Ultimate guide of cast resin dry type transformer

With the rapid development of the power industry, the requirements for industrial products such as power transmission and transformation and distribution equipment are getting higher and higher. The characteristics of cast resin dry-type transformers, which are pollution-free and environmentally friendly, meet the needs of modern economic development, so they have developed rapidly in recent years.


The annual output of China's dry transformers exceeds 40 million kVA, accounting for 30.47% of the output of distribution transformers. Let's learn about dry-type transformers today.

Table of contents

1. What is a cast resin dry type transformer?

2. How does a dry-type transformer work?

3. Which are the types of dry type transformers?

4. What are the advantages and disadvantages of using a cast resin transformer?

5. Which is the better oil type or dry type transformer?

6. What are cast resin dry-type transformers used for?

7. Can dry type transformers be used outside?

8. How are dry-type transformers cooled?

9. What is protection in dry-type transformer?

10. The difference between SCB dry transformer and GB dry transformer

11. Cast resin dry type transformer manufacturers in China

12. Cast resin dry type transformer manufacturing process

13. How do you test a dry-type transformer?

14. Maintenace of cast resin dry-type transformer

15. Cast resin dry-type transformer noise fault, judgment method, and solution analysis

16. What is the difference between indoor and outdoor transformers?

17. Are all transformers required to be insulated?

18. Do transformers have oil?

19. What is the Onan ONAF transformer?

What is a cast resin dry type transformer?

Cast resin dry type transformer is a type of SC(B) dry transformer with epoxy resin, which is a widely used dry type transformer. Its winding surface consists of high-quality protective material that forms the cover. Even in harsh environments such as dust and humidity, it will not affect the cast insulating dry-type transformer; the process characteristic of resin casting is that it must rely on molds and use special casting equipment to cast the coil in a vacuum state. The epoxy resin used is an insulating glue, so it is also called an epoxy cast resin dry type transformer. The epoxy resin is non-flammable, so it will not support combustion in the event of a fire; class. Suitable for dry and clean indoor


The heat capacity of the cast molding winding is large, so the epoxy cast resin dry type transformer has a strong overload capacity; no maintenance is required, so it is widely favored by people. With the development of the world economy, people's demand for resin dry-type transformers will increase rapidly.

Learn more: How to choose the best distribution transformer?


How does a cast resin dry type transformer work?

Like other transformers, cast resin dry-type transformers also use the principle of electromagnetic induction. When a conductive object is in a changing magnetic field, a current can be induced in the conductive body.


The AC voltage is input from both ends of the primary winding of the dry-type transformer, and the AC voltage is output from the secondary winding. After the AC voltage is input to the primary winding, the AC current passes through the primary winding, and the primary winding generates an alternating magnetic field. Most of the magnetic field lines of the magnetic field are formed by the iron core. Because the secondary winding is also wound on the iron core or the magnetic core, the changing magnetic lines of force pass through the secondary winding and induce electromotive force at both ends of the secondary winding. The voltage generated by the secondary winding is different from the input voltage, and its frequency and variation are the same as the input AC voltage.


Which are the types of cast resin dry type transformers?

The cast resin dry type transformer is divided into two types: filled resin casting and unfilled resin casting. In filled resin casting winding, adding quartz powder as filler in the resin can increase the mechanical strength of the resin, reduce the expansion coefficient, and improve the thermal conductivity, thereby reducing the material cost, and the winding appearance is good.

Try for free: How Much You Know For The Different Types Of Transformers?


What are the advantages and disadvantages of using a cast resin transformer?


Advantages of cast resin dry type transformer

1. Safe, fire-retardant, moisture-proof, explosion-proof, and can be directly installed in the electrical terminal site;


2. The insulation grade can reach F grade and H grade, so the electrical performance is good, the lightning impact resistance is strong, and the short circuit resistance is strong.


3. High mechanical strength, strong electrical performance, and long life;


4. Low loss, energy-saving, and environmental protection, no maintenance;


5. Only the temperature detection and protection device can provide a safe and reliable guarantee for the operation of the transformer.


6. Dry transformer is light in weight, small in size, occupies less space, and has low transportation and installation costs.


