There are several kinds of protection for dry-type transform

 

Dry type transformer is cooled by air convection, which is generally used for local lighting and electronic circuits. Dry type transformer is widely used in local lighting, high-rise buildings, airports, docks, CNC mechanical equipment and other places. In short, dry type transformer refers to the transformer whose core and winding are not immersed in insulating oil. The cooling methods are divided into natural air cooling (an) and forced air cooling (AF). Under natural air cooling, the transformer can operate continuously for a long time under rated capacity. When forced air cooling, the output capacity of transformer can be increased by 50%. It is suitable for intermittent overload operation or emergency overload operation; because the load loss and impedance voltage increase greatly during overload, it is in non economic operation state, so it should not be in continuous overload operation for a long time.

 

 

 

Cooling mode of dry type transformer

 

When the temperature of dry-type transformer exceeds the insulation withstand temperature, the insulation will be damaged and the safe operation and service life of dry-type transformer will be affected. Therefore, the dry-type transformer should be equipped with appropriate cooling device to keep the winding temperature within the allowable range. The common cooling methods of dry-type transformer are natural air cooling and forced air cooling. The rated capacity of dry-type transformer is that of natural air cooling. The output capacity of dry-type transformer with forced air cooling can be increased by 40% ~ 50%, which can be used for intermittent overload operation or emergency overload operation. Forced air cooling fan is generally installed in the lower part of dry-type transformer.

 

 

 

Due to the increase of load loss and impedance voltage during overload, it is in non economic operation state, so it should not be allowed to operate continuously for a long time. A thermistor is set in the winding of the dry-type transformer to measure the temperature. When the temperature exceeds the alarm value, the winding overtemperature alarm signal will be sent out. If the temperature continues to rise to the trip value, the overtemperature trip signal will be sent out to trip the dry-type transformer.

 

Dry type transformer is a kind of transformer whose core and winding are not immersed in insulating liquid. It is used in general public distribution network or industrial power grid. It can be divided into the following three categories:

 

 

 

(1) Totally enclosed dry-type transformer: a transformer placed in a sealed enclosure without pressure and cooled by internal air circulation.

 

 

 

(2) Enclosed dry-type transformer: a transformer placed in a ventilated enclosure and cooled by external air circulation.

 

 

 

(3) Non enclosed dry-type transformer: transformer without protective enclosure, cooled by natural or forced air circulation.

 

 

 

Performance characteristics of dry type transformer

 

1. High safety: SG10 transformer has no combustible resin, can not support combustion in the process of use, can be flame retardant, will not explode and release toxic gases, will not cause harm to the environment, other equipment and human body, is not sensitive to humidity, dust and pollution; operation has no partial discharge and never "crack" possibility.

 

 

 

2. High reliability: both high and low voltage coils are made of Nomex insulating material, and impregnated with H-class solventless impregnating paint for many times by VPI vacuum pressure equipment, and cured by high temperature baking for many times. The product is grade H (180 ℃), and the main insulating material is grade C (220 ℃), with strong overload capacity and good short circuit resistance. Under the condition of good ventilation, 20% overload operation is allowed.

 

 

 

3. Energy saving and environmental protection: after the end of product life, steel, iron and other materials are easy to recycle; all Nomex Paper used will not release toxic substances when burning; other insulating materials can degrade and do not pollute the environment; the product has low loss, energy saving, low noise and flexible design.

 

 

 

Application field of dry type transformer

 

In recent years, the dry type has become more and more widely used

 

 

 

1. Distribution transformer:

 

 

 

1) Voltage: 10kV is the most high voltage side voltage, the highest is 35kV; low voltage side voltage is usually 0.4kV, which can not only provide 380V AC power for low-voltage electrical equipment, but also provide 220V power for urban and rural lighting and household appliances.

 

 

 

2) Capacity: from 30KVA to 2500kVA.

 

 

 

3) The most widely used: for power distribution of power equipment and lighting in urban distribution network, industrial and mining enterprises, transformers account for more than 80-90% of the dry transformer market.

 

 

 

2. Power transformer:

 

 

 

For 10, 20 and 35kV Power transformers, the capacity is 630-20000kva. In 1996, China has manufactured 16000kva dry-type transformer with 35 / 10KV voltage. It is the first dry-type transformer with the largest capacity and the highest voltage in Asia. These transformers are mostly used in regional substations of power system.

