Resumen de Storage size for PV power ramp rate control
During the next years, a large share of PV power is expected to be integrated in worldwide electricity grids by virtue of the increase of its competitiveness against conventional energy sources earlier than expected. In order to achieve a large-scale integration of PV, grid-stability and power quality problems must be avoided, whereas the role of PV power in the grid must be shifted to becoming an active agent. In particular, short-term PV power fluctuations caused by clouds are raising the attention of operators of grids with high renewable energy penetration rates, leading them to impose ramp rate limitations. Such fluctuations are mitigated coupling modern energy storage systems such as batteries, but these devices still have a high cost and their use may hinder the feasibility of PV projects. In this context, the success of a project lies in the correct sizing of the battery and in creating advanced strategies and techniques to minimise the storage requirements.
This thesis addresses the sizing of energy storage systems used for mitigating short-term power fluctuations of PV plants to comply with a prescribed ramp rate limitation. The study is based on the fact that strong fluctuations scarcely occur, so battery power and capacity requirements must be minimised according to a trade-off between an adequate grade of compliance and a practical battery size. It analyses the effects of reducing the size of a battery designed to absorb every fluctuation by taking into consideration, both, the fluctuation occurrence and the penalties in case of non-compliance of the ramp-rate limitation.
A deep analysis assesses the relation between size reduction and ramp rate compliance, obtaining as result a model for predicting the probability of non-compliances with a reduced battery. Additionally, the size reduction analysis is applied to the particular grid code currently proposed for Puerto Rico, implementing three alternatives. The potential for reducing the size without falling into penalties is high, but it increases if part of the fluctuations can be absorbed by limiting PV inverters during upwards fluctuations. Therefore, an advanced control strategy has been created to team up PV inverter limitations with a reduced battery in order to improve the ramp rate compliance of the plant. Besides, PV array oversizing technique is studied as an alternative to mitigate part of the fluctuations. The results show that for the same storage size, a proper combination of these solutions increases the yearly PV production, creating new tools for selecting a battery with reduced power and energy capacity.
Finally, the reduced size battery control strategy is implemented and validated in a laboratory, addressing the accuracy and control response required for implementing the method in a utility-scale PV plant.
