Power Transformer

Active part is defined as the magnetic circuit and the windings.

Winding

The winding is made of copper (or aluminum) wires

insulated with materials (paper, varnish, kelvar, mylar, etc.)

a winding is resinated in several layers and the insulation

is made between the various layers.

The primary and the secondary are placed on top of each other

But insulated between layers of insulation.

Winding of a small transformer

The windings are vacuum dried and impregnated with varnish

to reduce oxidation.

Magnetic Core

The magnetic circuit of a three-phase transformer has

  general three columns.

A phase winding is placed on each column.

The primary and secondary windings are placed on top of each other and insulated from each other.

Large transformers use layer windings.

The insulated and varnished coils are placed in a metal tank.

The tank is filled under vacuum with an oil at the terminal blocks

The oil circulates through pumps and passes through the radiators.

The transformer is equipped with radiators cooled by forced ventilation. The fans are installed under the radiators. The large size terminal blocks allow connection to the network. The oil is circulated by pumps. Oil temperature and pressure are measured to predict transformer performance.

Insulating terminal

Insulating terminals are made with a porcelain insulator

Perforated. The insulator has a disturbed surface to increase the leakage path and the lightning strike voltage. A copper or aluminum bar goes through the porcelain. The isolator is filled with oil

transformer to improve insulation.

Tank

Transformer tanks are made of sheet steel. They must be able to withstand the forces exerted during transport of the transformer. Their cover is removable and sealed with bolts or welding. It is inclined at least 1° in order to evacuate rainwater. In order to guarantee tightness, synthetic rubber seals are used. The seal must be perfect, the tank is tested under vacuum to check this point. In addition, to withstand outdoor conditions, the tanks are painted with a corrosion-resistant coating.

Moreover, for high power transformers, the leakage flux becomes quite significant, to avoid excessive heating of the windings or the tank, linked to the eddy current inducing in their breasts, screens are placed inside walls of the tank. These conduct the flux and thus prevent its passage into the other parts and thus their heating by eddy current. As with magnetic circuits, in order to prevent current flow, it is important that they are only grounded at one point.

Components

List of parts and components constituting a power transformer

In addition to its magnetic core, windings and insulation, a transformer has many secondary components. Some are essential as a device for oil expansion (the conservator is presented here), others are not, such as the tap changer. Those presented here correspond to a “classic” power transformer.

On the below diagram are represented:

1. Tank
2. Cover
3. Conservator
4. Oil level indicator
5. Relay Buchholz
6. Oil pipe
7. Tap changer
8. Tap-changer electric motor
9. Mechanical tap-changer transmission
10. Primary feedthrough, with connection at its end
11. Dome with current transformers inside
12. Secondary terminal
13. Secondary connection with the outside
14. Dome with current transformers inside
15. Windings
16. Magnetic core
17. Mechanical element holding the magnetic core and the windings together exerting a compressive force
18. (not shown)
19. Connecting the tap-changer to the windings
20. Oil valve
21. Air valve

We can add the oil, the paper insulation and the cooling system which are not represented. Surge protectors are present at the level of the connections with the lines as already mentioned in the overvoltage chapter.

Tap changer

The tap changer allows the transformer to vary its conversion ratio by varying the value of the inductance of its windings (primary or secondary). This adjusts the voltage level of the electrical network.

It is usually located on the side of the transformer and has a separate oil pan. The electric arcs occurring during the switching of sockets indeed break down the oil and harm its dielectric properties, so it is necessary not to mix it with the healthy oil.

Conservator

The conservator is an oil tank overhanging the transformer. Since the temperature of the oil is not constant: it depends on the outside temperature and the load on the transformer, it expands more or less depending on the circumstances. Thus an increase in temperature of 100°C leads to an increase in the volume of oil of the order of 7 to 10. It is therefore necessary in the event of high temperature to store the surplus, in the case of so-called “breathing” transformers. this role is fulfilled by the curator.

This metal cylinder can be separated into two distinct parts using a rubber membrane, called “diaphragm”, on one side the transformer oil, on the other dry air (moisture could over time pass through the membrane and degrade the dielectric properties of the oil). Sometimes the oil is in direct contact with the air. This is made dry thanks to a desiccator located between the air pocket and the outside. The oil can therefore rise or fall in the conservator without being in contact with the air.

Alternative solutions to the preservative exist. For example, the top of the transformer can be filled with a neutral gas for the oil. It is also possible to design the cooling radiators so that they adapt their size according to the temperature, as Alstom does for medium power transformers: 10 to 136 MVA. This is called a “hermetic” transformer. This system is popular for distribution transformers.

Bushings

The bushings make it possible to link the windings of the transformer to the electric lines without risk of electric shock between the phases and the tank of the transformer which is on the ground.

There are also systems for direct connection of power transformers to electrical substations in metal enclosures, as well as transformer-cable connections.

Current transformer

A transformer can be equipped with current transformers placed in the domes of the bushings (that is to say at their foot). On large power transformers, there is often one measuring current transformer per phase, and several protective ones for each phase and the neutral. Measurement current transformers are used to measure the current flowing through the transformer. Those of protection give information to the protections. They can measure the current with good precision even when the latter exceeds the nominal current by several order factors, unlike measurement transformers which are only precise for values close to the nominal value. A winding is often provided to connect a so-called “thermal image” system which makes it possible to evaluate the heating of the windings.

After manufacture, the transformers are tested to verify their good quality and the validity of the design. These tests include various parts: the dielectric tests, which make it possible to guarantee the quality of the dielectric insulation; measurements of losses, noise emissions and transformer parameters, i.e. winding resistance, short-circuit impedance, capacitance between windings, between windings and tank, as well as thermal constants and temperature rises during transformer operation; verifications of the conversion factor, of the coupling; and fingerprinting for comparisons later in the component’s life. The details of the tests to be carried out are always the result of an agreement between the manufacturer and the processor’s customer.