The Prescient Simulation Cycle


Prescient simulates the operation of a power generation network throughout a study horizon, finding the set of operational choices that satisfy demand at the lowest possible cost.

A Prescient simulation consists of two repeating cycles, one nested in the other. The outer cycle is the Reliability Unit Commitment (RUC) planning cycle, which schedules changes in dispatchable generators’ online status during the cycle’s period. The inner, more frequent cycle is the Security Constrained Economic Dispatch (SCED) cycle, which determines dispatch levels for dispatchable generators.

The RUC Cycle

The RUC cycle periodically generates a RUC plan. A RUC plan consists of two types of data: a unit commitment schedule and optionally a pricing schedule (when compute-market-settlements is True). The unit commitment schedule indicates which dispatchable generators should be activated or deactivated during upcoming time periods. The pricing schedule sets the contract price for expected power delivery and for reserves (ancillary service products). The RUC plan reflects the least expensive way to satisfy predicted loads while honoring system constraints.

A new RUC plan is generated at regular intervals, at least once per day. A new RUC plan always goes into effect at midnight of each day. If more than one RUC plan is generated each day, then additional RUC plans take effect at equally spaced intervals. For example, if 3 RUC plans are generated each day, then one will go into effect at midnight, one at 8:00 a.m., and one at 4:00 p.m. Each RUC plan covers the time period that starts when it goes into effect and ends just as the next RUC plan becomes active.

A RUC plan is based on the current state of the system at the time the plan is generated (particularly the current dispatch and up- or down-time for dispatchable generators), and on forecasts for a number of upcoming time periods. The forecasts considered when forming a RUC plan must extend at least to the end of the RUC’s planning period, but typically extend further into the future in order to avoid poor choices at the end of the plan (“end effects”). The amount of time to consider when generating a RUC plan is known as the RUC horizon. A commonly used RUC horizon is 48 hours.

The simulation can be configured to generate RUC plans some number of hours before they take effect. This is done by specifying a time of day for one of the plans to be generated. The gap between the specified generation time and the next time a RUC plan is scheduled to take effect is called the RUC gap. Each RUC plan still covers the expected time period, from the time the plan takes effect until the next RUC plan takes effect, but its decisions will be based on what is known at the time the RUC plan is generated.

The SCED Cycle

The SCED process selects dispatch levels for all active dispatchable generators in the current simulation time period. Dispatch levels are determined using a process that is very similar to that used to build a RUC plan. The current state of the system, together with forecasts for a number of future time periods, are examined to select dispatch levels that satisfy current loads and forecasted future loads at the lowest possible cost.

The SCED cycle is more frequent than the RUC cycle, with new dispatch levels selected at least once an hour. The SCED honors unit commitment decisions made in the RUC plan; whether each generator is committed or not is dictatated by the RUC schedule currently in effect.

Costs are also determined with each SCED, based on dispatchable generation selected by the SCED process, the commitment and start-up costs as selected by the associated RUC process, as well as current actual demands and non-dispatchable generation levels.