## 1 Introduction
### FRT Capability and DRES Reaction During Grid Faults  
#### 3.5.1 Proposed Definition and Its Justification  
It is well known that the protection systems in distribution grids were designed under the assumption of unidirectional power flow. The increasing penetration of DRES however changes this situation and poses a number of challenges for the proper operation of the legacy protection systems, such as: false or sympathetic tripping [87], protection blinding [88], increase or decrease in short-circuit levels [89], undesirable network islanding [90], loss of protection coordination [91]. To avoid those problems, the DSOs and the relevant grid codes requested-in the early stages of DRES penetration- the immediate disconnection of the DRES at the occurrence of a fault. Their reconnection was allowed after some minutes from the instant the fault was cleared. However, this situation caused a significant active power mismatch as the DRES penetration increased significantly. The revised grid codes and even the recent standards request for certain FRT capability of the DRES both in the LV and MV parts of the distribution grid [11; 12]. Apart from the FRT capability, the same standards now request that the DRES inject reactive currents (in both the positive and negative sequence) in linear proportion to the voltage drop sensed at the DRES terminals. The RMS value of the injected current is limited to the rated current of the generating unit. Although the reason for injecting reactive currents during faults is not justified in the standards and grid codes, we may assume that the reactive currents will help in maintaining a larger voltage at the DRES terminals, thereby increase the time it should remain connected for a specific FRT curve. However, the injection of reactive currents by DRES may cause the aforementioned protection system problems. Moreover, in distribution grids with relatively high \(R/\)X ratio, the injection of active currents would be more helpful in reducing the voltage drop during faults. A lot of research has been conducted in past few years in order to address the protection systems in distribution grids and microgrids under high DRES penetration (see Table 1). However, the research focused on the use of more advanced protection systems and methods, which implies additional costs for the DSO. These costs will eventually be transferred to all consumers/prosumers in the distribution grid. Further research has shown that, by suitably controlling the injected currents according to locational information within an islanded microgrid, the legacy protection system can remain fully functional and maintain its selectivity even under high DRES penetration [104]. However, to fulfill this task, the DRES need to have a rather extended FRT capability [37] being also able to inject active currents during the fault. Both the adjustable FRT capability and the injection of active currents when the primary source is not available (lack or wind or during the night for PVPPs) can be achieved by the implementation and proper control of a FSS at the DC link of converter-interfaced DRES. Therefore, \begin{table}
\begin{tabular}{l l l} \hline \hline  & \(\bullet\) & voltage-based protection [92] \\ modification of the existing protective philosophy by & \(\bullet\) & distance protection [93] \\ applying alternative conventional protective & \(\bullet\) & differential protection [94; 95] \\ methodologies & \(\bullet\) & directional overcurrent protection [96] \\ \hline Mitigation of the adverse behavior of the DRES in & \(\bullet\) & by applying fault current limiters [97; 98] \\ case of short-circuit faults & \(\bullet\) & by disconnecting the DRES, [99] \\ \hline Advanced methodologies (communication technologies \& adaptive systems), [100] & \(\bullet\) & adaptive protection [101] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Fault-clearing methods in distribution systems- a categorization.