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Oct 31 2019

Three-phase Power Transformer

 The Principle and Structure of Three-phase Power Transformer

1. The Basic Structure of the Transformer

The power transformer works by the principle of electromagnetic induction, so its most basic structural composition is the circuit and the magnetic circuit part. The circuit part of the transformer is its winding. For the step-down transformer, the connection with the system circuit and the power supply is called the primary winding, and the connection with the load is called the secondary winding; the core of the transformer constitutes its magnetic circuit, it is composed of the iron yoke and the core column, and the winding is placed on the core; in order to reduce the eddy current and hysteresis loss of the transformer, silicon lamination coated with an insulating paint film are alternately formed into a core.

1.1 Common Three-phase Oil-immersed Power Transformer 

The structure of it is shown in figure 1.

Figure 1. Structure of Three-phase Oil-immersed Power Transformer

Figure 1. Structure of Three-phase Oil-immersed Power Transformer

(1) Fuel tank: The fuel tank is composed of a tank body, a tank cover, a heat sink and a drain valve. Its main function is to connect the transformer into a whole and to dissipate heat. The inside is the windings, core and the oil of transformer. Transformer oil has the functions of circulating cooling and heat dissipation as well as insulation. The winding has a certain distance from the tank (tank wall, tank bottom) and is insulated by the oil in the tank. The fuel tank generally has four structures:

a. The radiating pipe oil tank, the inner two ends of the radiating pipe are connected with the tank body. After the oil is heated, it flows through the pipe on the upper part of the radiating pipe and is cooled, and then flows back to the tank through the lower port to form a circulation for the transformer of 1600kV·A and below.

b. Fuel tank with radiator for transformers above 2000kV·A.

c. Flat top fuel tank.

d. Corrugated fuel tank.

(2) High and low voltage bushings: The bushings are porcelain insulated pipes with conductors inside, which are used for the connection and insulation of the primary and secondary windings of the transformer.

(3) Gas relay: Only oil-immersed transformer with a capacity of 800kV·A and above (the indoor transformer capacity is 400kV·A and above) needs to install a gas relay for protection when a fault occurs inside the transformer tank.

(4)Oil Conservator: It has a certain amount of oil stored in it. One of its function is to supplement the oil of the transformer due to decline in oil caused by the oil leakage caused and oil temperature change. The other is to maintain a balance with the surrounding atmospheric pressure when the transformer oil is inflated and contracted. The accessory moisture absorber communicates with the space above the oil level in the oil conservator to absorb moisture in the air entering the transformer to ensure the insulation strength of the oil.

(5) Explosion-proof tube: Its function is to prevent the oil tank from exploding. When a serious short circuit fault occurs inside the fuel tank, the oil in the transformer tank is rapidly decomposed into a large amount of gas, so that the internal pressure of the tank increases sharply. At this time, the glass at the exit of the explosion-proof tube will rupture by itself, release the pressure, and cause the oil to flow in a certain direction.

(6) Tap-changer: It is used to change the winding turns of the transformer to adjust the output voltage of the transformer.

1.2 Three-phase Dry-type Transformer Cast by Epoxy Resin

 Figure 2. Three-phase Dry-type Power Transformer Insulated by Epoxy Resin Casting

Figure 2. Three-phase Dry-type Power Transformer Insulated by Epoxy Resin Casting

Three-phase dry-type power transformer insulated by epoxy resin casting is also called resin insulated dry-type transformer. Its high and low voltage windings are individually cast with epoxy resin and coaxially placed on the core column. There are cooling air passages between the high and low voltage windings to make the winding dissipatheat. The wiring between the three-phase windings is also cast by epoxy resin, so that all live parts are not exposed. Its capacity ranges from 30kV·A to several thousand kV·A, up to tens of thousands of kV·A. High voltage side voltage is 6, 10, or 35kV, and low voltage side voltage is 230/400V.


Ⅱ The Function of Three-phase Power Transformer

A transformer is a stationary electrical device that exchanges voltage or current between two or more windings at the same frequency by means of electromagnetic induction.

The electric energy emitted from the power plant must be transported to distant users (such as factories, mines, hospitals, schools, agriculture, forestry, animal husbandry, etc.) through long transmission lines. In order to reduce the power loss on the transmission line, high voltage or ultra high voltage transmission must be used. At present, the voltage emitted by a general power plant cannot be too high due to the limitation of the insulation level. This requires a transformer to pump the power voltage from the power plant to the power grid. Such transformers are collectively referred to as boost power transformers.

