Results from the power flow analyses are fed back to the EMPS model to ensure that active power flow limits are not violated. Roughly speaking, Samlast resembles the market clearing process used in the Nordic power market today, whereas Samnett resembles the flow-based market clearing principle.
The EMPS model comprises a strategy evaluation and a system simulation part. Samnett and Samlast integrate with the system simulation part.
Operational decisions are evaluated for a number of hydrological years. In a first step, hydro and thermal production are determined for each time step in a optimization problem, based on the water values for each area (aggregate regional subsystem) obtained in the strategy evaluation part. Aggregate hydro production within each area is distributed among the available plants using a rule based reservoir drawdown model, which contains a detailed description of all plants and reservoirs.
Second, the results obtained so far are distributed to a detailed transmission grid model, and power flow studies are performed for each time step to compute losses and check for overloads. In case of overloads,
- Samnett generates linearized power flow constraints providing a functional relationship between active power injections (production – demand) in all areas. These constraints are added to the optimization problem.
- Samlast reduces available transmission capacities between areas according to a set of user-defined rules. The optimization problem is updated with new capacities.
The two steps are solved until all overloads have been alleviated.
Enabling detailed modelling of the hydro system, thermal power plants, wind power and the transmission grid, Samnett and Samlast are the most comprehensive type of models for power system simulations available in Scandinavia today. The range of uses comprises those traditionally covered by the EMPS model, in addition to identification of transmission grid bottlenecks, and studies of flows and losses in the transmission grid for a large number of realistically obtained power flow cases.
Either one or both of the two models are in use by most of transmission system operators in the Nordic countries. Samlast is currently used to compute marginal losses in Norway.