Abstract
Approximations made in traditional day-ahead unit commitment model formulations can result in suboptimal or even infeasible schedules for slow-start units and inaccurate predictions of actual costs and wind curtailment. With increasing wind penetration, these errors will become economically more significant. Here, we consider inaccuracies from three approximations: the use of hourly intervals in which energy production from each generator is modeled as being constant; the disregarding of startup and shutdown energy trajectories; and optimization based on expected wind profiles. The results of unit commitment formulations with those assumptions are compared to models that: (1) use a piecewise-linear power profiles of generation, load and wind, instead of the traditional stepwise energy profiles; (2) consider startup/shutdown trajectories; and (3) include many possible wind trajectories in a stochastic framework. The day-ahead hourly schedules of slow-start generators are then evaluated against actual wind and load profiles using a model real-time dispatch and quick-start unit commitment with a 5min time step. We find that each simplification usually causes expected generation costs to increase by several percentage points, and results in significant understatement of expected wind curtailment and, in some cases, load interruptions. The inclusion of startup and shutdown trajectories often yielded the largest improvements in schedule performance.
Highlights
Many power systems worldwide face a sustained and significant growth of variable and uncertain generation, such as wind and solar, driven by concerns for the environment, energy security and rising fuel prices
We have shown how the performance of traditional energybased Unit Commitment (UC) formulations could be more affected by inaccurate system representations than by wind uncertainty itself
This was demonstrated by comparing alternative formulations of the dayahead hourly commitment problem, and evaluating the quality and accuracy of their schedules through simulation of real-time dispatch and quick-start unit commitment in response to wind and load variations with a 5 min granularity
Summary
Many power systems worldwide face a sustained and significant growth of variable and uncertain generation, such as wind and solar, driven by concerns for the environment, energy security and rising fuel prices. There may be a high amount of energy that is not allocated by the UC but which is present in real time operations, affecting the total load balance Disregarding these trajectories can result in inefficient realtime operations, and even endanger power system security [19,20]. The goal of this paper is to reveal and quantify the impact of the above theoretical drawbacks of traditional UC formulations This is done by following the day-ahead UC with a simulated real-time dispatch stage to check if UC solutions are able to supply demand in every real-time interval. Our results demonstrate that even such an ‘‘ideal” stochastic UC formulation imposes a hidden system inflexibility by incorrectly representing ramp capabilities and by ignoring the units’ startup and shutdown trajectories This leads to a failing to optimally exploit the actual system flexibility and inefficiencies in dispatch.
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