Скачать или смотреть Expt.3 Simulation of 6 pulse converter in rectifier mode

Aim: Simulation of 6-pulse converter.
(a) With R-load, R=20 Ω.
(b) With R-L-Load R=20Ω, L=100mH.
The three phase fully controlled bridge converter has been probably the most widely used power electronic converter in the medium to high power applications. Three phase circuits are preferable when large power is involved. Control over the output de voltage is obtained by controlling the conduction interval of each thyristor. This method is known as phase control and converters are also called "phase controlled converters"

In phase controlled rectifiers though the output voltage can be varied continuously the load harmonic voltage increases considerably as the average value goes down. Of course the magnitude of harmonic voltage is lower in three phase converter compared to the single phase circuit. Since the frequency of the harmonic voltage is higher smaller load inductance leads to continuous conduction. Input current wave shape become rectangular and contain Sth and higher order odd harmonics, A three phase fully controlled converter is obtained by replacing all the six diodes of an uncontrolled converter by six thyristors as shown in Fig.1 (a)
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. It can be argued as in the case of an uncontrolled converter only one device from these two groups will conduct. Then 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 TI → 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, T6TI. Each conduction mode is of 60° duration and appears in the sequence mentioned. The conduction table of Fig. 1 (b) shows voltage across different devices and the dc output voltage for each conduction interval If the converter firing angle is α each thyristor is fired "a" 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 table of fig. 1 (b).Similarly line currents can be drawn from the output current and the conduction diagram. It is clear from the waveforns that output voltage and current waveforms are periodic over one sixth of the input cycle as shown in fig.2. 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.


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