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Control Approaches for Parallel Source Converter Systems 123

Figure 5.6 ISPS I L15L25L3 LSF for ideal CPL Step 30.9 - 53.4 MW Synergetic
Control.

5.2 SYNERGETIC CONTROL
In this section, a centralized nonlinear control method is considered
and later applied to MVDC system. It was developed by Kolesnikov [12]
and called synergetic control. This control technique has found successful
application in the stabilization of CPLs in [13 16]. In this control the
nonlinear part of the controlled system is not compensated as the LSF
was in Section 5.1.1, but a nonlinear control is designed which uses a
model of the system and forces the system onto a manifold. Once the sys-
tem is on the manifold the order is reduced [17]. For switching conver-
ters, the control variable is the state δ of the active switch, which has two
states, on (or 1) and off (or 0); they are therefore variable-structure sys-
tems. The synergetic control applies to the converter the duty cycle d
which is the averaging of the two states over time. It uses constant
switching frequency; which makes it appealing to implement in power
converters. The Synergetic control uses a model of the system to be con-
trolled in order to synthesize the control law. This approach follows the
internal model principle which says that an accurate control can only be
achieved if the control system contains some representation of the process
to be controlled [18]. In this chapter, the procedure of synergetic control
design with the resulting synergetic control law is presented in
Section 5.2.1. The application of the technique to a multiconverter test