Safe and Stable Supply

Utilization of corrosion prognosis map

The rate of deterioration of a steel tower varies greatly depending on the facility environment, as illustrated for example by the fact that rust is more likely to develop in coastal areas due to salt deposition.
In order to perform maintenance that reflects these differences, and with particular regard to the problem of rust (corrosion state), we are calculating the expected rate based on factors such as distance to the coast, direction of prevailing winds, humidity etc. This allows us to identify the timing for effective maintenance and renewal depending on the expected rate of deterioration for the location.

Utilization of new technology (drones)

For more effective monitoring and inspection of the state of aging in overhead power transmission facilities, we are in the process of harnessing advanced drone technology which will make it possible to check facilities while eliminating the need for personnel to hike mountains and climb towers. Preparations for enhancing the efficiency of operations by using such tools are under way.

Deterioration analysis of underground cables

There are two types of underground cables: OF cables that use paper and oil for the insulator and CV cables that use cross-linked polyethylene. We analyze the state of deterioration while taking into consideration the different characteristics of each.

With OF cables, a specific gas is generated in the oil as the deterioration progresses. This gas is sampled and its volume and components are analyzed to determine the state of deterioration.

A characteristic of CV cables is that the insulator deteriorates over time. A voltage is applied to the removed CV cable, and a sampling survey is performed to measure the breakdown voltage, thereby identifying deterioration trends.

In addition, to find a degradation phenomenon called water tree that expands in a dendritic manner, starting from very small foreign substances and voids generated when the CV cable is used for a long time in an environment with high moisture, the loss current method is used. This involves measuring and analyzing the loss current flowing in the insulator to assess the water tree type deterioration.

Origin and spread of water tree phenomenon

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CV cable
structural drawing

(1) Water ingress

Moisture penetrates the insulator over time

(2) Water condenses on core

Water accumulates on electric field concentration point (core)

(3) Water tree forms

Water tree forms

(4) Water tree expands

Water tree expands and damages insulator

Life extension by painting

Regular patrols and inspections are conducted to ascertain the state of deterioration and abnormalities in steel towers. Appropriate maintenance is performed according to collected data, in order to delay the progress of deterioration and extend the life of the tower as much as possible.

Example of steel tower painting

Overall rust occurrence less than 20%	Overall rust occurrence 20% or more
Continuous monitoring of rust condition by patrols etc.	Determining paintability by film thickness measurement etc.
If the tower is not painted, parts will corrode and a replacement costing about 30 times more will become necessary. In order to enable prolonged use of the tower, it is important to extend its service life by applying paint before corrosion occurs to reduce costs.

Sampling survey and life extension evaluation by loss current method

OF cable is considered to be resistant to deterioration and highly reliable, but the use of oil requires maintenance. CV cable is now the mainstream, because of advantages such as low manufacturing cost and easy maintenance.

With CV cable, to evaluate the deterioration status specifically according to usage, separately from its design life, we employ sampling surveys and water tree deterioration diagnosis to determine how deterioration is progressing and whether the service life can be extended. This results in increased accuracy of service life prediction.

Life prognosis with average polymerization evaluation (Remaining life evaluation)

One of the tools used when considering transformer renewal is life prognosis with average polymerization evaluation (remaining life evaluation). The average degree of polymerization is a value that indicates the average number of connected molecules of the main constituents in the paper material.

As the average degree of polymerization of the winding insulating paper used inside a transformer significantly affects its service life, we utilized data from the Electric Technology Research Association to develop our own unique analysis methods for assessing the degree of deterioration, and we are using the results for service life prediction.

A large volume of actual measurement data of transformers accumulated by Kansai Transmission and Distribution enabled a revision of conventional life estimation methods, contributing to an extension in estimated life expectancy.

Service life estimation
Source: Electric Technology Research Association, Vol. 54, No. 5

Relationship between furfural and average degree of polymerization
(Kansai Transmission and Distribution, 77 kV and below)

Appropriate maintenance measures according to deterioration state

For concrete poles with problems such as cracks and delamination, suitable maintenance steps carried out at a stage when deterioration is still minimal will reduce the necessity for renewal of electric poles and extend the service life as much as possible.

Deterioration of electric poles

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Step	Deterioration process	Strength reduction	Countermeasure
1	Cracks develop	Almost none	Repair cracks (immediately when found)	Repair deteriorated part before renewal of electric pole becomes necessary
2	Moisture and oxygen penetrate to internal rebar from cracks (Reduced anti-rust effectiveness of concrete for rebar)
3	Internal rebar rusts and expands, increasing pressure from inside
4	Surface concrete lifts and peels off due to internal pressure	Slight reduction in strength	Repair delamination
5	Surface concrete peels off and drops, increasing likelihood of water and oxygen ingress
6	Further enlarged delamination area causes excessive thinning or breaking of rebar	Excessive reduction in strength	Rebuild electric pole
7	Strong moisture and oxygen ingress leads to excessive delamination
8	Excessive reduction in strength makes recovery impossible

Before delamination repair

Repair of delamination

After delamination repair

Slimming down of transmission and substation facility network

As power demands decrease, we are actively working on ‘slimming down’ our facilities, through measures such as consolidation of facilities, reduction of capacity, and re-assignment of facilities with low utilization rates, to match the scale of local demand. With regard to transmission and substation facilities, we compare the “renewal” versus “consolidation” options at the point when aging requires investment, and we take measures to reduce costs while ensuring stable electricity supply.

Optimization of power distribution transformers

Slimming down of power distribution facility network

At the point where new construction or rebuilding of concrete poles becomes necessary, we redesign the facility layout to reduce the number of required poles while maintaining appropriate strength. In addition, concrete poles that have become unnecessary due to the abolition of electricity contracts are removed after checking aspects such as whether a subsequent supply schedule is in place.

(Old configuration)

(After optimized reconfiguration)