How often should oil in a transformer (say 0.5 to 5MVA) be changed?

Question

1. How can you tell whe it is time to change it?
2. What is the approx. cost to build a new one in that size range? Is there a relationship for cost v. size?
3. How long does it take to build one these days? I hear that steel is hard to get beacuse of the chinese building lots of plants.
4. When is it easier and practical to rebuild one? What do they rebuild inside and why?

Thanks.

Answer 1

Transformers of this size generally do not have their oil replaced unless they are returned to shops for repairs.

1. Take oil samples and perform disolved gas analysis (DGA). The readings will indicate various problems.

2. The cost of transformers is very volatile lately. The materials that make up a transformer have been flucuating tremendously. Core steel, copper, aluminum, mild steel, and even mineral oil. I think $14 to $20 per kVA is a reasonable ball park estimate.

3. The lead-time to buy a North American made transformer of that size is greater than 40 weeks. There are only two companies that make core steel in North America. Their production capacity can not keep up with demand. Many transformer manufacturers are using foreign steel.

4. Many transformers removed from service only have minor issues unrelated to the core/coil assembly. These are simple repairs.

Answer 2

Testing of the dielectric strength (breakdown of the oil at a given voltage) is the traditional method of determining the inherent "condition" of the oil.

Transformer oil is not subject (in normal operational, unfaulted conditions) to the stresses of other applied "oils". Unlike many other oils, transformer oil is not a lubricant and such is not normally expected to act as such, and will not require a change as a result of deterioration as would be expected in a lubricant.

A DGA (conducted on an oil sample collected from the transformer) will give you a very good idea of the activity (if any) that the transformer windings have experienced. Suffice to say, the analysis and interpretation of the dissolved gases can determine the activity of the windings (arcing etc).

Cost and time frame of transformer manufacture is variable, and certainly dependent on the size. To generalize, cost is proportional to size.

Build time is also as variable, but, due to the demand of developing nations for resources, manufactures generally have lead times of up two years.

The determination of whether to buy a new or to repair a transformer is a matter of economics and many strategic factors may come into play, such the length of time to purchase a new transformer vs the urgency of outage etc. Generally, if we consider a faulted winding, the repair would constitute the localized repair of the existing winding (perhaps involving the removal of primary coils), or, a the manufacture of new coils (windings)

Once again, to generalize wildly, many transformer faults involve on load tap change type issues (due to the mechanical nature of switching), which can be relatively easy and much less expensive to fix, as compared with winding faults.

Answer 3

Transformer Oil Replacement

Answer 4

1)when we hear humming sound frm the transformer ,its time to change the oil.
2)yes,cost is proportional to size.

Answer 5

test the oil for it's dielectric strenght(breakdown voltage)