Nitrogen Injection Fire Protection (NIFPS) System for Transformers

Fire Risk of Oil-Immersed Power Transformers

Oil-immersed power transformers are inherently at risk of fire. Continuous overheating of the iron core or transient high temperatures caused by internal arc faults (due to short circuits or overvoltage) can raise the oil temperature above 400 °C. At this point, transformer oil decomposes into various flammable gases, increasing internal tank pressure. When the pressure exceeds the mechanical limits of weak points such as bushings, welds, or explosion vents, hot oil and gas are expelled, ignite upon contact with air, and may lead to fire or even explosion.

Product Overview

The OR-NIFPS Series Oil Drainage and Nitrogen Injection Fire Extinguishing System (hereafter referred to as the System) is designed to automatically extinguish transformer fires by removing overheated oil and injecting nitrogen gas to suppress combustion and cool the oil surface below its flash point.

System Composition

The system consists of the following major components:
- Fire control cabinet
- Electrical control panel
- Oil drainage and nitrogen injection pipelines
- Temperature detectors
- Shut-off and control valves
- Nitrogen cylinders with pressure regulation

Interal Structure and Key Components of Nitrogen Injection Fire Protection System (OR-NIFPS):

Operation Principle

When a fault occurs inside the transformer, the gas relay trips due to a sudden pressure increase. If the internal temperature continues to rise, the thermal detector activates at its preset temperature (93 ± 2 °C), closing its circuit and triggering the electromagnetic actuator to open the fast oil drain valve, releasing the upper hot oil layer. After a short delay (0–20 s), the control valve opens the nitrogen cylinder. Nitrogen passes through the pressure regulator and injection line into the tank bottom, promoting oil circulation and cooling. This mixing eliminates the hot oil layer and lowers the surface temperature below the flash point, extinguishing internal flames automatically. The shut-off valve prevents oil from the conservator from feeding into the tank during discharge, avoiding fire escalation.

5. Technical Parameters

6. Product Advantages

The OR-NIFPS series replaces traditional electromagnetic and counterweight mechanisms with a mechatronic control structure, ensuring precise and reliable action. This innovation eliminates issues such as corrosion-induced jamming or malfunction, and allows reactivation without component replacement—significantly reducing maintenance costs and extending service life.

7. Notes and Limitations

This system is not applicable for extinguishing external fires on transformer surfaces or inside tap-changer compartments.

Here is the revised, professional version of the technical document, updated with standard international terminology (Nitrogen Injection Fire Protection System (NIFPS)) and enhanced engineering precision.

How Does a Nitrogen Injection Fire Protection System （NIFPS）Work?

1. Fault or Fire Detection and System Initiation

The Nitrogen Injection Fire Protection System (NIFPS) relies on a dual-fault or double-contingency voting logic to prevent inadvertent discharge. The system monitors inputs from electrical protection relays, mechanical sensors, and fire detection loops. Standard initiation logic requires simultaneous signals from both an internal electrical fault and an external or pressure-based threshold:

1.Primary electrical protection (Transformer Differential Protection or Sudden Pressure/Buchholz Relay)

2.Circuit breaker status (Trip feedback / Electrical isolation)

3.Fire detection network (Linear heat-detection cables or optical flame detectors)

4.Tank overpressure condition (Pressure Relief Device or Rapid Pressure Rise Relay)

Automatic discharge proceeds only after electrical isolation of the transformer is verified to ensure operator safety and prevent secondary electrical arc sustainment.

2. Rapid Oil Drainage (Pressure Relief)

Upon confirmation of initiation criteria, the control unit energizes the quick-opening oil drain valve.

1. A calculated volume of insulating oil is rapidly evacuated from the upper section of the transformer tank via a dedicated drain line directly into a designated accident oil pit or oil-water separator.

2. Rapid drainage immediately relieves internal hydrostatic and dynamic pressure, mitigating the risk of tank rupture or catastrophic structural failure.

3. The system is calibrated to lower the oil level below top-cover level while keeping the transformer core and active windings fully submerged to prevent thermal damage to active materials.

3. Conservator Isolation

Concurrently with oil drainage, the Conservator Isolation Valve (CIV / shutter valve) automatically actuates to a closed position.

1. This seals the oil path between the conservator (expansion tank) and the main transformer tank.

2. Shutting off this conduit stops the gravity-fed flow of fresh, oxygenated insulating oil from feeding the fault zone, isolating the main fuel reservoir.

4. Nitrogen Injection and Inerting

Following the controlled drawdown of the oil level, high-purity gaseous nitrogen (N₂ ≥ 99.99%) is released from high-pressure cylinder banks.

1. Passed through a precision pressure-regulating manifold, dry nitrogen is injected under controlled dynamic pressure into the lower region of the transformer tank through manifolded injection nozzles.

2. The injected nitrogen immediately expands into the upper headspace vacated by the drained oil, displacing atmospheric oxygen and establishing an inert blanket (O₂ ≤5%) above the oil surface to suppress surface flaming.

5. Oil Stirring and Rapid Thermal Cooling

Beyond oxygen displacement, the primary mechanical mechanism of bottom-injected nitrogen is forced oil circulation:

1. As nitrogen bubbles rise rapidly through the fluid column, they induce vigorous thermal turbulence and stirring within the remaining oil volume.

2. This forced convection rapidly mixes the hot upper oil layer with the colder bulk oil from the bottom of the tank, dropping the top-oil temperature below its flash point within seconds.

3. The combined action of rapid thermal suppression and upper headspace inerting prevents re-ignition.

6. Sustained Purging and System Monitoring

Nitrogen delivery is sustained over a predefined holding period (typically 10 to 30 minutes) to maintain active cooling and blanket integrity while emergency response teams secure the site. Throughout the discharge cycle, the NIFPS control panel continuously monitors critical parameters:

1. Nitrogen cylinder pressure and manifold distribution status

2. Actuator positions (Drain valve OPEN, Conservator valve CLOSED)

3. Oil level and tank pressure feedback

4. System supervisory loop alarms and interlock integrity

7. Summary of Operating Sequence

Fault/Fire Signal----->Electrical Isolation----->Rapid Drain & Conservator Isolation----->Nitrogen Injection----->Forced Cooling & Inerting----->Sustained Purge

Note: Specific valve opening delays, oil drain volumes, nitrogen mass flow rates, and hold times are derived from transformer tank geometry, oil mass, dielectric fluid flash points, and governing standards such as NFPA 15 / IEEE / IEC equivalents.