JPS61277307A - Management of hot line for installed power cable - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field to Which the Invention Pertains) The present invention relates to an improvement in a live line management method for power cables.

(Prior art) Power cables (hereinafter abbreviated as cables), especially OF cables, are required to have extremely high reliability due to their use.
Abnormalities may occur in the insulation layer within the cable due to aging deterioration during use, mechanical damage, or defects in the manufacturing process. This abnormality often appears as an increase in dielectric loss, and as a result, the amount of heat generated inside the cable increases, internal temperature rises, and thermal deterioration of the insulating layer is accelerated. The dielectric loss of the thermally deteriorated insulating layer further increases, causing a positive feedback effect in which the internal temperature rises and further accelerates thermal deterioration, which may eventually lead to thermal breakdown.

To prevent such accidents, it is necessary to periodically measure the dielectric loss of installed cable lines to estimate the remaining life and check for abnormalities in the insulators.

Conventional methods for measuring dielectric loss include electrical methods and thermal methods. In the electrical method, a lossless capacitor is connected in parallel with the cable and the loss is determined using a shearing bridge, etc., but it requires a great deal of cost and labor to transport the lossless capacitor and configure the measurement circuit. In addition to this, it also requires a long power outage, so this method is not used in actual installations of cables, except for power distribution cables.

On the other hand, the thermal measurement method is a method in which a thermal sensor is attached to the cable surface to detect the amount of heat dissipated from the cable surface and estimate the dielectric loss.The measurement device is simple and it is also possible to detect local abnormalities. However, with this conventional method, when the cable temperature changes in a complex manner due to fluctuations in the outside temperature or the current flowing, a heat flow other than the dielectric loss is generated due to the action of the current flow loss and the heat capacity of the cable, causing the thermal sensor to respond. Therefore, it was difficult to measure when the cable was actually placed. For this reason, a method has been proposed in which a dummy cable with the same structure as the cable under test is installed in the same environment as the cable under test, and the difference in the surface dissipation heat flow rate of both cables is determined to determine the dielectric loss. Carrying dummy cables onto the actual installed line is not a practical method because it requires a large amount of work and involves a long power outage.

(Specific Object of the Invention) The present invention eliminates the drawbacks of the conventional thermal dielectric loss measurement method, safely, easily, quickly, and accurately measures the dielectric loss of a cable in actual use without interruption. The purpose of this invention is to provide a power cable management method that makes it possible to estimate the remaining life of the cable or detect abnormalities in the insulator.

(Structure and operation of the invention) For convenience of explanation, the main points of the present invention will be listed first.

The management system of the present invention measures the surface temperature and surface radiant heat flow of the outermost layer of a power cable, or the surface temperature and ambient temperature from time to time, and performs special calculation processing on the measured values to calculate the temperature and heat generation amount inside the cable. It is characterized by estimating the state of deterioration and remaining life of power cables, and detecting abnormalities.

This specific means will be explained below.

Figure 1 is a cross-sectional view showing the internal structure of the cable, where l is the cable conductor, 1a is the insulator, and 1b is the metal shield (sheath).
Layer 1c is an anti-corrosion layer. The heat capacity and radial thermal resistance of each layer can be calculated from the specific heat and specific thermal resistance of the material. These thermal constants are defined by heat capacity C as capacitance and resistance R as
can be expressed by the thermal equivalent circuit inside the cable as shown in Figure 2 by replacing it with electrical resistance. In Figure 2, W,...
L-, W.

is the heat source inside the cable, Wc is the conductor loss, ! A
- is dielectric loss and 6 is sheath loss, both of which are expressed as current sources. Among these, -6 can be determined by multiplying a constant determined by the structure and installation form of the cable by the square of the conducting current.

When using the equivalent circuit shown in Figure 2, the conductor temperature is TCs, the average temperature of the insulator is Ta, the sheath temperature is T, the cable surface temperature is T0, and the surface dissipated heat flow is Ho, then the dielectric loss is -4
is given by the following equation.

Here, -0 can be actually measured using a sensor attached to the surface of the cable. Furthermore, when the surface heat dissipation resistance of the outermost layer is known, it may be determined from the difference between the cable surface temperature and the outside temperature.

Tc and T can be calculated using the following equations. ((2), (3
Regarding the one-car type guidance, the present inventor filed a patent application in 1988=8.
9260) Tt −T(1+ R1W6 −・−・・−・・−−
−−−−−−・−・−・−−−−−−−・・−−−−
−−1・−−−−(3) Equation (2) includes the unknown number −4, but the time change dL of − due to the temperature change of the insulator
/dt is not a problem, differentiate equation (2) and find dTc
- when calculating /dt.

will be deleted, so! Assuming an appropriate expected value for /44, calculate and store Tc, T, once.
The time differential of these values may be calculated, and the value of 6 may be obtained from equation (1) and corrected. Note that the time differential calculation required for calculation can be easily and accurately determined by storing momentary measurement data or calculation results in an internal memory of a microcomputer or the like, and calculating the changes over time.

Next, if the value of -4 is affected by its temperature characteristics and changes significantly over time with fluctuations in cable temperature, substitute the temperature characteristics of W4, which has been corrected using the method described above, into equation (2). Find Tc again and calculate dTc/dt (1)
The estimation accuracy can be improved by repeating the operation of substituting into the equation and re-correcting the value of 6.

