Basic Concept Of Transformer Capacity
Transformer capacity refers to the maximum power that a transformer can safely transmit under rated operating conditions for an extended period. It is typically expressed in kilovolt-amperes (kVA) or megavolt-amperes (MVA). The capacity is determined by the transformer’s design and operating environment and serves as a key parameter to evaluate the transformer’s load-handling capability.
The units used are kilovolt-amperes (kVA) or megavolt-amperes (MVA), with the following conversions:
1 MVA = 1000 kVA = 1,000,000 VA.
Components of Transformer Capacity
kVA – This is commonly referred to as the transformer capacity, such as a 5000 kVA transformer.
The formula is: S=U×I
Where:
- S: Apparent power (transformer capacity, unit: kVA or MVA)
- U: Rated voltage (unit: kV)
- I: Rated current (unit: A)
Transformer capacity considers only the apparent power and does not directly reflect the power factor.
kW – Active power is the actual power used by the load.
kVAR – Reactive power is associated with the establishment and maintenance of the electromagnetic field.
Power triangle relationship
Formulas for Calculating Transformer Capacity
The primary-side capacity is equal to the secondary-side capacity:
√ S=V×I
Relationship formula：
The capacity is calculated as:
√ 
Where:
- V: Line voltage (unit: kV)
- I: Line current (unit: A)
Rated capacity of transformer
- Rated capacity
The maximum capacity of a transformer that can operate safely for a long time under rated conditions
- Rated conditions
Rated voltage, rated frequency, and temperature rise during full load operation do not exceed the standard
| Temperature rise limit | IEC standards (K) | IEEE standards(K) |
| Oil top | 60 | 65 |
| Winding average | 65 | 65 |
| Winding hot spot | 78 | 80 |
Example: a 5000 kVA transformer, when the temperature rise test, the temperature rise limit exceeds the standard provisions, it is considered that the capacity of the transformer under rated conditions can not reach 5000 kVA, may not exceed the temperature rise limit under 4500KVA, then the actual capacity of the transformer should be 4500KVA
Power Factor
Power factor (PF) is a dimensionless parameter that indicates the nature of the load. It is the ratio of active power (P) to apparent power (S), defined as: 
Where:
- P: Active power, measured in kilowatts (kW);
- S: Apparent power, measured in kilovolt-amperes (kVA);
- ϕ: Phase angle between current and voltage.
The typical range of power factor values is 0 to 1:
- Pure resistive loads: Power factor equals 1 (voltage and current are in phase).
- Inductive loads (e.g., motors, transformers): Power factor is less than 1 (current lags behind voltage).
- Capacitive loads: Power factor is less than 1 (current leads voltage).
Relationship Between Capacity and Power Factor
Transformer capacity is designed to handle the maximum apparent power and is not directly affected by the power factor. However, during actual load operation, the power factor affects the transformer’s active power output capability. The relationship is: P=S×PF This means that with a low power factor, although the apparent power remains constant, the active power decreases, potentially limiting the transformer’s utilization.
Example:
- If a transformer has a capacity of 100 kVA and the power factor is 0.8, then: P=100×0.8=80kW This indicates that the transformer can only supply 80 kW of active power.
Therefore, under low power factor conditions, adding compensation devices (such as capacitor banks) to improve the power factor can fully utilize the transformer’s capacity and reduce losses.
