Three Phase Transformer Technical Parameters
As a cornerstone of modern power transmission and distribution systems, the three-phase transformer leverages the principle of electromagnetic induction to regulate alternating current (AC) voltages with high efficiency. Beyond its primary voltage conversion function, this critical equipment integrates precision-engineered components to deliver reliable performance across industrial, commercial, and utility applications.
At the heart of every three-phase transformer lie three essential components: the primary coil (which receives input power), the secondary coil (which delivers adjusted output power), and the iron core (a magnetic circuit that enhances flux linkage between coils). Together, these parts enable the transformer to fulfill multiple vital roles: converting voltage levels to optimize power transmission (stepping up for long-distance lines or stepping down for end-user devices), adjusting current magnitudes to match load requirements, modifying impedance to ensure efficient power transfer, providing electrical isolation to enhance safety, and regulating voltage stability (as seen in magnetic saturation transformers).
To help industry professionals, buyers, and technical teams gain a comprehensive understanding of transformer performance, we’ve compiled a detailed breakdown of the key technical parameters—data that is typically inscribed on the transformer’s nameplate and serves as the foundation for selecting the right equipment for specific applications.
Core Technical Parameters of Three-Phase Transformers
Transformers are designed to operate within strict operating conditions, and their technical parameters are standardized to ensure compatibility, safety, and performance consistency. Below is an in-depth explanation of each critical specification:
Rated Capacity (kVA)
Rated capacity, measured in kilovolt-amperes (kVA), represents the maximum electrical power a transformer can continuously deliver without exceeding thermal or electrical limits. It is calculated based on the transformer’s rated voltage and rated current, reflecting the equipment’s ability to support connected loads over extended periods. This parameter is non-negotiable for system design, as undersizing can lead to overheating and premature failure, while oversizing results in unnecessary energy waste and higher costs.
Rated Voltage (kV)
Rated voltage denotes the maximum voltage a transformer can safely withstand during long-term operation, specified for both high-voltage (HV) and low-voltage (LV) windings. To adapt to fluctuations in grid voltage, most three-phase transformers are equipped with tap changers on the high-voltage side. By adjusting the number of turns in the HV winding, operators can fine-tune the LV output voltage, ensuring it remains within acceptable ranges even as grid conditions change. This flexibility is crucial for maintaining stable power supply to sensitive equipment and minimizing downtime.
Rated Current (A)
Rated current refers to the maximum continuous current that the transformer’s windings can carry without exceeding temperature rise limits. This parameter is directly derived from the rated capacity and rated voltage (following the formula: Current = Capacity / Voltage) and is a key factor in selecting conductor materials and winding dimensions. Operating above the rated current for prolonged periods can cause insulation degradation, leading to reduced lifespan or catastrophic failure.
No-Load Loss (kW)
No-load loss—also known as iron loss—occurs when the transformer is energized at its rated frequency and voltage, with one winding connected to the power source and the remaining windings open-circuited. This loss is primarily attributed to two factors: hysteresis loss (caused by the reversal of magnetic flux in the iron core) and eddy current loss (induced currents in the core material). The magnitude of no-load loss depends on the quality of the core’s silicon steel sheet (e.g., low-loss grain-oriented steel), the precision of the manufacturing process (such as core lamination and assembly), and the applied voltage. Minimizing no-load loss is critical for improving energy efficiency, especially in transformers that operate at partial load or continuously.
No-Load Current (%)
When a three-phase transformer operates at rated voltage with no load connected to the secondary side, a small current—known as no-load current—flows through the primary winding. This current is typically expressed as a percentage of the transformer’s rated current, ranging from 1% to 5% for most modern transformers. No-load current serves two purposes: establishing the magnetic flux in the iron core and supplying the no-load loss. A lower no-load current percentage indicates better core efficiency and reduced energy waste during standby or light-load operation.
Load Loss (kW)
Load loss—also called copper loss—occurs when the transformer is under load, with the secondary winding short-circuited and the primary winding supplied with current at the rated tap position. This loss stems from the resistance of the transformer’s windings (primary and secondary) and increases with the square of the load current (I²R loss). Factors influencing load loss include the cross-sectional area of the winding conductors, the length of the windings, and the operating temperature (as resistance increases with heat). Load loss is a key contributor to overall energy consumption, particularly in transformers that operate at or near full load for extended periods.
