Analysis of the effectiveness of non-unit protection methods within DC microgrids

The characteristic behaviour of physically compact, multi terminal DC networks under electrical fault conditions can produce particularly demanding protection requirements. This represents a significant barrier to more widespread adoption of DC power distribution for microgrid applications. Whilst a number of methods have been proposed in the literature for such networks, which are based around the use of non-unit protection, there are limitations to the levels of fault discrimination which can be achieved. To illustrate the challenges in effectively implementing non-unit protection, this paper analytically describes the current, voltage, di/dt, dv/dt and impedance profiles of a representative network, with particular emphasis on the response for varying fault resistances. The paper highlights that the non-unit methods analysed are sub-optimal where fault location is required to be discriminated over the transient period after fault inception. Therefore, it is concluded that to achieve high levels of fault discrimination within future DC microgrid systems, more robust protection approaches are required.