How to inject transformer oil at field

Mainly supervise the two kinds of transformer oil to be injected in the transformers: new oil and the residual oil in the N₂ filled transformer.

1. First, inspect the two kinds of oil according to the oil sampling method.

a) New oil, whether it is new oil sent from a transformer factory or directly shipped from an oil refinery, as well as commercial oil purchased by yourself (although there is a qualified report), should be re-inspected: PH value, acid value, impurities content, interfacial tension, flash point, breakdown voltage and tan δ, etc. The quality should meet the relevant international transformer oil standards, such as China (220 ~ 330KV transformer) GB2536-1990 "Transformer oil" standard, (500KV transformer) SH- 0040-1991 "Ultra High Transformer Oil" standard.  If the inspection oil quality is unqualified, you should contact the manufacturer.

b) Residual oil shall be sampled for breakdown voltage, water content and tanδ (90℃) tests before exhausting N2. When 220 ~ 330KV transformer residual oil breakdown voltage <30kv or="" water="" content="">30 ppm; 500KV transformer residual oil breakdown voltage <40kv or="" water="" content="">30ppm, oil injection and N2 discharge are not allowed, and the transformer manufacturer should be notified to deal with it.

c) Treatment of unqualified residual oil: If the residual oil breakdown voltage is >30KV and tanδ(90℃)<0.5%, it can be discharged from the transformer, injected into the oil storage tank, and then reused after being purified by a vacuum oil degassing machine ; If the residual oil breakdown voltage>40KV and tanδ(90℃)>0.5%, the residual oil must be regenerated by the oil reconditioning plant before it can be used.

2. Secondly, before filling into the transformer, all new oil must be vacuum dehydrated and degassed to remove moisture, dissolved gas and impurities in the transformer oil. The quality of the new oil meets international or local standards before it can be injected with N2, vacuum oil injection and hot oil circulation. . The 500KV transformer should use a Vacuum Oil Purifier to continuously circulate the hot oil, and control the outlet temperature of the Vacuum Oil Purifier to maintain 60°C. After the cycle is over, a sample test is required, and the oil quality must meet the index requirements in the table below.

Table 1 Inspection index after new oil purification
Item	Voltage Level（kV）
	500	220~330	66~110
Oil Breakdown Voltage（KV）	≥60	≥55	≥45
Water Content（ppm）	≤10	≤15	≤15
Gas Content（%）	≤1	≤1	-
Dielectric Loss Factor（90℃，%）	≤0.2	≤0.5	≤0.5

Table 2 Oil quality index of 500kV transformer oil after hot oil circulation
Item	Voltage Level（kV）
	500	220~330	66~110
Oil Breakdown Voltage（KV）	≥60	≥50	≥45
Water Content（ppm）	≤10	≤15	≤20
Gas Content（%）	≤1	≤1	-
Dielectric Loss Factor（90℃，%）	≤0.2	≤0.5	≤0.5

3. Subsequently, before the transformer is put into running, due to contact with transformer components and insulating materials in the transformer oil, the oil quality may undergo some changes, such as moisture, dissolved gas, and impurities. At this time, the oil is neither new nor operating oil. It is called "oil before putting into operation", and oil samples must be taken for test again, and it can be put into operation only after reaching the "Quality Standard of Transformer Oil in Operation". The results of this test confirmed the initial oil quality data of the transformer, and the oil results of future operation will be compared with the original data.

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