JPH0351775A - Method for detecting water tree deterioration of power cable - Google Patents

Abstract

PURPOSE:To accurately detect water tree deterioration by removing the effect of a local battery by monitoring the transmission voltage of a power cable and detecting the water tree deterioration from the change degree of DC component quantity accompanying the change of the transmission voltage. CONSTITUTION:A voltage measuring device 3 for monitoring the transmission voltage to a power cable 1 is connected between a conductor L and the earth. In a cable wherein no water tree deterioration is present, there is no change in the DC component quantity measured by a measuring device 2 even when transmission voltage changes. However, in a cable wherein water tree deterioration is present, the DC component quantity is changed corresponding to deterioration and, therefore, the value of the measuring device 3 and the DC component quantity of the measuring device obtained when the transmission voltage boosts or drops by the load change of one day are measured and the measured values are compared with each other. By this method, even when a local battery is generated between an anti-corrosion layer and the earth, water tree deterioration can be accurately detected.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to water tree deterioration detection of power cables for detecting water tree deterioration of power cables, particularly cross-linked polyethylene power cables called Cv cables. It is about the method.

[Prior Art] Generally, the insulation performance of the electric insulator of a power cable deteriorates due to aging after installation. In particular, it is known that in Cv cables, dendritic cracks occur in the crosslinked polyethylene insulator, and the occurrence of so-called water trees, in which water enters the cracks, is the main cause of insulation deterioration. If left untreated, such deterioration in insulation performance may progress and sooner or later lead to a major dielectric breakdown accident. Therefore, it is extremely important to understand changes in cable insulation resistance and discover deterioration early. For this reason, various insulation measurement methods have been known for a long time, but in recent years in particular, several methods have been proposed for diagnosing live wires without stopping power transmission during measurement, making it possible to constantly monitor the condition. It is attracting attention because of its many advantages.

Conventionally, as such a method, for example, Japanese Patent Application Laid-Open No. 59-202
As disclosed in Japanese Patent No. 075, etc., a device utilizing the current rectification effect of a water tree has been proposed. FIG. 3 is an explanatory diagram of the principle of this method, and shows an equivalent electric circuit when inspecting a deteriorated cable in a water tree. A power cable 1 constituting a three-phase electric line is brought into a live state by a three-phase AC power source P. Further, the voltage of one of the three phases is applied to the conductor of the power cable 1, and cable conductors other than the conductor are omitted. A grounding transformer GPT is connected to this three-phase electric line, and its neutral point is grounded. Here, if a water tree occurs in the insulating layer (2) between the conductor and the shielding layer S in the power cable 1, in addition to the AC coupling due to the capacitance Ci between the conductor and the shielding layer S, A rectifier by the water tree and a DC coupling by the DC resistance Ri occur. This rectifier and DC resistance Ri are
Therefore, in the conventional method, a low impedance measuring device 2 is connected between the shielding layer S and the earth to measure the DC component of the flowing ground line current 11. By doing so, we are evaluating the state of deterioration caused by water trees.

[Problems to be Solved by the Invention] However, power cables are provided with an anti-corrosion layer C made of insulating vinyl etc. on the outside of the shielding layer S, and when actually laid by being buried in the ground, , the insulation resistance Rc of the anticorrosion layer C between the shielding layer S and the ground becomes a problem. Similarly to the weight of the insulating layer, the anti-corrosion layer C also deteriorates over time after installation, resulting in a decrease in insulation resistance Re. However, in general, the deterioration of the anti-corrosion layer C is worse than the deterioration of the insulating layer. I
In addition, it is known that a so-called local battery occurs where electric charge is induced at the interface where the anticorrosive layer C contacts the earth, and the electromotive force Ec of one local battery is
is applied between the shielding layer S and the ground in series with the insulation resistance Rc. When this phenomenon occurs, a current 12 due to the electromotive force Ec flows in a closed circuit between the shielding layer S, the insulation resistance Re, the electromotive force Ec, the measuring device 2, and the shielding layer S. The electromotive force Ec is unrelated to the AC voltage applied to the conductor, but if the insulation resistance Re decreases due to deterioration, the current 12 increases. Therefore, it is difficult to accurately measure the DC component of the ground line current 11 with the measuring device 2, and the power cable 1
However, there is a problem in that accurate diagnosis of water tree deterioration is impossible.

By the way, when transmitting and distributing power using power cables, the power transmission voltage increases and decreases in accordance with fluctuations in demand on the demand side, that is, fluctuations in load. In other words, for example, during the day, demand is generally high and the load is large, and the transmission voltage tends to drop, while at night, the demand is low and the load is small, causing the transmission voltage to rise. It becomes a trend. This fluctuation is not favorable for the power demand side, and the power supply side constantly monitors and records this voltage value, and deals with it by adjusting the amount of generated power, etc., so that it stays within a predetermined range, such as ±5%. The case is normal. It is difficult to completely eliminate this voltage fluctuation, and each power equipment used by the demand side is designed to handle voltage fluctuations within the above-mentioned predetermined range. There is.

