JPH0752126B2 - Maximum temperature evaluation method for underground power cables - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention generally relates to safe and efficient operation of a power cable line buried in the ground whose thermal constant is unclear.
It relates to a method for evaluating the maximum temperature of a cable.

[Prior Art] Power cables are often buried underground, but the thermal conditions of the surrounding soil are uncertain, and the thermal effects of other cables and buried objects are difficult to predict. Cable temperature control is extremely difficult. The main purpose of temperature control of power cables is to suppress chemical deterioration and mechanical deterioration due to temperature rise within a certain limit over many years, and for this purpose, current measures are sufficiently effective. . For example, the specific thermal resistance of the surrounding soil and the base temperature (soil temperature when no heat-generating material is buried in the soil) are set as numerical values on the safe side, and the target cable is operated under those conditions. There is a method of calculating the permissible current so that the temperature at that time is within the permissible value and operating within this current range (see JCS-168 etc.).

However, this method is uneconomical if safety is overlooked, and if another cable or heating element is added in the vicinity, the temperature will rise above the calculated value due to the influence, resulting in dangerous operation. Thermal conditions of soil cannot be expected to remain constant over many years. Recently, it has also been attempted to directly or indirectly cool the cable to remove a part of heat generation, suppress the temperature rise, and increase the allowable current, but this is also caused by excessive cooling due to the surrounding thermal conditions. Dangerous operation due to uneconomical operation and insufficient cooling may occur.

A method for alleviating such an uncertain state and performing safe and efficient cable design and operation is also considered. Basically, the temperature of the cable, the surroundings of the cable, or the soil around it is monitored, and the temperature of each cable is calculated directly or indirectly.

When the temperature of each cable is directly measured, for example, the surface temperature and the heat generation amount are measured, and the maximum temperature of the conductor is obtained from the cable constituent material and the thermal resistance calculated from the dimensions.

Also, if the cable temperature cannot be directly measured with the underground cable, monitor the temperature of other adjacent pipes and soil in the vicinity, and based on this and the separately measured cable heat value, the cable heat The conductor temperature is calculated from the resistance and the constant coefficient of mutual heat effect (mutual thermal resistance) between the cable and the temperature measurement point, which is calculated from the buried arrangement.

[Problems to be Solved by the Invention] It is possible to obtain the conductor temperature by the above method and use it for control and management of cable operation. But,
This method has the following problems.

(A) It is necessary to separately measure the heat generation amount of the cable by a complicated method, and it is necessary to attach a sensor to each and install a transmission system that collects the signal at one place.

(B) Cables are laid over many kilometers, and the thermal conditions of the soil, the placement of the soil, the presence of other cables and the presence of heat-generating substances, etc. during this period vary greatly in the length direction. Therefore, in order to obtain the temperature of the target power cable, it is necessary to monitor the soil temperature in the vicinity over the entire length, but there is no appropriate measuring method for it.

The present invention solves the above-mentioned problems of the prior art, measures the temperature of each part with a series of optical fibers, and calculates the heat generation amount of the power cable from this, further calculates the temperature of the cable conductor, It is to provide a new method for evaluating the maximum temperature of a power cable, which also seeks the maximum value.

[Means for Solving the Problems] In the present invention, a series of optical fibers are laid almost along a power cable buried in the ground, and the temperature is used as a sensor for measuring temperature distribution by detecting backscattered light. Using a distribution measuring device, determine the amount of heat generated by the power cable from the measured values, and by determining the temperatures of at least two points in the cross section of the power cable laying, the specific thermal resistance of the unknown surrounding soil and the base temperature Then, the cable temperature within the laying section is calculated from these values, and this operation is repeated to obtain the maximum cable temperature over the entire length of the line.

To determine the amount of heat generated by the power cable, the corrosion resistance layer inner surface, outer surface, peripheral location separated from the cable surface, or the outer surface of the heat resistance layer attached to the cable outer peripheral surface at approximately the same position in the length direction of the power cable. A part of the optical fiber as the temperature distribution measuring sensor is attached in a spiral shape with a length equal to or more than a certain length λ at any two positions, and the average temperature in the circumferential direction of the power cable is determined for each spiral portion. It is preferable to calculate the amount of heat generated by the power cable based on the temperature difference between two points in the radial direction measured by the temperature distribution measuring device and the known thermal resistance therebetween.

