July 8, 2022

Investigating core influence on transformers insulation diagnostics

Historical data suggests that oil-filled power transformers should have insulation PF of 0.5 % or less at 20 °C for all main insulation segments.

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Impact of core laminations on transformer C_L power factor

Historical data suggests that oil-filled power transformers should have insulation PF of 0.5 % or less at 20 °C for all main insulation segments, e.g., C_H, C_HL and C_L, to be considered in good condition. Normally, the effect of the core design would be negligible during PF measurements, as it is effectively grounded across the complete core stack. In most cases, a single-core ground lead, placed between two adjacent laminations, typically located in the centre of a core, is used to obtain an effective grounding of all core laminations. The interlaminar resistivity should be high enough to avoid circulating currents between core laminations during service, but additionally, it should be low enough to allow an effective core stack grounding. A combination of proper interlaminar resistivity and suitable burrs height at the core steel lamination edges is usually sufficient to produce a low-resistance path for currents flowing to the ground during the C_L measurement. This results in PF < 0.5 % for units with healthy C_L insulation.

Elevated PF caused by excessive core lamination insulation was first reported in 1979 in [1]. Moreover, in recent cases, a combination of shorter burrs (i.e., shorter than interlaminar space) and a much higher and non-uniform interlaminar insulation resistance forced the current in the core to flow through a much higher resistance. This condition may result in PF >  0.5  %, thus misrepresenting the condition of a healthy insulation segment. The analysis presented hereby addresses the C_L PF issue based on experience with core-type transformers.

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