Boosting transformer a, the secondary side (low, high voltage side) in general, there is no electrical connection, is through the electromagnetic coupling boosting, in short, is the primary side of the electrical energy is first converted to magnetic energy, and then the magnetic energy is converted to the secondary side of the electrical energy, the generator sends out a certain voltage of the electrical energy, but through the transformer can be changed into other voltage levels. For example, the generator end voltage is 13.8kV, can be transformed into 35kV, 110kV, 220kV and other voltages through the transformer according to the need to transmit out.
Sliding varistor is actually a laboratory device, is the use of series resistance principle of voltage division made in the high-current, high-voltage, high-capacity power system, it can not afford such a large capacity, he can only regulate the voltage, can not boost. For example, the generator end voltage of 13.8kV, then the maximum variable range of the sliding varistor is 0-13.8kV. can not raise the voltage to a higher level
I. The working principle of transformer
Transformer - the use of the principle of electromagnetic induction, from one circuit to another circuit to transfer electrical energy or transmission of signals an electrical appliance is the transmission of electrical energy or as a signal transmission of the important components
1. Transformer ---- stationary electromagnetic device
Transformer can be a voltage of alternating current electrical energy into the same frequency of the other voltage of the
The main components of a voltage apparatus are a core and two windings set on the core.
Transformer schematic diagram (Fig. 3.1.2)
The coil connected to the power supply, which receives the alternating current energy, is called the primary winding
The coil connected to the load, which sends out the alternating current energy, is called the secondary winding
Set
Primary winding of the secondary winding of the primary winding
Voltage phasors U1 Voltage phases U2
Current phase I1 Current phase I2
Electromotive force phase E1 Electromotive force phase E2
Turns N1 Turns N2
Simultaneously interlinking the primary and secondary windings, the phase of magnetic flux of the secondary windings is φm ,which is called the main flux
Please note that Fig. 3.1.2 The reference direction of each physical quantity is determined.
2. Ideal transformer
Not counting the resistance and iron consumption of the primary and secondary windings,
The transformer whose coupling coefficient K=1 is called an ideal transformer
The equation of balance of potential describing an ideal transformer is
e1(t) = -N1 d φ/dt
e2(t) = -N2 d φ/ dt
E2(t) = -N2 d φ/ dt
This is the phase of flux. dt
If the instantaneous values of voltage and electromotive force of the primary and secondary windings change according to sinusoidal law,
then there are
Not counting the core loss, according to the principle of conservation of energy can be obtained
This leads to the relationship between the voltage of the primary and secondary windings and the rms value of the current
Turns ratio (also known as the voltage ratio) is known as K = N1/N2, then
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II. Introduction to the structure of the transformer
1. Core
The core is the main magnetic circuit part of the transformer. Usually high silicon content, thickness of 0.35 or 0.5 mm,
Surface coated with insulating varnish hot-rolled or cold-rolled silicon steel sheet stacked
Core is divided into the core column and the yoke of the two parts of the core column sets of windings; yoke to close the magnetic circuit
The basic form of the core structure of the core and shell-type two kinds of
Cardiac Transformer Structure Schematic diagram (Figure 3.1.6)
Core transformer structure (Figure 3.1.6)
Core transformer structure (Figure 3.1.6)
Core transformer structure (Figure 3.1.4) 3.1.6)
2. Winding
The winding is the circuit part of the transformer,
It is made of paper-covered insulated flat or round wire winding
The basic principle of the transformer is the principle of electromagnetic induction, single-phase double-winding transformer as an example to illustrate the basic principle of its operation (as shown in the figure above): when the primary winding with a voltage of ?1, the flow of current of ?1. In the core of the alternating flux is generated ?1, these fluxes are called the main flux, under its action, both sides of the windings were induced potential ?1, ?2, the induced potential formula: E = 4.44fN ?m
System: E - RMS value of the induced potential
f - frequency
N - the number of turns
m - the maximum value of the main magnetic fluxBecause the secondary winding and the primary winding, the secondary winding is the same as the primary winding. secondary winding has different number of turns than the primary winding, the induced potentials E1 and E2 are of different magnitudes, and the voltages ?1 and ?2 are of different magnitudes when the internal impedance voltage drop is omitted.
When the secondary side of the transformer is unloaded, the primary side only flows through the main magnetic flux current (?0), this current is called the excitation current. When the secondary side loaded with load current flowing through the load current ?2, but also in the core to generate flux, trying to change the main flux, but the primary voltage is unchanged, the main flux is unchanged, the primary side of the primary side will have to flow through the two parts of the current, part of the excitation current ?0, part of the current is used to balance the ?2, so that this part of the current with the change of the ?2 changes and changes. When the current is multiplied by the number of turns, it is the magnetic potential.
The above balancing action is essentially a magnetic potential balancing action, the transformer is through the magnetic potential balancing action to realize the primary and secondary side of the energy transfer.
Transformer working principle animation
Three, the type of transformer
Transformer is a static motor, it can be converted from one voltage of electrical energy to another voltage of electrical energy.
I. Transformer Classification and Uses
Power Transformer: step-up transformer/ step-down transformer/distribution transformer etc. for transmission of electrical energy in power system.