In addition, when a transportation pipe for cooling or heating is provided near the power cable, a part of the optical fiber as the temperature distribution measuring sensor is provided at least at a certain length of the transportation pipe with a certain length λ or more. The temperature of the electric power cable in the cross-section direction and the length direction of the line is calculated by considering the temperature of the transport pipe measured by the temperature distribution measuring device at this contact portion at the same time by making contact with the length and evaluating the maximum temperature. Is preferably added to

By taking the soil base temperature in the ground sufficiently distant from the power cable line as one of the temperatures of at least two locations measured in the power cable laying cross section, and by using only the soil specific thermal resistance as an unknown constant related to soil, It is possible to simplify the calculation of the power cable temperature.

[Operation] The temperature distribution measuring device measures a linear temperature distribution on the optical fiber by using a series of optical fibers installed almost along the power cable as a sensor. From the measured value of the temperature distribution measuring device, the amount of heat generated by the power cable is obtained, and at the same time, the specific thermal resistance and the base temperature of the unknown surrounding soil at that portion are obtained from the temperatures of at least two points in the cross section of the power cable laying, From these, the cable temperature within the installed cross section is calculated. In this case, the cable conductor temperature is indirectly obtained by obtaining the cable heat generation amount and the soil heat condition, and using these, the buried position dimension, and the thermal constant of the cable body. Then, this operation is repeated to obtain the maximum temperature in the cable length direction over the entire length of the line.

In order to determine the amount of heat generated by the power cable, according to the second aspect of the present invention, the power cable is attached to the inner surface, the outer surface, a peripheral location apart from the cable surface, or the outer surface of the cable at substantially the same position in the length direction. An optical fiber as a temperature distribution measuring sensor is attached in a spiral shape to any two locations on the outer surface of the heat resistance layer. By thus winding the optical fiber in a spiral shape, the temperature distribution measuring device measures each of the above two points as an average temperature in the circumferential direction of the power cable, and the circumference of the power cable caused by the convection in the surroundings. The distortion of the heat quantity in the direction is corrected. In this case, the length of the optical fiber of each spiral portion is determined by the constant length λ for measuring the average temperature determined by the time width of the incident light pulse.
With the above length, the sampling points of the temperature distribution measuring device are always included in this spiral section, and the measurement error is reduced. Then, the amount of heat generated by the power cable is calculated from the temperature difference between the two points in the radial direction of the power cable measured by the temperature distribution measuring device and the known thermal resistance therebetween.

In the third aspect of the invention, the temperature distribution measuring optical fiber is brought into contact with at least one location of the transport pipe by at least λ, and the temperature of the transport pipe is simultaneously measured. As a result, even in the forced cooling (heating) line, it is possible to calculate the temperature of the power cable in the cross-sectional direction and the length direction of the line and evaluate the maximum temperature. Therefore, it is suitable for a case where a transportation pipe for cooling the power cable or for multi-purpose cooling and heating is provided side by side.

When the soil base temperature in the ground sufficiently distant from the power cable line is taken as one of the two temperatures measured in the buried power cable laying cross section as in the form of claim 4, an unknown constant related to soil is obtained. Is only the soil specific heat resistance, which simplifies the calculation of the power cable temperature.

[Example] First, the principle of the optical fiber type temperature distribution measuring device as a premise will be briefly described.

FIG. 1 schematically shows a temperature distribution measuring device using an optical fiber. The optical pulse emitted from the optical pulse light source 1 is sent to the optical fiber 2
When incident on, it propagates at the speed of light while generating scattered light. Part of the scattered light returns to the incident end of the optical fiber 2 as backscattered light. Of the scattered light, the light intensity of Rayleigh scattered light, Raman scattered light, etc. depends on the temperature of the optical fiber 2. Therefore, the optical demultiplexer 3 is inserted on the incident end side to take out the necessary backscattered light, and this is guided to the light receiver 4 to detect the time change of the backscattered light after the incidence. The time can be converted into “position” and the light intensity can be converted into “temperature”, and the temperature distribution in the length direction of the optical fiber 2 can be measured. In addition, in order to improve the measurement accuracy of weak backscattered light and the measurement temperature accuracy, an averaging processing device 5 that performs averaging processing on pulsed light of tens of thousands of times, data that calculates the correlation of multiple components of backscattered light. The processing device 6 and the like are also used.

