Wiring method for temperature gauges on dry-type transformers
1. Short-circuit the "input" and "output" terminals of the transformer and test their insulation resistance to ground using a megohmmeter. When measured with a 1000V megohmmeter, the resistance value should be greater than 2M ohms.
2. The cross-sectional area of the transformer's input and output power lines should meet the requirements of its current value; it is advisable to configure it according to a current density of 2-2.5A/min2.
3. The input and output three-phase power lines should be connected to phases A, B, and C respectively, according to the yellow, green, and red colors of the transformer terminal block busbars. The neutral wire should be connected to the transformer neutral wire, and the grounding wire should be connected to the transformer casing (if the transformer has a box, it should be connected to the grounding wire marking on the box). Check the input and output lines to confirm that they are correct.
4. First, power on the machine under no-load and observe whether the input and output voltages meet the requirements. At the same time, observe whether there are any abnormal phenomena such as unusual noises, arcing, or odors inside the machine. If any abnormality is found, please disconnect the input power immediately.
5. Load can only be connected after the no-load test is completed and normal. Overload capacity The overload capacity of dry-type transformers is related to the ambient temperature, the load condition before overload (initial load), the insulation and heat dissipation of the transformer, and the heating time constant. If necessary, the overload curve of the dry-type transformer can be obtained from the manufacturer. How to utilize its overload capacity? Here are two points for reference: (1) When selecting and calculating the transformer capacity, it can be appropriately reduced: fully consider the possibility of short-term impact overload of certain steel rolling, welding and other equipment - try to utilize the strong overload capacity of dry-type transformers to reduce the transformer capacity; for some uneven load places, 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) The standby capacity or number of units can be reduced: in some places, the standby coefficient of the transformer is required to be high, which makes the transformer capacity and number of units selected for the project large. The overload capacity of dry-type transformers can be reduced when considering their standby capacity; the number of standby units can also be reduced when determining the number of standby units. When a transformer is operating under overload, it is essential to monitor its operating temperature: if the temperature rises to 155℃ (an alarm is triggered), load reduction measures should be taken (reducing some secondary loads) to ensure the safe power supply to the main load. The safe operation and service life of selected dry-type transformers largely depend on the safety and reliability of the transformer winding insulation. The winding temperature exceeding the insulation withstand temperature and causing insulation failure is one of the main reasons why transformers cannot work normally. Therefore, monitoring the operating temperature of transformers and its alarm control are very important. (1) Automatic fan control: The temperature signal is measured by a Pt100 thermistor embedded in the hot part of the low-voltage winding. When the transformer load increases and the operating temperature rises, the system automatically starts the fan for cooling when the winding temperature reaches 110℃; when the winding temperature drops to 90℃, the system automatically stops the fan. (2) Over-temperature alarm and tripping: The winding or core temperature signal is collected by a PTC nonlinear thermistor embedded in the low-voltage winding. When the transformer winding temperature continues to rise, if it reaches 155℃, the system outputs an over-temperature alarm signal; if the temperature continues to rise to 170℃, the transformer can no longer continue to operate and must send an over-temperature trip signal to the secondary protection circuit to 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 large value display, and historical high temperature can be recorded). The high temperature can be output as a 4-20mA analog signal. If it is necessary to transmit to a computer at a 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 trip of the system can also be activated by the Pt100 thermistor signal, further improving the reliability of the temperature control protection system.
