The two outages were on 19 and 23 January and affected the same group of almost 8,000 customers, extending from Clyde to Ettrick and Lauder Flat.
At the time of the outage on 19 January, one of the two lines that feed the Central Otago network from the Clyde grid exit point (where Aurora Energy takes electricity from Transpower’s national grid) was out of service to enable Transpower to undertake essential maintenance on their grid assets.
“This meant we were supplying electricity to the region via a single line, a line that is designed to do this,” said Matt Settle, Aurora Energy’s General Manager for network operations.
“Unfortunately, we experienced a fault on this line, at a location on the line where it had the most impact – cutting supply to all customers. Under normal operating conditions, with two lines operating, this fault would not have been noticed as the second line would have taken over supply. We know the outcome but the cause of the fault has not been identified. Having patrolled the line, we do know that there were no visible faults with the line itself and at this stage we suspect an outside interference such as a tree or animal contact.”
Mr Settle said it’s not unusual for lines companies to be unable to pinpoint the cause of every fault because some fault conditions occur only momentarily, making them difficult to detect.
“This can be from vegetation touching lines, or animals/birds having brief contact with the lines, where any evidence of this has usually disappeared by the time the lines are patrolled by the fault crews. The feeder is designed to isolate/trip the faulted area, to make that part of the network safe. In these circumstances, once we validate that it is safe to do so we will start restoring power.”
Mr Settle said there was more than enough capacity to supply all customers via a single line and that there was no connection between the planned maintenance being undertaken by Transpower and the fault on the remaining line; it was an unfortunate coincidence.
Both of the lines were in service at the time of the second outage on 23 January, however this time a fault occurred on the Aurora Energy line that had been out of service on 19 January.
“When we have two lines in service, a fault on one line should not cause an outage to customers because ‘protection systems’ switch the load onto the in-service line. On the 23rd of January, the primary protection system didn’t operate as planned, causing a trip to both lines and loss of supply to all customers. It is important to note that there was no heightened ‘safety risk’ at the time but the protection didn’t manage the fault and load transfer as intended. We have since corrected and tested the protection settings,” Mr Settle said.
“Although we didn’t find the direct causes of the two outages, to back up our detailed visual inspection of the lines we will undertake a specialist acoustic inspection to ensure we haven’t missed anything and this is being planned for mid-late February. We are also reviewing the animal protection mitigations in place, such as possum guards.
Aurora Energy thanks customers for their understanding and acknowledges any unplanned outages are inconvenient.
“We’d like to reassure people we are pulling out all the stops to review and learn from the recent outages so we can provide a level of service that meets their expectations,” said Mr Settle.