1. Temperature control system of three-phase isolation transformer

 (1) Automatic fan control: The temperature signal is measured by the Pt100 thermistor temperature measuring resistor embedded in the hottest part of the low-voltage winding. The transformer load increases and the operating temperature rises. When the winding temperature reaches 110°C, the system automatically starts the fan to cool; when the winding temperature drops to 90°C, the system automatically stops the fan.

 (2) Over-temperature alarm and trip: The temperature signal of the winding or core is collected through the PTC nonlinear thermal temperature measuring resistor embedded in the low-voltage winding. When the temperature of the transformer winding continues to rise, if it reaches 155°C, the system outputs an over-temperature alarm signal; if the temperature continues to rise to 170°C, the transformer can no longer continue to operate, and an over-temperature trip signal must be sent to the secondary protection circuit, which should make the transformer Trip quickly.

 (3) Temperature display system: The temperature change value is measured by the Pt100 thermistor embedded in the low-voltage winding, and the temperature of each phase winding is directly displayed (three-phase inspection and maximum value display, and the highest historical temperature can be recorded). The maximum temperature is output as 4-20mA analog quantity, if it needs to be transmitted to a remote computer (distance up to 1200m), a computer interface can be added, one transmitter can monitor up to 31 transformers at the same time. The over-temperature alarm and tripping of the system can also be actuated by the signal of the Pt100 thermistor, which further improves the reliability of the temperature control and protection system.

 Two, three-phase isolation transformer protection method

 According to the characteristics of the use environment and protection requirements, the three-phase isolation transformer can choose different enclosures.

 The IP20 protective casing is usually used, which can prevent solid foreign objects larger than 12mm in diameter and small animals such as mice, snakes, cats, and sparrows from entering, causing vicious failures such as short circuit and power failure, and providing a safety barrier for live parts. If the transformer must be installed outdoors, the IP23 protective enclosure can be selected. In addition to the above IP20 protection function, it can also prevent water droplets within an angle of 60° from the vertical line. However, the IP23 enclosure will reduce the cooling capacity of the transformer, so pay attention to the reduction of its operating capacity when selecting it.

 Three, three-phase isolation transformer cooling method

 Three-phase isolation transformer cooling methods are divided into natural air cooling (AN) and forced air cooling (AF). Under natural air cooling, the transformer can run continuously for a long time under the rated capacity. When forced air cooling, the output capacity of the transformer can be increased by 50%. It is 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 used. Make it in continuous overload operation for a long time.

 The overload capacity of the three-phase isolation transformer is related to the ambient temperature, the load before the overload (initial load), the insulation and heat dissipation of the transformer and the heating time constant. If necessary, the overload curve of the dry transformer can be obtained from the manufacturer.