The rated technical data of a transformer defines the technical limitations that enable it to operate reliably and perform well over a long period. It also serves as the basis for manufacturers to design, manufacture, and test transformers. The rated technical data of a transformer is all marked on its nameplate and mainly includes the following:
1. Rated capacity SN
Rated capacity refers to the output capability that ensures long-term operation under rated conditions. For a single-phase transformer, it is the product of the rated current and the rated voltage. For a three-phase transformer, it is the sum of the capacities of the three phases. The unit is generally expressed in kilovolt-amperes (kVA) or megavolt-amperes (mVA).
2. Rated voltage UN
Rated voltage is the voltage specified by the manufacturer at the rated tap of the transformer under no-load conditions. This voltage ensures long-term safe and reliable operation, and is expressed in volts (V) or kilovolts (kV). When the transformer is under no-load conditions, the rated voltage U1N is applied to the primary side at the rated tap, and the voltage at the secondary side is the rated secondary voltage U2N. For three-phase transformers, unless otherwise specified, the rated voltage on the nameplate refers to the line voltage, while for single-phase transformers it refers to the phase voltage.
3. Rated current IN
The rated current on each side of a transformer is the line current value calculated by dividing the rated capacity of the corresponding side by the rated voltage of the corresponding winding, and the unit is expressed in amperes (A) or kiloamperes (KA).
4. No-load loss (also called iron loss), P0
No-load loss is the power loss when the load is not in use, and the unit is expressed in watts (W) or kilowatts (KW).
5. No-load current, I0
No-load current refers to the percentage of the excitation current of a transformer during no-load operation relative to its rated current.
6. Short-circuit voltage (impedance voltage percentage), UK
Short-circuit voltage refers to the percentage of the voltage applied to the primary winding relative to the rated voltage when the secondary winding of a transformer is short-circuited and a voltage is applied to the primary winding, and the secondary winding carries the rated current.
The percentage of short-circuit voltage is numerically equal to the percentage of transformer impedance, indicating the magnitude of the transformer's internal impedance.
7. Short-circuit loss, PK
Short-circuit loss refers to the power consumed by the transformer winding conductors when the secondary winding of the transformer is short-circuited and the primary winding current is at its rated current. The unit is expressed in watts (W) or kilowatts (KW).
8. Connection Group
A set of letters and clock ordinal numbers are used to represent the phase displacement relationship between the low-voltage winding and the high-voltage winding of a transformer, as well as the connection method between the primary and secondary windings. The phase displacement of different voltage windings of the transformer, i.e., the connection group, is generally represented by clock ordinal numbers.
Because the phase difference between the line voltages corresponding to the high and low voltage windings is always an integer multiple of 30°, which is exactly the same as the angle between the hours on a clock face, the method is to treat the primary side line voltage phasor as the long hand of the clock, fixed at 12 o'clock, and the corresponding secondary side line voltage phasor as the short hand. The position indicated by the short hand determines the group of the connection. For example, if the secondary side line voltage is in phase with the primary side line voltage, the short hand should also point to 12 o'clock, and the group of the connection is defined as 12.
If the secondary line voltage leads the primary line voltage by 30°, the short pin should point to the 11 o'clock position, and the group of its wiring group is specified as 11.
9. Rated frequency, f
my country stipulates that the standard industrial frequency is 50 Hz, therefore the frequency of power transformers is 50 Hz.
10. Rated temperature rise
The temperature difference between the winding or upper oil temperature inside a transformer and the ambient air temperature is called the winding or upper oil temperature rise. The temperature rise limits for each transformer are indicated on its nameplate. Chinese standards stipulate that the winding temperature rise limit is 65℃, the upper oil temperature rise limit is 55℃, and the maximum ambient temperature is 40℃. Therefore, during normal operation, the maximum temperature of the upper oil in a transformer should not exceed 95℃.
11. Rated cooling medium temperature
For air-cooled transformers, the rated cooling medium temperature refers to the maximum ambient air temperature that should not exceed 40℃ during transformer operation. This ensures that the winding and oil temperatures do not exceed the rated allowable values when the transformer is operating under rated load. Therefore, the nameplate specifies the ambient temperature requirements.
