Course EP-3: Transformers Types

I need to mention before going through different transformer types that all transformers are classified generally into two main categories:

1- Power transformers: transformers used in transmission of electrical power for long distances in high voltage ranges.

2-Distribution transformers: transformers used in distribution of electrical energy to users for short distances in medium and low voltage ranges ( note that traditionally , we use the term distribution transformers for only transformers that transform power to the domestic user at 400 V or Less).

In contrary, the IEC didn’t distinguish between power and distribution transformers, it deal with them as transformers only used to transmit power from one voltage to another.

now we can proceed explaining of different transformer types as follows:

A-According to transformed voltage level:

1. Step up transformer 
2. Step down transformers 

1- Step up transformer  see fig (2)
A step-Up Transformer is one whose secondary voltage is greater than its primary voltage. This kind of transformers “step up” the voltage applied to it. see fig(1)

fig(1)

The power source of this type of transformers on primary usually be a generator ,so it is usually star-delta connected and there are many reasons to connect the secondary winding in delta as follows:

• The Zero sequence impedance of the transformers will be reasonably low. 
• The delta connected winding has a smaller bundle of wires than that of star connected due to that the current in delta connected winding will be 1/√3 that of star connected one. 

fig(2)

The generator can be connected to the transformers by cables or bus ducts , the last is preferable to minimize the risk of short circuits between phases.

Points to be taken into consideration when procuring a step up transformer:

• Overheating due to strong magnetic fields originating from high current passing through low voltage conductors. 
• Over voltage protection as surge arresters must be used between each low voltage terminal and earth and between each phase.
• The transformer must be selected to undergo the maximum duration and magnitude of any over voltage occurring in the system network.

2- Step down transformer 
A step-down Transformer is one whose primary voltage is greater than its secondary voltage. This kind of transformers “step down” the voltage applied to it. see fig (3)

fig (3)

Step down transformer come in sizes of a thumb nail to huge sized units for power transmissions or national power grids. No matter what their size, they all operate with the same basic principles. They can transform voltage for any application requirement, from running a hair dryer to running large power grids. 

Now an important question can be raised as follows:

Is it possible to operate either of these transformers types backwards (powering the secondary winding with an AC source and letting the primary winding power a load) to perform the opposite function: a step-up can function as a step -down and visa-versa?.

And the answer will be as follows:

Efficient operation of a any transformer requires that the individual winding inductances be engineered for specific operating ranges of voltage and current, so if a transformer is to be used “backwards” like this it must be employed within the original design parameters of voltage and current for each winding, lest it prove to be inefficient.

in the next Topic ,we will show more transformer types.