and soil heat transfer coefficient of the soil related to the single-circuit cable, and construct two Finite element physical model of back cable heat dissipation;

2) Determine the parameters in the finite element physical model, generate a transient temperature rise model between the two cables, and adjust the parameters after verification;

3) Take the ambient temperature as the temperature reference point to obtain the cable thermal load of the actual operation of the first cable, and calculate the transient temperature rise between the two cables according to the transient temperature rise model between the two cables, that is, the first cable The transient temperature rise of the heat load on the core wire of the second cable.

Described step 2) specifically comprises the following steps:

21) Given the step thermal load of the first cable, use finite element calculation to obtain the response curve of the thermal load of the first cable and the temperature rise of the second circuit core;

22) Obtain the specific parameters of the transient temperature rise model between the two cables according to the response curve, including the comprehensive thermal resistance R ₁ of the second cable core to the environment and the transition parameters R ₂ , L ₁ and C ₁ .

Described step 22) specifically comprises the following steps:

221) Calculate the comprehensive thermal resistance R ₁ of the second cable core to the environment according to the value of the steady-state response in the response curve;

222) Substitute the comprehensive thermal resistance R ₁ of the second cable core to the environment into the finite element physical model of the heat dissipation of the two cables, and perform response curve fitting to obtain the specific values of the transition parameters R ₂ , L ₁ and C ₁ , Under the same given step thermal load condition of the first circuit cable, make its response curve coincide with the response curve of the thermal load of the first circuit cable and the temperature rise of the second circuit core.

The transient temperature rise model between the two cables is:

Among them, U ₃ is the transient temperature rise between the two cables, U ₁ is the nominal temperature rise of the thermal resistance R ₂ , U ₂ is the nominal temperature rise of the thermal sensor L ₁ , I ₁ is the thermal load of the cable, and I ₂ is the Heat capacity C is the heat flow of the ₁ branch, I ₃ is the heat flow flowing through the integrated thermal resistance R ₁ branch, and t is time.

In the step 222), the transition parameters R ₂ , L ₁ , and C ₁ are obtained through a genetic algorithm.

In the described step 3), the calculation formula of cable heat load _I1 is:

I ₁ =I ² *R

R=R'(1+Y _s +Y _p )

R'＝R ₀ [1+α(θ-20)]

Among them, I is the cable current, R is the AC resistance of the cable, R′ is the DC resistance of the conductor at the highest operating temperature, Y _s is the skin effect factor, Y _p is the proximity effect factor, R ₀ is the conductor DC resistance at 20°C, α is the temperature coefficient of the copper conductor at 20°C, and θ is the operating temperature.

Compared with the prior art, the present invention has the following advantages:

1. Fast calculation: This method proposes a time-domain response-based heat transfer response model between cables for the first time, which overcomes the shortcomings of poor timeliness of numerical calculation methods, poor reliability of real-time monitoring methods, and cumbersome traditional thermal path methods, and can quickly realize the heat transfer between cables. l Response transient calculations.

2. Convenient calculation: The determination of this model does not depend on the calorific value or current of the cable itself, but only on the thermal characteristics of the materials around the cable, and such characteristics can be considered basically unchanged in the general operating temperature range, so when changing the cable current There is no need to repeat numerical calculations such as finite elements, and satisfactory results can be obtained directly through simple matrix and iteration.

3. Easy to expand: This method combines the transient temperature rise model of single-circuit cable, and through a certain combination, the rapid realization of multi-circuit cable temperature rise calculation can be realized.

Description of drawings

Figure 1 is the thermal circuit diagram of the lumped parameters of the existing single power cable.

Figure 2 shows the thermal response model between cables.

Figure 3 is the finite element calculation model.

Figure 4 is the finite element calculation results.

Figure 5 shows the evolution process of genetic algorithm for parameter extraction.

Figure 6 is the mutual response model.

Figure 7 is a comparison of the calculation results of the transient temperature rise of cable 2.

Figure 8 is the calculation model of CYMCAP.

FIG. 9 shows the temperature rise process of the cable 1 .

FIG. 10 shows the influence of each cable on the temperature rise process of the cable 1 .

