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 Description
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.

31st March 2009

From the 31st March 2009

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.

When in 2019?

In an effort to improve the functioning of the Semi-Scheduled category, suitably approved Self-Forecasts were allowed to substitute for the AEMO-developed forecasts in AWEFS and ASEFS.

Will flesh this out with a short history as time permits.

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…