By Jason Deign

Experts say there was no obvious reason why the blade snapped on a GE turbine owned by DTE Energy at Thumb Wind Park, Sigel Township. The accident in the US state of Michigan, occurred during a low-wind period and there was no immediate evidence of an impact.

Significantly, though, it was a rare enough event to make the local news. The Huron Daily Tribune reported just two other blade breaks in the past year, both in East Central Illinois with turbines made by GE, the market leader in the US and worldwide.

Matt Rowland, of the renewable energy insurer GCube, confirms that failures big enough to put a turbine out of commission are indeed rare events.

“When we look at our book of business, and that equates to roughly about 40,000MW worldwide, we generally get three to four turbine fires a year,” he says. “Those are generally total loss.”

Among other types of accident, a recent example was a turbine that got blown over during strong January winds in Devon, England.

“It is understood to be the first such reported incident in the UK, although blades have fallen from turbines in a small number of cases,” reported British daily The Guardian.

Design parameters

“In the same way that a normal building is designed to withstand certain wind, a wind turbine will only withstand its design parameters,” observes Rowland. “It looks bad because it is a wind turbine, which is supposed to produce energy from wind.

“But if it is designed to withstand 30 mile-an-hour winds and it gets a 40 mile-an-hour gusts then, in the same way that any other building will, it will struggle to withstand that if it is not designed to.”

Rowland says fires are the most common cause of ‘total loss’ claims. “We have had very few turbine collapses or tower collapses on our book,” he comments. “When they do happen they are quite expensive.

“The day-to-day things that cause us issues are gearbox issues, lightning strikes on blades or just general blade issues.”

Rowland calls these: “Our main bread-and-butter losses, much lower value but very ‘attritional’ in their nature. Over time the amount of claims can add up to a lot of money.”

He adds that the chances of blade losses have decreased with improved build quality in newer-model turbines. “The three-bladed design is not really an issue,” he states. “From an external perspective, they are pretty robust.”

Minor problems

Of course, instances where a turbine is completely put out of action are fairly extreme, and it is commoner for machines to suffer minor problems that require maintenance but do not result in an insurance claim.

“Fires are not that frequent,” says Aris Karcanias of BTM Consult, a part of Navigant Consulting: You are more likely to see bearings going, yaw and pitch systems going, control and communications systems going, and so on.”

At the module level, he adds, the main source of faults in current designs tends to be the power module, and within that the highest level of failures is found, in decreasing order, with frequency converters, generator assemblies, low-voltage switch gears and medium-voltage switch gears.

Converter problems alone account for between 12% and 15% of outages.

The next-most failure prone part of a turbine is the rotor module, where pitch systems can cause problems, followed by control and communications systems, where the yaw system can have a tendency to pack up.

The gearbox, frequently blamed for service issues, is no longer one of the main culprits for downtime, notes Karcanias. And overall, most turbines today are pretty reliable, with an average uptime of 97%.

Preventive maintenance

This uptime is being improved upon all the time, using two different approaches. One is to make the most of planned downtime to carry out preventive maintenance, which stops problems from cropping up in the first place.

The second is continued research and development into measures that can improve reliability.

An area that has received significant attention from manufacturers is the implementation of sophisticated sensor systems that can alert operators of potential problems, allowing maintenance crews to carry out remedial action before a major fault occurs.

These systems are now installed pretty much as standard and in many cases it will also pay an operator to retrofit them to older machines.

Finally, an important consideration when looking into turbine failure rates is the fact that a single turbine is only part of the power generation plant. “You should look at a wind farm as a generator as a whole,” argues Karcanias.

“If one turbine goes down you lose some power but the whole farm doesn’t go down.”

In contrast, stated The Guardian’s article on the Devon incident: “In 2011, a jellyfish bloom caused both reactors at Torness nuclear power plant to close down, taking 1,200MW of capacity off the national electricity grid.”