Summary of reliability forecasting in the NEM

I’ve written this article as a guide to the different reliability forecasts that AEMO undertakes in the National Electricity Market (NEM). These forecasts help AEMO identify supply shortfalls, and plan for the future. For example, AEMO’s annual Summer Readiness Plan draws on the outputs of most of the models described here.

This article is under active revision. My plan is to continue to tweak and expand on this content as I continue to learn more about these models. At the moment, this article covers the following topics:

• LT PASA
• MT PASA
• ST PASA
• EAAP

Long Term Projected Assessment of System Adequacy (LT PASA)

Purpose

• Published annually looking out 10 years as part of the Electricity Statement of Opportunities (ESOO)
• The forecast is used to identify reliability gaps in the first five years (which helps inform the RRO)
• The final five years indicates whether there will be any future gaps in reliability
• Informs the market on the need for new investment
• Helps AEMO determine whether it needs to procure additional reserves

Inputs

• Demand traces
  • Two scenarios: Probability of Exceedance (PoE) 10 and 50 (expect to exceed every 10 or 2 years)
• Weather traces – 8 historic weather patterns:Wind/solar generation traces
  • Rooftop solar generation traces
  • Generator ratings and forced outages
• Demand-side participation (DSP)
• Transmission constraint equations (e.g. transmission line thermal ratings)

Outputs

• ESOO report flagging breaches to the reliability standard
• The ESOO Plexos model and input traces is available for download from AEMO’s site

Medium Term Projected Assessment of System Adequacy (MT PASA)

Purpose

• Published weekly looking out 2 years
• Produce daily supply and demand forecasts over the next 2 years
• Compare expected unserved energy (USE) to the reliability standard (informs the RRO)
• Identify whether additional reserves need to be procured
• Inform market participants about:
  • Revenue opportunities
  • Shortfalls in supply
  • How they should engage with the contract markets
  • Their exposure to spot prices

Inputs

• Market participants submit their expected availability over the next 36 months to AEMO

Outputs

• AEMO produces daily supply and demand forecasts over the next 2 years that are published to their site

Short Term Projected Assessment of System Adequacy (ST PASA)

Purpose

• Published every 2 hours looking out 6 days following the next trading day
• Identify periods where short-falls are occurring
• Identify the level of reserves required by the market
• If there are not adequate reserves, AEMO can issue a Lack of Reserve (LOR) notice to incentivise market participants to respond (e.g. by delaying maintenance). LOR notices inform the market about the expected level of short-term capacity reserve and allows AEMO to potentially intervene in the market via the reserve trading provisions.

Inputs

• Demand traces
  • The Demand Forecasting System (DFS) automatically generates 10%, 50% and 90% POE load forecasts every half-hour
  • The DFS has multiple inputs including historic and real-time load data, historic and forecast weather data, non-scheduled wind generation forecasts, rooftop PV forecasts, and date information (e.g. weekday, holidays, etc).

Outputs

• Produce half-hourly supply and demand forecasts for the next week

Energy Adequacy Assessment Projection (EAAP)

Purpose

• Published at least once a year looking out 2 years
• Similar to the LT & MT PASA modelling
• Focuses on the impact of energy constraints on reliability, including
  • Water availability (e.g. droughts) – which impacts hydro and thermal (as its used for cooling) generation
  • Constraints on fuel supply for thermal generators

Inputs

• Three rainfall scenarios:
  • Low rainfall – data from 2006-2007
  • Short-term average rainfall – average rainfall over the past 10 years
  • Long-term average rainfall – average rainfall over the past 50 years
• Demand data is the same as the LT PASA modelling

Outputs

• AEMO publishes a report documenting the potential impact these constraints may have on the NEM

Sources

• PASA Outputs
• Projected Assessment of System Adequacy (PASA)
• Short Term PASA Process Description
• Medium Term PASA Process Description
• Modeling Reliability in the Australia NEM
• NEM Electricity Statement of Opportunities (ESOO)
• Load forecasting in pre-dispatch and STPASA
• Energy Adequacy Assessment Projection (EAAP)
• Summer Operations
• Forecasting and planning methodologies and guidelines