Les Services Électrotechniques Inc.

Nondestructive testing – Calibration – Modernization
Oil & gaz analysis – Oil degassing – Engineering – Infrared thermography

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Oil analysis
for power transformers


On site analysis
  • On site sample (ASTM D3613)
  • Dielectric breakdown test (ASTM D877 and D1816)
  • Power Factor test (ASTM D924)
  • Water content test (ASTM D1533)
  • BPC dielectric test (ASTM D877)
  • Visual inspection (ASTM D1500 and D1524)
  • Interpretation and recommendations
In laboratory analysis
  • Dielectric breakdown (ASTM D877 and D1816)
  • Power Factor 25°C and 100°C (ASTM D924)
  • Water content (ASTM D1533)
  • Acidity index (ASTM D974)
  • Specific density (ASTM D1298)
  • Color (ASTM D1500)
  • Superficial tension (ASTM D971)
  • Visual exam (ASTM D1524)
  • PCB tests (ASTM D4959)
  • PCB particules count (ASTM D6786)
  • Particules count (ASTM D6786)
  • DBPC tests (ASTM D4768)
  • Polymerase tests (ASTM D4768)
  • Furans tests (ASTM D5837)
  • Dissolved gas analysis (ASTM D3612)

To know your insulating oil quality, your oil and dissolved gas sample will be analysed in laboratory and our engeneering service will offert you, in a certified report, a detailed interpretation and the appropriates recommendations.

What are these analyses:

Dielectric strength, acidity index, superficial tension, color and sedimentation analysis is used to evaluate the condition of the oil. For example, an acid oil deteriorates insulating paper in the transformer.

Dielectric strength is dielectric break point in ASTM standard test cut. An inferior value to 24 Kv requires a deeper verification; an inferior value to 15 Kv is considered dangerous. This test indicates insulate quality of the liquid.

Acidity of an insulating oil for transformer is determined by titration of a standard oil in a benzolisopropanol solution with a standard solution of potassium-isopropanol hydroxide. The indicator is benzo-naphtol-alpha. The value done is the necessary mg of KOH to neutralise acidity in 1 gm of oil. Organic acids are just a group of oxidation product which cause the gradual deterioration of oil and confirm the gradual disappearance of the anti-oxidant, which leads to the formation of silt and reduce the lifetime of your transformer. When value is more than 0,1 mg of KOH this indicates the decline in anti-oxidant, when the value is more than 0,3 mg of KOH this indicates the disappearance of anti-oxidant.

Superficial tension inbetween face of oil sample on a surface of distilled water is the measurement of sediment concentration of all oxidation products in oil. It is determined by measuring the force required to draw a fine platinum circle through common inbetween face of oil and water. The value is in dynes/cm. The value of a new insulating oil lies between 42 and 50 dynes/cm. Inferior values to 25 dynes/cm indicate start of polar compounds formation; if these values descend below 17 dynes/cm, it indicates mud and silt formation.

The recommanded methods to correct acidity and superficial tension when they are below standards are fuller earth treatment or oil change.

The color give an idea of type and condition of oil.
This ASTM abbreviations and value are:

ColorASTMColorASTMColorASTM
VPY0,5DY3,0DA6,0
PY1,0VDY4,0VDA7,0
Y2,0PA6,0B8,0

VPYVery pale yellowDYDark yellowDADark amber
PYPale yellowVDYVery dark yellowVDAVery dark amber
YYellowPAPale amberBBrown

Sediments indicate presence of silt or mud. These molecular deposits are oxidation product. To determine their presence, the sample must be retained in dark for a 12 hours period at room temperature.

Water quantity analysis in P.P.M./Vol. allows to determine if there's water condensation or infiltration in the transformer.

Oil PCB quantity analysis in P.P.M./Vol. determines if mineral oil of transformer is contamined with PCB.

Dielectric analysis on PCB determines the insulating quality of PCB liquid. For example, an inferior result to 24 Kv represent a risk of failure.

Dissolved gas analysis in oil allow to detect the presence of problem in apparatus and determine the type. For example, a bad connection on tap changer (formation of carbon by pyrolysis).

The antioxidant (DBPC) presence is the first defense against the oxidation of oil and development of muds, acids or other polar compounds because the antioxidant is eliminated as it performs its protective task. It's important to keep a concentration up to 0,02%.

Antioxidant interpretation

Did you know that your transformer oil contain antioxidant? It prevent oxidation of the oil. The decrease of antioxidant causes polar compounds formation, that causes the formation of soluble acids resulting in the formation of mud.

The normal rate of antioxidant according to ASTM standard is 0,08% for new oil, and 0,3% for revitalized oil, with the addition of the antioxidant. According to the experience, when the rate reach 0,04% adding an antioxidant is required to prevent a deterioration of the oil. Any further decrease entail considerable costs until rate 0,02% when fuller earth treatment or oil changement is mandatory

We recommand antioxidant adding to your oil if rate down under 0,05% to reach rate of 0,1% to 0,3%. Taking into account these recommendations your will prolong the life of your transformer and minimise risk of failure.