What is a Transformer?
A power transformer is a static electrical device. It transforms power from one circuit to another circuit without any direct electrical connection. The working principle of a transformer is based on mutual induction. Ideally, it transforms the entire power without changing its frequency. There are mainly three types of transformers: step-up transformers, step-down transformers, and unity transformers.
A step-up transformer increases the voltage level of the same power. A step-down transformer does the reverse. A unity transformer does not change the voltage level. However, it provides isolation between two electrical circuits.
Where are Transformers used in Power Systems?
Actually, power generation happens at a much lower voltage level. This is economical compared to high-voltage generation. But transmitting this low-voltage power to a far receiving end is not economical. So, for electrical power transmission purposes, the voltage level of the power must be stepped up. Here, we use a step-up transformer to increase the voltage level. But for distribution purposes, we need to step down the voltage level of the power. This is required for proper distribution. So, in distribution, we use step-down transformers.
In generating substations, we use step-up transformers. In EHV substations, we use step-down transformers. These reduce the voltage level of the power to sub-transmission and distribution levels. At the end-user point, we use 11 kV distribution transformers. These distribute the power at the 220-volt level.
Types of Transformers
Depending upon the phases of the system, there are two types of transformers. These are single-phase transformers and three-phase transformers. Sometimes, three-phase transformers are formed by three separate single-phase transformers. This arrangement is called a bank of single-unit transformers.
Depending upon the design of winding, there are two types of transformers. These are two-winding transformers and auto-transformers. In a two-winding transformer, there are separate primary and secondary windings. But in an auto-transformer, a common winding is used for both primary and secondary windings. The secondary winding is taken by tapping the common winding.
When the transformation ratio is greater than 2, a two-winding transformer is economical. When the transformation ratio is less than 2, the auto-transformer provides the most economical approach.
Power transformers are generally oil-immersed. Here, we use transformer oil for insulation and cooling purposes. Another type of transformer is also used nowadays in power systems. That type is the dry-type transformer.
Depending upon the design of the core, there are two types of transformers available. One is a core-type transformer. Another is a shell-type transformer. In a core-type transformer, the windings are wound on the limb of the core. But in a shell-type transformer, the windings are also wound on the limb of the core. In this case, two limbs cover the winding.
Cooling System of Transformer
Due to the operation of the transformer, heat is always generated inside the transformer. This loss includes all losses of a transformer, including no-load loss.
Depending upon the cooling system, transformers are classified as ONAN, ONAF, OFAF, and OFWF. ONAN means oil natural air natural cooling transformer. ONAF means oil natural air forced cooling transformer. OFAF means oil forced air forced cooling transformer. OFWF means oil forced water forced cooling transformer. Although all the cooling systems can be implemented in a single transformer, the system is activated according to the loading and temperature conditions.
Transformer Core and Windings
Transformer cores are normally made of laminated cold-rolled grain-oriented silicon steel sheets. Nowadays, an amorphous core is also used for the same purpose.
There are two types of windings mainly used in transformers. These are concentric winding and sandwich winding. Concentric winding means the LV windings are placed around the core leg. Then the HV windings are placed over the LV winding. We need to remember that the LV winding is always placed closer to the core. The HV winding is placed over the LV winding. This approach is economical. Windings are formed by either aluminum or copper. Paper insulation is used to insulate the winding.
Transformer Complete Assembly
The entire core and winding assembly is placed inside a transformer tank with a proper cover. This tank is filled with transformer oil. The oil serves the purpose of insulation and cooling. To take out the winding terminals from the earthed tank body, we use transformer bushings. Bushings are of different types. In power transformers, we normally use condenser bushings like RIP bushings, OIP bushings, etc. Also, for lower voltage class power transformers, we often use synthetic resin bonded paper bushing (SRBP or RBP).
Due to changes in load, the voltage regulation of transformers changes. To improve the output voltage level, we use a tap changer. In other words, we use it to improve the voltage regulation of the transformer. There are two types of tap changers. One is an off-circuit tap changer. The most popularly used tap changer in bigger power transformers is OLTC. OLTC means on-load tap changer.
Transformer tanks are equipped with radiators, coolers, fans, and pumps. Fans are used for forced-air purposes in ONAF and OFAF cooling systems. Pumps are used to circulate the oil during OFAF and OFWF cooling systems. This means oil is forced to circulate during temperature variation. It also depends on varying weather conditions. It further depends on the changing loading conditions of the transformer.
The volume of the oil inside the transformer also changes. So, sufficient space must be provided for this changing oil volume. For this reason, an additional tank is provided on top of the main transformer tank. This tank is called the conservator tank.
Nowadays, the conservator tank is provided with a bellow. This ensures proper breathing of the transformer without direct contact with air. A silica gel breather is fitted in the conservator tank. The transformer breather allows air to pass during changes in the oil volume. This volume change happens due to temperature variation.
There are some mechanical relays used in the transformer. The main mechanical relay used in the transformer is the Buchholz relay. If any internal arcing or similar type of fault occurs in the transformer, the oil creates gases. This gas gets collected in the Buchholz relay. Depending upon the volume of collected gas, the Buchholz relay gives an alarm. A major fault can also create an oil surge in the transformer. Oil surges activate the trip contact of the Buchholz relay.
There is an oil surge relay connected to the diverter tank. The diverter tank is a separate oil tank provided in the transformer for this purpose. It is attached to the main tank. The diverter tank contains oil separated from the main tank. The tap changer switchings are performed in the diverter tank. Due to these switching operations, the chance of arching is higher in the diverter tank.