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

ABSTRACT:

Direct power control with space vector modulation (D PC-S V M) features simple structure, fast dynamic performance and little tuning work. However, conventional D PC-S V M can not achieve accurate power control under unbalanced grid conditions. A modified D PC-S V M is thus proposed for accurate power control under both ideal and unbalanced grid conditions. Though power control accuracy is improved when compared with conventional D PC S V M, 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

because 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 D PC-S V M in ideal grid requires only additional compensation of power reference. As a result, significantly improves the control performance without substantial increase of complexity. The simulation and experimental results validates the superiority of the proposed method over the prior D PC-S V M obtained on a two-level P WM voltage source rectifier.

KEYWORDS:

  1. Predictive power control
  2. Power compensation
  3. Unbalanced grid

SOFTWARE: MAT LAB/SIM UL INK

BLOCK DIAGRAM:

Fig. 1. Control diagram of the proposed D PC-S V M.

 EXPECTED SIMULATION RESULTS:

 

Fig. 2. Simulation results of U a b c, Pin, Pref , Q in, Q ref and I ab c for (a) the MD PC-S V M and (b) CD PC-S V M.

Fig. 3. Results of U a b c, Pin, Pref , Pout, U dc and I a b c for (a) MD PC-S V M-PC and (b) MD PC-S V M

Fig. 4. Simulation results of MD PC-S V M-PC when 50% voltage dip in phase A is suddenly applied.

Fig. 5. Results of MD PC-S V M-PC when both R and L in the controller are (a) 50% and (b) 150% of their actual value.

Fig. 6. Simulated results of MD PC-S V M-PC under one phase grounding fault.

CONCLUSION:

In  the existing literature, most studies on D PC-S V M were carried out under balanced grid voltage conditions. Under unbalanced grid voltage conditions, the steady-state performance of D P C-S  V M 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 DP C-S V M 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 grid side active power and reactive power.

DP C-S V M

The proposed DP C-S V M is compared to conventional DP C-S V M and simulation results confirmed its effectiveness. Due to additional calculation of power compensation, complexity of the proposed DP C-S V M is higher than conventional power control schemes. However, the proposed method completely eliminated  the twice grid voltage frequency oscillations in theory 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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