*4.1. Radial Networks*

In this part, an IEEE 9-bus test system and IEC MG benchmark have been carried out as a radial network. Figure 7 shows the flowchart for the implementation of NSTCC in radial networks. Afterwards, the short-circuit calculations in various modes and locations were achieved. In this section, the GA method has been used to obtain the optimal setting (TMS and the coefficient A) for OCRs based on STCC [25], NSS [25], and the proposed NSTCC in this paper. Furthermore, the NSTCC has been compared with the STCC and NSS in terms of reducing the relays' operation time, OT, and ensuring the CTI selectivity. For solving the OCR protection coordination problem, the GA technique is used in this section based on the presented network configurations in Figure 7.

**Figure 7.** Flowchart of the Implementation of the NSTCC in Radial Networks.

#### 4.1.1. The Radial 9-Bus Test Systems

The proposed network system is developed based on the Canadian Urban Benchmark 4-bus feeder distribution system [49]. As shown in Figure 8, the IEEE 9-bus consists of one DG and 10 OCRs as well as 2 directional OCRs, DOCRs, which are R8 and R10. A utility main source feeds this radial distribution capacity of short circuit = 500 MVA as well as the ratio of X/R = 6 and all lines with length = 500 m. The system is associated with the utility throughout a transformer of 20 MVA, 115 kV/12.47 kV. The simplified network, as shown in Figure 8, presents the OCRs and DOCRs. The plug setting (PS) and the current transformer ratio (CTR) for each OCR are stated as follows: for R1 and R5, the PS and CTR are 1.128 and 100/1, respectively, while 1.130 and 200/1 are the values of the PS and CTR for the R2 and R6. For all R3, R7, R8, and R9, the values of PS and CTR are 1.132 and 300/1, respectively. 1.135 and 400/1 are the PS and CTR values for all R4, R10, and R11. Lastly, the PS and CTR of the R12 are 1.140 and 600/1, respectively [25]. In the following subsection, the results of the proposed NSTCC, STCC, and NSS schemes are presented over different fault and power network model scenarios, as discussed in the previous section.

**Figure 8.** The IEEE 9-Bus MG system.

• Radial IEEE 9-Bus System without DGs: Mode 1 test results

This mode represents a conventional power network, which is fed only via the main utility feeder without DGs, as shown in Figure 8. The GA optimization technique is used to evaluate the performance of the NTSCC method and compare it with the conventional STCC and the NSS [25]. In general, the optimized values of TMS, the overall OT, and coefficient A in mode 1 for all OCRs are presented in Table 3. The acquired settings and the total tripping time were computed by utilizing MATLAB software and GA methodology. As shown in Table 3, the NSTCC approach achieved the minimum overall OT of all OCRs which equals 8.224 s, compared to the STCC and NSS methods which are equal to 9.352 and 8.848 s, respectively. In addition, the optimized value of coefficient A for the NSTCC approach has been chosen as approximately 5 for all OCRs as shown in Table 3, which is a suitable value for maximum current faults in this conventional power network case.


**Table 3.** The Overall OT for STCC, NSS, and NSTCC Curves in IEEE 9-Bus (DGs- Mode 1).

• Radial IEEE 9-Bus System with DGs: Mode 2 test results

Integration of the DGs in the network leads to raising the complexity of obtaining the optimal OCR coordination. This mode tests and evaluates the NSTCC on the network that is fed by all types of DGs, as illustrated in Figure 8. Table 4 shows the optimized values of TMS, the overall OT, and coefficient A in mode 2 for all OCRs. The total OT of the NSTCC in mode 2 of all OCRs equals 9.327 s, while the overall OTs for the STCC and NSS are 11.282 and 10.353 s, respectively. As with mode 1, an appropriate optimized value of coefficient A has been selected in mode 2 for the NSTCC approach, which is approximately 5 for all OCRs as illustrated in Table 4. As a result, the NSTCC scheme has recorded the lowest overall OT in Table 4 and optimal optimized value of the coefficient A.


**Table 4.** The Overall OT for STCC, NSS, and NSTCC Curves in IEEE 9-Bus (DGs- Mode 2).

• Radial IEEE 9-Bus System under the islanding condition: Mode 3 test results

The grid's operational way in this mode is called islanding mode. Applying the NSCC on the network with this mode shows the reliability and effectiveness of the proposed approach with a low fault current. Over the islanding mode, a comparison has been made between the proposed approach and other approaches in the Table 5, in terms of the optimized values of TMS and the overall OT. Similarly, the NSTCC approach achieves the minimum overall operational time of all OCRs compared to STCC and NSS approaches. The overall OT of all OCRs in mode 3 is 1.3728, 1.34, and 1.187 s for STCC, NSS, and NSTCC, respectively. It can be noticed that the optimized value of coefficient A in the Table 5 (islanding mode) for the NSTCC approach has been chosen to be approximately 2, which is suitable for detecting the minimum fault currents, and it is considered the key contribution of this NSTCC approach without delaying time during the optimization task.

