ABSTRACT
Microgrid converters are required to have the
capability of both grid-tied mode and islanding mode operation. For this
dual-mode operation, large shunt capacitors are often used in the interfacing
converter output LCL filter, as it can help to stabilize supply voltage and to
reduce switching ripple pollutions to sensitive loads during autonomous
islanding operation. At the same time, this modification causes a few
challenges, including the low frequency harmonic distortions, the steady-state
tracking errors and the slow dynamic response, to the line current regulation
during grid-tied operation. To overcome these drawbacks, a modified weighted
average current controller is developed. First, to realize a fast line current
response, a deadbeat control of weighted average current is developed based on
a reduced-order virtual filter plant. Second, a grid voltage feed-forward term
is added to the weighted average current reference to mitigate the steady-state
line current tracking errors. Note that this compensation term is directly
added to the current reference, thus, it is very well decoupled from the
closed-loop current regulator. In addition, it can be seen that the low-order
line current harmonics caused by grid voltage distortion is inherently
compensated by this proposed corrective feed-forward control.
KEYWORDS:
1. Virtual filter
2. Deadbeat control
3. Weighted average current control
4. Active damping
5. LCL filter
6. Microgrid.
SOFTWARE: MATLAB/SIMULINK
CIRCUIT
DIAGRAMS:
Fig. 1. Diagram of a grid-tied
converter controlled by conventional weighted average current feedback.
Fig. 2. Diagram of the proposed control deadbeat
scheme with weighted average current feedback and line current tracking error
compensation.
EXPECTED SIMULATION RESULTS:
Fig.3. Performance of
the system using the proposed deadbeat control method (compensation term is
activated in 0.5sec). (from top to bottom: (1) grid voltage Vgrid;
(2) line current I2 ; (3) output current I1 ; (4) current
tracking errors ( Iref-I2).)
Fig. 4. Performance of
the system using the proposed deadbeat control method and the method in [14].
(from top to bottom: (1) grid voltageVgrid ; (2) line current I2;
(3) output current I1 ; (4) current tracking errors ( ).)
Fig. 5. Performance of the system
using the proposed method, operating in a distorted grid with grid impedance
variation
Fig. 6. Performance of the system using the proposed
deadbeat control method with feed-forward control. Grid frequency changes from
50Hz to 50.15Hz at 0.2sec. (from top to bottom: (1) Grid voltage Vgrid;
(2) line current I2 ; (3) output current I1; (4) weighted
average current I12 .)
Fig.
7. Performance of the system using the proposed deadbeat control method but
without feed-forward control. Grid frequency changes from 50Hz to 50.15Hz at
0.2sec. (from top to bottom: (1) Grid voltage Vgrid ; (2) line
current I2; (3) output current I1; (4) weighted average
current I12 .)
Fig. 8. Performance of the system using the PI
control for weighted average current regulation, with feed-forward control.
Grid frequency changes from 50Hz to 50.15Hz at 0.2sec. (from top to bottom: (1)
Grid voltage Vgrid ; (2) line current I2; (3) output
current I1; (4) weighted average current I12.)
CONCLUSION
An enhanced
current controller is proposed in this paper. The research work of this paper
is summarized here as:
1) In order to realize rapid control of converter current, the deadbeat
control is applied to regulate the weighted average current based on a virtual
filter plant.
2) The feed-forward compensator is developed to mitigate the steady-state
fundamental current tracking errors caused by conventional weighted average
current control.
3) The
frequency-selective capacitor leg current estimation is proposed and the
corresponding compensation term can be used to increase the robustness of the
converter against grid harmonic distortions. The design and implementation of
this compensator are highly decoupled from the closed-loop deadbeat current
regulator. Thus, both the current regulator and the compensator can be
independently designed.
REFERENCES
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