Sammendrag
The method used to calculate insertion indexes plays
an important role in determining the overall performance of the
Modular Multilevel Converter (MMC). Direct voltage control,
which is the simplest option, results in a large circulating current
ripple because this modulation technique does not account for the
arm voltage ripples. This led to the development of compensated
modulation techniques that compensate for the arm voltage ripples
thereby eliminating the circulating current ripple. There are
two variants of compensated modulation: closed-loop and openloop.
The closed-loop version requires measurement of the arm
voltages without distortion and delay, which is difficult to achieve
in practice. The open-loop method overcomes this challenge by
using estimated arm voltages. However, accurate knowledge of
the system parameters is needed for effective removal of the
circulating current ripples. This is a limitation because the
parameters change with time and operating conditions. This
paper presents a modified version of the open-loop method, which
includes a scheme for correcting parameter errors online. The
method estimates the parameters, the arm capacitances and time
delay, by using feedback controllers acting on the circulating
current ripples. Mathematical derivation of the method, together
with its validation using simulation and experimental tests, is
presented in detail.
an important role in determining the overall performance of the
Modular Multilevel Converter (MMC). Direct voltage control,
which is the simplest option, results in a large circulating current
ripple because this modulation technique does not account for the
arm voltage ripples. This led to the development of compensated
modulation techniques that compensate for the arm voltage ripples
thereby eliminating the circulating current ripple. There are
two variants of compensated modulation: closed-loop and openloop.
The closed-loop version requires measurement of the arm
voltages without distortion and delay, which is difficult to achieve
in practice. The open-loop method overcomes this challenge by
using estimated arm voltages. However, accurate knowledge of
the system parameters is needed for effective removal of the
circulating current ripples. This is a limitation because the
parameters change with time and operating conditions. This
paper presents a modified version of the open-loop method, which
includes a scheme for correcting parameter errors online. The
method estimates the parameters, the arm capacitances and time
delay, by using feedback controllers acting on the circulating
current ripples. Mathematical derivation of the method, together
with its validation using simulation and experimental tests, is
presented in detail.