Protecting Power Transformers

Power transformers deal with high voltage electric power and there is always a possibility of fire incidents. Because of the fire risks and the important role power transformers play in supplying electricity to the community, they must have a reliable fire protection system in place.

Current fire protection systems for power transformers using sprinklers require a large quantity of water, which may cause a problem to their electrical function as well as creating water damage and environmental impacts. Cleaning up after the fire suppression is another problem. Power transformers contain hazardous materials, and any run-off water from the fire suppression activities has to be collected. It is a costly proposition to provide infrastructure to contain this run-off water.

The National Research Council of Canada (NRC) has developed a means of producing Compressed- Air-Foam (CAF) in a fixed pipe system. This system provides superior quality foam with uniform distribution and high momentum. In previous studies, NRC has proven by full-scale tests that CAF also has superior fire suppression performance compared to current foam or sprinkler systems. As well, CAF has low water requirements, thus minimizing the clean-up problem.

For these reasons, CAF may be an ideal solution for the fire protection of power transformers. To evaluate this potential, a study was recently carried out at NRC.

A mock-up power transformer was constructed which was a full-scale representation of an actual power transformer at Hydro Quebec’s Berri Sta- tion in Montreal. The mock-up (see above) was built to represent the front half of the transformer, including the oil reservoir and cooling fins.

The fire scenario envisioned in the test was an explosion in the main transformer body due to internal arcing, resulting in the blowing of a high voltage bushing through the top of the power transformer, or the rupturing of the transformer oil reservoir leaking oil onto the top of the transformer. The oil on top of the transformer would be involved in fire and would overflow the side and front of the power transformer, producing cascading fires as well as pool fires around the bottom of the transformer.

A CAF distribution system was developed for the test using several configurations to determine the most efficient way to distribute foam around the vertical and horizontal obstacles of the power transformer. The final CAF system selected incorporated two types of nozzles: a Large Flow Gear Driven Rotary (GDR) Nozzle and a Small Flow Turbine Action Rotary (TAR) Nozzle.

As a comparison, a test was also conducted using a water deluge system similar to the one at the Berri Station. It consisted of a ring of 21 nozzles, with a total flow rate of 910 litres/min.

The CAF system, either with 2 large GDR nozzles or with 3 or 4 small TAR nozzles, performed much better than the water deluge system with 21 sprinkler heads. The CAF system with 3 TAR nozzles using 1% Class A foam concentrate extinguished the test fire in 4 min 2 s, which is almost the same as the results of the water deluge system. However, the 3 TAR CAF system used less than 8% of the total water flow rate of the water deluge system.

The CAF system with 2 GDR nozzles using 2% Class B foam concentrate extinguished the test fire in 1 min 58 s, which is almost one half of the extinguishment time of the water deluge system. And, the water usage was less than 18% of the total flow rate of the water deluge system. The CAF system with 8 TAR nozzles using 2% Class B foam concentrate extinguished the test fire in 1 min 29 s, with far less water requirement than the water deluge system.

The study shows that a CAF system can provide the required fire protection for power transformers, more effectively and with much less water compared to a traditional water deluge system.

A.K. Kim, Ph. D. is a senior research officer with the NRC Institute for Research in Construction, Ottawa, Andrew. kim@nrc.gc.ca.This article is adapted from his paper NRCC-51235, December 2009.