What does the manufacturer's maintenance manual have to say on the subject?

Thanks for this note, whoever i have studied more than 4 manual from different manufactures, and all of these gauges are maintenance free, only they are tested to make alarm/trip.

Ahem!

This is homework.

My search results:

1. The [Oil Level gauge] measures the oil level in the conservator using arm connected to floating element, connected to the pointer through magnetic coupling.
2. For the following cases
• At normal operation: Level is at (20C): OK
• At Hot operation: the level increases to be near Max (the design should assure that the expansion of oil will not exceed the space between 20C and Max, otherwise the oil will be spilled from the conservator through breather), or if the temprature increases to this limit the Temp. protection should trip (or the Bochlez if the cause is short circuit). so how to differentiate between hot oil and excess oil?
• At Cold operation: the level decreases to be near the Min. (at this moment how can i differentiate the cause of low level whether it is Cold oil, or oil leakage/shortage)?

Hot operation is up around 60 C for you I imagine. The coefficient rate expansion of your fluid can be found on your fluid data sheet. A simply formula is, Δ volume of oil=Δtemperature X original volume of oil X γ coefficient of volume expansion. Volume of oil will be found on the name plate and just do the math. You will find that the volume can change quite a bit from early morning to mid afternoon.

Transformers will use the base of 20C for proper level (normal) if your oil is at this line at 20C, I wouldn't worry too much. Low levels can be detected by walking around the the beast and looking for leaks. In Canada we get -40C ambient, do you think we are worried about the low level of the cooling oil during that time frame. If you have spillage out of the conservator, you might have to go to a larger one to take up the expansion of your oil.

Thanks for the detailed calculations about the expansion volume of oil.

For the oil spillage, i had already made inspection around the transformer, and i found a leak form the [Pressure relief device]. when i try to add oil to the conservator to compensate the missed volume (and it was day shift 38C ambient) i was hesitated when filling, would i fill to level 20C exactly or above? (because the oil is now expanding)

the oil level varies according to its temperature so if the oil temperature is zero for example the oil level indicator will show that the transformer needs to fill up .and if the degree is 100c the indicator will show that the transformer needs to drain some oil .

so the result of ignoring the oil temperature is incorrect decisions whether fill or drain some oil from the transformer.

So the correct indication of oil level refers to the level and oil temperature

Example :

The gauge pointer points to low level oil but we cant judge that the transformer needs to fill up without referring to the oil temperature, if the temperature is more thane the 20c so the transformer really needs to fill up