Page 43 - Modern Control of DC-Based Power Systems

8 Modern Control of DC-Based Power Systems

r L
d I

E C V P/V

Figure 1.7 Constant power load fed by a DC DC converter.

the stability of the system in the design of the control laws, with the
exception of Ref. [10] where an adaptive state feedback was implemented
which can handle disturbances up to a certain level. For example, Ref.
[35] relies on the passivity properties of the system, while Refs.
[36,38,39] use sliding mode control to increase the stability of the system.
A boundary controller is applied in Ref. [40] to stabilize a CPL circuit.
This boundary controller implements a state dependent switching. In
Ref. [39] the authors use a nonlinear sliding surface control law based on
state dependent switching to stabilize a microgrid.
The application of a synergetic control is presented in Refs. [41] and
[42], while stabilizing CPL circuits. This nonlinear control has similarities
with sliding mode, but offers an additional parameter T which defines the
speed of convergence toward the manifold. Both sliding mode control
and synergetic control exhibit the model order reduction characteristic
while the system is on the sliding surface/manifold. One main difference
between the synergetic control and sliding mode control is that the syner-
getic control operates with a fixed switching frequency.
In Refs. [34] and [37] the authors propose the implementation of a
gain which directly compensates the nonlinear term in the CPL and
therefore cancels out the source of instability. This technique is called
Linearization via State Feedback, and consists of injecting through the
converters a signal on their output voltage, which is capable of compen-
sating the nonlinearity of the CPL. Other methods and techniques are
also presented in Refs. [43] and [44].
In these methods it is not necessary to perform an adjustment on the
CPL, either in damping, impedance change, or power change, to ensure
the stability of the system. In those types of control the DC bus voltage
response will be controlled to meet the constraints imposed by the CPLs
for maintaining stability. Nevertheless, these methods only apply to cases
of systems containing a single DC DC converter and one CPL con-
nected to the bus.