## 3 Test Network
### Distance Protection Operation  
Only distance protection was analyzed, since it is the primary protection for 110-kV networks. The criterion for the operation of distance protection is the positive-sequence impedance, which was calculated using line voltage and current phasors measured on both sides of the faulted line [35]. The cases discussed in the previous section show a small error in calculated impedance from both sides of the faulted line, regardless of which FFC concept was employed. The error in calculated impedance was up to 20% in the \(R\)-direction and only up to 2.5% in the \(X\)-direction. Consequently, the correct operation of distance protection was expected, because the fault was supplied equally from both sides of the line. To test distance protection operation properly, two more challenging tests were assumed. In the first test, the SSC of the aggregated network supply was reduced to only 12 MVA and the line A-D was out of service, thus leading to the radial topology. Different fault types, i.e., three-phase (3-Ph), phase-to-phase (Ph-Ph), and phase-to-ground (Ph-G), were considered on the line C-D at the distances that correspond to 20% and 80% from the bus C. Furthermore, fault resistance of 2 \(\Omega\) was considered. Figure 8 shows the impedances seen by the relays C and D, i.e., \(\underline{Z}_{\text{C-D}}\) and \(\underline{Z}_{\text{D-C}}\), respectively. Furthermore, line impedance \(\underline{Z}_{\text{L}}\) is also shown, together with protection zones. Note that the first zone covers 85% of the line length, while the second zone is extended to the next line by covering 120% of the protected line length. The protection operation for faults in the first zone is instantaneous, while it is delayed for faults in the second zone to achieve selectivity [35]. As shown in Figure 8, the distance relay at side C would operate correctly in all cases, since the impedance \(\underline{Z}_{\text{C-D}}\) was seen in the first zone. The operation of the relay at side D would be delayed in some situations, due to the impedance \(\underline{Z}_{\text{D-C}}\) seen in the second zone. However, after the faulted line was disconnected from side C, the measured impedance of the relay at side D moved into the first operating zone in all cases, as indicated by the dotted arrow line. To avoid delays in distance protection operation, the introduction of communication with the appropriate logic scheme (tripping or blocking) is necessary [36]. In the last test, the SSC of an aggregated network was increased significantly to 1500 MVA. Again, the line A-D was out of service, representing the typical situation of a strong network that operates against a weak network with a significant impact on distance protection. Different fault types were assumed on line C-D at a distance of 20% from bus D. Again, fault resistance of 2 \(\Omega\) was considered. The distance protection relay at side D would fail to operate in cases of 3-Ph and Ph-Ph faults, as is shown in Figure 9, since the seen impedance \(\underline{Z}_{\text{D-C}}\) was out of all the protection zones. Only after the faulted line was disconnected from side C, the relay at side D calculated short-circuit impedance correctly, as indicated by the dotted arrow line. This would cause some unacceptable delay in the fault clearance; therefore, the build-in function \"Echo and Tripping in the Event of Weak Infeed\" should be implemented in such cases [36].