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Master of Science Thesis in Electric Power Engineering - page 18 / 114

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THREE PHASE CONTROLLED RECTIFIERS For ideal, positive sequence space vectors (voltage is reference)

and

where

and

are amplitude, the active power is given by

3

3

3 2

(2.23)

Where design.

and

are rms values. This previous relation will be use in the DC link voltage controller

2.2.3 Limitations MINIMUM DC LINK VOLTAGE

For proper operation of the rectifier, a minimum dc link voltage is needed to obtain undistorted current waveforms. To have a full control of the rectifier, its six diodes must be polarized negatively at all value of ac voltage supply. To keep the diodes blocked, we need to ensure a dc link voltage higher than the peak dc voltage generated by the diodes alone. Theoretically for diode rectifier, the maximum dc output voltage is the peak value of line to line RMS voltage [9] .

√2

√2 √3

(2.24)

VDC LINK

VDIODE RECTIFIER

15 20%

Figure 2.5 : DClink voltage condition

It will be better to select a DC link voltage about 15 20% more than √2

.

IMPORTANT : The previous voltage line impedance taking in account here.

correspond to the converter voltage (Us). There is no

Nevertheless, if there is no line impedance (R = 0, L=0H) we can continue to write the equation

(2.24) according to the amplitude of supply voltag √2 √3

e Em :

√3

(2.25)

ATTENTION : This is a true definition but doesn’t apply in all situation [9] . The DC link voltage depends on the PWM method. In our case, we will use a sinusoidal PWM. In this case the maximum reference voltage is Vdc/2 (Figure 2.6 [23] ).

9

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