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THREE PHASE CONTROLLED RECTIFIERS

2.3.4 Comparison and discussion

The following table summarizes the main characteristics of the different control strategies.

TECHNIQUE

ADVANTAGES

VOC

Fixed switching Freq. (easier design input filter) Advanced PWM strategies can be used Cheap A/D converters

DPC

  • No separate PWM block

  • No current regulation loop

  • No coordinate transformation

  • Good dynamics

  • Simple algorithm

  • Decoupled active and reactive power control

  • Instantaneous variables with all harmonics components estimated (improve power factor and efficiency)

DISADVANTAGES

  • Coordinate transformation and decoupling between active and reactive components is required

  • Complex algorithm

  • Input power factor lower than for DPC

  • High inductance and sample frequency needed

  • Power and voltage estimation should be avoid at the moment of switching

  • Variable switching frequency

  • Fast microprocessor and A/D converters required

VFOC

  • Fixed switching Freq. (easier design input filter)

  • Advanced PWM strategies can be used

  • Cheap A/D converters

  • Coordinate transformation and decoupling between

active and reactive components is required

  • Complex algorithm

  • Input power factor lower than for VF DPC

VF DPC

  • Simple and noise resistant power estimation algorithm, easy to implement in a DSP

  • Lower sampling frequency than for DPC

  • Low THD of line currents with distorted or unbalanced supply

  • No separate PWM block

  • No current regulation loop

  • No coordinate transformation

  • Good dynamics

  • Simple algorithm

  • Decoupled active and reactive power control

  • Variable switching frequency

  • Fast microprocessor and A/D converters required

Table 2.3 : Control strategies comparison [9]

VOC vs. VFOC One of the only advantages of VFOC against VOC is that VFOC provides improved rectifier control under non ideal line voltage condition (because ac voltage sensorless operation is much less noisy thanks to the natural low pass behavior of the integrator used in the flux estimator).

Switching Frequency A FIXED switching frequency is required in our project mainly because it means : easier design (input filter for eg.) and the sample frequency can be lower. Only VOC or VFOC can provide this feature.

Power factor DPC has better power factor than VOC but has variable switching frequency. DPC also means complicated calculations for p and q (need differentiation of line current, risk of instability). A good power factor can be achieved with VF DPC and the method has lots of advantages but as we want a fixed switching frequency, VOC or VFOC should be chosen. If the switching frequency can be variable, the VF DPC strategy is obviously the best solution (but the cost can become a problem. The method need fast microprocessors and A/D converters).

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