Under the specified operating conditions and environment, a transformer's key technical parameters are typically indicated on its nameplate. These include rated capacity, rated voltage and tap settings, rated frequency, winding connection group, and essential 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 that the transformer can continuously deliver under normal operating conditions at its rated current. It is a critical factor in determining the transformer’s size and application.
B. Rated Voltage (kV): This is the voltage level 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 for adjustments in the secondary voltage by changing the number of turns in the primary winding.
C. Rated Current (A): This is the maximum continuous current the transformer can carry without overheating or damaging its components, under the condition of rated capacity.
D. No-Load Loss (kW): This is the active power consumed when the transformer is energized but not supplying any load. It primarily results from core losses due to hysteresis and eddy currents, influenced by the quality of the core material and manufacturing techniques.
E. No-Load Current (%): This is 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): Also known as copper loss, this is the power consumed when the transformer is loaded. It occurs when the rated current is passed through the windings under short-circuit conditions, reflecting the resistance of the conductors.
G. Impedance Voltage (%): This is the percentage of the rated voltage required on the primary side to produce the rated current in the secondary winding when it is short-circuited. It indicates the internal impedance of the transformer and affects its performance during fault conditions.
H. Phase Number and Frequency: Transformers are either three-phase (denoted as S) or single-phase (denoted as D). In China, the standard frequency is 50Hz, while in countries like the U.S., it is 60Hz.
I. Temperature Rise and Cooling: This refers to the difference between the temperature of the transformer’s windings or oil and the ambient temperature. For oil-immersed transformers, the winding temperature rise is limited to 65K, and the oil surface to 55K. Cooling methods vary, including natural oil cooling, forced air cooling, Water Cooling, and more advanced types like tube or chip-based systems.
J. Insulation Level: This defines the transformer’s ability to withstand electrical stress. It is usually expressed in terms of lightning impulse and power frequency withstand voltages. For example, a transformer with a high-voltage rating of 35kV and low-voltage of 10kV may have an insulation level of LI200AC85/LI75AC35. This means the high-voltage side can withstand 200kV lightning impulse and 85kV power frequency, while the low-voltage side can handle 75kV impulse and 35kV power frequency. Some modern transformers, like those from Oaks Hi-Tech, may have a lower insulation level, such as LI75AC35, depending on their design and voltage requirements.
K. Winding Connection Group: This describes how the primary and secondary windings are connected, such as delta or star. The connection group is often denoted using a clock method, where the high-voltage side line voltage is considered as the 12 o’clock position, and the low-voltage side is positioned accordingly. For instance, Dyn11 indicates a delta-connected primary and a star-connected secondary with a neutral point, with a phase shift of 330 degrees (or 11 o’clock).
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