In view of the above-mentioned facts, it is an object of the present invention to provide a water tree system for power cables that allows accurate detection of water tree deterioration even when a local battery effect occurs between the corrosion protection layer of the installed cable and the ground. An object of the present invention is to provide a deterioration detection method.

[Means for Solving the Problems] In order to achieve the above object, in the method for detecting water tree deterioration of a power cable according to the present invention, there is provided In the method of detecting water tree deterioration of the power cable by detecting a DC component of the flowing current, the power transmission voltage to the power cable is monitored, and the degree of change in the amount of the DC component due to a change in the power transmission voltage is The present invention is characterized in that the deterioration of the water tree is detected from the above.

[Operation] The water tree deterioration detection method for power cables having the above configuration detects the rise and fall of the voltage by monitoring the power transmission voltage, and determines the amount of DC component in the ground wire current due to fluctuations in the power transmission voltage. The degree of deterioration of the water tree is detected based on the change. In other words, in a cable without a water tree, the amount of DC component does not change even if the transmission voltage fluctuates, but in a cable with a water tree, the amount of DC component changes depending on the degree of deterioration. Water tree deterioration can be detected by measuring the DC component amount when the power transmission voltage values are different and comparing these measured values.

[Example] The present invention will be explained in detail based on the example illustrated in FIGS. 1 and 2.

FIG. 1 is a circuit diagram for carrying out the method according to the present invention, in which the voltage transmitted to the power cable 1 is connected between the conductor and the ground in a circuit similar to that in the conventional method shown in FIG. Connect a voltage measuring device 3 for monitoring. Note that the voltage measuring device 3 may be installed separately on each power cable 1 for measurement, or it may be installed on several lines collectively in order to grasp the transmission voltage on the power supply side. It may be used as is, and furthermore, the power transmission voltage can be determined on the low voltage side of the grounding transformer GPT.

In this configuration, when the power supply side starts power transmission, the AC voltage applied to the conductor and measured by the measuring device 2 fluctuates over a relatively long period within a predetermined tolerance range of, for example, V±α, as described above. It turns out.

Here, the electromotive force Ec of the local battery, that is, the current 12, is independent of the AC voltage applied to the conductor as described above. On the other hand, the DC component of the ground line current 11 detected by the rectifying action of the water tree increases monotonically with respect to the increase in the applied AC voltage. If the current value of the measuring device 2 is determined at this point in time, the influence of the current 12 can be removed.

That is, in a graph such as that shown in FIG. 2, in which the horizontal axis shows the AC voltage value applied by the voltage measuring device 3 and the vertical axis shows the DC current value measured by the measuring device 2, in the case of a cable with no water tree deterioration, , a straight line parallel to the horizontal axis is obtained like a, and in this case, the DC current value is equal to the current 12. On the other hand, in a cable with water tree deterioration, the DC current increases monotonically as the applied AC voltage increases, as shown in b. Further, the ratio of the amount of current change to the amount of voltage change becomes larger for a cable with a small DC resistance Ri, that is, for a cable that is severely degraded by water tree. Therefore, by measuring the DC current value at at least two different AC voltage values within a predetermined allowable range and comparing these DC current values, it is possible to accurately determine the presence and extent of water tree deterioration. 0 For example, if there is a difference between two current values, it can be determined that water tree deterioration exists, and further, the degree of water tree deterioration can be determined quantitatively based on the ratio of these differences to the difference in applied AC voltage value. can be judged.

In one or more of the embodiments, it is assumed that fluctuations in the applied AC voltage occur passively due to load fluctuations, but in cases where measurement is urgently required, fluctuations in the applied voltage may be actively controlled within a predetermined fluctuation tolerance range. You may wake up.

[Effects of the Invention] As explained above, the method for detecting water tree deterioration of power cables according to the present invention eliminates the influence of deterioration of local batteries and anticorrosive layers, and can further evaluate the magnitude of the insulation resistance of the insulating layer. Therefore, water tree deterioration can be accurately detected, insulation breakdown accidents due to misdiagnosis can be prevented, and damage can be significantly reduced.

[Brief explanation of drawings]

Drawings 1 and 2 show an embodiment of the method for detecting water tree deterioration of power cables according to the present invention. FIG. 3 is a graph diagram of an AC voltage and a measured DC component, and FIG. 3 is a circuit configuration diagram for implementing a conventional detection method. Code 1 is a power cable, 2 is a measuring device, 3 is a voltage measuring device,
L is a conductor, ■ is an insulating layer, S is a shielding layer, and C is a corrosion protection layer.

Claims (1)

[Claims] 1. In a method for detecting water tree deterioration of the power cable by detecting a DC component of the current flowing between the shielding layer of the power cable in a live state and the earth, the power transmission voltage to the power cable is detected. A method for detecting water tree deterioration of a power cable, characterized in that the water tree deterioration is detected from the degree of change in the amount of direct current component caused by a change in the power transmission voltage.