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Monday 31 December 2018

Direct Power Control of Pulse Width Modulated Rectifiers without DC Voltage Oscillations under Unbalanced Grid Conditions



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.
REFERENCES:

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