This article describes the types of transformer in detail, transformer working principle, parts of transformer, construction of transformer.
A transformer is a static electrical device that can increase and decrease the voltage and change the current under a certain power. Various transformers are widely used in daily production and life fields.
Transformers have a wide range of uses. With the rapid development of economic construction, various types of transformers are widely used in industrial and agricultural production and people’s daily life in factories, mines and urban construction, such as subways, sewage treatment and other major engineering projects. Entered the port, power plant, university town, commercial and residential buildings.
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Transformers are indispensable equipment in power systems. Generally, the electrical energy used by people is mainly generated by AC motors in power plants, and the outlet voltage of large generators is generally 6.3kV.
The installed capacity of the transformer is 6 to 8 times the installed capacity of the generator. After the electric energy reaches the power consumption area, transformers of various capacities and voltages are needed for electric energy distribution.
The electric power department uses the transformer to convert the high voltage into a low voltage level suitable for the actual load on-site before it can be used.
For example, the lighting voltage is generally 36V, 48V, 110V, 220V, the power voltage is 380V, and the system voltage level is 10kV, 35kV, 60kV, 110kV. 220kV, 330kV, 500kV.
Transformers have a wide range of uses, and different uses also produce different needs. Various types of transformers are produced from this. The classification of the transformer can be divided according to its usage, structure, winding and number of phases. The classification of the transformer is shown in the table.
The distribution transformer is a commonly used ordinary power transformer, its structure is simple, the basic components have oil tank, iron core, winding and insulation, oil meter, gas relay, etc.
The S11 laminated core transformer is a new type of transformer developed on the basis of the original distribution transformer technology. It has the characteristics of low loss, low noise, strong short-circuit resistance, good impact resistance and economic operation. It is especially suitable for places with low load rates such as rural power grids.
The coiled core transformer is made of thin, high-permeability cold-rolled silicon steel sheet, giving full play to the advantages of low sheet eddy current loss, so that no-load loss can be reduced.
Therefore, it has the characteristics of small size, low loss and low noise, and is a new generation of environmentally friendly, high-efficiency and energy-saving products.
It is suitable for power grid transformation of urban and rural, industrial and mining enterprises, and more suitable for combined transformers and pre-installed substations.
The high-voltage winding of the three-dimensional wound core transformer is layered, the low-voltage winding is layered for the capacity of 500kV•A and below, and the foil winding is used for the capacity of 630kV «A and above.
The epoxy resin cast dry-type transformer is a dry-type transformer product with less filler and thin insulation. It has the advantages of small partial discharge, low noise, low loss, good heat dissipation performance, strong moisture resistance, strong resistance to sudden short circuits, and large overload capacity. .
The Open dry-type transformer adopts DuPont Nomec paper-based insulation system, H-class heat-resistant insulation grade, does not require fan cooling, and allows long-term overload operation of 20%. Low loss, small local discharge, low noise, no harmful gas, insensitive to humidity and dust. It is suitable for high fire protection requirements, large load fluctuations and dirty and humid environments.
Mine transformers are suitable for explosive and hazardous locations containing methane gas and coal dust in coal mines. The mine transformers are designed, manufactured and tested in accordance with relevant national explosion-proof standards.
Pad mounted transformer uses medium and high voltage cables to be connected to the transformer through cable connectors in the high voltage warehouse, and low voltage cables are connected to the low voltage terminals by bolts and low voltage terminals in the low voltage warehouse. The oil tank of the Pad mounted transformer is equipped with high-voltage fuses and load switches, so that the transformer can be used for terminal operation or ring network operation, and protect and control the power supply status of the high-voltage side of the transformer.
Transformers are static electrical equipment made by the principle of electromagnetic induction, which can convert alternating current with a certain voltage value. The working principle of the transformer is shown in the figure.
The winding connected to the power supply is the primary side, and the winding connected to the load is the secondary side. Under the action of the applied voltage, an alternating current flows through the primary side, and an alternating magnetic field is established in the winding.
Since the permeability of the iron core is much greater than that of air and transformer oil, most of the magnetic flux passes through the iron core, and at the same time interlinks the primary and secondary windings, and the electromotive force is induced in the primary and secondary windings.
According to the law of electromagnetic induction, the induced electromotive force of the primary and secondary windings is proportional to the number of turns. By reasonably arranging the number of turns of the primary and secondary windings, the required voltage can be obtained on the secondary side to achieve the purpose of changing the voltage.
