Capacitive Grounding for DC Distribution Grids with Multiple Grounding Points

Abstract

DC Microgrids could play an important role in a renewable and sustainable future. Unfortunately, there are not enough standards or guidelines for the implementation of DC Microgrids or DC distribution grids in general. Nevertheless, reliability and security of such a system must be assured at all times. As a consequence, an important challenge lies in terms of protection.

This thesis project focuses on capacitive grounding and their respective ground fault detection and protection scheme for a general DC distribution grid. The protection scheme proposed aims to discriminate currents caused by load shifts from ground fault currents with as little communication between the protective relays as possible.

The safety requirements needed for a DC distribution system are analyzed and threshold values for current that may endanger the human body are derived from literature. A scheme using capacitive grounding is analyzed further through a sensitivity analysis for capacitor size. This analysis serves as a starting point to determine an important challenge for the discrimination of currents caused by load shifts and human faults. A detection scheme is proposed to differentiate these currents and demonstrated with Matlab Simulink.

An analysis showing the behavior of the system to increasing number of grounding points using this detection scheme is further discussed and studied by means of state space representation. This analysis helps as a starting point for the scalability of DC distribution systems with capacitive grounding.