Sammendrag
This work studies the performance of the Modular
Multilevel Converter (MMC) under unbalanced conditions when
the internal circulating currents are controlled to follow a
reference value given by Lagrange-based optimization applied in
the abc frame. The Lagrange-based current reference calculation
is constrained to ensure that the MMC is providing constant, nonoscillatory,
power flow at the DC-side even the case the AC grid
voltage is unbalanced. Such operation can be achieved by the
investigated Lagrange-based control while either controlling the
differential currents of the MMC to have only a DC-component
or while minimizing the sum energy oscillations in each phase
of the MMC. The objective of preventing DC power oscillations
can also be achieved independently of the power control strategy
applied to control the three-phase currents on the AC side
of the converter. The operation of the MMC is studied with
three different objectives for the control of the AC currents:
1) Constant instantaneous three-phase power with sinusoidal
currents, 2) Balanced sinusoidal three-phase currents, and 3)
Constant instantaneous reactive power with sinusoidal currents.
The impact of these different AC power control strategies on the
oscillations of capacitor voltages and stored energy in the MMC
is then analyzed and discussed, verifying how the Lagrange-based
control is always able to keep the DC power flow free of second
harmonic oscillations.
Multilevel Converter (MMC) under unbalanced conditions when
the internal circulating currents are controlled to follow a
reference value given by Lagrange-based optimization applied in
the abc frame. The Lagrange-based current reference calculation
is constrained to ensure that the MMC is providing constant, nonoscillatory,
power flow at the DC-side even the case the AC grid
voltage is unbalanced. Such operation can be achieved by the
investigated Lagrange-based control while either controlling the
differential currents of the MMC to have only a DC-component
or while minimizing the sum energy oscillations in each phase
of the MMC. The objective of preventing DC power oscillations
can also be achieved independently of the power control strategy
applied to control the three-phase currents on the AC side
of the converter. The operation of the MMC is studied with
three different objectives for the control of the AC currents:
1) Constant instantaneous three-phase power with sinusoidal
currents, 2) Balanced sinusoidal three-phase currents, and 3)
Constant instantaneous reactive power with sinusoidal currents.
The impact of these different AC power control strategies on the
oscillations of capacitor voltages and stored energy in the MMC
is then analyzed and discussed, verifying how the Lagrange-based
control is always able to keep the DC power flow free of second
harmonic oscillations.