Efficiency, Cooling Systems, and Protection Methods of Oil-Immersed Transformers

Oil-immersed transformers are widely used in power transmission and distribution systems due to their reliability, long service life, and high efficiency. To ensure stable operation, three aspects are critical: efficiency, cooling system, and protection methods. This article explains how efficiency is calculated, compares common cooling methods, and introduces protection systems that safeguard oil-immersed transformers in operation.

Efficiency of Oil-Immersed Transformers

The efficiency of an oil-immersed transformer refers to the ratio of output electrical power to input electrical power, expressed as a percentage:

Efficiency = Output Power ÷ Input Power × 100%

Here:

- Output power = Rated capacity × Load factor

- Input power = Rated capacity × Load factor + Iron losses + Copper losses

In practice, the efficiency of oil-immersed transformers is typically above 94%, with modern high-efficiency models exceeding 98%.

Key Factors Affecting Efficiency

Load Factor: The ratio of actual output power to rated capacity. A higher load factor increases copper losses, reducing efficiency.

Operating Temperature: Excessive temperatures accelerate insulation aging and increase electrical losses.

Iron and Copper Losses: Iron losses occur in the core due to AC induction, mostly under no-load conditions. Copper losses occur in windings and increase with higher load and temperature.

Transformer Quality: Design, materials, and manufacturing processes significantly affect efficiency, even among transformers of the same capacity.

Cooling Systems of Oil-Immersed Transformers

Transformer losses are converted into heat, raising the temperature of the windings and core. Effective cooling is essential to maintain insulation performance and extend service life. Oil-immersed transformers use different cooling systems depending on capacity and application:

1. ONAN (Oil Natural Air Natural)

Natural oil convection circulates heat inside the tank. Hot oil rises, cooler oil sinks, and heat is dissipated naturally into the air.

Features: Simple structure, quiet operation, low maintenance, reliable performance.

Applications: Small and medium-sized distribution transformers.

2. ONAF (Oil Natural Air Forced)

Cooling fans are added to the radiator to accelerate heat dissipation, enhancing oil circulation.

Features: 30%–40% higher cooling efficiency than ONAN, supports increased transformer capacity.

Applications: Medium and large transformers. Fans are usually controlled automatically based on oil temperature and load, saving energy and reducing noise.

3. OFAF (Oil Forced Air Forced)

An oil pump forces oil circulation while fans blow air over the radiator, providing strong cooling.

Features: Very high cooling efficiency, but requires additional pumps and fan systems.

Applications: Large power transformers requiring enhanced cooling capacity.

4. OFWF (Oil Forced Water Forced)

The oil pump circulates oil through an oil-to-water heat exchanger, transferring heat to cooling water.

Features: Excellent cooling effect, with oil temperature approaching water temperature. Requires a dedicated water system with pumps and cooling towers.

Applications: Extra-large transformers in power plants or hydroelectric stations near water sources.

Protection Methods of Oil-Immersed Transformers

To ensure safe and stable operation, oil-immersed transformers rely on a combination of primary protection and backup protection.

1. Primary Protection (Fast-Acting)

Buchholz Relay (Gas Protection): Detects gas generated by internal faults. Minor gas accumulation triggers an alarm, while severe faults cause oil flow that activates tripping.

Differential Protection: Compares primary and secondary currents. An imbalance indicates an internal fault, triggering fast isolation. It is the main protection against internal short circuits.

Pressure Relief Device: Opens to release excess pressure during internal faults, preventing tank rupture. Simultaneously issues an alarm or trips breakers.

2. Backup Protection (Time-Delayed)

Overcurrent Protection: Provides backup for differential protection, tripping in case of external short circuits.

Overload Protection: Alarms when current exceeds rated value for a prolonged period, prompting load adjustment.

Zero-Sequence Current Protection: Detects external ground faults.

Temperature Protection: - Oil Temperature Indicator (OTI): Monitors top oil temperature, activates cooling or alarms. - Winding Temperature Indicator (WTI): Simulates hot-spot winding temperature for overheating protection, often with multi-level alarms and trip functions.

Oil Level Indicator: Ensures sufficient oil to cover windings and core, preventing exposure and potential insulation failure.

Relationship Between Efficiency, Cooling, and Protection

Efficiency, cooling, and protection are interdependent in oil-immersed transformers. Higher efficiency means fewer losses and less heat, reducing the burden on the cooling system. An effective cooling system ensures safe operating temperatures, prolonging transformer life. Comprehensive protection systems act as the final safeguard, detecting and isolating faults when efficiency drops or cooling fails, ensuring safe and reliable operation of the transformer.