For the backup user, the voltage required by various electrical equipment is not too high, and the high voltage of the power system is also converted into a rated voltage that meets the requirements of various electrical equipment of the user through a transformer. The transformer used for this purpose is collectively referred to as a step-down power transformer. As can be seen from the above, the power transformer is the main electrical equipment used to change the voltage in the power system.

From the point of view of the power system, a power grid links many power plants and users into a main system and several sub-systems. The voltages of the sub-systems are not necessarily the same, but the main system must be a unified voltage level, which also requires transformers of various specifications and capacities to connect the various systems. Therefore, the power transformer is an indispensable electrical equipment in the power system.


Ⅲ The Types, Main Parameters and Uses of Three-phase Transformers Commonly Used in China

3.1 According to the function, there are step-up transformers and step-down transformers. In a long-distance transmission and distribution system, in order to raise the lower voltage generated by the generator to a higher voltage level, a step-up transformer is required; and for a terminal substation that directly supplies power to various users, step-down transformers are used.

                     Figure 3.Step-up Transformer            

         Figure 3.Step-up Transformer 

          Figure 4. Step-down Transformer

          Figure 4. Step-down Transformer

3.2 According to the number of phases, there are two different types: single phase and three phase. Among them, three-phase transformers are widely used in substations of power distribution systems, while single-phase transformers are generally used for small-capacity single-phase equipment.

        Figure 5. Three-phase Transformer

                Figure 5. Three-phase Transformer                

                 Figure 6. Three-phase Transformer        

Figure 6. Three-phase Transformer

3.3 According to the material of the winding conductor, there are copper winding transformer and aluminum winding transformer. In the past, most of the factory substations in China used aluminum windings, but now low-loss copper winding transformers, especially large-capacity copper winding transformers, have been more widely used. 

3.4 According to the winding type, there are two winding transformers, three winding transformers and auto transformers. A two-winding transformer is used to transform a voltage; a three-winding transformer is used in a place where two voltages are required, and it has one primary winding and two secondary windings. Auto transformers are mostly used in the laboratory for voltage regulation.

 Figure 7. Two Winding Transformer 

Figure 7. Two Winding Transformer

  Figure 8. Three Winding Transformer     

Figure 8. Three Winding Transformer

  Figure 9. Auto Transformer

Figure 9. Auto Transformer   

3.5 According to the capacity series, at present, China mostly adopts the R10 series recommended by IEC to determine the capacity of the transformer, that is, the capacity is increased by multiples of R10=fig 1=1.26. The commonly used ones are kV·A of 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150, etc. Among them, the capacity below 500kV·A is called small sizethe capacity between 630~6300kV·A is called medium size, and the capacity of 8000 kV·A or more is called large size. This capacity series is densely graded for easy selection. 

3.6 According to the voltage regulation mode, there are unloaded voltage regulating transformers and on-load voltage regulating transformers. Among them, the no-load voltage regulating transformer is generally used in places where the voltage level is not high, especially the distribution transformer of 10kV and below; the power system above 10kV and the place with higher voltage level mainly adopt the on-load voltage regulating transformer.

                      Figure 10. Unloaded Voltage Regulating Transformer     

Figure 10. Unloaded Voltage Regulating Transformer

                                                                           Figure 11. On-load Voltage Regulating Transformer

Figure 11. On-load Voltage Regulating Transformer

3.7 According to the installation location, there are indoor type and outdoor type. 

3.8 According to the cooling method and winding insulation, there are oil-immersed type, dry type and inflatable type(SF6), among which oil-immersed transformers are oil-immersed self-cooling type, oil-immersed air-cooled type, oil-immersed water-cooled type and forced oil circulation cooling method, etc. The dry type transformer has a pouring type, an opening type, a closed type, and the like.

1) Oil-immersed transformers have good performance of insulation and heat dissipation, and are low in price and easy to overhaul. Therefore, they are widely used, but due to the flammability of oil, it is inconvenient for occasions with high flammability, explosiveness and high safety requirements.

 Figure 12. Oil-immersed Type Transformer

Figure 12. Oil-immersed Type Transformer

2) Dry type transformers are simple in structure, small in size, light in weight, and fireproof, dustproof, and moisture-proof. Although the price is higher than that of oil-immersed transformers of the same capacity, it is widely used in places with high safety and fire protection requirements, especially in substations in large buildings, underground substations and mine substations.

Figure 13. Dry Type Transformer 

Figure 13. Dry Type Transformer

3) Inflatable transformer is insulated and dissipated by using filled gas. It has excellent electrical performance. It is mainly used in places with high safety and fire protection requirements, and often cooperates with other inflatable appliances to form a complete set of devices.

