Dennis Matthews, 18 Dec 16 , 1 DECEMBER 2016.
The “separation event” was the disconnection of the Heywood interconnector into South Australia.
The following uses the same headings as the AEMO preliminary report.
A short-circuit in a Victorian 500 kV (kilovolt), alternating current (AC) transmission line connected to the Heywood Victorian-SA interconnector resulted in the SA electricity network being disconnected from the Heywood interconnector.
At the time of the “incident” the Victorian electricity network was highly vulnerable to disruption. One of the two circuits served by the Heywood interconnector had been taken out of operation for maintenance. To make matters worse, one of the circuits supplying the Alcoa aluminium smelter at Portland was also out of service. Like all aluminium smelters, the Portland smelter had a very heavy electricity demand (about 480 MW).
The vulnerability of the Victorian electricity network meant that the SA network was also vulnerable to an abrupt loss of 230 MW. Nevertheless, no measures had been put in place to immediately replace power supply from Victoria in the event of disconnection from the Haywood interconnector. As with the SA state-wide blackout two months earlier, there was more than sufficient generating capacity available in SA but it was not on standby.
A short circuit in the remaining transmission line in Victoria to the Heywood interconnector resulted in SA and the Portland smelter being disconnected and the shutdown of two wind farms in Victoria.
The “incident” in Victoria, together with inadequate contingency plans resulted in the loss of 230 MW to SA, BHP’s Olympic Dam project losing 100 of its 170 MW for 3 hours, Portland smelter being disconnected for 4½ hours and disconnection of two wind farms (Portland generating 3MW, and Macarthur generating 4MW) in Victoria.
2. Pre-event Conditions
“Immediately prior to the incident there were two planned outages.”
Use of terms such as “incident” and “event” is reminiscent of the nuclear industry’s avoidance of terms such as “failure” , “accident”, and “meltdown”.
“Planned outage” refers to deliberate disconnection of parts of the system for maintenance or repairs. Such deliberate disconnections should be permitted only if they do not expose the system to serious disruption and only if there is sufficient backup in case of a fault developing in the remaining parts of the system. For SA no backup was put on standby in the case of SA being disconnected to the Heywood interconnector.
One of the “outages” referred to was that one half of the Heywood supply to SA (a 500 kV busbar) was out of service. This left SA and Victoria vulnerable to a fault developing in the remaining half of the Heywood supply. The other “outage” was the Heywood to Portland 500 kV transmission line servicing the Alcoa aluminium smelter.
Both outages were given permission by the Australian Energy Market Operator (AEMO).
These two decisions left the aluminium smelter vulnerable to a fault developing in the remaining half of the Heywood transmission line in Victoria. There was no backup plan for maintaining supply to the smelter in this contingency.
At the time, SA was importing about 240 MW from Heywood in Victoria.
“A single phase to earth fault occurred on the Morabool-Tarrone 500 kV transmission line causing the line to trip out of service.” In other words, there was a short circuit in the only remaining transmission line in Victoria to the Heywood interconnector.
“It is believed that the line tripped as a normal response to this type of fault”. The short circuit caused the transmission line to Heywood to be disconnected (trip).
The short circuit was caused by the breaking of an electrical cable. The reason for the cable breaking was not known to the Australian Energy Market Operator (AEMO).
The “trip” of the transmission line left the Portland smelter still connected to SA, the power flow reversed so that instead of 240 MW into SA from Victoria there was 480 MW from SA to Victoria to supply the Portland smelter. A control scheme then disconnected the smelter from SA.
5 Operation of SA when Islanded
Islanded means that SA was on its own as far as power supply was concerned, in particular, it means that it was not receiving power from Victoria. In fact, SA was still receiving about 220 MW through the high voltage, direct current (DC), Victoria-SA, Murraylink interconnector.