Detailed ways

The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

Example:

1. The principle of this method is as follows:

The heating of multi-circuit cables mainly depends on the cable loss and the thermal characteristics of the section where it is located. The former has a clear correspondence with the operating current and operating temperature and can be directly applied; while the latter mainly depends on the geometric parameters of the section and the physical parameters of each part. , it can be considered that the physical parameters remain unchanged in the operating temperature range, which provides a theoretical basis for the establishment of the model.

The temperature reference point of the model proposed in the present invention is the ambient temperature, and the model is shown in Figure 2 below (taking the heat transfer from cable 1 to cable 2 as an example).

Among them, I1 is the thermal load of cable 1, which can be obtained through electrical knowledge; R1 is the comprehensive thermal resistance of cable 2 cores to the environment; L1 and C1 have no clear physical meaning, and are used to produce different transition processes; R2 is damping, It is used to suppress the possible overshoot caused by L1 and C1 components.

The parameters in this model are divided into two categories, the first category is R1, which has a clear physical meaning, and the second category C1, R2 and L1 parameters have no clear physical meaning, and the two categories together constitute the transient temperature rise model. After the model structure is confirmed, for each group of parameters (C1, R1, R2 and L1), the determined thermal load will correspond to a transient temperature rise process in the time domain, and the genetic algorithm is used to adjust the parameter values to gradually approach the convergence target, so that The transient process of the model is as consistent as possible with the transient process obtained by numerical calculation or experiment.

2. The main steps of this method include:

(1) Finite element calculation

1) Calculation model

This method adopts finite element calculation, and other numerical calculation or test methods can also be used in practical applications. The finite element calculation model selected in this example is shown in Figure 3.

In the figure, A1 and A2 are cable sections; heat current carrying capacity is a step wave; boundary conditions 1, 2, 3, and 4 are all set at a temperature of 30 degrees Celsius; the soil density is 1500kg/m3, and the cable density is 8900kg/m3; the specific heat capacity of the soil is 855J/(kg*K), the specific heat capacity of the cable is 400J/(kg*K). Considering that high-voltage power cables often contain multi-layer structures, and some structural layers are very thin. Since the cable is a cylindrical axisymmetric structure with the same thermal resistance in all directions, the multilayer cable structure can be equivalent by using the harmonic average method, and the structure of each layer outside the conductor in the multilayer cable is equivalent to an equivalent outer sheath , in the above example, the harmonic thermal conductivity is set to 23.3W/m2*K, and the soil heat transfer coefficient is 1W/m2*K.

2) Calculation working condition setting

The calculation conditions are: cable 1: 0-100(*1000s), 74.96W; cable 2: no load for now.

3) Calculation result

Figure 4 shows the calculation results of the temperature rise process of cables 1 and 2.

(2) Extraction of model parameters

1) Comprehensive thermal resistance R1 of 2 cores of the cable to the environment

According to the finite element calculation results, after the transition process, the temperature rise of the cable 2-core wire is 0.983K, and the heat load of the cable 1 is 74.96W, so the comprehensive thermal resistance of the cable 2-core wire to the environment R1=0.983/74.96=0.0132.

2) R2, L1, C1

Since the above parameters have no clear physical meaning, they are only used to reflect the transition process of cross-section heat transfer, and genetic algorithms are used to obtain them.

A) Set the parameter range: take L1∈(0,500), C1∈(0,500), R2∈(0,1000), binary code, the initial population size is 200, the maximum genetic algebra is 100, the crossover probability is 0.75, and the mutation probability is 0.15.

B) Set the fitness function: according to the transient temperature response m (i) of the model shown in Figure 2 and the deviation of the two curves of the transient temperature fem (i) shown in Figure 4 as the fitness function, take

C) Set the convergence criterion: the fitness function is less than 10 or the number of genetic algebra exceeds 100, and the evolution process is shown in Figure 5.

4) Overall model

The overall model is shown in Figure 6.

(3) Model verification

1) Calculation example 1

Apply the above model to calculation conditions (cable 1: 0-100(*1000s), 74.96W, step load; cable 2: no load for now), and compare it with finite element calculation, as shown in Figure 7.

