Influence of external factors on load of transformers

1. Ambient temperature:
If a transformer, choke or power supply is designed for operation at the ambient temperature of 40°C and the actual ambient temperature is higher, then the transformer load should be reduced according to the table below. If the load power is not reduced, then the equipment service life will be shorter or it may result in equipment damage.

Ambient temperature 40°C 45°C 50°C 55°C 60°C
Load power reduction coefficient 1 0,95 0,91 0,87 0,84

2. Operation altitude above 1,000 m above sea level.
If a transformer, choke or power supply is installed at the altitude H >1000 above sea level, then the load power should be reduced according to the table below:

Installation altitude (above sea level) 1000 1500 2000 3000 4000
Load power reduction coefficient 1 0,97 0,95 0,90 0,85

3. Discontinuous operation (S3). 
 If a transformer is intended for discontinuous operation (S3), then the relationship between the operational duty cycle and power drawn from the load is given in the following table. Hence a transformer with lower continuous rated power (S1) can be selected for discontinuous operation.

Discontinuous operation S3 in % 100 80 70 60 50 40 30 20
Load power reduction coefficient k3 1 0,89 0,84 0,77 0,71 0,63 0,55 0,45

Power drawn by load = 10 kVA
Operating period = 2 minutes
Idle period = 8 minutes
Therefore discontinuous operation S3 = [2/(2+8)]x100% = 20%.
From the table power reduction coefficient k 3 = 0.45,
So the corrected load power : 10 kVA x 0.45 = 4.5 kVA.
In order to supply a receiver with power of 10 kVA, for discontinuous operation S3-20%, we can select a transformer of design power = 4.5 kVA or typical, higher = 5.0 kVA.

4. Power frequency. 
 A transformer designed for a rated frequency of 50Hz may work at higher frequencies (up to 150-200 Hz). This is because the total core losses are inversely proportional to the power frequency at a constant voltage. However, when the frequency is lowered than rated, the supply voltage should be lowered in order to avoid an increase in core losses and no-load current. This relationship is given in the formula below. Overheating and damage to the transformer may result if this voltage reduction is not observed.

where: fN – rated frequency; fp – operating frequency (lower than rated);
UN – rated voltage; Up –voltage obtained with supply frequency fp.

5. Enclosure.
The transformers produced by Eland Transformatory with IP00 protection are designed to operate without restriction in ventilated IP23 enclosures having dimensions detailed in the ELHAND catalogue or provided the ambient temperature of the transformer is in accordance with that given on the rating plate. Where transformers are installed in non-ventilated IP44 or IP54 enclosures, transformer selection should be made after discussion with the the manufacturer to ascertain power loss and power dissipation requirements.