Impact of converter equivalent impedance on distance protection with the MHO characteristic

With the rapid increase of Converter-Based Resources (CBRs) and the decommission of conventional Synchronous Generators (SGs), recent studies have found that distance protection can face significant challenges. One of the key factors that can impact the protection performance is the CBRs’ fault behaviour being different from SGs, leading to the different characteristics of their equivalent internal impedance, which will have a particular impact on distance protection with the memory-polarised Mho characteristic. This paper presents a comprehensive investigation of the characteristics of the equivalent internal impedance of CBRs with different control strategies, based on which, their impact on the memory-polarised Mho distance protection is analysed in detail. In the paper, the equivalent impedance of CBRs with virtual impedance-based Grid Forming Control (GFM) and balanced current injection-based Grid Following Control (GFL) are calculated and plotted against time throughout the faults. These are then compared with the internal impedance of a reference voltage source. It is found that, unlike the SGs, which can be considered as voltage sources and have constant source impedance, CBRs’ internal impedance have dynamic and time-varying characteristics, which are governed by the implemented Fault-Ride-Through (FRT) control strategies. Such characteristics will lead to dynamical changes in the expansion levels of the Mho protection zones, which could lead to increase risks of protection failure and/or maloperation. It is also revealed that, by better understanding the internal impedance characteristics of CBRs, the control strategies of CBRs could potentially be refined to mitigate the negative impact on Mho distance operation, thus presenting a potential solution to mitigate the risks of compromised performance of memory-polarised Mho distance protection.