Disadvantages in using cast resin transformer

1. The cost of a cast resin transformer is higher, and the price of a dry transformer with the same capacitance is much higher than that of an oil transformer;


2. The voltage level and capacity are limited, the maximum voltage level is 35KV, and the maximum capacity is 25000kVA;


3. Dry transformer is generally only used indoors. If it must be used outdoors, it needs to be equipped with a shell with a higher protection level, and the cost is higher;


4. Once the dry transformer is damaged during operation, it cannot be repaired and can only be replaced with a new transformer.

Read more: The Ultimate Guide to 1500kVA Transformer


Which is better oil type or dry type transformer?


Before solving this problem, we need to know the difference between an oil transformer and a dry transformer:

1. The appearance and packaging forms are different. The dry transformer can see the iron core and the coil, and the oil transformer can only see the transformer shell;


2. The lead forms are different, most of the dry transformers use silicone rubber sleeves; most of the oil transformers use porcelain sleeves;


3. Different insulation and cooling media. The insulating medium of the dry transformer is epoxy resin. Resin insulation is used for natural cooling, and the large capacity is cooled by fans. The insulating medium of the oil transformer is oil, and the insulating oil circulates inside the oil tank. The heat sink required by the tropical to change ah to dissipate heat;


4. The applicable capacity and voltage are different. The dry transformer is generally used in the distribution networks, the voltage is lower than 35kV, and the power rating does not exceed 2500kva; an oil transformer can be used for all voltages and rating


5. The place of use is different. Dry transformers are generally used in high-rise buildings, hospitals, airports, stations, subways, supermarkets, schools, and other indoor places with densely populated areas, requiring fire and explosion protection. Oil transformer has a wide range of applications. Can be indoors or outdoors. The oil transformer has the risk of oil leakage, so is prone to explosion and fire. It is especially suitable for outdoor use, where the space is large and the accident oil pool needs to be dug


6. Environmental requirements are different. Dry change has requirements on the environment, such as not being too humid, not having too much dust, good ventilation, etc.


7. Different load capacities. The load capacity of an oil transformer is stronger than that of a dry transformer.


8. Prices vary. The price of a dry transformer is high, the price of an oil transformer is low, and maintenance is convenient, but it is flammable and explosive.

Get it now: How much do you know about oil-immersed distribution transformers?


The difference between oil type transformer and dry type transformer introduced above, let us know that they each have their own advantages and disadvantages. In humid and hot areas, it is suitable to use the oil transformer. When the space is crowded, it is better to choose a dry transformer. Dry transformers are generally used in shopping malls because they are energy-saving, environmentally friendly, and fireproof; dry-type transformers allow high and low voltages to be in the same room; the distance between high and low voltages of oil transformers shall not be less than five meters. Oil transformer adaptability is relatively strong, whether it is windy, rainy, etc., it can operate normally outdoors, so it is not said which transformer is better, but it can only be said that it is the best in its own suitable place. Therefore, you need to choose the most suitable type of transformer according to the use site, environment, voltage capacity, and budget of the transformer.


What is cast resin dry type transformers used for?

Cast resin transformers are widely used in lighting, shopping malls, airports, docks, and other places in areas with fire and explosion-proof requirements.


Can dry type transformers be used outside?


The answer is yes. Dry-type transformers can be used outdoors, but when used outdoors, the transformer must be equipped with a waterproof and moisture-proof casing. Then let's learn about the knowledge of dry-type transformer housing.


The dry-type transformer casing can protect the transformer body from being struck by foreign objects and prevent personal electric shock. There are IP00, IP20, and IP23 protection grades. IP00 is the transformer without an enclosure. Enclosure IP20 can prevent solid foreign objects more significant than 12mm and animals such as mice, snakes, cats, birds, and other animals from entering and destroying the performance of the transformer.


Dry-type transformer enclosures are divided into three categories according to different structural materials: aluminum alloy grinding board transformer enclosures, stainless steel transformer enclosures, and steel plate spraying transformer enclosures.