Generally, the current and voltage generated by large and medium-capacity generators are not only unsuitable for general electricity consumption, but also a large amount of electric energy needs to be transmitted to a distant place, and it is impossible to use lower voltage transmission.
Due to the low voltage, the current delivered by it is very large, and the large current will cause a lot of power loss and voltage drop on the transmission line, and it cannot be economically transported to far-reaching places.
The use of higher power transmission voltage enables us to appropriately and conveniently deliver the power to the appropriate places.
It usually takes a long transmission line to transmit the power of AC power from the power plant to the user.
When the transmission power P and the power factor cos are fixed values, the higher the voltage U, the smaller the current I in the line, and the smaller the cross-section of the transmission line can be obtained, which can save a lot of material.
Conversely, most or all of the electrical energy may be consumed on the transmission line.
In order to reduce energy loss on transmission lines and improve transmission efficiency, transformers are often used to increase the voltage to the required value before power transmission.
AC power is connected to the primary winding of the transformer, the secondary winding is not connected to the load, and the secondary current is zero. This state is called the no-load operation of the transformer.
The current of the primary winding is represented by I0. The number of turns of the primary winding is N1, then ION1 is the magnetomotive force of the primary winding when there is no load.
Under the action of this magnetomotive force, the main magnetic commuter closed by the iron core is produced.
Because the primary and secondary windings are on the same iron core, the main magnetic flux in the iron core passes through the primary and secondary windings at the same time, generating self-induced electromotive force E1 in the primary winding and mutual induction electromotive force E2 in the secondary winding.
According to the basic law of electromagnetic induction, the magnitude of this electromotive force is proportional to the number of winding turns linked by the magnetic flux and the maximum value of the magnetic flux.
If the internal voltage drop of the transformer is ignored, that is, U1≈E1, U2≈E2, the following formula can be approximately written:
This formula is one of the basic formulas of the transformer. It means that when the transformer is no-load, the ratio of the voltage of the primary and secondary windings is approximately equal to the ratio of the number of turns, and the ratio k is called the transformation ratio of the transformer.
AC power is connected to the primary winding of the transformer, and the load is connected to both ends of the secondary winding. This state is called transformer load operation.
The secondary side of the transformer is connected to the load under the action of the secondary winding U2, and the current I2 passing through the load is the load current of the transformer. The size of I2 depends on the load impedance.
The relationship between the primary and secondary winding currents of the transformer can be analyzed by the magnetomotive force balance relationship.
If the number of turns of the secondary winding is N2 and the passing current is I2, the magnetomotive force is m. The main magnetic flux in the iron core when the transformer is operating with a load. It is produced by the two magnetomotive forces I2N1 and I2N2 of the primary and secondary windings.
When the power supply voltage does not change, the main magnetic flux is basically unchanged.
The magnetic flux produced by the magnetomotive force of the primary and secondary windings is approximately equal to the magnetic flux produced by the no-load magnetomotive force. This is the flux balance relationship of the transformer load operation.
Since the no-load current is very small, the no-load magnetomotive force, Ni, is also very small and can be omitted. Therefore:
This shows that when the secondary winding passes current, the corresponding current will also pass through the primary winding, and the ratio of the current in the primary and secondary windings is approximately equal to the inverse ratio of the number of turns of the primary and secondary windings.
This shows that the transformer not only changes the voltage, but also changes the current.
The construction of transformer is roughly the same.
Power transformers are the most commonly used transformers.
Take it as an example to illustrate the construction of transformer.
The main construction of transformer includes the body, the oil tank, the voltage regulating device, the cooling device, the protection device, and the outlet device.
The iron core is the magnetic circuit part of the transformer. It is manufactured according to the principle of electromagnetic induction. There is no direct electrical connection between the primary coil and the secondary coil of the transformer, only the magnetic connection is formed through the iron core. The iron core of the transformer can be used to obtain a strong magnetic field, enhance the electromagnetic connection between the primary and secondary coils, and generate electromagnetic induction. Phenomenon.
There are core type and shell type, single-phase and three-phase, plane and three-dimensional type, laminated core and wound core. The two basic structural forms of iron core are shell type and core type.
The coil is the circuit part of the transformer, which is wound with insulated flat wire or round wire.
From the perspective of the relative position between the high and low voltage windings, the windings of the transformer can be divided into two types: concentric and overlapping.
