Improving Wind Power Utilization in System Dispatch Considering Output and Ramp Rate Dependent Generator Costs

Abstract

Making use of the steam heat storage in thermal generators enables them to operate in a "fast mode" to ramp up or down faster than regular, so as to better catch up with the fluctuations of wind power to improve system wind utilization. In such fast mode, generators have output dependent ramp rates and, distinguished from regular units, output and ramp rate dependent coal consumption costs. These fast generators cannot be properly described by using existing economic dispatch models, where generators usually have output dependent cost functions and constant ramp rate limits. This paper presents a new formulation and solution methodology of dynamic economic dispatch for wind-thermal power systems, to take into account ramping capabilities and costs of generators in their fast mode. In our model, the objective is to minimize a two-variable quadratic generator cost function depending on both output levels and ramp rates, and generator ramp rate limits are output dependent piece-wise linear functions. The model is solved by using existing quadratic programming methods, and is demonstrated by using numerical examples on the IEEE 30-bus system containing two 600MW thermal units with practical data. Results show that by using our model, unit ramping capabilities are better utilized in system dispatch to substantially save curtailed wind energy, and total generator costs are reduced.

Keywords: economic dispatch, generator cost, MW-dependent ramp rate, quadratic programming, wind utilization