**6. Conclusions**

Conventional GILs are comprised of a hollow conductor which, owing to its intrinsic rigidity, restricts several application perspectives of conventional GILs specifically in metropolitan areas. Thus, the incorporation of structural flexibility in GILs is essential in order to curtail the operational intricacies of conventional GILs. In this research, FGIL models based on flexible simple stranded and flexible field graded stranded conductors were developed and analyzed regarding electrostatic and dielectric aspects through simulation and experimental assay.

Simulation results revealed that the simple stranded conductors had regions of objectionably high electric fields which ultimately resulted in 31% and 23% degradation of the FUF regarding circular strand and trapezoidal strand conductors respectively in comparison to the conventional GIL. Thus, simple stranded conductors may result in dielectric breakdown due to their surface irregularity, and require contour stress minimization. Possible solutions regarding stress minimization include enclosure enlargement and the suppression of conductors' irregularity. However, enclosure enlargement significantly deviated the FUF of the FGIL from its allowable range, which is highly objectionable according to GIL standards. Thus, field-graded stranded-conductor-based FGIL models were developed and analyzed through simulation and experimental investigations.

Simulation results revealed that SiC-coated stranded conductors resulted in the achievement of analogous electrostatic characteristics compared to the conventional GIL, with a trivial deviation of 7.2% and 1.8% in the FUF for circular strand and trapezoidal strand conductors, respectively, which are quite acceptable per the allowable FUF range for GILs. Further, critical comparison regarding dielectric aspects revealed that the breakdown voltage for the proposed scheme was approximately 23% above the required standard BIL value for GILs. In addition, electric fields for the proposed scheme regarding standard BIL voltage were approximately 38% below the critical field value and were well within the standard allowable range for electric fields in GILs.

Additionally, experimental investigations of fabricated FGIL models revealed that in comparison to the simple stranded-conductor-based model, the field-graded stranded-conductor-based model

exhibited approximately 10–20% and 5–15% higher discharge voltages in power frequency and lightning impulse discharge tests. Moreover, the (*E*/*P*)*Breakdown* for the fabricated pliable models were observed to be relatively lesser than the (*E*/*P*)*Critical* at (α – η) = 0 for the respective dielectric gases.

Consequently, simulation and experimental analysis revealed that the proposed conductor scheme could facilitate the achievement of the required dielectric and electrostatic characteristics for FGILs as described by GIL standards. However, the next step of this research is to perform similar high-voltage investigations on a full-scale 132-kV FGIL demonstrator.

**Author Contributions:** Conceptualization, T.I. and A.A.Q.; Data curation, M.J.A. and H.S.K.; Formal analysis, M.J.A.; Investigation, M.J.A. and A.S.; Methodology, M.J.A.; Project administration, T.I. and A.A.Q.; Software, M.J.A. and H.S.K.; Writing—original draft, M.J.A.; Writing—review and editing, T.I., A.A.Q., and A.S.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
