The Kansai Electric Power Co., Inc.
Mitsubishi Electric Co.

Each nuclear power plant uses various types of transformers. Condition monitoring technologies applicable to transformers are essential to assure stable plant operation. The major condition monitoring technologies used for the diagnosis of transformer conditions are shown in Table 1.

Among these, this paper addresses a diagnostic technology to detect insulating paper deterioration and another one to analyze dissolved gas in oil because both of these technologies are essential in determining the operability of a transformer through planned maintenance programs and detection of deterioration.

Table. 1 Diagnostic technologies applicable to transformers

(1) Diagnostic technologies to detect deterioration of insulating paper¹⁾,²⁾

A decrease in the average degree of polymerization (i.e., a decrease in tensile strength) of coil insulating paper of a transformer, which is caused by thermal deterioration occurring with operating years, is supposed to be one of the main factors which determine the average service life of a transformer. Figure 1 shows the flow of a transformer fault due to deterioration of coil insulating paper.

The coil insulating paper plays an important role as an insulator between coil conductors. Figure 2 shows the internal structure of a shell-type transformer, including the location of insulating paper.

Figure 3 shows the relationship between the average degree of polymerization, temperature and heating time. Figure 4 shows pictures taken by the electronic microscope.

The service life of a transformer can be estimated by calculating the time when the average degree of polymerization of the insulating paper on the hot spot coil area reaches the end of service life using a regression equation, which was obtained by the deterioration characteristics shown in Figure 3, considering operating data.

The average degree of polymerization can be determined by sampling insulators (press boards) from the concerned transformer, or estimated based on the amount of furfural in oil, which is a decomposed product of insulating paper, and the amount of CO₂+CO.

Fig.1 Fault flow due to deterioration of insulating paper

　Fig.2　 Internal structure of shell-type transformer

　　　

Fig.3 Deterioration characteristics of insulating paper　Fig.4　 Electron micrograph of insulating paper

Insulating paper has a polymeric molecular structure consisting of cyclic hexagonal glucose rings. The number of glucose ring cycles is called the average degree of polymerization. An average degree of polymerization at 450 is considered to be the life time of insulating paper.

Fig. 5 Molecular constitution of cellulose

(2) Diagnosis of possible faults in the transformer main body

a. Dissolved gas analysis method
The analysis of dissolved gas in oil is widely utilized to determine the existence of faults, characterize faults, if any, and determine the degree of a fault inside a transformer by analyzing gases generated from the decomposition of insulating oil due to partial discharge or overheat, and measuring the amount of CO₂ and CO resulting from damage to the solid insulation nearby.
An on-line gas analysis system has been developed for the early detection of faults and streamlined maintenance. This system is able to automatically and periodically analyze the 6 chemical ingredients* of combustible gas, which is required for the diagnosis of transformer condition.
Figure 6 shows the on-line dissolved gas analysis system, which is installed adjacent to a transformer.

＊6 chemical ingredients：H₂,CH₄,C₂H₆,C₂H₄,C₂H₂, and CO

Fig. 6 On-line dissolved gas analysis system

b. Advanced dissolved gas analysis method³⁾
An advanced method of analyzing dissolved gas has been recently developed. The advanced analysis method is able to determine the defective locations (insulating materials), which conventional analytical techniques were not able to discriminate, by comparing decomposed products specific to each insulation material and those detected in the concerned transformer.
Heating tests were conducted on the 11 types of insulation material, which are used inside a transformer. It was revealed that the heat resistant paper, varnish coated paper and glass epoxy generated decomposed products specific to each material as shown in Table 2.
The advanced dissolved gas analysis method may fail to detect some decomposed products resulting from the aging process. With less data accumulated through the application of this new technique to actual plants, any target value for decomposed products has been set yet. It is planned to set the target value for each decomposed product in future.
The decomposed products specific to each insulation material are not likely to be easily gasified because of its high molecular weight and high boiling point. In addition, measurement to ppb order is required for the analysis. To meet these conditions, gas chromatography-mass spectroscopy (GCMS) is used to analyze qualitatively and quantitatively the ingredients extracted by the cold concentration technique as shown in Figure 7. Figure 8 shows the advanced gas analysis system.
The introduction the advanced diagnostic technologies mentioned above to operating plants has contributed to improving and maintaining stable plant operation.

Table. 2 Decomposed products specific to each insulation material

　　　

Fig.7 Method of extracting trace
ingredients dissolved in oil (Cold trap method)　Fig.8　 Advanced gas analysis system

1. [1]“Maintenance management of oil-immersed transformers”, Electric Technology Research Association, No.5 (Part 1), Vol. 54, February 1999
2. [2]“Diagnostic and replacement technologies for electric equipment”, Technical Report of the Institute of Electrical Engineers of Japan, No.831, July 2001
3. [3]“Guideline for the refurbishment of electric power transformers”, Electric Technology Research Association, No.1, Vol. 65, September 2009

Japan Society of maintenology (ejam@jsm.or.jp)