Admittance Margin: A Guideline for Placement of Grid-Following and Grid-Forming Inverters Regarding Small-Signal Stability

The drive towards fully decarbonized power systems leads to the widespread replacement of synchronous generators by inverter based resources (IBRs). IBR installations bring new dynamics to the power systems and affect them in various ways when connected to different locations in the system. The use of short-circuit ratio (SCR) for assessing system strength appears inadequate, especially against small-signal stability such as sub/supersynchronous oscillations that are caused by interactions between IBR. This paper presents a new metric, namely Admittance Margin (AM), that indicates the maximum allowed change in the admittance observed at a bus for a mode of interest, where for changes less than the maximum, the mode will remain stable. AM is proposed as a companion to the recently proposed small-signal system strength metric, impedance margin ratio (IMR), and is tailored for use to assess small-signal system strength at buses which do not yet have equipment connected but where a resource development is planned. It can identify weak buses in the system, and guide accordingly the installations and placement of future IBR in the system. It is shown that introducing grid-following (GFL) inverters at buses with low AM should be avoided as it significantly destabilises the system, while on the other hand, introducing grid-forming (GFM) inverters at buses with low AM could significantly damp specific oscillatory modes. The usefulness of the AM metric is validated on a modified IEEE 14-Bus test system that contains multiple IBRs.