Operating principle of 3 phase fully controlled bridge converter
Operating principle of a 3 phase fully controlled bridge converter:
Fig: 3 phase fully controlled bridge converter (a) circuit diagram (b) conduction Table (c) phaser diagram of line voltages
Fig: waveforms of 3 phase fully controlled converter: Analysis of converter in the rectifier mode
- A three phase fully controlled converter is obtained by replacing all the six diodes of an uncontrolled converter by six thyristors. For any current to flow in the load at least one device from the top group (T1, T3, T5) and one from the bottom group (T2, T4, T6) must conduct.
- From symmetry consideration it can be argued that each thyristor conducts for 120° of the input cycle. Now the thyristors are fired in the sequence T1 → T2 → T3 → T4 → T5 → T6 → T1 with 60° interval between each firing. Therefore thyristors on the same phase leg are fired at an interval of 180° and hence cannot conduct simultaneously.
- This leaves only six possible conduction mode for the converter in the continuous conduction mode of operation. These are T1T2, T2T3, T3T4, T4T5, T5T6, T6T1. Each conduction mode is of 60° duration and appears in the sequence mentioned.
- The conduction table shows voltage across different devices and the dc output voltage for each conduction interval. The phaser diagram shows each of these line voltages can be associated with the firing of a thyristor with the help of the conduction table-1.
- For example the thyristor T1 is fired at the end of T5T6 conduction interval. During this period the voltage across T1 was vac. Therefore T1 is fired α angle after the positive going zero crossing of vac. To arrive at the waveforms it is necessary to draw the conduction diagram which shows the interval of conduction for each thyristor and can be drawn with the help of the phaser diagram.
- If the converter firing angle is α each thyristor is fired “α” angle after the positive going zero crossing of the line voltage with which it’s firing is associated. Once the conduction diagram is drawn all other voltage waveforms can be drawn from the line voltage waveforms and from the conduction.
- Similarly line currents can be drawn from the output current and the conduction diagram. It is clear from the waveforms that output voltage and current waveforms are periodic over one sixth of the input cycle. Therefore this converter is also called the “six pulse” converter. The input current on the other hand contains only odds harmonics of the input frequency other than the triplex (3rd, 9th etc.) harmonics.