Soon after the state-wide blackout, people in SA were subjected to unedifying performances from Federal politicians who tried to blame the blackout on the renewable energy industry, wind energy in particular. Months later, and in the face of factual evidence supplied by AEMO, these politicians still show no sign of repenting.
Dennis Matthews, December 2016 BLACK SYSTEM, SOUTH AUSTRALIA, 28 SEPTEMBER 2016.
3rd Preliminary Report, December 2016 Australian Energy Market Operator (AEMO)
Introduction
The “Black System” referred to by AEMO is what is more commonly known as the South Australian state-wide blackout. AEMO also refers to it as “the event”. The AEMO report contains considerable technical jargon and use of acronyms. Constant referral to a list of terms and abbreviations at the beginning of the report is necessary.
AEMO Executive Summary
According to the executive summary, the SA blackout was “initiated by the loss of three transmission lines involving a sequence of faults in quick succession”. These electricity transmission lines are the high voltage power lines that feed into the low voltage distribution system that services homes and many small to medium businesses.
The damaged transmission lines were in the mid-north of SA.
The sequence of faults led to many wind turbine electricity generators in the mid-north initially trying to continue to generate. Within 7 seconds, these initial attempts to “ride through” the problems caused by transmission line damage were followed by wind turbines deliberately shutting down (tripping), or decreasing their output, in order to protect them from serious damage. This caused a decrease of power generation by about 460 megawatts (MW). Prior to the transmission line damage, the total generated grid power available to SA was about 1830 MW. Domestic, off-grid, solar photovoltaic power was about 50 MW.
Although it seems reasonable that wind turbines should have an ability to shut down to protect against serious damage, according to the report “AEMO was not aware of the protective feature of these generating units”. Consequently, AEMO had not taken steps to replace the lost power in such a situation.
The loss of about 460 MW of generating capacity resulted in an attempt to import extra power through the Heywood, high voltage, alternating current (AC), connector with Victoria. Such connectors between states are essential for the operating of an electricity market. Without interconnectors there would be no National Electricity Market (NEM).
The Heywood interconnector was already supplying about 610 MW to SA and was unable to supply an extra 460 MW. The Heywood interconnector then also “tripped” in order to protect the interconnector from serious damage. This resulted in a loss of about 900 MW from Victoria to SA leaving only 330 MW of gas-fired (thermal) generation to cope with a demand for about 1830 MW. This thermal power was being produced at three power stations at Torrens Island and a 40 MW thermal power station at Ladbroke Grove in the south-east of SA.
Corrective measures such as disconnecting major demand (load shedding) at places like Roxby Downs (170 MW demand) were too slow to stop the remaining power stations from “tripping” to protect them from serious damage. This resulted in a state-wide blackout.
SA had considerably more thermal power than was on-line at the time of the blackout but they were not on standby or ready to take up shortfalls in power supply. Some thermal power stations, such as one of the relatively new Pelican Point power stations, have been mothballed because of low wholesale electricity prices.
The cause of the loss of the electricity transmission lines appears to have been two widely separated (170 km apart), 190-260 km/h, tornados which brought down more than 20 towers supporting the transmission lines. The report makes no mention of actual sightings of the towers being brought down by the tornados. There is an unanswered question as to whether the collapse of a tower brought down other towers in a domino-like effect or whether is was just the tornados.
In the late 1900’s almost all of SA’s electrical power came from fossil fuel (coal and gas) thermal power stations that produced “synchronous” alternating current (AC) flow. According to the report, the increasing proportion of generators (solar and wind) producing “non-synchronous” direct current (DC) is leading to a “lower resilience” of the electricity networks in SA.
Considering the performance of the electricity transmission grid, the Heywood interconnector, thermal power stations in SA, and AEMO, it would seem that “low resilience” is not a characteristic peculiar to solar and wind power.
Soon after the state-wide blackout, people in SA were subjected to unedifying performances from Federal politicians who tried to blame the blackout on the renewable energy industry, wind energy in particular. Months later, and in the face of factual evidence supplied by AEMO, these politicians still show no sign of repenting.
The following is a more detailed discussion of relevant chapters in the report.
2. Pre-event
AEMO procedures depend very heavily on the distinction between “credible contingencies” and “non-credible contingencies”. For credible contingencies, plans are put in place to take corrective measures, whilst for non-credible contingencies no corrective measures are considered necessary. A contingency may be switched from credible to non-credible as circumstances change but if the switch is left too late then any corrective plans may be too slow to be effective.
The loss of certain groups of wind farms is considered a credible contingency if they are “connected to the grid by a single transmission line” and the disconnection of a single transmission line “is always treated as a credible contingency event” but the loss of both Heywood interconnector lines “was considered a non-credible contingency”. Consequently, AEMO was unprepared for either the loss of multiple transmission lines in SA or for the total loss of interconnection through the Heywood interconnector.