The coil is the circuit part of the transformer, which is wound with insulated flat wire or round wire. From the perspective of the relative position between the high and low voltage windings, the windings of the transformer can be divided into two types: concentric type and overlapping type. The high and low voltage windings of the concentric winding are sleeved on the iron core column concentrically. In order to facilitate insulation, the low voltage winding is generally close to the iron core, and the high voltage winding is sleeved outside the low voltage winding. The concentric winding structure is simple and easy to manufacture. Overlapping windings have low leakage reactance and are easy to form multiple parallel branches. They are mainly used in low-voltage and high-current electric welding, electric furnace transformers and shell-type transformers.
Since the transformer is subjected to various working voltages for a long time in operation, in order to ensure that there is no flashover or breakdown between the live parts, and between the live parts and the iron core and the fuel tank, a good insulation structure must be provided.
The lead wire of the transformer refers to the connecting wire between the coils, between the coil and the outlet bushing, and between the coil and the tap switch.
The lead insulation of the transformer is an important part of the internal insulation. Some leads pass between the coils or between the coil and the iron pivot clamp and the wall of the fuel tank.
Therefore, it is necessary to ensure that these leads have sufficient insulation strength, that is, the insulation distance.
The transformer tap switch should also have strong insulation.
The 10kv no-load tap changer is generally insulated with a rolled insulating paper tube as the ground insulation.
The contact part of the no-load tap-changer with a voltage level of 35kV and above is installed in a phenolic insulating paper tube, and the insulating paper tube is then installed on a wooden frame, and the operating handle is connected to the moving contact of the tap-changer through a wooden insulating rod.
The insulation of the on-load tap-changer to the ground is composed of the insulating paper tube and the insulating connecting plate of the switch itself.
The transformer oil tank mainly circulates insulating oil for cooling and heat dissipation of the transformer.
Transformer oil is a refined insulating mineral oil. Its main function is to insulate, cool and extinguish the arc of the transformer.
Because the electric strength of transformer oil is much higher than that of air, and the transformer core and coil are put into the oil tank to avoid the influence of moisture in the air on the coil.
Through the oil temperature difference, the heated oil rises to the upper part of the oil tank.
It flows to the oil pipe through the upper port of the oil pipe, dissipates heat in the oil pipe, and flows back to the bottom of the oil tank after cooling. The transformer achieves the purpose of cooling through oil circulation.
When an electric arc occurs in the oil, the transformer oil can extinguish the electric arc within a certain range.
Transformer cooling devices can be roughly divided into the following types.
The bushing of the transformer is an insulating device that leads the leads of the transformer coil to the top of the oil tank respectively.
It is the insulation of the lead wire to the ground (fuel tank) and the fixing device of the lead wire. Therefore, the casing must have insulation strength, mechanical strength, and thermal stability.
In addition, moisture absorbers, oil purifiers, oil conservators, thermometers, gas relays, etc. are indispensable parts of transformers.
The winding is the circuit part of the transformer, and generally transformers have primary and secondary windings.
The winding connected to the power supply, that is, the coil that receives external AC power, is called the primary winding.
The winding connected to the load, that is, the coil that outputs electrical energy to the outside, is called the secondary winding.
According to different winding methods, transformer windings can be divided into layer windings, continuous windings, spiral windings and foil windings.
Mainly used for the voltage regulating winding of small and medium transformers and large transformers.
It can be wound with a single wire or with multiple flat wires in parallel.
Can be made into single-layer, double-layer or multi-layer barrel type.
The insulation paper or oil passage between the layers depends on the voltage between the layers and the heat dissipation of the windings.
It is composed of several wire cakes wound by paper-wrapped flat wire, and each wire cake is continuously wound by several turns in sequence.
The continuous winding has high mechanical strength, good heat dissipation, and can adapt to the capacity and voltage requirements, but the winding process is more complicated and there are more connections from the outside.
Single or multiple wires can be wound in parallel, leaving gaps between the turns, and padding between the gaps to form the radial oil passage of the winding.
The spiral winding is simple to wind and has good cooling conditions.
It is not applicable if the winding has a large number of turns.
Generally used for three-phase capacity of 630kV•A or higher, voltage of 35kV and below high current windings and voltage regulating windings of on-load voltage-regulating transformers.
It is a winding that uses aluminum foil or copper clamps as a conductor.
The clamp structure is made up of a layer of conductor clamps and a layer of thin insulation intertwined uniformly and tightly.
The contact surface of the conductors between layers is large and the spacing is small, and the ability to withstand short circuits is strong.