From the perspective of response comparison, the model described in Figure 6 can better realize the simulation of the temperature rise process of cable 2 under the condition of cable 1 loaded and cable 2 unloaded. It should be noted that, 1) Although the model described in Figure 6 is not necessarily optimal, it can meet the actual needs of the project; 2) According to the characteristics of the response process, the model may be adjusted, but the response-based model building method is in line with actual.

2) Calculation example 2

In order to verify the feasibility and effectiveness of the proposed method, the method was compared with the general software CYMCAP.

The calculation model of CYMCAP is shown in Fig. 8.

The method and process of solving the model parameters are as described above.

The calculation conditions are shown in Table 1:

Table 1 Calculation working conditions

0-24h 24-48h 48-72h 72-96h cable 1 500A 500A 500A 500A cable 2 500A 1000A 1000A 750A cable 3 500A 500A 750A 1000A cable 4 500A 1000A 1000A 500A

The calculation results are shown in Figures 9 and 10. It can be seen from the above comparison that the effectiveness of the method proposed in this method can separate the influence of adjacent cables on its temperature rise, thus ensuring the reliability and accuracy of the overall temperature rise calculation. sex.

Claims (2)

1. A method for obtaining transient temperature rise between two cables, used to obtain the thermal load of the first cable to the transient temperature rise of the second cable core wire, it is characterized in that, comprising the following steps: 1) Obtain the intrinsic coefficient of each of the two cables, including cable density, cable specific heat capacity, harmonic thermal conductivity, and the soil density, soil specific heat capacity, and soil heat transfer coefficient of the soil related to the single-circuit cable, and construct two Finite element physical model of back cable heat dissipation; 2) Determine the parameters in the finite element physical model, generate the transient temperature rise model between the two cables, and adjust the parameters after verification, specifically including the following steps: 21) Given the step thermal load of the first cable, use finite element calculation to obtain the response curve of the thermal load of the first cable and the temperature rise of the second circuit core; 22) Obtain the specific parameters of the transient temperature rise model between the two cables according to the response curve, including the comprehensive thermal resistance R ₁ of the second cable core to the environment and the transition parameters R ₂ , L ₁ and C ₁ , specifically including the following steps : 221) Calculate the comprehensive thermal resistance R ₁ of the second cable core to the environment according to the value of the steady-state response in the response curve; 222) Substitute the comprehensive thermal resistance R ₁ of the second cable core to the environment into the finite element physical model of the heat dissipation of the two cables, and perform response curve fitting to obtain the specific values of the transition parameters R ₂ , L ₁ and C ₁ , Under the same given step thermal load condition of the first circuit cable, make its response curve coincide with the response curve of the thermal load of the first circuit cable and the temperature rise of the second circuit core; The transient temperature rise model between the two cables is: Among them, U ₃ is the transient temperature rise between the two cables, U ₁ is the nominal temperature rise of the thermal resistance R ₂ , U ₂ is the nominal temperature rise of the thermal sensor L ₁ , I ₁ is the thermal load of the cable, and I ₂ is the Heat capacity C ₁ branch heat flow, I ₃ is the heat flow flowing through the integrated thermal resistance R ₁ branch, t is time; The calculation formula of cable thermal load I ₁ is: I ₁ =I ² *R R=R'(1+Y _s +Y _p ) R'＝R ₀ [1+α(θ-20)] Among them, I is the cable current, R is the AC resistance of the cable, R′ is the DC resistance of the conductor at the highest operating temperature, Y _s is the skin effect factor, Y _p is the proximity effect factor, R ₀ is the conductor DC resistance at 20°C, α is the temperature coefficient of the copper conductor at 20°C, θ is the operating temperature; 3) Take the ambient temperature as the temperature reference point to obtain the cable thermal load of the actual operation of the first cable, and calculate the transient temperature rise between the two cables according to the transient temperature rise model between the two cables, that is, the first cable The transient temperature rise of the heat load on the core wire of the second cable. 2. A method for obtaining transient temperature rise between two cables according to claim 1, characterized in that, in the step 222), the transition parameters R ₂ , L ₁ , and C ₁ are obtained through a genetic algorithm.