Changes of Oil-immersed transformers

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Dissolved gas analysis (DGA) for oil-filled transformers and testing of insulating oil properties

Among equipment in substations, the percentage of aging and deteriorated devices continues to increase. Noticing deterioration conditions of aging devices and managing failure spot became important for maintaining and also for improving a reliability of power supply.
Internal faults and the degree of aging deterioration of oil-filled transformers can be determined by analyzing their insulating oil.

１．Dissolved gas analysis

Inside an oil-filled transformer, localized overheating and discharge causes thermal breakdown of the insulating oil and insulation materials, and the decomposed gas that is generated gets dissolved into the insulating oil. Dissolved gas analysis is a method of diagnosing the presence or absence of internal faults in the transformer by sampling and analyzing the concentrations of the gases dissolved in the insulating oil.

Dissolved gas analysis

Dissolved gas volume and criteria for determining deterioration

Caution I level	Total combustible gas TCG	Hydrogen H₂	Methane CH₄	Ethane C₂H₆	Ethylene C₂H₄	Acetylene C₂H₂	Carbon monoxide CO
Caution I level	500	400	100	150	10	0.5	300
Caution II level	1.　C₂H₂≧0.5 2.　C₂H₄≧10, TCG≧500
Abnormal level	1.　C₂H₂≧5 2.　C₂H₄≧100, TCG≧700 3.　 C₂H₄≧100, TCG rate of increase≧70/per month

Diagnosis flowchart of maintenance management using dissolved gas analysis

Examples of report details

1. Trends in dissolved gas

Sampling date (Year, month, day)	Oxygen (O₂)	Nitrogen (N₂)	Hydrogen ◆ (H₂)	Carbon monoxide ■ (CO)	Carbon dioxide (CO₂)	Methane ▲ (CH₄)	Ethane × (C₂H₆)	Ethylene ＊ (C2H4)	Acetylene ● (C₂H₂)	Total combustible gas (TCG)	TCG rate of increase per month (ppm)
1994.6.5	1321	8801	14	88	144	5	7	n	nd	114	－
1996.6.16	2163	14540	6	78	145	4	14	t	nd	102	0
2000.6.4	1709	66031	6	92	239	43	83	4	n	228	3
2000.6.4	1053	62953	-3	79	128	53	136	4	n	275	0
2004.5.30	1120	76332	31	102	412	89	245	8	n	475	4
2007.5.17	712	74592	8	102	469	110	348	9	n	577	3
2008.5.31	484	77131	5	66	625	84	310	9	n	474	-8
2008.10.1	1322	87451	5	76	521	104	345	11	n	541	17
2009.2.16	1659	87212	6	77	345	107	356	10	n	556	3
2009.8.25	911	74437	-4	53	405	83	324	9	n	473	-13
2010.2.22	1259	82253	5	53	331	83	320	9	n	470	0
2011.2.21	921	88712	5	53	444	85	346	10	n	499	2
2012.2.24	665	77104	7	55	428	88	370	11	n	531	3
2012.2.24	665	77104	7	55	428	88	370	11	n	531	3

Determination criteria

CautionⅠ level		400	300	100	150	10	0.5	500
CautionⅡ level	1.　C₂H₂≧0.5ppm 2.　C₂H₄≧10ppm and TCG≧500ppm
Abnormal level	1.　C₂H₂≧5ppm 2.　 C₂H₄≧100ppm and TCG≧700ppm 3.　 C₂H₄≧100ppm and TCG rate of increase≧70ppm per month

2. Gas pattern

3. Abnormality diagnosis chart (Composition ratio)

4. Specific gas

IEC-60599 (1999)	CO₂/CO<3，cellulose is degraded.
IEEE Std.C57.104-1991	CO₂/CO>7, normal.
International Council on Large Electric Systems (CIGRE) materials 15/21/33-19 (TF15.01.01) (1996)	CO₂/CO<10, cellulose is overheated.
	CO₂/CO<3, cellulose is degraded by electrical abnormalities.

２．Insulating oil property testing services

This is a method for diagnosing the health of insulation properties achieved by managing the moisture content, insulation breakdown voltage, total acid number, and volume resistivity in order to determine the deterioration level of insulating oil during operation.