**5. Conclusions**

Simulator development is essential when developing or testing new protective relays because it is time consuming and costly to verify performance on the real system; moreover, the test space is limited. To solve these real-world engineering problems, this paper presented and validated the modern testing simulator for protective relay in the TPSS. The developed simulator has the appropriate amplification rate to inject analog signals into the integrated relay and have a function to input external COMTRADE files and generate the simulator's self-waveform. It was also confirmed that the relay behaves the same as in the actual system under various contingency fault simulations, and the harmonics generation function also works accurately through the relay's malfunction test.

As a result, by using this proven digital simulator, it is possible to replicate in the laboratory electrical faults that have been detected in the real field without running the test on the real infrastructure, which can significantly speed up testing. The simulator developed in this study is expected to be used in the future to develop new protective relays or validate protective relay algorithms.

**Author Contributions:** J.K. (Joorak Kim) conceived and designed the main concept; J.K. (Joorak Kim), J.K. (Jaewon Kim), and G.-J.C. performed test and simulation; J.K. (Joorak Kim) and J.K. (Jaewon Kim) wrote the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Ministry of Land, Infrastructure and Transport's Railway Technology Research Project, "Development of Real Time Simulator and Analysis Model for Railway Power System (19RTRP-B146034-02)".

**Acknowledgments:** The authors are grateful for the support by Korea Agency for Infrastructure Technology Advancement and Korea Railroad Research Institute.

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