Introduction of transformer
The power transformer is static electrical equipment, that is used to change a number of AC voltage (current) into the same frequency of another or several values of voltage (current) equipment. When the primary winding is fed with alternating current, alternating flux is generated. The alternating flux will induce the alternating electromotive force in the secondary winding through the core magnetic action. The level of secondary induction electromotive force is related to the number of turns of the primary and secondary winding, that is, the voltage is proportional to the number of turns. The main function is to transmit electrical energy, therefore, the rated capacity is its main parameter. Rated capacity is a conventional value representing power, which represents the amount of power transmitted, represented by kVA or MVA. When the rated voltage is applied to the transformer, it is used to determine the rated current that does not exceed the temperature rise limit under specified conditions.
Application of transformer
Transformers have a wide range of applications. From the view of the power system, a power network connects many power plants and users. The electric energy generated by power plants needs to be transmitted over a long distance to reach the area where electricity is consumed.
The reason is that the voltage drop is proportional to the current, and the power loss is proportional to the square of the current. Therefore, high transmission voltage can reduce the voltage drop and power loss in the power line.
It is very difficult to manufacture high-voltage generators by current technology, so special equipment must be used to increase the voltage of the generator before it is sent out. Such special equipment is transformers. On the other hand, step-down transformers must be used at the receiving end to reduce the high voltage to the voltage of the distribution system, so a series of distribution transformers must be used to reduce the high voltage to an appropriate value.
As can be seen from the above, the transformer is a static induction electrical appliance that transmits ac electric energy by changing the voltage. In the power system, the transformer plays a very important role in the power system.
Transformers are not only used in power system, but also in industrial and mining enterprises which need a special power supply. For example electric furnace transformers for smelting, rectifier transformers for electrolysis or chemical engineering, welding transformers, test transformers for testing, traction transformers for transportation, as well as compensation reactors, arc suppression coils for protection, transformers for measurement, etc.
Classification of transformers
Transformers can be classified according to different applications, different capacities, number of windings, number of phases, voltage regulation, cooling medium, cooling method, core form, and so on, different types of transformers are used in different environments in order to meet the needs of different industries for transformers.
This article we will introduce about the difference of three-phase transformers and single-phase transformers, also the Dry-type and Oil-type transformer.
The difference between three-phase transformers and single-phase transformers
Generally speaking, three windings are wound around an iron core of a three-phase transformer, which can transform the three-phase power supply to the secondary side windings at the same time, and its output is also a three-phase power supply. However, there is only one winding in the iron core of a single-phase transformer, which can only transformer the one-phase power supply to the secondary side for output. In large substations and power plants, three single-phase transformers are combined into a three-phase transformer, which is called a “combined three-phase transformer”.And general grid transmission and industry are usually using three-phase power supply, and single-phase transformer is generally used for civil where needs single-phase power supply, such as household appliances and so on, its capacity is relatively small.
1. Different applications
Single-phase transformer: suitable for low voltage distribution network with low load density application and promotion, single-phase transformer is the input of single-phase ALTERNATING current, the output of single-phase alternating current, but it can only provide single-phase power supply for a load of single-phase electricity. Three-phase power can also be provided if three single-phase transformers are connected to each other.
Three-phase transformer：three-phase transformer is widely used in ac 50 Hz to 60 Hz, the circuit of voltage is up to 660 v and widely used to import equipment, precision machine tools, machinery, and electronics equipment, medical equipment, rectifier equipment, lighting, Three-phase transformer is input three-phase symmetric ALTERNATING current, output three-phase symmetric alternating current, it can provide three-phase power supply for industrial electrical equipment which need three-phase power, and also can provide single-phase power supply for civil electrical appliances that only need single-phase power.
All kinds of input and output voltage, connection groups, the number and position of regulating taps (generally ±5%), the distribution of winding capacity, the provision of secondary single-phase winding, the use of rectifier circuit can be manufactured according to the customers’ requirement.
Three coils are required on the high and low voltage sides of three-phase transformers, and the three coils on the high and low voltage sides are connected according to certain rules respectively. This is the transformer connection group. There are many kinds of transformers, such as Yyn0, Dyn11, Yd11, etc., which could provide users with the power they need.
While Single-phase transformer only need one coil on the high and low voltage side and connected to the power supply and load.
Difference between dry-type transformers and oil-immersed transformers
The two types are power transformers, the biggest difference is between “oil” and “dry”.That is to say, the two cooling medium is different, for the oil type, the transformer oil is used to be the insulation medium, while for the other type, it is used air or other gases such as SF6 as a cooling medium. The oil transformer that consists of the core and windings is in a tank filled with transformer oil. The iron core and winding are usually coated with epoxy resin, but there is also a kind of non-enveloping type that is widely used. The winding is impregnated with special insulating paint, such as insulation paper, to prevent the winding or iron core from moisture.
Different forms of packaging, dry-type transformers can see the core and coil on the surface, while oil transformers can just see the transformer shell.
2、Different lead forms
Most dry-type transformers use silicone rubber bushings, while most oil-type transformers use porcelain bushings.
3、Different capacity and voltage
Dry-type transformers are generally used for power distribution, the capacity is always below 1600KVA, and the voltage is below 10KV, there is also some individual reaching to 35KV voltage level; while the oil transformer can be custom-made arranging from low to high capacity.
4、Different Applicable places
Dry-type transformers are mostly used in places that need “fireproof, explosion-proof”, generally used on large buildings, high-rise buildings; and oil transformers are always applied outdoor places due to “accident” ( oil spray or leakage), resulting in fire, and there are sites to dig the “accident oil pool” places.
In terms of output and dosage, the dry voltage level is up to 35kV, and the capacity is smaller than the oil variation, about 2500kVA. As the complex manufacturing process of the dry type transformer, which due to the high cost. But as the advantages of environmental protection, flame retardant, impact resistance, and others, the dry-type is often used in indoor and other high requirements of power supply and distribution places, such as hotels, office buildings, high-rise buildings, and so on.
While Oil-immersed transformer is generally installed in a separate transformer room or outdoor to prevent the fire, and dry type transformer must be installed in the room, generally installed in the low-voltage distribution room.
Notes for the operation of power transformer
(1) the body, cooling device and all accessories should have no defects, and no leak oil;
(2) accident oil drainage facilities should be in good condition, complete fire facilities;
(3) grounding lead down and its main grounding network connection should meet the design requirements, grounding should be reliable.Grounding lead out bushing of iron core and clamp, grounding small bushing of bushing and voltage extraction device should be connected to ground.The secondary terminals of the standby ct should be short grounded.The contact and sealing of the casing top structure should be good;
(4) the location of the sub should meet the operation requirements;
(5) all electrical tests of transformers should be qualified;The setting value of the protection device conform.
In recent years, with the increased demand for electricity, the power grid construction by leaps and bounds, as the core of the power system, the transformer plays an important role in our life.
Urban grid power transformers will develop towards high impedance. High reliability, energy saving, environmental protection, low noise, miniaturization, and other characteristics will be the top priority in the development of urban power transformers. The urban substation will also present personalized design development, according to the geographical location and other factors, which makes the transformer the combination, modular, intelligent development