Impedance Voltage (%)
Impedance voltage (often referred to as short-circuit voltage) is measured by short-circuiting the secondary winding and gradually increasing the voltage applied to the primary winding until the secondary short-circuit current equals the rated current. The applied primary voltage at this point, expressed as a percentage of the rated primary voltage, is the impedance voltage. This parameter is critical for power system design, as it determines the transformer’s ability to withstand short-circuit currents, influences voltage regulation, and affects the parallel operation of multiple transformers. Typical impedance voltage values range from 4% to 10% for medium-voltage transformers, depending on the application and design requirements.
Phase and Frequency
Three-phase transformers are designated with a phase identifier: “s” for three-phase systems and “d” for single-phase configurations (though the latter is less common in high-power applications). Frequency is another fundamental specification, with the Chinese national standard specifying 50Hz for power systems. Internationally, 60Hz is widely used in countries such as the United States, Canada, and Japan. Transformers must be designed to match the frequency of the power grid they serve, as frequency deviations can lead to increased losses, reduced efficiency, and potential equipment damage.
Temperature Rise and Cooling Methods
Temperature rise refers to the difference between the temperature of the transformer’s windings (or top oil) and the surrounding ambient temperature. For oil-immersed transformers—the most common type in utility and industrial settings—international standards specify a maximum winding temperature rise of 65K and a top oil temperature rise of 55K (relative to a 40°C ambient temperature). Excessive temperature rise accelerates insulation aging, so effective cooling systems are essential to maintain safe operating temperatures.
Common cooling methods for three-phase transformers include:
- Oil Natural Air Natural (ONAN): Relies on natural convection of oil and air for heat dissipation (suitable for small to medium capacities).
- Oil Natural Air Forced (ONAF): Uses fans to force air flow over cooling fins, enhancing heat transfer (for medium to large capacities).
- Oil Forced Air Forced (OFAF): Employs pumps to circulate oil and fans for air cooling (for high-capacity transformers).
- Water Cooling: Uses water as a cooling medium for oil (ideal for compact installations or high-heat environments).
- Cooling Structures: Includes tube-type and fin-type designs, which increase the surface area for heat dissipation.
Insulation Level
The insulation level of a transformer defines its ability to withstand voltage surges (such as lightning strikes) and continuous operating voltages. It is denoted by a standardized code that specifies the lightning impulse withstand voltage (LI) and power frequency withstand voltage (AC) for both high-voltage and low-voltage windings.
For example:
- A transformer with a 35kV HV rating and 10kV LV rating might have an insulation level of LI200AC85/LI75AC35. This indicates the HV winding can withstand a 200kV lightning impulse and 85kV power frequency voltage, while the LV winding resists 75kV lightning impulse and 35kV power frequency voltage.
- For low-voltage oil-immersed transformers (e.g., 400V LV side), the insulation level is typically LI75AC35, with the LV side’s withstand voltage considered negligible due to its low operating voltage.
Insulation level is a critical safety parameter, as inadequate insulation can lead to arcing, short circuits, and transformer failure.
Connection Group Label
The connection group label describes the phase relationship between the primary and secondary windings of a three-phase transformer. It is standardized using a clock notation system, which simplifies the identification of winding configurations.
Here’s how it works:
- The line voltage vector of the high-voltage winding is fixed at the 12 o’clock position (representing the “long hand” of the clock).
- The line voltage vector of the low-voltage winding is represented by the “short hand” of the clock, pointing to the corresponding hour mark (e.g., 0, 1, 2, …, 11).
- The connection group is denoted by a combination of letters (indicating winding connection type, such as Y for wye or Δ for delta) and a number (indicating the phase shift in clock hours).
For example, a Yyn0 connection group means the HV winding is connected in wye (Y), the LV winding is connected in wye (yn, with a neutral point), and the LV line voltage vector aligns with the HV line voltage vector (0 o’clock position). This notation ensures compatibility when paralleling transformers, as mismatched connection groups can cause destructive circulating currents.