Under the specified operating conditions and environment, a transformer's key technical parameters are typically listed on its nameplate. These include rated capacity, rated voltage and tap settings, rated frequency, winding connection group, and performance data such as impedance voltage, no-load current, no-load loss, and load loss, along with the total weight of the unit.
A. Rated Capacity (kVA): This refers to the maximum power output the transformer can continuously deliver when operating at its rated current and voltage.
B. Rated Voltage (kV): The voltage that the transformer is designed to handle over long periods. To accommodate variations in grid voltage, transformers often have taps on the high-voltage side, allowing adjustment of the low-voltage output by changing the number of turns in the high-voltage winding.
C. Rated Current (A): The maximum current the transformer can carry continuously under its rated capacity without overheating or sustaining damage.
D. No-Load Loss (kW): This is the active power consumed by the transformer when one winding is supplied with rated voltage, while the others are open-circuited. It mainly results from core losses due to hysteresis and eddy currents, which depend on the core material and manufacturing quality.
E. No-Load Current (%): The percentage of the rated current that flows through the primary winding when the secondary is open. It reflects the magnetizing current required to establish the magnetic field in the core.
F. Load Loss (kW): This is the power lost when the transformer is under full load. It occurs when the secondary is short-circuited, and the primary is supplied with rated current at the rated tap position.
G. Impedance Voltage (%): Measured during a short-circuit test, this is the percentage of the rated voltage applied to the primary side when the secondary is shorted and the current reaches its rated value. It indicates the internal impedance of the transformer.
H. Phase Number and Frequency: Transformers are usually marked as S for three-phase or D for single-phase. In China, the standard frequency is 50Hz, whereas in countries like the US, it is commonly 60Hz.
I. Temperature Rise and Cooling: This refers to the difference between the temperature of the transformer’s windings or oil and the surrounding ambient temperature. For oil-immersed transformers, the winding temperature rise is limited to 65K, and the top oil temperature rise to 55K. Cooling methods vary, including oil-immersed self-cooling, forced-air cooling, Water Cooling, and more advanced systems like tube-type or chip-type cooling.
J. Insulation Level: This defines the transformer’s ability to withstand electrical stress. For example, a transformer with a high-voltage rating of 35kV and a low-voltage rating of 10kV may have an insulation level marked as LI200AC85/LI75AC35. This means the high-voltage winding can withstand a lightning impulse of 200kV and a power frequency voltage of 85kV, while the low-voltage side can handle 75kV and 35kV respectively. For lower voltage transformers, such as those with 400V on the low side, the insulation level may be simplified to LI75AC35, as the low-voltage side’s requirements are less stringent.
K. Winding Connection Group: This describes how the transformer’s windings are connected—either in star (Y), delta (D), or other configurations. The connection group is identified using the clock method, where the high-voltage side line voltage is considered as the 12 o’clock position, and the low-voltage side’s line voltage is used to determine the group number. For example, Dyn11 indicates a delta-connected primary and a star-connected secondary with a neutral point, and a phase shift of 11 o’clock.
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