An increased reliance on renewable energy could threaten reliability of the nation’s electric transmission grids by reducing the rotational mass and rotational inertia of on-line turbine...
Dynamic Scheduling: The Forgotten Issue
reality. Many utilities have used it for at least two decades, and it works. One system operator at the receiving end of a dynamically scheduled generator said that it operates "as if it was in our backyard."
We uncovered a range in the size of loads and generation that is dynamically scheduled. Generally speaking, the generation applications are for larger sizes than are the load applications. Several generation applications involved more than 1,000 MW, whereas many load applications were less than 100 MW. The number of points from which data need to be collected and telemetered may emerge as more important, especially from a cost standpoint, than the size of the load or generation. The loads typically involved several metering points, whereas the generation examples generally involved only the generating station itself.
None of our survey respondents offered clear rules concerning the maximum distance or maximum number of control areas over which or through which dynamic scheduling could be conducted. Our cost analysis suggests that the costs of transmission service and of loss replacement dominate the direct costs of dynamic scheduling. The need to compensate multiple control areas for transmission and losses would limit such applications of either dynamic or static scheduling.
Typically, data are telemetered at the same rate as that used in the control area's automatic generation control, or AGC, system once every two or four seconds. We learned of no special problems associated with the accuracy and reliability of the meters and
telemetry equipment used for dynamic scheduling. Indeed, most respondents indicated that problems associated with accuracy and reliability were no different from those associated with other systems for data collection and transmission, such as those used to record and transmit tieline flows and voltages to the control center. In a similar fashion, the control centers have prepared contingency plans that specify the actions to take when equipment fails.
The costs of dynamic scheduling appear quite low compared with both the cost of power and the costs of transmission and transmission losses. In addition, the many applications of dynamic scheduling we found suggest it is cost effective.
Although our findings are generally quite positive about dynamic scheduling, we need to be cautious about its widespread use, given its likely increase in the complexity of control-area operations and its possible adverse effects on reliability. We should be mindful of concerns about metering complexity, metering and telemetry failures, metering and computing errors, and possible confusion during emergency conditions over which entities are responsible for meeting which loads. These concerns, however, need not limit the cooperative and coordinated expansion of dynamic scheduling.
The number and complexity of dynamic-scheduling transactions will likely increase in the future as generation becomes more competitive, open access of transmission networks becomes a reality, pancaking of transmission rates is reduced, and market participants (customers, suppliers, marketers, brokers and others) seek more choices. t
Eric Hirst and Brendan Kirby are senior researchers in the Energy Division, Oak Ridge National Laboratory. Their research focuses on electric-industry restructuring. The work described here was sponsored by the Detroit Edison Co. and the