DUAL CONVERTERS IN DC DRIVES
In many variable-speed drives, the four-quadrant operation is required, and three-phase dual converters are extensively used in applications up to 2 MW level. Figure shows a three phase dual converter, where two converters are connected back-to-back.
In the dual converter, one rectifier provides the positive current to the load, and the other the negative current. Due to the instantaneous voltage differences between the output voltages of the converters, a circulating current flows through the bridges. The circulating current is normally limited by circulating reactor,
LD, as shown in figure. The two converters are controlled in such a way that if a+ is the delay angle of the positive current converter, the delay angle of the negative current converter is a- = 180° - a+.
The figure shows the instantaneous dc voltages of each converter, vD+ and vD-. Despite the average voltage VD is the same in both the converters, their instantaneous voltage differences given by the voltage vr, generates the circulating current ir, which is superimposed with the load currents iD+ and iD-.
To avoid the circulating current ir, it is possible to implement a “circulating current free” converter if a dead time of a few milliseconds is acceptable. The converter section not required to supply current remains fully blocked. When a current reversal is required, a logic switch-over system determines at first the instant at which the conducting converter’s current becomes zero.
This converter section is then blocked and the further supply of gating pulses to it prevented. After a short safety interval (dead time), the gating pulses for the other converter section are released.
This converter section is then blocked and the further supply of gating pulses to it prevented. After a short safety interval (dead time), the gating pulses for the other converter section are released.
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