Grid maintenance faults exposed by report on fire incident that caused Heathrow disruption
In a stark reminder of the importance of regular maintenance, a catastrophic failure of a bushing in a high-voltage transformer at the National Grid substation in North Hyde caused a fire that disrupted power to over 70,000 customers and resulted in 1,300 flight cancellations at Heathrow Airport.
The paper-foil bushing, an older type of insulating component used in high-voltage transformers, is made of several layers of paper wrapped around alternating layers of metal foil, all soaked in insulating oil. This design enables the bushing to carry high voltages, like the 275 kV seen in the National Grid transformer. However, the insulating power of this design depends on the paper remaining dry and uniformly saturated with oil.
The history of paper-foil insulation in transformers dates back decades, particularly in older units like those installed in the 1960s and earlier. The insulating paper is highly absorptive and is kept impregnated with oil to enhance dielectric strength and cooling. However, this technology has been gradually replaced by more modern materials and designs, but many legacy transformers still depend on paper-foil bushings.
Regular inspection and maintenance are critical to prevent moisture or air bubbles trapped in the paper from degrading the insulation, leading to partial discharge or short circuits. Utilities must ensure that seals and oil quality are maintained to prevent moisture ingress. Aging bushings require monitoring as deterioration can lead to catastrophic failure.
In the case of the North Hyde fire, moisture entering the paper-foil layers caused the insulation to lose its dielectric strength, leading to a short circuit inside the bushing. The spark produced by the short circuit ignited the oil inside the bushing and the larger volume of transformer oil, causing fires and total transformer failure.
Recent analysis shows that all electricity network operators have underspent their budgets for replacing grid hardware. If air bubbles or moisture get into the absorbent paper, it loses its insulating power, leading to the tragic events at North Hyde.
The report finds that the highest "category 1" moisture reading was identified during inspection of the doomed bushing in 2018. According to National Grid's procedures, the moisture reading in 2018 should have resulted in the immediate replacement of the bushing. However, routine maintenance of the failing transformer was deferred in 2022.
The fire at the National Grid substation at North Hyde destroyed not just one transformer, but also its sister transformer next to it. The transformer installed at North Hyde in 1968 was insulated with a special oil called "bushing oil."
The failure not only resulted in costly equipment damage but also caused major power outages. National Grid has carried out a review of its inspection and maintenance processes in response to the report, but Ofgem has ordered an enforcement investigation into National Grid.
A key question for Ofgem's investigation into National Grid is whether money for maintenance and replacement was instead going toward profits, at the expense of customers and the resilience of the electricity supply. Other infrastructure companies, particularly water firms, are accused of "sweating" ageing assets to increase their profits.
The underspent funds are part of "network charges" that are set to rise in Ofgem's latest review. The incident at North Hyde serves as a stark reminder of the importance of regular maintenance and the potential consequences of neglecting it.
The paper-foil bushing's failure, due to moisture accumulation in the insulating layers, led to a short circuit and subsequent fire at the National Grid substation in North Hyde, stemming from league-like consequences of disregarding regular maintenance. Despite the ongoing trend of replacing older technologies with modern materials, the recycled paper-foil insulation in older transformers, like the one installed in 1968, continues to pose potential risks if not meticulously maintained.
