The basic structure and working principle of the transformer
Transformer is a kind of electric appliance that uses the principle of electromagnetic induction, or uses the mutual inductance to transmit energy from one circuit to another circuit. Two windings are wrapped on the same iron core which are insulated with each other. There is a magnetic coupling between them, and there is no direct contact of the circuit.
When the original contact is connected to the AC power, the original contact has alternating current and produces alternating magnetic flux in the core. According to the law of electromagnetic induction, the original and deputy windings induct potential e1, e2 respectively. The deputy contact has the potential to supply power to the load, so as to realize the energy transfer. Adjust transformer ratio, k to achieve the goal of voltage transformation.
The classification of the transformer
According to the purpose
Power transformer, rectifier transformer, instrument transformer, special transformer
According to the number of phase
Single-phase, three-phase, polyphase transformer
According to the structure
Double winding transformer, three winding transformer, multi-winding transformer, auto-transformer, etc
According to the cooling way
Dry, oil-immersed, inflatable transformer
The basic structure of the transformer and main components
Main components: the core and winding (the body); and oil tank, insulation bushing, tap switch and explosion-proof pipe, etc
Explosion-proof pipe
High pressure bushing
Low pressure bushing
Tap switch
Transformer oil
Gas relay
Low pressure bushing
Oil storage tank
Fuel meter
Oil purifier
Signal thermometer
Winding
Valve
Oil drain valve
Moisture absorption device
Radiator
Nameplate
Oil sample gate
Iron core
Three-phase oil-immersed power transformer
Fuel meter
Explosion-proof pipe
Gas relay
High pressure bushing
Tap switch
Low pressure bushing
Oil storage tank
Moisture absorption device
Signal thermometer
Iron core
It is magnetic circuit and skeleton for winding combination
Include: heart column (wrapped with winding) and iron yoke (form a closed magnetic circuit)
Made by 0.35 to 0.5 mm thick silicon steel sheets which are piled up or by amorphous alloy
The structure is divided into: heart and shell type, and power transformer mainly adopts the heart type.
Voltage balance equation
Taking into account the resistance drop of original and deputy windings, i1R1, i2R2, voltage equations for the primary and secondary windings are as follows:
Original winding: leakage impedance of the original side.
Deputy winding: leakage impedance of the deputy side.
Summary
There is a magnetic potential balance in transformer and voltage balancing are two basic relations, the influence of load on the primary side works through the secondary side of the magnetic potential.
When the secondary side current is added, secondary side magnetic potential tends to change the primary magnetic flux in the core and the induced potential, E1, destroy the voltage balance relation of primary side. Primary side automatically generates a current component at this time to counter the effect of secondary side magnetic potential, reaching a new balance between the primary side potential and voltage. Through the process of the electric potential and magnetic potential reaching balance, energy is transmitted from primary side to the secondary side.
The parallel operation of transformer
The parallel operation of transformer refers to: the primary winding and secondary winding parallel to the public bus of the primary side and secondary side respectively.
Load capacity is very big, so a transformer can not meet the requirements.
Load change is relatively big, so with the parallel operation of more than one transformer, at any time the number of transformers on operation can be adjusted.
Reduce the reserve capacity of transformer.
The parallel operation of transformer should meet the requirements
The voltage ratio is the same, so the no-load voltage at secondary side will be the same.
Regulation: the difference between the voltage than < 0.5% (average voltage ratio).
The connection group is identical, so as to ensure that the phase of no-load voltage is the same at the secondary side.
The per-unit value of short circuit impedance is equal, and the impedance angle should be the same.
The equivalent circuit diagram of two transformers in parallel operation . The dashed part means the ideal transformer, current and impedance are calculated to the secondary side.
When the voltage ratio is not the same, the circulation between the transformer is caused
Circulation has nothing to do with the size of the load.
Leakage impedance of voltage transformer is very small. The small difference can cause large circulation
During ideal parallel operation, it is necessary to ensure that the voltage ratio of transformer is equal. For three-phase transformer, connection group number must be the same.
The load distribution when voltage ratio is the same and the equivalent leakage impedance is not the same
The load current is distributed inversely proportional to its equivalent leakage impedance.
Multiply both sides of the above equation by . Considering that the two transformers have the same rated voltage, the load distribution relation expressed by the per-unit value is:
Three winding transformer, the coupling transformer and instrument transformer
There are three windings and three voltage grade.
Low voltage, medium voltage, high pressure
Step-down transformer
Medium voltage, low voltage, high-voltage
Step-up transformer
Voltage transformer
Step-down transformer running with no load.
Domestic transformer:
Use note:
The short circuit of secondary winding is prohibited.
The secondary winding and iron core must be grounded.
Current transformer
Ni is very small
Step-up transformer running at short circuit
Domestic transformer
Use Note:
The secondary winding is forbidden to open circuit
When open circuit: I2=0, I1 is unchanged
When the working voltage of the primary winding is high,
the secondary winding and iron core must be grounded
