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Master of Science Thesis in Electric Power Engineering - page 21 / 114

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THREE PHASE CONTROLLED RECTIFIERS

2.3.2 Direct Power Control and Virtual Flux Direct Power Control

The DPC method is similar to Direct Torque Control (DTC) for induction motor. Instead of torque and stator flux the instantaneous active and reactive powers are controlled. The following figure shows the scheme of the DPC method.

Figure 2.9 : Block scheme (DPC, VFDPC)[9]

The power estimation of DPC is based on the line voltage. An important disadvantages of the method id the need of current differentiation to estimate this power. Other relevant points can be found ([9] ) :

  • Need high sample frequency because the estimated value are changing all the time.

  • Need high inductance value because the switching frequency is not constant.

  • A non constant switching frequency means trouble to design an input filter.

  • Calculation of power and voltage should be avoid during switching (errors).

The virtual flux method is an improvement of Voltage Oriented Control (VOC). The virtual flux is an integration of line voltage uL. For the VF DPC, we can summaries the following characteristics.

  • No line voltage sensors required. Furthermore, a voltage sensor less line power estimation is much less noisy due to natural low pass behavior of the integrator.

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

  • Lower sampling frequency (as conventional DPC).

  • Sinusoidal line currents (low THD).

  • No separate PWM voltage modulation block.

  • No current regulation loops.

  • Coordinate transformation and PI controllers not required.

  • High dynamic, decoupled active and reactive power control.

  • Power and voltage estimation gives the possibility to obtain instantaneous variables with all harmonic components, which has an influence for improvement of total power factor and efficiency.

  • Easier calculations for p and q than voltage based method, no differentiation of line current.

The typical disadvantages of VF DPC are :

  • Variable switching frequency.

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