Question 5-1 Why is it necessary to use high-voltage transmission for long-distance power transmission?
Electric Furnace Transformer (Specialized)
To supply power to electric furnaces (e.g., steel furnaces).
Welding transformers (specialized)
To supply power to welding machines.
Rectifier transformer (special):
Powers DC electric locomotives.
Instrument transformer: used in measuring equipment.
Electronic transformer: used in electronic circuits.
Two, the principle of operation of the transformer
(1) schematic
A core: provides a closed path of magnetic flux.
Two windings:1 secondary side winding(primary side)N1,2 secondary side winding(secondary side)N2.
(2)Working principle
When the primary winding is connected to the AC voltage, the current i0,which generates an alternating main flux Φ in the core.
Фinduced potentials e1 and e2 in the two windings respectively
e1= -N1dФ/dt?e2=-N2dФ/dt
If the winding resistance and leakage voltage drop are omitted, then
u1/u2≈(-e1)/(-e2)=N1/N2?
u1/u2≈(-e1)/(-e2)=N1/N2=k, k is defined as the transformer ratio. k is defined as the ratio of the transformer.
5-2 Types and structures of transformers
1. Types
In addition to the above categorization according to the purpose, transformers can also be categorized according to the number of phases/number of windings/core form/cooling method and other characteristics.
By the number of phases: single-phase / three-phase / multi-phase, etc.
By the number of windings: double-winding / autocoupled / three windings / multi-winding
Core form: heart / shell
Cooling: dry / oil-immersed, etc.
2, structure (power transformer)
The main parts of the transformer is the windings and the iron core (body).
The winding is the circuit of the transformer, the core is the magnetic circuit of the transformer. The two constitute the core of the transformer that is the electromagnetic part.
Besides the electromagnetic part, there are components such as the oil tank/cooling unit/insulation bushing/voltage regulator and protection device.
(1) core
Type: heart (simple structure and process is simple and widely used) / shell (used in small capacity transformers and furnace transformers).
Materials: generally by 0.35mm/0.5mm cold rolled (also used hot rolled) silicon steel sheet stacked.
Core interlocking: adjacent layers are stacked in different ways, the seams are staggered. The even number of layers just pressed against the seams of the odd number of layers, thus reducing the magnetic resistance and facilitating the flow of magnetic flux.
Core column cross-section shape: small transformers into square or rectangular; large transformers into stepped. Large capacity is more stages. Stacked pieces leave a gap between the oil channel (longitudinal/horizontal). (Longitudinal oil channel see textbook figure 5.13)
(2) winding
Generally insulated flat copper wire or round copper wire in the winding mold winding.
The windings are set on the core column of the transformer, with the low-voltage windings on the inner layer and the high-voltage windings set on the outer layer of the low-voltage windings for insulation.
(3) Oil/tank/cooling/safety devices
The body is mounted in an oil tank, which is filled with transformer oil.
Transformer oil is a mineral oil with good insulating properties. Transformer oil plays two roles: ① in the transformer windings and windings, windings and core and between the tank to play an insulating role. ② transformer oil is heated to produce convection, the transformer core and windings play a role in heat dissipation.
The oil tank has many cooling oil pipe to increase the heat dissipation area.
In order to speed up the heat dissipation, some large transformers use internal oil pump to force oil circulation, external transformer fan blowing or using tap water to flush the transformer tank. These are transformer cooling devices.
1 oil tank / 2 oil storage cabinet / 3 gas relay / 4 for the safety airway.
The heat generated during the operation of the transformer causes the transformer oil to expand and flow into the oil conservator.
The oil storage cabinet makes the transformer oil contact surface with air smaller, slowing down the oxidation of transformer oil and absorbing air moisture. Thus slowing down the deterioration of oil.
Fault, the heat will make the transformer oil vaporization, touch the gas relay alarm signal or cut off the power supply.
If it is a serious accident, the transformer oil is vaporized in large quantities, and the oil and gas breaks through the sealing glass of the safety airway pipe opening and rushes out of the transformer tank to avoid the tank bursting.
5-3 Transformer rating
(1) rated voltage U1N/U2N
Unit V or kV. U1N is the voltage that should be added to the primary side during normal operation. U2N is the voltage that is added to the primary side with the rated voltage, and the secondary side is in no-load condition.
Three-phase transformer, the rated voltage refers to the line voltage.
(2) rated capacity SN
Unit VA/kVA/MVA
SN is the apparent power of the transformer. Usually the rated capacity of 1 and 2 secondary sides of the transformer are designed to be the same.
(3) rated current I1N/I2N
Unit A/kA. It is the current that the transformer can bear in normal operation, in the three-phase transformer all represent line current.
For single-phase: I1N=SN/U1N I2N=SN/U2N
For three-phase:
I1N=SN/[sqrt(3)U1N]
I2N=SN/[sqrt(3)U2N]
(3)Rated Frequency fN
Unit Hz,fN=50Hz
In addition, the nameplate will also give three-phase connection group and the number of phases m/impedance voltage Uk/model/mode of operation/cooling mode/weight and other data