The fully controlled converter can produce a reversible direct output voltage with output current in one direction, and in terms of a conventional voltage/ current diagram(Fig.1:) is said to be capable of operation in two quadrants, the first and fourth. Such a range of operation is useful for certain purposes, examples being the control of a dc torque motor, i.e. a motor used to provide unidirectional torque with reversible rotation (Fig.2), and a DC transmission link between two AC systems in which power can be transmitted in either direction according to the polarity of the voltage with current flows always in one direction. Equally a converter may be used under steady- stage conditions in the first quadrant only but transiently in the second quadrant in order to extract energy from the load quickly and thereby improve the response of the system to changing command signals.
If four-quadrant operation of a DC motor is required, i.e. reversible rotation and reversible torque, a single converter needs the addition of either a change - over contractor to reverse the armature connections or a means of reversing the field current in order to change the relationship between the converter voltage and the direction of rotation of the motor. Both of these are practicable in suitable phase and three-phase dual-converter using bridge type converter circuits. The output terminals of each converter having the same potential are connected together through a reactor. The four possible quadrants of converter operation thus resulting can be translated into four (steady-state) combinations of motor-torque and rotation as shown in Fig. 3
The basic principle of operation of dual converter can be explained with reference to the simplified equivalent diagram of the DC circuit shown in Fig.4. In this simplified representation, assumption is made that the dual converters are ideal and they produce pure DC output terminals. As shown in fig.4, each two-quadrant converter is assumed to be a controllable direct voltage source, connected in series with a diode. Diode D1 and D2 represent the unidirectional current flow characteristics of the converters. The current in load circuit can, however, flow in either direction.
In a dual converter without circulating current operating mode, the flow of circulating current is completely inhibited through automatic control of the firing pulses, so that only that converter which carries the load current is in conduction and the other converter is temporarily blocked. Since only one converter operates at a time and the other is in blocking state, no reactor is required between the converters.
At a particular instant, suppose converter 1 (fig.5 :) is operating as a rectifier and is supplying the load current while pulses to second converter are blocked. For the inversion operation, converter 1 is first blocked by removing its firing pulses and load current is reduced to zero. Converter 2 is made to conduct by applying the firing pulses to it. The current in converter 2 would now build up through the load in the reverse direction. So long as converter 2 is in operation, converter 1 is in the blocking state since the firing pulses are withdrawn from it.
The pulses to converter 2 are applied after a delay time (current-free safety interval) of 10 to 20ms. This delay time ensures reliable communication of thyristors in converter 1. If the converter 2 is triggered before the converter 1 has been completely turned-off, a large circulating current would flow between the two converters.
Irregular jumps in the level of the DC terminal voltage at the point of current reversal must be avoided in order to achieve a smooth change over of current from one converter to the other. Thus the firing pulse control should, ideally, be such that the mean DC terminal voltage of the converter 2, at the instant of current reversal, is the same as that of the converter 1.
From the above discussion, it becomes clear that such a mode of operation requires sophisticated control system which automatically blocks and unblocks the individual converters in accordance with the direction of load current suitably with a safety intervals.
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