Advisory on Fire Protection of Power Transformers

Contents

• Challenges in Fire Protection of Power Transformers in an Industrial Plant.

• Diverse Choices Available as per Codes.

• Pros & Cons of Diverse Options Offered by Indian National Codes.

• Recommendations of International Codes 05.

• Conclusions & Advisory.

Fire Safety of Power Transformers
Author is Fire, Life Safety, MEP Design & Engineering Consultant who is also an Electrical System as well as Fire system designer and expert, he has taken a Pragmatic View of National & International Standards and has Valued his opinion based on the criticality of this key & critical asset in any plant.

Challenges in Fire Protection of Power Transformers
The usual approach to getting a fire protection system for a power transformer is either need based, project-based or time based in a green field project or there were audit observations by an Fire & Electrical Audit company in the brown field project and the NC (Non-Compliance needs to be closed). The perception of going in for the system by Electrical System end user who is expert on electrical systems but is seldom well informed on types of Fire Systems availability in the market, Fire Safety Codes and what will best protect his critical asset, many a times he gets carried away by possibly incorrect recommendations given by a particular system supplier without doing a proper due diligence in weighing pros & cons of different systems available in the market. This paper does a thorough analysis and then leaving a decision to the end user to take a legitimate call.

Most industries like Power Plants, Cement Plants, Steel Plants, Textile Plants usually are based in remote locations and usually have only one grid supply as power is usually coming from a source which may be several kilometers apart and cost of another standby supply or transmission line is significant, which a business rarely invests for dual supply.

The cause of fires in Power Transformers are usually due to:
Short Circuit or External Flashover and is rarely due to Internal arcing in the tank as seldom there could be fire in tank as dissolved oxygen in oil is not sufficient enough to cause fire, and tanks being sealed will not have possibility of external oxygen supporting combustion.

The major apprehension of the electrical system user is how they can use water in transformer fires( but transformers do operate live in rainy seasons), which is sort of illusion as while transformer is on fire no systems are kept live and both grid supply and secondary supplies are isolated, though occurrence or probability of transformer fire is seldom, but the risk involves losing and gutting the transformer completely and associated risk of fire & explosion causing a catastrophe for risk to life and other asset protection. Power Transformer is a critical asset for every business as loss of the transformer is the main cause for business disruption risk and if this risk is monetized it will be a major risk for business as usually there is only unit installed.

Intense Submission:
If You are Buying a Fire Protection System for a Critical Asset like a Power Transformer whose importance is very significant for a Business Continuity Proposition You should read this advisory from start to finish and then take a informed decision.

Diverse Choice Available As per NBC Codes
There are 2 systems accepted by the National Building Code 2016 of India one is High-Velocity Water Spray System also termed as HVWS and th e other is Nitrogen Injection System. These systems are necessary and only applicable to transformers of 10MVA or more capacity or transformers having a n Oil tank of more than 2000 liters. Both these systems are so diverse and their application needs to be understood before selecting them as appropriate to the environment where the transformer is presently installed. Also, it needs to be seen if a Risk Assessment was carried out for the power transformer, all hazards were identified then an appropriate system was selected post-risk assessment and mitigation measures were planned for each hazard type.

There are many types of fires in power transformers it could be mainly due to :

• HV Bushing or HV lead Failures ( highest cause of fires).

• Transformer Cable termination failure, or oil filled cable termination box fires, failures ( second highest cause of fires).

• OLTC ( on load tap changer ) failure.

• Internal Fault causing Arc generation inside causing gas generation inside and pressure buildup which can exceed the pressure handling capacity of tank, causing oil tank rupture, which is rare, as less oxygen inside will not support combustion.

• Pool fires in transformer yard or soak pits catching fire.

• Transformer overused more than its life and never maintained.

It will be seen that one of the systems permissible in the NBC code is not able to address 50% of the risks related to hazards of fire in an open switchyard or MRSS /MPSS areas as power transformers are mostly installed in open.