Now, the time width of the incident light pulse is approximately τ (ns).
The backscattered light received at a certain time 2t after the incidence of the light pulse has a minute time Δτ and a minute distance Δλ (where the speed of light is v,
When it is decomposed into Δλ = v · Δτ), “scattered light generated at distance 1 at time t” (solid line in FIG. 2) and “time t + Δ”
Scattered light generated at a distance of l-Δλ at τ "(dotted line in Fig. 2)
And "scattered light generated at distance l-2Δλ at time t + 2Δτ" ... That is,
The scattered light received at a certain time 2t is a λ width (λ
= V · τ / 2), which is the overlap of the scattered light, which can be said for the received light at all times, and equivalently corresponds to the average value between the λ widths. The temperature distribution obtained from such a signal is not a temperature distribution with connected points, but λ
It is the one that connects the average temperatures in the range.

(1) Cable calorific value Next, a method for obtaining the cable calorific value using the optical fiber type temperature distribution measuring device will be described.

Generally, as shown in FIG. 3, when the amount of heat w flows through an object having a certain thermal resistance r, the temperature difference ΔT between both ends is ΔT = w · r. That is, if the temperature difference ΔT on both sides of the known thermal resistance r is measured, the heat flow flowing there can be measured.

Also in the case of the power cable, as shown in FIG.
The temperature difference ΔT ₁ between the outer surface and the outer surface of the anticorrosion layer 8 or the temperature difference ΔT ₂ between the surface of the anticorrosion layer 8 and the ambient air 9 is measured, and the thermal resistance of the anticorrosion layer or the surface radiation heat resistance is known. Therefore, the heat flow can be obtained. However, real cables require extremely complex measurements. The reason is that when the outer surface of the cable is covered with a fluid such as air or water, convection occurs. For example, when the cable is horizontal, a large amount of heat flow collects in the upward direction as shown by the arrow in FIG. As a result, the heat flow flowing in the radial direction of the cable is not uniform in the circumferential direction, and a different heat flow is obtained depending on which two points in the circumferential direction the temperature difference is measured.

Therefore, in order to obtain a heat quantity that is as accurate as possible (correct as the total heat quantity flowing out from the entire cable), the temperature of the cable sheath 7, the surface of the anticorrosion layer 8 and the ambient air 9 should be at least raised and lowered in the circumferential direction. Measure at 4 points on the left and right,
The measurement system and the data processing system are complicated because it is necessary to perform the operation of obtaining each average value.

In addition, a sheet or block having a known thermal resistance is attached to the cable surface and a heat flow meter that measures the temperature on both sides of the cable is measured. Also, the cable current is measured to calculate the conductor Joule loss, and the sheath loss and dielectric loss are calculated. There are methods such as calculating body loss to obtain the total amount of heat generated by the cable, but in all cases, measurement other than temperature becomes complicated, and the conductor loss and sheath loss of the power cable change depending on the mutual positional relationship in the length direction. Therefore, since measurement is required for each section where the laying arrangement changes, a large number of measuring instruments are required, which is not economical.

However, these problems can be solved all at once by using the above-mentioned optical fiber type temperature distribution measuring device and carrying out the following.

That is, as shown in FIG.
Spirally wound around the surface of the anticorrosion layer 8 of
The extended portion from is further spirally arranged around the spiral portion 2a and folded back. Then, the length of each spiral portion 2a, 2b is set to λ or more as described above. By doing so, the temperature in the circumferential direction of the power cable 10 can be easily measured as an average value from the respective spiral portions 2a and 2b. Therefore, it is not necessary to measure four points on the top, bottom, left and right of the cross section of the power cable. In addition, in order to obtain a more accurate average temperature in the circumferential direction, it is desirable that the length of the spiral portions 2a and 2b be 3λ or more. In addition, one optical fiber 2 is laid over the entire length of one power cable line, and it is necessary to find an accurate average temperature in the circumferential direction on the way (every time cable arrangement changes, each type of different cable, etc.), By forming the spiral portion 2a in FIG. 5, all the required amount of heat generated by the cable can be accurately measured by one measuring device and one processing device.

In FIG. 5, the extension from the spiral portion 2a is folded back while being spirally arranged on the spiral portion 2a, but it may be extended further or folded back without forming the spiral portion 2b. . Further, the optical fiber of one spiral portion 2a or 2b is arranged on the sheath 7 inside the anticorrosion layer 8, or both spiral portions 2a, 2b
It is also possible to arrange the sheet in a cylindrical shape between the optical fibers of b to form a known thermal resistance layer.