Read my article on: How To Purchase 3000kVA Transformer In USA & Canada


The stainless steel enclosure will be more expensive. Dry-type transformer enclosures protection grades are divided into: ip20, ip21, ip23, ip30, ip31, ip33 and other specifications.

Classification Description of the protection level of dry transformer enclosures:

1.IP20 prevents spherical objects with a diameter greater than 12.5mm from entering, and provides a safety barrier for live parts;


2. While Ip21 has the performance of ip20, it can also prevent vertical water droplets from entering, and play a waterproof and moisture-proof role;


3. While Ip23 has the performance of ip20, it can prevent the entry of water within an angle of 60° from the vertical line, and play a waterproof and moisture-proof role;


4. Ip30 prevents spherical objects with a diameter greater than 2.5mm from entering;


5. While Ip31 has the performance of ip30, it can also prevent vertical water droplets from entering;


6. While Ip33 has the performance of Ip30, it can prevent water splashing within a range of 60° from the vertical line from entering the transformer;


7. IP40 can prevent the entry of linear objects with a diameter greater than 1mm and has a dust-proof effect.


You can choose the dry-transformer enclosure for outdoor use according to your actual situation.

Learn more: 5+FAQ About 15kV Transformer


How are cast resin dry-type transformers cooled?

Cast resin dry-type transformer cooling methods are divided into natural air cooling (AN) and forced air cooling (AF).

When natural air is used for cooling, the transformer needs proper ventilation during normal operation. Generally, the transformer needs 4m3/min of ventilation per 1kW loss. At this time, the transformer can continuously output 100% of the rated capacity. Therefore, when the transformer is installed in the basement or other environment with poor ventilation conditions, forced air cooling (mechanical ventilation) should be adopted. When the transformer is equipped with a casing, the cooling capacity is reduced in order to ensure the circulation of cooling air, and the bottom of the transformer of 800kVA and below should be at least 150mm from the floor, and the bottom of the transformer above 800kVA and below 1600kVA should be at least 200mm from the ground; 1600kV. The bottom of the transformer above A should be at least 240mm from the ground, and the output capacity of the transformer should be reduced by about 5% to 10% according to its capacity. If the transformer has a high load rate and maybe overloaded for a short time, it must be equipped with a low-noise radial fan or a curtain fan. In order to prevent the fan from starting frequently, a thermostat should also be configured for automatic control. When the transformer is installed in the basement or other environment with poor ventilation conditions, the HVAC professional can be invited to set up a centralized fan and air duct system to mechanically supply and exhaust air to the environment to achieve the effect of cooling and cooling the transformer.


Under the condition of natural air cooling, the dry transformer can run continuously for a long time under the rated capacity. When forced-air cooling, the output capacity of the transformer can be greatly improved, but it can only be used for short-term or intermittent overload.


What is protection in a dry-type transformer?

The general dry-type transformer protection includes quick-break, over-current, zero-sequence, and other protections.

Read more: Electrical Transformer Protection Knowledge Guide

1. Quick break protection

The quick-break protection is to overcome the long action time limit of the protection equipment near the power supply end of the overcurrent protection. It adopts the method of increasing the setting value and limiting the action plan to protect the line. There are three types of protection plans: voltage quick-break protection, current quick-break protection, and transformer differential quick-break protection.


2. Overcurrent protection

Many electronic devices have a rated current, and it is not allowed to exceed the rated current, otherwise, the device will be burned out. Therefore, some devices have made a current protection module. When the current exceeds the set current, the equipment will automatically power off to protect the equipment.


3. Zero sequence protection

Zero-sequence protection means that after a grounding fault occurs in a large short-circuit current grounding system, there will be zero-sequence current, zero-sequence voltage, and zero-sequence power. These electrical quantities are used to form a general term for relay protection equipment that protects grounding short-circuits.


4. Temperature protection

The dry transformer measures the winding temperature through only the temperature controller, and when the temperature of the transformer in operation is abnormal, an alarm sound will be issued.


5. Indoor and outdoor protection of dry-type transformers

When the dry transformer is used outdoors, we need to be moisture-proof and waterproof to prevent small animals from entering and destroying it. The transformer shell can be used for protection.

