Implications of reduced fault level and its relationship to system strength : a Scotland case study

Gordon, Samuel and Bell, Keith and Hong, Qiteng (2022) Implications of reduced fault level and its relationship to system strength : a Scotland case study. In: CIGRE Session 2022, 2022-08-28 - 2022-09-02, Palais des Congrès.

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The integration of Inverter Based Resources (IBRs), which displace traditional Synchronous Generators (SGs), presents new challenges to power system operation. SGs naturally provide voltage source behaviour, allowing them to be represented by a voltage source behind an equivalent impedance. In contrast with SGs, IBRs' behaviour during faults is typically like a controlled current source, and their responses to network faults are driven by their control strategies and the current limited capacity of the converter, resulting in a reduced fault current infeed during network faults. As a fault level can be seen as a measure of the equivalent system impedance, based on the 50 Hz Thevenin equivalent, it has also been used as a measure of "system strength", e.g. through the use of various Short Circuit Ratio (SCR) metrics. As a result, the terms "fault level" and "system strength" are sometimes used interchangeably, which may be largely valid for SG dominated systems. However, the fault level metric does not always capture all aspects of system behaviour in the context of high IBR penetration. Hence, particularly in systems which have a high number of IBRs relative to SGs, this paper argues that there is a need to make a clearer distinction between the low fault level issues and other challenges. The fault level remains a useful metric in the context of power system protection and to assess system characteristics during faults, such as the voltage depression. Meanwhile, other system challenges include a high voltage sensitivity during quasi-steady state system conditions, such as IBR control interactions. This paper has assessed fault levels in the Scottish transmission area of the Great Britain (GB) power system, an area which has a high number of IBRs and very few large SGs, some of which are close to retirement. These factors have motivated the system operator (SO) to launch a tender exercise – the Stability Pathfinder (SPf) Phase 2 – that is seeking new sources of fault current to connect to the system. Although the SPf has been motivated by closure of SGs, it is shown in this paper that, at many locations, equipment outages can reduce local fault levels more than the status of the available large SG units, and that fault level contributions from SGs are relatively localised. In addition, a sub-synchronous oscillation event in August 2021 suggests that challenges related to a high voltage sensitivity also exist in Scotland. Moreover, the fault levels during the event were not unusually low. Hence, considering the limitations of fault level as a metric for quasi-steady state voltage sensitivity challenges and that the observed fault levels can already be low and are not very sensitive to the anticipated closure of SGs, it is concluded that procurement of fault level alone may not be adequate to address some of the operational issues in the Scottish transmission system with further integration of IBRs. More work should be done on defining the exact metrics for quantifying the system needs in order to ensure security of supply in the most cost-effective manner.