Dry Type Transformer Selection

The safe operation and service life of dry-type transformer largely depend on the safety and reliability of transformer winding insulation. The winding temperature exceeds the insulation withstand temperature and the insulation is destroyed, which is one of the main reasons that the transformer can not work normally. Therefore, the monitoring of the transformer’s operating temperature and its alarm control are very important.
⑴ Fan automatic control: The temperature signal is measured through the Pt100 thermistor embedded in the hottest part of the low-voltage winding. The transformer load increases, the operating temperature rises, when the winding temperature reaches 110℃, the system automatically starts the fan cooling; When the winding temperature is as low as 90 ° C, the system automatically stops the fan.
(2) Overtemperature alarm and tripping: the winding or core temperature signal is collected through the PTC nonlinear thermal resistance embedded in the low-voltage winding. When the temperature of the transformer winding continues to rise, if it reaches 155℃, the system will output an overtemperature alarm signal; If the temperature continues to rise to 170 ° C, the transformer can no longer continue to operate, it is necessary to send the overtemperature trip signal to the secondary protection circuit, and the transformer should be quickly tripped.
⑶ Temperature display system: The Pt100 thermistor embedded in the low-voltage winding measures the temperature change value, and directly displays the temperature of each phase winding (three-phase inspection and maximum display, and can record the historical highest temperature), the highest temperature can be output with 4 ~ 20mA analog, if you need to transmit to a distant (distance up to 1200m) computer, you can add a computer interface, 1 transmitter, Up to 31 transformers can be monitored simultaneously. The overtemperature alarm and tripping of the system can also be acted by the Pt100 thermistor signal, which further improves the reliability of the temperature control protection system.
Protection mode
According to the use of environmental characteristics and protection requirements, dry transformer can choose different enclosures.
IP20 protective shell is usually used to prevent solid foreign bodies with a diameter greater than 12mm and small animals such as mice, snakes, cats and finches from entering, resulting in malignant failures such as short circuit power failure, and providing a safety barrier for the live part. If the transformer must be installed outdoors, you can choose IP23 protective housing, in addition to the above IP20 protection features, but also to prevent water droplets within 60° Angle of the vertical line. However, the IP23 shell will reduce the cooling capacity of the transformer, and attention should be paid to the reduction of its operating capacity when selecting.
Cooling mode
Dry transformer cooling methods are divided into natural air cooling (AN) and forced air cooling (AF). When air cooled naturally, the transformer can run continuously for a long time under the rated capacity. When forced air cooling, the transformer output capacity can be increased by 50%. Suitable for intermittent overload operation, or emergency overload operation; Due to the large increase in load loss and impedance voltage during overload, it is in a non-economic operation state, so it should not be kept in continuous overload operation for a long time.
Overload capacity
The overload capacity of the dry transformer is related to the ambient temperature, the load before overload (initial load), the insulation and heat dissipation of the transformer and the heating time constant, etc. If necessary, the overload curve of the dry transformer can be obtained from the manufacturer.
How to take advantage of its overload capability? The author puts forward two points for reference:
(1) When choosing to calculate the transformer capacity, it can be appropriately reduced: fully consider the possibility of short-term impact of some rolling, welding and other equipment – try to use the strong overload capacity of dry transformers to reduce the transformer capacity; For some places with uneven load, such as residential areas mainly for night lighting, cultural and entertainment facilities, and shopping malls mainly for air conditioning and daytime lighting, its overload capacity can be fully utilized, and the transformer capacity can be appropriately reduced, so that its main operating time is at full load or short-term overload.
(2) Can reduce the spare capacity or the number of units: in some places, the reserve coefficient of the transformer is higher, making the transformer selected by the project large capacity and more units. The overload capacity of dry transformation can be compressed when considering its reserve capacity. It can also be reduced when determining the number of spare units. When the transformer is in overload operation, it is necessary to pay attention to monitoring its operating temperature: if the temperature rises to 155 ° C (there is an alarm), load reduction measures should be taken (minus some minor loads) to ensure the safe power supply of the main load.
Application status
Dry transformer because there is no oil, there is no fire, explosion, pollution and other problems, so electrical codes, regulations, etc. do not require dry transformer placed in a separate room. Loss and noise are reduced to new levels, creating conditions for the transformer and the low-voltage panel to be placed in the same distribution room.
At present, the annual output of resin-insulated dry transformers in China has reached 10,000 MVA, becoming one of the countries with the largest production and sales of dry transformers in the world. Dry transformers are now widely used in power stations, factories, hospitals and other almost all electrical. With the promotion and application of SC(B)9 series with low noise (distribution transformer noise below 2500kVA has been controlled within 50dB) and energy saving (no-load loss reduced by 25%), the performance indicators and manufacturing technology of dry transformers in China have reached the world’s advanced level.