Try for free: Three-phase Transformer Industry Basic Guidelines 2022


The difference between SGB(VPI) and SCB dry transformer

We know that there are two types of dry-type transformers, resin-cast dry-type transformers (SCB) and vacuum varnished dry-type transformers (SGB), SGB transformers are also called vpi transformers; resin-cast dry-type transformers are mostly F-class insulation, and varnished dry-type transformers are H-class insulation.




Structure outside the winding


Resin casting

Insulation Materials

NOMEX Insulating paper

Epoxy resin

Heat resistant temperature



Design life



Cooling method



Conditions of Use



Overload capacity



Maintenance cost

High environmental requirements, regular maintenance

No pollution and maintenance-free

Installation requirements

Specific environment and foundation

Easy to install, no special fire protection requirements. Can be installed near the load

Operating environment

Normal operation under 95% humidity, insulation strength not less than 80%

Under normal operation under 100% humidity, the dielectric strength is greater than 99%

Heat dissipation capacity




As can be seen from the above table:

Same place

All are dry-type transformers, and the national standards to follow are the same.


Different places:

1. SGB and SCB, the former is a non-encapsulated dry-type transformer, and the latter is an epoxy resin cast dry-type transformer. For non-encapsulated dry-type transformers, the structure of the coil is a continuous cake winding, and for epoxy cast dry-type transformers, the structure of the coil is a segmented layer winding;


2. There is a difference in the overload capacity. The dry change of SGB can be overloaded by 20%, while the dry change of SCB can be overloaded by about 40% when the fan is turned on for cooling;


3. There is a difference in the maintenance cost. The dry change of SGB is because the coil adopts a cake-type structure, which is easy to accumulates dust and causes insulation breakdown. Therefore, it needs to be regularly maintained and kept clean; and the dry change of SCB, because it is cast as a whole. , can be pollution-free, maintenance-free;


4. The requirements of the installation environment are different. The dry change of SCB can be installed near the load, which greatly reduces the loss of the line and reduces the installation cost; while the SGB must be installed in a specific environment;


5. In terms of operating environment requirements, SGB is more demanding than SCB. The dielectric strength of SCB can be maintained above 99% under 100% humidity, while the dielectric strength of SGB under 95% humidity is not less than normal operation. 80%;


6. In terms of heat dissipation capacity, SGB is worse than SCB.

To sum up, the application of SCB is wider than that of SGB.

Read more: How to purchase a 2 MVA power transformer?


Cast resin dry type transformer manufacturers in China

Daelim is a professional transformer manufacturer in China. The dry transformers produced mainly include Cast resin dry-type transformers with voltage levels of 10kV, 20kV, and 35kV, up to 5000kVA. Dalin's dry transformers fully comply with IEC 60076, ANSI, GB/T10228, and other standards, the insulation grade can reach F grade and H grade. The enclosure protection grades are IP00, IP20, and IP23.


Daelim dry transformer has been exported to Vietnam, Chile, Algeria, Kazakhstan, and other regions.


Cast resin dry type transformer manufacturing process

The dry transformer production process is as follows:

1. First of all, to purchase raw materials;

2. Stacked iron cores;

3. Making clips and insulating parts;

4. Winding the coil

5. Use the mold for coil casting;

6. Dry change general assembly;

7. Dry transformer testing


How do you test a dry-type transformer?


Same as oil-immersed transformers, various tests are required after production is completed. Only after all tests are passed can they be transported and sent to customers for use. The main test items are:

1. Measure the DC resistance of the winding

Check the welding quality of the internal wires, whether the contact between the tap changer and the leads is good, whether the phase resistance is unbalanced, etc. Generally, the line resistance should not exceed 2%, and the phase resistance should not exceed 4%. If the DC resistance is unbalanced, a circulating current between the three phases will occur, increasing the circulating current loss, and the transformer heating and other undesirable phenomena;

Get it now: How to test a power transformer?


