Transformer Nameplate Interpretation
Under specified operating conditions and environmental requirements, the main technical parameters of a transformer are usually indicated on the transformer nameplate. These parameters provide essential information for selection, operation, and maintenance. They mainly include rated capacity, rated voltage and tapping range, rated frequency, winding connection group, rated performance data, and total weight.
A. Rated Capacity (kVA)
Rated capacity refers to the maximum apparent power that a transformer can continuously deliver under rated voltage and rated current conditions without exceeding permissible temperature rise.
B. Rated Voltage (kV)
Rated voltage is the operating voltage that the transformer can withstand during long-term operation.
To adapt to power grid voltage fluctuations, the high-voltage side of a transformer is usually equipped with tap changers, which regulate the output voltage on the low-voltage side by adjusting the number of turns in the high-voltage winding.
C. Rated Current (A)
Rated current is the maximum current allowed to flow continuously through the transformer windings when operating at rated capacity.
D. No-Load Loss (kW)
No-load loss is the active power absorbed by the transformer when rated voltage at rated frequency is applied to one winding while the other winding remains open-circuited.
It mainly depends on:
- Core material (silicon steel performance)
- Manufacturing process
- Applied voltage level
E. No-Load Current (%)
No-load current refers to the current flowing in the primary winding when the secondary side is open-circuited at rated voltage.
It is generally expressed as a percentage of the rated current.
F. Load Loss (kW)
Load loss is the power consumed by the transformer when the secondary winding is short-circuited and rated current is applied to the primary winding at the rated tap position.
G. Impedance Voltage (%)
Impedance voltage is defined as the percentage of rated voltage applied to the primary winding that produces rated current in the secondary winding when it is short-circuited.
This parameter is critical for short-circuit current limitation and system coordination.
H. Phase and Frequency
Three-phase transformers are denoted by the letter “S”
Single-phase transformers are denoted by the letter “D”
The standard frequency in China is 50 Hz, while some countries (such as the United States) use 60 Hz.
I. Temperature Rise and Cooling Method
Temperature rise is the difference between the winding temperature or top oil temperature and the ambient temperature.
For oil-immersed transformers:
- Maximum winding temperature rise: 65 K
- Maximum top oil temperature rise: 55 K
Common cooling methods include:
- ONAN (Oil Natural Air Natural)
- ONAF (Oil Natural Air Forced)
- Water cooling
- Radiator type or tubular cooling systems
J. Insulation Level
Transformers are designed according to standardized insulation levels.
For example, a transformer with:
- High-voltage rated voltage: 35 kV class
- Low-voltage rated voltage: 10 kV class
Its insulation level may be expressed as:
LI200 AC85 / LI75 AC35, where:
- LI200: Lightning impulse withstand voltage of the HV side (200 kV)
- AC85: Power-frequency withstand voltage of the HV side (85 kV)
- LI75: Lightning impulse withstand voltage of the LV side (75 kV)
- AC35: Power-frequency withstand voltage of the LV side (35 kV)
For oil-immersed transformers produced by AUX High Technology Co., Ltd., the insulation level is typically LI75 AC35, indicating a lightning impulse withstand voltage of 75 kV and a power-frequency withstand voltage of 35 kV on the high-voltage side. Since the low-voltage side is 400 V, it is generally not considered.
K. Winding Connection Group (Vector Group)
The winding connection group describes the phase relationship between the primary and secondary windings.
It is commonly represented using the clock notation method, where:
- The high-voltage line voltage phasor is fixed at 12 o’clock
- The low-voltage line voltage phasor indicates the group number
For example:
Dyn11 means:
- Primary winding: Delta (D) connection
- Secondary winding: Star (Y) connection with neutral (n)
- Phase displacement: 11 o’clock position