ABSTRACT:
This article proposes a series compensator with
unbalanced voltage sag ride-through capability applied to grid connected induction
motors. A conventional three-phase voltage source inverter (VSI) is intended to
regulate the motor voltages. The VSI is connected in series with the grid and a
three-phase machine with open-ended windings. The proposed system is suitable
for applications in which no frequency variation is required, like large pumps
or fans. The VSI dc-link voltage operates as a floating capacitor through the
energy minimized compensation (EMC) technique, in which there is no dc source
or injection transformer. The motor load condition determines the minimum grid
voltage positive component (sag severity) to keep EMC operation. Meanwhile, a voltage
unbalance may increase the dc-link voltage requirements. A 1.5-hp four-pole
induction motor has been used to verify the ride-through capability of the
proposed compensator under grid voltage disturbances. A total harmonic
distortion (THD) analysis of grid currents demonstrates that the proposed
system provides low THD even if no passive filter is used. The operating
principle, converter output voltage analysis, pulse width modulation technique,
control strategy, and components ratings are discussed as well. Simulation and
experimental results are presented to demonstrate the feasibility of the
system.
KEYWORDS:
1.
Floating
capacitor
2.
Induction
motor
3.
Series
compensator
4.
Unbalanced
voltage sag
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM:
Fig.
1. Block diagram of feedback small-signal model.
EXPECTED SIMULATION RESULTS:
Fig.
2. Simulation waveforms at the steady state and half load with perphase grid (vga
) and load (vla ) voltages, as well as the converter’s
line-to-line voltage (vcab ).
Fig. 3. Simulation waveforms with the proposed series compensator under balanced voltage sag at half load. (a) Rated grid voltages. (b) Three-phase voltage sag of 80%.
Fig. 4. Simulation waveforms with the proposed series compensator under unbalanced voltage sag:Fd = 15%and half load. (a) Grid voltages and currents. (b) DC-link voltage, torque, and speed.
CONCLUSION:
The
proposed system has unbalanced voltage sag ride-through capability, being
suitable for grid-connected induction motors applications. Indeed, the
simulation and experimental results supported the theoretical analysis. A
conventional three-phase VSI using a floating dc-link capacitor has been
applied as a series compensator. Besides that, the proposed system does not require
any additional passive filter, injection transformer, or extra power supply. A conventional
three-phase H-bridge converter to compensate balanced grid voltage disturbances
has recently been proposed in the literature. Compared to the conventional solution,
the proposed one has a lower number of components, a single dc link, and can
deal with unbalanced voltages without a complex control strategy. The higher
dc-link voltage requirement of the proposed series compensator was highlighted
as its main drawback. Although the proposed solution provided higher THD of
grid currents, its levels were acceptable. Hence, the proposed system can be
easily integrated along with standard squirrel-cage induction motors when no
frequency variation is required.
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[2]
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[3]
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