Basic Concept: Rated capacity refers to the standard apparent power value of a transformer at its principal tap position. It is marked on the nameplate, typically in kVA or MVA.
Calculation: For a three-phase transformer, the formula is √3 × Rated No-Load Line Voltage × Rated Line Current.
Design Significance: It represents the maximum capacity that can be continuously output throughout its normal service life (e.g., 30 years).
Actual Output: The actual output capacity depends on the voltage and current under load. For inductive loads, the load voltage is usually lower than the rated no-load voltage.
Transformers with Off-Circuit Tap Changers: Can output rated capacity at the -5% tap position; output capacity must be reduced for positions below -5%.
Transformers with On-Load Tap Changers (OLTC): Generally can output rated capacity down to the -10% tap position; output must be reduced for positions below -10%.
Variable Flux Voltage Regulation Transformers: For applications like furnace or rectifier transformers, the rated capacity indicates the maximum output. Output at most tap positions is less than this rated value.
Essential Difference: Load capability is the actual capacity a transformer can deliver for a specified period without compromising safe operation or significantly accelerating aging. It can temporarily exceed the rated capacity.
Critical Temperature Limits:
Winding Hotspot Temperature Limit: 140°C. Exceeding this may release gases, jeopardizing safety.
Oil Temperature Limit: 115°C. To avoid impairing the oil's dielectric strength.
Lifespan Impact Temperature: A hotspot temperature above 98°C accelerates insulation aging, reducing normal service life.
Emergency Overload Operation: Short-term operation above rated capacity is permitted, provided the hotspot temperature does not exceed 140°C. The resulting sacrificed service life must be compensated for by subsequent operation below rated capacity. Note: Load losses increase, output voltage drops, and efficiency decreases in this state.
Auto-transformers:
Rated Capacity (Throughput Capacity) is greater than its Design/Inherent Capacity (Electromagnetic Capacity).
Output capacity comprises both directly conducted and electromagnetically induced power.
The low-voltage winding capacity is often lower than the rated value (e.g., labeled 100%/100%/50%).
Three-Winding Transformers:
Rated capacity is often expressed as percentages for each winding, for example:
100%/100%/100%: All windings can achieve the full rated capacity.
100%/100%/60%: The low-voltage winding can only achieve 60% of the rated capacity.
For transformers with multiple cooling modes, the rated capacity corresponds to the value under the maximum cooling capability.
Switching cooling modes requires corresponding capacity adjustment. A common three-mode example:
Forced Oil, Forced Air Cooling (OFAF): Can deliver 100% of rated capacity.
Oil Natural, Air Forced Cooling (ONAF): Output reduces to approximately 80% if cooling pumps stop.
Oil Natural Air Natural Cooling (ONAN): Output reduces to approximately 60% if both pumps and fans stop.
Important Note: The capacity percentages depend on the cooler design. Some designs require rapid load reduction to zero if pumps fail. Always follow the manufacturer's specific instructions.
A transformer's Rated Capacity is the benchmark for long-term continuous operation, while its Load Capability is a dynamic value considering actual operating conditions and time factors. Correctly understanding the distinction between the two, and comprehensively considering limitations from voltage regulation, winding configuration, and cooling conditions, is key to achieving safe, efficient, and economical transformer operation. Always refer to the specific product's technical specifications for selection and maintenance, and consult professional engineers when necessary.
