Electricity + Control March 2018

PLANT MAINTENANCE, TEST + MEASUREMENT

Consider FULL PICTURE when Selecting, Sizing Transformers Ronaldo Bertoldi, WEG Transformers Africa (WTA)

Many factors are involved in the selection and sizing of transformers.

Take Note!

Different scenarios have to be considered when deciding how best to de- sign a system with the appropriate transformers. This includes the choice between single phase and three phase require- ments. It also includes the phys- ical constraints that a transformer installation must accommodate.

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W hile the sizing of a transformer for a par- ticular application can be worked out using a simple equation, there are many other factors that also need to be taken into account when making a selection. Determining the right transformer for the job requires an in-depth assess- ment of the conditions under which the transformer will operate. As one of the largest manufacturers of distribution transformers, power transformers and mini substations in South Africa, WTA is an ex- pert in designing, engineering and manufacturing a complete range of transformers for various sectors including mining, industry and agriculture. The technical sizing of single phase and three phase transformers is certainly based on the load voltage, the load current and the line voltage but this does not give the full picture of what will be demanded on the transformer over its lifespan. In an environment in which motors are being fed by a transformer, for instance, one of the vari- ables is the way in which these motors are started up. The user may easily identify a motor as a 100 Amp unit, but there will be a difference between the full load current and the start-up current if there is a direct online starting process. If the 100 Amps refers to the full load current, then a transformer that delivers 100 Amps will be sufficient for running that motor. However, if the motor is initiated by a direct online start, this could

demand six times higher current during start-up. A transformer will also usually feed a range of equip- ment, so it is also important to know about the starting sequence – whether the user starts up all this equipment at once, or one unit at a time. Another factor is the distance between the source of the power and the equipment being fed, as there will be a voltage drop to take into account in the sizing of the transformer. Transmit- ting low voltage over long distances can also be more expensive due to the thicker cables required by the higher current. Users can achieve a more cost effective solution in many cases by consider- ing a step-up, step-down configuration, where the voltage is increased to facilitate longer distances, then reduced to the requirement of the equipment at its point of location. Different scenarios have to be considered on their merits, in order to decide how best to design a sys- temwith the appropriate transformers.This includes the choice between single phase and three phase requirements, as well as the physical constraints that a transformer installation must accommodate. Certain applications will demand that a trans- former is elevated out of reach on a pole, for in- stance, while others may be at ground level on a concrete plinth. This affects the design, as the bushings may need to be enclosed for safety pur- poses. There can only be open conductors where

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A thermal image showing the effects of efficient cooling.

The WTA 100 MVA 132 kV test bay, which meet both IEC and BS standards.

The WTA mini substation production line.

32 Electricity + Control

MARCH 2018