**Table 5.** The Overall OT for STCC, NSS, and NSTCC Curves in Radial IEEE 9-Bus—Mode 3.


• Discussion of the Radial IEEE 9-Bus System results

In this section, the performance of proposed NSTCC, NSS, and STCC approaches on the radial IEEE 9-bus system over the different operation modes is presented. The overall OT for operation modes shown in Figure 9 was obtained by using the GA algorithm. The previous subsections show that the NSTCC approach reduced the overall OT of OCRs for all modes compared to NSS and STCC approaches. For example, the NSTCC reduced the overall OT in Mode 2 by 17.32% and 9.91% compared to NSS and STCC approaches, respectively.

**Figure 9.** The overall OT in Modes 1, 2, and 3.

4.1.2. The Radial IEC MG Test System

IEC MG benchmark connected to various DG technology types has been used to evaluate the NSTCC in this section. As shown in Figure 10, it has 4 DGs (two wind turbines and two synchronous generators), 5 transformers, and uses 15 OCRs as well as 5 DOCRs which are R1, R3, R5, R8, and R9, Refs. [49,50] give all details about IEC MG. The PS and CTR for each OCR and DOCR are described as follows: for R1, R2, R3, R4, R5, R6, R8, R9, R10, R12, and R15, the PS and CTR are 0.5 and 400/1, respectively. Whereas for R11 and R14, the PS is 0.65 and CTR is the same as the previous OCRs, which is 400/1. The PS and CTR values of the R13 are 0.88 and 400/1. Finally, for R7, the PS and CTR are 1 and 1200/1, respectively [25]. A short circuit on different lines was calculated for each mode in this study. The three-phase faults at different transmission lines in the MG are illustrated in Figure 10. In Figure 10, the DG is considered as a PMSG system where the contribution of fault current is low, similar to our case with the PV system, compared to the DFIG system, which is considered as high fault: around 7 times the full load. However, the DFIG condition will be protected by the first relay after the generator in this work.

These fault cases are: a fault on the line DL-5, named F1; a fault on the line DL-4, named F2; a fault on the line DL-2, named F3; a fault on the line DL-3, named F4; and a fault on the line DL-1, named F5. To optimize the TMS for the coordination of OCRs, GA optimization processes have been carried out to test the NSTCC using MATLAB simulations. For fault conditions, the primary OCR should isolate the fault firstly. If it fails to trip the fault, after allowable CTI, the backup OCR must be operated; it is assumed to be operated between 0.2 s and 0.5 s. Three operational modes have been implemented in this IEC MG.

**Figure 10.** IEC MG Benchmark (Large-Scale Network).

• The radial IEC MG Simulation Results in Mode 1

In this case, the main source is only connected to the IEC MG; however, all the DGs are off. For comparative purposes, the results of the TMS and the operating times for all relays that were obtained from the literature [29,36,51,52] are presented in Table 6. The authors in ref. [25] did not evaluate the IEC MG as a radial system. It can be noticed that the proposed obtained OT equals 2.42 s for the proposed NSTCC, which is a considerable reduction from those reported in the Table 6.


**Table 6.** The Overall OT for the radial IEC MG—Mode 1.

• The radial IEC MG Simulation Results in Mode 2

In this operational mode, the MG is connected to the main grid and the DG units. The results of the TMS and the operating times for all relays that were obtained from the

literature [29,36,51,52] are presented in Table 7. It can be guaranteed that the value of the coordination between the primary and backup relays is achieved by obtaining the lowest tripping time for the NSTCC among the approaches reported in Table 7. The total OT was equal to 4.69 s for NSTCC compared to 11.6 s as the lowest OT value obtained from literature, which equals a 60% reduction for using NSTCC. The coefficient A's optimized value equals approximately 4.8 too according to the maximum fault current in this mode.


**Table 7.** The Overall OT for the radial IEC MG—Mode 2.

• The radial IEC MG Simulation Results in Mode 3

In mode 3, the MG operates in islanding mode, in which the main grid is in off-grid mode and the load is supplied by all the DG units. The results of the OCR coordination obtained from the literature [29,36,51,52] are shown in Table 8. In this case, too, the proposed approach NSTCC outperformed the other approaches that are reported in Table 8. The proposed NSTCC achieved an OT equal to 4.055 s as the lowest value among others in Table 8. Consequently, the coordination problems may not exist, which means the selectivity is guaranteed. The optimized value of coefficient A has been chosen as 6 in this mode to obtain the lowest OT value in Table 8.

**Table 8.** The Overall OT for the radial IEC MG—Mode 3.

