Choosing the right oil-immersed transformer capacity is essential for stable and efficient operation in power systems. This guide provides a detailed explanation of key capacity parameters and practical selection strategies to help users eliminate selection confusion and optimize transformer performance.
Definition and Unit of Capacity
The capacity (or rated capacity) of an oil-immersed transformer refers to the apparent power it can continuously deliver under rated voltage, current, and frequency. It is measured in kilovolt-amperes (kVA) or megavolt-amperes (MVA).
1. Apparent Power
Apparent power is a combination of active power (useful work) and reactive power (to maintain the electromagnetic field). The transformer's load-carrying ability depends on both.
2. Meaning of Capacity
Greater capacity allows the transformer to carry more load. However, oversizing results in inefficiency, while undersizing may cause overload and shorten equipment life.
Core Capacity Parameters of Oil-Immersed Transformers
1. Rated Capacity
This is the standardized capacity provided by the manufacturer (e.g., 100kVA, 630kVA, 10MVA). It should align with actual load requirements, plus a 10%–20% margin for growth or overload.
2. Load Rate
The load rate is the ratio of actual load to rated capacity. An economic load rate of 60%–80% ensures minimal losses and maximum efficiency. Temporary overloads should not exceed 1.3 times the rated capacity and must follow IEC time limits.
3. Temperature Rise Limit
The temperature rise of the top oil is typically limited to 55K. At 40℃ ambient temperature, the oil temperature should not exceed 95℃. Overload increases heat, accelerating insulation aging.
4. Short-Circuit Impedance
Short-circuit impedance affects voltage regulation and fault currents. Larger transformers typically have lower impedance. For instance, a 10MVA unit may have 6%–8% impedance, while a 100kVA unit may have around 4%.
Key Points for Selecting Oil-Immersed Transformer Capacity
1. Identify Load Type and Characteristics
Resistive Loads: Lighting and heating have a power factor close to 1, making selection straightforward.
Inductive Loads: Motors and compressors have a power factor of 0.7–0.9. Include compensation and possibly oversize the transformer.
Impact Loads: Equipment like welders and cranes require larger capacity margins to handle fluctuations.
2. Estimate Initial Capacity Demand
First, sum all equipment power (kW). For inductive loads, divide by the power factor (e.g., 200kW ÷ 0.8 = 250kVA). Then add 10%–20% margin to ensure headroom and reliability.
3. Account for Environmental and Operational Conditions
High Altitude: Reduce transformer capacity by 5% for every 1,000 meters of elevation due to decreased cooling efficiency.
High Temperature: Strengthen cooling or lower load rates to prevent overheating and ensure safe operation.
Parallel Operation: Keep capacity ratio within 3:1 when using multiple transformers to maintain balanced loading.
4. Evaluate Efficiency and Scalability
Economic Load Rate: Select models that operate efficiently between 60%–80% load to balance cost and performance.
Future Expansion: Reserve additional capacity for future energy needs, including the integration of solar, wind, or energy storage systems.
Understanding and correctly applying transformer capacity parameters—such as rated capacity, load rate, temperature rise, and impedance—are essential for selecting an oil-immersed transformer that meets your operational needs. With proper planning and selection, you can ensure long-term system reliability, energy efficiency, and cost-effectiveness.
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