Grid current-feedback active damping for lcl resonance in grid-connected voltage-source converters

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Grid-Current-Feedback Active Damping
for LCL Resonance in Grid-Connected
Voltage-Source Converters
Introduction:
CL resonance has always been an important concern for LCL-
filtered voltage-source converters. A wide variety of resonance
damping techniques have thus been developed with the trend
generally favoring active damping techniques because of the
additional power losses experienced by passive damping techniques.
Active damping techniques can also be broadly divided into those
realized by cascading a digital filter with the current controller, and
those realized by feeding back the filter state variables. The former
represents a sensor-less technique realized by plugging in a digital
filter, which unfortunately, is sensitive to parameter uncertainties and
variations. It is, therefore, not as popular as the feedback of filter state
variables for damping purposes.
Existing system:
The state variables fed back, the filter capacitor current has been
widely chosen, and is usually realized with a proportional resistive
gain. The proportional resistive gain is, however, easily modified by
transport delays found in a digital system. The outcome is a negative
virtual resistance, which upon introduced, will add open-loop right-
half-plane (RHP) poles to the system. These open-loop poles will, in
turn, introduce a non-minimum-phase closed-loop behaviour to the
system. Such a non-minimum has behaviour has subsequently been

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resolved by replacing the usual proportional gain with a high-pass
filter (HPF) along the capacitor current feedback loop. The capacitor
current must, however, still be measured, which in practice, will
demand an additional sensor or a complex software-based observer
Disadvantages:
 Positive virtual resistance, is not implementable because of the
possible noise amplification.
 Lag to non-minimum-phase response of the system
Proposed system:
An impedance-based analysis in s-domain for generalizing
physical circuit property of grid current feedback active damping is
analysed. The analysis specifically demonstrates that the grid current
active damping is equivalent to the insertion of virtual impedance in
parallel with the grid-side inductance. In case of an HPF with a
negated output along the damping path, the virtual impedance can
further be notated as a series RL branch in parallel with a negative
inductance. The resistive part of the RL branch may become negative
when influenced by transport delays, which may then cause non-
minimum-phase response that can impair the overall system stability
and robustness. The non-minimum-phase problem can, however, be
minimized by the negative virtual inductance in parallel with the
series RL damper. This mitigation effect is not inherited by other
existing active damping techniques based on the feedback of the filter
capacitor current, and has presently not been discussed in the
literature. It is thus the intention of this paper to study the combined
negative resistive and inductive effects introduced by the grid current
active damping.

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Advantages:
 Lead to no minimum-phase response of the system
 Overlooked presence of negative virtual resistance.

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Block diagram:
AC Unbalanced
Load
Three phase
inverter
Input DC
supply
Gate driver circuit 12 V
DC
Buffer circuit
Micro-controller
circuit
5V
DC
Supply
LCL filter
Z-network

Grid current-feedback active damping for lcl resonance in grid-connected voltage-source converters

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Grid current-feedback active damping for lcl resonance in grid-connected voltage-source converters