A Comparison of Fault Behaviour of Bipolar vs. Unipolar LVDC Grids

Over the past years, there has been an increasing scientific interest in LVDC microgrids as an alternative for AC microgrids. Ongoing research mainly focusses on the two-wire unipolar architecture and, although the bipolar structure may offer several advantages, it is often regarded as just a double version of its unipolar equivalent. This paper compares the behaviour of unipolar and bipolar LVDC microgrids under fault conditions to demonstrate that this generalisation is not always valid. The differences in the fault behaviour of a unipolar and bipolar structure are first introduced by calculating the first fault transient in a fundamental example of both structures. Subsequently, PSCAD simulations of faults in a unipolar and bipolar LVDC microgrid are compared to demonstrate the differences between the two structures in a realistic LVDC microgrid context. These simulations are then applied to show the potential impact of generalising conclusions from one structure to the other on the protection algorithm of the LVDC grid. It is shown that a protection algorithm designed for one of the structures does not necessarily work in the other structure. Finally, an experimental validation of the differences observed regarding the fault behaviour of both structures is presented.