Assessing the ramping behavior and system impact of wind-based hybrid power plants

Abstract

Renewable energy sources have become a cost-competitive and green option for supplying power to the grid in recent years. Nonetheless, their variable nature poses a problem to the regular operation of the electrical grid by introducing severe fluctuations of large magnitudes and/or short-duration known as ramps. There is a lack of research in the literature on characterizing ramp events induced by wind-based hybrid power plants. The main research question of this study is how to characterize the ramping behaviour of wind-based hybrid power plants and what impact they have on the system. The application of the different methods to detect and assess the implications of ramps were presented in this thesis using wind and solar power based on the reference location.

Due to dependence on threshold values that vary across the literature and the limitations associated with calculating thresholds as a percentage of installed capacity, it was demonstrated that binary ramp definitions are not ideal and result in under-reporting. On the other hand, the wavelet approach extracts ramp events from the generation using statistically determined threshold values. As a result, the problem of under-detection of ramp events is mitigated. The proposed approach of "significant ramps" allows the evaluation of which ramp events are important and which are far less disruptive and may be ignored.

It was demonstrated that anti-correlation between wind and solar resources alone is not adequate to promise a smoother output as it does not provide sufficient information about ramp events. Anti-correlations at shorter time resolutions, such as 15 minutes or an hour, could be preferable. While seasonal anti-correlation may benefit national system adequacy, it does not benefit daily ramping events.

The optimal wind-PV capacity size for decreasing the total number of ramps was such that wind turbines filled the grid capacity, as solar power would result in extra ramps. It was observed that solar over-planting leads to a significantly increased number of ramp events, whereas wind over-planting results in a minimal change in ramp events. A penalty price was proposed to internalize the severity of ramp events, which could influence the choice between wind and solar over-planting. A solution was presented to mitigate ramp incidents in a hybrid power plant using a battery which was found to be more effective and/or more economical in minimizing ramps compared to over-planting.

The proposed "significant wavelet ramp approach" is shown to be a useful metric for characterizing wind-based hybrid power plant ramp occurrences. For a future in which variable renewable energy sources account for a substantial portion of the energy mix, it is proposed that demand information be considered when defining ramp events. More attention must be paid to power ramp occurrences, either by penalizing ramps or enforcing tougher grid codes. The ramp events must be considered at the sizing and development stage, with the possibility of including a ramp-mitigating battery strategy. A thorough examination of ramp events in hybrid power plants demonstrates the importance of minimizing and managing ramp events for both the system operator and the producer.