Selection of Transformer Rating

BIS had given detailed format in IS 1180 where they have specified the Energy Efficiency Levels at 50% load and 100% load. This has been given as per enclosed Table 1.

It is observed that the emphasis is on meeting total Iron and Copper Losses at 50% load. The maximum difficulties in achievement of Transformer design is observed in meeting this requirement.

In line with global practices, BIS has prescribed that selection of the Transformer should be such that it mostly operates at 50% load. The balance should be spare redundant capacity.

When a transformer operates at 50% load, the consumption of power is significantly lower. From the enclosed Table 2 & 3 we can see example where 200 KVA transformer is used in a site where the loading is 100 KVA, hence the consumption will be at 50% load, and the Transformer will consume only 890 watts. Whereas if 100 KVA Transformer is fitted, then the consumption becomes 1800 watts.

In the enclosed Table 3, the savings are shown. It can be observed that over 7900 units are saved every year by using higher capacity transformer of 200 KVA. Even with the generous price difference of Rs. 75,000 between the price of 200 KVA and 100 KVA Transformer, pay back period is less than 2 years. In India, the overloading on Transformers is rampant, often 150%. In a situation where 100 KVA Transformer is used on 100 KVA load, such overloading will result in the Transformer operating at 150% load or 150 KVA. The comparison between 100 KVA & 200 KVA transformer, in such situation, becomes even more important. It will be observed from enclosed Table 4, that the saving is over 18600 units per year in such situation. These figures have been arrived through work out I square R as per enclosed Table 4.

The total saving over 10 years comes to more than Rs. 8 lacs, simply by using 200 KVA Transformer instead 100 KVA Transformer. Other important advantage of selecting a Transformer of 200 KVA Transformer instead of 100 KVA on a load of 100 KVA are –

Transformer does not get heated up

Consequently life of the transformer is much more

Overloading failures are largely avoided .

Selection is as per BIS guidance.

Encl. Table 1 – BIS table of Energy Efficiency Level

Table 2 – Comparison of 100 KVA and 200 KVA as per BIS

Table 3 – Comparison of 100 KVA and 200 KVA at load of 100 KVA

Table 4 – Comparison of 100 KVA and 200 KVA at load of 150 KVA

Table 2

Table 4