In the early days of electric power the use of DC power was promoted by Edison. The use of AC power was in competition to the use of DC power. AC power prevailed with the invention of the transformer, which allowed power to be carried over large distances. The increased use of AC power has occurred since the 1890s to the present day.
There has been significant technological development of AC systems as a result of research into AC arc faults, AC protection systems and AC fault interruption.
DC power has made a comeback in recent times with the use of power electronics and photovoltaic solar panels. The increased use of DC power has caused a need for DC technology which provides the same protection functions as that used in AC power systems.
A fundamental problem for DC power systems is the detection and interruption of DC arc faults.
DC arc faults pose a significant hazard in photovoltaic solar systems. This is firstly because of the characteristics of the DC arc and secondly because of the difficulty in isolating the fault.
The DC current does not have the zero crossing point characteristic of AC current. Therefore the characteristics of the DC arc fault are different to those of an AC arc fault. The lack of a zero crossing makes it is more difficult to interrupt the DC arc fault.
If a DC arc fault occurs, the next problem is how to detect the presence of the arc fault. In applications such as variable frequency drives, the DC arc fault may occur together with an AC arc fault. The AC fault level may be limited by the use of reactors or earthing transformers. However, these devices may not be suitable for the limitation of DC fault currents.
AC earth leakage protection relays may be used to detect an AC earth fault. The detection of a DC earth fault requires DC earth leakage protection relays.
The photovoltaic solar panel acts as a generator and an energy source. Exposure of the solar panel to sunlight produces a DC output voltage. The solar panel is switched off when the sunlight is removed.
The solar panel may be connected with many other panels in series into a solar array to produce a suitable DC output voltage. The presence of a DC arc fault either inside the individual panel or within the array can present a series hazard. The risk to the array and to the whole photovoltaic system from the DC arc fault depends on the fault scenario.
BESST is able to provide DC power system load flow and protection studies for solar power generation and power electronics applications.