Frequently Asked Questions

Yes. Facilities with large supply contingency sizes are charged more for Contingency Reserve Raise (CRR).

CRR costs are recovered on a “causer-pays” model; those who cause the most need for the service (i.e. Facilities with large supply contingencies) are charged the most. See the WEM Metering, Settlement & Prudential Calculations for the formal details, or this article for some more intuitive diagrams. Note that while WEMDE operates on a least cost principle, it does not consider who those costs are charged to, or whether a particular Facility could increase their profit by running lower to avoid these costs.

Yes. However, there is no hard limit on allowed supply contingency sizes.

The Wholesale Electricity Market Dispatch Engine (WEMDE) calculates dispatch on a least-cost basis, balancing the size of the Largest Credible Supply Contingency (LCSC) with the Contingency Reserve Raise (CRR) requirement. When the system is insecure for the potential loss of  the LCSC, WEMDE has two options: it can either increase the CRR requirement or curtail the Facilities contributing to the LCSC. WEMDE will ultimately select whichever option minimises overall costs.

No. What is considered a credible supply contingency is not determined by how projects are registered as Facilities in the market, or how many connection points the project has.

It can be credible to lose multiple Facilities in the same contingency, for example if the Facilities are connected to the SWIS via the same single circuit line. Similarly, it can be credible to only lose part of a Facility in a single contingency, for example if a single Facility is split in half with each half electrically separate. If a Network Reinforcement Scheme disconnects several Facilities at once, this will also be considered a credible contingency, regardless of how each Facility is connected. In all these cases, Defined Contingency constraints are created to represent these contingencies to WEMDE. To avoid a whole project being considered as a credible contingency, ensure a single credible contingency (such as network fault or communications outage) can’t result it the loss of the whole project.

Defined Contingencies are special constraints that tell WEMDE how to calculate the size of various contingencies.

They can be found in the Operational Constraints Library by searching “LargestContingency”. Constraints with a “Constraint Type” of “Facility Risk” define the size the supply contingency associated with a single Facility, whereas constraints with a “Constraint Type” of “Network Risk” define the size of the supply contingency associated with multiple Facilities.

The below plots show the Largest Credible Supply Contingency (LCSC) sizes for the period from 1st Oct 2023 to 1st March 2025. Note that these contingency sizes include DPV shake-off as well as contributions from the Facilities themselves.

DPV shake-off is the portion of Distributed Photovoltaics (DPV) that disconnect following a network fault. DPV shake-off is considered part of the credible supply contingencies, so its contribution is added into Defined Contingencies.

Network faults cause a voltage disturbance to be propagated around the nearby network (see Behaviour of distributed resources during power system disturbances). Many DPV inverters installed prior to the introduction of the AS/NZS4777.2:2020 standard cannot ride through these voltage disturbances. There is also load shake-off that offsets some of the DPV shake-off. Hence DPV shake-off depends on network location, underlying load, current DPV generation, and the proportion of that DPV generation without ride-through capability. On a sunny low-load day, DPV shake-off can be as high as 150 MW in some parts of the network.