AEMO had also assumed that all wind farms were capable of riding through multiple faults “provided the faults were within the size and duration specified in the generator performance standards”. AEMO did not know that some wind farms had limits on the number of successive faults that they could ride through.
The loss of “any single element (generator, transmission, etc)” is considered a credible contingency, but the “coincident loss of multiple generating units or transmission lines, are termed non-credible contingency events”.
The above classifications may be changed if circumstances change but no changes were made during the events leading up to the state-wide blackout. Bureau of Meteorology wind speed forecasts were upgraded during the day of the blackout but AEMO did not see the need to change its classifications or procedures.
3 Events Resulting in Black System
The loss of three transmission lines resulted in low voltage being experienced at wind turbines. The wind turbines responded as designed by remaining connected to the network and helping to restore the voltage. This “ride-through” response varied according to the turbine settings. Ten of the thirteen on-line wind farms activated their ride-through response three to six times.
“All wind turbines successfully rode-through faults until the pre-set protection limit applied to most on-line wind turbines was reached or exceeded.”
“If the pre-set limit was exceeded in the event, each wind turbine either disconnected from the network, stopped operating (remained connected with zero output), or reduced its output.”
“Five wind farms successfully rode through the faults, they did not recover to the pre-disturbance level immediately and took several hundred milliseconds to recover.” “All wind turbines that exhibited this behaviour remained connected and operational until the SA power system was fully lost”.
In regards to the shutting down and separation and of the Heywood connector from SA the report notes “unforseen separation and complete loss of the Heywood Interconnector has occurred five times in the 17 years since 1999”. Prior to the SA blackout, this amounts to an average of once every 4 years. Since the SA blackout there has been another instance of disconnection of the Heywood Interconnector on 1st December which means there were two disconnections in two months. Disconnection of the Heywood interconnector in 2016 has occurred at a rate 24 times the average in the 16 years up to 2016. The latest disconnection occurred as a result of a transmission cable breaking in Victoria plus a Victorian transmission system made vulnerable by simultaneous maintenance work on transmission lines and power stations in the vicinity of Heywood and Portland.
The report states that the key difference between the SA blackout and previous Heywood disconnections was the lower number of on-line thermal generators prior to the SA blackout. As mentioned earlier, there was sufficient thermal generation available at the time of the SA blackout, but it was not operating.
It is not commonly known that there are two SA-Victoria interconnectors, The Heywood, high voltage, alternating current (AC) interconnector and the Murraylink, high voltage direct current (DC) interconnector. The Murraylink DC connector is much more resilient than the Heywood AC interconnector.
There was negligible effect of the SA transmission line failures and subsequent loss of wind generator output on transmission through the Murraylink DC connector, which continued to transmit 114 MW until the SA system collapsed. The eventual disconnection of the Murraylink interconnector occurred because of AC connections at both the SA and Victorian ends of the interconnector.
In regards to preventing overloading of thermal generators in SA by disconnecting large electricity users (load shedding) the report states 1150 MW could have been disconnected but the disconnection mechanism was too slow; “the amount of load shed and the timing at which these loads were disconnected was not sufficient to avoid system collapse”.
There was “practically no changes in the overall operational demand for the last 10 seconds before the event” including the initial loss of the transmission towers in the SA mid-north.
7. Preliminary Recommendations
“During extreme weather conditions, more rigorous processes to be put in place to monitor weather warnings for changes in order to trigger reassessment of reclassification decisions where necessary.”
Reclassification refers to change from a non-credible to credible event. Such a change, if initiated several hours before the blackout, would have led to more SA thermal power stations being on-line, preventing overloading of the Heywood interconnector.
As with many of the recommendations, classification as a credible event involves a trade-off between maintaining integrity of the system and short term financial benefits to power producers. It would appear that the classification system was too heavily weighted towards the latter. If so, given the financial losses caused by the blackout, then it was very short-sighted.
The report also noted “the level of risks associated with wind turbine over-speed protection, while not a major issue in this event, needs to be considered more closely.” Over-speed protection refers to the well-known dependence of wind turbine output on the wind speed and to the fact that they may have to shut down if the wind speeds are too high or too erratic. The wind turbines rode through the extreme weather as well as faults in the transmission lines. Multiple faults eventually caused large changes in wind farm output.
The report also looked at the possibility of “constraining flows through the Heywood Interconnector”. Importation of electricity from Victoria is usually maximised for short term economic gain, due to the relatively inexpensive nature of electricity from coal-fired power stations. This may change after the closure of the Hazelwood coal-fired power station in Victoria. In any event, over-reliance on importing power through the Heywood interconnector has proved to be short-sighted and costly, not just during the SA blackout but also with the recent power disruption caused by transmission line failure in Victoria.
It is noteworthy that SA’s biggest electricity user BHP has flagged that it is negotiating a wholesale electricity contract to purchase electricity from one of the thermal power stations (Pelican Point) that was not on-line during the events leading up to the SA blackout. This may decrease the dependence of SA on imported electricity, provided existing power stations stay on-line.
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