Transformer Failure II

In an earlier issue of Reason, we examined the biggest equipment losses among FM Global clients within a five-year period (2008 – 2013). Transformer loss ranked third among the top five losses and cost FM Global clients a combined US$339 million dollars in lost revenue.  

While the numbers prove that transformer loss is a substantial risk for businesses, oftentimes business decision-makers deem the risk remote and unworthy of attention.

A recent indoor transformer failure at a 1.6-million-square-foot (150,000-square-meter) automotive-related manufacturing facility provides an eye-opening example of the risks involved in transformer maintenance and operation, and how quickly things can go awry.   

It was mid-morning when the piercing sound of fire alarms notified the guardhouse at the automotive facility, and the local fire service, that something was wrong. A maintenance manager happened to be in the vicinity of the building—an unsprinklered two-story electrical building—which contained four transformers on the top floor, each housed in a separate room. Despite seeing smoke and hearing a muffled explosion, the manager entered the building in an attempt to isolate the problem transformer (Unit A) and saw thick black smoke engulfing the area.

At this point, the main 63kV supply to the entire plant was interrupted and on-site facility firefighters began to deploy on the lower floor. Unfortunately, despite this rapid response by the maintenance manager and plant firefighters, the situation continued to deteriorate. A second explosion was felt, followed by a fiery burst of an oil-fueled flame.  

It took only 15 minutes for the local fire service to arrive and an hour for the blaze to be fully extinguished, but the damage was substantial.  

As is often the case with transformer failures, the problem began with an internal short circuit and electric arc. In this particular case, a mineral oil-insulated transformer suffered an internal arc and ruptured, resulting in the fire that destroyed an adjacent transformer and electrical cabinets.

In situations like these, the extremely high temperature of the arc rapidly heats up the insulating fluid, which is often combustible mineral oil. The result is a physical explosion of the transformer’s outer shell which occurs from the insulating liquid being rapidly heated, vaporized and decomposed, causing a pressure buildup.

In this particular instance, and others like it, the liquid is then ejected as a spray and, if ignited, forms a fireball capable of forcing open weak construction such as metal doors and hatches, as well as igniting cable insulation and any other combustibles in the room. Mineral oil leaking from the rupture point can also form a pool fire that may spread to adjacent equipment if adequate containment is not provided.

At the automotive plant, two access doors to the transformer room were blown out and the interior of both floors of the electrical building was heavily contaminated by smoke and soot, with spalling of the concrete. A second transformer (Unit B) became seriously damaged by heat. The fire spread through cable insulation and floor penetrations to equipment linked with Unit A and Unit B circuit breakers and attached to electrical cabinets on the ground floor. Equipment was also burned in the rooms directly adjacent to the Unit A transformer room.

And that was only the direct damage. The plant was down for five days until temporary equipment was rented and installed. Once production ramped back up, it reached approximately 80 percent of the norm within three days, and was back to nearly 100 percent of capacity within two weeks following the loss; this was accomplished by operating with limited outside power using diesel generators and mobile transformers, which incurred additional costs to bring on-site and operate.

The fire necessitated the rental of 15 large generators to power the plant, as well as the rental of several propane-powered forklift trucks, to temporarily replace battery-powered units that could not be reliably recharged.

The long lead time required to purchase and install replacement transformers and electrical equipment entailed keeping that equipment in place for weeks. Similarly, the lack of sprinklers and the cable penetrations and floor openings allowed the fire to spread to areas outside the room of origin.

It should be noted that the separation and enclosure of transformers, the presence of smoke detectors, and the prompt action by plant personnel in response to the fire, helped to mitigate damage.

“This incident shows that a fire involving a relatively small amount of mineral oil released by an indoor transformer explosion can cause millions of dollars in damage and lost production if not properly isolated from the surrounding occupancy,” notes Glenn Mahnken, former senior engineering specialist for engineering standards at FM Global.

According to Mahnken, it is important to understand the potential ripple effects that transformer breakdowns may have on production downtime. “Recognize that any transformer could fail and establish a contingency plan to expedite obtaining and installing a replacement, especially when high-value production operations would be impacted by the breakdown,” he says.

When designing rooms for mineral oil-insulated transformers, Mahnken advises to anticipate the potential for violent arcing scenarios that may lead to rupture of the shell and cause fire to spread outside the room, if the room is not properly sealed to contain the ejected oil and fireball following the rupture. “If feasible,” says Mahnken, “use a dry transformer, an FM Approved transformer or a transformer insulated with FM Approved transformer fluid.”

Preventing electrical problems before they start is also critical. Shuzhen Xu, senior staff engineering specialist in engineering standards at FM Global, explains that transformer problems can arise from many different factors. Trouble can start with poor handling of the transformer in delivery and during installation, which can potentially damage or weaken the internal structure itself, overloading a poor operational environment. In addition to internal contamination, external threats such as lightening strikes and grid disturbance can pose issues. “To detect and anticipate any resulting problems, as well as those associated with normal usage, it is important to institute a program of regular inspection and ongoing testing and maintenance,” Xu says.

Testing is particularly important because it can provide early indicators of internal problems before they become dangerous. “Preparing to mitigate aftereffects of transformer failures is part of a sound loss prevention strategy, but paying close attention to transformer function and performance through maintenance and testing can help avoid failures in the first place,” says Xu. “Every transformer has electrical protective relay devices designed to act within a second to isolate the transformer when faults arrive. It is critical that they are properly maintained and regularly tested.”  

For more information on transformer loss prevention, consult FM Global Property Loss Prevention Data Sheet 5-4, Transformers, or talk to your FM Global loss prevention consultant about site-specific advice and assistance.