Low viscosity means easy circulation of transformer oil in the winding and better cooling.

Question

Examoogle · Accepted Answer

Option A: True

Explanation & Concepts:
Transformer oil must have low viscosity to ensure effective heat dissipation via natural or forced convection. Low viscosity allows the oil to flow freely through the cooling ducts of the transformer windings, effectively transferring heat to the radiator surfaces.

Working Principle: Heat transfer in a transformer occurs primarily through convection. According to the principles of fluid dynamics, the convective heat transfer coefficient ($h$) is influenced by the kinematic viscosity ($\nu$) of the fluid. A lower viscosity reduces the resistance to flow (drag), thereby increasing the velocity of oil circulation. Faster circulation rates promote a more uniform temperature distribution and efficient heat removal from the hot spots in the windings.

Key Formulas: $Q = h A \Delta T$ — Heat transfer equation where $h$ increases as viscosity decreases; $
u = \frac{\mu}{ho}$ — Kinematic viscosity definition where $\mu$ is dynamic viscosity and $ho$ is density

Key Points: Transformer oil serves both as an insulating medium and a cooling agent.; High viscosity at low temperatures can impede initial startup circulation.; Standard testing for viscosity is typically performed using a Redwood or Saybolt viscometer.; Low viscosity is essential for maintaining the thermosyphon effect in naturally cooled transformers.

Advantages: Efficient heat transfer; Reduced winding temperature; Prevents formation of sludge through proper circulation

Disadvantages: Extreme low viscosity may compromise the dielectric strength if the oil becomes too volatile; Increased risk of leaks if oil is too thin (low viscosity)

Applications: Power Transformers; Distribution Transformers; Instrument Transformers (CT/PT)

Additional Info: The viscosity of transformer oil is generally specified to be below 12 cSt (centistokes) at 40°C.. Option B is incorrect because high viscosity would restrict flow, creating 'dead zones' of stagnant oil that lead to insulation overheating.

Answer: A is correct — Low viscosity ensures efficient oil circulation, which is critical for transporting heat away from the transformer core and windings to the cooling surface.

Exam Tip: Always remember that transformer oil serves two masters: insulation and cooling. Viscosity is the primary property governing the cooling effectiveness.

Examoogle · Answer

Option B: False

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