Page 42 - Modern Control of DC-Based Power Systems

Overview—Voltage Stabilization of Constant Power Loads 7

Inequalities [21] or the Brayton Moser Criterion [24]. The authors in
[21] studied the influence of a “virtual capacity” on the large signal stabil-
ity. They use the Takagi Sugeno (TS) multimodel theory which permits
to obtain a set of linear models whose fuzzy weighted combination is
equivalent to the original nonlinear system [25].
Other methods available in the literature may present the implementa-
tion of a linear correction as it has been presented in Refs. [20,26].In
these cases, the impact of the stabilizing action on the stability of the new
system is assessed by studying the influence of the control on the input
impedance of the load.
Another approach has been proposed in Ref. [17] where an
input output linearization of the original nonlinear system was intro-
duced. The authors claim that the advantage of this method is to offer a
stabilizer whose size is independent of the operating point, thus ensuring
the overall stability of the system.
Another approach relies on advanced load shedding techniques to
assure stable operation of electric power systems during inadequate gener-
ation situations or voltage disturbances [27]. A real-time adaptive Load
Shedding Scheme was proposed in Refs. [28,29] to consider a wide range
of perturbations using real-time data. This decentralized action will be
triggered by a control center which has knowledge about how vital cer-
tain loads are for operation. Most real-time load shedding techniques
guarantee a fast response to the system instability [30].

1.4 COMPENSATING CPLS WITH DC BUS CONTROL
When the control of a DC DC converter which feeds the CPL
can be modified, a so-called generation side control is possible (see
Fig. 1.7). In this case, the system to be stabilized possesses a new control
variable—the duty cycle d. It is then possible to integrate the constraints
on the stability of the system in the control of the upstream DC DC
converter. The idea of this method was introduced for the first time sev-
eral years ago while considering the small signal stability of the system
[31,32] but there are also applications of this idea which account for the
large-signal stability [33].
More recently, this work has been complemented by new control
approaches dealing with large signal stability of these systems
[10,22,34 44]. These proposed methods of nonlinear control incorporate