In the actual measurement, since the signal is digitally processed, sampling is performed to obtain the temperature distribution. Therefore, the sampling point and the time 2t (position) do not always match. Therefore, it is necessary to secure the length of the optical fiber 2 to be wound so that the sampling point is always included in the spiral section for measuring the heating value of the cable.

(2) Temperatures of All Cables Next, a method of obtaining the temperatures of all cables by measuring the temperatures at two points in the vicinity of the underground cable will be described.

The cross section of the cable laying in Fig. 6 shows the pipeline 11 buried in the ground.
Up to 13 are pipes that do not contain cables, cooling pipes,
It is a pipe line or the like in which a cable is stored, and may be the cable line itself. In FIG. 6, 11 is a cable conduit, and the conduits p and q located at two points above and below the cable conduit are the temperature measurement points by the optical fiber. The temperature measurement points at the p and q2 locations may be the same positions as the pipelines (.

The intrinsic heat resistance of the surrounding soil is g, the base temperature is Ta, the unknown temperatures of the pipes are T1, T2, T3, ... Tn, and the known calorific value per unit length of those cables is W1, W2, W3. Wn, pipeline
If the temperature of p, q is Tp, Tq, and the proportional constant of self-heat resistance between the pipelines of ○ and ○ and the proportional constant of mutual heat resistance between the pipelines of ○ and ● are [R] in matrix display, The following equation holds for. ● If there is also fever, let it be Wp and Wq.
The thermal resistance proportional constant is shown in JCS-168.

The unknowns are (n + 2) pieces of the temperatures T1 to Tn of the pipelines, the specific thermal resistance g of the surrounding soil, and the base temperature Ta, and the solution is obtained by the simultaneous equation of (n + 2) pieces. That is,
Two extra equations are needed to obtain the unknown thermal constants g, Ta of the soil. Therefore, the temperature of the pipes p, q Tp, Tq 2
Measure the points and add the two equations. Since the specific thermal resistance g of the surrounding soil and the base temperature Ta can be obtained from the two equations at the bottom of the above equation, the g and Ta are used to determine the necessary temperature T1 to Tn of the pipeline. You can only ask for things.

In addition, when the temperature of two or more points is measured, the number of equations is larger than that of unknowns. In this case, use only two points or combine two points. Then, g and Ta can be obtained and the average value of them can be used. Since the temperatures T1 to Tn of the pipes obtained in this way correspond to the peripheral temperature of the pipe or the cable, the temperature of the conductor is obtained from this, and the thermal resistance and heat generation amount of each part of the cable.

On the other hand, when the refrigerant is flown through any of the conduits in FIG. 6 and is forcibly cooled, the cable temperature can be calculated in the same manner as above by the following method.

For example, in the equation (1) in FIG. 6, when the cooling water is flowing through the nth conduit, if the amount of heat generated inside the cooling water is Wc and the amount of heat taken by the cooling water is Ww, then the heat generation of the conduit The quantity Wn is Wn =
Wc-Ww, but the amount of heat Ww taken by the cooling water is an unknown number, so it is necessary to reduce the unknown number in equation (1) by one.

Therefore, the pipes inside the heat insulation layer are exposed at a part of the cooling water pipe, for example, at the f and k parts of the human hole parts 21 and 22 in FIG. 7 and the inlet / outlet (d part) of the circulation device. The optical fiber is wound directly on the wire, and the temperature Tn of the n-th conduit is obtained using the temperature. As a result, Tn becomes a known number.

As described above, in the temperature distribution measurement using the optical fiber, the average temperature between the λ widths is measured. Underground cables can be expected to have an effect of relaxing the temperature distribution due to the thermal resistance and heat capacity of the surrounding soil, so the average value between the λ widths will be sufficient for the purpose. It is desirable that λ be small, but since the amount of heat generated by the cable is also measured by the same measuring system, it is necessary to make it larger than a size capable of forming one spiral circumference in FIG.

Further, the temperature of the pipe having the cooling water (for example, the cooling pipe 13 in FIG. 7) causes a temperature difference in the lengthwise direction due to the flow, and therefore, depending on the degree of cooling, d, f, k in FIG. Like the point, by measuring the cooling temperature for each section, it is considered that there is a uniform change in the length direction, or it is considered that it is changing exponentially, The temperature Tn of the cooling pipe 13 at an arbitrary cross section in the length direction can be obtained. When there are a plurality of cooled pipes, the equation is solved by measuring the temperature of each cooling pipe and compensating for the increase in the number of heat removal amounts, which is an unknown number.

