  We consider a DC electricity grid composed of transmission lines connecting
power generators and consumers at its nodes. The DC grid is described by
nonlinear equations derived from Kirchhoff's law. For an initial distribution
of consumed and generated power, and given transmission line conductances, we
determine the geographical distribution of voltages at the nodes. Adjusting the
generated power for the Joule heating losses, we then calculate the electrical
power flow through the transmission lines. Next, we study the response of the
grid to an additional transmission line between two sites of the grid and
calculate the resulting change in the power flow distribution. This change is
found to decay slowly in space, with a power of the distance from the
additional line. We find the geographical distribution of the power
transmission, when a link is added. With a finite probability the maximal load
in the grid becomes larger when a transmission line is added, a phenomenon that
is known as Braess' paradox. We find that this phenomenon is more pronounced in
a DC grid described by the nonlinear equations derived from Kirchhoff's law
than in a linearised flow model studied previously in Ref. \cite{witthaut2013}.
We observe furthermore that the increase in the load of the transmission lines
due to an added line is of the same order of magnitude as Joule heating.
Interestingly, for a fixed system size the load of the lines increases with the
degree of disorder in the geographical distribution of consumers and producers.