2. Check the voltage ratio of all taps

Check whether the number of turns of the transformer is correct, whether the taps are correct, and whether the output of the transformer can reach 400V when the high-voltage input is 1000 and the voltage of each tap;


3. Check the three-phase wiring group and polarity of the transformer;


4. Measure the insulation resistance of each fastener and iron core insulated from the iron core;


5. Measure the insulation resistance of the windings

Check the insulation level between the high and low voltage of the transformer and the ground. Generally, the insulation resistance of a 2500V shaker to the high and low voltage and the ground is greater than the standard value detected by the 2500V shaker;


6. AC withstand voltage test of winding

The main insulation degree between the high and low voltage of the transformer and the ground is assessed by the electric strength test. It plays a decisive role in inspecting the local defects of transformers in the production process. Generally, the withstand voltage of the 10KV side of the dry-type transformer is 35KV, and the withstand voltage of 0.4KV is 3KV. It is qualified if there is no breakdown within 1 minute;


7. Jumping, closing, and interlocking tests of switches on each side of the transformer. Inspect the reliability of each protection action of the transformer and the integrity of the switchgear;

Impact closing test

(1) When the no-load transformer is opened, it may generate operating overvoltage. When the neutral point of the power system is not grounded or grounded through the arc suppression coil, the overvoltage amplitude can reach 4 to 4.5 times the phase voltage; when the neutral point is directly grounded, it can reach 3 times the phase voltage. In order to check whether the insulation strength of the transformer can be To withstand full voltage or operating overvoltage, an impact test is required;


(2) When the no-load transformer is powered on, an inrush current will be generated, and its value can reach 6 to 8 times the rated current. The magnetizing inrush current begins to decay quickly, generally reducing to 0.25 to 0.5 times the rated current value after 0.5 to 1 second, but the entire decay time is long, and the large-capacity transformer can reach tens of seconds. Because the magnetizing inrush current generates a large amount of electric power, in order to assess the mechanical strength of the transformer and whether the initial stage of the magnetizing inrush current decay can cause the malfunction of the relay protection device, an impact test is required. Generally, 5 impulse tests should be carried out for newly installed transformers and 3 times for overhauled transformers.


No-load test

During the test, the high-voltage side is open, and the low-voltage side is pressurized. The test voltage is the rated voltage of the low-voltage side, and the no-load loss is mainly iron loss.


Through the no-load test, the following defects of the transformer can be found: poor insulation between silicon steel sheets; partial short-circuit burning between the iron core and pole pieces; insulation parts of core bolts or binding steel strips, pressure plates, upper yokes, etc. are damaged and form short circuits; magnetic The silicon steel sheet in the road is loose, dislocated, and the air gap is too large. The multi-point grounding coil of the iron core has an inter-turn, inter-layer short circuit or the number of parallel branch turns is not equal, the ampere-turn is unbalanced, etc.; the high-consumption and inferior silicon steel sheet is misused or the design calculation is wrong.


Short circuit test

The transformer short-circuits test mainly measures short-circuit loss and short-circuit impedance. Check the correctness of the coil structure when it is put into production, and check whether there is a significant change from the last test value after replacing the winding.


The test power supply can be three-phase or single-phase, added to the high-voltage side, and the low-voltage side is short-circuited; during the test, the high-voltage side current is up to the rated value, and the low-voltage side current is controlled at the rated current.

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Maintenance of cast resin dry-type transformer

In order to prolong the service life of cast resin dry-type transformers, we should conduct detailed inspections on the transformers on a regular basis and do a good job in maintenance work.

1. From the perspective of the use environment of dry-type transformers, in some relatively clean and dry places, the inspection time can be appropriately extended, and a detailed inspection can basically be carried out once a year; if the environment where the equipment is located is very harsh, it is necessary to check and maintain every three months.


2. When inspecting the equipment, if it is found that there is a lot of dust on the equipment parts, it should be cleaned, but try to avoid the use of highly volatile cleaning agents. During the cleaning process, special attention should be paid to the insulators of the transformer, and three to five can be used. Atmospheric pressure to blow away the dust in the ventilation holes to ensure the unobstructed ventilation channels.


3. The parts of the dry-type transformer should be checked regularly to see if there is any looseness. If the parts are found to be rusted or severely corroded, they must be replaced in time. In addition, pay attention to whether there is carbonization on the insulation surface of the equipment. There should be timely measures to solve this.