The same applies when other cooling pipes or heat supply pipes are provided along the cable line.

(3) Specific Example FIG. 7 shows a specific example of the maximum temperature evaluation device.

The time width of the optical pulse emitted from the optical pulse light source 1 (FIG. 1) of the optical fiber type temperature distribution measuring device 16 is 20 ns.
The optical fiber 2 is a double spiral structure (2a, 2a) as shown in FIG.
After being wound around b) and further wound around the surface of the cooling pipe 12 at the d portion, at the e portion, the conduit 12 (8th line) beside the cable line 10 is provided.
(See the figure) and enter the next human hole 21. Further, the optical fiber 2 is wound around the surface of the cooling pipe 13 again at the f portion in the human hole 21 and passes through the same conduit 11 (FIG. 9) as the cable 10 at the g portion in the next buried portion, Enter the next hole 22.
The optical fiber 2 is further wound into a double spiral structure at the h, j, and i portions of the human hole 22, and is wound around the surface of the cooling pipe 13 at the k portion. After that, it is folded back in the opposite direction and the cable line
It passes through the pipeline 12 (FIG. 9) beside 10 and passes through the human hole 21, and passes through the pipeline 14 (FIG. 8) beside the cable line 10 and ends 15
Has been laid.

Since the speed of light in the used optical fiber 2 is 20 cm / ns,
λ = 20 × 20/2 = 200 cm, and a 6 m optical fiber was wound around the spiral portions 2a and 2b.

In this way, the required temperature is measured by one optical fiber 2 and one measuring device 16.

Since the distance between the optical fibers 2 and the positional relationship of the equipment are known in advance, in the maximum temperature evaluation processing device 17, a, b, c, h, i,
The respective temperatures of the double helix at the j portion and the temperature difference between the inside and the outside can be known, and the heat generation amount of each cable 10 in each section can be obtained.
Further, the temperature distribution of the pipe 13 containing the cooling water can be obtained from the temperatures of the parts d, j, k, and the temperatures at two points in the buried cross section can be obtained from the temperature distributions of e, u, p, q. These are further processed in the maximum temperature evaluation processing device 17 and compared with various preset conditions. This allows the maximum temperature of the cable to be evaluated and enables economical and safe operation of the cable.

In the above-described embodiment, the optical fiber 2 may be installed in any order as long as it is a so-called one-stroke drawing, and it is also possible to form the spiral parts individually and connect the long parts. is there.

Further, as shown in FIG. 7, it may be installed almost reciprocally, or in consideration of the convenience of installation, a plurality of fibers that share and measure the necessary points are installed, and the optical fiber is output at the temperature distribution measuring device outlet. The fibers may be switched by a switch, the temperature distribution of each fiber may be obtained, and the entire temperature distribution may be equivalently measured as a series of fibers.

Furthermore, in the example of Fig. 7, the temperature (e, u; q, p) at two points near the cable line is measured, but one point is installed far away so that it is not affected by the heat of the cable line. By considering this as the base temperature Ta of the soil, the base temperature Ta of the equation (1) can be set to a known number, and one equation can be omitted. In such a case, depending on the environmental conditions, the temperature of one point per cross section is measured along the entire length of the line, a part of the optical fiber is placed at a position away from the line, and the temperature is measured as the base temperature Ta. It is also possible to have two temperatures.

Finally, based on an actual example, this device will be compared with the conventional device. However, all of them are applicable to two AC pipes with a total length of about 3000m and 6 cable lines with two cooling pipes installed side by side. The total cable length is 3
There are various types, the middle of the cable line includes the crossing part under the subway tunnel, the intersection with other backbone cable lines,
The burial depth and soil quality change intricately from the mountain side to the urban area. The comparison results for the total length of the cable line are as follows.

Only these temperature measuring lines and measuring instruments are used, and the conventional method is 3-4.
While 100 million yen is required, this device requires less than 1/10 of that. In the conventional method, a dedicated pipeline is required to lay a temperature compensating lead wire for several hundred points, and a huge labor cost is required to lay it and connect it. Investing hundreds of millions of yen in cable temperature management is a wasteful investment considering the efficient operation of cables, so there is no example of applying the conventional method at present. In order to maintain the power supply reliably and economically in the future,
Efficient operation of major cable lines is essential. In this respect, this device enables the maximum temperature evaluation of underground power cables, which is economically and comprehensively advantageous for the first time.

