FCAS constraints
Four fifths of all binding rows in the data are frequency-control requirements, and almost none of them cost anything. Understanding why keeps you from mistaking bookkeeping for congestion.
What FCAS is
The grid runs at 50 Hz. When a large generator trips, supply drops below demand and the frequency falls; when a large load trips it rises. Frequency control ancillary services are the megawatts held in reserve to arrest and recover those excursions. There are ten of them, and NEMDE procures all ten every five minutes alongside energy:
| Service | Raise codes | Lower codes | What it does |
|---|---|---|---|
| Contingency, very fast | R1 | L1 | Responds within 1 second: batteries and fast inverters. |
| Contingency, fast | R6 | L6 | Within 6 seconds. |
| Contingency, slow | R60 | L60 | Within 60 seconds. |
| Contingency, delayed | R5 | L5 | Within 5 minutes, holding until dispatch catches up. |
| Regulation | RREG | LREG | Continuous trimming under automatic generation control (AGC), not tied to any single event. |
A unit offers FCAS the same way it offers energy, in price bands, and its FCAS capability is a trapezium: how much reserve it can provide depends on where its energy output sits. That coupling is why energy and FCAS are co-optimised in one solve, and why an FCAS shadow price can leak into an energy price.
The requirement as a constraint
A contingency requirement is sized to the largest credible contingency: the biggest single generator (for raise) or load (for lower) that could trip, less the load relief the grid gives for free because motor loads draw less power as frequency falls. Written as a constraint it reads: the sum of enabled reserve across the covered regions must be at least the requirement. Regulation requirements are set empirically from observed frequency performance, and regulation can substitute for the 5-minute contingency service.
Here is the deck's worked example, F_MAIN++NIL_BL_R6, the mainland 6-second raise requirement for a Basslink trip:
Three things to notice. Every service term carries factor 1, which is the FCAS rule (a megawatt of reserve is a megawatt, wherever it sits). The Basslink term enters negatively: reserve delivered across the link from Tasmania is netted off what the mainland must hold itself, which is what the ++ in the name means. And the RHS is a demand term with coefficient minus 0.01: the load-relief allowance, one percent of mainland demand.
Why FCAS binds are nearly free
An FCAS requirement binds whenever the requirement is exactly met, and NEMDE meets requirements exactly because holding a megawatt more reserve than needed costs money. So FCAS constraints bind almost all the time, and the marginal value of one more megawatt of requirement is simply the price of the next megawatt of reserve, which in a well-supplied market is a few cents. A bind count is therefore not a cost. The exception is Tasmania's regulation requirement, which has few suppliers and occasionally prices in the hundreds or thousands of dollars.
The Portland smelter constraint is the cleanest illustration: F_I+NIL_APD_TL_L60, the NEM-wide 60-second lower requirement covering loss of the aluminium potlines,
Regional FCAS prices are sums of marginal values
Energy prices come from offer-band attribution (Chapter 5). FCAS prices do not: a region's price for a service is the sum of the marginal values of every FCAS constraint for that service that covers the region. The scope token in the name (I, MAIN, T, and so on) says which regions a constraint covers. Two consequences follow. FCAS prices are additive over constraints, so several small requirements stack. And a change in which scope family AEMO invokes changes the bookkeeping of prices and bind counts without any change in physics: when Basslink's ability to transfer FCAS was restored in July 2026, the split-market F_MAIN+ and F_T+ requirement families handed over to NEM-wide F_I+ ones, and total bound hours fell by more than a quarter while the grid did nothing different.
An FCAS constraint bound in 98% of intervals over two months with a median marginal value of one cent. The best reading is:
Two raise-regulation constraints cover the mainland in an interval, with marginal values $1.24 and $0.25. What is the mainland raise-regulation price?
In F_MAIN++NIL_BL_R6, why does the Basslink term carry a negative sign on the LHS?
True or false: a change in which FCAS requirement families AEMO invokes can change total bound hours substantially without any change in physical conditions.