supagridElectricity dispatch and pricing in the NEM are not only determined by bids and regional demand. The physical limits of the power system play a crucial role in shaping outcomes. Network and market constraints ensure that electricity is produced, transported, and consumed safely.
Understanding constraints is essential for interpreting prices, dispatch decisions, and why the "cheapest generator" is sometimes not dispatched.
What Are Constraints
Constraints are rules applied during dispatch to ensure the power system operates within safe limits.
Constraints include:
- Thermal limits on transmission lines
- Voltage and reactive power requirements
- System security requirements during outages or contingencies
- Minimum and maximum operating limits for generators
Constraints act as boundaries for the dispatch process. They prevent the market from instructing generators to produce electricity that the network cannot physically deliver.
Constraints explain why low-cost generators may not be dispatched and why regional prices can diverge.
How Constraints Affect Prices
Constraints can influence prices in several ways:
- Binding constraints reduce the flow of electricity between regions, creating price separation
- Generators in constrained regions may set the marginal price, even if cheaper generators exist elsewhere
- Congestion and limits can create volatility, particularly during high-demand periods or renewable fluctuations
In effect, constraints translate the physical realities of the network into economic signals.
The NEMDE Engine
All dispatch and pricing decisions are calculated by the National Electricity Market Dispatch Engine (NEMDE). NEMDE is the optimisation engine that solves the dispatch problem in real time.
Key features of NEMDE:
- Uses a linear programming model to determine the lowest-cost dispatch that meets demand while respecting all constraints
- Takes into account generator bids, ramp rates, availability, and network limits
- Solves a set of linear equations representing the network, generation, and demand constraints for every 5-minute interval
- Produces dispatch targets for each unit and calculates regional prices
NEMDE is effectively a real-time solver that balances economics and physics to ensure both system security and market efficiency.
How NEMDE Works in Practice
The process can be summarised as:
- Collect all generator bids and unit capabilities
- Incorporate network limits and interconnector capacities
- Apply system security and contingency constraints
- Solve the optimisation problem using linear programming
- Output dispatch targets for each unit and the resulting regional prices
Because the problem is solved mathematically, it ensures the most cost-effective solution while respecting the physical limitations of the grid.
Types of Constraint Impacts
Constraints manifest in different ways in the market:
- Transmission congestion: Limits flows between regions, leading to price separation
- Local network limits: Can reduce output from individual units or entire stations
- Security constraints: Maintain system stability under contingencies, even if it increases cost
Understanding these impacts is critical when analysing why prices and dispatch sometimes appear counterintuitive.
Not all constraints bind all the time. Only active (binding) constraints directly influence dispatch and prices in a given interval.
Why This Matters
Constraints are the bridge between the physical grid and market outcomes. Without constraints:
- Dispatch could instruct generators to produce electricity that cannot reach demand
- Prices would fail to reflect local scarcity
- System security could be compromised
Constraints make the market outcomes realistic and safe, reflecting both economics and physics.
What to Take Away
- Constraints ensure electricity is delivered safely and efficiently
- The NEMDE engine solves dispatch using linear programming every 5 minutes
- Binding constraints can create regional price separation and volatility
- Understanding constraints is key to interpreting real-time market behaviour and generator performance
With this understanding, the core NEM Concepts section is complete. You now have a foundation in how the market works, how regions interact, what units are, how dispatch and prices are set, and how constraints shape outcomes.
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