HVWS System Covers 90- 95% of Power Transformers

Pros & Cons of the HVWS & Nitrogen Injection Systems

HVWS System Using Deluge Valves: The deluge system can immediately extinguish a fire externally to a transformer due to cable termination boxes or HV bushing or Porcelain Insulators by cooling the fire and the fuel and also by reducing the oxygen available for the combustion. It can also cool the transformer tank, oil pipes and cooler bank to prevent rupture form being caused by the fire and possible against explosion caused by heat from the fire. A HVWS system is a fixed fire protection system in which the pipe system is empty or sometimes filled with water if hydraulic trim is used and is always kept pressurized, until the a fire condition is detected and the deluge valve is operated to distribute pressurized water to the nozzles or sprinklers directing the water onto the protected object(s).

Various types of sensors can be used in the detection systems. The common ones include sensors for detection of: heat, smoke, infrared and/or ultraviolet radiation. Some common types of detection sensors use a low melting point material at the spray nozzles, where the melting of the sensor causes a loss of pressure in pipe downstream from the deluge valve. The loss of downstream pressure causes opening of the deluge valve and activation of the deluge system. Other types of activation can also be used.

The Nitrogen Injection System is more suitable for transformers used indoor as whenever you use inert gas suppression systems there shall be closed enclosure available as nitrogen is only effective in enclosed areas. Nitrogen can reduce oxygen to prevent combustion or fire happening if it is working in enclosed environment. In this system Inert gas cylinders are installed outside the building enclosure or an insulation panel around a transformer. Inert gas is injected into the enclosure by smoke detection signal and transformer fault detection signal.

Selection and combination of trigger signal depends upon each user’s practice and they are formed AND gate circuit for alarm evacuation. Transformer fault detection signal can be by a pressure relief device or Buchholz Relay. This system is designed to prevent tank explosion and fire during internal faults, usually these systems cannot address fire in bushing, cable termination boxes. It has been noticed that Nitrogen Injection System is not a full proof design as it only tries to address internal fires which are seldom but the fires due to bushing, cable termination boxes, pool fires which are more common do not get addressed by this system as very well done by HVWS system. Also the cost of Nitrogen injection system is significantly more than water based system and there are issues Pros & Cons of HVWS & Nitrogen Injection System like Nitrogen Stability, Gas leakage from cylinder, gauges, joints, hydro test of cylinders every 5 years which is a compliance requirement and which is never complied in reality.

Important: It shall be noted that International standards like NFPA 15 which advocate use of HVWS system for Openly located transformers so as to achieve:
a) Extinguishment of Fire
b) Control of Burning
c) Exposure Protection
d) Prevention of fire spread

NFPA 15 advocates use of only HVWS Systems for outdoor transformers as it addresses all potential risks & hazards and there is no mention of Nitrogen Injection System as part of NFPA 15 codes. Nitrogen Injection system meant for enclosed areas is not offering solutions for bushing fires, cable termination assemblies, OLTC, pool fires and fires in soak pits but it can only address requirements of Internal arcing within the tank which are seldom faults due to lack of oxygen.

NFPA & IEC standards also advocate further protection for transformer apart from HVWS Systems to further increase transformer defense by recommending following:

Transformers shall have fast, good quality, duplicate protection and preferably 2 cycle circuit breakers. This will reduce the impact of through faults and minimise the arcing energy within transformer if an internal failure does occur.

The tank design shall be very flexible so it can provide the volume expansion, but also withstand the pressure and deformation from the pressure generated by the arc for the time required by the protection and the circuit breaker to clear the fault and the arc.

Nitrogen Injection System in Use just addressing Internal tank fires

Conclusion & Advisory
HVWS System offers better protection abilities compared to Nitrogen Injection System as it can help address fire scenarios with bushings, porcelain insulators, cable termination boxes, conservator tanks, main oil tank cooling and exposure protection, pool fires, soak pit fires as against Nitrogen injection system which can only protect oil tank internally where major arc faults are necessary to cause fire and absence of oxygen prevents fire happening in this area. HVWS system is economical in cost by 15 – 25% as water is abundantly available than Nitrogen Injection systems and basic fire pump house infrastructure available can be always used with this system.