Figure 14. Inflatable Transformer 

Figure 14. Inflatable Transformer

Ordinary small and medium capacity transformers adopt self-cooling structure, that is, the loss heat generated by the transformer is naturally ventilated and radiated; the large-capacity oil-immersed transformer adopts water-cooled and forced oil circulation cooling mode; the air-cooled type uses the ventilator to enhance the heat dissipation and cooling of the transformer, it is generally used in large-capacity transformers (2000kV·A and above) and places with poor heat dissipation conditions. 

3.9 According to the purpose, there are ordinary transformers, lightning protection transformers, etc. A transformer of 6 to 10 kV/0.4 kV is often called a distribution transformer, and a transformer installed in a total step-down substation is often called a main transformer.

Figure 15. Oordinary Transformer      

Figure 15. Oordinary Transformer

      Figure 16. Lightning Protection Transformer

Figure 16. Lightning Protection Transformer

Ⅳ The Relevant Calculation of Three-phase Transformer

4.1 Calculation of Transformation Ratio of the Transformer

4.1.1 Basic Concept

Transformation ratio of three-phase transformer: The ratio of the induced electromotive force generated by the primary and secondary windings of the three-phase transformer is approximately equal to the ratio of the voltages on the primary and secondary windings, and is also equal to the ratio of N1 and N2, which are the number of turns of the primary and secondary windings. That is to say, U1/U2≈E1/E2= N1/N2=KU

In the formula, KU is the transformation ratio of the transformer.

When KU > 1, the transformer reduces the power supply voltage. It is called a step-down transformer.

When KU < 1, the transformer raises the power supply voltage. It is called a step-up transformer.

Note: When calculating the ratio, the connection group of the primary side and the secondary side of the transformer should be the same. If they are inconsistent, one is the Y connection method and the other is the △ connection method, then the phase voltage of the Y connection should be compared with the line voltage of the △ connection.

4.1.2 Application Examples

1) It is known that the rated capacity of a three-phase transformer is SN=100KVA, U1/U2=10/0.4KV. What is the transformer ratio?

The answer is: KU =U1/U2=10/0.4=25

2) It is known that the rated capacity of a three-phase transformer is SN=100KVA, U1/U2=10/0.4KV, please use Y/△ connection method to calculate the transformer ratio.

The answer is: KU =U1/U2=10/(fig 2×0.4)=14.43

4.2 Calculation of Voltage and Current of the Primary and Secondary Sides of the Transformer

4.2.1 Basic Concept

Converter: The converter formula of a three-phase transformer is the same as that of a single-phase transformer, that is, I1/I2= U2/ U1=1/ KU = KI

In the formula, KI is the variable current ratio of the three-phase transformer and is inversely proportional to the transformation ratio.

4.2.2 Application Examples

It is known that the rated capacity of a three-phase transformer is SN=100KVA, U1N/U2N =10/0.4KV, I1N=10A, please calculate the ratio of the transformer and the current on the secondary side.

The answer is: KU =U1/U2=10/0.4=25I1N = KU×I1N =25×10=250A

4.3 Calculation of Transformer Power

4.3.1 Basic Concepts

(1) Rated capacity: it indicates the maximum output power of the transformer under rated working conditions, also known as apparent power SV, the unit is KVA.

(2) Rated power: The actual output power at full load, also known as active power PN, the unit is KW.

4.3.2 Calculation Formula

(1) rated capacity

SVfig 2U2N I2N =fig 2 U1N I1N

(2) rated power

PN = SV cosΦ=fig 2U2N I2N cosΦ

In the formula, cosΦ is the power factor of the load.

4.3.3 Application Examples

It is known that the rated capacity of a three-phase transformer is SN =100KVA,U1N =10KV,U2N =380V, cosΦ=0.8, N2 = 200, What are PNI1NI2N and N1?

The answer is: PN= SV cosΦ=100×0.8=80KW

              I1N =SN /(fig 2 U1N)=100/(fig 2×10)=5.77A

              I2N = SN /(fig 2 U2N)=100000/(fig 2×380)=151.8A

Or:           I2N = (U1N/U2N) I1N =(10000/380) ×5.77=151.8A

              N1= (U1N/U2N) N2=(10000/380) ×200=5263 turns


 Use, Maintenance and Common Faults of Three-phase Transformers

In order to ensure the safe and reliable operation of the transformer, necessary inspections and tests should be carried out before operation. Strict monitoring and regular maintenance should be carried out during operation. When the transformer is abnormal, it should be discovered and dealt with in time.

