Effect of Source Inductance - Power Electronics

What is the Effect of Source Inductance?

Most of the converter analysis is generally simplified under the ideal conditions (no source impedance). But this statement will not be justified as source impedance is basically inductive with a small resistive element.

Source inductance will have major impact on the converter performance as its existence will change the output voltage of the converter. Consequently, output voltage will be decreased when the load current decreases. Additionally, input current and output voltage waveforms will be changing considerably.

Source inductance effect on a converter can be analyzed in below two ways.

Effect on Single Phase

Assume that the converter is operating in conduction mode and the ripple from the load current is going to be negligible. Here, open circuit voltage will become equal to average DC output at a firing angle of α. Below diagram describes a fully controlled converter with the source in single phase. Thyristors T3 and T4 will be assumed to be in conduction mode when t = 0 and alternatively, T1 and T2 fire when ωt = α

effect_on_single_phase

Here

  • Vi = input voltage
  • Ii = input current
  • Vo = output voltage
  • Io = output voltage

When the source inductance is not available, commutation will be occuring at T3 and T4 and the thyristors T1 and T2 will be switched ON instantly. This will result in the input polarity for changing immediately. When s is available, polarity change and commutation will not occur immediately. Therefore, T3 and T4 will not be commutating as soon as T1 and T2 are switched ON.

At some point of time all the four thyristors will conduct and this conducting interval is known as the overlap interval (μ).

Overlap at the time of commutation will reduce the DC output voltage and the angle of extinction γ will result in failed commutation when αis close to 180°. This is shown by the waveform below.

waveform.jpg

Effect on Three Phase

Similar to the single-phase converter, there will not be any immediate commutations because of the presence of the source inductances. Considering source inductances, the effects (qualitative) on the converter performance will be same as in a single phase converter and this is shown in below diagram.

effect_on_three_phase.jpg

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