The Basslink subsea cable outage of 2015-16 was caused by Basslink (BPL) exceeding its design limit, which then degraded the cable, according to reports by two global experts provided to Hydro Tasmania.
The findings on the subsea cable, which provides energy and wholesale high-speed telecommunications services to Tasmania, were made by a power cable failure expert and a thermal modelling expert from international engineering consultancy DNV GL.
"The DNV GL analysis indicates that the cable had been operated by BPL in a manner that allowed it to exceed its temperature design limits during a number of periods in its service life. This overheating and subsequent cooling of the cable has resulted in degradation of the cable," Hydro Tasmania explained.
"DNV GL concluded that the cable failure was the probable result of electrical energy discharge within the cable as a result of polarity reversal and cooling shortly before the 20 December 2015 cable failure."
Hydro Tasmania, a state government business enterprise that is responsible for a majority of the state's energy generation, said the findings "vindicate" its decision to have the outage investigated.
"We're confident this expert investigation solves the mystery of the Basslink failure for all Tasmanians," Hydro Tasmania CEO Steve Davy said.
"BPL believed its cable could safely and reliably operate at 630MW for extended periods without overheating the copper and insulation and causing an unreasonable likelihood of failure. Unfortunately, they were wrong. The expert reports note that the cable, as designed and constructed, cannot meet the minimum operating requirements."
Basslink responded by saying it is reviewing the reports, but has refuted that the outage was caused by anything other than a chance occurrence.
"Basslink stands by the independent investigation that was undertaken by Cable Consulting International (CCI), one of the world's leading submarine power cable experts, who described the exact cause of the subsea cable fault as 'cause unknown'," Basslink said.
"Based on the findings of CCI, Basslink maintains the cable fault was a force majeure event."
For the short term, the two experts recommended BPL stick to an export limit of 500MW in safely and reliably contributing to Victoria's energy supply over the summer season to prevent the temperature design limits from being exceeded.
"Tasmania's energy security is not affected, because Basslink cannot import at higher than 500MW in any case. Electricity imports to help supplement on-island generation over summer can therefore proceed as normal, as required," Hydro Tasmania added.
Reducing DC voltage while the cable flow is ramping down and increasing the time for deionisation before a change in polarity from two minutes to five minutes are the medium-term recommendations to reduce stress on the cable.
"The experts believe the cable can safely and reliably operate above 500MW in future, if BPL adopts a new control system and extends the cable's 'rest period' between polarity reversals (between export/import and vice versa) from two minutes to five minutes," Hydro Tasmania explained.
The Australian Energy Market Operator has also been advised of the information, with the redacted reports to be released publicly at the start of 2018.
The Basslink Interconnector was down between December 2015 and June 2016, leaving many Tasmanians without energy and internet services during that period. The fault in the cable was discovered 90.5km from the Tasmanian coastline and removed and capped three months after going down.
The outage had lasted so long that the Tasmanian government got involved, with Minister for Information Technology and Innovation Michael Ferguson also reprimanding TPG for not buying additional capacity on Telstra's cables during the outage.
Telstra had commented that it is the duty of telcos to provide continued services for their customers despite outages, with TPG thereafter negotiating for extra bandwidth over Telstra's fibre-optic cable.
Basslink finally completed its cable jointing repairs in June last year following months-long delays due to excess water damage.
At the time, Basslink had said it would require six days of consecutive clear weather to repair each of the three joints; in May, it announced losing more than 20 days to inclement weather.
Updated at 4.05pm AEDT, December 20: Added comments from Basslink
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