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Examoogle • User • info@examoogle.com • EE-2024-8821
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In annually inspection which is mainly inspected in less than 1000 kva transformer.
Fire system
Core
Coil
Oil
Coil
Quick Summary: In annually scheduled maintenance for distribution transformers rated below 1000 kVA, the insulation oil is the primary component inspected. Oil quality assessment is critical because the dielectric strength and cooling efficiency of the transformer directly depend on the chemical and physical condition of the oil.
In annually scheduled maintenance for distribution transformers rated below 1000 kVA, the insulation oil is the primary component inspected. Oil quality assessment is critical because the dielectric strength and cooling efficiency of the transformer directly depend on the chemical and physical condition of the oil.
Vb∝ρoil — where Vb is dielectric breakdown voltage and ρoil represents oil purity/resistivity
ΔT=Ploss×Rth — where the oil medium helps manage the temperature rise ΔT
The transformer oil performs dual functions: it acts as an electrical insulator between live parts and as a coolant for heat dissipation. Over time, moisture ingress, oxidation, and contamination lower the dielectric breakdown voltage (Vb) and increase acidity. Annual inspection includes testing for dielectric strength and moisture content to prevent internal flashovers or insulation degradation.
Oil dielectric strength (BDV) is tested using an oil test set.
Common tests include BDV, water content (PPM), and acidity.
Transformers < 1000 kVA are typically oil-immersed distribution types.
Oil color indicates the presence of oxidation or degradation products.
Prevents premature failure of transformer windings.
Ensures safety by monitoring insulation integrity.
Requires scheduled downtime for sampling.
Oil handling requires environmental compliance.
Distribution transformers in substations.
Industrial power supply units.
For transformers > 1000 kVA, the frequency and depth of inspection (e.g., Dissolved Gas Analysis - DGA) increase significantly.
Option B (Core) and Option C (Coil) are internal components that do not require annual dismantling for inspection unless there is a fault detected via electrical testing.
D is correct — The insulating oil is the most sensitive medium in a transformer and requires annual monitoring to ensure its dielectric properties have not degraded.
Always remember that while the core and coils are robust, the oil is the 'blood' of the transformer and is the most prone to environmental aging.