Download convergence of south island reserve requirements 1. 2.

TRANSPOWER REPORT: Convergence of SI reserve requirements
CONVERGENCE OF SOUTH ISLAND RESERVE REQUIREMENTS
(11/04/16 – 20/04/2016)
1.
SUMMARY
Recently (mid-April 2016) there have been some large changes to South Island
(SI) reserve requirements for particular trading periods, between successive
solves in forward looking schedules.
SI reserve procurement is not typically co-optimised1 with energy procurement.
Therefore it is common for reserve requirements in forward looking schedules to
take a number of iterations2 to converge on a stable solution.
This is especially the case when changes in schedule input data are made that
affect the calculations of Net Free Reserve (NFR) values – such changes include
changes to offered generation and reserve, risk MW, system conditions or the
load forecast.
The impact of this has been exacerbated recently due to a transmission outage
which put the Ohau A station on lower security which meant it was classified as
a single ACCE risk (rather than a single unit within that station, as is standard
for hydro generation).
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2.
DETAILS
On the 18th of April 2016 there was higher-than-usual ACCE risk and instantaneous
reserve requirements in the South Island, and a step change in SI reserve requirements
appeared in forward looking schedules.
Higher-than-usual SI reserve requirements
A continuous outage on the OHA_TWZ_1 circuit (11 April 2016 06:30 to 19 April 2016
12:30) required the Ohau A (OHA) station to be modelled as a contingent event (CE)
risk. This risk was often larger than the usual SI CE risk of 125 MW.
Step change in SI reserve requirements
When a large change is made to reserve offer quantity or type, forecast generation,
system conditions or forecast load levels, the first market solve using the new
information may result in quite a large change to NFR values. As subsequent schedules
solve, the NFR values tend to converge to new values. This occurred on the 18th of April
and represented the normal process by which RMT-SPD3 iterations find the most efficient
procurement of reserve and energy. These iterations can be observed in the chart below,
which shows the NFRs calculated for the 09:30 trading period in the forward looking
Non-Responsive Schedule Long (NRSL):
1
This is because the binding risk is usually a manually entered AC system Contingent
Event (ACCE) risk.
2
Iterations occur between forward schedule runs; for example the Fast Instantaneous
Reserve (FIR) requirement in a trading period could be forecast to be 100 MW in the
schedule produced 4 hours out from real-time, but drop to 80 MW in the next schedule
30 minutes later (numbers are illustrative).
3
Reserve Management Tool, and Scheduling, Pricing and Dispatch.
1
TRANSPOWER REPORT: Convergence of SI reserve requirements
Average of NFR Six Sec ACCE
80
70
SI FIR ACCE NFR
60
50
40
30
Trading Period ID
2016041920
20
10
2
18-Apr-16
NRSL time (RMT solve time + 02:00)
Date
8:00
6:00
4:00
2:00
0:00
22:00
20:00
18:00
16:00
14:00
12:00
10:00
8:00
6:00
4:00
2:00
0:00
0
19-Apr-16
Time
Illustration of convergence in RMT-SPD as a result of a change to some of the schedule inputs.
Further information on the determination of reserve requirements
The OHA risk is different to many North Island CE risks which are known to SPD and are
co-optimised. If SPD sees a risk plant (say a large North Island generator) is requiring
overly expensive reserves to be purchased, it may back-off that generator if it
contributes to a lower interval cost. The modelling of the OHA risk is currently a manual
risk, so SPD cannot back OHA off to reduce reserve cost. A market system enhancement
under development will address this as part of National Market for Instantaneous
Reserves, scheduled for release later in 2016.
Reserve procurement in the SI throughout the duration of the OHA_TWZ_1 outage has
been higher than usual. This is expected behaviour, with RMT working as intended, and
was a direct result of the larger risk (OHA CE).
As the risk gets bigger you expect not only more reserves procured, but also an increase
in the amount of reserves procured compared to the size of the risk. This is due to the
faster fall in frequency following a contingent event when risk is higher, requiring
proportionally more MW of FIR to arrest the frequency fall caused by an additional MW of
risk. At times where FIR requirements are higher than usual, the solver can be
particularly sensitive to changes in the input variables.
The recent addition of secondary CE risks (which are in effectively added to the risk
quantity) has also increased FIR and SIR requirements in the South Island. This is
discussed in the Customer Advice Notice #2042142173 “Change to the Reserve
Management Tool (RMT) for the quantity of reserves purchased in the South Island”.
Please contact Market Operations for more information:
(04) 590 7470
[email protected]