In the world of transmission & distribution there are a number of sites whose pollution severity is of such a scale as to represent the pinnacle of challenges when it comes to performance of electrical insulation. One of these is the Koeberg Insulator Pollution Test Station (KIPTS) near Cape Town, whose infamous environment of maritime and industrial pollution has for years challenged some of the most advanced insulator designs. Another is the east coast of Algeria, where the impact of petroleum refining and windy seas have combined to create a deadly mix of contaminants that deposit on insulators. Yet another ‘torture chamber for insulators’ is located halfway around the globe in a seemingly unlikely place – beautiful, pastoral New Zealand.
Almost from the instant it was built decades ago, the cable termination substation at a quiet inlet west of the nation’s capital, Wellington, has preoccupied engineers and maintenance staff at Transpower, the government- owned transmission network operator. The importance of the line that passes through this transition point only added to the urgency to win this battle.
The ± 350 kV Inter-Island tie is New Zealand’s only DC system and links the 220 kV grids of the North and South Islands. Beginning far to the south at the Benmore Hydroelectric Plant, the line travels 535 km to Fighting Bay on the shores of windy Cook Strait. It then crosses 40 km via undersea cables to Oteranga Bay, where it resumes overhead transmission for the remaining 35 km to the large Haywards Converter Station.
INMR traveled to the Oteranga Bay cable termination station to report on this unique ongoing battle against pollution.
According to Andrew Renton, power engineers in New Zealand have traditionally confronted a number of challenges not usually encountered in most other countries. Says Renton, a senior member of Transpower’s asset development engineering team, “ours is an isolated system which means that we cannot rely on other grids to supply us in the event of a serious outage. That makes us especially cautious when it comes to the design and operation of our network.” A good example of this vulnerability occurred in 1998, when the city of Auckland, the country’s commercial hub, suffered power disruptions for a full five weeks due to the thermal runaway failure of a critical cable link.
“These bushings are so valuable that money plays little role in purchasing or maintaining them.”
Another key issue, notes Renton, is that New Zealand is geographically far from almost everywhere, meaning that failed equipment cannot be quickly replaced. This has motivated a long-standing policy of stocking replacement inventory of virtually everything, from simple line hardware and components to very expensive HVDC wall bushings and even huge power transformers. Laughs Renton, “we’re literally a land of spares.”
Apart from these basic facts of life, Renton notes that the environment in New Zealand presents enormous challenges to electrical infrastructure due to corrosion, since the country is surrounded by vast stretches of open, wind-blown seas. Sulphur emissions from numerous geothermal sources as well as widespread use of fertilizers only make the situation worse. Corrosion has been a recurring problem for the copper pipe work of certain designs of 110 kV and 220 kV breakers and also affected alignment of contacts on disconnectors.
The pollution problem also impacts insulators, which in New Zealand are typically dimensioned for heavy to very heavy contamination, even though there is comparatively little industrial activity. For example, the flashover of a voltage transformer at the 220/110 kV Marsden Substation three years ago prompted plans for a major re-build as well as the application of RTV coatings on all the porcelains. “Now,” notes Renton, “all insulation at Marsden must be either composite or 31 mm/kV creepage porcelain that has been coated.”