Page 133 - Modern Control of DC-Based Power Systems

Background 97

V V

V
V 0 0

I I ref I
Figure 3.5 V I characteristic of an ideal voltage source (left) and real voltage source
droop curve (right).

the natural internal resistance of the source [6]. The resulting curve is also
called V I-characteristic or Droop-characteristic and is referred to as
such in this work. In a nonideal source, the droop characteristic is a slop-
ing straight line passing through a reference point, often referred to as a
set point. This point specifies how much current or power is to be gener-
ated at the nominal voltage. The slope of these straight lines corresponds
to the serial output resistance, as shown in Fig. 3.5.
To explain more precisely the natural load distribution of several paral-
lel converters with steep droop characteristic, a simple example network
as shown in Fig. 3.6 is used. It consists of two converters Conv1 and
Conv2, which are connected in parallel to each other on a busbar. The
voltage on the busbar is V 0 , and a load is connected to the same busbar
that requires the current I 0 5 P load . In this example cables are treated as
V 0
resistances.
If both voltage sources have exactly the same droop characteristic,
both will deliver the same current I 0 under load. However, if the droop
2
characteristic is different, e.g., at different operating points of the sources,
the load distribution is as shown in Fig. 3.7.
Due to the unequal reference power, i.e., the generated power at
nominal voltage, the droop characteristics of the sources are shifted rela-
tive to each other. In this example, this leads to an unequal distribution
of the supplied currents with the voltage V 0 . Fig. 3.7 also shows that the
slope of the droop characteristic determines the load distribution. The
steeper the curve is, the narrower the currents I 1 and I 2 are. However, a
steep droop characteristic also has disadvantages: due to the steepness, the
voltage regulation of the sources becomes worse [7]. From this consider-
ation it follows that in the case of the voltage droop control, a compro-
mise between voltage regulation and load sharing has to be made.