ABSTRACT:
Direct power control with space vector modulation (DPC-SVM)
features simple structure, fast dynamic performance and little tuning work.
However, conventional DPC-SVM can not achieve accurate power control under unbalanced
grid conditions. A modified DPC-SVM is thus proposed for accurate power control
under both ideal and unbalanced grid conditions. Though power control accuracy is
improved when compared with conventional DPCSVM, it still suffers highly
distorted grid current and DC voltage oscillations with an unbalanced network.
Therefore, a power compensation method is subsequently derived aiming at the
following targets: eliminating DC voltage oscillations, achieving sinusoidal
grid current and obtaining unity power factor. To that end, average grid-side
reactive power and oscillations in converter-side active power are controlled
as zero by simply adding a compensation to original power reference.
Additionally, the proposed method does not require extraction of positive
sequence or negative sequence component of grid voltage. Compared with
conventional DPC-SVM in ideal grid, only additional compensation of power
reference is required. As a result, control performance can be significantly
improved without substantial increase of complexity. The superiority of the proposed
method over the prior DPC-SVM is validated by both simulation and experimental
results obtained on a two-level PWM voltage source rectifier.
KEYWORDS:
1.
Predictive
power control
2.
Power
compensation
3.
Unbalanced
grid
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM:
Fig.
1. Control diagram of the proposed DPC-SVM.
EXPECTED SIMULATION RESULTS:
Fig.
2. Simulation results of Uabc, Pin, Pref , Qin, Qref and Iabc for (a) the
MDPC-SVM and (b) CDPC-SVM.
Fig.
3. Simulation results of Uabc, Pin, Pref , Pout, Udc and Iabc for (a) MDPC-SVM-PC
and (b) MDPC-SVM
Fig.
4. Simulation results of MDPC-SVM-PC when 50% voltage dip in phase A is
suddenly applied.
Fig.
5. Simulation results of MDPC-SVM-PC when both R and L in the controller are
(a) 50% and (b) 150% of their actual value.
Fig.
6. Simulated results of MDPC-SVM-PC under one phase grounding fault.
CONCLUSION:
In
existing literature, most studies on DPC-SVM were carried out under balanced
grid voltage conditions. Under unbalanced grid voltage conditions, the
steady-state performance of DPC-SVM are seriously deteriorated by exhibiting highly
distorted current and oscillations in the DC-link voltage. To cope with these
problems, this paper proposes a novel DPC-SVM method, which is able to work
effectively under both balanced and unbalanced grid conditions. An appropriate power
compensation is derived, which only requires the grid/converter voltages and
their delayed values. By adding this power compensation to the original power
references without modifying the internal control structure, constant DC-link
voltage and sinusoidal grid currents are achieved simultaneously without
affecting the average value of gridside active power and reactive power. The
proposed DPC-SVM is compared to conventional DPC-SVM and its effectiveness is
confirmed by the presented simulation and experimental results.
Due to additional calculation of power compensation,
complexity of the proposed DPC-SVM is higher than conventional power control
schemes. However, twice grid voltage frequency oscillations can be completely
eliminated in theory by the proposed method under unbalanced grid conditions,
which is beneficial to the lifetime and maintenance of capacitors. Although
using a larger capacitor can also reduce DC voltage ripples, it may increase
hardware cost and volume of the system. In this sense, the proposed method is
more suitable for the application where a high quality DC voltage is required under
unbalanced grid conditions.
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