Page 63 - Modern Control of DC-Based Power Systems

28 Modern Control of DC-Based Power Systems

Normally, a cascaded converter architecture is designed in such a way
v
that it delivers a defined voltage ^ out2 to the load under the assumption of
v
a predefined reference voltage ^ ref 1 . For the initial loop design the effect
v
of possible perturbations in the input voltage of the cascaded system ^ in1
and the load current ^ i Load2 are neglected. On further inspection of the
control block diagram in Fig. 2.9, it becomes clear that for the path ^v ref 1
to ^ out2 (marked in red (light gray in print version)) it is not necessary to
v
^ ^
consider the inductor currents ði L1 ; i Þ. The positive feedback loop con-
L2
Z out1 can be condensed to 1 which makes it
taining the block Z out1
Z IN CL2 1 1
Z IN CL2
possible to write the reference to output voltage transfer function (2.23)
for the first converter:
1 1
G C1 G vd1 Z out1
V m1 1 1
5 (2.23)
^ v out1 sðÞ Z IN CL2
1 1 H 1 1
^ v ref 1 sðÞ G C1 G vd1 Z out1
V m1 1 1
Z IN CL2
Inspecting the second converter loop yields the following equation:
1

v
^ v out2 sðÞ 5 ^v ref 2 sðÞ 2 H 2 ^v out2 G C2 G vd2 1 G vg2 ^ out1 (2.24)
V m2
From the Eqs. (2.19), (2.22) and (2.23) it can be clearly seen that there
exists interdependence on how the selection of parameters of the control-
lers G C1 and G C2 influence each other. It also can be seen how the
dynamics of the load supplied by the second converter influences the
voltage of the DC bus. This interdependence poses some interesting
questions on the nontrivial design process which was further elaborated
in [16].

2.4 LOAD MODELS

In this chapter the context of the canonical converters and their
cascading is extended to the use case of analyzing particular load types
which are connected on the POL converter. First, the CPL model is con-
sidered; afterwards, the presented methodology is used to analyze further
typical load characteristics which may be connected at a POL. This work
is related to the generalized load impedance model which was described