Report blames Basslink failure on ‘operational stress’

Hydro Tasmania says its internal investigation has shown the Basslink cable failed in December 2015 because Basslink Pty Ltd (BPL) exceeded its design limit, despite BPL claiming the failure was an unavoidable catastrophe.

The company says a report conducted by two experts concluded BPL operated the cable in a way that degraded it, and issues related to heating and cooling of the cable under operational stress “probably caused” the December 2015 failure.

The experts engaged by Hydro Tasmania recommended a short-term export limit of 500MW, while BPL considers and responds to the report.

“The experts are confident Basslink can safely and reliably operate at 500MW. Adopting that level will ensure safe and reliable operation of the cable to support Tasmania’s contribution to Victoria’s energy supply over summer,” the statement read.

“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.

“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 CEO Steve Davy says the findings vindicate Hydro Tasmania’s decision to conduct its own investigation.

“We’re confident this expert investigation solves the mystery of the Basslink failure for all Tasmanians,” he says.

“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.

“We received the expert reports in recent days, and have submitted them to BPL. The Basslink cable is the sole responsibility of its owner. It’s now incumbent on BPL to consider the reports and take the necessary steps to improve its cable to the standard and specifications promised.

“The expert investigation identifies the probable cause of the cable failure. But most importantly, it also identifies both interim and long-term solutions to ensure Basslink can operate as reliably as it was supposed to in future.”

Hydro Tasmania says it advised the Australian Energy Market Operator and operated a temporary trading halt while privately in possession of the information.

The company’s internal investigation was conducted by a power cable failure expert and a thermal modelling expert from the international engineering consultancy firm DNV GL.

“The DNV GL analysis indicates 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,” the report says.

“These changes make the cable susceptible to failure when it is operationally stressed, such as during polarity reversals and when the cable cools.

“DNV GL concluded 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.

“The recommendations include a short-term (and interim) measure of limiting the capacity of Basslink to 500MW (the continuous rating of the cable). This would mean that maximum export capacity of Basslink of 630MW would not be available for a period of time. This step would mitigate risk of temperature design limits being exceeded.”

Hydro Tasmania says the recommendation will have no impact on the 500MW import capacity of the cable, and so will not reduce existing Tasmanian energy supply or have any impact on Tasmanian energy prices.

“Medium-term recommendations that would reduce stress on the cable include reducing DC voltage when the cable flow is ramping down; and increasing the time for deionisation before a change in polarity (direction of flow) can occur from two minutes to five minutes. In combination, these steps would enhance the reliability of Basslink without impacting upon its capacity in any significant way,” the report states.

“Hydro Tasmania has proposed to BPL an interim arrangement whereby BPL can operate continuously at up to 500MW without an impact on the facility fee payments made by Hydro Tasmania. This will allow the cable to be operated safely and reliably while BPL considers the expert reports and their medium term recommendations.”

Hydro Tasmania says it will release the DNV GL report for public scrutiny “once commercially sensitive and confidential information has been redacted”.