Power Up! Building a Distribution Transformer Quiz

Quiz Answer Key and Fun Facts

1. Distribution transformers are important in that they convert huge voltages running through power lines to lower, more useful electricity that we can use in our homes. What is the main factor that determines the ratio that the transformer will reduce the voltage to?

Answer: Number of conductor turns in the primary and secondary winding

Simply put, a transformer works by applying the high voltage input to windings around the steel core in the active part. This creates an alternating magnetic field which induces voltage into the secondary winding. The ratio that the voltage is stepped down or up is determined by the ratio of turns from between the primary and secondary winding- if there are 200 turns (primary) and 100 turns (secondary), there will be a step down ratio of 2:1.

2. Distribution transformers generally consist of two main parts - a tank that holds everything together and an active part inside that does the work. In most cases the tank is also filled with a liquid. What do we fill the tank with to insulate the active part?

Answer: Mineral Oil

Mineral oil is used because of its excellent insulating properties. It is important that your oil is filtered to remove any water, impurities and dissolved gases, as all of these may cause arcing in the oil when current is applied. This in turn will break down the insulators causing expensive electrical loss and a reduced lifespan of the unit. Besides the insulating properties of the oil it also serves as a cooling agent, in that it fills all areas in the transformer including the corrugated panels that provide a large surface area. External factors like natural cold and wind affect this area, cooling the oil which in turn cools the active part.

3. We are now building the active part of the transformer. Which of these metals would we under normal circumstances consider using as conductors for our windings?

Answer: Copper

Although silver is considered the best conductor and gold is also very highly rated in this regard, both are prohibitively expensive in everyday applications.
Steel is a very poor conductor of electricity.
Copper and aluminium are generally the best conductors in terms of efficacy and cost-effectiveness.
Aluminium is generally used in higher voltage systems as it has a very high conductance to mass ratio, which saves on weight in the transformer. It does however require greater cooling areas because of its higher resistance which leads to higher temperatures.

4. We now have our core and conductor. The final step in the building of the active part is to wind the conductor on a drum which will fit over the core. However, the windings need to be insulated. Which substance should the insulation board that we use be covered in to be sufficient for our application?

Answer: Resin Epoxy

In order for the copper windings to be sufficiently insulated, we use a cellulose transformer board that is covered in a resin epoxy layer. Cellulose has excellent electrical properties, and the resin protects the board from decaying to a point where windings could come into contact with each other, while also providing sufficient space between the windings.

5. After attaching all the cabling correctly, our active part is now almost ready to be placed inside the tank. The last step would be to provide our customer with a way to make subtle changes to the output voltage. This is done by connecting a type of switch to the active part. What is this device called?

Answer: Tap Changer

A tap changer works by physically changing the position of the inflow voltage on the winding. This changes the ratio of the winding, which in turn leads to an adjustment in the output voltage.
Generally, "off-circuit" tap changers will be able to accommodate a 6% change up or down, and require an interruption of the power supply. "On-circuit" tap changers are generally expensive and require specialized equipment to be used effectively, but do not require the grid to be switched off in order to make an adjustment.

6. Our active part is now all set to be placed in the tank. Once this is done, the insulation liquid can be added. What test do we perform to ensure that the liquid is acceptable?

Answer: Di-electric Test

A di-electric test is performed on our oil to determine if the quality is of the correct specification and there are no impurities that could affect the operation of the transformer. A specialized machine tests the oil for insulation effectiveness across two poles - if the reading is lower than required the oil may not be used.

7. Once the active part has been fitted in the tank, we need to connect it to the outside of the tank via a conductive wire surrounded by a bushing. Bushings are the protrusions visible on the outside of a transformer where the power lines are connected. What is it made of?

Answer: Porcelain

Glazed porcelain is impervious to moisture and has no electrical conductive properties, so it is ideal as insulation on the outside of a transformer. The spaces inside the bushing are also filled with oil to provide additional insulation and assist in cooling.

8. We have now placed the active part in the tank, made all the connections, filled it, and finally welded the cover in place. Before sending it to the lab for testing we perform a final check at the welding station. What do we need to check for?

Answer: Leaks

As we have only welded the cover on, there may be leaks that are not immediately visible. This is why we place the transformer contents under pressure for 16-24 hours- if there are no leaks after this period we can safely assume the welding has been done correctly.
If leaks are detected we need to repair and redo the leak test on the transformer.

9. Our transformer is now complete and can be sent to the lab for extensive testing. Each unit should be tested as a test certificate is issued with every sale. Which test do we do to check if the transformer can withstand the voltage that will be applied in the field?

Answer: Induced Voltage Test

All the tests mentioned above are performed during the lab portion of our manufacturing process. However, during the Induced Voltage test the transformer is loaded with at least double the voltage that it is rated for and checked.
We also test for load and no-load losses and separate source.
Only a transformer that passes all the tests will be allowed to go to stock.

10. We now have a complete, fully tested transformer. What do we attach to the unit that shows what specifications it is designed for?

Answer: Rating Plate

A rating plate is a small metal plate that is permanently fixed to the unit. The plate shows what voltages the transformer is rated for, as well as its vector group, mass, date of manufacture, serial number and any other pertinent info.

Source: Author strudi74

This quiz was reviewed by FunTrivia editor WesleyCrusher before going online.
Any errors found in FunTrivia content are routinely corrected through our feedback system.

Distribution transformers are critical in our modern world, bringing usable electricity to your home. See if you can help me build a transformer!

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