Rapid bidirectional power flow of supercapacitor energy storage systems through grid-tied inverters for improved renewables integration

The variable output of renewables presents integration challenges, especially at high levels of penetration. The coordination of multiple energy storage solutions can provide an effective approach to integrate these variable resources. Since supercapacitor energy storage systems (SESS) are designed to meet fast response energy demands, examination of the effect of frequency of power flow reversal on system operation is crucial, as is development of suitable SESS control algorithms. This paper presents experimental and modeling work carried out using a SESS integrated in a research scale lab grid through a high frequency grid-tied inverter. Operation of the SESS at high frequencies shows poor performance, with effective SESS energy rating significantly decreased for power flow reversals faster than 0.3Hz. Two approaches to control power flow from a SESS are developed and tested, with one aimed at power smoothing and the other aimed at limiting ramp rates. Both effectively reduce the time the system spends in a ramp failure, with the ramp rate algorithm most effective at reducing the time spent in a ramp failure state.