The Nitrogen Injection Fire Protection System (NIFPS) is designed to prevent and extinguish fires in oil-filled transformers, thereby safeguarding critical power infrastructure. This system operates on a “drain and stir” principle, effectively mitigating fire hazards through a coordinated sequence of actions.
Working Principle of NIFPS
As it is an protection system it first needs to detect situations for which it will be activated. So, an NIFPS must have full proof capability of detecting the actual abnormal condition. Here, abnormal condition means fire hazard in a transformer or the condition of the event which can bring a fire hazard in the transformer. This system prevents the probability of fire hazard or extinguishes fire once it is started.
Fault Detection: The system continuously monitors the transformer for internal faults using protective relays such as Differential, Buchholz, Pressure Relief Valve (PRV), or Rapid Pressure Rise Relay (RPRR). In simple and most common logic a differential relay NO contact of the transformer is placed in series with a parallel combination of Buchholz trip and PRD relay MO contacts. This combination is then connected to an NO contact of master trip relay. Now let us understand the logic step by step. Suppose, differential relay operated and due to which the transformer gets isolated by master trip relay but Buchholz and PRD not operated. That means differential operated without any major oil surge hence the fault is not related to fire hazard. Again suppose the differential is operated, either Buchholz or PRD or both are operated but due to a defect in system master trip does not operate. Although there may be major fault but NIFPS should not be operated on a live transformer.
Fire Detection: Heat or fire detectors strategically placed on the transformer detect external fires. Sometimes it may so happen that there may be fire hazard in a bushing for some external fault. For these fault differential relay may not be operated. So, differential relay will not be operated in this event. Therefore, it is unnecessary to include differential relay contact here. Instead NO contact from all fire detectors are connected in parallel and that combination is connected in series with a parallel combination of Buchholz trip and PRD trip relay contacts. This entire logic circuit is connected with master trip relay NO contact. Upon detection of fire, and confirmation of Buchholz or PRD trip also on isolation (circuit breakers tripped viq master trip relay), the NIFPS is activated automatically.
Oil Draining: As soon as NIFPS is activated, it triggers quick-opening drain valve theteby releases a predetermined volume of hot oil from the transformer’s top layer. This reduces internal pressure. It is always recommended to drain oil from upper portion of the tank because hot oil is always at the top. Also, if oil is drained from bottom, there may be obstruction in drainage pipe due to oil slug of the bottom oil.
Conservator Isolation: Simultaneously, the Transformer Conservator Isolation Valve (TCIV) closes to prevent oil flow from the conservator tank to the main tank, averting potential escalation of the fire. Also, if the flow of oil from conservator is not stopped immediately, the drain volume and time will be unnecessarily increased.
Nitrogen Injection: Nitrogen gas is injected under pressure into the bottom of the transformer tank. The nitrogen injection is always from the bottom part of the main tank. This is because, when the injected nitrogen comes up through oil it creates a stirring effect, cooling the oil and lowering its temperature below the ignition point. Also, nitrogen occupies the space created by the drained oil, forming an insulating layer that separates the oil from oxygen, thus preventing combustion.
External Fires: In cases of external fires, the system ensures rapid suppression, typically within 30 seconds by spraying water jet from all sides.