Protection Challenges in a Stand-alone Microgrid: Case Study of Tsumkwe Microgrid

This paper reports on the development of an effective overcurrent protection system for Tsumkwe standalone microgrid, in Namibia as currently it does not have a well-coordinated protection scheme. The Tsumkwe microgrid architecture comprises of a Solar PV plant and 1 Diesel Generator rated, 300kWp and 300 kVA respectively. Short circuit current analysis is a very important part of micro grid protection design and analysis. In this study the IEC 60909 method was used to determine the maximum fault current contributions by the static and rotational generators at all the busbars within the Tsumkwe Microgrid in DIgSILENT Powerfactory using the short circuit calculation tool. The results obtained from the simulation indicates that static and rotational generators contribute different fault currents to the microgrid depending on the rating of the sources which subject the microgrid protection system to different fault currents. Also, currently the Tsumkwe microgrid is only protected with traditional circuit breakers and thermal fuses. The current method of protection subjects the microgrid to unreliable coordination between the protection devices as well as longer fault clearing times which consequently risks costly microgrid equipment and operating personnel. To overcome these protection challenges, a relay coordinated overcurrent protection scheme using an SEL751A relay is proposed. The proposed overcurrent protection scheme will be compared to the current protection system under test cases to analyze its ability to offer reliable coordinated protection and fault clearing within the shortest possible time.