Page 268 - Modern Control of DC-Based Power Systems

Simulation 229

hand, when selecting the ideal CPL model, the interaction between the
generation side and load side converter is neglected, and the dynamics of
the generation side are projected on the load. Thus, the ideal CPL model
does not necessarily represent the worst case in terms of system stability as
explained in [12].
A very interesting case shows the PI-controlled POL, which due to
the missing derivative part is slower than the PID control; therefore, the
load is not as tightly controlled as with the PID; in this case the Adaptive
Backstepping controller performs very differently, this is because it uses a
load model which differs too much from the imposed behavior.

6.2 SHIPBOARD POWER SYSTEM
A shipboard power system consists typically of many LRC in paral-
lel which supply the common MVDC bus as depicted previously in
Figs. 2.1 and 6.3. Prior to the simulation, it is necessary to mention
which load sharing strategy was chosen in shipboard power systems.

6.2.1 Load Sharing
The strategies used in parallel converter systems for load current
sharing can be categorized into droop control and active current shar-
ing. Active current sharing can be either master slave or democratic
current sharing. The main difference between the master slave and
democratic methods is providing a variable or fixed ratio of current
during transient and static operation [5];and thedifferencebetween
droop and active current sharing is that droop uses r d as a virtual
resistance for current sharing while active current sharing uses the
voltage of the source, hence, for realizing droop the converters are
controlled independently [4].
In this study, democratic current sharing is applied in synergetic
control. While droop was used for all other stabilizing control
methods.
The effect of the droop control is to create a virtual voltage drop on
the converters output proportional to the converters output current [13],
I
so the controller increases or decreases the setpoint by r dk k , to maintain