 

 

 

3. Rectifier traction transformer:

 

 

 

1) Rectifier excitation transformer: the excitation system of generator is developing to static equipment, dry type transformer has the tendency to replace excitation generator. The high voltage is 13.8kv ~ 20kV, and the low voltage is about 1KV. Usually single-phase structure, high-voltage enclosed bus for many. The excitation transformer developed for the Three Gorges generator is expected to have a single-phase capacity of 3000kva.

 

 

 

2) Metallurgical EAF transformer: it is characterized by low voltage and high current (over 20000 amperes produced in China), which is used for EAF smelting.

 

 

 

3) Traction transformer: it is a dry type traction transformer suitable for urban subway and rail transit. It will be widely used with the development of urban rail transit. The voltage is 10 kV, 20 kV and 35 kV, and the capacity is 800 KVA, 2500 KVA and 3300 kVA. From 12 pulse rectifier to 24 pulse rectifier, the harmonic pollution is reduced.

 

 

 

4. Dry transformer for various special purposes:

 

 

 

Dry transformer is suitable for various fields and special purposes, such as nuclear power station, marine and offshore platform, steel rolling and other special transformers, which have been designed and manufactured in China. As long as the relevant technical requirements are put forward, the dry transformer can be submitted to the manufacturer for development, design and production.

 

According to the different transformer capacity, differential protection may be added as the main protection.

 

 

 

General dry-type transformer protection: quick break, over-current, zero sequence protection.

 

 

 

1. Quick break protection

 

 

 

The purpose of quick break protection is to overcome the long action time limit of over-current protection in the protection device near the power supply end. The setting value is increased to limit the action range to protect the line. There are three kinds of protection scope: voltage quick break protection, current quick break protection and transformer differential quick break protection.

 

 

 

2. Over current protection

 

 

 

A lot of electronic equipment have a rated current, not allowed to exceed the rated current, otherwise it will burn the equipment. Therefore, some devices have made current protection module. When the current exceeds the set current, the device will automatically power off to protect the device.

 

 

 

3. Zero sequence protection

 

 

 

Zero sequence protection refers to the occurrence of zero sequence current, zero sequence voltage and zero sequence power in the large short-circuit current grounding system. These electrical quantities are used to form the general name of relay protection devices to protect the grounding short circuit.

 

Maintenance method of dry type transformer

 

 

 

1. Put in standby transformer, disconnect the circuit breaker at the low voltage side of the transformer for maintenance, take off the operation safety of control power supply, and hang the "no closing" sign at the switch handle.

 

 

 

2. Disconnect the breaker on the high voltage side of the maintenance transformer, close the grounding switch, lock the high-voltage cabinet after fully discharging the transformer, and hang the "no closing" sign at the switch handle.

 

 

 

3. For maintenance of dry-type transformer, first clean the porcelain sleeve and shell, and then check whether the shell, gasket and porcelain sleeve have any crack, discharge trace or aging rubber gasket, whether the cable and bus are deformed, and replace the broken ones.

 

 

 

4. Check whether the bus contact surface is kept clean, and the contact surface shall be removed with oxide layer and coated with power compound grease.

 

 

 

5. Check whether the grounding of transformer is good, whether the ground wire is corroded, and replace if the corrosion is serious.

 

 

 

6. Tighten the lead terminal, pin, grounding screw and connecting bus screw. If there is any looseness, remove the screw, or file the contact surface with a fine flat file, or replace the spring washer and screw until the contact is good.

 

 

 

7. Clean the dust around the transformer and accessories, and check whether the fire protection facilities and ventilation system are in good condition.

 

 

 

8. Disconnect the grounding switch on the high voltage side, lock the high-voltage joint bar, and use 2500V megger to measure the insulation resistance. Compared with the measured value before the transformer is delivered, the insulation resistance shall not be lower than 70% of the original data at the factory. If it is unqualified, it shall be reported to the factory for processing in time.

 

 

 

1. The insulation resistance must be measured by two persons.

 

 

 

2. Do not touch the transformer before charging and discharging.

 

 

 

3. Prevent reverse power transmission to transformer and power transmission from transformer to bus bar in operation.

 

 

 

4. Maintenance personnel should wear insulating shoes and gloves for operation.

 

 

 

5. It is strictly forbidden to switch on by mistake.