Transformer is a static electrical equipment. It transmits AC power by transforming AC voltage and current. According to the principle of electromagnetic induction, transformer can realize electric energy transmission. Transformers are important for power transmission and distribution to power users.
In short, both step-up and step-down must be completed by a transformer. In the process of transmitting electric energy in the power system, power loss is inevitable. When delivering the same power, the voltage loss is inversely proportional to the voltage, and the power loss is inversely proportional to the square of the voltage. At this time, the transformer increases the voltage, and reduces the power transmission loss.
Figure 1. Transformer Symbol
When using a transformer, select reasonably of the rated capacity of it. When the transformer is running at no load, you need a larger reactive power. And the power supply system should supply these reactive power. If the transformer capacity is too large, it will not only increase the initial investment, but also keep the transformer in no-load or light-load operation for a long time. This will increase the proportion of no-load loss, reduce the power factor, and increase the network loss. This operation is neither economical nor uneconomical. If the transformer capacity is too small, the transformer will overload for a long time and the equipment will damage easily. Therefore, select the rated capacity of the transformer according to the needs of the electrical load. In short, it should not be too large or too small.
Transformers are basic equipment for power transmission and distribution. You can see them in industries, agriculture, transportation, urban communities and other fields. Different applications will have different types of transformers. What are the specifications and models of transformers? What is the classification standard of transformers? Transformers can be classified by voltage level, insulation and heat dissipation medium, and core structure material. They can also be classified by phase number and capacity. So the specifications and models belong to many different types.
Figure 2. Transformer Structure
If you want to select a proper one, you should know what are the types of transformer. Here gives a details of transformer types according to different standards.
According to Applications
1) Power transformer, used for step-up or step-down of power system.
2) Test transformer generates high voltage to perform test on electrical equipment.
3) Instrument transformers, such as voltage transformers and current transformers, are popular for measuring instruments and relay protection devices.
4) Transformers for special purposes, electric furnace transformers for smelting, rectifier transformers for electrolysis, welding transformers for welding, voltage regulating transformers for testing, etc.
According to Phases
1) Single-phase voltage transformers, are very common in single-phase load and three-phase transformer group.
2) Three-phase transformer, used for step-up and step-down of three-phase system.
According to Winding
1) Autotransformers, connect ultra-high voltage, large-capacity power system.
2) Double-winding transformers, connect two voltage levels of power systems.
3) Three-winding transformers connect power systems with three voltage levels. And we are generally use them in regional substations of power systems.
According to the Core Form
1) Core transformers, are common in high-voltage power systems.
2) Shell-type transformers, also called high-current special transformers, such as electric furnace transformers and welding transformers, etc.; or used for power transformers such as electronic instruments, TVs, and radios. In addition, the shell structure is suitable for large-capacity power transformers.
According to Cooling Medium
1) Oil-immersed transformers, such as oil-immersed self-cooling, oil-immersed air-cooling, oil-immersed water-cooling, oil circulation air-cooling, and water-internal cooling, etc.
2) Dry-type transformers rely on air convection for cooling. Cool such transformers with low voltage and oil-free are usually by fans and are suitable for fire prevention and other occasions. For the low-voltage transformers used in the factory buildings of 600MW units, dry-type transformers are common for fire protection requirements.
3) Gas-filled transformers use special gas (SF6) instead of transformer oil to dissipate heat.
4) The evaporative cooling transformer uses special liquid instead of transformer oil for insulation and heat dissipation.
According to Capacity
1) Distribution transformer, voltage 35kV and below, three-phase rated capacity of 2500kVA and below, single-phase rated capacity of 83kVA and below, transformer with independent winding and natural circulation cooling.
2) Medium distribution transformer, the three-phase rated capacity does not exceed 100mVA or the capacity per column does not exceed 33.3mVA, and has independent windings. In addition, its rated short-circuit impedance meets the requirements.
3) Large transformer with three-phase rated capacity above 100mVA, or its rated short-circuit impedance meets the requirements.
Figure 3. Step Down Transformer
Causes of Transformer Faults
The main components of the transformer are the primary coil, the secondary coil and the iron core (magnetic core). In electrical equipment and wireless circuits, it has the functions of voltage adjustment, impedance matching, and safety isolation. In a generator, whether the coil moves through a magnetic field or a fixed coil, an electric potential can be induced in the coil. All the problems are also generated around these.
The following are the factors that obvious in actual feedback from users.
(1) Line Surge
It is also known as line interference, and is in the leading place among all factors that cause transformer failure. This category includes operating overvoltages, voltage peaks, line faults/flashovers, and other abnormalities in transmission and distribution caused by misoperation, unparalleled transformers, and arcing of on-load taps. This type of cause occupies the vast majority of transformer faults.
(2) Insulation Aging
Due to the factors of insulation aging, the average life span of the transformer is only 17.8 years, which is much lower than the expected life span of 35-40 years.
It includes flooding, pipeline leakage, roof leakage, water intrusion into the tank along the casing or fittings, and the presence of water in the insulating oil.
(4) Poor Maintenance
This factor includes protection devices without transformers or incorrect installation, coolant leakage, dirt accumulation, and corrosion.
This category includes faults that caused by overload. Overload often occurs when power plants or power users continue to slowly increase the load. Eventually, cause the overload operation of transformers, and the excessively high temperature leads to premature aging of the insulation. When the insulating paperboard of the transformer ages, the insulation strength of the insulating paper decreases. Therefore, the impact force of an external fault may cause insulation damage, which may lead to failure.
(6) Lightning Strike
Now, unless it is a lightning accident, and other general shock faults are classified as “line surge”.
(7) Unbalanced Three-phase Load
Due to the long-term overload of a phase caused by the unbalanced three-phase load, the temperature of the phase is too high and the insulation is aging, resulting in a short circuit between turns or short circuits between phases.
(8) Poor Connection
Poor connection belongs to the under-maintenance category, but there is enough data to list it independently, because it is different from previous studies. This category includes the manufacturing process and maintenance of electrical connections. One of the problems is the improper coordination between metals of different properties, which is fewer in recent years. Another problem is improper tightening between bolted connections.
(9) Poor Craft
Only a small percentage of failures belong to the process or manufacturing. For example, the outlet end is loose or unsupported, the spacer is loose, the welding is poor, the core insulation is poor, the short-circuit strength is insufficient, and there are objects in the fuel tank.
(10) Destruction and Damage
You can see the obvious vandalism, such as theft, vandalism, etc.
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