4. After the transformer has been used for five years, the performance of the transformer should be tested through the insulation resistance, and other aspects do not need to be tested.


Cast resin dry-type transformer noise fault, judgment method, and solution analysis

Read more about transformer noise: Analysis of electric transformer noise problem

1. Resonance of components such as fan motors and enclosure of dry-type transformers

Reason: The resonance of the fan, casing, and other components will generate noise, which is generally mistaken for the noise of the transformer.

Judgment method:

1) Enclosure: feel the aluminum plate (or steel plate) of the enclosure with your hand, and listen to whether the noise changes. If there is a change, it means that the enclosure is resonating.

2) Fan monitor: Top the casing of each fan with an insulating rod and observe whether the noise changes. If there is a change, the fan monitor resonates.



1)Check whether the enclosure's aluminum plate (or steel plate) is loose. If it is loose, you need to tighten the screws of the casing to fix the aluminum plate of the casing.

2) To see if the fan monitor is loose, it is necessary to tighten the fastening bolts of the fan, and put a small piece of rubber between the fan and the fan bracket, which can solve the vibration problem of the fan.

3) If the transformer parts are loose, they need to be fixed.


2. Installation problems

Reason: If the dry transformer is not installed properly, the vibration will increase and the noise will become louder during operation.

Judgment method:

1) Transformer foundation is not stable

2) Putting up the transformer with channel steel will increase the noise.



1) The original installation method shall be modified by the installation unit.

2) Add anti-vibration pads under the transformer to solve some noise problems


3. The problem of the vibration of the busbar bridge

Reason: Due to the large current passing through the side-by-side busbar, the busbar vibrates due to the leakage magnetic field. The vibration of the busbar bridge will seriously affect the noise of the transformer, which will increase the noise of the transformer by more than 15dB, which is difficult to judge. Generally, users and installation units will mistake it for the noise of the transformer.

Judgment method:

1) The noise varies with the size of the load.

2) Use a wooden stick to push the busbar bridge hard, if the noise changes, it is considered that the busbar bridge is resonating.

3) The busbar vibrates in the bridge, and it is useless to top it with a wooden stick. It is necessary to open the busbar bridge cover to check whether the busbar is fixed.



1) It is mainly to destroy the condition of the resonance of the busbar bridge and tighten or loosen the boom screws.

2) Open the busbar bridge cover and fix the busbar.

3) The low-voltage outlet adopts a soft connection.

4) Please ask the manufacturer of the busbar bridge to solve it.


4. Transformer core self-resonance

Reason: There is an electromagnetic attraction between the joints of the silicon steel sheets and the laminations due to magnetic flux leakage.

Judgment method:

1) The noise of the transformer is too large, and other noises are mixed with the normal noise.

2) Transformer noise is wavy.



1) Tighten the screws on the transformer, including the screws at both ends of the clip, the through-hole screws, and the spacer screw.

2) Add anti-vibration pads under the transformer trolley to solve some noise.


5. Transformer coil self-resonance

Reason: When a load current passes through the winding, the magnetic leakage generated by the load current causes the vibration of the winding

Judgment method:

1) The noise of the transformer is too large, and the noise is relatively low.

2) When the load of the transformer reaches a certain level, noise starts to appear, sometimes it occurs, and sometimes no phenomenon.


2) Solution:

1) Tighten all the screws of the spacer to increase the axial compression force of the coil.

2) Loosen all the screws of the spacer, loosen all the bolts on the outgoing copper bar and the neutral copper bar, shake the low-voltage coil, move the high-voltage coil by 3-5 mm, and then tighten all the bolts.

Learn more: 11kV Transformer Guide 2022-Daelimtransformer


What is the difference between indoor and outdoor transformers?

1. The difference between temperature rise and heat dissipation, especially for transformers with natural heat dissipation;


2. The insulation requirements will be different. Considering the poor outdoor insulation environment (pollution level), the outer insulation performance of the transformer will be required to be higher.


3. The protection level will be different. Considering the difference in outdoor safety performance, the transformer protection level will be slightly higher.


