Swing equation modelling of GFL inverter and comparison of its damping and inertia with GFM inverter

Abstract

The increasing integration of inverter-based renewable generation into the network as Grid-Following inverters (GFL), combined with the reduction in traditional Synchronous Generation (SG) leads to a reduction in inertia and to instabilities. To alleviate this, researchers have proposed using a mix of GFL and Grid-Forming controllers (GFM). However, there is a lack of simple system-wide analytical models. This paper proposes a swing equation to describe the transient frequency response of GFL that has droop regulation of real and reactive power. The swing equation for GFL with droop has an inertia and a damping term as do SG and GFM swing equations. Simulations show that the use of swing equations for GFM and GFL is accurate to study the transient frequency stability following a loss of infeed. GFM and GFL damping are equivalent, and their sum allows prediction of frequency deviation. However, although SG and GFM inertia terms limit the initial Rate of Change of Frequency (RoCoF), the GFL inertia term is found not to contribute towards limitation of the initial RoCoF. Instead, the GFL RoCoF is seen to follow adjacent GFM RoCoF and worsens slightly when the GFL Phase Locked Loop (PLL) bandwidth is reduced.