傳統配電系統為輻射式架構，故障電流為單一方向，其饋線保護方式為使用非方向性過電流電驛；但隨著分散式電源之開發與併入，可形成併網模式或孤島模式運轉之微電網，故障電流不再為單一方向，故其饋線之過電流保護需使用具備參考量與動作量之方向性過電流電驛，傳統上，參考量係透過比壓器取得電壓，造成成本上之增加。
當微電網運轉於孤島模式時，若故障點靠近比壓器位置，亦不能因電壓過低而導致方向判斷錯誤。基於此，本論文將開發以DSP為基礎僅利用電流訊號之演算法，適用於微電網饋線保護之電流型方向性過電流電驛。本演算法結合改良式離散傅立葉轉換之快速、準確濾波能力，可解決故障電流含有直流衰減成份之問題。因微電網中分散式電源的併入與現代化電子負載的日漸普及，都會產生諧波，為監測供電與用電品質，在本電驛上加入諧波分析之功能，最後透過Matlab/Simulink與Doble波形重現模擬器驗証本論文所研製的適用於微電網饋線保護之方向性過電流電驛的可行性。

Traditionally, the distribution system is radial type. The fault current presents in top-dowm direction; hence, a non-direction overcurrent relay may apply for feeder protection. Nowaday, Distributed Generations (DG) have been widely installed and fed into the distribution system. The system with DG may form as Microgrid and operate in Grid-Connected Mode or Islanded Mode. However, the fault current in Microgrid no longer presents in one-way. The directional function relay must be introduced instead of non-directional one. It will increase the installation cost since the additional Protecntial Transfomers (PT) are required to provide the reference quantity.
In case of the Microgrid operation in Island Mode, the fault location is close to the PT installation. The refrence quantity may be too low to provide the direction check for relay. To avoid this possibility a DSP-based Current-Only algorithm has been propsed. It can simply and easily apply feeder protection on Microgrids. Basically, the algorithm is base on modified DFT inherent in high calculation speed and fast filtering ability, the exponential decaying DC component in initial fault current can easily be removed within one more cycle. In addition, the more DGs and modern electronic loads in Microgrids the more harmonic issues need to be concerned. To monitor the power quality, the harmonic analysis function also be included in the proposed relay. Finally, the proposed relay is succeddfully tested and varified by Matlab/Simulink and Doble emulator. It concluded that the proposed relay is good for feeder protection in Microgrids.

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