4. Outdoor use should be considered rainproof, dustproof, antifouling, etc. Indoor use should not be used, but heat dissipation should be considered


5. The wiring method of outdoor transformers can be cable wiring or copper busbar wiring. Indoor transformers rarely use cable wiring.


6. Most the outdoor transformers are oil-immersed transformers, and dry transformers without a casing can only be used indoors

Try for free: How To Buy Power Transformers In Australia?


Are all transformers required to be insulated?

Transformers are mainly divided into main insulation, transverse insulation, and longitudinal insulation:

A.In general, the main insulation mainly refers to the insulation of the coil to other parts of the structure other than itself, including its insulation to the fuel tank, iron core, clamps and pressure plates, and the insulation of other coils in the same phase, as well as the insulation of different coils. insulation.


The longitudinal insulation of the B transformer includes three parts: inter-turn insulation, inter-layer insulation, and inter-segment insulation.


What needs to be considered in the design of longitudinal insulation are the various voltages acting on the longitudinal insulation and their gradient distribution; the degree of craftsmanship in the process of manufacturing the windings of the transformer; the mutual influence between the windings under special circumstances; the influence of the longitudinal insulation on the main insulation, the effect of the size of the oil gap between the segments on the heat dissipation and so on. Also, consider these aspects:

1. Insulation between turns. The windings of oil-type transformers are generally made of cable paper-wrapped wires. Because the paper is used as the turn insulation of the transformer winding because the dielectric constant of the paper is not much different from that of the oil, the electric field distribution can be made more uniform. Still, it should be noted that the turns cannot be selected according to the data of the complete breakdown of the oil gap. For insulation thickness, we have to keep enough degrees.


2. Insulation between layers and sections. Interlayer insulation is mainly suitable for cylindrical windings. When the working voltage between the two layers is high, the interlayer insulation must be thicker, which not only increases the radial dimension of the transformer winding but also is not conducive to heat dissipation, which increases the temperature of the transformer winding.


3. Longitudinal insulation structure of oil-type transformers: transformers of 35 kV and below; total insulation of transformers above 110 kV.


Do transformers have oil?

Transformers may or may not have oil. This is mainly based on the difference in the cooling medium. The cooling medium of the oil-immersed transformer is oil, so there must be oil. The cooling medium of dry-type transformers is air or other gas, so there is no oil.

Read more about transformer oil: What is the difference between mineral oil and vegetable oil?


What is the Onan ONAF transformer?

Both ONAN and ONAF are used to indicate the cooling method of oil-immersed transformers.


ONAN Transformer


The first two letters ON (O—oil, N—Nature) of ONAN represent that the internal cooling method of the transformer is the natural circulation of oil, which uses the thermosiphon effect to circulate the oil inside the transformer tank. The last two letters AN (A—Air, N—Nature,) represent the external cooling method of the transformer, and the outside of the transformer is cooled by the natural flow of wind. ONAF transformers have only one cooling method, oil immersion self-cooling, only heat sinks, and no fans.


ONAF Transformer

ON has the same meaning as above, and the oil inside the transformer is naturally circulated for cooling. AF (A—Air, F—Force,) forced air cooling, need to add a fan to the outside of the transformer for cooling. The ONAF transformer is cooled by adding a fan to the heat sink; when the load rate of the transformer is low, the fan can be turned off and rely on natural air cooling; when the power demand is great, the fan can be turned on to increase the load rate of the transformer. It is possible to switch between ONAN and ONAF. Therefore, in the ONAF transformer nameplate, we can see that two transformer capacities are marked, one is for self-cooling and the other is for forced air-cooling.


In addition to the two cooling methods of ONAN and ONAF, oil-immersed transformers also have ODAF, OFAF, OFWF, ODWF, etc. However, there are only two commonly used cooling methods for dry change, one is AN, natural air cooling; the other is AF, forced air cooling.


Daelim can not only do dry transformers but also transformers such as pad-mounted transformers, HV transformers, single transformers, and single-phase pad-mounted transformers. There are professional after-sales installation teams in North and South America. If you are looking for a new transformer supplier, please contact Daelim.

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