Newly installed or overhauled transformers should pay special attention to check whether the oil level of the oil conservator is normal before the injection, whether the desiccant in the moisture absorber is damp, whether the explosion-proof pipe is intact, whether the position of the tap changer is normal, whether the cooling device is normal and complete, whether the control circuit is good, and whether the grounding device is intact, etc.; in the test project, special attention is paid to measuring insulation resistanceabsorption ratio and connection group; in operation monitoring, special attention is paid to the physical quantity of the transformer within the rated range.

5.1 Inspection and Test Items Before the Transformer IPut Into Operation

Newly installed or overhauled transformers must be carefully tested before they are ready for operation:

5.1.1 Inspection Items Before the Transformer Is Put Into Operation

(1) The surface of the transformer body and its accessories should be clean and there should be no debris nearby.

(2) The components of the transformer are fastened, the surface is not damaged, and there is no oil leakage.

(3) The grounding device is in good condition and the fire fighting equipment is fully equipped.

(4) The oil level and oil color in the oil conservator and the oil-filled bushing are normal.

(5) The desiccant in the moisture absorber is not damp, and the protective film of the explosion-proof pipe is intact.

(6) The pipeline valves of gas relays, radiators and oil purifiers should be opened.

(7) The lead wires on the high and low voltage bushings are fastened, and the three-phase AC power is in the correct phase and the mark is obvious.

(8) The tap changer is in the correct position and the set screw is tightened.

(9) The cooling device is complete, the control circuit is good, and the thermometer indicates normally.

(10) There are no left grounding wires, nameplates, tools, materials, etc. on the transformer.

5.1.2 Test Items Before the Transformer Is Put Into Operation

(1) Measurement of insulation resistance and absorption ratio.

(2) Measure the DC resistance of each winding of the transformer.

(3) Measure the transformation ratio on each tap of the tap-changer.

(4) Determine the connection group of the three-phase transformer.

(5) Determine the no-load current and no-load loss of the transformer.

(6) Withstand voltage test.

5.2 Operation Monitoring and Maintenance of Transformer

5.2.1 Other Test Items of Operation Monitoring Content CaRefer to the Followings:

(1) Monitor and record the meter indication on the transformer control panel. The power meter can monitor the load of the transformer and whether it is overloaded. The three-phase ammeter can reflect the load and check whether the three-phase load is balanced. The voltmeter indicates the operating voltage of the transformer. If the power supply voltage is too high or too low for a long time, adjust the tap changer so that the output voltage of the transformer can be normal.

(2) Visually observe the oil level, oil color and transparency in the oil conservator and oil-filled bushing. Generally, the oil level should be within the scale and the transformer oil should be transparent and slightly yellow. When the transformer is equipped with a resistive telemetry thermometer, the upper oil temperature should be monitored at the same time and recorded.

(3) Use the ear test method to hear whether the running noise of the transformer is normal. Under normal conditions, the noise is light and stable.

(4) Observe the color of the color changing silica gel in the moisture absorber, and the dry silica gel should be dark blue. If the silica gel has turned pink, it means that the silica gel has a moisture absorption failure, and the silica gel should be taken out and dried before use.

(5) Monitor the transformer shell, oil-filled bushing and cooling device for oil leakage.

(6) Monitor the operation of the cooling system. For oil-immersed air-cooled and forced oil circulating air-cooled transformers, are there any individual fan that stops, whether the fan motor is overheated, and whether the sound is abnormal. For the forced oil circulating water-cooled transformers, whether the operation of the submersible pump is normal, whether the oil pressure and the flow rate change, whether the cooling water pressure meets the requirements, whether the cooling water inlet and outlet temperature meets the requirements, whether the cooler leaks oil or the like.

5.2.2 Contents of Operation and Maintenance

Most transformers are installed in open or semi-open spaces, and are subject to various climatic conditions such as rain, snow, wind, frost, lightning, high temperature, severe cold, fog, dust and so on. In the design and manufacture of each transformer, in accordance with national standards and technical conditions, although it is considered to withstand the above various harsh conditions, after the transformer has been operated for a period of time, its resilience will be reduced, so regular maintenance must be carried out to restore the resilience of the transformer.

Under normal circumstances, maintenance once every six months is sufficient, but in areas with dirty environment and bad weather, the maintenance cycle should be shortened appropriately such as once every four months, or even once every quarter.

The items for regular maintenance of transformers are as follows:

(1) Clean the transformer shell and its accessories.