The method described above can estimate the dielectric loss under any cable installation conditions if the surface heat dissipation flow rate from the cable can be accurately determined.

Next, we will show how to estimate the remaining life of an OF cable from the rate of progress of thermal deterioration and the thermal stability inside the cable. (
(Refer to the flowchart in Figure 3) According to previous research, in the temperature range where the thermal stability of the cable is a problem, the dielectric loss tangent δ of the insulator (oil-impregnated paper) is recently calculated as tanδ=Be+B1 e −”t/kT− ,---
It is known that it is represented by --, ----- (4). Here Bo, B.

is a constant, k is the Boltzmann constant, T4 is the average temperature of the insulator, and El is the activation energy of ionic electrical conduction in the insulator. In addition, the cable insulator deteriorates due to temperature rise due to energization, and ionic impurities increase, resulting in 8 in equation (4).
The value of 1 increases and the value of tan δ increases. The value of Bo hardly changes. It is known that if the deterioration temperature Td is constant, the rate of increase in B is expressed as pa. AO is a constant, and E is a constant.

is the activation energy of the thermal deterioration reaction. If the deterioration temperature T4 changes due to load fluctuations, etc., calculate the periodic change in T4 from the daily load pattern and the temperature characteristics of tan δ at that point, and set the temperature T4 that gives the same amount of deterioration.
Find eq. That is, the rate of change of B is calculated by substituting the equal bonding temperature Teq obtained with L as time in place of T4 in equation (5), and the minute time Δ is expressed as the value of B after and janδ - temperature. Find characteristics. If similar calculations are repeated thereafter, the transition of tan δ-temperature characteristics due to deterioration will be calculated. It is necessary in this case, B.

The initial value of and the activation energy of ionic conduction E,
is the dielectric loss Wd estimated by the method described above in the installed line.
It is determined from the temperature characteristics of the value of tan δ, which is back calculated from the value of the formula %.

Here, ω is the angular frequency of the applied voltage, ■ is the applied voltage, and C is the capacitance. Since it is known that in an OF cable, the value of C hardly changes due to deterioration of the insulator, the value of tan δ can be easily converted from -. Further, the constant of the deterioration reaction and the activation energy Ea may be determined using laboratory data, but if there are results of several measurements on an actually installed cable, they can be estimated from the time course.

As a method of determining cable life based on the degree of increase in the value of tanδ, if the life is defined as the point at which thermal stability cannot be maintained when the rated load current is continuously applied, then the expected tanδ at each point of deterioration can be calculated. The thermal stability at rated load is checked from the temperature characteristics (this allows the remaining life to be estimated.

Thermal stability is determined as follows. The amount of heat generated inside the cable IAa and the amount of heat dissipated from the cable -0 are expressed by the following equation using the average temperature T4 of the insulator.

Wr,<? a, = ωCV” tan δ (t, Ta) +
W. 10W, ・---------------------- (8) where T9 is the soil base temperature, and R is the thermal resistance on the outside of the cable. The conditions for the cable to become thermally unstable and thermally destroyed are always 0°74) > L (Tdl above room temperature), which can be determined from (8) and +91 Ryo2.

FIG. 3 is a flowchart of the remaining life estimation process, and the contents of each step are as described above. FIG. 4 is a side view of the configuration of a measuring circuit for managing the installed power cable according to the present invention.
1. Ia, lb, and lc are the same as in Figure 1, 2 is a heating medium for temperature measurement, 3 and 4 are thermocouples, 5 is a current transformer for cable current measurement, 6 is an interface circuit, 7 is a microcomputer, and 8 is a cable. 9 is a display device, 10 is a recording device, and 2 is the voltage of the applied power source.

(Effects of the Invention) As explained in the previous section, according to the present invention, the dielectric loss of a cable can be safely and easily measured without performing any work on the inside of the cable to be measured or on the power supply side. In addition, using a calculation method that takes into account the delay in heat transfer due to heat capacity, internal temperature and dielectric loss can be estimated immediately from the start of measurement regardless of past temperature history. Accurate estimation is possible in a short time even when complex temperature changes occur due to fluctuations in temperature.
Furthermore, it is possible to understand the temperature characteristics of dielectric loss.

Furthermore, it is possible to deal with local abnormalities in cables, and since these can be implemented at low cost, the present invention has a significant practical effect on live line management of installed cables, especially in estimating remaining life and detecting abnormalities. It is expected.

[Brief explanation of drawings]

Figure 1 is an example cross-sectional view of a cable, Figure 2 is a thermal equivalent circuit diagram of a cable, Figure 3 is a flowchart for estimating the remaining life of a cable, and Figure 4 is a measurement circuit for managing installed cables according to the present invention. FIG. 1... Cable conductor, 1a... Insulator, lb.
...metal shielding layer, lc...corrosion protection layer, 2...thermal medium, 3.4...thermocouple, 5...current transformer, 6...
・Interface, 7...Microcomputer, 8...Memory, 9...Display device, 1o...
・Recording device.

Claims (1)

[Claims] The surface temperature and surface dissipated heat flow of the outermost layer of the live power cable or the ambient temperature of the power cable are measured from time to time, and these measured values are calculated along with predetermined constants of the cable to determine the inside of the power cable. A live line management method for installed power cables, characterized by estimating the temperature and calorific value of the power cables, estimating the state of deterioration and remaining life of the power cables, and detecting abnormalities.