Transformer Forced Oil Circulation Air Cooler

Transformer Forced Oil Circulation Air Cooler

When the transformer is running, the hysteresis eddy current loss of the iron core and the resistance loss of the windings will continue to generate heat, and if the heat accumulates, it may lead to accelerated aging of the insulating material, decrease in insulation strength, and even cause short-circuit failure.

The core function of the strong oil air cooler is: through the oil pump to force the transformer oil circulation, so that the hot oil flow through the cooler and air efficient heat exchange, after cooling the oil and then return to the tank to absorb heat, the formation of closed-loop cooling.

Applicable scenarios:

Power transformers with large capacity (usually 110kV and above, capacity above 10MVA), natural cooling or ordinary air cooling can not meet the heat dissipation requirements;

Transformers with high load and continuous operation, such as main transformers in substations, industrial large transformers, etc.

Strong oil air-cooler usually “cooling unit” as the basic module (multiple parallel use), a single group mainly consists of the following components:

Hot oil circulation system

Submersible oil pump: the core power components, forced extraction of hot oil from the bottom of the transformer tank (or from the upper part of the tank, according to the design) sent to the cooler, to provide the power of the oil circulation, the flow rate can be adjusted according to the load. The flow rate can be adjusted according to the load.

Oil flow relay: monitor the state of oil circulation, if the oil pump failure or oil blockage caused by oil flow interruption, it will send out a signal and trigger the protection mechanism (such as stopping the transformer).

Inlet and outlet oil piping: connects the transformer oil tank with the cooler, usually equipped with valves to facilitate isolation of a single group of coolers for maintenance.

Air cooling system

Heat exchanger tube bundle and fins: the core heat exchanger parts through which the hot oil flows, mostly copper or aluminum finned tubes (fins to expand the heat dissipation area), the tube bundle is arranged densely in order to enhance the efficiency of heat exchanging.

Axial fan: installed under the heat transfer tube bundle or side, forced inhalation of cold air and high-speed blowing through the fin surface, and the bundle of hot oil for heat exchange, heat away. The number of fans is configured according to the heat dissipation requirements (usually 2-4 for a single group).

Wind deflectors / deflector shields: optimize the airflow path to avoid mixing of hot and cold air and to ensure efficient flow of cold air over the fins.

Control and protection components

Temperature controller: monitor the oil temperature through the temperature sensor (such as platinum resistance) in the oil tank, automatically start and stop the fan or oil pump (such as the oil temperature reaches 65 ℃ to start, 45 ℃ to stop the operation);

Overload protection: the fan and the oil pump are equipped with overload, overcurrent protection, automatic power-off and alarm in case of failure;

Pressure gauge / Vacuum gauge: monitor the pressure of the oil circuit, to determine whether there is a blockage or leakage.

Workflow (closed-loop cooling cycle)

hot oil extraction: submersible oil pump from the hot oil area of the transformer tank (usually the bottom of the tank, due to the small density of hot oil will float, the bottom of the relatively low-temperature zone, part of the design from the upper part of the hot oil pumping) to extract high-temperature transformer oil;

Forced Oil Circulation: hot oil in the oil pump pressure into the cooler's heat exchanger tube bundles, heat transfer through the tube wall and the fins fins;

Forced air-cooled heat exchanger: axial flow fan Inhalation of ambient cold air, blowing over the surface of fins at high speed, heat exchange with fins, and heat will be emitted into the air;

Cold oil reflux: Cooled transformer oil flows out from the cooler, returns to the lower part of transformer oil tank through the pipeline, and then flows to the windings and cores through the oil channels in the oil tank to absorb the new heat, completing the cycle.

Compared with natural oil circulation air cooler (self-cooled or air-cooled), the core advantages of strong oil air cooler are:

High heat dissipation efficiency:

Forced oil circulation makes the speed of oil flow increase (usually 0.8-1.5m/s), which shortens the time of heat exchange;

Forced air-cooling avoids the inefficiency of natural convection, and the coefficient of heat exchange can be up to 3-5 times of self-cooled, and the power of single-group cooler heat dissipation can be up to dozens to hundreds kW.

Adaptation to the fluctuation of high load:

Can be Adapt to high load fluctuation: the cooling capacity can be adjusted through “group switching” (e.g. stopping part of the cooling unit when the transformer load is low, and putting it into all of the cooling unit when the load is high), which can not only save energy but also cope with the sudden overload;

Cooling speed is fast, and it can quickly respond to the temperature rise caused by the change of the transformer load.

Structural modularization:

multiple cooling units are installed in parallel (e.g., 6-8 groups), and a single group can be isolated for maintenance when it fails, without affecting the operation of the overall cooling system, with high reliability.