(2) Wipe the outer surface of the high and low voltage bushings.

(3) For the transformer body and oil-filled accessories, take the oil sample and do the oil sample test.

(4) Check the conductive joints of the insulating bushing and the cap of the conductive plate.

(5) Before the thunderstorm season, maintain the lightning protection device and put it into the system in advance.

(6) During the maintenance of power outages, some sporadic small defects should be eliminated.

5.3 The Common Faults of Transformers, the Causes of Faults and Treatment Methods

During the operation of the transformer, the most common faults are winding faults, core faults, and partial faults such as bushings and tap-changers. According to the phenomenon of the fault, find the cause and take the corresponding treatment method. The following are the common faults of the transformer, the cause of the fault, and the method of disposal.

— Winding or inter-layer short circuit


(1) abnormal heating of the transformer

(2) The oil temperature rises

(3) The current on the power supply side increases

(4) DC resistance imbalance of three-phase windings

(5) High-voltage fuse blown

(6) Gas relay action


(1) Winding insulation aging

(2) Winding insulation is damp

(3) Improper winding of the windings, causing partial damage to the insulation

(4) The oil channel is filled with debris, which makes the oil passage blocked and partially overheated.

Treatment Methods:

(1) Replace or repair damaged windings, gaskets and insulation cylinders

(2) Dipping and drying treatment

(3) Replace or repair the winding

(4) Clearing debris in the oil passage

— Winding grounded or interphase short circuit


(1) High voltage fuse blown

(2) Explosion-proof pipe film rupture, fuel injection

(3) Gas relay action

(4) Transformer oil burning

(5) Transformer vibration


(1) Major defects such as aging or damage of the main insulation of the winding

(2) The transformer is flooded, and the insulating oil is seriously damped.

(3) The oil level is too low, and the lead wire exposed to the oil surface has no  sufficient insulation distance and is broken down.

(4) The debris falls into the winding

(5) Overvoltage breaks down the winding insulation

Treatment Methods:

(1) Replace or repair the winding

(2) Replace or treat transformer oil

(3) Overhaul the part that leaks oil and fill the oil to the normal oil level

(4) Remove debris

(5) Replace or repair the winding insulation and limit the amplitude of the overvoltage

— Winding deformation and disconnection


(1) The transformer makes an abnormal sound

(2) No current indication of disconnected phase


(1) Poor manufacturing assembly, winding is not compressed

(2) Electromagnetic force effect of short-circuit current

(3) Poor wire bonding

(4) Lightning strikes cause disconnection

(5) Manufacturing defects, insufficient strength

Treatment Methods:

(1) Repair the deformation part and replace the winding if necessary

(2) Tighten the clamping ring screw to tighten the loose gasket stays

(3) Cut off the wire that has been ablated or reduced in cross section or replace it with a new one.

(4) Repair the insulation and dry the paint

(5) Repair and improve structure and improve mechanical strength

— Insulation damage between iron chips


(1) The no-load loss is large

(2) The core is heated, the oil temperature rises, and the oil color becomes darker.

(3) Lifting the body to check that the silicon lamination film is peeling off or heating

(4) An abnormal sound is emitted inside the transformer


(1) Insulation aging between silicon laminations

(2) Subject to severe vibration, displacement friction between the laminations

(3) Core fasteners loose

(4) After the core is grounded, the heat burns the insulation between the laminations.

Treatment Methods:

(1) Repainting the insulating silicon laminations with insulation damage

(2) Fastening the core clip

(3) According to the core grounding fault treatment method

— Multi-point grounding or poor grounding of the core


(1) High voltage fuse blown

(2) The core heats up, oil temperature rises, oil color turns black

(3) Gas relay action

(4) Lifting the body to check that the silicon lamination is partially melted.


(1) Insulation aging between the core and the through-core screw rod, causing the core to be grounded at multiple points

(2) Core grounding piece is disconnected

(3) The core grounding piece is loosely connected

Treatment Methods:

(1) Replace the insulation tube and insulation gasket between the through-core screw rod and the core

(2) Replace the new grounding piece or compress the grounding piece

— Bushing flashover


(1) High voltage fuse blown

(2) There are discharge marks on the surface of the bushing


(1) Bushing surface area is dirty

(2) The bushing is cracked or damaged

(3) The bushing is not tightly sealed, and the insulation is damped.

(4) The debris falls between the bushing

Treatment Methods:

(1) Remove the dust and dirt on the surface of the bushing

(2) Replace the bushing

(3) Replace the gasket

(4) Remove debris

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