Damage of Distribution Transformers

Destruction by overcurrent can be prevented by proper circuit protection techniques.

Destruction by military action is more difficult to prevent.

Is the damage by natural cause? (e.g lightning, flooding etc)

Education may help for damage caused by persons. People can be taught that damaging transformers are not considered acceptable pastime.

The rate of damage extremely go up in hot weathers; ambient temperature is about 50 deg centigrade and in Monsoon when the atmosphere is extremely humid

So the transformers in question need to be de-rated according to the original equipment manufacturer's recommendations to allow for the ambient temperatures and humidities.

In unpredictible load demand the transformers run overload upto 130% or more during peak load load hours and at day time they are 20% to 25 loaded. So how to cope with such a diversified load. we pecieve some of the main reasons for damage of transformers is loose jumpers and connection, unbalance load, improper sealing which can not stop the entry of moisture particularly where bushing enters into the transformer tank, improper neutral and its earthing, improper fuse and many others which can be brought up in this forum.

Some of the perceived difficulties can therefore be attributed to shortfalls in the maintenance regimes applicable to these transformers. These matters are best addressed at a local level. Improvements in maintenance may have an immediate and significant effect on the failure rate and will, undoubtedly, present an excellent business case for increased activity in this domain.

Based upon the patterns you mentioned (hot weather and humidity), there are probably multiple reasons for failure. An analysis of the various types of failure and the resultant damage to the transformer would be beneficial.

The 130% peak load is a problem, particularly with the high ambient temperature. Transformer capacity is based primarily on the ability of the enclosure and cooling system to remove heat from the windings. However, the cooling system for a distribution transformer depends on transferring that heat to the surrounding environment, and the rate of heat transfer is directly proportional to the temperature difference between the hot item and the cooling medium (in this case, air). The distribution transformers I'm familiar with are designed for a maximum temp. rise of 65° C, with a max. ambient of 40° C. At 50° C ambient and 130% of design load, the temperature inside the transformer is probably 140° C or more. Unless the transformer is specifically designed for that type of temp rise, failure is inevitable. There are ways to determine how much useful life is lost for a specific overload condition, but I doubt you have metering installed on a distribution unit to record the magnitude and duration of each overload (I certainly don't).

If you are using standard loading assumptions to determine what size transformer to install, you may need to revise your calculations. Sizing calculations assume a diversity of load use, where the time distribution of loading between customers means that actual loading is less than the sum of the individual service capacities. The capacity factor may be anywhere from 20-80%. It is likely that all, or nearly all of the customers on a given transformer are at max capacity at the same time. Unfortunately, since bulk electricity cannot be stored, the distribution system must be designed for the peak load, not the average.

For the humidity issue, I suspect tracking across the bushings leads to failure. The humidity itself may not be the primary factor. In many regions, dust or pollution in the air lands on the insulator and over time provides a path across the surfact of the bushing to the grounded case.

If this is a long-term improvement issue, I suggest inspecting each failed transformer after replacement, and documenting the type and location of damage, as well as the environmental conditions and loading. That should build a database & allow you to pinpoint several causes.

use fuse protector to provent from over load.

Are you using three phase 220/380 volt transformers? Or; are you using single phase transformers? The Iraqi system here is using three phase 220/380 volt transformers that feed into blocks long secondaries. The load growth is more than these transformers can handle. Can you split your secondaries between more transformers to reduce loading on the transformers?

The signature analysis of failure of various transformers can definitely concludes on the required measures to minimize the break down:-

1. The selection of Transformer

2. The installation & protection provision

3. The load pattern and the temp inside to be less than 140 ^o C ( IS 6600 to be refereed)

4. The termination cabling on load side along with load side protections for various out goings feeders.

5. Over all preventive maintenance

How ever, the same required in length details / data to get practical solution.

nascon

How to solve the moisture problem in transformer conservator tanks.

http://www.h2ocontrol.com/productsTransformerBreatherKit.php

In our case the transformers are hermetically sealed and are without coservator tanks.