Three phase fully controlled converters are very popular in many industrial applications particularly in situations where power regeneration from the dc side is essential. It can handle reasonably high power and has acceptable input and output harmonic distortion. The configuration also lends itself to easy series and parallel connection for increasing voltage and current rating or improvement in harmonic behavior. However, this versatility of a three phase fully controlled converters are obtained at the cost of increased circuit complexity due to the use of six thyristors and their associated control circuit. This complexity can be considerably reduced in applications where power regeneration is not necessary. In those case three thyristors of the top group or the bottom group of a three phase fully controlled converter can be replaced by three diodes. The resulting converter is called a three phase half controlled converter. Replacing three thyristors by three diodes reduces circuit complexity but at the same time prevents negative voltage appearing at the output at any time. Therefore the converter cannot operate in the inverting mode.
The three phase half controlled converter has several other advantages over a three phase fully controlled converter. For the same firing angle it has lower input side displacement factor compared to a fully controlled converter. It also extends the range of continuous conduction of the converter. It has one serious disadvantage however. The output voltage is periodic over one third of the input cycle rather than one sixth as is the case with fully controlled converters. This implies both input and output harmonics are of lower frequency and require heavier filtering. For this reason half controlled three phase converters are not as popular as their fully controlled counterpart.
Although, from the point of view of construction and circuit complexity the half controlled converter is simpler compared to the fully controlled converter, its analysis is considerably more difficult. In this lesson the operating principle and analysis of a three phase half controlled converter operating in the continuous conduction mode will be presented.
Fig. 1 shows the circuit diagram of three phase half controlled converter supplying an R-L-E load. In the continuous conduction mode only one thyristor from top group and only one diode from the bottom group conduct at a time. However, unlike fully controlled converter here both devices from the same phase leg can conduct at the same time. Hence, there are nine conducting modes as shown in Fig. 2.
Now consider the conducting and blocking state of D2. In the blocking state the voltage across D2 is either vac or vbc. Hence, D2 can block only when these voltages are negative. Taking vbc as the reference phasor (i.e., bcLv= 2Vsinωt) D2 will block during 2∏/3ωt2∏≤≤ and will conduct in the interval 0ωt2π/3≤≤. Similarly it can be shown that D4 and D6 will conduct during 2∏/3ωt4∏/3≤≤ and 4∏/3ωt2∏≤≤ respectively.
Next consider conduction of T1. The firing sequence of the thyristor is T1 → T3 → T5. Therefore before T1 comes into conduction T5 conducts and voltage across T1 is acLv= 2Vsin (ωt + ∏/3). If the firing angle of T1 is α then T1 starts conduction at ωt = α - π/3and conducts upto α + ∏/3. Similarly T3 and T5 conducts during α + ∏/3ωtα + ∏≤≤ and α + ∏ωt2∏ + α - ∏/3≤≤. From this discussion the following conduction diagrams can be drawn for continuous conduction mode.
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