EFFECTS OF THE INVENTION As described above, the present invention includes a series of optical fibers installed around or around the power cable in the monitor section, one temperature distribution measuring device, and one processing device. Since the cable heat value and soil heat conditions necessary for monitoring the conductor temperature of the underground power cable are obtained, the cable conductor temperature is indirectly obtained from these, and the maximum value in the length direction is obtained. It is much more economical than the above, and for the first time it became possible to evaluate the maximum temperature of underground power cables.

[Brief description of drawings]

FIG. 1 is a principle diagram of a temperature distribution measuring device using an optical fiber,
FIG. 2 is an explanatory view of the average value measurement of the temperature distribution measurement by the optical fiber, FIG. 3 is an explanatory view of the relationship between the temperature difference between both ends of the known thermal resistance and the heat flow, and FIG. 4 is an explanation of the cross section of the horizontal laying cable and the surrounding convection. 5 and 5 are principle diagrams of the cable heat generation amount measurement according to the present invention, FIG. 6 is an explanatory diagram of an underground cable and a temperature distribution measuring unit, FIG. 7 is a diagram showing an embodiment of the present invention, and FIG. Is that
VIII-VIII sectional drawing, FIG. 9 is IX-IX sectional drawing. In the figure, 1 is an optical pulse light emitting source, 2 is an optical fiber, 3 is a demultiplexer, 4 is a light receiver, 5 is an averaging processing device, 6 is a data processing device, 7 is a cable metal sheath, and 8 is cable corrosion protection. Layer, 9 is the cable periphery (air etc.), 10 is the cable, 11 ~
Reference numeral 14 is a pipe, 16 is a temperature distribution measuring device, and 17 is a maximum temperature evaluation processing device.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Satoshi Yamamoto 5-1-1 Hidaka-cho, Hitachi-shi, Ibaraki Hitachi Cable, Ltd. Electric Wire Research Laboratory (72) Inventor Juno Ando 5-chome, Hidaka-cho, Hitachi-shi, Ibaraki No. 1 in the Electric Wire Research Laboratory, Hitachi Cable Ltd.

Claims (4)

[Claims] 1. A temperature distribution measuring device in which a series of optical fibers are laid along a power cable buried in the ground, and the optical fibers are used as a temperature distribution measuring sensor by detecting backscattered light. The amount of heat generated by the power cable is calculated from the values, and at the same time the temperature at at least two points in the cross section of the power cable is sought to determine the specific thermal resistance and the base temperature of the unknown surrounding soil at that portion, and the lay cross section A maximum temperature evaluation method for underground power cables, characterized by calculating the cable temperature inside the building and repeating this operation to find the maximum cable temperature over the entire length of the line. 2. To determine the amount of heat generated by the power cable, heat attached to the inner or outer surface of the anticorrosion layer, a peripheral location distant from the cable surface or a cable outer surface at substantially the same position in the length direction of the power cable. A part of the optical fiber as the temperature distribution measuring sensor is spirally attached to any two positions on the outer surface of the resistance layer with a length of a certain length λ or more. 2. The underground burial according to claim 1, wherein the heat generation amount of the power cable is calculated based on a temperature difference between two points in the radial direction measured by the temperature distribution measuring device as an average temperature and a known thermal resistance therebetween. Maximum temperature evaluation method for power cables. 3. When a transportation pipe for cooling or heating is provided near the power cable, a part of an optical fiber as the temperature distribution measuring sensor has a fixed length at least at one location of the transportation pipe. The temperature of the electric power cable in the cross-section direction and the length direction of the line is calculated by considering the temperature of the transport pipe measured by the temperature distribution measuring device at this contact portion at the same time by contacting for more than λ. The method for evaluating the maximum temperature of an underground power cable according to claim 1 or 2, which is added to the evaluation of the temperature. 4. As one of at least two locations measured on the cross section of the power cable laying, the soil base temperature in the ground sufficiently distant from the power cable line is taken, and only the soil specific thermal resistance is taken as an unknown constant related to the soil. The method for evaluating the maximum temperature of the underground power cable according to claim 1, 2 or 3, wherein the calculation of the temperature of the power cable is simplified.