Optimal control of PV ramp rate using multiple energy storage system

In addition to random power fluctuation, the unpredictable high ramp rate is another uncertainty of Photovoltaic (PV) power output. It increases the requirement of both the amount and response speed of operation reserves at different time scale in the grid. The fast fluctuation of PV needs to be compensated by frequent ramp up and down reserves. Traditional generators will face heavy wear and tear to provide such support. While energy storage system (ESS) can provide fast bidirectional power support, ESS' response time and duration differs for various technology and material, and are suitable to provide reserve at different time scale. In this paper, a multiple ESS solution is proposed to suppress the PV ramp rate at different time scale. An optimal coordinated control strategy is developed to minimize the operation cost of the multiple storage system. Instead of suppressing ramp rate at individual time scale, the proposed control strategy coordinates multiple storage to control ramp rate at different time scale in one problem. In this way, the contribution of one time scale's ramp rate control to other time scale's ramp rate is considered. Using the coordinated control strategy, the required ESS capacity can be reduced. It can also minimize the PV and ESS system operation cost subject to grid regulation on ramp rate. The control algorithm has been demonstrated using a triple-input ESS example and its effectiveness have been studied. The results show the effectiveness of the proposed control in regulating ramp rate at different time scale.