Abstract
This paper presents a control system implementation
in dqz-coordinates for equalizing the average energies
stored in each arm of a Modular Multilevel Converter (MMC);
a control objective that is typically referred to as horizontal
and vertical energy balancing. The proposed control scheme is
obtained from analysis and simplification of a detailed timeinvariant
dqz-frame state-space representation of the MMC. The
state variables of the model are the equivalent arm capacitor
energies and the current components, and it will be shown
that this representation is very suitable for designing outer-loop
energy controllers in dqz coordinates that rely on linear inner
current control loops. Moreover, a series of justified assumptions
on the energy dynamics will be presented, providing significant
insight that simplifies the control design. Finally, by proving that
the unbalances of the average values of the converter equivalent
arm capacitor energies in abc coordinates appear as undesired
oscillations in dqz coordinates, active filtering is proposed as
a mean to dissipate them and, therefore, achieve the desired
balanced operation. Operation of the proposed control strategy
is demonstrated by time-domain simulation of a 1 GW MMCbased
HVDC converter terminal.
in dqz-coordinates for equalizing the average energies
stored in each arm of a Modular Multilevel Converter (MMC);
a control objective that is typically referred to as horizontal
and vertical energy balancing. The proposed control scheme is
obtained from analysis and simplification of a detailed timeinvariant
dqz-frame state-space representation of the MMC. The
state variables of the model are the equivalent arm capacitor
energies and the current components, and it will be shown
that this representation is very suitable for designing outer-loop
energy controllers in dqz coordinates that rely on linear inner
current control loops. Moreover, a series of justified assumptions
on the energy dynamics will be presented, providing significant
insight that simplifies the control design. Finally, by proving that
the unbalances of the average values of the converter equivalent
arm capacitor energies in abc coordinates appear as undesired
oscillations in dqz coordinates, active filtering is proposed as
a mean to dissipate them and, therefore, achieve the desired
balanced operation. Operation of the proposed control strategy
is demonstrated by time-domain simulation of a 1 GW MMCbased
HVDC converter terminal.