The output of wind farms in South Australia was constrained over two periods on Sunday on Monday after their combined output jumped above 1200MW and above new limits placed by the Australian Energy Market Operator.

The constraints were put in place for some 10 hours on Sunday, and another six hours early on Monday morning, both at times of relatively low demand, and at a time when the combined output of wind and gas power was significantly higher than state demand.

A ttimes, wind output in a state with some 1698MW of wind capacity jumped above 1400MW, but had to be constrained back to 1200MW, because there were not enough gas generators in the system under the AEMO’s new guidelines.

These new guidelines doubles the number of gas generators require to be online – in November it suggested only two were required, but in December in a separate document it lifted this to four.

This graph above (courtesy of the Climate and Energy College) shows the most dramatic intervention, at around 1.15am on Monday morning, when the second constraint was invoked, pulling the output of the wind farms back from more than 1400MW to 1200MW.

An AEMO spokesman confirmed: Peak wind output last night (2 July 2017) reached an unconstrained 1442 MW at 23:00. The 1200MW constraint limit was applied from 01:30 through to 06:50 this morning, Monday 3 July 2017.

The constraint applies to all turbines, but AEMO’s NEM dispatch engine reduces output of the wind in the most economical way, reducing from the most expensive first.

AEMO did not say which these were, but Dylan McConnell at the Climate and Energy College produced this graph which shows that the Waterloo, Bluff and North Brown Hill wind farms copped the brunt of it.

It had the effect of keeping prices high. Normally, when wind farms increase output, particularly at night, the pool price falls sharply, and sometimes goes into negative territory.

The trading rights of all three wind farms are owned by EnergyAustralia and AGL, the providers of much of the synchronous generation in the state. Neither company provided any comment by the time of publication. The Clean Energy Council also did not respond.

AEMO says the constraint – flagged in its National Transmission Network Development Plan (NTNDP) issued in December – is designed to maintain “power system strength, more specifically system fault current.””

As a result of that, it introduced new operational procedural changes, resulting in the requirements below:

When there is between 0 – 1,200 MW of wind generation, South Australia must have three capable synchronous generating units available and in the market to maintain sufficient power system strength.

generating units available and in the market to maintain sufficient power system strength. When there is more than 1,200 MW of wind generation, South Australia must have four capable synchronous generating units available and in the market to maintain sufficient power system strength.

“In instances where there is not the required number of synchronous generating units available and in the market within South Australia, intermittent generation will be constrained to maintain a safe and secure fault current,” it says.

As it turned out on Sydney, only three “synchronous” units could be encouraged to bid into the market, meaning that the wind output had to be constrained at 1200MW.

AEMO says the measures are only interim as it decides on the best path forward, and pending the introduction of new requirements on wind and solar plants. The introduction of battery storage will not directly affect the provision of fault current, but could serve to store the excess output.

“AEMO strongly believes this is a required and necessary initiative to maintain power system security during a period of rapid transformation,” a spokesman said in an emailed statement.

AEMO is assessing whether further Network Support and Control Ancillary Services (NSCAS) are required in the next five years. These are non-market ancillary services designed to maintain power system security and reliability, and to maintain or increase the power transfer capability of the transmission network.

In the 2016 NTNDP, AEMO identified an NSCAS gap to provide system strength in South Australia, and stated that the gap would be confirmed in 2017 following completion of more detailed analysis.

The industry is going through a period of rapid reappraisal of exactly how much security is needed in the system, particularly the level of synchronous generation, inertia and other grid services. These requirements are being rapidly re-assessed as the potential of new technologies and smart controls becomes evident.

“In instances where there is not the required number of synchronous generating units available and in the market within South Australia, intermittent generation will be constrained to maintain a safe and secure fault current.”