Impact of Battery Ramp Rate Limit on Virtual Synchronous Machine Stability During Frequency Events

The fast growing penetration of renewable energy sources has led to increasing retirement of conventional power plants, which affects operational stability of electricity networks. Therefore, researchers have proposed virtual synchronous machine (VSM) techniques for inverter-based renewable generators to address the related instability issues. However, most of the studies consider a stiff or infinite DC source of VSMs, which does not reflect the reality of battery storage with limited ramp rates. In this article, a thorough investigation has been conducted to evaluate the impacts of VSM battery ramp rates on system stability during frequency events. Then, the mechanism of VSM instability with slow ramp rates has been analysed. The results show that the system may lose stability during the frequency events since the ramp rate significantly impacts the energy delivered during such contingency. The eigenvalues analysis has also been performed to investigate the impact of ramp rate limit on system stability. Potential solutions are proposed after analysing the mechanism of this instability, such as reducing the AC voltage control speed. This also reveals that VSMs with DC ramp rate restriction can not achieve a high AC voltage loop bandwidth as the traditional synchronous machines.