The Semi-Scheduled Category

Page to be fleshed out, about the Semi-Scheduled category

(A) Summary of current arrangements

In short-hand, the operation of a DUID that is registered as Semi-Scheduled is part-way between the fully Scheduled category (with a very prescriptive set of requirements specified in the NEM Rules) and the Non-Scheduled category (where the plant can essentially run as it likes, without following AEMO’s dispatch targets).

A Semi-Scheduled plant essentially switches operations between two states:

State What the Semi-Scheduled plant is required to do
Semi-Dispatch Cap (SDC) flag is OFF

In practice this covers the majority of dispatch intervals.

When the SDC is OFF, the DUID can operate as it likes without needing to listen to* the AEMO’s Dispatch Targets.

* Whilst this is true, in terms of the way the rules are written (i.e. the Semi-Scheduled plant Conformance Status is automatically set to ‘Normal’ when the SDC flag is OFF), there is still a cost applied indirectly to Semi-Scheduled plant operating away from their Dispatch Targets during this period – which comes about through recovery of Regulation FCAS costs using the ‘Causer Pays’ method.

Semi-Dispatch Cap (SDC) flag is ON

Historically this might have been a very low percentage of dispatch intervals – however in practice this is increasing for some Semi-Scheduled plant as a result of both:
Case 2a)  Being ‘constrained down’ – due to transmission constraints being bound, affecting the unit
Case 2b)  Being ‘dispatched down’ – due to the increasing incidence of negative prices in dispatch (i.e. below the bid prices of some Semi-Scheduled assets).When the SDC flag is ON, then the unit needs to take account of the Dispatch Target when it operates.Readers need to understand that ‘taking account of the Dispatch Target’ in these cases is still different from what’s required for fully Scheduled plant:

1)  For a Scheduled plant, the Target means what the name suggests (i.e. the unit needs to meet the ‘target’ or it is deemed Non-Conforming):
(a)  This test applies if the unit is higher than the Target by a small tolerance (the operations procedures talk about ‘Triggers’); and
(b)  This test also applies if the unit is lower than the Target by a small tolerance (the operations procedures talk about ‘Triggers’).

2)  For the Semi-Scheduled plant, the Target is perhaps mis-named and would be better named as a ‘Cap’:
(a)  The Unit can be deemed Non-Conforming if the unit is higher than the Target by a small tolerance (the operations procedures talk about ‘Triggers’); but
(b)  The unit cannot be Non-Conforming if the output is below the Target (even if the difference is a large MW amount).

Because there are so many dispatch intervals in which a Semi-Scheduled unit can essentially ignore its Dispatch Target, there are many more larger deviations in ‘Raw Off-Target‘ experienced for Semi-Scheduled plant than there are for Scheduled plant.  In various articles on WattClarity (such as this article of 5th May 2020) we explore some of the potential implications of this, as the energy transition continues.


(B) Chronology of evolution

Will add something in here, as time permits.

Date Milestone
(re the Semi-Scheduled category)
13th December 1998
(Start of the NEM)

At the start of the NEM generators supplying the NEM were registered as Scheduled or Non-Scheduled. The Semi-Scheduled registration category did not exist.

September 2003

The first* wind farm (that’s visible in production data) commences operations in the NEM.

* re ‘first’ we mean the first wind farm that is visible in production data (Starfish Hill Wind Farm).

The earliest wind farms were (smaller and) Non-Scheduled, but others came later that were (larger and) fully Scheduled.  These pre-dated the commencement of the Semi-Scheduled category (which began in 2009, as noted below).

31st March 2009

The Semi-Scheduled category came into existence.


The effect this had on the number of rebids submitted by wind farms that existed at the time (i.e. those then registered as Scheduled) can be seen in the chart included in this article here.


At this time, utilising AEMO’s AWEFS forecast (which was outsourced at the time) to provide Availability forecasts for time horizons including:

Time Horizon #1)  In the ST PASA  time horizon (out 8 days into the future);

Time Horizon #2)  In the P30 predispatch time horizon (out until 04:00 tomorrow or the day after);

Time Horizon #3)  In the P5 predispatch time horizon (out eleven dispatch intervals into the future); and

Time Horizon #4)  In the dispatch interval time horizon (i.e. a forecast, at the start of the dispatch interval, of what was possible for the end of the interval).

A similar ASEFS system was deployed for when the first Semi-Scheduled Large Solar Farms came into existence.

28th March 2018

Trials of Self-Forecasting were commenced with the coordination of AEMO and ARENA – with ARENA calling for expressions of interest on 28th March 2018 from various parties.  That Media Release notes:

‘In partnership with the Australian Energy Market Operator (AEMO), ARENA is seeking to demonstrate wind and solar farms can provide more accurate forecasts of their output into AEMO’s central dispatch system.’

Further details, including some Knowledge Reports, are provided here on the ARENA website.

In particular it’s worth highlighting ARENA’s report from January 2020 ‘Short-Term Forecasting Trial on the NEM:  Progress Report (April to October 2019)’ which they said (p5/15) is …

‘to summarise insights and progress from initial reports submitted by the 11 participants of the Short-Term Forecasting (STF) trial that is taking place between March 2019 to mid 2021.’

31 May 2019

We released the Generator Report Card 2018 (GRC2018).

This widely read report analysed aspects of, and posed questions about, the Semi-Scheduled category.

September 2019

From September 2019, suitably approved Self-Forecasts were allowed to substitute* for the AEMO-developed forecasts in AWEFS and ASEFS.

*  It’s important for readers to note that these Self-Forecasts only provided an alternate forecast for Time Horizon #4 (i.e. within the current dispatch interval), whereas AWEFS or ASEFS continued providing inputs for the other 3 x Time Horizons.

15th December 2021

On 15th December 2021 we released GenInsights21, which explored and discussed several aspects of the Semi-Scheduled category including as follows:

1)   With respect to the Semi-Scheduled nature of their operations …


1)  With respect to auto-bidding …


3)  With respect to Self-Forecasting:

(a)  In Appendix 6  within the report, we explored in some detail; and

(b)  We also discussed self-forecasting (summarising from the Appendix) in Key Observation #15 (of 22) within Part 2 of the GenInsights21 report….

 … we’ve subsequently chosen to publish that Key Observation in its entirety here on WattClarity.

23rd Nov 2022

As noted in this article at the time, AEMO began to use its improved ASEFS and AWEFS forecasts.

Coincident with this:

1)  the AEMO paused any new suppressions for Semi-Scheduled plant that failed the ongoing self-forecasting ongoing assessment; and

2)  notice was given that this ‘Grace Period*’ would end on Tuesday 28th February 2023:

(a)  ~3 months after the upgrade.

(b)  which has almost completely elapsed now.

* note that the term ‘Grace Period’ is our term, not the AEMO’s.

Tue 16th Feb 2023

We released GenInsights Quarterly Update for Q4 2022, which looks specifically at the period covering 1st October 2022 to 31st December 2022 (so including the period since 23rd November 2022).

Tuesday 28th Feb 2023

AEMO 3-month ‘Grace Period’ following the upgrade to ASEFS and AWEFS ends.


Future? There are other changes to the Semi-Scheduled category being explored by some parties.


Will add more here, as time permits.

(C) Other useful references

On this WattClarity ® site, any articles tagged with ‘Semi-Scheduled Generation’ can be found here.

Will add in other links to useful references as time permits…