Transformer Efficiency- Effect of Loading

Importance of Transformer Efficiency-

Distribution transformer is a part of every electrical power network at user end. If a facility is being supplied on HT from the utility, transformer and its losses comes under user’s responsibility. Like any other equipment its efficiency is calculated as the ratio of output power and input power. As transformer operates on the magnetizing phenomenon and it has no rotating parts, its efficiency is very high. It usually stays above 95% but as the power flow across a transformer is very high even small changes in terms of efficiency percentage gives significant energy savings.

For the purpose of energy wastage calculations, considering two transformers of 2500KVA each and operating at 95% & 96% efficiency. For supplying a load of 1000KW, the input power for them will be-

Transformer1-> 1000/0.95= 1052.6KW

Transformer2-> 1000/0.96= 1041.6KW

This implies extra losses of 11kw and 264 units in one day. For the purpose of illustration, the calculation has been made too simple and it is only to prove the significance of transformer’s efficiency.

Type of Losses in Transformer-

Further for understanding the losses in a transformer, they can be divided in two different parts.[1]

1. No-load losses or Core losses.

2. On-load losses or winding losses.

No-load losses represents for the energy dissipated for charging the transformer. Energy consumed in setting up a magnetizing field in the core of the transformer and eddy current flowing in its iron plates, accounts for these losses. As these losses depend only on the core size and frequency of the power supply, they stay constant and are independent of transformer’s loading.

Pcore= Ph+Pe (CONSTANT)

Where Ph =hysteresis losses and Pe= Eddy current losses.

Copper losses are produced in the winding of transformer, current flowing both in primary and secondary windings of the transformer dissipate energy in the form of heat due to I2R losses. Being directly proportional to the square of current, these losses vary with transformer loading.

PCopper= Pp+Ps

Where Pp Primary copper losses and Ps is secondary copper losses.

Efficiency of a transformer is maximum when these two type of losses are equal.

i.e. Core Losses = Winding Losses.[2]

The plot is prepared of core and copper losses along with percentage losses in a distribution transformer with 10KW as fixed losses. It is clear from the plot that minimum of percentage losses occur when both the core and copper losses are equal. Also for higher loading, heat losses inside the transformer increases which affect the life of insulation.

Effects of transformer loading-

As already mentioned, maximum transformer efficiency occurs when the core and the copper losses are equal. So, how do you find out the percentage loading , that corresponds to this point? Due to the continuous changing load on a transformer, they are designed for giving maximum efficiency at 50% of loading. Considering the practical constraints, a window of 50-60% loading is to be considered for optimum operation.[3]

Below is a plot of efficiency of the transformer over loading, representing the negative effect (which though is small in relative terms but can lead to significant money wastage in absolute terms) of higher loading on its efficiency.

Figure 3 is for better visualization of decrease in efficiency at higher loading, which is obtained by zooming in figure 2 with percentage loading varying between 40 to 100%.
Fig.2                                                                Fig.3

Thus the window of 50-60% loading can be considered for a transformer which has not been repaired on user site. If a transformer has been re-winded or repaired for faults in the core, its no-load losses do not remain the same. After such alterations in the transformer, point of ideal operation on percentage loading also get shifted.

Energy-Star Ratings of Transformers by BEE-

On 6th July 2009, bureau of energy efficiency released a circular about giving STAR ratings to distribution transformers. According to which, based on their losses at 50% loading and 100% loading, transformers will be rated between 1-STAR & 5-STAR [4]. For achieving better efficiency performance, every manufacturer is supposed to provide the STAR rating sticker on the transformer itself. As mentioned by BEE, transformers fitted at 11KV/433V or 33KV/433 point has to be of at least 3 STARS. [5] Chart below has been provided by BEE and can be referred as the standard data for the scaling.


Although standards provided by BEE are for distribution transformers of up to 200KVA rating but by calculating the efficiency percentage and extrapolating these readings, along with consideration towards increased no-load losses for large transformers, base line can be set for transformers above 200KVA.

Based on the basic requirement of 3-STAR distribution transformer, central authority of India has published the recommended levels of maximum total losses at 50% and 100% loading. Refer to the chart below for maximum losses in watts for transformers above 200KVA. [5]

The no-load losses and correspondingly the point of maximum efficiency can be obtained. This is possible with a meter on both HT and LT side configured with Zenatix’s Software, collecting high resolution power data. By analyzing the data for a long enough time, Zenatix calculates the constant and the variable part in the losses. Getting the constant losses, point of maximum efficiency can be calculated. At a facility with more than one transformer, loads can be adjusted across different transformers such that it gives maximum overall efficiency.

Fig1. No-Load, Copper Losses and percentage overall losses of a distribution transformer.

Fig2. Curve of efficiency of transformer over percentage loading.

Fig3. Zoomed in efficiency curve for visualizing decrease in efficiency from 50 to 100% loading.

Table.1 Maximum total loss baseline for different star ratings of transformer up to 200KVA

Table.2 Maximum losses for 3 STAR rated transformers


[1]- SEAD Distribution Transformers Report Part 3: Energy Efficiency Class Definitions