Failure of a large power transformer is probably the worst nightmare scenario faced by a transmission system operator. Repercussions can be enormous. First is the initial outage, which in major urban centres can cost tens of millions per hour. And that does not even begin to take account of the hardships faced by customers, potential liability issues or the damage to an energy supplier’s image.
What’s more, if large transformers fail, they often do so in ‘spectacular’ fashion. According to a 2015 study by CIGRE Working Group A2.37, for example, more than one in every 8 large transformer failures resulted in a fire or explosion – also putting public safety and lives at risk. Needless to say, such events also attract a lot of negative publicity in the media.
Then comes the huge cost and effort to replace the failed component. Transformers of this size cost millions and, more pertinently, are often custom-designed and built to order. Lead times for replacements can be up to 24 months during which the power system still has to operate reliably, but now with one transformer less.
While the standard N-1 approach to network design means that a reduced power mode is possible, the system will operate less effectively and with elevated risk. Not only are the remaining components under greater stress, another failure could even take down the entire grid.
Clearly, everyone in the power supply business knows that whatever is possible must be done to mitigate risk of large transformer failures. Yet, notwithstanding this, the number of failures remains high. Press reports of large failures that make headlines in the U.S. alone occur about once a week. CIGRE estimated the overall global rate is around one major failure per 200 transformers per year.
The rate and consequences of failures place large power transformers among the top five types of equipment covered by insurance claims. As a result, underwriting investment for them is a growing challenge.
So, how can the industry reduce the rate and impact of large transformer failures?
The two most common causes of failure, according to CIGRE, are ageing and external short circuits, each accounting for some 12% of failures. While ageing is best tackled through proper asset management, ensuring that a transformer can withstand an external short circuit is mostly a matter of quality assurance.
There are essentially two known ways to verify a transformer’s short circuit withstand capabilities: design review and physical testing. The first uses calculations based on design parameters to estimate how a transformer will behave during a short circuit situation. The second involves exposing the transformer to the conditions it would face during a real-life short circuit and measuring its response. Both techniques have their place. Design review is invaluable in the design phase, helping ensure the transformer meets general requirements, but can struggle to give sufficient verification of performance during fault conditions. There are a number of explanations for this, including over-simplification of a complex and highly dynamic situation within the transformer, reviewing an incomplete list of sub-components and the inability to cover material or production faults.
On the other hand, full power short-circuit testing looks at the actual, complete transformer under realistic fault conditions. This gives a more accurate verification of the transformer’s quality and is borne out by reviewing test results, such as those carried out at KEMA Laboratories. Over the 20-year period to 2015 – covering almost 300 power transformer short-circuit withstand tests – the initial failure rate was around 23%. All these transformers were designed using methods similar to those used in design review.
It is especially interesting to note that among those transformers that failed initially, the pass rate in re-testing after modification by the manufacturer was close to 100%. That means full power physical testing helped manufacturers improve the quality of their transformers. Testing is also a vital learning phase for new manufacturers, by allowing lessons from one failure to be applied to improve quality for subsequent products.
All this is reflected in statistics from countries across the globe, which show the proportions of short-circuit related failures fall after introduction of short-circuit testing. For example, the advent of short-circuit testing in China during the mid-90s led to a reversal of what had been a rising trend of short circuit failures. Similarly, while the U.S. was reporting that short circuit accounted for 50-85% of transformer failures in the 60s, this figure was down to 20% by 1979.
Key to the value of short circuit withstand testing is the capability to test full-scale transformers under the sort of fault conditions they actually could face in service. Therefore, with the dawn of super-grids and as transmission voltages continue to increase, industry needs access to testing facilities that deliver more power and higher voltages.