Shaanxi Hanzhong Transformer Co., ltd

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



Oil drain valve

Moisture absorption device



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.


  • 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

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