Input voltage waveform distortion & Three-phase converters
Input voltage waveform Distortion:
Fig: a single phase converter – circuit diagram Fig: Short circuit currents between incoming and outgoing SCRs for various trigger angles
- In a single-phase converter, a four-SCR fully-controlled converter operates into a load, which draws a constant current. The AC source includes the series (leakage) inductance LS.
- Waveforms are shown for (i) no overlap case (when LS = 0) and (ii) for a finite value of LS causing an overlap. It is evident from waveforms of ISCR 1,1’ ISCR 2,2’ that they take a finite time to rise and fall. In the intervening period all four SCRs are ON.
- The current in the incoming device rises till it equals the load current IL while that in the outgoing one falls to zero. All conducting SCRs can be considered to be short circuits and consequently the output voltage and thus also the input voltage is zero during this period.
- The output voltage is diminished and a ‘notch’ appears across the input. The input distortion affects other equipment connected to the same bus and protection must be provided against this cross-talk between two converters through this type of line distortion.
- The input voltage exhibits two notches in a single-phase converter both of which are identical and reach down to zero.
Fig: Three phase converters - circuit diagrams & waveforms
- In a three phase six-pulse converter, the triggering angle is α = 00. There are six notches per cycle. While two of the notches reach down to zero volts, the four other have different magnitudes.
- The three-phase converter has three inductances LS, each in series with each of the three phases. They are the leakage inductances of the transformer, which may supply other equipment of the plant too.
- The overlap time is dependent on the load current existing during the commutation period and also the voltage behind the short circuit current. This commutating voltage magnitude is dictated by the trigger angle.
- Thus for α = 00 this voltage is minimum. At α = 1800 too it would have been very low if successful commutation had been possible. However, without any allowance for an overlap time, the SCR current would just start to fall before it rises again.
- Note at α = 1800 the converter operates in the ‘inverter’ mode and if the outgoing SCR fails to turn off it is effectively triggered at α = 00 which pushes the converter from peak inversion to peak rectification mode.
- The resulting ‘commutation failure’ can cause severe short circuits. Thus the trigger angle must be restricted to values, which permit successful commutation of the SCRs.